On chemical kinetics of silicon deposition from silane (I). The first order chemical reaction

Theoretical expressions for silicon layer deposition in consequence of silane decomposition within an open isothermal reaction tube has been derived for the case of homogeneous as well as heterogeneous gas reaction. Layer growth distribution along reaction tube axis has been taken into consideration as well as average layer growth rate related to silane consumption during its passage through the tube on the one hand and to growth rate distribution along the tube on the other. Comparing the theoretical results with experimentally based data homogeneous rather than heterogeneous reaction mode might be preferred. In consequence, however, layer growth rate should be linearly effected by the ratio of reactor gas volume to total substrate surface area. In a practical sense average growth rate, and so axial growth homogeneity, should be expected the higher the lower silane consumption efficiency would be adjusted.     

两步法溅射中缓冲层厚度对Ge薄膜质量的影响

采用低温缓冲层技术制备Ge薄膜,利用AFM和Raman光谱研究缓冲层厚度对低温Ge缓冲层残余应变弛豫的影响.实验结果显示:随着缓冲层厚度的增加,残余应变弛豫度增大.在30 nm厚的低温Ge缓冲层上生长800nm厚的Ge外延层.Ge薄膜具有良好的结晶性,表面粗糙度RMS为2.06 nm.     

Microcrystalline silicon–germanium thin films prepared by the chemical transport process using hydrogen radicals

We propose microcrystalline silicon–germanium (μc-SiGe) as a bottom cell material of triple-junction solar cells in order to improve the conversion efficiency of thin film solar cells. The μc-SiGe thin films were prepared by the chemical transport process using Si and Ge targets exposed to hydrogen radicals. We successfully produced highly photosensitive μc-SiGe films with relatively low Ge composition by increasing the gas pressure, and observed the photovoltaic effect in pin solar cell structures. However, it was difficult to produce μc-SiGe films with higher Ge composition. We found that a small amount of argon introduction into the chemical transport process enables us to increase Ge composition at the high pressure. Moreover, the argon introduction seems effective to maintain the electrical properties in relatively high Ge composition. The results suggest that the μc-SiGe thin films prepared by the chemical transport process are one of the candidates for new photovoltaic materials.     

On chemical kinetics of silicon deposition from silane (III). LPCVD poly silicon formation in the temperature range 900–950 K

LPCVD poly Silicon deposition form silane has been investigated for limited conditions regarding temperature, silane input and pumping speed. It has been found that layer growth is controlled by a chemical reaction of 0.5^th-order in consequence of which growth rate linearly decays along the axis of an open isothermal reactor tube. The slope of that decay is determined not only by the reaction rate constant but also by linear gas velocity within the tube and that part of total substrate surface area that is effectively exposed to silane at each wafer position. In conseqence growth rate decay is the steeper not only the higher temperature will be chosen but also the slower gas velocity is adjusted and the smaller wafers are separated to each other. The kind of how axial layer growth rate distribution is effected by changing wafer spacing is a proof for the heterogeneous reaction mechanism. The silicon forming reaction is characterised by an activation energy of about 52 kcal/mole.     

Formation of Ge nanoislands before the completion of a wetting layer in the Ge/Si(1 1 1) system

The formation of Ge nanoislands directly on Si(1 1 1) surface before the completion of a wetting layer was studied by scanning tunneling microscopy and Raman scattering spectroscopy. The mechanism of the wetting layer formation in the Ge/Si(1 1 1) system depends on the rate of Ge deposition. Within the temperature range 350–500 °C, with Ge deposition rates of the order of 10⁻³ bilayers/min, the Ge wetting layer is formed by the multilayer growth mechanism. Therefore, the arrays of Ge islands with the densities of 10⁹–10¹² cm⁻², depending on the rate of Ge deposition, appear directly on the Si surface during the evolution of the wetting layer. The height of Ge islands is limited by 3 bilayers. The lateral dimensions depend on the coverage of Ge and on the growth temperature. A series of lines related to the quantization of the phonon spectrum along the growth direction [1 1 1] was observed in the spectra of Raman scattering by optical phonons of Ge nanoislands.     

