Trichlorosilane is the most used precursor to deposit silicon for photovoltaic applications. Despite of this, its gas phase and surface kinetics have not yet been completely understood. In the present work, it is reported a systematic investigation aimed at determining what is the dominant gas phase chemistry active during the chemical vapor deposition of Si from trichlorosilane. The gas phase mechanism was developed calculating the rate constant of each reaction using conventional transition state theory in the rigid rotor-harmonic oscillator approximation. Torsional vibrations were described using a hindered rotor model. Structures and vibrational frequencies of reactants and transition states were determined at the B3LYP/6-31+G(d,p) level, while potential energy surfaces and activation energies were computed at the CCSD(T) level using aug-cc-pVDZ and aug-cc-pVTZ basis sets extrapolating to the complete basis set limit. As gas phase and surface reactivities are mutually interlinked, simulations were performed using a microkinetic surface mechanism. It was found that the gas phase reactivity follows two different routes. The disilane mechanism, in which the formation of disilanes as reaction intermediates favors the conversion between the most stable monosilane species, and the radical pathway, initiated by the decomposition of Si2HCl5 and followed by a series of fast propagation reactions. Though both mechanisms are active during deposition, the simulations revealed that above a certain temperature and conversion threshold the radical mechanism provides a faster route for the conversion of SiHCl3 into SiCl4, a reaction that favors the overall Si deposition process as it is associated with the consumption of HCl, a fast etchant of Si. Also, this study shows that the formation of disilanes as reactant intermediates promotes significantly the gas phase reactivity, as they contribute both to the initiation of radical chain mechanisms and provide a catalytic route for the conversion between the most stable monosilanes.

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The results of a systematic investigation aimed at determining the dominant gas phase chemistry active during GaN MOVPE are reported and discussed in this work. This study was performed developing a thermodynamic database including the most stable GaN gas phase species and a gas phase mechanism that could efficiently describe their interconversion kinetics. The thermodynamic data and the kinetic mechanism were calculated combining density functional theory and ab initio simulations. Structures and vibrational frequencies of reactants and transition states were determined at the M062X/6-311+G(d,p) level, while energies were computed at the ROCBS-QB3 level. Rate constants were calculated using transition state theory using the rigid rotor - harmonic oscillator approximation and considering the possible degeneration of internal motions in torsional rotations. The thermodynamic analysis indicated that the Ga gas phase species formed in the highest concentration at the standard GaN deposition temperature (1300 K) is GaNH2, followed by GaH and Ga. The diatomic GaN gas phase species, often considered to be the main precursor to the film growth, is predicted to be unstable with respect to GaNH2. Among the gas phase species containing two Ga atoms, the most stable are GaNHGaH(NH2)3, GaNHGaH2(NH2)2, and GaNHGa(NH2)4, thus indicating that the substitution of the methyl groups of the precursor with H or amino groups is thermodynamically favored. Several kinetic routes leading to the formation of these species were examined. It was found that the condensation of Ga(R1)x(R2)3-x species, with R1 and R2 being either CH3, NH2, or H, is a fast process, characterized by the formation of a precursor state whose decomposition to products requires overcoming submerged energy barriers. It is suggested that these species play a key role in the formation of the first GaN nuclei, whose successive growth leads to the formation of GaN powders. A kinetic analysis performed using a fluid dynamic model allowed us to identify the main reactive routes of this complex system.

a Kinetic constants are high-pressure limits and are expressed as k = AT α exp(−E a /RT), with A in units consistent with cm, s, and mol and E a in kcal/ mol. b From Su and Schlegel. 39 c From Kunz and Roth. 40 d From Veneroni and Masi. 9 The rate constant is a collisional value. This reaction has been

