ZnSe Thin Films Research Articles

ZnSe/ITO thin films: candidate for CdTe solar cell window layer

The crystal structure, electrical and optical properties of ZnSe thin films deposited on an In2O3:Sn (ITO) substrate are evaluated for their suitability as the window layer of CdTe thin film solar cells. ZnSe thin films of 80, 90, and 100 nm thickness were deposited by a physical vapor deposition method on Indium tin oxide coated glass substrates. The lattice parameters are increased to 5.834 Å when the film thickness was 100 nm, which is close to that of CdS. The crystallite size is decreased with the increase of film thickness. The optical transmission analysis shows that the energy gap for the sample with the highest thickness has also increased and is very close to 2.7 eV. The photo decay is also studied as a function of ZnSe film thickness.

Study on the electrical properties of ZnSe/Si heterojunction diode

ZnSe thin film is e-beam evaporated on monocrystalline p-Si to fabricate n-ZnSe/p-Si heterojunction. The electrical properties were investigated by current–voltage (I–V), capacitance–voltage (C–V) and conductance–voltage (G/w–V) measurements. The forward bias I–V characteristics were analyzed in the temperature range of 220–360 K. The fabricated diode structure exhibited rectifying characteristics with a two order rectification ratio. The current transport in the junction was modeled by the modification of thermionic emission (TE) in which the observed anomaly was related to the interfacial disorder at the junction. From this analysis, the zero-bias barrier height and ideality factor at room temperature condition were determined as 0.775 and 3.195 eV, respectively. The TE anomaly was also evaluated by considering the fluctuations due to the barrier inhomogeneity and the assumption of Gaussian distribution in barrier height. Therefore, the forward bias I–V results were used to determine the density of interface states. The frequency dependence of C–V and G/w–V characteristics of the n-ZnSe/p-Si heterostructure were studied by taking into account of the effect of the series resistance and interface states at room temperature. According to the high-low frequency capacitance and Hill-Coleman methods, density of interface states was calculated and these experimental values were found in decreasing behavior with increasing frequency. The voltage and frequency dependence of series resistance values obtained from C–V and G/w–V measurements were also related to the insulator layer and the distribution density of interface states.

Influence of various complexing agents on structural, morphological, optical and electrical properties of electrochemically deposited ZnSe thin films

Electrochemical deposition (ECD) of ZnSe thin films from aqueous solution containing ZnSO4·7H2O, SeO2 and different complexing agents at bath maintained at 50 °C were investigated. The voltammetric curves were recorded in order to characterize the electrochemical behavior of Zn2+/SeO2 system with different complexing agent and to apply the appropriate range of potential for ECD. GAXRD studies confirms that as-deposited ZnSe thin films using with different complexing agents belongs to the hexagonal phase with (101) plane as the dominant peak. When EDTA, hydrazine hydrate and triethanolamine as complexing agents used, as-deposited films were shows that peaks due to the hexagonal phase along with hexagonal Se reflection observed. EDAX, AES, FESEM and UV–Visible spectrum were used to examine the compositions, morphologies and band gap of ZnSe thin films deposited on ITO glass substrate. A broad photoluminescence (PL) emission peak has been confirmed by a systematic blue-shift in emission peak due to the formation of nanocrystalline ZnSe thin films. In the Raman spectra, high intensity peak of LO phonon mode is observed at 251 cm−1 using ammonia, citric acid, ethylenediamine and polyvinyl alcohol as a complexing agents. For EDTA, hydrazine hydrate and triethanolamine as a complexing agents used, the phonon was shifted toward lower frequencies. The conductivity type, resistivity, carrier concentrations and electron mobility were measured for the as-deposited ZnSe thin films. As-deposited ZnSe thin films using all complexing agents have n-type conductivity. Electrochemical impedance spectroscopy (EIS) indicated that the ethylenediamine as a good complexing agent for ZnSe thin film deposition and the films demonstrate a less charge transfer resistance (Rct is 11 Ω) with excellent conductivity compared to other samples.

Investigation of effect of annealing on thermally evaporated ZnSe thin films through spectroscopic techniques

ZnSe thin films have been grown on clean glass substrates by thermal evaporation technique and deposited films have been annealed at 473K. William-Hall method has been adopted to extract information on crystallite size and internal strain in the film from X-ray diffractogram. Effect of annealing on ZnSe films has been analyzed by spectroscopic techniques which include optical absorption, Raman, and photoluminescence spectroscopy. From optical absorption, band gap has been estimated along with other optical parameters like refractive index and extinction coefficient. Also, Urbach tail, which originates near bad edge due to structural disorders, has been characterized. Raman spectra have been analyzed to get the information on the influence of crystallite size and strain effect on peak position, intensity and width. Photoluminescence spectra have been recorded and analyzed to get an insight on defect levels induced due to vacancies, interstadials, and impurity complexes.

