Author: Saketh Bharadwaja Publisher: ISBN: Category : Amorphous substances Languages : en Pages : 103
Book Description
Amorphous silicon (a-Si) thin-film solar cells grown via plasma-enhanced chemical vapor deposition (PECVD) are of significant technological interest. As a result, there is significant interest in understanding the physical processes which control the a-Si thin-film structure and morphology. In particular, since the early stages of a-Si growth on the silicon oxide substrate play a key role in determining the subsequent evolution, it is important to obtain a better understanding of this stage of a-Si growth. The key objectives of the work presented in this thesis are to obtain a better understanding of the structure and morphology of the silicon-oxide substrate used in a-Si growth via PECVD as well as of the key processes of Si diffusion on the substrate which control the nucleation of a-Si islands. In particular, motivated by experimental and simulation results, we have carried out molecular dynamics simulations of the formation of a thermal silicon oxide substrate (corresponding to oxide formation at high-temperature) as well as of the room-temperature oxidation of "native" silicon oxide thin-films. In addition, for the case of a native silicon oxide surface, we have studied the binding energies, binding sites, and diffusion barriers for Si diffusion in order to gain insight into the critical length-scales for a-Si island formation. In the case of thermal silicon oxide formed at high temperature, our molecular dynamics simulations were carried out using an effective Munetoh potential which takes into account the "average" charge transfer as well as bond angles and energies. In this case, due to the relatively high temperature the surface was found to be extremely rough and highly disordered, while the thin-film structure was found to be amorphous. In contrast, in our simulations of the formation of native silicon oxide thin-films at room temperature, a more sophisticated ReaxFF potential was used which properly takes into account the effects of O2 molecular dissociation and rebinding at the surface, as well as the long-range Coulomb interaction and local charge-transfer. We have also studied the binding and diffusion of Si atoms for this case in order to try to explain recent experiments and simulations in which it was shown that 3D a-Si islands with a typical island diameter of approximately 30 A are formed in the early stages of growth. For the case of native silicon-oxide our results for the oxygen penetration profile and surface roughness were found to be in good qualitative agreement with experiments. Our results also indicate that while the typical binding energies for Si adatoms on the SiO2 surface are significantly lower than for Si/Si(100), due to the disordered structure of the surface the barriers for diffusion are typically significantly higher. As a result, at the deposition temperature of 200oC used in low-temperature PECVD, these sites may act like "trapping sites" for deposited Si atoms. We note that these results are consistent with recent experiments on the relaxation of SiO2 microstructures at high temperatures. However, they also imply that the characteristic length-scale for 3D islands in the early stages of a-Si growth via PECVD cannot be explained by a combination of homogenous diffusion and a critical island-size, as is typically found in epitaxial growth.
Author: A. Borghesi Publisher: Newnes ISBN: 044459633X Category : Technology & Engineering Languages : en Pages : 580
Book Description
Containing over 200 papers, this volume contains the proceedings of two symposia in the E-MRS series. Part I presents a state of the art review of the topic - Carbon, Hydrogen, Nitrogen and Oxygen in Silicon and in Other Elemental Semiconductors. There was strong representation from the industrial laboratories, illustrating that the topic is highly relevant for the semiconductor industry. The second part of the volume deals with a topic which is undergoing a process of convergence with two concerns that are more particularly application oriented. Firstly, the advanced instrumentation which, through the use of atomic force and tunnel microscopies, high resolution electron microscopy and other high precision analysis instruments, now allows for direct access to atomic mechanisms. Secondly, the technological development which in all areas of applications, particularly in the field of microelectronics and microsystems, requires as a result of the miniaturisation race, a precise mastery of the microscopic mechanisms.
Author: Jun Wang Publisher: ISBN: Category : Languages : en Pages :
Book Description
The properties of a silicon adatom on Si(001) surface and around three single-layer steps, namely type A (S$rmsb A$), bonded type B (S$rmsb B$) and nonbonded type B (S$rmsb{Bspprime}$), on Si(001) have been studied, using a modified empirical and first-principle methods. The results presented in this thesis, providing the atomistic scale details of the adatom-surface interactions, represent an improved understanding to the growth of a technologically important material and surface, Si(001). These results indicate that single-layer steps on Si(001) do not serve as good sinks for the lone adatoms. The presence of the step on the surface may affect the growth behavior of the surface by changing the nucleation rate of the surface dimers and diffusivity of the adatoms. It is shown that there is a moderate additional energy barrier (0.2 $pm$ 0.1 eV) to cross the S$rmsb A$ step. The dimer-top lattice site on the lower terrace adjacent to the step edge is stabilized (by 0.15 $pm$ 0.1 eV) with respect to the flat surface result although the most stable binding sites near the step are unaffected. This behavior can be understood based on the disruption of dimer tilt near the step. The results suggest that adatoms are more likely to stop on lattice sites at the S$rmsb A$ step edge than on lattice sites on the open surface. This may affect the relative dimer formation rate near the step with respect to the behavior on the flat surface even in the absence of a clear change in binding energy. The effect of the S$rmsb A$ step terrace edge on adatom behavior is very short ranged and weak. This is consistent with the relatively small strain field and lack of change in dangling bond density associated with the step edge. The growth of the S$rmsb A$ step is proposed to be limited by nucleation of new dimer rows along the step edge. The results suggest that the S$rmsb{Bspprime}$ step should accumulate adatoms rapidly both from above and below. The energy barrier to cross the S$rmsb{Bspprime}$ step is $sim$0.2 eV greater than for diffusion on the flat surface. The binding sites along the S$rmsb{Bspprime}$ step edge are similar to those on the flat surface but are paired and connected by a low-energy diffusion pathway that may facilitate formation of dimers along the step edge. The S$rmsb B$ step attracts adatoms ${sim}0.5pm0.2$ eV more strongly than any other site on the surface. However, these sites are relatively inaccessible due to surrounding high energy barriers. Based on the results, the upper side of the S$rmsb B$ step should be repulsive to adatoms. The diffusion barrier for adatoms approaching the step rises and the binding sites become less favorable there. Hence, growth of the S$rmsb B$ step is probably much slower than the S$rmsb{Bspprime}$ step, which explains its observed predominance on Si(001) surfaces. It is proposed that growths of both B type steps are flux limited and hence are highly temperature dependent.
Author: David J. Fisher Publisher: Trans Tech Publications Ltd ISBN: 3038130311 Category : Technology & Engineering Languages : en Pages : 218
Book Description
This collection of abstracts of experimental and theoretical papers on the subject of diffusion in silicon is intended to complement earlier volumes (DDF153-155) which covered the previous decades work on the same topic. The abstracts are grouped according to the diffusing species in question. The latter comprise Ag, Al, As, Au, B, Ba, Be, C, Ca, Cl, Co, Cr, Cu, Er, F, Fe, Ge, H, He, Hf, In, Ir, K, Mg, Mn, Mo, N, Na, Nb, Ni, O, P, Pb, Pt, Rb, Sb, Se, Si, SiH3, Sn, Ti, V, Yb and Zn with regard to bulk diffusion, Ag, Au, Ba, Cl, Cu, Er, F, Ga, Ge, In, O, Pb, Sb, Si, SiH3, Sn and Y with regard to surface diffusion, H with regard to grain-boundary diffusion, and self-diffusion in liquid Si.
Author: Saketh Bharadwaja
Author: A. Borghesi
Author: Beat Sahli
Author: Jun Wang
Author: 
Author: Dan Mu
Author: Sweta Goel
Author: S. T. Pantelides
Author: 
Author: David J. Fisher