Formation of thin β-FeSi2 template layer for the epitaxial growth of thick film on Si (111) substrate

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.     

Formation of SiGe/β-FeSi2 superstructures from amorphous Si/FeSiGe layers

Growth of SiGe/β-FeSi₂ superstructures by annealing of a-Si/a-FeSiGe layered structures was investigated for control of the strains in β-FeSi₂ by Ge doping. The [a-SiGe/β-FeSi₂(Ge)]_n multi-layered structures were formed after annealing at 700 °C. From the analysis of the X-ray diffraction (XRD) spectra, it was found that β-FeSi₂(Ge) was strained by 0.4-0.5% for the sample with n=1. The strains decreased with increasing n, which was due to that the segregation of the Ge atoms from the a-Fe_0.4Si_0.5Ge_0.1 layers to the a-Si layers became large with increasing n. After annealing at 800 °C, agglomeration of β-FeSi₂ occurred, and nanocrystals of relaxed β-FeSi₂ and c-Si_0.7Ge_0.3 were formed. These demonstrate that the SiGe/β-FeSi₂ superstructures were formed by the Ge segregation. These new structures are useful for formation of opto-electrical devices.     

β-FeSi2 growth on Cu-mediated Si substrate and enhancement of photoluminescence

We have investigated the growth of β-FeSi₂ on Cu-mediated (100) Si substrates and photoluminescence (PL) behavior. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations revealed that the mediated surface became amorphous-like Si layers due to Cu atomic diffusion from the surface to the inside of Si and recrystallized during β-FeSi₂ deposition, and that the recrystallization may contribute to the improvement of crystallinity of β-FeSi₂ and the hetero-interface. We have observed pronounced enhancement of PL intensity from β-FeSi₂ grown on the Cu-mediated Si substrate. This implies that non-radiative recombination centers may be decreased by the improvement of hetero-interface.     

Semiconductor-metal phase transition of iron disilicide by laser annealing and its application to form device electrodes

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.     

Toward the β-FeSi2p-n homo-junction structure

β-FeSi₂ thin films were prepared on various substrates, and the influence of the thermal expansion coefficient (TEC) and the softening temperature on the film quality were discussed. It was clarified that a crack-free β-FeSi₂ film could be formed on a glass material substrate with a TEC close to that of β-FeSi₂, and when the softening point of the substrate is close to the crystal growth temperature of β-FeSi₂. A (β-FeSi₂)/(MoSi₂)/(Corning 1737 glass) stacked structure without leak current was prepared to demonstrate the possibility of a MoSi₂ back electrode layer. Furthermore, the (Al-doped p-β-FeSi₂)/(Ni-doped n-β-FeSi₂) homo-junction was also prepared by the vacuum evaporation and thermal diffusion method. We have succeeded in achieving current-rectification to a β-FeSi₂ thin film, although the anode current was small.     

Photoluminescence enhancement by isolating β-FeSi2 layers from defective layers

We have investigated an optimal annealing process in order to enhance 1.55 μm light emission from semiconducting β-FeSi₂ and found that two steps annealing at 600 °C and 800 °C is effective to its enhancement. Rutherford backscattering spectroscopy and SEM observations revealed that pronounced surface segregation of Fe atoms during annealing at 600 °C caused surface precipitate of β-FeSi₂. The enhancement of light emission is attributed spatial isolation of the surface β-FeSi₂ (light emitting layer) from damaged and defective layers with nonradiative recombination centers.     

Important research targets to be explored for β-FeSi2 device making

To place β-FeSi₂ 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-FeSi₂ 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-FeSi₂. Here we summarize the current status of film preparation technologies and describe their advantages and drawbacks. To explore the possibility of β-FeSi₂ for low cost and high conversion efficiency solar cells, high quality β-FeSi₂ 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-FeSi₂ template buffer layer on Si(111) substrate. The template served as a substrate for epitaxial growth of single crystal β-FeSi₂ film and restrains the Fe diffusion into Si at β-FeSi₂/Si interface. For n-β-FeSi₂/p-Si structure under air mass 1.5, an energy conversion efficiency of 3.7% was obtained, showing that β-FeSi₂ is practically a promising semiconductor for making solar cells.     

