Effects of surface adhesion of nonequilibrium charge carriers in the photoconductivity spectra of CdS crystals

The relaxation of low-temperature (T = 77 K) edge photoconductivity (PC) spectra of CdS crystals of the first group1 caused by the removal of the transverse electric field is investigated. A clearly manifested inversion of the fine (exciton) structure is found in the spectra during their relaxation to the initial form (existing before the application of the field). It is shown that the observed relaxation changes in the PC spectra of CdS are due to relaxation of the nonequilibrium surface charge appearing as a result of trapping of a part of the charge, induced by the transverse field into the sample, by slow surface states (adhesion levels). The experimental data on the effect of preliminary illumination by self-radiation on the edge PC spectra of the first-group CdS crystals indicating the existence of surface adhesion levels with a high density in these crystals are presented.     

“Anomalous” spectral photoresistive field effect in CdS crystals caused by the screening of the electron-hole interaction

The changes observed in low-temperature (T = 77 K) near-band-edge photoconductivity spectra of CdS crystals in response to an external transverse electric field applied to the sample surface have been investigated. An analysis of the “anomalous” character of these changes for a number of crystals has revealed a significant role of the near-surface effects of screening of the electron-hole interaction in the formation of near-band-edge photoconductivity spectra of CdS crystals with a technological excess of cadmium near the surface. It has been shown that the depletion (enrichment) transverse electric field leads to a weakening (enhancement) of screening effects in the photoconductivity spectra of the CdS crystals.     

Investigation of the excitonic structure in the photoconductivity spectra of CdS crystals

The changes in the low-temperature photoconductivity spectra of CdS crystals under the action of external influences are investigated. The high sensitivity of the fine (excitonic) structure of the spectra to changes in the conditions for the recombination of nonequilibrium carriers near the surface and in the bulk of the semiconductor is demonstrated. Fiz. Tverd. Tela (St. Petersburg) 40, 941–943 (May 1998)     

Photoreflection and photoconduction spectra of CdS crystals: Excitons in the electric fields of surface states

The exciton photoreflection spectra of CdS crystals are studied. It is found that the form of the exciton photoreflection spectrum is determined by a Stark shift of the exciton energy in the electric field of surface states. The dependences of the exciton photoreflection spectrum on temperature on the intensity and wavelength of the modulating radiation, and on the processes by which the photoreflection signal relaxes is determined. An energy scheme is proposed for the surface states which explains the observed effects of photoinduced changes in the surface field. A correlation is established between the exciton photoreflection spectrum and the form of the fine structure in the photoconductivity. Fiz. Tverd. Tela (St. Petersburg) 40, 875–876 (May 1998)     

Effect of IR illumination on photocurrent spectra in CdS crystals

Low-temperature near-band-edge photoconductivity (PC) spectra of CdS crystals were studied as a function of IR illumination intensity in the PC quenching interval. The photocurrent quenching by IR light of the PC response profile has been investigated. An analysis of these relationships permitted establishing a direct connection between the r photoconductivity centers and the near-band-edge structure of the spectrum. The effect of “pinning” of majoritycarrier lifetime on the semiconductor surface has been discovered and interpreted. It is proposed that surface-acceptor states in CdS crystals play the part of surface photoconductivity centers. Fiz. Tverd. Tela (St. Petersburg) 41, 1181–1184 (July 1999)     

Optical transitions of holes in uniaxially compressed germanium

Spontaneous emission and photoconductivity of germanium with gallium impurity are studied for determining the energy spectrum of hole states in this material in which radiation can be induced as a result of transitions of holes between these states. Holes were excited by electric field pulses with a strength up to 12 kV/cm at T = 4.2 K under uniaxial compression of samples up to 12 kbar. It has been found that hole emission spectra for transitions between resonant and local states of the impurity have a structure identical to the photoconductivity and absorption spectra. Transitions from resonance states, which are associated with the heavy hole subband, have not been detected. It has been found that in an electric field lower than 100 V/cm, a compressed crystal emits as a result of transitions of heavy holes. In a strong electric field (1–3 kV/cm), emission is observed in the energy range up to 140 meV, and transitions with emission of TA and LO phonons appear in such a field. The emission spectra under pressures of 0 and 12 kbar differ insignificantly. Hence, it follows that the contributions from heavy and light holes in a strong electric field are indistinguishable.     

