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Epitaxial lift-off process for gallium arsenide substrate reuse

and flexible electronics

Cheng-Wei Cheng*, Kuen-Ting Shiu, Ning Li, Shu-Jen Han, Leathen Shi,

Devendra K. Sadana

IBM T.J. Watson Research Center, Yorktown Heights, NY 10598, USA

*Chengwei@us.ibm.com

Supplementary Information

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Supplementary Figure S1 Photos of GaAs substrate surfaces after ELO (a)

conventional ELO and (b) Novel ELO process and the lifted GaAs thin film. The

surface usually becomes dark and rough after the conventional ELO process due to

the accumulation of etching residues and the attack of HF solution. On the other hand,

both the wafer surface and the lifted GaAs thin film surface are kept shiny and smooth

after our novel ELO process.

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Supplementary Figure S2 XPS spectrum of As 3d electrons All samples were

dipped in HCl or HF and kept in the air for 30 minutes or 1 day before loaded into

ultra-high vacuum chamber.

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Supplementary Figure S3 AFM images of the GaAs substrate surfaces (a) diluted

HCl for 1 day and (b) diluted HCl with pure nitrogen purged for 2 days

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Supplementary Figure S4 Measurements of etching rate of GaAs by HCl and HF

Depth profiles of GaAs samples dipped in 36% HCl, 49% HF, and 5% HF for 9 days.

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Supplementary Figure S5 Photos of small LED devices transferred to glass

substrate and flexible tape. Red light emission from these devices also suggests that

the integrity of the LED was preserved after the film transfer.

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Supplementary Figure S6 Schematic structure of SJ GaAs solar cell (left) before

and (right) after fabrication

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Supplementary Figure S7 Schematic structure of LED (left) before and (right)

after fabrication and transfer

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Supplementary Figure S8 Schematic structure of MOSCAP (left) before and(right)

after fabrication and transfer

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Supplementary Methods

1. Passivation of GaAs surface after HF and HCl dipping for oxidation

protection

It is well known that dipping the silicon wafer into HF solution removes the

native oxide on the silicon surface and causes the hydrophobic surface due to the

passivation of the silicon surface with hydrogn atoms. This hydrogen terminated

surface protects the silicon from being oxidized by the oxygen in the atmosphere for a

certain period at the room temperature. A similar phenomenon can also happen when

GaAs is dipped into HF or HCl solutions. HF or HCl dip can remove the native oxide

of GaAs and leave a hydrophobic GaAs surface. But the GaAs surface is mainly

passivated and terminated by F or Cl atoms, which has been discussed in the article

and several literatures.

To verify that F or Cl terminated surface could also protect GaAs surface from

being oxidized in the atmosphere, the native oxide covered GaAs wafers were dipped

into 49% HF and 36% HCl for 1 minute and were left in the air for 30 minutes or 1

day before loading into the high vacuum chamber for XPS examination.

Supplementary Figure S2 shows XPS spectrum of As 3d electrons from samples

dipped into HF or HCl and stored in the air for different periods. It can be seen that

arsenic oxide was significantly grown after storing the sample in the air for one day,

however, no arsenic oxide was observed for the sample dipped in the HF and stored in

the air for 30 minutes. For the sample dipped in HCl, a very weak arsenic oxide signal

was detected on the surface, and this may be caused by the partial oxidation of the

surface due to weaker Ga-Cl bonding compared to Ga-F bonding.

This result suggests that passivation of GaAs surface by HF or HCl dip can

protect GaAs from being oxidized in the air for a short period and implies that F or Cl

terminated GaAs can be kept in HF or HCl solution due to the continuously supply of

F or Cl atoms to the surface.

2. Nitrogen purged etching experiment

In order to prove that the dissolved oxygen in the etchant plays an important role

of roughening the wafer surface, a GaAs wafer was soaked into diluted HCl (5%)

with pure nitrogen purge for 2 days. Purging pure nitrogen could remove most of

dissolved oxygen and make an oxygen-free solution. Supplementary Figure S3 shows

AFM images of GaAs wafer surfaces soaked into diluted HCl with and without pure

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nitrogen purge. RMS of the sample soaked in oxygen-free diluted HCl is smoother

than that soaked in regular diluted HCl solution and is almost identical to that of the

sample soaked in concentrated HCl. This experiment proves that the dissolved oxygen

is responsible for roughening the wafer surface, and the passivated GaAs surface with

concentrated HCl or HF helps to protect the wafer surface from being oxidized by the

dissolved oxygen and keeps surface smooth.

3. Long-term (9 days) etching experiment

The purpose of this experiment is to verify the thermodynamic calculation to

prove that HF does attack and etch the GaAs surface and leaves a rough surface while

HCl does not attack the GaAs.

To quantify the etch rates of GaAs in HCl and HF, GaAs samples were masked

by the black wax and left in 36% HCl, 5% HF and 49% HF for 9 days. The black wax

was removed after the etching and etch depth was measured by the alpha-stepper and

the result is shown in Supplementary Figure S4. The etch depths were measured to be

3 nm, 12 nm and 100 nm for samples dipped in 36% HCl, 5%HF, and 49%HF,

respectively. The etch rate of GaAs in HF depends on the concentration of HF

solution.

4. Details of devices fabrication recipes

The schematic structures of a SJ GaAs solar cell, LED, and MOSCAP before and

after fabrication/transfer are shown in Supplementary Figure S6, S7, and S8,

respectively. The structures of devices were designed to prove the concept of reuse of

the substrate and flexible devices, and they may not represent the optimal structures

for high performance. The fabrication procedures are described in the Methods

section with some details shown below:

1. NH4OH/H2O2/H2O Etching Solution

NH4OH:H2O2:H2O = 2: 1: 30. The etching rate of GaAs is approximately 800

nm/min at room temperature.

2. Citric Acid/H2O2

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Citric Acid/H2O2 solution was used to selectively etch GaAs and stop on AlGaAs

layer. First, the citric acid solution was prepared by mixing 1g citric acid (anydrous)

and 1mL H2O. Then the citric acid solution and H2O2 was mixed with volume ratio of

4 to 1. The etching rate of GaAs is approximately 200nm/min at room temperature.

3. Buffered oxide etch (BOE) of Al2O3

1:100 BOE solution was applied to remove Al2O3 layer and the etching rate was

measured as approximately 33nm/min.

4. N- and p-ohmic contacts of GaAs

N- and p-ohmic contact of GaAs are AuGe/Ni/Au (20nm/15nm/100nm) and

Ni/Au (15nm/100nm).