Strain and crystalline defects in epitaxial GaN layersstudied by high-resolution X-ray diffractionRosa ChierchiaA dissertation submitted in partial satisfaction of therequirements of the degree of Doktor der Naturwissenschaften- Dr. rer. nat. -am Fachbereich 1der Universit¨ at BremenBremen, 20072ContentsIntroduction v1 Introduction to GaN 11.1 Crystalline structure ............................ 11.2 Optical properties.......... 31.3 Growth methods 31.3.1 HVPE.................................. 41.3.2 MBE.... 41.3.3 MOVPE.. 41.4 Substrates................................... 61.4.1 Sapphire . . 71.4.2 6H-SiC... 71.4.3 Si..................................... 81.4.4 Nucleationlayer ........ 81.4.5 Mosaicmodel.......... 91.5 Strain and stress.................... 101.5.1 Biaxialstres 11.5.2 Hydrostaticstrain........ 121.5.3 Hydrostatic+biaxialstress...................... 121.5.4 Uniaxialstres ......... 131.5.5 Influence of strain on bandgap energy........ 131.6 Crystal Defects ............................... 131.6.1 Pointdefects .......... 131.6.2 Linedefects 141.6.3 Planardefects.................... 151.7 Lateral overgrowth ......... 151.7.1 LEO............... 16iii CONTENTS1.7.2 Pendeoepitaxy............................. 161.8 Alloys.................. 171.8.1 AlGaN... 172 Experimental setup 192.1 Basicelementsofx-raydiffraction....................... 192.1.1 Bragg’s law ........... 192.2 Instrumentation.. 202.2.1 Crystalarangement........