ARL-TR-8228 ● DEC 2017

US Army Research Laboratory

Development of a Photoelectrochemical Etch 

Process to Enable Heterogeneous Substrate

Integration of Epitaxial III-Nitride 

Semiconductors

by Vijay Parameshwaran, Roy B Chung, Stephen Kelley, 

Kimberley Olver, Blair C Connelly, Anand V Sampath, and 

Meredith L Reed

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ARL-TR-8228 ● DEC 2017

US Army Research Laboratory

Development of a Photoelectrochemical Etch 

Process to Enable Heterogeneous Substrate

Integration of Epitaxial III-Nitride 

Semiconductors

by Vijay Parameshwaran, Roy B. Chung, Stephen Kelley, 

Kimberley Olver, Blair C. Connelly, Anand V. Sampath, and 

Meredith L. Reed

Sensors and Electron Devices Directorate, ARL

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December 2017

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Technical Report

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1 March 2016–28 February 2017

4. TITLE AND SUBTITLE

Development of a Photoelectrochemical Etch Process to Enable Heterogeneous 

Substrate Integration of Epitaxial III-Nitride Semiconductors

5a. CONTRACT NUMBER

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6. AUTHOR(S)

Vijay Parameshwaran, Roy B. Chung, Stephen Kelley, Kimberley Olver, 

Blair C. Connelly, Anand V. Sampath, and Meredith L. Reed

5d. PROJECT NUMBER

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U.S. Army Research Laboratory

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Adelphi, MD 20783

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ARL-TR-8228

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14. ABSTRACT

A process is developed to transfer epitaxially grown III-nitride films onto substrates more applicable to the desired 

application. The substrates for epitaxial growth of these films are limited to single-crystal sapphire, gallium nitride (GaN)-onsapphire templates, or free-standing GaN. However, the desired end application requires a different substrate for optical, 

electrical, or thermal reasons. This process is applied for a semipolar (20-21) GaN lasing stack, grown on free-standing GaN, 

and designed for emission at a wavelength of 369 nm. A sacrificial indium gallium nitride (InGaN) layer, with an emission 

wavelength of 410 nm, is grown below the lasing stack. The stack is processed into 200 mesa structures to minimize the 

amount of undercut etch-rate limitations. The etching is performed in a 0.1-M potassium hydroxide solution under backside 

irradiation from a 405-nm laser such that only the sacrificial layer absorbs the incident radiation. The photogenerated holes 

induce a surface oxidation current, etching the sacrificial InGaN layer and promoting the release of the epitaxial lasing stack, 

which can then be automatically transferred to a previously bonded heterogeneous substrate. 

15. SUBJECT TERMS

etching, substrate transfer, photoelectrochemistry, III-V nitrides, laser, heterogeneous integration

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Vijay Parameshwaran

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301-394-0927

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iii

Contents

List of Figures iv

Acknowledgments v

1. Introduction 1

2. Experiment 2

3 Results and Discussion 4

4. Summary and Conclusion 7

5. References 8

List of Symbols, Abbreviations, and Acronyms 9

Distribution List 10
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List of Figures

Fig. 1 Graphic (top) and SEM image (bottom) of the processed mesa array 

for GaN liftoff using an InGaN PEC etch layer.................................... 3

Fig. 2 Graphic (left) and photo (right) of the processed mesa array mounted 

on the electrochemical system to perform PEC etching liftoff............. 4

Fig. 3 SEM image of the mesa edge after the PEC etching process was run 

for 8 h.................................................................................................... 4

Fig. 4 SEM image of the mesa edge after the PEC etching process was run 

for 16 h.................................................................................................. 5

Fig. 5 SEM images of the mesa after sonication, showing partial liftoff near 

the edge as viewed zoomed out (left) and zoomed in (right)................ 5

Fig. 6 Schematic of the prebonding of both substrates across the mesa array 

with PEC etch layer .............................................................................. 6
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Acknowledgments

The authors would like to thank Professor Daniel Feezell of the University of New 

Mexico for discussions and helpful advice.
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INTENTIONALLY LEFT BLANK.
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1. Introduction

The III-nitride class of materials (aluminum nitride [AlN], gallium nitride [GaN], 

indium nitride, and corresponding ternary alloys) provide a basis for a variety of 

electronic and photonic devices across several different applications. Epitaxial 

growth techniques, such as molecular beam epitaxy (MBE) and metalorganic 

chemical vapor deposition (MOCVD), however, limit the growth of these materials 

on lattice-matched substrates. Due to the wurtzite hexagonal crystal structure of the 

III-nitrides, substrates on which these materials are grown have been mostly limited 

to aluminum oxide (Al2O3, single-crystal sapphire), a GaN-on-sapphire template, 

or free-standing GaN.

