Effect of Long Range Potential Fluctuations on the Transport Properties of Disordered Semiconductors

Abstract

The purpose of the present paper is to examine the effects of long range potential fluctuations (PF) on the transport properties by band carriers in disordered semiconductors. Such fluctuations may likely be produced , during the growth of the solid from the liquid or the vapor phase, by slow variations of temperature, pressure, gradients, occurring in times of the order of seconds or more, as well as by changes in local composition and flow of the liquid or (and) the gas phase. These variations are very difficult to avoid, even in carefully controlled processes, and they inevitably produce changes in the microscopic composition of the grown material. At usual growing speeds, these individual changes may extend over distances of tens or hundreds of atomic spacings in the grown solid. For instance microstructures of about 500 A diameter, resulting from columnar growth, have been observed2’3. In semiconductors, whatever their state (crystalline, glassy, amorphous) and their nature (inorganic, organic), such changes result in long range fluctuations of : the impurity density, the degree of compensation, the hydrogen (or other gas) content, the degree of stoechiometry in compounds, the respective proportions of several phases in polyphased materials. These fluctuations markedly differ from shorter range statistical fluctuations, on which they superpose. According to their nature, they produce covariant or contravariant fluctuations of the band edges.