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The fabrication of orthorhombic barium disilicide (BaSi2) thin-films on modified germanium (Ge) substrates by thermal evaporation method was demonstrated, in which the surface modification of Ge substrate was performed by a simple chemical etching method. The effects of etching time on crystalline quality and optical properties of the BaSi2 films were investigated. The results revealed that the substrate modification has positive impact in improving the crystalline quality, reducing the light reflection, and increasing the absorption of the BaSi2 thin-films. Etching time was optimized at 15 min, considering the trade-off between crystalline quality and optical properties. Minority carrier-lifetime of the evaporated film on Ge substrate achieved 3.17 μs, which is among the high value obtained for thin BaSi2 films.

The capping of a BaSi2 film with amorphous Si (a-Si) is a key technology for the solar cell applications of BaSi2. In this study, we have investigated the two-step evaporation process of BaSi2 to fabricate the BaSi2 film capped with an a-Si layer on a Si(100) substrate. The deposition of Si is possible as the second step because a small amount of Si remains after the first BaSi2 evaporation step. Raman and X-ray photoelectron spectroscopies show that a-Si is formed by the two-step evaporation on the sample surface while Ba and carbonate species are not present on the surface, which are detected when the BaSi2 film is fabricated without the second step. Depth composition profiles determined by X-ray photoelectron spectroscopy clearly demonstrate the formation of the layered structure of a-Si and BaSi2. Owing to the a-Si capping, O concentration in the BaSi2 film is lower than that without a-Si, as evidenced by the composition profiles and energy-dispersive X-ray spectroscopy results. In addition, the microwave-detected photoconductivity decay analysis shows that the carrier lifetime in the a-Si/BaSi2 samples is similar to or slightly higher than the BaSi2 film without a-Si layer possibly due to the surface passivation effect and the reduction of oxygen concentration.

A comparative experimental and theoretical study of the role of H incorporation in As-doped BaSi2 films has been carried out based on the experimental results that an optimal time of H treatment for the increase in photoresponsivity and carrier lifetime was in the range of 1 – 20 min. Adequate theoretical representation of the decay curves in the framework of the model for non-radiative processes accounted for various trap-related recombination mechanisms to estimate the trap concentration to be in the range of 1.9 × 1013 to 1.7 × 1014 cm−3. Additionally, the extended theoretical ab initio quantum-chemical simulation of the electronic structure of the studied systems was performed. It was revealed that interstitial As atoms can mostly provide trap states in the gap while H atoms neutralize such traps. The experimentally observed unexpected switching in conductivity from n-type to p-type and vice versa in As-doped BaSi2 with H incorporation was explained to specific configurations of point defects (an As impurity with a H atom in different positions and various interatomic As-H distances) which affect the position of states in the gap.

The joint impact of the Si/Al layer thickness on the growth kinetics, the crystalline orientation and the size of the poly-Si grains resulting from aluminum-induced crystallization process is analyzed. It is shown that the surface coverage of resulting poly-Si layers rapidly decreases together with annealing temperature and the Si/Al ratio. The surplus of a-Si over the Al needed to ensure continuity of the poly-Si thin film is in the range of 35%–50% for Al layers thicker than 225 nm, but rapidly goes up to 200% as the thickness of the Al layer decreases below 50 nm. It is demonstrated that the angular distribution of grain orientations is discrete and shifts towards the {111} direction as the Si/Al increases. It is reported that during an isothermal annealing, the nucleation of Si grains occurs in two steps. Finally, a simple model of the aluminum-induced crystallization process explaining the two-step nucleation is proposed.

Exploration of impurity elements for carrier density control is important for device application of the BaSi 2 semiconductor, which is a promising candidate for an earth-abundant solar cell absorber. In this study, we have investigated the doping of BaSi 2 films with alkali metals (Li, Na, and K) by deposition of alkali-metal fluorides followed by rapid thermal annealing. Electrical characterization by Hall measurement shows that LiF treatment increases electron density in BaSi 2 up to 10 20 cm − 3 after annealing at 500 °C while NaF and KF treatments have limited effects with electron densities lower than 10 18 cm − 3 . The mechanisms of electrical property modification are discussed from structural viewpoints. Secondary ion mass spectroscopy shows that Li atoms slightly diffuse into the film at 400 °C by prolonged annealing, which nevertheless does not accordingly increase the carrier density. Formation of barium fluoride, oxide, and silicon at 500 °C, which is revealed by energy-dispersive X-ray and Raman spectroscopies, brings about the conclusion that a high temperature is needed for the dissociation of LiF through the reaction with BaSi 2 and the incorporation of Li atoms probably into the interstitial sites of BaSi 2 , which would generate electrons. On the other hand, Na and K atoms are found to evaporate at the same time as the dissociation of NaF and KF, respectively, evidenced by energy-dispersive X-ray spectroscopy. BaF 2 formation accompanied by the NaF dissociation is detected by X-ray diffraction.

