This document discusses theoretical modeling and optimization of ultraviolet light-emitting diodes (UV LEDs). It begins by modeling an AlGaN-GaN multi-quantum well UV LED structure using simulation software. Next, it investigates ways to optimize UV LED design by reducing dislocation densities in the active region, such as using pseudomorphic or quasi-pseudomorphic growth on bulk AlN substrates instead of sapphire substrates. The document presents results of simulating current-voltage characteristics and electroluminescence spectra of the modeled LED structure. It concludes by discussing approaches to further improve UV LED performance, such as increasing carrier concentration or using quaternary quantum wells.

1. 
Abstract— Ultraviolet(UV) light emitting diodes (LED’s)
are replacing hazardous UV mercury lamps and find
their application in various sectors like water
purification, non-line of sight communication , germicidal
and medical instrument’s sterilization applications. In
this paper we start with the theoretical modelling of an
AlGaN-GaN multi quantum well UV LED using the Atlas
and Blaze module in SILVACO. Next we proceed to
optimizing the LED design as the potential of UV LED’s
has not yet been fully realized as the output power,
quantum efficiency and lifetime of UV-LEDs have been
limited by the large number of dislocations in the active
region of the devices, arising from the lattice mismatched
sapphire-substrate, which has been the substrate of
choice due to its high transparency to deep UV radiation
and easy and cheap availability. To reduce dislocations in
the active region of the DUV LEDs grown on sapphire,
AlN/AlGaN short period superlattice is usually grown to
manage strain and filter the dislocations. However, the
growth of these thick superlattices takes up a lot of time
and cause major substrate bowing which acts as a
deleterious effect. An alternative method for reducing the
dislocation density in deep UV-LEDs structure is the use
of low defect density bulk AlN substrates, which has more
than two orders of magnitude low defect density than
sapphire/AlN template. In spite of such a low order
density of defects, the quantum efficiency values for DUV
LEDs on bulk AlN substrate are very similar to that of
DUV LEDs on sapphire. Next we investigate the use of
quasi-pseudomorphic growth of UV LED’s and show how
it results in enhanced UV LED performance.
Index Terms—Theoretical modelling, ultraviolet light
emitting diode, pseudomorphic growth,
quasi-pseudomorphic growth.
I. INTRODUCTION
Ultraviolet Light emitting diodes emit light in the UV region
of electromagnetic spectrum. UV radiations can be further
subdivided into four distinct regions: UV-A (320–400 nm);
UV-B (290-320 nm); UV-C or deep UV (200-290 nm); and
vacuum UV (10-200 nm) [1].
In this paper we have initially concentrated on modelling a
multiple quantum well GaN-AlGaN structure LED structure
which emits light in the near ultraviolet regime [2]. The
III-nitride material system has direct band gap, making it an
ideal candidate for the development of optoelectronic
devices. The wide bandgap of this material system allows us
to tune the band gap energies from 0.7 eV for Indium nitride
(InN) to 3.4eV for GaN to 6.2eV for AlN to form a ternary or
quaternary III-nitride alloy system (AlInGaN) . This covers
the wavelength range from infrared to deep ultraviolet region
of spectrum, making it the only semiconductor material
system that covers the deep ultraviolet part of the spectrum
[3]. While modelling we have limited our design to a GaN
substrate due to software restrictions but the general substrate
of choice is sapphire which is transparent to incoming UV
light and is low cost. Until recently the research in UV LED’s
have concentrated more on increasing the carrier
concentration by adding electron and hole blocking layers.[4].
Another approach to improve device performance has been
the use of Quaternary AlInGaN Multiple Quantum Wells
[5].In this paper we have shifted our focus to reduce the
dislocation density and hence delve into newer novel ways to
enhance UV LED performance [7].
Fig1. Band diagram for GaN/InGaN LED with electron
blocking layer
Sapphire is the substrate of choice for deep ultraviolet light
emitting diodes (UVLEDs). Sapphire is primarily used due to
its low cost, high availability, and optical transparency to UV
radiations down to 150 nm range. The large lattice and
thermal mismatches between sapphire and high Al-content
AlGaN epilayers, however, lead to the formation of a large
number of threading dislocations and cracks in the LED
structure. Upon reaching the active region of the LED, these
defects act as non-radiative recombination centers for the
injected carriers, hence reducing the device output power. In
Atreyo Mukherjee
1
School of Electrical, Computer and Energy Harvesting , Arizona State University, Tempe Campus,
Phoenix-85281, United States Of America
ASU ID-1209101292
Theoretical modelling of Ultraviolet multiple quantum Well
GaN LED and optimization’s to improve I-V characteristic’s,
output power and reliability of device.

