Shuhei Yagi

Differences in SiC thermal oxidation process between crystalline surface orientations observed by in-situ spectroscopic ellipsometry

For a better understanding of the SiC oxidation mechanism, we investigated differences in the oxidation process for surfaces with different crystal orientations. Real-time observations of oxidation processes for (0001) Si-face, (11 2¯0) a-face, and (000 1¯) C-face substrates at various oxidation temperatures were performed using in-situ spectroscopic ellipsometry. Massouds empirical equation, which is composed of the classical Deal-Grove equation added by an exponential term, was applied to the observed growth rates and the oxidation rate parameters were extracted by curve fitting. The SiC oxidation mechanism is discussed in terms of the oxidation temperature dependence and surface orientation dependence of the oxidation rate parameters.

Fabrication of resonant tunneling structures for selective energy contact of hot carrier solar cell based on III–V semiconductors

In this study, double barrier (DB) resonant tunneling structures based on III–V semiconductors were fabricated and its potential for selective energy contacts (SEC) of hot carrier solar cells was evaluated. An AlGaAs/GaAs/AlGaAs quantum well (QW) based DB structure was fabricated by molecular beam epitaxy (MBE) on GaAs (001) substrate, which acts as SEC for electrons. The current-voltage (I–V) characteristics under light excitation shows a voltage shift of tunneling current tail to a lower bias and this result demonstrates an extraction of high energy photoelectrons through the DB structure. Furthermore, properties of quantum dot (QD) resonant tunneling structures were investigated as an ideal SEC. Photoluminescence (PL) measurements showed that controllable PL peak energy range of InAs QDs/AlxGa1−xAs structures well corresponds to the required carrier extraction energy, which is the difference between electron and hole extraction energies of SECs, for high conversion efficiency. In addition, resonant tunneling current peaks originate from the InAs QDs embedded in an Al0.6Ga0.4As barrier are clearly observed for both forward and reverse bias by conductive atomic force microscope (C-AFM). These results indicate that InAs QD/AlxGa1−xAs resonant tunneling structures are suitable for designing the optimum SEC structure.

Substitutional C Incorporation into Si1-yCy Alloys Using Novel Carbon Source, 1,3-Disilabutane

A new carbon source, 1,3-disilabutane (H3Si-CH2-SiH2-CH3:1,3-DSB), is proposed for use in the growth of epitaxial Si1-yCy films with high C substitutionality. The Si1-yCy films have been deposited by plasma-enhanced chemical vapor deposition (PECVD). The Si1-yCy films grown using C2H2 or SiH2(CH3)2 show a reduction in C substitutionality when total C content is more than 2%, whereas they show a marked improvement in C substitutionality for C contents up to 2.5% using 1,3-DSB as a C source.

Oxygen partial pressure dependence of the SiC oxidation process studied by in-situ spectroscopic ellipsometry

The oxygen partial pressure dependence of the Silicon carbide (SiC) oxidation process was investigated using in-situ spectroscopic ellipsometry at oxygen partial pressures between 1 and 0.02 atm for 4 H-SiC (0001) Si- and (000−1) C-faces. Analyses of the interface structure between the oxide and SiC indicate that the interface layer has a modified SiC-like structure around 1 nm thick accompanied by oxide growth; the structure and thickness do not change after an oxide growth of about 7 nm. The oxide thickness dependence of the growth rate at sub-atmospheric oxygen pressures is similar to that at 1 atm pressure, that is, just after oxidation starts, the growth rate rapidly decreases as the oxidation proceeds. After an oxide growth of about 7 nm thick, the deceleration of the growth rate suddenly changes to a gentle slope. The thickness at which deceleration changes depends slightly on both the oxygen partial pressure and surface polarity of the SiC substrate. The origins of these two deceleration stages, i...

