Crystal structure, carrier concentration and IR-sensitivity of PbTe thin films doped with Ga by two different methods

The comparison of the results of chemical composition, crystal structure, electronic properties and infrared photoconductivity investigations of PbTe/Si and PbTe/SiO₂/Si heterostructures doped with Ga atoms by two different techniques is presented in this work. One of these techniques is principally based on the vapour-phase doping procedure of PbTe/Si and PbTe/SiO₂/Si heterostructures, which were previously formed by the modified “hot wall” technique. The second method of PbTe(Ga)/Si and PbTe(Ga)/SiO₂/Si heterostructure preparation is based upon the fabrication of lead telluride films, which have been doped with Ga atoms in the layer condensation process directly. The lattice parameter and charge carrier density evolutions with the Ga impurity concentration show principally the different character of PbTe(Ga)/Si films prepared by these techniques. It has been proposed that complicated amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by different mechanisms of substitution or implantation of impurity atoms in the crystal structure of lead telluride.     

A structural study of Y2O3---Al2O3---SiO2 glass employing partial RDFs obtained by anomalous scattering

A structural study of Y₂O₃---Al₂O₃---SiO₂ glass has been performed using the partial radial distribution function (RDF) obtained by anomalous scattering at the Y absorption edge. The partial RDF consists of contributions from atom pairs, such as Y---O, Y---M(Si, Al) and Y---Y, related to the local arrangements around the Y atoms, while those of all of the constituent atom-pairs are convoluted into the ordinary RDF. The M(Si, Al)---O, Y---O, M---M, Y---M, and Y---Y interatomic distances and the coordination numbers around the Al and Y atoms are obtained from the two RDFs. The local structures around the Al and Y atoms in the glass are discussed.     

A magnetic resonance study on the structure of amorphous networks in the Si–B–N(–C) system

The amorphous networks Si₃B₃N₇ and ‘SiBN₃C' are studied by solid-state nuclear magnetic resonance (NMR), continuous-wave and pulse electron paramagnetic resonance (EPR), and by one- and two-dimensional electron nuclear double resonance spectroscopy. In both compounds, boron is found to be coordinated exclusively by nitrogen with close to trigonal planar geometry and close to equal bond lengths. Silicon is four-coordinated by nitrogen with the coordination tetrahedra being distorted to accommodate the coordination preferences of boron. REDOR measurements demonstrate that boron resides in the second coordination sphere of silicon. Carbon incorporation into the Si–B–N network does not lead to any observable changes in NMR parameters including the average dipolar coupling between ¹¹B nuclei which depends on the average distance of the boron atoms. Only spin–lattice relaxation of the nuclei is accelerated due to the generation of paramagnetic centers. The unpaired electrons appear to be delocalized over several carbon atoms and exhibit significant hyperfine couplings to boron, silicon, nitrogen, and some residual protons. In contrast to electron spectroscopic imaging experiments, the magnetic resonance results suggest formation of carbon clusters.     

Cracking assisted nucleation in chemical vapor deposition of silicon nanoparticles on silicon dioxide

Deposition of sub-monolayer silicon on SiO₂/Si(1 0 0) greatly facilitates nucleation in subsequent thermal chemical vapor deposition (CVD) of silicon nanoparticles. Sub-monolayer seeding is accomplished using silicon atoms generated via disilane decomposition over a hot tungsten filament. The hot-wire process is nonselective towards deposition on silicon and SiO₂, is insensitive to surface temperature below 825 K, and gives controlled coverages well below 1 ML. Thermal CVD of nanoparticles at 1×10⁻⁴ Torr disilane and temperatures ranging from 825 to 925 K was studied over SiO₂/Si(1 0 0) surfaces that had been subjected to predeposition of Si or were bare. Seeding of the SiO₂ surface with as little as 0.01 ML is shown to double the nanoparticle density at 825 K, and densities are increased twenty fold at 875 K after seeding the surface with 30% of a monolayer.     

