Vacuum Thermal Evaporation Technique Research Articles

Advances in fractal germanium micro/nanoclusters induced by gold: microstructures and properties.

Germanium materials are a class of unique semiconductor materials with widespread technological applications because of their valuable semiconducting, electrical, optical, and thermoelectric power properties in the fields of macro/mesoscopic materials and micro/nanodevices. In this review, we describe the efforts toward understanding the microstructures and various properties of the fractal germanium micro/nanoclusters induced by gold prepared by high vacuum thermal evaporation techniques, highlighting contributions from our laboratory. First, we present the integer and non-integer dimensional germanium micro/nanoclusters such as nanoparticles, nanorings, and nanofractals induced by gold and annealing. In particular, the nonlinear electrical behavior of a gold/germanium bilayer film with the interesting nanofractal is discussed in detail. In addition, the third-order optical nonlinearities of the fractal germanium nanocrystals embedded in gold matrix will be summarized by using the sensitive and reliable Z-scan techniques aimed to determine the nonlinear absorption coefficient and nonlinear refractive index. Finally, we emphasize the thermoelectric power properties of the gold/germanium bilayer films. The thermoelectric power measurement is considered to be a more effective method than the conductivity for investigating superlocalization in a percolating system. This research may provide a novel insight to modulate their competent performance and promote rational design of micro/nanodevices. Once mastered, germanium thin films with a variety of fascinating micro/nanoclusters will offer vast and unforeseen opportunities in the semiconductor industry as well as in other fields of science and technology.

Annealing effect on structural and optical properties of Se87.5Te10Sn2.5 thin films

Thin films of Se87.5Te10Sn2.5 were prepared by vacuum thermal evaporation technique. Various optical constants were calculated for the studied composition. The mechanism of the optical absorption follows the rule of direct transition. It was found that the optical energy gap (Eg) decreases from 2.26 to 1.79eV with increasing the annealing temperature from 340 to 450K. This result can be interpreted by the Davis and Mott model. On the other hand, the maximum value of the refractive index (n) is shifted towards the long wavelength by increasing the annealing temperature. In addition, the high frequency dielectric constant (εL) increased from 31.26 to 48.11 whereas the ratio of the free carriers concentration to its effective of mass N/m⁎ decreased from 4.3 to 2.09 (×1057(m−3Kg−1)). The influence of annealed temperature on the structure was studied by using the X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD studies show that the as-deposited films are amorphous in nature, but the crystallinity improved with increasing the annealing temperature. Furthermore the particle size and crystallinity increased whereas the dislocation and strains decreased with increasing the annealing temperature. SEM examination showed that the annealing temperature induced changes in the morphology of the as-deposited film.

Investigation of New Organic Photocathodes in Vacuum for Position Sensitive Gaseous Detectors

Position sensitive gaseous detectors with reflective photocathodes provide an attractive solution for photon localization over very large sensitive areas, under intense magnetic fields mainly due to their ease of construction. We report Quantum efficiency measurement of two new organic semiconductor photocathodes PEDOT/PSS and TECEB in spectral region 190-250nm, which are proposed for use in photosensitive gaseous detectors. Photocathodes were prepared by using resistive thermal vacuum evaporation technique. QE were measured in vacuum at about 10 -4 Torr and both the photocathodes exhibit non-negligible quantum efficiency up to 230nm.

Modification of some Optical and Mechanical Properties of Amorphous As-S-Se Thin Films by Copper Introduction

Abstract. In this research experimental investigation of the influence of copper introduction on some relevant parameters in As-S-Se amorphous thin films is performed. Copper is introduced into As2(S0.5Se0.5)3 amorphous thin film in concentration of 3 at.%. Samples of As2(S0.5Se0.5)3 and Cu3(As2(S0.5Se0.5)3)97 amorphous thin films are prepared by the vacuum thermal evaporation technique from previously synthesized bulk samples. Envelope method is applied for the determination of the optical constants, using the transmission and reflection spectra. The dispersion of the refractive index is discussed in terms of the single oscillator model proposed by Wemple–DiDomenico. Values of absorption coefficients in the high absorption region are discussed according to Tauc's law.Instrumented indentation testing is performed, using the Berkovich geometry indenter, for obtaining the value of nano-hardness.All the determined parameters have shown the increase with introduction of copper into amorphous thin film.

