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Kinetics of Silane Decomposition in High-Pressure Confined Chemical Vapor Deposition of Hydrogenated Amorphous Silicon Seyed Pouria Motevalian, Stephen C. Aro, Hiu Y. Cheng, Todd d. Day, Adri C.T. van Duin, John V. Badding, and Ali Borhan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03515 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017

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Industrial & Engineering Chemistry Research

Kinetics of Silane Decomposition in High-Pressure Confined Chemical Vapor Deposition of Hydrogenated Amorphous Silicon Seyed Pouria Motevalian1*, Stephen C. Aro2, Hiu Y. Cheng2, Todd D. Day2, Adri C. T. van Duin3, John V. Badding2, Ali Borhan1 1

Department of Chemical Engineering The Pennsylvania State University, University Park, PA 16802

2

Department of Chemistry

The Pennsylvania State University, University Park, PA 16802

3

Department of Mechanical and Nuclear Engineering The Pennsylvania State University, University Park, PA 16802

KEYWORDS High-Pressure CVD, silane pyrolysis, hydrogenated amorphous silicon

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We study the kinetics of silane pyrolysis via confined High-Pressure Chemical Vapor Deposition (HPCVD) at pressures of 20-33 MPa in a microcapillary of 5.9 µm inner diameter. We find the growth rate to be first order with respect to silane concentration, with an activation energy of 53.7 ± 2.9 kcal/mol and a pre-exponential factor of 1.5 × 1010 m/s. The obtained activation energy is in the range of activation energies reported for hydrogen desorption from cSi surfaces, suggesting that hydrogen desorption from the surface is the rate-limiting step in film growth. To further investigate this finding, reactive molecular dynamics simulations of thermal decomposition of silane on clean and hydrogen-passivated c-Si were performed. Homogeneous reactions were not observed in any of the simulations, in support of the hypothesis that heterogeneous silane decomposition on the silicon surface is the dominant mechanism for film deposition. In silane pyrolysis simulations on clean c-Si surfaces, almost all available silicon surface sites (i.e. dangling bonds) were occupied by silicon-hydrides (mostly tri- and dihydrides) upon exposure to gas-phase silane, whereas no reaction was observed during silane decomposition simulations on the hydrogen-passivated c-Si.

Therefore, the results of the

reactive molecular dynamics simulations indicate that the availability of dangling bonds resulting from hydrogen desorption from the surface is the rate-limiting step in film growth at high pressure.


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Introduction Hydrogenated amorphous silicon (a-Si:H), one of the most technologically important semiconductors1, is widely produced by the pyrolysis of silane (SiH4) at low pressure (mTorr) via chemical vapor deposition (CVD). The major challenge in synthesizing a-Si:H with CVD is to thermally decompose silane molecules at a low enough temperature to incorporate sufficient hydrogen to passivate the dangling bonds, while utilizing a high enough temperature to achieve rapid film growth. Since SiH4 precursor does not readily decompose until heated above 550°C in a conventional CVD process, several methods have been developed to activate the precursor molecules to decompose at temperatures suitable for sufficient hydrogen incorporation2. Examples include hot-wire CVD3,

4, 5

and plasma-enhanced CVD6, 7. Another method for

accelerating reaction kinetics at low temperature is high-pressure CVD at 70-200 atm. This method allows for deposition rates of 0.3 Å/s at 450°C, 30-fold higher than typical deposition rates of about 0.01 Å/s achieved in a conventional CVD reactor (with 13 Pa partial pressure of SiH4) at the same temperature8, 9.

However, high pressure accelerates the homogeneous

reactions that result in particle formation in the gas phase, leading to the deposition of low quality a-Si:H films. Gas-phase reactions can be avoided by confining the reaction to microscale spaces, thereby resulting in the formation of smooth films10. Recently, fabrication of singlecrystal silicon core fibers by laser crystallization of amorphous silicon deposited via highpressure CVD has been reported.11 The overall reaction for silane pyrolysis during confined HPCVD is SiH → Si + 2H

(1)

There are several homogenous and heterogeneous elementary reactions involved in this process. In the gas-phase, mono-silane (silane) is converted into other (poly) silanes (Si H ), silylenes (SiH), and disilenes (double-bonded isomers of silylene with the general chemical formula ACS Paragon Plus Environment

