Mid-Infrared Spectral Photoresponse of Semi-Insulating GaAs
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MID-INFRARED SPECTRAL PHOTORESPONSE OF SEMI-INSULATING GaAs G.J. BROWN AND W.C. MITCHEL Materials Laboratory (WRDCIMLPO), Wright Research and Development Center, WrightPatterson AFB, Ohio 45433-6533
ABSTRACT We have developed a characterization technique using spectral photoconductivity that is capable of identifying deep energy levels in gallium arsenide. Our technique requires less sample processing than deep level transient spectroscopy (DLTS) and can be used on semiinsulating (si) gallium arsenide. The technique uses a mid-infrared fourier transform spectrometer for rapid photoresponse versus wavelength measurements. Using this technique we have observed evidence of several below mid-gap energy levels in undoped si gallium arsenide by the spectral PC technique. Spectral PC measurements were made on si GaAs before and after illuminating the cooled samples with high intensity white light at 8K. The PC spectrum typically showed a broad photoresponse that was attributed to multiple energy levels being present in the material. The observed energy levels were: 0.54 eV, 0.44 eV, 0.25 eV, 0.17 eV, 0.14 eV and --0.10 eV. The shallow photoresponse from the carbon acceptor was not observed. The most striking spectral feature was a sharp ionization edge at about 0.44 eV in all of the samples studied. This energy level correlates to the 0.43 eV intrinsic defect level that has been observed by temperature dependent Hall effect measurements. The 0.44 eV level was also observed in our PC spectrum of a n-type Bridgman grown sample that had shown the 0.43 Hall effect level. The presence of additional deep levels in concentrations comparable to those of EL2 and carbon indicates that the simple model for compensation in si GaAs which invokes only EL2 and shallow impurities needs revision.
INTRODUCTION Semi-insulating (si) GaAs substrates are a key material in the development of improved compound semiconductor microelectronic devices. The semi-insulating property of these substrates is strongly dependent upon the nature of the native defects and impurities in the material. These defects and impurities, or their complexes, can introduce a wide range of energy levels in the GaAs bandgap. The midgap levels, such as EL2, are believed to have the dominant role in the compensation of the material. For instance, the standard compensation model for si GaAs uses only a deep donor level (EL2) and a shallow acceptor level (carbon)[ll. In this model, the shallow acceptor levels would become uncompensated when the EL2 donor is photoquenched to its metastable state, if this metastable state is electrically inactive. In order to study the uncompensated levels in si GaAs after photoquenching, we chose to use an FTIR spectral photoconductivity technique. Previously we had used this spectral PC technique to observe the acceptors levels in p-type GaAs[2]. Based on the above compensation model, we had expected to observe only the spectral photoresponse of the shallow acceptor impurities after the EL2 donor was photoquenched in si GaAs. However, the
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