Photoluminescence of Te isoelectronic centers in ZnS : Te under hydrostatic pressure

The photoluminescence of four epitaxial ZnS: Te samples with Te concentration from 0.5% to 3.1% was investigated at different temperature and ambient pressure. Two well-known emission bands related to the isolated Te-1 and Te-2 pair isoelectronic centers were observed for the samples with Te concentrations of 0.5% and 0.65%. For the samples with Te concentrations of 1.4% and 3.1%, only was the Te-2-related peak observed. The pressure behaviors of these emission bands, were studied at 15 K. The Te-1 -related band has faster pressure shift to higher energy than ZnS band gap. On the other hand, the pressure coefficient of Te-2 -related bands is smaller than that of the ZnS band gap. According to a Koster-Slater model, we found that the increase of the density bandwidth of the valence band with pressure is the main reason for the faster shift of the Te-1 centers, while the relatively large difference in the pressure behavior of the Te-1 and Te-2 centers is mainly due to the difference in the pressure-induced enhancement of the impurity potential on Te-1 and Te-2 centers.

Concentration effect of Mn2+ on the photoluminescence of ZnS : Mn nanocrystals

Mn-doped ZnS nanocrystals of about 3 nm diameter were synthesized by a wet chemical method. X-ray diffraction (XRD) measurements showed that the nanocrystals have the structure of cubic zinc blende. The broadening of the XRD lines is indicative of nanomaterials. Room temperature photoluminescence (PL) spectrum of the undoped sample only exhibited a defected-related blue emission band. But for the doped samples, an orange emission from the Mn2+ T-4(1)-(6)A(1) transition was also observed, apart from the blue emission. The peak position (600 nm) of the Mn2+ emission was shifted to longer wavelength compared to that (584 nm) of bulk ZnS:Mn. With the increase of the Mn2+ concentration, the PL of ZnS:Mn was significantly enhanced. The concentration quenching effect was not observed in our experiments. Such PL phenomena were attributed to the absence of Mn2+ pairs in a single ZnS:Mn nanocrystal, considering the nonradiative energy transfer between Mn2+ ions based on the Poisson approximation. (c) 2005 Elsevier B.V. All rights reserved.

Effect of fabrication conditions on I-V properties for ZnO varistor with high concentration additives by sol-gel technique

Polycrystalline nano-grain-boundary multi-doping ZnO-based nonlinear varistors with higher concentration additives have been fabricated by sol-gel and standard solid-state reaction method, of which the best sample has a very high threshold voltage of E-b = 3300 V/mm. The effect of sintering processes, sintering temperature and sintering time, and that of additive concentration of Bi2O3 on E-b of the samples are systematically investigated. The results show that the great merit of sol-gel method is its high threshold voltage obtained by a lower sintering temperature than the solid-state reaction method. The present work also shows that five phases including solid-state sintering, rich Bi liquid phase formation and ZnO as well as other additive dissolution, ZnO grain growth, the secondary phase sufficient formation and evolution have been experienced at different sintering temperatures. The hole type defect and nonhomogeneity of the microstructure will lead to the decrease of threshold voltage, i.e., the grain size and the homogeneity of the material will be important factors and directly affect the characteristic of the varistor. The sintering characteristic and the influence of Bi2O3 content on the threshold voltage are also discussed. (c) 2004 Elsevier B.V. All rights reserved.

Electronic states of a hydrogenic donor impurity in semiconductor nano-structures

We propose a method for uniformly calculating the electronic states of a hydrogenic donor impurity in low-dimensional semiconductor nano-structures in the framework of effective-mass envelope-function theory, and we study the electronic structures of this systems. Compared to previous methods, our method has the following merits: (a) It can be widely applied in the calculation of the electronic states of hydrogenic donor impurities in nano-structures of various shapes; (b) It can easily be extended to study the effects of external fields and other complex cases; (c) The excited states are more easily calculated than with the variational method; (d) It is convenient to calculate the change of the electronic states with the position of a hydrogenic donor impurity in nano-structures; (e) The binding energy can be calculated explicitly. (c) 2007 Elsevier B.V. All rights reserved.

