xSx Films Research Articles

Ge1−xSx chalcogenide alloys for OTS applications using magnetron sputtering

3D cross (X)–point memory arrays have attracted attention for future memory architecture due to their cost efficiency and high density. Chalcogenide–based materials are suitable candidates for 3D X–point memory because they can be stacked to form both memory and selector with a simple process. Conventional binary chalcogenide ovonic threshold switching (OTS) selectors such as GeXSe1−x or GeTe suffer from thermal instability. Furthermore, ternary OTS selectors are being investigated by doping various materials to overcome thermal instability. Here, we introduce a wide range of Ge1−xSx films to analyze and confirm the selector behavior for future applications. S–rich region (0.5 ≤ x ≤ 0.67) devices behaved as OTS selector with low leakage current (1.3 nA) and high thermal stability (amorphous up to 600 °C). To study the mechanism of devices with different compositions, the modified PF–model was used to calculate trap density, activation energy, subthreshold swing, and distance between traps. Additionally, extracted optical bandgap (1.65 – 3.45 eV) and urbach energy (166 – 103 meV) had good accordance with electrical characteristics of each device. This innovative research has potential to enable 3D X–point memory by combining conformal OTS selector and chalcogenide–based phase change memory.

Properties of CdSe1−xSx films by magnetron sputtering and their role in CdTe solar cells

CdS/CdSe composite window layer is a better candidate than CdS or CdSe single layer for CdTe solar cells. In order to find the underlying reason, the properties of co-sputtered CdSe1−xSx thin films with different compositions ( $$0\le x\le 1$$ ) were examined. XRD patterns show that all CdSe1−xSx films ( $$0\le x\le 1$$ ) are polycrystalline with hexagonal structures and the prominent peak position shows an increasing trend with the increase of x. This suggests that CdSe1−xSx window layer with a gradient composition, formed during the high-temperature growth and heat treatment of CdTe films, has the gradient lattice constant, meaning the less defect density. This is beneficial to the device performance. Hall measurements show that CdS1−xSex films (x ≥ 0.772) are n-type semiconductors with carrier concentrations ranging from 1015 cm−3 to 1016 cm−3. It reveals that CdS/CdSe bi-layers can form a strong built-in electric field with CdTe layer as CdS does. The electronic band structure of CdSe1−xSx window layer with a gradient composition suggests that the spikes formed in the conduction band are lower than 0.3 eV, which would not impede the electron transport. The valence band alignment is beneficial for the hole transport. Therefore, an appropriate band alignment can be formed between CdSe1−xSx and CdTe layers. These results reveal that CdSe1−xSx film is a good candidate as the window layer for high-efficiency CdTe solar cells considering its structural, optical and electrical properties as well as its electronic structure.

The fabrication of high-quality superconducting FeSe1−xSx films via pulsed laser deposition

Due to its unique properties, FeSe1−xSx superconductors have been prime candidates to investigate the unconventional superconducting mechanism in iron chalcogenides. In this study, we report the successful preparation of high-quality epitaxial FeSe1−xSx films with x ≤ 0.4 on different substrates via pulsed laser deposition. With different S content, it is found that CaF2 (100) single crystal is the most preferable substrate to use to grow superconducting FeSe1−xSx films. With the increment of S content, the nematic transition temperature (Ts) of FeSe1−xSx films decreases simultaneously and disappears at x ≈ 0.2. In contrast to FeSe1−xTex films, the superconducting transition temperature (Tc) of FeSe1−xSx films decreases continuously with S doping. And the Tc shows no enhancement after the disappearance of Ts, suggesting that the nematic transition does not positively impact the superconductivity in these films. In addition to the influence of different S content, it is found that the non-negligible in-plane tensile strain originating mostly from the mismatch between the FeSe1−xSx films and the substrate is the key factor suppressing the superconductivity. Our work can provide constructive guidance for preparing superconducting FeSe1−xSx films and gives a further understanding of the interplay of Ts, strain, and superconductivity in iron-chalcogenide films.

