Instability phenomena in microcrystalline silicon films

Microcrystalline silicon (μc-Si:H) for solar cell applications is investigated with respect to the material stability upon treatment of the material in various environments, followed by annealing. The material can be separated into two groups: (i) material with high crystalline volume fractions and pronounced porosity which is susceptible to in-diffusion of atmospheric gases, which, through adsorption or oxidation affect the electronic properties and (ii) compact material with high or low crystalline volume fractions which show considerably less or no influence of treatment in atmospheric gases. We report the investigation of such effects on the stability of μc-Si:H films prepared by plasma enhanced chemical vapour deposition and hot wire chemical vapour deposition

Elektronenspinresonanz und Untersuchungen transienter Photoströme an Mikrokristallinem Silizium

Title Page, Kurzfassung, Contents 1\. Introduction 2\. Fundamentals 3\. Sample preparation and characterization/a> 4\. Intrinsic microcrystalline silicon 5\. N-type doped μc-Si:H 6\. Reversible and irreversible effects in μc-Si:H 7\. Transient photocurrent measurements 8\. Schematic density of states 9\. Summary Acknowledgments Appendix Bibliography, List of publicationsThe 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 properties were determined by Raman spectroscopy. A wide range of structure compositions, from highly crystalline films with no discernable amorphous content, to predominantly amorphous films with no crystalline phase contributions, was investigated. 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, incorporated between the crystalline columns, appears to act as a passivation layer, leading to more effective termination of unsatisfied bonds at the column boundaries. Both reversible and irreversible changes in the ESR signal and dark conductivity 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 structure of highly crystalline material facilitates in-diffusion of atmospheric gases, which strongly affects the character and/or density of surface states. Two contributing 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 intensity of the db1 resonance (g = 2.0043) remains constant. With increasing amorphous 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, for doping concentrations up to the dangling bond spin density. At higher concentrations 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 electronic transport properties. By applying the TOF technique to study pin solar cells based on µc-Si:H, conclusive hole drift mobility data were obtained. Despite the predominant crystallinity of these samples, the temperature-dependence of hole transport is shown to be consistent with multiple-trapping in an exponential 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 1-2 cm2/Vs, were estimated from the data. These measurements support the predominance of mobility-edge transport for holes in these microcrystalline films, and extend the range of materials for which an apparently universal band mobility of order 1 cm2/Vs is obtained.In der vorliegenden Arbeit wurden die elektronischen Eigenschaften von mikrokristallinen Silizium (µc-Si:H) Dünnschichten mittels Elektronenspinresonanz (ESR), transienter Photoleitung (Time-of-Flight (TOF)) und Messung der elektrischen Leitfähigkeit untersucht. Es wurden Modelle und mögliche Erklärungsansätze hinsichtlich der Natur und der energetischen Verteilung der elektronischen Defekte als Funktion des Filmaufbaus diskutiert und deren Auswirkungen auf den elektrischen Transport erörtert. Dazu wurde µc- Si:H mit strukturellen Eigenschaften in einem Bereich von hochkristallinem bis zu vollständig amorphen Schichten abgeschieden. Der Grad der Kristallinität wurde jeweils mittels Raman Spektroskopie bestimmt. Es zeigt sich, dass die gemessenen Spindichten NS mit dem strukturellen Aufbau der µc-Si:H Schichten korrelierten. Während die höchsten 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 Absättigung von offenen Bindungen an den Säulengrenzen erklärt werden. Ferner fungiert die zusätzlich zwischen den kristallinen Säulen eingebaute amorphe Phase als Passivierungsschicht, was zu einer effektiveren Absättigung von dangling bond Zuständen an der Säulengrenzen führt. In Abhängigkeit von der Struktur der Filme, insbesondere der aktiven Oberfläche, zeigen sich deutliche reversible und irreversible Änderungen im ESR-Signal als auch in der Dunkelleitfähigkeit der µc-Si:H Schichten. Die poröse Struktur des hochkristallenen Materials begünstigt die Eindiffusion von atmosphärischen Gasen, welche sowohl den Charakter als auch die Dichte der Oberflächenzustände beeinflussen. Als wesentliche Ursache wurden zwei Prozesse identifiziert, Adsorption und Oxidation. Beide führen zu einer Zunahme der Spindichte. Bei der Adsorption konnte diese auf eine reversible Änderung der db2 Resonanz (g=2,0052) zurückgeführt werden, während die db1 Resonanz (g=2,0043) unverändert bleibt. Mit zunehmenden amorphen Anteilen in den Schichten nimmt die Größe der durch Adsorption und Oxidation hervorgerufenen Effekte ab, was auf eine zunehmende Kompaktheit der Filme zurückgeführt werden kann. Messungen an n-dotierten µc-Si:H Filmen wurden zur Untersuchung der Zustandsdichte in der Bandlücke benutzt und bestätigten, dass die gemessene Spindichte NS mit der Defektdichte zusammenhängt. Die Resultate legen nahe, das für einen weiten Bereich von Strukturkompositionen die Verschiebung des Fermi-Niveaus durch die Kompensation von Zwischenbandzuständen bestimmt wird. Dies gilt für Dotierkonzentrationen kleiner als die Defektkonzentration im intrinsischen Material, während für höhere Dotierungen eine Dotiereffizienz von eins beobachtet wird. Es lässt sich folgern, das die Spindichte den Hauptteil der Zwischenbandzuständen repräsentiert (NS = NDB). Die Kenntnis über Art und Dichte von Defekten ist von entscheidender Bedeutung beim Verständnis des Ladungsträgertransportes. Mittels TOF-Technik wurden pin- Strukturen auf der Basis von µc-Si:H untersucht, sowie Löcherdriftbeweglichkeiten und die zugrundeliegenden Transportmechanismen bestimmt. Trotz der sehr hohen Kristallinität der Proben zeigen temperaturabhängige Messungen, das der Löchertransport durch Multiple Trapping in einer exponentiellen Verteilung von Bandausläuferzuständen bestimmt ist, ein Verhalten das vorwiegend mit nichtkristallinen Materialien in Verbindung gebracht wird. Die Breite des Valenzbandausläufers konnte auf 31meV bestimmt werden, was zu Löcherdriftbeweglichkeiten von 1-2 cm2/Vs führt. Diese Werte bestätigen das Vorhandensein von Beweglichkeitskanten für Löcher in mikrokristallinen Filmen und erweitern die Bandbreite von Materialien, für die eine anscheinend universale Bandbeweglichkeit in der Größenordnung von 1 cm2/Vs gefunden wird

