Central cell effects in donor-acceptor pair spectra of polar semiconductors

The central cell effect in donor-acceptor (DA)-pair spectra is calculated by means of a semi-empirical short range potential and within the effective mass approximation (EMA). The electron-hole recombination energy is determined variationally as a function of the (DA)-pair separation R. The results are compared with previous theoretical work and with experiment in the case of ZnSe with In-donor and Li-acceptor dopants.     

Radiative recombination of donor-acceptor pairs in ZnTe

Lifetime of the first excited state of donor-acceptor pairs has been measured in ZnTe as a function of the donor-acceptor distance. The measured lifetime for the radiative recombinations of donor-acceptor pairs agrees well with the calculated one in which the central cell correction for the 1s state of the acceptors is taken into account. It has been found that the lifetime depends on the impurity concentration. The concentration dependence is discussed in connection with the non-exponential decay of the luminescence observed for more distant donor-acceptor pairs.     

Luminescence decay of distant donor-acceptor pairs in phosphorous-doped ZnTe

Luminescence decay of isolated donor-acceptor pairs has been measured over the range of pair distance R ～ 180 Å in phosphorous-doped ZnTe. Analysis of the recombination rate as a function of the pair distance suggests an asymptotic behaviour of the acceptor envelope function of the type exp(-r/a_A), with $\text{a}{}_{\mathrm{A}}\text{= 40 ± 10}\text{A}\text{?}$. This effective Bohr radius would correspond to an hydrogenic mass $\text{m}{}^1\text{= (0.13 ± 0.03)}\text{m}{}_{\mathrm{o}}$, which is close to the light hole mass as measured from cyclotron resonance experiments, $\text{m}{}^1\text{= (0.154 ± 0.005)}\text{m}{}_{\mathrm{o}}$. A discussion on the validity of the isolated donor-acceptor pair model is also given.     

Ti掺杂ZnTe体材料的优化光致发光光谱

针对光致发光光谱法研究ZnTe:Ti的困难,对包括激发能量、激发功率与激发光斑大小、水汽干扰和测量温度等实验条件进行了细致的优化.发现: (1)低于ZnTe禁带宽度的激发能量能给出相对强的光致发光光谱; (2) 水汽干扰既影响谱线的相对强度又增大谱线能量的测定误差; (3)相对强的激光聚焦有利于获得较好的光致发光光谱.可靠地观察到位于3903.5和3905.9cm^-1能量位置处的零声子光致发光谱线. 根据两谱线的能量间距和相对强度随 温度的变化关系,并借助于晶体场理论对四面体晶体场中 关键词： Ti掺杂 ZnTe 光致发光     

Hydrogen-acceptor interaction in CdTe and ZnTe studied by photoluminescence

Photoluminescence studies on CdTe and Znte implanted with hydrogen and deuterium at 150keV in the fluence range 10¹⁴–10¹⁵ cm^-2 are reported. In CdTe the introduction of hydrogen (deutrium) makes the recombination lines of excitons bounds to residual acceptors to disappear. The donor bound excitons are not affected. In ZnTe the intensity of the principal acceptor-bound exciton (Cu) is much reduced. In both crystals a new sharp line (H) appears with an adjacent structure on the low energy side. Short time annealings in the temperature range 150–250°C lead to the disappearance of the sharp lines (H) and to the recovery of the initial spectrum. These results shows that hydrogen is able to interact with acceptors in CdTe and ZnTe leading to the formation of a new exciton trapping centre. Possible microscopic models for this isoelectronic-like complex are proposedl     

Application of indium nanowires to donor-acceptor pair luminescence

A novel, self-supported nanostructured powder electroluminescence (EL) device, consisting of a dilute layer of conductive nanowires, which directly contacts a ZnS phosphor layer and functions as rear electrodes, is reported in this paper. Indium nanowires fabricated by using a porous alumina template and hydraulic pressure injection technique induce a localized field in the phosphor that is much higher than the average applied field and hence excite the phosphor to luminesce. The mechanism for light generation from the nanostructured contact EL device can be understood from the bipolar field-emission model.     

Evidence for a donor-acceptor pair band in CuInSe2

The broad-band emission, observed in p-type crystals, was studied as a function of excitation intensity and temperature. The band peak is usually in the energy range of 0.93–0.95 eV at 4.2°K. The band shifts to higher energy as the excitation intensity of temperature increases. This behavior is consistent with a donor-acceptor pair-band mechanism. The acceptor energy E_A is 85 ± 2 meV. The acceptors and donors involved in the pair band appear to be Cu and Se vacancies.     