Defect passivation by hydrogen reincorporation for silicon quantum dots in SiC/SiOx hetero-superlattice

The SiC/SiO_x hetero-superlattice (HSL) consisting of alternating near-stoichiometric SiC barrier layers for the electrical transport and silicon rich SiO_x matrix layers for the quantum dot formation is a promising approach to the realization of silicon quantum dot (Si–QD) absorbers for 3rd generation solar cells. However, additional defect states are generated during post deposition annealing needed for the Si–QD formation causing an increase in sub-band gap absorption and a decrease in PL intensity. Proper passivation of excess defects is of major importance for both the optical and electrical properties of the SiC/SiO_x HSL Si–QD absorber. In this work, we investigate the effectiveness of the hydrogen reincorporation achieved with hydrogen plasma in a plasma-enhanced chemical vapor deposition (PECVD) reactor, hydrogen dissociation catalysis in hot-wire chemical vapor deposition (HWCVD) reactor and annealing in forming gas atmosphere (FGA). Both the HSL samples and single layer reference samples are tested. The passivation quality of the hydrogen reincorporation was examined by comparing electrical and optical properties measured after deposition, after annealing and after passivation. In addition, the formation of Si–QDs in SiC/SiO_x HSL was evaluated using high resolution transmission electron microscopy. We demonstrated that hydrogen can be successfully reincorporated into the annealed HSL sample and its single layer reference samples. FGA passivation is most effective for SiO_1.2 single layers and HSL samples. Passivation with PECVD appeared to be only effective for SiC single layers.     

Study of ZnSe and CdSe thin film epitaxy on germanium and silicon substrates

The parameters of vaporization, mass-transfer, condensation, and epitaxial growth by hot wall technique (HWT) of ZnSe and CdSe thin films on monocrystalline Ge and Si substrates are studied (Bubnov et al.). It is shown, that the layers structure is improved as the mass transfer mechanism approaches to gasodinamical vapor flow. The influence of condensation temperature of the layers on their crystallographic structure is shown. The increase of the temperature gradient from the source towards the substrate as well as the substrate temperature conditions for growing layers of hexagonal modification. The decrease of the temperature gradient leads to cubic modification. The electron diffraction study revealed the stepwise character of the zinc selenide and cadmium selenide film growth. The knowledge of the parameters of ZnSe and CdSe thin films on monocrystalline Ge and Si by hot wall technique at relatively low substrate temperatures allows to obtain layers, suitable for formation of solid state devices for registration and reflection of optical information.     

Influence of phosphine flow rate on Si growth rate in gas source molecular beam epitaxy

As reported by other authors, we have also observed that the Si growth rate decreases with increasing phosphine (PH₃) flow rate in gas source Si molecular beam epitaxy using phosphorous (P) as a n-type dopant. Why small quantity PH₃ can affect Si growth rate? Up to now, the quantitative characterization of PH₃ flow influence on Si growth rate is little known. In this letter, the PH₃ influence will be analyzed in detail and a model considering strong P surface segregation and its absorption of hydrogen will be proposed to characterize the effect.     

InP heteroepitaxy on Si substrates in In-PH3-HC1-H2 systems

Heteroepitaxial growth of InP on Si in an In-PH₃-HC1-H₂ system is demonstrated. Injecting hydrogen chloride into the growth ambiance prevents indium droplets from precipitating and enables the growth of a featureless InP buffer layer. Morphologies of InP/Si grown by the two-step method vary from island growth at 650°C to continuous-layer growth at 550°C. The temperature required for growing continuous InP layers depends on the crystallinity of the InP under-layer on the Si. High-quality InP layers with 650 arc sec full width at half maximum of double X-ray diffraction are obtained by using a three-step growth method.     

Growth chemistry of nanocrystalline silicon and germanium films

We report on the growth of nanocrystalline Si:H and Ge:H films. The films were grown using plasma deposition and hot wire chemical growth techniques. Conditions such as pressure, temperature and hydrogen dilution were systematically varied. It is shown that excessive hydrogen dilution during growth leads to smaller grains in nanocrystalline Si and Ge. Films with very large grains (56 nm) could be obtained using hot wire growth techniques under appropriate conditions of growth. From the data, it is concluded that the natural growth direction for the films is 〈2 2 0〉, and that excessive bonded hydrogen leads to smaller grains.     