The BaSi 2 semiconductor is a promising candidate for an earth-abundant solar cell absorber. In this study, we have realized a crack-free BaSi 2 film by a simple thermal evaporation technique on a CaF 2 substrate at a growth temperature of 500 °C for electrical characterization. A Si layer preliminarily formed on the substrate by sputtering is a key to obtain stoichiometric BaSi 2 film. Detailed structural characterization of the evaporated films with different Si layer thicknesses by X-ray diffraction, scanning and transmission electron microscopy demonstrates that a crack-free 370-nm-thick BaSi 2 film is formed by consuming the Si layer. It is observed that the 90-nm-thick bottom part is microcrystalline and contains Ar atoms, which come from Si deposition atmosphere. The surface of the BaSi 2 layer is found to be covered by an amorphous Si layer due to Si-rich vapor at the last stage of evaporation. Electrical properties of the BaSi 2 film are revealed by Hall measurement and the electron density and mobility are found to be 6 × 10 20 cm −3 and 0.04 cm 2 /V⋅s, respectively. Owing to a better crystalline quality contributed by the preliminarily-deposited Si layer, minority-carrier lifetime of the evaporated film (0.6 μs) is twenty times longer than previous films on glass substrates.

Arsenic doping by ion implantation and thermal annealing of the BaSi 2 epitaxial films grown on Si(111) substrates has been studied. Raman spectroscopy shows that a structural change can occur by annealing at 500 °C after As implantation with as high dose as 1.0 × 10 15 cm − 2 . This structural change is shown to result in the formation of an altered layer after annealing at the surface by cross-sectional scanning electron microscopy. Secondary ion mass spectroscopy reveals that the altered layer contains a significant amount of O atoms. With the altered layer present, average electron density up to 6.0 × 10 19 cm − 3 has been realized, which is revealed by the Hall measurement. On the other hand, without the altered layer, high resistance of the BaSi 2 layer prevented the electrical characterization. Current path is discussed by theoretically calculating the band alignment and resistance, and possible highest electron density without the altered layer is extracted to be less than 2 × 10 17 cm − 3 .

We investigated inverted Al-induced crystallization (AIC) technique of amorphous Si films (thickness: 50–100 nm) for the formation of polycrystalline Si films on Al coated glass substrates at low-temperature ( 2 interlayer was inserted in between Al and Si layers in order to control the Al-Si diffusion rate. As a result, the crystal orientation of the AIC-Si layer strongly depends on the thickness of the SiO 2 interlayer: thin (1 nm) interlayer provided (100) orientation while thick (10 nm) interlayer provided (111) orientation. Meanwhile, the thicker the SiO 2 interlayer, the larger the grain size of the AIC-Si layer. In particular, for a sample with 10-nm-thick SiO 2 interlayer, the (111) orientation fraction reached 99% and the average grain size over 50-μm diameters. This AIC-Si layer holds promise as epitaxial templates for light-absorption layers of thin-film solar cells, as well as for functional silicide materials.

To address the carrier extraction mechanism that determines the fundamental characteristics, such as current density, open circuit voltage, and fill factor in nanostructure-based solar cells, we performed photoluminescence (PL) decay measurements of the Ge/Si quantum wells (QWs) in crystalline-silicon ( c -Si) solar cells. We found that the PL decay time of Ge/Si QWs depends on the temperature and the applied electric field; this dependence reflects the carrier separation characteristics of electron–hole pairs in Ge/Si QWs. Above ~ 40 K, the electron–hole pairs are rapidly separated by the thermal excitation and the built-in electric field of c -Si solar cells. In contrast, at 20 K the PL decay time remains almost unchanged for an applied electric field of up to ± 1 V. These results indicate that the electrons confined in the type-II band offsets could be thermally excited and then extracted by an applied electric field.

We demonstrate that maskless wet etching of self-assembled Ge quantum dot (QD) multilayers can create large-area photonic nanostructures, and the geometry can be tuned by changing wet etching conditions. It is found that the reflectance in the near-infrared wavelength can be decreased by controlling geometry, and an increase in the depth of the photonic nanostructures results in enhancement of photoluminescence intensity from Ge QDs. These results show that control of geometry in photonic nanostructures is useful for enhancement of optical absorption in the Ge QD multilayers.

