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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%.

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.

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.

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.

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.