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n-Mg2Si/p-Si heterojunction solar cell with a 1.4 µm thick unintentionally doped (n = 3 × 1017 cm−3) silicide epitaxial layer on p-Si(1 1 1) (p = 5 × 1014 cm−3) was grown by low temperature (250 °C) molecular beam epitaxy. Heterojunction demonstrated clear rectification and zero bias photoresponse in the (400–1400) nm wavelength range at room temperature. Under AM 1.5 illumination, an open-circuit voltage of 0.21 V, a short-circuit current density of 3.3 mA/cm2, fill factor of 0.36 were obtained while the conversion efficiency reached 0.24%, which is the pioneering demonstration of Mg2Si-based solar cell operation. Combined minority-carrier lifetime, Raman and AFM mapping together with TEM and XRD data revealed that carrier dynamics and photovoltaic performance are limited by the presence of non-epitaxial Mg2Si grains in the upper silicide film layer. However, minority-carrier lifetime up to 7.3 µs for Mg2Si demonstrates its great potential as absorbing material for Si-based solar cells.

Aluminum nitride (AlN) thin films were grown by pressure gradient sputtering (PGS) and conventional sputtering methods. A pressure gradient with low pressure at a substrate position was measured only in the PGS method. AlN films highly oriented along the c-axis were grown by both methods, but there were remarkable differences in the surface roughness and the piezoelectric properties. The average surface roughness (Ra) obtained by atomic force microscopy images was much smaller in the AlN grown by the PGS method. The piezoelectric constant (d 33) of the AlN grown by the PGS method was higher than that of the conventional method. These results revealed that the PGS technique has an advantage in the growth of AlN films that are highly c-axis oriented with a single dielectric domain.

Photoluminescence (PL) emission of colloidal PbSe/CdSe core/shell quantum dots (QDs, CdSe shell thickness: 0.2 nm) at the lowest exciton state was investigated at room temperature and varying inter-QD distance (L = 7-240 nm) by changing the QD concentration. A distinct enhancement of the valley splitting of PbSe QDs was observed upon reducing L. Simultaneously, there was a redshift in the emission due to Forster resonance energy transfer (FRET), when the L value was still sufficiently large (7 nm ≤ L ≤ 50 nm) so that the wave functions of different QDs do not overlap. The enhanced valley splitting under no apparent external field is quite interesting as a method to control the valley splitting. The electronic coupling leading to FRET may enhance the valley splitting, because it occurs in an identical range of L.

Sb-doped β-FeSi2 epitaxial films on Si(111) were grown by molecular beam epitaxy to control an electron density of β-FeSi2. After an optimization of donor activation conditions in the Sb-doped β-FeSi2, the electron density of 6 × 1018 cm-3 at 300 K was achieved by thermal annealing in a N2 ambient. In the temperature dependence of carrier density, the n-type conduction was changed to p-type conduction at low temperatures in the film annealed at high temperature (600 °C). Raman spectra of the annealed films showed that both Fe and Si sites were substituted by the doped Sb in β-FeSi2 lattice.

Infrared (IR) absorption and polarized Raman spectra were measured in BaSi2 epitaxial films to investigate the vibrational modes and the symmetry of Si4 cluster in BaSi2. By an analysis based on Raman and/or IR activity in the spectra, the symmetry of Si4 cluster was determined as Th-symmetry and the observed Raman lines and IR peaks were assigned to Ag, Eg, Fg, and Fu, respectively. In the three Raman lines of Fg-mode, one LO phonon line and two TO phonon lines were classified by the depolarization ratio of polarized Raman intensities.

Semiconducting ruthenium silicide (Ru2Si3) polycrystalline thin films were grown by solid phase epitaxy using Ru-Si amorphous layers on Si substrates. The formation of Ru2Si3 phase was confirmed by XRD and Raman measurements when the amorphous layers were annealed at 600−900 °C in a vacuum. The Ru2Si3 thin films showed a low electron density of 1 × 1016 cm-3 with a high mobility of 430−940 cm2V-1s-1. Photoluminescence (PL) at ~0.8 eV was observed in the Ru2Si3 films.