Effect of an electric field on the dissociation of electron-hole pairs interacting with triplet excitons in amorphous molecular semiconductors

The photoconductivity of amorphous molecular semiconductor films increases with the simultaneous photogeneration of singlet electron-hole pairs (EHPs) and triplet excitons but decreases when singlet EHPs are replaced by triplet EHPs. As the electric field increases, the influence of the triplet excitons on the photoconductivity of the films due to the dissociation of EHPs becomes less. It is concluded that as the electric field increases, the current-carrier mobility increases and the dissociation rate of EHPs becomes comparable to the spinconversion rate of EHPs interacting with triplet excitons. Fiz. Tverd. Tela (St. Petersburg) 39, 1020–1023 (June 1997)     

Effect of excitation intensity and electric field on the photoconductivity relaxation in CdxHg1−x Te/GaAs polycrystalline layers

The photoconductivity relaxation and the stationary photoconductivity in the n-Cd_0.8Hg_0.2Te compensated polycrystalline layers at T=300 K have been investigated as a function of the light intensity and the strength of applied electric field E. It is demonstrated that, at low excitation intensities, the saturation of stationary photoconductivity and a decrease in the relaxation time with an increase in E are caused by the minority carrier extraction. The effect of minority carrier extraction is analyzed with due regard for the internal electric field of potential barriers in intergranular layers. It is assumed that the features of nonequilibrium-carrier recombination, which proceeds through several channels and depends on the excitation intensity and extraction electric field strength, can stem from the polycrystalline structure of the Cd_0.8Hg_0.2Te layers.     

表面复合速度对于锗的光电导光谱分布的影响(二)

The continuity equation of minority carriers in n-type germanium is solved for the stationary case, in which the volume recombination is assumed to be of the recombination-center type and the surface recombination is introduced in the boundary condition. An expression for the photoconductivity is obtained from this solution.By substituting the known absorption data and the volume lifetime of minority carrier in the sample into the expression of photoconductivity and by choosing suitable values of surface recombination velocity, photoconductive spectra are calculated and are compared with the experimental curves. The agreement is satisfactory for wave-lengths greater than 1.2 μ. Below 1.2 μ, the experimental curves fall well below the calculated curves. It is thought that other types of volume recombination may exist.The photoconductive spectra usually have a maximum outside the absorption edge. By measuring the relative height of the maximum, the surface recombination velocity can be obtained. This method is suitable for medium and large values of surface recombination velocity.The photoconductivity drops sharply on both sides of the maximum in case of large values of recombination velocity. In such cases, the energy gap of the semiconductor can be obtained in a simple way. For germanium the energy gap is 0.83 eV for direct transition and 0.635 eV for indirect transition.From this investigation, the energy gap of CdS is reestimated to be 2.53 eV and that of Cu₂O greater than 2.10 eV.     

Characteristics of recombination in semiconductors with intercrystallite barriers

A model of nonuniform potential relief (modulated by adsorption of oxygen and an applied electric field) of a film containing centers of slow recombination distributed over the volume of a crystallite and centers of fast recombination localized on their boundaries is studied. The characteristic features of the current-voltage characteristics (CVC) of the photocurrent and the effect of adsorption of oxygen on the photoconductivity in semiconductors with intercrystallite barriers were studied. The saturation of the CVC is determined by the transformation of the surface trap levels into centers of fast recombination under the action of an electric field. The proposed model permits explaining from a unified viewpoint the sensitization and desensitization of a semiconductor as a result of oxygen chemisorption.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 56–60, March, 1989.     

