Hitoe Habuchi

Hydrogen structures and the optoelectronic properties in transition films from amorphous to microcrystalline silicon prepared by hot-wire chemical vapor deposition

Transition films from amorphous (a-) to microcrystalline (μc-) silicon were prepared by hot-wire chemical vapor deposition using silane decomposition with either varied hydrogen-to-silane ratio, R, or with fixed R=3 but a varied substrate temperature, Ts. Raman results indicate that there is a threshold for the structural transition from a- to μc-Si:H in both cases. The onset of the structural transition is found to be R≈2 at Ts=250 °C and Ts≈200 °C at R=3. The properties of the material were studied by infrared absorption, optical absorption, photoluminescence (PL), and conductivity temperature dependence. We observed that the peak frequency of the SiH wag mode remains at 630−640 cm−1 for all the films, but the hydrogen content shows two regimes of fast and slow decreases separated by the onset of microcrystallinity. When microcrystallinity increased, we observed that (a) the SiO vibration absorption at 750 cm−1 and 1050−1200 cm−1 appeared, (b) the relative intensity of the 2090 cm−1 absorption increased...

Optical and electronic properties of microcrystalline silicon as a function of microcrystallinity

Films were prepared by hot wire chemical vapor deposition at ∼240 °C with varied hydrogen dilution ratios R=H2:SiH4 from 1 to 20. The optical and electronic properties as a function of microcrystallinity were studied. We found: (a) At low H dilution R⩽2, there is no measurable crystallinity by Raman spectroscopy and x-ray diffraction in the a-Si:H matrix, but an optical absorption peak at ∼1.25 eV appears; when R=2, the film shows the lowest subgap absorption, the highest photosensitivity, and the largest optical gap. (b) When R⩾3, the c-Si phase is measurable by Raman and a low-energy photoluminescence (PL) band (0.84–1.0 eV) appears in addition to the high-energy band (1.3–1.4 eV). Meanwhile, all the absorption spectra show a featureless line shape. (c) An energy redshift is observed for both PL peaks as the film grows thicker. Finally, (d) the conductivity activation energy first decreases from 0.68 to 0.12 eV, then increases with increasing microcrystallinity. A mode of two sets of energy bands of ele...

Localized electronic states related to O2 intercalation and photoirradiation on C60 films and C70 films

Deep localized electronic states are created by O2 intercalation into C60 films and C70 films, which causes the Fermi level to shift down to the middle of gap. The states act as a trap level for charge carriers and as nonradiative recombination centers. It seems that prepared C60 films and C70 films have a shallow localized state. The shallow state is located at ∼0.2 eV under the conduction band and affects the electrical and optical properties. Furthermore, the photoirradiation of C60 films and C70 films causes polymerization of the O2-free sample and oxidization of the O2-intercalated sample. The quasistable electronic states at room temperature are created as a result of photo-oxidization of C60 films. C60 oxides create deep localized electronic states which cannot disappear under thermal annealing. The photoluminescence intensity of O2-free samples increases with photoirradiation for 1 h. It is found for the first time that this increase occurs along with a decrease of localized state density.

Highly photoconductive amorphous carbon nitride films prepared by cyclic nitrogen radical sputtering

We report on the growth of amorphous carbon nitride films (a-CNx) showing the highest conductivity to date. The films were prepared using a layer-by-layer method (a-CNx:LL), by the cyclical nitrogen radical sputtering of a graphite radical, alternated with a brief hydrogen etch. The photosensitivity S of these films is 105, defined as the ratio of the photoconductivity σp to the dark conductivity σd and is the highest value reported thus far. We believe that the carriers generated by the monochromatic light (photon energy 6.2eV) in the a-CNx:LL films are primarily electrons, with the photoconductivity shown to increase with substrate deposition temperature.

