Weak Disorder and Finite Temperature Effects on the Conduction Band Edge and Mobility Gap in Elemental and Compound Semiconductors.
Abstract
A understanding of the combined effects of weak disorder and finite temperatures on elemental group IV semiconductors and on IIIV compound semiconductors and their alloys is important because of their widespread technological use in electronic and optical devices. Consequently, in this thesis we have undertaken a detailed theoretical investigation of the effects of finite temperatures on weakly disordered hydrogenated amorphous silicon (aS:H) and hydrogenated amorphous germanium (aGe:H). We have also investigated the effect of weak disorder on more complex systems such as GaAs and its alloy Al_{rm x}Ga_{rm 1x} As. This series of investigations has been carried out in two nonoverlapping theoretical frameworks. In the elemental amorphous systems, we reconsider a previous selfconsistent treatment of localization on a Cayley tree for weak disorder described by a CauchyLorentzian distribution of siteenergies. The treatment is generalized to a system with n electronic states at each site and is an extension of the usually considered model for disordered systems. The effect of the electronphonon interaction on the siteenergy distribution is consistently taken into account, not only in the distribution of the selfenergy but also in that of the derivative of the selfenergy, which determines the mobility edge, E_ c . This important improvement leads to a temperature dependent E_ c which reduces to well known results in the zero temperature and zero disorder limits. The mobility gap and its temperature coefficient determined from these calculations is compared with optical gap data obtained from isoabsorption, photoluminescence, and optical transmission measurements on hydrogenated amorphous silicon and germanium. We find very good agreement over a wide temperature range with a judicious choice of physical parameters in the model. In our study of more complex systems, we have investigated the effects of disorder induced band tailing on deep levels in compound semiconductor alloys, such as GaAs and Al_{rm x}Ga _{rm 1x}As. In particular, we have eliminated the assumption of Gaussian broadening of the defect density of states proposed earlier by others on the basis of the central limit theorem. The expressions derived for the capacitance transients in the presence of the disorder induced band tailing are quite different from previously published results and indicate that the form of the broadening function is quite sensitive to, and should closely reflect, the underlying physical environment. We applied this theory to the investigation of the deep level transient spectra (DLTS) in GaAs and Al_{rm x}Ga _{rm 1x}As. Our results indicate that standard experimental measurements of DLTS, without appropriate corrections, in general, can underestimate the activation energy, capture crosssection, etc., even in the case of weak disorder.
 Publication:

Ph.D. Thesis
 Pub Date:
 February 1991
 Bibcode:
 1991PhDT........75T
 Keywords:

 Physics: Condensed Matter; Engineering: Materials Science; Engineering: Electronics and Electrical