Lateral Growth Direction Research Articles

Phase separation-suppressed and strain-modulated improvement of crystalline quality of AlGaN epitaxial layer grown by MOCVD

High crystalline quality AlGaN films were grown on GaN templates by metalorganic chemical vapor deposition (MOCVD). Inhomogeneous distributions of Al compositions in both the vertical and lateral growth direction caused by the strong gas-phase pre-reaction occurring between Al precursors and NH3 were suppressed by optimizing the growth conditions. The residual strain in AlGaN/GaN induced by lattice mismatch, which results in the degradation of crystallinity, was modulated by varying thickness of a high temperature (HT) AlN interlayer (IL) inserted between AlGaN and GaN layers. Both the crystalline quality and Al-incorporation into the AlGaN are influenced by the residual strain related to the AlN IL thickness. Lastly, the understanding of the AlGaN growth and of the strain modification was employed to grow an AlGaN/GaN HEMT structure showing good electrical characteristics and uniformity.

Synthesis and characterization of La(OH) 3 nanopowders from hydrolysis of lanthanum carbide

The lanthanum carbide alloy was induction melted in vacuum induction melting furnace from lanthanum and graphite with the mass ratio of 89:11. Lanthanum hydroxide (La(OH) 3) nanopowders were prepared by a simple hydrolysis of lanthanum carbide at room temperature under normal atmospheric pressure without any surfactant. X-ray diffraction (XRD) showed that the nanoparticles were with a hexagonal structure. The effect factors such as reaction time, reaction temperature and the mass ratio of lanthanum carbide powders to H 2O on BET surface area of the obtained nanopowders were investigated. Furthermore, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) showed that nanopowders behaved the morphology of short nanorod. The longitudinal growth direction of the nanorod had an angle of about 14° with the (101) lattice plane and the lateral growth direction had an angle of about 35° with the (200) lattice plane. The mechanism of formation of nanorod was discussed on the basis of the experimental results.

Post-Annealing Effects on a Laterally Grown Polycrystalline Silicon Layer

A polysrystalline silicon (poly-Si) layer prepared by selectively enlarging laser crystallization (SELAX) was post-annealed by excimer laser irradiation. The average grain width increased 26% because of partly merged grains, while the morphological trace of the laterally grown poly-Si layer was maintained. The difference in field-effect mobility resulting from thin-film-transistor (TFT) configuration was reduced by post-annealing. The field-effect mobility of TFTs parallel to the lateral growth direction decreased slightly (within 4.5% for n-type and 11.5% for p-type). On the other hand, TFTs configured perpendicular to the lateral growth direction increased as the energy density during post-annealing increased (up to 200% for both types). For n-type TFTs these results were attributed to the increase of the effective width of the grain and the reduction of the density of trapping states at grain boundaries.

Sequential Lateral Solidification Processing for Polycrystalline Si TFTs

The sequential lateral solidification (SLS) process is an excimer-laser projection-based scheme for crystallization of thin films on amorphous substrates. This method can be used to readily produce a wide range of microstructures through manipulation of grain boundary placement within the crystallized material. In this paper, we focus on the 2-shot SLS process for crystallization of thin Si films for thin-film transistor (TFT) applications. We have investigated the effect of process parameter variation on the resulting microstructure, as well as on the performance of TFTs fabricated on the material. The 2-shot SLS microstructure was further engineered to reduce anisotropy of the TFT performance relative to the lateral growth direction using additional laser scans. Through this method, we were able to improve the mobility directionality ratio between devices with majority carrier flow parallel and perpendicular to the lateral growth direction, respectively, from 0.3 to over 0.7. Post-SLS process thinning and planarization of the Si surface was used to improve the uniformity and performance of the TFT devices.

Effects of Light Intensity Distribution on Phase Modulated Excimer Laser Crystallization Characteristics

Microtexture was analyzed for Si films crystallized by a phase-modulated excimer laser annealing method, using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Electron Back-Scattering Pattern (EBSP). Threshold fluences were identified for crystallization, lateral growth and film braking, by comparing the surface morphology and the microscopic excimer laser light intensity profile. It was found that {001} and {110} orientations are preferable for lateral growth direction but the surface orientation is random. A possible origin of preferable orientations for laterally grown film was discussed.