We grew 500-nm-thick lightly As-doped n-BaSi2 epitaxial films at 600 °C by molecular beam epitaxy, and supplied atomic H in durations (t BaSi:H) of 0–30 min, followed by capping with a 3-nm-thick amorphous Si layer at 180 °C. The photoresponsivity of the BaSi2 films was enhanced by approximately five times by As doping. Deep-level transient spectroscopy measurement revealed the disappearance of two previously reported electron traps. The photoresponsivity was further enhanced by approximately six times after H supply. It reached a maximum at t BaSi:H = 1–10 min, owing to the reduction of defects.

We have succeeded in situ Eu doping into Al x Ga 1 − x N grown by organometallic vapor phase epitaxy and investigated the effects of the growth pressure and Al composition on the photoluminescence (PL) properties of Eu 3+ ions. The intensity of red emission due to the 5 D 0 – 7 F 2 transition of Eu 3+ ions decreased with increasing Al content in the case of growth at atmospheric pressure. The effect of the reactor pressure on the Eu concentration and Eu 3+ PL properties showed that low-pressure growth is effective in increasing the Eu doping concentration and luminescence efficiency. The PL intensity increased with the Al composition and reached a maximum intensity at approximately x = 0.24. At Al compositions of higher than 0.24, X-ray absorption near-edge structure analysis revealed that the concentration of Eu 2+ ions increased with increasing Al composition, leading to a reduction in the concentration of optically active Eu 3+ ions.

We have grown Yb-doped ZnO (ZnO:Yb) on c-plane sapphire substrates by sputtering-assisted metal-organic chemical vapor deposition (SA-MOCVD). X-ray diffraction (XRD) revealed that these ZnO:Yb films exhibit a c-axis orientation and that the lattice constant increases with Yb concentration. After annealing the samples at 600 °C for 0.5 h in O2, they showed a clear 980 nm emission due to the intra-4f shell transitions of 2F5/2–2F7/2in Yb3+ ions. It turns out that Yb lu-minescent centers with different local structures coexist, which exhibit different energy transfer processes from the ZnO host. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence (PL) measurements were carried out on 0.5-μm thick BaSi2 epitaxial films grown on Si(111) substrates with various Ba-to-Si deposition rate ratios (R Ba/R Si) in the range of 1.7–5.1. The samples were excited from both the frontside (BaSi2) and the backside (Si substrate), at temperatures in the range of 8–50 K. These measurements have highlighted the existence of localized states within the bandgap that result from defects in the BaSi2 films. The PL intensity is highly dependent on the excitation power, temperature, and R Ba/R Si. Of those studied, the BaSi2 film at R Ba/R Si = 4.0 showed the most intense PL and weak photoresponsivity, whereas the PL intensity was weaker for the other samples. Therefore, we chose this sample for a detailed PL investigation. Based on the results we determined the energy separation between localized states, corresponding to PL peak energies. The difference in PL spectra excited from the BaSi2-side and Si-side is attributed to the difference in kinds of defects emitting PL. The photoresponsivity of the BaSi2 was drastically enhanced by atomic hydrogen passivation, and the PL intensity of the sample decreased accordingly.

We have investigated the growth condition of Ge-doped β-FeSi 2 on Si(1 1 1) substrates by molecular beam epitaxy (MBE). By the optimization of growth temperature ( T s ) and Si/Fe flux ratio, the β-Fe(Si 1− x Ge x ) 2 with a flat surface was grown at T s =500 °C and Si/Fe=0.5, which were different from the optimized condition of the undoped β-FeSi 2 ( T s =670 °C, Si/Fe=2.0). In the dependence of lattice constants on Ge concentration, all the lattice constants expanded in the range of x =0–11%. In the direct transition energies ( E g ) measured by photoreflectance measurements, the E g decreased with the increase of Ge concentration. These results revealed that β-Fe(Si 1− x Ge x ) 2 was successfully grown on Si(1 1 1) substrate in the range of x =0−11%.

