Partner und Internationale Organisationen
(Englisch)
|
BP Solar, HMI, Uni Stuttgart, Uni Barcelona, CNRS Paris, Uni Wien
|
Abstract
(Englisch)
|
Micro crystalline thin films attract attention as potential photovoltaic material for stable high efficiency thin film solar cells. The scope of the ongoing project is the evaluation of deposition techniques for the formation of micro crystalline thin films on glass. Generally techniques avoiding damages of the growing films due to high energy ions during growth are preferred. In addition, the process of growth for microcrystalline films should yield a high deposition rate to achieve large scale industrial production of thin film solar cells. To serve these two purposes at the same time, one needs to use high plasma powers and low voltages to avoid high energy particles creating damage due to sputtering. For these reasons, here we have developed a remote PECVD using a low voltage high current DC plasma. With this configuration one may increase the power to the plasma without creating much damage to the growingˆ films so that simultaneous increase in the growth rate as well as high quality microcrystalline films are achieved.
In the present work, microcrystalline silicon films are grown on glass substrates using Remote DC Plasma Enhanced Chemical Vapour Deposition(RPECVD). The films were characterised for their surface, structural, electrical and optical properties. The nucleation and growth of microcrystalline films are analysed in detail by several characterisation techniques. The growth of microcrystalline films at different concentration of silane in the mixture of H2 and Ar is studied. The structural and surface properties of the films grown at different dilution ratios (SiH4/SiH4+H2) are studied. The substrate temperature was increased up to 400ºC. All the films grown are microcrystalline. A growth rate of 4Å/sec is achieved with the present set up for microcrystalline silicon films. The amount of amorphous matrix in the film varies depending on the ratio of dilution of silane. The general trend is the increase in the amount of amorphous tissues in the film with increase in silane concentration. The grain size increases as one decreases the amount of silane concentration. The structural properties are analysed in detail using X-ray, Raman, infra red(IR) spectroscopic measurements and TEM studies. The <220> orientation of silicon films can be influenced by changing the growth conditions. For a completely columnar growth, highly preferred orientation along <220> is necessary. The <220> orientation is enhanced at high dilution of silane. On the other hand the deposition rate is reduced considerably at this growth regime. To have both high deposition rate as well as columnar growth at the same time, it seems that one has to increase the plasma power and or substrate temperatures. From Raman spectroscopic measurement the amount of amorphous phase and crystalline phase present in the films are clearly manifested. From the shift in the wave number shift (cm-1), from 520 cm-1 of single crystalline Si towards 480cm-1 for amorphous silicon, we estimate the size of the grains. The trend shows that the increase in silane concentration during growth decreases the grain size. IR spectroscopic measurements show the presence of 'fingerprint' of microcrystalline silicon at 2100cm-1 to 2500 cm-1. This shows the presence of Si-H bonds in the film that occurs at the boundaries of microcrystalline grains. This indication of the grain size change is in agreement with the IR spectroscopic measurements. The incorporation of hydrogen is increased at high concentrations of silane during growth. Transmission Electron Microscopic (TEM) images clearly show the presence of microcrystalline domains in an amorphous matrix. Also it exhibits the presence of twins in the film. The cross TEM images show a narrow, with a width of 20nm typically, and columnar structure of the grains starting from the glass substrate to the top of the films. This is a required feature of the hydrogenated microcrystalline silicon to have an enhanced conduction along the columnar structure in order to fabricate thin film solar cells. The electrical properties of microcrystalline thin films are also analysed using a van der Pau as well as a two probe configurations.
Two different configurations of anode were investigated for a remote plasma enhanced chemical vapour deposition. The first one is a cylindrical anode and the later is a circular ring anode. In both cases mostly we get microcrystalline films.
Thin films of silicon were grown by varying the substrate temperature up to 450°C as well as the plasma power. With this, the deposition rate of microcrystalline films could be slightly improved. Microcrystalline film with a growth rate of 2.3 Å/sec were achieved with the circular ring anode configuration. The main effect of the increase in substrate temperature as well as the plasma power is the improvement of the quality or the microcrystalline content of the material. This is due to the increased surface mobility of dissociated atoms and molecules from silane.
Thin films of silicon were also grown by Molecular Beam Epitaxy (MBE) on glass, metallic as well as SiO2 substrates. The growth mechanisms of microcrystalline films grown by PECVD as well as MBE were compared. By MBE one gets a highly columnar microcrystalline films with a grain size of 0.5 micron as is seen from the cross TEM pictures. These grains with a complete orientation along <220> direction are columnar from the substrate up to the top surface of the film. When such a grain size and structure is an inherent feature of microcrystalline films by MBE, the disadvantage is that the grain boundaries of the films were not passivated with hydrogen. Certain microcrystalline films deposited by MBE as well as PECVD show surface texture. This is a required property of thin films for solar cells to achieve 'Light trapping'. Cross TEM analysis showed that surface texture in the present case is induced by metallic underlying layers.
Development of Transparent Conducting Oxides was carried out. The stability of certain TCOs were analysed under device processing conditions. Deposition system for the three important TCOs; ITO, SnO2:Sb, ZnO:Al, (Tin doped Indium Oxide, Antimony doped Tin Oxide, Aluminium doped Zinc Oxide) was established. With this one will obtain a clear picture of all three TCOs grown in the same chamber. Also the differences between RF and DC magnetron sputtering of these films will be obtained. The later is highly preferred for industrial fabrication of TCOs
|
