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Electron Spin Resonance and Transient Photocurrent Measure- Ments on Microcrystalline Silicon

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Electron Spin Resonance and Transient Photocurrent Measure- Ments on Microcrystalline Silicon Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Electron Spin Resonance and Transient Photocurrent Measure- ments on Microcrystalline Silicon Thorsten Dylla Energietechnik Energy Technology Schriften des Forschungszentrums Jülich Reihe Energietechnik / Energy Technology Band / Volume 43 Forschungszentrum Jülich GmbH Institut für Photovoltaik Electron Spin Resonance and Transient Photocurrent Measurements on Microcrystalline Silicon Thorsten Dylla Schriften des Forschungszentrums Jülich Reihe Energietechnik / Energy Technology Band / Volume 43 ISSN 1433-5522 ISBN 3-89336-410-2 Bibliographic information published by Die Deutsche Bibliothek. Die Deutsche Bibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available in the Internet at <http://dnb.ddb.de>. Publisher Forschungszentrum Jülich GmbH and Distributor: Zentralbibliothek D-52425 Jülich Telefon (02461) 61-5368 · Telefax (02461) 61-6103 e-mail: [email protected] Internet: http://www.fz-juelich.de/zb Cover Design: Grafische Medien, Forschungszentrum Jülich GmbH Printer: Grafische Medien, Forschungszentrum Jülich GmbH Copyright: Forschungszentrum Jülich 2005 Schriften des Forschungszentrums Jülich Reihe Energietechnik / Energy Technology Band / Volume 43 D 188 (Diss., Berlin, Freie Univ., 2004) ISSN 1433-5522 ISBN 3-89336-410-2 Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher. Kurzfassung In der vorliegenden Arbeit wurden die elektronischen Eigenschaften von mikro- kristallinen Silizium (µc-Si:H) Dunnschichten¨ mittels Elektronen-Spin-Resonanz (ESR), transienter Photoleitung (Time-of-Flight (TOF)) und Messung der elek- trischen Leitfahigkeit¨ untersucht. Es wurden Modelle und mogliche¨ Erklarungsan-¨ satze¨ hinsichtlich der Natur und der energetischen Verteilung der elektronischen Defekte als Funktion des Filmaufbaus diskutiert und deren Auswirkungen auf den elektrischen Transport erortert.¨ Dazu wurde µc-Si:H mit strukturellen Eigen- schaften im Bereich von hochkristallinem bis zu vollstandig¨ amorphen Schichten abgeschieden. Der Grad der Kristallinitat¨ wurde jeweils mittels Raman Spek- troskopie bestimmt. Es zeigt sich, dass die gemessenen Spindichten NS mit dem strukturellen Aufbau der µc-Si:H Schichten korrelierten. Wahrend¨ die hochsten¨ NS generell bei hochkristallinem Material gefunden werden, verringert sich die Spindichte mit zunehmenden amorphen Volumenanteil in den Schichten. Dies kann mit den zunehmenden Wasserstoffgehalt und der damit verbundenen Absattigung¨ von offenen Bindungen an den Saulengrenzen¨ erklart¨ werden. Ferner fungiert die zusatzlich¨ zwischen den kristallinen Saulen¨ eingebaute amorphe Phase als Pas- sivierungsschicht, was zu einer effektiveren Absattigung¨ von ”dangling bond” Zustanden¨ an der Saulengrenzen¨ fuhrt.¨ In Abhangigkeit¨ von der Struktur der Filme, insbesondere der aktiven Oberfla-¨ che, zeigen sich deutliche reversible und irreversible nderungen im ESR-Signal als auch in der Dunkelleitfahigkeit¨ der µc-Si:H Schichten. Die porose¨ Struktur des hochkristallenen Materials begunstigt¨ die Eindiffusion von atmospharischen¨ Gasen, welche sowohl den Charakter als auch die Dichte der Oberflachenzust¨ ande¨ beeinflussen. Als wesentliche Ursache wurden zwei Prozesse identifiziert, Ad- sorption und Oxidation. Beide fuhren¨ zu einer Zunahme der Spindichte. Bei der Adsorption konnte diese auf eine reversible nderung der db2 Resonanz (g=2,0052) zuruckgef¨ uhrt¨ werden, wahrend¨ die db1 Resonanz (g=2,0043) unverandert¨ bleibt. Mit zunehmenden amorphen Anteilen in den Schichten nimmt die Große¨ der durch Adsorption und Oxidation hervorgerufenen Effekte ab, was auf eine zuneh- mende Kompaktheit der Filme zuruckgef¨ uhrt¨ werden kann. iii Messungen an n-dotierten µc-Si:H Filmen wurden zur Untersuchung der Zu- standsdichte in der Bandlucke¨ benutzt und bestatigten,¨ dass die gemessene Spin- dichte NS mit der Defektdichte zusammenhangt.¨ Die Resultate legen nahe, das fur¨ einen weiten Bereich von Strukturkompositionen die Verschiebung des Fermi- Niveaus durch die Kompensation von Zwischenbandzustanden¨ bestimmt wird. Dies gilt fur¨ Dotierkonzentrationen kleiner als die Defektkonzentration im in- trinsischen Material, wahrend¨ fur¨ hohere¨ Dotierungen eine Dotiereffizienz von eins beobachtet wird. Es lasst¨ sich folgern, das die Spindichte den Hauptteil der Zwischenbandzustanden¨ reprasentiert¨ (NS = NDB). Die Kenntnis uber¨ Art und Dichte von Defekten ist von entscheidender Bedeu- tung beim Verstandnis¨ des Ladungstragertransportes.