The dependence of Eu3+ photoluminescence (PL) properties on Eu concentration was studied in Eu-doped ZnO (ZnO:Eu) thin films grown by sputtering-assisted metalorganic chemical vapor deposition (SA-MOCVD). The ZnO:Eu showed band-edge PL from ZnO and red emission lines due to the intra-4f transitions in Eu3+ ions at room temperature (RT). The intensities of band-edge and Eu3+ PL decreased with the increasing Eu concentration. In the temperature dependence of Eu3+ PL, the ZnO:Eu with high Eu concentration showed large thermal quenching of the PL intensity. In addition, the lifetimes of Eu3+ PL became short at high Eu concentration. The concentration quenching mechanism of Eu3+ PL using a model based on non-radiative recombination processes in ZnO and Eu3+ ions was presented.

Direct bandgap energy (Eg) and lattice deformations were investigated in β-FeSi2 epitaxial films grown by molecular beam epitaxy (MBE) with different growth condition. As Si/Fe flux ratio during the MBE growth became smaller than Si/Fe = 2.0, the lattice constants deviated from those of β-FeSi2 single crystal, which indicated an enhanced lattice deformation at the lower Si/Fe ratio. In photoreflectance (PR) measurements, the PR spectra shifted to lower photon energy with the enhanced lattice deformation. These results revealed that the Eg of β-FeSi2 epitaxial film was modified by the lattice deformation depending on the growth condition.

We have investigated perfection of atomic rows on iron-based Heusler alloy films on Ge(111) planes by using ion channeling technique in order to find the dominant factors for the perfection. Fe 3 Si/Ge(111) and Fe 2 CoSi/Ge(111) have a high quality of atomic rows at the heterointerface like that of perfect crystals. Fe 3−x Mn x Si/Ge(111) ( x = 0.84, 0.72 and 0.36) interfaces have imperfection of atomic rows which may be controlled by both the lattice mismatch with the Ge substrate and the Mn–Si pairs due to the site disorder in the film with the Mn content x > 0.75. Analysis of axial channeling parameters employed in this study is very useful for quantitative evaluation of perfection of atomic rows at the heterointerface.

The modifications of direct transition energies by lattice deformations were investigated in β-FeSi 2 epitaxial films, polycrystal films and single crystal, systematically. The lattice deformations depending on thermal annealing temperature ( T a ) were observed in β-FeSi 2 epitaxial films. In photoreflectance (PR) measurements, the direct transition energies of the epitaxial films shifted to lower energies as the T a increased. The polycrystal films did not show the lattice deformation and the shift of direct transition energies. These results show that the direct bandgap is modified by the lattice deformation originating from the lattice mismatch at the hetero-interface of β-FeSi 2 /Si.

We obtained room-temperature red emission from GaN-based light-emitting diodes (LEDs) using a Eu-doped GaN (GaN:Eu) as an active layer. The bright emission was observed under normal lighting condition, which is associated with the intra-4f shell transition of Eu 3+ ions. The LED properties depends on the growth condition of GaN:Eu layer. Since the high-quality GaN can be grown at higher growth pressure, the intense electroluminescence (EL) was observed in the LED with a GaN:Eu active layer grown at atmospheric pressure, which is due to the enhancement of the energy transfer efficiency from the GaN host material to the Eu ions. At a d.c. current of 20 mA, the light output power and external quantum efficiency were 17 μW and 0.04%, respectively. These results indicate the feasibility of GaN:Eu to realize a GaN-based red emitter for fabrication of nitride-based monolithic optical devices.

