Epitaxial β-FeSi2 Research Articles

Enhancement of photoluminescence from iron disilicide on Si(111) substrates with Au layers by controlling microstructures

The photoluminescence (PL) spectrum of iron disilicide (β-FeSi2) was enhanced by increasing the density of epitaxial grains grown at 650 °C on Si(111) substrates with a Au layer. The Au–Si liquidus phase obtained from the Au–Si eutectic reaction contributed to the formation of epitaxial β-FeSi2 grains on the Si(111) surface. The density of the epitaxial β-FeSi2 grains on the Si(111) surface decreased with increasing thickness of the Au layer, while the grain size increased from 0.5–2 to 20–50 µm. It is suggested that the excitation volume of the β-FeSi2 is a key point that determines the PL intensity.

Thermoelectric properties of epitaxial β-FeSi2 thin films grown on Si(111) substrates with various film qualities

Si-based epitaxial β-FeSi2 thin films are attractive as materials for on-chip thermoelectric power generators. We investigated the structure, crystallinity, and thermoelectric properties of β-FeSi2 thin films epitaxially grown on Si(111) substrates by using three different techniques: conventional reactive deposition epitaxy followed by molecular beam epitaxy (RDE+MBE), solid phase epitaxy (SPE) based on codeposition of Fe and Si presented previously, and SPE followed by MBE (SPE+MBE) presented newly by this work. Their epitaxial growth temperatures were fixed at 530 °C for comparison. RDE+MBE thin films exhibited high crystalline quality, but rough surfaces and rugged β-FeSi2/Si(111) interfaces. On the other hand, SPE thin films showed flat surfaces and abrupt β-FeSi2/Si(111) interfaces but low crystallinity. We found that SPE+MBE thin films realized crystallinity higher than SPE thin films, and also had flatter surfaces and sharper interfaces than RDE+MBE thin films. In SPE+MBE thin film growth, due to the initial SPE process with low temperature codeposition, thermal interdiffusion of Fe and Si was suppressed, resulting in the surface flatness and abrupt interface. Second high temperature MBE process improved the crystallinity. We also investigated thermoelectric properties of these β-FeSi2 thin films. Structural factors affecting the thermoelectric properties of RDE+MBE, SPE, and SPE+MBE thin films were investigated.

Effect of microstructures on electrical conduction properties of β-FeSi2 epitaxial films

We investigated the carrier concentration and Hall mobility of epitaxial β-FeSi2 films grown on semi-insulating 4H-SiC(001) single crystals to clarify the contribution of the microstructures to their electrical conduction properties. These electrical conduction properties of epitaxial β-FeSi2 films were changed by their grain size. The Hall mobility of the films consisting of the grains of 250–350nm in size was limited by the grain boundaries, while it was not limited by grain boundaries for the films consisting of the coarse grains of 20–40µm in size but limited by defects inside the grains.

Thermoelectric Properties of Epitaxial β-FeSi2 Thin Films on Si(111) and Approach for Their Enhancement

We have investigated the intrinsic thermoelectric properties of epitaxial β-FeSi2 thin films and the impact of phosphorus (P) doping. Epitaxial β-FeSi2 thin films with single phase were grown on Si(111) substrates by two different techniques in an ultrahigh-vacuum molecular beam epitaxy (MBE) system: solid-phase epitaxy (SPE), where iron silicide films formed by codeposition of Fe and Si at room temperature were recrystallized by annealing at 530°C to form epitaxial β-FeSi2 thin films on Si(111) substrates, and MBE of β-FeSi2 thin films on epitaxial β-FeSi2 templates formed on Si(111) by reactive deposition epitaxy (RDE) at 530°C (RDE + MBE). Epitaxial SPE thin films based on codeposition had a flatter surface and more abrupt β-FeSi2/Si(111) interface than epitaxial RDE + MBE thin films. We investigated the intrinsic thermoelectric properties of the epitaxial β-FeSi2 thin films on Si(111), revealing lower thermal conductivity and higher electrical conductivity compared with bulk β-FeSi2. We also investigated the impact of doping on the Seebeck coefficient of bulk and thin-film β-FeSi2. A route to enhance the thermoelectric performance of β-FeSi2 is proposed, based on (1) fabrication of thin-film structures for high electrical conductivity and low thermal conductivity, and (2) proper choice of doping for high Seebeck coefficient.