Phosphorus doping by P implantation and thermal annealing of the BaSi 2 epitaxial film grown on the Si(111) substrate has been studied. Raman spectroscopy results show that the structural damage due to P implantation can be almost removed by annealing within 30 and 1 s at 500 and 700 °C, respectively. The depth profile of P is investigated by secondary ion mass spectroscopy, which reveals considerably slower diffusion kinetics of P at 500 °C than 700 °C. The activation energy of the diffusion is roughly estimated to be 2 eV from the temperature dependence. The Hall measurement of the P-doped films clarifies that the P impurity is an electron donor in BaSi 2 . The average electron density up to the order of 10 18 cm − 3 is observed.

Al-induced crystallization (AIC) enables low-temperature crystallization of amorphous Ge thin films on insulators. We investigated growth promotion of Ge thin films using Ge membranes (1–10 nm thickness) that are initially inserted below the Al layer. These Ge insertion layers enhanced supersaturation of Al with Ge, which results in low-temperature AIC (275 °C). However, thick (≥ 3 nm) insertion layers result in small grains because of the high nucleation frequency. A 1-nm-thick insertion layer accomplished a growth promotion and yielded large grains of over 100 μm in diameter. Moreover, electron backscatter diffraction measurement revealed that the AIC-Ge layer was highly (111) oriented. This low-temperature crystallization technique opens up the possibility for developing Ge-based electronic devices on inexpensive glass substrates, as well as on flexible polymer substrates.

Uniaxially strained SiGe layers were fabricated with a newly developed selective-ion-implantation technique. The SiGe layer was grown on the Si substrate, into which laterally selective ion-implantation with stripe pattern was carried out prior to the SiGe growth. A strain-relaxation of the SiGe layer was largely enhanced due to ion-implantation-induced defects selectively in the ion-implanted area while it was hardly enhanced in the neighboring unimplanted area. However, micro-Raman mapping and X-ray diffraction reciprocal space mapping measurements obviously revealed that the relaxed SiGe in the implanted area remarkably influenced a strain state of the neighboring strained SiGe in the unimplanted area, that is, the strain along the stripe line direction was highly relieved due to the stress caused by the neighboring relaxed SiGe while the strain in the direction perpendicular to the line was well maintained. As a result, highly asymmetric strain state, that is, uniaxial strain was realized, where 4 times different relaxation ratios in the two directions were observed. These results indicate that the selective-ion-implantation technique developed in this study has a high potential to realize uniaxially strained Si/Ge channel devices with high mobility.

The formation of different types of twins were observed in SiGe/graded buffer/Si(110) heterostructures grown at different substrate temperatures using gas-source molecular beam epitaxy. It was found that the growth twins were dominant when the substrate temperature was lower than 700 °C. The growth at higher substrate temperature effectively suppressed the nucleation of growth twins and resulted in the formation of another type of defect, microtwin lamellas, which accompany the strain-relaxation process.

Polycrystalline silicon (poly-Si) films thicker than 1.5 μm, consisting of dense small grains called nano-grain poly-Si (ngp-Si), are formed by flash lamp annealing (FLA) of amorphous silicon (a-Si) films prepared by catalytic chemical vapor deposition (Cat-CVD) method. Crystallinity of the ngp-Si films can be controlled by changing lamp irradiance. Secondary ion mass spectroscopy (SIMS) profiles of dopants in the ngp-Si films after FLA shows no serious diffusion. A minority carrier lifetime of over 5 μs is observed from these ngp-Si films after defect termination process using high pressure water vapor annealing (HPWVA), showing possibility of application for high-efficient thin film solar cells.

The dark current–voltage ( J – V ) characteristics and the conversion efficiency of the solar cells with embedded stacked Ge islands in the intrinsic layer were investigated. These islands were grown by molecular beam epitaxy on Si substrates. We used a two-diode model to analyze the dark J – V characteristics of Ge island solar cells. Results showed that the minority carrier diffusion and the recombination current components increase as a function of the stacked Ge island layers. This increase of the minority carrier diffusion current was due to an increase of the intrinsic carrier density as a function of the number of stacked layers. Similarly, the increase in the recombination current components was due to the enormous recombination of carriers in the intrinsic region as the number of stacked layer increases. This phenomenon could lead to a decrease of the open circuit voltage, V oc . The decrease of V oc should be overcompensated by the increase of photocurrent, due to the presence of stacked Ge islands with higher absorption coefficient, in order to attain an optimum value of the conversion efficiency.

