ZnTe Thin Films Research Articles

Ovonic Threshold Switching Selection Device Based on Chalcogenide ZnTe for Cross-Point ReRAM Device

Cross-point array structure has attracted increasing attention for high-density ReRAM, owing to the development of ReRAM with promising scalability. Cross-point array of only resistors(1R) cannot function because of sneak current through unselected devices, which can be reduced by integrating resistive memory elements with bidirectional nonlinear selection devices(1S1R). Namely, the high value of Kw in ON states, which is the writing current @ Vwrite / the current @ 1/2Vwrite, was required in cross-point ReRAM memory industry. In this study, the high current density non-linear IV curve of Ovonic threshold switching(OTS) selection device based on Chalcogenide ZnTe was shown and the ALD HfO2 switching device has the linear properties of ON states and the compliance current of 100uA. In 1S1R unit cells based on ZnTe OTS selection device of low off current (< 400nA) and HfO2 switching device, excellent non-linear characteristics of ON states such as Kw of 522, Kr (reading current @Vread / current @ 1/2Vread) of 1430 and on/off ratio of 43 were shown. OTS behaviors with ZnTe film thickness, device area and annealing temperatures were investigated. Initial current during electro-forming process was decreased but OTS characteristics were not changed with increasing ZnTe thickness and decreasing device area. Initial current in electro-forming process and OTS characteristics at annealing temperature of below 200°C was not changed, but initial current and off current was rapidly increased and threshold voltage (Vth) in OTS was decreased at above 250°C because of Te diffusion in ZnTe film. To suppress the diffusion of Te atoms, the reactive N2during deposition which can improve the thermal stability of the chalcogenide glass was performed. And chemical compositions of ZnTe thin films were analyzed by Rutherford backscattering spectrometry(RBS). Also, crystallinity as a function of annealing temperature was investigated by Raman spectroscopy, X-ray diffraction (XRD) and plan-view transmission electron microscopy (TEM).

ZnTe thin films grown by electrodeposition technique on Fluorine Tin Oxide substrates

Zinc telluride (ZnTe) thin films are prepared potentiostatically from an aqueous solution bath containing ZnCl2, TeCl4 and LiCl on Fluorine Tin Oxide coated glass substrate to investigate its suitability as a material for solar cell technology. The appropriate potential region where formation of stoichiometric ZnTe semiconductor occurs is found to be close to −0.85V versus Ag/AgCl. X-ray diffraction results have indicated that all electrodeposited films have cubic structure. Quantitative analysis of energy dispersive X-ray analysis results has shown that the composition ratio (Zn:Te) of films is around (1:0.99). A direct energy gap of 2.19eV is determined by NIR–Vis–UV spectroscopy. The effect of annealing on the crystallinity and optical properties has been reported. Impedance measurements have revealed that all grown films present a p-type electrical conductivity with acceptor density NA=2.16×1019cm−3. Furthermore, the impedance results are adjusted to an equivalent circuit having two capacitors in series (Helmholtz, CH and space charge, CSC) and two resistances RCT and RSC, which are associated with charge transfer processes.

Concentration Dependent Optical and Structural Properties of Mo doped ZnTe Thin Films Prepared by e-beam Evaporation Method

Concentration Dependent Optical and Structural Properties of Mo doped ZnTe Thin Films Prepared by e-beam Evaporation Method

Growth and Optical Properties of ZnTe Thin Films Formed by Using Pulsed Laser Deposition

Growth and Optical Properties of ZnTe Thin Films Formed by Using Pulsed Laser Deposition

Electrical properties of p-type ZnTe thin films by immersion in Cu solution

ZnTe thin films were deposited on SiO2 substrate by pulsed-laser deposition (PLD) at room temperature. The ZnTe films were immersed in different concentrations of Cu(NO3)2–3H2O solutions for 1min, then heated at 200 and 300°C, both temperatures for 10min in a N2 atmosphere. The X-ray diffraction (XRD) showed the hexagonal and orthorhombic ZnTe phases when the copper-doped films were heated at 200 and 300°C for 10min respectively. The films immersed in 15 and 60mg of Cu(NO3)2–H2O solutions had similar values of sheet resistance ~104Ω/□, resistivity ~10−1Ωcm, specific contact resistance ~10−4Ωcm2, and hall mobility 5cm2/Vs. Also, the copper-doped conditions were used to dope the source and drain bias to make a thin-film transistor of ZnTe (TFT) by photolithography.

