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This paper reports a Fe 3 Si buffer layer for the growth of semiconducting β-FeSi 2 film on stainless steel (SS) substrate. It was formed through the silicidation reaction of Fe and Si at a Fe-rich interface at an elevated temperature of 600 °C. β-FeSi 2 film was deposited on it at low temperature around 400 °C. Auger electron spectroscopy depth profile showed that the inter-diffusions of Fe, Si and impurities contained in the SS substrate were effectively blocked by the buffer layer. The adhesive ability to SS substrate and crystallinity of β-FeSi 2 film were clearly improved by the introduction of the buffer layer, thus also induced the decrease of residual carrier concentration, which was revealed by capacity–voltage measurement.

β-FeSi2 films were prepared on non-silicon substrates by sputtering. The crystalline growth, stress induced cracks and adhesive ability to the substrate were investigated on substrate temperature and thermal expansion coefficient of substrate materials. It was found that crack formation in β-FeSi2 films was dependent on the thermal expansion coefficients of CaF2, MgO and quartz glass insulating materials. High-density cracks were observed from β-FeSi2 films on CaF2 and quartz glass substrates with large difference of the thermal expansion coefficient between β-FeSi2 film and substrate materials, and it was crack-free on MgO substrate with a thermal expansion coefficient close to that of β-FeSi2 films. Polycrystalline β-FeSi2 films grew on Mo, Ta, W, Fe and stainless steel (SS) substrates at low substrate temperature around 400 °C. There was no α-FeSi2 phase confirmed in the films. All the films had continuous structures without noticeable cracks even though they have different thermal expansion coefficients. Capacity–voltage measurements showed that β-FeSi2 films formed on SS substrates has n-type conductivity, with residual carrier concentrations of about 1.3∼6.4 × 1018 cm− 3. Auger electron spectroscopy depth profile measurements identified homogeneous distribution of Fe and Si atoms in the film region, but with a large interface region between the film and the substrate.

High-quality (1 1 0)/(1 0 1)-oriented epitaxial β -FeSi2 films were fabricated on Si (1 1 1) substrate by the sputtering method. The critical feature was the formation of a high-quality thin β -FeSi2 template buffer layer on Si (1 1 1) substrate at low temperature. It was demonstrated that the template is very important for the epitaxial growth of thick β -FeSi2 films and for the blocking of Fe diffusion into the Si at the β -FeSi2/Si interface. Hall effect measurements for β -FeSi2 films showed n-type conductivity, with residual electron concentration around 2.0 × 1017 cm−3 and mobility of 50–400 cm2/V s. A prototype thin-film solar cell was fabricated by depositing n- β -FeSi2 on p-Si (1 1 1). Under 100 mW/cm2 sunlight, an energy conversion efficiency of 3.7%, with an open-circuit voltage of 0.45 V, a short-circuit current density of 14.8 mA/cm2 and a fill factor of 0.55, was obtained.

We fabricated β-FeSi 2 films-based metal-insulator-semiconductor (MIS) devices to avoid influences of diffused iron in Si and at the same time to testify the ability of β-FeSi 2 thin film as a promising optoelectronic material. β-FeSi 2 films were prepared on Si(1 1 1) substrates using sputtering. MIS structures were fabricated by depositing a thin SiO 2 layer (thinner than 10 nm) and then an aluminum film on SiO 2 layer, in which Si was only used as the base for β-FeSi 2 epitaxial growth. I–V measurements showed that rectifying features depend on the thickness of SiO 2 layer and clear rectifying features were obtained from the devices with SiO 2 layer thickness than 7.5 nm. Obvious photo response peaked at a wavelength around 1460 nm was obtained from the devices having SiO 2 layer of about 5.0 and 7.5 nm. These results pave the way to the possible realization of a new β-FeSi 2 optical sensor operating at 1500 nm.

β-FeSi 2 can be used for various optoelectronic devices owing to its superior material features including high optical absorption coefficient and direct band gap of about 0.8 eV. Due to its high refractive index (>5.6), however, suitable antireflection coating (ARC) is necessary for practical device applications. In order to increase the effective areas of optoelectronic devices, transparent electrodes should be also developed. In this work, Al-doped ZnO (AZO) films were fabricated by sputtering on β-FeSi 2 thin films and were found suitable for both transparent electrodes and ARC films. Choosing optimum substrate temperature and sputtering rate, high quality AZO films were formed. The conductivity of AZO films was as high as 3×10 3 S/cm and ohmic contact was easily achieved between AZO and β-FeSi 2 films, indicating AZO film as an ideal transparent electrode for β-FeSi 2 . The transmittance of 400-nm-thick AZO films was >80% and >70% in the wavelength ranges 400–1400 and 1400–1600 nm, respectively. By changing the thickness of AZO film, the central wavelength of minimum reflectance was adjusted to 1550 nm where the total reflectance of AZO/β-FeSi 2 /Si structure was reduced below 2%.

