Fabrication of nano-crystalline silicon thin film at low temperature by using a neutral beam deposition method

Low temperature (<80 °C) neutral beam deposition (LTNBD) was investigated as a new approach to the fabrication and development of nano-crystalline silicon (nc-Si), which has better properties than that of amorphous silicon (α-Si). The difference between LTNBD and conventional PECVD is that the film formation energy of the nc-Si in LTNBD is supplied by controlled neutral beam energies at a low temperature rather than by heating. Especially, in this study, the characteristics of the nc-Si thin film were investigated by adding 10% of an inert gas such as Ne, Ar or Xe to SiH₄/H₂. Increasing the beam energy resulted in an increase in the deposition rate, but the crystallinity was decreased, due to the increased damage to the substrate. However, the addition of a higher mass inert gas to the gas mixture at a fixed beam energy resulted not only in a higher deposition rate but also in a higher crystallization volume fraction. The high resolution transmission electron microscopy image showed that the grown film is composed of about 10 nm-size grains.     

The effect of oxygen pressure on the microstructures of Co-doped rutile TiO2 thin films grown by pulsed laser deposition

Comprehensive microstructures of 7% cobalt-doped rutile TiO₂ thin films grown on c-plane sapphire by pulsed laser deposition were characterized using transmission electron microscopy (TEM). The effects of oxygen pressure during growth on the Co distribution inside the films were investigated, and the detailed growth mechanism of both TiO₂ and TiO₂+Co was discussed. The similar oxygen sublattices and low mismatch between (1 0 0) rutile and c-plane sapphire favors the rutile phase. However, the three-fold symmetry of the substrate surface resulted in three rutile domain orientation variants, and they grow adjacent to each other. Cobalt was found to precipitate out as nanocrystals inside the TiO₂ matrix as the growth pressure of oxygen was decreased. At 0.05 mTorr oxygen pressure, almost all of the Co segregates into crystallographically aligned nanocrystals with a particle size of 4.4±0.15 nm. All the samples have magnetic coercivity at room temperature. The magnetic moment per Co atom increased with decreased oxygen pressure, suggesting that the Co that replaced the Ti²⁺ in the TiO₂ lattice does not have a large magnetic moment.     

Characterization of polycrystalline GaN grown on silica glass substrates

The structural, optical, and electrical properties of GaN films grown on silica glass substrate by metalorganic chemical vapor deposition were studied. X-ray diffraction showed that the films were grown in hexagonal structure with a predominant (0 0 0 2) peak. A broad and strong band-edge emission and very weak yellow luminescence in photoluminescence (PL) spectra were observed. And the temperature dependence of the PL spectra was extensively studied. The thermal quenching activation energy was found to be very close to the donor activation energy determined from the temperature dependence of the carrier concentration. Longitudinal optical phonons were found to be responsible for the PL broadening above 100 K.     

The tri-methyl-Sb flow and the surfactant time effect on InGaAsN/GaAs-strained MQWs grown by MOCVD

The tri-methyl-Sb flow and the surfactant time dependence of photocurrent (PC) spectra was studied on InGaAsN/GaAs-strained multiple quantum wells (MQWs) structures grown by using metalorganic chemical vapor deposition (MOCVD). The structural properties of InGaAsN/GaAs-strained MQWs were investigated by using high-resolution X-ray diffraction (HRXRD). In the case of InGaAsN/GaAs-strained MQWs, an increase in compressive strain from an analysis of the satellite peaks in HRXRD was observed on increasing the tri-methyl-Sb flow and the surfactant time. For InGaAsN/GaAs-strained MQWs, the peaks observed in the photocurrent spectra were preliminarily assigned to electron–heavy hole (e₁–hh) and electron–light hole (e₁–lh) fundamental excitonic transitions. Their peaks are red-shifted with increasing tri-methyl-Sb flow and surfactant time. But the photocurrent peak is blue-shifted at the surfactant time of . It seems to be due to the improvement of structure properties at interface owing to a surfactant-suppressing surface diffusion phenomenon during growth. We compared this with the result of the experimental energies for InGaAsN/GaAs-strained MQWs.     