Electron-phonon coupling with donor-acceptor pair recombination in polar semiconductors

An analysis of the electron-LO phonon interaction function S(R) for donor-acceptor pair transitions in polar semiconductors is presented. Contradicting theoretical results (S is a monotonically increasing function of pair separation distance R, ref. 2) with reliable experimental findings (S needs to be a drecreasing function of R, ref. 5) are removed, if the interaction between LO-phonons and the donor (acceptor) is taken into account more rigorously.     

The effect of annealing procedures on photoluminescence and electroluminescence in ZnTe

The effects of elevated temperature annealing procedures on the photoluminescent properties of ZnTe crystals were investigated. The materials studied included nominally-undoped, phosphorus-doped and oxygen-doped crystals. As a result of annealing in Zn vapor at temperatures of 850°C for periods of 65 hr the radiative recombination associated with the isoelectronic oxygen center was enhanced by a factor of five times. Correlation existed between the photoluminescent properties of the crystals and the electroluminescent properties of M.I.S. devices. Photoluminescence measurements on the grown crystals are considered to provide a reliable indication of potential device performance. For M.I.S. devices fabricated from annealed crystals by a technique in which a compensated layer was produced by Al diffusion and a contact applied by an In alloying procedure, room-temperature external quantum efficiencies up to 10⁻³ photons/electron were obtained for the 1·8 eV emission associated with the isoelectronic oxygen center.     

Growth rate characteristics and photoluminescence properties of ZnTe in MOVPE system

The effects of total gas flow rate and transport rate of source materials on the growth rate of ZnTe layers grown on the (1 0 0) ZnTe substrates by atmospheric pressure metal organic vapor phase epitaxy have been investigated. The growth rate increases with the square root of the gas flow rate and then it deviates from this tendency with increasing total gas flow rate. The growth rate shows a sublinear increase with increasing the transport rate of the II or VI group source. From the results of photoluminescence property, it is found that epitaxial layers of good quality can be obtained under the growth regime in between mass transport and surface kinetic reaction ones.     

Temperature dependence of the photoluminescence of CdTe/ZnTe quantum-dot superlattices

The temperature dependence of the luminescence of CdTe/ZnTe quantum-dot superlattices (self-assembled quantum-dot multilayers) with ZnTe spacers of various thicknesses was studied. Luminescence quenching observed to occur with increasing temperature is shown to depend substantially on the thickness of the ZnTe spacer. Particular attention is focused on the temperature dependence of the luminescence of a structure with the smallest ZnTe layer thickness, containing clusters of regularly arranged quantum dots. The luminescence line of tunneling-coupled quantum dots appearing in this structure exhibits an unusual temperature dependence, more specifically, an anomalously large shift of the peak position and fast luminescence quenching with increasing temperature.     

Study of acceptor states in CdTe by donor-acceptor pair excitation luminescence

Excitation of donor-acceptor pair luminescence has been studied in CdTe doped with lithium or chlorine. The excitation spectrum of the lithium acceptor is determined and fitted with the effective mass theory of Baldereschi and Lipari. Revised values of the valence band parameters are deduced: μ = 0.8, δ = 0.054, Ry = 24 meV. The analysis of the 1.45 eV luminescence band in compensated Cl-doped crystals shows the existence of donor-acceptor pair transitions. Three acceptor centers are identified: E_A = 89, 111 and 119 meV, and the contribution of a deep donor (E_D > 40 meV) is demonstrated. Besides intracentre type excitation transitions of the 1.45 eV band have been observed in non-compensated chlorine-doped crystals. Thus several recombination channels and distinct acceptor states contribute to the composite 1.45 eV luminescence band.     

Stress effects on the photoluminescence energies and binding energies of excitons in ultra-thin ZnTe/CdTe/ZnTe quantum wells

Using the variational method and the effective mass and parabolic band approximations, electron and heavy-hole ground-state energies and exciton and photoluminescence energies are calculated in ultra-thin quantum wells of CdTe/ZnTe heterostructures. The results indicate dependencies on the well width, the barrier height, and stress-related effects and occur because the wave functions of both free carriers and those bound in exciton form determine the system energy and are shaped by the geometry of the well. Critical system thicknesses were estimated for the point at which stress effects become negligible: a value of five monolayers was obtained based on the exciton binding energy, and a value of seven monolayers was obtained based on the free-carrier ground-state energy.     