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Si H )12. The pioneering work of Purnell and Walsh13 proposed the conversion of silane to mono-silylene and hydrogen in the gas-phase as the initial step of the decomposition, followed by conversion of mono-silylene to higher silanes (di-silane and tri-silane) according to the following reactions: SiH → SiH + H

(2)

SiH + SiH ↔ Si H

(3)

SiH + Si H ↔ Si H

(4)

They postulated that silicon film formation proceeds via decomposition of silylenes at the surface. The simple reaction mechanism proposed by Purnell and Walsh was later modified by Coltrin et al.14 by adding several more reaction intermediates (mostly disilenes) to increase the number of reaction steps to 26 reversible reactions, with the rate constant for each reaction step calculated using Rice-Ramsperger-Kassel-Marcus (RRKM) theory.

They assumed that the

reaction of intermediates striking the surface results in the formation of the silicon film. The difference between the reactivity of different intermediate species at the surface was considered by assigning surface reaction probabilities to different gas-phase reaction byproducts. They concluded that in CVD with hydrogen and silane precursor, only mono-silane and mono-silylene from the gas phase contribute to film growth, with negligible contribution from the latter at low temperatures. Later, Ho et al.15 proposed a gas-phase reaction mechanism consisting of 10 elementary steps based on the study of Coltrin et al., and parameterized the temperature and pressure dependencies of the rate constant for each step in Troe form. Their predicted values of the density of silicon particles formed in the gas-phase agreed well with laser-induced fluorescence measurements. The silane pyrolysis model of Ho et al. has been widely used in the literature16, 17, 18 and details of their gas-phase mechanism are provided in Table 1.


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Table 1. The gas-phase reaction mechanism proposed by Ho et al. 15. ↔ +

(1)

Si H ↔ SiH + SiH

(2)

Si H ↔ H SiSiH + H

(3)

Si H ↔ SiH + Si H

(4)

Si H ↔ SiH + H SiSiH

(5)

H SiSiH ↔ H SiSiH

(6)

H SiSiH + H ↔ SiH + SiH

(7)

H SiSiH + SiH ↔ Si H +SiH

(8)

H SiSiH ↔Si+SiH

(9)

Si+Si H ↔ SiH+H SiSiH

(10)

Odden et al.16 studied the kinetics of silane decomposition at relatively high temperature and pressure ranges of 690-830 K and 0.1-3.7 MPa, respectively. Using gas chromatography, they analyzed the residual gas after pyrolysis and found that it only contained mono-silane and hydrogen, with traces of disilane in a few experiments. They adopted the reaction mechanism proposed by Ho et al., but added two more gas-phase reactions to regenerate mono-silane from di- and tri-silanes. They also modified the gas-surface interactions by considering only one dangling bond to be involved in the surface adsorption of silane in light of having a dense environment at high pressure. More recently, kinetic and thermodynamic data for silicon hydrides have been incorporated into Reaction Mechanism Generator (RMG), an open source software that generates kinetic models based on a general understanding of how molecules react. The resulting RMG-generated model reasonably predicts silane concentration profiles at relatively low pressure for different temperatures19.


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The heterogeneous decomposition of silane has been the subject of several studies. Robertson et al.20 performed several experiments at low pressure and concluded that silane pyrolysis on a silicon surface is completely surface-initiated. They also proposed that both hydrogen and silane adsorb at the surface by competing for free surface sites (i.e. dangling bonds), with silane adsorbing at the surface to form mono- and tri- hydrides that subsequently decamp to form silicon at the surface. Gates et al.21, 22 studied the kinetics of surface reactions in low-pressure CVD using static secondary ion mass spectroscopy. They performed silane pyrolysis experiments at temperatures below 500oC and at very low pressure to minimize the contribution of homogenous reactions to film growth. They observed three different silicon-hydride species (mono-hydride, di-hydride, and tri-hydride) at the surface, and proposed the reaction mechanism shown in Table 2. For experiments below 500oC, they reported an overall silane pyrolysis activation energy of 47 kcal/mol, a value that is close to the activation energy for hydrogen desorption from adsorbed monohydrides (reaction 4 in Table 2). Thus, they concluded that hydrogen desorption from the surface, resulting in the availability of new dangling-bonds, is the rate-limiting step; this result is also supported by other studies20, 23-25. Table 2. The surface reaction mechanism proposed by Gates et al. 21 22.

