Yoshihito Maeda

Photovoltaic properties of ion-beam synthesized β-FeSi2/n-Si heterojunctions

We present the first evident photovoltaic responses from ion-beam synthesized (IBS) polycrystalline p-type β-FeSi2/n-Si(100) heterojunctions. The triple ion implantation and subsequent annealing at 800°C provided polycrystalline continuous layers ∼60-nm thick with large crystalline grains of ∼10 μm. The high temperature and long annealing time were very effective in amplifying the photovoltaic responses from the heterojunctions. We achieved a maximum open-circuit voltage of 0.34 V by 5 mW/cm2 of white light illumination. Furthermore, we confirmed that the annealing procedure at 500°C induced the precipitation of the psuedomorphic metallic γ phase, which is detrimental to both rectification and the photovoltaic voltage at the p–n heterojunction.

Raman spectra of Ge nanocrystals embedded into SiO2

We start with an analysis of the Raman spectra of Ge nanocrystals obtained in previous studies and demonstrate that in many cases the observed experimental peak attributed to Ge in fact originates from the Si substrate. We further compare various experimental ways to separate the Ge signal from that of the substrate and suggest optimum conditions for such measurements. Finally, we demonstrate that upon the annealing of an amorphous Ge–Si–O film, Ge nanocrystals are formed. The nanocrystals are randomly oriented and Ge–Si mixing takes place only at the interface with the Si substrate.

Raman spectroscopic study of ion-beam synthesized polycrystalline β-FeSi2 on Si(100)

We examined effects of ion implantation doses, annealing temperature and time on ion-beam synthesis (IBS) of polycrystalline β-FeSi2 using Raman spectroscopy. It was confirmed that at very low Fe concentration doses (up to 1×1016 ions/cm2), fine grains of β-FeSi2 and a small amount of fluorite γ-FeSi2 may precipitate after annealing at 800°C. In the case of high dose (>1×1017 ions/cm2), the clear Raman lines showed that β-FeSi2 grows after annealing at 800°C. However, we observed an outstanding Raman line at 324 cm−1 and broad features at 300–450 cm−1 after annealing at 600 and 700°C. These Raman features can be considered to be due to presence of γ-FeSi2 and lattice imperfections in the samples. Furthermore, we evaluated improvement of crystalline quality with increasing the annealing time and temperature using a clear blue shift of the Raman line and increase of the intensity-ratio of two Raman lines at ∼192 and ∼246 cm−1, θ=(I192/I246).

Microstructure characterization of ion-beam synthesized β-FeSi2 phase by transmission electron microscopy

Abstract β-FeSi2 is a new material showing semiconductor properties, which has been characterized to consist of the abundant and not toxic constituents, iron and silicon. The ion implantation method is one of the most useful techniques to make a good quality β-FeSi2 phase on a Si wafer, although it is polycrystalline. Using field-emission transmission electron microscopy, the formation process of the β-FeSi2 particles and layer-grown β-FeSi2 phase was investigated. With annealing at 800°C after 56Fe+ ion implantation at energy of 100 keV, an amorphous-like damaged layer changed to the single phase of β-FeSi2 and Si single crystal phase. It was characterized that the precipitation always occurred from the Si wafer surface, resulting in the possibility of the formation of a layered β-FeSi2 structure on the Si wafer. By controlling the Fe concentration at the damaged layer with multiple ion implantation, a layer grown β-FeSi2 poly-crystals was obtained. The morphology of a grain of the layered poly-crystals is dendrite, of 5–10 μm in diameter. High resolution electron microscope observations further revealed that the each dendrite grain of β-FeSi2 consists of more fine domains of 50–100 nm in size, resulting in the release of a large lattice misfit strain for the Si single crystal.

Infrared-Photovoltaic Responses of Ion-Beam Synthesized β-FeSi 2 / n -Si Heterojunctions

Photoresponses of photovoltaic cells using ion-beam synthesized (IBS) polycrystalline p + -β-FeSi 2 /n-Si heterojunctions were examined in an infrared (IR) wavelength region. At room temperature, an evident photoresponse due to an internal photoemission from trap levels in β-FeSi 2 with the threshold energy Φ=0.62 eV was observed at 0.6-0.87 eV. The pronounced increase of a photoresponse corresponding mostly to an interband transition in β-FeSi 2 was observed at 0.87-1.1 eV. The maximum dominated by a surface recombination process appeared around ∼1.2 eV. The surface recombination rate of ∼10 4 cm/s was estimated. The quantum efficiency was ∼60 % in the 0.8-1.0 µm wavelength region and ∼14 % around the band-gap of βFeSi 2 .