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.     

Impact of amorphous Ge thin layer at the amorphous Si/Al interface on Al-induced crystallization

We investigated the impact of an amorphous Ge (a-Ge) thin layer inserted at the amorphous Si (a-Si)/Al interface on Al-induced crystallization. In situ observation of the growth process clarified that the nucleation rate is drastically reduced by insertion of a-Ge, which led to increase in the average size of crystal grains. This was interpreted as resulting from decrease in the driving force of crystallization, mainly due to the larger solubility of Ge in Al than that of Si in Al. The obtained films were SiGe alloys with lateral distribution of Ge content, and its origin is discussed based on the two-step nucleation process.     

Deposition of microcrystalline Ge films using hot-wire technique without toxic gases

To investigate the deposition of Ge films without toxic gas such as germane, we have studied the Ge films prepared by the hot-wire technique, which utilize the reaction between a Ge target and hydrogen atoms generated by the hot-wire decomposition of H₂ gas. The films deposited on Si substrate were microcrystalline Ge films and the mean crystallite size of the films increased from 13.3 to 24.8 nm with increasing the substrate temperature from 300 to 500 °C. Moreover, the deposition rate of Ge films deposited on Si substrate was higher than that of Ge films deposited on Corning 1737 substrate. It was found that the substrate temperature and the kind of substrate are key parameters for the preparation of microcrystalline Ge films by the hot-wire technique.     

离子束溅射制备Si/Ge多层膜及红外吸收性能研究

采用离子束溅射方法在Si衬底上制备Si/Ge多层膜.通过改变生长温度、溅射速率等因素得到一系列Si/Ge多层膜样品.通过X射线衍射、拉曼散射、原子力显微分析(AFM)等表征方法研究薄膜结构与生长条件的关系.在小束流(10mA)、室温条件下制备出界面清晰、周期完整的Si/Ge多层膜.通过红外吸收谱的测量发现薄膜样品具有较好的红外吸收性能.     

Effect of oxygen partial pressure on the growth of zinc micro and nanostructures

Zinc micro and nanostructures were synthesized in vacuum by condensing evaporated zinc on Si substrate at different gas pressures. The morphology of the grown Zn structures was found to be dependent on the oxygen partial pressure. Depending on oxygen partial pressure it varied from two-dimensional microdisks to one-dimensional nanowire. The morphology and structural properties of the grown micro and nanostructures were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Transmission electron microscopy (TEM) studies on the grown Zn nanowires have shown that they exhibit core/shell-like structures, where a thin ZnO layer forms the shell. A possible growth mechanism behind the formation of different micro and nanostructures has been proposed. In addition, we have synthesized ZnO nanocanal-like structures by annealing Zn nanowires in vacuum at 350 °C for 30 min.     

Defects in epitaxial layers of silicon-germanium grown on silicon substrates

Crystal defects of various kinds found in epitaxially grown Si/Ge alloy layers on Si substrate, may be either inherent to the material and originating from atomic radii misfit, or can be traced to the growth process and controlled or eliminated by varying its parameters. A network of slip lines, becoming more pronounced with increased Ge content, indicates plastic deformation resulting from partial relief of stresses during the high temperature growth process. Electron microprobe and X-ray diffraction analysis indicate some Ge segregation in the fault vicinity, and a slight anisotropy in the lattice constant expansion due to the Ge.     

Ga segregation during Czochralski-Si crystal growth with Ge codoping

The segregation of Ga during the growth of Czochralski-Si crystals with Ge codoping was investigated. The effective segregation coefficient of Ga in Ga/Ge-codoped Si crystal growth was nearly constant over a wide Ge concentration range, even at high Ge concentrations of about 10²¹ cm⁻³. In contrast, the effective segregation coefficient increased at high B concentrations in Ga/B-codoped CZ-Si crystal growth. The segregation behavior of Ga in Ga/Ge- and Ga/B-codoped CZ-Si crystal growth was theoretically compared. The difference in the segregation coefficients of Ga as a function of the codoped impurity (Ge or B) between the two Si crystals was attributed to a difference in the excess enthalpy due to impurity incorporation into the Si crystal between Ga–Ge pairs and Ga–B pairs     