2. addition to reduced efficiency, the non-radiative
recombination process is directly related to lifetime
degradation of LEDs [6]. The energy emitted through the
non-radiative recombination of carriers is usually in the form
of phonon (heat), which dissipates into crystal lattice and
accelerate the degradation of the device. Due to poor doping
efficiency of high Al-content n-AlGaN layer, DUV-LEDs
exhibit high series resistance, which subsequently leads to
higher operating voltage and results in low wall-plug
efficiency [7].
In order to reduce dislocations in the active region of the
DUV-LEDs grown on sapphire, an AlN buff er layer is first
deposited [8]–[10]. This deposition is followed by the growth
of AlN/AlGaN short-period superlattice, introduced in, to
manage strain in the epilayer and filter the dislocations. The
introduction of the superlattice allows for the growth of thick
n-AlGaN layer without the formation of cracks. The growth
of super lattice is an effective strain management techniques,
however, it suff ers from a number of issues. It is a complex
and time consuming process that suff ers from the problem of
reproducibility, resulting in lower yields and commercial
feasibility. Furthermore, the growth of thick superlattice
structures is often plagued by the introduction of bowing in
the resulting wafer which further complicates the fabrication
process[7].
Another source for reduction in the dislocation density in
DUV-LEDs structure is the use of low defect density bulk
AlN substrates instead of sapphire [11]. The defect density in
homo epitaxial AlN buff er layer grown on bulk AlN substrate
is more than 2 orders of magnitude lower than the AlN buff er
layers grown on sapphire. The bulk AlN based devices
generally employ pseudomorphic structures to avoid epilayer
relaxation. The pseudomorphic structures keep the layers
strained which otherwise could lead to increase in defect
density from less than 106
cm-2
to over 108
cm-2.
The resulting
pseudomorphic AlGaN multi-quantum well structures
exhibited dislocation densities well below 107
cm-2 [12].
The arrangement of this paper goes as follows : A)
Theoretical modelling of AlGaN-GaN multi quantum well
UV LED B) Pseudomorphic AlGaN UV LED C)
Quasi-pseudomorphic AlGan UV LED
A. Theoretical modelling of AlGaN-GaN multi
quantum well UV LED
The main objective was to get the basic I-V-L curve and
electroluminescence spectrum. The whole modelling was
basically divided into 8 parts required to make the basic input
file. These created the structure, specify the model
combination and bias in and finally plot the curves [13].
The substrate of choice was GaN as sapphire was not present
in the ATLAS module as a LED material. The device
structure is given in figure 2. Layers 1, 2 and 3 are P-type with
a doping of 1e20 cm-3. Layers 4 to 10 are the multi-quantum
well regions and are undoped. Layers 11 and 12 are N-type
with a doping of 2e18 cm-3. All doping profiles have a
uniform distribution
While modelling the mesh was first designed with appropriate
meshing which depended on doping concentrations. The
device diameter was taken as 120um. Strain in the device was
measured using the CALC.STRAIN, POLARIZATION
parameters in ATLAS. The ANODE and CATHODE
contacts are defined the top and bottom of the device.
Fig 2. Device Structure of AlGaN-GaN UV LED.
The following results were obtained after running the code:
Fig 3. Current Vs Voltage Plot
Fig 3 shows the anode current vs anode voltage characteristics
for the LED. As an LED is a forward biased PN junction the
I-V characteristics are similar to that of a PN junction. The on
voltage is about 3.38V which is high due to the higher
bandgap of GaN. The anode voltage was swept from 0 to 7
Volts. In theory at higher values of anode voltage the anode
current saturates as the quantum wells are filled with electrons
and any further injection would lead to carrier overflow.
Equation 1 gives the formula for carrier overflow. Nc
represents the effective carrier density. DelEc is the difference
between the fermi energy and the conduction band of the
quantum well.E0 represents quantized states in the quantum
well. WQW is the width of the well which is 3nm in our case.
Joverflow = [(m*
/π(h/2π)2
)(DelEc –Eo)]2
*(eB/WQW) (1)
GaN-p contact-p type- thickness 100nm (LAYER
1)
Al 0.1 GaN-p emitter-p type- thickness 200nm
(LAYER 2)
Al 0.2 GaN-p emitter-p type- thickness 100nm
(LAYER 3)
4 MQW- GaN – thicknes-3nm (LAYER 4,6,8,10))
3 Barriers- Al0.2GaN, thickness-7nm (LAYER
5,7,9)
Al 0.2 GaN-n emitter-n type- thickness 100nm
(LAYER 11)
GaN-n contact-n type- thickness 300nm
(LAYER 12)

3. Fig 4. Output light Vs Anode Current
In Fig 4 the output light vs the anode current can been plotted.
