Interplay between quantum well width and interface roughness for electron transport mobility in GaAs quantum wells

Abstract

We report transport mobility measurements for clean, two-dimensional (2D) electron systems confined to GaAs quantum wells (QWs), grown via molecular beam epitaxy, in two families of structures, a standard, symmetrically-doped GaAs set of QWs with Al0.32Ga0.68As barriers, and one with additional AlAs cladding surrounding the QWs. Our results indicate that the mobility in narrow QWs with no cladding is consistent with existing theoretical calculations where interface roughness effects are softened by the penetration of the electron wave function into the adjacent low barriers. In contrast, data from AlAs-clad wells show a number of samples where the 2D electron mobility is severely limited by interface roughness. These measurements across three orders of magnitude in mobility provide a road map of reachable mobilities in the growth of GaAs structures of different electron densities, well widths, and barrier heights.Two-dimensional electron systems (2DESs) in engineered quantum structures have proven to be a fruitful tool for discovering new fundamental physical phenomena, including the integer (IQHE) and the fractional quantum Hall effect (FQHE) [1,2]. Enabled by the progress in molecular beam epitaxy (MBE) technology [3], the GaAs system has become the benchmark for the highest material quality and has paved the way to very long carrier mean-free paths and high mobilities [4]. The present state of the MBE art allows precise control of a number of growth parameters, such as substrate temperature, rate, material composition, and growth interrupts, which critically affect scattering mechanisms and carrier mobilities [5][6][7][8][9]. Generally, quantum wells (QWs) doped from both sides have to be narrower than triangular modulation-doped single-sided GaAlAs/GaAs QWs to avoid second subband occupation at high carrier concentrations [10]. In such QWs, small variations of the QW width have a profound effect on the energy eigenvalues. Thus interface roughness takes on additional importance for MBE growth of high density, high mobility 2DESs. Despite the immense progress in MBE techniques, however, GaAs structures where interface roughness dominates the carrier scattering have not been fully delineated systematically [11][12][13][14][15]. More specifically, the emergence of significant interface scattering has been demonstrated only in isolated cases in samples with sufficiently narrow QWs [11] and for 2D systems with very low density [13]. The lack of systematic reports of carrier mobilities is surprising in view of the important role of interface roughness at high wave function confinement and its effect on carrier mobility. In QWs with AlAs cladding, where the carrier wave function is expected to have very little penetration in the barrier, the mobility was experimentally shown in a landmark paper by Sakaki et al.[11] to go as µ ∝ W 6 . This agrees well with the concept that interface scattering is the major mobility-limiting factor [11][12][13][14][15][16]. On the other hand, in QWs with no AlAs cladding, where wave f...

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