Investigations on doping of amorphous and nano-crystalline silicon films deposited by catalytic chemical vapour deposition

Hydrogenated amorphous and nanocrystalline silicon, deposited by catalytic chemical vapour deposition, have been doped during deposition by the addition of diborane and phosphine in the feed gas, with concentrations in the region of 1%. The crystalline fraction, dopant concentration and electrical properties of the films are studied. The nanocrystalline films exhibited a high doping efficiency, both for n and p doping, and electrical characteristics similar to those of plasma-deposited films. The doping efficiency of n-type amorphous silicon is similar to that obtained for plasma-deposited electronic-grade amorphous silicon, whereas p-type layers show a doping efficiency of one order of magnitude lower. A higher deposition temperature of 450°C was required to achieve p-type films with electrical characteristics similar to those of plasma-deposited films

Thin silicon films ranging from amorphous to nanocrystalline obtained by Hot-Wire CVD

In this paper, we have presented results on silicon thin films deposited by hot-wire CVD at low substrate temperatures (200 °C). Films ranging from amorphous to nanocrystalline were obtained by varying the filament temperature from 1500 to 1800 °C. A crystalline fraction of 50% was obtained for the sample deposited at 1700 °C. The results obtained seemed to indicate that atomic hydrogen plays a leading role in the obtaining of nanocrystalline silicon. The optoelectronic properties of the amorphous material obtained in these conditions are slightly poorer than the ones observed in device-grade films grown by plasma-enhanced CVD due to a higher hydrogen incorporation (13%)

Analysis of bias stress on thin-film transistors obtained by Hot-Wire Chemical Vapour Deposition

The stability under gate bias stress of unpassivated thin film transistors was studied by measuring the transfer and output characteristics at different temperatures. The active layer of these devices consisted of in nanocrystalline silicon deposited at 125 °C by Hot-Wire Chemical Vapour Deposition. The dependence of the subthreshold activation energy on gate bias for different gate bias stresses is quite different from the one reported for hydrogenated amorphous silicon. This behaviour has been related to trapped charge in the active layer of the thin film transistor

PEN as substrate for new solar cell technologies

The possible use of polyethylene naphthalate as substrate for low-temperature deposited solar cells has been studied in this paper. The transparency of this polymer makes it a candidate to be used in both substrate and superstrate configurations. ZnO:Al has been deposited at room temperature on top of PEN. The resulting structure PEN/ZnO:Al presented good optical and electrical properties. PEN has been successfully textured (nanometer and micrometer random roughness) using hot-embossing lithography. Reflector structures have been built depositing Ag and ZnO:Al on top of the stamped polymer. The deposition of these layers did not affect the final roughness of the whole. The reflector structure has been morphologically and optically analysed to verify its suitability to be used in solar cells

Analysis of bias stress on thin-film transistors obtained by Hot-Wire Chemical Vapour Deposition

The stability under gate bias stress of unpassivated thin film transistors was studied by measuring the transfer and output characteristics at different temperatures. The active layer of these devices consisted of in nanocrystalline silicon deposited at 125°C by Hot-Wire Chemical Vapour Deposition. The dependence of the subthreshold activation energy on gate bias for different gate bias stresses is quite different from the one reported for hydrogenated amorphous silicon. This behaviour has been related to trapped charge in the active layer of the thin film transistor.Peer ReviewedPostprint (published version

Amorphous silicon solar cells obtained by Hot-Wire Chemical Vapour Deposition

The photovoltaic conversion of sun light energy into electricity constitutes a viable, clean and renewable alternative for electric energy production. A wide variety of different materials have been proposed over the last decades for photovoltaic applications, although silicon seems to be the most suitable one for global implementation. Crystalline silicon, c-Si, is the most commonly employed material, although the elevated costs associated to its production have given place to new technologies based on thin film materials. Among thin film materials, hydrogenated amorphous silicon (a Si:H), either solely or in combination with other thin film materials, has been proven to be a very promising alternative due to the suitable properties and to the low costs associated to its production. Taking into account the promising properties of a-Si:H, its deposition and characterization was carried out at Universitat de Barcelona in the frame of the presented work. Plasma Enhanced Chemical Vapour Deposition (PECVD) is the most widely employed technique for the deposition of thin silicon layers. Nevertheless, certain limitations have led to the investigation of alternative deposition techniques, such as Hot-Wire Chemical Vapour Deposition (Hot-Wire CVD), which features higher deposition rates, simpler and cheaper deposition geometry and easier scalability to large area deposition than PECVD. Hot-Wire CVD has been analysed at Universitat de Barcelona since 1993 for the growth of nanocrystalline silicon (nc-Si:H) layers, this being the first work concerning a-Si:H based devices. Moreover, our research was focused on the low substrate temperature regime (Ts ~ 200ºC), thus allowing the future use of low cost substrates, such as flexible plastic substrates. In the present work, the influence of the different deposition parameters on the structural and optoelectronic features of both intrinsic and doped a-Si:H is presented and carefully analysed. The ability to grow intrinsic a-Si:H exhibiting satisfactory structural and optoelectronic properties after suitably tuning the deposition parameters is described. Subsequently, the properties of doped layers are shown, these results giving evidence of the satisfactory behaviour of the n-type layers and the limited properties of the p-type ones. Finally, the performance of our first a-Si:H based solar cells completely obtained by Hot-Wire CVD at low substrate temperatures is shown. Promising results are presented, especially when dealing with the properties of both the intrinsic and the n-type material, thus making the employed technique a promising alternative for industrial application

Investigations on doping of amorphous and nano-crystalline silicon films deposited by catalytic chemical vapour deposition

Hydrogenated amorphous and nanocrystalline silicon, deposited by catalytic chemical vapour deposition, have been doped during deposition by the addition of diborane and phosphine in the feed gas, with concentrations in the region of 1%. The crystalline fraction, dopant concentration and electrical properties of the films are studied. The nanocrystalline films exhibited a high doping efficiency, both for n and p doping, and electrical characteristics similar to those of plasma-deposited films. The doping efficiency of n-type amorphous silicon is similar to that obtained for plasma-deposited electronic-grade amorphous silicon, whereas p-type layers show a doping efficiency of one order of magnitude lower. A higher deposition temperature of 450°C was required to achieve p-type films with electrical characteristics similar to those of plasma-deposited films

Collection asymmetry in a drift-driven p-i-n solar cell

An analytical expression for the voltage dependence of the internal collection efficiency of amorphous silicon p-i-n solar cells is presented. The influence of arbitrary drift lengths (asymmetrical case) is taken into account and it is shown how this asymmetry affects the spectral dependence of the collection. A new experimental technique is proposed to determine the so-called collection voltage. Comparison between preliminary experimental data and theoretical results reveals the limitations of the analytical description

Studies on grain boundaries in nanocrystalline silicon grown by Hot-Wire CVD

The use of a tantalum wire in hot-wire chemical vapour deposition (HWCVD) has allowed the deposition of dense nanocrystalline silicon at low filament temperatures (1550 °C). A transition in the crystalline preferential orientation from (2 2 0) to (1 1 1) was observed around 1700 °C. Transmission electron microscopy (TEM) images, together with secondary ion mass spectrometry (SIMS) measurements, suggested that no oxidation occurred in materials obtained at low filament temperature due to the high density of the tissue surrounding grain boundaries. A greater concentration of SiH 3 radicals formed at these temperatures seemed to be responsible for the higher density