In some applications of grown III-nitride structures, the substrate can limit the 

performance or prevent successful operation. For example, applications that require 

an optically transparent substrate in the UV spectral region are either to perform 

optical experiments on the grown materials and/or in the application’s final 

implementation or a substrate with high thermal conductance to mitigate localized 

heat generation due to high carrier densities. These reasons, among others, 

necessitate the use of a different substrate due to sapphire’s low thermal 

conductivity and GaN’s limited optical transparency window. 

Specifically, the desired application is a GaN-based lasing stack with an emission 

wavelength of 369 nm and driven by an electron beam irradiation, which leads to 

high carrier densities. This necessitates the transfer/removal of the GaN substrate 

(or GaN-on-sapphire) to diamond, which has a high optical transparency and 

thermal conductivity. Therefore, a method has to be developed to enable the 

transfer of a GaN lasing stack onto a diamond submount. Although GaN liftoff from 

sapphire has been demonstrated with laser irradiation,

1 the physical principle is 

based on the absorption, heating, and dissociation of GaN at the GaN-sapphire 

interface, which causes some minimal surface damage that could negatively impact 

optical transmission of the emitted light. Since this work requires the isolation and 

transfer of a specific region, a more refined technique is required to transfer the 

grown material.

The method chosen to achieve substrate transfer is photoelectrochemical (PEC) 

etching, in which a region or a part of the material stack is selectively etched off in 

an electrolyte solution in response to photogenerated carriers from light absorption. 

Due to the photo-oxidation process at the surface of the release layer, corrosion 

occurs, and the release layer is dissolved in solution. PEC etching was first 

demonstrated for GaAs2 and then GaN3 to process wafers with a controlled 

material-removal technique. This process was then extended to creating surface 
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2

features on GaN patterned by cathodic metal deposition4 and creating pillar/disk 

microstructures by etching buried InGaN layers within GaN.

5,6 The ability to 

change the bandgap of the material across the III-nitride alloy system allows for 

light-absorption selectivity within the PEC etch layer, hence the design to use 

InGaN etch layer around a GaN release stack. Recently, this technique has been 

refined with band engineering within the release layer7 and extended to the point 

where it has been used to integrate metal-oxide distributed Bragg reflector mirrors 

on GaN-based vertical cavity surface emitting lasers.

8

In this work, PEC etching is used as a processing step to enable transfer of a 

GaN-based materials stack lasing at 369 nm to a diamond submount.

2. Experiment

The materials are grown on free-standing semipolar (20-21) GaN substrates using 

MOCVD. First a 20-nm InGaN layer is grown as designed to have an In/Ga alloying 

fraction such that its emission wavelength is longer than the 405-nm laser source 

being used to initiate the etch. On top of the InGaN a 250-nm GaN layer is grown 

to serve as the layer that will be transferred through the PEC etch process. 

After growth, the substrates are processed to enable liftoff. Mesas with a 200-μm 

diameter are lithographically defined and etched down to a depth of approximately 

450 nm using a plasma etching chemistry of chlorine (Cl2) and boron trichloride 

(BCl3) gases. The etch depth is chosen to expose the InGaN PEC etch layer from 

the side of the mesa. Around the mesas, a cathode metal stack of titanium (Ti)/gold 

(Au) is deposited on the GaN surface. The purpose of the cathode metal stack is to 

source photogenerated electrons in the PEC release layer to complete the 

electrochemical reaction (both oxidation and reduction processes) to drive the 

etching. It is also important to have the cathode metal in close proximity to the etch 

layer such that the diffusion path for photogenerated electrons is minimized; 

otherwise, the reaction will either slow down or completely stop before the etch 

layer is completely dissolved. Figure 1 shows a graphical picture and a scanning 

electron microscope (SEM) image of the completed structure. 
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Fig. 1 Graphic (top) and SEM image (bottom) of the processed mesa array for GaN liftoff 

using an InGaN PEC etch layer

An advantage of this design, which contributes to the facile nature of the PEC 

process, is that the reaction is self-limited: there is no necessity to monitor the 

current or voltage or to run the process for a set amount of time. The process is 

finished and the reaction stops once the InGaN etch layer is completely consumed. 

