Plasma-Induced Damage and Passivation of GaN in Electron Cyclotron Resonance Excited N 2 Plasma Source
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Plasma-Induced Damage and Passivation of GaN in Electron Cyclotron Resonance Excited N2 Plasma Source J. T. Hsieh, O. Breitschädel, M. Rittner, L. W. Fu and H. Schweizer 4. Physikalisches Institut, Universtät Stuttgart, D-70550, Stuttgart, Germany ABSTRACT Electron cyclotron resonance (ECR) excited N2 plasma exposure were performed to explore a possibility of a robust surface passivation on Ar+ ion beam etched GaN. We adopted Taguchi orthogonal design to optimize the ECR operating parameters with respect to reverse breakdown voltage (VB) and then used the results to determine the VB dependence on the parameters close to the optimum operating point. Rather consistent results were obtained by Taguchi and normal experimental methods. It is clearly indicated that the radio frequency (rf) chuck power play the most important role on the plasma passivation. After N2 plasma treatment, the diode reverse breakdown voltage is drastically restored from 15V to 95V. With the aid of N2 plasma exposure, the Schottky characteristics of ion beam etched GaN can be thus effectively improved.
INTRODUCTION With the commercialization of GaN-based laser diodes, increasing interest has been turned to fabrication of high power/high temperature electronic devices based on GaN and related alloys due to their attractive material properties. These properties included a band gap of 3.39 eV, a breakdown field of ~5×106 V/cm1,2, and a saturation drift velocity of 2.7×106 cm/s. The utility of GaN-based transistors has already been demonstrated through the realization of epitaxial metal semiconductor field effect transistors (MESFETs)3,4. Dry etching has proven an effective technique in the gate recessing for GaN MESFETs5. Despite this, it was found that defects can be induced on GaN during dry etching and this etchinduced damage has to be minimized in order to enhance the device performance. Several authors have found that the plasma etching of GaN will lead to a near-surface nitrogen deficiency and a higher n-type surface doping level6-7. Furthermore, Schottky contacts fabricated on reactively ion etched n-GaN showed severe degradation8. X. A. Cao et al.9 also studied inductively coupled plasma damage in GaN Schottky diodes and found that under all conditions there was a strong reduction in diode reverse breakdown voltage (VB) and an increase in forward and reverse currents. Either annealing at 750 oC under N2, or removal of ~500-600 A of the surface essentially restored the initial Current-Voltage (I-V) characteristics. To my best knowledge, however, none is reported on using plasma passivation as a damage removal technique after dry etching of GaN. In this work the passivation effects after exposing dry etched GaN surfaces to ECR excited N2 plasma were studied. I-V and VB characteristics in GaN Schottky diodes fabricated before and after N2 plasma treatment are examined. The VB dependence on ECR plasma operating parameters has been determined.
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EXPERIMENTAL The layer structure consisted of 1.5 µm of unintentionally-doped (n~1017 cm-3)
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