Charge Transport in Solid and Liquid Ar, Kr, and Xe

Abstract

This paper reports an investigation of the drift velocity of excess electrons in solid and liquid Ar, Kr, and Xe. After purification of the commercially available gas, thin crystal specimens (100-600 \ensuremath{\mu}m) were grown from the liquid between parallel electrodes in a chamber attached to a miniature cryostat. Pulses of 40-keV electrons were used to generate the charge carriers in both liquids and solids. This technique overcomes the limitations inherent in previously applied methods and has made it possible to investigate the drift velocity over a range of applied fields from 10 V ${\mathrm{cm}}^{\ensuremath{-}1}$ to 100 kV ${\mathrm{cm}}^{\ensuremath{-}1}$. Near the triple point, the low-field mobility ${\ensuremath{\mu}}_{0}$ in solid Ar, Kr, and Xe was found to be 1000, 3700, and about 4500 ${\mathrm{cm}}^{2}$${\mathrm{sec}}^{\ensuremath{-}1}$${\mathrm{V}}^{\ensuremath{-}1}$, respectively. In the liquids the corresponding mobilities were 475, 1800, and 2200 ${\mathrm{cm}}^{2}$${\mathrm{sec}}^{\ensuremath{-}1}$${\mathrm{V}}^{\ensuremath{-}1}$. The temerature dependence of ${\ensuremath{\mu}}_{0}$ has been measured on Ar crystals, and the results indicate that ${\ensuremath{\mu}}_{0}$ is determined by acoustic scattering. The electron lifetime appears to be controlled predominantly by oxygen impurities. Pronounced hot-electron effects are observed in drift-velocity---versus---field curves for both liquids and solids, and their fit to the Shockley hot-electron theory has been investigated. In the high-field region all the curves show an almost complete saturation with field. The theory of Cohen and Lekner applied to liquid Ar fits the results over most of the field range, suggesting that the deviations from the Shockley theory at higher electron temperatures are associated with an increase in the value of the structure factor. In solid Ar or Kr, positive holes do not appear to be mobile, but in Xe crystals a hole mobility of about 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}2}$ ${\mathrm{cm}}^{2}$${\mathrm{sec}}^{\ensuremath{-}1}$${\mathrm{V}}^{\ensuremath{-}1}$ was found. The implications of these results are briefly discussed.