Morphological modification of β-FeSi2 on Si(111) by high temperature growth and post-thermal annealing

β-FeSi₂ crystals have been grown on Si(111) substrates, and morphological modification of the β-FeSi₂/Si(111) by high temperature growth and post-thermal annealing was investigated. The morphological feature of the β-FeSi₂ crystals significantly depends on the growth conditions, especially, substrate temperature during growth. The β-FeSi₂ continuous layers with relatively smooth surfaces were grown at the low substrate temperatures of 650-700 °C with exposure of the grown layers to Sb flux during the growth. On the other hand, nano-scaled islands have been grown at the higher substrate temperature of 850 °C. The structural property, interfacial morphology and growth evolution of the β-FeSi₂ islands were examined, and compared with those for the layers grown at a lower substrate temperature. In addition, the morphological evolution of the β-FeSi₂/Si layers by post-thermal annealing was examined, and it was found that the interfacial smoothness between the β-FeSi₂ layers and the Si(111) substrates was improved by the post-thermal annealing on condition that a thin SiO_x amorphous overlayer should be formed on the β-FeSi₂ layer during the post-thermal annealing. The mechanisms of the morphological modification at the β-FeSi₂/Si(111) interface by the post-thermal annealing will also be discussed.     

Metalorganic chemical vapor deposition of β-FeSi2 on β-FeSi2 seed crystals formed on Si substrates

We have fabricated a β-FeSi₂ film by metalorganic chemical vapor deposition on a Si(001) substrate with β-FeSi₂ seed crystals grown by molecular beam epitaxy, and investigated the crystallinity, surface morphology and temperature dependence of photoresponse properties of the β-FeSi₂ film. The surface of the grown β-FeSi₂ film was atomically flat, and step-and-terrace structure was clearly observed. Multi-domain structure of β-FeSi₂ whose average size was approximately 200 nm however was revealed. The photoresponse was obtained in an infrared light region (~ 0.95 eV) at temperatures below 200 K. The external quantum efficiency reached a maximum, being as large as 25% at 100 K when a bias voltage was 2.0 V.     

Solid-phase crystallization of β-FeSi2 thin film in Fe/Si structure

Dependence of solid-phase growth of β-FeSi₂ thin films on the crystal orientation of Si substrates has been investigated by using a-Fe (thickness: 20 nm)/c-Si(100), (110) and (111) stacked structures. X-ray diffraction (XRD) measurements suggested that the substrate orientation dependence of the formation rate of β-FeSi₂ was as follows: (100)>(111)>(110). This dependence can be explained on the basis of the lattice mismatch between β-FeSi₂ and Si substrates, i.e., the lattice mismatch between β-FeSi₂(100) and Si(100), β-FeSi₂(110) or (101) and Si(111), and β-FeSi₂(010) or (001) and Si(110) of 1.4-2.0%, 5.3-5.5% and 9.2%, respectively. The substrate orientation dependence of solid-phase growth becomes relatively remarkable for very thin films.     

Carrier control of β-FeSi2 by 1.2 MeV-Au ion irradiation

We have investigated effects of 1.2 MeV-Au⁺⁺ ion irradiation into β-FeSi₂ samples which are synthesized by different processes. Such a high energy Au⁺⁺ ion irradiation can be expected to induce Si and Fe vacancies in the β-FeSi₂ lattice. After recrystallization of the lattice, we found that one of the samples showed conversion from the initial p-type conduction to the n-type one. RBS analysis revealed that irradiated Au atoms even after recrystallization by thermal annealing were not included in the β-FeSi₂ layers. These results suggest that the Au atoms cannot contribute to conversion of the electrical conduction type observed. One possible explanation is that the highly induced Fe vacancy can play donor and its annihilation rate is much slower than that of Si vacancy that surely plays acceptor, so that carrier's compensation balance shifts toward n-type conduction. These results imply that vacancy-induction by high energy ion irradiation can be employed to control a conduction type of β-FeSi₂.     