SIMS and Raman characterizations of ZnO:N thin films grown by MOCVD

Nitrogen was incorporated into ZnO films grown by metalorganic chemical vapour deposition (MOCVD) on ZnO substrates using DMZn-TEN, tert-butanol and diallylamine, respectively, as zinc, oxygen and doping sources. The carrier gas was either hydrogen or nitrogen and the partial pressure ratio (R_VI/II) was varied in order to favor the nitrogen incorporation and/or reduce carbon related defects. The ZnO films have been characterized by Micro-Raman scattering and SIMS measurements. SIMS measurements confirm the nitrogen incorporation with concentrations extending from ∼10¹⁹ cm⁻³ to ∼4×10²⁰ cm⁻³. Raman spectra show nitrogen local vibration modes in films grown at low R_VI/II ratio and using H₂ as carrier gas. However, a vibration band attributed to carbon clusters dominates the Raman spectra for films grown with N₂ carrier. The contribution of N complexes was discussed. The strain was calculated for the as-grown and annealed films and it changes from tensile to compressive after annealing.     

Room-temperature luminescence study on the effect of Mg activation annealing on p-GaN layers grown by MOCVD

An Mg-doped p-GaN layer was grown by the metalorganic chemical vapor deposition method. The dissociation extent of hydrogen-passivated Mg acceptors in the p-GaN layer through Mg activation annealing was estimated by using room-temperature cathodoluminescence (CL) spectroscopy. The CL measurement revealed that the CL spectra intensities tend to increase with increasing the activation annealing temperature. The sample annealed at 925 °C showed the most intense emission and the narrowest width among the emission peaks. Consequently, it was the most excellent dissociation extent of Mg–H complexes caused by the Mg activation annealing. The hole concentration under this optimum condition was 1.3×10¹⁷ cm⁻³ at room temperature. The photoluminescence (PL) measurement showed a 2.8 eV band having characteristically a broad peak in heavily Mg-doped GaN at room temperature. By analyzing the PL results, we learned that this band was associated with the deep donor–acceptor pair (DAP) emission rather than with the emission caused by the transition from the conduction band to deep acceptor level. The four emission peaks in the resolved 2.8 eV band were emitted by transiting from deep donor levels of 0.14, 0.26, 0.40, and 0.62 eV below the conduction band to the shallow Mg acceptor level of 0.22 eV above the valence band.     

Oriented polycrystalline α-HgI2 thick films grown by physical vapor deposition

This work investigates the growth of polycrystalline α-HgI₂ thick films from physical vapor deposition. By varying the growth conditions, the as-deposited thick films are characterized by scanning electron microscopy, X-ray diffraction, current–voltage and photoconductivity measurements. The growth mechanism and its effects to the properties of these polycrystalline α-HgI₂ thick films are then discussed. Finally, the best deposition conditions for polycrystalline α-HgI₂ thick films compactly formed by separated columnar monocrystallines with uniform orientation along c-direction and with good crystallinity are reported.     

Heteroepitaxial growth of Cu2O thin film on ZnO by metal organic chemical vapor deposition

Cuprous oxide (Cu₂O) thin films were grown epitaxially on c-axis-oriented polycrystalline zinc oxide (ZnO) thin films by low-pressure metal organic chemical vapor deposition (MOCVD) from Copper(II) hexafluoroacetylacetonate [Cu(C₅HF₆O₂)₂] at various substrate temperatures, between 250 and 400 °C, and pressures, between 0.6 and 2.1 Torr. Polycrystalline thin films of Cu₂O grow as single phase with [1 1 0] axis aligned perpendicular to the ZnO surface and with in-plane rotational alignment due to (2 2 0)_Cu2O(0 0 0 2)_ZnO; [0 0 1]_Cu2O[1 2¯ 1 0]_ZnO epitaxy. The resulting interface is rectifying and may be suitable for oxide-based p–n junction solar cells or diodes.     

Predicted nitrogen doping concentrations in silicon carbide epitaxial layers grown by hot-wall chemical vapor deposition

A simple quantitative model for the surface adsorption of nitrogen has been developed to simulate the doping incorporation in intentionally doped 4H–SiC samples during epitaxial growth. Different reaction schemes are necessary for the two faces of SiC. The differences are discussed, and implications to the necessary model adjustments are stressed. The simulations are validated by experimental values for a large number of different process parameters with good agreement.     