+ ∗ → ∗ + ∗

∗ + ∗ → ∗ + ∗

∗ → + ∗

(1) (2) (3)

(4)

(5)

∗ → + ∗ ∗ → + ∗

∗ → + ∗

(6)


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Whether the homogenous or heterogeneous mechanism is predominantly responsible for silicon deposition is subject to debate. Some researchers believe homogenous initiation of the growth process occurs wherein higher-order silanes (di-silane and tri-silane) and/or silylenes formed in the gas-phase stick to the surface to deposit a semiconductor film12-16,

26

. Others

believe heterogeneous decomposition of silane at the surface occurs to deposit the film20, 23. The relative contributions of the homogeneous and heterogeneous reactions to film growth depend on deposition temperature, pressure, and the surface-to-volume ratio of the reactor27

28

. In this

study, we present the kinetics of film growth via confined HPCVD in a microcapillary which results in formation of optical fibers. We also show via reactive molecular dynamics simulations that in the confined HPCVD process, heterogeneous thermal decomposition of silane at the amorphous silicon surface is the dominant mechanism for deposition of the a-Si:H film due to the very high surface area to volume ratio present in the confined geometry. The kinetic data obtained in this study for a-Si:H film growth can be utilized to optimize the confined HPCVD process for fabrication of high quality waveguide optical fibers with a plugged central channel over lengths of centimeters9.

Experimental Methods During each deposition experiment, a high-pressure (~35 MPa) gas mixture of silane (less than 5%) and helium was introduced into bare-silica glass microcapillaries with 5.9 µm inner diameter. The microcapillaries were heated by a single furnace consisting of a ceramic casing wrapped in resistive heating wire and insulated with a thick (2-3 inch) layer of fiberglass. Wall temperatures throughout the furnace were measured via a k-type thermocouple housed in the ceramic casing using a hole of 1/8-inch outer diameter. This allowed for the thermocouple to


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have good thermal contact with the interior of the furnace at each measurement point. A second, identical thermocouple was used along with a PID feedback loop to control the temperature of the furnace via pulse-width-modulation of the AC electric load (~3A). temperature profile is shown in Figure 1c.

A typical wall

At the desired time point, the deposition was

quenched by removing the microcapillary from the furnace, venting it, and flushing it with highpressure helium to purge the silane. The high-pressure helium and silane mixtures were prepared by first evacuating a stainless steel high-pressure vessel under high vacuum (9-10 Torr) via a turbomolecular pump (Pfeiffer HiPace 300) backed by a scroll pump (Edwards XDS series). Once evacuated, the desired amount of silane gas was cryogenically condensed into the stainless steel vessel from a pressuremonitored antechamber of known volume. Upon warming to room temperature, the silane pressure within the vessel was measured. The vessel was then pressurized to 35 MPa with helium carrier gas using a pneumatically-actuated diaphragm gas compressor. Film deposition was subsequently carried out for sufficiently short times to avoid mass transfer limitations. Cross-sections of the sample at different axial-positions along the microcapillary were mounted using standard electron microscopy techniques, and imaged using a FEI Nova NanoSEM 630 field-emission scanning electron microscope. Typical samples are shown in Figure 1a. This allowed measurement of the azimuthally-averaged a-Si:H film thickness within each cross-section, which was then used to determine an axial profile for film thickness such as that shown in Figure 1b. Film thickness profiles were obtained by performing depositions at various precursor gas pressures and furnace temperatures.

Results and Discussion


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The measured rate of a-Si:H film growth at different furnace temperatures and silane partial pressures is shown in Figure 2. The measured growth rates range from 0.008 Å/s to 0.272 Å/s, depending on the silane partial pressure and furnace temperature used in the experiment. Silicon film growth rate is an increasing function of both furnace temperature and silane partial pressure.

Figure 1. a) FESEM images of the a-Si:H film cross-section in a 5.9µm-diameter microcapillary. b) The axial profile of film thickness determined from FESEM images of the aSi:H film cross-section after 24 hours for a 2% silane mixture at 30 MPa. c) The axial profile of furnace temperature.


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1.5 % silane mole fraction 2% silane mole fraction

0.25

0.20 Growth Rate (Å/s)

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0.15

0.10

0.05

0.00 330

340

350

360 Temperature (°C)

370

380

390

Figure 2. Film growth rate for different furnace temperatures and silane partial pressures.

