spie-laser polishing GaN

and 2 tm thick, respectively.
*Email: jan.j.dubowskinrc.ca
442 Laser Applications in Microelectronic and Optoelectronic Manufacturing VI, Malcolm C. Gower, Henry Helvajian, Koji Sugioka, Jan J. Dubowski, Editors, Proceedings of SPIE Vol. 4274 (2001) © 2001 SPIE · 0277-786X/01/$15.00
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2. EXPERIMENTAL DETAILS
The GaN layers were grown on (0001) sapphire substrates in a custom built ammonia-molecular beam epitaxy system
[5]. The initial buffer/nucleation layer of A1N was grown by magnetron sputter epitaxy. The A1N and GaN layers were 20 nm
Samples were irradiated at normal incidence with 10 ns pulses from a XeCl excimer laser (X 308 nm) using an experimental setup described elsewhere [6]. The irradiation was carried out with laser pulse fluences between 99 - 231 mJ/cm2. The experiment was carried out in an air environment. The diameter of laser etched craters was about 375 tm. Following laser irradiation, the samples were etched in a 50% solution of HC1/H20 to remove excessive Ga formed at the irradiated surface. The ablation rate was determined from direct measurements of the depth of laser-produced craters by
Due to its attractive properties, gallium nitride (GaN) has received in recent years a steadily growing attention by fundamental and applied research communities. The interest is primarily driven by the potential this material has for the development of blue-emitting laser diodes and high-frequency, high-power devices. The rapid expansion of this material into optoelectronic application is, however, significantly hindered by the lack of large size lattice matched substrates. Sapphire (Al203) remains to be the most popular substrate applied for the hetero-epitaxial growth of this material. The growth has also been investigated on other substrates, such as SiC, Si, GaAs, MgO, ZnO [1]. A significant lattice mismatch between GaN and sapphire complicates the growth of a high quality (low-defect) material. It contributes to the increased surface roughness of GaN films and may result in, for instance, a reduced capability of high-definition patterning of such material. Consequently, a method improving smoothness of hetero-epitaxially grown GaN through the post-growth treatment has a potential practical application. Some recent investigations have indicated that such a process is feasible through the laser-induced thermal decomposition of the GaN surface. It has been demonstrated that the irradiation with several pulses of a KrF [2,3] and a F2 [4] lasers could lead to a significant planarization of GaN films grown on sapphire. The maximum effect, a reduction from the initial roughness of 0s = 1 0 nm to 2 nm, was observed for the 7-pulse KrF laser irradiation at the fluence of F 1 J/cm2 per pulse (i; = 34 ns) [3]. It is well known however that the laser interaction with solid surfaces at such high laser fluence can easily induce significant damage to the processed material. Clearly, the attractiveness of the laser-induced planarization process of GaN will depend on the ability to carry it out without inducing significant damage, or to confine the laser-induced damage to as small volume as possible. One possible route to achieve that would be by the application of a shorter wavelength laser, such as F2 (157 nm), as has been reported recently [4]. Short-pulse ablation, with a laser operating in the
Laser polishing of GaN
J.J. Dubowski*, S. Moisa, B. Komorowski, H. Tang and J.B. Webb Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada
ABSTRACT
The ablative decomposition of GaN films induced with a XeC1 excimer laser (? = 308 nm) was investigated for a potentially low-damage surface planarization process. Samples, 2-tm-thick, were grown on (0001) sapphire by ammonia molecular beam epitaxy. They had a characteristic micro-hillock type surface morphology with a roughness, averaged over 5 x 5 tm2 area, typically, of 13 nm. Following the laser irradiation, this roughness could be reduced to 3.6 urn. The results indicate that the ablation process follows the Lambert-Beer's law, with an absorption coefficient of 3 x iO cm'. The experiment was carried out with relatively short pulses ('r = 10 ns), which appear to be responsible for the observed onset of the laser-induced decomposition of GaN and surface planarization at significantly smaller laser fluences than reported in the literature. The ability to carry out decomposition of GaN with low laser fluences is of practical importance for achieving a low-damage GaN planarization process and/or intentional delaminating of this material from the sapphire substrate by the back side irradiation technique.