Si SiH Research Articles

Radical species involved in hotwire (catalytic) deposition of hydrogenated amorphous silicon

Threshold ionization mass spectroscopy is used to measure the radicals that cause deposition of hydrogenated amorphous silicon by “hotwire” (HW), or “catalytic,” chemical vapor deposition. We provide the probability of silane (SiH 4) decomposition on the HW, and of Si and H release from the HW. The depositing radicals, and H atoms, are measured versus conditions to obtain their radical-silane reaction rates and contributions to film growth. A 0.01–3 Pa range of silane pressures and 1400–2400 K range of HW temperatures were studied, encompassing optimum device production conditions. Si 2H 2 is the primary depositing radical under optimum conditions, accompanied by a few percent of Si atoms and a lot of H-atom reactions. Negligible SiH n radical production is observed and only a small flux of disilane is produced, but at the higher pressures some Si 3H n is observed. A Si–SiH 4 reaction rate coefficient of 1.65 ⁎ 10 − 11 cm 3/s and a H + SiH 4 reaction rate coefficient of 5 ⁎ 10 − 14 cm 3/s are measured.

Temperature programmed desorption of molecular hydrogen from a Si(100)-2×1 surface: Theory and experiment

New experimental temperature programmed desorption (TPD) data have been obtained under carefully controlled conditions for atomic deuterium on the single crystal Si(100)-2×1 surface. A wide range of coverages from Θ=1.5 to 0.05 ML was used. A kinetic lattice-gas model has been developed which describes atomic hydrogen (or deuterium) adsorbed on the Si(100)-2×1 surface in terms of four basic units: dihydride (SiH2), doubly occupied dimers (H–Si–Si–H), singly occupied dimers (Si–SiH), and unoccupied dimers (Si=Si). The equilibria between these species have been determined by considering both the lattice partition functions and the vibrational partition functions associated with the Si–H bonds. By using a quasiequilibrium approximation and two competing desorption routes corresponding to formation of the β1 and β2 peaks, the TPD spectra for hydrogen (deuterium) molecules are determined and compared with the new experimental data. Fitting the experimental curves with the simulated data from the aforementioned model showed that the desorption process which leads to the β1 peak obeys first-order kinetics with an A factor of 2×1015 s−1 and activation energy of 57 kcal mole−1, whereas the process giving the β2 peak follows second-order kinetics with an activation energy of 47 kcal mole−1 and an A factor (expressed in 1st order units) of 3×1015 s−1.

Unimolecular dissociation dynamics of disilane

The unimolecular dissociation dynamics of disilane are investigated using classical trajectory methods with a global potential-energy surface fitted to the available experimental data and the results of various ab initio calculations. The potential surface is written as the sum of 52 many-body terms containing 86 adjustable parameters which are fitted to experimental and/or calculated data for stationary point geometries, fundamental vibrational frequencies, reaction endo- and exothermicities, and potential-energy barrier heights for reactions of disilane and molecules derived from disilane. In general, the equilibrium bond lengths and angles for Si2 H6 , Si2 H5 , H3 Si–SiH, H2 Si=SiH2, H2 Si=SiH, H2 Si=Si, HSi=Si, Si2 , H2 , and SiH2 given by the global potential agree with ab initio results to within 0.03 A and 2°, respectively, or better. The predicted heats of reaction for 13 reactions involving disilane or its derivatives are in good accord with the experimental and ab initio results. The average absolute deviation is 3.55 kcal/mol. The average absolute difference between the normal-mode frequencies given by the global potential for Si2 H6 , Si2 H5, and H3 Si–SiH and those obtained from scaled MP4 calculations are 58.7, 52.1, and 62.8 cm−1 , respectively. If two low-frequency Si–Si–H deformation modes for each of these molecules are omitted from consideration, the average absolute differences are all in the range 34–36 cm−1 . The calculated barrier height for the hydrogen-atom transfer process leading to SiH4 +SiH2 products is 56.7 kcal/mol. For three- and four-center H2 elimination reactions, the barrier heights given by the global surface are 60.1 and 91.1 kcal/mol, respectively. These values are all within 1.2 kcal/mol of the results obtained by Ho et al. from MP4 calculations. The Si2 H6 dissociation dynamics at seven internal energies ranging from 5.31 to 9.31 eV have been investigated. At low internal energy, dissociation leading to SiH4 +SiH2 dominates the dynamics. At internal energies in the range 5.31≤E≤6.31 eV, the various Si2 H6 decomposition channels are, in order of importance, hydrogen-atom transfer leading to SiH4 +SiH2 , Si–Si bond rupture giving two SiH3 radicals, three-center H2 elimination, Si–H bond rupture to give Si2 H5 +H, and four-center H2 elimination to give H2 Si=SiH2 . At higher internal energies, entropy effects cause an inversion of this ordering such that Si–Si bond rupture becomes the major decomposition channel followed by three-center H2 elimination, SiH4 +SiH2 formation, Si–H bond rupture, and four-center H2 elimination. The present results suggest that the formation of disilene in disilane pyrrolysis occurs predominantly via the formation of H3 Si–SiH from three-center H2 elimination followed by a low-barrier hydrogen transfer process. For all decomposition channels, most of the available energy is partitioned in vibrational modes of the products. To a large extent, product energy partitioning is found to be governed by statistical considerations. Exceptions to this generalization are found in three-center H2 elimination and for any product which involves the formation of a new bond. We find that while three-center H2 elimination is a concerted reaction, it probably does not occur along a symmetric pathway. Hydrogen transfer to form SiH4 +SiH2 is found to be a concerted process, but four-center H2 elimination involves the rupture of one Si–H bond followed by hydrogen transfer and a subsequent H2 abstraction reaction to give the H2 +H2 Si=SiH2 products.