The Effect of Changing the Vapor Flux on Physical Properties of Nanocrystaline CdTe Thin Film, Prepared by Thermal Evaporation Method

In the present investigation, seven Cadmium Telluride (CdTe) thin films with thickness of 350nm have been prepared by vacuum thermal evaporation technique. One of them was deposited by conventional vacuum thermal evaporation technique, while six others were deposited with applying a novel vapor flow controlling system. Placing this equipment in the way of vapor flux from the source to the substrate cut the depositing flux of vapor periodically and can change the way of vapor flux. It led to considerable change of optical properties, nanocrystalline structure, and also stochiometery of the thin films. All the samples showed nanocrystalline structure. A considerable increase in absorption coefficient was observed at 520nm wavelength of light, from 5.5 ×104cm-1for the sample prepared with conventional vacuum thermal evaporation technique to 9.8×104cm-1for a sample which prepared by applying mentioned equipment. The most considerable change in the bandgap, grain size and Te/Cd ratio were 1.54 eV to 1.66 eV, 40nm to 15nm, and 56/44 to 60/40 respectively. The optical parameters such as absorption coefficient, bandgap, refractive index, and structural parameters such as texture coefficient, preferential orientation factor and crystallite size of samples were obtained. Also EDAX and FESEM result of samples were compared with each other in this study.

Light outcoupling efficiency enhancement in organic light emitting diodes using an organic scattering layer

AbstractCrystallized 4,7‐diphyenyl‐1,10‐phenanthroline (BPhen) films deposited by convenient vacuum thermal evaporation technique have been found to be an efficient means to extract the substrate wave guided light in organic light emitting diodes (OLEDs). The optimized BPhen film working as organic scattering layer was successfully used with OLEDs for light outcoupling efficiency improvement. Enhancement of 26%, 15% and 6% in efficiency of the blue, green and red OLEDs were obtained, respectively. The achievement was found to be advantageous in terms of simplicity of fabrication method and feasibility for large area OLED applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and optical properties of thermally evaporated Sb doped Ge–Se thin films

Ge25Se75−xSbx chalcogenide thin films have been prepared by thermal vacuum evaporation technique with thickness 400 nm on glass/Si wafer substrates. The effects of composition, thermal annealing near glass transition temperature and laser-irradiation on the optical band gap and structural properties of these thin films were investigated. The glass transition temperature of Ge25Se75−xSbx chalcogenide glass was measured by non-isothermal Differential Scanning Calorimetric measurements. The influence of Sb content in Ge25Se75−xSbx system results in a gradual decrease in indirect optical gap from 1.86 to 1.43 eV, this behavior can be explained as increased tailing. On increasing the annealing temperature and laser-irradiation time, the optical band gap also decreases gradually for the crystallized films; this behavior can be attributed to transformation from amorphous to crystalline and was explained in the light of dangling bond model. The effect of annealing/laser irradiation on the nature and degree of crystallization has been investigated by studying the structure using X-ray diffraction and scanning electron microscope.

