Si-Si Bond Breaking Research Articles

Thermodynamics and kinetics of hydrogen evolution in hydrogenated amorphous silicon films

Plasma Physics Corporation, P.O. Box 548, Locust Valley, NY 11650 The changes in enthalpy and entropy due to hydrogen evolution in hydrogenated amorphous silicon films were measured by differential scanning calorimetry (DSC). Hydrogen evolution was associated with an endothermic DSC peak, as supported by thermogravimetric analysis and evolved gas analysis. The en-thalpy and entropy changes of hydrogen evolution increased with heating rate and hydrogen content, because the evolution involved not only Si-H bond breaking, but also defect formation (such as Si-Si bond breaking), which was enhanced by a high flow of evolving hydrogen. In contrast, the activation energy of hydrogen evolution was controlled by the doping rather than the hydrogen content, because doping affected the Si-H bonding, which in turn affected the state before hydrogen evolution. Crystallization, which occurred at tempera-tures higher than hydrogen evolution, was delayed for the amorphous silicon film in a higher disordered state after hydrogen evolution, suggesting that hydrogen content influenced the crystallization process.

Thermodynamics and Kinetics of Hydrogen Evolution in Hydrogenated Amorphous Silicon Films

ABSTRACTThe enthalpy (endothermic) of hydrogen evolution from p-type (boron doped) amorphous silicon with 17 at. % H was 4.8, 10.3, 15.8 and 17.3 kJ/g, the evolution temperature was 585, 606, 625 and 644 °C and the entropy of evolution was 5.6, 11.7, 17.5 and 18.9 J/g.K at heating rates of 5, 10, 20 and 30 °C/min respectively. That the enthalpy and entropy increased with heating rate means that the evolution involves not only Si-H bond breaking, but also Si-Si bond breaking and other defect formation. The Si-Si bond breaking and defect formation were enhanced at high heating rates, which caused high rates of hydrogen evolution. For n-type (phosphorous-doped) and intrinsic amorphous silicon with 25 and 23 at. % H respectively, the enthalpy and entropy of hydrogen evolution were higher than the p-type case, due to severe defect formation resulting from the higher hydrogen content. The activation energy of hydrogen evolution was 1.38, 2.5 and 4 kJ/g for the p-type, intrinsic and n-type materials respectively. Crystallization which occurred at temperatures higher than hydrogen evolution, was delayed for the amorphous silicon film in a higher disordered state after hydrogen evolution, suggesting that hydrogen evolution influenced the crystallization process.

Hydrogen desorption rate and surface hydrogen coverage during isothermal annealing for Si 2H 6-adsorbed Si(100) surfaces

The hydrogen desorption kinetics were investigated from the viewpoint of hydrogen-unoccupied site distributed on Si 2H 6-adsorbed Si(100) surfaces during isothermal annealing. The surface electronic state due to Si dimer dangling bonds was observed in situ by ultraviolet photoelectron spectroscopy. The hydrogen desorption was found to be a first-order reaction with an small activation energy of 21.9 kcal/mol, in comparison to the value obtained by thermal desorption spectroscopy (TDS), 47–58 kcal/mol. To interpret this discrepancy, we proposed a desorption reaction model, in which H 2 formation from a paired monohydride on a dimer, which proceeds dominantly during isothermal disorption, is disturbed by dimer Si-Si bond breaking due to rapid temperature elevating in TDS, and therefore an additional energy for hydrogen to hop between resultant isolated monohydrides and form dihydride, 34.5±4.6 kcal/mol, is necessary in TDS for H 2 desorption.

Hydrogen Diffusion in Undoped and B-Doped a-Si1-xCx:H

ABSTRACTA deuterium secondary ion mass spectrometry (SIMS) study of hydrogen diffusion in undoped and boron-doped a-Si0.86C0.4:H deposited by an electron cyclotron resonance plasma is described. The undoped films deposited at 250°C clearly indicated deuterium-hydrogen interdiffusion at T ≥ 350°C. The dispersion parameter a of the power-law time dependent diffusion constant D = D00(ωt)−α decreased from ∼0.3 at T = 350 and 400°C to ∼0.1 at 450°C, and the activation energy for a diffusion length of 1000 Å was ∼1.0 eV. These results are discussed in relation to previous studies of a-Si:H. The diffusion in ∼0.2 and ∼0.6 at.% B-doped a-Si0.86C0.14:H sharply differs from that in B-doped a-Si:H, where an enhancement of up to ∼103 was previously observed. In doped a-Si0.86C0.14:H, the diffusion of most of the H atoms is strongly suppressed, but a small fraction undergoes fast diffusion. IR Measurements indicate that the B-doping reduces the bulk-like Si-H stretch vibration at ∼2000 cm1. Upon annealing, the Si-CHn and C-H wag modes at ∼780 and ∼1000 cm−1, resp., increase, while the 640 and ∼2000 cm1 Si-H wag and stretch modes, resp., weaken, indicating transfer of hydrogen from Si- to C-bonds, in which the H atoms are apparently deeply trapped. As in a-Si:H, the fast diffusion component is apparently due to carrier recombination-enhanced weak Si-Si bond breaking. The results suggest that B-doping also induces microvoids and enhances the rate of breaking of weak Si-C bonds, leading to enhanced trapping of H.

