A.W.R. Leitch

Properties of CuIn(Se,S)2 thin films prepared by two-step growth processes

Abstract The relatively small band gap approximately 1 eV of CuInSe 2 (CIS) restricts the efficiencies of completed CIS/CdS/ZnO solar cell devices. In this study, CuIn(Se,S) 2 thin films were deposited by the reaction of copper–indium alloys with H 2 Se and H 2 S gases in Ar at elevated temperatures approximately 450 °C. The concentration of the respective reaction gases and exposure periods were optimized in order to produce single-phase material with a controlled distribution of sulfur through the depth of the thin films. X-ray fluorescence in-depth profiling revealed uniform Cu/In atomic ratios and a systematic increase in the sulfur concentration towards the near-surface region of the samples under optimum reaction conditions. The increase in band gap of the absorber layers after sulfurization was also confirmed by low temperature photoluminescence, X-ray diffraction studies as well as optical absorption measurements.

Investigation on the I-V characteristics of a high concentration, photovoltaic array

The aim of this study was to highlight the effects on the I-V characteristics of concentrator arrays due to mismatch between series-connected high concentration PV modules and between single junction cells within a module. The mismatch between cells was caused by a number of factors including: misalignment of optical elements and cells, uneven shading due to dust or delamination of the cell-secondary lens interface, degradation of the main Fresnel lens and non-uniform cell material parameters. Small amounts of mismatch that would go unnoticed at one-Sun insolation levels are vastly amplified at high concentration ratios. This paper reports on, and interprets the regular I-V measurements that were recorded over a period of two years under various conditions. The effect of bypass diodes on the module I-V curves is also investigated. The general characteristics of the measured resultant I-V curves are explained. The degradation of the PV concentrator modules over a period of two years is also confirmed from the I-V measurements.

Structure and morphology of CuGaS2 thin films

Abstract The growth of CuGaS2 epilayers by the simultaneous evaporation of the constituent elements in vacuum is investigated. Growth under Cu-rich conditions resulted in the formation of a CuS secondary surface phase. This phase, recognised as dark-blue crystallites, resulted in a number of additional X-ray diffraction lines. It was possible to selectively remove this phase using a potassium cyanide etchant. Transmission electron microscopy analysis proved that epitaxy is indeed possible despite the simplicity of the growth method. Raman spectroscopy revealed lines inherent to CuGaS2 and an additional, unidentified mode at ∼472 cm−1 was observed for Cu-rich material.

The growth of Zn3As2 on InP by atmospheric pressure MOVPE

Abstract Atmospheric pressure MOVPE was used to grow Zn 3 As 2 layers on (001) and (111) InP. The Zn 3 As 2 layers were found to be polycrystalline when grown at 300°C, while growth at 450°C resulted in single crystalline material, for both types of substrate orientations. At 600°C no growth occurred. For a growth temperature of 450°C, the growth rate on (001) InP was calculated to be 0.40 nm/s, while it was 0.42 nm/s for growth on (111) InP. Further, it was found that a linear dependency exists between the Zn mole fraction and the growth rate for growth on both orientations of the substrate. Hall measurements at 295 K indicated that the Zn 3 As 2 epilayers were unintentionally doped p-type, with an average carrier concentration of 2.8 × 10 18 cm −3 and a Hall mobility of 50 ± 4 cm 2 /V · s.

Influence of growth interruption on the heterointerface morphology ofInGaAsGaAs strained quantum wells

Abstract A study of the influence of growth interruption on the photoluminescence properties ofInGaAsGaAs strained quantum wells grown by metalorganic vapour phase epitaxy is presented. The effect of misorientation of the GaAs(001) substrate on the heterointerface properties is also considered. Growth interruption is shown to cause a considerable broadening of the photoluminescence line widths. The line-width broadening is related to changes in the interface morphologies upon growth interruption. For the misoriented substrates, these changes are illustrated by transmission electron microscopy. In particular, step bunching is shown to be enhanced during growth interruption on the misoriented surfaces. The results are discussed by considering the growth modes dominating on the different types of surfaces, as well the influence of strain on the surface morphology of the InGaAs layers.

Correlation of transport and optical properties of Si-doped Al0.23G0.77N

The properties of Si-doped Al0.23Ga0.77N grown by metalorganic vapor-phase epitaxy have been investigated by photoluminescence and Hall effect measurements. Nonintentionally doped samples were found to be insulating, while the room temperature carrier concentration of the doped layers did not vary linearly with the silane flow. The Si donor activation energy decreases with the doping level and the results correlate very well with those of Si-doped GaN. The temperature dependence of the photoluminescence spectra of the samples, in particular, their quenching with increasing temperature, can be explained by considering characteristic energies deduced from the Hall effect measurements. Our study shows that Al0.23Ga0.77N is a well-behaved semiconductor alloy, in which Si is a standard hydrogeniclike donor.

PHOTOLUMINESCENCE PROPERTIES OF METALORGANIC VAPOR PHASE EPITAXIAL ZN3AS2

The photoluminescence (PL) properties of the II3V2 compound semiconductor Zn3As2, grown by metalorganic vapor phase epitaxy, is investigated in the temperature range of 4.2–350 K. Several lines are reported, believed to be due to acceptors and donor-acceptor complexes in the epitaxial material. From variable temperature PL, the band gap is deduced to be indirect in nature (in contrast to most experimental findings to date which have indicated the band gap to be direct in nature) and ∼25–30 meV larger than the indirect band gap. The direct band gap energy has been extracted from the PL spectra in the range of 110–350 K. A linear relationship is observed in this temperature range with slope dEg/dT=−4.00×10−4 eV/K. PL yielded a value of 1.005 eV for the direct gap at room temperature in good agreement with values between 0.989 and 0.999 eV obtained from transmission and reflection measurements.

Non-grid electrification of 45 schools in the Eastern Cape, South Africa: An assessment

This paper reports on the electrification using photovoltaic power of 45 schools in the rural areas near Queenstown, in the Eastern Cape region of South Africa. The project was funded by the state, coordinated by Eskom and implemented by Siemens Limited (Solar Division). The University of Port Elizabeth (Department of Physics) independently monitored the electrification process. As solar power is a relatively new concept for many of the rural people in South Africa, the University also gauged the opinion of the school teachers and local community on this new form of electricity.

The characterization of GaAs AND AlGaAs by photoluminescence

Abstract The basic fundamental principles of photoluminescence are briefly reviewed in this paper. A detailed description of a typical home-built system will be given. Results to be presented will be restricted to GaAs and AlGaAs epitaxial layers grown by MOVPE. In particular, the photoluminescence response from high purity undoped GaAs, Se-doped GaAs, AlGaAs as well as GaAs/AlGaAs quantum wells will be described. These results will clearly demonstrate the capabilities and limitations of this nondestructive technique in the characterization of semiconductor material.

Scanning photoluminescence study of the spatial uniformity of the growth rates of OMVPE grown GaAs Quantum wells

Abstract This paper describes a low temperture scanning photoluminescence study to determine the spatial uniformity of the growth rates of undoped GaAs quantum wells sandwiched between Ga 0.7 A 1 0.3 As layers which were grown in a horizontal OMVPE reactor at atmospheric pressure. The observed spatial uniformity of the growth rates confirm Moffat and Jensens predictions that under certain conditions bouyancy driven secondary flows can adversely effect the spatial uniformity of the growth rates in a horizontal organometallic vapour phase epitaxial reactor.