Structure and thermal stability of MOCVD ZrO2 films on Si (1 0 0)

The structure and thermal stability of ZrO₂ films grown on Si (1 0 0) substrates by metalorganic chemical vapor deposition have been studied by high-resolution transmission electron microscopy, selected area electron diffraction and X-ray energy dispersive spectroscopy. As-deposited films consist of tetragonal ZrO₂ nanocrystallites and an amorphous Zr silicate interfacial layer. After annealing at 850°C, some monoclinic phase is formed, and the grain size is increased. Annealing a 6 nm thick film at 850°C in O₂ revealed that the growth of the interfacial layer is at the expense of the ZrO₂ layer. In a 3.0 nm thick Zr silicate interfacial layer, there is a 0.9 nm Zr-free SiO₂ region right above the Si substrate. These observations suggest that oxygen reacted with the Si substrate to grow SiO₂, and SiO₂ reacted with ZrO₂ to form a Zr silicate interfacial layer during the deposition and annealing. Oxygen diffusion through the tetragonal ZrO₂ phase was found to be relatively easier than through the monoclinic phase.     

Influence of oxygen contamination during Si low presure vapour phase epitaxy on epitaxial layer quality

The influence of oxygen contamination on Si low pressure vapour phase epitaxy (LPVPE) at 800°C in the SiCl₂H₂−H₂ system has been investigated. O₂ was added intentionally with partial pressures between 10⁻⁸ and 2×10⁻⁴ mbar. The quality of the epitaxially grown silicon layers was determined by comparing surface morphology, defect density, Schottky diode characteristics and SIMS measurements. These four parameters are correlated and they reveal a drastic decrease in epitaxial layer quality for O₂ pressures above 15×10⁻⁶ mbar. The critical oxygen pressure which has been until now considered as a limit for epitaxial growth can therefore be exceeded by one order of magnitude.     

The structure of alkali silicate glasses

Pulsed neutron diffraction has been used to study the structure of three alkali silicate glasses of nominal composition SiO₂ · (M₂O)_0.5, with M = K, K_0.46Li_0.54 and Li. The data are quantitatively consistent with the modified random network model. Lithium is coordinated by two oxygen atoms with an interatomic distance of 1.97 Å. The potassium-oxygen distance is similar to the tetrahedral oxygen-oxygen distance, 2.66 Å, with potassium being coordinated by approximately four oxygen atoms.     

Structural characterization of amorphous silicon nitride by molecular dynamics simulation

Computer simulation, using the molecular dynamics (MD) technique, has been carried out on amorphous silicon nitride (a-Si₃N₄) with simple Busing-type potentials. From the MD simulation, the following points have been deduced. (1) The average Si---N bond length obtained from MD results is r_Si---N=1.74 Å, and its coordination number, N_Si---N, is 3.95. The bond angles around a Si and a N atom, N---Si---N and Si---N---Si, are found to be 109.8° ± 12.36° and 127.08° ± 16.63°, respectively. The N---Si---N value obtained is in very good agreement with the tetrahedral bond angle (= 109.47°). Hence, the short-range structural arrangement of a-Si₃N₄ comprises tetrahedral SiN₄ units. The MD results presented in this study also indicate that there exist only a small number of defects such as dangling bonds. (2) These MD results are in good agreement with the reported X-ray and neutron data. The a-Si₃N₄ structure can be reproduced by the MD simulation given in this study.     

Low-temperature synthesis of C60 thin films by ionized cluster beam deposition technique

C₆₀ films on Si(1 1 1) have been grown at low substrate temperature of 100°C by ionized cluster beam deposition technique. Fourier transform infrared (FTIR) transmission spectroscopy, Raman measurements and X-ray diffraction are employed to investigate the structure of deposited films. The results show that the acceleration voltage plays an important role in the growth of the films. As the acceleration voltage is moderate (100 V), a pristine C₆₀ film with face-centered-cubic (fcc) crystal structure can be grown. Further increase of the acceleration voltage leads to the formation of amorphous carbon (a-C) in the grown films. When the acceleration voltage is increased to 600 V, the deposited film has a complete amorphous carbon (a-C) structure.     

