describe las caracteristicas del silicio poroso

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2. Semicond. Sci. Technoi. 10 (1995) 1187-1207. Printed in the UK
~ ~ ~
TOPICAL REVIEW
Porous silicon
B Hamilton
Department of
M60 IQD, UK
Physics, UMIST, PO Box 88, Sackville Street, Manchester
Received 5 January 1995, accepted for publication 17 March 1995
Abstract. This paper attempts to review the field of research into light emission
from porous silicon. The driving force behind such research is the tantalizing goal
of adding optoelectronic functions to the already impressive array of electronic
functions provided by silicon-based devices. A silicon technology with included light
emission would move even closer to complete dominance of the electronics
market. After several years of research effort. the fundamental mechanisms of light
emission are still not completely resolved. This is not surprising: porous silicon has
many attributes of a new and complex material, and its study requires a truly
interdisciplinary effort involving electrochemistry, surface science, structural and
chemical microscopy on the atomic scale and detailed optical spectroscopy. This
paper tries to connect these various threads; inevitably what emerges will only
serve as a rather selective 'snapshot' of a still developing and often perplexing field.
1. Introduction
The dominance of silicon in the electronics industry
is almost complete, at least in terms of volume: the
worldwide market for silicon-based devices and systems
depending on them is huge. The comparatively small
but important markets which silicon does not^ fulfil are
those of ultra high-speed devices and optoelectronics, in
particular optical communications. In fact virtually all
optoelectronic functions requiring high-speed modulation
rely on compound semiconductor devices; fibre-optic-based
optical communications systems rest firmly on InP-based
lasers and modulators, whereas GaAs-based devices supply
the near-infrared and visible emission required for short-range
communication and disc redwrite functions.
It is a curious fact that although silicon is the
material which essentially fed the information technology
revolution, much of the highly successful international
research effort into semiconductor physics during the past
15 years has been devoted to II-V semiconductors. The
search for novel physical phenomena based on reduced
dimensionality-superlattice, quantum well and latterly
quantum wire and dot structures-has been a major driving
force for condensed matter physics. Improved device
functionality has emerged both in the fields of high-speed
transport and optics, which have strengthened the III-V
industry in these areas.
A picture emerges of a silicon industry dealing with
a rather mature technology, able to fulfil many of the
growing demands of an information-dependent culture.
Materials-based research which will underpin a future
silicon indushy currently centres around silicon-germanium
heterojunction devices, novel configurations for reduced
power consumption in portable systems, cheap thin film
0268-1242/95/091187+21519.50 6 1995 IOP Publishing Ltd
devices and nanoscale fabrication. Of course the latter
topic holds out the possibility of novel functionality which
exploits the quantum regime of electron behaviour, and so
connects with some of the work reviewed here. Porous
silicon burst into this arena several years ago, offering at
least a possibility that silicon technology might eventually
yield light-emitting devices. The fact is that under optical
excitation. porous silicon does produce light with high
efficiency, and furthermore with an emission spectrum
which can be 'tuned' from the near-infrared to the green
by varying porosity. Further processing by rapid oxidation
extends the emission into the bludviolet region of the
spectrum.
Clearly then, one driving force behind research into
light emission from porous silicon is the hope that having
finally understood the basic mechanisms, it might be
possible to make an electrically excited LED or laser which
could, ultimately, be integrated into a complex chip. This
notion should not be seen as one of simply enlarging the
functionality of silicon; discoveries of new functionality
traditionally end up finding applications in semiconductor
technology. In their turn, both the rapidly oxidizing
silicon surface and the semiconductor laser respectively
were dismissed either as a nuisance which would prevent
development or as lacking in applications! It would
be wrong therefore to rule out, say, optical interconnect
applications for porous silicon, provided that it could
be developed into a stable electroluminescent system,
compatible with integrated circuit processing.
In order to even contemplate real applications for
porous silicon devices we must understand the basic
radiative processes and must have a clear view of how
to optimize the porous skeleton and how to control and
perhaps to take advantage of its enormous surface area.
1187

3. B Hamilton
Finally we must learn how to electrically excite the
luminescence. In the final analysis it may prove impossible
to achieve these goals, or some other less complex form of
optically functional silicon may emerge. Many issues are
currently being pursued, including modification of porous
silicon and new ways to process material and contacts, and
these will be touched upon below. However, the central
issue remains the origin of the light, and its relationship
to the atomic-scale structure and the associated electronic
structure of the porous layer. The main aim of this review,
then, is to try to draw together the threads of evidence which
are guiding workers in the field towards understanding the
physics of the material. The story to date, although largely
qualitative, is complicated and has generated lively debate.
This being so, it is politic to simply state at the outset the
four most commonly held views on the origin of the visible
luminescence:
(i) The visible and near-infrared light is the result of of
quantum confinement shifts of the silicon energy gap due
to particle localization in nanometre scale structures (wire
or dot) which make up the porous skeleton.
(ii) The luminescence originates from surface molecular
species which coat the porous skeleton, and which result
from the electrochemical processing.
(iii) The light originates from radiative decay at
surfacdinterface states, the character of which are partly
determined by nanocrystaltine particles within the porous
layer.
(iv) Hydrogenated amorphous silicon is a product of
the invasive electrochemistry and is responsible for the
emission.
In attempting to revicw the field, it is necessary to
subdivide the information; first, in section 2, an overview
of the main optical phenomena is presented. Section 3
thcn deals, in a.simple way, with the electrochemical
process involved in pore formation, leading on to a brief
review of pore morphology and microstructure. In section
4 some of the issues involved with the surface of porous
silicon are discussed in order to provide a firmer basis
for the review of the debate surrounding luminescence
mechanisms in section 5. Section 6 deals with some issues
concerning oxidized porous silicon which shed light on
some fundamental aspects of the material. Finally section
I outlines some attempts to make simple device stmctures
and also the considerable problems involved.
2. An overview of the optical phenomena
associated with porous silicon
Highly porous Si, processed using electrochemical etching
methods, exhibits strong photoluminescence; efficiencies
of several per cent have been routinely reported. Spectra
are broad, but peak wavelengths can be 'tuned' over a
wide range in the near-infrared and visible, by varying
the porosity. These facts, which were first noted by
Canham in 1990 [l], remain the key points underpinning
a wide-ranging and interdisciplinary research effort. It is
interesting to note though that the observation that ultra
small silicon crystallites, passivated by hydrogen, and with
1188
Wavevector
Figure 1. The energy band structure of crystalline silicon.
The indirect energy gap leads to slow band to band
radiative decay transitions which require the participation of
momentum-conserving phonons.
emission wavelengths which depend on size, pre-dates the
first report of porous silicon luminescence [2].
A natural starting point for a review of porous silicon
is a comparison with the optical emission properties
associated with crystalline silicon. The energy band
structure of any semiconductor dictates many of the
observed luminescence properties. Silicon has an indirect
enera gap, shown in figure 1. A well known consequence
of this that the radiative efficiency of Si is low at room
temperature. The indirect gap dictates that electron-hole
recombination across the gap requires the involvement of
momentum-conserving phonons; the matrix element for the
transition is thus small. Note that this is not a fundamental
limit to the radiative efficiency, it simply results in a
long radiative lifetime; the calculated radiative lifetime of
moderately doped silicon at room temperature is in the
millisecond regime. With such a slow radiative decay
process, injected carriers inevitably recombine through non-radiative
shunt paths, and the net recombination lifetime
(though very sample dependent) is orders of magnitude
shorter than the radiative lifetime. However, if all
competing shunt paths like deep electron states or surfaces
did not exist, then silicon would be a perfect emitter at
close to 1 pm. Unfortunately this remains a hypothetical
case, though data exist which demonstrate clearly that
effective removal of the surface shunt path by hydride
passivation results in long minority carrier decay times and
large increases in radiative efficiency [3].
At low temperatures, Si becomes more optically active.
This is principally because certain optical decay channels
become thermally stabilized. For example, the free exciton
population under injection conditions grows, and more
importantly shallow impurities or defects can stably bind
excitons. The initial capture event for an exciton into an
impurity state may be fast; if the exciton can remain trapped
for a sufficient time, i.e. is not thermally ionized from the

