Selective Area Etching of GaN and AlGaN by Thermally Chemical Reaction in Hydrogen Ambient
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ABSTRACT The selective area etching of wurtzite GaN (0001) and AlGaN (0001) with Si0 2 masks is investigated for different temperatures of 800 to 1100IC and different ambient gases of H2, N 2 and Ar including NH 3. The etching rate of GaN increases with increasing annealing temperature under H2 ambient. This etching is attributed to the chemical reaction between Ga-N and H 2. On the other hand, the etching of GaN does not occur in the N 2 or Ar ambient gas. The NH3 gas in H2 ambient suppresses the chemical reaction, while the NH 3 gas in N 2 enhances it. The surface etching of Al0. 1Ga 0.9N is not observed even in H2 ambient. INTRODUCTION Significant progress has recently been made in the areas of crystal growth and device processing of the III-V nitrides and their ternary alloys. This has resulted in nitride-based blueIUV light emitting and electronic devices. However, GaN and InGaN films were reported to be unstable at high temperature under H2 ambient experimentally [1, 2] and theoretically [3]. This causes surface degradation during growth procedure of nitride films [2]. For example, the regrowth surface of GaN, which is utilized in selective area epitaxy [4, 5], is damaged just before the growth. It might be harmful to crystalline quality of the epitaxial layer via selective area epitaxy such as ELO (epitaxially lateral overgrowth) GaN layers and hexagonal GaN pyramids. Therefore, it is important to investigate the chemical reaction mechanisms in ambient gases such as H2 and N 2 including NH 3, that are normally used in MOVPE. The etching rate of cubic GaN in H 2 ambient has been reported recently [6]. In this paper, the etching rate of wurtzite GaN and AlGaN in different ambient gases of H 2, N2 and Ar including NH 3 is measured qualitatively by using selective area etching technique. In addition, the effects of NH3 on the chemical reaction in above ambient gases are investigated. EXPERIMENT Selective area etching was performed on a 2 gim GaN (0001) and AIGaN (0001) thick films grown on a sapphire substrate by MOVPE. The technique was reported elsewhere [3, 4]. The 50 - 60 nm thick Si0 2 mask was used as masks of the selective area etching to prevent etching of the nitride films. The Si0 2 mask was patterned by conventional photolithography and NH4 HF2 chemical etching. The mask pattern with 10 jrm-line is used to observe the cross-sectional etching pattern. We etched the nitride samples at various temperatures from 800 to 1100°C using the different ambient gases (atmospheric pressure): H2, N 2 and Ar including NH3 . These conditions are similar to conventional MOVPE growth conditions although a TMG gas is not supplied in this experiment. After the selective area etching, we observed the surface morphology and the cross section of the line pattern by SEM. The etching depth was measured at the middle part of the line 991
Mat. Res. Soc. Symp. Proc. Vol. 482 0 1998 Materials Research Society
pattern by a stylus instrument. was measured.
The surface was observed by AFM and the surface roughness
RESULTS AND D
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