Effect of process parameters on the structural characteristics of laterally grown, laser-annealed polycrystalline silicon films

In this work, we have conducted a systematic study aiming at assessing the effects of process parameters on the microstructural characteristics of laterally grown polycrystalline silicon (poly-Si) films. Poly-Si films were formed by the sequential lateral solidification (SLS) method. The Si film thickness was found to affect significantly the quality of the poly-Si microstructure, manifested by a decreased crystal-growth defect density and increased subboundary spacing in thicker films. A weak (100) texture was observed in the lateral growth direction, except for very thin films (<30 nm) where (110) texture was observed. No specific texture was identified in the normal and transverse directions. Lateral crystallization proceeds by seeded, lateral epitaxial growth at an advancing pitch. We investigated the quality of lateral growth as a function of the advancing, substrate pitch. We found that an optimum pitch range exists, bound on the low end by the detailed shape of the beam-edge profile and on the high end by the quality of crystal formed under deep, interfacial undercooling. In that sense, only a fraction of the laser beamlet can be effectively used for lateral growth and the process should be tuned towards maximizing this fraction. Thin film transistor data were used to further refine the considerations driving the selection of optimum process parameters for SLS lateral crystallization providing the least sensitivity to process-induced variations.

Surface crystallization of rare-earth aluminosilicate glasses

Surface crystallization in a rare-earth aluminosilicate glass (Nd 2O 3–Al 2O 3–SiO 2–TiO 2) was studied using an isothermal method and the crystal growth rate of the glasses was evaluated as a function of the composition. For measuring the surface crystal growth rate, two different methods: measurement of the crystal layer in the longitudinal and lateral growth direction. It was found that crystallization proceeded by surface crystallization only and TiO 2 did not act as a nucleating agent. The growth rate was strongly dependent on the viscosity of glass and agreed with prediction from the Preston model using the known viscosity and melting temperature. As the Si/Nd and Si/Al ratios decreased, the crystal growth rate increased. TiO 2 and Nd 2O 3 played the role of network modifier, which decreased the viscosity of the glass, facilitating crystallization of the rare-earth aluminosilicate glass.

Carbon growth by thermal laser-assisted chemical vapour deposition

The heated area in thermal laser-assisted chemical vapour deposition (LCVD) is much smaller than in large area CVD. This affects the growth conditions drastically. One important growth aspect is that CVD processes requiring homogeneous reactions in the vapour are not effective for thermal LCVD. In this work LCVD of carbon from a non-adsorbable precursor (CH 4) and an adsorbable precursor (C 2H 4) in combination with cold wall and hot wall reactors was investigated. The combination with the hot wall reactor also means that adsorbable carbon-carrying species can be generated homogeneously in the vapour before deposition. For laser CVD of carbon from CH 4 a high deposition temperature was needed. The threshold laser power density was 9.0 × 10 4W cm −2 (3500 K) for a hot wall reactor temperature of 1270 K. For carbon LCVD from C 2H 4 carbon lines could be deposited already under cold wall conditions at the same laser power density as was used in the CH 4 case. The thickness profile perpendicular to the grown lines was close to gaussian for deposition temperatures below 3600 K. Bimodal profiles, which were observed for higher temperatures, indicate evaporation or surface diffusion of carbon. The deposited lines were fine grained with a nodular appearance. Crystalline carbon was found to grow in a layer structure parallel to the substrate surface. The lines deposited from CH 4 were considerably wider than the laser spot diameter. This may be due to the thermal anisotropy of the material. The temperature profile and also the line will be broadened since the heat conduction is much better along the graphitic planes than perpendicular to them. The lateral growth direction of the lines as well as the orientation of the graphite layers indicate that the edge atoms of the graphitic basal planes are very active during the growth.

Extremely thin and defect-free Si-on-insulator fabrication by tunnel epitaxy

A novel technique for extremely thin and defect-free Si-on-insulator (SOI) fabrication, called tunnel epitaxy, is demonstrated. A lateral/vertical growth ratio of 20 is achieved with precisely controlled SOI film thickness, where the minimum thickness for the fabricated SOI is 0.05 μm. No defect was observed in the fabricated SOI with 〈001〉 lateral growth direction. The growth front is a flat (001) plane. The lateral growth length is limited to 1–2 μm by tunnel clogging, due to poly-Si grain growth. The tunnel epitaxy, however, has a potential for larger area SOI fabrication, because the fabricated tunnel length is 10 μm.