||Yu Zhang, Qian Sun, Chris Yerino, Benjamin Leung, Qinghai Song, Coung Dang, Sang-wan Ryu, Hui Cao, Arto Nurmikko, Jung Han
The importance of GaN devices in display, data storage, and lighting applications is clearly established by now. In a more forward-looking manner, one may rightly ask about new opportunities the wide bandgap III-nitride family could offer, and new dimensions one could introduce to the existing technologies. In this talk we describe an electrochemical procedure to selectively etch conducting n-type GaN layers. We also present a few proof-of-concept demonstrations of device structures facilitated by this simple method of etching. Wet etching of Ga-polar GaN at room temperature can only be done through photoelectrochemical (PEC) etching method where photoexcited holes as minority carriers mediate an oxidation-etching mechanism. This procedure has been incorporated into a selective etching technique with a combination of epitaxial heterostructures (InGaN/GaN) and bandgap selective photoexcitation. Recently, a few groups have pursued the wet-etching of “GaN-compatible” layers, layers that have sufficiently different chemical compositions and are more amenable to wet-etching. Two of such GaN-compatible layers are AlInN and CrN, and there have been already interesting demonstrations. Our work in based on a recent observation that a heavily doped (N<SUB>D</SUB>>2x10<SUP>18</SUP>cm<SUP>-3</SUP>) GaN can be selectively etched in an oxalic-acid electrochemical etching. Distinct advantages of using n-GaN as the “sacrificial layer” for etching include (1) complete lattice matching to conventional GaN structures with negligible degradation in microstructure or morphology, (2) complete freedom in thickness design toward 1D, 2D, and 3D photonic-electronic-mechanical structures, and (3) the possibility of tuning the etching pathways at microscale through conductivity, bias, and solution, thus offering a new tool set to etch, to undercut, and to “texture” GaN. Figure 1(a) and (b) show GaN microdisk array and preliminary result of mode pattern of a microdisk (10 μm diameter) under optical pumping, Figure 1(c) shows the formation of GaN microdisk where the lower cladding is nanoporous GaN in stead of air, thus greatly improving the structural robustness. The effect of optical confinement can be seen from the contrast of annular rings under Nomarski (Figure 1(d)), figure 1(e) shows another example of lateral undercut etch in forming distributed Bragg reflector (DBR) structures. It is worth noting that under reduced bias or doping level, nanoporous GaN is produced as a new class of GaN material with tunable index of refraction for photonic structure. Figure 1(f) illustrates the use of nanoporous GaN for form a three-period DBR. Figure 2 show (a) a GaN cantilever prepared by the EC etching and (b) Spectral response of a GaN cantilever compared with a reference spectrum. The reference was obtained at un-etched wafer surface. Figure 3 shows the nanocrystal GaN obtained by sonicating nanoporous GaN in DI water.