Growth and morphology of 6H-SiC epitaxial layers by CVD

Single crystals of 6H-SiC were epitaxially grown on 6H-SiC substrates in the temperature range of 1500 to 1750°C with gas composition: H₂ ≈ 1 l/min, SiCl₄ ≈ 1 ml/min, C₃H₈ ≈ 0.05 ml/min. The grown layers were transparent and mirror-like. The morphology of the grown layer was strongly influenced by the polarity of the substrate surface. Aggregates of trapezoidal crystals were observed on the (000$\text{1}$)C surface and a mosaic pattern was observed on the (0001)Si surface. By observing the initial stage of the crystal growth, the growth mechanism of 6H-SiC is discussed. On (000$\text{1}$)C surfaces the vertical growth dominates, while on (0001)Si surfaces the lateral growth dominates.     

Mechanism of epitaxial ferrite-spinel layer formation on magnesium oxide substrate

The initial stage of small gap heteroepitaxial growth of magnesium-manganese ferrite films during chemical gas transportation reactions on orientation (100) magnesium oxide substrate was studied. Film growth would begin with origination of parallely oriented three-dimensional nuclei, their shapes at lower (1170 K) and at appear (1420 K) film synthesis temperature limits being different. After little islands had grown together and continuous layer formed the secondary nuclei would appear, their shapes resembling cut-up pyramids. Continuous layer minimum thickness was 0.15 μm. The growing together of little islands results in film block structure. Block sizes could be as big as 300–400 μm if films grow at the speed of 2 μm/min, the density of islands being 10⁶ cm⁻². Linear density of unconformity dislocations at film-substrate boundary would be (3.7 ÷ 7.4) 10³ cm⁻¹. It would depend on magnesium content for a film. Elastic stresses in films would come to 7 · 10⁸ ÷ 3 · 10⁹ dyne/cm². X-ray topography pictures of block structures and of accumulations of admixtures at block boundaries are presented.     

Theoretical analysis of equilibrium adsorption layers in CVD systems (Si-H-Cl,Ga-As-H-Cl)

Adsorption of Cl, GaCl, As, As₂, As₄, Ga and AsGaCl species on GaAs(111) in the temperature range of 973–1223 K, and Cl, H, Si, SiCl and SiCl₂ on Si(111) in the range of 1425–1525 K, has been considered for different gas phase compositions. All the faces are covered mainly with Cl and H atoms. The fraction of vacancies on GaAs(111)Ga varies in the range of 4–13% reaching ?50% on GaAs(111)As. On the Si(111) the typical fraction of the vacancies is ?1%. The main adsorbed molecules containing Ga and As atoms are GaCl and As₄. For example, at and T = 1073 K, the surface concentration of these molecules reaches ?4 and 2% on GaAs(111)Ga and ?26 and 6% on the GaAs(111)As faces, respectively. Under these conditions, AsGaCl complexes occupy only ?10^-4?10^-5 adsorption sites on both faces. The set of other data is presented. Surface diffusion in the dense adsorption layer is seriously hindered by the lack of vacancies. A rough estimate gives a value of the order of 10^-8 cm²/sec for the diffusion coefficient and diffusion length of several tens of distances for AsGaCl or As₄ complexes on the GaAs(111)Ga face. The local electric field caused by adatoms on the interface may seriously influence the reaction rate.     

On the development of single and multijunction solar cells with hot-wire CVD deposited active layers

We present an overview of the scientific challenges and achievements during the development of thin film silicon based single and multijunction solar cells with hot-wire chemical vapor deposition (HWCVD) of the active silicon layers. The highlights discussed include the development of Ag/ZnO coatings with a proper roughness and morphology for optimal light trapping in single and multijunction thin film silicon solar cells, studies of the structural defects created by a rough substrate surface and their influence on the performance of nc-Si:H n–i–p single junction solar cells, and studies of the phase change during the growth of nc-Si:H by HWCVD and the use of a ‘reverse’ H₂ profiling technique to achieve nc-Si:H single junction n–i–p cells with high performance. Thus far, the best AM1.5 efficiency reached for n–i–p cells on stainless-steel with HWCVD i-layers is 8.6% for single junction nc-Si:H solar cells and 10.9% for triple junction solar cells. The opportunities for further improvement of cell efficiency are also discussed. We conclude that the uniqueness of HWCVD and of the i-layers deposited with this technique require some adjustments in the strategy for optimization of single or multijunction solar cells, such as using a reverse H₂ profiling technique for the deposition of nc-Si:H i-layers. However, the output performance of solar cells with HWCVD deposited i-layers is close to those with i-layers deposited by other techniques. The difference between the best nc-Si:H n–i–p cells obtained so far in our lab and the reported best n–i–p cells with PECVD i-layers can be mainly attributed to the differences in the rough substrates and to the use of rather thin i-layers.     

