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Effects of nitrogen impurity on ZnO crystal growth on Si substrates have been investigated. The quantitative analysis on the surface morphology deriving height-height correlation function indicates that adsorbed nitrogen atoms suppress the secondary nucleation and enhance adatom migration. The resultant films have smooth surface as well as large grain size up to 24 nm even for small thickness of 10 nm. ZnO films fabricated by using such films as buffer layers possess high crystal quality, where the full width at half maximum of (002) rocking curve is 0.68°, one-fourth of that for films fabricated without nitrogen.
We have recently developed a novel semiconductor, (ZnO)x(InN)1-x (abbreviated to ZION). In this study, we have succeeded in direct epitaxial growth of ZION films on 19–21%-lattice-mismatched c-plane sapphire by radio-frequency (RF) magnetron sputtering. X-ray diffraction analysis showed that there is no epitaxial relationship between ZION films fabricated at room-temperature (RT) and the sapphire substrates, while the films fabricated at 450oC grow epitaxially on the sapphire substrates. From the analysis of time evolution of the surface morphology, the process for the epitaxial growth of ZION on sapphire is found to consist of three stages. They are (i) initial nucleation of ZION crystallites with crystal axis aligned to the sapphire substrate, (ii) island growth from the initially formed nuclei and subsequent nucleation (secondary nucleation) of ZION crystallites, and (iii) lateral growth of ZION islands originated from initially formed nuclei. On the other hand, non-epitaxial ZION films fabricated at RT just grow in 3D mode. From these results, we conclude that the substrate temperature is the key to control of nucleation and subsequent epitaxial growth of ZION films on the lattice-mismatched substrate.
We have fabricated ZnInON (ZION), which is a pseudo-binary alloy of wurtzite ZnO and wurtzite InN and has a tunable band gap over the entire visible spectrum and a high optical absorption coefficient of 105 cm-1. ZION films grow two dimensionally at Ts = room temperature (RT) and 150°C, whereas they grow three dimensionally at Ts = 250 and 450°C. These films at RT and 150°C show a step-terrace structure with the step height of 0.27 nm, which corresponds to the height of a single-atomic-layer step and the half length of the c-lattice parameter of ZION. ZION film has the same a-lattice parameter of 0.325 nm as ZnO and a longer c-lattice parameter of 0.536 nm, indicating the coherent growth of ZION films on ZnO templates. ZION film grown at RT shows blue (2.89 and 3.08 eV) photoluminescence at RT.
ZnO films were fabricated by RF magnetron sputtering with nitrogen mediated crystallization (NMC) under various gas pressures. X-ray diffraction measurements show that the NMC-ZnO films are highly crystalline regardless of the gas pressure, and the full width at half maximum values of the (0002) rocking curves range from 0.032 to 0.044°. In contrast, atomic force microscopy (AFM) reveals that the gas pressure plays an important role in determining the surface morphology of the films. The root-mean-square (RMS) roughness decreases monotonically from 1.05 to 0.60 nm with increasing pressure from 0.2 to 0.7 Pa. However, the RMS roughness increases with further increases in the pressure, reaching 2.15 nm at 2.1 Pa. The height distribution of the NMC-ZnO films derived from the AFM images is narrowest at 0.7 Pa, indicating that the smooth surface obtained at 0.7 Pa can be attributed to spatially uniform nucleation occurring in a short time period. These results indicate that the sputtering gas pressure is a key parameter for controlling the surface morphology of NMC-ZnO films.
We study effects of deposition temperature on growth mode and surface morphology
of hetero-epitaxial (ZnO)x(InN)1-x (ZION) films on ZnO
templates. ZION films deposited at low temperature of RT-250oC grow
two dimensionally, whereas ZION films deposited at high temperature of
350-450oC grow three dimensionally. Growth mode is changed from
two-dimensional growth mode to three-dimensional one, because the critical
thickness where film strain begin to relax decreases with increasing the
deposition temperature. At high deposition temperatures, the number of point
defects in ZION films decreases because migration of adatoms on the growing
surface is enhanced. The strain energy in ZION films increases with increasing
the deposition temperature, since the strain energy is not released by point
defects. Therefore, lattice relaxation for the higher deposition temperature
begins at the smaller film thickness to release the strain energy. As a result,
ZION films with atomically-flat surface were obtained even at RT.
We succeeded in photovoltaic power generation of p-i-n solar cells utilizing epitaxial ZnInON film with a wide band gap of 3.1 eV as the intrinsic layer, suitable for a top cell of tandem solar cells. The solar cell shows a high open circuit voltage (Voc) of 1.68 V under solar simulator light irradiation of 3.2 mW/cm2. The solar cell performance becomes worse under 100 mW/cm2, which is mainly attributed to the leakage current caused by crystal defects and grain boundaries. X-ray diffraction analysis reveals that the ZnInON film has rather large tilt and twist angles and a high dislocation density of 7.62×1010 cm-2. Such low crystallinity is a bottleneck for high performance of the solar cells. Our results demonstrate a potential of epitaxial ZnInON films as an intrinsic layer of wide band gap p-i-n solar cells with a high Voc.
