CdTe Target Research Articles

CdTe:Sn thin films deposited by the simultaneous laser ablation of CdTe and Sn targets

CdTe:Sn thin films were grown by pulsed laser deposition on glass substrates at room temperature. The study of the changes in the chemical, structural and optical properties of the films as a function of the density of Sn ions calculated from Langmuir planar probe measurments is presented. Four films were grown by the simultaneous ablation of CdTe and Sn targets in vacuum. The Sn ion density was modified by varying the fluence on the Sn target, while the CdTe plasma density was kept constant for all the deposits. One CdTe film was grown as control sample using the same experimental conditions as in the CdTe:Sn films. The chemical composition was analyzed by XPS, where a dependence of the Sn incorporation into the CdTe lattice, on the Sn ion plasma density was demonstrated. The crystalline structure analysis by XRD showed a hexagonal structure for all the films. When the CdTe and Sn plasmas were combined, a preferential orientation in the plane (110) was observed for the resulting films. Furthermore, as Sn plasma density increased, the intensity of the (110) peak increased as well, suggesting that crystalline re-orientation is an effect of Sn incorporated into the CdTe lattice. Vibrational behavior was analyzed by Raman spectroscopy. A vibrational mode appeared at 118 cm−1 and was related to a Sn-Te vibration, suggesting the incorporation of substitutional Sn into the CdTe lattice. The Sn-Te bonding was confirmed by XPS by the appearance of signals centered at 485 and 494 eV, which correspond to Sn2+ in SnTe. Thin films thicknesses values were between 320 and 460 nm increasing with Sn ion density. The band gap calculated using UV–vis spectrophotometry, resulted in values ranging from 1.42 to 1.46 eV. PL measurements showed a slight blue shift of the near-edge emission as Sn plasma density increased.

Effect of the combination of Cu and CdTe plasmas on the structural and optical properties of CdTe:Cu thin films deposited by laser ablation

In this work, thin films of semiconducting copper doped cadmium telluride (CdTe:Cu) were obtained by laser ablation. The effect of the combination of CdTe and Cu plasmas by the simultaneous ablation of Cu and CdTe targets, on the properties of CdTe:Cu films, when the copper plasma parameters (mean kinetic ion energy and density) are varied while CdTe plasma parameters were kept constant, is analyzed. The mean kinetic ion energy of Cu plasmas was increased from 73 to 100 eV. Cu plasma density increased linearly with mean kinetic ion energy. Mean kinetic ion energy and density of CdTe were kept constant at 75 eV and 2 × 1014 cm−3, respectively.The chemical composition was measured by energy dispersive X-ray spectroscopy. It was observed that Cu content incorporated into the CdTe lattice varied from 1.4 to 1.6 at%. Structural characterization revealed the growth of a CdTe hexagonal structure with no preferential orientation for a reference CdTe film. For samples grown combining Cu and CdTe plasmas, it was observed that increasing density of Cu plasma produces a crystalline orientation of the films in the (110) direction. Optical properties of the samples were obtained by UV–Vis spectroscopy, the data was used to estimate band gap using Tauc plots yielding values that decreased slightly form 1.47 eV for the reference CdTe film, to 1.41 eV for the film grown using the highest Cu plasma density. Scanning electron microscopy was used to study surface morphology, were rougher surfaces were observed as the Cu content in the films was increased.

Influence of plasma parameters and substrate temperature on the structural and optical properties of CdTe thin films deposited on glass by laser ablation

In the pulsed laser deposition of thin films, plasma parameters such as energy and density of ions play an important role in the properties of materials. In the present work, cadmium telluride thin films were obtained by laser ablation of a stoichiometric CdTe target in vacuum, using two different values for: substrate temperature (RT and 200 °C) and plasma energy (120 and 200 eV). Structural characterization revealed that the crystalline phase can be changed by controlling both plasma energy and substrate temperature; which affects the corresponding band gap energy. All the thin films showed smooth surfaces and a Te rich composition.