It can be seen that an increase in current leads to an increase in
the output light. Fig 5 shows the power spectral density vs
wavelength plot. The peak wavelength comes out to be
351nm which is in the near ultraviolet range. The full width at
half maximum (FWHM) values are typically related to the
crystalline quality of the material. A better quality epilayer
will have a higher intensity spectrum and narrower FWHM.
The FWHM was this design was calculated to be 9.1nm.
Fig 5. Power spectral Density Vs Wavelength Plot
Next we have shown the electron and hole concentration for
anode voltages of 4V and 5V in Fig 6 and Fig 7.We can see
that the electron concentration increases with increase in
anode voltage. Fig 6 and 7 shows an increase in the electron
and hole concentration in the depth region of 400nm – 443nm.
This shows the active region of the LED structure. We can
further see that increase in anode voltage causes an increase in
electron concentration and hole concentration in the quantum
well rejoin. Thus carrier concentration increases with increase
in anode voltage.
Fig 6.Electron and hole concentration Vs Depth for
anode voltage of 4V
Fig 7. Electron and hole concentration Vs Depth for anode
voltage of 4V
B. Pseudomorphic Growth of bulk AlGaN
Since the dislocations generated from the heterostructure
interface can propagate to the overlaying layers, a smooth and
low defect density bottom layer is needed to reduce the
overall defect density in the LED structure, particularly in the
active region of LED. Hence, a thick AlN layer has been
successfully used to serve as a buffer layer between the
n-AlGaN layer and the substrate, in order to reduce the defect
density and improve the crystal quality of the subsequent
layers.
When growing n-AlGaN layer on AlN buffer layer, the
in-plane lattice parameter of the AlGaN conforms to that of
AlN if the thickness of the AlGaN layer is below a

4. critical value. In this regime, the growth of AlGaN is referred
to as pseudomorphic [14], and all of the strain induced at the
hetero-interface is contained within the AlGaN film, keeping
the crystalline quality of the active layer close to that of the
underlying AlN template. Once the AlGaN is grown beyond
its critical thickness, layer cracking and misfit dislocations are
introduced due to the strain in the film, leaving numerous
nonradiative centers within the quantum well. These
imperfections results in a decline in the LED internal quantum
efficiency and lifetime affecting the device output power
and reliability.
Fig 8: Schematics of the UV-LED epilayer structures:
standard Short period superlattice (relaxed) LED (left) and
pseudomorphic LED (right).
Fig 8 shows the difference in structure between a short period
superlattice and a psuedomorphically grown AlGaN epilayer
on bulk AlN buffer layer.
Fig 9: I-V characteristics of pseudomorphic and standard
SPSL LEDs measured in dc mode [7].
Fig 10: L-I characteristics measured in dc mode for
pseudomorphic and standard SPSL LEDs [7] .
Though a pseudomorhically grown structure reduced
dislocation by two orders we find that the output power and
current is lesser than that of short period superlattice’s. This
shown in Fig 9 and Fig 10 happens due to the use of thin
epilayers (< 0.8um) which leads to high amount of self
heating [7]. The thin n-AlGaN layer in pseudomorphic
growth introduces a serious concern in the
form of high sheet resistance due to its reduced thickness.
This high sheet resistance proved to be a deterrent to device
output power and reliability.
The problems of the psuedomorphic structure was solved
using the quasi –psudomorphic structure.
C. Quasi-Pseudomorphic Growth of bulk AlGaN
It is imperative to address the heating issues by growing
thicker n-AlGaN while maintaining low dislocation density.
This goal must be achieved by keeping the epilayers mostly
strained, so that the dislocations density remain close to that
of the starting template. The thicker n-AlGaN layer exhibits
better sheet resistance, which alleviates the current crowding
and device self-heating issues, thereby, increasing the
Device output power and lifetime.
The main advantage of this approach over comparable strain
management techniques comes from the straightforward
growth process. For comparison, short-period superlattice
structures, an alternate strain management scheme, require
complex and time consuming steps that can lead to increased
fabrication cost and lower commercial feasibility.
Furthermore, thick superlattice structures also suffer from
issues such as severe substrate bowing, further reducing the
yield of these structures

5. Fig11: Schematic diagram of DUV-LED epilayer structures:
standard SPSL LED (a) and Quasi-pseudomorphic LED (b) .
The schematic structures for standard (SPSL) and
quasi-pseudomorphic LEDs are illustrated in Fig 11. The
initial steps in the growth process for both techniques are
similar. Both the samples were grown on c-plane sapphire
substrate. A 2.4 mm thick, high quality AlN buffer layer was
first deposited at ~1200 ºC using pulsed MOCVD . The
dislocation density was measured to be 2 x108cm-2.
Quasi-pseudomorphic LEDs have a simpler epilayer
structure, with the AlN buffer layer followed by 2 mm thick
silicon-doped n-Al0.60Ga0.40N layer. A four-period
multiple quantum wells is grown over the n-AlGaN layer.