Our previous studies on the Ga doping of β-FeSi 2 films proved that Ga atoms could be an effective p-type dopant by using the molecular beam epitaxy (MBE) method. However, a crucial problem was the generation of a great number of pinholes when a high concentration of Ga atoms was introduced into the β-FeSi 2 films. In this report, we perform a qualitative investigation into Ga diffusion in β-FeSi 2 layers with secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy (AES). We found that Ga atoms diffuse much faster in the β-FeSi 2 layers than in Si, and they tend to congregate on the front surface of β-FeSi 2 films during the post thermal annealing at 800 °C. However, the Ga is supposed to evaporate from the surface. Based on these results, we assumed that the formation of pinholes may be closely related to the Ga diffusion and evaporation in β-FeSi 2 layers during the annealing process. In order to suppress the evaporation of Ga, we deposited an SiO 2 capping layer (∼500 nm in thickness) terminating on Ga-doped Fe/Si multilayers prior to the annealing. Preliminary results demonstrated that pinhole density was reduced by the SiO 2 capping layer.

We fabricated impurity-doped β-FeSi 2 thin films with boron as p-type and arsenic as n-type dopants by sputtering method. The doping source materials were elemental boron chips and heavily arsenic-doped Si chips. They were put on two separate silicon targets and were co-sputtered with silicon during Fe/Si multilayer deposition. For boron-doped p-type β-FeSi 2 films, microstructures were affected by the temperature decrease rate after annealing at 800 °C, and cracks were observed when the temperature decrease rate was high as 20 °C/min. It was found that cracks were eliminated by slow cooling at the rate of 2 °C/min. By changing boron concentration, net hole concentration from 3.0 × 10 17 to 1.0 × 10 19 cm −3 and Hall mobilities from 100 to 20 cm 2 /Vs were successfully achieved. SIMS measurements showed homogeneous distribution of boron dopant in β-FeSi 2 film. For arsenic-doped n-type β-FeSi 2 films, the activation of arsenic dopant requested longer annealing time at 800 °C than for boron. The microstructures were independent on the cooling rate after annealing. The doping level of net electron concentration from 2.0 × 10 17 to 4.0 × 10 17 cm −3 and mobility from 250 to 160 cm 2 /Vs were obtained when the arsenic concentration was changed from about 1.2 × 10 18 to 3.2 × 10 18 cm −3 . β-FeSi 2 thin film p/n homojunctions were formed by successive deposition of p- and n-type β-FeSi 2 films on Si substrates in the same sputtering chamber. The diodes showed rectifying I – V characteristics and photoresponse to 1.3–1.6 μm near-infrared light.

β-FeSi 2 is expected as a new material for light emitting diode and optical sensor operating at 1.5 μm. However, doping techniques suited for various fabrication methods of β-FeSi 2 films have not been established. In this report, we present the results of Ga doping as a p-type impurity into β-FeSi 2 films using three molecular beam epitaxy (MBE)-associated growth methods. They are, (i) standard MBE method by depositing simultaneously Fe, Si and Ga molecular beams, (ii) alternating Fe/Si multilayers deposition method with an interval time between the Fe and Si depositions (migration enhanced epitaxy, MEE) in which Ga was deposited only in Si layer, (iii) standard alternating Fe/Si multilayers deposition (superlattice, SL) method in which Ga was deposited only in Si layer. All β-FeSi 2 films were prepared on Si substrates at room and elevated temperatures in MBE chamber and were subjected to post-annealing. Ga concentration was adjusted by changing its Knudsen effusion cell temperature or opening time of shutter for K-cell. Unintentionally doped β-FeSi 2 films prepared by SL method were n-type, having residual net electron concentration, |N D -N A | of ∼3×10 16 cm -3 and mobility of ∼400 cm 2 /Vs. When these β-FeSi 2 films were doped with Ga above a critical concentration, they exhibited p-type conductivity. By varying Ga concentration, net hole concentration, |N A -N D | ranging from 7×10 16 to 2x ×10 18 cm -3 with hole Hall mobility, μ h from 200 to 10 cm 2 /Vs were obtained. These results demonstrate that Ga is an effective p-type dopant for β-FeSi 2 films to fabricate various electronic and optoelectronic devices.

