The cost of crystalline silicon solar cells must be reduced in order for photovoltaics to be widely accepted as an economically viable means of electricity generation and be used on a larger scale across the world. There are several ways to achieve cost reduction, such as using thinner silicon substrates, lowering the thermal budget of the processes, and improving the efficiency of solar cells. This thesis examines the use of plasma enhanced chemical vapour deposited silicon nitride to address the criteria of cost reduction for n-type crystalline silicon solar cells. It focuses on the surface passivation quality of silicon nitride on n-type silicon, and injection-level dependent lifetime data is used extensively in this thesis to evaluate the surface passivation quality of the silicon nitride films. The thesis covers several aspects, spanning from characterisation and modelling, to process development, to device integration. The thesis begins with a review on the advantages of using n-type silicon for solar cells applications, with some recent efficiency results on n-type silicon solar cells and a review on various interdigitated backside contact structures, and key results of surface passivation for n-type silicon solar cells. It then presents an analysis of the influence of various parasitic effects on lifetime data, highlighting how these parasitic effects could affect the results of experiments that use lifetime data extensively. A plasma enhanced chemical vapour deposition process for depositing silicon nitride films is developed to passivate both diffused and non-diffused surfaces for n-type silicon solar cells application. Photoluminescence imaging, lifetime measurements, and optical microscopy are used to assess the quality of the silicon nitride films. An open circuit voltage of 719 mV is measured on an n-type, 1 Ω.cm, FZ, voltage test structure that has direct passivation by silicon nitride. Dark saturation current densities of 5 to 15 fA/cm2 are achieved on SiN-passivated boron emitters that have sheet resistances ranging from 60 to 240 Ω/□ after thermal annealing. Using the process developed, a more profound study on surface passivation by silicon nitride is conducted, where the relationship between the surface passivation quality and the film composition is investigated. It is demonstrated that the silicon-nitrogen bond density is an important parameter to achieve good surface pas-sivation and thermal stability. With the developed process and deeper understanding on the surface passivation of silicon nitride, attempts of integrating the process into the fab-rication of all-SiN passivated n-type IBC solar cells and laser doped n-type IBC solar cells are presented. Some of the limitations, inter-relationships, requirements, and challenges of novel integration of SiN into these solar cell devices are identified. Finally, a novel metallisation scheme that takes advantages of the different etching and electroless plating properties of different PECVD SiN films is described, and a preliminary evalua-tion is presented. This metallisation scheme increases the metal finger width without increasing the metal contact area with the underlying silicon, and also enables optimal distance between point contacts for point contact solar cells. It is concluded in this thesis that plasma enhanced chemical vapour deposited silicon nitride is well-suited for n-type silicon solar cells.

One of the poly-Si thin-film cells developed at the University of New South Wales (UNSW) is the EVA cell. In this work, SiN films for EVA cells as an antireflection/barrier coating were investigated. In addition, the effect of hydrogenation pre-treatment of solid phase crystallisation (SPC) on grain size and open-circuit voltage (Voc) was investigated. The SiN films deposited by PECVD were examined for uniformity of the thickness and the refractive index of the films across the position of the samples in the PECVD deposition system. A spectrophotometric analysis was used to determine these film properties. It was found that these properties were very uniform over the deposition area. Good repeatability of the depositions was also observed. A series of SiN film depositions by reactive sputtering were also performed to optimize the deposition process. Parameters adjusted during the deposition were nitrogen flow rate, substrate bias, and substrate temperature. By investigating the deposition rate, refractive index, and surface roughness of the films, the three deposition parameters were optimised. The effects of post SiN deposition treatments (a-Si deposition, SPC, RTA, and hydrogenation) on thickness and refractive index of both SiN films deposited by PECVD and reactive sputtering were investigated by using samples which have the same structure as the EVA cells. The thickness of the PECVD SiN films decreased about 6 % after all the treatments. On the other hand, the thickness reductions of the reactively sputtered SiN films were very small. The refractive index of the PECVD SiN films increased about 0.6 % after the treatments, whereas that of the reactively sputtered SiN films decreased 1.3 % after the treatments. As a possible method to improve the performance of EVA cells, hydrogenation of a-Si was investigated as a pre-treatment of SPC process. There were no obvious differences in the grainsize and the Voc of the EVA cells with and without the hydrogenation. Therefore it is likely that the hydrogenation pre-treatment of SPC does not have a beneficial effect on the performance of EVA cells.