DEFECTS IN AMORPHOUS Si:H — THE REHYBRIDIZED TWO-SITE (RTS) MODEL

A comprehensive model for the metastable defects in amorphous Si:H is developed by adapting a recent theory for several kinds of defects in crystalline semiconductors, particularly the DX center in AlGaAs. This new model accounts in a unified way for all of the major observations of defects induced by light, quenching, doping, or compensation; as well as for their anneal. The stretched-exponential time dependence of defect densities with light exposure or annealing, and saturation of the density are also explained. This model is based on foreign atoms rather than on breaking of Si-Si bonds, and in undoped materials it is suggested that unintentional impurities are the source.

The adsorption of disilane on Si(111)-7 × 7 as studied by multiple internal reflection spectroscopy

The adsorption of Si 2H 6 on Si(111)-7 × 7 has been studied between 120 and 500 K using multiple internal reflection infrared spectroscopy. Si 2H 6 adsorbs dissociatively on the Si(111)−7 × 7 surface at temperatures as low as 120 K. The adsorption occurs by Si-Si bond scission resulting in the formation of SiH 3 species. Evidence for molecular multilayer formation at 120 K is indicated by the linear increase in integrated intensity of the absorption features attributed to molecular Si 2H 6 with increasing exposure and by significant lineshape differences as compared with chemisorbed SiH 3. By use of the polarization selection rules for infrared spectroscopy, we can identify the SiH 3 symmetric stretch at 2154 cm −1 and the asymmetric stretch at 2130 cm −1 at 150 K. Ab initio cluster calculations suggest that a highly strained SiH 3 surface molecule with an expanded Si-Si-H bond angle will result in the observed splitting that we observe for the two modes. At or above 250 K the infrared spectra suggest that Si-H bond scission occurs in conjunction with Si-Si bond breaking. Exposures of disilane at 400 K result in a surface that is dominated by surface monohydrides with some higher hydrides possibly present. At an adsorption temperature of 500 K the infrared spectra show only one strong absorption feature at 2090 cm −1 with significant intensity parallel to the surface indicating the presence of only tilted silicon monohydride.

Defects in amorphous silicon — Extrinsic or intrinsic?

Competing types of models of dangling-bond defects in hydrogenated amorphous silicon are analyzed in the light of recent advances in descriptions of light-induced defects such as saturation of their density and stretched-exponential kinetics. The familiar intrinsic models (Si-Si bond breaking) are found to have several serious problems—more than generally recognized—while extrinsic models have few. In particular, for the rehybridized two-site model based on foreign atoms, the one important need is to explain defects in undoped material.

Time and temperature dependence of instability mechanisms in amorphous silicon thin-film transistors

We have measured the time and temperature dependence of the two prominent instability mechanisms in amorphous silicon thin-film transistors, namely, the creation of metastable states in the a-Si:H and the charge trapping in the silicon nitride gate insulator. The state creation process shows a power law time dependence and is thermally activated. The charge trapping process shows a logarithmic time dependence and has a very small temperature dependence. The results for the state creation process are consistent with a model of Si dangling bond formation in the bulk a-Si:H due to weak SiSi bond breaking stabilized by diffusive hydrogen motion. The logarithmic time dependence and weak temperature dependence for the charge trapping in the nitride suggest that the charge injection from the a-Si:H to the nitride is the rate limiting step and not subsequent conduction in the nitride.

Weak bond-dangling bond conversion in amorphous silicon

Abstract A microscopic model based on charge-induced breaking of weak Si-Si bonds is proposed as an explanation for the experimentally observed increase of the dangling-bond density in a-Si: H upon illumination, charge injection, and doping. Energetic aspects of the conversion between weak and dangling bonds and implications for the electronic density of states of a-Si: H are described, and the relation of the model to negative4 defects in this material is discussed.

Capacitance Studies of Metastable Defect Creation in Hydrogenated Amorphous Silicon

ABSTRACTWe have studied the light induced instability problem in hydrogenated amorphous silicon using junction capacitance techniques. These techniques are used to examine specific changes in the density of gap states, and occupation of gap states, for undoped a-Si:H samples after light saturation and for a series of partial anneal “states” which culminate in the original dark annealed state (state A). We find that the observed changes in the metastable occupied and unoccupied defects contradict the Si-Si bond breaking model and indicate at least two defect creation processes. In several samples we also find clear evidence that the metastable defect distribution near midgap has a slightly different energy distribution than the stable deep state (dangling bond) distribution. At the same time, these results seem to be qualitatively consistent with many aspects of recent ESR and optical absorption studies of metastable defect creation. We discuss these findings in terms of alternative possible microscopic models for metastable effects in a-Si:H.