Journal of Crystal Growth 189/190 (1998) 730—733

Etching of cubic GaN by annealing in hydrogen ambient

Hidenao Tanaka*, Atsushi NakadairaNTT Integrated Information and Energy Systems Laboratory, 3-9-11 Midori-cho, Musashino-shi, Tokyo 180, Japan

Abstract

Cubic GaN was annealed in different ambient gases at various temperatures from 800 to 950°C. It remained stable ininert gases, but it sublimated in hydrogen ambient. This sublimation was suppressed in H

2#NH

3and H

2#TEGa

mixtures. The sublimation rate in H2

can be controlled by changing the annealing temperature; the activation energy ofthe sublimation was estimated to be 37 kcal/mol. Observation of the anisotropic properties at the cross section of a stripeetched by this sublimation showed that (1 1 1)B surfaces were stabilized during the etching. To estimate the damageduring etching, we measured the photoluminescence from etched and rested surfaces. There was no difference in the shapeof the spectrum, and the decrease in intensity with etching was only 18% when the film thickness was reduced by15%. ( 1998 Elsevier Science B.V. All rights reserved.

PACS: 81.40.!z; 81.60.!j; 81.60.Cp; 82.65.Dp

Keywords: GaN; Cubic; Etching; Anisotropy; Sublimation; Thermal stability

1. Introduction

The wurtzite structure is a stable configurationfor group III nitrides. In addition, we can growmetastable cubic (zinc-blende structure) group IIInitrides on such substrates as GaAs, Si, 3C—SiC,and MgO, which have cubic symmetry. Cubic GaNis promising because its higher symmetry gives itseveral superior properties, such as easy cleavabil-ity for fabricating laser cavities. Although its crystal

*Corresponding author. Tel.: #81 422 59 3682; fax: #81422 59 2340; e-mail: htanaka@ilab.ntt.co.jp.

quality has been improved to enable stimulatedemission by optical pumping [1], most of its phys-ical and chemical characteristics are still unclear.Determining its thermal stability is important foroptimizing its growth conditions and device pro-cesses. In this paper we report on the etching ofcubic GaN by sublimation in hydrogen ambient.

2. Experimental procedure

We grew cubic-GaN layers on (0 0 1) GaAs sub-strates by low-pressure metalorganic vapor-phaseepitaxy [2]. Triethylgallium (TEGa) and NH

3were

0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved.PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 2 7 4 - 7

used as source materials, and the substrate temper-ature was 950°C. The thickness of the GaN layerwas about 1.5 lm. After cleaving the cubic-GaNalong with the GaAs substrate to form a 5]5 mmsquare, we coated SiO

2all over the specimen by RF

sputtering. This SiO2

coating was important, be-cause it enabled us to reproduce the experiment.The SiO

2was patterned by photolithography, then

used as a pattern mask for thermal annealing. Tomeasure the sublimation rate, an SiO

2hole pattern

with a 1.5 mm diameter was used. To observe thecross-sectional etching pattern, a 3 lm wide linepattern was used. We annealed the specimens atvarious temperatures from 800 to 950°C using dif-ferent ambient gases: N

2, Ar, H

2, H

2#NH

3mix-

ture, and H2#TEGa mixture. The pressure in

a furnace was kept 76 Torr. In case of N2

and Ar,the gas flow rate was 4 lm. The total gas flow rate ofH

2mixture was 7 lm. After annealing, we removed

the SiO2

mask by HF solution. We then observedthe surface morphology and the cross section of thestripe pattern by scanning electron microscopy.The etching depth was measured at the edge of thepattern by a stylus instrument. We used photo-luminescence excited by a frequency-doubled Arion laser (257 nm) at room temperature to measurethe optical properties of the etched layers.

3. Results and discussion

The results of 15 min annealing at 950°C in sev-eral ambient gases are shown in Table 1. We foundno trace on the cubic GaN when annealed in inertgases, indicating that cubic GaN is stable at 950°Calthough it has a metastable configuration com-pared to hexagonal GaN. However, the cubic GaNdisappeared during annealing in H

2ambient. This

enhanced sublimation of cubic GaN by H2

is sim-ilar to that of hexagonal GaN reported by Mori-moto [3]. Apparently, the GaN#H

2changed into

Ga#NH3

and the Ga evaporated. This sublima-tion was suppressed in a mixture of gases contain-ing such reaction products as NH

3and Ga

precursor (TEGa) as shown in Table 1.The dependence of the sublimation rate on the

TEGa carrier gas-flow rate in H2

when annealed at900°C is shown in Fig. 1. When the gas-flow rate

Table 1Surface morphology of cubic-GaN after annealing in differentambient gases at 950°C for 15 min

Ambient gases Surface morphology

Ar No traceN

2No trace

H2

GaN disappearedH

2#NH

3(0.2%) No trace

H2#TEGa (5 ppm) Except Ga droplet no trace on GaN

Fig. 1. Sublimation rate dependence on TEGa carrier gas-flowrate during annealing in H

2ambient at 900°C.

was large, there was no etching trace on the cubic-GaN surface after removing the Ga droplets byHCl solution. When the flow rate was less than 6 cc,decreasing the flow rate increased the sublimationrate monotonically. The sublimation rate can thusbe controlled by changing the supply of reactionproduct. Although we did not test the sublimationfor different NH

3flows, it seems apparent that the

sublimation rate can be changed by precisely con-trolling the NH

3flow.

