Resonance tunneling of charge carriers in photoexcited type-II ZnSe/BeTe heterostructures

In is shown that, at high densities of spatially separated electrons and holes in type-II ZnSe/BeTe heterostructures, the conditions for resonance tunneling of photoexcited holes from the ZnSe layer to the BeTe layer are attainable. Nonlinear behavior of the intensity of the photoluminescence band corresponding to spatially direct optical transitions with photoexcitation intensity is observed. Numerical calculations are carried out, and the results are in good agreement with the experimental data in a wide region of variation of the optical pumping intensity.     

Hot phonon effects in ZnSe

Femtosecond pump-probe differential reflectivity spectroscopy is used to investigate the ultrafast cooling dynamics of hot photoexcited carriers in a high-quality ZnSe epilayer grown on GaAs. Comparison with a theoretical model based on a balance equation approach indicates that the observed reduction in the electron cooling rate with increasing carrier density (for carrier densities greater than ～3×10¹⁷cm^?3) is due to both screening of the Fröhlich interaction and a non-equilibrium hot longitudinal optic-phonon population generated by the cooling electron distribution.     

Charge accumulation effects in quantum-well structures

The effect of accumulation of charges produced owing to the absorption of light in quantum-well structures under longitudinal electric fields is considered. It is shown that, in such two-dimensional structures, the charge accumulation process differs substantially from that in three-dimensional systems. The charge distributions in the structures are calculated for various electric fields and total numbers of accumulated charges. It is also shown that, at considerable amount of accumulated charges, the Fermi-type degeneracy begins to play an important part in the distributions of charges and potentials. Such accumulation of charges can result in nontrivial luminescence in which the processes of charge carrier generation and subsequent recombination can be essentially separated in time from each other. In addition, this luminescence is not masked by emission on the transitions involving the tails of the density of states in the band gap and by emission from the substrate.     

Field dependence of the capture and re-emission of charge carriers by shallow levels in germanium and silicon

The capture and re-emission of charge carriers by shallow traps in lithium-compensated samples of gallium-doped germanium and boron-doped silicon have been studied by a transient charge drift method. The field dependence of the capture coefficient for hot carriers was observed at low temperatures and high electric fields; capture coefficients of the order of 10^?6 to 10^?7 cm³ sec^?1 were found. The re-emission of the trapped carriers was strongly affected by the electric field. This effect is well explained by a lowering of the Coulomb potential barrier about the trapping centres (the Poole-Frenkel effect) which enhances thermal re-emission. At the highest fields and lowest temperatures, the re-emission from the shallow traps in germanium appeared to be dominated by tunnelling.     

CdSe/ZnSe quantum dot structures grown by molecular beam epitaxy with a CdTe submonolayer stressor

A procedure for formation of CdSe quantum dots (QDs) in a ZnSe matrix is suggested. The procedure is based on the introduction of a CdTe submonolayer stressor deposited on the matrix surface just before deposition of the material of the QDs. (For CdTe/ZnSe structure, the relative lattice mismatch is Δa/a ≈ 14%.) The stressor forms small strained islands at the ZnSe surface, thus producing local fields of high elastic stresses controlling the process of the self-assembling of the QDs. According to the data of transmission electron microscopy, this procedure allows a considerable increase in the surface density of QDs, with a certain decrease in their lateral dimensions (down to 4.5 ± 1.5 nm). In the photoluminescence spectra, a noticeable (～150 meV) shift of the peak to longer wavelengths from the position of the reference CdSe/ZnSe QD structure is observed. The shift is due to some transformation of the morphology of the QDs and an increase in the Cd content in the QDs. Comprehensive studies of the nanostructures by recording and analyzing the excitation spectra of photoluminescence, the time-resolved photoluminescence spectra, and the cathodoluminescence spectra show that the emission spectra involve two types of optical transitions, namely, the type-I transitions in the CdSeTe/ZnSe QDs and the type-II transitions caused mainly by the low cadmium content (Zn,Cd)(Se,Te)/ZnSe layer formed between the QDs.     