Oxygen incorporation in wide band gap semiconductor ZnSe thin films

Oxygen incorporation into ZnSe thin films in Ar/O2 or O2 ambient during radio-frequency sputtering was studied in this paper. With the increase of oxygen partial pressure ratio, the poor crystalline quality occurred in the films accompanying with a SeO2 secondary phase. The values of optical band gap for the films increased after light oxygen doping, ranging from 2.65 to 2.88 eV, which indicates the formation of ZnSeO ternary compounds. The near-band edge emission was observed in all samples, i.e., electronic transitions from the valence band to the conduction band. Nevertheless, heavy incorporation of O in ZnSe thin films led to the formation of the thin films with wide energy band gap (∼5.61 eV) and high optical transmittance (∼90%). X-ray photoelectron spectroscopy spectra indicate that oxygen atoms ionized by plasma enhanced the formation of SeO2 bonds with Se of ZnSe under the non-equilibrium conditions and many oxygen ions incorporated in the random sites, resulting in the formation of amorphous states.

Optical, magnetic, and photoelectrochemical properties of electrochemically deposited Eu3+-doped ZnSe thin films

The various mole percent (1–5%) of Eu3+-doped ZnSe thin films were fabricated on the indium-doped tin oxide (ITO) conducting glass substrate by single-step electrochemical deposition (ECD) process in an aqueous medium at 50 °C. The structural, optical, magnetic, and electrochemical properties were characterized as a function of the Eu3+ ion concentration. The X-ray diffraction (XRD) analyses evidenced that the films were hexagonal wurtzite structure along with the (101) preferential orientation. High-resolution scanning electron microscopy (HRSEM) results revealed that the thin films show a spherical like structure for 1–3% of Eu3+-doped ZnSe films. Further, increasing of Eu3+ concentration (4 and 5%), the surface morphology of thin films was observed as agglomerated grain-like structure. The band gap energy of Eu3+-doped ZnSe thin films (2.35 to 2.49 eV) determined by UV-Vis spectra showed a blue shift of absorption edge compared to the pure ZnSe thin film (2.33 eV). The increased band gap by doping of Eu3+ is due to the quantum size effect. The PL emission intensity enhanced by increasing Eu3+ concentration which revealed the enhanced radiative recombination in the luminescence process. The magnetic study revealed that Eu3+-doped ZnSe thin films were ferromagnetic in nature. Electrochemical impedance analysis indicated that 4% of Eu3+-doped ZnSe thin films showed a lower charge transfer resistance (352 Ω) and excellent properties compared to the other samples. Further, the photoelectrochemical measurements carried out for the optimized 4% Eu3+-doped ZnSe thin film revealed the faster migration of photoinduced charge carriers. The present investigation demonstrates that the electrochemically deposited Eu3+-doped ZnSe thin film is a promising candidate for electrochemical device applications.

Electrical and photoresponse properties of vacuum deposited Si/Al:ZnSe and Bi:ZnTe/Al:ZnSe photodiodes

The paper reports fabrication and characterization of Bi:ZnTe/Al:ZnSe and Si/Al:ZnSe thin film photodiodes. The characteristics of the devices were studied under dark and illuminated conditions. The normalized spectral response, speed of photoresponse and variation of photocurrent with power density were studied in detail. Many vital parameters, such as diode ideality factor, barrier height, the thickness of the depletion region, trap depth, rise and decay times of photocurrent, were determined. Conduction mechanism in the photodiodes is discussed with the help of widely accepted theoretical models.

Study of structural, optical and electrical parameters of ZnSe powder and thin films

Nanocrystalline ZnSe powder and thin film forms have been synthesized via chemical bath deposition technique. The ZnSe thin films are deposited onto ultrasonically clean glass substrates in an aqueous alkaline medium using sodium selenosulphate as Se2− ion source. The ZnSe powder and thin film are characterized by structural, optical and electrical properties. It is confirmed from X-ray diffraction study that cubic phase is present in ZnSe thin film form with (111) as preferred orientation and hexagonal phase is present in ZnSe powder form with (100) as preferred orientation. Optical absorption measurement indicates the existence of direct allowed optical transition with a wide energy gap and blue shift in the fundamental edge has been observed in both cases. The optical band gap of ZnSe powder is greater than the thin film. The electrical conductivity (both dark and photoconductivity) measurements are also carried out in different temperature range and variation in activation energy has been calculated.

