Far Infrared Spectroscopic Study of Bulk and Epitaxial Iii-V Iv-Vi and II-Vi Binary, Ternary and Quaternary Semiconductors.

Far infrared (FIR) spectroscopy, in the frequency range 10 cm('-1) to 500 cm('-1), has been used to study the free carriers and their spatial distribution at the epitaxial film/substrate interface in PbSe/NaCl, GaAs/GaAs and InAs/GaAs; the optical phonons in In(,1-x) Ga(,x)As(,y)P(,1 -y); and the interband transitions, free carriers and phonons in Hg(,1-x)Mn(,x)Te. The measurement of both reflection and transmission spectra, using a Fourier Interferometer and a CO(,2) laser pumped FIR laser spectrometer, combined with detailed computer fitting of the spectra yield reliable information on many parameters. A classical multioscillator model, for the dielectric constant is found adequate to describe the optical properties of all the materials except Hg(,1-x)Mn(,x)Te, where the contribution due to the interband transitions had to be taken into account. The epitaxial film/substrate interface in three different systems were studied at 300(DEGREES)K. They are: a 2 (mu)m PbSe film on a highly ionic NaCl substrate; three GaAs films, with thickness ranging from 2 to 3 (mu)m, on high resisitivity GaAs substrates; and a (TURN) 15 (mu)m thick InAs film on a high resistivity GaAs substrate. All the samples were grown by vapor and liquid phase epitaxy. The interface is found to be abrupt, on a scale measurable using FIR wavelengths, in PbSe/NaCl and GaAs/GaAs. Significant interface effects are present in InAs/GaAs. A mixed In(,x)Ga(,1 -x)As interface region is found to be present, the thickness, free carrier density and mobility are determined to be .074 (mu)m, (TURN) 6 x 19('17) cm('-3) and 400 cm('2)/V.sec respectively. The free carrier density is much higher than 4 x 10('15) cm('-3) and the mobility much lower than 16400 cm('2)/V.sec, the values in the epitaxial film. The presence of a (TURN) 0.1 (mu)m thick surface accumulation layer on the InAs film was also detected. The 300(DEGREES)K FIR spectra of the quaternary alloy, In(,1-x)Ga(,x)As(,y)P(,1-y) were analyzed to determine the optimal phonon structure. All the samples were n-type and were grown from the liquid phase on high resistivity InP substrates and ranged in thickness from 1 to 6 (mu)m; the range of y values extended from 0.22 to 0.66, with y/x (TURNEQ) 2.2 to ensure lattice matching to the substrate. The observed longitudinal and transverse optical phonons could be assigned to InP-, GaAs- and InAs-like vibrations. The assignments were made using the phonon and relevant impurity mode frequencies in the constituent binaries. Such a model satisfactorily describes the variations of the phonon frequences with the compositional parameter, y. The FIR reflection spectra of two samples of bulk Hg(,1-x)Mn(,x)Te, with x values of 0.015 and 0.082, were measured in the temperature range 5(DEGREES)K to 300(DEGREES)K. The transmission of a sample with x = 0.094 was measured at 6(DEGREES)K. The transmission was also measured as a function of the temperature at fixed FIR laser frequencies. All the low temperature (5(DEGREES) to 30(DEGREES)K) show strong evidence of interband transitions between the (GAMMA)(,8) valence and conduction bands. The interband transition effect calculated using a non-parabolic conduction band and a finite heavy hole mass of 0.55 m(,o); m(,o) is the free electron mass. The energy gap, E(,o), determined from the fit to the spectra shows a semimetal-semiconductor transition at x (TURNEQ) 0.10. The Fermi energies and the conduction band bottom effective mass values are also determined. The high temperature spectra (77(DEGREES)K to 300(DEGREES)K) were analyzed to determine the free carrier and phonon parameters. The results are in substantial agreement with previous studies.