Temperature dependence of direct transition energies ( E g ) was investigated in β - FeSi 2 epitaxial films on Si(111) substrate. The lattice volume of the epitaxial films was reduced as the annealing temperature ( T a ) increased. In photoreflectance measurements, the samples annealed at higher T a showed a larger temperature dependence of E g . These results revealed that the temperature dependence of E g depended on the lattice deformation by the thermal annealing. The fact supports the band gap modifications by the lattice deformation.

Photoluminescence (PL) and photoreflectance (PR) properties were investigated in Si/ β - FeSi 2 /Si(001) double heterostructure (DH) samples. The as-grown sample did not show clear PL, but the annealed one showed the 1.54 μ m PL. In PR spectra, the direct transition energies ( E g ) were observed at 0.910–0.935 eV in the samples. The E g shifted to lower photon energy with the increase of annealing temperature, which indicates a modification of the band structure. These results revealed that the 1.54 μ m PL was related to the transition at the indirect band gap. The effect of thermal annealing on the 1.54 μ m PL was discussed.

We have investigated the exciton spin dynamics in an ultrathin CdTe/ZnTe single quantum well (QW) and self-assembled quantum dots (QDs) by observing time-resolved Kerr rotation (TRKR). At temperatures low enough to suppress the thermal escape of holes from the quantum-confinement structures, excitons dominate the TRKR signal. It shows characteristic dependence on the magnetic field, which is distinctly different from that of isolated electrons. From the data, we have estimated the electron–hole exchange energy to be 137 µeV in the QW, and larger than 190 µeV in the QDs.

Polarized Raman spectra of a β-FeSi2(100)//Si(001) epitaxial film grown by molecular beam epitaxy were measured to identify the Raman mode of the observed Raman active lines. Twelve of the observed 18 Raman lines showed a clear dependence of the Raman intensity on the crystal rotation angle. By factor group analysis using the orthorhombic symmetry D2h18 of β-FeSi2, five Raman lines (193, 200, 249, 401, 494 cm-1) and seven lines (175, 277, 284, 298, 327, 410, 442 cm-1) were completely assigned to the Ag and B3g modes, respectively. The depolarization ratio of Raman scattering intensities was obtained from polarized Raman spectra measured in two polarization configurations. The values of the depolarization ratio also support the assignment of the Ag and B3g modes in β-FeSi2.

We investigated the luminescence properties of the Eu-doped GaN (GaN:Eu) grown at atmospheric (100 kPa) and low (10 kPa) pressures by organometallic vapor phase epitaxy (OMVPE). Although Eu concentration of atmospheric pressure GaN:Eu (AP-GaN:Eu) is lower than that of low pressure GaN:Eu (LP-GaN:Eu), the integrated photoluminescence (PL) intensity of the AP-GaN:Eu was 10 times higher than that of the LP-GaN:Eu (A. Nishikawa et al., Appl. Phys. Lett. 97, 051113 (2010)). Temperature dependent PL and time-resolved PL measurements revealed that the improved PL intensity was attributed to the higher crystal quality of the AP-GaN:Eu compared to that of the LP-GaN:Eu, which resulted in the enhancement of the energy transfer efficiency from the GaN host material to the Eu ions and in the increase in the number of optically active Eu ions.

