Amorphous Silicon Carbide for Photovoltaic Applications Dissertation zur Erlangung des akademischen Grades Doktor der Naturwissenschaften (Dr. rer. nat.) an der Universität Konstanz Fakultät für Physik vorgelegt von Stefan Janz geb. in Leoben/Stmk. Fraunhofer Institut für Solare Energiesysteme Freiburg 2006 Referenten: Prof. Dr. Gerhard Willeke Prof. Dr. Elke Scheer Tag der mündl. Prüfung: 8. Dezember 2006 „Der wahre Zweck des Menschen ist die höchste und proportionierlichste Bildung seiner Kräfte zu einem Ganzen.“ Wilhelm v. Humboldt Amorphous Silicon Carbide 5 AMORPHOUS SILICON CARBIDE FOR PHOTOVOLTAIC APPLICATIONS ......... 1 1 AMORPHOUS SILICON CARBIDE ............................................................................. 17 1.1 INTRODUCTION......................................................................................................................17 1.2 AMORPHOUS STRUCTURE OF SIC..........................................................................................18 1.2.1 The tetrahedral network ............................................................................................... 18 1.2.2 Hydrogenated SiC......................................................................................................... 20 1.2.3 Vibrational spectroscopy (Infrared spectra) ................................................................ 21 1.2.4 Silicon and carbon content ........................................................................................... 22 1.3 PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION (PECVD).........................................23 1.3.1 Basics of plasma physics .............................................................................................. 24 1.3.2 Methods of plasma excitation ....................................................................................... 25 1.3.3 Optical Emission Spectroscopy (OES) ......................................................................... 31 1.4 SIC LAYER TYPES – DEPOSITION REGIMES...........................................................................33 1.4.1 SiC layer deposition...................................................................................................... 33 1.4.2 SixC1-x layer deposition ................................................................................................. 35 2 ETCHING AND MECHANICAL BEHAVIOUR ......................................................... 39 2.1 ETCHING SIC.........................................................................................................................39 2.1.1 Plasma etching of SiC................................................................................................... 39 2.1.2 Plasma etching experiments ......................................................................................... 40 2.1.3 Wet chemical etching of SiC......................................................................................... 42 2.2 STRESS BEHAVIOUR ..............................................................................................................43 2.2.1 Stress measurements..................................................................................................... 43 2.2.2 Stress development with deposition temperature ......................................................... 44 2.2.3 Stress development with annealing temperature .......................................................... 45 2.3 MECHANICAL STABILITY ......................................................................................................46 3 OPTICAL BEHAVIOUR................................................................................................. 49 3.1 INTRODUCTION......................................................................................................................49 3.2 REFRACTIVE INDEX...............................................................................................................50 3.2.1 Experiments and results................................................................................................ 51 3.3 BAND GAP TUNING ...............................................................................................................53 3.3.1 Optical transmission measurements............................................................................. 53 3.3.2 Experiments and results................................................................................................ 55 3.3.3 Photothermal deflection spectroscopy (PDS)............................................................... 57 6 Introduction 3.3.4 Experimental setup and results..................................................................................... 59 4 ELECTRICAL BEHAVIOUR......................................................................................... 63 4.1 ELECTRICAL CONDUCTIVITY OF AMORPHOUS SEMICONDUCTORS ........................................63 4.2 DOPING .................................................................................................................................65 4.3 HIGH TEMPERATURE DEPOSITION.........................................................................................67 4.4 INFLUENCE OF ANNEALING ...................................................................................................70 4.5 ACTIVATION ENERGY............................................................................................................71 4.6 TEMPERATURE DEPENDENT CONDUCTIVITY MEASUREMENTS..............................................75 4.6.1 Experimental setup ....................................................................................................... 75 4.6.2 Metal contact-semiconductor junction (Al-SiC)........................................................... 75 4.6.3 Results........................................................................................................................... 77 4.7 CONDUCTIVITY OF THE RECRYSTALLISED WAFER EQUIVALENT (REXWE)..........................80 4.7.1 First principle measurements ....................................................................................... 80 4.7.2 Advanced experimental setup: the “contows” ............................................................. 81 4.7.3 Measurements of the “contows” .................................................................................. 82 5 ANNEALING OF SIC LAYERS (HIGH-TEMPERATURE BEHAVIOUR) ............ 85 5.1 ANNEALING CONDITIONS......................................................................................................85 5.1.1 Temperature regions..................................................................................................... 86 5.1.2 Different atmospheres................................................................................................... 86 5.1.3 Process time.................................................................................................................. 86 5.2 RE-ORGANISATION AND CRYSTALLISATION..........................................................................87 5.2.1 Layer density and refractive index ............................................................................... 87 5.2.2 Network re-organisation (FTIR measurements)........................................................... 89 5.2.3 X-ray diffraction (XRD) measurements ........................................................................ 92 5.2.4 Raman measurements ................................................................................................... 95 5.3 APPEARANCE OF CRACKS......................................................................................................98 5.3.1 Internal stress ............................................................................................................... 98 5.3.2 Unequal thermal expansion coefficients....................................................................... 99 5.3.3 Influence of substrate’s surface roughness................................................................. 100 5.3.4 Blistering coming from hydrogen ............................................................................... 102 5.3.5 Contamination on substrate and in SiC layer............................................................. 103 6 DIFFUSION IN SIC........................................................................................................ 107 6.1 BASICS OF DIFFUSION .........................................................................................................107 Amorphous Silicon Carbide 7 6.1.1 Macroscopic access.................................................................................................... 107 6.1.2 Diffusion constant....................................................................................................... 110 6.1.3 Segregation coefficient ............................................................................................... 111 6.2 DIFFUSION BARRIER PERFORMANCE ...................................................................................111 6.2.1 Experimental setup ..................................................................................................... 112 6.2.2 Simulations ................................................................................................................. 116 6.2.3 Results......................................................................................................................... 117 6.3 OUT-DIFFUSION