Perylene diimide: Synthesis, fabrication and temperature dependent electrical characterization of heterojunction with p-silicon

A novel, n-type, organic semiconductor N-Butyl-N′-(6-hydroxyhexyl)perylene-3,4,9,10-tetracarboxylic acid diimide (N-BuHHPDI) has been successfully synthesized in high yield. The compound has been characterized by atomic force microscopy (AFM) to understand the morphological properties of a new n-type organic semiconducting material. A 120nm thin film of N-BuHHPDI has been sandwiched between Al and p-Si to form Al/N-BuHHPDI/p-Si device using the vacuum thermal evaporation technique. The electrical properties of sandwich type Al/N-BuHHPDI/p-Si device have been investigated. The current voltage (I–V) characteristics of the device, in dark, have been measured in the temperature range of 300–330K. At room temperature, the device exhibits rectifying behavior with a rectification ratio of 51.5 at ±6.8V. The device parameters such as ideality factor, barrier height, series and shunt resistances have been extracted using the conventional I–V characterization method. The effect of temperature on these parameters is also studied. Alternative electrical characterization methods such as Cheung's functions and Norde's techniques have been employed to measure the device parameters for comparison. The conduction mechanisms are investigated through the interface of N-BuHHPDI and p-Si.

Investigation of structural and optical properties of the sulfosalt SnSb4S7 thin films

Sulfosalt SnSb4S7 thin films could be used as a potential candidate in many technological applications such as photovoltaic solar cells due to its high absorption coefficient. SnSb4S7 thin films were obtained by the thermal vacuum evaporation technique. Here we investigate the effect of the annealing process on the structural, morphological and optical properties of these thin films. The films were annealed in air atmosphere in the temperatures range 100–250°C for 1h. Absorption coefficients higher than 104cm−1 were found.

Effects of annealing on the optical and electrical properties of CdO thin films prepared by thermal evaporation

Cadmium oxide (CdO) thin films with a thickness of approximately 298nm were prepared on glass substrates at room temperature via thermal evaporation technique in vacuum (∼2.2×10−5mbar). X-ray diffraction results show that the films were polycrystalline structure and had a cubic crystal. CdO thin films and peaks became more crystalline and sharp after annealing at 500°C for 1h. The optical direct band gap energy was determined by optical absorption using an UV/vis spectrophotometer. The band gap energy was 2.48eV before annealing, and slightly decreased to 2.42eV after annealing. Electrical measurement results show that the resistivity decreased with increasing carrier concentration after annealing. Hall-effect measurement results reveal that all of the thin films were n-type.

The Effect of Sulfurisation Temperature on Structural Properties of CuAlS2 Thin Films

CuAlS2 thin films of the same thickness are deposited on suitably cleaned coning 7059 glass substrate by two stage vacuum thermal evaporation technique at room temperature and sulfurised at different temperatures. The structural properties studied by means of XRD revealed that the films are of crystalline in nature having tetragonal structure. For all the films the preferential orientation is (112). The other orientations like (220), (312) and (400) were also observed in the films depending upon the sulfurisation temperatures. The values of lattice constants, grain size, micro strain and dislocation density of the films are calculated and the values agree strongly with ICDD data. Visual inspection of (SEM) micrographs of the films showed that crystallite size increase with increase in sulfurisation temperature.

Electrical and optical properties of copper-complexes thin films grown by the vacuum thermal evaporation technique

In this work, the synthesis and characterization of molecular materials formed from K2[Cu(C2O4)2], 1,8-dihydroxyanthraquinone and its potassium salt are reported. These complexes have been used to prepare thin films by vacuum thermal evaporation. The synthesized materials were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), fast atomic bombardment (FAB+) mass and ultraviolet–visible (UV–vis) spectroscopy. Electrical transport properties were studied by dc conductivity measurements. The electrical activation energies of the complexes, which were in the range of 0.36–0.65 eV, were calculated from their Arrhenius plots. Optical absorption studies in the 100–1100 nm wavelength range at room temperature showed thin films' optical band gaps in the 2.3–3.9 eV range for direct transitions. On the other hand, strong visible photoluminescence (PL) at room temperature was noticed from the thermally-evaporated thin solid films. The PL of all investigated samples were observed with the naked eye in a bright background. The PL and absorption spectra of the investigated compounds are strongly influenced by the molecular structure and nature of the organic ligand.

