Thermal treatments are crucial steps for improving the quality of semi-insulating InP substrates. The result of these treatments depends on the starting material and the specific annealing conditions. The best results in terms of the electrical properties and homogeneity are obtained by annealing Fe-doped InP wafers. The influence of the specific annealing conditions is discussed. The best results are obtained for long annealing times (50 hours) and slow cooling rates, which give good homogeneity, high carrier mobility and high enough electrical compensation.
A. IntroductionSemi-insulating InP is a basic material for high speed electronics and OEIC's (Optoelectronic Integrated Circuits) for fibre optic communication systems. The growth and preparation of substrates with precise requirements is a crucial step in the development of these technologies. Among the major requirements of these substrates are high resistivity, high mobility and whole wafer uniform properties [l]. Commercially available LEC (Liquid Encapsulated Czochralski) substrates are prepared by adding iron to the polycrystalline charge [1,2]. Iron enters the In site in the InP lattice giving a midgap acceptor (Ea=0.64 ev) [3], that compensates the residual shallow donors. However, the presence of iron raises several challenging problems. These problems are normally related to its small distribution coefficient (< 0.001) [l], its high diffusivity [4] and its incomplete electrical activation [1,4]. The consequence of the low segregation coefficient is a pronounced axial distribution profile [5]. The low electrical activation demands that high amounts of iron be added to the charge, above that strictly necessary for compensating the shallow donors. The high diffusivity together with the high doping amounts are responsible for the iron contamination of the epilayers grown on these substrates [6], which is detrimental for the device performance.From the point of view of the radial uniformity the problems are not smaller, since typical macroscopic and microscopic lateral fluctuations appear. Doping growth striations are commonly observed [7,8]. These inhomogeneities arise from non convective transport in the melt, the curvature of the growth interfaces and other growth features.In addition to these macroscopic and mesoscopic non uniformities, a number of microdefects are present, e.g. grown-in dislocations, microprecipitates and grain boundaries [9]. These microdefects play an important role in the properties of the substrates and in the thermal treatments as we will see later [lo-121. Following these considerations, a great effort is necessary in order to improve the quality of the wafers actually available. As it was shown for other substrates, e.g.GaAs [13], adequate thermal treatments seem to be the best way for achieving advances in the optimization of the semi-insulating InP substrates. Strong research has been carried out in the last years about thermal treatments to obtain high quality semi-insulating InF' substrates. In spite of this, there is ...