Thermal-Hydrogen Promoted Selective Desorption and Enhanced Mobility of Adsorbed Radicals in Silicon Film Growth

Car-Parrinello simulations and static density-functional theory calculations reveal how hydrogen promotes growth of epitaxial, ordered Si films in plasma-enhanced chemical vapor deposition at low-temperature conditions where the exposed $\mathrm{Si}(001)\mathrm{\text{−}}(2\times 1)$ surface is fully hydrogenated. Thermal H atoms, indeed, are shown to selectively etch adsorbed silyl back to the gas phase or to form adsorbed species which can be easily incorporated into the crystal down to $T\sim 200°\mathrm{C}$ and start diffusing around $T\sim 300°\mathrm{C}$. Our results are well consistent with earlier experiments.

Figure 1

Cross section for the processes induced by a H atom hitting a ${\mathrm{SiH}}_3$ adsorbed on a dangling bond (inset 2Ha). The number preceding the minimum label indicates the number of H atoms in excess with respect to $\mathrm{Si}(001)\mathrm{\text{−}}(2\times 1):\mathrm{H}$. Different symbols and different colors (in the online version) are used to distinguish among the mechanisms. □ (light blue): a ${\mathrm{SiH}}_4$ molecule is abstracted by the incoming H. ▵ (yellow): a ${\mathrm{SiH}}_2$ adsorbed on the surface (1Hb) and a desorbed ${\mathrm{H}}_2$ are produced. × (red): the incoming H removes a H atom from an adjacent dimer leaving a reactive dangling bond (1Hc). ○ (white): recoil. The area associated with each single point is $0.25{\AA }^2$. The path for incorporation in the crystal or for diffusion along the dimer row is shown at the bottom. The energies of the displayed minima and of the transition states (in bold) are reported in eV and are computed relatively to the lowest-energy state with the same number of H atoms.

Figure 2

(a) Cross section for the processes induced by a H atom hitting a ${\mathrm{SiH}}_3$ adsorbed on the second layer. □ (light blue): the incoming H atom induces the ${\mathrm{SiH}}_3$ etching by formation of silane. × (red): a H atom from a near dimer is abstracted without affecting the adsorbed ${\mathrm{SiH}}_3$. + (green): a ${\mathrm{SiH}}_3$ adsorbed on a dangling bond (configuration 2Ha) is formed after the abstraction of a H atom from the adsorbate or from nearest dimers as sketched in panel (b). ○ (white): recoil. (b) From top to bottom: snapshots of a Car-Parrinello simulation where ${\mathrm{SiH}}_3$ immediately jumps to occupy the newly formed dangling bond.

Figure 3

Cross section for the processes induced by a H atom hitting a ${\mathrm{SiH}}_2$ molecule adsorbed in configuration 1Ha (inset). ▵ (yellow): a H atom is removed forming ${\mathrm{H}}_2$ and the configurations 0Ha or 0Hc, the latter immediately decaying to 0Hb after surmounting a barrier of 0.05 eV. ● (dark blue): the impinging H atom breaks a Si-Si bond forming configuration 2Hc (which can evolve to 2Ha surmounting a barrier of only $\sim 0.5\mathrm{eV}$) or 2Hd. ▿ (green): configuration 2Hb is obtained due to the adsorption of incoming H on the dangling bond. × (red): a H atom of the neighboring dimers is abstracted by the incoming H. ○ (white): recoil. + (purple) corresponds to a simulation where configuration 2Ha was directly reached in dynamics. The energies of minima and transition states (in bold) are indicated with respect to the corresponding lowest state, as in Fig. 1.