Raman E 1, E 1+Δ1 resonance in unstrained germanium quantum dots

Raman scattering by optical phonons in unstrained Ge quantum dots obtained in GaAs/ZnSe/Ge/ZnSe structures was studied using molecular beam epitaxy. A shift in the E ₁, E ₁+Δ1 resonance energy due to the quantization of the spectrum of electron and hole states in quantum dots was observed. The properties observed were explained with the use of a simplest model of localization with allowance for the spectrum of Ge electron states.     

Valence band contributions to photoluminescence excitation spectra of tightly bound holes in zincblende semiconductors

Photoluminescence excitation (PLE) spectra of deep acceptor states in ZnSe, for example the Cu-related luminescence band at ≈1.95 eV, contain a prominent excitation band at ≈3.25 eV. This band lies above the structure marking the lowest direct E_O band gap E_g by the spin-orbit splitting energy Δ of the valence bands at Γ. The higher energy feature is either absent or greatly de-emphasised in the PLE spectra of shallow acceptor states in ZnSe and of the oxygen iso-electronic trap in ZnTe, where the electron rather than the hole is tightly bound. However, a significant PLE component at E_g + Δ is observed for deep acceptor-like states in ZnTe, where Δ is ≈0.95 eV. Efficient PLE at E + Δ for luminescence from deep acceptor-like states is shown to be consistent with the extended wave-vector contributions to the bound state wave-functions of holes of binding energies ≈Δ.     

Study of persistent photoconductivity effect in n-type selectively doped AlGaAs/GaAs heterojunction

A detailed study of the persistent photoconductivity effect (PPE) at selectively doped Al_0.3Ga_0.7As/GaAs interface was carried out at low (4.2 and 77 K) temperatures on samples with different original channel concentrations and mobilities. The observed selectiveness of the PPE to the photon energy allowed us to identify two independent mechanisms of the PPE making almost equal contributions to the total effect. These two mechanisms are: (i) electron photoexcitation from DX centers in AlGaAs layer, (ii) electron—hole generation in bulk GaAs with a charge separation at the interface. It has been found that the behavior of the mobility as a function of the channel concentration (altered by light) depends on a setback thickness d. For a sample with small d a marked mobility drop has been found. The well-resolved structure in the dependence of the electron mobility on the channel concentration has been observed. The first peculiarity is explained by free electron population of AlGaAs layer due to the electron photorelease from DX-centers. The second feature, occurring at higher charge densities in the channel is attributed to the effect of intersubband scattering arising due to the electron occupation of an excited subband at the interface.     

Selective pair luminescence in semiconductors

A novel, highly sensitive spectroscopic technique for the investigation of excited impurity states in semiconductors is demonstrated for Li- compensated n-type ZnSe and for p-type ZnTe:P. Donor/acceptor pairs with one partner in an excited state are pumped selectively to assure optimal radiative re-emission after fast de-excitation of the electron or hole, respectively. Luminescence is confined to a sharp line riding on the high energy shoulder of the pair band.     

Lateral electric field effects on quantum size confinement in cylindrical quantum dot under parabolic potential

Within the effective mass approximation, we investigated theoretically the ground-state energy of a single particle and the binding energy of the neutral donor impurity (D⁰) affected by a lateral electric field in a parabolic quantum dot (QD). The results show that the electron and the hole ground-state energy and the band to band transition energies shift to lower values (red shift) by increasing the field intensity. The quantum Stark shift (QSS) for the electron increases rapidly in the quasi spherical QD (QSQD) by increasing the lateral field, whereas for the hole it increases monotony. In the cylindrical QDs (CQDs), we found that the QSS for electron and hole increase monotonically. The quantum size, lateral electric field and impurity position effect on the binding energy of neutral donor (D⁰) is studied. Unexpected behavior of D⁰ in quantum well limit (QW), the binding energy of D⁰ is increasing (blue shift) with increasing QD radius R$R$ at the presence of a lateral electric field. It appears that for a fixed size of the QD, the off-center binding energy decreases when the impurity ion is displaced from the center to the QD borders, while it is shifted to lower energy with increasing the field.     