Evidence for Negative Electron Affinity in Laser Irradiated ZnTe Thin Films

Local transport properties of laser irradiated ZnTe thin films are reported. By rastering the laser beam (of 532 nm wavelength) appropriately, ZnTe decomposes into n-type ZnTe and Te and a grating like struct ure with micro-stripesof ZnTe separated by grooves of Te is obtained. Conductive atomic force microscopy studies and local I -V measurements made on thestripesand grooves show that the film properties are mainly determined by chemical composition, ratherthan by the topography of the film. When the tip is positively biased, the current images closely match the topography images. In contrast, when the tip is negatively biased the current and topography images are very different and a large negative curren t was also observed in the grooves. This is attributed to the variation in charge separation (interface capacitance) caused by the rough surface. It is shown that Te forms ohmic contact with Au tip, but the junction exhibits Schottky diode behavior under l ow biasing voltages. The large current at both high positive and negative tip biasing may arise due to semi-metallic properties of Te. The I -V characteristics measurement reveals formation of Schottky barrier between ZnTe-Au junctions with a very low valu e (32.4 �78.3 meV) of barrier which indicates the presence of negative electron affinity . OriginalResearch Article

Optical Properties and Surface Morphology of ZnTe Thin films Prepared by Multiple Potential Steps

In this work, the ZnTe thin films were electrodeposited using potentiostatic steps, on Pt substrate. The effect of steps number, the deposition time for each element (Zn or Te) and layer order (Zn/Te or Te/Zn) in the morphology, composition, band gap energy and photocurrent was evaluated. Microanalysis data showed that the ratio Zn/Te ranged from 0.12 and 0.30 and the film was not stoichiometric. However, the band-gap value obtained from in all experimental conditions used in this work was 2.28 eV, indicating film growth of ZnTe. The samples with higher Zn showed higher photocurrent, which was of the order of 2.64 µA cm-2 and dendritic morphology.

Effects of Cu incorporation on physical properties of ZnTe thin films deposited by thermal evaporation

The present paper reports on a systematic study of the Cu doping effect on the optical, electrical and structural properties of ZnTe:Cu (Cu=0, 6, 8, and 10at%) thin films. Polycrystalline Cu-doped ZnTe thin films were deposited on glass substrates at room temperature by thermal evaporation. A detailed characterization of the Cu-doped ZnTe films were performed by X-ray diffraction (XRD), Spectrophotometry, Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. XRD of the as-deposited Cu-doped ZnTe films belong to single-phase cubic structure of ZnTe with preferential orientation along (111) planes revealed minor effect of Cu content. The interference pattern in optical transmission spectra was analyzed to determine energy band gap, refractive index, extinction coefficient and thickness of the films. Wemple–DiDomenico and Tauc's relation were used for the determination and comparison of optical band gap values. The formation of ZnTe and Cu-doped ZnTe phase was confirmed by FT-IR. AC conductivity in a frequency range of 0–7MHz has been studied for investigation of the carriers hoping dynamics in the films. Raman spectra indicated merely typical longitudinal optical (LO) phonon mode of the cubic structure ZnTe thin film at 194cm−1 because the excitation energy is well above of the optical band-gap of the material and exhibited a blue-shift from 194 to 203cm−1 with Cu which could be associated to the substitution of Zn atom with Cu at the lattice sites.

Oxygen incorporation in ZnTe thin films grown by plasma-assisted pulsed laser deposition

We studied oxygen incorporation into ZnTe thin films with nitrogen and oxygen plasma during a plasma-assisted pulsed laser deposition (PA-PLD). It was shown that ZnTe:O layer formed with oxygen plasma exhibits an enhancement of optical transparency in visible spectral region due to the formation of amorphous TeOx. Especially, the ZnTe:NO deposited by PA-PLD under nitrogen and oxygen partial pressures with N2:O2 of 10:3 sccm showed p-type semiconducting characteristics and the formation of intermediate band at about 0.5–0.8 eV below the ZnTe band edge. These results for oxygen incorporation in ZnTe thin film such as the enhancement of optical transparency in visible spectral region and the intermediate band formation will be useful for optoelectronic devices or intermediate band solar cells.

Comparative study of ZnTe thin films prepared using close space sublimation (CSS) and electron beam evaporation (EBE) thin film fabrication techniques for optoelectronic applications

ZnTe thin films were deposited onto soda lime glass substrates by the close space sublimation (CSS) and electron beam evaporation (EBE) techniques under vacuum (as deposited, annealed at 300 °C and Ag doped for 10 min). Structural, morphological, optical and electrical properties of ZnTe films were studied before and after doping. The ZnTe thin films fabricated by EBE technique showed improvement in microstructure, morphology, crystalline orientation and physical parameters such as porosity and density as compared to CSS fabricated thin films. The highest density with very small porosity is observed in EBE fabricated films. The improvement in electrical parameters such as resistivity, mobility, sheet concentration, type of semiconductor (p or n type), bulk concentration and magnetoresistance were observed in EBE fabricated thin films. The highest decrease in electrical resistivity (8·34×102 Ω cm) was observed by incorporating Ag dopant via EBE technique. The main purpose of this was a comparative study between EBE and CSS techniques and an improvement in the electrical and optical properties such as electrical conductivity, mobility, sheet resistance/concentration and the band gap of the fabricated thin films. The doping of Ag was performed to observe their effects on morphology, structure, electrical and photovoltaic properties of the as deposited thin films. It was observed that electrical and optical properties of the fabricated films enhanced with dopant and annealing temperature.

