1. Post-etching mesa surface composition investigation of InAs/GaSb
type-II strained layer superlattices using XPS characterization
B. Klein a,⇑
, K. Artyushkova b
, E. Plis a
, A. Jamus a
, S. Maji a
, L. Casias a
, M.N. Kutty a
, S. Krishna a
a
Center for High Technology Materials, 1313 Goddard Street SE, Albuquerque, NM 87106, United States
b
Advanced Materials Laboratory, 1001 University Blvd. SE, Albuquerque, NM 87106, United States
h i g h l i g h t s
InAs/GaSb superlattice p-i-n and bulk samples were fabricated to compare H3PO4-based and HCl-based treatments.
Spectral and imaging X-ray photoelectron spectroscopy (XPS) analysis were performed on the bulk sample.
Dark current measurements were performed on the p-i-n sample.
XPS and electrical characterization results were compared.
a r t i c l e i n f o
Article history:
Received 25 July 2014
Available online xxxx
Keywords:
MBE
Photodiode
Superlattices
III–V semiconductors
Infrared detectors
a b s t r a c t
XPS characterization was used to determine the surface chemistry of a mid-wave infrared T2SL treated by
both an HCl-based and an H3PO4-based etching solution. This analysis, performed over both the etched
and unetched portions of the sample, revealed that the HCl-based etch removed Ga and Sb oxides while
the H3PO4-based etch removed In and As oxides. XPS imaging was also done on 200 lm  200 lm areas
of the sample, and showed that HCl solution (Ga, and O) produced surfaces that were less stoichiometric
than the H3PO4 solution (Ga2O3, Sb2O5, Sb in GaSb). Single-pixel, p-i-n test structures were fabricated
using either etching solution, and an electrical comparison revealed over an order of magnitude improve-
ment in dark current for the sample treated with the H3PO4 solution, compared to the HCl sample.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
There are many advantages to using InAs/GaSb type-II superlat-
tices (T2SL) as infrared detectors. They are shown to have low tun-
neling currents [1], as well as suppressed Auger recombination [2],
and can be designed to suppress dark currents through hetero-
structure engineering using such structures as nBn [3], CBIRD [4],
M-structure [5], or pBiBn [6]. However, one of the main challenges
still facing this technology is the lack of a widely-accepted surface
passivation technique [7]. Surface leakage paths are formed during
mesa etching [8]; unsatisfied bonds on the newly etched surface
react with the atmosphere [9], forming oxides such as Ga2O3,
Sb2O3 [9], In2O3, As2O3 [10]. These oxides and their elemental
components can facilitate higher surface leakage, surface recombi-
nation velocity, and Fermi level pinning. For example, elemental
antimony and antimony oxides were shown to be a major contri-
bution to diode leakage current [11].
Many passivation methods and materials for T2SL have been
explored in an attempt to alleviate this problem, including
dielectric coatings [12], organic solutions [13], chalcogenides
[14], electro-chemical passivation [15], epitaxial overgrowth [16],
and atomic layer deposition [8]. Generally, these techniques are
performed by first removing the oxides, and then applying a mate-
rial that satisfies the dangling bonds, prevents re-oxidation, and/or
encapsulates the device. Thus, oxide removal is crucial step in the
passivation process, and one of the common ways to do oxide
removal is by chemical etching. However, while much effort has
gone into finding materials for passivation, not as much effort
has gone into studying the effects of a chemical etchant on surface
chemistry. For successful pre-passivation oxide removal on T2SL, it
is unclear what oxides must be removed. Also, it is important that
the etching solution maintains surface stoichiometry, otherwise
surface state density will be increased [17].
We report on our study of the surface chemistry before and
after etching T2SL. Two etching solutions were tested, an HCl-
and an H3PO4-based solution. HCl was selected because it has been
shown to remove surface oxides on GaSb while maintaining
http://dx.doi.org/10.1016/j.infrared.2014.10.010
1350-4495/Ó 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding author.
E-mail address: bklein01@unm.edu (B. Klein).
Infrared Physics Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Infrared Physics Technology
journal homepage: www.elsevier.com/locate/infrared
Please cite this article in press as: B. Klein et al., Post-etching mesa surface composition investigation of InAs/GaSb type-II strained layer superlattices using
XPS characterization, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/j.infrared.2014.10.010

2. stoichiometry [17], and H3PO4 was selected because it had been
used previously in pre-passivation oxide removal [13]. First,
X-ray photoelectron spectroscopy (XPS) was used to determine
the surface chemistries of T2SL etched by the HCl and H3PO4 solu-
tions. Then, to correlate this information with device performance,
a set of single-pixel p-i-n devices was fabricated using either solu-
tion, dark current data were collected, and device performance was
compared between the etching treatments.
