ADVANCED ELECTRON-BEAM BASED TECHNIQUES FOR SOLAR CELL CHARACTERIZATION MENG LEI (B. Eng. (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGPAORE 2014 DECLARATION DECLARATION I hereby declare that this thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. MENG LEI 04 May 2014 i ii Advanced Electron-Beam Based Techniques for Solar Cell Characterization Acknowledgements My first and also my most sincere gratitude goes to my Ph.D. supervisors, Professors Charanjit Singh Bhatia and Jacob Phang from Department of Electrical and Computer Engineering, National University of Singapore (ECE, NUS), for their continuous guidance and support throughout my doctoral studies. Professor Bhatia is someone you will instantly love and never forget once you meet him. His mentorship has always been paramount in providing a well-rounded experience consistent with my long-term career goals. He has given me the freedom to pursue various areas that I am interested in and has been very supportive in all my Ph.D. projects. Professor Phang had always been motivating and inspiring me to take up new challenges and had made one of the biggest difference in my life. His attitude of living every moment to its fullest and his strong determination has helped me come a long way and will always guide me in future. My special thanks also go to my Ph.D. mentor, Alan Street, for always being so kind, helpful and motivating. I have always enjoyed the personal discussion with him and the time I spent with him during dry runs of my presentations. His technical inputs and friendly nature has always made me feel at ease with him. I would like to express my deep gratitude to Professor Armin Aberle, Dr. Bram Hoex and Dr. Johnson Wong from Solar Energy Research Institute of Singapore (SERIS); and Professors Aaron Danner and Yang Hyunsoo from Spin Energy Lab (SEL). The discussion and suggestions from them are always valuable to me. My special appreciation goes to Johnson for his kind help in reviewing my thesis chapters on short notices. Acknowledgements I am very much thankful to Dr. Steven Steen, Dr. Satyavolu S. Papa Rao, Dr. Ron Nunes and Dr. Harold Hovel from IBM Thomas J. Watson Research Centre for their valuable support and collaboration with Professor Bhatia (ECE, NUS) during the period of NUSIBM Joint Study Agreement # W0853529. It provided me with the unique opportunity to gain a wider breadth of research experience while I was still a graduate student. I would like to thank the ECE and SERIS for offering me the NUS Research Scholarship as well as equipment support during my Ph.D. candidature. My acknowledgement will never be complete without the special mention of my lab seniors at the Centre of Integrated Circuits Failure Analysis and Reliability (CICFAR): Dr. Xie Rongguo, Dr. Hao Yufeng, Dr. Huang Jinquan, Dr. Wong Chee Leong, Dr. Jason Teo, Dr. Zhang Huijuan, Dr. Pi Can, Dr. Wang Ziqian, Dr. Wang Rui and Dr. Ren Yi for all their personal and professional help during the initial days of my stay in the lab. I would also like to extend my sincere thanks to Mrs. Ho, Mr. Koo and Linn Linn for keeping a friendly and healthy lab atmosphere and bearing with me all these days. I am grateful to my fellow lab mates and friends: Liu Dan, Yihong, Jiayi, Wei Sun, Bai Xue, Yuya, Yunshan, Dr. York Lin, Dr. Ma Fusheng, Baochen, Mridul, Fajun, Cangming, Yang Yue for always being there and bearing with me for the good and bad times during the wonderful days of my Ph.D. life. I find myself lucky to have friends like them. Finally, I would like to acknowledge my parents, grandparents and all elders to me in my family for their constant support and strong faith in me. I cannot imagine a life without their love and care. iii iv Advanced Electron-Beam Based Techniques for Solar Cell Characterization Table of Contents DECLARATION i Acknowledgements . ii Table of Contents . iv Abstract vii List of Figures . viii List of Tables xiv List of Symbols xv Chapter Introduction and Motivation . 1.1 Photovoltaic Technology and Challenges 1.2 Current Characterization Techniques for Solar Cells 1.3 Strengths of Electron-Beam Based Techniques . 1.4 Organization of thesis . Chapter Theory and Literature Review 2.1 Introduction 2.2 Electron Beam and Sample Interaction 2.3 Secondary Electron Imaging in SEM . 2.4 Scanning Electron Acoustic Microscopy (SEAM) 10 2.4.1 Physical Principles 10 2.4.2 Applications of SEAM 14 2.5 Conventional Electron Beam Induced Current (EBIC) 19 2.5.1 Physical Principles 19 2.5.2 Applications of EBIC Imaging . 20 2.5.3 Quantitative EBIC Measurements 24 2.6 Single Contact Electron Beam Induced Current (SCEBIC) . 