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 111-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 of continuing innovation in this area, to indicate present trends and material problems which may arise in the near future. T h e emphasis of the review is on the physics of the methods: detailed discussion of results on specific materials is avoided and it is assumed that the intrinsic material properties are well known. The review is therefore concerned with the assessment of the electrical effects of impurities and defects.Routine characterisation using the resistivity, Hall coefficient and magnetoresistance effects is reviewed and methods for determining the donor and acceptor impurity concentrations from the 77 K mobility or from the temperature dependence of the free carrier density are described. Current knowledge of the Hall scattering factor is summarised. A review is given of rapid resistivity measurement by four-point probe, spreading resistance and contactless methods. Depth profiling by spreading resistance probe on bevelled structures, and by layer removal using chemical etching, anodic oxidation and ion beam etching are also considered. A major development in the last 10 years has been the use of capacitance methods for material characterisation. T h e principal C-V methods for measuring dopant profiles are compared, and the limitations are examined in detail. Capacitance and thermally stimulated current methods for studying deep traps are reviewed and the interpretation of their results is discussed. There has been increasing concern in recent years with the minority carrier diffusion length and lifetime. Measurement of these properties by photoluminescence, cathodoluminescence, and diode current collection methods is reviewed and the MOS capacitance method for lifetime measurement is also described. The review concludes with a section on optical methods in which the use of luminescence for chemical identification of electrically active impurities is emphasised and direct measurement of properties such as carrier density and epitaxial layer thicknesses is described.

T h e characterisation of the transport properties of semiconducting material through a combination of Hall effect and resistivity measurements is as important in understanding the electrical transport properties of polycrystalline and powdered semiconductors as it is for single-crystal semiconductors. However, the interpretation of these measurements for polycrystalline and powdered semiconductors is more complicated due to the presence of grain boundaries and trapped interface charges which lead to inter-grain band bending and potential barriers. I n recent years a resurgence of interest in these materials, due to their potential for large-area device applications, has led to a much better understanding of the influence of grain boundaries on these properties.This review surveys the development of this subject from its origins in the early 1950s to the recent advances of the last few years, showing the extent to which a considerable body of experimental results can now be understood in terms of simple theoretical models.We give a critical review of idealised two-phase geometrical models which, though first considered nearly 30 years ago, still form the basis of the subject. These treatments derive expressions for the resistivity and Hall coefficient of a composite material in terms of the properties of its constituents. We 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. When the depletion region extends only partially through the grain, the Hall effect measures the carrier concentration in the bulk of the grain while the Hall mobility should be given by p= po exp (-+b/kT) where +b is the bandbending and ,uo is related to the grain diameter 1. When the depletion region extends right through the grain, the carrier concentration measured may be very much lower than the bulk doping level but is still close to the carrier concentration in the centre of the grains. Band-bending is related to doping level N and interface trap density Nt. It reaches a maximum value when Nt=Nl, and ,u therefore shows a minimum.Consideration of the possibility of the mean free path being greater than the grain size 0034-4885/80/111263

The group III nitrides (AlN, GaN and InN) represent an important trio of semiconductors because of their direct band gaps which span the range 1.95-6.2 eV, including the whole of the visible region and extending well out into the ultraviolet (UV) range. They form a complete series of ternary alloys which, in principle, makes available any band gap within this range and the fact that they also generate efficient luminescence has been the main driving force for their recent technological development. High brightness visible light-emitting diodes (LEDs) are now commercially available, a development which has transformed the market for LED-based full colour displays and which has opened the way to many other applications, such as in traffic lights and efficient low voltage, flat panel white light sources. Continuously operating UV laser diodes have also been demonstrated in the laboratory, exciting tremendous interest for high-density optical storage systems, UV lithography and projection displays. In a remarkably short space of time, the nitrides have therefore caught up with and, in some ways, surpassed the wide band gap II-VI compounds (ZnCdSSe) as materials for short wavelength optoelectronic devices. The purpose of this paper is to review these developments and to provide essential background material in the form of the structural, electronic and optical properties of the nitrides, relevant to these applications. We have been guided by the fact that the devices so far available are based on the binary compound GaN (which is relatively well developed at the present time), together with the ternary alloys AlGaN and InGaN, containing modest amounts of Al or In. We therefore concentrate, to a considerable extent, on the properties of GaN, then introduce those of the alloys as appropriate, emphasizing their use in the formation of the heterostructures employed in devices. The nitrides crystallize preferentially in the hexagonal wurtzite structure and devices have so far been based on this material so the majority of our paper is concerned with it, however, the cubic, zinc blende form is known for all three compounds, and cubic GaN has been the subject of sufficient work to merit a brief account in its own right. There is significant interest based on possible technological advantages, such as easier doping, easier cleaving (for laser facets) and easier contacting.

Defects induced in magnesium oxide by neutron irradiation are largely the result of atom displacements. The vacancies which are left behind are sites at which either electrons or holes may be trapped, depending upon the sign of the displaced ion. Such bound electrons or holes are paramagnetic entities which may give rise to e.s.r. absorption. We suggest that negative-ion vacancies in three different environments are trapping sites for electrons, and positive-ion vacancies of two types are trapping sites for holes. These two hole centres are in addition to those types found in crystals which have not been neutron-irradiated. Some of these centres may be induced by grinding and X-irradiation of MgO or other alkaline earth oxides, sulphides or selenides which have the MgO structure. Mechanical working is believed to distribute negative-ion vacancies by dissociation from dislocations which are swept through the crystals. In the neutron-irradiated crystals, there is evidence that some local regions are so disrupted that they rearrange to bodycentred instead of face-centred packing. Extended heating largely anneals out the neutronproduced structural defects. One is then enabled to see e.s.r. spectra of defects which are highly sensitive to distortions of cubic symmetry.

We report on the growth of GaN with a zinc-blende, wurtzite, or a mixed phase structure on (001)GaP and (001)GaAs substrates by a low-temperature modified molecular beam epitaxy technique. By systematically varying the incident arsenic overpressure, films grown at a moderate substrate temperature of ≊620 °C show predominately wurtzite α-GaN, zinc-blende β-GaN, or a mixed phase of the two. Films containing only the metastable phase β-GaN were achieved by using a relatively high growth temperature of ≊700 °C and with an arsenic overpressure of ≊2.4×10−5 Torr. X-ray diffraction measurements indicate an improved crystalline quality for the layers grown at ≊700 °C compared to those grown at ≊620 °C as evident by a narrower full width at half-maximum of 35 min for β-GaN, which is among the narrowest reported to date.