Date of Award
Doctor of Philosophy (PhD)
Dr. T. Timusk
Dr. A.A. Berezin
Far-infrafred Fourier transform spectroscopy is used to investigate the excited states in semiconductors. Two different research projects are presented. The first involves an investigation of the far-infrared absorption of compensated p-type Ge at a temperature of 3 K. The absorption mechanism which is of interest is due to photon-induced hopping transitions of charge carriers between impurity centers. The samples were prepared by neutron transmutation doping (NTD). It is found that the absorption spectra for samples with carrier concentrations (Na-Nd) ranging from 2.3x10(15) to 2.6x10(16) cm(-3) show a broad absorption with the maximum occuring at a frequency between 10 and 24 cm (-1). The absorption coefficient of this maximum ranges between 2 and 87 cm (-1). The dynamical change of absorption due to the evolution of the Ga impurity is also presented. The absorption and the frequency of its maximum increase asymptotically with respect to time. It is found that at low frequencies, the absorption coefficient is proportional to frequency. The overall behaviour of the absorption spectra is found to be consistent with a theory based on the localized pair model. E. Kaczmarek and Z.W. Gortel have theoretically predicted a sharp peak a 20.4 cm(-1) on the absorption curve. Its position is independent of the compensation or the impurity concentration of the sample. However, this peak has not been observed in the experimental absorption spectra.
The second project involves the investigation of shallow impurities in selectively boron-doped GexSi(1-x)/Si strained-layer heterostructures using far-infrared photothermal ionization spectroscopy (PTIS). The spectra ar obtained under various experimental conditions with and without band-edge light and by varying the temperature of, and voltage applied to, the sample. The transport properties, resistivity, sheet charge density and Hall hole mobility of these samples are presented. It is found that a sample with a two-dimensional hole gas (2-DHG) behaviour at the GexSi(1-x)/Si interface has a weak photoresponse. This is due to the small number of photogenerated carriers in comparison with the residual hole carriers. For samples which do not show 2-DHG behaviour, the photoresponse from the substrate plays the dominant role for the low applied voltages. At higher voltages, the response from the epitaxial layers becomes evident. The majority impurity was identified as the intentional dopant boron and the dominant minority impurity was found to be phosphorous. It is concluded that although the free carriers are generated in the heavily B-doped layer, those carriers conducted through the low energy, high mobility GexSi(1-x)/Si interface layer, will dominate the photoresponse.
Jang, Ho Fan, "Far-Infrared Fourier Transform Spectroscopy of NTD Ge and GexSi(1-x)/Si Heterostructures" (1989). Open Access Dissertations and Theses. Paper 2038.