HIGH FIELD MAGNETO-RESISTANCE IN GaAs-GaAlAs HETEROJUNCTIONS

D.R. Leadley¹, R.J. Nicholas¹, W. Xu², F.M. Peeters², J.T. Devreese², C.T. Foxon³, and J.J. Harris⁴

1. Deptment of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK
2. Departement Natuurkunde, Universiteit Antwerpen (UIA), Universiteitsplein 1, B-2610 Antwerpen, Belgium
3. Department of Physics, University Park, Nottingham, NG7 2RD, UK
4. Deptment of Electronics and Electrical Engineering, University College London. WC1E 6BT UK

In this paper we present measurements and calculations of the high field linear magnetoresistance in GaAs-GaAlAs heterojunctions over the temperature range 1.5K to 300K and for a number of different samples. In high quality GaAs-GaAlAs heterojunctions the electron mobility, in the absence of a magnetic field, is well understood in terms of scattering by impurities and phonons. The mobility varies between samples due to the differences in electron concentration, impurity density and undoped spacer layer thickness, and may change by factors of ~500 between 1K and 300K due to the temperature dependence of the various scattering mechanisms. However, there is also a large magneto-resistance which has not been well described.

We have measured this magnetoresistance rxx in heterojunctions with carrier densities from 0.3 to 5.5 x10¹⁵m^-2 and find it is linear in field in the magnetic quantum limit. To allow comparison of the scattering mechanisms dominant in the high and low field limits we have evaluated (analogous to the zero field mobility) at fixed fields over a wide temperature range. At 9T all the electrons in the low density samples are in the lowest Landau level and as the temperature is increased remains remarkably constant until ~50 K when, surprisingly, it increases. This is explained by elastic impurity scattering making the dominant contribution to the conductivity in high magnetic fields, since inelastic acoustic phonon scattering will be restricted by the narrow Landau levels and their large separation in energy. However, the large self broadening associated with optic phonon scattering means it is only weakly affected by magnetic fields (which leads to the small oscillatory terms of the magnetophonon effect), and so above ~150 K this becomes dominant, leading to a large reduction in the magnetoresistance, as in zero field.

In contrast to previous calculations of high field magnetoresistance, which have been restricted to low temperatures or the magnetophonon resonance region, these calculations are valid at high field over a full range of temperatures. We assume a triangular potential well, Fermi-Dirac statistics, take the Landau levels to be Lorenzians with self consistently calculated broadening and include scattering from impurities, acoustic and optic phonons. The results reproduce the experimental results extremely well and indeed show a linear magnetoresistance and increasing with temperature due to impurity scattering.

Proc. High Magnetic Fields in Semiconductor Physics, Chiba, Japan,
Physica B 184, 197 (1992)