Spin Effects in quasi-1D Systems of Narrow Gap Semiconductors
Final Report Abstract
Fundamental research in spin electronics focuses on using the spin degree of freedom of charge carriers to develop new paradigms for electronic devices and circuits. There is great interest in exploiting the spin-orbit interaction (SOI) to use electric fields to manipulate spin states. The main property of interest is inversion asymmetry, particularly structural inversion asymmetry (the Rashba effect). There are interesting and unique electrical properties of quantum point contacts (QPCs) and quantum wires (QWRs) in the presence of SOI that make them very attractive. In particular, recent reports provide evidence that a QPC made from a semiconductor with high intrinsic SOI can generate a completely spin-polarized current, observed as a shift in the 1D conductance plateaus when the lateral confinement is made highly asymmetric. The quantum Hall effect (QHE) is one of the most prominent phenomena in modern solid state physics and is characterized by the quantization of the Hall resistance in integral multiples of h/e and by the formation of edge channels in two-dimensional electron gases (2DEGs). There has been no discussion of using the quasi-1D edge channels to isolate and manipulate spins in quasi-2D systems, and little work has been carried out on the effects of SOI on the edge channels in systems with large SOI. Our research involved 2DEGs in a high-quality InGaAs/InAlAs quantum well (QW) with an inserted InAs layer, provided by the group of Prof. Hansen, Universität Hamburg, and focused on two areas: 1) quantum Hall edge channels in Hall-bars, and 2) lithographically fabricated nanoscale quasi-1D quantum wires. We developed fabrication methods and expertise to produce high-quality InAs-based Hall-bars with back-gate and semi-transparent top-gates and to combine them with QPCs and QWRs with in-plane gates and semi-transparent top-gates. We determined quantum transport properties in the temperature range 0.3-4.2 K and beyond, including measurements to determine g-factors. In collaboration with the group of Prof. McCombe, University at Buffalo (UB), THz magnetospectroscopy was performed on Hall-bars and in particular the photoresponse (PR) was investigated. The magnetotransport measurements of Hall-bars show that we successfully fabricated structures with micro-laser photolithography. At low temperatures and high magnetic fields the 2DEGs are in the QH regime and the back-gate allows control of the electron mobility (n 5.5 6.0 10 cm ) without affecting the mobility (μ 2.1 10 cm2 /Vs). Although a direct observation of SOI-induced effects was hindered on behalf of the the low carrier density we estimate an upper limit to the Rashba parameter of αR 10 eVcm using the broadening of Landau levels in the SdH oscillations. Similar heterostructures had been investigated previously, but therein also, direct SOI-induced effects could only be observed with particular back- and top-gate configurations. Joint magnetotransport at HUB and THz investigation at UB were carried out on these Hall-bars. Modelling of the PR curves yields several parameters of the 2DEG: electron density, cyclotron effective mass, g-factor, SdH scattering time, cyclotron resonance scattering time. Electron density, mobility and g-factor are consistent with the magnetotransport. The most important result is the g-factor: combining the coincidence method with the modelling of the PR curves, it is possible to estimate the g-factor components parallel (about 6.4) and normal (about 20) to the 2DEG. The conclusion is that the g-factor is larger in magnitude and more anisotropic than theoretical estimates based on single-particle band calculations. Additional theoretical work shows that the anisotropy is in agreement with wavefunction penetration into the (anisotropic) QW barrier material, enhanced by manybody effects. Our results are consistent with previous reports, where only the perpendicular component of the g-factor was determined and found to agree with wavefunction penetration into the barriers. Quasi-1D channels were successfully fabricated with in-plane gates: the in-plane gates are electrically insulating and applying an increasingly negative voltage constricts the 2DEG until “pinch-off”. Features consistent with quasi-1D conductance quantization are observed at low temperatures and measurements were performed with regards to symmetric and asymmetric in-plane gate voltages. We find no observable shift in height of conductance plateaus, indicating that asymmetric transverse electric fields do not give rise to substantial spin-polarized transport in our experiments. Clear indications for quasi-one dimensional spin-polarized transport, instead, can be successfully studied in the quantum Hall regime. QPCs in the QH regime are currently under investigation at HUB, to explore manipulation of Quantum Hall edge channels with a QPC in a SOI-material. For filling factor 2, we see clear signs of transmission of spinresolved electrons through the QPC. Our results are promising for the study of the influence of SOI in lower dimensions on InAs-based structures (large SOI) and will be compared to GaAs-based structures (small SOI).
Publications
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Electrical and THz magnetospectroscopy studies of laser-patterned micro- and nanostructures on InAs-based heterostructures
Chiatti, Olivio / Buchholz, Sven S. / Heyn, Christian / Hansen, Wolfgang / Pakhmer, Mehdi / McCombe, Bruce D. / Fischer, Saskia F.
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Characterization of high mobility InAs/InGaAs/InAlAs composite channels by THz magnetophotoresponse spectroscopy. International Journal of High Speed Electronics and Systems, Vol. 24, No. 01n02, 1520004 (2015)
Pakmehr, Mehdi / McCombe, Bruce D. / Chiatti, Olivio / Fischer, Saskia F. / Heyn, Christian / Hansen, Wolfgang
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THz magneto-photoresponse spectroscopy of two-dimensional electrons in an InAs/InGaAs/InAlAs inserted-channel. Journal of Infrared, Millimeter, and Terahertz Waves, March 2015, Volume 36, Issue 3, pp 291–297
Pakmehr, Mehdi / Khaetskii, Alexander / McCombe, Bruce D. / Chiatti, Olivio / Fischer, Saskia F. / Heyn, Christian / Hansen, Wolfgang