41 2.6.1 Physical Principles 41 2.6.2 Applications of SCEBIC . 44 Table of Contents 2.6.3 2.7 Limitations and Challenges of SCEBIC . 45 Strength and Challenges for Solar Cell Characterization . 46 Chapter Experimental Setup 48 3.1 Introduction 48 3.2 Experimental Setup (SEAM, EBIC and SCEBIC) 49 3.3 Summary 52 Chapter SEAM Imaging on SDE Multicrystalline Silicon Wafers . 53 4.1 Introduction 53 4.2 SEAM Signal Detection . 55 4.3 Sample Procedures of Saw Damage Etch (SDE) . 56 4.4 Defect Characterization of Saw-Damage-Etched Wafers 56 4.5 Optimization of SDE Duration . 60 4.6 Summary 64 Chapter 5.1 Defect Characterization of Solar Cells 65 Morphological and Electrical Defects in Multicrystalline Silicon Solar Cells 65 5.1.1 Principle of Signal Detection 65 5.1.2 Defect Characterization in Isolation Trenches 68 5.1.3 Distinguishing Morphological and Electrical Defects 70 5.2 Defect Characterization of Amorphous Silicon (a-Si:H) Thin Film Solar Cells 76 5.2.1 Device Fabrication and Performance 77 5.2.2 Defect Characterization Using LBIC Imaging and FIB Cross-Sectioning . 79 5.3 Studies of Photon Emission at Defects in Multicrystalline Silicon Solar Cells . 90 5.4 Summary 93 Chapter SCEBIC Imaging on Solar Cells 95 6.1 Introduction 95 6.2 SPICE Model of SCEBIC 96 6.2.1 SCEBIC Transient Phenomenon . 97 v vi Advanced Electron-Beam Based Techniques for Solar Cell Characterization 6.2.2 Factors of SCEBIC Transient Signals . 99 6.3 Experimental Verification of SCEBIC Model . 101 6.4 SCEBIC Imaging on Multicrystalline Silicon Solar Cells . 104 6.5 SCEBIC Imaging on Partially-Processed Solar Cells 106 6.6 Summary 107 Chapter Extraction of Surface Recombination Velocity 108 7.1 Introduction 108 7.2 One-dimensional Numerical Approach for SRV . 110 7.3 Three-dimensional Simulative Approach for SRV 115 7.4 Sample Preparation and Experiment Setup 118 7.5 Results and Discussion . 120 7.6 Summary 127 Chapter Conclusions 129 8.1 Summary 129 8.2 Future Work . 131 References 134 Appendix A: List of Publications . 148 Abstract Abstract This dissertation presents a detailed comparative study of advanced electron-beam based techniques for solar cell characterization. Firstly, the advantage of the subsurface imaging of scanning electron acoustic microscopy (SEAM) was utilized to characterize the structural properties of saw-damage-induced defects and the non-destructive nature of SEAM could enable accurate optimization of saw-damage etch process duration. SEAM was also employed together with electron beam induced current (EBIC) to investigate defects in photovoltaic devices. It was found that combination of these two techniques could provide complementary information that clearly distinguishes the morphological and electrical nature of the defects. The first demonstration of single contact EBIC (SCEBIC) on solar cells is then reported and the experimental results were supported with an analytical model and clearly explained using SPICE simulations. The requirement on only one contact enables SCEBIC to be performed on partially processed solar cells, thus allowing a high degree of flexibility of SCEBIC and its potential applications in photovoltaic industry. Lastly, highly localized quantitative EBIC were demonstrated to measure surface recombination velocity (SRV) for solar cells with different surface passivation conditions. A three-dimensional Monte Carlo simulation for electron-beam sample interaction was first employed to create a three-dimensional carrier generation profile for accurate modelling of EBIC using Sentaurus TCAD. These simulation results were then verified using experimental data that were almost perfectly matching, clearly demonstrating the capability and benefit of the high resolution and accuracy of quantitative EBIC for the extraction of SRV for solar cells. vii viii Advanced Electron-Beam Based Techniques for Solar Cell Characterization List of Figures Figure 2-1. Electron scattering in silicon using CASINO Monte Carlo simulation at an electron beam energy of 10 keV . Figure 2-2. Schematic of SEAM thermo-elastic mode . 11 Figure 2-3. Schematic comparison of (a) SEAM (< MHz), whose acoustic wavelength is longer than the sample thickness, and (b) conventional SAM (~ few GHz), whose acoustic wavelength is much smaller than the sample thickness. . 13 Figure 2-4. (a) Secondary electron (SE) and (b) SEAM images (at 165 kHz) of the domain structure in Polycrystalline Mn50Ni28Ga22 alloy. . 