Photo-thermovoltaic effect induced by CO₂ laser illumination of PbTe crystals

The study of the thermovoltaic effect in the presence of a high temperature gradient within structures with a potential barrier (in particular, a p-n junction) is of special interest. The photo-thermovoltaic effect (PTVE) is caused by the generation of electron-hole pairs due to the local temperature increase and the carrier separation at a potential barrier. In contrast to the photovoltaic effect in which the nonequilibrium electron-hole pairs move towards the junction, a temperature gradient causes a flux of charge carriers from the hot towards the cold region. Usually the direction of the temperature gradient is the same both in the p- and the n-region. The thermovoltage at the barrier (junction) is, thus, the difference between the two components, V_B = V_Bp, - V_Bn, where V_Bp and V_Bn are the contributions of the pand n-regions, respectively. However, in the case of an asymmetric p-n junction, a metal-semiconductor Schottky barrier, or a heterojunction, the contribution of these components can be different. The barrier Seebeck coefficients, S_Bp = V_Bp/ΔT_Bp and S_Bn = V_Bn/ΔT_Bn with ΔT_Bp and ΔT_Bn the near-barrier temperature difference in the p-type and n-type regions, respectively - are proportional to E_g/qT (E_g is the energy gap of semiconductor material, q is the electronic charge). The present study of the PTVE over a wide temperature range (80 - 300 K) in PbTe was undertaken with the purpose of checking the validity of the above considerations, namely: (i) the near surface local heating due to the absorption of the far IR laser pulses by the free electrons; (ii) the low thermal conductivity of PbTe crystals. For measurements of the PTVE was used a laser pulse produced by a CO₂ laser source (laser power ∼ 1.26 kW) with pulse duration of 200 ns which was directed normal to the external surface in the vicinity of the metallic contact. The diameter of illuminated area (area over which the laser pulse intensity is > 50% of its peak value) was about 0.4 mm. The PTVE measurements were carried out in. an optical cryostat over the 80-300 K temperature range. The thermovoltaic effect in p-type PbTe with a Ni-semiconductor Schottky barrier is by a factor of 30 at least higher than in homogeneous (ohmic contact) PbTe.