B. N. Davidson

Incorporation of polyhydride bonding groups into thin films of hydrogenated amrophous silicon (a-;Si:H)

Abstract We show for several different deposition methods, including conventional glow-discharge (GD), reactive magnetron sputtering (RMS), and remote plasma-enhanced chemical-vapor deposition (remote PECVD), that the distribution of hydrogen between monohydride and polyhydride groups is a function of the total amount of bonded hydrogen in the film, and is not uniquely determined by the substrate temperature, or any other single deposition process variable. We demonstrate that RMS and remote PECVD offer advantages over the GD process in control of polyhydride formation in a-Si:H films.

Ultrafast recombination and trapping in amorphous silicon

Abstract We have studied time-resolved reflectivity changes induced by femtosecond laser pulses in a-Si and a-Si:H thin-films. By varying pump-power, we have identified a non-radiative recombination process which controls the free-carrier density, N, on a picosecond time scale for N>5 ×1018cm−3 in a-Si:H and >5×1019 cm−3 in a-Si. At lower carrier densities, transients are controlled by trapping of free-carriers.

Low-temperature deposition of hydrogenated amorphous silicon (a-Si:H): Control of polyhydride incorporation and its effects on thin film properties

It is shown how remote plasma-enhanced chemical vapor deposition (remote PECVD) and reactive magnetron sputtering (RMS) can be adopted to limit the relative fraction of polyhydride bonding groups for low substrate temperature depositions (Ts < 200 °C) of a-Si:H. These process modifications provide new options for the processing of device structures by combinations of low-temperature deposition and post-deposition, short-term thermal annealing. It is shown for several different deposition processes, including conventional glow-discharge deposition (GD), RMS, and remote PECVD, that the distribution of bonded hydrogen between monohydride and polyhydride bonding groups is determined by the total amount of bonded hydrogen [H] in the film and is not intrinsically related to Ts, or any other deposition parameter. A statistical model is presented, which provides a basis for translating process-dependent representations of data for the relative monohydride and polyhydride fractions to a universally obeyed scaling relationship in which the independent variable is [H]. Finally, selected properties of low-Ts films formed by RMS and remote PECVD are briefly discussed.

Effect of the Local Disorder in a-Si on the Electronic Density of States at the Band Edges.

We have systematically investigated the formation of electronic states in the region of the conduction and valence band edges of a Si as functions of variations in the bond angle distributions. Local Density of States (LDOS) for Si atoms in disordered environments have been calculated using the cluster Bethe lattice method with a tight-binding Hamiltonian containing both first and second nearest neighbor interaction terms. LDOS for atoms with bond angle dis ortions in the nearest neighbor and second neighbor shells are compared and contrasted, both showing an influence on the LDOS near the gap. We also consider the role of the second neighbor term in the Hamiltonian by comparing the DOS for a distoned infinite Bethe lattice using Hamiltonians with and without the second neighbor interactions. It is found that in this case the second neighbor interaction terms cause greater conduction band tailing than using the nearest neighbor interaction terms alone.

Energy Differences Between the Si and the Ge Dangling Bond Defects in a-Si 1-x Ge x Alloys

We have investigated the difference in the electronic energies of neutral Si and Ge dangling bond states in undoped a-Si 1-x Ge x alloys as a function of the alloy composition, x, and local bond-angle distortions. The local density of states, LDOS, in a-Si 1-x Ge x alloys has been calculated using nearest-neighbor interactions, and employing the Cluster Bethe Lattice method. We conclude that for ideal, tetrahedrally bonded amorphous semiconductors alloys, the Ge dangling bond energy is lower than that of Si dangling bonds by ∼ 0.13 eV, independent of the specific nearest neighbors to the dangling bond (3 Si-atoms, 2 Si-atoms and 1 Ge-atom, etc.), but that the spread in dangling bond energies associated bond-angle variations of the order of 6–8 degrees can be larger than this energy difference (∼0.3 eV or greater). This means that structural disorder, rather than chemical disorder causes Si and Ge-atom dangling bond states to overlap in their energy distributions.