15 Figure 2-5. SE images of a multi-level IC (a) before and (b) after removing the top metal layer; and corresponding SEAM amplitude images prior to the top-down de-processing at electron beam energy of 30 keV and electron beam modulation frequency of (b) 25 kHz, (c) 60 kHz, (d) 173.8 kHz and (e) 200 kHz. . 16 Figure 2-6. (a) SE image of an IC; and SEAM phase images at modulation frequency of 173.2 kHz and different phases respect with the reference signals when b(1) θ = 40o, b(2) θ = 80o, b(3) θ = 100o, b(4) θ = 120o, b(5) θ = 160o 17 Figure 2-7. (a) SEAM image taken at 71.9 kHz of a multi-level IC, (b) SE image of the cross-section of the sample after focus ion beam (FIB) milling at the highlighted location indicated at the SEAM image. 18 Figure 2-8. EBIC images of (a) a continuous junction; and (b) a discontinuous junction regions created by different laser diode currents. . 21 Figure 2-9. Temperature dependence of EBIC contrasts of dislocations for different concentrations of contaminating impurities 22 Figure 2-10. Comparison of EBIC (30 keV) and band-to-band luminescence or SiPHER (532 nm) on block-cast mc-Si. 23 134 Advanced Electron-Beam Based Techniques for Solar Cell Characterization References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] V. Devabhaktuni, M. Alam, S. S. 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Hombourger, et al., "Shallow As dose measurements of patterned wafers with secondary ion mass spectrometry and low energy electron induced x-ray emission spectroscopy," Journal of Vacuum Science & Technology B, vol. 28, pp. C1C54-C1C58, Jan 2010. 147 148 Advanced Electron-Beam Based Techniques for Solar Cell Characterization Appendix A: List of Publications A1. Journal Publications [1] L. Meng, F.-J. Ma, J. Wong, B. Hoex, C.S. Bhatia, “Extraction of Surface Recombination Velocity at Highly Doped Silicon Surfaces Using Electron Beam Induced Current”, IEEE Journal of Photovoltaics (J-PV). (Submitted) [2] L. Meng, A.G. Street, J.C.H. Phang, C.S. Bhatia, “Application and Modelling of Single Contact Electron Beam Induced Current Technique on Multicrystalline Silicon Solar Cells”, Solar Energy Materials and Solar Cells. (Under review) [3] L. Meng, S. S. Papa Rao, C. S. Bhatia, S. E. Steen, A. G. Street, J.C.H. Phang, “Nondestructive Defect Characterization of Saw-Damage-Etched Multicrystalline Silicon wafers Using Scanning Electron Acoustic Microscopy”, IEEE Journal of Photovoltaics (J-PV), vol. 3, pp. 370-374, 2013. ** Shortlisted for the best student paper in the area of Characterization in the 38th IEEE PVSC, USA. [4] L. Meng, D. Nagalingam, C.S. Bhatia, A.G. Street, J.C.H. Phang, “Distinguishing Morphological and Electrical Defects in Polycrystalline Silicon Solar Cells Using Scanning Electron Acoustic Microscopy and Electron Beam Induced Current”, Solar Energy Materials and Solar Cells, vol. 95, pp. 2632-37, 2011. Appendix A: List of Publications A2. Conference Publications [1] L. Meng, A.G. Street, J.C.H. Phang, C.S. Bhatia, “Single Contact Electron Beam Induced Current Technique for Solar Cell Characterization”, 39th IEEE Photovoltaic Specialists Conference (PVSC), 16-21 Jun. 2013, Tampa, Florida, USA. [2] L. Meng, S. Steen, C.K. Koo, C.S. Bhatia, A.G. Street, P. Joshi, Y.H. Kim, J.C.H. Phang, “Characterization of Hydrogenated Amorphous Silicon Thin-Film Solar Cell Defects Using Optical Beam Induced Current Imaging and Focused Ion Beam CrossSectioning Techniques”, 37th IEEE Photovoltaic Specialists Conference (PVSC), 19-24 Jun. 2011, Seattle, Washington, USA., pp. 79-84. ** Shortlisted for the best student paper in the area of Characterization. [3] L. Meng, D. Nagalingam, C.S. Bhatia, A.G. Street, J.C.H. Phang, “SEAM and EBIC Studies of Morphological and Electrical Defects in Polycrystalline Silicon Solar Cells”, 2010 IEEE International Reliability Physics Symposium (IRPS), 2-6 May 2010, Anaheim, California, USA, pp. 503-507. [4] L. Meng, A.G. Street, J.C.H. Phang, “Subsurface imaging of multi-level integrated circuits using scanning electron acoustic microscopy”, 35th Inter. Symp. Testing and Failure Analysis (ISTFA), 15-19 Nov. 2009, San Jose, California, USA, pp. 27-32. ** Highlighted as the Feature Article of Electronic Device Failure Analysis (EDFA) e-News on 24 March 2011 [5] P. Joshi, S. Steen, K. Sivakumar, W. K. Yang, S. Rossnagel, S. Mittal, M. Steiner, D. Neumayer, Y. H. Kim, D. Nagalingam, L. Meng, C. S. Bhatia, J. C. H. Phang, "Development, Characterization and Interface Engineering of Films for Enhanced Amorphous Silicon Solar Cell Performance", 35th IEEE Photovoltaic Specialists Conference (PVSC), 20-25 Jun. 2010, Honolulu, Hawaii, USA, pp. 3686-3691. 149 [...]