Luminescence of CdS crystals due to near-surface potential fluctuations

The nature of the wide emission band observed in the low-temperature (T ≤ 60 K) near-edge photoluminescence spectra of CdS crystals placed in water and irradiated by a He-Cd laser has been investigated. The spectral and temporal characteristics of the band and its dependence on temperature, excitation intensity, and transverse electric field are considered. The relationship between the band and the radiative recombination of carriers localized on near-surface potential fluctuations, which are due to the defect-forming character of the joint effect of laser irradiation and water on near-surface layer of semiconductor is grounded.     

Surface studies by modulation spectroscopy

Surface barrier and electronic surface states are important parameters for characterizing semiconductor surfaces. Qualitative information on the surface electric field can be deduced from electroreflectance studies. Energetic positions of surface states have been determined by spectroscopic methods using effect modulation techniques. Besides the field effects of surface conductivity and absorption, which are limited to low densities of surface states, new information on surface states was gained by investigating the spectral dependence of surface photoconductivity. Also surface phonons were detected in “spectral oscillations” of photoconductivity. The measurement of surface photovoltage at photon energies where the bulk is transparent promises a new tool for surface state research in the future. To demonstrate these modulation techniques examples are given for Ge, Si, ZnO, and CdS surfaces.     

Excitons and multi-exciton complexes bound to a two-dimensional hole layer at a silicon surface: the Kondo effect,the Coulomb blockade and a negative photoconductivity

The recombination radiation line of surface excitons and the recombination radiation line of multi-exciton complexes bound to a two-dimensional hole layer are observed in luminescence spectra of [100] silicon metal–oxide–semiconductor structures at low two-dimensional hole density. The circular polarization of these two lines in a transverse magnetic field is defined by the average electron spin. The hole spin contribution to the circular polarization is very small due to Kondo spin correlations of holes in the excitons and complexes and holes in the two-dimensional hole layer. The Coulomb blockade excludes a direct contribution of the complexes to a surface photoconductivity. Moreover, a significant negative photoconductivity of the two-dimensional holes is observed at high excitation levels, presumably as a result of the quantum scattering of the two-dimensional holes by the complexes. A shell model of surface multi-exciton complexes is introduced.     

Exciton photoluminescence, photoconductivity and absorption in GaSe0.9Te0.1 alloy crystals

The photoluminescence, photoconductivity and absorption in GaSe_0.9Te_0.1 alloy crystals have been investigated as a function of temperature and external electric field. It has been observed that the exciton peaks shift to lower energy in GaSe_0.9Te_0.1 alloy crystals compared to GaSe crystals. The long wavelength tails of interband photoluminescence, photoconductivity and absorption spectra are determined by the free exciton states and show an Urbach-Martienssen-type dependence to the photon energy. The maxima of the extrinsic photoluminescence and photoconductivity spectra were found to be determined by the acceptor centers with an energy of E_A=E_V+0.19 eV formed by the polytypism and defects complexes that include Se and Te anions.     

Electro-chemical manifestation of nanoplasmonics in fractal media

Electrodynamics of composite materials with fractal geometry is studied in the framework of fractional calculus. This consideration establishes a link between fractal geometry of the media and fractional integrodifferentiation. The photoconductivity in the vicinity of the electrode-electrolyte fractal interface is studied. The methods of fractional calculus are employed to obtain an analytical expression for the giant local enhancement of the optical electric field inside the fractal composite structure at the condition of the surface plasmon excitation. This approach makes it possible to explain experimental data on photoconductivity in the nano-electrochemistry.     

