Rapid Thermal Processing of Silicon Solar Cells - Passivation and Diffusion INAUGURAL-DISSERTATION zur Erlangung des Doktorgrades der Fakultät für Mathematik und Physik der Albert-Ludwigs-Universität Freiburg i. Brsg. vorgelegt von Ji Youn Lee aus Wonju, Süd-Korea 2003 Fraunhofer Institut für Solare Energiesysteme (ISE) Freiburg i. Brsg. Dekan: Prof. Dr. Rolf Schneider Leiter der Arbeit: Prof. Dr. Wolfram Wettling Referent: Prof. Dr. Wolfram Wettling Korreferent: Prof. Dr. Matthias Weidemüller Tag der Verkündung des Prüfungsergebnisses: 10. Oktober 2003 To my grandmother for faith my parents for setting me on the path toward my dream and encouragement my elder sister for her love, patience and sacrifice my other sister and brothers for their love and support Contents 1 Introduction 1 2 Device physics of silicon solar cells 3 2.1 The p-n junction model 3 2.1.1 Basic equations 3 2.1.2 The p-n junction at equilibrium 4 2.1.3 The p-n junction at non-equilibrium 6 2.1.4 The p-n junction under illumination 8 2.2 Solar cell characteristics 9 2.2.1 Solar cell parameters 9 2.2.2 Illuminated current-voltage (I-V) characteristics 10 2.2.3 Dark current-voltage (I-V) characteristics 11 3 Recombination mechanisms 13 3.1 General theory 13 3.2 Radiative recombination 13 3.3 Auger recombination 15 3.3.1 Traditional Auger recombination 15 3.3.2 Coulombic enhanced Auger recombination 15 3.4 Shockley-Read-Hall recombination 17 3.4.1 Shockley-Read-Hall recombination in bulk 17 3.4.2 Shockley-Read-Hall recombination at surfaces 19 3.4.3 Reducing the surface recombination rate 20 3.5 The effective lifetime 21 4 Rapid thermal processing 25 4.1 Fundamental properties of radiant heating systems 25 4.2 Characteristics of RTP 26 4.2.1 Fundamental difference between CTP and RTP 26 4.2.2 Role of photoeffects in RTP 27 4.2.3 Emissivity of Radiation 28 4.3 Structure of the RTP used in this work 29 4.4 Temperature measurements 31 4.4.1 Thermocouple 31 4.4.2 Pyrometer 31 4.4.3 Calibration 32 5 Czochralski (Cz) silicon-specific defects during RTP 35 5.1 Czochralski silicon 35 5.1.1 Chemical refining of silicon 35 5.1.2 Czochralski crystal growth 36 5.2 Cz-Si specific metastable defects 38 ii contents 5.2.1 Influence of Boron and interstitial oxygen 39 5.2.2 Influence of process steps 42 5.3 Influence of high-temperature steps on the carrier lifetime in Cz silicon 44 5.3.1 Selection of a barrier layer to avoid external contamination 44 5.3.2 Optimization of process parameters 46 5.3.3 Effect of plateau temperature on the stable effective lifetime 50 5.4 Optimization of rapid thermal firing (RTF) for screen printed contacts 55 5.5 Influence of two subsequent high-temperature steps 57 5.6 Effect of improving processes on the RTP solar cells 58 5.6.1 Solar cell structure and process sequence 58 5.6.2 Solar cells results and analysis 60 5.7 Conclusions 64 6 Optimal passivation for solar cells 65 6.1 Thermally grown silicon dioxide 65 6.1.1 Growth mechanism 65 6.1.2 Silicon thermal oxidation model 66 6.1.3 Fundamental properties of Si-SiO2 68 6.1.4 Oxidation process 74 6.2 Silicon nitride by PECVD 78 6.2.1 Growth mechanism 79 6.2.2 Fundamental properties of silicon nitride 80 6.2.3 Plasma enhanced chemical vapor deposition (PECVD) 83 6.3 SiO2/SiNx stacks 87 6.4 Passivation of solar cells 89 6.4.1 Passivation of the rear surface 89 6.4.2 Passivation and reflection of the front surface 90 6.5 Solar cells 93 6.5.1 Cell Design and fabrication 93 6.5.2 Results 94 6.5.3 Analyses and discussions 95 6.6 Conclusion 105 7 Rapid thermal diffusion with spin-on dopants 107 7.1 Basic diffusion theory 107 7.1.1 Models of diffusion 107 7.1.2 Diffusion equations 108 7.1.3 Intrinsic Diffusion 109 7.1.4 Extrinsic Diffusion 112 7.2 Back surface field 115 7.3 Spin-on technique for diffusion 116 7.4 Characteristics of rapid thermal diffusion 118 7.4.1 Experiment procedures 118 7.4.2 Phosphorus diffusion 119 Contents iii 7.4.3 Boron diffusion 121 7.5 Solar cells 124 7.5.1 Cell design and fabrication 124 7.5.2 Results 125 7.5.3 Analyses and discussions 126 7.6 Conclusion 131 8 Summary 133 Appendix A Physical constants and properties 137 Appendix B Methods for measurement 143 B.1 Microwave-detected photo conductance decay (MW-PCD) 143 B.2 Carrier density imaging (CDI) 146 B.3 Profile Measurements 147 Bibliography 151 Publication 159 Acknowlegements 161 Curriculum Vitae 165 iv contents 1 Introduction Today the demand for energy is increasing steadily and rapidly. However, the conventional sources of energy such as fossil and nuclear fuels are limited. Moreover, the associated negative effects on the environment such as the green house effect, the hole in the ozone layer, acid rain and smog can no longer be