Transmission electron microscope analysis of epitaxial growth processes in the sputtered β-FeSi2/Si(001) films

The crystallographic orientation relationships and the formation process of β-FeSi₂/Si(001) films were investigated by transmission electron microscopy. A film produced by sputtering pure iron onto a silicon substrate at 600 °C consists of α- and β-FeSi₂ particles. The crystallographic relationships obtained are: (112)_α‖(111)_Si and (101)_β‖(111)_Si or (110)_β‖(111)_Si. The grains of α- and β-FeSi₂ grown inside the substrate adopt the epitaxy to Si(111), irrespective of the surface orientation of the substrate. At 500 °C, on the contrary, there are few α-FeSi₂ grains and some grains of β-FeSi₂ with (100)_β‖(001)_Si [010]_β‖[110]_Si. These results demonstrate that the lower temperature and the higher Fe concentration suppress the formation of α-FeSi₂ and promote the formation of β-FeSi₂ on/below the substrate surface.     

Characterization of a β-FeSi2 p-n junction formed by the PECS method

We have fabricated semiconducting β-FeSi₂ bulks without doping and with Mn and Co doping by using a pulse electric current sintering (PECS) method, and explored the possibility of a direct bonding of n-type and p-type β-FeSi₂ bulks to form a p-n junction structure. P-type Mn-doped and n-type Co-doped β-FeSi₂ bulks were obtained by an annealing process at 800-850 °C for 100 h. The PECS was applied to bond the n-type and p-type bulks together for forming a p-n junction structure. We confirmed that the bonding was processed without any change in the β-FeSi₂ phase and was strongly joined with each other. Although we could not obtain the electrical characteristics of the p-n junction, Seebeck coefficients for n-type and p-type β-FeSi₂ in the bonded sample were determined to be −356 and 778 μV/K, respectively. We propose that these results should lead to an expanded use of the sintered β-FeSi₂ bulks in thermoelectric devices.     

Photoabsorption properties of β-FeSi2 nanoislands grown on Si(111) and Si(001): Dependence on substrate orientation studied by nano-spectroscopic measurements

Photoabsorption properties of β-FeSi₂ nanoislands epitaxially grown on Si(111) and Si(001) have been discussed using photoabsorption nano-spectroscopy based on scanning tunneling microscope. The obtained spectra exhibit clear features around 0.86-0.91 eV and around 0.71-0.74 eV, which are explained as a direct and an indirect photoabsorption edge of β-FeSi₂, respectively. We also observed a blue shift of spectrum obtained from β-FeSi₂ nanoislands on Si(111) substrates, compared to those on Si(001) substrates. We attributed the dependence on Si-substrate orientation not to a quantum confinement effect but to an effect of elastic strain in the? β-FeSi₂ nanoislands epitaxially grown on the substrate.     

Growth of plate-type β-FeSi2 single crystals by optimization of composition ratio of source materials

The importance of the β-FeSi₂ bulk single crystals has increased not only to investigate the intrinsic properties of β-FeSi₂ but also to use it as substrate of β-FeSi₂ thin films for optical devices. Though single crystals of β-FeSi₂ are grown by chemical vapor transport (CVT) method, most of those crystals are needle-like and widths of those flat surfaces are 0.5 mm or less. In order to understand the mechanism of the growth process of β-FeSi₂ by the CVT method and to obtain the conditions for large size crystal growth, we have carried out in-situ observations of the crystal growth by using a transparent electric furnace. Based on the experimental data, we have proposed the most likely reaction process, FeI₂(g) + 2SiI₄(g)→FeSi₂(s) + 5I₂(g), and we found that the crystal growth progresses under the environment where the FeI₂ gas is insufficient compared with a suitable ratio of FeI₂/SiI₄. Then, to raise the partial pressure of FeI₂ gas, the composition ratio of Fe to Si for the source material was increased and we have obtained the plate-type β-FeSi₂ crystals that exceeded a few square millimeters in size.     

Growth of β-FeSi2 thin films on β-FeSi2 (110) substrates by molecular beam epitaxy

We have investigated the preparation of β-FeSi₂ substrate and growth condition of β-FeSi₂ thin film on β-FeSi₂ (110) substrate by molecular beam epitaxy. The surface of the substrate was prepared by a wet-etching using HF(50%):HNO₃(60%):H₂O = 1:1:5 solution at 25 °C. It is clear that the optimal etching period to obtain a flat surface was 3 min. The β-FeSi₂ thin film with streak RHEED pattern was obtained at Si/Fe flux ratio of 2.9. Average surface roughness (Ra) of the β-FeSi₂ film was about 0.5 nm in 5 × 5 μm² area.     