不同退火条件对PEALD制备的Ga2O3薄膜特性的影响

利用等离子增强原子层沉积技术(PEALD)在c面蓝宝石衬底上制备了氧化镓(Ga₂O₃)薄膜,研究了退火气氛(v(N₂)∶v(O₂)=1∶1(体积比)、空气和N₂)及退火时间对Ga₂O₃薄膜晶体结构、表面形貌和光学性质的影响。研究结果表明,退火前的氧化镓处于亚稳态,不同退火气氛下退火后晶体结构发生明显改变,而且退火气氛中N₂比例增加有利于Ga₂O₃重结晶。在N₂气氛下退火达到30 min,薄膜结构已由亚稳态转变成择优取向的β-Ga₂O₃。而且表面形貌分析表明,退火30 min后表面形貌开始趋于稳定,表面晶粒密度不再增加。另外实验样品在 400~800 nm的平均透射率几乎是100%,且光吸收边陡峭。采用N₂气氛退火,对于富氧环境下沉积的Ga₂O₃更利于薄膜表面原子迁移,以及择优取向Ga₂O₃重结晶。     

Laser precipitation of SnO2 nanocrystals in glass and energy transferred-fluorescence of Eu ions

SnO₂ nanoparticles having an average particle size of 5 nm were precipitated in a transparent glass by irradiation with an 800 nm femtosecond laser. Glass was prepared to co-dope Sn⁴⁺ and Eu³⁺ ions by a sol–gel method, followed by heating in H₂ gas atmosphere to form the Sn⁰₂ point defects with a molecular-like electronic structure in the twofold-coordinated Sn atoms. Upon laser irradiation, the Sn⁰₂ defects react with oxygen, forming SnO₂ nanocrystals. When excited at the energy corresponding to the absorption edge of the SnO₂ nanocrystals, the energies, which are absorbed by the quantum-dot effect in the nanosized crystals, are efficiently transferred to the Eu³⁺ ions, resulting in the large enhanced fluorescence intensities from the Eu³⁺ ions. The observed phenomenon suggests application to high-density memory devices.     

In-situ RHEED study on the effect of light irradiation on Ge/Si heteroepitaxial growth by GeH4 source MBE

Effects of light irradiation on growth processes in the initial stage of growth of Ge films on (100)Si substrates by gas source molecular beam epitaxy using GeH₄ have been investigated by in-situ RHEED observations. It has been found that the growth rate is enhanced by an Ar⁺ ion laser and a D₂ lamp irradiation, and, moreover, that the growth process sequence of the Ge films irradiated by the Ar⁺ laser and D₂ lamp is the same as that without irradiation. The irradiation effect on the photolysis of GeH₄ gas has been examined by mass spectroscopy. The increase of the growth rate is due to the photolysis of GeH₄ and the excitation of substrate surfaces by vacuum-ultraviolet light in the D₂ lamp irradiation, and is due to only the excitation of substrate surfaces in the Ar⁺ laser irradiation.     

Effects of rapid thermal treatments on the electrical properties of thin SiO2 gate oxide for DRAM p-channel MOS transistors

Silicon oxide has been grown by rapid thermal processing. The growth rate, in the range of very thin films (<10 nm), has been studied as a function of the oxidation temperature. Combined films composed by conventional thermal silicon oxide growth over SiO₂ passivation layer deposited by rapid thermal processing onto Si(1 0 0) substrates have been used as gate oxide of p-channel metal-oxide semiconductor (p-MOS) transistors of dynamic random access memory (DRAM). The effect of rapid thermal annealing treatments on these films has also been experimented. Improvements in the electrical performances of transistors have been observed.     

