Study Of GaN Epitaxial Growth On Si-based Micro Structures

GaN is one of the most important direct-gap semiconductors with 3.4eV midgap width, and widely used in optoelectronic,high power,and high frequency devices.Since it is difficult to grow bulk GaN crystal,heteroepitaxial growth is mainly employed to obtain GaN thin film.Silicon has been drawing more and more attention as the substrate for GaN epitexial growth,due to its low cost and the potential of integrating optoelectronic GaN devices with Si electronic devices.Unfortunately,the large lattice constant(～17%)and thermal expention coefficient(～60%)mismatch between GaN and Si will lead to high dislocation density(about 10^9-10cm^-2)and huge in-plane thermal stress(0.8GPa),even cracks,in GaN epitaxial layers.To reduce the thermal stress in Si-based GaN layers,this paper fabricated several Si-based micro-structures with less thickness,and then studied the GaN epitaxiai growth on these structures.The major results are summerized as follows:1.The thermal stress distribution of GaN/SOI structure was simulated,and it was found that reduction of the substrate thickness would release part of the stress in GaN layers.The idea of using ICP(Inductively coupled Plasma)to back etch the SOI wafer was implemented to form isolated Si membranes for GaN epitaxial growth.The GaN layers of 250μm×250μm with thickness of 1μm was obtained crack-free on the suspended parts of the membranes,indicating that the thinner substrates could help to decrease the thermal stress in GaN layers.2.To further reduce the constraint from substrate,SBM(Surface/Bulk Micromachining)process was used to fabricate 5μm-thick cantilevers on Si(111)substrate. GaN layers with thickness of 1μm obtained on these cantilevers were crack-free,showing the Si cantilevers could help to relax part of the thermal stress by the bending of their free ends.3.The top Si layer of SIMOX-SOI wafer was patterned to isolated rectangle Si islands for ELOG(Epitaxial Lateral Over Growth)of GaN,and the coalescence of GaN on neighboring Si islands was realized.It was found that both dislocation density and stress was reduced in ELOG region.Besides,the SOI substrate could also work as compliant substrate to release the lattice constant and thermal expention coefficient mismatch,by deforming the lattice of top Si.To conclude,by micro-machining the substrates,the thermal stress of GaN layers were reduced,and their crystal qualities were improved.It also offered another method of integrating GaN with Si-based micro structures for advanced micro transducers.