Naotaka Otogawa

Reduction of iron diffusion in silicon during the epitaxial growth of β-FeSi2 films by use of thin template buffer layers

We fabricated continuous highly (110)/(101)-oriented β-FeSi2 films on Si (111) substrates by the facing-target sputtering method. An epitaxial thin β-FeSi2 template buffer layer preformed on the silicon substrate was found to be essential in the epitaxial growth of thick β-FeSi2 films. It was proved that the template reduced the iron diffusion into the silicon substrate during thick β-FeSi2 film fabrication. Even though the annealing was performed at high temperature (880 °C) for a long duration (10 h), iron diffusion was effectively hindered by the template. By introducing this template buffer layer, an abrupt interface without appreciable defects between the β-FeSi2 film and the silicon substrate formed. The mechanism for the reduction of iron diffusion by the template buffer layer is discussed.

β-FeSi 2 as a Kankyo (environmentally friendly) semiconductor for solar cells in the space application

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.

Boron doping for p-type β-FeSi2 films by sputtering method

High quality epitaxial β-FeSi2 thin films prepared by alternate Fe/Si multilayers stacking were doped for p-type by co-sputtering of silicon and boron, in which elemental boron chips were placed on silicon target. The starting β-FeSi2 films before doping were n-type with residual electron concentration of about 2 ×1017 cm-3 and mobility of about 200 cm2/Vs. After doping with boron, β-FeSi2 films showed the same epitaxial crystallinity with continuous structure as that of non-doped one. Doping level of p-type β-FeSi2 films with net hole concentration from 3 ×1017 to 1 ×1019 cm-3 and mobility from 100 to 20 cm2/Vs were successfully achieved. Desired net hole concentration was obtained by varying the area ratio of boron chips on silicon target.

Prototype infrared optical sensor and solar cell made of β-FeSi2 thin film

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.

Arsenic doping of n-type β-FeSi2 films by sputtering method

High quality epitaxial n-type β-FeSi2 thin films prepared by alternate Fe/Si multilayer deposition were doped with arsenic as impurity by sputtering method. Doping sources were heavily arsenic-doped silicon chips placed on the surface of silicon target. The starting β-FeSi2 films before doping were typically n-type with a residual electron concentration of about 1.0 ×1017 cm-3 and a mobility of about 260 cm2/Vs. When arsenic concentration changed from 7.0 ×1017 to 1.9 ×1018 cm-3, net electron concentration increased from about 2.0 ×1017 to 4.0 ×1017 cm-3, and electron mobility decreased from 250 to 160 cm2/Vs. Secondary ion mass spectroscopy (SIMS) measurements showed a homogeneous arsenic distribution in β-FeSi2 films and a small diffused region (about 50 nm) at the interface between β-FeSi2 and a Si substrate. Arsenic-doped β-FeSi2 films exhibited epitaxial growth in the (110)/(101) orientation and a continuous structure without cracks. However, other crystalline orientations together with pinholes appeared when the arsenic doping concentration increased.

Characterization of β-FeSi2 films as a novel solar cell semiconductor

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%.

Growth of beta-iron disilicide (β-FeSi 2 ) on flexible metal sheet substrates for solar-cell application

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.

Formation of β-FeSi2 Microstructures by Reactive Ion Etching Using SF6 Gas

The reactive ion etching (RIE) technique, using SF6 as reaction gas, was applied for the formation of microstructures of semiconductor β-FeSi2 films. The etching mask is an Al film patterned by photolithography and wet chemical etching. The maximum etch rate of β-FeSi2 is 0.1 µm/min and the etch selectivity of β-FeSi2 to the Al mask is about 1. This attempt suggests that existing RIE techniques for Si process may be applied directly to the micro fabrication of β-FeSi2.