Laser produced amorphous phases in NbSi and VSi thin films

The binary systems NbSi and VSi are investigated by laser induced melting and quenching of vapor deposited thin films. Glassy phases of various compositions are produced. The established compositional glass forming ranges support that the equilibrium compounds Nb₄Si is stable only at high temperature. Thermal decomposition of the amorphous NbSi films proceeds via several intermediate stages. In particular, the AuCu₃-type configuration of Nb₃Si was found to be formed. In addition, amorphous NbSi films around 20 at.% si show the formation of an undentified metastable compounds upon post-irradiation annealing. Amorphous VSi films proved to be stable up to at least 500°C. Above this temperature they decompose directly into their respective equilibrium phases.     

Order and disorder in edge-supported pure amorphous Si and pure amorphous Si on Si(001)

We report results from an investigation into hidden anisotropy in pure fully-dense amorphous silicon. For amorphous silicon in intimate contact with a crystalline Si(001) substrate, one can reasonably expect that the interface with the substrate may impose anisotropy in the form of distorted ordering within the film. Indeed, we found four-fold periodic intensity variations, with bimodal intensity centered along the substrate c-Si < 110> directions, in the X-ray scattering from a-Si on Si(001). These well-defined intensity variations disappeared entirely in X-ray scattering from edge-supported a-Si films, where there was no detectable anisotropy.     

Photoconductivity and absorption in amorphous Si

The paper deals with photoconductivity and absorption in aSi specimens, prepared mainly by the decomposition of silane in a glow discharge. Substrate temperatures, T_d, between 300 K and 650 K were used during deposition. The normalised photoresponse was measured at room temperature as a function of photon energy and T_d in a spectral range from 0.5 eV to 3 eV. The absorption coefficient was determined for evaporated, sputtered and glow discharge specimens.The main features of the results are in agreement with conclusions drawn from previous electrical transport and field effect measurements and can be interpreted on the basis of the proposed model for the localised state distribution. It is confirmed that ?_c??_v is 1.5 to 1.6 eV, and that there is a local density of state maximum at about 1.2 eV below ?_c. At room temperature the steady photocurrent is carried predominantly by electrons in states above ?_c, whether excitation is from localized or extended states. Specimens prepared at T_d > 500 K are highly photosensitive, with electron recombination lifetimes, τ, of up to 10^?5s. Rise and decay times of the signal lie in the millisecond range. For T_d < 500 K there is a drastic decrease in τ, which falls to 10^?11 s at $\text{T}{}_{\mathrm{<mtext>d</mtext>}}\text{? 300}\text{K}$ and is even less for evaporated specimens. These results are discussed in some detail.     

Seebeck coefficient in amorphous chalcogenide films

The Seebeck coefficient determined for a number of amorphous chalcogenide films is linear in 1/T. Slopes were typically 0.1 to 0.2 eV less than the conductivity activation energies. A parameter A in the expression for thermopower appears to be a measure of the disorder, being large for highly disordered materials and small for annealed stoichiometric materials.     

Photoconductivity of amorphous CdS films

The photoconductivity of amorphous CdS films, vacuum evaporated onto cooled substrates, has been investigated. The conditions and possible reasons for the appearance of a low temperature maximum in the photoconductivity temperature dependence of chalcogen rich samples have been found. The presence of a slow recombination centre related to the lone pair level of the one-fold coordinated negative chalcogen has been assumed. The energy necessary for both the electron (2 eV) and hole (0.4 eV) escape from this centre has been determined. Photoinduced changes in photoconductivity have been examined as well.     

Crystallization of amorphous germanium films

Kinetics of isothermal crystallization has been studied in the temperature range from 375 to 525°C. The kinetic curves are obtained and the rate of isothermal transformation of amorphous films into crystalline ones has been determined. Using experimentally determined kinetic curves the stability diagram of the amorphous films has been plotted in the temperature range from 400 to 525°C. The value of effective activation energy has been defined.     