There are 2 reasons for creating mesa structures for the PEC etch: 1) it allows for a 

region on which to apply the cathode metal and 2) it minimizes rate-limiting 

processes associated with undercutting a relatively large area. 

The processed structure in Fig. 1 is adhered on a Teflon submount with Apiezon W 

(Black Wax) then placed an electrochemical cell filled with 0.1-M potassium 

hydroxide (KOH) electrolyte, with the processed mesas and cathode metal in 

contact with the solution. KOH is chosen as an electrolyte because it is relatively 

benign in the context of etching the cathode metal, and solvates III-nitrides more

easily than acids.

9 The mesas are then illuminated by a 405-nm laser operating at 

100 mW for the etching process. The reaction is left running for 8 h, occasionally 

refilling electrolyte that was lost to evaporation. A schematic of the process on the 

mount with a corresponding photograph is shown in Fig. 2. As the structure is 

illuminated in solution, the photogenerated holes in the PEC etch layer diffuse to 

the solution interface, where they drive an InGaN oxidation. The oxide is dissolved 

in solution. The corresponding photogenerated electrons diffuse to the cathode 

metal on which the hydrogen evolution is run. 
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Fig. 2 Graphic (left) and photo (right) of the processed mesa array mounted on the 

electrochemical system to perform PEC etching liftoff

3 Results and Discussion

Figure 3 shows an SEM image of the mesa after the PEC etch process was run to 

completion. The interface between the substrate and the epitaxial GaN is clearly 

delineated as being etched, showing that this experiment successfully removed the 

InGaN PEC layer. 

Fig. 3 SEM image of the mesa edge after the PEC etching process was run for 8 h
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Two aspects of this experiment require further investigation. First, even though 

etching occurred across the PEC InGaN layer, the epitaxial GaN did not lift off. 

The initial thought was that 8 h was not enough time to fully undercut across the 

200-μm-diameter mesa. The etch process was run for another 8 h, with the resulting 

SEM shown in Fig. 4, where the epitaxial layer remained. After the second etching 

process, the structures were subjected to sonication in acetone for 10 min. Partial 

liftoff of the epitaxial GaN on the mesa was achieved (Fig. 5), showing that the etch 

undercut was successful. However, much of the layer was not lifted off. 

Fig. 4 SEM image of the mesa edge after the PEC etching process was run for 16 h

Fig. 5 SEM images of the mesa after sonication, showing partial liftoff near the edge as 

viewed zoomed out (left) and zoomed in (right)
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This brings up the question as to whether the etch completely ran under the time of 

the experiment (16 h total). This is not likely the case, as the empty space within 

the etched layer is still viewable. The likely scenario is that mechanical forces or 

adhesion are preventing much of the liftoff process.

This problem can be addressed by prebonding the substrate onto which the material 

is to be transferred, as shown in Fig. 6. The bonded “sandwich” can then be 

mounted on the Teflon submount in the electrochemical cell, and as the etch 

proceeds, the mechanical forces from the bonding substrate can assist to pull off 

the epitaxial GaN from the PEC etch interface. This work is already in progress for 

using AlN for bonding between GaN and diamond. 

Fig. 6 Schematic of the prebonding of both substrates across the mesa array with PEC etch 

layer

The second aspect is that from the SEM images, the etching interface between the 

lifted-off GaN and the substrate is rough and serrated, which is not ideal for using 

epitaxial GaN for a lasing or electrical application, and is also not ideal in terms of 

reusing a substrate for future growth experiments. This interface is due to GaN 

being susceptible to a small amount of etching in an aqueous base solution, which 

has also been previously observed in PEC etching experiments.

4 This issue can be 

resolved by incorporating aluminum gallium nitride (AlGaN) as barrier regions 

around the InGaN PEC etch layer. Even a dilute amount of Al alloying improves 

AlGaN etch resistance. Therefore; the PEC etch process will produce clean 

interfaces that preserve the material quality of both the initial growth substrate and 

the transferred epitaxial film.