Investigation of void formation beneath thin AlN layers by decomposition of sapphire substrates for self-separation of thick AlN layers grown by HVPE

Void formation at the interface between thick AlN layers and (0 0 0 1) sapphire substrates was investigated to form a predefined separation point of the thick AlN layers for the preparation of freestanding AlN substrates by hydride vapor phase epitaxy (HVPE). By heating 50–200 nm thick intermediate AlN layers above 1400 °C in a gas flow containing H₂ and NH₃, voids were formed beneath the AlN layers by the decomposition reaction of sapphire with hydrogen diffusing to the interface. The volume of the sapphire decomposed at the interface increased as the temperature and time of the heat treatment was increased and as the thickness of the AlN layer decreased. Thick AlN layers subsequently grown at 1450 °C after the formation of voids beneath the intermediate AlN layer with a thickness of 100 nm or above self-separated from the sapphire substrates during post-growth cooling with the aid of voids. The 79 μm thick freestanding AlN substrate obtained using a 200 nm thick intermediate AlN layer had a flat surface with no pits, high optical transparency at wavelengths above 208.1 nm, and a dislocation density of 1.5×10⁸ cm⁻².     

The photoluminescence of ZnO thin films grown on Si (1 0 0) substrate by plasma-enhanced chemical vapor deposition

High-quality ZnO thin films have been grown on a Si(1 0 0) substrate by plasma-enhanced chemical vapor deposition (PECVD) using a zinc organic source (Zn(C₂H₅)₂) and carbon dioxide (CO₂) gas mixtures at a temperature of 180°C. A strong free exciton emission with a weak defect-band emission in the visible region is observed. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption peak in the absorption spectra, are closely related to the gas flow rate ratio of Zn(C₂H₅)₂ to CO₂. Full-widths at half-maximum of the free exciton emission as narrow as 93.4 meV have been achieved. Based on the temperature dependence of the PL spectra from 83 to 383 K, the exciton binding energy and the transition energy of free excitons at 0 K were estimated to be 59.4 meV and 3.36 eV, respectively.     

The influence of the intensity of melt stirring on the radial impurity distribution in proustite single crystals grown by the Stockbarger method using ACRT

The influence of the intensity of melt stirring on the radial impurity distribution and optical quality of proustite single crystals grown by the Stockbarger method using ACRT is studied by the example of Cu, Sb and Mg impurities. We report results obtained in a wide range of Taylor numbers (1.9×10⁵<Ta<7.12×10⁷). The studies revealed that the ACRT can be applied validly to decrease the impurity content during the growing of high-quality single crystals.     

X-ray and cathodoluminescence study on the effect of intentional long time annealing of the InGaN/GaN multiple quantum wells grown by MOCVD

GaN-based InGaN/GaN multiple quantum wells (MQWs) structure having a high-quality epilayer and coherent periodicity was grown by metalorganic chemical vapor deposition. After thermal annealing of InGaN/GaN MQWs, the increase in temperature and annealing time caused the intermixing between the barrier and the wells, which in turn caused a decrease in periodicity on the high-resolution X-ray diffraction patterns. Thereby, we confirmed that the structural performance of InGaN MQWs is successively degrading with increasing thermal annealing temperature. Especially, InGaN MQWs of the sample annealed at 950 °C were profoundly damaged. The cathodoluminescence (CL) measurement indicated that MQWs emission intensity decreases with increasing thermal annealing temperature. Thus, the integrated CL intensity ratio of InGaN MQWs to GaN dramatically decreased while thermal annealing temperatures increased. This result caused the intermixing in MQWs to deteriorate the active layer performance. Furthermore, the peak position of MQWs showed a tendency of the red shift after high thermal annealing. It is suggested that the annealing-induced red shift in MQWs is attributed to the reduction of the inhomogeneity of the In content in the MQWs leading to the reduction of the quantized energies. Consequently, it indicates that the high temperature and the long-time thermal annealing would be inevitably followed by the structural destruction of InGaN MQWs.     