Biexciton in II–VI quantum dots with different localization potentials

We present a comparative study of the influence of the form of a localization potential on the binding energy of the biexciton in spherically symmetric quantum dots based on II–VI compounds. The proposed criterion for the comparison of potentials of different forms—the box potential, the harmonic oscillator, and the Gaussian potential—is based on the identical localization of charge carriers of the same sign in these potentials. Calculations of the biexciton binding energy have been performed using the variational method within the framework of the kp-perturbation theory taking into account additional polarization terms in the wave functions of the electron and hole subsystems, as well as the complex structure of the valence band. The obtained results have demonstrated that the presence of a smoothly varying finite-height potential in Cd(Zn)Se/ZnSe quantum dots can lead to a more efficient localization in the case of the biexciton in comparison with the exciton, which is of interest for the implementation of fast-acting quantum light emitters.     

Dual band enhanced transmission through a subwavelength aperture based on two split-ring-resonators

Enhanced transmission through a subwavelength aperture is observed at two frequency bands with employment of two split-ring-resonators (SRR) of different sizes. Each of the SRR is excited to produce resonance and the electric field energy localized in its gap and split region can be coupled into a small hole with a radius of 2.3 mm around the respective resonance frequency. The simulation results show that the energy through the small hole is increased at 5.94 GHz (r/λ ₁=0.045) and 7.03 GHz (r/λ ₂=0.054), where 1042-fold and 88-fold enhancements are obtained, respectively, in comparison with the case of a single isolated hole. Moreover, it is found that placing two identical SRR structures in front of the hole can realize higher enhanced transmission with respect to the case of only one SRR utilized. The electric field coupling-enhancement mechanism is well described by studying the electric field distribution.     

Low-temperature heat conductivity of ZnSe:Ni crystals: An unusually strong resonant change caused by phonon scattering from the anharmonic modes induced by charged impurities

An anomalously strong resonancelike change with a sharp minimum at a temperature of 15±1 K was observed for the heat conductivity of ZnSe:Ni crystals for the first time. The heat conductivity of one of the samples decreased by more than 200 times, as compared to its maximal value in pure ZnSe. A new phonon scattering mechanism including the A processes is suggested to interpret the unusual temperature dependence of heat conductivity. These are the umklapp processes accompanying the phonon scattering from the anharmonic modes induced by charged nickel impurities. As a result, the efficiency of A processes may be very high and comparable at 15 K to the efficiency of the U processes in ZnSe at a temperature of approximately 1000 K.     

Self-activated luminescence in zinc-sulfo-selenide crystals

The orange luminescence at 2.15eV of As-grown ZnS_0.52Se_0.48crystals has been studied. The peak energy, halfwidth and Gaussian band shape exhibit a temperature dependence which is similar to that of the self-activated (SA) luminescence of ZnS at 2.66 eVand to that of ZnSe at 2.03 eV, and which can be described satisfactorily by a configurational coordinate model. However, optically detected magnetic resonance experiments at 1.8 K show that the luminescence is due to the recombination of an electron thermally released from a donor state with a hole trapped at an A-center acceptor state. The temperature dependence of the thermoluminescence spectra above 77 K and their spectral shift under additional IR exposure are also consistent with radiative recombination involving distant donor-acceptor pairs.     

The electron trap associated with an anion vacancy in ZnSe and ZnSxSe1−x

A dominant electron trap in ZnSe and ZnS_xSe_1?x has been studied by DLTS. The trap depth of about 0.3 eV is independent of the crystal growth, doped impurities and S mole fraction x in ZnS_xSe_1?x, while the electron-capture cross-section decreases with x. These properties indicate that the electron trap is due to the anion vacancy in ZnSe and ZnS_xSe_1?x.     

