Quasi-closed Volume Technique Research Articles

On the mechanism of electronic transport in polycrystalline CdO thin films

Cadmium Oxide (CdO) thin films ( d = 0.16−0.62 μm) were deposited onto glass substrates by thermal evaporation under vacuum (quasi closed volume technique) of high purity (99.99%) CdO polycrystalline powders. The substrate temperature was 300 and 473 K, respectively. After a post-deposition heat treatment, the temperature dependence of the electrical conductivity becomes reversible. The electronic transport mechanism in studied samples is explained in terms of Seto’s model for polycrystalline semiconducting films. The values of optical bandgap have been determined from absorption spectra.

Synthesis of thermally evaporated ZnSe thin film at room temperature

Zinc selenide (ZnSe) thin film on glass substrates were prepared by thermal evaporation under high vacuum using the quasi-closed volume technique at room temperature (300 ± 2 K). The deposited ZnSe properties were assessed via X-ray diffraction, atomic force microscope (AFM), UV-Vis specrophotometry, Raman spectroscopy, photo-luminescence, Fourier transform infrared spectroscopy (FT-IR) and spectroscopic ellipsometry. The X-ray diffraction patterns of the film exhibited reflection corresponding to the cubic (111) phase (2θ = 27.20°). This analysis indicated that the sample is polycrystalline and have cubic (Zinc blende) structure. The crystallites were preferentially oriented with the (111) planes parallel to the substrates. The AFM images showed that the ZnSe films have smooth morphology with roughness 6.74 nm. The transmittance spectrum revealed a high transmission of 89% in the infrared region (≥ 600 nm) and a low transmission of 40% at 450 nm. The maximum transmission of 89.6% was observed at 640 nm. Optical band-gap was calculated from the transmission data of specrophotometry, photo-luminescence and ellipsometry and was 2.76, 2.74 and 2.82 eV respectively. Raman spectroscopic studies revealed two longitudinal optical phonon modes at 252 cm -1 and 500 cm -1. In photoluminescence study, the luminescence peaks was observed at 452 nm corresponding to band to band emission. FT-IR study illustrated the existence of Zn-Se bonding in ZnSe thin film. The optical constants were calculated using spectroscopic ellipsometry and were determined from the best fit ellipsometric data in the wavelength regime of interest from 370–1000 nm. These results manifested excellent room temperature ZnSe synthesis and characteristics for opto-electronics technologies.

Influence of the oxidation conditions on the structural characteristics and optical properties of zinc oxide thin films

ZnO thin films were obtained by two-step oxidation processes of Zn films: heating up to 575 K for 1–2 h followed by heating at higher temperature (775 K) for 1 h. Zinc thin films (d=450 nm) were deposited by the quasiclosed volume technique under vacuum onto glass substrates maintained at room temperature. The investigation of film structure, performed by x-ray diffraction technique, corroborated with atomic force microscopy, confirmed that as-prepared zinc films are polycrystalline. The as-prepared ZnO films have a polycrystalline wurtzite (hexagonal) structure, with film crystallites preferentially oriented with (002) planes parallel to the substrate. Transmission and absorption spectra were recorded in the spectral domain from 300 to 1400 nm. The energy band gap for respective films determined from absorption spectra ranged between 3.02 and 3.06 eV.

Structural and electrical properties of zinc oxides thin films prepared by thermal oxidation

We report on zinc oxide (ZnO) thin films ( d = 55–120 nm) prepared by thermal oxidation, at 623 K, of metallic zinc films, using a flash-heating method. Zinc films were deposited in vacuum by quasi-closed volume technique onto unheated glass substrates in two arrangements: horizontal and vertical positions relative to incident vapour. Depending on the preparation conditions, both quasi-amorphous and (0 0 2) textured polycrystalline ZnO films were obtained. The surface morphologies were characterized by atomic force microscopy and scanning electron microscopy. By in situ electrical measurements during two heating–cooling cycles up to a temperature of 673 K, an irreversible decrease of electrical conductivity of as flash-oxidized Zn films was revealed. The influence of deposition arrangement and oxidation conditions on the structural, morphological and electrical properties of the ZnO films is discussed.

Structural characterization and optical properties of ZnSe thin films

Zinc selenide (ZnSe) thin films ( d = 0.11–0.93 μm) were deposited onto glass substrates by the quasi-closed volume technique under vacuum. Their structural characteristics were studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The experiments showed that the films are polycrystalline and have a zinc blende (cubic) structure. The film crystallites are preferentially oriented with the (1 1 1) planes parallel to the substrate surface. AFM images showed that the films have a grain like surface morphology. The average roughness, R a = 3.3–6.4 nm, and the root mean square roughness, R rms = 5.4–11.9 nm, were calculated and found to depend on the film thickness and post-deposition heat treatment. The spectral dependence of the absorption coefficient was determined from transmission spectra, in the range 300–1400 nm. The values of optical bandgap were calculated from the absorption spectra, E g = 2.6–2.7 eV. The effect of the deposition conditions and post-deposition heat treatment on the structural and optical characteristics was investigated.