Luminescence properties of Si-capped β-FeSi2 nanodots epitaxially grown on Si(001) and (111) substrates

We studied the luminescence properties of Si-capped β-FeSi2 nanodots (NDs) epitaxially grown on Si substrates by using photoluminescence (PL) and electroluminescence (EL) spectroscopies. Codepositing Fe and Si on ultrathin SiO2 films induced the self-assembly of epitaxial β-FeSi2 NDs. The PL spectra of the Si/β-FeSi2 NDs/Si structure depended on the crystal orientation of the Si substrate. These structures exhibited a broad PL peak near 0.8 eV on both Si(001) and (111) substrates. The PL intensity depended on the shape of the β-FeSi2 NDs. For the flat NDs, which exhibited higher PL intensity, we also recorded EL spectra. We explained the luminescence properties of these structures by the presence of nanostructured Si offering radiative electronic states in the Si cap layers, generated by nano-stressors for upper Si layer: the strain-relaxed β-FeSi2 NDs.

Necessity of Epitaxial Growth of β-FeSi2 Thin Films in Formation of n-Type β-FeSi2/p-Type Si Heterojunction Photodiodes

n-Type β-FeSi2 thin films were epitaxially and non-epitaxially grown on p-type Si(111) substrates by facing-targets direct-current sputtering (FTDCS) at a substrate temperature of 600{degree sign}C. Whereas the n-type epitaxial β-FeSi2/p-type Si heterojunctions exhibited a typical rectifying behavior with a rectification ratio of more than two orders of magnitude at bias voltages between {plus minus}1 V, the heterojunctions comprising non-epitaxial β-FeSi2 films rarely exhibited rectifying action. The epitaxial growth of β-FeSi2 is an significant factor for forming the heterojunction diodes.

Epitaxial growth of (010)-oriented β-FeSi2 film on Si(110) substrate

ABSTRACTHigh-quality (010)-oriented epitaxial β-FeSi2 films were grown on Si(110) substrates by coating silver thin layer. The full width at half maximum of the rocking curve of β-FeSi2040 was 0.14o for the film deposited at 800°C on Si(110) substrates with 95 nm-thick silver layer. Moreover, this epitaxial β-FeSi2 film was constructed with single domain structure, and the lattice parameter of a-axis was extended by 0.7%. The photoluminescence spectrum from this epitaxial β-FeSi2 indicated that the band-gap was modulated by lattice strain of a-axis.

Influences of Hydrogen Passivation on Near-Infrared Light Detection of n-Type β-FeSi2/p-Type Si Heterojunction Photodiodes

not Available.

Growth of β-FeSi2 thin film on textured silicon substrate for solar cell application

(202)/(220)-oriented epitaxial β-FeSi2 thin films were deposited on textured Si (100) substrate by magnetron sputtering. The influences of thickness and annealing temperature on the β-FeSi2 crystallization were studied to find the optimal condition. The results of surface morphology and optical property measurements showed that the inverted pyramid array in the surface of β-FeSi2 thin films could reduce the surface reflection of β-FeSi2. In dark condition, the β-FeSi2/textured-Si heterojunction showed diode property with rectifying ratio of 2.89×105 and built-in potential of 0.58V. These results indicated the potential application of textured Si substrate in β-FeSi2 solar cells.

Epitaxial Orientation of β-FeSi2 on 3C-SiC/Si(111)

(100)-oriented β-FeSi2 films were expitaxially grown on a SiC (3C-SiC)-buffered Si(111) substrate by cosputtering iron and silicon. The full width at half maximum (FWHM) of the rocking curve of the β-FeSi2 800 diffraction peaks was 0.37° for the films deposited at 750 °C, which is the same as that for epitaxial β-FeSi2 films on the Si(111) substrate. These epitaxial films had a triple-domain structure. The photoluminescence spectrum from β-FeSi2 was observed at 10 K, and roughly had four luminescence bands.