Effect of the local disorder in a-Si on the electronic density of states near the band edges

We have systematically investigated the formation of electronic states in the region of the conduction and valence band edges of a-Si as functions of variations in the bond and dihedral angle distributions. Local Densities of States (LDOS) for Si atoms in disordered environments have been calculated using the cluster Bethe lattice method with a Hamiltonian containing both first and second nearest neighbor interaction terms. We conclude that the changes in orbital overlap, incurred from rotations about the axes defining the dihedral angle distortions, are the origin of the effect of dihedral angle disorder on the electronic states near the band gap.

Localized states in amorphous Si and Si,Ge alloys

This paper discusses applications of the tight‐binding method to: i) localized anti‐bonding states of Si‐H groups in a‐Si:H, and ii) dangling bond defect states in a‐Si,Ge:H alloys. The a‐Si:H calculations demonstrate that anti‐bonding states of Si‐H groups with bond‐angle distortions of ∼16°–20° or more can be localized below the conduction band edge, and display the trapping properties of floating‐bond defect states. The calculations for the a‐Si,Ge:H alloys show that the average‐alloy bond‐angle disorder, ∼6–8°, can induce a spread in the energies of Si and Ge dangling bond states of ∼0.3 eV, that is larger than that due to chemically‐induced splittings, 0.11–0.15 eV, which include the effects of different distributions of nearest‐neighbor Si and/or Ge‐atoms. The bond‐angle disorder induced dispersion leads to a spectral overlap of the Ge and Si dangling bonds, so that both types of defects can be observed in alloy samples.

QUANTUM MOLECULAR DYNAMICS OF CLUSTERS

Recent quantum molecular dynamics studies of Al and carbon clusters are described. For Al, we focused on the 13- and 55-atom clusters, which can assume perfect icosahedral and cubic structures. However, the distortions from these ideal structures are substantial. For the 55-atom cluster, several inequivalent but nearly energetically degenerate structures are found, due to the short range of the screened interatomic interactions. For solid C60, it is found that the soccerball structure is well-preserved in the solid. The intermolecular interactions are so weak that the individual C60 can rotate at relatively low temperatures. At high temperatures vibrations cause large distortions, but the cage structure is still preserved. The C60 isomer containing two pairs of adjacent five-fold rings has a binding energy only 1.6 eV smaller than that of perfect C60, but the transformation between these two structures is hindered by a 5.5 eV barrier. It thus requires high temperatures and long annealing times. High temperatures are also needed for the transformation of the lowest energy C20 isomer, a dodecahedron, to a corannulene structure, which can be thought of as a fragment of C60. The corannulene structure is a natural precursor for the formation of C60. These results are consistent with the experimental findings that high temperatures are necessary for the formation of substantial quantities of C60. A formulation and the first applications of a new, real space quantum molecular dynamics method, particularly suitable for cluster calculations, are also described.

STRUCTURE, DYNAMICS, AND FORMATION OF CARBON AND ALUMINUM CLUSTERS

The results of recent ab initio molecular dynamics studies of C and Al clusters are presented. The simulations have shown that C60 molecular structure is well preserved in the solid and that the individual C60 molecules start to rotate at relatively low temperatures. Our results are in very good agreement with NMR, photoemission, and neutron scattering data. At high temperatures C60 undergoes large amplitude soccerball-rugbyball oscillations, but the cage structure is still preserved. The C60 isomer containing two pairs of adjacent pentagons has a binding energy only 1.6 eV smaller than that of perfect C60, but high temperatures and long annealing times are required for the transformation between these two structures. Its activation energy is 5.4 eV. We have also studied the various isomers of C20, since it could form the smallest possible fullerene. At T=0, the lowest energy isomer is indeed a dodecohedral structure. However, high temperatures favor the corannulene structure, which is a perfect precursor...

Free carrier absorption and the transient optical properties of amorphous silicon thin films: A model including time dependent free carrier, and static and dispersive interband contributions to the complex dielectric constant

Abstract A model for free-carrier relaxation, including dispersion in the dielectric function of the host is developed for a-Si and a-Si:H. The model is applicable for probe-beam pulse energies in the spectral regime of the absorption edge, from ∼1–3 eV.