Ultrafast photoconductive response of semiconducting diamond

The photoconductive response of natural p-type semiconducting diamond has been investigated using nanosecond 1.06μm laser pulses. The photocurrent indicates the presence of fast (τ ? 1 nsec) and slow (τ ～ 10 msec) mechanisms which contribute to the free hole lifetime. The fast decay component is attributed to valence band-acceptor recombination. The slow component results from bound holes optically excited from donors to acceptors being thermally promoted to the valence band. These holes return to the donors via the acceptors at a rate determined by the long acceptor-donor recombination time.     

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。     

Linearly polarized terahertz radiation in uniaxially deformed Ge(Ga) upon the electric breakdown of an impurity

The polarization spectra of spontaneous terahertz radiation in uniaxially deformed germanium have been measured upon the electric breakdown of shallow acceptors. Lines with various degrees of polarization with respect to the compression axis have been observed in the radiation spectrum. These lines are associated with the optical transitions of holes between the excited and ground states of the acceptor, as well as with the transitions of holes from the valence band to the ground state of the impurity. At a pressure of about 3 ± 0.3 kbar in the [111] direction near the impurity breakdown, the linear polarization degree reaches ～80–90% in the main lines of terahertz radiation. As the electric field intensity increases, the depolarization of radiation is observed, which is caused by the heating of nonequilibrium holes by the electric field.     

Far infrared absorption by acceptor states in tellurium

Transmission spectra of slightly antimony-doped tellurium were measured at 2K. Absorptions due to transitions of holes from the acceptor ground state to the excited state and to the valence band were observed. Reflecting the double-maximum structure of the valence band, the ground state splits into two levels, the binding energies of which are found to be 1.31 and 1.47 meV, respectively.     

Infrared radiation of zinc selenide single crystals

Long-wave photoluminescence (PL) spectra of both as-grown and Au-doped n-ZnSe single crystals are studied in the temperature range from 81 to 300 K. A narrow band of infrared (IR) radiation centered at 878 nm (1.411 eV) manifests itself in the low-temperature PL spectrum. It is established that this band intensity first increases and then decreases with increasing concentration of doping impurity. With increasing excitation radiation intensity, spectral position of the IR PL band is unchanged and its intensity increases under the linear law. With increasing excitation radiation wavelength, the IR PL band intensity increases, it becomes narrower and shifts towards long wavelengths. It is shown that the observed IR radiation is caused by recombination of free electrons with holes localized on associative acceptors in the ZnSe:Zn:Au crystals or in the undoped crystals.     

Two-carrier transition in radiative recombination of two-dimensional electrons in GaAs/AlGaAs

We observed photoluminescence (PL) and photoluminescence excitation (PLE) spectra due to shake-up processes of recombination of two-dimensional electrons and free excitons in a modulation-doped GaAs quantum well at He temperatures. One of the processes is that when an electron recombines with a hole, another electron is excited from the conduction band in GaAs to that in AlGaAs. The other process is that a hole is excited from an acceptor level or the valence band in GaAs to the valence band in AlGaAs during recombination. The electron process is observed in both PL and PLE spectra while the hole process only in the PL spectra. The excitation-intensity dependence of the peak intensity of hole-excited PL is almost quadratic, indicating three-carrier process in the shake-up process. The band offsets of the conduction and valence bands are estimated to be 220 and 146 meV, respectively.     

Multicomponent Structure of an Electron-Hole Liquid in Shallow SiGe/Si Quantum Wells

The fine structure of the emission spectrum lines is studied for a quasi-two-dimensional electronhole liquid (EHL) embedded in SiGe/Si quantum wells. It is shown that the patterns observed in the spectra of the two-particle (IR band) and four-particle (visible band) recombination can be explained by the presence of both heavy and light holes in the condensed phase. Comparison of the experimental data and calculated photoluminescence spectra of EHL allow determination of its main parameters, e.g., the equilibrium density, work function for pairs of particles, and light hole–heavy hole splitting at the G-point of the Brillouin zone for quantum wells that are 3.7–5.1% Ge.     

