Electronic devices based on semiconductors such as diodes, transistors and integrated circuits can be found everywhere. For many of these applications, if the cost of the device is significantly reduced, many more applications can be envisaged. This is especially true for the photovoltaic or solar energy application, for a whole range of sensors as well as for other application areas such as ferroelectric devices, light emitting diodes for solid state lighting applications, storage applications such as computer hard disc drives, magnetoresistance based devices, photoluminescence based devices, non-volatile memory applications, dielectric devices, thermoelectric devices,
The use of renewable energy sources is essential for the future of the world we are living in. There is an unlimited potential for solar energy to power all the world's energy needs. However, for the past two decades, the promise of solar energy has remained unrealized. This is primarily because of the unfavorable price/performance metric of solar cells being manufactured today. Further technological innovations have the potential to cause the economic and commercial breakthrough necessary to lower prices to make solar energy cheaper than or equal to fossil fuels in cost.
Thin-film photovoltaics (PV) have a significant advantage over the traditional wafer-based crystalline Si cells. The primary advantage of thin films is cheaper materials and manufacturing costs and higher manufacturing yields compared to single-crystal technologies. Thin films use 1/20 to 1/100 of the material needed for crystalline Si PV and appear to be amenable to more automated, less expensive production. Currently, three film technologies are receiving significant interest from the industry for large scale PV: amorphous Si, CuInSe2 and CdTe. In most cases, module efficiencies are closely related to cell efficiencies, with minor losses (˜10%) due to some loss of active area and some electrical resistance losses. In order to further increase the efficiency and to be able to reproducibly fabricate thin-film based, high efficiency cells, microstructural features which limit the performance need to be controlled. While a complete understanding of the microstructural features which limit the performance are still unclear, it is reasonably well established that recombination at grain boundaries, intragrain defects and impurities is critical. In an effort to minimize the effect of grain boundaries, films with large grains or only low-energy GB's are an objective.
Most thin-film solar cells are based on polycrystalline device layers since the cost of single crystal substrates is prohibitively expensive. Because they are polycrystalline, they do not have a well-defined crystallographic orientation (both out-of-plane and in-plane). Crystallographic orientation can have two important effects. The first is the effect of orientation of the growth surface on incorporation of dopants, intrinsic defects, and other impurities. Previous studies on a wide variety of dopants have shown that variations of 1 to 2 orders of magnitude can occur based on crystallographic orientation. An extreme effect of anisotropic doping is Si doping in GaAs films. Si doping in GaAs films, causes n-type conduction on (111) B-type GaAs, but p-type on (111) A-type GaAs. The second effect of crystallographic orientation is a variation in growth rate of the film being deposited. Both experiments as well as simulations have shown that under certain conditions growth rates can vary by 1 to 2 orders of magnitude as a function of crystallographic orientation. Uncontrolled crystallographic orientation in PV materials with large grain sizes may therefore result in reproducibility problems and hence lower yields during high volume production. Of course, grain boundaries at the intersection of grains in the polycrystalline film act as detrimental, recombination centers.
Most of the microstructural features currently thought to be limiting polycrystalline, thin-film, solar cell performance can be avoided by growing epitaxial films on lattice-matched, single crystal substrates. However, the high costs of single crystal substrates, prohibits their use for realistic applications. The effect of grain boundaries can be circumvented in polycrystalline photovoltaic thin films if the grain sizes are large enough (grain size at which effects on properties are minimal depend among other things on the doping level). However, in thin-films, grain growth is typically restricted to only twice the thickness of the film. Hence, grain boundaries in polycrystalline films have a dominant effect on efficiencies. A large number of studies have reported the effects of grain boundaries on photovoltaic properties.
While much of the discussion above has focused on the solar cell application, there are numerous applications where a low cost, practically scalable method of fabricating single crystal-like semiconductor films is required where the effective size of the single crystal required is about a 100 μm or a few hundred microns in diameter. Furthermore, for certain applications, the semiconductor surfaces/films/wafers need to be flexible, thus enabling applications where a curved semiconductor may be desirable. For example, for a solar cell application it may be desirable to conform the PV module to the contour of a roof upon where it is placed. Thin-film transistors are used for fabricating displays. In this application one can also easily appreciate the use for flexible and large-area displays.
For electronic devices, an ordered array of three dimensional nanodots and nanorods promises to extend device physics to full two- or three-dimensional confinement (quantum wires and dots). Multidimensional confinement in these low dimensional structures has long been predicted to alter significantly the transport and optical properties, compared to bulk or planar heterostructures. More recently, the effect of charge quantization on transport in small semiconductor quantum dots has stimulated much research in single-electron devices, in which the transfer of a single electron is sufficient to control the device. The most important factor driving active research in quantum effect is the rapidly expanding semiconductor band-gap engineering capability provided by modern epitaxy. Possible applications include spin transistors and single electron transistors. Other possible applications of three dimensionally ordered nanodots and nanorods include potential applications in optoelectronics and sensors. For example, an array of luminescent ordered nanodots within a transparent matrix can be used for devices using the photoluminescence effect. Other applications include those in highly efficient photovoltaics, solid-state lighting devices, etc.