The Effect of Hydrogen-Based, High Density Plasma Etching on the Electronic Properties of Gallium Nitride
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ABSTRACT Development of devices based on the wide gap semiconductor gallium nitride (GaN) requires the realization of reliable, high fidelity, low damage pattern transfer processes. In this work, GaN thin films grown by OMVPE have been subjected to both chlorine- and methane/hydrogen-based etch chemistries in an electron cyclotron resonance microwave plasma reactive ion etching system. Both n-type and semi-insulating thin films have been utilized to examine the effect of these etch processes on the electronic properties of the materials. The methane/hydrogen-based etch system (CH 4 /H2 /Ar) induced considerable changes in the electrical properties of both n-type and semi-insulating films, causing the former to become more insulating and the latter to become conducting. In both cases, the original electrical properties were recoverable after a short, high temperature anneal. In the chlorine-based etching system (C1 2), no changes in the electrical properties were observed and etch rates five times greater than in the methane/hydrogen-based system were achieved. Proposed mechanisms responsible for the observed behavior will be discussed. These results show that pattern transfer processes based in chlorine etch chemistries are more suitable for the generation of high performance GaN devices. INTRODUCTION The wide band-gap, compound semiconductor gallium nitride (GaN) continues to increase in importance for the development of high temperature electronic and blue/green optoelectronic device structures. Consistent with this trend, the ability to pattern GaN using dry etching techniques has been an area of increasing research[ 1-7]. As with most semiconductor processing techniques, the objective is to achieve the desired result (here, the delineation of small areas of semiconducting thin film) with minimal modification to the properties of the material, namely, the electronic properties. In dry etch processing, the electronic properties of the remaining thin film structures can be affected in several ways: 1) the direct ion bombardment of the surface can cause structural defects to form near the surface of the semiconductor, 2) reactive specie from the plasma can penetrate into the remaining structure and alter the electronic properties through passivation of carriers, and 3) the creation of recombination centers at the surface as a result of unpassivated semiconductor bonds. Carrier passivation, particularly when hydrogen is present in the process, has been shown to be a significant problem for III-V materials[8]. Several studies have shown that at elevated temperatures, hydrogen exposure can result in significant passivation of carriers in GaN to depths of up to a micrometer[9,10]. Previous works have examined GaN etching in high density discharges using chlorine- [1-3,5,7], methane/hydrogen-based [1-4], and mixtures of these chemistries[6]. These works have shown that both discharges are effective in performing anisotropic pattern transfer on GaN even down to nanometer-scale feature sizes, but none of these works evalu
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