Real-time reflectance anisotropy spectroscopy of GaAs homoepitaxy.

Reflectance anisotropy spectroscopy (RAS) is a highly sensitive optical probe for the real-time study of the epitaxial growth of zincblende semiconductors. Here we report on (1) non-equilibrium RAS spectra acquired in real time during the homoepitaxial growth of GaAs, and (2) RAS spectra for GaAs surfaces under equilibrium with several arsenic overpressures. We show that in both cases RAS spectra can be decomposed into two basic components, each with a characteristic line shape. We further show that both dynamic and equilibrium RAS spectra are described by the same pair of basic components. We conclude that the time evolution of non-equilibrium RAS spectra acquired during the epitaxial growth can be described in terms of RAS spectra for equilibrium surfaces. The results reported here should be useful for the interpretation of the physics underlying the rapid time evolution of dynamic RAS spectra during the first monolayer growth. Thus, we show that RAS constitutes a valuable tool for the study of epitaxial growth mechanisms.

Temperature-dependent infrared ellipsometry of Mo-doped VO2 thin films across the insulator to metal transition.

We present a spectroscopic ellipsometry study of Mo-doped VO2 thin films deposited on silicon substrates for the mid-infrared range. The dielectric functions and conductivity were extracted from analytical fittings of Ψ and Δ ellipsometric angles showing a strong dependence on the dopant concentration and the temperature. Insulator-to-metal transition (IMT) temperature is found to decrease linearly with increasing doping level. A correction to the classical Drude model (termed Drude-Smith) has been shown to provide excellent fits to the experimental measurements of dielectric constants of doped/undoped films and the extracted parameters offer an adequate explanation for the IMT based on the carriers backscattering across the percolation transition. The smoother IMT observed in the hysteresis loops as the doping concentration is increased, is explained by charge density accumulation, which we quantify through the integral of optical conductivity. In addition, we describe the physics behind a localized Fano resonance that has not yet been demonstrated and explained in the literature for doped/undoped VO2 films.

A rapid reflectance-difference spectrometer for real-time semiconductor growth monitoring with sub-second time resolution.

We report on a rapid, 32-channel reflectance-difference (RD) spectrometer with sub-second spectra acquisition times and ΔR/R sensitivity in the upper 10(-4) range. The spectrometer is based on a 50 kHz photo-elastic modulator for light polarization modulation and on a lock-in amplifier for signal harmonic analysis. Multichannel operation is allowed by multiplexing the 32 outputs of the spectrometer into the input of the lock-in amplifier. The spectrometer spans a wavelength range of 230 nm that can be tuned to cover E(1) and E(1) + Δ(1) transitions for a number of III-V semiconductors at epitaxial growth temperatures, including GaAs, InAs, AlAs, and their alloys. We present two examples of real-time measurements to demonstrate the performance of the RD spectrometer, namely, the evolution of the RD spectrum of GaAs (001) annealed at 500 °C and the time-dependent RD spectrum during the first stages of the epitaxial growth of In(0.3)Ga(0.7)As on GaAs (001) substrates.