Sign reversal of the photon drag effect in p type germanium

Under open circuit conditions a longitudinal emf is generated in a bar of p type germanium exposed to CO 2 laser radiation due to the transfer of photon momentum to mobile holes. At room temperature the polarity corresponds to positive holes being driven down the bar by radiation pressure but in some samples the polarity reverses on cooling. The authors describe this effect and the way in which the reversal temperature varies with doping concentrations greater than about 10 15 cm -3 the reversal temperature decreases rapidly so that with hole concentrations greater than 2*10 16 cm -3 no sign reversal occurs. These results are accounted for theoretically in terms of the known band structure of p type germanium and the effects of impurity and phonon scattering of light and heavy holes.