A final consideration is if the PEC etch layer requires optimization to effectively 

lift off the film above it in a reasonable time frame as well as achieving a clean 

break and a pristine surface in both the lifted-off film and the substrate. Bandgap 

engineering7 has been shown to provide an improved method for etching near the 

materials interfaces, and using quantum wells6 provides an alternative way to have 

a light absorber that etches in an electrochemical environment. 
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4. Summary and Conclusion

We have demonstrated a PEC etch process for epitaxial III-nitride materials by 

using InGaN as a sacrificial etch release layer that allows for the transfer of epitaxial 

GaN films from a growth substrate to a more appropriate substrate for the desired 

application. The scope of this project is to enable the use of a semipolar (20-21) 

GaN-based laser to be pumped with an electron beam for optical emission, but this 

process is versatile enough to enable III-nitride substrate transfer for a variety of 

electrical and optical applications. The range of materials is extended to most 

semiconductors provided the following criteria are met: 1) a lower bandgap alloy 

can be epitaxially integrated in the growth stack as an etch release layer and 2) a

high-power optical source whose wavelength is such that the lifted-off stack is 

transparent, but the lower bandgap etch release layer is not. 

This capability serves as a valuable processing step within the US Army Research 

Laboratory for a variety of research projects in which a substrate transfer of 

epitaxially grown materials is required. For each project, the specifics of etch 

chemistry, bonding, and other materials processing vary, but the setup created for 

this project can be applied to others as well. 
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5. References

1. Wong WS, Sands T, Cheung NW. Damage-free separation of GaN thin films 

from sapphire substrates. Applied Physics Letters. 1998;72:599–601.

2. Khare R, Hu EL. Dopant selective photoelectrochemical etching of GaAs 

homostructures. Journal of the Electrochemical Society. 1991;138:1516–1519.

3. Minsky MS, White M, Hu EL. Room-temperature photoenhanced wet etching 

of GaN. Applied Physics Letters. 1996;68:1531–1533.

4. Tamboli AC, Schmidt MC, Rajan S, Speck JS, Mishra UK, DenBaars SP, Hu 

EL. Smooth top-down photoelectrochemical etching of m-plane GaN. Journal 

of the Electrochemical Society. 2009;156:H47–H51.

5. Tamboli AC, Schmidt MC, Hirai A, DenBaars SP, Hu EL. 

Photoelectrochemical undercut etching of m-plane GaN for microdisk 

applications. Journal of the Electrochemical Society. 2009;156:H767–H771. 

6. Haberer ED, Sharma R, Meier C, Stonas AR, Nakamura S, DenBaars SP, Hu 

EL. Free-standing, optically pumped, GaN/InGaN microdisk lasers fabricated 

by photoelectrochemical etching. Applied Physics Letters. 2004;85:5179–

5181.

7. Tamesh P, Krishnamoorthy S, Rajan S, Washington GN. Energy band 

engineering for photoelectrochemical etching of GaN/InGaN heterostructures.

Applied Physics Letters. 2014;104:243503.

8. Holder CO, Leonard JT, Farrell RM, Cohen DA, Yonkee B, Speck JS, 

DenBaars SP, Nakamura S, Feezell DF. Nonpolar III-nitride vertical-cavity 

surface emitting lasers with a polarization ratio of 100% fabricated using 

photoelectrochemical etching. Applied Physics Letters. 2014;105:031111.

9. Walker P, Tran WH, editors. The CRC handbook of metal etchants. Boca 

Raton (FL): CRC Press; 1991.
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List of Symbols, Abbreviations, and Acronyms

Al2O3 aluminum oxide (sapphire)

AlN aluminum nitride

AlGaN aluminum gallium nitride

Au gold

BCl3 boron trichloride (gas)

Black Wax Apiezon W

Cl2 chlorine (gas)

GaN gallium nitride

InGaN indium gallium nitride

InN indium nitride

KOH potassium hydroxide

MBE molecular beam epitaxy

MOCVD metalorganic chemical vapor deposition

PEC photoelectrochemical

SEM scanning electron microscopy

Ti titanium

UV ultraviolet
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