¨ Mittels TOF-Technik wur- den pin-Dioden auf der Basis von µc-Si:H untersucht, sowie Locherdriftbeweglich-¨ keiten und die zugrundeliegenden Transportmechanismen bestimmt. Trotz der sehr hohen Kristallinitat¨ der Proben zeigen temperaturabhangige¨ Messungen, das der Lochertransport¨ durch ”Multiple Trapping” in einer exponentiellen Verteilung von Bandauslauferzust¨ anden¨ bestimmt ist, ein Verhalten das vorwiegend mit nicht- kristallinen Materialien in Verbindung gebracht wird. Die Breite des Valenzban- dauslaufers¨ konnte auf 31 meV bestimmt werden, was zu Locherdriftbeweglich-¨ keiten von 1-2 cm2/Vs fuhrt.¨ Diese Werte bestatigen¨ das Vorhandensein von Beweglichkeitskanten fur¨ Locher¨ in mikrokristallinen Filmen und erweitern die Bandbreite von Materialien, fur¨ die eine anscheinend universale Bandbeweglich- keit in der Großenordnung¨ von 1 cm2/Vs gefunden wird. iv Abstract The electronic properties of microcrystalline silicon (µc-Si:H) films have been studied using electron spin resonance (ESR), transient photocurrent time-of-flight (TOF) techniques, and electrical conductivity measurements. Structural proper- ties were determined by Raman spectroscopy. A wide range of structure compo- sitions, from highly crystalline films with no discernable amorphous content, to predominantly amorphous films with no crystalline phase contributions, was in- vestigated. Models and possible explanations concerning the nature and energetic distribution of electronic defects as a function of film composition are discussed. It is shown that the spin density NS in µc-Si:H films is linked strongly to the structure composition of the material. The highest NS is always found for material with the highest crystalline volume fraction. With increasing amorphous content, NS decreases, which is attributed to increasing hydrogen content and improved termination of dangling bonds. Moreover, the amorphous phase content, incor- porated between the crystalline columns, appears to act as a passivation layer, leading to more effective termination of unsatisfied bonds at the column bound- aries. Both reversible and irreversible changes in the ESR signal and dark conductiv- ity due to atmospheric effects are found in µc-Si:H. These are closely connected to the structure composition, in particular the active surface area. The porous struc- ture of highly crystalline material facilitates in-diffusion of atmospheric gases, which strongly affects the character and/or density of surface states. Two con- tributing processes have been identified, namely adsorption and oxidation. Both processes lead to an increase of NS . In the case of adsorption the increase is identified as arising from changes of the db2 resonance (g=2.0052), while the in- tensity of the db1 resonance (g=2.0043) remains constant. With increasing amor- phous content the magnitude of both adsorption and oxidation induced changes decreases, which may be linked to the greater compactness of such films. Measurements on n-type µc-Si:H films were used as a probe of the density of gap states, confirming that the spin density NS is related to the density of defects. The results confirm that for a wide range of structural compositions, the doping induced Fermi level shift in µc-Si:H is governed by compensation of defect states, v for doping concentrations up to the dangling bond spin density. At higher concen- trations a doping efficiency close to unity was found, confirming that in µc-Si:H the measured spin densities represent the majority of gap states (NS = NDB). The nature and density of defects is of great importance in determining elec- tronic transport properties. By applying the TOF technique to study pin solar cells based on µc-Si:H, conclusive hole drift mobility data were obtained. De- spite the predominant crystallinity of these samples, the temperature-dependence of hole transport is shown to be consistent with multiple-trapping in an expo- nential distribution of band tail states, behavior that is frequently associated with non-crystalline materials. A valence band tail width of 31 meV, and hole band mobilities of 1 − 2cm2/Vs, were estimated from the data. These measurements support the predominance of mobility-edge transport for holes in these microcrys- talline films, and extend the range of materials for which an apparently universal band mobility of order 1 cm2/Vs is obtained. vi Contents 1 Introduction 1 2 Fundamentals 7 2.1 Structural Properties of Microcrystalline Silicon .......... 7 2.2 Electronic Density of States ..................... 9 2.2.1 Band-Tail States ...................... 10 2.2.2 Deep Defects ........................ 11 2.3 Charge Carrier Transport ...................... 14 2.3.1 Barrier Limited Transport ................. 15 2.3.2 Dispersive Transport in Disordered Semiconductors . 15 3 Sample Preparation
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