Free hole - neutral donor recombination in high purity GaAs

We report results of photoluminescence measurements in high purity n- and p-type GaAs at low temperatures which substantiate the identification of the 1.5133 eV emission line with the recombination of free valence band holes with electrons bound to shallow donors.     

Photostimulation of photoconductivity in electron-irradiated CdS:Cu single crystals

We have studied the photoconductivity of CdS:Cu single crystals irradiated by fast electrons. When the density of fast recombination centers (s-centers) due to the electron irradiation is high, one observes photostimulation of photoconductivity (PSP). PSP is due to the photoexcitation of holes from the s-centers into the valence band and their subsequent decay at slow recombination centers. The energy location of the s-centers can be found from the peak in the PSP spectrum; we find that E_s=E_v+(1.67 ± 0.02) eV.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 50–53, January, 1989.     

Electrical properties of α-MnS

Electrical data including thermoelectric power, Hall effect and resistivity on iodine-grown crystals of p-type α-MnS are reported. A study of the temperature dependences reveals that the conductivity occurs by holes in a 3d-band (Mn³⁺) and that the mobility of the holes is not thermally activated. Photoelectron spectra confirm the 3d character of the top of the valence band. Accurate analysis of the electrical data gives a satisfactory explanation of the extrinsic and intrinsic behaviour; the simultaneous presence of donors (substitutional iodine) and acceptors (manganese vacancies) is responsible for the observed phenomena.     

Mechanism of infrared stimulation and quenching in ZnS: Cu,Al Phosphors

To investigate the mechanism of the stimulation and quenching of the green luminescence in ZnS: Cu, Al phosphors by infrared (IR) light of 0.7–1.5 μm, stimulation and quenching spectra, IR effects on time-resolved emission spectra, and IR-induced photoconductivity are measured at 4.2 K, 77 K, and room temperature. Both the stimulation and quenching are caused by the IR transitions ascribed to excited copper acceptors. It is concluded that the stimulation is induced by the process in which holes produced by IR light migrate among copper acceptors, so that the statistical distribution of the intrapair separations of excited copper-aluminum pairs are changed to shorter distances. It is found that at low temperatures the holes migrate from one copper acceptor to another without being thermally released to the valence band. It is confirmed that the quenching is caused by the recombination of holes released to the valence band with electrons in the conduction band via some kind of nonradiative recombination centers.     

A model of hopping and band DC photoconduction in doped crystals

Expressions for the screening length and the ambipolar diffusion length are derived, for the first time, for the case where hopping conduction and band conduction coexist in semiconductors with hydrogen-like impurities. A method is proposed for calculating the diffusion coefficient of electrons (holes) hopping between impurity atoms from data on the Hall effect, in the case where the hopping and band conductivities are equal. An interpretation is given of available experimental data on hopping photoconduction between acceptors (Ga) and donors (As) in p-Ge at T=4.2 K doped by a transmutation method. It is shown that the relative magnitude of the mobilities of electrons hopping between donors and holes hopping between acceptors can be found from the hopping photoconductivity measured as a function of the intensity of band-to-band optical carrier excitation.     

Screening effect on the binding energies of shallow donors,acceptors and excitons in finite-barrier quantum wells

The conduction and valence subband energies in the presence of an electric field are calculated using the fifth-order Runge–Kutta method. The binding energies of shallow donors, acceptors and excitons in finite-barrier GaAs/Ga_1−xAl_xAs quantum wells are then obtained variationally in the presence of a magnetic field. The effects of a spatially dependent screening function ϵ (r) on the calculation of binding energies are specifically investigated. The use of ϵ (r) in comparison with the use of a constant ϵ₀increases the binding energy of acceptors as the increase on shallow donors and excitons is quite small.     

Optical and luminescent VUV spectroscopy of La2Be2O5 crystals

Electronic excitations and the processes of their radiative relaxation are studied in pure and Ce³⁺ ion-doped crystals of lanthanum beryllate excited by synchrotron radiation in the x-ray and VUV ranges by methods of optical and luminescent vacuum ultraviolet time-resolved spectroscopy. Manifestations of excitons of the valence band are absent in the reflection spectra. However, a fast (τ=1.7 ns) and a slow (microsecond range) channel corresponding to two possible types of self-trapped excitons (STE) are found in radiative relaxation of intrinsic electronic excitations at T=10 K. The slow channel corresponds to emission of STE formed through recombination, the fast channel corresponds to emission of relaxed metastable excitons from the STE state. In the energy region higher than 14 eV (E>2E _g), the effect of multiplication of electronic excitations due to generation of secondary electron-hole pairs resulting from inelastic scattering of both hot photoelectrons and hot photoholes is exhibited.     