For an axially-uniform a-Si:H film, the time-evolution of the thickness of the deposited film () can be related to the precursor pressure (P) and silane mole fraction ( ) according to " '( ) =% & -, (1) "# *+, where t, k, R, T, Mw, and ρ denote the time, reaction rate constant, universal gas constant, temperature, and molecular weight and density of the a-Si:H film, respectively. Here, we have assumed the precursor to be an ideal gas mixture, and the overall silane decomposition reaction to be first-order with respect to silane concentration. Equation 1 was used to calculate the reaction rate constant (k) at different temperatures from the corresponding experimental measurements of film growth rate shown in Figure 2. Assuming an Arrhenius temperature-


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dependence for the reaction rate constant, the activation energy (01 ) and pre-exponential factor (21 ) were then obtained from a semi-log plot of the reaction rate constant versus 1/T. . The Arrhenius plot of the surface reaction rate constant versus 1/T is shown in Figure 3. The vertical error bars represent the combination of errors involved in the calculation of the reaction rate constant (i.e. the errors associated with pressure and temperature variations, as well as azimuthal variations of the a-Si:H film thickness measured by field-emission scanning electron microscopy), while the horizontal error bar represents temperature controller error of 0.5°C. The slope of the linear regression fit in Figure 3 is proportional to the activation energy (01 ) and yields a value of E0 = 53.7 ± 2.9 kcal/mol, while the intercept corresponds to a preexponential factor of 1.5 × 1010 m/s. The error bar for the activation energy is calculated based on a 99% confidence interval. The R2 value of 0.99 for the linear regression shows that the experimentally measured reaction rate is well described by first-order kinetics. The predicted activation energy falls in the range of activation energies reported for hydrogen desorption reactions in heterogeneous silane pyrolysis (e.g., 45, 47, and 55 kcal/mol for reactions 3, 4, and 5 in Table 2, respectively)29,

30

. This suggests that hydrogen desorption from the surface to

produce new dangling-bonds is the rate-limiting step in HPCVD of a-Si:H within micron-sized confinements. This is consistent with the finding of previous kinetic studies at pressures below 1300 Pa, where hydrogen desorption was reported as the rate-limiting step for silicon film growth20. Additional insight into the silane pyrolysis reaction was obtained by performing reactive molecular dynamics simulations of thermal decomposition of silane at a silicon surface using ReaxFF

31-35

. In particular, ReaxFF simulations were performed to provide additional support

for the notion that hydrogen desorption from the surface is the rate-limiting step for film growth


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at high pressures in confined microcapillaries. ReaxFF is a bond-order based potential that includes a geometry-dependent, polarizable charge as well as van der Waals interactions - which together enable the method to simulate both the covalent and the non-bonded interactions involved in a system comprised of a silicon surface and silane molecules. Additional details about ReaxFF and force-field testing is provided in Appendix 1.

Figure 3. Arrhenius plot for the surface reaction rate constant versus furnace temperature.

Dynamic simulations of silane decomposition at a silicon surface were performed for a system comprised of 15 silane molecules (at 7 MPa pressure and 400 oC temperature in a 22X22X50 Å box) and a c-Si slab with 256 silicon atoms (as shown in the upper left hand side


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panel embedded in Figure 4). Details of silane energy minimization and c-Si slab preparation via NVT simulation are provided in Appendix 1. Starting with the initial configuration shown in the upper left hand corner of Figure 4, canonical ensemble simulations of silane decomposition on clean and hydrogen-passivated c-Si surfaces were performed and the number of unreacted silane molecules was monitored throughout the simulations.

Homogeneous reactions were not

observed during ReaxFF simulations of high-pressure silane pyrolysis on either clean or hydrogen-passivated surfaces, supporting the notion that, unlike silicon film growth in reactor geometries with very small surface-to-volume ratios, high-pressure CVD of silicon in confined domains is predominantly due to a heterogeneous reaction mechanism. Figure 4 shows the comparison between the simulation results for clean and hydrogenpassivated c-Si surfaces in terms of the percentage of unreacted silane molecules in the gas phase as a function of time. Starting from the initial configuration shown in the upper left hand side panel, almost all silane molecules decompose at the clean c-Si surface to form silicon-hydrides (mostly di- and tri-hydrides) within 10 ps of simulation time, as shown by the final configuration in the lower right hand side image in Figure 4. However, no reaction was observed on the hydrogen-passivated c-Si surface during NVT simulations, as shown by the upper right hand side panel in Figure 4.