C Stability in Si1-yCy Epitaxial Films Grown by Low-Temperature Chemical Vapor Deposition

Thermal stability of the substitutional C (Csub) in the Si1-yCy film has been investigated. The Si1-yCy films were grown on Si(001) substrates by low-temperature plasma-enhanced chemical vapor deposition using SiH4 and H2. C2H2 or SiH2(CH3)2 were used as C source gases. The simulation taking into account the chemical reactions of the complexes of the Si–C–H system in the Si1-yCy well explained the annealing behavior and thermal stability of the Si1-yCy films. During high-temperature annealing, thermal diffusion of Csub occurred resulting in 3C-SiC precipitation. The activation energy for the precipitation was estimated at 3.18–3.23 eV from the simulation. Both the calculated and experimental results showed that thermal stability of the Si1-yCy decreased with increasing initial Csub content in the film.

Biexciton Luminescence from Individual Isoelectronic Traps in Nitrogen \delta-Doped GaAs

We report on the observation of biexciton luminescence from single isoelectronic traps formed by nitrogen–nitrogen pairs in nitrogen δ-doped GaAs. The biexciton luminescence intensity showed a quadratic dependence on the excitation power while the exciton luminescence intensity increased linearly with increasing excitation power. The biexciton binding energy was found to be 8 meV, which is considerably larger than that reported for single InAs quantum dots in GaAs. We have also found that both the biexciton and exciton emission lines show completely unpolarized and no fine-structure splitting. This is suitable for the application to polarization-entangled photon pairs.

Conversion Efficiency of Intermediate Band Solar Cells with GaAs:N δ-Doped Superlattices

The performance of intermediate band solar cells using a GaAs:N δ-doped superlattice (SL) as the optical absorber is analyzed. In GaAs:N δ-doped SLs, both of the E+ and E- bands formed around the N δ-doped layers form SL potentials with the conduction band of the spacer GaAs layers, resulting in the formation of multiple minibands. The conversion efficiency limits of the solar cells are calculated using the detailed balance model based on intermediate band structures composed of the valence band and two minibands formed respectively in the E-- and E+-related SL potentials. A high efficiency of 62.6% for full concentration is expected by properly adjusting the structural parameters of the SL. Alloying other elements such as In to a GaAs:N δ-doped SL is a possible way to further facilitate the development of intermediate band materials for high-efficiency solar cells.

Control of intermediate-band configuration in GaAs:N δ-doped superlattice

GaAs:N δ-doped superlattices (SLs) consisting of alternating layers of undoped and N δ-doped GaAs were fabricated by molecular beam epitaxy (MBE) as possible candidates for the light-absorbing material of intermediate-band solar cells (IBSCs). Since the energy gaps in IBSCs need to be adjusted to optimum values to achieve sufficiently high conversion efficiency, it is important to control precisely the band configuration of intermediate-band (IB) materials. In this study, we demonstrated the control of the IB energy configuration in GaAs:N δ-doped SLs by changing their structural parameters. Optical transitions due to the SL minibands related to the N-induced conduction subbands E+ and E− were clearly observed and the transition energies depended systematically on the N area density and period length of the SLs. Conversion efficiency calculations based on the detailed balance model indicated that IBSCs with an efficiency of nearly 60% are achievable by using the fabricated GaAs:N δ-doped SLs.

Enhanced optical absorption due to E+-related band transition in GaAs:N δ-doped superlattices

The photoabsorption characteristics of GaAs:N δ-doped superlattices (SLs) are investigated. Periodic insertion of N δ-doped layers into GaAs induces the formation of conduction subbands E+ and E−, and each conduction subband forms SL minibands with the GaAs conduction band between the δ-doped layers. In addition to an optical absorption related to the E− band, an abrupt absorption edge originating from the electron transition between the valence band and an E+-related miniband is observed at 1.6 eV in a photoluminescence excitation (PLE) spectrum, indicating that GaAs:N δ-doped SLs are promising candidates for the absorber of intermediate-band solar cells.

Thermal Oxidation Mechanism of Silicon Carbide

Recently, semiconductor devices that work normally for a long time under harsh environ‐ ments are demanded such as in nuclear power application or in space development field. Especially, after the disaster of Fukushima Dai-ichi nuclear power plant caused by the EastJapan great earthquake on March 11, 2011, the importance of such a hard electronic devices has been growing. However, it is difficult to achieve the performance using conventional semiconductors such as Si semiconductor because of its physical property limit.