Self-aggregated Si quantum dots in amorphous Si-rich SiC

Amorphous silicon quantum dots (Si-QDs) self-aggregated in silicon-rich silicon carbide are synthesized by growing with plasma-enhanced chemical vapor deposition on (100)-oriented Si substrate. Under the environment of Argon (Ar)-diluted Silane (SiH₄) and pure methane (CH₄), the substrate temperature and RF power are set as 350 °C and 120 W, respectively, to provide the Si-rich SiC with changing fluence ratio (R = [CH₄ ]/[SiH₄] + [CH₄]). By tuning the fluence ratio from 50% to 70%, the composition ratio x of Si-rich Si_1 ? xC_x film is varied from 0.27 to 0.34 as characterized by X-ray photoelectron spectroscopy (XPS), which reveals the component of Si_2p decreasing from 66.3 to 59.5%, and the component of C_1s increasing from 23.9% to 31% to confirm the formation of Si-rich SiC matrix. Annealing of the SiC sample from 650 °C to 1050 °C at 200 °C increment for 30 min induces the very tiny shift on the wavenumber of the crystalline Si (c-Si) related peak due to the precipitation of Si-QDs within the SiC matrix, and the Raman scattering spectra indicate a broadened Raman peak ranging from 410 to 520 cm^? 1 related to the amorphous Si accompanied with the significant enhancement for SiC bond related peak at 980 cm^? 1. From the high resolution transmission electron microscopy images, the critical temperature for Si-QD precipitation is found to be 850 °C. The self-assembly of the crystallized Si-QDs with the size of 3 ± 0.5 nm and the volume density of (3 ± 1) × 10¹⁸ (#/cm³) in Si-rich SiC film with R = 70% are observed after annealing at higher temperature.     

Optical absorption in amorphous InN thin films

Amorphous indium nitride (a-InN) thin films were deposited onto different substrates at temperatures <325 K using RF magnetron sputtering at a rate 0.3–0.4 Å/s. X-ray diffraction patterns reveal that the films grown on the substrates are amorphous. The optical absorption edge, ‘bandgap’ energy, E_g, of a-InN has been determined by spectroscopic ellipsometry over the energy range 0.88–4.1 eV. The absorption coefficient was obtained by the analysis of the measured ellipsometric spectra with the Tauc–Lorentz model. The E_g was determined using the modified Tauc and Cody extrapolations. The corresponding Tauc and Cody optical bandgaps were found to be 1.75 and 1.72 eV, respectively. These values are in excellent agreement with the values of the bandgap energy obtained as fitting parameters in the Tauc–Lorentz model: 1.72 ± 0.006 eV as well as by using spectrophotometry (1.74 eV) and photoluminescence (1.6 eV). The spectral dependence of the polarized absorptivities was also investigated. We found that there was a higher absorptivity for wavelengths <725 nm. This wavelength, ∼725 nm, therefore indicates that the absorption edge for a-InN is about 1.70 eV. Thus, the average value of the measured optical absorption of a-InN film is approximately 1.68 ± 0.071 eV.     

Dark conductivity in amorphous undoped silicon films

The temperature dependence of the dark conductivity was investigated in amorphous undoped silicon films deposited by glow-discharge in a SiCl₄H₂ mixture. Different transport processes were indentified according to the investigated temperature range. The dependence of the dark conductivity was also examined as a function of some deposition parameters. The experimental results are discussed in terms of the two-phase structure of the film.     

Holographic recording in amorphous chalcogenide thin films

Holographic recording by He–Ne laser (line 632.8 nm) light in amorphous As_0.55Se_0.45 thin films for different film thickness and grating period was studied. A strong dependence of the diffraction efficiency of the gratings on the readout light wavelength (650 nm, 805 nm and 1150 nm) was observed. A decrease in diffraction efficiency for longer wavelengths is explained by a decrease in the photoinduced changes of refractive index. It is shown that high efficiency gratings can be recorded in As_0.55Se_0.45 films with a thickness of ∼1 μm.     

Magnetic behaviors of amorphous Fe78Si9B13 thin films prepared by pulsed laser deposition

Amorphous magnetic thin films have been deposited onto BK-7 substrates using the pulsed laser deposition technique. The source material irradiated by the laser was a pack of amorphous ribbons of composition Fe₇₈Si₉B₁₃. The structural properties of thin films were investigated and a large number of droplets were observed in the magnetic layers. Electric current passing through the films causes significant deformation of droplets and consequently changes the magnetic thin films characteristics. Magnetic properties evolution after electrical processing was investigated using magneto-optical Kerr effect.     

Thermopower of amorphous Si(Al)

Thermopower data for amorphous Si(Al) alloys from 230 to 500 K are reported and shown to be consistent with a two conduction mechanism model. Complementary optical absorption and electrical conductivity data are reported.     

The crystallization of amorphous germanium films

Germanium thin films were prepared in an amorphous form by vacuum deposition onto room temperature fused silica substrates. The amorphous—crystalline transition was studied as a function of time and temperature by measuring the optical transmission near 0.65 μm, where the absorption constant is most sensitive to the phase transformation. At a fixed temperature, the time for half the volume of the sample to become crystallized was found to be consistent with the relation t_c = τ exp(E₀/kT), with an activation energy E₀ = 2.96 eV.     

Structure of multicomponent amorphous semiconductor films

Composition of multicomponent amorphous semiconductor films varies with the preparation technique and the chemical nature of component atoms. Inhomogeneous structures due to phase separation of compositional fluctuation have been observed in these films. It seems that there are some regularities governing the film growth and the structure of liquid phase condensation.