We fabricate BaSi2 epitaxial films on Si(111) substrates by molecular beam epitaxy and investigate point defects inside the films using Raman spectroscopy with the help of first-principles calculation. Point defects such as Ba substituted for Si antisites, Si vacancies, and Si interstitials are considered as candidates for native point defects in BaSi2. Vibration analysis based on first-principles calculation suggests that local vibrational modes caused by these point defects appear at around 430, 480, and 560 cm−1, respectively, and are in good agreement with Raman peak positions. Comparing calculations with Raman spectra of the films formed with different Ba to Si deposition rate ratios RBa/RSi from 1.0 to 5.1, we conclude that the density of point defects reaches a minimum at RBa/RSi = 2.2. Furthermore, the position of Raman peaks at approximately 490 cm−1 shifts to a lower wavenumber, depending on RBa/RSi and thereby the density of point defects.We fabricate BaSi2 epitaxial films on Si(111) substrates by molecular beam epitaxy and investigate point defects inside the films using Raman spectroscopy with the help of first-principles calculation. Point defects such as Ba substituted for Si antisites, Si vacancies, and Si interstitials are considered as candidates for native point defects in BaSi2. Vibration analysis based on first-principles calculation suggests that local vibrational modes caused by these point defects appear at around 430, 480, and 560 cm−1, respectively, and are in good agreement with Raman peak positions. Comparing calculations with Raman spectra of the films formed with different Ba to Si deposition rate ratios RBa/RSi from 1.0 to 5.1, we conclude that the density of point defects reaches a minimum at RBa/RSi = 2.2. Furthermore, the position of Raman peaks at approximately 490 cm−1 shifts to a lower wavenumber, depending on RBa/RSi and thereby the density of point defects.

CuPt-ordering and phase separation were directly investigated in In1-xGaxAsyP1-y with a low arsenic content grown by organometallic vapor phase epitaxy on GaAs substrates. CuPt-ordering and phase separation in samples grown at the substrate temperatures of 630 and 690 °C were characterized by transmission electron diffraction and transmission electron microscopy. Although the immiscibility of InGaAsP was enhanced at the lower substrate temperature, the sample grown at 630 °C showed less phase separation than the 690 °C-grown sample. The degree of CuPt-ordering was significantly enhanced in the sample grown at 630 °C. The results demonstrated that the CuPt-ordering originating from surface reconstruction of P(2×4) suppressed the phase separation even in the miscibility gap. The detailed characterization of the phase separation clearly revealed a vertical composition modulation (VCM) in InGaAsP for the first time. The mechanism of the VCM formation is discussed based on the modulated-strain field on the surface.

Photoluminescence (PL) properties of Eu-doped ZnO (ZnO:Eu) grown by a sputtering-assisted metalorganic chemical vapor deposition technique were investigated. In PL measurements at 300K, the samples annealed at 600°C for 30min showed clear red-emission lines due to the intra-4f shell transition of 5D0→7FJ (J = 2, 3) in Eu3+. In photoluminescence excitation (PLE) spectra, the PL were observed under the high-energy excitation above the band-gap energy of ZnO (indirect excitation) and the low-energy excitation resonant to the energy levels of 7F0-5D3 and 7F0-5D2 transitions in Eu3+ (direct excitation). These results revealed that the energy transfer from ZnO host to Eu3+ was accompanied under indirect excitation in ZnO:Eu.

We investigated the growth temperature dependence of luminescence properties in Eu-doped GaN layers grown by organometallic vapor phase epitaxy (OMVPE). The dominant photoluminescence (PL) peak intensity at 621 nm, due to the intra-4f shell transitions of 5D0-7F2 in Eu3+ ions, became the highest when the sample was grown at 1000°C. Above 1000°C, the PL peak intensity decreased because of the lower Eu concentration associated with the surface desorption of Eu ions. On the other hand, although the Eu concentration of the layer grown at 900°C was only half of the layer grown at 1000°C, the pronounced decline in the PL peak intensity was observed with decreasing growth temperature from 1000°C to 900°C, which results from the modification of the local structure around Eu ions. These results indicate that the growth temperature strongly influences the Eu concentration and the local structure around Eu ions. Therefore, an optimized growth temperature exists for strong Eu-related luminescence from Eu-doped GaN layer grown by OMVPE.

