Influence of Intersubband Scattering on the Magnetic Field Dependence of the Conductivity Tensor

Abstract

Transport properties of semiconductor structures with several subbands occupied have been attracting scientific interest for a long time because of, among other things, the appearance of intersubband scattering. It is known that this extra scattering mechanism affects both classical as well as quantum transport. In this paper we show that intersubband scattering can lead to apparent inconsistency of the experimental results obtained by means of classical and quantum transport measurements and this discrepancy is entirely connected with the usage of the classical fomulae to describe the magnetic field dependence of the conductivity tensor. We have studied electron transport of two-dimensional electron gas (2DEG) in the modulation Si δ-doped, MOVPE grown pseudomorphic GaAs/InGaAs/AGaAs quantum well (QW) at temperature T = 4.2 K and in magnetic field up to 7 T. The persistent photoconductivity effect was used to control the free electron concentration. Shubnikov de Haas (SdH) oscillations seen in ρxx indicated the existence of 2DEG. When electron sheet density exceeded the value of 1.3·10 12 cm -2 , the so-called magneto-intersubband (MIS) oscillations appeared. These oscillations are related to scattering between Landau levels of two occupied subbands. This means that quantum lifetimes of both first and second subband should be sufficiently high. However, at the same time classical analysis of the conductivity tensor revealed very low mobility of electrons on the second subband which seemed to rule out the possibility of the appearance of MIS oscillations. We prove that there is no contradiction in our observations. First we show that even if intersubband scattering is present in the sample with multi-subband occupation, one still can use the classical multilayer approach to analyse the non-oscillatory part of the conductivity tensor. However, one has to be very cautious in using this approach, since all the parameters describing the carriers of the contributing conduction channels (subbands) must be redefined in that case. This redefinition applies both to density and mobility of carriers. That is why a difference between electron density values obtained from quantum oscillatory effects and classical analysis of magnetic field dependence is usually observed in such cases. The same applies to the values of mobilities extracted from the classical analysis of magnetic field dependences of conductivity tensor – these values may substantially differ from the real ones determined by intraand intersuband scattering processes. Hence we show that there is really no contradiction in our observations. Low electron mobility obtained in classical analysis for the second subband in our sample does not imply low electron quantum lifetime for this subband. Thus the observation of MIS oscillations is absolutely possible.