Electronic and Vibrational States of Point Defects in Semiconductors

Citation

Feenstra, Randall Meindert (1982) Electronic and Vibrational States of Point Defects in Semiconductors. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/p5ph-6811. https://resolver.caltech.edu/CaltechTHESIS:05142018-171221657

Abstract

This thesis deals with the properties of defects in tetrahedrally-bonded semiconductors. The detects which will be studied here are impurity atoms substitutional for host atoms in the crystal. In particular, those defects which produce localized electronic states in the middle of the electronic energy gap ("deep levels") will be discussed. The main experimental technique used is photoluminescence. The crystals are excited with a laser, and they emit light due to various electronic transitions at the defects. The energy of this luminescence yields information about the nature of the excited electronic states. Also, excited vibrational states of the defects are apparent in the luminescence, and these vibrational states yield structural information about the defect.

The major system studied here is GaP containing Zn and O impurities. The Zn and O ions experience an attractive Coulomb interaction, so that they tend to occupy lattice sites which are near to each other, forming defect pairs. The energy or luminescence emitted from a (Zn,O) pair depends on the separation of the impurities. Thus, a luminescence spectrum contains information about the number or pairs of each possible separation. I have used this phenomenon to monitor the relative positions of Zn and O impurities in the lattice. I have observed reactions in which the impurity atoms move through the lattice under the influence of laser excitation. Specifically, I observe the dissociation of nearest-neighbor (Zn,O) pairs, and the subsequent formation of further separated pairs. The dissociation of the nearest-neighbor pairs can occur thermally, or by a photoinduced mechanism. At temperatures near 200 C, the intensity of the (Zn,O) luminescence spectra changes with time, a direct observation of the photoinduced reactions in progress. The (Zn,O) pairs are observed to dissociate by purely thermal means at temperatures near 900 C. From the rates of these two types of reactions, I identify the photoinduced pair dissociation as being a "recombination-enhanced defect reaction". In the reaction, electron-hole recombination puts the defect into a highly excited vibrational state, leading to the dissociation. This is the first observation of this sort of reaction in a system with known defect types. Thus, my study provides unique information about the electron-phonon interaction at defects. This study also has some practical application. The material GaP:(Zn,O) is used for fabricating red light-emitting-diodes, and the dissociation or the pairs provides an explanation for the degradation of these diodes. Presumably the degradation of some other semiconductor devices proceeds by mechanisms similar to those observed here.

This thesis deals with several other topics aside from GaP:(Zn,O). The geometry or impurity pairs in zinc-blende crystals is discussed. For a given separation or the impurity atoms, there is some number or different possible relative orientations of the impurities. I have derived an analytic form for this distribution of impurity pair separations, and I show how this formula can be used to interpret the observed luminescence spectra of GaP:N. Another system studied here is Si containing In and B impurities. Recombination of excitons bound onto the impurities produces luminescence. From the observed decay times of these luminescence lines, I deduce values of the cross sections for free exciton capture onto In and B impurities. The magnitude of the In cross section indicates the presence of excited states of the In bound exciton. Finally, a theoretical treatment of the vibrational modes of substitutional defects in zinc-blende crystals is presented. The defects consist of an impurity atom, with springs or variable strength connecting it to its neighbors. For the case of oxygen in GaP, the theory predicts the existence or two defect vibrational modes, in agreement with experiment. From the energies of the observed vibrational modes, it appears that the oxygen impurity is quite weakly bonded to its neighboring atoms.

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