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Semiconducting BaSi₂ has attractive features for thin-film solar cell applications because both a large absorption coefficient and a long minority-carrier diffusion length can be utilized. In this article, we explore the possibility of semiconducting BaSi₂ films for thin-film solar cell applications. Recent experimental results on the optical absorption coefficient, minority-carrier diffusion length, and minority-carrier lifetime in undoped n-BaSi₂ films are presented. After that, the photoresponse spectra are calculated for a BaSi₂ p⁺n abrupt homojunction diode based on the one-dimensional carrier continuity equation using previously reported experimental values. The individual contributions of the three layers, that is, the neutral p⁺-type layer, the depletion region, and the neutral n-type layer, to the total photoresponse are discussed. The photoresponse depends on parameters such as layer thickness, minority-carrier diffusion length, and surface recombination velocity. We further estimate the photocurrent density and the open-circuit voltage under AM 1.5 illumination.

The aim of this paper is to summarize considerable experimental efforts undertaken within the last decades in the investigations of transport properties of β-FeSi₂. The β-FeSi₂ compound is the most investigated among a family of semiconducting silicides. This material has received considerable attention as an attractive material for optoelectronic, photonics, photovoltaics and thermoelectric applications. Previous reviews of the transport properties of β-FeSi₂ have been given by Lange and Ivanenko et al. about 15 years ago. The Hall effect, the conductivity, the mobility and the magnetoresistance data are presented. Main attention is paid to the discussion of the impurity (defect) band conductivity, the anomalous Hall effect, the scattering mechanisms of charge carriers, as well as to the hopping conduction and the magnetoresistance.

Results of our ab initio calculations have revealed changes in electronic properties in Ca₂Si semiconducting silicide when reducing dimensionality from bulk to slabs and, eventually, to nanowires. In the case of the bulk, Ca₂Si is found to be a direct band-gap semiconductor with the band-gap value of 0.30, 0.60, and 0.79 eV by using the generalized gradient approximation, the modified Becke–Johnson exchange potential and the screened hybrid functional, respectively. We have also identified that among Ca₂Si(001), (010), and (100) surfaces the (100) one has the lowest surface energy. Ca₂Si slabs with (010) or (100) surfaces are predicted to be semiconductors, while (001) surface provides metallic properties due to surface states. The role of the surface states in the band-gap variation is also discussed. In the case of Ca₂Si nanowires with 〈001〉, 〈010〉, and 〈100〉 axes and different morphologies only the 〈001〉 orientation guarantees semiconducting properties because of absence of {001} facets which induce metallic properties as for the corresponding slab.

The growth, properties and applications of metal silicide nanowires (NWs) have been extensively investigated. The investigations have led to significant advance in the understanding of one-dimensional (1D) metal silicide systems. For example, CoSi is paramagnetic in bulk form, but ferromagnetic in NW geometry. In addition, the helimagnetic phase and skyrmion state in MnSi are stabilized by NW morphology. The influencing factors on the growth of silicide phase have been elucidated for Ni–Si, Pt–Si, and Mn–Si systems. Promising results were obtained for spintronics, non-volatile memories, field emitter, magnetoresistive sensor, thermoelectric generator and solar cells. However, the main thrust has been in microelectronic devices and integrated circuits. Transistors of world-record small size have been fabricated. Reconfigurable Si NW transistors, dually active Si NW transistors and circuits with equal electron and hole transport have been demonstrated. Furthermore, multifunctional devices and logic gates with undoped Si NWs were reported. It is foreseen that practical applications will be realized in the near future.

There are few silicides that could be used for thermoelectric energy conversion, following higher silicides of transition metals: CrSi₂, MnSi_1.75, β-FeSi₂, Ru₂Si₃, ReSi_1.75, and solid solutions based on compounds of Mg₂X (X = Si, Ge, and Sn). Some of them have very high figures of merit (ZT). It can be shown that, in some silicides, a high ZT is the result of energy spectrum optimization besides the decrease in thermal conductivity. This is very difficult to achieve in some materials, because the density of states is typically dependent only on the band structure of a material, for which there is no means to produce such a change. However, in solid solutions, if they have a special band structure of components, it is possible to alter the band structure to increase ZT.

We report and compare the luminescence, both photo- and electroluminescence, in the near-infrared of a wide range of rare earths (Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm) doped dislocation engineered silicon light emitting devices. The rare earths are introduced using ion implantation into standard Czochralski (CZ) n-type silicon wafers pre-implanted with boron to form both the p–n junction and an engineered dislocation loop array. Rare earth internal transitions are observed in samples co-doped with Dy, Ho, Er, and Tm. We show that for each rare earth optimizing the optical activity depends critically on the rare earth implant parameters and post-implant process conditions. Room temperature operation in the 1.5 and 2.0 µm spectral regions is observed from the internal rare earth transitions in Er and Tm.