We carried out a kinetic analysis of metallorganic vapor phase epitaxy (MOVPE) of GaN to investigate the dependence of the growth rate on the process conditions as a function of residence time of the precursors in the reactor. The wafer was not rotated during growth, allowing us to analyze the thickness profile of the film in the direction of gas flow, and hence the dependence of the growth rate on the residence time. The growth rate is determined mainly by the concentration of the growth species and mass transfer of the growth species to the wafer surface. The growth rate peaked in the flow direction, and the position of this peak could, in most cases, be explained by considering a combination of the linear gas velocity and the time constant for vertical diffusion of trimethylgallium (TMGa) and/or growth species across the NH 3 feed stream to the wafer surface. In some cases this was not possible, indicating that more complex effects were significant. This work is expected to contribute to understanding of the reaction pathways for GaN-MOVPE, and the growth rate data reported here are expected to provide useful benchmarks for growth simulations that combine computational fluid dynamics and reaction models. Gallium nitride (GaN) is a III/V compound semiconductor material with considerable potential for optoelectronic and high-power electronic devices due to its wide bandgap and high breakdown voltage.1-5 For these reasons, GaN is currently used for mass-produced light-emitting diodes (LEDs), lasers, and high-frequency devices. 6-10Metalorganic vapor phase epitaxy (MOVPE) is commonly employed to manufacture GaN films using trimethylgallium (TMGa) and NH 3 as group-III and group-V precursors, respectively. Research into the growth mechanisms involved in GaN-MOVPE in both academic and industrial institutions has found that the reaction chemistry is relatively complicated, consisting of gas-phase reactions followed by surface reactions. [11][12][13][14][15][16][17] This intrinsic complexity of the reactions suggests that it is not straightforward to design optimal reactors for mass production as well as settling the optimal growth conditions via conventional empirical approaches. Therefore, a variety of studies have been carried out with the aim of understanding GaN-MOVPE. Initially, attention mainly focused on identification of intermediate species that are generated from the gas-phase precursors, which contribute to the layer growth. It then became apparent that parasitic reactions in the gas phase resulted in the formation of adducts, even in the non-heated area of the reactor, in addition to those formed in the hot zone in the vicinity of the heated substrate.18,19 Some of these reaction products led to particle formation without contributing to layer growth. [20][21][22] 39 However, a model that can describe the growth behavior consistently in all reactor configurations and for all process conditions remains elusive. This is our concern, and necessitates comprehensive investigation on the behavior of GaN growt...

The heating and evaporation of single component spherical and spheroidal drops in gaseous quiescent environment are predicted, accounting for the effect of a non-uniform distribution of the temperature at the drop surface. The analytical solution of the species conservation equations in the proper coordinate system (spherical/spheroidal) is implemented to numerically solve the energy equation in a rectangular domain. The effect of temperature non-uniformity on the local Nusselt number and global heat and evaporation rates is calculated for different species, drop deformation and gaseous temperature. KeywordsDrop evaporation, spheroidal coordinates, non-uniform Dirichlet Boundary conditions. IntroductionMost of the models predicting the drop heating and evaporation to be implemented in CFD codes for dispersed phase applications rely on the assumption that drops are spherical, thus allowing a simpler solution in spherical coordinates of the energy and species conservation equations. However, experimental investigation on liquid drops in multi-particle systems has revealed that they are subject to significant shape deformations while interacting with the carrier phase [1-3], due to the interaction of surface tension and fluid-dynamic stresses on the drop surface [3]. Numerical investigations on oscillating drops [4,5] have shown that the vapour and heat fluxes on the drop surface are not uniform and they were empirically correlated to the local mean curvature of the surface [1,6]. Analytical modelling of the heating and evaporation of spheroidal drops have shown that the local vapour and heat flux scale with the fourth root of the Gaussian curvature [7,8] and later the same result was extended to a wider class of drop shapes [9]. When dynamical simulation of droplet heating and evaporation is necessary, uniform drop temperature is often assumed, on the basis of a commonly accepted belief that the internal recirculation would maintain uniform conditions. However a more accurate simulation can be obtained by using the concept of effective conductivity, firstly introduced by [10], to account for the effect of recirculation (see also [11] and [12]) and, although this cannot properly describe the temperature field inside the droplet, it can give a better estimation of the droplet surface temperature [13] . Recent modelling of heating and evaporation of spheroidal droplets [14] revealed that the uneven distribution of fluxes on the drop surface causes a corresponding uneven distribution of temperature on the drop surface, during most of the drop lifetime. This non-uniform temperature distribution affects the heat and vapour flow fields in a non neglectful way. The motivation of the work reported here is to investigate, through a combined analytical-numerical solution of the species and energy conservation equations, the effect of non-uniform Dirichlet boundary conditions at the drop surface (for spheroidal liquid drops) on the local heat and mass transfer coefficients.