Parametric Optical Property Database for CdSe1−xSx Alloys

A comprehensive database of the optical response in the form of complex dielectric function (e) spectra of magnetron co-sputtered CdSe1−xSx alloy thin films is developed spanning the full 0 ≤ x ≤ 1 range of compositions. A parametric model is presented and used to determine e describing each film while in both as-deposited and thermally annealed states. This model combines a critical point electronic transition lineshape and an Urbach tail in the above- and below-bandgap portions of the measured spectrum, respectively, while maintaining first derivative continuity. Additionally, this hybrid parametric description of e automatically determines the Urbach energy (EU) describing the width of the sub-bandgap absorption tail, thereby providing a relative measure of the defect density of the modeled material. These as-deposited CdSe1−xSx films are generally found to be the most defective at intermediate compositions with some EU reaching energies > 200 meV. Annealing reduces EU in all films to a relatively uniform value < 100 meV for all compositions. Leveraging the full e database in modeling CdS-CdSe bi-layer stacks is demonstrated to be effective in detecting the composition gradients resulting from inter-diffusion upon annealing. The accuracy of this technique is verified through excellent agreement with cross sectional energy dispersive X-ray spectroscopy measurements.

Study of the optical properties of amorphous As–Se–S thin films

This study investigates the effects of substitution of selenium by sulfur on the optical constants of amorphous (a-)As20Se80 − xSx films (x = 0, 4, 8, 12, 16, and 20 at%), where the coordination number for both Se and S is the same. The absorption coefficient, α, and the index of refraction, n, were derived using the transmission spectra in a wide range of wavelengths from 400 to 2500 nm. This revealed that the index of refraction for the a-As20Se80 − xSx system decreases with increase in sulfur content throughout the range under study. The effective coordination number of As has been determined based on the refractive index dispersion analysis. A decreasing in Nc from 4.022 to 3.752 with increase content of sulfur from 0 to 20 at% has been recorded. In addition, the optical bandgap, Eg, of the a-As20Se80 − xSx thin films increases with increase in S content which can be explicated in terms of the chemical bond approach.

Growth of p-type ZnOS films by pulsed laser deposition

ZnO1−xSx films were deposited on quartz substrates by pulsed laser deposition (PLD) of ZnO1−xSx targets. The ZnO1−xSx films with S-contents of 0.03–0.17 were grown from the ZnO1−xSx targets sulfured at temperatures of 200 and 500°C. The resistivity of the ZnO1−xSx films is slightly increased with the S-content. An increase of the O2-partial pressure in an atmosphere reduces the S-content in the films and drastically enhances the resistivity of the films. However, the carrier type of the films is still n-type. In order to incorporate excess S atoms into films, evaporation of Sulfur was performed during the PLD process. As a temperature of the S-evaporation is raised, the resistivity of the films is significantly enhanced and hole-conductivity appears in the films grown by the S-evaporation at 80 and 90°C. By X-ray photoelectron spectroscopic measurements, the presence of SOx species is confirmed for the p-type ZnO1−xSx film. Both interstitial SO3 or SO4 clusters and complexes of Zn-vacancy with H are considered to be appropriate acceptors responsible for the hole-conductivity at room temperature.

Growth and characterization of ZnO1−xSx highly mismatched alloys over the entire composition

Alloys from ZnO and ZnS have been synthesized by radio-frequency magnetron sputtering over the entire alloying range. The ZnO1−xSx films are crystalline for all compositions. The optical absorption edge of these alloys decreases rapidly with small amount of added sulfur (x ∼ 0.02) and continues to red shift to a minimum of 2.6 eV at x = 0.45. At higher sulfur concentrations (x &amp;gt; 0.45), the absorption edge shows a continuous blue shift. The strong reduction in the band gap for O-rich alloys is the result of the upward shift of the valence-band edge with x as observed by x-ray photoelectron spectroscopy. As a result, the room temperature bandgap of ZnO1−xSx alloys can be tuned from 3.7 eV to 2.6 eV. The observed large bowing in the composition dependence of the energy bandgap arises from the anticrossing interactions between (1) the valence-band of ZnO and the localized sulfur level at 0.30 eV above the ZnO valence-band maximum for O-rich alloys and (2) the conduction-band of ZnS and the localized oxygen level at 0.20 eV below the ZnS conduction band minimum for the S-rich alloys. The ability to tune the bandgap and knowledge of the location of the valence and conduction-band can be advantageous in applications, such as heterojunction solar cells, where band alignment is crucial.

