Article No. sm960225 Superlattices and Microstructures, Vol. 23, No. 6, 1998 Photogalvanic effect in asymmetric quantum wells and superlattices H. S CHNEIDER, S. EHRET, C. S CH ¨ ONBEIN, K. S CHWARZ, G. BIHLMANN, J. F LEISSNER, G. TR ¨ ANKLE, Fraunhofer-Institut f¨ ur Angewandte Festk¨ orperphysik, Tullastrasse 72, D-79108 Freiburg, Germany G. B ¨ OHM Walter-Schottky-Institut, T. U. M¨ unchen, D-85748 Garching, Germany (Received 15 July 1996) We have investigated the influence of the final states of bound-to-continuum transitions within the conduction band of asymmetric quantum well structures on the photocurrent. This influence manifests itself by an energy-dependent oscillation of the current direc- tion. We observe pronounced oscillations at zero bias voltage in a double quantum well structure, induced by an asymmetric excitation into continuum states with positive and negative momentum, i.e. by a photogalvanic effect (PGE). If this effect is superimposed on an asymmetric backrelaxation, similar oscillations are observed in the spectrum when the latter asymmetry is compensated by an external electric field. Theoretically, we find a strong relation between the PGE and a quantum interference effect occurring in the continuum. c  1998 Academic Press Limited Key words: intersubband transition, photogalvanic effect, quantum well, superlattice, infrared detector, transport. 1. Introduction A pronounced photo-electromagnetic force has been observed in many bulk crystals with reduced sym- metry [1, 2]. In LiNbO 3 :Fe, this effect is observed at an excitation energy of about 2.8 eV, shortly below the absorption bandedge of the crystal. It is accompanied by a dc photocurrent without external volt- age and gives rise to electric fields of up to 100 kV cm −1 along the polar axis [3], strong enough to induce photorefraction and electrical breakdown. This photovoltage is due to a transition asymmetry in k-space upon excitation of the electron system, i.e. to a bulk photogalvanic effect (PGE). We point out that this photovoltaic effect is different from optical rectification. Indeed, the latter cannot give rise to a dc photocurrent since the resulting signal is proportional to the temporal derivative of the incident power. The PGE in LiNbO 3 :Fe has been attributed to asymmetric electronic orbitals induced by an asymmetric crystal potential in the vicinity of the Fe impurities. In order to explain the effect theoretically, the potential distributions shown in Figs 1A–C have been proposed [1]. The single-barrier potential in Fig. 1A induces a preferential carrier emission towards the right-hand side of the well, since the tunneling probability across the barrier can be very small. A similar effect is expected to occur in the case of the triangular potential of Fig. 1B, such that the asymmetry of the well induces different occupation probabilities for the 0749–6036/98/061289 + 07 $25.00/0 c  1998 Academic Press Limited