4. Porous silicon
1090 1110 113 1150
2
Photon energy h"el
Figure 2. An example of a low-temperature photoluminescence spectrum
of high-quality p-type bulk clystalline silicon. The sharp structure is due
to the decay of excitons which are stably bound to the boron impurities at
low temperatures
impurity potential, decay may occur with a matrix element
determined partially by the impurity. Radiative decay times
for such transitions vary considerably, but even though
non-radiative 'branching' usually occurs for such impurity-localized
excitons it is a relatively simple matter to measure
the associated luminescence spectra. Such spectroscopy is
an active field of semiconductor physics, and the reader
is referred to a comprehensive review by Davies for more
detail [4].
The luminescence spectra associated with bulk
crystalline Si are typically highly structured and well
defined, whereas porous Si luminescence is strikingly
different. Figure 2 shows the photoluminescence spectrum
from a lightly boron-doped sample [4], this is a high-quality
version of the sort of wafer which might be used as a
starting point for processing into porous material. Whilst
the emission from the p-type wafer shows the characteristic
sharp features due to the decay of excitons trapped at boron
acceptors, that from porous material is both broad and
significantly shifted above the three dimensional gap; this
is clear from figure 3. which shows some typical examples
of spectra measured on freshly prepared porous material.
Although the porous Si emission is blue shifted, it is clear
from figure 1 that the emission energies lie far below the
lowest direct gap at the r point in the Brouillon zone. In
fact it is clear that none of the light emission observed
from porous Si, even the blue emission discussed later, is
associated with the three-dimensional direct gap.
The spectral emission from porous silicon is not
confined to a single band. The band shown in figure 3 [5]
is often called the visible or slow band. and is the one first
reported. This band can actually be shifted systematically
between the near-infrared and the yellowlgreen region of
the visible spectrum. Other radiating systems exist and are
crucial to the emerging story of porous Si, but we shall use
the properties of the visible band to obtain an overview of
the luminescence properties. Table 1 gives a brief resume
of the various bands observed to date with comments on
their origin.
g1.1 1
0.9 oi 0.2 o.i 0.6 o:8 1
(1-Porosity)
I
Figure 3. (a) An illustration of the way in which the 'visible
band' vanes with porosity. All samples in this data set
came from the same p+ substrate and were processed in
the same electrochemical cell. (b) The variation of peak
energy with porosity for the same band. Although porosity
is only an indirect assessment of size, it is clear that the
blue shill becomes faster at the highest porosities.
Although all conductivity types of Si wafer have now
been demonstrated to yield the visible band, most work
has been canied out on pt material. This is largely to do
with the fact that the anodic electrochemical dissolution,
which requires a large supply of holes, is most readily
and precisely established in p-type material (see section 3).
1189

5. B Hamilton
Table 1. Luminescence bands obselved for porous silicon.
Energy range (ev) Key properties
1.2-2.2
('visible band')
Sensitive to porosity in most reported cases:
sensitive to surface passivation,
especially but not exclusively hydride passivation;
strong temperature dependence of decay time;
convolution of at least two components;
dominant band in freshly anodized material.
Usually weak compared with visible band;
sensitive to surface oxidation condition.
strong in high temperature oxidized material
fast decay time,
0.8-1.3
('IR band') sensitive to porosity;
2.5-2.8 Weak dependence on porosity
('blue band')
The visible band displays a quite remarkable variation of
peak wavelength with porosity. Figure 3 shows this for
small pieces of the same p substrate, processed in the same
electrochemical cell to different degrees of porosity and
measured under identical excitation conditions [SI, It is
common practice to measure the visible band in freshly
etched material. This behaviour has been reproduced by
many groups, and though there are small variations the key
features are now established. The onset of luminescence
requires a threshold porosity of around 45%; below this
value only the luminescence attributes of crystalline, non-porous
silicon are observed. The visible band moves
smoothly from its threshold peak energy of 1.3 eV to 2.0 eV
at a porosity of 90%. At and beyond this porosity, the
film is mechanically fragile and porosities are difficult to
reproduce and to measure. The sensitivity of wavelength
to porosity increases dramatically as the porosity increases.
Since in a general sense the characteristic size of the porous
skeleton reduces with increasing porosity, this wavelength
sensitivity was one of first attributes to underpin the search
for quantum confinement effects.
It was rapidly discovered [7] that the visible band is not
completely stable following anodization; both wavelength
shifts and efficiency changes occur when porous Si layers
are stored in ambient conditions. In general blue shifting
of the emission occurs, with a peak shift of 0.5 eV being
recorded for a three year storage period, though all effects
seem to saturate after around one year. Interestingly
the quantum efficiency often changes little with ambient
storage and may increase [7]. Another early discovery
was the optical fatigue of the visible band which is
especially pronounced for short-wavelength, high-power
Laser excitation [SI. These instability problems relate to
surface chemical and electronic structure, issues of vital
importance for the understanding and control of porous Si.
In addition to the issues of stability, the visible band
turns out to have rather complex spectral and temporal
properties. The emission contained within the spectral
envelope consists of more than one emission band [91. By
using time-gated detection it has been demonstrated that
both fast and slow components are typically present with
distinctive spectral shapes. Figure 4 [9] demonstrates this,
and shows that the fast component peaks at significantly
higher photon energies. The data of figure 4 were obtained
1190
I I r
Photon energy [eVI
Figure 4. The 'visible' emission band is not a single
system. This figure shows that time domain measurements
reveal that at least two spectral bands are present.
from a p- layer of around 80% porosity which had a fully
stabilized native oxide (i.e. is fully aged). More recently,
fast high-energy components have been associated with
specific types of oxidation of porbus Si. However, for
porous silicon which has received no additional surface
treatment, aside from ambient aging, the slow component
is by far the most important and accounts for almost all of
the measured quantum efficiency.
The detailed temporal behaviour of the slow component
of the visible band varies according to which spectral
bundle of the rather broad band is measured, and also
on the measurement temperature. Also when measured
over several decades, the decay profile is never completely
exponential. There is broad agreement about the general
form of the temperature variation of the decay time for this
band. At low temperatures ( 4 0 K) the decay can be very
long, of the order of milliseconds; as the temperature is
increased the lifetime quenches to the microsecond regime.
There are some differences in detail from sample to sample,
which makes it difficult to fit the data to detailed kinetic
models; however, figure 5 demonstrates the trend which
has been established by many workers.

6. Porous silicon
10
1 10 100
Temperature (K1
Figure 5. The temperature dependence of the decay time
of the main component of the visible band measured at
1.8 eV. The data are taken from t'Hoofl G W et a/ 1992
Appl. Phys. Lett 61 2344.
0.16-
c 0 , 0 8 i I
- j L 0 -0
0 50 100 150 ZOO 250 3 0
Temperature IKI
Figure 6. The spectral peak of the visible band moves in a
very uneven way with temperature. The detailed movement
is sample dependent. The effect is illustrated here for p+
material of around 85% porosity. This trend is typical of a
disordered system, but can be very marked for porous
silicon.
This shortening of the total lifetime at high temperatures
is always accompanied by a quenching of the luminescence
efficiency. This shows in a rather unambiguous way that
non-radiative channels are opening up to the excited carrier
populations as the temperature is raised. This is rather a
familiar picture in semiconductors and is often associated
with disorder, or more specifically particle localization
within the potential energy minima resulting from disorder.
It suggests that at low temperatures excitons (for example)
created by the optical pumping rapidly localize into sites
which have good radiative efficiency. that is to say the local
non-radiative channel is slow compared with the measured
lifetime of typically 1 ms. As the temperature is raised. the
thermalization time out of these 'radiative sites' becomes
shorter and the exciton is free to explore larger volumes
of the porous skeleton, and to find much more efficient
non-radiative paths.
The notion that disorder plays a role in the
luminescence mechanisms of porous Si is compelling
given the enormous complexity of the porous skeleton,
and indeed other simple observations support this view.
One such observation is the spectral shift of the visible
band as the temperature of the sample is raised and an
example is shown in figure 6. This occurs because at
the lowest temperatures particles bind efficiently into the
deepest potential fluctuations and the luminescence signal
i o 4
1.0 20 3.0 4.0 5.0 6.0 70
Excitation Energy (eV)
Energy (eV)
Figure 7. The photoluminescence excitation spectrum of a
porous silicon layer. There is some similarity to that of bulk
silicon especially in the higher energy regions towards the
direct gap
is weighted in favour of the lower energies characteristic
of the deep fluctuations. At higher temperatures shallower
potential fluctuations become statistically more significant,
shifting the mean of the spectral distribution to higher
energies. This is exactly what is observed in disordered
alloy quantum wells [IO], but the effect is much more
dramatic in porous silicon [ I l l ,
Other spectral features may reveal the presence of
disorder phenomena, such as the Stokes shift between
emission and absorption bands, and the relationship
between the Stokes shift and luminescence linewidth. For
any situation in which luminescence is dependent on the
details of localizing potentials, the absorption process itself
is more representative of the band structure of the solid;
it is not unevenly weighted by the defect phenomena. A
practical difficulty in obtaining absorption data from porous
silicon is that one of the spectral regions of interest is
well above the three-dimensional gap of the underlying
substrate. The strong substrate absorption inevitably
masks the processes in the porous layer. For this reason
many measurements rely on photoluminescence excitation
spectroscopy (PLE) which is well suited to the measurement
of thin surface layers. One of the first reported PLE
measurements is shown in figure 7 [12].
The first impression of such data is that it is rather
reminiscent of the absorption of bulk silicon, with strong
absorption above 3.4 eV corresponding to the three-
1191