Absorption and photoionization of the donor level in CdF2 semiconductor crystals

A model of strong vibronic interaction is proposed to interpret the specific features of infrared absorption and photoionization in CdF₂ semiconductor crystals. The model takes into account the polaronic nature of the conductivity in these crystals and the profound configuration shift of the free and bound polaron states. It is shown that the intense infrared absorption band in the crystals is not due to the transitions of charge carriers from hydrogen-like donor levels to the conduction band, but is caused by the phonon replicas of intracenter transitions. The low-temperature photoconductivity (in the temperature range 0–70 K) is a result of tunneling transitions between the phonon states of bound and free polarons, since these states are separated by rather high potential barriers. Overcoming the barriers in both directions is responsible for equilibration in the polaron subsystem upon the photoexcitation of charge carriers. The tunneling character of this process is responsible for the slight variation in the equilibration time in the above-indicated temperature range.     

Intensification of electroluminescence of ZnSe:(Te,O) crystals as a result of irradiation with γ-ray photons

The effects of irradiation with γ-ray photons of the ⁶⁰Co isotope and of treatment in zinc vapors on the electroluminescence of ZnSe:(Te, O) crystals were studied with the aim of revealing the potential for fabrication of light-emitting structures. The broadband electroluminescence with a peak at 600 nm is excited in the initial samples at voltages higher than 70 V. The threshold voltage is decreased to several volts irrespective of polarity after treatment of the crystals in zinc vapors. A similar effect is observed after irradiation. The position of the peak in the electroluminescence band does not depend on the magnitude of voltage or on irradiation. This position is related to the recombination of charge carriers at the centers of interstitial zinc according to the mechanism of excitation of the prebreakdown type. Treatment in zinc vapors and irradiation with γ-ray photons of ZnSe:(Te, O) bring about an increase in the electroluminescence intensity at both polarities of applied voltage.     

Type-II Ge/Si quantum dots

The electronic structure of spatially indirect excitons, multiparticle excitonic complexes, and negative photoconductivity in arrays of Ge/Si type-II quantum dots (QDs) are considered. A comparison is made with the well-known results for type-II III-V and II-VI QD heterostructures. The following fundamental physical phenomena are observed in the structures under study: an increase in the exciton binding energy in QDs as compared with that for free excitons in homogeneous bulk materials, a blue shift in the excitonic transitions during the generation of multiparticle complexes (charged excitons, biexcitons), and the capture of equilibrium carriers to localized states induced by the electric ield of charged QDs.     

The effect of intense laser light on the absorption-edge region of the spectrum of a CdCr2Se4 ferromagnetic semiconductor

The effect of intense incident laser light on the spectral dependence of the absorption of circularly polarized light is investigated in single crystals of CdCr₂Se₄ in the neighborhood of the absorption edge for temperatures in the range 100–130 K. The observed large changes in the shape of the edge are related to peaking of the exciton resonance, which is caused by photoexcited charge carriers that screen the internal electric fields in the crystal. Fiz. Tekh. Poluprovodn. 33, 583–585 (May 1999)     

Periodic formation of transient population inversion for intersubband laser transitions in quantum wells

A method of periodic short-term inversion for intersubband laser transitions in a quantum well with a repetition frequency of ～1 GHz and inversion retention time ～1 ps is suggested. The method is based on fast population of the upper transition level as a result of resonance tunneling of charge carriers from neighboring quantum wells as the electric field applied to the heterostructure is varied. As a consequence, the peak value of the population inversion can be as large, for example, as 8 × 10¹⁰ cm^?2 for the transition corresponding to the wavelength λ ≈ 10 μm and 2 × 10¹⁰ cm^?2 in the case of λ ≈ 25 μm. This method can be implemented at room temperature, which makes it attractive for fabrication of amplifiers of picosecond pulses for the middle and far infrared optical regions and the terahertz band.     

Far-infrared radiation from n-InGaAs/GaAs quantum-well heterostructures in high lateral electric fields under injection conditions

The infrared radiation emitted by hot electrons in n-InGaAs/GaAs quantum-well heterostructures subjected to a lateral electric field is investigated under conditions of carrier injection from the current contacts. In structures with double tunneling-coupled wells one of which is δ-doped, a pronounced increase in the intensity of far-infrared radiation upon the onset of carrier injection is observed. At the same time, this effect is lacking in single-quantum-well structures with doped wells or barriers. The observed increase in the radiation intensity is associated with the direct intersubband transitions of electrons which contribute to emission upon the real-space transfer of charge carriers between wells. The intensity of these transitions increases due to compensation of the space charge existing between the wells by the injected holes.     