The electrical transport properties of β-FeSi2 epitaxial films grown on Si(001) or Si(111) substrates were investigated to clarify the growth-orientation dependence of the electrical conduction mechanism in the β-FeSi2 epitaxial films with low electron density of ∼1016 cm-3. As temperature was decreased, the transition from band conduction to hopping conduction was observed in both the epitaxial films. The β-FeSi2 on Si(111) film showed the transition at higher temperature than the β-FeSi2 on Si(001). When the epitaxial films were annealed, the transition temperature shifted to lower temperature. The β-FeSi2 on Si(001) film annealed at 800oC showed the largest electron mobility of 14800 cm2/V·s at 70 K. These results were understood by the conduction through the defect levels in the β-FeSi2 epitaxial films. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-temperature Er-related electroluminescence (EL) properties have been investigated in Er,O-codoped GaAs (GaAs:Er,O) light-emitting diode (LED) grown by organometallic vapor phase epitaxy. Under forward bias at 77 K, the EL spectra from a cleaved edge of the LED were dominated by the luminescence due to an Er–2O center, indicating that injected carriers contribute effectively to the excitation of the Er–2O center. Excitation cross-sections of Er ions by current injection were obtained by the current density dependence of the EL intensity and its time response. The excitation cross-sections depended on temperature and the active layer thickness of the LED.

Al-doped β-FeSi2 [β-Fe(Si1-xAlx)2] thin films with a thickness of 100 nm were epitaxially grown on Si(111) substrate by reactive deposition epitaxy (RDE). The reflective high-energy electron diffraction (RHEED) images showed epitaxial growth of β-Fe(Si1-xAlx)2 (101)(110)//Si(111) in the composition range of 0 < x < 2.3%. In high-resolution X-ray diffraction (HRXRD) measurements, the β(800) and β(406)(460) peaks shift to lower diffraction angles. The peak shifts linearly increase with Al composition. From an analysis of the peak shifts, Vegard's law of a, b, and c-axis in Al-doped β-FeSi2 was proven for the investigated Al-composition range. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Highly-insulating silicon oxide which contained hydrocarbon (CHn) groups (SiOCH) was deposited at room temperature by plasma-enhanced chemical vapor deposition using tetraethoxysilane. The carbon-rich SiOCH film deposited at 50W, 27 °C showed the high resistivity of 6 × 1015Ω cm at 0.5MV/cm. The insulating properties strongly depended on the deposition temperature and radio frequency (RF) power. The SiOCH films deposited at 300W, 53 °C showed the low carbon content and low resistivity due to the incorporation of hydroxyl (OH) groups generating electron traps. The room-temperature deposition at low RF power enabled the high-CHn addition, resulting in the suppression of OH incorporation into the films. The highly-insulating properties of the room-temperature deposited SiOCH originated from the suppression of OH incorporation.

Energy-transfer processes in Er,O-codoped GaAs (GaAs:Er,O) have been investigated by means of a pump and probe reflection technique. Time-resolved reflectivity exhibited a characteristic dip; a negative signal due to bandgap renormalization in less than 1 ps and then a gradual recovery. In the recovery process, there were two components, fast and slow. The fast recovery time (several ps) was inversely proportional to Er concentration. The analysis based on a rate equation indicated that it is due to the captare of electrons by charged traps. The slow recovery (30-60 ps) was well coincident with the time (54 ps) predicted theoretically in the fiamework of a multiphonon-assisted model. Optical excitation cross section of Er ions in GaAs: Er,O has also been studied in various samples and shown to depend strongly on Er concentration.

Dynamics of photoexcited carriers in Er-doped GaAs (GaAs:Er) and Er,O-codoped GaAs (GaAs:Er,O) have been systematically investigated by means of a pump and probe technique. A characteristic ps-scale relaxation was clearly observed in transient reflectance (dR/R) and transmission (dT/T) curves. The relaxation was due to the trapping of photoexcited carriers by an Er-related trap, which was closely related to efficiency of Er3+ luminescence.

