[en] The nonlinear optical response of excitons in a single ZnSe quantum well is investigated by pump-probe measurements. In the counter-circular polarization configuration at negative delay, Δt, spectral oscillations are observed the period E_PP of which is energy dependent. Whereas at energies above the 1S exciton the usual E_PP=h/|Δt| dependence is found, the period strongly deviates from that value at energies below the exciton. Model calculations reveal that this behavior is caused by the bound biexciton state in the four-particle correlations. (c) 2000 The American Physical Society

[en] Contactless thermally stimulated lifetime measurements were performed on detector-grade Cd_1-xZn_xTe (x∼0.1) crystals using a pulsed laser microwave cavity perturbation method. The carrier lifetime decreased from approximately 30 μs at 110 K to 4 μs at 160 K, and then remained relatively constant from 160 to 300 K. The sudden drop in carrier lifetime within a particular temperature range is consistent with the thermal activation of a charge trap with a detrapping time longer than the carrier lifetime. The maximum trap activation temperature and the minimum detrapping time are estimated from the lifetime versus temperature curve to be approximately 160 K and 10^-6 s, respectively. (c) 2000 American Institute of Physics

[en] We report a strongly nonlinear pressure dependence of the band gaps and large downward shifts of the conduction band edges as functions of composition in ZnS_xTe _1-x and ZnSe_yTe _1-y alloys. The dependencies are explained by an interaction between localized A₁ symmetry states of S or Se atoms and the extended states of the ZnTe matrix. These results, combined with previous studies of III-N-V materials define a new, broad class of semiconductor alloys in which the introduction of highly electronegative atoms leads to dramatic modifications of the conduction band structure. The modifications are well described by the recently introduced band anticrossing model. (c)

[en] We studied systematically the structural and electronic properties of binary CdX (X=S, Se, and Te) semiconductors in both zinc-blende (ZB) and wurtzite (WZ) structures, the band alignment on the ZB/WZ interfaces, and carrier localization induced by the band offsets. We show, by first-principles band-structure calculation that at low temperature, CdS is stable in the wurtzite structure, while CdSe and CdTe are stable in the zinc-blende structure. However, coherent substrate strain can change CdTe to be more stable in the wurtzite form. We find that CdX in the wurtzite structure has a larger band gap than the one in the zinc-blende structure. The band alignment on the ZB/WZ interface is found to be type II with holes localized on the wurtzite side and electrons on the zinc-blende side. (c) 2000 The American Physical Society

[en] Spin clustering in diluted magnetic semiconductors (DMS) arises from antiferromagnetic exchange between neighboring magnetic cations and is a strong function of reduced dimensionality. Epitaxially grown single monolayers and abrupt interfaces of DMS are, however, never perfectly two dimensional (2D) due to the unavoidable intermonolayer mixing of atoms during growth. Thus the magnetization of DMS heterostructures, which is strongly modified by spin clustering, is intermediate between that of 2D and 3D spin distributions. We present an exact calculation of spin clustering applicable to arbitrary distributions of magnetic spins in the growth direction. The results reveal a surprising insensitivity of the magnetization to the form of the intermixing profile, and identify important limits on the maximum possible magnetization. High-field optical studies of heterostructures containing ''quasi-2D'' spin distributions are compared with calculation. (c) 2000 The American Physical Society

[en] CaS:Eu²⁺,Tm³⁺ is a persistent red phosphor. Thermoluminescence was measured under different excitation and thermal treatment conditions. The results reveal that the charge defects, created by substituting Tm³⁺ for Ca²⁺, serve as hole traps for the afterglow at room temperature. Tm³⁺ plays the role of deep electron trapping centers, capturing electrons either through the conduction band or directly from the excited Eu²⁺ ions. These two processes, in which two different sites of Tm³⁺ are involved, correspond to two traps with different depths. (c) 2000 American Institute of Physics

[en] We explore Cu electronic states in CdTe using photoluminescence as the main investigative method. Our results are consistent with some Cu atoms occupying substitutional positions on the Cd sublattice and with others forming Frenkel pairs of the type Cu_i⁺-V_Cd^- involving an interstitial Cu and a Cd vacancy. In addition, we find that Cu-doped CdTe samples exhibit a significant ''aging'' behavior, attributable to the instability of Cu acceptor states as verified by our Hall measurements. The aging appears to be reversible by a 150-200 degree sign C anneal. Our results are used to explain efficiency degradation of some CdTe solar-cell devices which use Cu for the formation of a backcontact. (c) 2000 American Institute of Physics

[en] A variational calculation of the ground-state energy of neutral excitons and of positively and negatively charged excitons (trions) confined in a single-quantum well is presented. We study the dependence of the correlation energy and of the binding energy on the well width and on the hole mass. The conditional probability distribution for positively and negatively charged excitons is obtained, providing information on the correlation and the charge distribution in the system. A comparison is made with available experimental data on trion binding energies in GaAs-, ZnSe-, and CdTe-based quantum well structures, which indicates that trions become localized with decreasing quantum well width. (c) 2000 The American Physical Society

[en] Local electronic property variations at individual interfaces have been determined using scanning surface potential microscopy, a variant of atomic force microscopy in conjunction with locally applied electric fields. Micropatterning is used to isolate individual interfaces and position contacts so that biases can be controlled locally. Positional variations in the voltage dependent interface charge and density of states in polycrystalline zinc oxide are determined from surface potential imaging. (c) 2000 American Institute of Physics

[en] We report direct observation of the existence of buried thin wurtzite CdTe layers in nominally pure zinc-blende CdTe thin films using high-resolution transmission electron microscopy. The formation of the buried wurtzite layers is a result of the formation of high density of planar defects in the zinc-blende films--the wurtzite layers are formed by closely spaced lamellar twins. First-principles calculations reveal that the presence of the buried wurtzite layers may be responsible for the poor electrical properties of the polycrystalline zinc-blende CdTe films. (c) 2000 American Institute of Physics