7. B Hamilton
dimensional direct gap. Once again, though, we run into
the complexity of the material in the interpretation of the
data. 'we do not know in detail the macroscopic optical
constants of the porous silicon layer, though the refractive
index has been shown to decrease with increasing porosity.
Clearly some caution must be exercised in assigning an
optical thickness w. The role of internal light scattering
is likely to complicate the estimate of optical thickness.
Calculation of the dielectric functions of the altered layer
is difficult because of the complexity of the layer, though
some attempts have been made [13]. Even in the limit of an
optically very thin surface layer, for which the PLE method
truly measures the absorption processes, the experiment will
average the whole ensemble of size distributions; this will
inevitably lead to a smearing of the data. Such smearing
would be particularly enhanced if size effects were present
in the optical density of states functions.
In order to get a better feeling for the material structure
which gives rise to these very distinctive optical properties
of porous silicon, we now turn to a review of the fabrication
process: this includes some insight into the crucial role that
high-resolution microscopy has played in the interpretation
of the material properties.
3. Porous Silicon formation and microstructure
For many years porous silicon formation has been used
as one of the mahy processing techniques for device
isolation. The FIPOS process, (full isolation by porous
oxidized silicon) makes use of hydrofluoric acid (HF)
as an electrolyte in an anodic electrochemical reaction;
HF, it seems, is the only known electrolyte which can
anodically dissolve silicon in an efficient manner. The basic
electrochemical phenomena involved in the FIPOS process
and optically active porous silicon are essentially the
same, except that the latter usually has significantly higher
porosity and in the limit of such high porosity additional
electrochemical reactions may occur. The electrochemistry
of nanostructured silicon is still a developing field. and only
the simplest of views can be presented here.
In principle the production of a porous silicon layer
is not demanding; a carefully constructed electrochemical
cell along the lines of that illustrated in figure 8 [14]
is all that is required. The cell and the electrolyte
system must be formed from high-purity material, and
good control of the operating characteristics is required.
Nevertheless. processing of centimetre-size samples with
good macroscopic uniformity is not difficult. Ironically, the
ease of fabrication is in stark contrast to the complex range
of characterization methods which have been employed in
an attempt to understand the porous material.
In the cell, the SifHF interface forms an elec-trode/
electrolyte barrier system. The potential harriers and
electric field distributions across even the equilibrium sys-tem
are rather involved, depending on the doping character-istics
of the semiconductor and the chemical composition
of the electrolyte. However, the gross feature of the barrier
is its 'double layer' attribute. There exists finite regions of
space over which the interfacial electric fields are spread
and the potential barrier evolves; these are the Hehnoltz
1192
Ammeter I -
Cathode- h -Anode
-Magnetic
stirrer
. , , . . . . . . ,
U ca
e:
0
*
- -U Silicon ca
Potential Distribution
.s ................................ U
Figure 8. A simple schematic diagram of a basic
electrochemical cell used for anodization. The potential
distribution across the electrolytelsilicon system is shown
below.
layer in the electrolyte, and the depletion layer in the semi-conductor.
A schematic diagram [ZO] of the barrier system
is shown in figure 8.
The dissolution of silicon occurs only under anodic
conditions, and the primary process leading to massive
removal of Si atoms is considered to be the formation
of silicon fluoride molecules, SiF,. Various routes are
possible in principle 115-171. Perhaps the simplest example
proposed are for Si dissolution involves the divalent state
[I81
Si + 2HF + ne' + ,352 + 2H' + (2 - n)e-.
Here it is assumed that holes take part, i.e. holes are
freely available in the silicon to feed the reaction. This
requirement is easily fulfilled by p' material, but low or
even n-type conductivity is not a fundamental barrier to the
process because optical excitation can always be used to
generate an excited hole population. The SiFz formed in
the above reaction may then be removed by other chemical
reactions [18, 191. It has emerged, however [19], that the
number of electrons consumed in the initial electrochemical
reaction, that is the number n in the above equation, is

8. Figure 9. The electrochemical regimes available for silicon
processing as a function of the I-V characteristic of the
electrochemical cell. Region A: pore formation, region B:
transition, region C: electropolishing.
greater than 2. It seems therefore that both the divalent
and tetravalent Si dissolution occur simultaneously
Si + 4HF + (4 - n)e+ + SiF4 + 4H+ +ne-.
Again, several routes are possible for the removal of the
SiF4.
What is achieved in practice depends on the precise
anodizing conditions, for example the anodic potential,
and .ranges from a layer of uniform porosity to
complete removal or electropolishing. The current-voltage
relationship of the sample-cell system reveals the various
regimes of electrochemistry. This is shown in figure 9 [ZO].
In the region of low applied potential (A) the current is
generally exponential with voltage (the Tafel region), with
a slope of typically 60 mV per decade; this value is clearly
an indication of the physics of the potential banier and the
in this region, the silicon removal being driven primarily
by the above reactions. At significantly higher potentials,
the electropolishing regime is entered, resulting in complete
removal of the porous layer, Electropolishing results from
the formation of an anodic oxide which is dissolved by
the HF, any irregularities in the silicon topography being
removed due to the divergence of th e electric field lines at
regions of dielectric with a consequent enhancement at any
Si features [221. One proposal for the electrochemistry of
this oxidation process [23] is the following reaction
carrier transport [20, 211. Pore formation occurs ................
Si + 40H- +ne+ -+ Si(OH)4 + (4 - n)e-associated
~i~~~~1 0, The simpleS. model for pore formationb, ased
essentially on impedance to current flow, leads to columnar
pores (a). A more complex model based a
diffusion-controlled mechanism of pore formation leads to
the sort of multiply interconnected or spongy porous layers
ofien obselved in TEM measurements (b),
the processing of porous silicon sensitive to the HF
concentration in the electrolyte, low HF concentrations and
hence low oxide removal rates favouring electropolishing;
these trends have been established. experimentally [20].
Whilst the simplified electrochemistry discussed so
far can explain Si removal, it does not account for the
spatial selectivity which results in pore formation. In
fact Pore morphologY does depend on conductivity type
and several models have been proposed to explain this
crucial feature of the processing, most of them resting
Si02 + 6HF + H2SiF6 + 2H20. on built-in inhomogeneities in the original Si wafer as
the trigger for pore formation. The wafer conductivity
At low potentials, in the Tafel region, the oxide formation and electrochemical details then dictate the detailed pore
rate is too low to compete with Si removal and porous evolution.
silicon results. At high potentials oxide formation is One of the earliest attempts to explain pore formation
enhanced and surpasses the oxide dissolution rate, resulting is due to Beale and co-workers [24, 251, and is based on
in electropolishing. It is to be expected that this interplay the barrier properties coupled with the spatial variation
between oxide formation and removal rates should make of impedance to current flow. The essence of the
1193
4.
Si02 + 2Hz0.
me oxide formation rate is in competition with its
dissolution rate governed by

9. B Hamilton
model is that small inhomogeneities on the wafer surface
cause enhanced current flow and locally rapid removal
of Si. The original depression is enlarged, leading to
pore formation. The nature of the inhomogeneity is not
specified; it could be some macroscopic perturbation of
surface morphology, or even a defect at the atomistic level.
In its simplest form this model led to the expectation that
silicon between the pores will ultimately become depleted,
simply because the dimensions of the remaining silicon
'columns' is insufficient to support the space charge width.
The impedance offered to the current path into the silicon
column is then held to grow rapidly and current flows
preferentially down the electrolyte and into the wafer at the
bottom of the pore, as illustrated in figure 10(a) [271. This
provides a possible mechanism for producing columnar
structures.
This way of describing the pore evolution does seem
to go some way towards explaining the gross morphology
of the porous skeleton in p+ silicon. It is suggested
that the heavily doped wafer leads to a narrow space
charge layer in the semiconductor, tunnelling phenomena
are enhanced Gust as in Schottky baniers to degenerate
semiconductors) and the impedance to current at the base
of a pore is significantly lowered. However, the model does
not explain the pore morphology observed in p- material;
this typically consists of massively interconnected network
which is uniformly distributed across the film.
Smith et al [26] have shown that a more complex pore
morphology may be explained if pore evolution is limited,
at least partially by the rate of diffusive transport of the
hole to the reaction point at the electrolytic interface. The
diffusion-limited case arises because the impedance offered
by the barrier system is much higher than in the pf case,
tunnelling being much weaker in the wide, low-field barrier
system of the lightly doped semiconductor. This analysis
still accounts for a faster than average reaction rate at a
pore tip, and hence elongation of the pore. It also makes
the interconnected network a more reasonable expectation,
as shown in figure IO@). A pore E, initiated on the sidewall
of an existing pore A, will be in better communication with
the diffusing hole flux in that region of silicon between
pores A and B until the tip of pore E approaches the tip
of B within two hole diffusion lengths. This is rather a
complex, at simplest. two-dimensional diffusion problem,
but the significant sidebranching is a fundamental feature
of the observed morphology.
Other models of pore formation have been discussed.
The possible effect of quantum confinement in residual
silicon structures has been proposed [27] as way of
enhancing carrier depletion effects and hence limiting pore
growth in the limit of very small structures. Alternative
electrochemical schemes have also been proposed for the
Si removal process [19, 281, which draw closer analogies
with the formation of porous aluminium. The theoretical
simulation of pore structure for a variety of possible
electrochemical conditions is given by Parkhutik etal 1291.
No complete understanding of pore morphology exists,
and it is likely that improvements in our understanding
will come about through the application of high-resolution
microscopy. Transmission electron microscopy has already
1194
proved essential in probing the porous structure. Porous
silicon layers are inevitably fragile and this makes their
evaluation more difficult, and necessitates the development
of some novel approaches to specimen preparation.
Measured pore sizes can vary from -100 nm (macroporous)
down to c2 nm (mesoporous). As a very approximate
guide to published data it appears that lightly doped p-type
silicon produces a fine network of pores whereas
heavily doped p-type material produces more of a columnar
structure [24, 25, 30, 311. For lightly doped n-type silicon,
the pores in general take up a more crystallographic form
with typical dimensions of several tens of nanometres
propagating in the (100) direction. This attribute has
even played a role in VLSI device isolation by trench
formation. Un l i e the case of p-type silicon, as n-type
doping level increases, the pore dimension increases and
hence the interpore spacing decreases.
These general comments on pore morphology must
be taken as a rough guide only. The detail form of
the layer depends on the precise anodization conditions
used, and very high resolution imaging can often real
more complex geometry, leading to a fractal view of the
altered layer. For example it has been known since the
early work of Beale et al [32] that the columnar pore
arrangement in pt silicon is heavily branched. It is also
possible to produce mesoporous n+ silicon with -5 nm
pore dimensions (33). The key question which high-resolution
electron microscopy has attempted to address
concerns the detailed relationship between porosity and
luminescence. This has been reviewed by Cullis [34], who
highlighted the need to avoid ion beam milling or other
invasive specimen preparation methods for the preparation
of electron transparent samples of porous material, which
is easily amorphized and chemically modified. Figure
11 is taken from that review; it demonstrates well
the key issues concerning the microstructure of p-type
porous silicon in the transition from relatively low-porosity
weakly luminescent material to high-porosity strongly
luminescent material. The pictures represent bright field
(001) projections. For the weakly emitting material, the
Si skeleton comprises mainly rod-like structures with a
range of diameters, the smallest being around 5 nm.
The corresponding electron diffraction patterns indicate
completely crystalline material. Figure 1 l(b) illustrates
material of higher porosity than (a) which gave stronger
luminescence. The microstructure is now finer with silicon
structures down to 3 nm clearly visible. Arcing of the
electron diffraction spots indicates misalignment of the Si
columns. The electron diffraction pattern now shows more
severe misalignment of the Si skeleton, but still indicates
crystalline material. As porosity grows, these trends are
continued. These TEh4 data, then, point to a correlation
between a reduced characteristic size distribution of the
silicon porous skeleton and the switching on of strong
luminescence. In particular, column or particle sizes of
around 3 nm or smaller are present in highly luminescent
material.
Ultra high-resolution microscopy will continue to play a
key role in porous silicon research and it can be anticipated
that advances in microscopy will add more vital information

