Measurements

More documents

Recommendations

Info

Chapter 8 Schematic Density of States In Chapter 4-7, a study of electronic states located in the band gap of µc-Si:H using different experimental techniques and conceptional methods was described. However, all these experiments have certain limitations and therefore provide information only in a limited range. The aim of this section is to combine this information into a schematic picture of the density of states in both a spatial and energetic sense. From the results obtained in the study of hole drift-mobilities in highly crystalline µc-Si:H it could be deduced that the disorder within the investigated material is sufficient enough to strongly alter the band-edge states and affects the charge carrier transport. Applying the model of multiple trapping it could be shown that the band-tail falls exponentially towards the gap with a band width of ∆E V ≈ 31 meV. The existence of conduction band-tail states could be observed by the well known CE resonance found in n-doped µc-Si:H material, but neither the shape nor a typical width can be derived from these data. However, recent investigations using photoluminescence suggest that an exponential distribution also applies for the conduction band-tail [75]. The ESR signal of intrinsic microcrystalline silicon shows an asymmetric line shape which can be described by two Gaussian distributions db 1 and db 2 at g- values of g=2.0043 and g=2.0052, respectively. The results presented in this work suggest that these contributions are independent states located in different microscopic environments. So far the exact location of the states remains unknown. However, combining the results obtained in this work some speculations can be made. The energetic position within the band gap can be determined from the study of n-type material presented in Chapter 5. Together with the data derived from transient photocurrent measurements presented in Chapter 7, a schematic picture of the density of states for highly crystalline material can be drawn, as shown in Fig. 8.1 (a). To provide a comprehensive view on the DOS, the energetic distribution of the db 1 and db 2 states within the band gap has been included, 101
Chapter 8: Schematic Density of States Figure 8.1: (a) Schematic picture of the DOS of highly crystalline material as derived from the results of this work. The distribution of the db 1 and db 2 states below mid-gap has been taken from reference [30] and [40]. (b) Schematic band diagram of the transition region between the crystalline and disordered phase in µc-Si:H (see the text for details). that have been taken from reference [30] and [40], respectively. It can be seen that the energy distribution of the db 1 signal is rather concentrated within the crystalline gap of E G = 1.1 eV, while the db 2 resonance strongly overlaps with the conduction band-tail. It was therefore suggested, that the db 2 resonance in highly crystalline material are spatially separated from CE states presumably located at the columnar boundaries. This was also concluded in earlier investigations [165]. Additional support for this thesis comes from the study of intrinsic µc-Si:H prepared at different T S . Due to the higher deposition temperatures hydrogen desorbs during the process. As there are a number of indications that the bonded hydrogen is located at the column boundaries, terminating dangling bond defects, it is therefore tempting to relate the higher N S to a poor surface passivation caused by the hydrogen desorption. In Chapter 4 it was argued that the increasing N S is borne by an increasing number of db 2 states, suggesting that these states are located at the column boundaries. The assignment of the db 2 resonance to surface states located at the column boundaries is supported by the reversible effects on the ESR signal investigated in Chapter 6. These processes were attributed to the adsorption of oxygen at the columnar clusters. As the adsorption mainly effects the occupation of near surface states, the change of the db 2 resonance indicates that the origin of this line are states within this region. Quite surprisingly, the db 1 resonance at g=2.0043 stays unaffected by these processes. On might therefore conclude that the origin of these states is within the crystalline columns, which is in agreement with an energetic distribution confined within the crystalline band gap. In conclusion, one can draw a schematic band diagram for microcrystalline 102
