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.     

GaAs,InP, and CdSe photovoltage transients

The response of the photovoltage of GaAs, InP, and CdSe to changes in the incident light intensity are examined with the retarding potential electron beam technique. It is demonstrated that the dynamics of the photovoltage transients are sensitive to the surface conditions. This is discussed in terms of a theoretical model which involved the capture of both electrons and holes at the semiconductor surface.     

Non-equilibrium surface state properties at clean cleaved silicon surface as measured by surface photovoltage

Surface photovoltage transients were measured at clean cleaved silicon (111) faces in ultrahigh vacuum. The temperature and doping of the samples, the intensity of the stimulating light pulses (energy less than band gap), and the surface coverage (clean and adsorbed water vapor) were varied systematically. The results yield information on the charge transfer at the surface and on surface recombination. The calculation of the surface photovoltage (using only the generation rates into and out of the surface states and data of thermal equilibrium) shows, that only one bulk band (conduction band for n-doped samples and valence band for p-doped samples) controls completely signal height and its relaxation via charge transfer to the surface states. The determined surface state parameters are: relaxation time constants, capture cross-sections for photons and transition probabilities. On the basis of the model all decay curves can be reproduced quantitatively.     

Surface state properties of clean cleaved silicon as derived from the temperature dependence of the surface photovoltage

The temperature dependence of the surface photovoltage of clean silicon surfaces cleaved in UHV was investigated with a special emphasize on that temperature where surface photovoltage changes its sign. The surface photovoltage was measured by the electron beam method. Chopped light with an energy larger than the bandgap of silicon was used. The ratio of the transition probabilities $\text{r}{}_{\mathrm{V}}\text{r}{}_{\mathrm{C}}$ for transitions from and into the surface states has been derived from the surface photovoltage and the value for the band bending at the clean silicon surface. It is concluded from model calculations that at temperatures below the sign change of the surface photovoltage the signal height of the surface photovoltage is essentially determined by the surface recombination whereas the contribution due to band bending is negligible.     

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.     

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 properties by means of large signal photovoltage pulses and the influence of trapping

Large signal photovoltage pulses, measured on real surfaces of high-resistivity p-type silicon as a function of excitation intensity and induced charge, show characteristic features, especially smooth minima in the negative pulses. It is shown how the results can be used for a determination of surface potentials (band bendings) and surface- or interface-state distributions. Comparisons between theoretically and experimentally determined photovoltages show that the exchange of charge carriers between surface states and space charge layer during electron-hole nonequilibrium was not negligible and has to be taken into account for an accurate determination of surface potentials. The influence of such trapping processes is analysed graphically and analytically for continuously distributed surface states and the modifications due to trapping are determined. It is shown that the bands become asymptotically flat for the limit of large excitation even when strong trapping in a system of continuously distributed surface states with arbitrarily large concentrations prevails.     

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.     

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.     

Influence of trap filling and junction capacitance charging on photovoltage transients in HgCdTe-based infrared photodiode

Open circuit transient photovoltage (TPV) decay measurements have been carried out on HgCdTe-based infrared photodiode at different bias-light power illumination. By employing a picosecond pulsed laser excitation with wavelength of \(4.5\,\upmu \hbox {m}\) on steady background illumination, the TPV decay varying behavior has been observed. The effect of junction capacitance and carrier traps on TPV decay can be decreased to the minimum saturation values by increasing the bias illumination level. The study indicates the TPV decay time constants are dominated by the discharging of junction capacitance, trap emission and photocarrier recombination. The minority carrier lifetime are affected by the carrier injection level.     

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.     

GaAs surface oxidation-related photoluminescence transients

Evidence is presented for the involvement of surface oxidation phenomena in the photoluminescence response obtained on GaAs(110) surfaces. By using high intensity HeNe laser illumination on freshly cleaved (110) n-type GaAs surfaces, room temperature photoluminescence transients are observed having magnitudes determined by excitation intensity. The observed transients are in excess of the steady state photoluminescence signal on surfaces exposed to normal ambient air conditions. A model based on photodesorption and photoadsorption processes, is proposed to explain this phenomenon.     

Quantum state measurement using coherent transients

We present the principle and experimental demonstration of time resolved quantum state holography. The quantum state of an excited state interacting with an ultrashort chirped laser pulse is measured during this interaction. This has been obtained by manipulating coherent transients created by the interaction of femtosecond shaped pulses and rubidium atoms.     

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.     

Surface photovoltage spectroscopy and surface piezoelectric effect in GaAs

“Real” (111) surfaces of n-type GaAs were investigated employing surface photovoltage spectroscopy and the surface piezoelectric effect. Surface states at the energy position E_c ? E_t ? 0.72 eV were found on both the Ga and the As surfaces. Both types of surfaces exhibited a barrier of about 0.55 V. No variations in the surface barrier or the energy position of the surface states were observed in various ambients at atmospheric pressure (dry air, wet air, ammonia and ozone). However, the capture cross-section of the surface states for electrons, as determined from the surface piezoelectric effect transients (of the order of 10^?13 cm²), was found to be sensitive to the ambient. It decreased in wet air and increased in ozone. This effect was more pronounced on the As than on the Ga surfaces. Additional surface states were found to be present in the energy region of 0.9 to 1.0 eV, below the bottom of the conduction band. However, their exact energy positions could not be determined due to interference caused by the carrier trapping of the surface states at E_c ? E_t ? 0.72 eV.     

Determination of the surface parameters for aspheric aphakic lenses

Two methods are proposed for aspherical surface evaluation — scanning and interferometry. Each of these methods enables the determination of the radius of curvature at the lens vertex, the asphericity, astigmatism, rotation asymmetry, surface topography and the deviation of the surface from following the theoretically stated shape. While the scanning technique is low cost and simple, the interferometric technique (which needs optical arrangements) gives more information about the surface quality. These methods, combined with a computer, establish powerful instruments for controlling lens production.     

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.     

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.