Thermally-evaporated Films Research Articles

Microstructural, nanomechanical, and microtribological properties of Pb thin films prepared by pulsed laser deposition and thermal evaporation techniques

In this work, the authors compare the morphological, structural, nanomechanical, and microtribological properties of Pb films deposited by thermal evaporation (TE) and pulsed laser deposition (PLD) techniques onto Si (111) substrates. Films were investigated by scanning electron microscopy, surface probe microscopy, and x-ray diffraction in θ-2θ geometry to determine their morphology, root-mean-square (RMS) roughness, and microstructure, respectively. TE films showed a percolated morphology with densely packed fibrous grains while PLD films had a granular morphology with a columnar and tightly packed structure in accordance with the zone growth model of Thornton. Moreover, PLD films presented a more polycrystalline structure with respect to TE films, with RMS roughness of 14 and 10 nm, respectively. Hardness and elastic modulus vary from 2.1 to 0.8 GPa and from 14 to 10 GPa for PLD and TE films, respectively. A reciprocal friction test has shown that PLD films have lower friction coefficient and wear rate than TE films. Our study has demonstrated for first time that, at the microscale, Pb films do not show the same simple lubricious properties measured at the macroscale.

Preparation of As–In–S:Sm thin films and a study of their properties

The study examines the physico-chemical properties of thin films of the As–In–S:Sm system. Thin amorphous films were prepared by thermal evaporation (TE) from two sources (As–In–S glass and metallic Sm) and by pulsed laser deposition (PLD). The films prepared with PLD contained more Sm and In than the TE films. The optical gap of the PLD films is lower than that of the TE films (Egopt=2.29eV for TE, Egopt=1.88eV for PLD), both are dependent on exposure of films and on their annealing. The as-deposited PLD films did not contain any separated As4S4 molecules; overstoichiometric arsenic was incorporated into the amorphous matrix. The index of refraction of the prepared PLD films (nћω→0=2.48) is higher than that of the TE films (nћω→0=2.35). The corresponding coefficient of the third order non-linear optical susceptibility (χ(3)) is also higher (PLD: 6.83×10−20m2/V2, TE: 4.02×10−20m2/V2). The structural changes induced by exposure and by annealing, determined by Raman spectroscopy, can be described through photochemical reactions (induced by exposure) and chemical reactions (induced by annealing). The strong luminescence bands were assigned to inner f–f electron transitions. The material is promising for a number of applications.

Comparison of the optical, thermal and structural properties of Ge–Sb–S thin films deposited using thermal evaporation and pulsed laser deposition techniques

Thin films of Ge 23Sb 7S 70 glass were prepared by thermal evaporation (TE) and pulsed laser deposition (PLD) techniques. We measured their thermal, optical and structural properties and compared with those of the parent bulk. The probe penetration temperature ( T p) of the bulk glass, measured using a micro-thermal analyzer, was found to be 412 ± 10 °C, while those of the TE and PLD films were 468 and 470 ± 10 °C, respectively. The refractive index of the bulk and thin films was measured by ellipsometry and ultraviolet–visible–near infrared transmission spectroscopy, and we show that the films have similar refractive indices, which are lower than those of the parent bulk glass. Using micro-Raman spectroscopy, the structure of the film was investigated. The films contain homopolar Ge–Ge bonds and a lower number of homopolar S–S bonds compared to bulk material, which leads to an increase in the proportion of corner shared GeS 4/2 units in the films as compared to the bulk glass. Comparison of structural entities associated with each deposition process showed that the TE film possesses a lower number of S–S bonds and a slightly higher number of SbS 3/2 units compared to the PLD film. These structural changes lead to a more interconnected glass network, and therefore to a higher viscosity at elevated temperatures, as well as a higher refractive index, presumably through increased density. Post-deposition of annealing of these films causes their thermal and optical properties to revert to a more bulk-like state, suggesting that these property differences are due to a difference in the thermal histories of the bulk and film glass networks. For the first time to our knowledge, the refractive index of the bulk glass in the 0.6–10.6 μm range, measured using the prism coupling technique, is also presented.

Influence of the oxidation level on the electronic, morphological and charge transport properties of novel dithienothiophene S-oxide and S,S-dioxide inner core oligomers

Novel inner-core dithienothiophene S-oxide and S,S-dioxide oligomers were designed and synthesized. The controlled oxidation at the central sulfur of the dithienothiophene core allows the combined tailoring of the HOMO–LUMO energies, thermal properties, and thin film morphology. Electrochemical measurements indicate that a marked decrease in electron affinity is obtained by addition of the first oxygen at the central sulfur, while addition of the second oxygen induces only minor effects on both the oxidation and reduction potentials. The newly synthesized materials exhibit high solubility in common organic solvents together with good film forming properties when deposited on SiO2 by solution-based techniques. Similar hole field effect transistor (FET) mobility for solution-deposited and thermally-evaporated films together with FET performances independent of film morphology and molecular orientation call for non-conventional charge transport mechanisms.