Using solid phase epitaxy of thin Fe films and molecular beam epitaxy of Si, p-Si/β-FeSi₂ nanocrystallites/n-Si(001) diode structure was fabricated. The diode exhibited a current responsivity of 15 mA/W and external quantum efficiency of about 1% at a wavelength of 1300 nm at 120 K without bias and 200 mA/W and 10%, respectively, at −30 V. The device specific detectivity calculated at 120 K in zero bias conditions of 2.1 × 10¹¹ cm·Hz^1/2/W at a wavelength of 1.3 µm is the highest ever reported for Si/β-FeSi₂ systems. The Franz–Keldysh effect gives grounds for applying such systems not only for the development of optrons but also for that of electro-optical modulators.

Photonic crystals (PhCs) composed of β-FeSi₂ with amorphous Si (a-Si) cladding layers are systematically studied to realize silicon photonics devices incorporating Si-based light-emitting layers. The bandgap characteristics of two types of triangular-lattice-type PhC in the telecommunication wavelength region of approximately 1.55 µm are calculated. They are composed of a-Si circular holes surrounded by β-FeSi₂ (hole type) and β-FeSi₂ circular columns surrounded by a-Si (column type). As a result, the bandgap for the hole-type PhC is obtained for TE polarization, while that for the column-type PhC is obtained for TM polarization. Furthermore, the PhC lattice constant range giving a bandgap for the column-type PhC is much wider than that for the hole-type PhC. The column-type PhC can be easily fabricated compared with the hole-type PhC. Thus, the column-type PhC is useful for actual applications from the viewpoint of fabrication and the bandgap characteristics themselves.

Pronounced enhancement of photoluminescence (PL) intensity was observed from β-FeSi₂ by using metal–organic chemical vapor deposition (MOCVD) on (100) Si substrates coated with a silver (Ag) layer. X-ray diffraction analysis revealed modifications to the crystal structure near the surface of Si, where the in-plane lattice parameter had been elongated, by Ag atomic diffusion from the surface to inside the Si during the heating process before deposition. This modified Si surface contributed to decreasing the non-radiative recombination centers at the β-FeSi₂/Si interface and in the β-FeSi₂ film, which led to the pronounced enhancement of PL intensity.

Temporal decay characteristics of 1.54 µm photoluminescence (PL) were investigated in β-FeSi₂ and Si-implanted Si samples grown by ion-beam-synthesis (IBS). In the samples, the band-edge PL of β-FeSi₂ (A-band) and the dislocation-related PL (D1-band) of Si were both observed at ∼0.8 eV. Regarding the dependence of the PL decay curves on excitation power density (P), PL decay curves without extrinsic effects were obtained at a low P of P ≤ 4.3 mW/cm². The PL decay times obtained at a low P showed clear differences between the A-band and the D1-line. The result showed that the band-edge PL of β-FeSi₂ was distinguished from the dislocation-related PL of Si by the PL decay times. The intrinsic PL decay times of β-FeSi₂ were determined to be τ₁ = 70–100 ns and τ₂ = 550–670 ns at 5 K.

We have investigated the melt growth of Mg₂Si crystal and its electrical and optical properties. Progress in Mg source purity and stoichiometric control during the growth enabled the development of a high purity Mg₂Si crystal with low carrier density and a high stable Mg₂Si with good doping controllability. The Mg₂Si crystal grown by the pressure controlled Bridgman method using 5N purity or 6N purity of Mg source and purified PG crucible showed low electron density (∼10¹⁵ cm⁻³) and high electron mobility (485 cm² V⁻¹ s⁻¹ at 300 K and 21900 cm² V⁻¹ s⁻¹ at 40 K). Silver doping in the high purity crystals performed the low-hole density of p-type Mg₂Si (∼3 × 10¹⁶ cm⁻³). Ionization energy of residual Al donor in the high purity crystal and Ag acceptor in the Ag doped crystals was determined as 8–9 meV and 26 meV, respectively. Indirect band gap energy E_g of approximately 0.61 eV at 300 K and 0.69 eV at 4 K were estimated by the optical transmission measurements on the high purity crystals. It is also found that the Sb-doped melt grown crystal had good power factor around room temperature (26 µW cm⁻¹ K⁻² at 270 K).

(110)-oriented epitaxial Mg₂Si films were grown on (100), (110), and (111) MgO single crystals by RF magnetron sputtering. Two, one, and three types of in-plane variants were observed for (100), (110), and (111) MgO single crystals, respectively. In addition, it was also demonstrated that epitaxial Mg₂Si films can be grown on (001) Al₂O₃ substrates using an epitaxially grown (111) MgO buffer layer. Transmission electron microscopy studies revealed a clear interface between Mg₂Si and the MgO buffer layer with an epitaxial relationship. This result indicates that Mg₂Si films can be epitaxially grown on other substrates by using an epitaxial buffer layer of MgO.