... in new manufacturing facilities for 1 2 Advanced Electron- Beam Based Techniques for Solar Cell Characterization monocrystalline and multicrystalline wafer based solar cells, as well as for the closely related silicon ribbon and sheet approaches A “second generation” of thin-film solar cell technology has also emerged during the past 15 years [11-13] Thin-film solar cells offer strong advantage as a... SCEBIC and SEAM for solar cell characterization Following the present chapter (Chapter 1) on the background and motivation of the project, Chapter 2 gives a detailed literature survey together with an in-depth discussion on the theories and working principles of the key electron- beam based characterization 5 6 Advanced Electron- Beam Based Techniques for Solar Cell Characterization techniques mentioned... Figure 7-10 Comparison of experimental and simulated EBIC gain (IEBIC/Ibeam) for the solar cell with AlOx/SiNx passivation Surface recombination velocity is 2.8 × 105 cm/s 126 xiii xiv Advanced Electron- Beam Based Techniques for Solar Cell Characterization List of Tables Table 5-1 Summary of performance of the three solar cell samples 79 List of Symbols List of Symbols Cj Zero-bias... chapter, advanced electronbeam based characterization techniques including scanning electron acoustic microscopy (SEAM), electron beam induced current (EBIC) and single contact EBIC (SCEBIC) are discussed in detail When applied on solar cell characterization, these techniques are capable for detailed localized analysis with a relatively high resolution given the small probe size of the electron beam In... Ig = 100 µA, Rsh = 5 kΩ, Rs = 1Ω, Cj = 200 nF, Cs = 100 pF xi xii Advanced Electron- Beam Based Techniques for Solar Cell Characterization The values assigned for each parameter are typical for solar cells with a sample size of about 1 cm2 98 Figure 6-3 SCEBIC transient characteristics of a typical single-junction solar cell using SPICE simulations with the same model parameters as Figure... general, the electron beam is scanned in a raster scan pattern and the beam' s position is combined with the detected signal to produce an image The most common mode of detection is by secondary electrons emitted by atoms at or near the surface of the sample excited by the electron beam In the most common or standard detection mode, secondary 9 10 Advanced Electron- Beam Based Techniques for Solar Cell Characterization. .. 45 Figure 3-1 Overview of the electron- beam based characterization techniques: (a) Conventional EBIC, (b) single contact EBIC (SCEBIC), and (c) SEAM 50 Figure 3-2 Block diagram of the experimental setup of EBIC and SEAM 51 Figure 3-3 Modification of the setup for single-contact EBIC (SCEBIC) 52 ix x Advanced Electron- Beam Based Techniques for Solar Cell Characterization Figure 4-1 SEAM... fundamental origin behind a specific material characteristic 3 4 Advanced Electron- Beam Based Techniques for Solar Cell Characterization 1.3 Strengths of Electron- Beam Based Techniques Apart from the various methods above-mentioned, there has been an increasing trend of extending electron- beam based techniques for characterization of PV material properties, such as carrier recombination activities within defects,... same sample at electron beam energy of 3 keV 121 Figure 7-8 Comparison of experimental and simulated EBIC gain (IEBIC/Ibeam) for the solar cell without passivation Surface recombination velocity is equal to the maximumpossible value of 107 cm/s 123 Figure 7-9 Simulated EBIC gain (IEBIC/Ibeam) as a function of electron beam energy for a n-type silicon wafer solar cell (a) with... upper limit of 33% [14] for a standard solar cell This suggests that the performance of solar cells could be further improved 2 - 3 times if different concepts are used to produce a “third generation” of high-performance cells [15, 16] For example, novel structural design was employed to produce what is best known as a tandem cell, where efficiency can be increased by adding more cells of different band . quantitative EBIC for the extraction of SRV for solar cells. viii Advanced Electron- Beam Based Techniques for Solar Cell Characterization List of Figures Figure 2-1. Electron scattering. 200 nF, C s = 100 pF. xii Advanced Electron- Beam Based Techniques for Solar Cell Characterization The values assigned for each parameter are typical for solar cells with a sample size of about. (I EBIC /I beam ) for the solar cell with AlO x /SiN x passivation. Surface recombination velocity is 2.8 × 10 5 cm/s. 126 xiv Advanced Electron- Beam Based Techniques for Solar Cell Characterization