Study of the impurity photoconductivity and luminescence in ZnSe:Ni crystals in the visible spectral region

The photoconductivity and photoluminescence spectra of ZnSe:Ni crystals in the visible spectral region are studied. It is established that the high-temperature impurity photoconductivity of ZnSe:Ni crystals is controlled by the optical transitions of electrons from the ground state ³ T ₁(F) to high-energy excited states, with subsequent thermally activated transitions of electrons to the conduction band. A photoconductivity band associated with the photoionization of Ni impurities is revealed. The intracenter luminescence of ZnSe:Ni crystals is efficiently excited with light corresponding to the intrinsic absorption region of Ni²⁺ ions.     

Photoelectric spectroscopy of InAs/GaAs quantum dot heterostructures in a semiconductor/electrolyte system

The photovoltaic effect in the semiconductor/electrolyte junction is an effective method for investigation of the energy spectrum of InAs/GaAs heterostructures with self-assembled quantum dots. An important advantage of this method is its high sensitivity. This makes it possible to obtain photoelectric spectra from quantum dots with high barriers for the electron and hole emission from quantum dots into the matrix even if the surface density of the dots is low (～10⁹ cm^?2). In a strong transverse electric field, broadening of the lines of optical transitions and emission of electrons and holes from quantum dots into the matrix directly from the excited states are observed. The effect of the photovoltage sign reversal was detected for a sufficiently high positive bias across the barrier within the semiconductor. This effect is related to the formation of a positive charge at the interface between the cap layer and electrolyte and of the negative charge on impurities and defects in the quantum dot layer.     

Persistent photoconductivity and electron mobility in In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As/InP quantum-well structures

The influence of the width of the quantum well L and doping on the band structure, scattering, and electron mobility in nanoheterostructures with an isomorphic In_0.52Al_0.48As/In_0.53Ga_0.47As/In_0.52Al_0.48As quantum well grown on an InP substrate are investigated. The quantum and transport mobilities of electrons in the dimensionally quantized subbands are determined using Shubnikov-de Haas effect measurements. These mobilities are also calculated for the case of ionized-impurity scattering taking into account intersub-band electron transitions. It is shown that ionized-impurity scattering is the dominant mechanism of electron scattering. At temperatures T < 170 K, persistent photoconductivity is observed, which is explained by the spatial separation of photoexcited charge carriers.     

Hot electrons and phonons under high intensity photoexcitation of semiconductors

It has become well established during the last few years that intense photoexcitation of a semiconductor leads to the heating of the carriers and the generation of nonequilibrium phonons. These phenomena which result from the relaxation of photoexcited carriers to the band extrema by interaction with other carriers and by emission of phonons, are reviewed in this paper. At relatively low intensities (<10⁵W/cm² for GaAs) the photoexcited carrier distribution is Maxwellian with a carrier temperature T_e different from the lattice temperature. T_e as high as 150K and effective phonon temperatures as high as 3700K have been observed in GaAs. The observed variation of T_e with excitation intensity leads to the conclusion that in semiconductors like GaAs the polar optical mode scattering is the dominant energy loss mechanism from the electron gas to the lattice. Similar results are obtained in CdSe and CdS. At higher intensities (>10⁵W/cm² for GaAs), the carrier dist0ribution becomes non-Maxwellian for reasons not well understood at present. We will also discuss some recent measurements of variation of T_e with excitation wavelength and of the transmission spectra of photoexcited GaAs.     

Optical an photoelectric properties odf ZnSe:Ti crystals

The photoconductivity and photoluminescence spectra of ZnSe:Ti crystals in the visible and infrared spectral regions are studied. It is established that the high-temperature impurity-induced photoconductivity of ZnSe:Ti crystals is defined by the optical transitions of electrons from the ³ A ₂(F) ground state to highenergy excited states, with the subsequent thermal transitions of electrons to the conduction band. The efficient excitation of intracenter luminescence of ZnSe:Ti crystals is achieved by light from the region of the intrinsic absorption of Ti²⁺ ions.     