Substrate effects on the structural properties of Cd1-x Mn x Te thin films grown via magnetron Co-sputtering

Cd1-xMnxTe thin films were prepared by co-sputtering CdTe and Mn targets on to a variety of substrates at room temperature. The sputter powers of the CdTe and the Mn targets were fixed at 75 W and 65 W, respectively, and the influences of the substrate on the structural properties of the deposited thin films were investigated. All of the films were polycrystalline in nature, and X-ray diffraction showed that the lattice constant varied between 6.3912 and 6.4518 Ǻ depending on the substrate. The incorporated Mn mole fraction varied between 12.796% and 13.046%; this variation may be attributed to a change in the band-gap. The band-gap energy increased from 1.6767 to 1.6799 eV and the grain size decreased from 133 to 74 nm as the strain increased from 14.12% to 34.12% for the various substrates.

Studies on CdTe films deposited by pulsed laser deposition technique

Abstract Polycrystalline cadmium telluride films were successfully deposited on glass substrates by ablating a CdTe target by pulsed Nd–YAG laser. Microstructural studies indicated an increase in the average crystallite size from 15 nm to ∼50 nm with the increase in substrate temperature during deposition. The films deposited here were slightly tellurium rich. X-ray diffraction pattern indicated that the films deposited at 300 K had wurtzite structure while those deposited above 573 K were predominantly of zinc blende structure. Residual strain in the films deposited at 300 K was quite low as compared to those deposited at higher temperatures. PL spectra of all the CdTe films were dominated by a strong peak at ∼921 nm (∼1.347 eV) followed by a low intensity peak at ∼863 nm (∼1.438 eV). Characteristics Raman peaks for CdTe indicated a peak at ∼120 cm −1 followed by peaks located at ∼140 cm −1 and 160 cm −1 .

Spectroscopic ellipsometry study of CuCdTeO thin films grown by reactive co-sputtering

The complex dielectric function of CdTeOx and CuCdTeO thin films was determined by spectroscopic ellipsometry in the photon energy range of 1.5 to 5eV. The films were grown onto glass slides substrates by reactive rf co-sputtering using CdTe and Cu targets in an Ar+O2 atmosphere. Films with different Cu concentrations were obtained by varying the power on the Cu target. The dielectric function of the films is represented by a generalized Lorentz harmonic oscillator expression. Three-dimensional type line-shapes for the critical points E1 and E1+Δ1 of CdTe were identified in CdTeOx and CuCdTeO films even for Cu and O concentrations above 20at.%. This latter result can be indicative of CdTe alloying with those elements widening the possibilities for new photovoltaic materials.

Structural analysis of Cd–Te–O films prepared by RF reactive sputtering

Crystalline and microcrystalline Cd–Te–O samples have been obtained by RF reactive sputtering from a CdTe target using N 2O as oxidant. The growth conditions were substrate temperatures of 323 K, 573 K and 773 K and cathode voltage of −400 V, corresponding to 30 W of forward power. The samples were studied by micro-Raman spectroscopy, X-ray diffraction and optical transmittance. The films are remarkably transparent in the visible range, with transmittances about 88% at 400 nm and band gap energies above the absorption edge of the glass substrates. Although only the samples prepared at 773 K present defined diffraction peaks, the analysis of the Raman spectra indicate that samples prepared at 323 K and 573 K have a defined microstructure indeed. The spectra fitting performed by comparison with pattern compounds demonstrate that Cd–Te–O films are formed of Te–O units similar to those present in metal oxide-doped tellurite glasses, such as TeO 3 and TeO 3 + 1 linked through Cd–O bonds. As the substrate temperature increases the microstructure evolves from a γ-TeO 2 richer state to Cd x Te y O z . In the crystalline sample the main phase identified was CdTeO 3 even though evidence of other phases was observed.

