Properties,Polishing And Epitaxy Of Bulk AlN

Bulk AIN possesses a wide direct bandgap of about 6.2 eV,a high thermal conductivity,and lattice and thermal parameters that are closely matched with those of all-Al-content AlxGa1-xN alloys.Therefore,bulk AIN substrates with low dislocation densities are required to fabricate high-performance AlGaN-based deep UV optoelectronic devices and high-power electronic devices.However,due to its unavailability in industry-relevant sizes at reasonable costs,the study on growth of AlGaN using AlN substrates is still in its infancy.Physical Vapor Transport(PVT)is a promising method for fabricating large-scale and high-quality AIN single crystals.In this paper,the material properties,chemical mechanical polishing(CMP)and epitaxy of bulk AIN have been studied.The main contents and results are shown as flows:1)The defects,optical and structural properties of bulk AIN crystal prepared by PVT method have been investigated.The results show that the bulk AIN used in our experiment possesses a very high quality,which means it can be a promising substrate for the epitaxy of device.2)The CMP process of bulk AIN crystal has been studied in detail.Due to the small size and irregular shape of AIN single-crystal,a sapphire-aided method has been used to improve thickness uniformity of the AIN substrate after CMP process.High-quality surface with root-mean-square(RMS)roughness of 0.267nm and thickness deviation of under 1 μm was obtained after CMP.3)AlGaN epi-layers with Al composition of 18.2%have been grown on CMP-AlN substrates by metal-organic chemical vapor deposition(MOCVD).The tensile stress in nominally compressed AlGaN layer and the formation mechanism of surface cracks are explained by strain gradients model.