xO Thin Films Research Articles

Role of substrate and annealing temperature on the structure of ZnO and AlxZn1−xO thin films for solar cell applications

This paper reports on the deposition of pure and 5at% Al doped ZnO (AZO) prepared by sol–gel and applied to the substrates by spin-coating, and the role of annealing temperature on the crystallinity of these layers. It is found that both ZnO and AZO are largely amorphous when coated on glass compared to n-Si(111), as substrates. On both substrates, X-ray diffraction (XRD) shows that the crystallinity improves as annealing temperature is raised from 200 to 600°C with better crystallinity on Si substrates. The thickness of the films on substrates was determined as 120nm by Rutherford backscattering spectroscopy (RBS). Specular ultra-violet visible (UV–vis) gives the direct transition optical band gaps (Eg) for AZO as-deposited films are 2.60 and 3.35eV while that of 600°C annealed films are 3.00 and 3.60eV. The Eg calculated from diffuse reflectance spectroscopy (DRS) UV–vis are more diverse in ZnO- and AZO-Si than the ZnO- and AZO-glass samples, although in both sets the Eg tend to converge after annealing 600°C. The Raman spectra of samples show multiphonon processes of higher order from the AZO and substrates. It is found that residual stresses are related to E2 Raman mode.

Microstructural, electrical, and optical properties of sol–gel derived HfMgZnO thin films

This paper reports the characterization of sol–gel derived HfxMg0.05Zn0.95−xO (x = 0, 0.025, 0.05, 0.1) thin films. The films show high transparency (∼90%) in the visible light wavelength region. The incorporation of Hf in the thin films increases the bandgap and decreases the crystallinity in the as-deposited films. This may be due to the high bandgap of HfO2 and the crystal frustrating effect caused by mixing atoms of different sizes. As the annealing temperature increases, the bandgap changes because of the alteration of the stress status, grain growth, and atomic rearrangement. The resistivity also decreases slightly with an increase in the annealing temperature.

Growth of highly transparent CdxZn1−xO (CZO) thin films: Structural and optical studies

The deposition of CdxZn1−xO thin films with different cadmium concentrations i.e., x = 0.0, 0.05, 0.20 by sol–gel spin coating is reported. The doping fraction of Cd in ZnO films was measured by Rutherford backscattering spectrometry (RBS), while the surface morphology was studied by scanning electron microscopy (SEM). Grazing incidence X-ray diffraction (GIXRD) study was carried out for the structural investigations and reveals hexagonal wurtzite structure with polycrystalline nature. The various structural parameters are calculated including the lattice constants ‘a’ and ‘c’, stress (σ), strain (ε) and internal parameter (u). For x = 0.20 Cd content, the formation of secondary phase of the cubic CdO at 33.12° (111) and 38.41° (200) is observed and this is further confirmed by micro-Raman studies, where the TO mode emerges at ∼261.5 cm−1. The basic wurtzite structure is maintained as ‘c/a’ ratio and internal parameter ‘u’ found to be closer to the ideal values. All the films are found to be highly transparent in the visible region and a bending of the near band edge (NBE) absorption is observed with Cd doping. It is further confirmed by calculating the Urbach energy (Eu), which is found to be increased from 0.14 eV (for x = 0.0) to 0.26 eV (for x = 0.20) with maximum value for the lightly doped films i.e. x = 0.05. However, the optical band gap is found to decrease from 3.26 eV (for x = 0.0) to 3.08 eV (for x = 0.20).

Investigations on the structural and optical properties of band gap engineered Zn1−x(Cd, Mg)xO thin films deposited by sol–gel technique

Cadmium and magnesium co-doped zinc oxide (ZnO) thin films have been deposited on sapphire substrates using the sol–gel method and the spin coating technique. X-ray diffraction studies confirmed that the deposited films exhibit a hexagonal wurtzite ZnO structure. A strong ultraviolet near-band-edge emission and a deep level emission were observed in the room temperature photoluminescence (PL) spectra of all the films. The PL spectra were also investigated as a function of temperature to examine the emission mechanism of the deposited films. The ability to vary the band gap energy from 3.1 to 3.6 eV was realised by adjusting the cadmium and magnesium concentration in the co-doped ZnO films. X-ray photoelectron spectroscopy studies revealed the successful incorporation of cadmium and magnesium in the ZnO films.