Enhanced performance of p-GaN by Mg delta doping

The electrical and structural properties of Mg delta-doped GaN epilayers grown by MOCVD were investigated. Compared to uniform Mg-doping GaN layers, it has been shown that the delta-doping (delta-doping) process could suppress the dislocation density and enhance the p-type performance. The influence of pre-purge step on the structural properties of GaN was also investigated. The hole concentration of p-GaN decreases when using a pre-purge step. These results can be explained convincingly using a simple model of impurity incorporation under Ga-free growth condition. (C) 2007 Elsevier B.V. All rights reserved.

Binding energy of a hydrogenic donor impurity in a rectangular parallelepiped-shaped quantum dot: Quantum confinement and Stark effects

We calculate the binding energy of a hydrogenic donor impurity in a rectangular parallelepiped-shaped quantum dot (QD) in the framework of effective-mass envelope-function theory using the plane wave basis. The variation of the binding energy with edge length, position of the impurity, and external electric field is studied in detail. A finite potential model is adopted in our calculations. Compared with the infinite potential model [C. I. Mendoza , Phys. Rev. B 71, 075330 (2005)], the following results are found: (1) if the impurity is located in the interior of the QD, our results give a smaller binding energy than the infinite potential model; (2) the binding energies are more sensitively dependent on the applied electric field in the finite potential model; (3) the infinite potential model cannot give correct results for a small QD edge length for any location of the impurity in the QD; (4) some degeneracy is lifted when the dot is no longer cubic. (C) 2007 American Institute of Physics.

Hydrogenic impurity in double quantum dots

The ground state binding energy and the average interparticle distances for a hydrogenic impurity in double quantum dots with Gaussian confinement potential are studied by the variational method. The probability density of the electron is calculated, too. The dependence of the binding energy on the impurity position is investigated for GaAs quantum dots. The result shows that the binding energy has a minimum as a function of the distance between the two quantum dots when the impurity is located at the center of one quantum dot or at the center of the edge of one quantum dot. When the impurity is located at the center of the two dots, the binding energy decreases monotonically. (c) 2006 Elsevier B.V. All rights reserved.

Self-consistent analysis of double-delta-doped InAlAs/InGaAs/InP HEMTs

We have carried out a theoretical study of double-delta-doped InAlAs/InGaAs/InP high electron mobility transistor (HEMT) by means of the finite differential method. The electronic states in the quantum well of the HEMT are calculated self-consistently. Instead of boundary conditions, initial conditions are used to solve the Poisson equation. The concentration of two-dimensional electron gas (2DEG) and its distribution in the HEMT have been obtained. By changing the doping density of upper and lower impurity layers we find that the 2DEG concentration confined in the channel is greatly affected by these two doping layers. But the electrons depleted by the Schottky contact are hardly affected by the lower impurity layer. It is only related to the doping density of upper impurity layer. This means that we can deal with the doping concentrations of the two impurity layers and optimize them separately. Considering the sheet concentration and the mobility of the electrons in the channel, the optimized doping densities are found to be 5 x 10(12) and 3 x 10(12) cm(-2) for the upper and lower impurity layers, respectively, in the double-delta-doped InAlAs/InGaAs/InP HEMTs.

Electronic structure and binding energy of a hydrogenic impurity in a hierarchically self-assembled GaAs/AlxGa1-xAs quantum dot

We calculate the electronic structures and binding energy of a hydrogenic impurity in a hierarchically self-assembled GaAs/AlxGa1-xAs quantum dot (QD) in the framework of effective-mass envelope-function theory. The variation of the electronic structures and binding energy with the QD structure parameters and the position of the impurity are studied in detail. We find that (1) acceptor impurity energy levels depend more sensitively on the size of the QD than those of a donor impurity; (2) all impurity energy levels strongly depend on the GaAs quantum well (QW) width; (3) a donor impurity in the QD has only one binding energy level except when the GaAs QW is large; (4) an acceptor impurity in the QD has two binding energy levels, which correspond to heavy- and light-hole quantum states; (5) the binding energy has a maximum value when the impurity is located below the symmetry axis along the growth direction; and (6) the binding energy has a minimum value when the impurity is located at the top corner of the QD. (c) 2006 American Institute of Physics.

