The valence bands of model amorphous semiconductor structures

Using a basis of bonding orbitals, and tight-binding matrix elements which depend on the local topology, we calculate the valence band electronic structure of silicon on the diamond and ST12(GeIII) lattices, as well as on the continuous random networks of Polk and Boudreaux and Connell and Temkin. Results indicate the need for odd-membered rings to account for the experimentally observed features.     

Optical absorption in sputtered amorphous phosphorus

Optical absorption measurements are reported on r.f. sputtered films of a-P prepared at variable substrate temperatures. With decreasing substrate temperature the absorption spectra shift to lower frequencies and the energy gap decreases. The form of the spectra are, however, similar both in the band edge and Urbach tail regions. The Urbach slope is found to differ substantially from that of a-As, indicating that factors other than coordination number may influence this parameter. The changes in the absorption spectra are suggestive of a possible role of intermediate range order on the electronic states in a-P.     

A time-resolved photoluminescence study of amorphous phosphorus

Radiative recombination in amorphous phosphorus is studied by the technique of time-resolved photoluminescence. In the first 200 ns following excitation the luminescence band moves from 1.36 to 1.18 eV. The position of the band is also shown to depend on the energy of exciting photons. These observations are discussed in terms of Coulomb interaction and thermalisation, and their implication for a detailed modelling of excitation and radiation assessed.     

Nucleation of quasi-crystalline and amorphous structures

Summary The structural characteristics of quasi-crystalline phases obtained by ion irradiation are compared with those of the corresponding amorphous structures. Chemical forces drive the formation of atomic clusters and their spatial organization under specific geometric constraints. The same elementary structural units exist in quasi-crystalline and glassy phases which display identical local order, their main difference being the degree of defect organization at the interface between locally ordered atomic groups. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Effect of phosphorus content on structural properties of phosphorus incorporated tetrahedral amorphous carbon films

With phosphorus incorporated tetrahedral amorphous carbon (ta-C:P) films prepared using filtered cathodic vacuum arc technique with PH₃ as the dopant source, we investigate the effect of phosphorus content on the structural properties of the films by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. XPS analysis indicates that a function is established between the atomic fraction of phosphorus in the samples and the flow rate of PH₃ during deposition, and that phosphorus implantation increases the graphite-like trihedral sp² bonds deduced from fitted C 1s and P 2p core level spectra. Raman spectra of a broad range show that there are two notable features for all ta-C:P films: the first-order band centered at about 1560 cm^-1 and the second-order band between 2400 and 3400 cm^-1. The broad first-order band demonstrates that the amorphous structure of all samples does not remarkably change when a lower flow rate of PH₃ is implanted, while a higher concentration of phosphorus impurity enhances the clustering of sp² sites dispersed in sp³ skeleton and the evolution of structural ordering. Furthermore, the second-order Raman spectra confirm the formation of small graphitic crystallites in size due to a finite-crystal-size effect. PACS 81.05.Uw; 81.15.Ef; 63.50.+x     

Valence-band structures of phosphorus allotropes II

Densities-of-states calculations are reported for the phosphorus allotrope known as Hittorf's phosphorus, and for two closely related compounds, KP₁₅ and BaP₁₀; all three contain pentagonal tubes of covalently bonded phosphorus atoms. The experimental photoelectron energy distribution curves from amorphous red phosphorus suggest that these tubes could be the dominant structural unit there also.     

The microscopic structure of bulk amorphous red phosphorus: A Raman scattering investigation

The microscopic structure of bulk amorphous red P has been investigated by Raman scattering. It is concluded that the phosphorus tubes of pentagonal cross-section present in Hittorf's P and KP₁₅ are the structural units of the amorphous red P network. Intratube and intertube correlations are responsible for intermediate range order effects. The Raman data indicate that a continuous transition from the crystalline to the amorphous state takes place for P.     

Anisotropic x-ray scattering from simulated amorphous structures

Two 800-atom cluster models of amorphous copper were constructed utilizing the method of continuous static relaxation of nonrigid spheres developed by the authors. The Morse and Ballog potentials were used to construct and relax the structures. The angular dependence of the x-ray scattering was simulated for both clusters, and a rather appreciable anisotropy was found for them. In order to elucidate the causes of the observed anisotropy, a distribution of atoms in thin sections perpendicular to the direction of the main peak was investigated. Quasiperiodic fluctuations in the occupation numbers of the sections with a period roughly corresponding to the radius of the first coordination sphere were found. A special numerical—graphical analysis showed that the atoms contributing to the main peak form pseudoplanes corresponding to fluctuations in the occupation numbers of the sections. In spite of appreciable differences in the character of interatomic interactions, the density, and the microstructure of the modeled clusters, the pseudocrystalline motifs in both clusters were very similar. A comparison was performed of the results obtained from the scattering pattern for an 800-atom unrelaxed stochastic cluster especially generated for the investigation of the influence of relaxation on the anisotropy. It is shown that the relaxation somewhat increases the anisotropy and order the displacement of the atoms within the sections although on the whole the scattering pattern in all cases is similar. The results obtained correlate well with published data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 7–12, August, 1993.     