Structural and electrical properties of SrFexTi1−xO3 (x=0.001, 0.005 and 0.01) thin films prepared by pulsed laser depositions

Three different SrFexTi1−xO3 (x=0.001, 0.005 and 0.01) thin films were deposited on Pt/Ti/SiO2/Si (100) substrates by pulsed laser depositions at different substrate temperatures. The crystalline structure and surface morphology were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The dielectric properties of the films were studied by using Agilent 4980A LCR meter. The XRD results indicate that the films deposited above 650°C shows a cubic perovskite structure with (110) orientation. Leakage current tests show that the conduction mechanism in the three different SrFexTi1−xO3 (x=0.001, 0.005 and 0.01) thin films display ohmic behavior at low electric field and schottky emission at high electric field, respectively.

Phonon dynamics and Urbach energy studies of MgZnO alloys

The MgxZn1−xO alloy system is emerging as an environmentally friendly choice in ultraviolet lighting and sensor technologies. Knowledge of defects which impact their optical and material properties is a key issue for utilization of these alloys in various technologies. The impact of phase segregation, structural imperfections, and alloy inhomogeneities on the phonon dynamics and electronic states of MgxZn1−xO thin films were studied via selective resonant Raman scattering (SRRS) and Urbach analyses, respectively. A series of samples with Mg composition from 0–68% were grown using a sputtering technique, and the optical gaps were found to span a wide UV range of 3.2–5.8 eV. The extent of the inherent phase segregation was determined via SRRS using two UV-laser lines to achieve resonance with the differing optical gaps of the embedded cubic and wurtzite structural domains. The occurrence of Raman scattering from cubic structures is discussed in terms of relaxation of the selection rules due to symmetry breaking by atomic substitutions. The Raman linewidth and Urbach energy behavior indicate the phase segregation region occurs in the range of 47–66% Mg. Below the phase segregation, the longitudinal optical phonons are found to follow the model of one-mode behavior. The phonon decay model of Balkanski et al. indicates that the major contributor to Raman linewidth arises from the temperature-independent term attributed to structural defects and alloy inhomogeneity, while the contribution from anharmonic decay is relatively small. Moreover, a good correlation between Urbach energy and Raman linewidth was found, implying that the underlying crystal dynamics affecting the phonons also affect the electronic states. Furthermore, for alloys with low Mg composition structural defects are dominant in determining the alloy properties, while at higher compositions alloy inhomogeneity cannot be neglected.

Obtaining strong ferromagnetism in diluted Gd-doped ZnO thin films through controlled Gd-defect complexes

We demonstrate the fabrication of reproducible long-range ferromagnetism (FM) in highly crystalline GdxZn1−xO thin films by controlling the defects. Films are grown on lattice-matched substrates by pulsed laser deposition at low oxygen pressures (≤25 mTorr) and low Gd concentrations (x ≤ 0.009). These films feature strong FM (10 μB per Gd atom) at room temperature. While films deposited at higher oxygen pressure do not exhibit FM, FM is recovered by post-annealing these films under vacuum. These findings reveal the contribution of oxygen deficiency defects to the long-range FM. We demonstrate the possible FM mechanisms, which are confirmed by density functional theory study, and show that Gd dopants are essential for establishing FM that is induced by intrinsic defects in these films.

Lattice spacings and domain sizes of room-temperature epitaxial LixNi1−xO (0 ≤ x ≤ 0.48) thin films grown on ultra-smooth sapphire substrates

The effects of heavy Li doping on lattice spacings, out-of-plane, and in-plane domain sizes of room-temperature epitaxial LixNi1−xO (0≤x≤0.48) thin films on ultra-smooth sapphire (0001) substrates were investigated. The pseudocubic-on-hexagonal epitaxial relationship between the LixNi1−xO epitaxial thin films and the sapphire substrates was verified. The (111) lattice spacing of the film was larger than the (11¯1) lattice spacing of the film regardless of the Li content. It indicated that all the crystal structures deviated from the ideal cubic structure and elongated along the [111] out-of-plane growth direction. The crystal domain sizes of LixNi1−xO thin films in the in-plane direction were found to be very similar, while the out-of-plane domain size increased with a Li content up to 48mol%. It suggested that the out-of-plane growth can be improved by heavy Li doping. Moreover, the crystal quality of the films was compared with that grown by high temperature pulse laser deposition in view of the domain size information.

