Dislocation Etching Solutions for Mercury Cadmium 

Selenide

by Kevin Doyle and Sudhir Trivedi

ARL-CR-0744 September 2014

prepared by

Oak Ridge Associated Universities

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Belcamp MD 21017

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Army Research Laboratory

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ARL-CR-0744 September 2014

Dislocation Etching Solutions for Mercury Cadmium 

Selenide

Kevin Doyle and Sudhir Trivedi

Sensors and Electron Devices Directorate, ARL

prepared by

Oak Ridge Associated Universities

4692 Millennium Drive, Suite 101

Belcamp MD 21017

under contract

W811NF-12-2-0019

Approved for public release; distribution unlimited. 
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Dislocation Etching Solutions for Mercury Cadmium Selenide

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Kevin Doyle and Sudhir Trivedi

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14. ABSTRACT

Mercury cadmium selenide (Hg1-xCdxSe) is a possible alternative material to mercury cadmium telluride (Hg1–xCdxTe) for 

infrared (IR) sensor applications, but etch pit density (EPD) measurements are required to measure dislocations that affect 

device performance. No EPD solutions have been reported for Hg1–xCdxSe, and standard EPD solutions for Hg1–xCdxTe have 

proved ineffective. Thus, a new etching solution is required for EPD measurements of Hg1–xCdxSe. Samples were etched in 

various solutions and the resulting pits were observed using Nomarski microscopy and scanning electron microscopy (SEM). 

Solutions consisting of nitric and hydrochloric acid produced mainly trapezoid-shaped pits, but with flat or rounded bottoms 

rather than converging to a single point as expected. One solution consisting of nitric, hydrochloric, and phosphoric acid 

produced hexagonal pits that converged at a single point as expected, but this solution was unstable and these pits could not be 

repeated on any other sample. Further experiments are required to produce an etching solution that consistently forms pits that 

converge on a single point and then transmission electron microscopy (TEM) measurements will need to be performed to 

confirmed that these pits correspond to a dislocation—thus enabling EPD measurement of Hg1-xCdxSe.

15. SUBJECT TERMS

Mercury cadmium selenide, etch pits, dislocations, preferential etching solutions

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iii

Contents

List of Figures iv

Acknowledgments v

1. Introduction 1

2. Etch Solutions 1

2.1 Nitric and Hydrochloric Acid..........................................................................................1

2.2 Nitric Acid, Hydrochloric Acid, and Complexing Agents..............................................5

2.3 Nitric, Hydrochloric, and Phosphoric Acid.....................................................................7

2.4 Polisar Etch......................................................................................................................9

3. Etch Pit Summary 10

4. Etching of ZnTe Buffer Layer 10

5. Further Work 11

6. References 13

List of Symbols, Abbreviations, and Acronyms 14

Distribution List 15
iv

List of Figures

Fig. 1 Hg0.84Cd0.16Se sample SZ48-E2 viewed under Nomarski 100×, a) unetched and b) 

etched 20 s in HNO3:HCl (2:1), then 20 s 50% H2SO4 .............................................................2

Fig. 2 Hg0.84Cd0.16Se sample SZ48-E3 a) unetched, and etched 20 s in HNO3:HCl (5:4) 

viewed under, b) Nomarski 100×, c) SEM 4×,485, and d) SEM 25×,374 ................................3

Fig. 3 Pieces of Hg0.79Cd0.21Se sample SZ56 viewed under Nomarski 100× after etching in 

HNO3:HCl (2:1) for a) 5, b) 10, c) 15, and d) 20 s. The red lines were a measure of pit 

size. ............................................................................................................................................4

Fig. 4 Pit size (top) and etch depth (bottom) vs. etch time for pieces of SZ56 etched in 

HNO3:HCl (2:1).........................................................................................................................4

Fig. 5 Hg0.81Cd0.19Se sample SZ50-GR1 viewed under Nomarski 100× a) pre-etch and b) 

etched 5 s in HNO3:HCl:H2O (2:1:3).........................................................................................5

Fig. 6 Hg0.82Cd018Se sample SZ45-E5 under Nomarski 100× a) pre-etch and b) etched 10 s 

in HNO3:HCl:HF (4:2:1)............................................................................................................5

