The contribution of surface effects to the surface photovoltage dependence on temperature for the real Si(111) surface

The dependences of the surface photovoltage and photoconductivity on temperature (in the range of 300–430 K) for the real p- and n-type Si(111) surface have been measured. The results of the experiments have shown the strong influence of the surface treatment (etching in HF solution or baking in vacuum) on the temperature dependences of the surface photovoltage and photoconductivity. Good qualitative agreement has been obtained between experimental and theoretical plots of the surface photovoltage as a function of temperature calculated on the basis of the Lile theory.     

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.     

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.     

Optical spectroscopy of electronic surface states

In this paper a number of optical spectroscopic methods for investigating surface electronic structure are discussed, including reflectance techniques, ellipsometry, surface photoconductivity and surface photovoltage spectroscopy. In addition to electron scattering techniques and UV-photoemission, optical spectroscopic methods have contributed much in recent times to the understanding of electronic surface states on solids. A discussion and comparison is given of the nature and significance of information obtained by these methods and exemplary experimental results are presented to illustrate the contribution of the optical techniques to the present knowledge about surface states. The relation between information obtained from optical measurements and electron spectroscopy is considered.     

The influence of atomic steps on the photoelectric properties of clean cleaved silicon as derived from the surface photovoltage

The correlation of structural and electrical properties of clean silicon surfaces cleaved in UHV was investigated quantitatively by the surface photovoltage, using light with an energy larger than the band gap of silicon. The surface photovoltage, which is a function of band bending and recombination probability, depends strongly on the appearance of atomic steps. The additional surface states vary with density and crystallographic orientation of the steps as well as with adsorption of oxygen. The experimental facts can be explained by accepting a shift of the Fermi level at the surface towards the valence band due to edge atoms. By measuring the change of sign of the surface photovoltage of crystals with various dopings an exponential temperature dependence of the ratio of the recombination probabilities r_v/r_c for transitions from and into the Surface states has been derived.     

Characterization of photovoltage evolution of ZnO films using a scanning Kelvin probe system

Work function (WF) and surface photovoltage evolution of films can be measured using the Kelvin probe technique, and further analysis of the photoelectronic behavior can provide information on the energy level structure. In this paper, a theoretical analysis to measure surface photovoltage using Kelvin probe technique is presented. Based on this analysis, the surface photovoltage and its time-resolved evolution process as well as the energy level structure of ZnO films are determined using a scanning Kelvin probe. The present study therefore provides a simple and practical methodology for the characterization of the electronic behavior of films.     

Experimental studies on the spectral sensitization of photoconductivity on zinc-oxide crystals

The photoconductivity at the prism-surfaces(1$\text{1}$00) of ZnO-crystals was studied before and after adsorption of a merocyanine-dye under well defined experimental conditions in ultrahigh vacuum.The undyed crystal exhibited a pronounced photoconductivity only for fundamental absorption (UV-light, band-band excitation). The dyed crystal showed a very enhanced photoconductivity in the visible surmounting even the values measured for band-band light. The two-component system merocyanine dye/ZnO-surface is thus well suited for studies of spectral sensitization.The influence of different parameters was studied, e.g. pretreatment of the surface with oxygen or hydrogen (variation of surface conductivity) and the effect of temperature between 90 and 298 K. The conductivity of the crystals in the dark may be lowered or increased during dye-deposition. The spectrally sensitized photoconductivity was found negative or positive. No decrease was found for samples with low surface conductivity before dyeing.An activation energy of 34 meV was deduced from the temperature dependence of the initial slope of the sensitized photoconductivity. The sign of the photosignal may change between 90 and 298 K. Pretreatment or treatment of the dyed surface with oxygen or atomic hydrogen did not influence the spectral distribution of photoconductivity in the sensitization region.     

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.     

On the correlation of geometrical structure and electronic properties at clean semiconductor surfaces

In their LEED patterns most clean semiconductor surfaces show extra spots in addition to the normal beams as expected for a continuation of the bulk lattice up to the topmost layer. This means, that these surfaces are reconstructed. The (111) surfaces of silicon and germanium crystals exhibit another interesting feature: the cleaved surfaces show a metastable superstructure which irreversibly converts to a new modification upon heat treatments above about $\text{1}\text{3}$ of the melting temperatures. The structural changes are correlated with changes in the electronic properties of the surfaces. This has been shown by investigations of surface conductivity, surface photoconductivity, surface photovoltage, internal reflection, electron energy-loss spectroscopy, photoemission, and work function. Recent theoretical studies have also revealed a distinct correlation between geometrical arrangements of the surface atoms and the corresponding band structures of the surface states. However, the atomic positions of the surface atoms used in these calculations are only a model since a detailed analysis of the LEED-intensity-versus-energy curves have not been carried out up to now.     