ANÁLOGOS CUÁNTICOS Y ELECTRODINÁMICOS APLICADOS AL SISTEMA ÓPTICO DIELÉCTRICO-SUSTRATO PARA CURSOS DE FÍSICA

Se establece la analogía entre la propagación de electrones en pozos cuánticos, con la propagación de ondas electromagnéticas entre medios dieléctricos. Al ser estudiados los fenómenos de transmisión en medios estratificados; pozos o dieléctricos, se encuentran resultados análogos. En primer lugar, se describe la transmitancia para el caso cuántico como electrodinámico. Luego, las relaciones de dispersión mediante los coeficientes de transmisión en ambos casos (electrones-luz) relacionando estos dos sistemas. Y finalmente se compara esta equivalencia con los resultados experimentales en la zona transparente (baja absorción 800-2500 (nm)) del espectro de transmisión de la película delgada de ZnSe, utilizando el método de la matriz de transferencia (MMT).

Optical quality ZnSe films and low loss waveguides on Si substrates for mid-infrared applications

Zinc selenide (ZnSe) is a promising mid-infrared waveguide material with a high refractive index and wide transparency. Optical quality ZnSe thin films were deposited on silicon substrates by RF sputtering and thermal evaporation, and characterized and compared for material and optical properties. Evaporated films were found to be denser and smoother than sputtered films. Rib waveguides were fabricated from these films and evaporated films exhibited losses as low as 0.6 dB/cm at wavelengths between 2.5 µm and 3.7 µm. The films were also used as isolation/lower cladding layers on Si with GeTe4 as the waveguide core and propagation losses were determined in this wavelength range.

Composition and Band Gap Controlled AACVD of ZnSe and ZnS<sub>x</sub>Se<sub>1-x</sub> Thin Films Using Novel Single Source Precursors

Polycrystalline thin films of ZnSe and ZnSxSe1-x have been deposited on glass substrates by Aerosol Assisted Chemical Vapour Deposition (AACVD) from bis(diethyldiselenocarbamato)zinc(II) and a 1:1 and 1:0.75 mixtures of bis(diethyldiselenocarbamato)zinc(II) and bis(diethyldithiocarbamato)zinc(II) as precursors. All films were characterized by p-XRD, SEM, EDX, Raman spectroscopy, photoluminescence (PL and UV/Vis spectroscopy. The band gap of pure ZnSe thin films was found to be 2.25 whereas the band gap of ZnSxSe1-x films varied from 2.55 to 2.66 eV depending on the sulfur content in the films. PL emission spectra showed a clear blue shift for ZnSxSe1-x films compared to ZnSe due to the sulphur content in the films which increase the band gap. The band gap of ZnSSe can be controlled by sulfur to selenium ratio in the alloy. The morphology of the ZnSe thin films changed from small randomly shaped crystallites to triangles whereas the morphology of ZnSxSe1-x was mainly based on cuboids.

Optical and structural characterization of ZnSe thin film fabricated by thermal vapour deposition technique

Zinc-selenide thin film was prepared on a soda-lime glass substrate by thermal vapor deposition technique. The X-ray diffraction data confirmed the cubic zincblende structure with a preferred orientation along (111) plane. Different models such as Debye-Scherrer, Size-Strain plot, Williamson-Hall, uniform stress deformation model and uniform deformation energy density model and the so called approximation model were adopted to analyse the XRD data. Detailed microstructural parameters such as crystallite size, strain, stress, isotropic energy density, dislocation denstiy were reported and discussed in comparison with scanning electron microscopy data. The transmission spectra was obtained by UV-Vis-NIR spectrometer in the range of 250–2500 nm at room temperature. Important optical constantssuch as refractive index and dielectric constant was estimated using Swanepoel’s envelope method. The same method was used to check the thin film thickness which was found to be ~374.9 nm and this thickness was further confirmed by Scanning Electron Microscopy (368.3 ± 19.1 nm). The optical band gap was estimated to be 2.64 eV. The dispersion of refractive index was discussed interms of empirical Cauchy dispersion relation and the dispersion was further analysed by Wemple-DiDomenico Single oscillator model which provide physically meaningful parameters such as static refractive index, oscillator energy and dispersion energy. Both the structural and optical data not only complemented each other but also implied that good quality Zinc-selenide thin film could be deposited on ordinary glass substrates.

