Hybrid Quantum Oscillations in a Surface Space-Charge Layer

Abstract

We report the observation of quantum oscillations in a surface space-charge layer for the crossed-field geometry. In this geometry the magnetic field H is applied parallel to the spacecharge layer, in contrast to the usual configuration in which both electric and magnetic fields are normal to the layer. For the usual configuration, electric and magnetic quantization is decoupled, leading to complete quantization of the two-dimensional electron gas. 1 For the crossedfield geometry, electric and magnetic quantization is mixed, giving hybrid subbands whose dispersion depends on the relative strengths of electric and magnetic fields. 2 The number of observable oscillations in our experiment corresponds to the number of electric subbands occupied at H = 0; thus a space-charge layer with multiply filled subbands is best suited for studying the effect.The accumulation layer on low-concentration ntype InAs is such a multiply filled system. 3 ' 4 Because of the small (isotropic) effective mass (ra* = 0.024m e ) 5 the space-charge layer is -100 A thick in the usual density range (~10 12 cm" 2 ). In relatively modest magnetic fields (^10 T), the cyclotron radius achieves a comparable value, so that a perturbation approach 6 is not expected to be applicable.Effects of a parallel magnetic field on a spacecharge layer have been observed in other experi-5 T" Morita, . ^See D. Weaire and B. Kramer, J. Non-Cryst. Solids 35/36, 9 (1980), and references therein.ments. In a fixed, high-density accumulation layer on rc-type InAs, Tsui 7 used a tunneling technique to observe the decrease of the binding energy of the (single) bound state with increasing magnetic field. In this high-density case (i.e., strong electric field) perturbation theory 6 predicted the observed H 2 dependence. Beinvogl, Kamgar, and Koch 8 observed the (plasma-shifted) intersubband resonant frequency to increase with magnetic field for an accumulation layer on (100) Si. Their result, expected qualitatively from perturbation theory, 6 was analyzed quantitatively by Ando. 2 ' 9 Although the effect of H on the occupied groundstate subband can be treated as a small perturbation, the excited (unoccupied) subbands are severely distorted by the magnetic field. In PbTe this strong-field condition is easily realized for all subbands, and recently 10 inversion-layer cyclotron resonance was shown to behave three dimensionally in a parallel magnetic field. The present experiment is capable of describing features of the hybrid subbands not directly accessible by these other methods.Our sample is a (100) epitaxial layer of InAs {n ~2 xlO 15 cm"" 3 ) overlaid with a Si0 2 insulating layer and a gate electrode. Further details maybe found elsewhere. 3 ' 4 We monitor the low-temperature (4.2 K) transconductance do/dV G of the induced accumulation layer as a function of V G and H. (a is the channel conductivity and V G is Thejtransconductance of an accumulation layer on n-type InAs is studied in a magnetic field H parallel to the layer. Structures are observed, nonperiodic in l/#...

Magnetic-field-induced indirect gap in a modulation-doped quantum well

“…At higher fields, as the magnetic confinement dominates (a^ < d), each subband develops some of the characteristics of a Landau level: The energy tends asymptotically towards a linear shift with field, and there is an increase in the effective mass, and hence density of states. This type of behavior has been observed in quantum wells and accumulation layers, in transport measurements [2,3], in intersubband magnetoabsorption [4,5], and in photoluminescence (PL) [6].…”