Determination of AlGaN/GaN HFET Electric Fields using Electroreflectance
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L10.4.1
Determination of AlGaN/GaN HFET Electric Fields using Electroreflectance S. R. Kurtz, A. A. Allerman, D. D. Koleske, A. G. Baca, and R. D. Briggs Sandia National Laboratories Albuquerque, NM 87185-0601, U.S.A. Abstract A contacted electroreflectance technique was used to characterize the electronic properties of AlGaN/GaN heterostructure field-effect transistors (HFETs). By studying variations in the electroreflectance with applied electric field, spectral features associated with the AlGaN barrier, the 2-dimensional electron gas at the interface, and bulk GaN were observed. Barrier-layer composition and electric field were determined from the AlGaN FranzKeldysh oscillations. Comparing HFETs grown on SiC and sapphire substrates, the measured AlGaN polarization electric field (0.25±0.05 MV/cm) approached that predicted by a standard model (0.33 MV/cm) for the higher mobility HFET grown on SiC.
Introduction Large piezoelectric and spontaneous polarization fields occurring in AlGaN/GaN heterostructures can generate a 2-dimensional electron gas (2DEG) without doping. The unusually large 2DEG carrier densities (>1x1013/cm2), electron saturation velocities, breakdown fields, and thermal conductivities found in these structures make them attractive as heterostructure-field-effect-transistors (HFETs) for high voltage, high power microwave amplifiers.[1-5] In this work, we use a contacted electroreflectance measurement technique to identify distinct spectral features associated with the 2DEG and the AlGaN barrier. We show that electroreflectance augments conventional electrical characterization of GaN-based HFETs by providing a direct measurement of the electric field in the AlGaN barrier. Experiment AlGaN/GaN samples were grown by metal-organic chemical vapor deposition (MOCVD). An AlN nucleation layer was used to initiate GaN growth on sapphire or SiC substrates. Thickness of the top AlGaN barrier, the 2DEG sheet concentration, and the 2DEG depletion or pinch-off voltage were determined from Hall mobility and capacitance measurements for each sample. Gated van der Pauw contacts (1-4 mm2) were used for electroreflectance. Ohmic Ti/Al/Ni/Au source and drain contacts at corners of the sample penetrated to contact the 2DEG. Oxidized Ni(75 Å)/Au(75 Å) formed an optically transparent, Schottky-barrier gate contact. Electroreflectance measurements were performed with a 150W Xe lamp. Typical gate voltage modulation amplitudes were 0.2 Vpp @ 100 Hz, with grounded source and drain.
L10.4.2
Results and Discussion An electroreflectance spectrum for an AlGaN/GaN HFET, grown on sapphire, is shown in Fig. 1. As reported previously for thicker InGaN [6] and AlGaN [7] layers, Franz-Keldysh oscillations (FKOs) from the AlGaN barrier, polarization-induced electric field were clearly observed. The period of the FKO increased as the barrier electric field increased with negative gate bias, enabling unambiguous identification of those features associated with the AlGaN barrier.[8] In addition, a broad 2DEG feature appeared at energ
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