Effect of Indium nano-sandwiching on the structural and optical performance of ZnSe films

Abstract In the current study, we attempted to explore the effects of the Indium nanosandwiching on the mechanical and optical properties of the physically evaporated ZnSe thin films by means of X-ray diffractions and ultraviolet spectrophotometry techniques. While the thickness of each layer of ZnSe was fixed at 1.0 μm, the thickness of the nanosandwiched Indium thin films was varied in the range of 25–100 nm. It was observed that the as grown ZnSe films exhibits cubic and hexagonal nature of crystallization as those of the ZnSe powders before the film deposition. The cubic phases weighs ∼70% of the structure. The analysis of this phases revealed that there is a systematic variation process presented by the decreasing of; the lattice constant, compressing strain, stress, stacking faults and dislocation intensity and increasing grain size resulted from increasing the Indium layer thickness in the range of 50–100 nm. In addition, the nanosandwiching of Indium between two layers of ZnSe is observed to enhance the absorbability of the ZnSe. Particularly, at incident photon energy of 2.38 eV the absorbability of the ZnSe films which are sandwiched with 100 nm Indium is increased by 13.8 times. Moreover, increasing the thickness of the Indium layer shrinks the optical energy band gap. These systematic variations in mechanical and optical properties are assigned to the better recrystallization process that is associated with Indium insertion which in turn allows total internal energy redistribution in the ZnSe films through the enlargement of grains.

Chemical bath deposition (CBD) of zinc selenide (ZnSe) thin films and characterisation

The Chemical Bath Deposition (CBD) technique was used to synthesise Zinc Selenide (ZnSe) thin films. To characterise ZnSe thin films for their structural, compositional, electronic and optical properties, XRD, SEM, EDX, optical absorbance and dielectric studies were conducted. To study the structure and crystallite size of ZnSe thin film, X-ray diffraction (XRD) analysis was used. By Scanning Electron Microscopic (SEM) and energy-dispersive X-ray analysis (EDX) studies, the surface morphology of these films was studied. Using UV–Visible absorption spectrum, the optical properties of the ZnSe thin films were determined. The optical band gap for a ZnSe thin film was found to be 2.8 eV. Photoluminescence emission peaks of the as-deposited ZnSe thin films were observed. At different frequencies and at room temperature for the prepared ZnSe thin films, the dielectric studies were carried out.

Ellipsometric and reflectometric characterization of thin films exhibiting thickness non-uniformity and boundary roughness

In this paper epitaxial ZnSe thin films prepared by molecular beam epitaxy onto GaAs single crystal substrates exhibiting two defects, i.e. boundary roughness and thickness non-uniformity, are optically characterized using a combination of spectroscopic ellipsometry and near-normal spectroscopic reflectometry. The influence of boundary roughness is included into optical quantity formulae by the combination of the scalar diffraction theory and Rayleigh–Rice theory. Very thin overalyers modelled by rough thin films with identically rough boundaries are taken into account on the upper boundaries of the ZnSe thin films. Two approximations are used to express the local reflection coefficient of the rough ZnSe thin films covered with the overlayers within combination of both the theories. Thickness non-uniformity is incorporated by means of averaging the elements of the unnormalized Mueller matrices. The universal dispersion model of the optical constants of the ZnSe thin films based on parametrization of the joint density of electronic states is used. The spectral dependencies of the optical constants of the ZnSe thin films are determined within the wide spectral range (0.12–8.7eV). Moreover, the mean thickness of the ZnSe thin films and thickness of overlayers are determined together with the other structural parameters characterizing the defects. The values of roughness parameters, determined by the optical method, are verified by a comparison with results achieved by atomic force microscopy. It is also shown that the approximations of the local reflection coefficient presented are usable for processing the experimental data.

Structure Analysis and Optical Parameters of Nano-scale ZnSe/Flexible Substrate Thin Film

The ZnSe thin films with different thicknesses have been deposited on polymer substrates for flexible optical devices applications. The XRD of different thicknesses for ZnSe films reveals the cubic structure of the films oriented along the (1 1 1) direction. The structural parameters such as particle size (40.41–105.24 nm) and lattice strain (6.5 × 10−3–14.7 × 10−3 lin−2m−4) were evaluated. Also AFM was used in order to obtain quantitative information on microstructure properties. The optical constants, the refractive index n and the absorption index k have been calculated from transmittance T and reflectance R through the spectral range of 400–2500 nm using Swanepoel’s method. The optical constants (n, k) were calculated in medium and transparent regions. The energy gap of direct transition for polycrystalline ZnSe thin films was calculated in the strong absorption region and found to be increased from 2.55 eV to 2.70 eV with the increasing the film thickness. ZnSe/flexible substrates are good candidates for optoelectronic devices.

