Electrical and optical properties of amorphous boron and amorphous concept for ß-rhombohedral boron

Measurements of the conductivity (σ), thermoelectric power (S) and thermal conductivity (κ) of amorphous boron are made over wide temperature ranges (T = 77–850 K for σ, T = 300–850 K for S and T = 80–1100 K for κ). The room temperature spectral dependencies of the reflection (R) and absorption (α) coefficients are determined for the wavelength intervals 2–25 μm and 1.3–25 μm respectively. The I–V characteristics are also studied and shown to be consistent with the Poole-Frenkel law.The value obtained for the thermal energy gap of amorphous boron (1.3 eV) is slightly smaller than that of crystal ß-rhombohedral boron (1.4 eV). The temperature dependence of the electrical conductivity can be satisfactorily described by the Mott law $\text{ln}\text{σ ≈ ?(T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$, where $\text{T}{}_0\text{? 10}{}^8\text{K}$. This gives an estimate, N ≈ 10¹⁸ cm^?3, for the concentration of trapping levels responsible for the hopping conduction. The value $\text{?}{}_0\text{? 9}$ is found from the spectral dependence of R while α has Urbach-like character $\text{? α ≈}\text{exp}\text{(}\text{h}\text{?}\text{ω/Δ)}$, where $\text{Δ ? 0.19}\text{eV}$.A comparison is made between amorphous boron and crystalline ß-rhombohedral boron. Because of the very complex crystal structure and the large dimensions of the unit cell of ß-boron, some of its physical properties could be qualitatively described on the basis of the so-called ‘amorphous concept’.     

Structure of amorphous GeSe thin films

Amorphous films of GeSe_0.7 and GeSe_2.4 have been examined by energy-filtered scanning electron diffraction and by electron microscopy. Radial distribution analysis of the GeSe_0.7 intensity curves indicates that the local atomic order of these films differs considerably from the distorted rocksalt structure of bulk crystalline GeSe. Radial distribution studies also indicate a change in the structure of GeSe_0.7 and GeSe_2.4 films supported on copper mesh as they are heated with an electron beam. Gross structural features are not observable in the electron micrographs of the amorphous films. This does not eliminate, however, the possibility of the presence of glassy phase separation on a fine scale in the heat-treated films.     

Spectral response of amorphous–nano-crystalline silicon thin films

A representative set of amorphous–nano-crystalline Si thin films was deposited by radio-frequency plasma enhanced chemical vapor deposition using silane highly diluted by hydrogen. By Raman spectroscopy it was found that the variation of silane to hydrogen ratio resulted in films with crystal fraction between 0 and 55 vol.% and individual crystal sizes between 2 and 20 nm with bi-modal, broad size distribution. High resolution transmission microscopy, done on certain number of samples, confirmed the nano-meter size of crystallites and bi-modal size distribution. The optical properties measured by Fourier transform photocurrent spectroscopy and photo thermal deflection spectroscopy correspond to the material with structure between amorphous and crystalline. The spectral distribution of relative quantum efficiency of photovoltaic solar cell made from this material shows ‘blue shift’ with increase of crystal to amorphous fraction. This result is discussed as a possible consequence of quantum effects accompanied with actual size and size distribution of crystals.     

The study of transformation kinetics of the amorphous PdSi alloys

The kinetics of transformation behavior of roller-quenched amorphous Pd₈₃Si₁₇ and Pd₈₀Si₂₀ alloys after linear and isothermal heating is reported. The transformation was examined with electron microscopy, electron diffraction and electrical resistivity measurement. The crystallization of amorphous alloys with 17 at% Si begins with metastable ordered fcc solid solution. The ordered fcc solution is transformed to a ordered metastable phase with, probably, orthorhombic structure. The crystallization of amorphous alloys with 20 at% Si begins by formation of spherulites with lamellar structure. Using electron diffraction we found that spherulites consist of two phases - orthorhombic Pd₃Si silicide and Pd-rich silicide. From resistivity measurements, activation energies of 28.5 kcal/mol for Pd₈₃Si₁₇ and 80 kcal/mol for Pd₈₀Si₂₀, respectively, were calculated.     

