Laser-Assisted Selective Chemical Etching of GaAs/AlGaAs Layered Structures
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LASER-ASSISTED SELECTIVE CHEMICAL ETCHING OF GaAs/AlGaAs LAYERED STRUCTURES R. T. BROWN, J. F. BLACK, R. N. SACKS, G. G. PETERSON, and F. J. LEONBERGER United Technologies Research Center, East Hartford, CT 06108 ABSTRACT Laser-assisted etching techniques have been developed for alloyselective patterning of GaAs/AlGaAs layered structures. Such techniques are potentially very important for the fabrication of a large variety of heterostructure devices. Studies have been carried out with several liquid etchants and have utilized a CWdye laser tuned to a wavelength above the band edge of GaAs, but below that of the AlGaAs layer (e.g., X = 0.85 ý±m) to etch GaAs. Using the laser focal spot to spatially define the etched area, features exhibiting very smooth interface surfaces were produced in MBE-grown epitaxial layers. A high degree of selectivity for GaAs over AI(0.4)Ga(0.6)As, low background etch rates, and high feature etch rates (> 2 1 ±m/min) were demonstrated. Patterned etching with AZ 1350J photoresist exhibited good pattern resolution and nearly vertical sidewall profiles. Etchant mixtures of dilute H2S04:H202:H20 and HNO3:H20 produced qualitatively different results, with the HNO3:H20 mixture exhibiting a tendency to leave a residual GaAs layer at the interface, even for long etch times. Surface analysis techniques, including Auger spectroscopy, profilometry, and interference contrast microscopy have been used to characterize the etched structures, and a simplified analytical model has been used to predict the qualitative dependence of etch rate on layer thickness. INTRODUCTION A problem of considerable technological importance for the fabrication of a variety of heterostructure devices is the selective etching of compound semiconductor layers with differing chemical composition, e.g, GaAs over Al(x)Ga(l-x)As. Conventional techniques typically utilize differences in surface chemistry to achieve this selectivity, e.g., the formation of a nonvolatile aluminum-fluoride compound at the Al(x)Ga(l-x)As interface in the case of dry etching of GaAs over Al(x)Ga(l-x)As [1]. As pointed out by Ashby [21, photo-assisted etching can be made material selective by utilizing the variation in band-gap energy with material composition. The present study examined selective wet chemical etching of GaAs over AI(O.4)Ga(0.6)As as a means of obtaining both fast etch rates and a high degree of material selectivity. EXPERIMENT Based on the absorption coefficient of Al(x)Ga(i-x)As at 300 0 [3K, a wavelength of 0.85 jtmwas chosen to provide a high level of absorption in the GaAs layer and essentially no absorption in the Al(O.4)Ga(O.6)As layer. The 0.85 aLmsource was a CWdye laser (CRL 599-01, Exciton LDS 820 dye) pumped by a CWAr+ laser (Innova 18). Experiments were also carried out with the Ar+ laser operating on the 0.34-0.36 jm UV transitions. The reaction cell was a UV-grade fused silica spectrophotometer cell, with the sample mounted vertically within - 1.0 mm of the inside surface of the laser input cell wall. The samples and cell
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