10. Porous silicon
regarding the relationship between microstructure and
luminescence. Whatever new insight is gained from
microscopy, though, it remains true that increasing the
porosity of the material inevitably increases the surface
area. Surface chemical interactions and the influence of
the surface on electronic-properties area key areas of
investigation.
4. Surface effects on porous silicon
The 'internal' surface area of porous silicon is very large;
several hundred square metres per cubic centimetre of
porous material is typical. It is reasonable therefore to
expect that the surface itself might play a direct role in some
of the observed luminescence behaviour, or that the surface
would exert important effects on the 'bulk' behaviour of
the material. A good deal of effort has been expended on
investigating these issues, which still remain at the heart of
the debate on the origin of the light emission. One of the
earliest [2] and most graphic attributes of surface chemistry
is the role of hydrogen coverage. After anodization in the
HF-based electrolyte, a surface rich in Si-H bonds can be
routinely observed using infrared local mode absorption
spectroscopy. Bonds involving one (Si-H), two (Si-H2)
and three (Si-H3) hydrogen atoms are normally present
and both stretching and scissor vibrational modes can be
seen [35-381. Figure 12(n) [5] shows a typical absorption
spectrum measured for a freshly prepared porous layer
(curve (a)). Compared to that of, say, an unprocessed
Si wafer, the H bond-related absorption is dramatically
increased. Other Vibrational features can be seen which
are common to both porous layers and bare wafers; these
correspond to SiSi stretching modes and to (probably bulk)
Si-0-Si, interstitial 0 asymmetric stretching modes. These
and other 0-related modes assume much more significance
for oxidized material. The figure shows the evolution
of the local mode structure as the sample is annealed
in vacuum (curve (b)) and also in nitrogen at 300 "C
for 5 (curve (c)) and 10 min (curve (d)). The vacuum
anneal completely removes the H-related features, whilst
the nitrogen anneals promote 0-related modes, probably
due to weak 0 contamination.
It was noted above that the process of atmospheric
aging has an indeterminate effect on the luminescence,
and may cause it to increase. Such aging causes a
broadening of the H-related absorption modes and also
a growth of 0-related modes. However, by far the
most dramatic phenomenon associated with H coverage
is observed following desorption on a large scale during
vacuum anneal. This causes a complete quenching of the
luminescence. Figure 12(b) illustrates the luminescence
spectrum for a freshly prepared sample. After vacuum
anneal no luminescence can be seen, though some weak
recovery is observed if the vacuum anneal is followed
by a nitrogen anneal. This recovery is significant, even
though no H-related absorption can be measured. By
immersing the annealed sample in HF for a few seconds,
both the luminescence and surface H bonds measured by
absorption are dramatically restored. Very small shifts in
peak wavelength are seen due to this cycle, and these are
1195
Figure 11. High-resolution data obtained from TEM
measurements of porous p-type silicon. The trend in
characteristic sizes of the remaining silicon skeleton as a
function of porosity is clear. The associated optical
characteristics are described in the text.

11. B Hamilton
(4 si.0-si
related mods
J-defamation
I
IIIP 1 1
I I I I I
500' lMXl 1500 2m 2500
Wavenumbers cni'
Energy (eV)
Figure 12. (a) The infrared absorption spectrum of
prepared 4550% porosity silicon is rich in Si-H
bond-related transitions curve (a), vacuum anneal for 2 min
at 400 'C removes these modes completely. Further
annealing in nitrogen at 300 'C for 5 (curve c) or 10 min
serves only to weakly promote 0-related bonds. (b). The
luminescence spectrum of the same sample: as-prepared
(full curve), after the vacuum plus the 10 minute nitrogen
anneals (dotted line) and finally after immersion in HF
(broken curve)
not surprising since one expects small changes in the silicon
skeleton to Lake place; we don't have precisely the same
sample at the end of the sequence. There is no doubt.
however. that surface hydrogen coverage plays a key role in
the luminescence behaviour of freshly prepared porous Si,
and that removal and replacement of the H coverage leads
to reversible quenching and restoration of the luminescence.
The electronic role of H is not fully understood, but
removal of H has been shown to increase the Si dangling
1196
bond density measured by electron spin resonance [39].
Since the dangling bond is known to be a powerful non-radiative
recombination centre [40], a straightforward role
of H as a passivating centre is suggested. This notion is
much connected with the debate surrounding the radiative
mechanisms which operate in porous silicoa
There is no doubt then that the surface plays an
important role and that H bonding is necessw to sustain
the luminescence. Furthermore the vibrational assignments
suggest that simple Si-H, bonds account for some, possibly
most, of the surface hydrogen. It is unrealistic, however, to
expect that this bonding arrangement accounts for all of the
surface chemistry and some considerable effort has been
devoted to probing for other surface constituents which
might bear on electronic processes. There are several
important candidates for surface bonding, based simply
on the processing environment of the wafer; to date most
reported work has been aimed at probing the involvement of
oxygen, fluorine, or organic radicals of varying complexity.
Although surface Si-F bonds play an important role in
the dynamics of pore evolution, it seems that they are not
stable on the free surface after processing. Probably they
are replaced via a hydrolysis reaction, by Si-OH bonds
which themselves can dissociate into Si-H or Si-0-Si
bonds by reaction with the atmosphere [41].
The role of oxygen is important. Simple exposure to
air causes surface oxidation of all silicon, and the effect is
enhanced for porous silicon. After all, this fact led directly
to the development of the FIPOS process mentioned above.
The luminescence aging of porous silicon is connected the
incorporation of 0 into the surface bonding arrangement.
The detailed form of 0-modified surface bonding has been
analysed by Kat0 et al [36]. Low-temperature ( d o 0 "C,
for times of less than 50 min) oxidation was used; this
might be regarded as a sort of accelerated aging process.
Using IR local mode absorption, the Si-H transitions are
seen to broaden and shift somewhat. These spectral changes
were attributed to the incorporation of 0 into the Si back
bond(s) associated with the S-H, atomic arrangement. For
example if 0 is incorporated into one of the three back
bonds of S-H, the Si-H stretching mode transition was
calculated to shift from 2090 cm-l to 2127 cm-', and by
considering all possible sites for 0 incorporation into back
bonds the general changes which occur in 'lightly' oxidized
material are accounted for. This picture of 0 incorporation
leaves all surfaces terminated with an Si-H bond; only the
back bonds are broken. The spectral deconvolution leading
which led to this picture rests on the assumption that the
Si-H stretching mode transition is located at 2090 cm-',
but it should be noted that some debate exists regarding thc
precise vibrational nature of this transition.
Aging, and non-aggressive oxidation also cause
increased absorption in all peaks that relate to Si-0
vibrational modes. The effect of further increasing the
0 content of porous Si causes yet more changes, and is
currently being used as a modification process to stabilize
the material. Oxidation is also potentially useful in helping
to evaluate the luminescence mechanisms and the role of
the dangling bond in quenching luminescence; these issues
are discussed further in section 7.