Low-temperature plasma-enhanced chemical vapor deposition of amorphous carbon (a-C:H) films was investigated for surface passivation of carbon-doped silicon oxide (SiOCH) films. The a-C:H films were deposited using CH4 and Ar gases at 40–65°C. FT-IR results showed that the deposited films are a-C:H which incorporates hydrocarbon groups. In current−voltage measurements, the a-C:H showed a low leakage current of ~10–10 A/cm2 in air, indicating that the a-C:H films have a potential as a surface passivation layer to prevent moisture absorption in air. The insulating properties of room-temperature deposited SiOCH covered by the a-C:H strongly depended on radio frequency (RF) power in the SiOCH deposition. In the SiOCH film deposited at high RF power of 200 W, the resistivity in air was improved by the a-C:H passivation.

Ion-beam-synthesized β-FeSi2/Si interfaces were investigated by carrier-injection photoluminescence (CPL) measurements. The CPL intensity of Al-doped β-FeSi2/Si sample was larger than that of non-doped one. We obtained a carrier injection efficiency (γ) of γ = 0.41 in the Al-doped sample and γ = 0.19 in the non-doped one. These results revealed that Al-doping into β-FeSi2 is an effective technique for the improvement of β-FeSi2/Si interface which is defective due to ion implantation damages.

We have investigated dry-etching properties of polycrystalline β-FeSi2 films formed by ion-beam sputter-deposition (IBSD) and the films epitaxially grown on Si by metal organic chemical vapor deposition (MOCVD) in order to realize fabrication of photonic crystals with several hundreds nanometers in dimension. Using reactive ion etching with magnetic neutral loop discharge (NLD) plasma, we succeeded in realizing a comparatively higher etching rate than that obtained by inductively coupled plasma (ICP). Both reactive ion etching and impact ion etching modes may contribute to etching of β-FeSi2. We have fabricated a two-dimensional photonic crystal of β-FeSi2 on Si substrates and confirmed its predicted photonic properties in a reflectance spectrum of polarized light.

Carbon-doped silicon oxide (SiOCH) films were deposited by plasma-enhanced chemical vapor deposition (PECVD) using tetraethoxysilane (TEOS) at low temperatures (27–53 °C). The structural and insulating properties of the films deposited with Ar or N2 plasma were investigated. In the deposition with low plasma density and low substrate temperature, both plasmas produced films with high hydrocarbon (CHn) content. The films prepared using Ar plasma showed a low leakage current of 7×10-10 A/cm2 at 1 MV/cm due to the incorporation of CHn groups, while the films with high CHn content prepared using N2 plasma showed poor insulating properties. The deposition using N2 plasma formed films with a defective structure, resulting in a higher etch rate than that of the films deposited with Ar plasma. The deposition with the low-density plasma of inert Ar gas is suitable for the low-temperature deposition of SiOCH films with high resistivity.

High-quality epitaxial Si films have been grown at low temperatures (500–600 °C) by atmospheric pressure plasma chemical vapor deposition (AP-PCVD) with a high growth rate (0.31 µm/min at 600 °C averaged over the 4-in. wafer). In contrast to the previous reports on other low-temperature CVD Si, the films in the present study are defect-free as observed by transmission electron microscopy and selective etching, and show no oxygen and carbon pileups at the film/substrate interface. To characterize defect-free epitaxial films with high sensitivity, we employed a photoluminescence (PL) method. When the epitaxial layer has better quality than the substrate, PL spectrum at 4.2 K mainly reveals the property of the substrate. On the other hand, room temperature PL measurements clearly show that the PL intensities of the surface-passivated epitaxial Si samples are much higher than that of Czochralski (CZ)-Si, which indicates a longer minority carrier recombination lifetime in the epitaxial Si layer than the bulk lifetime in CZ-Si.

GaAs epitaxial layers were grown by organometallic vapor phase epitaxy (OMVPE) using TEGa and TBAs over a wide range of growth temperature (350°C-700°C) and V/III ratio (2-9). GaAs with excellent surface morphology was obtained when the growth temperature was higher than 600°C. The GaAs surface tended to be rough with decreasing growth temperature below 550°C. When the growth temperature was decreased to 400°C, p-type GaAs was obtained with specular surface. The hole concentration increased with decreasing V/III ratio. The hole concentration was as high as 5.5×1017 cm–3 for the sample grown at 400°C with V/III ratio = 2.