neglected. Therefore, the development of renewable energy is necessary. The conversion of sunlight into electricity using photovoltaics (PV) is a very attractive way to produce renewable energy. Photovoltaic systems are environment friendly, use abundant material (especially in the case of silicon solar cells), require no fuel, can be used everywhere, are reliable, almost maintenance-free, flexible in scale from miliwatt to megawatt and aesthetically pleasing. In spite of these various advantages, PV competes with the conventional energy since the production cost of PV electricity is still higher compared to the traditional methods. Therefore, the aim of most of the research is to increase the competitiveness of photovoltaics. The cost of industrial solar cell modules is at present around 3.5 Euro/Wp. This cost can be reduced to near or below 1 Euro/Wp, which is the present target price [1] by • High volume of production (≥ 500 MWp/year) • Reduced material costs • High throughput processing • Improved efficiencies In this thesis, to contribute to the achievement of these goals, cost effective Czochralski (Cz) crystalline silicon materials and process technologies such as rapid thermal processing, plasma- enhanced chemical vapor deposition (PECVD) and spin-on dopants are investigated. Cz silicon materials represent 40 % in the world-wide solar cell production and are three times cheaper than FZ wafers. Also, Cz silicon materials can be treated at high temperatures and can yield high efficiencies well above 20 % [2]. Rapid thermal processing (RTP) uses tungsten-halogen lamps in the range of ultraviolet and infrared wavelengths as heating sources. RTP is a promising technique to replace the classical thermal process which only uses infrared radiation. RTP can achieve high throughputs due to short annealing times of a few seconds and high ramping rates of over 100 °C/s, and opens the possibility of simultaneous diffusion of both sides of a silicon wafer. In addition, the low thermal budget, and the low power consumption are some of the attractive properties of RTP. Surface passivation is crucial for high-efficiency solar cells. Silicon nitride (SiNx) deposited by PECVD is a very attractive technique to fabricate passivation and antireflection layers. The deposition at low temperatures, the high deposition rate, the very good passivation quality and the adjustable refractive index are the reasons, why SiNx layers are increasingly used for solar cells. 2 1 Introduction Using spin-on dopants, allows to adjust easily the doping concentration and junction depth of the emitter. Spin-on dopants also give the possibility of simultaneous diffusion. Furthermore, spin-on dopants used in conjunction with RTP only require low thermal budgets. This thesis is organized in the following way: In chapter 2 the device physics of solar cells is presented. From the fundamental device equations, a p-n junction cell is described. From this simple model the ideal form of the dark and illuminated characteristics of silicon solar cells are obtained. In chapter 3 the basic recombination mechanisms in bulk and at the silicon surface are described. The photoconductance decay method used in this thesis to measure the effective lifetime is presented. The method for measuring the injection level dependence of the effective lifetime is discussed. In chapter 4 rapid thermal processing is introduced. The general properties and mechanisms of RTP are compared to those of the classical conventional furnace. The different methods for the temperature measurement are presented and the problems associated with such measurements in RTP are discussed in detail. In chapter 5 the fundamentals of the Cz-metastable defects are introduced. Various models and theories for the Cz-metastable defect are also discussed. The effects of RTP-process parameters on the stable lifetime after degradation are systematically investigated using the ‘design of experiment (DOE)’ method. The influence of two sequent high temperature processes for diffusion and for oxidation in the fabrication of solar cells on the stable lifetime is investigated. Finally, the optimized and non-optimized processes are applied to the fabrication of solar cells. In chapter 6 the fundamental properties of silicon oxides (SiO2) fabricated using classical thermal oxidation and rapid thermal oxidation and of silicon nitrides (SiNx) using PECVD are discussed. The surface passivation qualities of a single layer and a double layer SiO2/SiNx stack composed of the thermally grown SiO2 and SiNx by PECVD are investigated on pure p-type and on emitters. Furthermore, silicon solar cells passivated with such SiO2/SiNx stacks are also analyzed.