Effects of metal doping on the physical properties of ZnTe thin films

Zinc telluride (ZnTe) thin films were sublimated on a glass substrate using closed space sublimation (CSS) technique. ZnTe thin films of same thickness were tailored with copper (Cu) & silver (Ag) doping, considered for comparative study. X-ray diffraction (XRD) patterns of as-deposited ZnTe thin film and doped ZnTe samples exhibited polycrystalline behavior. The preferred orientation of (111) having cubic phase was observed. XRD patterns indicated that the crystallite size had increased after silver and copper immersion in as-deposited ZnTe thin films. Scanning electron microscopy (SEM) was used to observe the change of as-deposited and doped sample's grains sizes. EDX confirmed the presence of Cu and Ag in the ZnTe thin films after doping respectively. The optical studies showed the decreasing trend in energy band gap after Cu and Ag-doping. Transmission also decreased after doping. Resistivity of as-deposited ZnTe thin film was about 106 Ω cm. The resistivity was reduced to 68.97 Ω cm after Cu immersion, and 104 Ω cm after Ag immersion. Raman spectra were used to check the crystallinity of as-deposited, Cu and Ag-doped ZnTe thin film samples.

(211) oriented ZnTe growth on m‐plane sapphire by MBE

AbstractSingle‐crystalline and single domain ZnTe thin films are sought for high‐performance terahertz wave detectors, and ZnTe/sapphire heterostructures were considered since the Electro‐Optical (EO) effect could be obtained only from epilayers. ZnTe epilayers were grown on m‐plane sapphire substrates by molecular beam epitaxy, and the potential of single domain epilayers was explored. Through the X‐ray diffraction pole figure measurement it was confirmed that one (100) oriented ZnTe domain along with two kinds of (211) oriented domains were formed on the m‐plane sapphire when the layer was grown at 340 °C. When the layer was grown at 350 °C, the (211) oriented domain dominated the film. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Influence of Deposition Temperature of ZnTe Films on Silicon by Thermal Evaporation

The microstructure and electrical properties of ZnTe films were investigated by using thermal evaporation with emphasis on the effects of a deposition temperature. Microstructure, crystallinity, carrier concentration, resistivity, and mobility are shown to be dependent on the deposition temperature. The highest carrier concentration of 9.1×1014 cm-3, the lowest resistivity of 9.9 Ω-cm and the largest mobility of 667 cm2V-1S-1 are presented at a deposition temperature of 580oC, respectively. The ZnTe thin films using thermal evaporation can find applications in solar cell or light emitting diodes.

Physical properties of sublimated zinc telluride thin films for solar cell applications

Zinc telluride (ZnTe) thin films were fabricated by using closed space sublimation (CSS) technique on glass substrate under vacuum. Pre-fabricated ZnTe thin films were doped with silver (Ag) by ion exchange method. X-ray diffraction showed the preferred orientation (111) of ZnTe thin film with polycrystalline behavior. Scanning electron microscope images were taken to estimate the grain boundaries; energy dispersive X-ray results confirmed the Ag composition in doped-ZnTe samples. Electrical measurements were performed to determine the resistivity, mobility and carrier concentrations of un-doped thin films and Ag-doped samples. The electrical resistivity was of the order of 106Ω-cm before doping. Ag-doped ZnTe samples exhibits low resistivity of the order of 103Ω-cm along with a change in the carrier concentrations and mobility as well at room temperature. The angle resolved optical transmission data, taken by spectrophotometer, was used to find the optical properties before and after Ag doping. Energy band gap showed decreasing trend with increasing Ag doping time.

Optical characteristics of wurtzite ZnTe thin films

The growth of wurtzite ZnTe thin films with thickness between 250 and 1000nm on borosilicate glass substrates by electron beam evaporation is reported. The formation of the wurtzite structure was confirmed using X-ray diffraction. The films showed diffraction peaks originating from the (110), (016) and (116) planes, indicating absence of any preferred orientation. The transmission of all the films was of the order of 80% in the near IR region. The refractive index of the wurtzite ZnTe phase increased with increase in thickness from 3.0 at 250nm to 4.2 for the 1000nm thickness film at a wavelength of 1800nm. The optical band gap of these films increased with thickness showing values of 0.85, 0.9 and 0.98eV at 250, 400 and 1000nm thickness, respectively. Chemical composition studies revealed that the films were mildly non-stoichiometric with excess Te. Comparison with the zinc blende structure of ZnTe shows that the wurtzite structure has a higher refractive index, lower band gap and lower charge carrier concentration.

