Structure and Properties of Quasi-Monocrystalline Silicon Thin-Films
- PDF / 1,498,021 Bytes
- 6 Pages / 390.24 x 621.9 pts Page_size
- 37 Downloads / 175 Views
ABSTRACT This contribution describes the preparation of a single crystalline Si thin-film separable from a reusable Si wafer. The method relies on: i) etching of a porous silicon layer ii) hightemperature annealing and iii) transfer of the recrystallized film to a foreign substrate. As a result of the process we obtain 1 to 30 jim thick monocrystalline Si films that contain voids with a size of several 100 nm. Due to its "swiss-cheese-like" structure the material is termed as "quasi-monocrystalline Si". Sub micrometer thin layers are almost compact, while in several micron thick films voids cause scattering of incident light. This effect increases the effective absorption coefficient by light trapping and seems promising for the application of our quasimonocrystalline films in thin film solar cells. Quasi-monocrystalline p-type silicon reaches a hole mobility of 78 cm2 / Vs measured by room-temperature Hall-effect. High carrier mobility and adjustable optical characteristics make these films suitable for display and photovoltaic applications. Quasi-monocrystalline films are processed using conventional high-temperature Si processing; finished devices can be transferred to a foreign substrate such as glass, while the starting wafer can be reused several times. INTRODUCTION Requirements on electronic properties of Si films for photovoltaic and for display technology are quite similar: low defect density, low contamination level and large-area processing at low cost are essential ingredients for these applications [1]. However, the
fabrication of device grade semiconductor films on cheap substrates, such as glass, is still a challenge. One approach is the crystallization of thin amorphous or micro-crystalline Si films by laser annealing leading to crystalline Si on glass with a grain size of several 4im [2]. On the other hand, the transfer of monocrystalline thin-films to glass offers the possibility to obtain material of improved quality, without the limitations imposed by grain boundary effects. During the last years several layer transfer techniques have been developed: Isolated Silicon Epitaxy (ISE) uses an oxidized silicon wafer as host wafer for the subsequent crystallization of a polycrystalline Si layer. Here the wafer edge serves as a seed for the crystallization process. The resulting monocrystalline film is released from the wafer and transferred to glass for the use in active matrix liquid crystal displays (AMLCDs) [3]. The "smart cut" method [4] as well as the "smarter cut approach" [5] or a recent modification of this process [6] use thermal annealing of a hydrogen implanted silicon wafer to peel-off a thin monocrystalline Si film. Other groups use a porous silicon film as a seeding layer for epitaxy and a subsequent release and transfer of the epitaxial layer [7,8,9]. Unfortunately, these techniques make use of expensive process steps such as epitaxy or implantation, or they suffer from low crystalline quality or low throughput. In contrast to previous methods, the process demonstrated here makes use o
Data Loading...