Growth and process modeling studies of nickel-catalyzed metalorganic chemical vapor deposition of GaN nanowires

A combination of experimental and computational fluid dynamics-based reactor modeling studies were utilized to study the effects of process conditions on GaN nanowire growth by metalorganic chemical vapor deposition (MOCVD) in an isothermal tube reactor. The GaN nanowires were synthesized on (0 0 0 1) sapphire substrates using nickel thin films as a catalyst. GaN nanowire growth was observed over a furnace temperature range of 800–900 °C at V/III ratios ranging from 33 to 67 and was found to be strongly dependent on the position of the substrate relative to the group III inlet tube. The modeling studies revealed that nanowire growth consistently occurred in a region in the reactor where the GaN thin-film deposition rate was reduced and the gas phase consisted primarily of intermediate species produced by the reaction and decomposition of trimethylgallium–ammonia adduct compounds. The GaN nanowires exhibited a predominant [1 1 2¯ 0] growth direction. Photoluminescence measurements revealed an increase in the GaN near-band edge emission intensity and a reduction in the deep-level yellow luminescence with increasing growth temperature and V/III ratio.     

Phase diagram of growth mode for the SiGe/Si heterostructure system with misfit dislocations

The strain, surface and interface energies of the SiGe/Si (SiGe grown on Si) heterostructure system with and without misfit dislocations were calculated for the Frank–van der Merwe (FM), Stranski–Krastanov (SK) and Volmer–Weber (VW) growth modes essentially based on the three kinds of fundamental and simple structures. The free energies for each growth mode were derived from these energies, and it was determined as a function of the composition and layer thickness of SiGe on Si. By comparison of the free energies, the phase diagrams of the FM, SK and VW growth modes for the SiGe/Si system were determined. The (1 1 1) and (1 0 0) reconstructed surfaces were selected for this calculation. From the phase diagrams, it was found for the growth of SiGe on Si that the layer-by-layer growth such as the FM mode was easy to be obtained when the Ge composition is small, and the island growth on a wetting layer such as the SK mode was easy to be obtained when the Ge composition is large. The VW mode is energetically stable in the Ge-rich compositional range, but it is difficult for the VW mode to appear in the actual growth of SiGe on Si because the VW region is right above the SK region. The regions of the SK and VW modes for the (1 1 1) heterostructure are larger than those for the (1 0 0) one because the strain energy of the (1 1 1) face is larger than that of the (1 0 0) face. The regions of the SK and VW modes for the heterostructure with misfit dislocations are narrower than those for the one without misfit dislocations because the strain energy is much released by misfit dislocations. The phase diagrams roughly explain the behavior of the FM and SK growth modes of SiGe on Si.     

In-process monitoring and control of doping gas concentration during vapor phase silicon epitaxial growth by using a flame photometric detector

An in-process monitoring and control method of the doping gas concentration during epitaxial growth of Si was developed. A flame photometric detector (FPD) can be used as a monitor for the PH₃ and B₂H₆ dopant concentrations in the injected doping gases. A combination of this dopant monitor with an automatic control system of the silicon source (SiHCl₃) gas concentration using an infrared spectrophotometer as a monitor, makes possible an automatic in-process control of the concentrations of dopant and of silicon source gas supplied to the reactor. The present system provides an accurate and reproducible control of impurity concentrations in Si epitaxial layers. Good correlation between the monitored signal (or the doping gas concentration) and the impurity concentration incorporated into the growth layers was confirmed for PH₃ (n-type) and B₂H₆ (p-type) doping. For the B₂H₆ doping, a divergence from the linear relationship between the doping gas concentration and the impurity concentration in the layers was observed in the level of acceptor concentration below about 10¹⁵ atoms/cm³. The transient response of the present system was measured by growing epitaxial layers with increasing and decreasing step-changes in the dopant gas flow during continuous deposition of the layers. Some interesting, but complicated, transient responses of impurity concentration in the growth layer were observed. The responses are different between the PH₃ doping and the B₂H₆ doping, and also different between increasing and decreasing steps especially for the B₂H₆ doping.