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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 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.

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.