Fig. 7 Hg0.81Cd0.19Se sample SZ50-GR12 a) pre-etched, Nomarski 100× and then etched 40 

s in HNO3:HCl:CH3COOH (2:1:1) viewed under, b) Nomarski 100×, c) SEM 6,246×, 

and d) SEM 64,802×..................................................................................................................6

Fig. 8 Hg0.81Cd0.19Se sample SZ50-GR17 etched 20 s in HNO3:HCl:C3H6O3 (2:1:2) viewed 

under a) SEM 39,749× and b) SEM, 51,548× ..........................................................................7

Fig. 9 Hg0.71Cd0.29Se sample SZ40-E3 etched 20 s in HNO3:HCl:H3PO4 (20:10:5) viewed 

under a) Nomarski 20×, b) Nomarski 100×, c) SEM 2,554 ×, d) SEM 13,526×, e) SEM 

9,572×, and f) SEM 16,098× .....................................................................................................8

Fig. 10 Sample SZ6 viewed under Nomarski 100× a) unetched, b) etched in Polisar for 1 

min, c) etched in Polisar for 2 min, and d) etched in Polisar for 2 min then Br2-methanol 

for 2 s .........................................................................................................................................9

Fig. 11 Piece of ZT072406N viewed under Nomarski 20× a) unetched, and etched 

HNO3:HCl:C3H6O3 (2:1:2), b) 5 s, c) 10 s (center), and d) 10 s (near edge)..........................11
v

Acknowledgments

We would also like to thank Dr J David Benson at the Night Vision and Electronic Sensors 

Directorate for his input on the process. Research was sponsored by the US Army Research 

Laboratory and was accomplished under Cooperative Agreement # W911NF-12-2-0019.
vi

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1

1. Introduction

Mercury cadmium selenide (Hg1–xCdxSe) is a possible alternative material to mercury cadmium 

telluride (Hg1–xCdxTe) for infrared (IR) sensor applications. The bandgap of Hg1–xCdxSe can be 

tuned across the same spectral regions as Hg1–xCdxTe, and Hg1–xCdxSe is also closely latticematched to gallium antimonide (GaSb). Since GaSb is available as a large-area substrate, 

Hg1–xCdxSe epitaxial layers can potentially be grown on GaSb substrates with fewer misfit 

dislocations, which have been shown to be detrimental to long wave infrared Hg1–xCdxTe

devices.

1

In order to verify this, a reliable technique for measuring dislocations needs to be established for 

Hg1–xCdxSe. For samples where the dislocation densities are too low (<108 cm–2) to measure with 

tunneling electron microscopy (TEM), this is typically done with etch pit density (EPD) 

measurements. EPD measurements are performed by placing samples in a solution that etches 

slowly for the polarity and main orientation of the sample, but etches at a faster rate for other 

crystallographic orientations. As a result, the etch produces a pit where dislocations intersect the 

crystallographic surface, so the density of dislocations can be measured from the density of pits 

observed after etching.

The morphology and size of etch pits will vary depending on the etch solution and the 

crystallography of the material, but in general the etch pits will form geometric shapes that 

emanate from a single point where the dislocation is located.2 No EPD solutions have been 

reported for Hg1–xCdxSe, and standard EPD solutions for Hg1–xCdxTe and cadmium zinc 

telluride, such as the Benson etch3 and the Everson etch,

4 proved ineffective. In order to establish 

an EPD solution for Hg1–xCdxSe, Hg1–xCdxSe samples were etched in various solutions and the 

resulting pits were observed using Nomarski microscopy and scanning electron microscopy 

(SEM). The samples were grown via molecular beam epitaxy on silicon substrates with zinc 

telluride buffer layers (ZnTe/Si).5

2. Etch Solutions

2.1 Nitric and Hydrochloric Acid

Previously, solutions consisting of nitric acid (HNO3) and hydrochloric acid (HCl) had been 

shown to produce etch pits on mercury selenide (HgSe) and cadmium selenide (CdSe),

6 and so 

solutions consisting of these two acids were tested. It was expected that the pits would be 

triangular, which is typical for the Benson etch on Hg1–xCdxTe. However, a solution of 

HNO3:HCl (2:1) produced trapezoid-shaped pits, as seen in Fig. 1. Previous reports suggested 

that these etchants could leave an Se-film on the surface, so initially the samples were etched 
2

briefly dilute sulfuric acid (50% H2SO4) to remove any remaining films. The pits had the same 

shape and orientation, which is expected for a dislocation etch pit, but they appeared to have 

curved bottoms rather than the expected faceted walls emanating from a single point.