Therefore, the results of these ReaxFF simulations suggest that silane

decomposition is mainly controlled by the availability of surface reaction sites (i.e. dangling bonds), which further supports the notion that hydrogen desorption from the surface to produce new dangling-bonds is the rate-limiting step in confined HPCVD of a-Si:H. The experimental study of Odden et al.16 on silane pyrolysis at pressures of 0.1-3.7 MPa showed that concentrations of the reactive gas-phase intermediates, i.e. Si2H6, Si3H8, and SiH2, were two, four, and twelve orders of magnitude smaller than silane concentration, respectively.


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The reaction rate constants for adsorption of Si2H6 and Si3H8 at the surface (reactions 2 and 3 in Table 2, respectively) are only one order of magnitude larger than that for adsorption of silane 27. Hence, assuming first-order kinetics, the rate of silane adsorption at the surface was 10 and 1000 times the adsorption rates of di- and tri-silane, respectively, in the experiments of Odden et al.16. Therefore, heterogeneous decomposition of silane was the dominant source of the deposited aSi:H film in their experiments, consistent with the results of our ReaxFF simulations.

Figure 4. Percent of unreacted silane molecules in the gas phase as a function of time for ReaxFF simulations on clean and hydrogen-passivated c-Si surfaces. The initial configuration of the ReaxFF system is shown in the upper left hand side panel. The final configurations for simulation with hydrogen-passivated and clean c-Si slabs are shown in the upper and lower right hand side panels, respectively.

Geometric confinement can play an important role in determining the dominant film growth mechanism. Previous studies have demonstrated that decomposition of SiH4 carried out in reactors with centimeter dimensions leads to formation of silicon particles in the gas phase that subsequently deposit at the substrate surface16, 36, 37. It is possible, however, to avoid particle


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formation in the gas phase and promote heterogeneous film growth by confining the precursor in extreme aspect-ratio reactors, such as silica fiber capillaries, as reported previously9,

38-39

.

Therefore, confining HPCVD reactors to capillaries with diameters on the order of 100 µm or smaller can lead to the predominance of heterogeneous pathway in a-Si:H film growth.

Conclusions We studied the kinetics of silane pyrolysis at pressures of 20-33 MPa within the extreme aspect-ratio confinement of a 5.9 µm-diameter microcapillary reactor with elevated wall temperature. Silane flowing in the reactor decomposed at the microcapillary wall to deposit a thin film of hydrogenated amorphous silicon. Film growth rates at different temperatures were determined by measuring the thickness of the deposited films using Scanning Electron Microscopy. The overall decomposition reaction was found to be first-order with respect to silane concentration, with activation energy and pre-exponential factor of 53.7 ± 2.9 kcal/mol and 1.5 × 1010 m/s, respectively. The resulting activation energy falls in the range of activation energies reported for hydrogen desorption reactions at c-Si surface, suggesting that hydrogen desorption from the surface is the rate-limiting step in film growth. To further investigate this observation, reactive molecular dynamics simulations of the thermal decomposition of silane on silicon surfaces were performed at 7 MPa pressure and 400°C temperature using ReaxFF. No gas phase reactions were observed during the ReaxFF simulations of high-pressure silane pyrolysis, in agreement with previously reported experimental results highlighting the effects of confinement. Furthermore, upon silane exposure of the simulated silicon surface, almost all of the available silicon surface sites (i.e. dangling bonds) were rapidly occupied with silicon-


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hydrides (mostly tri- and dihydrides), with further progress in film growth being contingent on the availability of surface dangling bonds resulting from hydrogen desorption from the surface.


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Corresponding Author *

Seyed Pouria Motevalian, Department of Chemical Engineering, The Pennsylvania State

University, University Park, PA, 16802 Email: [email protected]

Funding Sources NSF DMR-1420620

Acknowledgments This work was supported by the Penn State Materials Research Science and Engineering Center (NSF DMR-1420620). SPM wish to thank Mohammad Haghighi Parchini for insightful comments and valuable suggestions. Supporting Information Reactive Molecular Dynamic Simulations Using ReaxFF


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Kinetics of Silane Decomposition in High-Pressure ... - ACS Publications

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