2. Experiment
Both the XPS and p-i-n dark current samples (see Fig. 1) were
grown using a solid source molecular beam epitaxy (MBE) VG-80
system equipped with valved cracker sources for the group V Sb2
and As2 fluxes, and Ga/In SUMOÒ
cells. The two XPS samples were
grown on Te-doped (n-type) GaSb epi-ready substrates and con-
sisted of a non-intentionally doped layer 0.6 lm thick of superlat-
tice of 10 monolayers (MLs) InAs/10 MLs GaSb. These samples
were grown consecutively and on pieces from the same substrate,
in order to minimize variation in growth. The p-i-n detector struc-
ture was grown on nominally undoped GaSb (100) epi-ready sub-
strate and was composed of the 8 monolayers (MLs) InAs/8 MLs
GaSb T2SL non-intentionally doped (n.i.d.) absorber layer with
thickness of $3.6 lm (700 periods) grown on top of the 200 nm
thick p-type GaSb layer serving as a bottom contact. The structure
was finished by a 144 nm thick n-type top contact layer formed by
the T2SL with the same thickness and composition as the absorber
layer. Doping concentrations were 2 Â 1018
cmÀ3
for n-type top
and p-type bottom contact layers. To improve the transport of
photo-generated carriers, the doping of top 50 periods and bottom
10 periods of T2SL absorber region was graded. This sample was
cleaved into two pieces after growth, with one half used in the
H3PO4-based solution, and the other for the HCl-based solution.
After growth, half of each of the two XPS samples were covered
in photoresist and the other half was left exposed (defining an
unetched area, an edge, and an etched area), and then each was
etched in either an HCl-based solution (HCl:H2O2:H2O = 1:1:4) or
an H3PO4-based solution (H3PO4:H2O2:H2O = 2:1:20). After etch-
ing, the photoresist was removed with acetone, isopropyl alcohol,
and dried with nitrogen.
For the electrical performance study, device fabrication was ini-
tiated with a standard optical photolithography to define
410 lm  410 lm square mesa devices with apertures ranging
from 25 to 300 lm. With the same mask variable area diode
(VADA) arrays were defined with mesa side size varied from
30 lm to 400 lm. Detector mesa delineation was performed using
inductively coupled plasma (ICP) reactor with BCl3 gas. Next,
ohmic contacts were evaporated on the bottom and top contact
layers using Ti (500 Å)/Pt (500 Å)/Au (3000 Å) in both cases.
Finally, devices were passivated by SU-8 2007 photoresist. Prior
to passivation, the devices were immersed into phosphoric acid
based (H3PO4:H2O2:H2O = 1:2:20) or hydrochloric acid based
(HCl:H2O = 1:10) solutions for 30 s to remove any native oxide film
formed on the etched mesa sidewalls. Note that the recipes for
these acid solutions have changed compared to the ones used for
the XPS samples. The spectral response of the samples was taken
to confirm device operation; a representative plot of the spectral
response (from the H3PO4 sample) is presented in Fig. 2.
The XPS characterization was performed with a Kratos Ultra
DLD spectrometer using two different analytical methods, spectral
XPS and imaging XPS analyses. Small area XPS spectra were col-
lected in 55 lm areas, analyzing areas in increments of 50 lm from
the edge on both the etched and unetched sides. High resolution
200 by 200 lm photoelectron images with the edge in the center
were acquired. These images were acquired within As, Ga, In and
Sb/O spectral regions with 0.5 eV increments. All the images were
combined into multispectral data sets and principal component
analysis (PCA) was applied. The output of multivariate analysis
are images that show spatial distribution of different phase. The
type of chemical bonds or elements that have similar spatial distri-
bution are combined into one chemical map showing this particu-
lar spatial distribution. Spectral XPS analysis provides percentages
of elements and a quantitative comparison, while photoelectron
imaging indicates which elements are present on a surface and
allows for a qualitative comparison.
3. Results and discussion
First, we studied the surface chemistries of T2SL samples before
and after H3PO4- and HCl-based treatments. The results of XPS
spectroscopic analysis are plotted in Fig. 3. Plots a and b are the
H3PO4 results and c and d are the HCl results. Each plot has a ver-
tical line running down the center, representing the edge between
the etched (right) and unetched (left) sides of the sample. Legends
for all data are shown in plots c and d. Plots a and c are for the Ga
and Sb components, while b and d are for the In and As compo-
nents. Comparing the surface chemistries before and after etching
for both solutions reveals that the etched surfaces are very similar
in composition, while the unetched sides are not. This may indicate
that there was variation in growth or sample handling prior to
etching. Plot a shows little change in Ga and Sb before and after
etching with H3PO4, while the plot c shows a large reduction in
Ga and Sb oxides after etching with HCl (Sb–GaSb and GaSb both
Fig. 1. MBE-grown structures used for this study. Fig. 2. Representative plot of spectral response from fabricated devices at 77 K.