Fig. 1 Hg0.84Cd0.16Se sample SZ48-E2 viewed under Nomarski 100×, a) unetched and b) etched 20 s in HNO3:HCl 

(2:1), then 20 s 50% H2SO4

At first, the HNO3:HCl ratio and etch time were varied to control the etching speed and 

formation of the pits. However, changing the ratio started changing the shape of the pit, making 

the shape less defined, as seen in Fig. 2. Based on this, it was decided that the (2:1) HNO3:HCl 

ratio was preferred. An estimate for the etch rate for this solution was determined by etching 

pieces of an Hg0.79Cd0.21Se sample SZ52 for different etch times, and then using a Tencor step 

profilometer to determine the change in sample height. The base-to-height length of the 

trapezoidal pits was also estimated from Nomarski 100× images, as shown in Fig. 3.

Based on these rough measurements, given in Fig. 4, it appears that HNO3:HCl (2:1) etched at a 

rate of roughly 0.14 μm/s. Measurements from sample SZ56-GR5 (the sample etched for 25 s) 

are probably not trustworthy as at that point as the etch had been heavily used on the previous 

samples, so the etch was remixed for the 30-s etch. Not counting SZ56-GR5, the pit size 

appeared to increase at a rate of 0.82 μm/s for 20 s, then it leveled off at around 16 μm.

In order to control the etch rate, it was decided to keep the HNO3:HCl ratio at (2:1) and add 

complexing agents to slow down the etch process. 
3

Fig. 2 Hg0.84Cd0.16Se sample SZ48-E3 a) unetched, and etched 20 s in HNO3:HCl (5:4) viewed under, b) Nomarski 

100×, c) SEM 4×,485, and d) SEM 25×,374
4

Fig. 3 Pieces of Hg0.79Cd0.21Se sample SZ56 viewed under Nomarski 100× after etching in HNO3:HCl 

(2:1) for a) 5, b) 10, c) 15, and d) 20 s. The red lines were a measure of pit size.

Fig. 4 Pit size (top) and etch depth (bottom) vs. etch time for pieces of SZ56 

etched in HNO3:HCl (2:1)
5

2.2 Nitric Acid, Hydrochloric Acid, and Complexing Agents

Complexing agents were then added to the HNO3:HCl (2:1) solution. These include de-ionized 

water (H2O), hydrofluoric acid (HF), lactic acid (C3H6O3), acetic acid (CH3COOH), and

phosphoric acid (H3PO4). Solutions with H2O produced some discoloration, as seen in Fig. 5, 

which at first was thought to be an oxidation effect, but could also be the etch solution reaching 

the ZnTe layer as discussed in section 4. A solution of HNO3:HCl:HF (4:2:1) only appeared to 

roughen the surface (Fig. 6), so H2O and HF were dropped as complexing agents. 

Fig. 5 Hg0.81Cd0.19Se sample SZ50-GR1 viewed under Nomarski 100× a) pre-etch and b) etched 5 s in 

HNO3:HCl:H2O (2:1:3)

Fig. 6 Hg0.82Cd018Se sample SZ45-E5 under Nomarski 100× a) pre-etch and b) etched 10 s in HNO3:HCl:HF 

(4:2:1)

Samples etched with an acetic acid and lactic acid produced the trapezoidal pits, but with the 

same curved bottoms and pits-within-pits observed before, as shown in Fig. 7. Some samples 

etched in solutions with lactic acid appeared to produce pits with well-defined walls, as seen in 

Fig. 8, which is to be expected for a pit corresponding to a dislocation. However, these pits still 

had flat bottoms, rather than converging on a single point, as should be the case.
6

Fig. 7 Hg0.81Cd0.19Se sample SZ50-GR12 a) pre-etched, Nomarski 100× and then etched 40 s in 

HNO3:HCl:CH3COOH (2:1:1) viewed under, b) Nomarski 100×, c) SEM 6,246×, and d) SEM 64,802×
7

Fig. 8 Hg0.81Cd0.19Se sample SZ50-GR17 etched 20 s in HNO3:HCl:C3H6O3 (2:1:2) viewed under a) SEM 

39,749× and b) SEM, 51,548×

2.3 Nitric, Hydrochloric, and Phosphoric Acid

Samples etched in solutions containing H3PO4 mostly produced trapezoidal pits, but the exact 

shape of the pits was inconsistent from sample to sample or even across the surfaces of the same 

sample. The color of the etch solutions was observed to change rapidly, and the etching rate 

quickly deteriorated once the solution was made, making these etching results very inconsistent.