Band-gap engineering of ZnO1−xSx films grown by rf magnetron sputtering of ZnS target

Structural and optical properties of ZnO1−xSx (0 ≤ x ≤ 1.0) thin films grown onto sapphire substrates (≿-Al2O3) at 300 °C by radio frequency (rf) magnetron sputtering of ZnS ceramic target are studied. A possibility of purposeful controlling sulfur content and, as consequence, ZnO1−xSx band gap energy via changing the ratio of the partial pressures of argon and oxygen are revealed. Linear dependence of ZnO lattice parameter c on S content suggests that structural properties of single-phase ternary alloys in the composition range between ZnO and ZnS obey Vegard's law. The mechanisms of influence of gas mixing ratio on film growth and band gap energy are discussed. Cu(In,Ga)Se2 (CIGS)-based heterojunction solar cells with ZnO1−xSx buffer layers were fabricated by one-cycle magnetron sputtering procedure. Electrical characteristics of Cd-free devices are comparable to those of CdS-containing photovoltaic heterostructures, thereby indicating prospects of using ZnO1−xSx layers for fabrication of CIGS solar cells.

The S concentration dependence of lattice parameters and optical band gap of a-plane ZnOS grown epitaxially on r-plane sapphire

High quality non-polar a-plane ZnO1−xSx films were deposited epitaxially on r-sapphire substrates by pulsed laser ablating a ZnS ceramic target in O2 atmosphere. The in-plane orientation relationship between ZnO1−xSx films and sapphire substrates was revealed by X-ray diffraction φ-scans as ZnO1−xSx[1¯100] || sapphire [1¯1¯20] and ZnO1−xSx [0001] || sapphire [1¯101]. Both lattice constants a and c expand with increasing S content in the ZnO1−xSx alloys. Large lattice misfit along either ZnO1−xSx [0001] or ZnO1−xSx[1¯100] indicated a domain matching rather than lattice matching epitaxy mode for a-plane ZnO1−xSx on r-plane sapphire. Optical measurements show high transparency of the films in visible range. The band gap of ZnO1−xSx becomes narrower with S fraction increasing up to 43%, which behaves similar to the band gap of c-plane ZnO1−xSx films grown on c-plane sapphire.

Tuning the composition and optical band gap of pulsed laser deposited ZnO1−xSx alloy films by controlling the substrate temperature

High-quality ZnO1−xSx thin films were grown on (001) sapphire substrates in the temperature range of 300–800°C by pulsed laser deposition (PLD) with a ZnS ceramic target and O2 as reactive gas. By increasing the substrate temperature, the crystalline quality of the films is enhanced. The S content in the single-phase ZnO1−xSx films can be systematically adjusted from 0.556 to 0.202 via changing the substrate temperature. The maximum S content in the film grown at 300°C reaches 0.556 without phase separation, which is significantly higher than the solid solubility limits reported previously for the ZnOS alloys. The narrowed band gap of the ZnO1−xSx film (2.63eV) grown at the low substrate temperature will extend the application of ZnO-based optoelectronic devices to the blue light region. As the composition, structure, and band gap energy of the ZnOS films were found to depend critically on the growth temperature, this work suggests a simple and flexible means of tuning the composition and optical band gap of ZnOS alloy films by controlling the substrate temperature during the PLD process.

Growth of non-polar a-plane ZnO1−xSx films by pulsed laser deposition

A series of non-polar ZnO1−xSx thin films with different S contents were grown on r-plane sapphire substrates by pulsed laser deposition. The S content in the ZnO1−xSx thin films was adjusted by changing the growth temperature. Up to 20at% S was introduced into ZnO film without changing the single-phase hexagonal structure. Based on the X-ray diffraction analysis, the ZnO1−xSx thin films with S content below 19at% exhibit unique non-polar 〈112−0〉 orientation, while the films with S content above 19at% show 〈0001〉 and 〈112−0〉 mixed orientations. X-ray photoelectron spectroscopy confirmed the chemical states of Zn, O and S elements in the ZnO1−xSx films.