Hexagonal AlN films grown on nominal and off-axis Si(0 0 1) substrates

Nucleation and growth of wurtzite AlN layers on nominal and off-axis Si(0 0 1) substrates by plasma-assisted molecular beam epitaxy is reported. The nucleation and the growth dynamics have been studied in situ by reflection high-energy electron diffraction. For the films grown on the nominal Si(0 0 1) surface, cross-sectional transmission electron microscopy and X-ray diffraction investigations revealed a two-domain film structure (AlN¹ and AlN²) with an epitaxial orientation relationship of [0 0 0 1]_AlN || [0 0 1]_Si and AlN¹ || AlN² || [1 1 0]_Si. The epitaxial growth of single crystalline wurtzite AlN thin films has been achieved on off-axis Si(0 0 1) substrates with an epitaxial orientation relationship of [0 0 0 1]_AlN parallel to the surface normal and 0 1 1 0_AlN || [1 1 0]_Si.     

Coordination of aluminum in amorphous sodium aluminosilicate films

Amorphous sodium aluminosilicate thin films containing large amounts of Al₂O₃ were deposited on fused silica substrates by rf-sputtering, and their aluminum K-band X-ray emission spectra were measured by using an electron probe X-ray microanalyser in order to determine the coordination number of aluminum ions in the amorphous thin films.

The chemical shifts for the amorphous films with Al₂O₃/Na₂O<1 were almost identical with those of tetrahedrally coordinated aluminum ions in microline. On the other hand, for the amorphous films with Al₂O₃/Na₂O>1, the chemical shifts increased with increasing Al₂O₃/NA₂OF ratio, approaching that of amorphous alumina. From the comparison with the chemical shifts of -Al₂O₃ and mullite, the coordination state of aluminum ions in amorphous alumina was found to be about 5, and its structure was found similar to be that in crystalline Al₂O₃ with spinel-type structure. These results indicate that in amorphous sodium aluminosilicate thin films aluminum ions exist in the tetrahedrally coordinated state when the Al₂O₃/Na₂O ratio is nearly equal to or less than unity. However, when the Al₂O₃/Na₂O ratio exceeds unity, some of the aluminum ions begin to assume the octahedrally coordinated state and increase in number with increasing Al₂O₃/Na₂O ratio.     

A new method for fabricating water repellent silica films having high heat-resistance using the sol–gel method

There has been a great demand in the field of kitchen appliances to develop transparent water repellent films which have high heat-resistance around 300°C. However, those films have not been obtained by conventional sol–gel methods. In this paper, we propose a new method for fabricating transparent water repellent films with high heat-resistance using the sol–gel method, in which silicon or germanium substrates were coated with a solution including tetraethoxysilane (Si(OC₂H₅)₄) and (2-perfluorooctyl)ethyltrimethoxysilane (CF₃(CF₂)₇C₂H₄Si(OCH₃)₃), followed by ‘ammonia-treatment' and annealed at 300°C. The contact angles of water on the ammonia-treated film maintained its initial value, 110° after the heat treatment at 300°C for 250 h while those on the untreated film decreased to 70°, indicating that the ammonia-treatment improves heat-resistance on the film. The mechanism of ammonia-treatment was inferred from FT-IR results; the ammonia-treatment should accelerate hydrolysis and polymerization of FAS and TEOS molecules, resulting in high density of siloxane bonds between FAS and silica glass. These bonds suppress the evaporation of FAS molecules from the film during the heat treatment at 300°C, thus the film has high heat-resistance.     

Synthesis of fluorine doped silica gel and its properties at high temperature

Fluorine doped silica gels were synthesized by using the sol-gel processes of (A) SiF₄(g) and H₂O(1) and (B) the mixed solution of Si(OC₂H₅)₄, C₂H₅OH, H₂O and H₂SiF₆. By the former process we obtained a gel of relatively high fluorine content (8–12 at.%F), while we could synthesize the gel of 0–12 at.% F by adjusting the F/Si ratio of the starting solution mixtures by the latter process.