Effects of lattice deformations on Raman spectra were investigated in β-FeSi2 epitaxial films grown on Si substrates. The lattice of the epitaxial film was deformed depending on thermal-annealing temperature (Ta). In Raman spectra, Raman lines of Ag-mode (iron displacements) showed a frequency shift with increasing Ta. The shift of Raman lines showed that the residual strain in the epitaxial films was changed by the lattice deformation. In the temperature dependence of Raman spectra, the temperature shifts of the Raman lines were found to be different between β-FeSi2(100)//Si(001) and β-FeSi2(110)(101)//Si(111) epitaxial films.

Magnetic properties of Er-doped GaAs (GaAs:Er) and Er,O-codoped GaAs (GaAs:Er,O) have been investigated by using superconducting quantum interference devices (SQUID) magnetometer at low temperature. In M-H curves of GaAs:Er,O, a characteristic magnetization due to the codoping with Er and O was observed at 2 K, while GaAs:Er exhibited much smaller magnetization. The saturation magnetization of GaAs:Er,O slightly increased with Er concentration. These results were discussed in relation to an atomic configuration formed selectively around Er3+ ions in GaAs:Er,O. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Bulk β - FeSi 2 crystals were grown by annealing of unidirectionally solidified structures with rods of the e -FeSi in the α - FeSi 2 matrix. The crystallographic orientation of annealed samples was investigated by means of electron-backscattered pattern analysis. It was found that there were fibrous single crystals of β - FeSi 2 in the sample. The β - FeSi 2 crystal was easily grown along the solidification direction. The formation of the fibrous β - FeSi 2 single crystals resulted from limited nucleation at the initial stage.

Carbon-doped silicon oxide (SiOCH) thin films were deposited by room-temperature plasma-enhanced chemical vapor deposition (PECVD) using tetraethoxysilane (TEOS). The deposition rate and composition of the films strongly depended on radio frequency (RF) power. The films deposited at low RF power contained more CH n groups. The SiOCH films showed high etch rate and low refractive index in proportion to the carbon composition. The deposition with low plasma density and low substrate temperature is effective for SiOCH growth by PECVD using TEOS.

Dynamics of photo-excited carriers in Er,O-codoped GaAs have been systematically investigated by means of a pump and probe reflection technique. Time-resolved reflectivity exhibited an abrupt increase in amplitude, followed by a steep decrease to negative in less than 1 ps and then a gradual recovery. There were two components, fast and slow, in the recovery process. The fast recovery depended strongly on Er concentration and measurement temperature. The quantitative analysis indicated that the fast recovery corresponds to electron capture by a trap formed by Er and O codoping.

We have succeeded in enhancing the photovoltaic properties of p-n heterojunctions using β-FeSi 2 /Si(100) by doping Al into β-FeSi 2 . Raman spectroscopy, Rutherford backscattering spectroscopy (RBS) and transmission electron microscope (TEM) observations revealed that Al doping contributes to the synthesis of a less defective interface and epitaxial growth of β-FeSi 2 on Si(100). This feature of epitaxial growth may reduce the recombination rate of electron-hole pairs near the depletion region, such that the photoresponse can be enhanced. We observed a significant photoelectric response corresponding to the interband transition near the band-gap energy of β-FeSi 2 . This effect of the Al doping can be used in IR-photoelectric cells.

Effects of impurity-doping on photoluminescence (PL) properties of ion-beam synthesized (IBS) β-FeSi2 have been investigated. The Al- and B-doped β-FeSi2 showed increase of both PL intensity and activation energy (Ea) for non-radiative recombination path. On the contrary, the Mn and Co doping reduced the PL intensity and Ea. We found that the doping of Al and B atoms which occupy the Si sites in β-FeSi2 is a technique to enhance the 1.54 μm photoluminescence.