Gives a critical review of idealised two-phase geometrical models. These treatments derive expressions for the resistivity and Hall coefficient of a composite material in terms of the properties of its constituents. The authors show that these models can be applied to the interpretation of transport measurements in polycrystalline films and powder layers. Important distinctions are made depending on whether the depletion layers extend completely or partially through the grains, whether the Debye length is greater or less than the grain size and whether the mean free path is greater or less than the grain size. The authors discuss the theoretical treatment of the Hall effect in percolative systems, as geometrical models neglect percolation. The modulation of Hall coefficient and conductivity by illumination and the adsorption and desorption of ambient gases are also considered.

https://iopscience.iop.org/article/10.1088/0034-4885/43/11/001/pdf

The Hall effect in polycrystalline and powdered semiconductors

Semi‐insulating GaAs materials, undoped or doped with concentration of chromium varying from 6×1015 to 4×1017 cm−3, have been studied using both Hall effect measurements and optical absorption measurements. It is definitively concluded that compensation comes from the presence of the deep donor EL2 in undoped materials, and from both this deep donor and the deep acceptor related to chromium in Cr‐doped materials. Sets of curves are given which allow the determination of ND‐NA, the concentration of shallow donors and acceptors, knowing the Hall mobility and the Cr concentration in a given sample. Such curves can be a working tool for assessing any piece of semi‐insulating GaAs in a routine way.

Compensation mechanisms in GaAs

Requirements for infrared focal plane arrays (IRFPAs) for advanced infrared imaging systems are discussed, and an overview is given of different IRFPA architectures. Important IR detector structures, including photoconductive, photovoltaic, metal-insulator-semiconductor (MIS), and Schottky barrier, are reviewed. Infrared detector materials and related crystal-growth techniques are discussed, emphasizing applicability to IRFPA designs and performance. Three types of input circuit used to couple the detector to the readout circuitry are discussed, namely, direct injection, buffered direct injection, and gate modulation. An overview is given of several readout techniques, including the CCD, MOSFET switch, CID, and CIM. Also discussed are related onchip signal processing topics as well as questions regarding producibility and array implementation. >

Infrared focal plane array technology

MgCl2 is shown to be a cheap and non-toxic replacement for the costly and environmentally unfriendly salt CdCl2 that has long been used as the ‘activation’ step in the production of cadmium telluride solar cells. Solar cells based on cadmium telluride, CdTe, are among the most cost-efficient photovoltaic systems currently in use. But according to Jonathan Major et al., there is still plenty of room for improvement. Specifically, they show that it is possible to replace the costly and environmentally unfriendly cadmium-containing salt (CdCl2), which has long been used to 'activate' the CdTe during processing, with a cheap and non-toxic alternative, MgCl2. This change does not appear to be detrimental to device performance, yet shows great potential for reducing processing costs and environmental risk. Cadmium telluride, CdTe, is now firmly established as the basis for the market-leading thin-film solar-cell technology. With laboratory efficiencies approaching 20 per cent1, the research and development targets for CdTe are to reduce the cost of power generation further to less than half a US dollar per watt (ref. 2) and to minimize the environmental impact. A central part of the manufacturing process involves doping the polycrystalline thin-film CdTe with CdCl2. This acts to form the photovoltaic junction at the CdTe/CdS interface3,4 and to passivate the grain boundaries5, making it essential in achieving high device efficiencies. However, although such doping has been almost ubiquitous since the development of this processing route over 25 years ago6, CdCl2 has two severe disadvantages; it is both expensive (about 30 cents per gram) and a water-soluble source of toxic cadmium ions, presenting a risk to both operators and the environment during manufacture. Here we demonstrate that solar cells prepared using MgCl2, which is non-toxic and costs less than a cent per gram, have efficiencies (around 13%) identical to those of a CdCl2-processed control group. They have similar hole densities in the active layer (9 × 1014 cm−3) and comparable impurity profiles for Cl and O, these elements being important p-type dopants for CdTe thin films. Contrary to expectation, CdCl2-processed and MgCl2-processed solar cells contain similar concentrations of Mg; this is because of Mg out-diffusion from the soda-lime glass substrates and is not disadvantageous to device performance. However, treatment with other low-cost chlorides such as NaCl, KCl and MnCl2 leads to the introduction of electrically active impurities that do compromise device performance. Our results demonstrate that CdCl2 may simply be replaced directly with MgCl2 in the existing fabrication process, thus both minimizing the environmental risk and reducing the cost of CdTe solar-cell production.

A low-cost non-toxic post-growth activation step for CdTe solar cells

This article is a personal review of the principles, capabilities, limitations and potential of the technique of electrochemical capacitance-voltage (C-V) carrier concentration profiling of compound semiconductors and the associated technique of photovoltage absorption spectroscopy. The profiling technique was developed by Ambridge and co-workers (1973-5, 1980) to overcome the depth limitation in depletion C-V profiling by using an electrolyte barrier to measure the carrier density and to etch the material in a controlled electrolytic process. The electrolyte also provides a transparent barrier which facilitates observation of absorption spectra, hence providing the added capability of band-gap profiling. The author summarises the basic principles of C-V profiling and analyses the balance between measurement accuracy and instrumental depth resolution, and considers the effect of series resistance. In reviewing the principles of electrochemical C-V profiling he pays particular attention to the electrolyte (Helmholtz) capacitance, the high electrolyte resistance and the definition of contact area. In considering these problems, and those of depth resolution and the influence of deep states, he takes account of the use of a fixed low reverse bias in electrochemical C-V profiling compared with an increasing bias in depletion profiling. The interpretation of photovoltage spectra from single layers and heterostructures is described and examples are given of band-gap profiling of laser structures. The article concludes with examples of the characterisation of multiple quantum-well structures including carrier density profiles and photovoltage spectra on structures with periods less than 200AA.

https://iopscience.iop.org/article/10.1088/0268-1242/1/1/002/pdf

Capacitance-voltage profiling and the characterisation of III-V semiconductors using electrolyte barriers

An important aspect of the large expansion in the development and production of solid-state devices has been the demand for more sophisticated techniques for determining the electrical properties of semiconductors, especially silicon and the III-V compounds. A very wide range of measurement techniques now exists and it is the purpose of this article to review those techniques which are in widespread use or which show promise for future application, and at a time for continuing innovation in this area, to indicate present trends and material problems which may arise in the near future.

https://iopscience.iop.org/article/10.1088/0034-4885/41/2/001/pdf

P Blood

Papers

The electrical characterisation of semiconductors