The sublimation rate also can be controlled bychanging the annealing temperature. Fig. 2 showsan Arrhenius’s plot of the sublimation rate during

H. Tanaka, A. Nakadaira / Journal of Crystal Growth 189/190 (1998) 730–733 731

Fig. 2. Sublimation rate versus reciprocal of temperature dur-ing annealing in H

2ambient.

annealing in H2

ambient. The activation energywas estimated to be 37 kcal/mol from the slope. Incomparison with hexagonal GaN, this activationenergy is similar to that of sublimation in H

2ambi-

ent (34 kcal/mol) [4] and smaller than that in a vac-uum (75 kcal/mol) [5]. The similar activationenergy indicates that the enhanced sublimation byH

2depends on the chemical reaction between the

Ga—N bond and H2.

In our experiments, although we used differentepitaxial films, the sublimation rate did not dependon the epitaxial lot number. But when we did notcoat the GaAs substrate with SiO

2, the rate was

often smaller, apparently because the Ga evapor-ated from the GaAs substrate. Therefore, the SiO

2coating is very important for reproducing the subli-mation.

Anisotropic properties are usually observedwhen the chemical reaction is the limiting factor inthe etching. Cross-sectional scanning electronmicrographs of a cubic-GaN stripe etched by subli-mation in H

2ambient are shown in Fig. 3. A mesa

shape was observed in the [1 1 0] direction, and an

Fig. 3. Cross-sectional scanning electron micrographs of cubic-GaN stripe in different directions.

inverse-mesa shape was observed in the [1 11 0]direction. These results show that the (1 1 1)B sur-face was stabilized during annealing in H

2. In the

anisotropic etching of III—V materials using chem-ical solutions, generally the (1 1 1)A surface is sta-bilized because the etching is limited by thereaction of the group-III materials. In contrast,group-V materials limit the reaction in sublimationetching in H

2. In other words, N disconnection

limits sublimation etching.To use this etching for device fabrication, dam-

age to the crystal during etching must be mini-mized. We measured the photoluminescence fromthe crystal to estimate the damage caused by subli-mation etching. As shown in Fig. 4, there was nodifference in shape in the spectra of photolumines-cence from etched and remained surfaces. There-fore, the radiative defects were very small. Thedecrease in intensity with etching was only 18%,and the film thickness was reduced by 15% byetching. The film thickness was relatively thin com-pared to the carrier diffusion length, therefore, theintensity decrease is mainly due to the difference in

732 H. Tanaka, A. Nakadaira / Journal of Crystal Growth 189/190 (1998) 730–733

Fig. 4. Photoluminescence from etched and remained surfacesof cubic-GaN. 1.36 lm thick GaN was remained from 1.6 lmthick epitaxial layer.

emitting volume. The increase in nonradiative de-fects should also be small. These results indicate themildness of sublimation etching, making it suitablefor device fabrication.

4. Summary

In summary, cubic GaN was found to be stablewhen annealed in inert gases at temperatures between

800 and 950°C. Sublimation was observed inH

2ambient, but it was suppressed in H

2#NH

3and H

2#TEGa mixtures. The sublimation rate in

H2

can be controlled by changing the annealingtemperature; the activation energy of the sublima-tion was estimated to be 37 kcal/mol. This activa-tion energy is almost the same as that of hexagonalGaN. The anisotropic properties were observed ata cross section of a stripe etched by this sublima-tion; the (1 1 1)B surfaces were stabilized during theetching. Apparently, in the sublimation, theGaN#H

2changes into Ga#NH

3, and the Ga

evaporates. To estimate the damage caused byetching, we compared the photoluminescencefrom etched and rested surfaces. We found nodifference in the shapes of their spectrums. Thedecrease in intensity with etching was only 18%when the film thickness was reduced by 15%. Theseresults indicate that sublimation etching causeslittle damage, making it promising for devicefabrication.

References

[1] A. Nakadaira, H. Tanaka, Appl. Phys. Lett. 71 (1997)812.

[2] A. Nakadaira, H. Tanaka, J. Electronic Materials 26 (1997)320.

[3] Y. Morimoto, J. Electrochem. Soc. 121 (1974) 1382.[4] H. Hanai, H. Matsushima, K. Hiramatsu, N. Sawaki,

Technical Report of IEICE ED 96-29 (1996) 1 (inJapanese).

[5] R. Groh, G. Gerey, L. Bartha, J.I. Pankove, Phys. Stat. Sol.A 26 (1974) 353.

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