Properties of amorphous silicon films— Dependence on deposition conditions

We report measurements on the electrical conductivity, optical absorption, electron spin resonance, Raman spectrum and electron diffraction of a set of a-Si films vaccum deposited at room temperature. As revealed by absorption at about 10 μm some of the films grown at deposition rates of about 0.4 Å/sec contain considerable amounts of oxygen. All the properties, except Raman spectra and electron diffraction, are found to vary strongly with the deposition rate and the background pressure during evaporation. Qualitatively, these variations show significant correlations. For instance, if the electrical conductivity is higher so is the spin density, the optical absorption and the low-frequency refractive index. Also, increasing oxygen content leads to lower conductivity. In addition, we have tried to establish a rough quantitative relation between the shift in the optical gap and the change in the spin density by connecting each of these changes to the variation in the low-frequency refractive index. The temperature dependence of the electrical conductivity was also measured and it was found that below about 150 K the data are consistent with recent theoretical predictions, i.e., log σ exhibits a ($\text{1/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ dependence.     

Microstructural evolution of amorphous silica following alkali–silica reaction

Combining X-ray diffraction, NMR (CP-MAS and ²⁹Si NMR-MAS) and diffuse reflectance infra-red Fourier transformed spectroscopy (DRIFTS), the local structural changes induced during the reactivity of amorphous (a-SiO₂) under alkali–silica reaction (ASR) have been investigated. XRD patterns provide useful and new qualitative information concerning the reactivity of a-SiO₂. A shift of the maximum of the halo towards the high values of 2θ is observed when the reaction progresses. NMR shows that ASR is accompanied in a-SiO₂ by the diminution of Q₄ species and by the formation of Q₃ (and to a less extend to Q₂) species. This importance played by Q₃ species confirms the results obtained with natural alkali–silica gels. The formation of OH bonds observed by DRIFTS can be followed at ca. 950 cm^?1 and confirms NMR results. The microstructural state of silica (here represented by the medium- and short-range order (MRO) of amorphous silica) plays a crucial role in governing the kinetics of the ASR.     

Mössbauer characterization of an amorphous steel with optimal Mo content

Three amorphous steel alloys, Fe_71.2?xC_7.0Si_3.3B_5.5P_8.7Cr_2.3Al_2.0Mo_x (x = 0, 4.5 and 6.5 at.%), have been produced by the melt-spinning technique and characterized by X-ray diffraction and transmission Mössbauer spectrometry (TMS). TMS allows us to study the local environments of the Fe atoms in the glassy state, showing the changes in the amorphous structure due to the addition of Mo. A reduction of the mean hyperfine field is observed as the amount of Mo increases. With intermediate Mo content, this reduction is associated to the substitution of Fe by Mo in a disordered magnetic Fe-rich structure, whereas for high Mo content, this structure is destroyed leading to an increase of paramagnetic environments. Finally, the relationship between the glass forming ability (GFA) of these alloys and their local structure determined by TMS will be discussed.     

Effects of post-annealing on electrical properties of amorphous Ga-doped Zn–Sn–O semiconductor films

We investigated the effect of post-annealing on the electrical properties of amorphous gallium-zinc-tin oxide (a-GZTO) films with different Ga contents. The films were deposited at room temperature by sputtering and annealed in air for 1 h. It was found that the doping with Ga, which acts as the carrier suppressor, contributes to the thermal stability of characteristic properties of a-GZTO thin films. The film with a small amount of Ga showed significant variations in carrier concentration according to the annealing temperature. Increases in carrier concentration and mobility can be ascribed to the reduction of subgap density of states by annealing. After annealing at 400 °C, however, the enrichment of Zn cations in surface region resulted in considerable changes in chemical bonding states and consequently, the carrier concentration decreased by two orders of magnitude for the low Ga-doped ZTO film.     

