Impurity Conduction inp-Type Silicon at Microwave Frequencies

Abstract

The microwave conductivity of boron-doped silicon was studied at low temperature at \ensuremath{\sim}9000 Mc/sec. Samples measured had boron concentrations ranging from 6.7\ifmmode\times\else\texttimes\fi{}${10}^{15}$ to 1.6\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. In the impurity conduction range, the microwave conductivity varied much more slowly than the dc conductivity, becoming orders of magnitude larger in comparison at 4.2\ifmmode^\circ\else\textdegree\fi{}K. Calculations based on the hopping model give microwave conductivity of the right order of magnitude as measured. However, pronounced nonohmic behavior was observed in samples of impurity concentrations less than \ensuremath{\sim}${10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, and the measured temperature dependence of conductivity for the samples of high-impurity concentrations was too strong. These observations cannot be accounted for by the hopping model. It is shown that the nonohmic behavior may be attributed to the direct absorption of microwave power for which phonon interaction is not the essential part of the process. The strong temperature dependence is explained on the basis of carrier trapping by closely spaced impurity centers. Some measurements were made at lower frequencies. The results are consistent with the interpretation.