A. Sibille

dc and microwave negative differential conductance in GaAs/AlAs superlattices

Negative differential conductance (NDC) at 300 K in n+‐nn+‐GaAs/AlAs superlattice structures biased perpendicularly to the layers is demonstrated, and shown to be strongly enhanced at microwave frequencies close to the inverse transit time of electrons. The deduced electron velocities are in fair agreement with those independently determined in undoped superlattices where NDC was inhibited by the electric field nonuniformity. From the analysis of the experimental data, we show that NDC is a bulk superlattice effect, not related to ‘‘quantum defects,’’ e.g., enlarged barriers.

High-field perpendicular conduction in GaAs/AlAs superlattices

Miniband conduction in undoped GaAs/AlAs superlattices (SLs) has been investigated through current‐voltage measurements on n+‐SL‐n+ structures. From the comparison with simulations based on an effective medium approximation for the conduction through the superlattice, we directly obtain the field dependence of the electron velocity perpendicular to the layers. Our data show strong evidence of negative differential velocity in a 35.5/20 A (well/barrier width) SL.

Coexistence of Wannier–Stark localization and negative differential velocity in superlattices

We show the coexistence of Wannier–Stark localization as monitored by photoconduction experiments, and of negative differential velocity (NDV) for electrons in a perpendicularly biased GaAs/AlAs superlattice. The critical electric field for the onset of NDV, and that for the appearance of localization as monitored optically are nearly the same, which exemplifies the fundamental link between Wannier–Stark quantization [Phys. Rev. 117, 432 (1960)], and Esaki–Tsu nonlinear transport [IBM J. Res. Dev. 14, 61 (1970)].

Perpendicular transport in superlattice bipolar transistors (SBT)

Abstract Diffusion-limited electron transport in superlattices is studied by gain measurements on heterojunction bipolar transistors with a GaAs GaAlAs superlattice base. In the case of thin barriers, Bloch conduction is observed, while hopping between localized levels prevails for large barriers. A transition occurs between these two regimes, localization being achieved when the energy broadening induced by the electron-phonon coupling added to the disorder due to imperfect growth is of the order of the miniband width. This interpretation is supported by temperature dependence measurements of the perpendicular mobilities in relation with theoretical calculations of these mobilities.

Negative differential conductance frequency resonances in X valley superlattice minibands

Electron transport along the growth axis of a series of slightly indirect GaAs/AlAs superlattices (SL) is experimentally investigated. Our results, based on current‐voltage and frequency resonance measurements in agreement with calculated characteristics, reveal unambiguously the existence of negative differential velocity at 80 K. Miniband transport in the lower Xxy miniband appears to explain this effect, owing to the low effective mass of Xxy states along the quantization axis.

Excitonic transitions in GaAsGaAlAs superlattices under a weak electric field

Abstract We report a study of the photoluminescence spectra of GaAsGa0.65Al0.35As superlattice under an electric field of about 10 kVcm−1, from 9 K up to 80 K; the ratio ϱ = I −1 I 0 of the intensities of the −1 and 0 peaks of the Wannier-Stark ladder varies with the temperature T. To fit these variations, we assume for both transitions, a trapped exciton density of states with a gaussian distribution of eigenenergies, and a two dimensional density of states for free excitons. We show that above about 50 K there is a thermal equilibrium between direct and crossed free excitons whereas at low temperatures, the equilibrium statistics are not respected: the excitons trap into the direct state rather than to the crossed one.

High field perpendicular transport in GaAs/AlAs superlattices

Abstract Perpendicular transport is investigated in undoped GaAs/AlAs superlattices (SL) as a function of temperature and electric field. Experimental current-voltage (I–V) characteristics are measured and compared to numerical simulations based on an effective medium approximation for the conduction in the SL. The validity of this procedure is confirmed by its applications to a reference GaAs sample whose transport properties are well known. We thus deduce the temperature and electric field dependence of the perpendicular electron velocity in the SL. In particular our data shows evidence of negative differential velocity (NDV) in one sample, in a large temperature range.

Negative resistance switching in superlattices: Resonant tunneling or hot electron transfer?

Abstract Negative differential resistance (NDR) has been observed at low temperature on the current-voltage (I–V) characteristics of an undoped GaAs Ga 0.7 Al 0.3 As superlattice. Abrupt switching events towards a low conduction state can also occur, and are shown to result from spontaneous oscillations in the NDR region. Under hydrostatic pressure, the I–V curves do not exhibit the behavior typical of Gunn transfer towards the X or L valleys. The quenching of resonant tunneling conduction under large electric fields in the superlattice appears the source of NDR in the samples studied.