Aligned defects behaviour of β-FeSi2 thin film on Si(100) substrate prepared by ion beam sputter deposition

β-FeSi2 is one of the possible candidates for a compound semiconductor. Epitaxial β-FeSi2 films grown on a Si substrate are technologically important for microelectronic and silicon-based optoelectronic applications. In earlier studies, we clarified that the treatment of the Si substrate surface greatly influences the crystal structure of the film. Among several surface treatment procedures, it has been reported that the sputter etching (SE) of the substrate with subsequent thermal annealing is fairly effective in obtaining a film that has a favorable epitaxial orientation relationship of β-FeSi2(100)//Si(100) with atomically smooth interface by choosing better conditions. Even though the obtained films have good quality, anomalous patterns were sometime observed in the dark field image of the obtained film by cross-sectional TEM observation. They formed a horizontal line along the β-FeSi2/Si interface. Higher magnification image shows that these spots are aggregated defects. The ‘lines’ tend to appear in the samples, which are irradiated in higher energy. However, they are also occasionally observed in 1 keV Ne+ irradiated sample. It can be related to the diffusion process of the silicon atoms and the defects formed in the silicon through Ne+ irradiation. Formation behavior of these ‘aligned’ defects will be discussed in the present study.

Formation of ferromagnetic interface between β-FeSi2 and Si(111) substrate

Epitaxial β-FeSi2 thin films were grown on Si(111)7×7 clean surfaces by solid phase epitaxy in ultrahigh vacuum: iron deposition at low temperature and subsequent annealing. We found that a ferromagnetic interface layer of iron-rich silicides forms between a β-FeSi2 surface layer and a Si(111) substrate spontaneously from transmission electron microscopy observations and magnetization measurements.

Cross-sectional Transmission Electron Microscopy of Interface Structure of β-FeSi2/Si(100) Prepared by Ion Beam Sputter Deposition

The effect of a silicon substrate surface pretreatment on epitaxial iron silicide film formation on a Si(100) substrate with ion beam sputter deposition (IBSD) was investigated. In this study, two surface pretreatment methods namely, thermal etching (TE) and sputter etching (SE) with subsequent thermal annealing, were employed. The interface structure between a β-FeSi2 film and a Si substrate was analyzed by cross-sectional observation using a transmission electron microscope (XTEM). High-resolution XTEM images showed that the lattice of the Si substrate is an almost perfect crystal immediately after the TE treatment. However, the TE treatment results in an undulated interface, and the deposited silicide contained coalesced β-FeSi2 islands. On the other hand, the dislocations and stacking faults produced by radiation damage were observed near the Si substrate surface for the SE treatment. Even though this treatment produced defects, the interface of the SE-treated epitaxial β-FeSi2(100) film had a smooth interface after the deposition at 973 K. It can be concluded that a moderate disorder of the silicon substrate surface treated by SE may well enhance the mixing of Fe and Si atoms for the epitaxial growth of β-FeSi2.

Horizontal growth of epitaxial (1 0 0) β-FeSi 2 templates by metal–organic chemical vapor deposition

An epitaxial β-FeSi 2 thin film with continuous flat surface was grown by metal–organic chemical vapor deposition on (1 0 0) Si substrate with an aggregated β-FeSi 2 template. The horizontal growth on the aggregated β-FeSi 2 template made smaller voids among islands and became the continuous film in metal–organic chemical vapor deposition (MOCVD)-overgrowth process. The higher growth rate on the sidewall planes than on the (1 0 0) β-FeSi 2 plane is considered to be the origin of the horizontal growth.

