Density of phonon states in amorphous germanium and silicon

The density of phonon states in amorphous germanium and silicon is calculated by statistically averaging the crystalline phonon density of states according to the radial distribution function. A simple rigid ion model is used to calculate the density of phonon states at various lattice spacings. The appropriate model parameters are obtained from the pressure dependent elastic constants and the Raman frequency. The calculated results compare favorably to experimental data obtained by infrared and Raman scattering and the results of other theoretical calculations.     

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.     

Interaction of ultrasonic phonons with defects in hydrogenated amorphous germanium

The attenuation of acoustic surface waves of 300 MHz has been measured in sputtered films of amorphous Ge and a-GeH_x (0x0.25) at temperatures between 0.5 K and 475 K. A strong absorption maximum due to structural relaxation is found at 135 K in pure a-Ge. This absorption maximum is shifted by annealing and is suppressed by hydrogen incorporation. Below 20 K the attentuation varies linearly with temperatures in all films investigated so far.On leave from: Department of Physics, Maharshi Dayanand University, Rohtak, 124001, India     

Paramagnetic states in doped amorphous silicon and germanium

ESR-results for doped hydrogenated amorphous silicon and germanium are discussed. g-values and linewidths of the resonances as well as effective correlation energies of the corresponding energy states are compared for both materials. It is shown that the existing large differences of these quantities between a-Si : H and a-Ge : H can be explained by the differences of spin-orbit coupling and degree of localization.     

Localized and nonlocalized impurity states in amorphous germanium

The purpose of this study was to investigate which impurities form localized states in amorphous Ge and to study the concentration dependence of the effect. The low temperature resistivity of amorphous alloy films deposited at 77°K was well fitted by the relation $\text{ρ = ρ}{}_{\mathrm{<mtext>0\; </mtext><mtext>exp</mtext>}}\text{[(}\text{T}{}_0\text{T}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{]}$. The increase in the number of localized states resulting from an increase in the number of impurities is evidenced by a decrease in T₀ Impurities which lead to shallow energy levels in the crystalline form do not form localized states near the Fermi level while impurities with deep lying levels do.     

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.     

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.     

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.     

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.     

退火处理对掺铒氢化非晶硅悬挂键密度和光致荧光的影响

采用磁控溅射技术制备了铒掺杂的氢化非晶硅(a-Si∶H(Er))样品.进一步通过200—500℃温度递增的后退火处理，获得了不同的Si悬挂键(Si-DBs)密度，并在此基础上研究了Si-DBs密度改变对其Er光荧光(Er-PL)的影响.退火温度低于350℃时，Er-PL强度持续增加，但Si-DBs密度的变化显得较复杂；350℃以上时，Er-PL强度随Si-DBs密度的增加而减小.在200—250℃的退火温度范围内，Si-DBs是由于结构弛豫而减少；在250—500℃的退火温度范围内，可能由于Si—H键的断 关键词： 氢化非晶硅 铒掺杂 Si悬挂键 光荧光     

Density of states effect on resonant tunneling

We study the effect of a gaussian type of density of states distribution on the conductance of a thin amorphous junction in the presence of localized states. The absolute value of the conductance is strongly affected by the width of the distribution but is sensitive to the center of the distribution only in the case of narrow distributions. All these effects disappear once we normalize the conductance to its zero field value.     

Quantum efficiency of light-induced defect creation in hydrogenated amorphous silicon and amorphous As2Se3

The quantum efficiency (QE) of light-induced metastable defect creation in hydrogenated amorphous silicon (a-Si?:?H) and amorphous As₂Se₃ (a-As₂Se₃) by bandgap and subgap illumination has been deduced from photocurrent measurements. The QE decreases with increasing number of absorbed photons. A higher QE for a-As₂Se₃ than for a-Si?:?H has been observed and this is interpreted in terms of the higher structural flexibility of a-As₂Se₃. We have also found that, for both materials, subgap illumination yields a higher QE than does bandgap illumination.     

Direct role of hydrogen in the Staebler-Wronski effect in hydrogenated amorphous silicon

We report a hydrogen-related defect that establishes the direct role of hydrogen in stabilizing the silicon dangling bonds created in the Staebler-Wronski effect in hydrogenated amorphous silicon. A specific NMR signal due to paired hydrogen atoms occurs only after optical excitation, exists at an intensity that is consistent with the density of optically induced silicon dangling bonds, and anneals at temperatures that are consistent with the annealing of the optically induced silicon dangling bonds. At this defect the hydrogen atoms are 2.3+/-0.2 A apart.     

Nb 3d and O 1s core levels and chemical bonding in niobates

A set of available experimental data on binding energies of Nb 3d_5/2 and O 1s core levels in niobates has been observed with using energy difference (O 1sNb 3d_5/2) as a robust parameter for compound characterization. An empirical relationship between (O 1sNb 3d_5/2) values measured with XPS for Nb⁵⁺-niobates and mean chemical bond length L(NbO) has been discussed. A range of (O 1sNb 3d_5/2) values possible in Nb⁵⁺-niobates has been defined. An energy gap ∼1.4–1.8 eV is found between (O 1sNb 3d_5/2) values reasonable for Nb⁵⁺ and Nb⁴⁺ states in niobates.