IR transmission and reflection spectra of structures with ZnTe and ZnTe/ZnMgTe quantum wires

The IR transmission and reflection spectra of ensembles of one-layer (ZnTe) and two-layer (ZnTe/ZnMgTe) quantum wires have been measured in s- and p-polarized light. The optical parameters of the structures and the material/vacuum volume ratio have been found by dispersion analysis. The frequencies of IR-active modes have been calculated using a quasi-electrostatic approximation and models of prolate ellipsoid and two-layer cylinders. The spectra of ensembles of ZnTe nanowires differ little from the spectrum of bulk ZnTe, exception for some softening of the LO-like mode. The spectra of two-layer ZnTe/ZnMgTe nanowires contain interface modes along with the TO-like mode. The calculated and measured data agree well.     

Resonant light scattering spectra from polycrystalline ZnTe films

We have studied the feature of the resonant light scattering (RLS) spectra from polycrystalline ZnTe films of different crystallite size by exciting above the fundamental absorption edge. It is found that decreasing the crystalline size, higher order LO Raman-like lines are quenched due to increase in the nonradiative damping of exciton-like polaritons as intermediate states of RLS.     

Excitation of photoluminescence in Er-doped oxide and fluoride crystals

It has been established that laser excitation of infrared radiation (2.9 μm) in oxide and fluoride crystals doped by Er³⁺ ions gives rise to excitation of concomitant luminescence in the visible region according to the two-step nonlinear mechanism.     

The kinetics of donor-acceptor pair transitions with strong phonon coupling in GaP

Donor-acceptor pair luminescence in GaP was studied by time-resolved spectroscopy, by measuring and analysing the integral band decay over many decades of intensity and time and by measuring and analysing the temperature dependence of the luminescence intensity. We report here the results obtained with some or all of these techniques for pairs involving deep as well as shallow centres. Those with a deep centre are: S_P-Si_P and Si_Ga-Si_P, in which the acceptor is the deep centre, and O_P-C_P and O_P-Zn_Ga, in which the donor is the deep centre. These pairs all have a broad, phonon-dominated luminescence band. The pairs involving shallow centres include Si_Ga-C_P, S_P-C_P and Te_P-C_P. Due to the different way of momentum conservation in these pairs, those involving a donor on a Ga-site have strong phonon co-operation and weak zero-phonon (ZP) transitions, whereas those with a donor on a P-site have strong ZP transitions.With time-resolved spectroscopy, well-resolved structure, due to a ZP pair band and its phonon replicas, is obtained for nearly all pairs. A comparison is made with the structure obtained by measuring in the stationary state at very low excitation densities. Some trends in the strength of phonon co-operation are noted. In the case of Si_Ga-C_P pairs, sharp replicas of ZP pair lines with a momentum-conserving phonon are also resolved.Using the integral band decay technique and results from time-resolved spectroscopy we have determined the transition probabilities for the total luminescence band as well as for the ZP band for nearly all pairs mentioned. The influence of incomplete saturation of the pairs during excitation with a focussed beam of an argon-ion laser is discussed and approximately corrected for. Extrapolated to zero pair separation, the total transition probability ranges from ∽ 10⁵s^-1 for Si_Ga-C_P pairs to ∽ 15 × 10⁵s^-1 for O_P-C_P and O_P-Zn_Ga pairs. The ZP probability ranges from ∽ 4 × 10³s^-1 for Si_Ga-C_P and Si_Ga-Si_P pairs to ∽ 4 × 10⁵s^-1 for S_P-C_P pairs. The transition probability for pairs in GaP is discussed in relation to the site occupied by the donor and the depth of the centres involved. The relatively high dope concentrations used in some cases, ∽ 1 to 2 × 10¹⁸cm^-3, are discussed briefly in relation to “concentration quenching”.The temperature dependence of the luminescence intensity of S_P-Si_P pairs is satisfactorily analysed with a simple linear model. The same model is applied to similar literature data on the S_P-C_P pair luminescence. Using in addition data on the average pair transition probability, trapping rate constants for hole capture by the Si_P and C_P acceptors were calculated to be ∽ 10^-9 cm³s^-1. Evidence is presented that the trapping rate constant for holes by the Zn_Ga-O_P complex is about 2 × 10^-9 cm³s^-1.