Epitaxial Growth of (100)-Oriented β-FeSi2 Thin Films on Insulating Substrates

Epitaxial β-FeSi2 thin films were grown on insulating substrates by rf magnetron sputtering. (100)-Oriented β-FeSi2 films were epitaxially grown on (100) yttria-stabilized zirconia (YSZ) and (001) Al2O3 substrates. The epitaxial relationship of the β-FeSi2 film with the (100) YSZ substrate was the same as that with the (100) Si substrate. On the (001) Al2O3 substrate, the epitxial film had a threefold symmetry domain structure, which has not been reported on a film grown on a Si substrate. Epitaxial growth of (100)-oriented β-FeSi2 film was affected by not only lattice mismatch but also atomic or ionic matching between β-FeSi2 and substrate and requires substrates consisting of either a single element or only cations.

Formation of epitaxial β-FeSi 2 nanodots array on strained Si/Si 0.8Ge 0.2 (001) substrate

Epitaxial β-FeSi 2 nanodots were grown on strained Si/Si 0.8Ge 0.2 (001) substrates by solid phase epitaxy (SPE) method. High quality β-FeSi 2 nanodots were grown at 800 °C by employing strained Si/Si 0.8Ge 0.2 substrates, owing to a decrease of the in-plane lattice mismatch between the lattice parameters of the β-FeSi 2 [001]and [010] directions and that of Si substrate. Ordered β-FeSi 2 arrays along 〈110〉 direction were observed to form on surfaces of strained Si/Si 0.8Ge 0.2 substrate. It is shown that dislocation slip originated from compositionally graded Si 1− x Ge x layers can produce local surface-strain and local thickness variation. The surface features are used for the fabrication of epitaxial β-FeSi 2 nanostructures on strained Si/Si 0.8Ge 0.2 substrate.

Surface analysis of β-FeSi 2 layer epitaxially grown on Si(100)

The epitaxial growth process of β-FeSi 2 on Si(100) surface under ultrahigh vacuum condition has been studied by low energy electron diffraction (LEED) and low energy ion scattering spectroscopy (LEIS). The LEED pattern of Si (100)-2×1 changes into amorphous structure with Fe deposition of about 10 Å at room temperature. With annealing at 540 °C, the LEED pattern shows 2×2 structure corresponding to the formation of the epitaxial β-FeSi 2 (100) template layer. The α-scan in Li +-LEIS and X-ray diffraction (XRD) study strongly suggest that the topmost surface of the 2×2 structure is terminated by Si atoms. By XRD, it is shown that the β-FeSi 2 develops with characteristic orientation even if iron reactant is deposited onto the template surface.

Preparation of β-Fesi 2 thin film by metal organic chemical vapor deposition using iron-carbonyl and mono-silane

β-FeSi 2 films were deposited at 750 °C by a supplying Fe(CO) 5 and SiH 4 simultaneously during metal organic chemical vapor deposition (CVD). Films could be deposited using this precursors system, even though film deposition was not ascertained by the single supply of Fe(CO) 5. Fe(CO) 5 was probably decomposed in gas phases before it reached to substrate surface. It was suggested that a kind of intermediate reactant which was more stable than Fe(CO) 5 created by the simultaneous supply of Fe(CO) 5 and SiH 4 makes Fe–Si films. Epitaxial β-FeSi 2 films were obtained on Si(111) substrates, and neither carbide nor oxide phases were detected on XRD patterns.

Effect of substrate temperature and deposited thickness on the formation of iron silicide prepared by ion beam sputter deposition

Ion beam sputter deposition (IBSD) method was employed to find optimum conditions for the formation of epitaxial β-FeSi 2 films on Si(100) substrate. It was found that crystal structure of the films as determined by X-ray diffraction (XRD) analysis is dependent on the substrate temperature as well as on the deposited thickness of sputtered Fe. The film with best crystal properties was obtained either at 873 K with the deposited Fe thickness of 15 nm, or at 973 K with the deposited Fe thickness of 30 nm. The obtained results indicate the importance of Fe and/or Si diffusion in determining the crystal properties of β-FeSi 2 film.

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.