Thesis (M. S.)--Materials Science and Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Cochran, Joe; Committee Member: Berczik, Doug; Committee Member: Sanders, Tom; Committee Member: Sandhage, Ken; Committee Member: Thadhani, Naresh.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

abstract: As crystalline silicon solar cells continue to get thinner, the recombination of carriers at the surfaces of the cell plays an ever-important role in controlling the cell efficiency. One tool to minimize surface recombination is field effect passivation from the charges present in the thin films applied on the cell surfaces. The focus of this work is to understand the properties of charges present in the SiNx films and then to develop a mechanism to manipulate the polarity of charges to either negative or positive based on the end-application. Specific silicon-nitrogen dangling bonds (·Si-N), known as K center defects, are the primary charge trapping defects present in the SiNx films. A custom built corona charging tool was used to externally inject positive or negative charges in the SiNx film. Detailed Capacitance-Voltage (C-V) measurements taken on corona charged SiNx samples confirmed the presence of a net positive or negative charge density, as high as +/- 8 x 1012 cm-2, present in the SiNx film. High-energy (~ 4.9 eV) UV radiation was used to control and neutralize the charges in the SiNx films. Electron-Spin-Resonance (ESR) technique was used to detect and quantify the density of neutral K0 defects that are paramagnetically active. The density of the neutral K0 defects increased after UV treatment and decreased after high temperature annealing and charging treatments. Etch-back C-V measurements on SiNx films showed that the K centers are spread throughout the bulk of the SiNx film and not just near the SiNx-Si interface. It was also shown that the negative injected charges in the SiNx film were stable and present even after 1 year under indoor room-temperature conditions. Lastly, a stack of SiO2/SiNx dielectric layers applicable to standard commercial solar cells was developed using a low temperature (< 400 °C) PECVD process. Excellent surface passivation on FZ and CZ Si substrates for both n- and p-type samples was achieved by manipulating and controlling the charge in SiNx films. / Dissertation/Thesis / Ph.D. Electrical Engineering 2013

abstract: Silicon (Si) solar cells are the dominant technology used in the Photovoltaics industry. Field-effect passivation by means of electrostatic charges stored in an overlying insulator on a silicon solar cell has been proven to be a significantly efficient way to reduce effective surface recombination velocity and increase minority carrier lifetime. Silicon nitride (SiNx) films have been extensively used as passivation layers. The capability to store charges makes SiNx a promising material for excellent feild effect passivation. In this work, symmetrical Si/SiO2/SiNx stacks are developed to study the effect of charges in SiNx films. SiO2 films work as barrier layers. Corona charging technique showed the ability to inject charges into the SiNx films in a short time. Minority carrier lifetimes of the Czochralski (CZ) Si wafers increased significantly after either positive or negative charging. A fast and contactless method to characterize the charged overlying insulators on Si wafer through lifetime measurements is proposed and studied in this work, to overcome the drawbacks of capacitance-voltage (CV) measurements such as time consuming, induction of contanmination and hysteresis effect, etc. Analytical simulations showed behaviors of inverse lifetime (Auger corrected) vs. minority carrier density curves depend on insulator charge densities (Nf). From the curve behavior, the Si surface condition and region of Nf can be estimated. When the silicon surface is at high strong inversion or high accumulation, insulator charge density (Nf) or surface recombination velocity parameters (Sn0 and Sp0) can be determined from the slope of inverse lifetime curves, if the other variable is known. If Sn0 and Sp0 are unknown, Nf values of different samples can be compared as long as all have similar Sn0 and Sp0 values. Using the saturation current density (J0) and intercept fit extracted from the lifetime measurement, the bulk lifetime can be calculated. Therefore, this method is feasible and promising for charged insulator characterization. / Dissertation/Thesis / M.S. Electrical Engineering 2014