With the aim of producing functional materials based on earth-abundant elements, we examined the synthesis of the ternary type-I clathrates A₈Al_xSi_46−x (A = Na and K). The type-I Si clathrate K_7.9(1)Al_7.1(1)Si_38.9(4), having a lattice parameter of 10.434(1) Å, was successfully synthesized via the direct reaction of K, Al, and Si by optimization of both the synthesis temperature and the molar ratios among the raw ingredients. K₈Al₇Si₃₉ exhibited metallic conduction: its electrical resistivity increased with increasing temperature. The high pressure synthesis of Na₈Al_xSi_46−x was also examined, using a belt-type apparatus and employing a mixture of NaSi, Al, and Si as the reagents. In this manner, the type-I Si clathrate Na_8.7(9)Al_0.5(1)Si₄₅₍₂₎, having a lattice parameter of 10.211(1) Å, was synthesized at 5.5 GPa and 1570 K.

The mechanical properties of Sb-doped Mg₂Si were measured by various mechanical methods, and the effects of four incorporated metallic binders (Ni, Cu, Zn, and Al) on the mechanical properties were investigated. The measured Young's modulus, bending strength, Vickers hardness, and fracture toughness of Mg₂Si without binders were 105 GPa, 57 MPa, 700, and 1.2 MPa m^1/2, respectively. These values, except for Vickers hardness, were improved by the addition of the metallic binders. Moreover, the thermoelectric performance of Mg₂Si was also slightly improved by the incorporation of the metallic binders. These results suggest that the addition of a metallic binder to Mg₂Si is effective in improving its mechanical properties, in addition to improving its thermoelectric property and sinterability.

Novel ternary stannides and a plumbide, Na₂Mg₃X₂ (X = Sn, Pb) and Na₄Mg₄Sn₃, were synthesized by heating the corresponding elements. The crystal structures were determined by single-crystal X-ray diffraction analysis, and the electrical conductivities and Seebeck coefficients were measured. The crystal structures of Na₂Mg₃X₂ [orthorhombic, a = 7.3066(9) Å, b = 14.4559(13) Å, c = 6.6433(7) Å for X = Sn, a = 7.4272(11), b = 14.770(3), c = 6.6852(11) Å for X = Pb] are based on the Mg₅Ga₂-type structure (space group Ibam, Z = 4). Na₄Mg₄Sn₃ crystallizes in an orthorhombic cell [a = 6.879(3) Å, b = 7.154(2) Å, c = 22.285(7) Å, space group Fmmm, Z = 4] with layers of disordered Na atom arrangement with defects. The electrical conductivities measured for the polycrystalline sintered samples of Na₂Mg₃Sn₂, Na₄Mg₄Sn₃, and Na₂Mg₃Pb₂ were 1.9 × 10⁵ S m⁻¹ at 300 K, 1.6 × 10⁵ S m⁻¹ at 307 K and 3.3 × 10⁵ S m⁻¹ at 300 K, respectively. The Seebeck coefficients (S) of Na₂Mg₃Sn₂, Na₄Mg₄Sn₃, and Na₂Mg₃Pb₂ were +47 to +72, +29 to +67, and +10 to +24 µV K⁻¹, respectively, and increased with increasing temperature of 300–600 K.

We theoretically investigate the impurity doping effects on the structural parameters such as lattice constant, atomic positions, and site preferences of impurity dopants for Al-doped magnesium silicide (Mg₂Si) crystal using the first-principles calculation methods. We present comparison between several codes: ABCAP, Quantum Espresso, and Machikaneyama2002 (Akai KKR), which are based on the full-potential linearized augmented plane-wave method, the pseudopotential method, and KKR/GGA Green's function method, respectively. As a result, any codes used in the present study exhibit qualitative consistency both in the dependence of the lattice constants on the doping concentration and the energetic preference of the Al atom for the following sites; substitutional Si and Mg sites, and interstitial 4b site; in particular, ABCAP, which is based on the all-electron full-potential method, and Quantum Espresso, which is a code of the pseudopotential method, produce closely-resemble calculation results. We also discuss the effects of local atomic displacement owing to the presence of impurities to the structural parameters of a bulk. Using the analytical method considering the local atomic displacement, moreover, we evaluate the formation energy of Na- and B-doped systems as examples of p-type doping in order to examine the possilbility of realizing p-type Mg₂Si.

