EL2 deep level in sub-bandgap surface photovoltage spectra in GaAs bulk crystals

Experimentally observed surface-photovoltage-method (SPV) spectra in the subbandgap energy range are presented for a real (100)GaAs surface, treated with preepitaxial procedures. Conductive, n-type GaAs and semi-insulating GaAs are studied. It is shown that SPV spectra are formed as a result of the simultaneous action of both surface states and deep bulk levels. The spectral shape of the surface-state photoionization cross-section is qualitatively determined. The influence of the deep bulk levels on the SPV spectra is explained, and the photoionization cross-section for both Cr and EL2 levels is qualitatively determined.     

Some photoeffects on the semiconductor surface under sub-bandgap illumination

Surface photovoltage as well as surface photoconductivity response on the light intensity under sub-bandgap illumination is treated. The influence of the surface barrier height, the density of surface traps, the bulk electron density as well the temperature on the surface photovoltage (surface photoconductivity) is analysed. The detectability conditions are included. The consequences of detectability conditions are discussed.     

Photoelectricity of β-Agl under sub-bandgap illumination

An experimental study of the spectral, intensity and temperature dependences of the photoconductivity and contact photovoltage of β-AgI single crystals under sub-bandgap illumination is carried out. Both dc and ac photoconductivity experiments are performed and dark-to-light relaxation characteristics of the contact photovoltage are investigated. It is found that the processes under consideration are of a unified character in the spectral region below 2.20 eV, in which the photoconductivity spectrum follows Urbach's rule, temperature behaviour is opposite to that of dark conductivity and dependence on exciting light intensity is linear. A simple model is proposed to explain qualitatively the results obtained, based on the assumption for photoexcited ionic-type conductivity.     

Surface photovoltage in semiconductors under sub-band-gap illumination: continuous distribution of surface states

Surface photovoltage spectra in semiconductors are analyzed when the sub-band-gap illumination induces the electron transitions from surface states to the conduction band under the assumption that distribution of surface states is continuous. From analysis performed it follows that the fictitious densities of surface states can be induced due to the wavelength dependence of the photoionization capture cross-section of surface states for electrons and by the electron recombination capture cross-section of surface states which depends on the energy position of surface states in the energy gap. The high illumination intensity (laser illumination), which makes completely empty the surface states, can eliminate the fictious surface states when the density of surface states is not very large, the temperature of measurements is low, and the surface potential barrier is high. Received: 24 June 1998 / Accepted: 29 March 1999 / Published online: 14 June 1999     

Influence of sub-bandgap illumination on space charge distribution in CdZnTe detector

The space charge accumulation in CdZnTe crystals seriously affects the performance of high-flux pulse detectors.The influence of sub-bandgap illumination on the space charge distribution and device performance in CdZnTe crystals were studied theoretically by Silvaco TCAD software simulation.The sub-bandgap illumination with a wavelength of 890 nm and intensity of 8×10?8 W/cm2 were used in the simulation to explore the space charge distribution and internal electric field distribution in CdZnTe crystals.The simulation results show that the deep level occupation faction is manipulated by the sub-bandgap illumination,thus space charge concentration can be reduced under the bias voltage of 500 V.A flat electric field distribution is obtained,which significantly improves the charge collection efficiency of the CdZnTe detector.Meanwhile,premised on the high resistivity of CdZnTe crystal,the space charge concentration in the crystal can be further reduced with the wavelength of 850 nm and intensity of 1×10?7 W/cm2 illumination.The electric field distribution is flatter and the carrier collection efficiency of the device can be improved more effectively.     

Nonstationary photovoltage induced in tin disulfide crystals under strong surface excitation

The specific features of nonstationary photovoltage excitation in molecular crystals of tin disulfide are investigated. A substantial change in the dependence of the photocurrent amplitude on the spatial frequency of the interference pattern is revealed. The observed behavior is explained in terms of the model of a photoconductor with different-type charge carriers of the same sign.     