Orientador: Jose Alexandre Diniz / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Eletrica e de Computação / Made available in DSpace on 2018-08-10T02:20:44Z (GMT). No. of bitstreams: 1 Zoccal_LeonardoBreseghello_D.pdf: 6734188 bytes, checksum: 05f6c64d923bafb5e071d89514d0fa43 (MD5) Previous issue date: 2007 / Resumo: Este trabalho apresenta um método simples de passivação de superfícies semicondutoras III-V de substratos de arseneto de gálio (GaAs) e de heteroestruturas de fosfeto de gálio-índio sobre arseneto de gálio (InGaP/GaAs), que são utilizados em transist res de efeito de campo, MESFET (Metal-Semiconductor Field Effect Transistor) e MISFET Metal-Insulator-Semiconductor Field Effect Transistor), e transistores bipolares de heterojunção (HBT), respectivamente. O processo de passivação visa à máxima redução da densidade de estados de superfícies semicondutoras para níveis menores que 1012 cm-2. A alta densidade de estados na superfície do GaAs provoca corrente de fuga nas regiões ativas dos transistores MESFET e HBT, reduzindo o desempenho destes dispositivos. Além disso, impossibilita a formação de dispositivos MISFET sobre os substratos de GaAs, devido à alta densidade de estados na região da interface isolante-semicondutor. Para o estudo da passivação de superfícies, filmes de nitreto de silício (SiNX) são depositados diretam nte por plasma ECR-CVD (Electron Cyclotron Resonance - Chemical Vapor Deposition) sobre substratos de GaAs e heteroestruturas do tipo InGaP/GaAs. Os plasmas ECR foram analisados por espectroscopia de emissão óptica (OES), e identificou-se baixa formação de espécies H e NH na fase gasosa para pressão de processo de 2,5 mTorr. Os filmes de SiNX foram caracterizados estruturalmente por espectroscopia de absorção do infravermelho (FTIR) e por elipsometria, que indicaram, respectivamente, a formação de ligações Si-N e valores de índice de refração es de nitreto de silício. Capacitores MIS e transisto T foram fabricados para avaliar os efeitos da passivação sobre os dispositivos. Os excelentes resultados obtidos, tais como transist o e em torno de 2,0 nos filmres MISFET e HB ores HBT passivados apresentando maiores ganhos de corrente do que os não-passivados, e os transistores MISFET apresentando maiores valores de transcondutância do que os MESFET (que foram usados como dispositivos de controle), indicam que o nosso processo de passivação é muito eficiente, sendo completamente compatível com a tecnologia de fabricação de circuitos integrados monolíticos de microondas (MMIC) / Abstract: This work presents a simple passivation method for III-V semiconductor surfaces of gallium arsenide (GaAs) substrates and indium-gallium phosphide on gallium arsenide (InGaP/GaAs) heterostructures, which are us in field effect transistors MESFET (Metal-Semiconductor Field Effect Transistor) and MISFET (Metal-Insulator-Semiconductor Field Effect Transistor) and heterojunction bip lar transistors (HBT), respectively. The passivation process aims the maximum reduction of semiconductor surface state density at levels lower than 1012 states/cm2. The high surface state density on GaAs surface produces current leakage in active regions of MESFET and HBT transistors, reducing the device performance. Furthermore, the MISFET device formation on GaAs substrate is not allowed, passivation study, silicon nitride films (SiNX) are deposited by ECR-CVD (Electron Cyclotron Resonance - Chemical Deposition Vapor) plasma directly over GaAs substrate and InGaP/GaAs heterostructures. The ECR plasmas were analyzed by optical emission spectroscopy, (OES), and low formation of H and NH molecules in the gas phase was detected at process pressure of 2.5 mTorr. The SiNX film structural characterization was obtained by infra-red absorption spectrometry (FTIR) and ellipsometry, which, respectively, indicate the Si-N bo tive index values of about 2.0 at the silicon nitride films. MIS cap BT transistors were fabricated to verify the passivation process effect on devices. The excellent results obtained, such as higher and formation and refracacitors, MISFET and H current gain of passivated device compared to unpassivated HBTs and higher transconductances of MISFET devices compared to MESFET (which were used as control devices), indicate that our simple passivation process is very efficient, being fully compatible with monolithic microwave integrated circuits (MMIC) / Doutorado / Eletrônica, Microeletrônica e Optoeletrônica / Doutor em Engenharia Elétrica

Orientadores: Peter Jurgen Tatsch, Stanislav A. Moshkalyov / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Eletrica e de Computação / Made available in DSpace on 2018-08-05T15:04:36Z (GMT). No. of bitstreams: 1 Nunes_AlcineiMoura_M.pdf: 4164375 bytes, checksum: 7cc7a3a84a2aa99efc455551e270bc57 (MD5) Previous issue date: 2005 / Resumo: Este trabalho apresenta os resultados e as discussões dos mecanismos de corrosão por plasma de filmes de silício policristalino e nitreto de silício para aplicações em dispositivos MEMS e CMOS. A corrosão foi feita em um reator convencional de corrosão por plasma em modo RIE (Reactive Ion Etching). Para aplicação em MEMS, corrosões de silício policristalino com perfis anisotrópicos e seletividade maior que 10 para óxido de silício foram obtidos. As misturas gasosas utilizadas na corrosão foram: SF6/CF4/CHF3 e SF6/CF4/N2. Processos híbridos, utilizando duas etapas de corrosão em condições diferentes, num mesmo processo, foram feitos para possibilitar a obtenção de perfis altamente