Real time spectroscopic ellipsometry of sputtered CdTe, CdS, and CdTe1−xSx thin films for photovoltaic applications

AbstractReal time spectroscopic ellipsometry (RTSE) has been applied to study the deposition of polycrystalline CdTe, CdS, and CdTe1–x Sx thin films on crystalline silicon wafer substrates as well as the formation of CdS/CdTe and CdTe/CdS heterojunctions, all using a magnetron sputtering process. For CdTe and CdS, the key process variable is deposition temperature (T). The nucleation and growth behaviors of CdTe and CdS show strong variations with T and this influences the ultimate grain size of the polycrystalline films. For the co‐sputtered CdTe1–x Sx alloys, the key variables are the rf power levels applied to the CdTe and CdS targets and thus the resulting composition x. The dielectric functions of the alloys have been used as a database for real time investigation of inter‐diffusion at the CdS/CdTe and CdTe/CdS heterojunctions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Physical properties of Bi-doped CdTe thin films deposited by cosputtering

The structural, morphological, electrical, and optical properties of CdTe–Bi cosputtered thin films related with composition are presented. The films were grown on Corning glass substrates at room temperature from a CdTe–Bi target. The composition measurements show that the Bi content in the films ranges from x = 0.0 to x = 6.37 at%, depending on the area fraction covered by the Bi piece attached to the CdTe target. The structure of the annealed films was determined from X-ray diffraction measurements. Two kinds of structures were observed, depending on the Bi content: (1) CdTe polycrystalline films containing a small amount of Bi that is probably incorporated in the Cd and Te sites of the CdTe lattice. (2) Amorphization of the polycrystalline films, with higher Bi content. From the experimental results, we concluded that using this deposition method n/p-type Bi-doped CdTe polycrystalline films can be produced with electrical resistivity between 102–103 Ω cm and electron mobility between 101 and 102 cm2 V–1 s–1. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural properties of thin films of the novel Cu x(CdTe) yO z semiconductor system

This work presents a study on the structure of Cu x (CdTe) y O z thin films grown by reactive RF co-sputtering of Cu and CdTe targets, as a function of the Cu concentration (0.1–12 at%) and the oxygen partial pressure ( P O 2 ) during growth (5.6×10 −5, 9.5×10 −5, 3.0×10 −4 and 6.4×10 −4 Torr). When O 2 is introduced in the growth chamber, at the two lower partial pressures and in the absence of Cu, XRD peaks of the hexagonal (H) phase are observed with increased intensity, including the (1 0 1)H peak, which becomes more intense than the normally dominant (1 1 1)C/(0 0 2)H peak; this is very rarely observed in CdTe-based films. The presence of Cu, however, tends to return the material to the crystalline state usually observed in pure CdTe films. The samples grown with higher P O 2 values, 3.0×10 −4 and 6.4×10 −4 Torr, are amorphous, regardless of the Cu concentration.

Quantum confinement effects of CdTe nanocrystals sequestered in TiO2 matrix: effect of oxygen incorporation

Thin films of TiO2 with high volume fraction ∼(40–55%) and crystallite size ∼(6–40nm) of CdTe nanoparticles had been prepared by rf magnetron sputtering from a composite TiO2:CdTe target at room temperature and 373K. A detailed optical properties of nanocrystalline CdTe:TiO2 films as-deposited and after thermal treatment (⩽300°C) are studied. The absorbance of the TiO2 films with CdTe nanocrystallite dispersions depends both on the nanocrystallite size and volume fraction. The blue-shifts of the optical absorption edge concurrent with the CdTe nanocystal size reduction for as-deposited and after thermal treatment of nanocrystalline CdTe:TiO2 thin films with respect to the bulk semiconductor agrees quite well with the strong quantum confinement theory. A slight deviation in absorption edge values than the predicted values from the strong quantum confinement model can be attributed to change in interplanar distance due to oxygen incorporation and inhomogeneous size distribution of CdTe nanocrystallites in these films.

Role of cadmium-related defects on the structural and electrical properties of nanocrystalline CdTe:TiO2 sputtered films

Abstract CdTe:TiO 2 nanocrystalline films with varying volume fraction of CdTe were prepared by rf magnetron sputtering from a composite TiO 2 :CdTe target. The structural and electrical properties of the films were analyzed as a function of their Cd concentration. It was found that thermally treated films were Cd-rich and were stress free and had lower resistivity values. Field dependent current measurements showed that nanocrystalline films upon thermal treatment in the range 250–300 °C exhibit an unusual current peak under certain conditions of field and temperature. The current peak was associated with the presence of Cd-related defects in the CdTe lattice. The threshold temperature for the formation of Cd-related defects shifted to lower values for Cd-deficient films.

