We investigated a red to yellow color change observed in α-HgI2 when cooled below 150 K. A phase transformation to β-HgI2, which has a yellow color, was ruled out by a variable temperature x-ray diffraction study. Instead the color change at cryogenic temperatures is caused by a shift in the transmission edge to shorter wavelengths which we attribute to a widening band gap at low temperatures. Using optical transmission spectroscopy the width of the band gap was measured between 10 and 330 K. At room temperature the gap was 2.11 ± 0.03 eV which is significantly smaller than the most recently published values of ~ 2.3 eV. This smaller band gap was further verified by measuring the thermoelectric current at elevated temperatures.
Thermoelectric Effect Spectroscopy and Thermally Stimulated Current measurements were used to investigate trapping levels in a semi-insulating CdTe and Cd1-xZnxTe crystals from multiple ingots grown by vertical Bridgman with over pressure control and high-pressure Bridgman methods. The crystals from different growth methods have different dislocation densities as well as Zn concentrations. The thermal ionization energies of these levels were extracted using both the variable heating rate and initial rise methods; the trapping cross sections were then calculated using the temperature maximum method. We report here that the shallow levels observed at E1=0.11+/- 0.02 and E2=0.17+/- 0.02 eV are intrinsic and the latter level is most likely related to the dislocation density.
Thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) were used to study the deep trapping levels in undoped and Sn doped Cd1-xZnxTe crystals. Temperature maximum, varying heating rate and initial rise methods were used to extract activation energies and trapping cross sections of the deep trapping levels in the samples. The concentration of deep levels was estimated from the resistivity data using Neumark model. The pure sample had deep trapping levels with ionization energies of EV + 0.73 eV and EV + 0.74 eV. The concentration of the deep levels was estimated at 500 ppb. These deep levels were associated with intrinsic defects due to Cd vacancies and Te antisites. In the Sn doped samples deep levels at EV + 0.34 eV, EV + 0.55 eV and EV + 0.73 eV were observed. The level at EV + 0.55 eV was associated with Sn, while the EV + 0.34 eV and EV + 0.73 eV levels were associated with Cd vacancies. The concentration of these levels was estimated a 10000 ppb. The pulse height measurements on these samples indicated that Sn doping did not improve the detector performance.
The molecular beam epitaxy growth of GaAs homoepitaxial films has been studied. Scanning tunneling microscopy images show that in the earliest stages of growth the surface morphology oscillates between one with two-dimensional islands and flat terraces. Concomitant with the decay of the RHEED oscillations, the surface morphology evolves to a dynamical steady state characterized by a constant value of the step density. Numerical models of the growth allow a prediction to be made for the asymtotic value of the step density. On a larger length scale (approximately 10 micrometers ) the surface morphology is found to consist of large mounds. The angle of the mounds with respect to the substrate is fixed and is determined by the separation between nucleating islands. This growth instability is found for singular surfaces and is absent for vicinal surfaces grown under step flow conditions.
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