Influence of laser-irradiation on structural and optical properties of phase change Ga25Se75−xTex thin films

Ga25Se75−xTex chalcogenide thin films of thickness 400nm were deposited by thermal vacuum evaporation technique. These films were irradiated by transversely-excited atmospheric-pressure (TEA) nitrogen laser for 15min. As-prepared and laser-irradiated thin films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and UV/VIS/NIR spectroscopy. The XRD results revealed the amorphous nature of as-prepared films whereas, the laser-irradiated thin films showed the polycrystalline nature. The results were discussed in terms of the structural change, i.e. amorphous to polycrystalline phase change after laser irradiation. These structural changes were associated with the interaction of the incident photon and the lone-pair electrons, which affects the band gap of the Ga25Se75−xTex thin films. It was observed that the optical band gap decreases whereas, the absorption coefficient increases with increasing Te concentration and also inducing laser-irradiation. This decrease in the optical band gap was explained on the basis of changes in the structural behavior of films, i.e. from amorphous to polycrystalline state.

Unique hierarchical structure and high thermoelectric properties of antimony telluride pillar arrays

The p-type antimony telluride (Sb2Te3) pillar arrays with unique hierarchical architecture have been self-assembled in large scale by a simple vacuum thermal evaporation technique. The composition and the microstructure of the films are studied by X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy. The results show that the hierarchical film with multi-scale and multi-dimensional structure is well-oriented pillar arrays perpendicular to the substrate. A large number of nanowires (NWs) are assembled into a submicro/micro-scaled pillar, while antisite defects and dislocations as well as other defects are found in the one-dimensional NWs. The growth mechanism of such nanostructure is proposed and investigated. The thermoelectric (TE) properties, i.e., electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (κ) of the films are measured. The properties of the hierarchical Sb2Te3 film have been greatly enhanced in comparison with those of the ordinary Sb2Te3 films. A TE dimensionless figure-of-merit ZT = 0.88 in the novel hierarchical Sb2Te3 film is obtained at room temperature.

Al-Induced Crystallization of Amorphous Ge and Formation of Fractal Ge Micro-/Nanoclusters

Results on Al-induced crystallization of amorphous Ge (a-Ge) deposited by vacuum thermal evaporation techniques under thermal annealing in N(2) atmosphere are presented in detail. The a-Ge crystallization and fractal Ge pattern formation on the free surface of annealed Al/Ge bilayer films deposited on single-crystal Si (100) substrates were investigated by using scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), energy dispersive X-ray spectrometry (EDS), and Raman spectra. It is found that the temperature field effects play an extremely crucial role in a-Ge crystallization and fractal Ge formation process. The open branched structure of fractal Ge clusters in Al/Ge bilayer films was effectively prepared by Al-induced crystallization when they were annealed at 400 °C for 60 min. These films with fractal Ge clusters exhibit charming noninteger dimensional nanostructures, which differ from those of conventional integer dimensional materials such as one-dimensional nanowires/nanorods, nanotubes, nanobelts/nanoribbons, two-dimensional heterojunctions, thin films, and zero-dimensional nanoparticles. The SEM image shows that a big Al grain was found located near the center of a fractal Ge cluster after the films were annealed at 400 and 500 °C for 60 min. This suggests that the grain boundaries of polycrystalline Al films are the initial nucleation sites of a-Ge. It also validates the preferred nucleation theory of a-Ge at triple-point grain boundaries of polycrystalline Al at the interface. This discovery may be explained by the metal-induced nucleation (MIN) mechanism.

