Two‐dimensional electrons occupying multiple valleys in AlAs

Shayegan, M.; Poortere, E. P. De; Gunawan, Oki; Shkolnikov, Y. P.; Tutuc, Emanuel; Vakili, K.

doi:10.1002/pssb.200642212

Cited by 128 publications

(216 citation statements)

References 64 publications

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“…This improvement results from removing the narrow GaAs QW as a scattering mechanism and an intervening barrier. We note that for all AlAs-clad samples with W < 5.66 nm, we see bilayer AlAs-like characteristics, namely a weak ν = 1 state and absence of minima at odd fillings [20,23]. The magnetoresistance data from the lower density set of samples (not shown), on the other hand, lack any unusual behavior because of the deep penetration of the wave function into the low barriers and the effective larger quantum well width.…”

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confidence: 82%

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

Kamburov

Baldwin

West

et al. 2016

Applied Physics Letters

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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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confidence: 82%

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

Kamburov

Baldwin

West

et al. 2016

Applied Physics Letters

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“…Evidence for a QW width crossover from double-to single-valley occupation has been shown for (001)AlAs wells 8,9 , as has evidence for single valley occupancy in wide (110)AlAs wells 5 . Dynamic control of the valley degeneracy has been realized with uniaxially strained (001)AlAs QWs to induce valley degeneracy splitting 1,10,11 . Such studies can quantify interaction effects, calibrating valley strain susceptibility and valley effective mass 10,[12][13][14] .…”

Section: Introductionmentioning

confidence: 99%

Valley degeneracy in biaxially strained aluminum arsenide quantum wells

et al. 2011

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This paper describes a complete analytical formalism for calculating electron subband energy and degeneracy in strained multi-valley quantum wells grown along any orientation with explicit results for AlAs quantum wells. In analogy to the spin index, the valley degree of freedom is justified as a pseudospin index due to the vanishing intervalley exchange integral. A standardized coordinate transformation matrix is defined to transform between the conventional-cubic-cell basis and the quantum well transport basis whereby effective mass tensors, valley vectors, strain matrices, anisotropic strain ratios, piezoelectric fields, and scattering vectors are all defined in their respective bases. The specific cases of (001) (411)-oriented QWs and we define and solve for a shear-to-biaxial strain ratio. The notation is generalized to address non-Miller-indexed planes so that miscut substrates can also be treated, and the treatment can be adapted to other multi-valley biaxially strained systems. To help classify anisotropic intervalley scattering, a valley scattering primitive unit cell is defined in momentum space which allows one to distinguish purely in-plane momentum scattering events from those that require an out-of-plane momentum component.

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“…To understand the result of the calculations, we first concentrate on the influence of strain in AlAs/GaAs/InAs heterostructures. In contrast to GaAs QWs, strain modifies the electronic bandstructure significantly in AlAs QWs [12]. Furthermore, strain is the important parameter for the growth of InAs nanostructures.…”

Section: Calculationsmentioning

confidence: 99%

“…All energies in the plot are normalized to the F , E while the z-axis has its origin at the cleavage plane. From the plot it is apparent that the X-valley splits into a doublet X xy and a singlet X z [11,12] due to the pseudomorphically strained AlAs bulk layer. Close to the cleavage plane the strain condition changes due to the presence of the highly strained InAs and InAlAs, and the valley splitting increases.…”

Section: Calculationsmentioning

confidence: 99%

“…1a). As AlAs is an indirect semiconductor with the lowest conduction band minium at the X-point, the GaAs spacer needs to be replaced by an Al 0.45 Ga 0.55 As barrier to keep the electrons in the AlAs layer [11,12]. Reference [6] points out that the AlAs layer thickness is usually below 50 nm, in order to obtain an aligned chain of QDs, and below 15 nm in order to obtain QWR.…”

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Investigation of a contacting scheme for self‐assembled cleaved edge overgrown InAs nanowires and quantum dot arrays

Fehr

Uccelli

Dasgupta

et al. 2009

Physica Status Solidi (a)

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A contacting scheme to measure the transport properties into self‐assembled InAs Quantum Wires (QWRs) or Quantum Dots (QDs) is presented. The nanostructures are formed on the (110) cleaved edge of a AlAs/AlGaAs heterostructure substrate by means of the Cleaved Edge Overgrowth (CEO) technique and Molecular Beam Epitaxy (MBE). The InAs nanostructure grows directly on top of the AlAs layer, which hosts a two dimensional electron gas (2DEG). In a transistor‐like schematic of the device, the 2DEG acts as a contact to the InAs nanostructure. A top gate is used to deplete the 2DEG, thereby defining the InAs nanostructure as a channel between source and drain. Measurements confirm that the device can be operated as a field‐effect transistor, but no evidence of a current flow through the InAs QWRs can be found. Numerical calculations of the electron density and the device band structure confirm that a depletion zone is present in the AlAs layer close to the cleaved edge and the InAs QWR seems electrically isolated from the AlAs 2DEG leads. Possible solutions could be an additional Schottky gate contact on the CEO side or selective doping inside the CEO barrier. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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Two‐dimensional electrons occupying multiple valleys in AlAs

Cited by 128 publications

References 64 publications

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

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

Valley degeneracy in biaxially strained aluminum arsenide quantum wells

Investigation of a contacting scheme for self‐assembled cleaved edge overgrown InAs nanowires and quantum dot arrays

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