anisotrópicos, com seletividade elevada. Para avaliar melhor a evolução do perfil de corrosão, foram utilizadas amostras com filmes espessos de silício policristalino (>3mm). Para aplicação em eletrodo de transistores CMOS, afinamento de linhas de 5mm para 1mm de largura foram obtidos com perfil anisotrópico (A~0,95) em processos híbridos, utilizando uma primeira etapa de corrosão com condições de maior bombardeio iônico e menor seletividade, e uma segunda etapa com menor bombardeio iônico e maiores seletividades para o óxido e fotorresiste. Corrosões de nitreto de silício (SiNx), com seletividade elevada são imprescindíveis para aplicação em tecnologia LOCOS. Resultados de corrosões e filmes de nitreto de silício para esta aplicação foram feitas utilizando as seguintes misturas gasosas: CF4/O2/N2, CF4/H2, SF6/CF4/N2 e CHF3/O2. As Maiores seletividades obtidas foram de aproximadamente 10 para óxido e aproximadamente 7 para o substrato de silício. Os filmes foram caracterizados com vários equipamentos. Um Perfilômetro foi utilizado para medir a profundidade das corrosões, e por conseguinte, calcular a taxa de corrosão através da divisão pelo tempo. Um Elipsômetro foi utilizado para medir as espessuras e índices de refração dos filmes utilizados. O FTIR (Fourier Transform Infrared Spectrocopy) foi utilizado para caracterizar a composição do filme de nitreto de silício após o tratamento térmico. Imagens SEM (Scanning Electron Microscopy) dos filmes corroídos foram feitas para analisar o perfil e mecanismo de corrosão para cada mistura / Abstract: This work presents the results and the discussion about mechanisms of plasma etching of polysilicon and silicon nitride films for applications in MEMS and CMOS devices. The etching was performed in a conventional reactor of plasma etching in a RIE mode (Reactive Ion Etching). For application in MEMS, polysilicon etching with anisotropic profile and high selectivity (>10) for silicon oxide was obtained. The mixtures used in etching were SF6/CF4/CHF3 and SF6/CF4/N2. Hybrid processes, using two etching stages in different conditions, in the same etching process, were done as possible solutions for highly anisotropic profiles with elevated selectivity. The evolution of the etching profile is better evaluated using polysilicon thick films (>3mm). For application in CMOS transistors electrode, 5mm to 1mm thinning was obtained with anisotropic profile (A~0,95) in hybrids processes, using the first stage in conditions with elevated ionic bombardment and reduction selectivity, and the second stage with reduction ionic bombardment and elevated selectivity to the oxide and photorresist. Highly selective silicon nitride etching (SiNx) is necessary for application in LOCOS technology. Results of the etching and silicon nitride films for this application was performed using the following mixtures: CF4/O2/N2, CF4/H2, SF6/CF4/N2 and CHF3/O2. High selectivity was obtained for silicon oxide (>10) and silicon substrate (~7). The films were characterized by various equipment. The Profiler was used to measure the etching depth, and herewith, the etching rate was evaluated. The Elipsometer was used to measure the refractive index and width of the films. FTIR (Fourier Transform Infrared Spectroscopy) was used to characterize the composition of the nitride film after the thermal treatment. SEM (Scanning Electron Microscopy) images of the etched films were done to analyse the profile and the etching mechanism for each mixture / Mestrado / Microeletronica / Mestre em Engenharia Elétrica

The effects of direct plasma chemistries on carbon removal from silicon nitride (SiNx) and oxynitride (SiOxNy ) surfaces and Cu have been studied by x-photoelectron spectroscopy (XPS) and ex-situ contact angle measurements. The data indicate that O2,NH3 and He capacitively coupled plasmas are effective at removing adventitious carbon from silicon nitride (SiNx) and Silicon oxynitride (SiOxNy ) surfaces. O2plasma and He plasma treatment results in the formation of silica overlayer. In contrast, the exposure to NH3 plasma results in negligible additional oxidation of the SiNx and SiOxNy surface. Ex-situ contact angle measurements show that SiNx and SiOxNy surfaces when exposed to oxygen plasma are initially more hydrophilic than surfaces exposed to NH3 plasma and He plasma, indicating that the O2 plasma-induced SiO2 overlayer is highly reactive towards ambient corresponding to increased roughness measured by AFM. At longer ambient exposures (>~10 hours), however surfaces treated by either O2, He or NH3 plasma exhibit similar steady state contact angles, correlated with rapid uptake of adventitious carbon, as determined by XPS. Surface passivation by exposure to molecular hydrogen prior to ambient exposure significantly retards the increase in the contact angle upon the exposure to ambient. The results suggest a practical route to enhancing the time available for effective bonding to surfaces in microelectronics packaging applications.