2 B. Klein et al. / Infrared Physics Technology xxx (2014) xxx–xxx
Please cite this article in press as: B. Klein et al., Post-etching mesa surface composition investigation of InAs/GaSb type-II strained layer superlattices using
XPS characterization, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/j.infrared.2014.10.010

3. increased by $25% and 57%, with an equal but opposite change in
Sb2O5, and Ga2O3, respectively), with very little change in the In
and As components. The opposite is true for the H3PO4, where
the As and In have large changes for the H3PO4 etch (As–In and
In–As both increase by $25% and 40%, while As2O3 and In2O3
reduced by the same amounts, respectively), but little change
when using HCl.
The PCA for XPS showed that unetched surfaces for both sam-
ples were similar (HCl: In, As, Sb in SbGa/Sb2O5 and H3PO4: In,
As, O in In2O3, Sb2O3) while the etched sides had drastically differ-
ent surface chemistries (HCl: Ga, O and H3PO4: Ga2O3, Sb2O5, Sb in
GaSb). This qualitative characterization shows evidence that the
HCl-based solution produces a surface which does not preserve
stoichiometry as well as the H3PO4.
Next, we studied effectiveness of HCl-based and H3PO4-based
native oxide removal treatments on T2SL detector performance.
Temperature-dependent dark current densities of large-area pin
detectors, 400 lm  400 lm, treated with HCl-based and
H3PO4-based native oxide removal solutions are presented in
Fig. 4. Measurements were performed in close-cycle cryostat under
true dark current condition. At 80 K and small negative bias
(À0.1 V), detector treated with H3PO4-based solution demon-
strated dark current density by factor of 19 lower compared to
the device treated with HCl-based solution (mean = 4.5 Â 10À4
A/
cm2
, standard deviation (r) = 3.8 Â 10À4
A/cm2
and mean = 8.6 Â
10À3
A/cm2
, r = 8.8 Â 10À3
A/cm2
, respectively, averaged over
three devices per sample). Moreover, current–voltage (IV) charac-
teristics of detector treated with H3PO4-based solution were less
symmetric throughout all the range of measured temperatures.
To further investigate effect of two different native oxide
removal treatments on T2SL detector performance, Arrhenius plots
were made for each sample and dark current densities of several
VADA devices were evaluated. Arrhenius plots in Fig. 5 show that
each sample has two different dominating mechanisms for dark
current. At high temperatures (above 80 K), the dark current is
generation–recombination dominated, while at low temperatures
it is dominated by some other mechanism such as tunneling or sur-
face current. From the spectral response data, the bandgap is
approximately 0.25 eV. The black lines are fits to each of these sec-
tions using Jd / eEa=kT
, where Jd is dark current and Ea is activation
Fig. 3. HCl and H3PO4 etching XPS results.
Fig. 4. Dark current densities for H3PO4- and HCl-based etching solutions.
B. Klein et al. / Infrared Physics Technology xxx (2014) xxx–xxx 3
Please cite this article in press as: B. Klein et al., Post-etching mesa surface composition investigation of InAs/GaSb type-II strained layer superlattices using
XPS characterization, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/j.infrared.2014.10.010

4. energy. Extracted activation energies were 0.19 eV, or 3/4 of Eg
(H3PO4) and 0.1 eV, or 0.4 of Eg (HCl) within the diffusion limited
regime and 1 meV (H3PO4) and 2 meV (HCl) within the low tem-
perature regime. The H3PO4 sample had lower dark current and a
sharper knee than the HCl sample. Fig. 6 presents the dark current
densities measured at 77 K and different values of applied bias
(À0.1 V, À0.2 V, À0.3 V) for devices treated with HCl-based and
H3PO4-based solutions. Whereas data scattering, attributed to the
growth and fabrication non-uniformity, is observed for both treat-
ments, it is less notable for devices treated with the H3PO4-based
solution.
4. Conclusion
We have investigated the effect of two etching solutions, H3PO4
and HCl based, on the surface chemistry and electrical properties of
T2SL. XPS analysis of the etched and not etched samples showed
that H3PO4 reduced the amount oxides of In and As, while HCl
reduced the oxides of Ga and Sb. Imaging XPS indicated that HCl
produced a less stoichiometric surface than the H3PO4 etch,
because the HCl-etched surface had Ga and O, while the
H3PO4-etched surface had Sb, Ga, and oxides. This implies that
the surface quality of a sample etched in H3PO4 has fewer surface
states than HCl. A dark current comparison between two samples
produced with the two surface treatments also showed
approximately an order of magnitude better performance for the
sample treated with the H3PO4-based solution than the HCl-based
solution. Dark current curves were more symmetrical on the
HCl-treated sample.
Conflict of interest
There is no conflict of interest.
Acknowledgments
We would like to thank AFOSR and the United States
Government for their support of this project.
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Fig. 5. Arrhenius plot of dark current density for the HCl and H3PO4-based etching
solutions at À0.3 V bias. Activation energies are shown for the fitted lines.
Fig. 6. Dark current density as a function of perimeter over area for several reverse
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Please cite this article in press as: B. Klein et al., Post-etching mesa surface composition investigation of InAs/GaSb type-II strained layer superlattices using
XPS characterization, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/j.infrared.2014.10.010