Pt Schottky contacts on Ga- and N-face surfaces of free-standing GaN
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Pt Schottky contacts on Ga- and N-face surfaces of free-standing GaN U. Karrer, C.R. Miskys, O. Ambacher, and M. Stutzmann Walter Schottky Institut, Technische Universität München, Am Coulombwall, 85748 Garching, Germany
ABSTRACT Thick GaN films, grown by hydride vapor phase epitaxy (HVPE), were separated from their sapphire substrate with a laser-induced lift-off process. After cleaning and polishing, these films offer the most direct way to investigate and compare the influence of crystal polarity on the electronic properties of Ga-face and N-face surfaces, respectively. Different barrier heights for Pt Schottky diodes evaporated onto Ga- and N-face GaN are determined from the dependence of the effective barrier height versus ideality factor by I-V measurements to 1.15 eV and 0.80 eV, respectively. The charge neutrality condition at the surface is modified by the spontaneous polarization due to the polarization induced bound sheet charge. This effect has to be included in the electronegativity concept of metal induced gap states (MIGS) and can also be illustrated by different band bending of the conduction and valence band, inferred from the self-consistent solution of the Schrödinger-Poisson equation. Furthermore, temperature dependent I-V characteristics are compared to simulated behavior of Schottky diodes, exhibiting excellent agreement in forward direction, but showing deviations in the reverse current.
INTRODUCTION A key property of wurtzite group III nitrides is their large spontaneous and piezoelectric polarization1,2, which strongly influences the performance of electronic and optoelectronic devices3. In relaxed GaN, the total polarization is dominated by the spontaneous polarization, whose orientation is determined by the crystal polarity. It has been shown for GaN epilayers grown by plasma-induced molecular-beam epitaxy (PIMBE) on c-Al2O3 substrates that different Schottky barrier heights are obtained for Pt/GaN Schottky diodes with different polarity4. The explanation of this finding is based on the presence of polarization induced bound sheet charge that has to be considered in the condition of charge neutrality at the metal-semiconductor interface. In this contribution, a comparison is made between Pt Schottky contacts deposited on the Nface and Ga-face of the same free-standing GaN sample. Schottky barrier heights are determined for both polarities with current voltage measurements. The observed differences in the electronic properties of the diodes prepared on samples with opposite polarities are discussed in the context of the MIGS-model. On temperature dependent I-V measurements, deviations between the simulated and experimentally determined reverse currents are observed and explained by the influence of threading dislocations5.
EXPERIMENTAL E6.8.1
An alternative for single crystal GaN substrates are thick bulk-like GaN layers grown by HVPE on sapphire substrates. The samples used in this study have a thickness of 110 µm. These films still suffer from stress and macroscopic curvature due to
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