La fonctionnalisation de la surface du verre par des molécules organiques permet de modifier son énergie de surface ou d’améliorer l’adhésion d’un revêtement. La méthode classique de fonctionnalisation directe du verre repose sur une réaction de silanisation, via la formation de ponts siloxanes Si O Si. Ces ponts ont tendance à s’hydrolyser en milieu salin ou alcalin, entrainant la perte de la fonctionnalité du verre. Une solution envisagée consiste à déposer une couche de nitrure de silicium (SixN4) sur le verre, permettant de greffer des molécules organiques via des liaisons covalentes robustes : Si C ou N C. Le nitrure de silicium présente l’avantage d’être un matériau très souvent utilisé dans l’industrie verrière en raison de sa capacité à bloquer la diffusion des ions sodium et de protéger ainsi le verre de la corrosion.L’objectif de ce travail de thèse est de caractériser et contrôler la surface du nitrure de silicium, puis d’optimiser et de comprendre la modification de sa surface par le greffage covalent de molécules organiques.Lorsque le nitrure de silicium est exposé à l’air, une couche d’oxynitrure est formée en surface. L’optimisation et la compréhension du décapage de cette couche d’oxynitrure natif en milieu liquide est l’objet de la première phase de ce travail. La composition chimique de la surface est finement caractérisée et quantifiée en combinant des mesures de spectroscopie infrarouge en mode de réflexion totale atténuée (IR-ATR), de spectroscopie de photoélectrons X (XPS) et des dosages chimiques de surface. Le décapage dans des solutions fluorées (HF et NH4F) permet de retirer efficacement la couche d’oxynitrure et laisse majoritairement en surface des liaisons Si-F et dans une moindre mesure des liaisons N H et Si OH. La composition chimique de la surface peut toutefois être modifiée pour former des groupements Si H, soit en enrichissant la couche du SixN4 en silicium, soit en soumettant la surface à un traitement par plasma d’hydrogène à l’issue du décapage. A partir des observations expérimentales, une proposition décrivant les mécanismes mis en jeu lors du décapage est présentée.Dans la seconde partie de la thèse, la surface du nitrure de silicium est modifiée par l’immobilisation de molécules organiques, plus spécifiquement par la réaction d’un 1 alcène sous activation thermique ou photochimique. La composition chimique de la surface et les conditions d’activation de la réaction modifient la réaction de greffage et la densité des couches organiques. En particulier, la présence de liaisons Si-H et l’enrichissement de la couche en silicium sont étudiés en détail. Dans une dernière partie, dans une visée plus applicative, des couches denses fluorées présentant un caractère hydrophobe naturel sont greffées sur la surface du nitrure de silicium. / Covalent grafting of organic molecules on glass can modify its surface physico-chemical properties or improve the adhesion of a coating. Such a functionalization usually relies on a silanisation reaction, bonding molecules to the surface through Si-O-Si bonds. Unfortunately, the resulting molecular layers do not exhibit long-term stability due to the hydrolysis of siloxane groups. One solution would consist in depositing a silicon nitride layer on glass, allowing the glass surface to be functionalized through more stable bonds N-C or Si-C. Silicon nitride layers are frequently used in glass industry. They are well-known for their durability properties and are often used as a protective layer against glass corrosion.The aim of this project is to characterize and control the non-oxidized silicon nitride surface, then to optimize and understand the surface modification by covalent grafting of organic molecules.When silicon nitride is exposed to atmosphere, an oxynitride layer is formed on its surface. Several efficient ways to remove this native oxynitride are first studied and optimized. The quantitative characterization and control of the surface chemical composition provide a reliable starting point for the functionalization step. The surface chemical composition is quantitatively investigated by combining Attenuated Total Reflection InfraRed spectroscopy (ATR-IR), X-ray photoelectron spectroscopy (XPS) and chemical dosing. The etching in HF-based solutions efficiently removes the oxynitride layer and leads to a surface mainly covered with Si-F bonds and smaller amounts of Si-OH and N-H bonds. The surface composition can be modified by a H2 plasma treatment performed after the wet etching or by changing the silicon nitride layer composition (silicon enrichment), leading in either case to the formation of Si-H bonds on surface. An etching mechanism is suggested from these experimental observations.The second part of this work is focused on the grafting of the alkyl chains on the silicon nitride surface. The surface is reacted with a 1-alkene, using photochemical or thermal activation. The grafting efficiency depends on the surface composition and the activation conditions. The presence of surface Si-H bonds and the effect of Si enrichment are considered in details. In a final part, in an applicative view, functional hydrophobic molecules are grafted on the silicon nitride surface.