Direct observation of a Γ−X energy spectrum transition in narrow AlAs quantum wells

Abstract

Spectra of magnetoplasma excitations have been investigated in a two-dimensional electron systems in AlAs quantum wells (QWs) of different widths. The magnetoplasma spectrum have been found to change profoundly when the quantum well width became thinner than 5.5 nm, indicating a drastic change in the conduction electron energy spectrum. The transformation can be interpreted in terms of transition from the in-plane strongly anisotropic Xx − Xy valley occupation to the outof-plane isotropic Xz valley in the QW plane. Strong enhancement of the cyclotron effective mass over the band value in narrow AlAs QWs is reported.

Mobility and quantum mobility of modern GaAs/AlGaAs heterostructures

“…In our SPSL-doping layers, the much lighter effective mass in GaAs produces a much larger size quantization energy than in AlAs, raising the Γ-minimum in GaAs above the X-minima in AlAs. 41,42 Thus, all donated electrons which are not transferred to the 2DEG (excess electrons) are stored in the AlAs side wells. Each excess electron pairs with a donor in a compact dipole atom, so that their low-temperature parallel-to-2DEG conductance is negligible.…”

Achieving balance of valley occupancy in narrow AlAs quantum wells

“…The corresponding anisotropic, elliptical Fermi contours are characterized by a heavy longitudinal mass m l = 1.1m 0 and a light transverse mass m tr = 0.2m 0 , where m 0 is the electron mass in a vacuum. The valley occupancy in AlAs quantum wells (QWs) can be tuned via uniaxial strain [12] or confinement [13][14][15]19].…”

“…The interplay between these two factors establishes the actual distribution of electrons between the valleys. In the quantum wells of width W > 5 nm, the contribution from the biaxial strain is dominant and the in-plane X x−y valleys are occupied, whereas electrons confined in a narrow (W < 5 nm) AlAs QW occupy a single out-of-plane X z conduction-band valley [13,14,19].…”

“…Contacts to the 2DES were made by soldering indium to the Hall bar contact pads. The magnetoplasma excitations were detected using a double lock-in technique [19,22]. We measured the microwave-induced part of longitudinal resistance ∆R xx , which appears due to resonant microwave absorption.…”

Piezoplasmonics: Strain-induced tunability of plasmon resonance in AlAs quantum wells

“…The collective plasma excitations in the 2DES with anisotropic energy spectrum possess a number of unique properties. In particular, the discovery of a gap in the spectrum of plasma excitations in perfectly symmetric circular samples [16][17][18]. It was shown that inter-valley energy splitting significantly influences the plasmon spectrum through redistribution of charge carriers between X x and X y valleys.…”