β-FeSi2 has many attracting properties as a semiconductor not consisting of toxic chemical elements and is an
ideal semiconductor as a thin film solar cell owing to its extremely high optical absorption coefficient. To evaluate
β-FeSi2 as a solar cell, photo-response measurement is critically important and useful. Since β-FeSi2 thin films are
normally deposited on Si substrates, intrinsic photo-response of β-FeSi2 is usually difficult to be collected due to the
strong contribution from Si substrates. We here present the photo-response from bulk β-FeSi2 crystals, expecting that we
can eliminate the contributions coming from the Si substrates and the crystallographic defects existing at the β-FeSi2/Si
interfaces when we use β-FeSi2 thin films. We prepared bulk specimens by chemical vapor transport method (CVT) in
which needle-like and plate-like β-FeSi2 crystals were obtained. We chose the former specimens for the formation of
Al/n-β-FeSi2 Schottky contacts to measure their photo-responses. These contacts were found to form Schottky diodes
even though there are large series resistances and leakage currents. Under laser light illumination of 1.31 μm through
optical fiber, the positive voltage was observed between the Al contact and the In solder glued to the back-surface of
β-FeSi2 bulk specimen. Two-dimensional distribution of photo-responses were measured by scanning the above optical
fiber with the spot size of 50 μm. The highest photo-response was obtained in the vicinity of Al wire, and was 7.7 mA/W
for the as-grown sample, and 31 mA/W for the annealing one, respectively. These observations state that β-FeSi2 holds
appropriate optical features to be used as a solar cell.
β-FeSi2 is an attractive semiconductor owing to its extremely high optical absorption coefficient (α>105 cm-1), and is expected to be an ideal semiconductor as a thin film solar cell. For solar cell use, to prepare high quality β-FeSi2 films holding a desired Fe/Si ratio, we chose two methods; one is a molecular beam epitaxy (MBE) method in which Fe and Si were evaporated by using normal Knudsen cells, and occasionally by e-gun for Si. Another one is the facing-target sputtering (FTS) method in which deposition of β-FeSi2 films is made on Si substrate that is placed out of gas plasma cloud. In both methods to obtain β-FeSi2 films with a tuned Fe/Si ratio, Fe/Si super lattice was fabricated by varying Fe and Si deposition thickness. Results showed significant in- and out-diffusion of host Fe and Si atoms at the interface of Si substrates into β-FeSi2 layers. It was experimentally demonstrated that this diffusion can be suppressed by the formation of template layer between the epitaxial β-FeSi2 layer and the substrate. The template layer was prepared by reactive deposition epitaxy (RDE) method. By fixing the Fe/Si ratio as precisely as possible at 1/2, systematic doping experiments of acceptor (Ga and B) and donor (As) impurities into β-FeSi2 were carried out. Systematical changes of
electron and hole carrier concentration in these samples along variation of incorporated impurities were observed through Hall effect measurements. Residual carrier concentrations can be ascribed to not only the remaining undesired impurities contained in source materials but also to a variety of point defects mainly produced by the uncontrolled stoichiometry. A preliminary structure of n-β-FeSi2/p-Si used as a solar cell indicated a conversion efficiency of 3.7%.
Semiconductor iron-disilicide (β-FeSi2) is expected to be used for thin film solar cells owing to its direct band gap (around 0.85eV) feature and high optical absorption coefficient (α) that is higher than 105cm-1. To fabricate β-FeSi2 solar cells on Si substrates, thick Si substrates are needed, and cost reduction is hard to be accomplished. This paper shows the possibility to use non-Si substrates such as insulating materials or metal sheets for replacing the Si substrates. SOI, fused quartz and CaF2 single-crystal were used as non-metal substrates, and Mo, Ta, W, Fe and stainless steel sheets were used as metal substrates. Growth of β-FeSi2 thin films was carried out with changing substrate temperature by facing-target sputtering (FTS) method. Formation of β-FeSi2 thin film was characterized by XRD and Raman scattering observations. Adhesion force of the films to the substrates was evaluated by pealing test and electrical properties were examined by Seebeck and Hall effects measurements. Results showed that stainless steel and iron sheets become good substrates for the growth of β-FeSi2 thin films. Peeling tests and SEM surface observations of these films stated that the adhesion force of these films to iron sheet and to stainless steel sheet is satisfactorily strong. Results of films deposited on the remaining substrates indicated that formation of β-FeSi2 thin films was not clearly identified, and those films were easily removed from the substrate.
β-FeSi2 defined as a Kankyo (Environmentally Friendly) semiconductor is regarded as one of the 3-rd generation semiconductors after Si and GaAs. Versatile features about β-FeSi2 are, i) high optical absorption coefficient (>105cm-1), ii) chemical stability at temperatures as high as 937°C, iii) high thermoelectric power (Seebeck coefficient of k ~ 10-4/K), iv) a direct energy band-gap of 0.85 eV, corresponding to 1.5μm of quartz optical fiber communication, v) lattice constant nearly well-matched to Si substrate, vi) high resistance against the humidity, chemical attacks and oxidization. Using β-FeSi2 films, one can fabricate various devices such as Si photosensors, solar cells and thermoelectric generators that can be integrated basically on Si-LSI circuits. β-FeSi2 has high resistance against the exposition of cosmic rays and radioactive rays owing to the large electron-empty space existing in the electron cloud pertinent to β-FeSi2. Further, the specific gravity of β-FeSi2 (4.93) is placed between Si (2.33) and GaAs ((5.33). These features together with the aforementioned high optical absorption coefficient are ideal for the fabrication of solar cells to be used in the space. To demonstrate fascinating capabilities of β-FeSi2, one has to prepare high quality β-FeSi2 films. We in this report summarize the current status of β-FeSi2 film preparation technologies. Modified MBE and facing-target sputtering (FTS) methods are principally discussed. High quality β-FeSi2 films have been formed on Si substrates by these methods. Preliminary structures of n-β-FeSi2 /p-Si and p-β-FeSi2 /n-Si solar cells indicated an energy conversion efficiency of 3.7%, implying that β-FeSi2 is practically a promising semiconductor for a photovoltaic device.
A prototype infrared optical sensor has been fabricated by using a 0.21 μm-thick β-iron disilicide (β-FeSi2) thin film prepared by reactive deposition epitaxy (RDE) on an n-type (100) Si substrate (ρ approximately 1.5 Ωcm). Manganese ions (Mn+) were implanted into the β-FeSi2 thin film as p-type dopants with a total dose of 5.5 x 1018 cm-3. Al and AuSb thin films were metallized on β-FeSi2 and Si surfaces respectively as electrodes. A circle area of the FeSi2 film was left naked as the illumination window. The good diode characteristic confirmed the high quality of the pn junction. The spectroscopic spectrum indicated a clear photoresponse at room temperature. As evaluated by a standard solar simulator, the device provided an open-circuit voltage of voc = 261 mV and a short-circuit current density of Jsc = 3.1 mA/cm2, suggesting a large potential of such devices in solar energy conversion. Rutherford backscattering spectroscopy (RBS) measurements found a large volume of oxygen in the surface of the β-FeSi2 thin film and severe Fe/Si interdiffusion at the silicide-Si interface. These unwanted effects may be responsible for the unideal device performance. Methods to solve these problems are discussed including a proposal of an all-iron-silicide structure.
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