Preparation and characterization of ZnO thin films prepared by thermal oxidation of evaporated Zn thin films

In this paper, the experimental results regarding some structural, electrical and optical properties of ZnO thin films prepared by thermal oxidation of metallic Zn thin films are presented. Zn thin films (d=200–400 nm) were deposited by thermal evaporation under vacuum, onto unheated glass substrates, using the quasi-closed volume technique. In order to obtain ZnO films, zinc-coated glass substrates were isochronally heated in air in the 300–660 K temperature range, for thermal oxidation. X-ray diffraction (XRD) studies revealed that the ZnO films obtained present a randomly oriented hexagonal nanocrystalline structure. Depending on the heating temperature of the Zn films, the optical transmittance of the ZnO films in the visible wavelength range varied from 85% to 95%. The optical band gap of the ZnO films was found to be about 3.2 eV. By in situ studying of the temperature dependence of the electrical conductivity during the oxidation process, the value of about 2×10−2 Ω−1 m−1 was found for the conductivity of completely oxidized ZnO films.

On the structural changes during thermal oxidation of evaporated Zn thin films

Metallic Zn thin films ( d = 200 nm–500 nm) were deposited in vacuum onto unheated glass substrate by quasi-closed volume technique at a source temperature of 723 K. Sets of samples simultaneously deposited on horizontally and vertically arranged substrates were prepared. The as-deposited Zn films were heated under ambient atmosphere at various temperatures ranged between 300 K and 650 K. By XRD and AFM techniques, the microstructural characteristics and their changes during Zn films heating were investigated. The influence of the deposition conditions on the structural changes during the oxidation process is also discussed.

Microstructural characterization and optical properties of ZnSe thin films

Zinc Selenide (ZnSe) thin films (d=0.09–1.25μm) on glass substrates were prepared by physical vapour deposition under vacuum, using the quasi-closed volume technique. X-ray diffraction (XRD) analysis indicated that the samples are polycrystalline, have a cubic (zinc blende) structure and that the crystallites are preferentially oriented with the (111) planes parallel to the substrates. The microstructure of ZnSe films examined by transmission electron microscopy (TEM), including electron diffraction (ED) and high-resolution electron microscopy (HRTEM), confirms the preferential orientation of crystallites. The HRTEM micrographs show that preferred orientation of crystallites, along (111) planes, increases if, after deposition, the samples are subjected to a heat treatment. The spectral dependences of the transmission and absorption coefficients were studied in the range 300–1400nm. The values of the bandgap energy, Eg, calculated from the absorption spectra, ranged between 2.50eV and 2.80eV.

ON THE OPTICAL CHARACTERISTICS OF CdS THIN FILMS DEPOSITED BY QUASI-CLOSED VOLUME TECHNIQUE

Cadmium sulfide (CdS) thin films (d=0.15–1.15 μ m ) were deposited onto glass substrates by the quasi-closed volume technique under vacuum. The investigations shown that the films are polycrystalline and have a hexagonal structure. It was experimentally established that the films with stable structure can be obtained if they are submitted to a heat treatment, consisting of several succesive heating/cooling cycles within a given temperature range (ΔT=300–600 K ), the temperature dependence of the electrical conductivity becomes reversible. For heat-treated samples, the values of thermal activation energy calculated from the temperature dependence of the electrical conductivity, ranged between 2.30–2.45 eV. The spectral dependences of the transmission and absorption coefficients were studied in the range 500–1400 nm. The influence of heat treatment on the shape of the absorption spectra and dispersion index of refraction is studied for samples with different thickness. Optical energy gap, calculated from the absorption spectra was in the range 2.30–2.5 eV.

On the electronic transport properties of polycrystalline ZnSe films

Zinc selenide (ZnSe) thin films were deposited onto glass substrates by the quasi-closed volume technique under vacuum. The film structure was studied by X-ray diffraction (XRD) technique, scanning electron microscopy and atomic force microscopy (AFM). The investigations showed that the films are polycrystalline and have a cubic (zinc blende) structure. The films with stable structure can be obtained if they, after deposition, were submitted to a heat treatment. The mechanism of electrical conduction is discussed in terms of Seto’s model for polycrystalline semiconducting films. The values of optical energy gap have been determined from the absorption spectra.

Optical properties of CdSe films deposited by the quasi-closed volumetechnique

Polycrystalline CdSe thin films (d = 0.08-1.65 µm) have beenprepared by the quasi-closed volume technique under vacuum on glasssubstrates. The film structure was studied by the x-ray diffraction techniqueand atomic force microscopy.The values of the optical parameters (refractive index, n, and absorptioncoefficient, α) were determined from transmission spectra. The opticalenergy gap for the obtained CdSe films was also determined. The influence ofdeposition conditions (substrate temperature, evaporation source temperature)on the values of optical parameters is investigated.