M. Leibovitch

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 diffusion like currents. However, it is found that the specific case of photovoltage sign reversal, observed after etching highly quantized CdSe QD films, is governed by diffusion like 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.

Structure and conductance evolution of very thin indium oxide films

The conductance of transparent conducting oxide films as a function of their coverage has been investigated in situ. The films have been prepared by means of reactive evaporation of In in the presence of oxygen on the glass substrate at different substrate temperatures. The analysis shows that island growth, percolation, coalescence, and ohmic stages can be identified. Critical parameters of the films can be determined during the growth, such as anisotropic and percolative growth modes, resistivity, a lower limit of the effective dopant concentration. The technique shows a potential for in‐depth characterization of very thin film growth.

Band diagram of the polycrystalline CdS/Cu(In,Ga)Se2 heterojunction

Contact potential difference measurements in the dark and under illumination are used to derive the conduction band offset (ΔEc) in a solar cell quality junction formed by chemical bath deposition of CdS on a polycrystalline thin film of Cu(In,Ga)Se2. Our experimental measurements and the estimates made for dipole contributions show that the junction is of type II, i.e., without a spike in the conduction band ( ΔEc=80 meV±100 meV). This is consistent with the high performance of the actual solar cell. However, it differs from most previous results on junctions based on single crystals and/or vacuum deposited CdS, which indicated the existence of a conduction band spike.

Quantitative assessment of the photosaturation technique

Abstract The photosaturation technique is a well-known method for measuring the band-bending at semiconductor surfaces. It is based on the assumption that the bands can be flattened upon sufficiently intense illumination. The validity of this approach has been a subject of considerable dispute. A rigorous, quantitative examination of the method is presented. The physical mechanisms governing the photosaturation experiment are identified and analyzed using both an analytical and a numerical model. We show that while the technique is essentially valid, the illumination intensity required to obtain band flattening may be unrealistically high. Criteria for attaining photosaturation are formulated in terms of surface state parameters. Numerous pitfalls and sources of misinterpretation are pointed out. Specifically, a previously undiscussed pseudo-saturation due to surface states with significantly different thermal cross-sections, is described. A systematic approach to future experiments is suggested.

Quantitative surface photovoltage spectroscopy of semiconductor interfaces

A comprehensive and quantitative method for extracting the important parameters of interface states is presented. The method is based on wavelength-, intensity-, and time-resolved surface photovoltage spectroscopy, as well as on measurements as a function of the thickness of an overlayer. Data analysis provides detailed information about interface state properties, including their energy position and distribution, density, and the transition probabilities, i.e. their thermal and optical cross sections. It is also possible to distinguish between surface and bulk states, and determine the spatial site of the states in the case of a heterostructure. Experimental examples for various III-V and II-VI compound semiconductors are given.

Surface photovoltage spectroscopy of thin films

The surface photovoltage (SPV) spectrum due to subband‐gap illumination of thin films is theoretically studied. It is shown that this SPV is inherently sensitive to buried interfaces just as it is sensitive to the external semiconductor surface. The different contributions to the SPV from all the optically active gap states present within a sample, consisting of a bulk substrate covered by a thin film, are analyzed. Analytical expressions are obtained in the low illumination intensity and the depletion approximation regime. The evolution of the SPV spectrum with film thickness is examined and is found to depend on both site and population of the gap states. Three modes of evolution are found, according to the relative importance of gap state population changes with film thickness. These modes are confirmed by a numerical simulation of a thin film of pseudomorphic InAlAs on InP substrates and by experiments conducted on the same system. The approach is also applied to the InP/In2O3 system, revealing gap st...

Surface photovoltage spectroscopy of an InGaAs/GaAs/AlGaAs single quantum well laser structure

An InGaAs/GaAs/AlGaAs single quantum well graded-index-of-refraction separate-confinement hetero-structure laser has been analyzed using surface photovoltage spectroscopy (SPS) in a contactless, nondestructive way at room temperature. Numerical simulation of the resulting spectrum made it possible to extract growth parameters, such as the InGaAs well width, the well and cladding compositions, as well as important electro-optic structure data of this device, including the lasing wavelength and built-in electric field. The results highlight the power of SPS in obtaining performance parameters of actual laser devices, containing two-dimensional structures, in a contactless, nondestructive way.

Indium oxide Schottky junctions with InP and GaAs

Junctions of transparent conducting oxides on III–V semiconductors have been prepared by deposition of indium oxide layers onto p‐type InP and n‐type GaAs by means of reactive evaporation of In in the presence of oxygen at different substrate temperatures. The electrical properties and chemical composition of these junctions have been investigated using current‐voltage measurements in the dark at room temperature, capacitance‐voltage measurements, and depth profiling by Auger electron spectroscopy. The best diodes were obtained by deposition at a substrate temperature near 250 °C and oxygen pressure of 5×10−4 Torr. These diodes exhibit a Schottky barrier height of 0.80 eV for n‐type GaAs and 0.87 eV for p‐type InP with an ideality factor of 1.04. The Schottky barrier height decreases with decreasing deposition temperature for both substrates. The roles of the tunneling–transparent interface layer and interface region are theoretically considered. It is shown that as the deposition temperature is increased, the barrier height increases due to the accompanying reduction in the density of surface states, which are induced by elemental In at the interface.

Surface photovoltage spectroscopy of a GaAs/AlGaAs heterojunction bipolar transistor

The electronic properties of a GaAs/AlGaAs heterojunction bipolar transistor (HBT) structure have been studied by surface photovoltage spectroscopy. The p-base band-gap narrowing has been determined and confirmed by numerical simulation. Based on the shape of the surface photovoltage spectrum, it is possible to monitor the doping level and evaluate the minority-carrier mobility. This work demonstrates the power of the technique as a precision tool for HBT quality control.

In-situ monitoring of surface chemistry and charge transfer at semiconductor surfaces

Abstract A simple method for in-situ distinction between the effect of dipole formation/annihilation and charge transfer to/from surface gap states on the semiconductor work function is described. The technique is based on simultaneous monitoring of the work function and photovoltage at the semiconductor surface. The approach is illustrated by experiments performed on single crystalline InP(100) surfaces and polycrystalline Cu(In,Ga)Se 2 .