However, one section of Hg0.71Cd0.29Se sample SZ40-E3 etched 20 s in HNO3:HCl:H3PO4

(20:10:5) produced pits that were hexagonal, rather than trapezoidal, as seen in Fig. 9.

Furthermore, while some of these pits appeared to be filled with debris, the clear pits appeared 

to converge on a single point with faceted walls, which is what is expected for pits emanating 

from a dislocation. Moreover, Figs. 9a and 9b shows pits along slip lines, indicative of revealing 

the dislocations. These hexagonal pits were the most likely to represent dislocations, but due to 

the volatility of the etch solution these pits could not be reproduced on any other samples. 

Additionally, while many of the hexagonal pits are clear, others appear to be filled with debris. 

This could be from material being redeposited back into the etch pits rather than being removed 

in the etch process. By extension, this redeposition back into the pits could be the reason the 

trapezoidal pits appear to have curved uneven bottoms rather than the expected single-point 

bottoms that would indicate a dislocation.
8

Fig. 9 Hg0.71Cd0.29Se sample SZ40-E3 etched 20 s in HNO3:HCl:H3PO4 (20:10:5) viewed under a) Nomarski 20×, 

b) Nomarski 100×, c) SEM 2,554 ×, d) SEM 13,526×, e) SEM 9,572×, and f) SEM 16,098×
9

2.4 Polisar Etch

Prior to any of the etch work discussed above, one attempt was made to use the Polisar etch on 

Hg1–xCdxSe. A previously reported etching study of bulk-grown Hg1–xZnxSe wafers cleaved 

along the [111] surface gave EPD measurements after soaking in an ambient solution of 

90:60:25:5 H2O, HNO3, HCl, and 0.1 cc bromine (Br2) in acetic acid. This etch produced a Se 

film, which was removed by placing the samples in a Br2-methanol solution for 1–2 s.7 

A couple Hg1-xCdxSe samples were etched in this solution, which produced some small pits after 

~2 min, as seen in Fig. 10. At the time, we thought the pits looked too circular, but perhaps they 

were not given time to form. 

Fig. 10 Sample SZ6 viewed under Nomarski 100× a) unetched, b) etched in Polisar for 1 min, c) etched in Polisar 

for 2 min, and d) etched in Polisar for 2 min then Br2-methanol for 2 s
10

3. Etch Pit Summary

Though the distinctness of the etch pits varied with the etch solution, many different solutions 

produced recognizably trapezoidal pits. These pits likely correspond to dislocations for the 

following reasons:

1. Shape is consistent: Pits are consistently trapezoidal, except for the one case where they 

were hexagonal.

2. Orientation is consistent: The trapezoids are always orientated perpendicular to the [211] 

wafer flat, suggesting they are sensitive to the crystallography.

3. Slip lines: As we went on, we began inducing slip lines by indenting the samples with a 

pin prior to etching. The pits would form along the lines of stress (the ones we created and 

other ones we didn’t), which was expected as dislocations also form along stress lines. 

However, the trapezoidal pits do have some features that are inconsistent with dislocation pits:

1. Walls are not faceted: A dislocation pit should have clearly faceted walls that correspond 

to the crystallographic faces, while the trapezoidal pits are often curved. 

2. The pits do not converge to single point: The bottom of a dislocation pit should be at a 

single point corresponding to the dislocation. Most of the trapezoidal pits have curved or 

flat bottoms. 

The hexagonal pits did have faceted walls and single-point bottoms, and some of these pits also 

appear to have debris leftover from etching. Thus it’s possible that one reason for the odd shape 

of the trapezoidal pits is that material is being redeposited in the pit during etching, covering up 

the faceted walls and producing the curved/flat bottoms instead. Further tests (possibly TEM) 

should be performed to confirm this. 

