Growth process of microcrystalline silicon (μc-Si:H) using plasma-enhanced chemicalvapor- deposition method under high-rate-growth condition has been studied for the control of optoelectronic properties in the resulting materials. We have found two important things for the spatial-defect distribution in the resulting μc-Si:H through a precise dangling-bond-density measurement, e. g., (1) dangling-bond defects are uniformly distributed in the bulk region of μc- Si:H films independent of their crystallite size and (2) large number of dangling bonds are located at the surface of μc-Si:H especially when the film is deposited at high growth rate. Starting procedure of film growth has been investigated as an important process to control the dangling-bond-defect density in the bulk region of resulting μc-Si:H through the change in the electron temperature by the presence of particulates produced at the starting period of the plasma. Deposition of Si-compress thin layer on μc-Si:H grown at high rate followed by thermal annealing has been proposed as an effective method to reduce the defect density at the surface of resulting μc-Si:H. Utilizing the starting-procedure-controlling method and the compress-layerdeposition method together with several interface-controlling methods, we have demonstrated the fabrication of high conversion-efficiency (9.27%) substrate-type (n-i-p) μc-Si:H solar cells whose intrinsic μc-Si:H layer is deposited at high growth rate of 2.3 nm/sec.

To achieve 520–532 nm green laser diodes (LDs), nonpolar and semipolar nitrides have attracted much attention because their usage leads to the elimination of the quantum-confined Stark effect and higher optical gains in this wavelength region. Since the breakthrough in the homoepitaxial growth technology for them, many nonpolar m -plane devices such as mW-class blue light-emitting diodes, violet 405 nm LDs, blue 460 nm LDs, and blue-green LDs beyond 490 nm have been announced. Advantages such as small blueshift and high slope efficiency (high output power to injected current ratio) have been confirmed for the first time in m -plane LDs beyond the blue region. On the other hand, the semipolar plane is also a candidate for green LDs. The pulsed operation of semipolar (1011) and (1122) violet LDs and lasing for a (1122) LD at 514 nm by optical pumping also have been reported. Such rapid progress in this research field will be reviewed.

We have already reported that Au or B doped SiGe amorphous thin films have superior thermoelectric properties which are attribute to amorphous phase. For the practical use, the bulk materials are required. In this work, we have tried and succeeded to fabricate Si-Ge-B amorphous bulk samples. First, fine particles were prepared by three kinds of mechanical process, such as roller milling method, mechanical alloying method and planetary milling method. It was intended to introduce a large amount of defects and strain into samples and/or gamorphouslization'h. Then, the prepared fine particles were pressed and formed into 2 x 5 x 15 mm^3 samples. Thermoelectric power and electrical resistivity of samples were measured by steady state measurement and four terminal method with temperature range of room temperature to 873 K in N2 flow at atmosphere pressure, respectively. Thermal conductivity was measured by photo-pyroelectric method in room temperature. Some samples have maximum value of thermoelectric power over 10-3 V/K. therefore≤ we were succeeded to prepare bulk materials which have higher thermoelectric power than that of conventional crystal materials. X-ray diffraction measurement and scanning electron microscope observation were performed on fabricated samples. From XRD measurement and SEM observation, the samples were in mixed condition of disordered microcrystals and amorphous state.

Linearly polarized electroreflectance (ER) measurements have been firstly performed on PSi with near-normal incident (NNI) and glancing-angle incident (GAI) light. In the NNI geometry, no significant polarization anisotropy is observed. Meanwhile, in GAI geometry, a clear polarization anisotropy is observed. The s-polarized ER spectrum consists of the ER features corresponding to those found in the NNI geometry, while in the p-polarized ER spectrum, the two features found in s-polarized spectrum in infrared-visible region are not found, while a new feature is observed at ∼2.1 eV. The mechanism of the unusual polarization anisotropy is discussed in association with structural anisotropy of PSi.

We have developed a new type of a thin-film electroluminescence (TFEL) device with nano- structured (NS)-ZnS:Mn utilizing its enhanced luminescent efficiency due to the quantum confinement (QC) effects. As NS-ZnS:Mn, ZnS:Mn/Si3N4 multilayers with thicknesses of 1.9–3.5 nm for ZnS were prepared by a rf-magnetron sputtering method. From the results of grazing incidence X-ray reflectometry and X-ray diffractmetry, formation of ZnS:Mn nanocrystals in the ZnS layers are confirmed. With a decrease in the ZnS:Mn layer thickness, the photoluminescence (PL) efficiency associated with the Mn2+ transitions is increased, and the PL excitation spectrum is shifted toward higher energies, indicating appearance of the QC effects. As the results of the application of NS-ZnS:Mn to the emission layer of the TFEL device, we have successfully observed reddish-orange emission above the threshold voltage of 12 V0-p, and the maximum luminance is 3.0 cd/m2 operated with the 1-kHz sinusoidal voltage of 20 V0-p.

