Suppression of electron injection into a finite superlattice in an applied magnetic field

Abstract

We present experimental and theoretical studies of the current-voltage characteristics, $I(V),$ of undoped $\mathrm{GaAs}/({\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x})\mathrm{As}$ superlattices (SL's) in a strong magnetic field $\mathbf{B}$ applied parallel to the growth axis. A series of $I(V)$ characteristics measured at $B=1,2,\dots{},10 \mathrm{T}$ shows that increasing the magnetic field gradually suppresses the current across the whole range of V. We show that at low V this suppression originates from a decrease in the rate of injection of carriers into the SL from the heavily doped emitter contact. Because the chemical potential in the emitter contact lies below the lowest miniband of the SL, electrons enter the SL by tunneling through a triangular potential barrier formed by the miniband edge. The tunneling rate depends on V and on the electron energy for longitudinal motion along the SL axis. The latter is reduced (by at least $\ensuremath{\Elzxh}{\ensuremath{\omega}}_{c}/2,$ where ${\ensuremath{\omega}}_{c}$ is the cyclotron frequency) in the magnetic field. Consequently, the tunneling rate decreases with increasing $\mathbf{B}.$ This mechanism of suppression dominates at low voltages $(Vl40 \mathrm{mV})$ when the barrier transmission coefficient is low.