Luminescence from hot electrons relaxing by LO phonon emission in p-GaAs and GaAs doping superlattices

Laser excited hot electrons in GaAs relax by LO phonon emission within a few hundred femtoseconds, leading to a series of peaks in the distribution of hot electrons in the conduction band, which we observe in luminescence. We find that the luminescence peaks shift according to the acceptor binding energy for C?, Ge?, Zn?, and Be-p-doped GaAs layers grown by MBE and LPE. Thus we prove that recombination is between hot electrons and neutral acceptors. The series of peaks due to electrons from the heavy hole band agree well with $\text{k}\text{.}\text{p}$ band structure, while peaks due to those from light holes are about 15 meV lower than expected from the band structure. We show that the discrepancy is not due to heating or surface fields. The peak separation in the luminescence ladder is about 6% larger than the LO energy suggesting emission of renormalised LO phonons. We find thermalisation by LO emission also in GaAs nipi doping superlattices. In nipi crystals the emission is shifted to higher energies (by 12 meV for light and by 6 meV for heavy holes) due to a change in band structure caused by the space charge fields.     

Terahertz electroluminescence under conditions of shallow acceptor breakdown in germanium

The spectrum of spontaneous terahertz electroluminescence was obtained near the breakdown threshold of a shallow acceptor (Ga) in germanium. The emission spectra were recorded by the Fourier spectroscopy method at a temperature of ～5.5–5.6 K. The emission spectrum exhibits narrow lines with maxima at ～1.99 THz (8.2 meV) and ～2.36 THz (9.7 meV), corresponding to the optical transitions of nonequilibrium holes from the excited impurity states to the ground state of impurity center. A broad line with a maximum at ～3.15 THz (13 meV) corresponding to the hole transitions from the valence band to the impurity ground state is also seen in the spectrum. The contribution of the hole transitions from the states of the valence band increases upon an increase in the electric-field strength. Simultaneously, the optical transitions of nonequilibrium holes between the subbands of the valence band appear in the emission spectrum. The integral terahertz-emission power is ～17 nW per 1 W of the input power.     

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     

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.     

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.     

晶格失配对InAsxP1-x/InP发光特性的影响

采用低压金属有机化学气相沉积(LP-MOCVD)技术,在掺Fe的半绝缘InP衬底上制备了InAs_0.157P_0.843 外延层。利用变温光致发光研究了InAs_0.157P_0.843外延层在13~300 K温度范围内的发光特性,通过理论分析与计算,证实了在应力作用下InAs_0.157P_0.843外延层价带顶的轻重空穴带发生了劈裂,并研究了导带底与价带顶轻空穴带之间形成的复合发光峰在应力作用下随温度的变化规律。     

Manifestation of auger processes in C1s-satellite spectra of single-walled carbon nanotubes

Using the equipment of the Russian-German beamline of the synchrotron radiation at the BESSY II electron storage ring, satellite spectra accompanying the C1s core lines in the cases of single-walled carbon nanotubes and highly ordered pyrolytic graphite have been measured with a high energy resolution. The Auger spectra corresponding to shaking of the valence system of carbon by the core vacancy have been found and investigated. The Auger spectra of the studied single-walled carbon nanotubes and highly ordered pyrolytic graphite are caused by annihilation of the excited π* electron with a hole in the π subband. It has been established that the electron states in the conduction band have 3π* (gT, K, M) symmetry; i.e., they correspond to flat 3π* subband, which is localized by 12–13 eV above the Fermi level. It has been revealed that the general regularities of the distribution of electron states in the valence system insignificantly change during its shake-up by the excited core.     

Optical investigations of Zn,Hg and Li doped GaN

The use of infrared quenching of photoluminescence to study the spectral dependence of the photoionization cross-section in GaN doped with Zn, Hg, and Li is reported. It is shown that these impurities produce deep level centres 0.48 eV, 0.41 eV, and 0.75 eV, respectively, above the valence band. In addition, excitation spectra are investigated for Zn and Li doped GaN, giving values of 3.17 eV and 2.86 eV at 78 K for the energy distance of these levels from the conduction band. Finally, from the temperature dependence of the excitation spectra, it is concluded that the levels are probably pinned to the valence band.     

