We have performed high-resolution x-ray diffraction measurements on vertically aligned InAs/GaAs quantum-dot nanostructures. The measurements were carried out for both the symmetric (004) and asymmetric (113) and (224) Bragg reflections. Theoretical simulations of the rocking curves indicate that the x-ray signal is primarily from the pseudomorphically strained (In,Ga)As wetting layers. The average thickness and indium composition in the wetting layers, as determined from simulations of the rocking curves, were, respectively, 0.72 nm and 88%. Transmission electron microscopy studies show the creation and annihilation of quantum dots with no observable dislocations.
We report on the polarization dependence of intraband photoresponse of (In,Ga)As/GaAs quantum-dot device structures for light polarized parallel and perpendicular to the layers. Strong photoresponse due to intersublevel transitions induced by both s- and p-polarized infrared light was observed. Within the plane of the layers, it is found that the photoresponse for s-polarized light aligned along the [110] crystallographic direction is virtually identical to that in the [1̄10] direction, suggesting that, at least in the x-y plane, the dots are symmetric. The devices studied were found to operate up to a temperature of around 100–105 K.
Hydrogen passivation effects in undoped p-ZnTe single crystals were studied by photoluminescence (PL) and photoconductivity (PC) measurements. Samples were exposed to r.f. hydrogen plasma at 250 • C for different durations. Before passivation PL peaks were observed at 2.06 eV, 1.47 eV, 1.33 eV and 1.06 eV. After 60 min of exposure, samples showed strong band edge green luminescence at 2.37 eV due to an exciton bound to a Cu acceptor. In PC studies the dark current decreased by a factor of 70 on passivation for 60 min. From the temperature dependence of PC gain, the minority carrier lifetime τ n was found to go through a maximum of 4.5 × 10 −7 s at 220 K before passivation. After 60 min of hydrogenation, τ n remained constant at 4.5 × 10 −7 s for T > 220 K and decreased for T < 220 K. The activation energies of τ n have been determined and show marked changes on passivation for T > 220 K. Comparison between PL and PC studies showed that the deep acceptor level O Te responsible for emission at 2.06 eV is passivated, giving rise to strong band edge emission at 2.37 eV, while emission due to the midgap impurity levels at 1.47, 1.33 and 1.05 eV remained unaffected. The thermal activation energies of PL peaks have also been determined and allow the construction of a defect energy level diagram for ZnTe.
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