Response time of photoconductivity of amorphous carbon nitride films prepared by a nitrogen radical sputter method

Abstract Amorphous carbon nitride a-CN X films prepared by a nitrogen radical sputter method have been studied and found as good material applicable as photoconductive devices from 2 up to ultraviolet 6.2 eV. Especially a-CN X made by the layer-by-layer method, LLa-CN X , shows photoconductivity larger than that of usual a-CN X . It is interesting to observe the response time τ of photoconductivity for LLa-CN X films, because response time τ is an important factor to design a device. The value of response time τ for the light-on experiments is found in 160 μs or less under the light source of 6 eV. Dependence of photoconductivity for LLa-CN X on preparation conditions is also studied. LLa-CN X prepared at higher substrate temperature T S shows large photoconductivity than that made at lower T S . The results are discussed with data of X-ray Photoelectron Spectroscopy (XPS) and Electron Spin Resonance (ESR).

Potical Energy Gap and Below Gap Optical Absorption of Fullerene Films Measured By Constant Photocurrent Method and Photothermal Deflection Spectroscopy

ABSTRACT CPM spectra of the fullerene film was measured to obtain the below gap absorption. The optical energy gap Eo was obtained by using the Taucs plots. Eo did not change so much with the intercalated impurities. The absorption due to intercalated impurities was found below 1.6eV.

Complex changes in the framework of endohedrally Na-doped type II Si clathrates with respect to Na content

Crystal structures of endohedrally Na-doped type II silicon clathrates with variable Na content were refined by Rietveld analysis. Type II Si clathrates have two types of cages: small and large. The large cages were preferentially occupied by Na atoms. Upon occupation by Na in the large cages, the lattice constant of the clathrates decreased slightly. The attractive interaction of the Na atoms in the large cages with a framework that caused displacement of the Na atoms from the center might influence lattice shrinkage. Moreover, in the region where almost large cages contained Na atoms and the small cages were partially occupied by Na, the lattice constant increased with the Na content; however, the relationship between these features was complex. The lattice constant demonstrated a linear relationship with the size of the small cages; however, the enlargement of the small cages was not linear with respect to its Na occupancy, resulting in complex changes in the lattice constant with respect to the Na content. It was inferred that enlargement of the small cages by Na insertion may be dependent on the Na occupancy of neighboring small cages.

Gas Effusion Spectra of Fullerenes

Abstract Gas effusion spectra of fullerenes are studied. Three peaks of toluene are found in gas effusion spectra dp/dT-T. Two peaks of oxygen are found in gas effusion spectra dN/dt-T of samples kept in the air after annealing to avoid the toluene. These phenomena confirmed to be reversible are discussed by treating fullerenes as intercalated materials.

Photoconductivity and Photoluminescence of Amorphous Carbon Nitride a-CN x Films Prepared by the Layer-by-Layer Method

Amorphous carbon nitride films a-CN x , which made in our laboratory, show high photosensitivity P s that is a ratio of photoconductivity σ p and dark-electrical conductivity σ d Maximum P s of a-CN x is about 5×10 6 which is about 50 times larger than that of hydrogenated amorphous silicon a-Si:H. We have succeeded to observe photoconductivity spectra where photoconductivity starts nearly at 2 eV and saturates at 3 eV. Then σ p increase again at 3.7 eV up to 6.2 eV. The dependence of σ p on excitation energy and intensity is observed. Photoluminescence spectrum is also obtained which have a spectrum from infrared of 0.8 eV up to ultraviolet of 3.5 eV. Using these data together with photothermal deflection spectra, we have presented a refined model of the electronic density of states for LLa-CN x .

Temperature dependence of photoluminescence spectra of C60 films and a luminescence related to oxygen molecules

Abstract The photoluminescence intensity has a maximum at 50 K and decreases as temperature increases rather rapidly above 90 K. The decrease of photoluminescence at main and at 0.968 eV bands on a C60 film is observed when it is exposed to air and irradiated by a laser. This phenomenon is explained by relating to the intercalation of O2.