The effect of quantum wells on electron transport across GaAs/AlGaAs graded barrier structures

Abstract

Electron transport over short n + GaAs/Al x Ga 1−x As/n + GaAs barrier structures containing doped GaAs quantum wells has been investigated. The barriers had graded interfaces at the contacts which inhibited tunnelling at low electric fields. It was found that the presence of wells pinned the Fermi level, eliminating the increase in the barrier height caused by background space charge seen in AlGaAs barriers. The wells also had the effect of magnifying any injected space charge, which meant that the electric field was non-uniform across the sample. Hence the three regions, graded interface/central region with wells/graded interface, functioned like three resistors in series and this behaviour has been modelled successfully using drift-diffusion theory. Increasing the density of wells, which allows tunnelling to occur at lower fields and higher temperatures, limits the range over which the thermionic current dominates.

Temperature and electric field dependences of the mobility of electrons in vertical transport in GaAs/Ga1−y AlyAs barrier structures containing quantum wells

“…The barrier-structure samples investigated in the present study were prepared from the same wafers as those used by Bishop et al [4]. The layer structure of the samples is shown in Fig.…”

“…, and the doping in the quantum wells was chosen to give a Fermilevel matching that of the contacts, in order to avoid band bending [4,5]. All barrier structures containing quantum wells in the central barrier were designed to have only one bound state in the well.…”

“…Barrier structures with rectangular potential barriers have been studied extensively (see [1]). Recently, Ridley and coworkers [2][3][4][5][6] investigated vertical transport in GaAs/Ga 1− Al As barrier structures with linearly graded barriers grown between the n + -GaAs contacts and the central barrier containing N (=0, 2, 4, 7 and 10) quantum wells. Bishop et al [4] measured the electron mobility as a function of temperature in the range 77-300 K by applying low voltage (0.01 V) to the samples with N =4, 7 and 10.…”

“…Nevertheless, the scattering mechanisms that determine the temperature and applied voltage dependences of the electron mobility in these barrier structures have not been investigated in detail. In this paper we present a systematic study of the effects of temperature and applied voltage on the mobility of electrons in vertical transport in barrier-structure samples, which were prepared from the same wafers as those used in [4]. The electron mobility was determined from geometric magnetoresistance (GMR) measurements carried out in the dark.…”

“…Brought to you by | MIT Libraries Authenticated Download Date | 5/9/18 11:58 AM is the magnetoresistance, and µ GMR is the GMR mobility, which is usually taken approximately equal to the Hall mobility [2,4,8,9,[22][23][24]. In order to use the GMR technique, the magnetic field must be low enough to ensure that (µ GMR B) 2 1, and the mesa diameter ( ) must be much greater than the total thickness (L) of the barrier layers [4,8,23].…”