The defluorination behavior and the structural change of these gels at high temperature were studied by heating-mass spectrometry, IR and ESR measurements. The results revealed the following: (1) defluorination by liberation of SiF₄(g) was admitted from temperatures at about 400°C and was controlled by the diffusion of fluorine in the gel bulk. (2) The peak separation analysis for the IR band of 1300-900 cm⁻¹, where the stretching vibrations of Si---O and Si---F appear, showed that the change of the band shape resulted from the increase or the decrease of the Si---F bonds and the change of the bond angle of Si---O---Si as well as the change of the force constant accompanied by fluorination or defluorination. (3) The defects of the Si E′ center were induced by X-ray irradiation depending on the degree of the defluorination, and were reduced by the heat treatment. However, with the heat treatment at temperatures higher than 1000°C, the E′ center increased again. The IR spectra suggest that this behavior might correspond to the gel-glass trasition.     

Microstructure and ferroelectric properties of BaTiO3 films on LaNiO3 buffer layers by rf sputtering

We have fabricated LaNiO₃ and BaTiO₃ films using the rf sputtering method. The LaNiO₃ were deposited on Si substrates, demonstrating a (1 0 0) highly oriented structure and nanocrystalline characteristic with a grain size of 30 nm. The BaTiO₃ thin films were deposited on the LaNiO₃ buffer layers, and have exhibited a (1 0 0) texture with a thickness of 400 nm. A smooth interface is obtained between the LaNiO₃ bottom electrode and the BaTiO₃ film from cross-section observations by scanning electron microscopy. The bi-layer films show a dense and column microstructure with a grain size of 60 nm. Ferroelectric characterizations have been obtained for the BaTiO₃ films. The remnant polarization and coercive field are 2.1 μC/cm² and 45 kV/cm, respectively. The leak current measurements have shown a good insulating property.     

TEM investigation on the structure of amorphous silicon monoxide

Commercially available powder samples of silicon monoxide have been investigated by methods of transmission electron microscopy: electron scattering, electron energy-loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). Pair distribution functions (PDFs) as well as EEL spectra can be shown to be a composition of the PDF and EEL spectra of elemental silicon and amorphous SiO₂. The distribution of the elements silicon and oxygen calculated from ESI images proof the silicon monoxide to be inhomogeneous, i.e. it consists of amorphous silicon and amorphous SiO₂. The phase separated regions measure ≈3–4 nm. One maximum in the PDF at 2.95 Å does not stem from either a-Si or a-SiO₂, and it is assigned to atomic configurations at the interphase boundary layer between Si and SiO₂. The portion of the interphase domain in the total composite material is estimated to be between 20% and 25%.     

磁控溅射衬底加热温度和后退火温度对制备β-Ga2O3薄膜材料的影响

作为宽禁带半导体材料的一员,结构稳定的β-Ga₂O₃具有比SiC和GaN更宽的禁带宽度和更高的巴利加优值,近年来受到科研人员的广泛关注。本文采用射频(RF)磁控溅射法在C面蓝宝石衬底上生长β-Ga₂O₃薄膜,探究溅射过程中衬底加热温度的影响。溅射完成后通过高温退火处理提升薄膜质量,研究衬底加热温度和后退火温度对氧化镓薄膜晶体结构和表面形貌的影响。利用X射线衍射(XRD)、原子力显微镜(AFM)等测试手段对β-Ga₂O₃薄膜晶体结构、表面形貌等进行分析表征。实验结果表明,随着衬底加热温度的升高,β-Ga₂O₃薄膜表面粗糙度逐渐降低,薄膜晶体质量得到显著提升;在氧气气氛中进行后退火,合适的后退火温度有利于氧化镓薄膜重新结晶、增大晶粒尺寸,能够有效修复薄膜的表面态和点缺陷,对于改善薄膜晶体质量有明显优势。