Substrate temperature dependence of electrical conduction in nanocrystalline CdTe:TiO2 sputtered films

Abstract TiO2 thin films with high volume fraction (∼50­70 %) of CdTe nanoparticles were prepared by radio frequency (rf) magnetron sputtering from a composite TiO2:CdTe target. With increase in substrate temperature Ts from room temperature (RT ∼300 K) to 373 K, a transition from an ordered structure exhibiting metallic-type conduction to a disordered structure exhibiting nonmetallic-type conduction was observed for annealed nanocrystalline CdTe:TiO2 films. The annealed RT-deposited films showed a large coalescence of distinct islands (size ∼0.3­0.7 µm) mainly of Cd and CdTe, and as result, a 3D network was realized. For metallic regime films, electrical conduction is essentially due to electrical percolation through Cd/CdTe crystallites embedded in an amorphous TiO2 matrix. However, the annealed high Ts films consisted of noncoalescent, small islands (size ∼0.15­0.3 µm) of Cd and CdTe embedded in amorphous TiO2 matrix. Here, the conduction is essentially by hopping mechanism via thermally activated tunneling.

Effects of Cd vacancies on the electrical properties of polycrystalline CdTe sputtered films

Polycrystalline CdTe films were grown on 7059 Corning glass substrates at room temperature by sputtering from a pure CdTe target and by co-sputtering from a composite CdTe–Cd target. The electrical, optical and structural properties of the films were analyzed as a function of their Cd concentration. It was found that the films grown from the pure CdTe target were Cd-deficient with electrical resistivity of about 10 7–10 8 Ωcm and those grown from the CdTe–Cd target were Cd-rich with electrical resistivity of the order of 10 3 Ωcm. Dark electrical current vs 1/K B T measurements showed that the CdTe films with low cadmium content (<50 at.%) exhibit an unusual current peak in the range of 1/K B T=40–60 eV −1. When the cadmium composition in the CdTe films approaches 50 at.%, this effect vanishes and the current has an exponential behavior for 1/K B T>40 eV −1. The current peak was associated with the presence of Cd vacancies in the CdTe lattice.

Size-dependent optical edge shifts and electricalconduction behaviour of RF magnetron sputtered CdTenanocrystals:TiO2 composite thin films

CdTe nanocrystals sequestered and passivated inan amorphous TiO2 thin film matrix have been prepared by RFsputtering from a composite TiO2:CdTe target. The CdTenanocrystal size and volume fraction increases from 15 to 40 nmand 2 to 20% respectively as the film thickness increases,typically from 0.05 to 0.25 µm. A systematic dependence of theoptical band edge on the CdTe nanocrystal size shows a strongquantum confinement effect. The optical edge shifts aresignificantly higher than the theoretical prediction based onsingle-particle confinement of decoupled electrons and holes.This is understood on the basis of nucleation-controlled growthof CdTe nanocrystals by direct vapour phase condensation, inwhich small nuclei are rapidly passivated by TiO2 depositingat much higher rates. The nano-sized CdTe growth island thus formedcomprises of several TiO2 passivated nanocrystals. Electricalconduction behaviour of these films show that tunnelling betweenthe CdTe nanocrystals is not a dominant mechanism, as athree-dimensional network is not realized due to smallthickness and lower coverage. The current transport isessentially space-charge-limited. The injection of electronsfrom nano-sized CdTe crystals follows spherical radial spacecharge flow which modifies the usual power law dependence fromquadratic to 3/2. The analytical description of the current conductionprocess in composite CdTe:TiO2 is discussed.

CdTe-Cd: A new semiconductor with polytype structure

Abstract Cd-doped CdTe thin films were prepared by sputtering on unheated glass substrates. Targets of stoichiometric CdTe with inclusions of elemental Cd were used to vary the Cd concentration of the films in the approximate range 47–55at.%. The amount of Cd in the films is directly related to the area of the Cd pieces on the CdTe target. The electrical and structural properties of the films were analysed using Hall measurements and X-ray diffraction. It is found that both properties strongly depend on the Cd concentration. For Cd concentrations below or equal to the stoichiometric value, the crystalline structure corresponds to the regular zincblende, and the electrical transport is dominated by the extended states. For Cd concentrations above the stoichiometric value, the X-ray data show an unusual 6H polytype structure and the electrical conduction takes place by hopping along localized states. It is proposed that this latter behaviour in the conductivity occurs because the excess of Cd atoms enter the CdTe lattice in interstitial sites, creating energy levels in the forbidden bandgap.