Wavelength tunable photoluminescence of ZnO1-xSx alloy thin films grown by reactive sputtering

ZnO1−xSx alloy thin films with various S contents were deposited on glass substrates by reactive sputtering. The films were grown in high crystalline quality and strong preferential crystallographic orientation. Variations of the lattice constant c followed Vegard's law. X-ray photoelectron spectroscopy confirmed the substitution of O by S in ZnO. The composition dependence of the band gap energy in ZnO1−xSx system was investigated and the band gap bowing parameter was estimated to be about 1.46 eV. The incorporation of S led to the expected redshift of the band gap related photoluminescence emission of ZnO1−xSx films up to 320 meV. The results indicate that ZnO1−xSx films could hold the prospect for the development of ZnO based quantum structures.

Structure and optical band gap of ZnO1−xSx thin films synthesized by chemical spray pyrolysis for application in solar cells

Highly crystalline c-axis oriented and homogeneous ZnO1−xSx films with sulfur composition 0.05 ≤ x ≤ 0.9 without phase separation were deposited using spray pyrolysis of aqueous precursors. A mechanism of film growth is proposed which envisages a slower kinetics of ZnO precursor decomposition and its specific by-products combined with S-precursor decomposition which enables homogeneous ZnO1−xSx phase formation over a wide S-composition range 0.05 ≤ x ≤ 0.9. This is achieved by controlling the substrate temperature to ≤ 300 °C and the spray rate at ∼ 3 ml/min. The ZnO1−xSx films primarily form by S2− substitution at the O2− lattice sites which is confirmed by detailed analysis of the Zn2p, S2p and O1s x-ray photoelectron spectroscopy peaks. With the increasing of S-content, a structural transformation is observed in ZnO1−xSx films; for x &amp;lt; 0.3, the ZnO1−xSx films in the oxygen-rich phase are in ZnO wurtzite crystal structure and for x ≥ 0.44, the ZnO1−xSx films lie in the sulfur-rich phase with β-ZnS structure. At threshold x = 0.3, in the structural transition state, diffraction peaks corresponding to both structural phases are observed. The optical transmission spectra at the band gap transition energy position shift to the red side for 0.05 ≤ x &amp;lt; 0.52 and towards the blue side with the further increase in S-content in the 0.52 &amp;lt; x ≤ 0.9 range. Optical band gap energy determined for ZnO1−xSx films show strong band gap bowing over the entire S-composition range, 0.05 ≤ x ≤ 0.9. The band gap modulation with increased S-content is inconsistent with the use of a single bowing parameter. A bowing parameter of 2.5 eV shows a closer fit for 0.05 ≤ x &amp;lt; 0.52. By accounting for additional effect of the strain energy on the band gap due to addition of excess S-content, a closer fit for the observed variation in the band gap is explained in the 0.52 &amp;lt; x ≤ 0.9 composition range. Electrical resistivity variation with S-content is consistent with the compensating effect of S2− substitution at the O2− sites in ZnO1−xSx films.

Structural properties and bandgap bowing of ZnO1−xSx thin films deposited by reactive sputtering

A series of ZnO1−xSx films with 0⩽x⩽1.0 was deposited by radio-frequency reactive sputtering on different substrates. The structural characterization by x-ray diffraction measurements revealed that the films have wurtzite symmetry and correlated investigations of the layer composition by photoelectron spectroscopy showed that the lattice constant varies linearly with x. The composition dependence of the band gap energy in the ternary system was determined by optical transmission and the optical bowing parameter was found to be about 3eV.

