Preface

Abstract

This special issue contains a selection of contributions to theWideGap2001 conference, centred on doping issues in wideband-gap semiconductors, held at the University of Exeter,United Kingdom, on 21--23 March 2001.The aim of the workshop was to bring togetherexperimentalists - interested in optimizing andcharacterizing the electrical activity of dopants in wideband-gap materials - and theoreticians who carry outcalculations in this area. The workshop focused mainly ondefects and impurities in III-nitrides,silicon carbide, zinc oxide, and diamond.There is a great deal of interest in overcoming problems relating toshallow dopants in these materials. For example, in galliumnitride, oxygen and silicon are known to be shallow donorsbut there are difficulties in finding efficient shallowacceptor dopants for creating p-type material - magnesium andberyllium are favoured but have high activation energies.In zinc oxide, an effective method to achieve p-type conduction isstill eluding current researchers. In diamond, boron is a shallowacceptor but there are difficulties finding suitable shallow donors.Phosphorus and sulphur are key candidates and there isintense interest in optimizing their activity. Nevertheless,dopants in all these materials suffer from problems relatingto solubility and the formation of complexes,for example with hydrogen, a common impurity,or with native interstitials and vacancies.A striking characteristic of the WideGap2001 workshop wasthe large number of theoretical groups represented.Physicists are well aware that first-principles modelling is a veryeffective way in which dopant activity can be predicted.Combined with leading-edge computational resources, so-called ab initio codes can determine theoretically, amongstother properties, the structural,electrical, optical and vibrational characteristics of crystallinedefects. The computed formation energies, equilibrium concentrations,diffusion paths, and behaviour under appliedstress of a variety of defects are often in excellent agreement withexperiment, as are the calculated results of electronparamagnetic resonance experiments.Such valuable information helps to understand a widevariety of experimental observations, and aims to predict the resultsof new ones. It is significant that the shallow oxygen donorin GaN was first suggested from theoretical results.In the fast-evolving field of wide band-gap semiconductors,the experimental techniques employed to probe dopant activityare becoming more and more sophisticated. Not only arephotoluminescence and photoconductivity studiesroutinely applied, but new methods such as positron annihilation,photoluminescence microscopy, and muon spin resonance can todayprovide equally valuable information.The WideGap2001 conference successfully brought together groupshaving interests in different materials and experience withdifferent techniques of doping. It will be judged successfulif the tricks used in one area are used in others. Certainly, co-doping should be tried in silicon carbideand theorists encouraged to analyse the difficultproblems of dopant incorporation and surface effects. Theseobstacles will need to be overcome in the drive towardsnanostructures built from wide band-gap materials.