This document summarizes an investigation of the surface composition of InAs/GaSb type-II strained layer superlattices (T2SLs) using X-ray photoelectron spectroscopy (XPS) characterization before and after mesa etching with two solutions: hydrochloric acid (HCl) and phosphoric acid (H3PO4). XPS analysis found that HCl etching reduced gallium and antimony oxides while H3PO4 etching decreased indium and arsenic oxides. Current-voltage measurements and dark current density versus perimeter-to-area ratio indicated better device performance when treated with the H3PO4 solution, suggesting it provides better surface passivation. Future work may explore different etching solutions or processing

1. Post-etching Mesa Surface Composition Investigation of InAs/GaSb
Type-II Strained Layer Superlattices Using XPS characterization
Status Quo and Importance:
Results:
Extensive literature has shown that T2SL
superlattices possess many advantages (low
tunneling currents, suppressed Auger
recombination, heterojunction engineering
minimizing dark current generation, and
bandgap tuneability) that make them useful as
structures for the creation of IR-detectors.
Unfortunately T2SL technology is limited by
carriers’ low vertical mobility and low lifetimes,
as well as a lack of an adequate passivation
scheme to make parts of the superlattice
(specifically the sidewalls) electrically inactive
such that dark-current generation is minimized,
as is its resulting negative impact on the accuracy
and effectiveness of the detector.
T2SL Structure:
Purpose:
An adequate surface passivation scheme
has yet to be identified. Dielectric coating,
organic-based and chalcogenide solutions,
and ECP-overgrowth have been attempted
as post-processing passivation treatments,
but little has been done with chemicals
involved in the processing itself.
Our goal is to characterize the sidewall
and surface chemistry for two etching
solutions (HCl and H3PO4) to determine
the sidewall composition before and after
mesa etching.
Conclusion and Future Work:
Acquire photoelectron images
in energy range of elements
of interest (0.5eV increments);
Combine images into
multispectral data-sets; Apply
principal component analysis;
Formulate resulting chemical
map.
Approach and Prior Work:
GaSb : UN
1/4 of 2” wafer
GaSb
P+ (2e18), Be-doped
200 nm
8/8 SLS (InAs/GaSb)
Graded doping
10 periods
8/8 SLS (InAs/GaSb)
NID (intrinsic-
doping)
700 periods
8/8 SLS (InAs/GaSb)
Graded doping
50 periods
8/8 SLS (InAs/GaSb)
N+ (2e18), Te-doped
30 periods
P-I-N Structure was grown and processed into single-pixel devices
Processing Steps:
1. Mesa etch (ICP): H3PO4 at 0.1 μm/min, HCl at 1.9 μm/min
2. Metalization (500 A Ti / 500 A Pt / 3000 A Au)
3. Acid dip: [H3PO4 : H2O2 : H2O (1 : 2 : 20), HCl : H2O (1 : 10)] and SU8
encapsulation
Sample
AZ 4330
1. AZ 4330
photoresist spun
on sample
2. Photoresist is
patterned using
lithography
3. Sample is
immersed in
etching solution
for 1 minute
4. Photoresist is
removed with
acetone and
isopropyl alcohol
1 2 3 XPS Characterization:
Unetched
Etched
 Sideview of Sample, 55mm-diameter areas
Unetched
Etched
Edge
50mm
Quantitative Analysis  Qualitative Analysis
*Etchant choice
based on the
fact that HCL
has been used
previously in
T2SL XPS
characterization
s and H3PO4 is
commonly used
as an etchant
for GaAs
Binding Energy (eV)
CPS
Ga2O3, Sb2O5
Sb in GaSb
Etched
HCl Etch H3PO4 Etch
In, As, Sb in
SbGa/Sb2O5
Unetched
Ga,O
Etched
In, As, O in
In2O3, Sb2O3
Unetched
Aperture
Variable-Area Diodes
Single-pixel Test Structures
Bottom metal
Top metal
H3PO4 Solution HCl Solution
Current vs. Voltage: Device size: 400x400 mm squares, 20K – 240K
XPS Characterization:
Arrhenius Equation
(for plot below) Solution Qualitative Quantitative Comment
HCl Unetched In, As, Sb in SbGa/Sb2O5 Ga2O3, Sb2O5, AsIn, InAs Etching reduces percentage of
Ga and Sb oxides.
No change for InAs, AsIn,
As2O3, In2O3
Etched Ga, O GaSb, Sb/GaSb
H3PO4 Unetched In, As, O in In2O3, Sb2O3 AsIn, InAs, GaSb,
Sb/GaSb, Ga2O3, Sb2O5
Etching decreases percentage
of In and As oxides.
Sb/GaSb and GaSb remain
about the same.
Etched Ga2O3, Sb2O5, Sb in GaSb As2O3, In2O3, AsIn, InAs,
Sb/GaSb,GaSb
Anomalously
High?
Current Density vs. Perimeter/Area
Etching solutions: HCl : H2O2 : H2O (1:1:4), H3PO4 : H2O2 : H2O
(2:1:20)
Surface chemistries found on etched material:
H3PO4 removes oxides of As and In, HCl removes oxides of Sb and Ga
Device performance appears better when treated with
H3PO4-based solution, based on current-voltage data
and Jd vs P/A. For surface-related studies, less variance over
Perimeter/Area is a good indicator of device quality since a low and
uniformly predictable dark current is ideal. Future work in this area
may include experimentation with different types of etching solutions
or differing the chemical compositions of other components of
processing (provided they are still effective for the processing itself) to
see if such changes have any effect on the passivation of this device
and/or others like it. Other strategies for dark-current minimization
including defect characterization may constitute another approach for
attacking the dark-current problem.
By Shaleena Maji, Cornell University College of Engineering (Mentor: Brianna Klein; PI: Dr. Sanjay Krishna)