Thin (45–50 nm) non-doped and doped (by Sb and Al) polycrystalline Mg stannide films consisting mainly of Mg₂Sn semiconductor phase and containing small quantity of Mg₂Si phase have been grown by multiple layer deposition at room temperature and single step annealing at 150 °C of the (Sn–Mg) bi-layers on Si(111) n-type wafers with 7.5 Ω·cm resistivity. Optical spectroscopy data have shown that the grown Mg stannide films is a semiconductor with direct band gap of 0.17 ± 0.03 eV, with second and third direct interband transitions at 0.34 ± 0.02 and 0.45 ± 0.04 eV. An undispersed refraction index: n₀ = 3.78 ± 0.06 was calculated from phonon energy dependence of the refraction index of the grown films in the 0.12–0.20 eV energy range. Temperatures dependent Hall effect measurements have revealed about 0.28 eV electrical band gap value in the films.

We developed the epitaxial growth technique of various kinds of nanodots (NDs) on Si substrates using ultrathin SiO₂ film. Fe-based NDs were epitaxially grown on Si substrates without intermixing with Si atoms in Si substrates despite the easy reaction of Si and Fe, resulting in atomically sharp interfaces between NDs and substrates. The status control of the nuclei was important at the nucleation stage in terms of control of ND crystal structure. The proper nuclei also suppressed the undesirable reactions such as intermixing of Fe and Si in Si substrates. The epitaxial NDs formed by our technique can supply the prospect for the application to some devices.

Si and silicide nanowires/microrods were synthesized using metal-chloride-based sources by the CVD technique. In the case of Si nanowire synthesis, the morphological structures of the products depended on the distance from the source material. The distance dependence of the morphological and structural properties of the nanostructures was investigated to systemically clarify the shape modification phenomena in Si nanowire synthesis. The modification of the cross-sectional shape of the Si nanowires/microrods was successfully demonstrated. The triangular nanowire has a sawtooth faceting structure on its sidewall. In addition, the synthesis of Mn-silicide phases over the entire range of the Mn–Si system was examined. The effect of adding an impurity to the source materials on the structural modifications of the resulting nanowires/microrods was also investigated. It is expected that the morphological and structural control of the nanowires/microrods will be improved by a simple thermal treatment using a metal chloride as the source material.

Electrically active defect levels in 650-nm-thick undoped n-BaSi₂ epitaxial layers grown by molecular beam epitaxy were investigated by deep-level transient spectroscopy (DLTS) using undoped n-BaSi₂/p-Si heterojunction diodes. The layer structure was designed so that the depletion region extended toward the n-BaSi₂ layers under reverse bias conditions. DLTS revealed the presence of majority-carrier (electron) traps located at approximately 0.1 and 0.2 eV below the bottom of the conduction band. The densities of these trap levels were approximately 1 × 10¹⁵ cm⁻³. Photoresponse spectra are also discussed in relation to these trap levels. Minority-carrier traps were not detected in this work.

We have realized BaSi₂ films by a simple vacuum evaporation technique for solar cell applications. X-ray diffraction analysis shows that single-phase BaSi₂ films are formed on alkali-free glass substrates at 500 and 600 °C while impurity phases coexist on quartz or soda-lime glass substrates or at a substrate temperature of 400 °C. The mechanism of film growth is discussed by analyzing the residue on the evaporation boat. An issue on the fabricated films is cracking due to thermal mismatch, as observed by secondary electron microscopy. Optical characterizations by transmittance and reflectance spectroscopy show that the evaporated films have high absorption coefficients, reaching 2 × 10⁴ cm⁻¹ for a photon energy of 1.5 eV, and have indirect absorption edges of 1.14–1.21 eV, which are suitable for solar cells. The microwave-detected photoconductivity decay measurement reveals that the carrier lifetime is approximately 0.027 µs, corresponding to the diffusion length of 0.84 µm, which suggests the potential effective usage of photoexcited carriers.

The energy changes in the formation of interstitially doped BaSi₂, caused by doping with Na, Mg, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, B, C, N, O, F, and Ne, are calculated using the Perdew–Wang generalized gradient approximations of the density functional theory. It is predicted that the majority of the elements, apart from Na, Mg, Zn, and Ne, are capable of forming interstitially doped compounds with BaSi₂, if these elements are provided as an isolated atom. However, the energetic stabilities of the standard states of these elements (metals, diatomic gases, etc.) exceed the energy gain accompanying the formation of the interstitial compounds and, therefore, the conventional diffusion method using metals or gaseous source materials cannot produce the interstitial compounds. From the energetic perspective, B, C, N, O, and F appear to be favorably inserted into the BaSi₂ lattice, but the observed behavior of B-implanted BaSi₂ suggests that substitution of B for Si may occur.