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2D Transport Measurements in Undoped Electron Systems

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If you have a question about this talk, please contact Dr Kaveh Delfanazari.

The MBE growth of ultra-high-quality GaAs/AlGaAs epilayer structures has enabled the discovery of many novel physical phenomena – for example, the fractional quantum Hall effect (FQHE) in two- dimensional electron systems (2DESs)[1]. In order to attain the highest electron mobilities, it is necessary to understand and limit the dominant sources of disorder during the wafer growth. These include both intentional (i.e. doping) and unintentional charged impurities, interface roughness, and surface defect states, all of which cause electron scattering. The electron mobility in a modulation-doped 2DES is dominated by doping impurities. By contrast, the electron mobility in an undoped 2DES is dominated by unintentional background impurities and interface roughness. By measurement and numerical modelling of the mobility versus density characteristics of an undoped wafer, it is possible to determine the concentration of background impurities and the characteristic length and height scales of the interface roughness[2,3]. In this talk I will present experimental data of electron mobility in an undoped heterostructure at 0, 45 and 90 to the major flat of the wafer, and how numerical modelling of the electron mobility can be used to explain the dependence of mobility on angle to the major flat. I will also discuss how assumptions about the shape of the electron wave function4 in the potential well formed by the heterostructure may limit the accuracy of the mobility modelling, and how the model can be extended using numerical solutions for the wave function.

Reference: [1] D. C. Tsui, H. L. Stormer, and A. C. Gossard, “ Two- dimensional magnetotransport in the extreme quantum limit,” Phys. Rev. Lett., vol. 48, pp. 1559–1562, may 1982. [2] A. F. Croxall, B. Zheng, F. Sfigakis, K. Das Gupta, I. Farrer, C. A. Nicoll, H. E. Beere, and D. A. Ritchie, “Demonstration and characterization of an ambipolar high mobility transistor in an undoped GaAs/AlGaAs quantum well,” Appl. Phys. Lett., vol. 102, no. 8, p. 082105, 2013. [3] W. Y. Mak, K. Das Gupta, H. E. Beere, I. Farrer, F. Sfigakis, and D. A. Ritchie, “Distinguishing impurity concentrations in GaAs and AlGaAs using very shallow undoped heterostructures,” Appl. Phys. Lett., vol. 97, no. 24, p. 242107, 2010. [4] F. F. Fang and W. E. Howard, “Negative field-effect mobility on (100) Si surfaces,” Phys. Rev. Lett., vol. 16, pp. 797–799, may 1966.

This talk is part of the Semiconductor Physics Group Seminars series.

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