Phase transition of semiconducting β-FeSi2 to metallic α-FeSi2 has been realized by laser annealing (Nd:YAG laser, λ=1.064 μm) in order to form ohmic electrodes for β-FeSi2 devices. The starting samples were 200-nm-thick β-FeSi2 films formed on Si substrates by reactive deposition epitaxy method. XRD and micro-Raman spectroscopy measurements confirmed the successful phase transition. The electrical resistivity of the α-FeSi2 film was 2.0×10−4 Ω·cm, three orders lower than that of the original β-FeSi2 film (1.4×10−1 Ω·cm). Cross-sectional TEM images and electron diffraction patterns indicated that the upper ∼100 nm of the annealed film was changed to α-FeSi2 while the lower part remained to be β-FeSi2. α-FeSi2 layer had good ohmic contact with β-FeSi2. A testing p-β-FeSi2/n-Si diode with laser-annealed α-FeSi2 as the electrode on β-FeSi2 showed the same rectifying characteristic as that using Al-evaporated electrode.

To place β-FeSi 2 as a 3rd generation Kankyo (Environmentally Friendly) Semiconductor after GaAs, one should demonstrate its superior features by fabricating practical devices. One has to prepare high-quality β-FeSi 2 films with (1) precisely controlled Fe/Si ratio (1:2), (2) flat and cracks-free surface, (3) pinhole-free surface and interfaces, (4) flat interfaces, (5) well-controlled electron or hole concentration with residual carrier concentration as low as ∼10 16 cm −3 . One should accordingly explore novel thin film manufacturing technologies by considering specific properties of constituent Fe and Si atoms. Conventional growth methods used for III–V and II–VI compound semiconductor films are not suited for β-FeSi 2 . Here we summarize the current status of film preparation technologies and describe their advantages and drawbacks. To explore the possibility of β-FeSi 2 for low cost and high conversion efficiency solar cells, high quality β-FeSi 2 films have been formed on Si substrates by molecular beam epitaxy (MBE) and sputtering methods. The critical feature about the device structure is an optimized thin β-FeSi 2 template buffer layer on Si(111) substrate. The template served as a substrate for epitaxial growth of single crystal β-FeSi 2 film and restrains the Fe diffusion into Si at β-FeSi 2 /Si interface. For n-β-FeSi 2 /p-Si structure under air mass 1.5, an energy conversion efficiency of 3.7% was obtained, showing that β-FeSi 2 is practically a promising semiconductor for making solar cells.

For the epitaxial growth of thick β-FeSi2 films, we fabricated ultrathin β-FeSi2 template layers (thinner than 20 nm) on Si (111) substrates with different methods. Surface morphology and crystallinity of the template layers were found to be dependent on the surface conditions of the substrate and the fabrication method. It was revealed that to form a smooth and continuous template, a hydrogen-terminated surface was better than that covered with a several-nanometer oxide layer. Using this surface, continuous (110)/(101)-oriented epitaxial template was obtained by depositing 6-nm iron at 400 °C and subsequent in situ annealing at 600 °C in MBE chamber, namely, a reaction deposition epitaxy (RDE) method. Co-deposition of iron and silicon with atomic ratio of Fe/Si=1/2 allowed the forming of template layers at further low temperature. Co-deposited template layers exhibited better crystallinity and morphology than those prepared by RDE. By using the optimized template layer, we succeeded in growing high-quality thick β-FeSi2 films on Si (111) substrates with sharp β-FeSi2/Si interface.

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.

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.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Plate-like β-FeSi2 bulk crystals with size of 10×10 mm2 and thickness of 1 mm were fabricated by annealing CVT (chemical vapor transport)-grown plate-like α-Fe2Si5 at 800°C in Ar atmosphere. Before annealing, α-Fe2Si5 crystals were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM) to be single crystals with flat surfaces. XRD measurements of β-FeSi2 crystals subjected to annealing showed that they had a po lycrystalline structure. The mean Fe/Si co mposit ion rat io of β-FeSi2 crystal measured by energy dispersive x-ray spectroscopy (EDX) was 31/69 and it was the same as that of α-Fe2Si5 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-FeSi2 crystals. At annealing temperature of 800°C, the plate-like β-FeSi2 bulk was obtained even the annealing duration time was as short as 5 hours.

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.

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/V·s. 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/V·s. 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.

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.

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/V·s. 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/V·s were successfully achieved. Desired net hole concentration was obtained by varying the area ratio of boron chips on silicon target.