Effect of drifting carriers on longitudinal electro-kinetic waves in ion-implanted semiconductor plasmas

We present an analytical study of wave spectra of electro-kinetic waves propagating through semiconductor plasma, whose main constituents are drifting electrons, holes and non-drifting negatively charged colloids. By employing the hydrodynamical model of multi-component plasma, a compact dispersion relation for the same is derived. This dispersion relation is used to study slow electro-kinetic wave phenomena and resultant instability numerically. We find some important modifications in the wave spectra of the slow electro-kinetic branch. It is found that the drift velocities of electrons and holes are responsible for converting two aperiodic modes into periodic ones. The applied dc electric field increases the phase velocities of contra-propagating modes. The amplification coefficients of propagating modes can be optimized by tuning the amplitude of applied electric field and wave number. It is hoped that the results of this investigation should be useful in understanding the wave spectra of slow electro-kinetic waves in ion-implanted semiconductor plasma subjected to a dc electric field along the direction of wave propagation.     

The effect of an electrostatic field on the photoluminescence and photoconductivity of polymer films doped by cationic polymethine dyes with end groups with different electron-donating properties

The effect of an external electrostatic field on the photoluminescence spectra and photoconductivity of symmetric and asymmetric cationic polymethine dyes with end groups possessing different electron-donating properties (basicity) in polymer films based on photoconductive poly-N-epoxypropylcarbazole and nonphotoconductive polyvinylethylal is studied. It is found that application of an electric field to films can both decrease and increase their luminescence intensity. The strongest decrease in the luminescence intensity is observed for symmetric dyes in poly-N-epoxypropylcarbazole films with either a decrease in the electron-donating ability of the end groups or with a decrease in the length of the polymethine chain. Colored polyvinylethylal films exhibit both a negative and a positive effect of the field on the photoluminescence intensity. The character of changes in the luminescence spectra does not depend on the electron-donating ability of the end groups. The spectral effects are explained by a change in the probability of vibronic transitions due to the redistribution of the electron density in the chromophore of the dye in the ground and excited states in an external electric field.     

Optical, electrical, and photophysical properties of films of polycomplexes of azobenzene derivatives with cobalt

The absorption spectra, photoconductivity, and electrical conductivity of films of new polycomplexes of 4-methacyloyloxy-(4′-carboxy-3′-oxy)azobenzene and 4-methacryloyloxy-2-nitro-(4′-carboxy-3′-oxy)azobenzene with cobalt were investigated with switched-off and switched-on external electric fields. The effect of an external electric field on the transmission of linearly polarized light through films near the long-wavelength absorption edge is revealed. It is shown that the change in the light transmission under the action of an external electric field is due to the spatial reorientation of the dipole moments of azobenzene groups photoinduced by polarized light in an external electric field. The effect of an electric field on the light transmission is explained by the formation of forces acting on the dipole moments of azobenzene groups and on metal ions. The increase in the dipole moments of azobenzene groups as a result of introduction of nitro groups into their composition decreases the effect of cobalt ions on the electro-optical properties of polycomplex films.     

Experimentally observed orientation of C60F18 molecules on the nickel single crystal (100) surface

The angular dependence of near edge X-ray absorption fine structure (NEXAFS) spectra has been obtained in the vicinity of carbon and fluorine 1s absorption edges in a monolayer film of polar fullerene fluoride (C₆₀F₁₈) molecules on a Ni(100) substrate. The fine structure of the spectra has been identified according to experimental data via calculations based on the density functional theory, and the angular dependence of the spectra has been explained. The orientations of structural molecular fragments are determined from the angular dependence of the NEXAFS spectra. It is demonstrated that the electric dipole moment of a C₆₀F₁₈ molecule is oriented along the normal to the substrate surface with an accuracy of 5°.     

Current state of photoconductive semiconductor switch engineering

This paper presents the current state of a photoconductive semiconductor switch engineering. A photoconductive semiconductor switch is an electric switch with its principle of operation based on the phenomenon of photoconductivity. The wide application range, in both low and high-power devices or instruments, makes it necessary to take design requirements into account. This paper presents selected problems in the scope of designing photoconductive switches, taking into account, i.e. issues associated with the element trigger speed, uniform distribution of current density, thermal resistance, operational lifespan, and a high, local electric field generated at the location of electrodes. A review of semiconductor materials used to construct devices of this type was also presented.