We have investigated the crystal structures of SiGe layers grown on Si(110) substrates. A method was developed to analyze Ge composition and strain parameters along [1¯10] and [001] directions, considering anisotropic in-plane strain relaxation process. Samples grown by solid source molecular beam epitaxy (MBE) showed that strain in SiGe layers preferentially relaxed along [1¯10] direction. However, the samples grown by gas source MBE showed formation of domains that had different lattice orientations. Result of the analysis showed that the domains tilted towards [001]/[001¯] directions, by 0.4–0.6°. It was found that the crystal structure strongly depends on growth conditions and sample structures.

Liquid-phase epitaxial (LPE) growth on silicon carbide simultaneously covers macroscale defects, e.g. micropipes, and improves the quality of the crystal. In this study, an epi-layer grown over a macrodefect was evaluated by micro-Raman scattering spectroscopy. Before the growth process, the density of the stacking fault was high and the carrier density spatially inhomogeneous in the vicinity of the macrodefects. On the other hand, after growth, the layer over the macrodefect displayed good quality; the density of the stacking fault was less than that before growth and the homogeneity of the carrier density improved.

We fabricated Si-based solar cell with stacked Ge islands grown via the Stranski-Krastanov growth mode in the intrinsic layer of pin diodes. The onset of the external quantum efficiency in the near infrared regime was extended up to approximately 1.4 μm for the solar cells with stacked Ge islands. The quantum efficiency was found to increase with increasing number of stacking, showing that a part of electron-hole pairs generated within Ge islands was separated by the internal electric field and contributed to the photocurrent. Increase of the processing temperature for the impurity diffusion was found to bring blue-shift of the band gap through the intermixing of Si and Ge as well as the deformation of Ge islands. Therefore, low-temperature process was suggested to be necessary for further enhancement of quantum efficiency in the near infrared regime.

Growth characteristics and optical properties of ZnO films epitaxially grown on Al2O3 (0 0 0 1) substrates at temperatures of Tg=200–500 °C by metalorganic chemical vapor deposition were investigated. The in-plane orientation of ZnO unit cells was different for Tg=200 °C and Tg⩾300 °C. The surface morphology varied significantly: smooth surface with cracks at Tg⩽250 °C, facet and column growth at 300 °C⩽Tg

We systematically investigated ion energy and dose dependencies of strain relaxation for SiGe buffer layers using Si + ion implantation. It was found that for the SiGe grown on Si substrates implanted at the energy of 25 keV and with a dose of 5 × 10 14 cm − 2 , relaxation ratio was highly enhanced with very smooth surface. The obtained ratio was 65% higher than that of unimplanted samples. However, strain relaxation ratios were seen to reduce as ion energy increased or ion dose increased. This result indicated that the End-of-Range (EOR) defects generated by ion implantation and post annealing were responsible for strain relaxation and that the depth of the EOR defects from the SiGe/Si heterointerface was very crucial for strain relaxation of the SiGe layers.

We report on the molecular beam epitaxial (MBE) growth of 70 Ge n / 74 Ge n isotope superlattices composed of alternating layers of the stable isotopes 70 Ge and 74 Ge. Samples prepared in this work have atomic layers n =4, 8, 16, and 32. All superlattices are p-type having a net-impurity concentration of ∼10 16 cm −3 . Zone-folding of optical phonons due to the mass periodicity in the growth direction has been observed clearly for all samples using high resolution Raman spectroscopy. The corresponding phonon mode of each Raman peak has been indexed according to theoretical calculations using the linear-chain model and the planar bond-charge model. The frequency of the Raman peaks found by the experiment agree very well with those of the phonon modes calculated for each superlattice structure. A detailed analysis of the Raman spectra concludes that the degree of interface mixing between 70 Ge and 74 Ge layers for our typical growth condition is less than two monolayers.

The modification of the Stranski–Krastanov growth mode produced by the strain-field from buried islands has been non-destructively investigated by photoluminescence (PL) spectroscopy in lattice-mismatched Ge/Si system. The samples consist of double Ge layers with each coverage of 4.5 monolayers separated by a Si spacer layer with various widths. At an intermediate Si spacer thickness of 100–200 A, the modification of the growth mode of the second Ge layer was clearly evidenced in the PL spectral feature which was characterized by the superposition of two different PL spectra. In addition, a drastic increase of the density of Ge islands in the second layer was observed by atomic force microscopy. These facts show that the strain-field from the buried islands is very important to control the growth mode of lattice-mismatched semiconductor heterostructures.

The effects of introducing boron (B) on the growth of Ge islands are studied and quite small Ge islands were found to appear in the presence of B adlayer. On the other hand, cosupply of Ge and B brought no change in the island size and the decrement of the number of Ge islands. The average of lateral size and height of these small islands are 30 and 1.5 nm, respectively, and the density is 1.4×10 10 cm −2 which is much higher than that of typical Ge islands. These results indicate that pre-deposition of B can produce preferable surface morphologies that small Ge islands can be easily formed.