White electroluminescence from hybrid organic inorganic LEDs based on thermally evaporated nanocrystals

Light-emitting diodes (LEDs) with a broad spectral emission have been fabricated using co-evaporated monolayers of cadmium sulfide (CdS) and cadmium telluride (CdTe) nanocrystals in a hybrid organic-inorganic structure. The nanocrystals of CdS and CdTe were grown by vacuum thermal evaporation technique. The incorporation of 50:50 ratio of CdS and CdTe resulted in a broad emission spectrum with CIE coordinates (0.22, 0.33). The fine tuning of the emission spectrum to achieve pure white light (CIE coordinates (0.30, 0.33)) was achieved by incorporating a small quantity of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyan (DCM) dye in the device structure. The study indicates a promising route towards more stable and efficient light-emitting devices for lighting applications.

Studying the Optical Properties of CdS Thin Films Prepared by Thermal Vacuum Evaporation Technique with a Different Thickness

CdS thin films with a different thickness have been prepared on glass substrates by using the thermal vacuum evaporation technique at substrate temperature of 150oC. The optical characteristics of the prepared thin films have been investigated by UV-VIS spectrophotometer in the wavelength range (300-1100 nm) . The films have a direct allow electronic transitions and the optical absorption are shifted to the low energies by increasing the thickness . Also the optical energy gap (Eg) has decreased from 2.47 eV to 2.22 eV by increasing the thickness value. The extinction coefficient (k) , refractive index (n) and the real and imaginary dielectric constants (ε1 , ε2 ) are investigated. All of these constants increased with the increases in thickness and shifted to the low energies

Photoconductivity of Se85−xTe15Hgx thin films

The photoconductive properties such as dark conductivity, steady state and transient characteristics of a-Se85−xTe15Hgx thin films, prepared by thermal vacuum evaporation technique have been studied in the temperature range 312–380K. Analysis of data shows that the activation energy of dark current is greater as compared to the activation energy of photocurrent. The activation energy increases at higher concentration of Hg which shows that the defect density of states decreases. Analysis of intensity dependent photoconductivity shows that the bimolecular recombination is predominant. The transient photoconductivity shows that the carrier lifetime decreases with the increase in Hg concentration and increases at higher concentration of Hg. This decrease is due to the transition trapping process. Further the photosensitivity and carrier lifetime increases at higher concentration of Hg which also confirms that the density of defect states decreases.

Preparation of CdTe nanostructures with different crystal structures and morphologies

CdTe nanorods and various branched nanostructures with different crystal structures were have successfully prepared via the catalyst-assisted vacuum thermal evaporation (CVTE) technique using various experimental parameters. SEM and XRD studies were carried out on the as-prepared CdTe nanostructures. The results show that the morphologies and crystal structures of the products were strongly influenced by the growth conditions and the mole ratios of Bi and CdTe. In the high mole ratio (0.08:1) of Bi and CdTe, CdTe branched nanostructures of CdTe were obtained, while nanorods of CdTe were formed at a lower mole ratio of 0.05:1. The crystal structure of products is either Zinc blende or a two-phase mixture of zinc blende and wurtzite. The content of the wurtzite phase were found to increase with increasing growth temperature. Our results also reveal that high growth temperature tends to form the wurtzite phase, and stacking faults may exist in materials grown in higher temperatures. These nanostructures grow following the vapor–liquid–solid (VLS) mechanism.

Influence of annealing on the structure and optical properties of Zn 40Se 60 thin films

Thin films of Zn 40Se 60 were prepared by the vacuum thermal evaporation technique. The influence of annealed temperature on the structural and optical properties was investigated using the X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical transmission. The XRD studies show that the as-deposited film is amorphous in nature, but the crystallinity improved with increasing the annealing temperature. Furthermore the particle size and crystallinity increased whereas the dislocation and strains decreased with increasing the annealing temperature. SEM studies showed that the annealing temperature induced changes in the morphology of the as-deposited sample. Various optical constant have been calculated for as-deposited and annealed films. The mechanism of the optical absorption follows the rule of direct transition. It was found that, the optical energy gap ( E g ) decreased with increasing the annealing temperature. These results can be interpreted by the Davis and Motte model. On the other hand the maximum value of the refractive index ( n) is shifted toward the long wavelength by increasing the annealing temperature.