Surface Photovoltaic Effect Research Articles

Effect of photoenhanced minority carriers in metal-oxide-semiconductor capacitor studied by scanning capacitance microscopy

A scanning capacitance microscope was used to study the photoenhanced minority-carrier contribution to the capacitance of the metal-oxide-semiconductor (MOS) capacitor at high frequencies. When a light is induced over the semiconductor surface, electron-hole pairs are generated and recombined. This steady-state generation-recombination process yields the temporary source of minority carriers, and the inversion layer underneath the oxide layer can respond to very fast-varying ac bias. We measured the differential capacitance (dC/dV) of the MOS capacitor under various light intensities, and observed a peak at the inversion region where the amplitude increased as the irradiation intensity increased. By integrating dC/dV with respect to V, we obtained C–V curves in which the capacitance of the depletion region recovered its value up to that of the accumulation region as the light intensity increased. We also observed that the C–V curves shifted in one direction under irradiation which we believe is due to the surface photovoltaic effect.

Surface States and Photovoltaic Effects in CdSe Quantum Dot Films

Photovoltaic effects in CdSe quantum dot (QD) films have been studied using surface photovoltage spectroscopy and complementary methods. The results show that, contrary to previous studies, nonnegligible electric fields can exist in QD films. As a result, driftlike currents must be considered, in addition to the well‐known diffusionlike currents. However, it is found that the specific case of photovoltage sign reversal, observed after etching highly quantized CdSe QD films, is governed by diffusionlike transport. The latter is highly influenced by preferential trapping of one type of charge carrier. The preferential trapping is shown to be surface localized and is strongly ambient dependent. It is shown that the photovoltaic properties of these CdSe QD films are dominated by their surface state distribution.

Surface band-bending assessment by photocurrent techniques. Application to III - V semiconductors

In epitaxial layers with a large area to thickness ratio, surface and interface space charge regions behave as transverse resistance - capacitance systems modulating the effective volume that takes part in the electrical conductivity. In this work it is shown that in photoconductivity spectroscopy experiments these surface and interface photovoltaic effects can become dominant, therefore giving information about the sample surface and interface band-bending. In photoconductivity spectroscopy, procedures to identify the signatures of such band bending are described. The present technique to assess surface bending is first applied to gated and ungated GaAs samples, to validate the present model. N-type AlGaAs and undoped InGaAs strained buffer layers, with various degrees of strain and surface roughness, are also characterized by this technique.

Reverse surface photovoltaic effects in GaAs surface quantum wells

We observed that GaAs surface quantum wells on undoped AlGaAs substrates experienced a reverse surface photovoltage (RSPV) that caused the bands to bend upward when illuminated by femtosecond laser pulses for time-resolved two-photon photoemission spectroscopy (TRPES). The RSPV effect is created by trapping of photoexcited electrons in the quantum well. Secondary illumination creates a surface photovoltage opposite in magnitude that flattens the bands. The secondary illumination does not affect the width of the energy spectrum or the ultrafast dynamics with respect to the conduction band minimum. A simple model based on the recombination current from the AlGaAs fits the data excellently.

Optical gain in photoconductors made of compensated and partially compensated GaAs

The dc gain behaviour of GaAs photoconductors realized utilizing a partially compensated buffer layer of an epitaxial MESFET structure as well as a Cr-doped semiinsulating substrate is studied. The light-power dependence of the gain hints to the dominant role of the bimolecular recombination process and trapmediated gain, and only a minor role of the surface photovoltaic effect. The possible correlation between dark current and gain mechanism in the MESFET-like device is pointed out.

Gain Studies on Photoconductors Made on Partly Compensated GaAs

The gain behavior of GaAs photoconductors realized on the partly compensated channel of a MESFET is studied. The gain versus light power dependence hints at the domination of the bimolecular recombination and the trap-mediated gain, and only a minor role of the surface photovoltaic effect. The possible correlation between dark current and gain mechanism is pointed out. PACS numbers: 73.50.Pz, 85.60.Gz

Differential photoreflectance of Si-δ-doped GaAs

The signals from buried layer in Si-δ-doped GaAs of different undoped cap thickness have been studied by differential photoreflectance. The first-derivative-like line shape of differential photoreflectance is attributed to the energy transitions related to the modulations of two-dimensional electron gas density. We have observed a change of line shape at low temperatures. This change of line shape is probably due to the change of potential distribution in the conduction band, which is caused by the surface Fermi-level pinning and surface photovoltaic effect.

Space-charge layer, metallization, and collective excitations of the Bi/GaAs(110) interface.

The collective excitations and the metallization of the Bi/GaAs(110) interface grown at room temperature, up to the completion of a few tens of a monolayer, have been studied by means of the high-resolution electron-energy-loss spectroscopy (HREELS). Through analysis of the HREELS data, also by means of an appropriate semiclassical dielectric model, the modifications experienced by the substrate-related loss structures (Fuchs-Kliewer phonon and dopant-induced free-carrier plasmon) and by the quasielastic peak are related to changes in the dielectric response of the overlayer and in the semiconductor space-charge region. The influence of bismuth is effective in enlarging the depletion layer thickness, leaving the interface semiconducting at the monolayer-coverage scale. A band bending value of 0.56 eV is obtained at the coverage of one monolayer on the highly n-type doped sample (n\ensuremath{\sim}2.7\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$); a determination free from any possible surface photovoltaic effect. At coverages greater than two monolayers, which corresponds to a structural transition, the intermediate structural phase becomes metallic, thus marking a clear semiconductor-metal transition. This intermediate metallic stage further develops towards the formation of actual semimetallic crystalline bismuth layers oriented with the basal plane parallel to the substrate surface.