Charge trapping phenomena in high-efficiency metal-oxide-silicon light-emitting diodes with ion-implanted oxide

This work is a comparative study of the processes of charge trapping in silicon dioxide layers doped with different rare-earth (RE) impurities (Gd, Tb, Er) as well as with Ge. Diode SiO₂-Si structures incorporating such oxide layers exhibit efficient electroluminescence (EL) in the spectral range of UV to IR. Ion implantation was performed over a wide dose range with the implant profiles peaking in the middle of the oxide. Charge trapping was studied using an electron injection technique in constant current regime with simultaneous measurements of the EL intensity (ELI). High-frequency C/V characteristics were used to monitor the net charge in the oxides.Analysis of the charge trapping and the variation of the EL intensity during electron injection shows that the current density range can be divided in three portions: (i) low injection level, where electron/hole capture at traps with large capture cross-sections and low ELI occurs; (ii) medium injection level corresponding to the main operation mode of the devices (odd hole trapping depending on the injected current level is observed); and (iii) high injection level (electrical quenching of the EL that correlates with electron capture at traps of extremely small capture cross-sections takes place). The nature of specific hole trapping at the medium injection level in RE-doped devices is discussed. Mechanisms of EL quenching at the high injection level are proposed.     

ZnSe/SiO2复合薄膜光学常数与荧光光谱的研究

采用溶胶-凝胶工艺与原位生长技术，制备了ZnSe/SiO₂复合薄膜.X射线衍射分 析表明薄膜中ZnSe晶体呈立方闪锌矿结构.X射线荧光分析结果显示薄膜中Zn与Se摩尔比为1 ∶1.01—1∶1.19.利用场发射扫描电子显微镜观察了复合薄膜的表面形貌，结果表明复合薄 膜表面既存在尺寸约为400nm的ZnSe晶粒，也存在尺寸小于100nm的ZnSe晶粒.利用椭偏仪测 量了薄膜椭偏角Ψ，Δ与波长λ的关系，采用Maxwell-Garnett有效介质理论对薄膜的光学 常数、厚度、气孔率、ZnS 关键词： 2复合薄膜')" href="#">ZnSe/SiO₂复合薄膜 光学性质 椭偏光度法 荧光光谱     

Lattice instability induced by 3<Emphasis Type="Italic">d</Emphasis> impurities in a zinc selenide crystal

It was experimentally shown that, in a 3d-impurity-doped ZnSe crystal with the zinc blende structure, nanosized ionic shear displacements of the trigonal (ZnSe : Ni, ZnSe : V) and tetragonal (ZnSe : Cr) types arise at temperatures of 300 and 120 K. As the temperature decreases in the range 100.0–4.2 K, the shear elastic moduli C ₄₄ (ZnSe : Ni) and (C ₁₁–C ₁₂)/2 (ZnSe : Cr) are softened owing to the 3d impurities. A new peak at a frequency of 90 cm^?1 appears in the Raman scattering spectrum of ZnSe : Ni at 5 and 20 K.     

Electron—hole liquid in CdS crystals as revealed by non-equilibrium transmission spectroscopy

Transmission measurements in the excitonic region on thin platelets under high N₂ laser excitation showed the disappearance of exciton absorption into a strong absorption continuum. The low energy edge of the latter — about 12 meV below the free exciton A_n=1 — coincides with the high energy edge of the gain spectrum of stimulated emission under similar excitations. These results favour the assumption of electron—hole drop formation with a ground state of the e-h-pair about 12 meV below A_n=1.     

New results on the high field magnetoresistance of (TMTSF)2ClO4: Oscillatory effects and phase transitions

The transverse magnetoresistance of (TMTSF)₂ClO₄ has been investigated in magnetic fields as high as 32T, at low temperature down to 2.4 K. On the magnetoresistance, in relaxed state (R state), we observed many oscillations periodic in 1/B in a temperature range from 2.4 to 12 K, together with slope changes induced by phase transitions. At low temperature two series at the same frequency are observed, interpreted by the presence of equal area electron and hole pockets.     