12. Porous silicon
5.1. The quantum confmement mechanism
The idea that the surviving silicon skeleton contains within
it structures small enough to exhibit quantum confinement
effects such as opening up of the bandgap was the first
proposal for a mechanism for porous silicon luminescence.
This suggestion represents a simple explanation based on a
well established attribute of the material, i.e. the existence
of nanoparticles in the layer. Figure 13 is an example
of the way in which the quantum confinement mechanism
is often viewed. As we have seen, nanometre sizes for
crystalline Si particles are amply proven from TEM data
and are also confirmed by an analysis of the optic phonon
Raman lineshape [45]. Energy shifts due to confinement on
a scale comparable to the particle size are a universal feature
of quantum mechanics, but proof that such a mechanism is
correct must rest on a direct observation of luminescence
from the nanoparticles and supporting evidence on the
interconnection between the geometly of the nanoparticle
and the emission wavelength.
Such direct evidence does not exist for porous
silicon. However, a direct observation of red luminescence
from oxidized isolated Si nanoparticles with characteristic
sizes of below 5 nm has been reported [46, 471.
Such observations demonstrate that isolated particles can
luminesce, but the detailed chemical arrangement of the
oxidized nanoparticles complicates the interpretation of the
luminescence process
It remains the case that the blue shift of the visible
luminescence with increasing porosity in p+ material is
one of the key observations linking the light output
to nanoparticle size. However, porosity in itself is a
quantitative measurement of the fractional mass removed,
but is not a quantitative measure of nanopaaicle size.
So. for example one can imagine crudely that a film of
a particular porosity might consist of large Si particles
and extremely large voids, or of very smal1.nanopartick.s
and moderate size voids. In p+ material, in which the
morphology is known to exhibit size reduction of the
nanoparticles as porosity increases, the increased sensitivity
of the luminescence blue shift to porosity is precisely what
would be expected from quantum size effects provided
that the light originated from recombination within the Si
nanoparticles.
The slow decay rate of the visible band is really what
one might expect from an indirect semiconductor; and
if the interior of the residual silicon nanoparticles were
perfectly crystalhe and therefore presented a shunt-free
environment with no non-radiative recombination centres
and with completely passivated surfaces, they might offer
the perfect environment for light emission. However,
the non-exponential decay indicates that such an idealized
notion is unlikely. In fact this behaviour is not unlike that
observed in amorphous silicon, in which carrier trapping in
the tail states plays a dominant role.
The photoluminescence attributes are rather different
for n-type material. In general, for such material there is
no systematic variation of blue shift of the visible band with
porosity. For example, marginally porous layers, e.g. less
than 40%, have been shown to exhibit strong visible
luminescence with the same general character as that seen
1197
Although IR absorption has emerged as a powerful tool
in the surface analysis of porous silicon, other techniques,
principally x-ray photoelectron spectroscopy (XPS) and
SIMS, have been used. SIMS analysis has confirmed the
presence of hydrogen and Ruorine as the major surface
species of freshly anodized material, whilst oxygen, carbon
and nitrogen were detected at lower concentrations [42].
The SIMS data also c o n h that F is not stable but
reduces with atmospheric exposure, presumably due to
the hydrolysis reaction mentioned above, and indeed an
increase in surface hydrogen to be expected from this
process is also observed using SIMS. The other important
changes revealed by SIMS measurements are a build-up
of carbon and oxygen with prolonged exposure to the
atmosphere.
The XPS technique has revealed fluorine, carbon and
oxygen on porous silicon surfaces [43], in broad agreement
with the SIhlS data. Evidence in support of a fluorine-admixed
Si02 surface phase has also been claimed, based
on XPS analysis [44]. This possibility, of surface layers
with rather complex chemistry, for example an Si-0-F-H
system, though more difficult to analyse experimentally
than simple Si-H bonding arrangements, is the basis for one
of the models suggested for the luminescence, to which we
now turn.
5. The light emission process
Having reviewed the basic features of the material it is
possible. by looking in a little more detail at particular
pieces of experimental evidence, to try to glean what
is cumently understood about the basic light emission
mechanism. This of course is the pivotal question
surrounding porous silicon. Until it is answered progress
towards any technological goals will be limited.
It must be readily acknowledged that the complexity
of the material provides fertile ground for the proposal of
differing models for the light emission process. Porous
silicon is a richly interconnected system of small particles
and intricate surface topology. This fact alone leads
naturally on to the expectation of electronic disorder
with its associated defect and interface states; furthermore
the large surface area, generated in a chemically varied
environment adds the possibility of partial surface coverage
with complex molecular films. All of these attributes
have formed the basis for hypothesis regarding the light
emission, and at the time of writing all of these generic
schemes receive support, often zealous. In keeping with
the spirit of this lively debate, this section is presented
by analysing some of the data which either support or
undermine current models. In the current literature, by far
the largest attention has been paid to investigations which
have been designed around the hypothesis that quantum
confinement plays a key role; by far the largest number
of reports discuss this issue. Accordingly, this aspect is
given more emphasis here, though the correctness of the
hypothesis is not proven.

13. B Hamilton
Figure 14. The optical transmission spectrum of
free-standing porous silicon films.
to the PLE measurements reported above, there have
been successful attempts to measure absorption in free-standing
films, free of any complications associated with
the substrate. The absorption measurements often appear
to be rather more sensitive to the low-energy tail, near and
even below the three-dimensional gap of silicon. Several
such measurements seem to indicate that the threshold for
absorption in porous silicon is higher in energy than in
bulk silicon and that the threshold moves to higher energies
with increasing porosity. Figure 14 demonstrates this for
films originally processed from both p and p+ substrates
[27]. The up-shift in energy was more marked for the p+
material which was found to have smaller nanoparticle sizes
than the p material. These observations are consistent with
the opening up of the gap due to confinement and would
strongly support it if it were to be confirmed that silicon
nanoparticles and not some other phase of material were
dominating the absorption spectrum.
Looked at in more detail, the absorption edge of
porous silicon does not wholly support a simple quantum
confinement model. Photothermal deflection spectroscopy
has shown [49] that the absorption strength increases
roughly exponentially above the luminescence peak. Whilst
it might be argued that a size distribution of nanoparticles
might partly explain such data, the same experiments show
that absorption occurs significantly below the gap of three-from
Figure 13. Schematic diagrams illustrating the some of the
structures envisaged for the optically active material,
according to the quantum confinement hypothesis.
(a) The transition from quantum wire through oxidized
nanoparticles to porous glass [7].(b )T he aligned
nanocrystalline or wire structures consistent with EPR data.
(From Harvey J F et a/ 1993 NATO AS/ Series voI244,
p 179.) (c) An electronic view of how an exciton localized
in a nanoparticle might suffer three possible fates for
radiative decay giving rise to three luminescence bands.
(From Koch F 1993 Mat. Res. Soc. Symp. Proc. 298 319.)
P-tyPe material [481. This is an imPortant Point. and dimensional silicon. The existence of significant Urhach
is further highlighted by the fact that the pore morphology tails in the density of states is of course a qualitative
is much more macroscopic in nature with tYPicallY large measure of departure from crystallinity and is reminiscent
widely spaced and crystallographically oriented pores. This of the behaviour of amorphous silicon. Whatever the
picture is at variance with the quantum confinement model. origin of the density of states low-energy tails, its existence
However, the fact that the macropores have much smaller implies strongly some significant degree of electronic
structure on the sidewalls is a further complication which localization.
means that we cannot rule out a nanoparticle explanation Falling back on the evidence relating to the
for the luminescence. luminescence spectrum, the modification of the emission
A key test of low dimensionality in any electronic characteristics by post-anodization processing has been
system is a measure of the density of states functions for used to variously support or oppose confinement models,
electrons and holes, and it was noted above that, in the The blue shift of the luminescence peak as a result of
case of porous silicon, optical ahsorption is in principle oxidation followed by HF dipping was first suggested as
a fundamentally better measurement of these (or more supporting evidence, since the consumption (by oxidation)
precisely of the joint optical density of states). In addition and subsequent removal of silicon is expected to reduce the
1198