Structure and electrical studies on nanostructured ZnTe thin films

Nanostructured ZnTe thin films on glass substrate were obtained by using a thermal evaporator. Structure, topography, composition and optical transmittance of the film were analyzed using x-ray diffractometer, Scanning probe microscope, x-ray photoelectron spectroscopy and UV-Vis-spectrophotometer, respectively. Dielectric studies such as dielectric constant and loss tangent (tan δ) as a function of frequency for ZnTe film with Al/ZnTe/Al (MSM) configuration reveals dielectric constant of the ZnTe as 7.9. Electrical characterization of the prepared MSM structure was analyzed by measuring current-voltage. The conduction mechanism was deduced as Schottky type from the I–V study.

Structural and Optical Characterization of Zinc Telluride Thin Films

Group II-VI compounds have been investigated largely in last two decades due to their interesting optoelectronic properties. ZnTe, a member of this family, possesses a bandgap around 2.26eV. This material is now a day investigated in thin film form due to its potential towards various viable applications. In this paper, the authors report their investigations on the preparation of ZnTe thin films using vacuum evaporation technique and their structural and optical characterizations. The structural characterization, carried out using an X-ray diffraction (XRD) technique shows that ZnTe used in present case possesses a cubic structure. Using the same data, the micro strain and dislocation density were evaluated and found to be around 1.465×10-3lines-m2and 1.639×1015lines/m2respecctively. The optical characterization carried out in UV-VIS-NIR region reveals the fact that band gap of ZnTe is around 2.2eV in present case. In addition to this, it was observed that the value of bandgap decreases as the thickness of films increases. The direct transitions of the carries are involved in ZnTe. Using the data of UV-VIS-NIR spectroscopy, the transmission coefficient and extinction coefficient were also calculated for ZnTe thin films. Besides, the variation of extinction coefficient with wavelength has also been discussed here.

Structural and Electrical Properties of ZnTe Thin Films Deposited at Various Substrate Temperatures

The structural and electrical properties of ZnTe thin films were investigated as a function of substrate temperature. Vacuum evaporated thin films of Zinc Telluride (ZnTe) of 10kÅ thickness have been deposited on ultrasonically cleaned glass substrates at various substrate temperatures (303K, 373K 448K). Structural parameters were obtained using XRD analysis. It was observed that the films deposited were cubic in nature with a strong (111) texture. Electrical parameters (Hall Effect measurement) have been obtained and studied at various temperatures in the range 303-393K. It is observed that Hall coefficient remains positive throughout the whole temperature range indicating that holes are the majority carriers. The results obtained from structural and electrical parameters study have been correlated and it is found that the thin films deposited at higher substrate temperatures possess increasingly good crystalline structure with improved electrical conductivity along with an increase in carrier concentration and mobility of carriers.

Structural and electrical properties of ZnS/CdTe and ZnTe/CdTe heterostructures

We investigated the structural, substructural and electrical properties of ZnS/CdTe and ZnTe/CdTe heterostructures obtained by the close-spaced vacuum sublimation. It was found that the structural properties of CdTe and ZnTe thin films deposited on ZnS or CdTe sublayers are better than those of the films obtained on glass substrate at the same growth conditions. XRD-analysis has shown that ZnxCd1−xTe (x = 0.21–0.30) solid solutions having the cubic phase were formed near the films' interfaces. Furthermore, the saturation current, the ideality factor and the value of the potential barrier height were determined by the analysis of dark current–voltage characteristics. This makes it possible to establish optimal growth conditions of ZnS/CdTe heterojunctions.

Preparation and Characterization of The Chromium Doped ZnTe Thin Films

The chromium doped zinc telluride (ZnTe:Cr) as well as ZnTe thin films and their sandwich structures were prepared onto glass substrate by thermal evaporation method under the vacuum of 10-5 Torr. We have studied the structural, optical and electrical properties of thermally evaporated Cr-doped ZnTe thin films as a function of Cr concentration. XRD measurements show that Cr-doped ZnTe films possess the mix phase of cubical and hexagonal structure of ZnTe thin film. The optical energy band gap (Eg) calculated from the optical absorption spectra which was observed around 2.57 eV for undoped ZnTe, and reduced to 1.47 eV for the Cr-doped thin films. The result of I-V characteristics is also presented in this paper.