On the mechanism of conduction and photoconduction in PbTe films

Conductivity, photoconductivity, thermoelectric, Hall and photothermoelectric measurements have been carried out on vacuum evaporated, air exposed PbTe films of different thickness in the temperature range 300 K-130 K to probe into the mechanism of conduction and photoconduction in these films. The dark electronic transport properties have been explained considering accumulation layers on the free surface and substrate-film interface and also the diffusion of oxygen inside PbTe microcrystals. It appears that both bulk and potential barrier scattering predominate in the high temperature region (300 K-250 K) and only barrier scattering can explain the low temperature carrier mobility at lower temperatures (250 K-130 K). Photoconductivity can be described with trapping of minority carriers within the PbTe microcrystals in the low temperature region and in the high temperature region with diffusion of minority carriers to the surface. The same oxygen trap situated at a depth of 0·06 eV from the conduction band is found to be responsible for both conduction and photoconduction in the low temperature region.Notation N _t Total trap concentration - N _r Total recombination centre concentration - n _t Electron concentration at the trapping centres - n _r Electron concentration at the recombination centres - _n Capture coefficient of electrons by recombination centres - _p Capture coefficient of holes by recombination centres - Capture coefficient of electrons/holes by trapping centres - E _t, E ₁ ^* Trap energies - E _c Conduction band edge - E _v Valence band edge - f Number of electron-hole pairs generated per unit volume per unit time - N _c Effective density of states at the conduction band edge - Potential barrier height - _p Life time of non-equilibrium holes - _s Surface barrier height in dark - _sl Surface barrier height under illumination - n _b Free electron concentration - P _b Free hole concentration - _s Surface mobility of holes - _b Bulk mobility of holes The authors are grateful to Prof. H. N. Bose for helpful discussions.