Optical and Electrical Characterisation of Plasma Processed n-GaAs
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ABSTRACT
A Photoreflectance (PR), Photoluminescence (PL), Raman scattering and Ellipsometry optical characterisation study of He and SiCl4 reactive ion etched 1018 cm-3 n-type GaAs is presented. Ellipsometric and PL data reveal significant modifications to the GaAs substrates for the He plasma as a function of etch power and etch gas pressure. Raman data reveal an increase in the depletion depth while the surface electric field data, extracted from PR spectra, decrease with etch severity. This is accounted for by a plasma etch induced decrease in the near surface net carrier density. Ellipsometric and PL measurements of SiCI4 etching reveal evidence of an improvement in the GaAs surface crystallinity while PR and Raman data are consistent with an increase in the surface carrier concentration. Optical characterisation data are correlated with electrical measurements using capacitance-voltage profiling.
INTRODUCTION
Plasma etch processing of compound semiconductors is a key technology for producing optoelectronic integrated circuits (OEIC's), lasers and high speed devices. He ion and in particular silicon tetrachloride (SiC14 ) have been shown to be viable etchants of GaAs resulting in anisotropic, polymer free and low residual damaged surfaces [1]. Plasma etching however can directly impact the electronic, transport and optical properties in the near surface region of semiconductor materials. Such surface modification can adversely effect device manufacturing yields via degradation mechanisms such as increased junction leakage, threshold voltage shifts and variations in optical properties [2]. Furthermore the exact nature of etch induced damage and possible causes such as introduction and propagation mechanisms are not yet understood. One reason for this is the lack of suitable methods for identifying damage confined within the nearsurface region. Optical characterisation methods are important diagnostic tools due to their inherent noninvasive and non-destructive nature. Analytical techniques such as spectroscopic ellipsometry (SE), Raman scattering and photoluminescence (PL) have been employed for the characterisation of etched silicon [3], GaAs [4] as well as other semiconductor materials. Photoreflectance (PR) spectroscopy is also a valuable non-destructive optical diagnostic tool employed in the characterisation of plasma etched GaAs substrates [4]. In this work we report the application of PR to characteriseothe surface electric field of GaAs arising from etch induced modification to the electronic band structure. Experimental PR results are correlated with extracted etch damage optical model parameters from variable angle spectroscopy ellipsometry (VASE) as well as photoluminescence (PL) and Raman data. Finally, using capacitance-voltage (C-V) profiling, the surface carrier density was determined for the etched substrates. 327 Mat. Res. Soc. Symp. Proc. Vol. 406 © 1996 Materials Research Society
EXPERIMENTAL
(100) n-type orientated VGF (Vertical Gradient Freeze) 1018cm-3 Si doped GaAs substrates were uti
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