The authors present a systematic study showing the evolution of the defect morphology and crystalline quality in molecular beam epitaxially grown HgTe epilayers with substrate temperature. The authors have characterized the layers using optical microscopy, atomic force microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, and high-resolution x-ray diffraction. Four types of defects (microvoids, circular voids, hillocks, and high-temperature voids) have been characterized on epilayers grown in the substrate temperature range of 183.3–201.3 °C. The authors find that there is a minimum in the area covered by defects at a temperature just below the onset of Te precipitation, and they define this temperature as the optimal growth temperature. Above the optimal growth temperature the authors observe the appearance of high-temperature voids. By determining the onset of Te precipitation in HgTe, and performing thermodynamic calculations, the authors can also successfully predict the onset of Te precipitation in CdHgTe, which again is related to the optimal growth temperature in CdHgTe. Furthermore, the authors have found that the shape and density of the microvoids are particularly sensitive to the substrate temperature, and that these properties can be used to determine the deviation from the optimal growth temperature. From the shape and density of microvoids in one growth of HgTe, the authors can therefore determine the temperature correction needed to reach the optimal growth temperature for CdHgTe. The authors also suggest a mechanism for the formation of the microvoids based on the assumption of impurities on the substrate combined with a preferential Te diffusion in the [1 ¯11] direction across the steps.

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Spin relaxation times in GaAsxSb1−x quantum wells are measured at 295 K using time-resolved circular dichroism induced by 1.5 μm, 100 fs pulses. Values of 1.03 and 0.84 ps are obtained for samples with x=0 and 0.188, respectively. These times are >5 times shorter than those in InGaAs and InGaAsP wells with similar band gaps. The shorter relaxation times are attributed to the larger spin-orbit conduction-band splitting in the Ga(As)Sb system, consistent with the D’yakonov–Perel theory of spin relaxation [M. I. D’yakonov and V. I. Perel, Sov. Phys. JETP 38, 177 (1974)]. Our results indicate the feasibility of engineering an all-optical, polarization switch at 1.5 μm with response time <250 fs.

A systematic study of the evolution of the defect morphology and crystalline quality in molecular beam epitaxially grown CdxHg1−xTe epilayers with growth temperature is presented. The layers were characterized with optical microscopy, atomic force microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, and high-resolution x-ray diffraction. Four types of defects (microvoids, hillocks, high-temperature voids, and needles) were characterized on epilayers grown in the growth temperature range 188.9−209.9 °C. There is a minimum in the area covered by defects at a temperature just below the onset of Te precipitation, which is defined as the optimal growth temperature. Microvoids with various shapes, and at various stages of growth, were observed side-by-side in many of the CdxHg1−xTe layers, along with hillocks and needles. The defect density of microvoids changes by several orders of magnitude in the studied temperature range. A mechanism for the formation of microvoids and needles is suggested. High-temperature voids associated with Te precipitates appear above the optimal growth temperature. The onset of Te precipitation is well described by a thermodynamic model.

Spin relaxation times in GaAs x Sb 1Ϫx quantum wells are measured at 295 K using time-resolved circular dichroism induced by 1.5 m, 100 fs pulses. Values of 1.03 and 0.84 ps are obtained for samples with xϭ0 and 0.188, respectively. These times are Ͼ5 times shorter than those in InGaAs and InGaAsP wells with similar band gaps. The shorter relaxation times are attributed to the larger spin-orbit conduction-band splitting in the Ga͑As͒Sb system, consistent with the D'yakonov-Perel theory of spin relaxation ͓M. I. D'yakonov and V. I. Perel, Sov. Phys. JETP 38, 177 ͑1974͔͒. Our results indicate the feasibility of engineering an all-optical, polarization switch at 1.5 m with response time Ͻ250 fs.