Damage-Free Photo-Assisted Cryogenic Etching of GaN as Evidenced by Reduction of Yellow Luminescence
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W. H. Hung Synchrotron Radiation Research Center, Hsinchu, 300, Taiwan, R.O.C.
Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, 10.6 (1999) ABSTRACT Damage-free etching of GaN by Cl2 , assisted by an ArF (193 nrm) excimer laser, is demonstrated. At low temperatures, photo-assisted etching can provide a better etch rate and largely improve the surface morphology and quality. AFM results show that the etched GaN surface is obtained with a root-mean-square roughness of 1.7 nm. As compared with the photoluminescence spectra of photoelectrochemical wet etched GaN, the photo-assisted cryogenic etching is proved to be a damage-free dry etching technique. INTRODUCTION GaN and related substrates have good thermal stability and excellent chemical inertness because of their strong bond strengths. This has made it difficult to develop controlled etch processes to successfully realize 111-nitride based devices. Most of the previous work has been directed toward mesa formation in UV/blue/green laser diodes, where etch depths are relatively large (2-4 em) and the final surface morphology is relatively unimportant. Most attention is paid to a smooth vertical facet. On the other hand, the etching requirements for high power/high temperature electronics are quite different. With shallower etch depths, the fabrication of these devices is on retaining smooth surface morphologies and obtaining high etch selectivity for one material to another. In the past, chemically assisted ion beam etching (CAIBE)', reactive ion etching (RIE) 2 and inductively coupled plasma (ICP) 3 etching techniques have been used to etch GaN and related compounds. Despite high etch rates and good anisotropy etching have been obtained from these techniques, it is difficult to produce a perfect facet and damage-free sidewall and bottom surface due to the bombardment of energetic ions. Based on the above discussions, photo-assisted etching appears to be an alternative method to resolve these problems. Photo-induced etching is initiated as a result of direct absorption of photons by adsorbate molecules on the semiconductor substrate. Subsequently, the photostimulated chemical reaction occurs to form volatile products, which may desorb from the surface and are exhausted out by the pumping system to complete the etching process. Desorption of product species plays a key role in determining the etching characteristics. By cryogenic cooling of the substrate during photo-assisted etching, thermal reaction and desorption are suppressed. It provides a route for an anisotropic etching process with the damage-free surface. The advantage of etching at low temperatures also includes the fact that the sticking coefficient of C12 on the substrate is increased. In previous work, Shih et al. 4 presented a technique for cryogenic chlorine etching of GaAs. Leonard and Bedair5 also obtained successful realization of photoassisted etching of GaN in HC1 G 10.6 Mat. Res. Soc. Symp. Proc. Vol. 537 © 1999 Materials Research Society
by using a 193 nm ArF excimer laser. I
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