Yasuhiko Nakayama

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.

Photoresponse properties of Al/n-beta-FeSi2 Schottky diodes using beta-FeSi2 single crystals

We have clearly observed photoresponse properties in an Al∕n-β-FeSi2 structure using β-FeSi2 single crystals grown by chemical vapor transport. A photocurrent is observed for photons with energies greater than 0.68eV. It increases sharply with increasing photon energy and attains a maximum at approximately 0.95eV (1.31μm). The photocurrent originated from the photoexcited electrons in the Al and the band-to-band photoexcited carriers in the β-FeSi2 located under the Al contact. The photoresponsivity increased upon high-temperature annealing, reaching 58mA∕W at 0.95eV after annealing at 800°C for 8h.

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.

Magnetic properties and resonance linewidths of Zr- and Ti-subsituted Ca-V garnet

Neel temperature and ferrimagnetic resonance linewidth measurements were carried out in the composition Y 3-2x-y Ca 2x+y Fe 5-x-y M y V x O 12 , where M=Zr or Ti. It was revealed, that both of the Zr and Ti substitutions in this system reduced the value of K 1 /M s effectively, and this led to the remarkable linewidth reduction. The lowest value of linewidth obtained was 1.8 Oe at the composition of x=0.4 and y=0.4 for M=Zr.

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.

Preparation of Plate-Like Bulk Beta Iron-Disilicide Crystals Using Metal to Semiconductor Phase Transition by Heat Treatment

Plate-like β-FeSi 2 bulk crystals with size of 10×10 mm 2 and thickness of 1 mm were fabricated by annealing CVT (chemical vapor transport)-grown plate-like α-Fe 2 Si 5 at 800°C in Ar atmosphere. Before annealing, α-Fe 2 Si 5 crystals were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM) to be single crystals with flat surfaces. XRD measurements of β-FeSi 2 crystals subjected to annealing showed that they had a po lycrystalline structure. The mean Fe/Si co mposit ion rat io of β-FeSi 2 crystal measured by energy dispersive x-ray spectroscopy (EDX) was 31/69 and it was the same as that of α-Fe 2 Si 5 bulk crystal before annealing. SEM, Raman scattering and electron probe micro-analysis (EPMA) measurements identified that there existed small Si precipitates mixed in the matrix of β-FeSi 2 crystals. At annealing temperature of 800°C, the plate-like β-FeSi 2 bulk was obtained even the annealing duration time was as short as 5 hours.

Shot leveling and focusing with interferometry for optical lithography of sub-half-micron LSI

This paper presents a shot-by-shot leveling applied to an i-line stepper system. This shot leveler utilizes laser interferometry with a laser beam that has S-polarization and a large incident angle to the exposure surface of the LSI wafer. Thereby leveling and focusing accuracy is preserved regardless of the wafer surface condition under the photoresist. This system acquires interference fringe data with charge-coupled device (CCD) line image detector, and extracts the fringe frequency and phase information with a fast-Fourier-transformation (FFT). These correspond to the tilt and height information of the photoresist surface. The performance was evaluated with 4M-DRAM process wafers. Tilt and height measurement linearity was proved to be ±2 µrad and ±0.3 µm respectively, and total leveling control accuracy was proved to be better than ±7 µrad.

Epitaxial Growth of Garnet Films for Bubble Memory Devices

The distribution coefficient of Ca2+ and Ge4+ ions have been studied by a measurement of Curie temperature in LPE grown films of (Y, Re)3-xCaxFe5-xGexO12 system. It is seen that the substituent ions of Ca2+ and Ge4+ incorporate into the films as one pair by means of the charge compensation, but the distribution coefficient of each ions is quite different with each other. The distribution coeff., α(Ge) is considerably larger than the value, α(Ca). From the determination of the distribution coeff. of both ions, a proper concentration of the rate of GeO2/CaCO3 is found in order to get the films which show high quality.