Peculiarity of constant photocurrent method for silicon films with mixed amorphous–nanocrystalline structure

The results of conductivity, photoconductivity and constant photocurrent method absorption measurements by DC and AC methods in hydrogenated silicon films with mixed amorphous–nanocrystalline structure are presented. A series of diphasic silicon films was deposited by very high frequency plasma enhanced chemical vapor deposition technique, using different hydrogen dilution ratios of silane. The increase of hydrogen dilution ratio results in five orders of magnitude increase of conductivity and a sharp increase of grain volume fraction. The comparison of the absorption spectra obtained by DC and AC methods showed that they are similar for silicon films with the predominantly amorphous structure and films with high grain volume fraction. However we found a dramatic discrepancy between the absorption spectra obtained by DC and AC constant photocurrent methods in silicon films deposited in the regime of the structure transition from amorphous to nanocrystalline state. AC constant photocurrent method gives higher absorption coefficient than DC constant photocurrent method in the photon energy range of 1.2–1.7 eV. This result indicates the possibility of crystalline grains contribution to absorption spectra measured by AC constant photocurrent method in silicon films with intermediate crystalline grain volume fraction.     

Kinetics of self-bleaching in some photodarkened Ge–As–S amorphous thin films

Three amorphous Ge–As–S films with the average coordination numbers of 2.4–2.8 were prepared by thermal evaporation. True relaxation that is self-bleaching of photodarkened state has been studied. Considerable differences in kinetics of relaxation of photodarkened state were observed for Ge₁₂As₁₇S₇₁ (Sa 1) and Ge₁₅As₂₀S₆₅ (Sa 3) amorphous thin films. In both cases, the relaxation (self-bleaching) followed stretched exponential, however, flexible matrix of Sa 1 film relaxed significantly faster than the matrix of Sa 3 film. The amorphous film Ge_25.5As_29.5S₄₅ (Sa 8) was found to be insensitive to illumination. It is suggested that the network rigidity may significantly influence the magnitude of photodarkening and the rate of relaxation of photodarkened state.     

Effect of germanium on the microstructure and the magnetic properties of Fe–B–Si amorphous alloys

In this work, we present a systematic study on the crystallization kinetics and the magnetic properties of melt-spun Fe₈₀B₁₀Si_10 ? xGe_x (x = 0.0 ? 10.0) amorphous alloys. The activation energy for crystallization, determined by differential scanning calorimetry, displayed a strong dependence on the Ge content, reflecting a deleterious effect on the alloys' thermal stability and their glass forming ability with increasing Ge concentration. On the other hand, the alloys exhibited excellent soft magnetic properties, i.e., high saturation magnetization values (around 1.60 T), alongside Curie temperatures of up to 600 K. Complementary, for increasing Ge substitution, the ferromagnetic resonance spectra showed a microstructural evolution comprising at least two different magnetic phases corresponding to a majority amorphous matrix and to Fe(Si, Ge) nanocrystallites for x ≥ 7.5.     

Dependences of the crystallization behavior of Al–Ni–La amorphous alloys on La and Ni contents

Crystallization behaviors of Al–Ni–La amorphous alloys with a fixed Ni (La) content of 6 at.% were investigated by X-ray diffraction and differential scanning calorimeter when the La (Ni) content changes from 3 to 9 at.%. The results show that the thermal stability of the amorphous alloys monotonically increases with increasing the La (Ni) content. Glass transition only exists in the alloys with the La (Ni) content higher than 6 at.%. La plays a more significant role than Ni in promoting the glass-forming ability, improving the thermal stability, stabilizing the supercooled liquid region, depressing the precipitation of fcc-Al in the first crystallization process and increasing the apparent activation energy of the first reaction.     