14. Porous silicon
..;- i 10000
c
3
E 8000
.. P
-
5 -
6000
-
5 h
4000
- c
c 2000
850 800 750 700 650
Wavelength lnml
Figure 15. An example of one effect of immersing porous
silicon in ethanol: (1) is the ‘as-prepared spectrum’, (2) is
for 1 min of immersion, (3) for 3 min, (4) for 10 min and (5)
for 60 min. The luminescence was measured in situ
overall size of all silicon components in the porous skeleton.
To counterbalance this, reports of red shifts with hydrogen
loss [SO] would not be expected to affect the particle size,
though very small effects due to strain might be expected
to produce small wavelength shifts. The influence of low-energy
‘processing’, essentially immersion in a variety of
organic fluids, has been shown to have large and nearly
reversible effects on the emission spectrum. Effects due
to acetic acid, propanol and ethanol have been reported
[SI, 521. Figure IS [SI] shows the rather dramatic effect
of immersion in ethanol for times of up to 1 h. The
detailed chemical interaction with the porous skeleton has
not been analysed for these organic treatments, but it seems
reasonable to assume that they involve surface or near-surface
effects, and such effects are expected to impinge
only weakly on optical transitions with energies determined
mainly by size quantization.
The debate on quantum confinement has been
underpinned by attempts to calculate the electronic structure
of silicon nanoparticles, and hence to predict optical
properties, in particular the transition energies and matrix
elements. Effective mass theory (EMT), which has been
so successful in predicting the properties of epitaxially
grown low-dimensional smctures, has been applied to
small silicon structures typical of those know to exist
in porous silicon. One such calculation [53] was based
on the notion that for cubic structures with sides greater
than IO atoms long, bounded by (100) planes, EMT
represents a plausible approximation for the description of
wavefunctions. Simple envelope functions and confinement
energy shifts result. The infinite barrier approximation
at the cube boundary leads to an optical matrix element
which is an oscillatory function of the cube size. This is
difficult to test in a real system because the ensemble of
sizes present in a given film inevitably smears the effect.
However, the overall trend for radiative lifetime variation
with confinement energy shift for the optical transition
shift, which of course relates to cube size, is predicted
by EMT to vary rapidly: approximately as the inverse
cube of confinement shift. Some workers have noted that
the measured lifetime of the visible band can vary with
1 + +
0.5 1.5 2.5 3.5 4.5
Olnml
Figure 16. The optical gap predicted by LCAO theory for
nanometre-size silicon crystallites, as a function of size.
porosity and therefore with peak photon energy [54], and
there is rough agreement with the EMT prediction and the
experimental data.
Of course, the very small sizes of some crystallites
observed in porous siiicon must eventually limit the
applicability of EMT, and point to the need for first
principles calculations. One such calculation [5S] has been
performed for wire structures, spanning wire thicknesses
which vary from the thii, molecular, limit of polysilane to
structures which are essentially bulk-like. The calculation
was performed for wires with axes in the [OOI] direction,
bounded by (110) surfaces which were assumed to be fully
terminated with hydrogen atoms. A supercell approach
was used with the basic unit cell of the wire repeating
in space in order to retain three-dimensional periodicity.
Such a calculation is far removed from the effective mass
approach, using a first-principles pseudopotential for the
Si ions and a bare Coulomb potential for the H ions; the
exchange-correlation energy and potential were included
using a local density approximation. It is interesting to
compare the results of such a calculation with EMT. For
the confinement up-shift, agreement was good for wire
diameters of greater than 23 A; above this value the EMT
prediction appears to be an overestimate. The calculation
also yielded a radiative lifetime of around 380 ps for a
wire with 72 atoms in the unit cell, i.e. in broad agreement
with experiment based on the notion that small crystallites
yield the photon output of the visible band. The authors
noted, however. that such a long radiative lifetime implies
that the high quantum efficiency of this band is largely a
consequence of the small non-radiative competition rather
than the lifting of the momentum selection rule.
A recent calculation using the linear combination of
atomic orbitals (LCAO) technique has been used 1561 to
calculate the optical properties of wires and crystallites
(cylindrical and spherical shapes). This form calculation
should yield information on both conduction and valence
band properties of the structures. A key result, the
calculated optical gap as a function of diameter, is shown
in figure 16. The crystallites show the greatest sensitivity
to size; this is the intuitive result based on the fact that
confinement is in three dimensions. The authors also
illustrate the Coulomb electron-hole interaction energy
1199

15. B Hamilton
which makes only a small difference to the total calculated
energy gap. An interesting comparison is made with
the experimentally measured photoluminescence energy
measured not from porous silicon but from hydrogen-passivated
silicon crystallites produced by nucleation from
the gas phase [57].
The result for the wires is a little more complex,
showing anisotropy between different wire directions. The
authors note that the visible band, which is tunable between
1.4 and 2.2 V, would be consistent with characteristic
structure sizes of between 2.5 and 4.5 nm. The exponent
relationship between gap and diameter, in the visible band
energy window. was found to follow D-',39 rather than D-2
predicted by EMT. However, the calculation predicted an
inverse square law at larger D values where EMT is valid.
The LCAO calculation also dealt with recombination
and optical absorption. It was concluded that the strong
confinement in silicon (43 A) induces band mixing and
dipole allowed transitions. The optical matrix elements,
though, remain small and the radiative decay rates as
function of transition energy show strong scatter. Partly
this results from the oscillatory behaviour induced by the
dependence of the matrix element on the overlap in k space
of the electron and hole wavefunctions; this was a point
which emerged also from the EMT formalism. For the case
of the crystallites, the LCAO calculation also demonstrated
that the radiative rate was also sensitive to the symmetry
representation of the Td point group which varies greatly as
the size of the crystallite is varied. This effect was shown
to be more sensitive at lower temperatures.
The optical absorption coefficient based on the above
calculation, for a crystallite of 3.86 nm diameter is shown
in figure 17(a) [%I. This shows that the major absorption
strength is in the ultraviolet, with an absorption 'edge' near
to 3.5 eV, i.e. close to the direct edge of bulk silicon.
The spectral shape is also very structured and bears a
superficial resemblance to what might be expected from
a molecular system. When viewed on a more sensitive
scale. figure 17(b), the calculated absorption coefficient
for this crystallite does show that the transition is allowed
down to the calculated gap energy, but with small oscillator
strength. The absorption coefficient shows a quadratic
dependence on photon energy above threshold, unlike that
of bulk silicon which shows a linear dependence. The
blue shift of absorption edge with porosity (assuming an
attendant reduction of crystallite size) reported above is
then generally predicted by the LCAO calculation, and
the predicted non linear shape has also been recorded
experimentally [58].
These three illustrations serve only to review the trend
in calculations of small structures, and are by no means
exhaustive. They underline the point that in general there
is no fundamental disagreement between theory and the
quantum confinement model for the main emission band
observed from porous silicon. They also highlight the
fact that the regime of solid at the heart of the debate
is tantalizingly poised between one which is comfortably
crystalline and populated with electrons in Bloch states, and
one which is better described by a molecular framework
This notion is very much the theme of the surface film and
defect models reviewed next.
1200
E lev1
Figure 17. LCAO prediction for the optical absorption
coefficient of a 3.86 nm crystallite.
5.2. Molecular films, interfaces and defects
Since it is clear that surface hydrogen coverage is
an important criterion for light emission, at least in
unprocessed porous silicon, several groups have explored
the possibility that the hydrogen does not simply play
a passivating role (i.e. dangling bond saturation), but is
somehow involved directly in the radiative process. Two
main candidates have emerged; surface hydride species
and a class of compounds known generically as siloxenes.
Although other variations have been suggested, these two
examples are illustrative of the key ideas.
The idea that surface hydride species of the form
S a I are directly involved in the luminescence process
stems largely from the fact that particle size distribution,
and in particular size reduction attempts, are not
universally consistent with the quantum confinement model.
Luminescence from only moderately porous p+ silicon
(20%) has been reported (591 which did not show a
blue shift with repeated HF dipping, and with increased
porosity. On the other hand, this material did show all
of the well known attributes of surface hydride coverage,
i.e. luminescence could be quenched and restored by
hydride removal and replacement. A further report of
luminescence peaking at 1.7 eV measured for n-type
samples sample of less than 10% porosity bas been made
[60]. Particle sizes of around 200 nm were found in
this material and it was claimed that side pores did.not
exist. Such a size distribution is not appropriate for