Self-magnetism and Persistent Photoconductivity

Persistent photoconductivity has been investigated by various models, among which the Macroscopic Barrier model, Large-Lattice-Relaxation model, and Random Local Potential Fluctuations model are mostly well known. Although the three well-known models have played important roles in describing the persistent photoconductivity, they are not the principal cause of persistent photoconductivity. In this paper a classical model originated from “self-magnetism of electron gas” is proposed to illustrate the persistent photoconductivity phenomenon. This classical model is based on electron gas pulsation, which depends on the charge density. Different concentrations of current carriers create different frequencies in the system, and thus the system is sensitive to different wave lengths of incident light. Then the construction of different detectors can be possible for different wave lengths of incident light.     

Analysis of the surface photovoltage under sub-bandgap illumination: Effect of the bulk traps

The simple theory of the surface photovoltage induced by photostimulated electron transitions from the bulk impurity levels is given. The relevant calculations show that the surface barrier height, the concentration of the impurity levels and the trapping process at the surface effectively control this phenomenon. Under the proper conditions, a very low excitation level in the bulk can generate significant surface photovoltage signals.     

Highly reduced iron-doped lithium niobate for optoelectronic tweezers

We investigate the applicability of highly reduced lithium niobate samples doped with iron for the use as optoelectronic tweezers. Increasing the reduction degree of Fe-doped lithium niobate is well known to increase the photoconductivity and reduce the writing time of internal space-charge fields. Based on our measurements of the photorefractive properties, we determine the optimal conditions for dielectrophoretic trapping and present the application of Fe-doped lithium niobate as optoelectronic tweezers. For higher reduction degrees, an unexpected decrease in the photovoltaic current density and the saturation space-charge field is reported.     

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.     

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.     

Photovoltage studies of n-type InP (100)

The transient photovoltage signals of n-type InP(100) have been studied by the retarding potential electron beam technique and are a sensitive function of several experimental parameters. The photovoltage decreases as the duration of the light exposure, and/or the intensity of the light is decreased. The photovoltage decreases sharply at energies less than the bandgap energy, which is temperature dependent, and also shows considerable structure at energies above the bandgap energy. The photovoltage is also sensitive to changes in the surface composition. Both the photovoltage and the work function decrease sharply after Ar bombardment.     

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.     

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.     

Microwave-induced magnetic field state with zero conductivity in GaAs/AlAs Corbino disks and hall bars

The microwave photoconductivity of the 2D electron gas in GaAs/AlAs heterostructures has been investigated at a temperature of 4.2 K in magnetic fields up to 1.5 T. It has been found that the magnetic field state with zero conductivity appears in GaAs/AlAs Corbino disks irradiated by 130.70-GHz microwave radiation. This state was previously observed only in GaAs/AlGaAs Corbino disks with much higher electron mobility and lower density. It has been shown that the microwave photoconductivity measured in high magnetic fields on Corbino disks can significantly differ from the value calculated from the results of the measurements on Hall bars. This difference is explained by the fact that the conditions of the appearing magnetoplasmons that affect the magnitude and character of the microwave photoconductivity (photoresistance) in the Corbino disks are nonequivalent to those in the Hall bars.     

InAs/GaSb量子阱中太赫兹光电导特性

太赫兹技术由于具有重大的科学价值及应用前景而引起了广泛关注,其核心问题是性能优异的室温太赫兹辐射源和探测器研究.本文用半经典的玻尔兹曼方程方法研究了In As/Ga Sb量子阱系统中载流子对电磁场的响应,运用平衡方程方法求解玻尔兹曼方程得到了量子阱系统中的光电导,系统地研究了量子阱结构对光电导的影响,揭示了在该量子阱系统中光电导产生的物理机制.研究发现,量子阱结构主要通过调节载流子的能级、浓度和波函数的耦合影响光电导,对称性较好的量子阱结构(8 nm-8 nm)的光电导信号更强,其峰值落在太赫兹区(0.2 THz),并且在低温下器件的性能较好,温度升高则吸收峰略有降低,且光电导峰值发生红移.研究结果表明该量子阱系统可以用作室温太赫兹光电器件.     

ZnO/Si异质结的光电转换特性研究

利用直流反应溅射方法在p型Si衬底上生长掺Al的n型ZnO薄膜,测量了由n型ZnO薄膜和p型Si衬底组成的异质结在黑暗和光照条件下的I-V特性,结果表明该异质结具有优良的整流特性,而且在光照条件下的反向电流迅速增大并很快趋于饱和.通过测量ZnO薄膜的光电流和异质结的光电压的光谱响应,初步分析了异质结的光电转换机理.测量结果显示,在入射光波长为380nm时光电流强度明显下降,反映出光电流与ZnO薄膜禁带宽度的密切关系;同时还发现,在与ZnO禁带宽度相对应的波长前后所产生的光生电压方向相反.推测这一现象与异质结的能带结构密切相关. 关键词： ZnO薄膜 异质结 光电转换 光谱响应