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The effect of modulation doping by Be on the ferromagnetic properties of Ga 1-x Mn x As is investigated in Ga 1-x Mn x As/Ga 1-y Al y As heterojunctions and quantum wells. Introducing Be acceptors into the Ga 1-y Al y As barriers leads to an increase of the Curie temperature T C of Ga 1-x Mn x As, from 70 K in undoped structures to over 100 K with the modulation doping. This increase is qualitatively consistent with a multi-band mean field theory simulation of carriermediated ferromagnetism. An important feature is that the increase of T C occurs only in those structures where the modulation doping is introduced after the deposition of the magnetic layer, but not when the Be-doped layer is grown first. This behavior is expected from the strong sensitivity of Mn interstitial formation to the value of the Fermi energy during growth.
We have measured the cosmic ray spectrum above 10 17.2 eV using the two air fluorescence detectors of the High Resolution Fly's Eye observatory operating in monocular mode. We describe the detector, photo-tube and atmospheric calibrations, as well as the analysis techniques for the two detectors. We fit the spectrum to a model consisting of galactic and extra-galactic sources.
For modern semiconductor heterostructures containing multiple populations of distinct carrier species, conventional Hall and resistivity data acquired at a single magnetic field provide far less information than measurements as a function of magnetic field. However, the extraction of reliable and accurate carrier densities and mobilities from the field-dependent data can present a number of difficult challenges, which were never fully overcome by earlier methods, such as the multicarrier fit, the mobility-spectrum analysis of Beck and Anderson, and the hybrid mixed-conduction analysis. More recently, to overcome the limitations of those methods, several research groups have contributed to development of the quantitative mobility-spectrum analysis (QMSA), which is now available as a commercial product. The algorithm is analogous to a fast Fourier transform in that it transforms from the magneticfield (B) domain to the mobility (µ) domain. The QMSA converts the fielddependent Hall and resistivity data into a visually meaningful transformed output, comprising the conductivity density of electrons and holes in the mobility domain. In this article, we apply QMSA to both synthetic and real experimental data that are representative of modern multilayer HgCdTe structures. We discuss such features as the accuracy of the extraction of individual layer conductivities and average mobilities, reconstruction of the carrier mobility distribution within a particular layer, the resolution of two carriers with similar mobilities, and limits of the sensitivity.
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