Preparation and characterization of CdTeO 3 thin films

Cadmium telluride oxide CdTeO 3 thin films were prepared by r.f.-sputtering from a polycrystalline CdTe target in an atmosphere of nitrogen and oxygen. X-ray diffraction (XRD) analysis showed that the as-deposited samples were amorphous. X-ray photoelectron spectroscopy (XPS) measurements revealed that the films were nearly stoichiometric. The resistivity and the static dielectric constant of the as-deposited layers were found to be 3×10 6 Ω m and 16, respectively. Optical transmission measurements in the ultraviolet–visible–near-IR (UV–VIS–NIR) region allowed the determination of the optical constants of the prepared films. Post-deposition annealing of the layers at 420°C for 2 h favored their crystallization. Finally, we show that polycrystalline CdTO 3 films can be grown on CdTe films, deposited on C7059 glass by an appropriate heat-treatment of the CdTeO 3/CdTe/C7059 structure.

Interstitial Cd doping CdTe films by co-sputtering

A series of CdTe thin films were grown in a Cd enriched ambient by co-sputtering a Cd–CdTe target. Different concentrations were obtained by changing the relative area occupied by the metallic Cd pieces placed onto the CdTe target. It was observed that the process of electrical conduction is not the same for all the samples. For samples with Cd < 50 at% the electrical conductivity is due to the direct activation of charge carriers from the valence band or from impurity states to the conduction band. For samples with Cd > 50 at% electrical conduction is due to the hopping of carriers from state to state by optimizing the distance of tunneling. Conductivity ( σ) vs temperature ( T ) measurements indicate that electrical conductivity behaves as ln σ α ( T 0/ T ) [1, 4], the Mott law for variable range hopping (VRH) in disordered systems. We think that this change in conductivity occurs because Cd atoms place in interstitial sites, doping the CdTe lattice, after the Cd vacancy ( V cd) are filled.

Electrical and optical properties of rf-sputtered CdTe films

In this paper some electrical and optical properties ofn-type CdTe films prepared by rf sputtering at 180 W power have been reported. For doping the films a number of pellets of pure Cd placed on the CdTe target were simultaneously sputtered with the target material to get Cd-doped CdTe films. The films after doping were foundn-type. Maximum doping concentration obtained this way was of the order of 1014 cm−3. XRF spectra of target material and the rf-sputtered films were found to be more or less similar. All the films were found to have large number of defects indicated by profound aging effect in the initial stages of aging. The films became stable for measurements after about 8–10 days. Activation energy and band gap found from the temperature dependence of dark conductivity were 0–5 eV and 1.43 eV respectively. Photoconductivity of the films was studied and the photoconductive rise time, decay time and the decay constants were determined from the photoconductive rise and decay curves at 500 Lx and 1000 Lx of intensity of illumination.

Characterization of CdTe-Sb co-sputtered films

The structure and electrical properties of CdTe-Sb co-sputtered thin films and their relation with composition were studied. The films were grown on glass substrates by radio frequency sputtering deposition. The ternary alloys were deposited by co-sputtering of Sb and CdTe. The substrate temperature was varied from room temperature to 250 °C. The composition measurements show that the Sb content in the films ranges from 8 to 60 at. %, depending on the area fraction covered by the Sb attached to the CdTe target. The Sb content is found to be independent of the substrate temperature. The structure of the films was determined from x-ray diffraction measurements. Three kinds of structures were observed, depending on the Sb content and substrate temperature: (1) CdTe polycrystalline films containing small amounts of Sb, that is probably incorporated in the Cd and Te sites of the CdTe lattice; (2) amorphous CdSbTe films, with high Sb content and low substrate temperatures; and (3) polycrystalline films composed by a mixture of both Sb and CdTe crystallites, with high Sb content and high substrate temperatures. The room temperature electrical resistivity of the films varies from 109 Ω cm in the CdTe polycrystalline films to 10−3 Ω cm in the films composed of Sb and CdTe crystallites.