Yasuhiro Fukuzawa

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.

Effect of low-energy nitrogen molecular-ion impingement during the epitaxial growth of GaAs on the photoluminescence spectra

Low-energy (∼100 eV) nitrogen molecular ions (N2+) were impinged during molecular beam epitaxial growth of GaAs at the substrate temperature of 550u200a°C. In the low-temperature (2 K) photoluminescence (PL) spectra, extremely sharp N-related emissions (Xi, i=1, 2, and 5) were observed in as-grown condition. These emissions were roughly two orders of magnitude stronger than those formed by the impingement of nitrogen atomic ions (N+). The results indicate that nitrogen (N) atoms are in situ substituted at As sites without inducing large structural damages and become quite efficient radiative recombination centers as isoelectronic impurities in GaAs. Further, to study the substitutional condition of N isoelectronic impurity, N isotope (15N) doped GaAs was grown by 15N2+ ion impingement. When 15N is doped, PL peak energy of X5 shifted towards higher energy side by 1.8 meV. The value is fairly close to the expected one of 1.9 meV when 15N replaces 14N. Together from energy separation between X2 emission (∼60 meV...

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.

Highly Efficient Nitrogen Doping Into GaAs Using Low-Energy Nitrogen Molecular Ions

Low-energy N 2 + molecular-ions were irradiated during the epitaxial growth of GaAs. Ion acceleration energy and ion beam current density were varied in the range of 30-200 eV and 3-37 nA/cm 2 , respectively. GaAs growth rate was kept constant at 1µm/ h and the thickness of N-doped GaAs layer was about 1 µm. N concentration was obtained by using secondary ion mass spectroscopy. Strong N-related emissions were observed in the low-temperature photoluminescence spectra, which indicates that N atom is efficiently substituted at As site and is optically active as an isoelectronic impurity

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.

Effect of nitrogen ion impingement during molecular beam epitaxy growth of GaAs as a function of acceleration energy

Nitrogen ions were impinged during the molecular beam epitaxial growth of GaAs at 550°C, varying its acceleration energy in the range from 100 eV to 10 keV. No photoluminescence (PL) emissions were observed in as-grown condition when ion acceleration energy becomes higher than 500 eV. After high-temperature annealing at 750°C, structural defects were removed and incorporated nitrogen atoms became optically active. PL emissions that relate to isoelectronic impurity and dilute GaAsN alloys were observed with an ion acceleration energy of 10 keV.