Rachel S. Goldman

Professor

rsgold@umich.edu

2094 H.H. Dow Building

T: (734) 647-6821

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Structure-Property Correlations: Impurities in Semiconductors

Collaborators: V. Rotberg, NERS and P. Bahattacharya, EECS at UM
Sponsor: National Science Foundation DMR-9975701, National Renewable Energy Laboratory ACQ-1-31619, Air Force of Scientific Research F49620-00-0328, Office of Naval Research N000-021-0899
In many semiconductors, the introduction of impurities at dilute concentrations leads to dramatic changes in the electronic, optical, and magnetic properties. For example, the introduction of a few percent nitrogen into GaAs leads to a band gap reduction of hundreds of meV. Furthermore, the incorporation of a few percent manganese into GaAs enables a combination of semiconducting and ferromagnetic behavior. In dilute GaAsN alloys, the effects of N incorporation mechanisms on the GaAsN bandgap and electron mobility have apparently not been considered. Our studies reveal that substitutional N incorporation leads to the most significant band gap lowering, while simultaneously leading to the highest electron mobilities reported to date. These advances are due in part to our identification of a forbidden-window of growth for GaAsN. We are in the process of developing an understanding of the relative effects of N clusters and interstitials on the electronic states using a combination of transport studies of modulation-doped heterostructures in conjunction with calculations by a colleague in Ireland. Our long-term goals include tailoring heterostructures for high-performance heterojunction bipolar transistors and high-efficiency multi-junction (tandem) solar cells. In the dilute magnetic semiconductor, GaMnAs, we recently reported the Mn-concentration dependence of various point defect concentrations, revealing a substantial concentration of arsenic anti-sites and vacancies, which likely contribute to compensation of free carriers. We are in the process of expanding these studies in order to develop an atomic-level understanding of carrier compensation and its interplay with magnetism in dilute magnetic semiconductors.
Highlights (Click an image for more information)
  • Point Defects and Ferromagnetism in GaMnAs Alloys

    For many compound semiconductors, the introduction of impurities at dilute concentrations leads to dramatic changes in the electronic, optical, and magnetic properties. For example, the incorporation of a few percent manganese into GaAs enables a combination of semiconducting and ferromagnetic behavior. The resulting dilute magnetic semiconductors are promising for several applications including spin-electronics and spin-optoelectronics. In both cases, the nanometer-scale details of impurity incorporation are critical to understanding and controlling the observed properties. Thus, we are exploring the details of the incorporation of Mn into GaAs films and InAs QDs, using cross-sectional scanning tunneling microscopy (XSTM), in conjunction with scanning tunneling spectroscopy (STS). In GaMnAs alloys, we have identified several point defects, including substitutional Mn, arsenic antisites, and arsenic vacancies, as shown in the figure.  Our quantitative studies of the concentrations of these defects as a function of Mn composition suggest that As antisite and vacancy defects contribute to free carrier compensation in this system. In addition, our pair correlation analyses suggest that Mn is mainly randomly distributed in GaMnAs, for a variety of compositions. We are currently endeavoring to quantify the effects of the point defects and clusters on the ferromagnetic properties of GaMnAs and related alloys.

  • Mechanisms of Nitrogen Incorporation in GaAsN Alloys

    We have investigated nitrogen incorporation mechanisms in dilute nitride GaAsN alloys grown by plasma-assisted molecular-beam epitaxy. A comparison of nuclear reaction analysis and Rutherford backscattering spectrometry in channeling and nonchanneling conditions reveals significant composition-dependent incorporation of N into nonsubstitutional sites, presumably as either N–N or N-As split interstitials.