Synchrotron radiation x-ray topography of crystallographic defects in GaN

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Sakari Sintonen Name of the doctoral dissertation Synchrotron radiation x-ray topography of crystallographic defects in GaN Publisher School of Electrical Engineering Unit Department of Microand Nanosciences Series Aalto University publication series DOCTORAL DISSERTATIONS 187/2014 Field of research Nanotechnology Manuscript submitted 15 August 2014 Date of the defence 12 December 2014 Permission to publish granted (date) 22 October 2014 Language English Monograph Article dissertation (summary + original articles) Abstract In this thesis, the crystal structures of bulk, homoepitaxial and heteroepitaxial GaN were characterized by synchrotron radiation x-ray topography (SR-XRT), x-ray diffraction (XRD) and defect selective etching (DSE).In this thesis, the crystal structures of bulk, homoepitaxial and heteroepitaxial GaN were characterized by synchrotron radiation x-ray topography (SR-XRT), x-ray diffraction (XRD) and defect selective etching (DSE). The SR-XRT image contrast of threading screw dislocations and threading mixed dislocations in GaN were determined. The images caused by the strain fields of threading screw dislocations and threading screw dislocation clusters were simulated, and the simulated and experimental topograph images of low defect density ammonothermal GaN were in excellent agreement. Topograph images corresponding to strain fields of large dislocations with Burgers vector magnitudes equal to multiples of the elemental screw dislocation Burgers vector were observed. DSE experiments were performed to determine whether these large defect images originate from single dislocation strain fields, i.e. super screw dislocations or micropipes, or from combined strain fields of several elemental dislocations in close proximity. DSE images revealed that all observations but one were caused by groups of adjacent elemental threading dislocations. The determined topograph image contrasts of threading mixed and threading screw dislocations were utilized in large area studies of bulk GaN grown by the ammonothermal method. In one of the studies, threading mixed dislocations were observed to form arrays consisting of mixed dislocations with identical Burgers vectors. The minute lattice tilt and twist caused by the dislocation arrays were calculated based on dislocation spacing and confirmed with high resolution XRD measurements. SR-XRT analysis of grain boundaries and basal plane dislocations in ammonothermal GaN were also discussed. The structural quality of homoepitaxial GaN and heteroepitaxial GaN on patterned substrates was characterized by SR-XRT and XRD. The GaN layers were grown by metallo-organic vapour phase epitaxy (MOVPE) on ammonothermal GaN substrates, patterned sapphire substrates and substrates consisting of patterned MOVPE grown GaN layers on sapphire. SR-XRT is a technique especially suited for imaging the defect structure of materials with low crystalline defect density, whereas XRD is applicable to characterization of materials with higher defect density as well. SR-XRT measurements enabled imaging of individual dislocations and identification of different dislocation types.