Electron paramagnetic resonance and raman spectroscopy characterization of diamond films fabricated by HF CVD method

Thin polycrystalline diamond films were synthesized on silicon substrate by Hot Filament Chemical Vapor Deposition (HF CVD) technique from a mixture of hydrogen and different content of methyl alcohol. A comparative study on the Electron Paramagnetic Resonance (EPR), Raman spectroscopy and Scanning Electron Microscopy (SEM) were performed. It was shown that EPR signal, Raman spectra and morphology, studied by SEM, strongly depend on the ratio of CH₃OH/H₂ in the HF CVD reactor. The peak‐to‐peak line‐width in EPR signal varies from 0.09 to 0.8 mT depending on diamond quality. The Raman spectra of our diamond film showed, except well defined diamond Raman lines positioned at 1332 cm^‐1 with different Full Width at Half Maximum (FWHM), a broad band having maximum at around 1530 cm^‐1 which is characteristic for amorphous carbon phase. The obtained results show that EPR, SEM and Raman spectroscopy yield complementary results about the defects present in CVD diamond films. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

EXAFS study on poly-Si1−XGeX films prepared by reactive thermal CVD method

Extended X-ray absorption fine structure analyses were carried out on Si_1−XGe_X films of different thicknesses, prepared by the reactive thermal chemical vapor deposition (CVD) method. From a Rutherford backscattering measurement, the Ge fraction was found to be high near the substrate interface. The Ge coordination ratio, Ge–Ge bond length and Ge–Si bond length decreased with increasing film thickness. The Ge fraction dependences of these parameters were found to be different from the results of previous studies on Si_1−XGe_X films prepared by molecular beam epitaxy. Our results are considered to be caused by the local structure formation around the Ge atoms during the reactive thermal CVD process.     

以S、Se为原料的CVD ZnS、CVD ZnSe的化学反应分析

本文详细分析了用单质硫或单质硒为原料,在Zn-S-H2-Ar体系或Zn-Se-H2-Ar体系中化学气相沉积生长 ZnS和ZnSe晶体所发生的化学反应,认为在这两种化学气相沉积过程中所发生的化学反应是以锌蒸汽与硫或硒蒸汽反应来实现的.计算出了上述反应的△H、△S和△G这些热力学函数,并将该△G与采用H2S气体(Zn-H2S-Ar体系)和H2Se气体(Zn-H2Se-Ar体系)为原料的CVD ZnS和ZnSe做了对比.实验结果表明,以单质Se为原料生长的CVD ZnSe比以H2Se为原料的CVD ZnSe的努普硬度有显著的提高.     

Optical and magnetic properties of (Zn,Mn)O nanostructures synthesized by CVD method

We report on microstructural, optical and magnetic properties of (Zn,Mn)O nanostructures synthesized by a chemical vapor deposition (CVD) technique. Average diameters of the as grown (Zn,Mn)O nanorods and nanowires were ∼400 nm and ∼50 nm, respectively. X‐ray diffraction (XRD) and photoluminescence (PL) spectra provided the evidence that Mn was incorporated into ZnO lattice. PL spectra of the (Zn,Mn)O nanostructures showed shift in near band edge (NBE) emission at 396 nm together with a green band (GB) emission at 510 nm and a blue band (BB) emission at 460 nm. Magnetic measurements revealed mixed magnetic phases (ferromagnetic and antiferromagnetic) in the (Zn,Mn)O nanostructures. Vapor‐solid‐solid (VSS) mechanism was thought to be responsible for the growth of the nanostructures at low temperatures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

ZnS晶体的化学气相沉积生长

本文报道了用化学气相沉积(CVD)法生长ZnS透明多晶体的实验结果,对生长的晶体的物理化学性能进行了测试,讨论了化学气相沉积工艺中影响ZnS晶体质量的因素.结果表明:选用固体硫作原料,用化学气相沉积方法,可以沉积出透明ZnS多晶体;它的透过性能极其优异,在6.2μm处无吸收峰,在中、长波红外透过率可达70;以上.     

CVD法制备微螺旋炭纤维的研究

以乙炔为碳源,Ni粉为催化剂,含硫化合物噻吩为助催化剂,通过化学气相沉积法(CVD)制得了微螺旋炭纤维.利用扫描电子显微镜(SEM)考察了微螺旋纤维的微观形貌,发现所得的纤维几乎为双螺旋,同时通过对影响微螺旋炭纤维生长因素研究,发现有利于微螺旋炭纤维生长的最佳反应条件:反应温度为750℃,噻吩温度为30℃,H2/C2H2=3,反应时间为2h,石墨棒竖直起来放置.     