Then p-AlGaN/p-GaN p-contact layers were grown to
complete the LED structure. In comparison, for standard
SPSL LEDs, a short period superlattice is also de-posited
before the growth of 3 mm silicon doped n-AlGaN layer
[7].
The primary advantage of quasi-pseudomorphic epilayers are
rooted into a simpler growth and fabrication process that can
lead to significant reductions in manufacturing time and cost.
The sheet resistances have improved significantly over
pseudomorphic LED but still trail behind standard short
period superlattice LEDs. However, the thickness of
quasi-pseudomorphic n-AlGaN can be increased further to
further reduce the sheet resistance, albeit at the cost of higher
density of defects.
Fig12: I-V characteristics of standard SPSL and
quasi-pseudomorphic LEDs measured in dc mode [7].
Fig 13: Output Power Vs Current for quasi-pseudomorphic
and standard SPSL LEDs under dc pump currents [7].
Figure 12 and 13 shows the current Vs voltage characteristic’s
and Output power Vs Current characteristic’s for a
quasi-pseudomorphically grown LED as compared to a
standard one [7]. In both cases we find that the quasi LED
gives better device performance . The quasi-pseudomorphic
LEDs have a partially relaxed structure compared to the
fully-strained structures in pseudomorphic LEDs. The
reduced strain between the adjacent layers allow for the
growth of thicker n-AlGaN layers that can result in the
development of higher optical output devices.
II. DISCUSSION AND CONCLUSION
A major impediment to the development of commercially
feasible DUV-LED is the availability of a well-behaved
native substrate that is both readily available and cost
effective. III-nitride light emitting diodes are generally grown
on a sapphire substrate. Sapphire substrates are inexpensive,
abundant, and offer adequate transparency to radiations down
to 150 nm range. However, sapphire suffers from a number of
serious limitations. The large lattice mismatch between
sapphire and AlN/AlGaN layers creates high levels of strain
in the epitaxial layer of the device. The resulting strain
reduces the likelihood of growing crack-free AlGaN layers.
Devices produced using a mismatched AlN/AlGaN layer on
top of a sapphire substrate often exhibit large number of
dislocations. These defects lead to the creation of
non-radiative recombination centers for the injected carriers,
lowering the overall output power and efficiency of these
device. Short period superlattice structures have been
successfully used to minimize and mitigate the strain between
the mismatched adjacent layers, allowing for the growth
of thick n-AlGaN layers. The fabrication of the superlattice
structures is a complex process that involves lengthy
preparation and requires large amount of materials.
Furthermore, the growth of thick superlattice structures
introduces bowing problems for the resultant wafer. The
bowing of the wafer introduces alignment problems during
the application of masks in the lithography process, which
further reduces the yield during fabrication.

6. In this paper, DUV-LEDs have been developed
pseudomorphically on low defect density AlN/sapphire
template without the superlattice structures. This technique
was initially introduced for bulk AlN substrates but its
commercial applications are limited due to concerns related to
the cost and availability of bulk AlN. By replacing the bulk
AlN with a high quality AlN/sapphire template, the overall
Manufacturing cost of the resulting devices are significantly
reduced while retaining the many advantages offered by
pseudomorphic layers. These devices exhibit better reliability
scores as compared to SPSL relaxed LEDs owing to the lower
dislocation density in the pseudomorphically grown epitaxial
layers. The optical output power of pseudomorphic UV LEDs
is observed to be lower than standard UV LEDs with
superlattice structures. The thickness of n-AlGaN layer must
be increased to reduce its sheet resistance and hence, increase
its output power. The maximum thickness that can be
achieved in high-strain n-AlGaN layer is limited in
pseudomorphic LEDs [7]. Increasing the thickness of the
n-AlGaN layer beyond a certain threshold significantly
increases the dislocation density, reducing the reliability as
well as lowering the optical output power of the device.
To resolve the limitation of pseudomorphic LEDs, an
alternate approach, based on quasi-pseudomorphic n-AlGaN
over AlN/sapphire, has also been suggested here. The
thickness of n-AlGaN current spreading layer has been
increased to 2 mm in the proposed approach as compared to
the 0.6 mm for pseudomorphic LEDs. The suggested
quasi-pseudomorphic LEDs have a partially relaxed structure
compared to the fully-strained structures in pseudomorphic
LEDs. The reduced strain between the adjacent layers allow
for the growth of thicker n-AlGaN layers that can result in the
Development of higher optical output devices. The optical
output power achieved by quasi-pseudomorphic LEDs, is
hown to be greater than that of normal LED. Thus this
technique for growth can be used to enhance the performance
of UV LED’s.
.
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