4. Etching of ZnTe Buffer Layer

Greater control over the etch rate is needed because of the effect these etchants appear to have on 

the ZnTe buffer layer. After etching, some areas of the samples appeared to suddenly develop 

very rough and discolored surfaces. This was attributed to the fact that the acid-based etchants 

have a strong effect on ZnTe. A ZnTe/Si sample, ZT072406N, was etched for 5 s, then an

additional 5 s in a HNO3:HCl:C3H6O3 (2:1:2) solution. After 5 s, the surface became very rough, 

and after 10 s, the layer was actually dissolving leaving the Si substrate behind near the edges, as 

seen in Fig. 11. 
11

Fig. 11 Piece of ZT072406N viewed under Nomarski 20× a) unetched, and etched HNO3:HCl:C3H6O3 (2:1:2), 

b) 5 s, c) 10 s (center), and d) 10 s (near edge)

Given this effect, the etch rate for EPD measurements for these samples needs to be precisely 

controlled to ensure the etch solution does not reach the ZnTe buffer layer.

5. Further Work

While we now have etch solutions that can produce pits that could correspond to dislocations, 

further work is required to produce a standard EPD process for Hg1–xCdxSe. The final EPD 

solution for Hg1–xCdxSe should have the following features:

1. Consistent etch rate: While some decrease in etch rate is to be expected as the solution 

sits out in the hood, that decrease needs to be slow enough for the etch to be consistent 

from sample to sample (unlike the solutions with phosphoric acid).

2. Uniform across sample: The pits appeared to be slightly different across the sample, 

suggesting the etch rate was not consistent across the surface. It was already noted that the 
12

tweezers formed a partial etch-mask where they gripped the sample, so a slower motion 

was adopted when etching. 

3. Smaller pit size: The HNO3:HCl (2:1) solution had the pits growing at a rate of 

~0.82 μm/s. The rate at which the size of the pit grows needs to be slowed to minimize the 

pits overlapping.

4. Slower etch rate: The rate for the sample overall needs to be slowed so that the EPD 

versus depth can be more precisely measured. 

In addition to developing an EPD solution with the characteristics listed above, polishing etch 

solutions for Hg1–xCdxSe will also be investigated. Through the course of this work, a better 

understanding of the etching kinetics of Hg1–xCdxSe will be obtained in order to develop better 

etching solutions.
13

6. References

1. Brill G, Chen Y, Wijewarnasuriya P. J. Elect. Mater. 2011;40:1679. 

2. Wehyer JL, Kelly JJ. in Handbook of Crystal Growth. Berlin, Springer-Berlin Heidelberg, 

2012, pp. 1453–1476.

3. Benson JD, Bubulac LO, Smith PJ, Jacobs RN, Markunas JK, Jaime-Vasquez M, Almeida 

LA, Stoltz AJ, Wijewarnasuriya PS, Brill G, Chen Y, Lee U, Vilela MF, Peterson J, Johnson 

SM, Lofgreen DD, Rhiger D, Patten EA, Goetz PM. J. Elect. Mater. 2010;39:1080.

4. Everson WJ, Ard CK, Sepich JL, Dean BE, Neugebauer GT, Schaake HF. J. Elect. Mater.

1995;24:505.

5. Doyle K, Swartz CH, Dinan JH, Myers TH, Brill G, Chen Y, VanMil BL, Wijewarnasuriya

P. J. Vac. Sci. Technol. B 2013;31:03C124. 

6. Walker P, Tarn WH. in CRC Handbook of Metal Etchants; Boca Raton, FL, CRC Press LLC, 

1991, pp. 221 and 809.

7. Cobb SD, Szofran FR, Jones KS, Lehoczky SL. J. Elect. Mater. 1999;28:732. 
14

List of Symbols, Abbreviations, and Acronyms

Br2 bromine

C3H6O3 lactic acid

CdSe cadmium selenide 

CH3COOH acetic acid

CH3OH methanol

EPD etch pit density 

GaSb gallium antimonide 

H2O water

H2SO4 sulfuric acid

H3PO4 phosphoric acid

HCl hydrochloric acid

HF hydrofluoric acid

Hg1–xCdxSe mercury cadmium selenide 

Hg1–xCdxTe mercury cadmium telluride 

HgSe mercury selenide 

HNO3 nitric acid

IR infrared 

SEM scanning electron microscopy 

TEM tunneling electron microscopy

ZnTe zinc telluride
15

1 DEFENSE TECHNICAL

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RDRL CIO LL

IMAL HRA MAIL & RECORDS MGMT

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 KEVIN DOYLE 

 SUDHIR TRIVEDI
16

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