Experimental aspects of hot electron distribution functions

A review is presented on the experimental efforts to obtain hot electron distribution functions in bulk materials in high d.c. electric fields. Three optical methods will be discussed in detail: (i) inter- and intraband absorption measurements, (ii) emission due to radiative transitions between band states, and band- and impurity states, (iii) inelastic light scattering experiments. A distinction is made between methods which yield the energy distribution functions and those which give information on the anisotropic distribution of hot carriers in the momentum space. The latter experiments involve a determination of the electric field induced dichroism in absorption or emission and the dependence of the scattering cross section on scattering geometry in inelastic light scattering experiments. The influence of the nonequilibrium phonon distribution on the interpretation of the experimental results is discussed, too. In addition, current work on energy distribution functions of hot carriers in quantizing magnetic fields as obtained from absorption or emission of infrared radiation due to transitions of carriers between Landau states or magnetic field split impurity states will be presented.     

Photoconductivity in ZnSe under high electric fields

High voltage photoconductive switches utilizing polycrystalline ZnSe mere investigated. Experiments have been performed on polycrystalline ZnSe switches in a longitudinal geometry. Electrodes of perforated metal films, a transparent liquid electrolyte, plasma, and ultraviolet-light-generated carriers were used. High-bias fields of up to 100 kV/cm and current densities over 100 kA/cm² can be applied to the polycrystalline ZnSe switches. Nonlinear effects were observed at high fields with near band edge illumination. Applications of these effects are discussed     

Photoconductivity Amplification in a Type-II <Emphasis Type="Italic">n</Emphasis>-GaSb/InAs/<Emphasis Type="Italic">p</Emphasis>-GaSb Heterostructure with a Single QW

Significant photocurrent/photoconductivity amplification is observed at low reverse biases in a type-II n-GaSb/InAs/p-GaSb heterostructure with a single quantum well (QW), grown by metal-organic vapor phase epitaxy. A sharp increase in the photocurrent by more than two orders of magnitude occurs under exposure of the heterostructure to monochromatic light with a wavelength of 1.2–1.6 μm (at 77 K) and the application of a reverse bias in the range 5–200 mV. The optical gain depends on the applied voltage and increases to 2.5 × 10² at a reverse bias of 800 mV. Theoretical analysis demonstrated that the main role in the phenomenon is played by the screening of the external electric field by electrons accumulated in the deep InAs QW and by the mechanism of the tunneling transport of carriers with a small effective mass. It is shown that the effect under study is common to both isotype and anisotype type-II heterojunctions, including structures with QWs and superlattices.     

Ultrafast Dynamics of Photoexcited Charge Carriers in In<Subscript>0.53</Subscript>Ga<Subscript>0.47</Subscript>As/In<Subscript>0.52</Subscript>Al<Subscript>0.48</Subscript>As Superlattices under Femtosecond Laser Excitation

The results of experimental studies of the time dynamics of photoexcited charge carriers in In_0.53Ga_0.47As/In_0.52Al_0.48As superlattices grown by molecular-beam epitaxy on a GaAs substrate with a metamorphic buffer are reported. On the basis of the results of the numerical simulation of band diagrams, the optimal thickness of the In_0.52Al_0.48As barrier layer (4 nm) is chosen. At this thickness, the electron wave functions in In_0.53Ga_0.47As substantially overlap the In_0.52Al_0.48As barriers. This makes it possible to attain a short lifetime of photoexcited charge carriers (τ ~ 3.4 ps) at the wavelength λ = 800 nm and the pumping power 50 mW without doping of the In_0.53Ga_0.47As layer with beryllium. It is shown that an increase in the wavelength to λ = 930 nm (at the same pumping power) yields a decrease in the lifetime of photoexcited charge carriers to τ ~ 2 ps. This effect is attributed to an increase in the capture cross section of trapping states for electrons with lower energies and to a decrease in the occupancy of traps at lower excitation densities.