A narrow process window for the preparation of polytypes of microcrystalline silicon carbide thin films by hot-wire CVD method

Effects of deposition conditions on the structure of microcrystalline silicon carbide (μc-SiC) films prepared by hot-wire chemical vapor deposition (hot-wire CVD) method have been investigated. It is found from X-ray diffraction patterns of the film that a diffraction peak from crystallites from hexagonal polytypes of SiC is observed in addition to those of 3 C-SiC crystallites. This result is obtained in the film under a narrow deposition conditions of SiH₃CH₃ gas pressure of 8 Pa, the H₂ gas pressure of 80–300 Pa and the total gas pressure of 40–300 Pa under fixed substrate and filament temperatures employed in this study. Furthermore, the grain size of hexagonal crystallites (about 20 nm) on c-Si substrates becomes larger than that of 3 C-SiC crystallites (about 10 nm) for the films deposited under the total gas pressure of 36–88 Pa. The fact that microcrystalline hexagonal SiC can be deposited under limited deposition conditions could be interpreted in the context of a result for c-SiC polytypes prepared by thermal CVD method.     

CVD法生长ZnSe的工艺分析

采用单质Se为原料(Zn-Se-H2-Ar体系)来生长CVD ZnSe,初步分析了这种工艺的机理,并详细分析了各种工艺参数对生长ZnSe的影响.这些工艺参数包括沉积腔的温度和压力,Zn坩埚和Se坩埚的温度,各路载流气体的流量.对这些工艺参数进行调整和精确控制,并控制好Zn蒸气和Se蒸气气嘴处的ZnSe生长形态,生长出了质量良好的ZnSe多晶体,透过率超过70;.     

等离子体增强CVD法沉积的微晶硅薄膜的微结构研究

本文系统研究了PECVD法沉积μc-Si薄膜中衬低温度、氢气稀释率和射频功率等参数对μc-Si薄膜结构特性的影响.表明:随着衬低温度的增加、氢气稀释率的增大、射频功率的提高,薄膜的晶化率增大.沉积薄膜的晶化率最大可达80;,表面粗糙度大约为30nm.通过对反应过程中的能量变化进行了分析,得到反应为放热反应,且非晶结构对沉积参数比较敏感.     

Investigation of charge carrier lifetime in high-resistivity semiconductor layers by the method of small charge photocurrent

A method of determination of charge carrier lifetime from experimental data of small charge current transient under constant electric field strength is discussed. On the basis of experimental results on transient of small charge photocurrent of holes and electrons which were obtained at different electric field strengths, an analysis of charge carrier lifetimes in a-Se layer has been performed. For interpretation of experimental results the approximation, according to which photogenerated electrons or holes can be trapped in levels of three types, has been used. We found that the process of localized charge carrier release is influenced by the electric field strength and that in a-Se layer, in the vicinity of the substrate, there is a narrow region where the density of localized states exceeds its mean in a layer.     

A new method for determining the thickness and composition of thin layers by electron probe microanalysis

A new method is described which allows the determination of thickness and composition of thin layers by EPMA simultaneously. The method is based on new depth distribution functions of the characteristic X-rays and on a substrate correction which was developed by the authors. The test of this method with the aid of wellknown layers gives very good results. The method works with massive standards. Layers for calibration are not needed.     

Preparation and properties of titanium silicide coating glass by CVD

Titanium silicide thin films were prepared on glass substrates by chemical vapor deposition using SiH₄ and TiCl₄ as the precursors. The phase structure of the thin films was identified by XRD. The surface morphology of the thin films was observed by FESEM. The sheet resistance and optical behaviors of the thin films were measured by the four point resistivity test system and FTIR spectrometer, respectively. Titanium disilicide (TiSi₂) thin films with the face-centered orthorhombic structure are formed. The suitable formation temperature of the TiSi₂ crystalline phase is about 710 °C. The formation of TiSi₂ crystalline phase is dependent on the thickness of thin films and a quantity of the crystalline phase of TiSi₂ in the thin film is directly related to mole ratio of SiH₄/TiCl₄. The sheet resistance of the TiSi₂ thin films is dependent on the formation of the TiSi₂ crystalline phase. With the mole ratio of SiH₄/TiCl₄ of 3, the lowest sheet resistance (0.7 Ω/□) of titanium silicide thin film is formed at 710 °C. The maximum reflectance of the TiSi₂ thin films is about 0.95 on the broad IR heat radiation. A related reaction mechanism was proposed.     

化学气相沉积法制备平直多壁碳纳米管的研究

本文研究了用焦炭还原二氧化碳后的一氧化碳作碳源,化学气相沉积法合成平直的多壁碳纳米管的工艺.研究表明在化学气相沉积条件下,用焦炭还原二氧化碳得到的一氧化碳在适当的铁基催化剂作用下,采用催化剂/碳原子共沉积的工艺可以生产出平直、并且内径很大的多壁碳纳米管.碳源中催化剂的浓度影响着纳米管的形成,而沉积温度对其没有影响.