Effects of surface morphology of buffer layers on ZnO/sapphire heteroepitaxial growth have been investigated by means of “nitrogen mediated crystallization (NMC) method”, where the crystal nucleation and growth are controlled by absorbed nitrogen atoms. We found a strong correlation between the height distribution profile of NMC-ZnO buffer layers and the crystal quality of ZnO films. On the buffer layer with a sharp peak in height distribution, a single-crystalline ZnO film with atomically-flat surface was grown. Our results indicate that homogeneous and high-density nucleation at the initial growth stages is critical in heteroepitaxy of ZnO on lattice mismatched substrates.
We present here performance of Li ion batteries with SiC nanoparticle-film anode, which is fabricated by a double multi-hollow discharge plasma chemical vapor deposition (CVD) method. The first cycle of charge/discharge property of the Li ion battery with the SiC nanoparticle-film anode shows a high capacity of over 4,000 mAh/g, which is 12 times higher than the Li ion battery with the conventional graphite anode. The discharge capacity shows high stability for first 10th cycle, and is 3750 mAh/g for the 10th cycle.
We have carried out in-situ measurements of cluster volume fraction in silicon films during deposition by using quartz crystal microbalances (QCM’s) together with a cluster-eliminating filter. The cluster volume fraction in films is deduced from in-situ measurements of film deposition rates with and without silicon clusters using QCM’s. The results show that the higher deposition rate leads to the higher volume fraction of clusters.
We have developed a cluster-eliminating filter which reduces amount of amorphous silicon nanoparticles (clusters) incorporated into a-Si:H films. We have applied the filter to fabricate a-Si:H Schottky solar cells. The cells show a high initial fill factor FF=0.563 and a high stabilized value after light soaking FF=0.552 which light-induced degradation was quite low value of 1.95 %.
Quantum dot-sensitized solar cells (QDSCs) based on the multiple exciton generation (MEG) of QD are attractive in the field of photochemical cells because the improvement of conventional sensitized solar cells has been stagnant recently. The distinctive characteristics of QDs are their strong photo-response in the visible region and quantum confinement effects. Its theoretical efficiency is much higher than that of solar cell based on the single exciton generation (SEG). Moreover, QDs have tunable optical properties and band-gaps depending on the particle size. But QD materials widely used for QDSC have some disadvantages of toxicity and scarcity. On the other hand, Si as one of good QD materials is abundant and not toxic. Also, Si QD has high stability against light soaking and a high optical absorption coefficient due to quantum size effects. However, the research on Si QD is rare although the quantum effect of Si was already verified. It is one of reasons that the fabrication and collection of Si nano-particles are too difficult. Therefore, this work proposed multi-hollow plasma discharge chemical vapor deposition (CVD). It is possible to collect Si particles unlike conventional CVD and solve the problems of the wet process. The optical properties of Si particles were controlled by varying experimental conditions. In this work, Si particles were fabricated with various sizes and their characteristics were analyzed. Based on the results, Si QD was applied to Si QDSC.
A novel fabrication method of ZnO films utilizing solid-phase crystallized seed layers has been developed. In this method, solid phase crystallization (SPC) is performed by annealing amorphous ZnON films, which are prepared by sputtering of ZnO targets in Ar/N2 mixed gases, in an oxidization atmosphere. The grain size of ZnO films deposited on the seed layers is significant larger than that of ZnO films directly deposited on glass substrates, which is considered to be due to the low grain density of seed layers. By utilizing this technique, the resistivity of ZnO:Al (AZO) films is decreased from 20 × 10-4 Ωcm to 5 × 10-4 Ωcm at the film thickness of 30nm. Furthermore, we observed that SPC seed layers are in-plane aligned when Al2O3 substrates are used, which suggests that the fabrication method proposed here is also promising for synthesizing epitaxial ZnO films.
The device characteristics of thin-film transistors (TFTs) having amorphous In-Ga-Zn-O channel layers with various chemical compositions were studied by using combinatorial synthesis techniques. The In-Ga-Zn-O films were prepared by a radio-frequency magnetron sputtering method at room temperature in mixed-gas atmosphere of argon and oxygen. The TFT libraries enabled us to systematically survey the device characteristics of the TFTs in a wide compositional range of channel materials. It is found that the TFT characteristics are very sensitive to the chemical composition ratio of In:Ga:Zn and depend also on the oxygen partial pressure during deposition. Some devices exhibited good performance of the field-effect mobility of ∼10 cm2V−1sec−1 and on-to-off current ratio of ∼108.
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