High Magnetic Field Studies of AlGaN/GaN Heterostructures Grown on Bulk GaN, SiC, and Sapphire Substrates
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HIGH MAGNETIC FIELD STUDIES OF AlGaN/GaN HETEROSTRUCTURES GROWN ON BULK GaN, SiC, AND SAPPHIRE SUBSTRATES W. Knap 1,7, E. Borovitskaya 1, M. S. Shur 1, R. Gaska 1,8 G.Karczewski2, B.Brandt2, D. Maude 3, E. Frayssinet4, P. Lorenzini4, N. Grandjean4, J. Massies4 J. W. Yang5, X.Hu5, G. Simin5, M. Asif Khan5, C. Skierbiszewski6, P. Prystawko6, I. Grzegory6 and S. Porowski6 1 Rensselaer Polytechnic Institute, Troy, NY 12180, USA 2 NHMFL/ Florida State University, Tallahassee, USA 3 Grenoble High Magnetic Field Laboratory, Grenoble, France 4 CNRS – CRHEA, Valbonne, France 5 University of South Carolina, Columbia, USA 6 High Pressure Research Center, UNIPRESS, Warsaw, Poland 7 GES- CNRS-Universite Montpellier2 UMR5650 Montpellier, France 8. Sensor Electronic Technology, Latham, NY 12110, USA ABSTRACT We present the results of the high magnetic field studies of properties of two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures grown over high-pressure bulk GaN, sapphire, and insulating SiC substrates. The experimental results include the low field Hall measurements, cyclotron resonance measurements, and cryogenic temperature Quantum Hall Effect studies as well as room-temperature characteristics of High Electron Mobility Transistors fabricated on all these substrates. The room temperature high field measurements allow us to clearly separate the contributions of a parasitic parallel conduction from 2DEG conduction in all investigated heterostructures. The magnetotransport measurements are performed in the magnetic fields up to 30 Tesla for temperatures between 50mK-300K. This high magnetic field in combination with very high mobilities (over 60.000 cm2/Vs) in the sample on the bulk GaN substrates allow us to observe features related both to cyclotron resonance and spin splitting. The temperature dependence of this splitting determines the spin and cyclotron resonance energy gaps and, in combination with cyclotron resonance and tilted field experiments, allows us to determine the complete energy structure of 2DEG conduction band. We also present the first experimental results showing so called “the exchange enhancement” of the energy gaps between spin Landau levels. INTRODUCTION III-V nitride semiconductors have attracted much attention due to their potential applications in high power electronics and electro-optic devices [1]. Advances in the heteroepitaxial growth of GaN have led to high quality layers, with controlled doping. These advances have opened the way to the fabrication of microelectronics devices, such as heterostructure field effect transistors (HFETs) [2]. 2DEG can be obtained in modulation doped GaN/AlGaN heterojunctions in a similar way as in the very well known GaAs/AlGaAs system. GaN that replaces GaAs is a wide band gap semiconductor with an energy gap of 3.5 eV. It has a heavier effective mass and a lower mobility of the carriers. Therefore, high magnetic fields are required in order to obtain reliable data for GaN/AlGaN heterostructures from transport and optical measurements.
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