Determination of surface state parameters from surface photovoltage transients: CdS

A method was developed for determining surface state parameters such as density, fractional occupancy and capture cross section for electrons and for photons from surface photovoltage transients. These transients were found to be associated with the photostimulated transition of electrons from surface states to the conduction band. Suitable analytical expressions were derived from basic semiconductor surface equations. The application of the method to CdS surfaces is presented.     

Phonons in surface photovoltage of GaAs

Oscillatory surface photovoltage is reported in GaAs at 4.2°K, characterized by two series of minima. Dominating series is attributed to the capture of photoexcited electrons by surface states with emission of phonons. Second, weak series coincides with oscillations in photoconductivity.     

Determination of surface state parameters from surface photovoltage transients

Photo-stimulated electron transitions from the surface states into the conduction band which are involved in surface photovoltage spectroscopy are analyzed in terms of phenomenological surface state parameters. The surface state parameters are determined frolm photovoltage transients on the basis of relationships derived for a general case where the mechanism of electron transients is not specified, for the case where the surface states are in equilibrium with the bulk and the case where the surface states are not in equilibrium with tpe bulk. The procedure is illustrated utilizing experimental data obtained on CdS surfaces.     

Slow variations of the surface potential of CdS layers under illumination

Observations have been made of slow changes in the surface potential of CdS layers as a result of illumination, the kinetics and certain other regularities of this phenomenon have been investigated, and a phenomenological theory for the phenomen is presented. It is shown that the disruption of the adsorption equilibrium upon illumination does not play an important role in the mechanism of the surface photoemf, which is determined mainly by changes in the charge on fixed surface traps.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 79–84, December, 1970.     

Surface photovoltage due to photo-thermo-ionization of surface states: GaAs

Surface photovoltage spectroscopy was employed for studying the mechanism of subuand gap photoionization transitions from surface states in GaAs surfaces. It was found that the photoionization cross-section exhibits a maximum for a photon energy of about 0.9 eV. This finding indicates a photo-thermal mechanism of photovoltage, i.e., photo-induced transitions between surface state levels and the subsequent thermal ejection of electrons from the upper level into the conduction band.     

Polarization-enhanced bulk photovoltaic effect of BiFeO3 epitaxial film under standard solar illumination

In this Letter, we report the polarization-enhanced bulk photovoltaic effect (BPV) in pristine BiFeO₃ (BFO) epitaxial film under standard 1 sun AM 1.5 G illumination. High-quality epitaxial BFO films are grown on (001)-oriented niobium doped-SrTiO₃ substrates by pulsed laser deposition. The best BFO film based photovoltaic device achieves a power conversion efficiency of 0.0062% under standard illumination. Besides, it is found that the number of bipolar pulses plays a key role in improving the short-circuit current density and open-circuit voltage. These results are beneficial for further understanding of physical origin of the photovoltaic properties in ferroelectric oxides.     

The use of surface photovoltage measurements for the surface characterisation of photoconductors

Measurements of the surface photovoltage as a function of wavelength of the exciting radiation show a number of features which change with variations in the surface conditions. In the case of photoconducting cadmium sulphide, it is shown that the changing spectral features, rather than being directly associated with surface states, are due to bulk transitions which give rise to changes in the bulk carrier concentrations and (indirectly) to changes in the surface potential as a result of redistribution of charge between bulk and surface states.     

AC surface photovoltage of indium phosphide nanowire networks

Surface photovoltage is used to study the dynamics of photogenerated carriers which are transported through a highly interconnected three-dimensional network of indium phosphide nanowires. Through the nanowire network charge transport is possible over distances far in excess of the nanowire lengths. Surface photovoltage was measured within a region 10.5?C14.5?mm from the focus of the illumination, which was chopped at a range of frequencies from 15 Hz to 30 kHz. Carrier dynamics were modeled by approximating the nanowire network as a thin film, then fitted to experiment suggesting diffusion of electrons and holes at approximately 75% of the bulk value in InP but with significantly reduced built-in fields, presumably due to screening by nanowire surfaces.     