Raman and FT-IR studies of photodynamic processes of cholesteryl oleate using IRFELs

Chemical bond-changes of cholesteryl oleate in infrared free electron laser (IRFEL) photodynamic processes were examined by Raman spectroscopy and Fourier transform infrared microspectroscopy (FT-IR) as a function of the FEL exposure time. We found that the exposure of 5.79 μm-FELs induces not only dissociation of ester (RCOOR′) bonds but also the chemical changes from the ester (RCOOR′) bonds to aldehyde (RCHO), carboxylic acid (RCOOH), carboxylate (RCOO−) or ketone (R2Cue605O) bonds.

Ion beam synthesis of β-FeSi2 as an IR photosensitive material

The large sized and flat polycrystalline (beta) -FeSi2/n- Si heterojunction can be formed by a triple energy implantation method and the sample annealing at 800 degrees C. The polycrystalline (beta) -FeSi2 gains show good crystalline characteristics, a photoluminescence peak at 0.81 eV at 4.2 K and the optical direct band-gap of 0.84 eV. The (beta) -FeSi2/n-Si heterojunction shows good diode characteristics and high photovoltaic sensitivity for IR light. These results support that the ion beam synthesized (beta) -FeSi2/n-Si heterojunction is a promising IR sensitive materials.

Ion-Beam Synthesized Semiconducting β-FeSi 2 Controlled By Annealing Procedures And Phase-Transitions

The ion beam synthesis (IBS) of β-FeSi 2 was examined by Rutherford backscattering spectroscopy (RBS) and x-ray diffractometry (XRD), and the structural characterization was carried out by Raman spectroscopy and scanning electron microscopy (SEM). We found that the IBS of β- FeSi 2 is controlled by two different processes depending on the annealing temperature (Ta) and Fe surface concentration (Cs); (I) precipitation of β-FeSi 2 on the surface in Cs˜30 at% and Ta⩾700° C and (II) phase transition from γ -FeSi 2 to β-FeSi 2 in Cs 2 . The good crystalline β-FeSi 2 obtained above 800°C showed a clear reflectance maximum at 0.88 eV due to the optical transition at the direct band-gap of 0.84 eV observed in the characteristic plot ((ahv) 2 vs. hv) of the optical absorption.

Effect of the interface on the local structure of Ge–Si nanostructures

We first discuss the limitations of Raman scattering as applied to Ge/Si nanostructures. We further summarize our recent efforts to investigate the local structure of various Ge nanostructures, namely, Ge quantum dots grown by molecular beam epitaxy (MBE) on bare Si(100), on Si(111) with a 0.3 nm SiO2 coverage, and nanocrystals embedded in SiO2, by x-ray absorption fine structure spectroscopy. In particular, the MBE growth of Ge dots on bare Si(100) has been studied as a function of the growth conditions: in most cases strong alloying with Si takes place. Ge nanoislands on Si(111) with SiO2 coverage, on the other hand, may retain the local structure of bulk Ge and be very stable against oxidation. The Ge nanocrystals embedded in SiO2 possess the structure of relaxed bulk Ge without any Ge–Si bonding. The latter two kinds of Ge nanostructures possess visible photoluminescence.

Local structure of Ge/Si nanostructures: Uniqueness of XAFS spectroscopy

A discussion of the limitations of Raman scattering as applied to Ge/Si nanostructures is followed by a summary of our recent efforts to investigate the local structure of various Ge nanostructures, namely, Ge quantum dots MBE grown on bare Si(10 0), on Si(111) with a 0.3 nm SiO 2 coverage, and nanocrystals embedded in SiO2, by X-ray absorption fine structure spectroscopy. For the latter case, combined DAFS–EXAFS analysis has been applied to determine separately, for the first time, the structural parameters for intermixed nanocrystalline and amorphous phases. 2002 Elsevier Science B.V. All rights reserved.