On the Chemical Reaction Mode of Polysilicon Deposition from Silane

Silane decomposition and silicon layer growth will be described by way of theory taking into account heterogeneous as well as homogeneous reaction mode and 1st as well as 0.5th order of the chemical reaction. Comparing axial layer growth rate distribution and total substrate surface area effect on the latter with respect to theoretical and experimental results (for the process conditions investigated), it will be shown that the deposition of undoped poly silicon is characterized by a heterogeneous reaction mode, whereas chemical reaction is of first reaction order in the temperature range above 973 K and of 0.5th reaction order below 973 K. The deposition kinetics of strongly in-situ phosphorus doped poly silicon is shown to be in agreement with a homogeneous reaction mode of 0.56^th reaction order.     

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.     

In-situ doping of LPCVD poly silicon (II). Temperature influence

Poly silicon deposition by pyrolysis of silane under low pressure conditions has been investigated with respect to the influence of temperature when simultaneously in-situ doping of the deposited layer takes place. The growth rate of poly silicon is retarded in the presence of phosphine provided that a certain lower PH₃/SiH₄-ratio has been exceeded. It has been shown how that lower ratio depends on temperature. Increasing PH₃/SiH₄-ratio not only slows down layer growth rate but also the apparent activation energy of the layer forming reaction. An empirical equation describing the temperature dependence of that activation energy has been derived. Phosphine adsorption has been discussed as a cause of both layer growth rate and activation energy reduction. Additionally, incorporation of phosphorus during layer growth has been investigated with respect to the total amount and the electrically active concentration, the latter measured after a postdeposition anneal at 1000 °C.     

New insights in microcrystalline silicon deposition with expanding thermal plasma chemical vapor deposition

We report on further insights in the microcrystalline silicon (μc-Si:H) deposition using expanding thermal plasma chemical vapor deposition. We have shown before that the refractive index at 2 eV of μc-Si:H layers increased if the silane (SiH₄) was injected close to the substrate, while the deposition rate remained the same. We argued that at high injection-ring position, the SiH₄ travels a long way to the substrate and therefore has a long interaction time with the plasma, in particular atomic hydrogen. In this way, the SiH₄ injection position influences the number of hydrogen atoms stripped from the SiH₄ as well as the consumption of atomic hydrogen. In this paper, we present an analysis of the growth flux of depositing particles as function of the radical production rate. The data suggest that there is no dependence on the SiH₄ injection position, implying that the mix of depositing radicals is not changed. However, the data also show the microcrystalline-to-amorphous transition shifts to higher SiH₄ flows for lower injection positions. We therefore now think that it is not the interaction time between the SiH₄ and the arc plasma determining the material properties, but the interaction of excess atomic hydrogen with the μc-Si:H growth surface.     

On the deposition mechanism of boron in silicon CVD epitaxy

The deposition mechanism of boron doping in CVD silicon epitaxy has been investigated by exposing silicon substrates to B₂H₆ H₂ doping gas mixtures at epitaxy temperatures and examining the effect by dopant profile measuring in an afterwards intrinsically in-situ deposited epitaxial silicon layer. It has been shown that boron is deposited increasing its concentration on the surface linearly with prolonged exposition time and desorbed by purging the surface in pure hydrogen. In the latter case its content decreases linearly proportional to the predeposited concentration. The desorbed boron builds up a secondary doping source which maintains a parasitic boron flow for reincorporation during following layer growth.     

Heteroepitaxial growth of GaP on silicon

Epitaxial gallium phosphide layers have been grown on silicon substrates by the metal-organic process. This process involves the reaction between trimethylgallium (CH₃)₃Ga and PH₃ and gives a high density of nucleation sites on the silicon. The influence of the substrate orientation and of the deposition temperature on the crystallinity of the layers has been studied. Best results were obtained on (001) oriented substrate at a deposition temperature of 800°C. X-ray reflection topographs of the layers have revealed the formation of cracks extending along the [110] direction, which are explained by the lattice mismatch and the difference in thermal expansion coefficients. The cracking is asymmetric with the main direction parallel to [110]. The density of cracks can be reduced by a two stage epitaxy. The electrical properties of undoped and n-type doped layers have been assessed by Hall and C(V) measurements. It shows auto-doping with silicon coming from the substrate.     

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.     

Deposition Efficiency Reducing Reactions in Silicon Deposition from Silane at Low Pressure

Silane pyrolysis at polysilicon low-pressure-deposition conditions is analyzed along the reactor-tube axis with respect to the existence of competing decomposition reactions leading to the formation of silicon-containing gaseous byproducts. The presence of such a parallel reaction is indicated by a difference between silane decomposition efficiency η_SiH4 and silicon deposition efficiency η_si. The rate of such a competing parallel reaction is influenced by the variation of deposition temperature and/or substrate area of silicon deposition. It is drastically accellerated by the presence of phosphine in the gas phase. For the latter case the competing parallel reaction is shown to occur as a homogeneous reaction.     