16. Porous silicon
and clearly show that .the optical gap shrinkage of the
amorphous silicon matches well to the red shift of the
porous silicon sample.
A somewhat more complex model for the involvement
surface molecular species in the form of siloxene related
compounds has been suggested 1641. Siloxene in its
simplest form has the chemical composition SiaO& and
can be prepared from Cash via the reaction
3CaSiz + 6HCI + 3Hz0 = Si6O& + 3CaC12 + 3H2.
The existence of such compounds has been known for
some time, and their fluorescence in the green region of
the visible spectrum is also well known in the chemical
literature [64]. The initial suggestion was that siloxene
or closely related compounds are a by-product of the
electrochemical processing of silicon, which is rich in Si.
H and 0 atoms. The tuning of the luminescence was
suggested tentatively to result from chemical variations to
the basic structure, for example by substituting other ligands
for the H-terminated Si bonds in the sixfold Si ring of
the isolated molecule. Many of the features of porous
silicon luminescence were also seen in the fluorescence
of siloxene: tunability of wavelength, electroluminescence
during anodic oxidation (see section 7), luminescence
fatigue and non-exponential decay.
The tunability of the chemical structure of the siloxenes
and its link to emission wavelength were the main question
marks which militated against the siloxene explanation soon
after it was suggested. In part this shortcoming was due to
a lack of understanding of the physical chemistry of these
materials. More recently [65] it has been demonstrated how
crystalline films on silicon substrates can in principle be
produced by evaporation of calcium followed by reaction
with HCI, i.e. a potential planar technology. Perhaps more
importantly for the present debate, recent quantum chemical
simulations of siloxene [66] crystals have led to a better
understanding of the stability of the system and the way
in which modification by oxygen incorporation can change
the electronic properties.
The idealized crystalline siloxene structure is shown in
figure 19(n) [65], and consists of a silicon plane, terminated
by OH and H radicals on opposite side of the plane.
Calculations suggest that this form is metastable; insertion
of 0 into the bonds of the Si plane gains 1 eV per Si-
@Si bond. Therefore annealing the structure is likely to
transform it into that shown in figure 19(b) [651. The
stoichiometry remains the same, but now that all the 0
atoms have been incorporated into what was the Si plane
notice that isolated Si6 rings begin to appear.
The optical properties of the metastable and annealed
structures are quite different. The metastable structure
fluoresces near 2.6 eV, and has a relatively sharp absorption
edge at only a slightly higher energy, i.e. a fairly small
Stokes shift, The quantum chemical calculations predict
that the Si plane present in the metastable form is a
direct-gap semiconductor with a gap of 2.7 eV at the
point, broadly consistent with the experimental data. The
annealed structure fluoresces in the red, near 2 eV, and
shows a much broader absorption edge and a very much
larger Stokes shift. This is much less like the properties
1201
"18.705 7
1.70t v =
T
1,501 1 ohm-cm porous Si
1.45 T a6:H (Yamasakl et al.)
1.40
100 200 300 400 500 600
Temperature ["Cl
Figure 18. A comparison of the measured shift of the
luminescence spectra of amorphous and porous silicon as
a result of annealing. In both cases the loss of hydrogen
from the system is implicated.
producing quantum size effects. The same report also
detailed measurements of p-type material which was subject
to cyclic (atmospheric) oxidation and HF dipping. This is
without doubt an obvious way to thin the microstructure,
and pore enlargement with an associated reduction in
average particle size is to be expected. However, what
was in fact observed was a cyclic shift of peak wavelength
from 720 nm to 680 nm, the shorter wavelength reappearing
after the HF dip.
A futther observation [60] relating to size distribution
is that high-temperature (up to 1200 "C) annealing of
porous silicon under UHV conditions causes a collapse
of the microstructure, to .the point where the material
resembles a collection roughly spherical particles having a
dimension of a few hundred nanometres. The luminescence
is also quenched. When such material is dipped in HF,
luminescence is restored, even though the particle size
distribution is unchanged.
These inconsistencies' with the quantum confinement
model appear to leave the presence of the surface
hydride species as the only completely consistent factor
in determining whether or not luminescence is present.
The plausibility of this idea gains support from work on
hydrogenated amorphous silicon [61], deposited from the
vapour phase, with high H content. Luminescence from
such material is in the range 1.3 to 2.08 eV, and blue shifts
with increasing H content. This was explained in terms
of polysilane complexes: (SiHz)" or hydride complexes.
Wavelength variation may be a feature of the luminescence
of both entities because the 'gap' of the polysilanes depends
on the chain length, and the SiH, species, according to
tight binding calculations, produce bonding states deep in
the silicon at energies which depend on the H content of
the molecule [62].
On a more practical note, an interesting comparison has
been made [60] between the red shift of the luminescence
of porous silicon induced by annealing, (in an argon
atmosphere), compared with the red shift induced in the
luminescence of hydrogenated amorphous silicon by similar
processing [63]. The data are shown in figure 18 [60],

17. B Hamilton
expected of a direct gap semiconductor, and is similar
to the measured properties of porous silicon. Another
similarity between annealed siloxene and porous silicon
lies in the involvement of the triplet exciton in the low-temperature
luminescence, probed by optically detected
magnetic resonance [66]. These experiments also support
the idea that the exciton is strongly localized, on a scale
compatible with the size of the sixfold Si ring expected to
be present in annealed siloxene.
It has been proposed that the sixfold silicon ring is the
basic luminescence 'centre' for both annealed siloxene and
porous silicon [65]. The spectral properties of properties of
both are similar and the EPR measurements have identified
the Si dangling bond as the key non-radiative shunt for
both. It has been argued that coalescing pores will produce
fragmentation of monolayer silicon that could lead to the
formation of the ring structure. Furthermore it was noted
by the authors of [65] that such a process might be a
simple explanation for the fact that strong luminescence has
been reported from porous amorphous silicon [671, which
appears to relax crystallinity as an absolute prerequisite for
the luminescence.
The siloxene model, like the quantum confinement
model, has many appealing features. However, at the
present time it seems not to explain in a simple way
the smooth shift of wavelength with porosity which is
probably the key result. It must he remembered that this
band can be reliably tuned down to less than 1.4 eV. A
more complete statement seems to be required about the
way in which the sixfold silicon ring, or perhaps some
perturbation to it, might allow such gross tunability. In
contrast, wavelength tunability with size is a natural feature
of quantum confinement.
6. Oxidized porous silicon
Oxidation of porous silicon has received much attention
recently mainly because it produces stable material with
additional emission at short wavelengths, often referred to
as the blue or fast band. Not only is this luminescence
relatively immune to thermal degradation but it exhibits
decay times in the nanosecond regime, a potentially useful
attribute for devices. It must be stressed, though, that
the visible or slow band remains (with some spectral
modification) in oxidized material, and rapid oxidation for
typically 30 s at 900 "C can give stable material which emits
strongly at these longer wavelengths. Higher-temperature
processing than this tends to remove the visible band [5,
681, leaving only the fast band, and of course a much more
fully oxidized material structure.
Figure 20 shows representative spectra of oxidized
porous silicon. It has been noted by several groups that the
intensity of the visible band drops rapidly with oxidation
for oxidizing temperatures up to 600 "C, and then rises
with processing temperature until the melting point of
silicon is reached. Figure 20(a) [69] shows that the visible
band remains, in a broadened blue shifted form, but also
that a higher energy band emerges which extends into the
blue. Of course the latter band requires pumping with an
appropriately short wavelength source. Figure 20(b) shows
-,-
Figure 19. (a) The idealized structure of the Si planes in
as prepared siloxene: the planes are terminated by H or
OH radicals on opposite sides. (b) Siloxene after the
ordered insertion of 0. isolated SiB rings now appear, and
these may be the luminescent centres responsible for
emission from modified siloxenes.
1202

18. Energy (eV)
Figure 20. Photoluminescence spectra typical of rapidly
oxidized porous silicon. (a) The broadening and blue
shifting of the visible band and the appearance of the high
energy band (from 1691). (b) Besides the visible band (a)
measured at 10 K, a lower energy band, the infrared band,
is produced. There is strong competition between the
infrared and visible bands which depends on temperature;
spectra (b), (c) and (d) were measured at 10, 70 and 300 K
respectively.
another key result; the oxidation also produces a low energy
band known as the infrared band. This band which is below
the energy of the bulk gap competes with the visible band,
but at low temperawes becomes as efficient as, or more
efficient than, the visible emission.
Electron paramagnetic resonance has been applied to
most forms of porous silicon. By far the most important
defect to be observed is the dangling bond or Pbo centre.
This centre has been known for some time to be present at
the SiSi02 interface [70]; it is a [ill] axially symmetric
system with the unbonded orbital directed along one of the
four equivalent [ 11 I] directions. The density of this centre
increases when the visible luminescence is quenched by
annealing and it accordingly correlates with hydrogen loss
from the surface [711. The centre has also been detected
via its influence on the intensity of the visible band as the
magnetic field is swept through resonance [72]. The general
conclusion is that the dangling bond is a key non-radiative
shunt path for the visible band.
The optically detected magnetic resonance (ODMR)
experiment shows a large effect for the infrared band; the
intensity of the hand increases by around 15% at resonance.
This compares with values of typically 0.01% variation for
the visible band. The data, as shown in figure 21 [73],
indicate something slightly more complex than a simple P ~ o
Porous silicon
1.22 1.24 1.26
Magnetic field IT1
Figure 21. The strong ODMR signal measured for the
low-energy or IR band [73]. These data prove that the PbO
centre (together with another centre) is directly involved in
the luminescence process responsible for the IR band
centre. The lowest-field peak with an isotropic g value of
2.013 does not belong to the dangling bond, but may belong
to a localized hole. The two higher peaks are signatures of
the Pbo; their g values are 811 = 2.0017 and gl = 2.0085,
where the parallel direction of !he magnetic field is along
the [ 11 11 direction. This anisotropy is exactly that found in
the EPR spectrum of the dangling bond. The strong effect
in ODMR may imply that the dangling bond is directly
involved in the infrared band.
The origin of the blue band is of course of great
interest. One obvious question to he asked is that since
it is much faster than the visible band [74] could it be
the true signature of a direct energy gap in a quantum
confined system? This idea has been supported by several
groups who point to the similarities which exist between the
blue band and emission from direct-gap semiconductors.
Unfortunately the blue band is most easily seen in
highly porous (oxidized) material, and so its wavelength
dependence on porosity cannot be easily explored; i.e. even
this rather uncertain size control has not been explored for
particles which remain in the system. Besides which, the
rather aggressive oxidations used modify the skeleton of
the porous layer. Other workers have supported the view
that the blue band originates from SO?. For example, it
has been reported [69] that the luminescence does not shift
spectrally with increasing oxidation time (up to 50 min).
It has also been noted that oxidized planar silicon wafers
give similar luminescence [75]. In both these references,
the suggestion that defects in the Si02 glass might be
responsible for the emission was made.
7. Electroluminescence and devices
The goal of research into porous silicon is without doubt the
realization of an efficient LED. The wavelength range of
1203