Estimation of growth rate in unidirectionally solidified multicrystalline silicon by the growth-induced striation method

Multicrystalline silicon was grown by unidirectional solidification method using the accelerated crucible rotation technique. The application of the accelerated crucible rotation technique in unidirectional solidification method induced growth striations across the axial direction of the grown crystal. This striation pattern was observed from carbon concentration distribution, obtained by using Fourier transform infrared spectroscopy. The generated striation pattern was found to be weak and discontinuous. Some striations were absent, probably due to back melting, caused during each crucible rotation. From the growth striations and applied time period in crucible rotation, the growth rate was estimated by using Fourier transformation analysis.     

The effect of the AlxGa1−xN/AIN buffer layer on the properties of GaN/Si(1 1 1) film grown by NH3-MBE

We describe the growth of GaN on Si(1 1 1) substrates with Al_xGa_1−xN/AlN buffer layer by ammonia gas source molecular beam epitaxy (NH₃-GSMBE). The influence of the AlN and Al_xGa_1−xN buffer layer thickness and the Al composition on the crack density of GaN has been investigated. It is found that the optimum thickness is 120 and 250 nm for AlN and Al_xGa_1−xN layers, respectively. The optimum Al composition is between 0.3<x<0.6.     

Oxygen-pressure dependence of the crystallinity of MgO films grown on Si(1 0 0) by PLD

Effects of the oxygen partial pressure on pulsed-laser deposition of MgO buffer layers on silicon substrates were investigated. The overall growth process was monitored in situ by reflection high-energy electron diffraction (RHEED) method. It was found that the crystallinity and surface morphology of the MgO films were strongly affected by oxygen partial pressure in the deposition chamber. The oxygen-pressure dependence could be explained in terms of interactions of oxygen with species in the plume-like plasma. The MgO film obtained at an optimal oxygen-pressure range of 1×10⁻²–1 Pa exhibited an atomic-smooth and defect-free surface (the root-mean-square roughness being as low as 0.82 nm). For the metal–insulator–metal (MIM) structure of Au/MgO (150 nm)/TiN prepared at the optimal growth conditions achieved a very low leak current density of 10⁻⁷ A cm⁻² at an electric field of 8×10⁵ V cm⁻¹ and the permittivity (ε_r) of about 10.6, virtually the same as that of the bulk MgO single crystals.     

Structural and optical properties of CuGaSe2 layers grown by the hot wall epitaxy method

Copper gallium selenide (CuGaSe₂, CGS) layers were grown by the hot wall epitaxy method. The optimum temperatures of the substrate and source for growth turned out to be 450 and 610 °C, respectively. The CGS layers were epitaxially grown along the 1 1 0 direction and consisted of Ga-rich components indicating the slight stoichiometric deviations. Based on the absorption measurement, the band-gap variation of CGS was well interpreted by the Varshni's equation. The band-gap energies at low temperatures, however, had a higher value than those of other CGS. It suggests that the band-gap increase is influenced by the slightly Ga-rich composition. From the low-temperature photoluminescence experiment, sharp and intensive free- and bound-exciton peaks were observed. By analyzing these emissions, a band diagram of the observed optical transitions was obtained. From the solar cell measurement, an 11.17% efficiency on the n-CdS/p-CGS junction was achieved.     

Influence of hydrogen on structural and optical properties of low temperature polycrystalline Ge films deposited by RF magnetron sputtering

To improve the properties of polycrystalline Ge thin films, which are a candidate material for the bottom cells of low cost monolithic tandem solar cells, ∼300 nm in situ hydrogenated Ge (Ge:H) thin films were deposited on silicon nitride coated glass by radio-frequency magnetron sputtering. The films were sputtered in a mixture of 15 sccm argon and 10 sccm hydrogen at a variety of low substrate temperatures (T_s)≤450 °C. Structural and optical properties of the Ge:H thin films were measured and compared to those of non-hydrogenated Ge thin films deduced in our previous work. Raman and X-ray diffraction spectra revealed a structural evolution from amorphous to crystalline phase with increase in T_s. It is found that the introduction of hydrogen gas benefits the structural properties of the polycrystalline Ge film, sputtered at 450 °C, although the onset crystallization temperature is ∼90 °C higher than in those sputtered without hydrogen. Compared with non-hydrogenated Ge thin films, hydrogen incorporated in the films leads to broadened band gaps of the films sputtered at different T_s.