Thin films of guest-free type-II Si clathrate (Si136) were fabricated on Si(111) wafers in two steps: NaxSi136 thin-film formation by thermal decomposition of NaSi precursor films and Na removal from the NaxSi136 film by a heat treatment with iodine. Cross-sectional TEM observation and XRD and Raman measurements verified the formation of 1-µm-thick Si136 films on the Si wafer. Since the prepared films showed n-type conduction, pn junction devices were developed by a Si136/p-type Si structure. This device showed a photovoltaic (PV) response under white light illumination. The thin film formation and the PV response of Si136 indicated this Si allotrope to be the next-generation platform for semiconductor technology.

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Two nondestructive techniques of surface photovoltage (SPV) and piezoelectric photothermal (PPT) spectroscopies were adopted to investigate GaAs/AlAs multiple quantum well (MQW) heterostructures fabricated on a GaAs substrate, which is difficult to obtain the absorption spectra using a conventional technique. Excitonic and two-dimensional step-like subband absorptions corresponding to the density of states of MQW were obtained by subtracting the background signal from the substrate signal at room temperature. We also conclude that a difference between SPV and PPT spectra indicates the nonradiative component of the carrier relaxation processes in MQW. We found that the combination of SPV and PPT measurements is a powerful method of investigating the carrier confinement and recombination processes in MQW.

The optical properties of ZnO nanorods and nanowires grown by our newly developed nanoparticle-assisted pulsed-laser deposition (NAPLD) technique were studied by photoluminescence (PL) spectroscopy. This NAPLD technique, which does not require a catalyst for crystal growth, is expected to synthesize high-quality nanostructured ZnO crystals. The green luminescence (GL) band and ultraviolet luminescence (UVL) observed at low temperature were investigated for these crystals. The UVL band consists of several sharp emission lines due to free excitons, donor-bound excitons, donor-acceptor pair (DAP) transitions, and their longitudinal-optical (LO) phonon replicas. The free-exciton binding energy was estimated to be 59 meV and the band gap of these ZnO crystals was determined to be 3.435 eV at 10 K. The temperature variation for the energy positions of free-exciton emission was fitted by the Manoogian-Woolley equation and the Debye temperature was estimated to be 505 K. The low-temperature PL investigation of our nanostructured ZnO crystals that were easily synthesized by our newly developed NAPLD technique demonstrated their excellent crystallinity.

Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films possess the following specific characteristics: (a) the appearance of additional energy levels in diamond bandgap and (b) large absorption coefficients ranging from visible to ultraviolet, both of which might be due to large number of grain boundaries between UNCD grains and those between UNCD grains and a-C:H. Owing to them, UNCD/a-C:H films are expected to be applied to photovoltaics such as UV sensors. Actually thus far, we have fabricated pn heterojunction diodes comprising p-type UNCD/a-C:H films and n-type Si substrates, and confirmed their photovoltaic action. In this study, the minority carrier lifetime, which is an important factor for photovoltaics, was experimentally measured by microwave reflected photoconductivity decay, and it was estimated to be 0.21 and 0.43 µs for UNCD/a-C and UNCD/ a-C:H, respectively. In addition, on the basis of the previous work on the heterojunctions, the effects of hydrogenation on the photovoltaic action of the heterojunctions were studied. The photocurrent apparently increases with an enhancement in the hydrogenation of UNCD/a-C:H films, which might be because dangling bonds in the UNCD/a-C:H films, which act as photogenerated-carrier trap centers, are terminated by hydrogen atoms.