Growth and spectral characterization of β-Ga2O3 single crystals

Beta gallium oxide (β-Ga₂O₃) single crystals were grown by the floating zone technique. The absorption spectra and the luminescence of the crystals were measured. The absorption spectra showed an intrinsic short cutoff edge around 260 nm with two shoulders at 270 and 300 nm. Not only the characteristic UV (395 nm), blue (471 nm) and green (559 nm) lights, but also the red (692 nm) light can be seen in the emission spectra. The deep UV light was attributed to the existing of quantum wells above the valence band and the red light was owed to the electron-hole recombination via the vicinity donors and acceptors.     

Interband recombination dynamics in resonantly excited single-walled carbon nanotubes

Wavelength-dependent pump-probe spectroscopy of micelle-suspended single-walled carbon nanotubes reveals two-component dynamics. The slow component (5-20 ps), which has not been observed previously, is resonantly enhanced whenever the pump photon energy coincides with an absorption peak and we attribute it to interband carrier recombination, whereas we interpret the always-present fast component (0.3-1.2 ps) as intraband carrier relaxation in nonresonantly excited nanotubes. The slow component decreases drastically with decreasing pH (or increasing H+ doping), especially in large-diameter tubes. This can be explained as a consequence of the disappearance of absorption peaks at high doping due to the entrance of the Fermi energy into the valence band, i.e., a 1D manifestation of the Burstein-Moss effect.     

Picosecond carrier relaxation in type-II ZnSe/BeTe heterostructures

Relaxation of photoexcited carriers in the course of the formation of spatially separated layers of electrons and holes in type-II ZnSe/BeTe heterostructures has been studied based on a high time resolution investigation of fast luminescence kinetics. The hole escape times τ from the ZnSe layer have been measured in structures with different ZnSe layer thicknesses (τ = 2.5, 7.5, and 23 ps for thicknesses d = 10, 15, and 20 nm, respectively). It is shown that the increase in the time τ can be explained by the fact that the escape rate of photoexcited holes from the lowest above-barrier level in the ZnSe layer into the BeTe layer decreases as the thickness of the ZnSe layer increases.     

Impurity centers in ZnSe:Na crystals

The influence of sodium impurity on photoluminescence (PL) spectra of ZnSe crystals doped in a growth process from a Se+Na melt is investigated. It is shown that the introduction of the impurity results in emergence of emission bands in the PL spectra due to the recombination of exciton impurity complexes associated with both donors and hydrogen-like acceptors. Apart from that, four bands generated by donor-acceptor pairs recombination and a band produced by electronic transitions from the conduction band to a shallow acceptor are discussed. As a result of the analysis it is concluded that Na impurity forms in ZnSe lattice Na_Zn hydrogen-like acceptors with activation energy of 105±3 meV, Na_i donor centers with activation energy of 18±3 meV, as well as Na_ZnV_Se and Na_iNa_Zn associative donors with activation energy of 35±3 and 52±9 meV, respectively.     

Radiation-stimulated pulse conductivity of CsBr crystals

The radiation-stimulated pulse conductivity of CsBr crystals is investigated upon picosecond excitation with electron beams (0.2 MeV, 50 ps, 0.1–10 kA/cm²). The time resolution of the measuring technique is ～150 ps. It is shown that the lifetime of conduction band electrons is limited by their bimolecular recombination with autolocalized holes (V _k centers). A delay in the conduction current pulse build-up is revealed. This effect is explained within the proposed model, according to which the Auger recombination of valence band electrons and holes of the upper core band substantially contributes to the generation of conduction band electrons.     

Impurity-stabilized zinc-blende phase of wurtzite compounds

We propose here a new approach to stabilizing the cubic zinc blende phase of semiconductors that are usually more stable in the hexagonal wurtzite phase. We show that this can be done by taking advantage of the valence and conduction band offsets between the cubic and the hexagonal phases. Due to this band offset, it will cost less energy to insert electrons by shallow donors, or insert holes by 3d acceptors in the zinc blende structure, thus stabilizing the cubic phase.     

Luminescence property studies of α-Al2O3 by means of nanosecond time-resolved VUV spectroscopy

The luminescence spectra of corundum monocrystals grown by different methods are investigated by means of a time-resolved spectroscopy method at temperatures 90 K and 300 K. The existence of fast and slow emission in the VUV luminescence spectra of irradiated and nonirradiated crystals was observed. We observed luminescence bands with a maximum at 326 nm produced by F ⁺ centers. A new type of fast luminescence at the band of 270 nm was found. This is known as cross-luminescence and is connected with the recombination of valence band electrons with the holes in the low ground band. It was shown that the band of 410 nm isn't due to to anionic centers (F-centers), but is determined by the short lifetime center of emission (F ^- -centers). Received 20 October 1998 and Received in final form 20 January 1999     

Photoconduction of natural cinnabar (α-HgS)

The photoconductivity measurements presented in this paper permit us to reveal a sensitizing process in mercury sulphide (α-HgS) which is common to a number of II–VI compounds. The excitation spectra of the photocurrent as well as the results obtained in terms of excitation density and temperature allow us to locate the slow recombination center at 165 meV from the valence band. The ratio of capture cross sections for holes and electrons for this center is around 10⁵. Experiments on photoresponse to a cut off of the excitation show that, in the temperature range corresponding to the quenching of the photocurrent, the recombination process of free carriers is close to the bimolecular. However, the influence on our results of an electron trap located at 50 meV from the conduction band is pointed out.     