Amplitude-phase reflectance spectra of amorphous silicon-based Bragg structures

Amplitude-phase spectra of light reflection from distributed Bragg reflectors and Fabry-Pérot microcavities based on a-Si: H/a-SiO_x: H thin films have been studied. The frequency dependence of the phase difference between the amplitude p-and s-light reflection coefficients within the photonic band gap is measured. The phase spectrum exhibits predominantly a monotonic, close-to-linear frequency behavior, except for spectral regions near the stop band edges and near the singularities related to the microcavity eigenmodes. The experimental spectra are compared with theoretical calculations based on the transfer matrix method and approximate analytical relations. A method based on analyzing amplitude-phase reflectance spectra is proposed for structural characterization of multilayer microcavity systems.     

Distribution of boron and phosphorus ion paths in amorphous germanium

Distributions of the projected paths of boron and phosphorus ions with energies E = 25–150 keV in germanium are obtained experimentally and by means of Monte-Carlo calculations. Good agreement is established between the calculated and experimental data for boron ions. Phosphorus experimental paths are 30% less than calculated. It is shown that it is not possible to explain the discrepancy by the lack of correspondence of the true electronic deceleration to the value calculated according to the Lindhard-Scharff theory. The results are discussed from the viewpoint of the influence of shell effects on the elastic and inelastic deceleration of the ions.Translated from Izvestiya VUZ. Fizika, Vol. 15, No. 1, pp. 43–49, January, 1972.     

Dispersion of phonon excitations in model amorphous structures

The theory proposed by Takeno and Goda to explain phonon excitations in topologically disordered systems is used to calculate dispersion relations for two amorphous clusters of copper in a harmonic approximation. The structures are generated by the method of continuous static relaxation [11]. Atomic interaction in model clusters is specified by means of Morse and Ballog empirical potentials. The position of the roton minimum of the longitudinal mode coincides with the principal maximum of the static structural factor in both cases, which agrees with well-known results obtained in theories of elementary excitations in amorphous systems. It is found that the highest-frequency waves are those for which wavelength equals the radius of the second coordination sphere, while the nearest neighbors vibrate with the frequency corresponding to the roton minimum. A numerical estimate of the ratio of sonic velocity in the longitudinal and transverse directions shows complete agreement with theoretical predictions. Calculations show that due to weak dispersion and thus low transverse sonic velocities, longitudinal vibrations make the main contribution to the density of states.Tomsk State Architectural-Building Academy. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 31–36, April, 1994.     

Electronic structure of red amorphous phosphorus studied by X-ray spectroscopy

A SXS study of occupied and unoccupied 3p states of semiconducting red amorphous phosphorus is presented. It is shown that in the upper part of the valence band, partially hybridized sp spates are present among p pure states, whereas the bottom of the valence band is almost s-like. The results are discussed in connection with theoretical data available for black phosphorus.     

Modeling of three-dimensional structures of amorphous and nanocrystalline alloys

The possibilities of the program for modeling the structure of amorphous and nanocrystalline materials have been considered. Calculations within different models and their modifications have been performed to construct atomic structure images.     

Magnetoelectric effect in amorphous FeNiSiC ferromagnet-piezoelectric planar structures

The magnetoelectric (ME) effect is studied in composite two- and three-layer disk structures containing magnetostriction layers of an amorphous FeNiSiC ferromagnet and a lead zirconate titanate piezoelectric layer. Due to a high magnetostriction (∼33 × 10⁻⁶) and a low saturation field (∼200 Oe), an FeNiSiC layer has a high piezomagentic coefficient, which results in an effective ME coupling in low fields (∼25 Oe). The ME effect is ∼0.2 V cm⁻¹ Oe⁻¹ at a low frequency and increases to 11.9 and 13.2 V cm⁻¹ Oe⁻¹ when bending and in-plane mechanical vibrations are excited in a resonance manner in the structures at frequencies of ∼8.2 and ∼170.0 kHz, respectively. Structures containing amorphous FeNiSiC layers are promising for magnetic field transducers and electric energy generators and converters.     

Observation of bound exciton and distant pair resonances in amorphous phosphorus

For the first time in an amorphous semiconductor magnetic resonance of a bound exciton is reported. ODMR investigations of amorphous phosphorous show that the luminescence consists of two bands, a low energy emission which is consistent with triple exciton recombination at IVAPs and a high energy emission due to electron-hole recombination at distant IVAPs. The triplet exciton has g = 2.13 ± 0.05, and the distant pair resonance occurs at g = 2.0 ± 0.02.     

Lattice dynamics of Hittorf's phosphorus and identification of structural groups and defects in amorphous red phosphorus

Using the force constants and bond polarisabilities of black phosphorus the dispersion, eigenvectors, and density of states for phonons, the sound velocity and the Raman spectra are calculated for polymeric violet Hittorf's phosphorus. Good agreement with Raman and Brillouin (v_s,110,exp=6.95·10⁵cm s^?1) measurements reported for the first time on oriented single crystals is found. Thus we are able to assign specific vibrational modes, such as extended wave-like vibrations and modes localised on the P₈ and P₉ structural subunits. Comparison with vibrational spectra of amorphous red phosphorus shows that the latter contains tubes similar to those of Hittorf's P. The absence of the group of lines (around 373 cm^?1) due to P₈ cages in a-Pproves that the P₈ cages are present in much reduced numbers only there, or that the weaker bonds are broken or distorted. The features at 400 cm^?1 and 455 cm^?1 in a-P might be explained by the occurrence of P₇ cages. Broken P₈ units may also be the origin of the metastable intrinsic defects giving rise to the photo-induced ESR and axial luminescence centers in amorphous phosphorus.