表面光电压谱及其检测技术实验

利用锁相放大器、单色仪、Xe灯光源等搭建了表面光电压谱检测仪.基于表面光电压谱技术的原理,并借助场诱导表面光电压谱测定了半导体材料的光伏响应、导电类型、能带隙和表面电荷分布.     

Long-term changes in the surface potential of CdGeP2 single crystals under illumination

A study is made of the variation in the surface potential V_c of p-type CdGeP₂ single crystals during and after illumination. The sign of V_c corresponds to the increase of the work function of the semiconductor surface under illumination. Together with a sharp variation of V_c, a long-term component is observed whose value increases with the increase of the light intensity and the time of exposure of the specimens to light. Slow changes are explained by the presence on the semiconductor surface of an amorphous film, which impedes the electron exchange of the semiconductor with surface states. A model is proposed and a calculation carried out for the long-term kinetics in the small-signal approximation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 51–54, March, 1976.     

Surface photovoltage and internal photoemission at the anodized InSb surface

Surface photovoltage measurements have been made as a function of wavelength and temperature on a number of variously doped samples of n- and p-type InSb in the carrier concentration range of 8.9 × 10¹³ to 1.0 × 10¹⁸ cm^?3. The measurements were made using an MIS sandwich employing for the dielectric an anodically formed layer of In₂ O₃. Differential capacitance measurements have shown that, when cooled in the dark, the surface of the n-type material is near flat band whereas that of the p-type is depleted. Illumination with photons of energy in excess of ～ 1.5 eV leads to a shift of the surface potential to larger negative values presumably as a result of optical activation of electrons from fast interfacial surface states to slow states near the InSb surface. Internal photoemission measurements lend support to this model and suggest that, in the absence of any applied bias an internal field within the oxide causes the electrons excited from the semiconductor to move towards the metal. A theory for the surface photovoltage in the presence of a continuum of surface states is developed. It is concluded from theory, and supported by experiment, that surface trapping as well as recombination can exert a considerable influence on the photovoltaic response.     

Surface photovoltage,band-bending and surface states on a-Si : H

We have measured the surface photovoltage (SPV) of intrinsic (i.e., undoped) and phosphorus-doped amorphous Si : H between ?168 and 25°C in the spectral range from 0.5 to 2.5 eV. The a-Si : H was grown in a silane glow discharge. Vibrating Kelvin probe techniques were used for the SPV measurements; Auger spectroscopy was used for monitoring surface cleanliness and chemistry. At all temperatures and for both materials, (1) the SPV was invariably negative, (2) there was no correlation between the spectral, thermal and response-time properties of the SPV and the bulk photoconductivity, and (3) surface treatments such as sputtering and oxygen physisorption strongly affected the SPV but not the photoconductivity. These facts indicated that the SPV was due to the emptying of surface-states via surface transitions, and corresponded to the flattening of bands which, when unilluminated, were bent upwards. Intrinsic material showed a maximum SPV of about 0.2 V. The SPV was characterized at ?168°C by strong electronic isolation between surface-states and valence band (i.e., once light was removed, there was no surface-state refilling or decay of the SPV), slow rise times (～min), saturation at photon fluxes of about 10¹¹/cm² · s, and a SPV spectral threshold occurring at 0.7 eV. At 25°C, all SPV responses were much faster (<0.5 s) and the optical threshold was 0.9 eV. The thermal activation energies associated with the SPV were 0.11 eV for surface-state emptying and 0.22 eV for surface-state refilling. For P-doped material the maximum SPV at ?168°C was 0.3 V and its properties indicated less electronic isolation between surface-states and valence band. There was no SPV at room temperature. Our results are discussed in terms of an energy level scheme which contains a distribution of filled surface states isolated from both conduction and valence bands. The surface-state density is estimated to be about (1?2) × 10¹¹/ cm², a relatively low value which is consistent with the observed lack of Fermi level pinning. In both materials there is a very fast component of the SPV which suggests the presence of additional surface states below the valence band edge.     

Determination of the surface barrier height by combined measurements of surface photovoltage and field effect

A new technique is described for determining the surface barrier height in large-gap semi-conductors.