Search

SiGe thin films, such as hydrogenated amorphous silicon germanium (a-SiGe:H) and hydrogenated microcrystalline silicon germanium (μc-SiGe:H), are formed by plasma enhanced chemical vapor deposition (PECVD) methods from SiH4, GeH4, and H2. Thin film properties such as surface morphology, hydrogen content, and crystal structure are greatly affected by process parameters such as a substrate temperature, gas pressure, and gas composition. For example, SiH3 and GeH3 are dominant species when the SiGe thin film with low defect is deposited under low power density conditions. However, the structural defects are formed by SiH and SiH2 that are highly reactive species under high power density conditions. Although the density and flux of these highly reactive species are much lower than those of the dominant species, the electrical properties of the device are greatly affected by a few structural defects. Therefore, to understand the influence of the composition of each precursor on the thin film properties is important. The purpose of the present study is to clarify the influence of the composition of multiple precursors such as SiH3, GeH3, and H under different substrate temperatures on the surface morphology, hydrogen content, and crystal structure. We performed reactive force-field molecular dynamics (ReaxFF MD) simulations. An existing parameter set was modified to match the dissociation energies in gaseous species because bond formation and breaking are particularly important in this study. The substrate was Si (100)– (2×1), and the size of the simulation box was 30.72 Å× 26.88 Å× 45.00 Å. A periodic boundary condition was applied in the x and y directions, and a fixed boundary condition was applied in the z direction. The first bottom layer of the substrate was fixed to prevent vibrations of the substrate up and down. The second and third layers were controlled constantly at desired temperature by the Berendsen thermostat. The temperature was not controlled on the fourth and subsequent layers to prevent direct influences of changing the velocity of atom due to the temperature control on the surface reaction. The substrate temperature was set at 800 K, 1000 K, 1300 K, and 1600 K. 2000 SiH3 molecules with the velocity corresponding to 1300 K in the direction towards the substrate were deposited one by one onto the substrate surface every 7.5 ps. The initial position of the gaseous species was set at 10 Å above the top of substrate Si atoms, and the in-plane (x-y) position were set randomly. The Velocity-Verlet algorithm was used for time integrations corresponding to 0.25 fs time step. A pre-thermal annealing for 7.5 ps and a post-thermal annealing for 60 ps were performed before and after the deposition to fully equilibrate the substrate and deposited thin films. Figure shows front views of the final configurations and results of analysis when the substrate temperature is 800 K, 1000 K, 1300 K, and 1600 K. The yellow, blue, and red represent substrate Si atoms, added Si atoms, and added H atoms, respectively. Gray shown in the figure are gas-solid surfaces. The surface construction algorithm was used to calculate a surface area that is closely related to the surface morphology. The hydrogen content was calculated using the select type algorithm. The structure identification algorithm for the crystal structure was used to characterize the structure of simulated films. These analyses were carried out by the Open visualization tool (OVITO). The surface area decreases as the substrate temperature increases, indicating that the formation of smoother surface at high temperature. The diffusivity of a SiH3 molecule that reached to the surface increases as the substrate temperature increases, and the atomic scale valleys are filled. This tendency is in good agreement with relevant studies. The hydrogen content was calculated by dividing the number of added H atoms by the total number of atoms existing in the system. The hydrogen content decreases as the substrate temperature increases because a hydrogen desorption is promoted as the substrate temperature increases. Figure 1

We simulated the growth of SiGe alloys by using reactive force-field (ReaxFF) molecular dynamics simulations in binary systems of SiH x and GeH x and identify the effect of gaseous species on the compositions and crystallinity in alloys. The compositions in alloys could be estimated if compositions of gaseous species can be identified because of the observation of the linear increase in Ge content in alloys. Supplying GeH 2 decreased the crystallinity compared to GeH 3 . The mechanism is described by the mobility and reactivity of the gaseous species and surface structures. These results suggest that the mobility and reactivity of both surface and gaseous species are essential in identifying the effect of gaseous species on composition and crystallinity.

In order to form a SiGe thin film by chemical vapor deposition (CVD) with a suitable quality for advanced devices, the relationships between materials/process and structure/composition are needed to be clarified at the atomic level. We simulated SiGe CVD by using reactive force-field (ReaxFF) molecular dynamics simulations, especially on binary systems of SiHx + GeHx, and derived the influence of the substrate temperature and these ratios of gaseous species on the crystallinity and compositions in the thin films. The crystallinity increases as the substrate temperature increases, and the lowest crystallinity is obtained at the ratios of gaseous species 0.5 and 0.7 for the SiH3 and SiH2, respectively. As the substrate temperature increases, the hydrogen content decreases while Si and Ge content tend to increase. These trends can be seen in relevant studies. Through this simulation we successfully observe that the reactivity of gaseous species greatly affects the crystallinity and compositions in the thin films.

We calculate a deposition process of hydrogenated amorphous silicon (a-Si:H) films on a silicon (100) substrate by reactive force-field molecular dynamics simulations. The influences of (a) substrate temperatures and (b) coverage of hydrogen atoms on the substrate on the adsorption probability are investigated, and it is found out that (a) the adsorption probability is almost constant for SiH 2 and SiH 3 , but decrease with increase in the substrate temperature for SiH 4 , (b) it decreases with the increase in hydrogen coverage.

We consider a Couette flow of a rarefied Ar gas with heat transfer between two wall surfaces and investigate the scattering behavior of gas molecules reflected either at a clean Pt surface or at a surface contaminated with adsorbates. Water molecules abundantly present in the atmosphere were adopted as the adsorbates. The reflection of gas molecules on the lower wall surface was simulated by Molecular Dynamics (MD) method to obtain accommodation coefficients and velocity distribution functions of gas molecules. We applied the modified reflection model of gas molecule and investigated the velocity distribution functions of the model by comparing the MD results to verify the validity. The accommodation coefficients obtained by the MD method depend on the number of adsorbed water molecules on the lower wall surface. Specifically, tangential momentum accommodation coefficient (TMAC) tended to increase and then decrease with the increase in adsorbed water molecules, but normal momentum accommodation coefficient (NMAC) tended to decrease monotonically. The velocity distribution functions of the modified reflection model approximately show the good agreement with the MD calculation but the degree of coincidence depends on the speed difference between the upper and lower wall surfaces, and the number of adsorbed water molecules on the surface.