Lifetime study of N impurity states in GaAs1-xNx (x=0.1%) under hydrostatic pressure

The lifetimes of a series of N-related photoluminescence lines (A(2)-A(6)) in GaAs1-xNx (x=0.1%) were studied under hydrostatic pressures at similar to 30 K. The lifetimes of A(5) and A(6) were found to increase rapidly with increasing pressure: from 2.1 ns at 0 GPa to more than 20 ns at 0.92 GPa for A(5) and from 3.2 ns at 0.63 GPa to 10.8 ns at 0.92 GPa for A(6). The lifetime is found to be closely correlated with the binding energy of the N impurity states, which is shown either in the pressure dependence for a given emission line or in the lifetime variation from A(2) to A(6). (c) 2006 American Institute of Physics.

Microdefects and electrical uniformity of InP annealed in phosphorus and iron phosphide ambiances

Microdefects originating from impurity-dislocation interactions in undoped InP that had been annealed in phosphorus and iron phosphide ambiances have been studied using optical microscopy. The electrical uniformity of the annealed wafer is improved by removing impurity aggregation around dislocations and by eliminating impurity striations in the annealing process. Compared to as-grown Fe-doped semi-insulating (SI) material, SI wafers obtained by annealing undoped InP in iron phosphide ambiances have better uniformity. This is attributed to the avoidance of Fe aggregation around dislocations and dislocation clusters, Fe precipitation and impurity striations, and is related to the use of a low concentration of Fe in the annealed material. The influence of Fe diffusion on the migration of dislocations in the annealing process has been studied and reviewed. (C) 2003 Elsevier B.V. All rights reserved.

Effects and numerical analysis of argon gas flow on the oxygen concentration in Czochralski silicon single crystal growth

Argon gas, as a protective environment and carrier of latent heat, has an important effect on the temperature distribution in crystals and melts. Numeric simulation is a potent tool for solving engineering problems. In this paper, the relationship between argon gas flow and oxygen concentration in silicon crystals was studied systematically. A flowing stream of argon gas is described by numeric simulation for the first time. Therefore, the results of experiments can be explained, and the optimum argon flow with the lowest oxygen concentration can be achieved. (C) 2002 Elsevier Science B.V. All rights reserved.

The effects of rapid thermal annealing on the optical properties of Zn1-xMnxSe epilayer grown by MOCVD on GaAs substrate

Zn1-xMnxSe thin films with different Mn compositions are grown by metal-organic chemical vapor deposition on GaAs substrate. Good crystallinity of sample is evidenced by X-ray diffraction and rocking-curve measurements. Photoluminescence (PL) properties were carefully studied. A dominant PL peak close to the band edge is observed at low temperature for samples with higher Mn concentration. The temperature-dependent PL and time-resolved photoluminescence show that this emission peak is associated with the recombination of exciton bound to Mn-induced impurity bound states. It is found that rapid thermal annealing can induce reorganization of Mn composition in alloys and significantly reduce the density of impurity induced by Mn incorporation and improve the intrinsic interband transition. (C) 2002 Elsevier Science B.V. All rights reserved.

Carrier concentration profiling in magnetic GaMnSb/GaSb investigated by electrochemistry capacitance-voltage profiler

Depth profiles of carrier concentrations in GaMnSb/GaSb are investigated by electrochemistry capacitance-voltage profiler and electrolyte of Tiron. The carrier concentration in GaMnSb/GaSb measured by this method is coincident with the results of Hall and X-ray diffraction measurements. It is indicated that most of the Mn atoms in GaMnSb take the site of Ga, play a role of acceptors, and provide shallow acceptor level(s).

Pressure behavior of Te isoelectronic centers in ZnS : Te

ZnS:Te epilayers with Te concentration from 0.5% to 3.1% were studied by photoluminescence under hydrostatic pressure at 15 K. Two emission bands related to the isolated Te-1 and Te-2 pair isoelectronic centers were observed in the samples with Te concentrations of 0.5% and 0.65%. For the samples with Te concentrations of 1.4% and 3.1%, only the Te-2-related peak was observed. The pressure coefficients of all the Te-1-related bands were found to be unexpectedly much larger than that of the ZnS band gap. The pressure coefficients for all the Te-2-related bands are, however, rather smaller than that of ZnS band gap as usually observed. Analysis based on a Koster-Slater model indicates that an increase of the valence bandwidth with pressure is the main reason for the faster pressure shift of the Te-1 centers, and the huge difference in the pressure behavior of the Te-1 and Te-2 centers is due mainly to the difference in the pressure-induced enhancement of the impurity potential on the Te-1 and Te-2 centers. (C) 2002 American Institute of Physics.