Electrodeposition mechanism of ZnSe thin film in aqueous solution

ZnSe is one of the important and excellent II–VI semiconductor materials, which has direct transition band structure. In this paper, ZnSe thin films were prepared by an electrochemical deposition method, and the formation mechanism of ZnSe was studied systematically. Voltammetry and chronoamperometry combined with X-ray diffraction (XRD) and Raman techniques were used to analyze the deposition processes. It is found that the substrate and deposition potentials have a great influence on the phase composition of deposited thin film, and Zn substrate is beneficial to the preparation ZnSe films. Strong selenium–substrate interaction results in the formation of selenium compounds involving electrode materials. The addition of Zn(II) source can affect the reduction potential of Se, and results in the change of reducing mechanism of Se(0) from Se(IV). Se(0) formed from H2Se because the formation of H2Se is more active than forming Se(0) directly from Se(IV), and H2Se can recombine with the substrate material, forming selenium–substrate compounds more quickly.

Study of optical properties of thermally evaporated ZnSe thin films annealed at different pulsed laser powers

This research work is devoted to the study of optical properties of thermally evaporated and laser annealed ZnSe thin films. Thin films of zinc selenide were annealed by a CO2 pulsed laser at different powers (10, 20 and 30 Watt). As shown by X-ray diffraction analysis, all the studied films exhibit cubic zinc blende crystal structure with dominant preferred orientation along the plane (111). Optical transmission spectra, were investigated in the spectral range 300–2500 nm at room temperature. Swanepoel’s envelope method was employed to check the film thicknesses, as well as to evaluate the refractive index, absorption coefficient and the extinction coefficient by using transmission measurements only. Optical energy gap was determined using Tauc plot extrapolation. It was found to be increased from 2.67 to 2.90 eV with increasing the pulsed laser annealing power and it is due to allowed direct transition. Using Wemple-DiDomenico single oscillator model, the dispersion of the refractive index and energy dispersion parameters together with their dependences on the laser annealing power have been studied.

Constraints for ZnSe thin film growth and stoichiometry regulation

The main theme in thin film science is to control the stoichiometry of chemical constituents to form a quality yield supporting functional applications. Here, we have attempted the constraint formulation for stoichiometry regulation of ZnSe thin films by an industry preferred chemical deposition. The as-deposited films were characterized through the elemental, compositional, structural, morphological, optical and electrical transport studies. X-ray photoelectron spectroscopy confirmed the +2 and −2 oxidation states of Zn and Se in as-grown thin films. The compositional analysis further suggested near-stoichiometric film formation for TEA = 0.3 ml, NH3 = 17 ml and N2H4 = 6 ml. Structural assessment confirmed the formation of poly-microcrystalline thin films with cubic zinc blend structure. Spherical crystallites with uneven intercrystalline spacing were observed in microstructural studies. Minor variation in the bandgap values was attributed to stoichiometric deviations and microstructural amendments. The electrical transport studies revealed temperature dependence of electrical conductivity.

Post deposition annealing: a route to bandgap tailoring of ZnSe thin films

The ZnSe semiconductor thin films thermally evaporated on a glass substrate and an ITO substrate are studied and compared for their optical properties. Surprisingly, two different values of band gaps 2.49 and 2.91 eV in ZnSe thin films are observed, respectively deposited on glass substrate and ITO substrate. Similarly, the ZnSe films on ITO substrate has shown a hexagonal crystal structure with higher crystallinity as compared to that of ZnSe on glass. The ZnSe thin films deposited on glass substrate are also annealed at 350, 400, 450 and 500 °C under vacuum conditions, which has been resulted in improvement in crystallinity and increase in band gap energy from 2.45 to 2.54 eV. The ZnSe thin film deposited on ITO coated glass were annealed in vacuum and air at 450 °C. The air annealing has been resulted in a large increase in energy bandgap from 2.9 to 3.21 eV. It is also observed from these studies that the electrical conductivity of the films annealed in vacuum is higher than those of the annealed in air.