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.
CuInSe2 (CIS) is expected to be applicable to the solar cell materials. We prepared CIS thin films by pulsed laser ablation technique using CIS targets and investigated the influence of the laser energy density, laser repetition frequency and substrate temperature on the fabrication of CIS thin films. The characterization of CIS thin films were carried out by X-ray diffraction (XRD), scanning electron microscope (SEM), electron probe micro-analyzer (EPMA). Further, as-deposited CIS thin films were annealed in Se vapor in order to improve the crystallinity of CIS thin films. We obtained CIS single phase thin films deposited at laser repetition frequency of 10 Hz and deposition time of 120 min. It confirmed that crystallinity in CIS thin films is improved by increasing substrate temperature. In particular, CIS thin film deposited at substrate temperature of 600 degrees Celsius showed a good crystallinity and smooth surface morphology and few droplets. From the results of optical absorption spectra of CIS thin film deposited at 600 degrees Celsius, it showed high absorption coefficient of the order of 104 to approximately 105 cm-1 in the wide range of wavelength and determined optical band gap Eg equals 1.0 eV. It is found that crystallinity and chemical composition of CIS thin films improved by annealing in Se vapor. CIS thin films was constructed with column-like grains which were grown by annealing.
Under ultra high vacuum, (beta) -FeSi2 thin films were formed by laser ablation method using poly crystal (beta) - FeSi2 as target material that was prepared by horizontal gradient freeze method. In order to compare the physical properties of thin films prepared by laser ablation with those of bulk crystal, (beta) -FeSi2 single crystal was prepared by chemical vapor transport method. The (beta) -FeSi2 plate-like and needle-like crystals were formed at 7 mg and 1.0 g of iodine quantity, respectively. To check a crystal symmetry and orientation, Laue transmission patterns were taken. Anisotropic Raman signals were observed from polarized Raman scattering measurements. Further, electron spin resonance measurement was carried out to examine the residual impurities and to determine g values. From (beta) -FeSi2 films during laser ablation growth, streaky signals were obtained in the RHEED observation. Highly oriented (202)/(220) (beta) -FeSi2 films were predominantly identified in XRD measurements. Raman scattering and optical absorption measurements for these layers revealed that the grown samples are nearly epitaxially-like and have approximately 0.85 eV as its direct optical band-gap.
We report nanometer-sized silicon (Si) crystallites prepared by pulsed laser ablation in constant pressure inert gas ambient. Size distribution of the Si ultrafine particles depends on the pressure of inert gas ambient. It is verified that the size of the Si ultrafine particles is approximately 3 nm and greater in diameter. The nanoscaled ultrafine particles has a crystallinity similar to that of bulk Si. Furthermore, optical properties of the Si nanocrystallites have been studied in terms of the particle size. Visible photoluminescence (PL) bands in the red and green spectral regions appear at room temperature after an oxidation process. The red PL band is independent of the particle size and is stable without degradation by the irradiation of excitation light. In contrast, the green PL band depends on the particle size. The green PL intensity decreases during the irradiation of excitation light in air, and then recovers in the subsequent vacuum evacuation.
Semiconductive iron disilicide (beta) -FeSi2 is an attractive material for optoelectronic and thermoelectric devices that can be integrated on Si substrates. Advantages arise from the direct band-gap, high absorption coefficient and high thermoelectric power figure of merit. We present here the semiconductor properties of (beta) -FeSi2 films on Si(100) substrate prepared by laser ablation (LA) method. We compare these results with those obtained from (beta) - FeSi2 films prepared by ion beam synthesis using high- energy ion implantation and electron beam deposition methods. As for laser ablation, two independent growth processes were adopted using two different target materials, The first one was Fe deposition on Si (100) substrate by LA using Fe target and subsequent high-temperature annealing leading to solid phase epitaxy. The second was LA using (beta) -FeSi2 bulk polycrystal as a target material which was grown by horizontal gradient freeze method. (beta) - FeSi2 films prepared by the two processes were heat- treated as a function of annealing temperature and duration time. Structural characterizations were made by reflection high-energy electron diffraction, x-ray diffraction, Raman scattering and optical absorption spectroscopy measurements at room temperature, which revealed that high-quality semiconducting (beta) -FeSi2 films can be fabricated by two LA processes.
By using molecular beam epitaxy (MBE) technology, one can now fabricate routinely ultra-pure GaAs and Si layers with the background residual impurity concentration lower than approximately 1 by 1014 cm-3. We incorporated mass-separated impurities into these layers by means of high-energy ion-implantation (HE-I2) and low-energy ion beam impinging during MBE growth (CIBMBE). Low-temperature (2K) photoluminescence (PL) spectra from acceptor-doped GaAs layers prepared by HE-I2 and CIBMBE methods revealed that multiple shallow emission levels are formed with increasing net hole concentration, NA-ND. These emissions were theoretically interpreted as the pairs between ground- and excited-states acceptors. The absence of these energy levels in the conventional specimens was attributed to a strong optical compensation effect induced by an extremely small amount of shallow donors that were accidentally introduced during sample preparation. The majority of these energy levels were found to present strong red or blue energy-shifting with increasing NA-ND. These PL features were confirmed to be used for the determination of NA-ND and compensation ratio totally by an optical method. High-dose incorporation of Fe atoms into Si substrates was carried out by HE-I2 method for the synthesis of beta-FeSi2. In 2K-PL spectra, one noticed strong edge-emission from beta- FeSi2, indicating that HE-I2 method is powerful to synthesize high quality silicides.
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