Preparation of β-FeSi2 substrates by molten salt method

Large-sized β-FeSi₂ substrates were successfully prepared for the first time from the silicide bulk crystal grown by the molten salt method. The structural, electrical and optical properties of the as-grown β-FeSi₂ bulk crystals were also investigated. The crystal is single phase β-FeSi₂, and polycrystalline with no preferable growth crystallographic orientations. It was also determined that the β-FeSi₂ shows a p-type conduction, and the hole concentration and the Hall mobility at room temperature were about 10¹⁷ cm^− 3 and 10 cm²/Vs, respectively. In addition, the PL emission around 0.8 eV was realized from the β-FeSi₂ bulk crystal. This simple vacuum-free growth technique of β-FeSi₂ and the large-sized substrate preparation procedure encourage us to develop future silicide-based electronics.     

Electrical properties of p-type β-FeSi2 single crystals grown from Ga and Zn solvents

The electrical properties of β-FeSi₂ single crystals grown from solutions were investigated by the Hall effect measurements. The crystals grown from Ga solvent (Ga-β-FeSi₂) and Zn solvent (Zn-β-FeSi₂) showed p-type conduction. The resistivities at RT of Ga-β-FeSi₂ and Zn-β-FeSi₂ were 0.02-0.03 Ω cm (Ga) and 0.4-2 Ω cm (Zn), respectively. The hole concentration and Hall mobility at RT were (1-2)×10¹⁹ cm⁻³ and 14-16 cm²/V s for Ga-β-FeSi₂ and (2-4)×10¹⁷ cm⁻³ and 19-46 cm²/V s for Zn-β-FeSi₂. The temperature dependence of the hole concentration reveals that ionization energy E_A is approximately 0.02 and 0.12 eV for Ga-β-FeSi₂ and Zn-β-FeSi₂, respectively. We also found the intrinsic conduction of β-FeSi₂ above 500 K.     

Synthesis of β-FeSi2/Si heterojunctions for photovoltaic applications by unbalanced magnetron sputtering

We report here the possibility of the growth of semiconducting FeSi₂ layers on Si(100) substrates by depositing iron with unbalanced magnetron sputtering. The originality of the study is the achievement of heterojunction without any further treatment of the deposited films. Pure iron is deposited on Si(100) substrates with unbalanced magnetron sputtering for the production of β-FeSi₂/Si heterojunctions. Prior to coating process the substrates are cleaned with neutral molecular source. Microstructure of β-FeSi₂ films were investigated by X-Ray Diffraction analysis and Raman Spectroscopy. Dark current-voltage characteristic of the deposited coatings showed a rectifying behavior for the β-FeSi₂/Si heterojuctions. Open-circuit voltage (V_oc) and short-circuit current density (J_sc) were measured under 100 mWcm^− 2 illumination and a V_oc of 360 mV and J_sc of 180 μAcm^− 2 were measured. The illumination of the silicon side gave higher photosensitivity than the illumination of the iron silicide side.     

Effect of target compositions on the crystallinity of β-FeSi2 prepared by ion beam sputter deposition method

Targets with the elemental composition of Fe, Fe₂Si and FeSi₂ were employed in the present study to grow β-FeSi₂ film on Si (100) substrate by means of ion beam sputter deposition (IBSD) method. The results revealed that when FeSi₂ target was employed, a Si-rich phase, α-FeSi₂ (Fe₂Si₅), was predominant at temperatures above 973 K, while β-FeSi₂ phase was observed only in the limited temperature range at around 873 K. In this case, Si was originated both from the sputtered target and the substrate, thus, the supply of Si was considered to be excessive to sustain β structure. On the other hand, the films prepared with Fe target became polycrystalline as they grow thicker than 100 nm. In order to optimize the supply of Fe and Si for epitaxial growth, Fe₂Si target was employed, where highly (100)-oriented β-FeSi₂ layer of 120 nm in thickness was obtained at 973 K.