This thesis presents two different studies of the infrared optical properties of two different semiconductors. Chapter 2 describes the results of the first infrared photoluminescence (IRPL) measurements of a HgTe-CdTe superlattice. IRPL spectra of two different HgTe-CdTe superlattices from two different sources were measured from 100 to 270K. Sample 1 was grown on (111) Cd0.96Zn0.04Te, and was grown to have 250 repeats of 38 - 40 Å of HgTe followed by 18 - 20 Å of CdTe. Sample 2 was grown on (111) CdTe and was grown to have 75 repeats of 50 Å of HgTe followed by 50 Å of CdTe. Sample 1 exhibited a single asymmetrical luminescence line at all temperatures studied. (Low signal-to-noise ratio from sample 2 prevented detailed analysis of the lineshape.) The luminescence from both samples occured at significantly lower energies than that from Hg1-xCdxTe alloys with the same Cd concentrations as the average Cd concentrations of the superlattices. At 240 K, the luminescence peak from sample 1 was near 148 meV, with a full width at half-maximum intensity of 42 meV, while the peak from sample 2 was near 242 meV, with a full width at half-maximum intensity of 69 meV.Analysis of the luminescence lineshape from sample 1 showed it to be consistent with wave-vector conserving band-to-band recombination. In this case, the band-gap energy of the superlattice would be near the low energy threshold of the luminescence peak. This study therefore represents the first direct determination of the band gap of an HgTe-CdTe superlattice. A comparison of the lineshapes from both samples with those measured in GaAs-Ga1-xAlxAs super-lattices showed evidence for fluctuations in the layer thicknesses of both the HgTe-CdTe superlattice samples. A comparison was made between the data and a simple theory of the band gaps of HgTe-CdTe superlattices. The theory was shown to be consistent with the experiments, if there were small errors in the measurements of the superlattice layer thicknesses of each sample. The differences in the luminescence properties of the two samples show that it is possible to tailor the band gaps of HgTe-CdTe superlattices.
Chapter 3 describes the first observation of s-like excited states of a double acceptor in a semiconductor. Two experiments were performed to study the s-like excited states of the 78-meV acceptor in GaAs. The techniques used, selective excitation scattering (SEL) and electronic Raman scattering (ERS), are both sensitive to the detection of s-like excited states of single acceptors in semiconductors. Measurements on two different liquid encapsulated Czochralski GaAs samples showed two s-like excited state transitions of equal magnitude, separated by 4.0 meV. Only one s-like transition is expected in the energy range measured for a single acceptor. A simple effective mass-like model of a double acceptor was developed to account for the two s-like excited states. This model predicted a splitting of the 1s12s1 excited state of a double acceptor to be 2.6 meV, in good agreement with the observed value of 4.0meV. This proved that the 78-meV acceptor in GaAs is due to the first ionization of a double acceptor, the first such identification to be made based on the s-like excited state spectrum. It is therefore possible to identify the valency of an acceptor in a semiconductor by measuring the s-like excited state spectrum.
Appendix A describes a novel technique for performing infrared photoluminescence measurements using a Fourier transform infrared spectrophotometer. This technique was developed to perform the experiments described in Chapter 2.