Multiple layers of Ge self-assembled quantum dots were embedded into two-dimensional silicon photonic crystal microcavities fabricated on silicon-on-insulator substrates. Microphotoluminescence was used to study the light-emission characteristic of the Ge quantum dots in the microcavities. Strong resonant room-temperature light-emission was observed in the telecommunication wavelength region. Significant enhancement of the luminescence from Ge dots was obtained due to the resonance in the cavities. Multiple sharp resonant peaks dominated the spectrum, showing strong optical resonance inside the cavity. By changing the lattice constant of photonic crystal structure, the wavelengths of the resonant peaks are tuned in the wide wavelength range from 1.2 to 1.6 μm.

We have studied the early stage and successive stage of the growth of poly-Si1-xGex films in gas-source molecular beam deposition (GSMBD), in which disilane and germane are utilized as source gases. Solid-source molecular beam deposition (SSMBD) was also used for the film formation in comparison with GSMBD. In GSMBD, the grain size is larger than that in SSMBD in the early stage of the growth and the grains after growth have columnar structures. In SSMBD, there are many small grains in the early stage of the growth, and the successive growth formed grains with the shape of triangular pyramid.

We report on combination of crystal growth of bulk SiGe and thin film crystals on SiGe bulk substrates to prepare heterostructures with controlled stain in constituent materials. Muticomponent zone melting method to permit crystal growth with uniform composition and Czochralski growth were employed as growth techniques for SiGe bulk crystals. On homemade SiGe bulk substrates, we realized strained Si thin film as well as luminescent strained quantum wells with improved structural perfection compared with those on relaxed SiGe on commercially available Si.

Exposure of GeH4 on Si was attempted at various temperatures to establish growth conditions for high-quality Ge on Si or Si-on-insulator, which could be utilized for on-chip 1.3–1.55 μm photodetectors. The grown film was found to be not pure-Ge but Ge-rich SiGe, and Si composition in the film increased with the rise of the growth temperature. This behavior could be utilized for growth of Ge on a compositionally graded SiGe buffer layer just by decreasing growth temperature. In fact, a preliminary experiment in order to obtain such a structure was successfully demonstrated by decreasing temperature from 700 to 500 °C at a rate of − 6.7 °C/min.

Strain distribution in SiGe heterostructures and its influence on the roughness formation of the overgrown structures were investigated. The crosshatch surface roughness was found to appear after the homoepitaxial regrowth of SiGe on the planarized SiGe buffer layers. The morphology of this roughness was almost equal to the in-plane strain distribution observed by Raman mapping measurement, which clearly demonstrated that the crosshatch roughness is formed by the strain distribution with the mechanism associated with the growth kinetics. Strain distribution wavelength was found to strongly depend on buffer thickness and strain fluctuation still existed on the very thick buffer layers. It can be said that the buffer thickness should be carefully taken into account for device applications.

Anomalies have been observed in photoluminescence (PL) of pure-Ge/Si type-II coupled quantum wells (II-CQWs). Although the II-CQWs (LGe=1.5 ML) showed clear energy red-shift with decreasing Si spacer thickness (LSi) due to interwell coupling of the hole wave-function, an anomalous energy lowering was observed for smaller LSi (≤6 A). Moreover, PL intensity of the II-CQWs increased linearly with increasing excitation power, as opposed to a sublinear increase in type-II QWs due to exciton localization at the interfaces. These results seem to be the result of enhanced electron capture by the II-CQW potential profile. Furthermore, correlation of thickness fluctuation among the layers was observed by high-resolution transmission electron microscopy, which may be connected to the PL anomalies.

We present a systematic study of luminescence from strained SiGe/Si quantum well (QW) structures grown by gas source silicon molecular beam epitaxy (MBE) and the result is compared with luminescence from QWs grown by conventional solid source silicon MBE. Distinct excitonic photoluminescence (PL) was obtained both from single QW (SQW) and multiple QW (MQW) structures grown by either method. A systematic blue shift of PL energy was observed with decreasing well width and the well width dependence of PL emission energy was in good agreement with a square potential profile for QWs in the case of gas source MBE. In contrast, a considerable energy shift exceeding square potential eigenvalues was found for solid source MBE-grown QWs. We found that the asymmetric potential profile due to surface segragation of germanium atoms was responsible for this blue shift. Electroluminescence and PL data from (111)-oriented QWs are also presented.