Electron Spin Resonance and Transient Photocurrent Measurements on Microcrystalline Silicon

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 properties were determined by Raman spectroscopy. A wide range of structure compositions, from highly crystalline films with no discernable amorphous content, to predominantly amorphous films with no crystalline phase contributions, was investigated. 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 N$_{S}$ in μc-Si:H films is linked strongly to the structure composition of the material. The highest N$_{S}$ is always found for material with the highest crystalline volume fraction. With increasing amorphous content, N$_{S}$ decreases, which is attributed to increasing hydrogen content and improved termination of dangling bonds. Moreover, the amorphous phase content, incorporated between the crystalline columns, appears to act as a passivation layer, leading to more effective termination of unsatisfied bonds at the column boundaries. Both reversible and irreversible changes in the ESR signal and dark conductivity 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 structure of highly crystalline material facilitates in-diffusion of atmospheric gases, which strongly affects the character and/or density of surface states. Two contributing processes have been identified, namely adsorption and oxidation. Both processes lead to an increase of N$_{S}$. In the case of adsorption the increase is identified as arising from changes of the db2 resonance (g=2.0052), while the intensity of the db1 resonance (g=2.0043) remains constant. With increasing amorphous 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, [...

St. John's Daily Star, 1920-12-11

The St. John's Daily Star was published daily except Sunday between 17 April 1915 - 23 July 1921

Instability phenomena in microcrystalline silicon films

Microcrystalline silicon (μc-Si:H) for solar cell applications is investigated with respect to the material stability upon treatment of the material in various environments, followed by annealing. The material can be separated into two groups: (i) material with high crystalline volume fractions and pronounced porosity which is susceptible to in-diffusion of atmospheric gases, which, through adsorption or oxidation affect the electronic properties and (ii) compact material with high or low crystalline volume fractions which show considerably less or no influence of treatment in atmospheric gases. We report the investigation of such effects on the stability of μc-Si:H films prepared by plasma enhanced chemical vapour deposition and hot wire chemical vapour deposition