Large supercell molecular dynamics study of defect formation in hydrogenated amorphous silicon

We have performed molecular dynamics simulations of the defect formation associated with the Staebler-Wronski (SW) effect in a-Si:H using 224 and 231 atom supercells and employing semiempirical Si-Si and Si-H total energy functionals. The role of hydrogen in the defect formation within the bond breaking model of the SW effect has been investigated for both large supercells. The results suggest that, within this model, H can be important in weakening the normal Si-Si bonds which break to produce defects in the SW effect.     

Thermally induced defect photoluminescence in hydrogenated amorphous silicon

The intrinsic defect photoluminescence of hydrogenated amorphous silicon (a-Si:H) films has been investigated at high intensities of optical pumping that lead to heating of the film. It has been revealed that, for short heating times, the intensity of the defect photoluminescence increases exponentially with an increase in the temperature with an activation energy of 0.85 eV, which is considerably higher than the activation energy (∼0.2 eV) determined from experiments on classical annealing. This and other experimental results on the temperature dependence of the intensity and kinetics of the defect photoluminescence have been explained in terms of the “hydrogen glass” model by thermally induced generation of intrinsic defects in amorphous silicon. The results of the calculations are in good agreement with the experimental data on the defect photoluminescence that reflects the formation and annihilation of defects for short heating times under optical excitation.     

Density of states and charge distribution in hydrogenated amorphous silicon

The local density of states and the charge distribution in a cluster model of hydrogenated amorphous silicon is calculated using the recursion method. The results indicate an average charge transfer of 0.3 electrons from silicon to hydrogen. In addition Si-H dipoles carry small negative charges, which are neutralized by a positively charged bulk of silicon atoms. There are also static spatial charge fluctuations, which are due to the topological disorder.     

Transient photocurrent saturation and deep trapping in hydrogenated amorphous silicon

The transient photocurrent following laser impulse excitation has been measured as a function of laser excitation density for several samples of undoped hydrogenated amorphous silicon (a-Si:H). A feature is observed in these data which is interpreted as originating from complete saturation of the occupancy of deep electron traps. The trap density is estimated from this feature and compared to the neutral dangling bond defect density obtained from electron spin resonance; both measurements are of the same magnitude. Estimates of the density of states, g¹(E), using the measured trap density are also presented.     

Dispersive processes of light-induced defect creation in hydrogenated amorphous silicon

The growing curve of light-induced dangling bonds under illumination has been observed for various intensities of illumination in a-Si:H. It is fitted to a stretched exponential function and then two parameters β and τ involved in the function are estimated as a function of saturated dangling bond density . The experimental values of β, τ, and are compared with those calculated based on our model of light-induced defect creation in a-Si:H.     

Urbach edge and the density of states in hydrogenated amorphous silicon

Photoconductivity measurements of the optical absorption edge and time-of-flight measurements of the hole drift mobility, on the same amorphous silicon film, enable us to identify the origin of the Urbach edge in amorphous silicon with the density of states in the valence band tail.     

Ultrafast carrier dynamics in wide gap hydrogenated amorphous silicon

We report on picosecond and femtosecond pump and probe measurements of the dynamics of photoexcited carriers in wide gap a-Si : H prepared by microwave electron–cyclotron resonance plasma-enhanced chemical-vapour-deposition. We interpret the picosecond dynamics of transient absorption under strong picosecond excitation in terms of a bimolecular recombination process with the rate constant B≈5×10⁻¹⁰ cm³ s⁻¹, followed by a slower nanosecond decay. In the femtosecond measurements, we observed an initial decay with effective time constant ≈20 ps. We have not found any change in the picosecond dynamics when tuning the excitation wavelength through photoluminescence (PL) excitation spectrum profile. The ultrafast dynamics do not differ in the samples with PL efficiency differing in more than one order of magnitude, and they agree well with those in standard a-Si : H.     

Specificities of light-induced relaxation of metastable defects in amorphous hydrogenated silicon

The thermal relaxation kinetics of light-induced metastable defects in a-Si:H was studied prior to and after partial relaxation in the dark and in a dim light. The film lighting was found to change the relaxation rate of the defects and their distribution in relaxation time. This was demonstrated to be due to the concurrent light-induced relaxation and formation of the defects.     

Gap states in hydrogenated amorphous silicon: A comparison of photoemission and photoconductivity results

We report photoemission results from which we directly determined the density of states g(E) in the gap of a-Si:H between the top of the valence band E_v and the Fermi level. At 0.4 eV above E_v, g(E) was found to be ≈1×10²⁰ cm^-3 eV^-1 in the undoped film; P-doping increased g(E) in this region whereas annealing reduced it. The photoconductivity-derived optical absorption spectrum matched the shape of the photoemission spectrum, and thus supports the explanation that the photoconductivity shoulder at photon energies in the region of 1.3 eV is due to transitions from localized states above the valence band to the conduction band.     

A comparative study of muonium states in crystalline and amorphous hydrogenated silicon

A study of muons implanted into amorphous hydrogenated silicon (a-Si: H), using both transverse and longitudinal field μSR, is presented. Particular use is made of the muon repolarization curves in longitudinal fields. By comparison with the results of similar measurements on polycrystalline silicon, both the diamagnetic and Mu^* fractions are found to be substantially increased. We postulate that weak strained bonds in the amorphous structure are responsible. Little evidence has been found from longitudinal field measurements for isotropic muonium Mu', and a transverse field experiment on a-Si: D suggests that this state might not exist in the amorphous material.     

Features of the structure and defect states in hydrogenated polymorphous silicon films

The structural and electronic properties of thin hydrogenated polymorphous silicon films obtained by plasma-enhanced chemical vapor deposition from hydrogen (H₂) and monosilane (SiH₄) gas mixture have been studied by means of transmission electron microscopy, electron paramagnetic resonance (EPR) spectroscopy, and Raman spectroscopy. It has been established that the studied films consist of the amorphous phase containing silicon nanocrystalline inclusions with the average size on the order of 4–5 nm and the volume fraction of 10%. A signal was observed in the hydrogenated polymorphous silicon films during the EPR investigation that is attributed to the electrons trapped in the conduction band tail of microcrystalline silicon. It has been shown that the introduction of a small fraction of nanocrystals into the amorphous silicon films nonadditively changes the electronic properties of the material.     

Experimental elucidation of hydrogen local density of states in hydrogenated amorphous silicon

Ultraviolet and X-ray photoemission spectroscopies (UPS and XPS) are used to study the valence band in device quality hydrogenated amorphous silicon films (s-Si:H). Their results are combined to obtain estimated information about the hydrogen local density of states (HLDOS). The monohydride bonding configuration and the occurence of interhydride bonding between H atoms are confirmed by the experimental data.