Spectroscopy of electronic and vibrational excitations in semiconductors and oxide insulators

Jayprakash Bhosale, Purdue University

Abstract

A temperature tuned light emitting diode (LED) has several advantages over conventional sources for optical spectroscopy. The large radiation density of LEDs, concentrated in a small spectral region, is ideal for Spectroscopic techniques where a high signal-to-noise (S/N) ratio is desired. A simple, inexpensive LED source leads to a superior performance at high resolutions exceeding that of a tungsten halogen lamp, from near infrared to ultraviolet spectral region. A theoretical investigation with ab initio techniques of the electron-phonon interaction of semiconductors with chalcopyrite structure and its comparison with modulated reflectivity experiments yield a striking difference between those with (AgGaS2) and without (ZnSnAs 2) d electrons in their valence bands. The former exhibit a non-monotonic temperature dependence of the band gaps whose origin is not yet fully understood. The analysis of this temperature dependence with the Bose-Einstein oscillator model involving two oscillator terms having weights of opposite signs, provides an excellent agreement with the experimental data and correlates well with the characteristic peaks in the phonon density of states associated with the acoustical phonon modes. This work underscores the need for theoretical understanding of the electron-phonon interaction involving d electrons, particularly in ab initio investigations. Spectroscopic signatures of point defects serve as insightful characterizations in basic studies on semiconductors and their applications. In this context, localized vibrational modes (LVMs) revealed in their infrared absorption spectra the appearance of vacancies and interstitials originating from the lack of exact stoichiometry is a special feature of compound semiconductors. A striking manifestation of the LVMs of oxygen impurities substitutionally incorporated into CdSe is observed in which cation vacancies are either generated or suppressed deliberately by adopting specific crystal growth procedures. The complex thus formed can be attributed OSe- VCd with Cs symmetry. In addition the impressive LVMs related to oxygen antisite (O Cd) with host isotopic effect are reported. These identifications have been made on the basis of their infrared signatures recorded with an ultra-high resolution Fourier transform spectrometer. As in the case of CdSe, the occurrence of these signatures in CdTe was found to be influenced by stoichiometry. In order to explain these infrared signatures two defect structures namely OTe and OTe - VCd were proposed where the later exhibits a one step symmetry transformation as a function of temperature. Oxygen isotopic substitution enables to validate these defect structures. The signatures associated with OTe defect having Td symmetry show the expected shift of the LVM frequency upon isotopic substitution, confirming the structure of the defect. However, in the case of OTe-VCd defect structure, the LVM signatures associated with the 18O were surprisingly absent indicating that the defect structure is not yet fully understood. Therefore, to get more insight into the nature of this defect the influence of S and Se co-doping with oxygen on the LVM signatures is reported.

Degree

Ph.D.

Advisors

Ramdas, Purdue University.

Subject Area

Condensed matter physics|Optics

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