Analysis of structure, electronic and transport properties of heavily doped polycrystalline silicon thin films

U radu su prikazana i analizirana svojstva visokodopiranih filmova polikristalnog silicija s primarnim ciljem razvoja grijaćeg, odnosno termoelektričnog elementa. Predstavljen je model rasta zrna iz amorfne faze koji za razliku od poznatih modela predviđa stagnaciju rasta prije nego se dosegne monokristalna faza. Potreba za preciznim predviđanjem ponašanja električnih karakteristika nameće i nužnost točnog određivanja koncentracije slobodnih nosilaca. Osim određivanja koncentracije nosilaca iz vrijednosti slojnog otpora, alternativno su predstavljeni modeli određivanja koncentracije nosilaca iz parametara Fano interakcije prema ponašanju O(_) vrpce Ramanovog spektra te iz linearne ovisnosti termostruje o temperaturi. Postignuta su zadovoljavajuća međusobna slaganja dobivenih vrijednosti. Postojeći modeli raspršenja nosilaca u obzir uzimaju elektron-elektron raspršenje kao korekciju ostalim dominantnim mehanizmima raspršenja. Mjerenjem ovisnosti električnog otpora o temperaturi, opaža se da je na niskim temperaturama elektron-elektron raspršenje dominantno do čak 80 K, a temperaturni raspon je funkcija koncentracije primjesa. Teorijski model iz literature koji opisuje vođenje električne struje u polikristalnom siliciju uklopljen je u proračun ovisnosti termoelektrične snage o koncentraciji. Prema predstavljenom izračunu, maksimalna vrijednost TE faktora snage nije samo funkcija koncentracije primjesa nego je i obrnuto proporcionalna veličini zrna polikristalnog silicija. Uzorak s najvišom koncentracijom atoma bora bio bi najbolji kandidat pri izradi grijaćeg elementa zbog gotovo linearne ovisnosti električnog otpora o temperaturi i pozitivnog temperaturnog koeficijenta te za termoelektrične uređaje zbog najvišeg iznosa faktora termoelektrične snage.Microelectronic industry as we know it today relies on the second most abundant material in the Earth's crust - silicon. In contrast to single-crystal silicon, polycrystalline silicon films contain grain boundaries which greatly influence the material properties, making them particularly sensitive to deposition conditions and doping levels. The subject of this thesis is an investigation of structural and electronic properties of heavily doped polysilicon thin films. Potential applications of these layers lie in silicon heating elements and thermoelectric devices. A silicon heating element is a structure consisting of a technical ceramic material (preferably AlN) onto which an active polysilicon layer is deposited. The advantage of this kind of heater is that the heat losses are relatively small compared to other resistive heaters as heat is transferred to the heated medium mainly by conduction. Until recently, silicon has not been considered a promising thermoelectric (TE) material due to its high thermal conductivity. However, recent publications have proven that nanostructuring could influence/decrease thermal conductivity, therewith potentially leading to competitive values of the TE figure of merit. Poly- or nanocrystalline silicon could be a great compromise between silicon nanowires that are cumbersome to use in volume applications and bulk monocrystalline silicon. After a general introduction, the second chapter gives an overview of the main properties of polycrystalline silicon films. Doped polycrystalline silicon has values of the thermal and electrical conductivity below those of a single-crystal layer with comparable carrier concentration at all temperatures. This means that the dominant phonon and electron scattering mechanism comes from the scattering on the grain boundaries. In the third chapter the thermoelectrical effects of Seebeck, Peltier and Thomson are introduced. Common thermoelectrics are put into perspective, while the main focus is set on silicon based systems and recent advances in the field related to this material. The deposition and experimental methods used in this work for the structural and electronic analysis are presented in chapter four. Two groups of samples were analysed, both deposited in the low pressure chemical vapour deposition (LPCVD) furnace. The first set of samples was deposited on oxidized (111) silicon wafers at 750°C and in-situ boron δ-doped. To achieve structural relaxation and additional dopant activation, samples were annealed at 1200°C for 1h. The second group consists of samples deposited onto (100) p-type Si wafers at 530 and 580°C and in-situ phosphorus doped. Subsequently, samples were subjected to rapid thermal annealing (RTA) at 950°C for 10, 20, 30 and 45s. The fifth chapter summarizes the main results of the investigation. Structure and morphology of the obtained films were determined with the help of scanning electron microscopy (SEM), x-ray diffraction (XRD) and reflectivity (XRR). The grain sizes of the polycrystalline films were calculated from the SEM micrographs by measuring the length of the small and the big axes of the ellipse covering the visible grain silhouette. For samples heavily doped with phosphorus, the grain size is a function of the annealing duration. Available models assume a final single-crystal state which is in contradiction with experimental results. On the basis of the obtained data, a new theoretical model which takes into account the grain growth stagnation is developed. Carrier concentration is one of the most important parameters when controlling electrical characteristics. The carrier concentration was determined from sheet resistance measurements and compared to the overall impurity concentration obtained from secondary ion mass spectroscopy. Two new models of the carrier concentration calculation are presented. The first one relies on the Fano interaction visible in the O(_) peak of Raman spectra of heavily doped silicon samples, and the second one takes into account the linear temperature dependence of the Seebeck coefficient. The thus obtained values are in good agreement with the ones obtained from sheet resistance measurements. Low temperature resistivity measurements revealed that at the lowest temperatures the dominating scattering mechanism is electron-electron scattering, as is characteristic for disordered metals. This kind of behaviour can also be found in some quasi 2D systems (at lower temperatures the electron mean free path becomes comparable to the film thickness). The characteristic T1/2 resistivity dependence even spans up to 80 K in the case of a boron concentration of p = 4,86・1019 cm-3 (the sample with the highest boron concentration investigated). As found in the literature, the dominant electron-electron interaction for heavily doped silicon thin films was not observed at temperatures higher than 1K. In this work, the obtained results also imply that the grain boundary scattering mechanism is temperature independent. A well established model for the carrier conduction in polycrystalline silicon was used to determine the power factor dependence on the grain size and doping effect. It was found that if the doping level increases, the grain size should decrease in order to keep the maximum TE power factor value constant. The sample with the highest boron concentration (p = 4,86・1019 cm-3) is the most promising candidate to be used as an active layer in heating elements due to its positive temperature coefficient which is rather constant in the whole temperature range. The same sample also has the highest TE power factor. However, the obtained TE power factor is still rather small when compared to today’s state-of-the-art thermoelectric materials. The final chapter summarizes the findings and conclusions of the presented work, and gives guidelines for future research and advances. Apart from increasing the figure of merit through nanostructuring, other ways to improve the thermoelectric performance could be through the formation of mixed structural phases which would additionally introduce scattering centres. Furthermore, another way to obtain a good theremoelectric material could be through electrochemical etching of heavily doped polycrystalline silicon with different doping levels. Following these avenues of research, and given the abundance of silicon as well as established production schemes, silicon based devices may soon boast a very competitive cost-benefit ratio/analysis (converted energy/production cost) in thermoelectric applications