Evidence for interlayer donor–acceptor recombination in type II GaAs/AlAs superlattices

Steady-state and time-resolved photoluminescence of (GaAs)₇(AlAs)₉type II superlattices grown simultaneously by molecular beam epitaxy on (311)A and (100) GaAs substrates, intentionally undoped or uniformly doped with silicon, has been studied. It is shown that at temperatures T >  30 K, the dominant line in the photoluminescence spectra of superlattices is caused by donor–acceptor recombination between the donors located in the AlAs layers and the acceptors in the GaAs layers. The sum of the binding energies of the donors and acceptors in the pairs has been determined. A spectrally-dependent linear polarization of the donor–acceptor line along the direction of the interface corrugation of the superlattice has been discovered in the spectra of (311)A-oriented superlattices.     

Relaxation of femtosecond photoexcited electrons in a polar indirect band-gap semiconductor nanoparticle

A model calculation is given for the energy relaxation of a non-equilibrium distribution of hot electrons (holes) prepared in the conduction (valence) band of a polar indirect band-gap semiconductor, which has been subjected to homogeneous photoexcitation by a femtosecond laser pulse. The model assumes that the pulsed photoexcitation creates two distinct but spatially interpenetrating electron and hole non-equilibrium subsystems that initially relax non-radiatively through the electron (hole)-phonon processes towards the conduction (valence) band minimum (maximum), and finally radiatively through the phonon-assisted electron-hole recombination across the band-gap, which is a relatively slow process. This leads to an accumulation of electrons (holes) at the conduction (valence) band minimum (maximum). The resulting peaking of the carrier density and the entire evolution of the hot electron (hole) distribution has been calculated. The latter may be time resolved by a pump-probe study. The model is particularly applicable to a divided (nanometric) polar indirect band-gap semiconductor with a low carrier concentration and strong electron-phonon coupling, where the usual two-temperature model [1-4] may not be appropriate.     

Spectroscopy of composite scintillators

The spectral and temporal characteristics of X-ray luminescence of composites consisting of microparticles of “heavy” components (oxides, fluorides, sulfates) and an organic polymer binder containing optically active impurities have been investigated. It has been found that, in the case of pulsed X-ray excitation of the composites with a photon energy of 130–150 keV, the fast component (τ < 10 ns) of the luminescence arises whether or not the “heavy” component of the composite is doped with an optically active impurity. A mechanism has been proposed for the formation of the fast component of the luminescence: electrons and low-energy X-ray photons generated during the interaction of high-energy X-ray photons with the “heavy” component of the composite are effectively absorbed by the polymer binder and, thus, induce its luminescence. It has been shown that, in order to produce a composite-based fast scintillator with a high light yield, it is necessary to use a binder prepared from an organic material with a short scintillation decay time and another component prepared from a compound whose composition includes an element of a large atomic number Z.     

Fast Intrinsic Emissions of Wide-Gap Oxides Under Electron Irradiation

The emission spectra have been measured in the range of 1.6–9.0eV under irradiation of wide-gap oxides by single electron pulses (3 ns, 300kV). A fast (τ < 3 ns) continuous and temperature-independent emission, connected mainly with the transitions of hot holes between the levels of the valence band of oxides, can be separated in these spectra at 300–600 K, when the inertial emissions (5–7eV) of localized excitations undergo a strong thermal quenching. It is suggested that a drastic decrease of the intensity of this so-called hole intraband luminescence (IBL) in a short-wavelength spectral region is caused by the lowering of the density of states at the edges of the valence band and, therefore, supplies information on the width of an anion valence band E^v. The drastic decrease of the IBL intensity takes place at 6.4–8.6eV in BaMgAl₁₀O₁₇, SrAl₂O₄, MgAl₄O₇, MgO and BeO, that agrees satisfactorily with the values of E^v in these systems obtained by other methods.     

GaAs/AlGaAs超晶格的光致发光

在室温下测量了GaAs/A l0.3Ga0.7As超晶格的光致发光,发现在波长λ=761 nm处存在一较强的发光光峰,此发光峰目前尚未见报道。经理论分析表明,此峰是量子阱中的第一激发态电子与受主空穴复合发光。实验还观测到在λ=786 nm处,λ=798 nm处和λ=824 nm处分别存在一发光峰,分析表明λ=786 nm处的发光峰为量子阱阱中费米能级附近的电子与轻空穴复合发光;λ=798 nm处的发光峰为量子阱内的基态电子到轻空穴的复合发光;λ=824 nm处的发光峰为阱中激子复合复合发光。理论计算与实验结果符合的很好。