Undoped and arsenic-doped low temperature (∼165 °C) microcrystalline silicon for electronic devices process

Microcrystalline silicon films are deposited at 165 °C by plasma enhanced chemical vapor deposition (PECVD) from silane, highly diluted in hydrogen–argon mixtures. Ar addition during the deposition allows to increase the crystallinity from 24% to 58% for 20 nm thick films. The final crystallinity for 350 nm thick films reaches 72% with an increase in the grain size. A further increase, still 80%, is provided by substrate pre-treatment using hydrogen plasma before the deposition process. Arsenic doped μc-Si films, deposited on previous optimized (5 W power and 1.33 mbar pressure) undoped films without stopping the plasma between the deposition of both layers, show high electrical conductivity up to 20 S cm⁻¹.     

Preparation of microcrystalline silicon solar cells on microcrystalline silicon carbide window layers grown with HWCVD at low temperature

N-type microcrystalline silicon carbide layers prepared by hot-wire chemical vapor deposition were used as window layers for microcrystalline silicon n–i–p solar cells. The microcrystalline silicon intrinsic and p-layers of the solar cells were prepared with plasma-enhanced chemical vapor deposition at a very high frequency. Amorphous silicon incubation layers were observed at the initial stages of the growth of the microcrystalline silicon intrinsic layer under conditions close to the transition from microcrystalline to amorphous silicon growth. ‘Seed layers’ were developed to improve the nucleation and growth of microcrystalline silicon on the microcrystalline silicon carbide layers. Raman scattering measurement demonstrates that an incorporation of a ‘seed layer’ can drastically increase the crystalline volume fraction of the total absorber layer. Accordingly, the solar cell performance is improved. The correlation between the cell performance and the structural property of the absorber layer is discussed. By optimizing the deposition process, a high short-circuit current density of 26.7 mA/cm² was achieved with an absorber layer thickness of 1 μm, which led to a cell efficiency of 9.2%.     

The effects of annealing temperature on photoluminescence of silicon nanoparticles embedded in SiNx matrix

The effects of the annealing temperature on photoluminescence (PL) of non-stoichiometric silicon nitride (SiN_x) thin films deposited by plasma enhanced chemical vapor deposition (PECVD) using ammonia and silane mixtures at 200 °C were investigated. The optical property and the chemical composition of the films annealed at different temperatures were investigated by PL spectroscopy and Fourier transform infrared absorption spectroscopy (FTIR), respectively. Based on the PL results and the analyses of the bonding configurations of the films, the light emission is attributed to the quantum confinement effect of the carriers inside silicon nanoparticles and radiative defect-related states. These results provide a better understanding of optical properties of silicon nanoparticles embedded in silicon nitride films and are useful for the application of nanosize silicon semiconductor material.     

10.6 μm laser CVD for amorphous silicon films and its spectroscopic studies

CO₂ laser CVD of silane was performed to deposit amorphous silicon films on thin quartz substrates. The deposition is induced mainly by pyrolysis following the substrate heating by the laser. However, using CARS, we show that gas heating and internal excitation of silane are induced by photon absorption. These photo-induced effects may be responsible for the laser wavelength dependence of deposition rates.     

Transport properties of hydrogenated amorphous silicon deposited by dc glow discharge decomposition

Conductivity and thermoelectric power measurements have been made as a function of temperature on a series of hydrogenated amorphous silicon samples. The samples were prepared by the dc glow discharge decomposition of silane and silane phosphine mixtures. The activation energy for conduction varied with the substrate temperature and discharge condition for undoped specimens. The difference in the activation energy for conduction as well as the dependence of photoconductivity and optical gap on the activation energy for conduction among undoped specimens can be explained by introducing centers acting as donors or by change transfer between the island and hydrogen rich interfacial region. The kinks in the log σ versus inverse temperature curves always appear at about 430 K for the undoped specimens prepared at 300°C, while they are absent for low substrate temperature specimens. The downward kinks with increasing temperature can be explained by a two-phase material model. A revised two-channel conduction path model including material heterogeneity is applied to interpret the conductivity and thermopower versus inverse temperature curves of doped a-Si:H films, and to determine the position of phosphorus donor levels. The levels are found to lie at about 0.47 eV below E_c, the mobility edge at the conduction band.     

Low-temperature growth of epitaxial (1 0 0) silicon based on silane and disilane in a 300 mm UHV/CVD cold-wall reactor

Epitaxial (1 0 0) silicon layers were grown at temperatures ranging from 500 to 800 °C in a commercial cold-wall type UHV/CVD reactor at pressures less than 7×10⁻⁵ Torr. The substrates were 300 mm SIMOX SOI wafers and spectroscopic ellipsometry was used to assess growth rates and deposition uniformities. High-resolution atomic force microscopy (AFM) was employed to verify the atomic terrace configuration that resulted from epitaxial step-flow growth. Deposition from disilane exhibited a nearly perfect reaction limit for low temperatures and high precursor flow rates (partial pressures) with measured activation energies of ≈2.0 eV, while a linear dependence of growth rate on precursor gas flow was found for the massflow-controlled regime. A similar behavior was observed in the case of silane with substantially reduced deposition rates in the massflow-limited regime and nearly a factor of 2 reduced growth rates deep in the reaction limited regime. High growth rates of up to 50 μm/h and non-uniformities as low as 1σ=1.45% were obtained in the massflow-limited deposition regime. Silicon layers as thin as 0.6 nm (4.5 atomic layers ) were deposited continuously as determined using a unique wet chemical etching technique as well as cross-sectional high-resolution transmission electron microscopy (HRTEM). In contrast, epitaxial silicon deposited in RPCVD at 10 Torr using disilane within the same temperature range showed imperfect reaction limitation. While activation energies similar to that of UHV/CVD were found, no partial pressure limitation could be observed. Furthermore, layers deposited using disilane in RPCVD exhibited a large number of defects that appeared to form randomly during growth. We attribute this effect to gas phase reactions that create precursor fragments and radicals—an effect that is negligible in UHV/CVD.     