Horizon thermodynamics in <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>) theory

We investigate whether the new horizon first law proposed recently still work in f(R) theory. We identify the entropy and the energy of black hole as quantities proportional to the corresponding value of integration, supported by the fact that the new horizon first law holds true as a consequence of equations of motion in f(R) theories. The formulas for the entropy and energy of black hole found here are in agreement with the results obtained in literatures. For applications, some nontrivial black hole solutions in f(R) theories have been considered, the entropies and the energies of black holes in these models are firstly computed, which may be useful for future researches.     

Influence of polarization on the hole formation with picosecond laser

Linear polarization effects on laser drilling were investigated by the use of a picosecond laser system. It was observed that the exit shapes of stainless steel were placed perpendicular to the direction of polarization. According to theoretical analysis, the main factor that determines the exit hole shape is the difference between R _s and R _p. When this gap is large enough, the bulges on the exit hole will be easily observed. Inversely, when the gap is small, bulges can be barely spotted on the exit hole. This is further confirmed by comparative experiment and calculation conducted on a copper sheet.     

Negatively charged excitons and the optical properties of modulation-doped quantum wells

Optical and magneto-optical properties are studied for II-VI semiconductor multiple quantum wells (MQWs) doped with donors in the barriers to give electron concentrations of 2 10¹⁰ to 6 10¹¹ cm^-2 in the well layers. Following on from the recent identification of negatively charged excitons X^- (two electrons bound to one hole) in CdTe/Cd_1-xZn_xTe MQWs, this paper presents more specifically the circular polarisation of the luminescence associated with X^- and with the normal exciton X (one electron and one hole) in this type of structure. Very similar magneto-optical properties are observed for modulation doped Zn_0.9Cd_0.1Se/ZnSe MQWs, and X^- is identified in these wells with a binding energy as large as 7 meV for the second electron at 50Å, well-width.     

Velocity distribution of plasma electrons in the negative H2- and He-glow with superimposed longitudinal magnetic field

The negative glow plasma has been found nearly field free in axial direction. Therefore plasma electrons in the stationary glow can thermalize down to the temperature of the neutral gas, whenever their diffusion—and recombination—lifetime is high enough. Applying Boltzmann's equation to this problem, the conditions of thermalization of plasma electrons are derived as a function of the outer parameters of the plasma: vessel diameter 2R, neutral gas pressurep and longitudinal magnetic fieldB. — If plasma electrons have a too short diffusion—and recombination—lifetime to be in thermal equilibrium with the neutral gas, the electron energy increases. For this case the distribution function of plasma electrons is derived using Boltzmann's equation. Approximating the calculated energy distribution by a Maxwellian distribution function, the electron temperature in the glow is obtained as a function of the parameters:R, p, B. OurT _e -measurements carried out in the H₂- and He-glows of different tube diameters, neutral gas pressures and magnetic fields agree closely with the theoretical results. TheT _e -measurements have been performed with Langmuir probes and by the method of reversal of the radial ambipolar electric field in a longitudinal magnetic field.     

Coulomb effects in spatially separated electron and hole layers in coupled quantum wells

We report on the (magneto-) optical study of many-body effects in spatially separated electron and hole layers in GaAs/Al_xGa_1?x As coupled quantum wells (CQWs) at low temperatures (T = 1.4 K) for a broad range of electron-hole (e-h) densities. Coulomb effects were found to result in an enhancement of the indirect (interwell) photoluminescence (PL) energy with increasing the e-h density both for a zero magnetic field and at high fields for all Landau level transitions; this is in contrast to the electron-hole systems in single QWs where the main features are explained by the band-gap renormalization resulting in a reduction of the PL energy. The observed enhancement of the ground state energy of the system of the spatially separated electron and hole layers with increasing the e-h density indicates that the real space condensation to droplets is energetically unfavorable. At high densities of separated electrons and holes, a new direct (intrawell) PL line has been observed: its relative intensity increased both in PL and in absorption (measured by indirect PL excitation) with increasing density; its energy separation from the direct exciton line fits well to the X ^? and X ⁺ binding energies previously measured in single QWs. The line is therefore attributed to direct multiparticle complexes.