The crystallization kinetics of nanothin amorphous TlGa1 − x Ge x Se2 films

Crystallography Reports - The kinetics of phase transformations of amorphous Ge-doped TlGaSe2 films has been investigated by kinematic electron diffraction. It is shown that the crystallization of...     

Transition from amorphous semiconductor to amorphous insulator in hydrogenated carbon–germanium films investigated by IR spectroscopy

Thin a-Ge_XC_1?X:H plasma polymerized films, depending on deposition conditions, can be produced in two very different structures, namely amorphous semiconductor and amorphous insulator. The transition from amorphous insulator to amorphous semiconductor is related to the formation of germanium nanoclusters due to ions bombarding the surface of the growing material. This paper concentrates on investigations of the transition by means of IR spectroscopy. To this end a quantitative analysis of IR spectra obtained for thin films deposited on silicon substrate has been described and used for estimation of hydrogen atom concentration and bonding in the investigated material. It was found that the probability that a given H atom is bonded to a germanium or to a carbon atom is almost the same. This conclusion is true both for a-S and a-I films. The average concentration of hydrogen in the investigated material was found to be about 2.4–3.4 × 10²² cm^?2 which means that there are two times more atoms of the carbon family than hydrogen atoms in the film structure.     

Crystallization of sputtered amorphous silicon induced by silver–copper alloy with high crystalline volumeratio

Silver–copper alloy-induced crystallization of sputtered a-Si has been studied. In this alloy, Cu acts as a catalyst to accelerate the crystallization, while Ag acts as a new kind of buffer layer, different from Al₂O₃ and Si₃N₄, to obtain well-crystallized poly-Si films with short annealing time and free of post-treatment for ohmic contact. When the Cu content is limited to below 30%, Ag can effectively slow down the diffusion of Cu into Si and decrease the Cu–silicide nuclei density to improve the crystalline volume ratio from 80% to over 90%. A 1:4 ratio of Cu to Ag yields the best result. The crystalline volume ratio and Hall mobility reach nearly 100% and 29.4 cm²/V s, respectively. This high quality poly-Si film demonstrates a promising application in solarcells.     

The effects of annealing and rate of deposition on the electrical,optical and structural properties of amorphous GeSi alloy films

The electrical, optical and structural properties of rf-sputtered amorphous GeSi alloy films have been investigated as functions of the rate of deposition between 90 and 360 Å min^?1, and various annealing conditions. The stabilization evidenced in the electrical conductivity, optical absorption and density of these films deposited at higher rates and/or annealing temperatures may be explained in terms of the reduction of imperfections in the as-deposited films.     

Nanocrystallization effects on the specific heat of Fe–Co–Nb–B amorphous alloy

Specific heat at constant pressure, C_P, was measured on amorphous, nanocrystalline and fully crystalline samples of Fe₆₀Co₁₈Nb₆B₁₆ alloy. Magnetic and calorimetric measurements agree, describing a continuously decreasing Curie temperature of the amorphous phase. A clear enhancement of C_P over the Dulong–Pettit limit has been observed (from 14% to 25 %). Part of the enhancement is due to magnetic (mainly amorphous phase) and electronic contributions, although an excess volume can be inferred from the high value of the slope of C_P versus temperature.     

Acoustic study of low-energy activation processes in amorphous Se,Ge and SeGe compounds

The study of the elastic properties of amorphous selenium over a wide range of frequency (30 kHz−100 MHz) reveal a single internal friction peak below the glass transition temperature.This one involves low activation energies ranging down to very small values and is reduced when the chains are crosslinked. In amorphous germanium, there is no evidence for the existence of activation processes exhibiting very small values. We suggest that the underlying microscopic mechanisms responsible for the peak in a-Se are closely related to the polymeric structure and so should exist in any amorphous polymers. We also suggest that the numerous other peaks existing in most polymers are not intrinsically related to structure in chains.