Page 1 and 2:
Forschungszentrum Jülich in der He
Page 4 and 5:
Forschungszentrum Jülich GmbH Inst
Page 6 and 7:
Kurzfassung In der vorliegenden Arb
Page 8 and 9:
Abstract The electronic properties
Page 10 and 11:
Contents 1 Introduction 1 2 Fundame
Page 12 and 13:
CONTENTS C Abbreviations, Physical
Page 14 and 15:
Chapter 1 Introduction Solar cells
Page 16 and 17:
defect states provided that they ar
Page 18 and 19:
Chapter 8: In this chapter, the inf
Page 20 and 21:
Chapter 2 Fundamentals In this firs
Page 22 and 23:
2.2 Electronic Density of States St
Page 24 and 25:
2.2 Electronic Density of States ta
Page 26 and 27:
2.2 Electronic Density of States Fi
Page 28 and 29:
2.3 Charge Carrier Transport influe
Page 30 and 31:
2.3 Charge Carrier Transport functi
Page 32 and 33:
Chapter 3 Sample Preparation and Ch
Page 34 and 35:
3.1 Characterization Methods Figure
Page 36 and 37:
3.1 Characterization Methods Homoge
Page 38 and 39:
3.1 Characterization Methods µ Fig
Page 40 and 41:
3.1 Characterization Methods compar
Page 42 and 43:
3.1 Characterization Methods bution
Page 44 and 45:
3.1 Characterization Methods posite
Page 46 and 47:
3.2 Deposition Technique admixture
Page 48 and 49:
3.3 Sample Preparation 3.3.1 Sample
Page 50 and 51:
3.3 Sample Preparation reverse bias
Page 52 and 53:
Chapter 4 Intrinsic Microcrystallin
Page 54 and 55:
4.2 Electrical Conductivity Figure
Page 56 and 57:
4.3 ESR Signals and Paramagnetic St
Page 58 and 59:
4.3 ESR Signals and Paramagnetic St
Page 60 and 61:
4.4 Discussion - Relation between E
Page 62 and 63:
4.4 Discussion - Relation between E
Page 64 and 65: Chapter 5 N-Type Doped µc-Si:H In
Page 66 and 67: 5.2 Electrical Conductivity Figure
Page 68 and 69: 5.4 Dangling Bond Density Figure 5.
Page 70 and 71: 5.5 Conduction Band-Tail States Fig
Page 72 and 73: 5.6 Discussion concentration evalua
Page 74 and 75: 5.7 Summary Figure 5.7: Schematic b
Page 76 and 77: Chapter 6 Reversible and Irreversib
Page 78 and 79: 6.1 Metastable Effects in µc-Si:H
Page 88 and 89: 6.2 Irreversible Oxidation Effects
Page 94 and 95: 6.3 On the Origin of Instability Ef
Page 96 and 97: 6.3 On the Origin of Instability Ef
Page 98 and 99: Chapter 7 Transient Photocurrent Me
Page 100 and 101: 7.2 Transient Photocurrent Measurem
Page 106 and 107: 7.3 Temperature Dependent Drift Mob
Page 108 and 109: 7.4 Multiple Trapping in Exponentia
Page 110 and 111: 7.5 Discussion Table 7.2: Multiple
Page 112 and 113: 7.5 Discussion 7.5.3 The Meaning of
Page 116 and 117: silicon as shown in Fig. 8.1 b, whi
Page 118 and 119: Chapter 9 Summary In the present wo
Page 120 and 121: Appendix A Algebraic Description of
Page 122 and 123: Eq. A.7 then becomes L(t) F = sin(
Page 124 and 125: Appendix B List of Samples Table B.
Page 126 and 127: Table B.3: Parameters of n-type µc
Page 128 and 129: Appendix C Abbreviations, Physical
Page 130 and 131: µ d Drift mobility µ d,h Hole dri
Page 132 and 133: Bibliography [1] E. Becquerel. Mém
Page 134 and 135: BIBLIOGRAPHY [21] D. Will, C. Lerne
Page 136 and 137: BIBLIOGRAPHY [45] H. Overhof and M.
Page 138 and 139: BIBLIOGRAPHY [66] P.W. Anderson. Ab
Page 140 and 141: BIBLIOGRAPHY [93] J.W. Orton and M.
Page 142 and 143: BIBLIOGRAPHY [121] J.R. Haynes and
Page 144 and 145: BIBLIOGRAPHY [148] W. Luft and Y.S.
Page 146 and 147: BIBLIOGRAPHY [170] G.N. Advani, R.
Page 148 and 149: List of Publications 1. T. Dylla, R
Page 150 and 151: Acknowledgments At this point, I wa
Page 152 and 153: Schriften des Forschungszentrums J
Page 159: Forschungszentrum Jülich in der He
show all

Measurements

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?