Vertical Transport Properties of GaN Schottky Diodes Grown by Molecular Beam Epitaxy
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ABSTRACT Lateral and vertical electron transport parameters were investigated in lightly doped n-GaN films, grown by MBE. Diodes were fabricated by forming Schottky barriers on n--GaN films using a mesa-etched vertical geometry. Doping concentrations and barrier heights were determined, from C-V measurements, to be 8-9x1016 cm-3 and 0.95-1.0 eV respectively. Reverse saturation current densities were measured to be in the 1-10x10-9 A/cm2 range. Using the diffusion theory of Schottky barriers, vertical mobility values were determined to be 950 cm2/V-s. Lateral mobility in films grown under similar conditions was determined by Hall effect measurements to be 150-200 cm2/V-s. The significant increase in mobility for vertical transport is attributed to reduction in electron scattering by charged dislocations.
INTRODUCTION Lateral transport in lightly doped n-GaN thin films has been extensively studied, most often by using Hall effect measurements. These studies indicate that electron transport in the lateral direction is dominated by scattering from charged dislocations, which serves to reduce the electron mobility.[1-2] Based on this model, transport in the vertical direction should be relatively unaffected by the presence of the threading dislocations and electron mobility should be significantly increased. Since a large number of GaN devices, including lasers, LEDs, detectors and bipolar junction transistors [3-5], have vertical geometry, it is important to investigate electron mobility in this direction to better understand and engineer device performance. In this paper we have used Schottky barrier diodes, fabricated with a vertical geometry on lightly doped n-GaN films grown by ECR-MBE, to investigate electron mobility for vertical transport. The diodes were evaluated by I-V and C-V measurements to determine diode ideality, reverse saturation current, doping concentration and barrier height. Using the diffusion model for current transport in Schottky barriers, mobility for vertical transport was determined. The results are compared with lateral electron mobility determined by Hall Effect measurements.
EXPERIMENTAL METHODS The GaN films were grown on c-plane sapphire, by plasma-assisted molecular beam epitaxy, following procedures described earlier.[5] In this paper we present only a brief description. Prior to introduction into the growth chamber, the substrates were subjected to solvent degreasing. The Al2O3 surface was converted to AlN by exposing the substrate, held at 800 °C, to nitrogen plasma using an electron cyclotron resonance (ECR) microwave plasma source. This step was followed by the deposition of 1000 Å
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AlN buffer. Two types of devices (Figure 1) were investigated. In the first, 2.25 µm thick n+ GaN (2x1018 cm-3) was deposited first, followed by a n- GaN (1x1017 cm-3) of about the same thickness.The second device consists of 4 µm thick n- GaN (1x1017 cm-3). All the layers were grown under Ga rich conditions. RHEED patterns, taken during growth of the n-GaN layers, showed 2x2 surface recon
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