Employing electroreflectance (ER) spectroscopy, we have studied optical transitions in porous Si (PSi) with a thickness of 100±50 nm made from crystalline Si (c-Si) substrates with different resistivities, 4–10 ωcm (p -), 0.1–1 ωcm (p), and < 0.018 ωcm (p +). The ER features observed at 1.1–2.8 eV in p + PSi are analyzed with a simple effective mass approximation (EMA) model for confined electron-hole (e-h) pairs in a spherical quantum dot as we have previously done for those observed at 1.2–3.1 eV in p - PSi. From the ER analysis with the EMA model, the effective crystal size is estimated without destruction, and the kinetic energy and the Coulomb attraction energy of the confined e-h pairs are also deduced.

Furthermore, the ER features corresponding to the optical transitions at E 1(E 0') critical point (CP) are observed at 2.8–3.3 eV in p + PSi. With an increase in the crystal size, the transition energy of E 1(E 0') CP in p + PSi is decreased, while that in p - and p PSi is unchanged from that of 3.4 eV found in c-Si including the p +c-Si. From the Raman results, strain- and disorder-induced spectral changes are found to be negligible, so that the high doping induced effect is the most acceptable mechanism for the red-shift in E 1(E 0') transitions of Si nanocrystals with the crystal size of 2–3 nm.

Electroreflectance spectroscopy measurements have been performed on light-emitting porous Si (LEPSi). The transmission electron microscope measurements reveal that LEPSi includes Si nanocrystals with a mean crystal size of 1–2 nm. The ER features are observed at a transition energy of 3.4 eV in all of the samples, giving that LEPSi still keeps the threedimensional (3D) electronic structure. Changes in the transition energies are not found for LEPSi with the different mean crystal sizes. Furthermore, we directly observed interband transitions of quantized states due to quantum-confined electron-hole (e-h) pairs in LEPSi as two or three extra ER features being located between 1.2 and 3.1 eV which are never observed in bulk crystalline Si. Employing a simple effective mass approximation model, we have evaluated the reduced mass, the kinetic energies and the Coulomb attraction energies of the quantumconfined e-h pairs. We also found that the energy distance between the transition energies at the ground state and the photoluminescence (PL) peak energies basically corresponds to the Coulomb attraction energies. Finally, we propose a new luminescent model based on interband transitions involving the quantum-confined dense e-h plasma.

Clarifying the local amorphization on the grain boundaries, the in-situ observation during ion-irradiation was carried out for poly-crystalline Si film. The critical dose of amorphous formation increased exponentially with increasing temperature, where the local amorphization was developed at middle temperature. The critical dose was affected by the doped impurity and the grain size. The preferential amorphization on and near grain boundaries had two processes; first stage with rapid growth rate and second stage with almost constant growth rate. The importance of stress was demonstrated from the acceleration due to the stress on the first stage of amorphization.

A staircase structure reflecting quantum-size effect has been observed in differential optical absorption spectra in a-Si:H/a-SiC:H multilayer and a-Si:H ultrathin single layer. The threshold energies for the identified subband transition are found to be consistent with those expected from one-dimensional quantum-well model involving a conservation rule for the subband index. The experimental approaches introduced here will open up new possibilities for investigating the quantized band structure as well as for establishing the design concept of functional elements based on quantum size effects.

A series of systematic trials to improve the light emission efficiency has been made on thin film visible a-SiC LEDs. A wide variety of the approaches, such as a superlattice structure, p-i-n/p-i-n tandem structure, an efficient injection electrode with a wide-gap layer etc., have been done. From the results, two practically available new technologies for bright LEDs have been developed, that is; a) a hot carrier injection structure by inserting a highly- resistive layer between p-and i- or i- and n-layers and b) a wide-gap highly-conductive p-type injector layer prepared by ECR (Electron Cyclotron Resonance) CVD deposition. With these trials, the brightness of yellow LEDs have been increased by more than one order of magnitude to be about 5cd/m2