19. B Hamilton
Figure 22. An indium tin oxidelporous silicon
heterojunction LED, showing the device structure and
electrical characteristics.
any potential device is actually a secondary consideration
since any wavelength band has applications. It has become
painfully apparent that to fabricate such a device is hugely
difficult, and quantum efficiencies of 10-6 are typical; even
then large drive voltages, in excess of tens of volts for
example are often required.
A variety of structures with solid state contacts have
been tried, ranging from simple indium tin oxide, to
attempts at p n junction technology. Figures 22 and
23 show two devices which are representative of the
sort of structures appearing in the literature. Figure 22
demonstrates a simple heterojunction technology using
conducting (and transparent) indium tin oxide [761.
Rectification is observed for such contacts with light
emission occurring only for one polarity of voltage.
This particular diode structure produced a rather narrow
emission band, about 20 nm wide, centred on 580 nm,
significantly narrower than a typical room-temperature
photoluminescence spectrum. Although the reasons for
this are not clear, it may he that the electroluminescence
excitation excites only a subset of the porous layer.
A device based on a porous p-n technology is shown
in figure 23 [77]. This device is based on an n-type
wafer with a surface-implanted pf layer. Anodization was
carried out using illumination so that the n-type material
as well as the implanted p layer became porous. The
vertical porosity profile depends very much on the doping
profile, but the active region was thought to be a nanoporous
region which straddled the metallurgical p n junction.
The quantum efficiency of the device was measured to
be lO-4. An interesting attribute of this structure is
that the emission hand seen in electroluminescence can
he tuned by the wavelength used to excite the layer
during the anodization process; the shorter the excitation
wavelength, the shorter the emission wavelength. Although
the reasons for this are not completely established, the
authors point out that in order to supply holes in porous
1204
Figure 23. A porous Si p n ju nction device fabricated by
anodization under illumination of a boron implanted n-type
wafer. (a) Device structure and I-V characteristics.
(b) The electroluminescence output for devices anodized
with different wavelengths of light.
n-type material, and hence for anodization to proceed,
the exciting photon must be strongly absorbed within
the small silicon particles characteristic of nanoporous
material. Following the qualitative expectations of quantum
confinement, very small particles would require short
wavelengths for absorption by the opened up energy
gaps. Whatever the detail, it was demonstrated that
electroluminescence ranging from the infrared to blue could
be produced by varying the anodization excitation from
infrared to ultraviolet. The electroluminescence data are
also shown in figure 23.
A variety of device structures have been tried, but the
key result, the external quantum efficiency, remains low. It
seems an obvious conclusion that the contact technology to
date fails because it does not provide volume excitation of
the porous film. What is required is a contact technology
which transports energy into the whole of the film and
then facilitates local minority carrier injection. Such a
scheme would also demand a continuous current path to the
wafer. Of course optical excitation fulfils all of the required
conditions without the need for current continuity; it is the
perfect excitation source. It has become increasingly clear

20. Wavelength (nm)
Figure 24. (a) The transient (integrated) light emission
from porous silicon during anodization. (b) The spectral
detail of the light emitted during the transient period.
that some innovation in solid state contact philosophy is
required in order to achieve better success.
Liquid contacts represent a state of suitability which is
intermediate between photon beams and solid state contacts,
and the study of such systems has yielded some important
results for our understanding of luminescence processes in
porous silicon. This work has utilized electrolytic solutions
in order to transport charge carriers to the interior of the
porous film. In 'fact the anodic dissolution process used
to create the porous layer is accompanied by some rich
luminescence detail 178, 791. The key observations for
the light emitted during the anodic oxidation process are
shown in figure 24. Firstly there is a time delay before
any emission is observed and the authors ascribe this to
the fact that the porosity is simply too low in the early
stages of anodization for any light to be seen. After this
delay, the luminescence builds up to some peak value and
then decays to zero. The quenching is always associated
with a sharp rise in the required anodic potential (to
sustain a constant anodic current). Although it is estimated
from the total anodic charge transferred (Qo in figure 24)
that the film porosity is only around 50% at the point
of quenching, the authors conclude that at this point the
electrical connectivity between the small crystallites in the
film, which are assumed to be the source of light, is broken.
The electrochemical activity then switches to oxidation of
the base of the pores; a process which is associated with
the increased anodic potential. Actually, light emission is
Porous silicon
also observed in this high anodic potential regime, but it
is of short wavelength and is thought to be associated with
processes occurring in the oxide layers 1801.
The spectral distribution of the light transiently emitted
during the initial stages of anodization, though, shows
remarkable similarity to the visible or slow band, and
it would be surprising if its origin were not the same
as the optically pumped emission. This is shown in
figure 24, which also shows that as anodization proceeds
the luminescence peak blue shifts. This shift has been
analysed by the authors in terms of a tunnelling-limited
escape mechanism for excited carriers which leads to non-radiative
recombination. A model was described for a
tunnelling process occurring through small regions which
connect optically active crystallites to non-radiative bulk
silicon sinks. The yodel demonstrates that because these
regions, which may be even smaller than the crystallites,
are particularly sensitive to shape (and hence bandgap)
changes, the predicted variation of tunnelling flux with
time would produce the observed blue shift. They
also commented that the oxidation-induced thinning of
nanopdcles mentioned above cannot account for the blue
shift: it would be too small. These results then provide
some additional insight into the non-radiative processes,
and are supported by the qualitative observation that the
radiative efficiency of the fluorescent species, perhaps the
nanoparticles, seems to increase as the porosity and hence
the barriers to tunnelling transport increases.
Strictly speaking, many would wish to label these
observations of light emission during anodization as
chemiluminescence, rather than electroluminescence, and
there do remain some problems in understanding the
electron injection process in the overall ,scheme of the
luminescence. The hole of course is provided directly
by the anodic reaction. One possibility for the electron
supply channel may lie in the existence of intermediate
species being formed during the oxidation process of the
silicon wafer; such intermediate species may have energy
levels high enough to allow electron injection [SI]. The
key point here is that the structure is undergoing chemical
transformation during the light emission process. However.
luminescence using liquid contacts has also been achieved
under cathodic conditions [82], and cathodic injection does
not modify the chemistry of the material. The principle
of liquid contact electroluminescence is therefore firmly
established.
8. Conclusions
Research into the physics of light emission from porous
silicon is motivated by a very practical desire to extend
the functionality of silicon. The fact that enthusiasm
remains strong for research into this complex material is a
testimony to this huge technological prize and the challenge
of bringing together the impressively wide ranging
interdisciplinary tools needed to deal with structures of
almost fractal complexity. It is the sheer scale of the
complexity which has limited the interpretation of data and
the success of attempts to make devices.
1205

21. B Hamilton
As regards the basic luminescence mechanism. it
would be wrong to give the impression that views do
not remain divided; quantum structures, surface hydride
species, amorphous silicon and more complex molecular
arrangements like siloxene derivatives all have their
protagonists. It may be that a combination of some or
even all these are present in porous silicon, but it is
more satisfying to think that there is a single mechanism
operating. Such satisfaction is rooted in the traditional
approach of physics, which finds elegance in obtaining the
most complete solution possible for model systems. This
luxury, though, is not available to, say, biologists who deal
with systems in which complexity is the intrinsic dominant
feature.
On balance at the present level of knowledge, the
quantum confinement model has most support. It is easy to
understand why this should be the case: size confinement
and the associated energy shifts are absolutely fundamental
features of quantum mechanics. So, whatever additional
mechanisms might be invoked in porous silicon, quantum
size effects should occur if very small singlecrystal entities
are formed. The quantum particle or wire hypothesis for
the optical activity, then, is viewed as a development of
a straightforward and basic rule of physics. Of course, in
practice this intrinsic effect may be overwhelmed by the
optical activity of other chemical species, or there may
be reasons why quantum sized crystallites simply do not
fluoresce with adequate efficiency to explain the obsewed
light emission. Such objections, though, are details,
especially since the alternative hypotheses are on the whole
more complex than quantum confinement involving exotic
molecular species for which detailed knowledge of the
electronic structure may be missing.
The current debate therefore tends support the view
that whilst the jury is still out on all of the possible
mechanisms, the quantum confinement model remains the
simplest explanation: and until better or more innovative
experimental techniques prove otherwise, simplicity holds
sway.
Looking back over the past several years of research
into porous silicon, however, it seems that this effort
is very much part of a paradigm shift in condensed
matter physics, and particularly in materials research.
The movement towards understanding materials systems
of great complexity is now evidenr One can think of
numerous examples, including semicofiductor superlattices,
high-T, superconducting materials, hierarchical biological
structures characteristic of living matter, and many more.
Probably this research will in the end prove to be valuable
even if devices do not result, because at the very least it has
led to one of the most successful periods interdisciplinary
work and experimental development for many years.
Acknowledgments
The author wishes acknowledge the financial support of the
EPSRC and from the ESPRIT programme for maintaining
his involvement in porous silicon research. Special thanks
are due to his collaborators Ursel Bangert, Phil Dawson,
Spyros Gardelis and Robert Pettifer for their unstinting
efforts and critical approach.
1206
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