On the composition dependence of ZnO 1− x S x

We report on the optical and structural properties of a series of ZnO1−xSx films deposited by radio-frequency sputtering on float glass and sapphire substrates. In the range 0 < x < 0.4 we find that the films have wurtzite symmetry and the lattice constant varies linearly on the composition as confirmed by photoelectron spectroscopy and Rutherford backscattering. Raman measurements show that the spectral position of the A1(LO) mode is sensitive to the sulfur content in the films. The composition dependence of the band gap was evaluated by analyzing the absorption coefficient as a function of the square root of the photon energy (direct transition). The energy gap shifted by more than 500 meV to lower energies indicating a bowing of appr. 2.5 eV in contrast to previous predictions. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase behavior in the CdTe–CdS pseudobinary system

Polycrystalline thin films deposited by coevaporation of CdTe and CdS form metastable single-phase CdTe–CdS alloys. Subsequent heating in a kinetic-enhancing ambient segregates CdTe1−xSx and CdS1−yTey phases from the original alloy phase. The equilibrium miscibility gap between the resulting CdTe1−xSx and CdS1−yTey alloy phases is determined for CdTe1−xSx films with initial x ∼ 0.4 treated from 360 to 700 °C. At 625 °C, the equilibrium compositions correspond to published results for mixed crystals. Below 625 °C the miscibility gap widens asymmetrically due to different mixing free energies for S in CdTe and Te in CdS. The solubility thermodynamics are modeled with an excess mixing free energy to account for nonideal mixing behavior.

Comment on “Optical properties of CdTe1−xSx (0⩽x⩽1): Experiment and modeling” [J. Appl. Phys. 85, 7418 (1999)

Wei et al. [J. Appl. Phys. 85, 7418 (1999)] performed the room temperature spectroscopic ellipsometry to determine the dielectric function of CdTe1−xSx films. They have fit the obtained dielectric function using the Holden’s model dielectric function [Phys. Rev. B 56, 4037 (1997)], and derived conclusions about the line shape at the band gap E0. However, their description of the fitting procedure is ambiguous, and some model parameters in Table I [J. Appl. Phys. 85, 7418 (1999)] are missing which makes it impossible to reproduce their calculations. Furthermore, the results of Wei et al. [J. Appl. Phys. 85, 7418 (1999)] do not represent conclusive proof of the advantages of their approach over other models available in the literature.

Optical properties of CdTe1−xSx (0⩽x⩽1): Experiment and modeling

Spectral ellipsometry at 300 K, in the range 0.75–5.4 eV, has been used to determine the optical constants ε(E)[=ε1(E)+iε2(E)] of a series of CdTe1−xSx (0⩽x⩽1) films fabricated by a laser-deposition process. The measured ε(E) data reveal distinct structures associated with critical points (CPs) at E0 (direct gap), spin-orbit split E1, E1+Δ1 doublet and E2. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Phys. Rev. B 56, 4037 (1997)] based on the electronic energy-band structure near these CPs (also E0+Δ0 CP) plus excitonic and band-to-band Coulomb enhancement (BBCE) effects. In addition to evaluating the energies of these various band-to-band CPs, our analysis also makes it possible to obtain information about the binding energies of not only the three-dimensional exciton associated with E0 but also the two-dimensional exciton related to the E1, E1+Δ1 CPs. Our results will be compared to previous experiments and modeling (which neglect the BBCE terms) of ε(E) of CdTe and CdS as well as optical absorption measurements of E0 of CdTe1−xSx (0⩽x⩽1). The results of this experiment demonstrate conclusively that the band-to-band line shape at E0 is BBCE even if the exciton is not resolved.

Pulsed-laser ablation growth of epitaxial ZnSe1−xSx films and superlattices with continuously variable composition

We describe a new method to grow epitaxial semiconductor alloys with continuously variable composition, while using a single pulsed laser ablation target of fixed composition. Epitaxial ZnSe1−xSx films with continuously variable sulfur content, x, were grown by ablating a ZnSe target through low-pressure ambient H2S gas. The sulfur content was easily controlled in the range 0&amp;lt;x&amp;lt;0.18 by varying the H2S partial pressure from 0 to 45 mTorr, for films grown at 325 °C. ZnSe1−xSx films differing in composition by as much as x=0.52 from the pure ZnSe target have been grown at 400 °C. We have used this method to grow heteroepitaxial structures with continuously graded or periodically repeating, abrupt compositional changes (compositional superlattices). This development removes the principal barrier to pulsed-laser ablation (PLA) growth of compositionally graded semiconductor thin-film materials, namely that the film and target normally have the same composition. The method appears to have broad application for PLA growth of other compound semiconductor films and heterostuctures, as well as to dope individual layers.