流态化多晶硅化学气相沉积过程的数值模拟

利用基于欧拉-欧拉两相流模型,建立硅烷热分解的均相和非均相反应模型,模拟了二维流态化的多晶硅化学气相沉积过程,以及硅烷、硅烯和硅沉积速率在反应器中的分布规律.模拟结果表明多晶硅的沉积主要发生在流化床中的密相区及气泡的周围,浓度相对较小的硅烯非均相反应对多晶硅沉积的贡献约为硅烷的10;.分析了硅烷入口浓度和反应温度对硅沉积速率及转化率的影响,模拟的硅沉积速率与文献中的实验数据做了比较.     

Device grade hydrogenated polymorphous silicon deposited at high rates

Hydrogenated polymorphous silicon (pm-Si:H) thin films have been deposited by plasma-enhanced chemical vapor deposition at high rate (8–10 Å/s), and a set of complementary techniques have been used to study transport, localized state distribution, and optical properties of these films, as well as the stability of these properties during light-soaking. We demonstrate that these high deposition rate pm-Si:H films have outstanding electronic properties, with, for example, ambipolar diffusion length (L_d) values up to 290 nm, and density of states at the Fermi level well below 10¹⁵ cm^?3 eV^?1. Consistent with these material studies, results on pm-Si:H PIN modules show no dependence of their initial efficiency on the increase of the deposition rate from 1 to 10 Å/s. Although there is some degradation after light-soaking, the electronic quality of the films is better than for degraded standard hydrogenated amorphous silicon (values of L_d up to 200 nm). This result is reflected in the light-soaked device characteristics.     

Catalyzed-assisted growth of well-aligned silicon oxide nanowires

Bulk quantity and ultra-long silicon oxide nanowires on micrometer-sized solid tin balls have been synthesized by typical chemical vapor deposition via a vapor–liquid–solid process. Low melting point tin droplets can be used as an effective catalyst for the large-scale growth of highly aligned silicon oxide nanowires. Observations using scanning electron microscopy indicate that numerous nanowires simultaneously nucleate, grow at nearly the same rate, and simultaneously stop growing. The silicon oxide nanowires have a uniform diameter distribution about 60 nm and are well-aligned. A model for the growth of silicon oxide nanowires on the surface of the tin balls was proposed. The Sn balls on the substrate come from the thermal evaporating SnCl₂ powders, and one of the reactants, Si, on the surface of a Sn ball come from the silicon wafer. Silicon reacts with oxygen to form silicon oxide nanowires on the surface of a liquid Sn ball.     

Dark conductivity in amorphous undoped silicon films

The temperature dependence of the dark conductivity was investigated in amorphous undoped silicon films deposited by glow-discharge in a SiCl₄H₂ mixture. Different transport processes were indentified according to the investigated temperature range. The dependence of the dark conductivity was also examined as a function of some deposition parameters. The experimental results are discussed in terms of the two-phase structure of the film.     

Low-temperature deposition of crystalline silicon nitride nanoparticles by hot-wire chemical vapor deposition

The nanocrystalline alpha silicon nitride (α-Si₃N₄) was deposited on a silicon substrate by hot-wire chemical vapor deposition at the substrate temperature of 700 °C under 4 and 40 Torr at the wire temperatures of 1430 and 1730 °C, with a gas mixture of SiH₄ and NH₃. The size and density of crystalline nanoparticles on the substrate increased with increasing wire temperature. With increasing reactor pressure, the crystallinity of α-Si₃N₄ nanoparticles increased, but the deposition rate decreased.     

A First Approach to CVD Si1–xGex Layer Growth by Means of Chemical Reaction Kinetics

A reaction mechanism will be suggested for interpreting Si_1–xGe_x CVD layer deposition from a silicon source and germane reaction gas mixtures in order to explain the observed silicon layer growth increase in consequence of the presence of germane. A successive substitution of the hydrogen atoms available in germane molecules by silyl groups forming a silicon containing intermediate of germane will be assumed. It will further be supposed that both the original silicon source and the germane intermediates will independently be decomposed by chemical reactions leading to Si_1–xGe_x CVD layer deposition from dichlorosilane-germane-hydrogen reaction gas mixtures at temperatures in the range of 600 ≦ T(°C) ≦ 900 as recently published by KAMINS and MEYER .