The comparison of Ag–As 33S 67 films prepared by thermal evaporation (TE), spin-coating (SC) and a pulsed laser deposition (PLD)

Chalcogenide thin films could be prepared by many experimental methods resulting in some differences in structure and physicochemical properties of prepared films. In this work, the As 33S 67 amorphous films were prepared by three different preparation techniques: vacuum thermal evaporation (TE), pulsed laser deposition (PLD) and spin-coating (SC). A silver film was deposited on the top of the As 33S 67 films and photodoped. The X-ray diffraction analysis showed significant differences in arrangement between bulk glass and thin films and also among films themselves. The Raman spectroscopy showed that the Raman spectra of PLD film and bulk glass are almost similar. On the other hand, TE films contain higher amount of homopolar bonds As–As and S–S. The value of refractive index of As 33S 67 bulk glass was 2.31. All prepared films have lower index of refraction contrary to bulk glass, i.e. TE∼2.27, PLD∼2.20 and SC∼1.90. The increase of refractive index with silver concentration is shown either. The optical bandgap of undoped As–S prepared films was different: TE∼2.42 eV, PLD∼2.45 eV and SC∼2.54 eV.

Thin film deposition of Ge33As12Se55 by pulsed laser deposition and thermal evaporation: Comparison of properties

Thin films of Ge33As12Se55 were produced using two deposition techniques, ultra-fast pulsed laser deposition (UFPLD) and thermal evaporation (TE), and their properties have been investigated. The chemical composition of the UFPLD films was virtually identical to the composition of the chalcogenide glass targets, whereas the composition of films deposited by TE was significantly different from the target material. Heating of the substrate with a temperature gradient during deposition produced a gradient in composition in both UFPLD and TE films.

The Initial Growth Stages and Crystallization Mechanism of Bi-based films

AbstractThe present work deals with investigation of the initial stages of growth of (Bi, Bi-Sb) films deposited by the methods of different degrees of nonequilibrium, such as thermal evaporation (TE) and self-ion assisted deposition (SIAD). The comparative analysis of growth mechanisms and film island evolution (faceting and coalescence) was carried out depending on the type (crystalline, amorphous) and temperature of substrate, evaporation rate, film composition, etc. Ion bombardment during deposition leads to films of higher density and reduced porosity, and greater stability than those deposited without bombardment. As compared with the pure Bi films the facetted island morphology of the SIAD Bi–Sb (20%) films is not so clear. This is explained by partial realization of the coalescence process by the liquid fluidity mechanism. At all substrate temperatures the grain orientation (111) R is more pronounced for SIAD films than for the TE films.

High orientation CdS thin films grown by pulsed laser and thermal evaporation

Undoped CdS films have been grown by pulsed laser evaporation (PLE) and thermal evaporation (TE) techniques on Corning’s 7059 glass at a substrate temperature between room temperature and 250 °C. The deposition rates are 0.07 Å/s for PLE and 60 Å/s for the TE technique. These as-deposited films have high (002) preferred orientation by both techniques even without preheating the substrate. However, films deposited by PLE have more preferred orientation and larger grain size. These features make the PLE films have sharper Raman peaks, smaller Raman shift of surface mode and more overtones of longitudinal optical phonon mode observed than those in TE films. The transparency of PLE or TE films for wavelengths which are larger than that of absorption edge is around 90% on average.

Measurement of lattice strain in Au–Ni multilayers and correlation with biaxial modulus effects

Measurement of lattice strain within each atomic plane of sputter-deposited, gold–nickel multilayer films is accomplished using x-ray diffraction and high resolution electron microscopy. Experimental determination of strain along the composition modulation direction [1 −1 1] from x-ray diffraction scans fit to dynamical theory compare well with values measured from high-resolution electron micrographs and selected-area-diffraction patterns. The measured strain in-plane (2 2 0) and along the composition modulation (1 −1 1) reveal the gold layers to be in compression and the nickel layers in tension. The experimental strain values are used within the framework of the ‘‘coherency strain model’’ to predict an enhancement of the biaxial elastic modulus Y(111), which is in good agreement with the original experimental measurements on thermal-evaporated films.

Pulsed laser deposition of CdTe thin films for heterojunctions on silicon

Thin films of CdTe were deposited on Si substrates by pulsed laser evaporation (PLE) at substrate temperatures Ts of 25 and 200 degrees C. Films were also deposited by thermal evaporation (TE) for comparison. X-ray diffraction studies showed that while both films were polycrystalline with a zincblende structure, the PLE films showed a preferred (111) orientation. The composition of the films was checked by energy dispersive X-ray analysis (EDAX) and Cd:Te ratios of 1:1.03 (PLE) and 1:1.04 (TE) were obtained. The band gap of the former films was 1.5 eV but no sharp band edge was found for TE films. The resistivity values of PLE and TE films deposited on glass substrates at Ts=200 degrees C were 6-7*104 Omega cm and 3-4*105 Omega cm, respectively. The best ideality factor of the Si-CdTe heterojunction was found to be 1.56 with a barrier height of 0.5 eV from C-V analysis for laser-deposited films at Ts=200 degrees C. These films and heterojunctions were thus found to be superior to thermally evaporated films. Possible reasons for this are discussed.