Photocatalytic reduction of pollutant Hg(II) on doped WO 3 dispersion

The photocatalytic reduction of pollutant Hg(II) on photocatalyst ZnO—WO 3 or NiPc—WO 3 under visible light irradiation is investigated. The adsorbance of Hg(II) on the photocatalyst surface is determined. The Hg(II) (100 μg ml −1) contained in an aqueous solution can either be almost totally eliminated by using ZnO—WO 3 as catalyst or be 71.7% photoreduced using NiPc—WO 3 as catalyst on irradiation for 1 h. The gram molecular ratio 0.5ZnO—WO 3 and 5wt.%NiPc—WO 3 are chosen to be the optimal doping ratios. The photocatalytic activities remain constant at pH values in the range 3–4.5 and markedly decrease at pH<3 in accordance with thermodynamic criteria. Photocatalytic Hg(II) reduction is proved to be a first-order reaction. The solar photocatalytic reduction rate of pollutant Hg(II) is markedly improved. 99.1% of the Hg(II) can be photoreduced from a solution of 100 μg ml −1 concentration under sunlight irradiation for 110 min; from this, better prospects for the application of solar photocatalysis to environmental protection is developed. The photocatalyst is characterized by the surface photovoltaic effect, UV—visible absorption spectrum, X-ray diffraction and laser plasma time of the flight mass spectrum. All the experimental results are explained theoretically. A reaction mechanism has been proposed in detail.

Franz–Keldysh oscillations of δ-doped GaAs

Si-δ-doped GaAs (N2D ≊ 1011 cm−2) samples grown by molecular-beam epitaxy are investigated by using photoreflectance spectroscopy. The oscillations observed above the GaAs fundamental band gap are attributed to the Franz–Keldysh effect in the region between the δ-doped layer and the crystal surface. This ascription is confirmed by detailed studies through varying the cap thickness (250–2500 Å), temperature (10–450 K), and laser pump power (0.05–7 mW/cm2). The surface potential deduced from the Franz–Keldysh oscillations is found to be temperature and laser pump power dependent, which is explained by taking the surface photovoltaic effect into account. The surface Fermi level has been measured by this method and is found to have the value 0.73±0.02 V.

Systematic study of the surface photovoltaic effect in photoemission.

The effects of a surface photovoltage on band bending determined by photoemission were investigated experimentally and theoretically for Cs on p-type GaAs(110). Calculations based on carrier recombination by thermionic emission as well as tunneling are in excellent agreement with experimental data. Surface photovoltages are found to contribute noticeably only in the case of low dopant concentrations n and low temperatures (n${10}^{18}$/${\mathrm{cm}}^{3}$ and T280 K for a Schottky-barrier height of 0.7 eV). General criteria for the influence of surface photovoltages in photoemission studies of Schottky contacts are established.

Photovoltaic effects in photoemission studies of Schottky barrier formation

It is shown here that much of the recent photoelectron spectroscopy literature describing the onset of pinning in adsorbate–semiconductor systems at low temperature must be reinterpreted in light of surface photovoltaic effects. Two sources of surface charging are discussed, both of which are strongly enhanced at low temperature. The surface photovoltage resulting from separation of electron–hole pairs by the electric field in the depletion region is usually the dominant source of surface potential shifts, and causes flattening of the semiconductor bands. In addition, surface charging due to photoemission into the vacuum may reverse bias a p-type diode at low temperatures, causing increased band bending.

A study of light-induced charge transfer at interface of copper tetrasulphonatophthalocyanine molecular films and p-Si(111)

In this work, copper tetrasulphonatophthalocyanine (CuTsPc) supramolecular films and CuTsPc Langmuir-Blodgett (LB) films with space in the lipid chain were prepared on p-Si(111) using the LB technique. The surface photovoltage spectra of both film systems were measured for the first time. It was found that the surface photovoltaic effect of the film systems was at maximum when only one monolayer of CuTsPc molecules was modified on p-Si(111). Our experiments indicated that only the first CuTsPc monolayer adjacent to the semiconductor plays a key role in the light-induced interfacial charge transfer.

Sub-bandgap surface photovoltage in deep bulk impurity level semiconductors

A theoretical study of the surface photovoltaic effect in the sub-bandgap energy range is carried out for semiconductors with deep bulk impurity levels. The recombination kinetics assumed involves one deep bulk acceptor-like level and one or several surface donor-like levels communicating with the valence band. This situation is met in a number of materials, a particular example of which is indium-doped silicon. The surface photovoltage spectra are calculated at low temperatures for a variety of indium concentrations and light intensities, the surface level position being scanned both below and above that of the bulk level. Conditions under which the influence of the bulk trapping is negligible are found and possibilities to determine basic surface and bulk state parameters are discussed.

Analysis of the surface photovoltaic effect in photoconductors: CdS

A theoretical model of the surface space-charge layer in an illuminated photoconductor is analysed for the case in which bulk trapping is important. Sample calculations using parameters relevant to cadmium sulphide are presented to illustrate the conditions under which (a) significant surface photovoltages can be generated by very low intensities of illumination and (b) changes in the polarity of the surface photovoltage can occur with increasing illumination intensity. The effects of surface state structures associated with both clean and contaminated surfaces are examined.

Selective Photoetching of Gallium Arsenide

Etching of in an oxidizing etchant under intense local illumination was found to be selective and, moreover, to be a powerful method for the characterization of this material. A flat bottom hole is etched into the illuminated surface region of n‐type , whereas a mesa structure forms on p‐type material. Thus, it is possible to determine the conductivity type of very small surface areas. Height and depth of those structures both decrease with increasing doping level. Striations, precipitates, and decorated dislocation lines are revealed with excellent resolution even in undoped or semi‐insulating samples. By measuring photovoltages between an illuminated and a dark electrode in , the surface photovoltaic effect was shown to be the origin of selective photoetching.