Point defects related to 260 K thermostimulated luminescence in α-Al2O3

Abstract

Thermo- and photostimulated processes are studied in reduced hydrogen containing α-Al₂O₃ excited by UV light. It is found that UV excitation in F absorption band at 90 K results in a ionization of the F-centers and capture of released electrons at defects thus producing an anisotropy absorption band at 4.2 eV and the dominant thermoluminescence (TL) peak at 260 K. The 260 K TSL peak is accompanied by complete bleaching of the 4.2 eV absorption band and vice versa—by light stimulation in the region of the 4.2 eV band the 260 K TSL peak disappear and released electrons recombine with F⁺-centers. Both the effect of the preliminary high-temperature thermal treatment of samples on formation of 4.2 eV-centers and the observed dichroism characteristics allows to conclude that corresponding complex defect contains hydrogen and can involve vacancy pair.     

Radiative recombination in ZnTe-CdSe and ZnSe-CdTe heterojunctions

Electroluminescence in II–VI heterojunctions has been investigated. Three types of ZnTe-CdSe heterojunctions were studied depending on the preparation method: Red diodes obtained by doping with O₂, yellow Cu-doped diodes and green undoped heterojunctions. Radiation was observed only in the forward biased junctions. At 80 K the external quantum efficiency is about 1.3% for red ZnTe-CdSe heterojunctions and decreases by one order of magnitude at room temperature. The radiation intensity for the other heterojunctions is the same at 80 K but at 150 K the luminescence disappears. The band diagram and the electroluminescence spectra show that the two-directional injection takes place in the ZnTe-CdSe heterojunctions. The blue electroluminescence for ZnSe-CdTe heterojunctions is due to the injection of hot holes in ZnSe from the high-resistivity layer at the interface and the recombination of these holes with the free electrons through an accepto level at 0.124 eV from the valence band. At 80 K slow periodic current oscillations accompanied by in-phase oscillations of the luminescence have been observed in ZnSe-CdTe heterojunctions.     

Electric field effect on the mechanisms of recombination in KI doped with divalent ions

In KI crystals doped with divalent ions (Eu²⁺, Sr²⁺, Mn²⁺) a strong influence of the electric field is observed, after irradiation, on the carriers (electrons and holes) recombination kinetics. The phenomena are similar whether the electrons, distributed on traps bound to divalent ions, are excited by IR at 4 K, and recombine with trapped holes (V_k centers) or whether the holes are made thermally mobile at T>77 K. It is suggested that this is due to the recombination mechanism: the kinetics are simultaneously controlled by diffusion and tunneling. The tunneling range is a function of the applied field.     

Cathodoluminescence of ZnSe(Bi):Al crystals

The cathodoluminescence (CL) in ZnSe crystals annealed at T=1200 K in a Bi melt containing an aluminum impurity is investigated. The spectra are recorded for different excitation levels, temperatures, and detection delay times t ₀. As t ₀ is increased, the intensity of the orange band at λ _max=630 nm (1.968 eV) in the CL spectrum decreases in comparison to the intensity of the dominant yellow-green band at λ _max=550 nm (2.254 eV), whose half-width increases in the temperature range 6–120 K and then decreases as the temperature increases further. It is shown that such behavior of the yellow-green band is caused by the competition between two processes: recombination of donor-acceptor pairs and of free electrons with holes trapped on acceptors. The former mechanism is dominant at low temperatures, and the latter mechanism is dominant at high temperatures. At T∼120 mK the contributions of the two mechanisms to the luminescence are comparable. The resultant structureless band then achieves its greatest half-width, which is dictated by the interaction of the recombining charge carriers with longitudinal-optical and longitudinal-acoustic phonons and with the free-electron plasma. The mean number of longitudinal-optical phonons emitted per photon is determined mainly by their interaction with holes trapped on deep acceptors in the form of Al atoms replacing Se. The donor in the pair under consideration is an interstitial Al atom. Fiz. Tverd. Tela (St. Petersburg) 39, 1526–1531 (September 1997)     

Irreversible quenching of luminescence in porous silicon

The influence of applied voltage on photoluminescence (PL) in porous silicon was studied. A strong PL band around 680 nm was observed when excited by a 300 nm ultraviolet light with no voltage applied, but upon increasing the bias voltage, a strong and progressive decrease of the PL intensity was observed leading finally to a complete quenching of the emitted light at 1.80 V. The peak position of the emission appears to be stable. This effect is completely irreversible, and the spectra depend on the increased voltage to the sample and corresponding temperature increase. Nonradiative recombination resulting from the thermal oxidation was suggested to be responsible for the quenching.