Optical properties of MgxZn1−xO thin films deposited on silicon and sapphire substrate by rf magnetron sputtering

The index dispersion at UV–VIS range for polycrystalline MgxZn1−xO films on silicon with different Mg concentration was obtained by spectroscopic ellipsometry (SE) method. It decreases with the increase of the Mg content. Above the relative peak wavelength, they are well fitted by the first-order Sellmeier relation. The band gap of films on sapphire of different Mg content was determined from transmission measurements. Photoluminescence (PL) illustrated that for MgxZn1−xO films every PL peak corresponded to a special excitation wavelength. The wavelength of the PL peak was proportional to the special excitation wavelength. A strong peak was obtained in the blue band for the films due to the large amount of oxygen vacancies caused by excess Zn and Mg atoms, while weak peak at ultraviolet band.

Microstructure and blueshift in optical band gap of nanocrystalline AlxZn1−xO thin films

In this paper, we report the structural and optical properties of Al doped ZnO (AZO) thin films grown on glass substrates using the sol–gel process. To understand the effect of Al doping on the structural and optical response of ZnO nanoparticles thin films, the prepared samples have been characterized using X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), photoluminescence (PL), UV–vis absorption and Raman spectroscopy. X-ray diffraction results show that Al doped ZnO nanoparticles have hexagonal phase similar to ZnO nanoparticles. TEM images as well as XRD data exhibit the estimated size of nanoparticles to be in the range 35–45nm. The optical band gap has been determined from optical absorption spectra. The band gap varied from 3.27eV for undoped ZnO film to 3.87eV for AZO film having 3atwt% Al. The blue shift in energy band gap mainly related to carrier concentration induced by Al-donor doping, and to the degree of crystalline order. Photoluminescence study further confirms the blue shift in UV emission when Al doping concentration is increased, as a consequence of extension in band gap.

Fermi level stabilization and band edge energies in CdxZn1−xO alloys

We have measured the band edge energies of CdxZn1−xO thin films as a function of composition by three independent techniques: we determine the Fermi level stabilization energy by pinning the Fermi level with ion irradiation, measure the binding energy of valence band states and core levels by X-ray photoelectron spectroscopy, and probe shifts in the conduction band and valence band density of states using soft X-ray absorption and emission spectroscopy, respectively. The three techniques find consensus in explaining the origin of compositional trends in the optical-bandgap narrowing upon Cd incorporation in wurtzite ZnO and widening upon Zn incorporation in rocksalt CdO. The conduction band minimum is found to be stationary for both wurtzite and rocksalt alloys, and a significant upward rise of the valence band maximum accounts for the majority of these observed bandgap changes. Given these band alignments, alloy disorder scattering is found to play a negligible role in decreasing the electron mobility for all alloys. These band alignment details, combined with the unique optical and electrical properties of the two phase regimes, make CdZnO alloys attractive candidates for photoelectrochemical water splitting applications.

Anomalous thickness-dependent strain states and strain-tunable magnetization in Zn-doped ferrite epitaxial films

A series of ZnxFe3−xO4 (ZFO, x = 0.4) thin films were epitaxially deposited on single-crystal (001)-SrTiO3 (STO) substrates by radio frequency magnetron sputtering. The anomalous thickness-dependent strain states of ZFO films were found, i.e., a tensile in-plane strain exists in the thinner ZFO film and which monotonously turns into compressive in the thicker films. Considering the lattice constant of bulk ZFO is bigger than that of STO, this strain state cannot be explained in the conventional framework of lattice-mismatch-induced strain in the hetero-epitaxial system. This unusual phenomenon is proposed to be closely related to the Volmer-Weber film growth mode in the thinner films and incorporation of the interstitial atoms into the island's boundaries during subsequent epitaxial growth of the thicker films. The ZFO/STO epitaxial film is found in the nature of magnetic semiconductor by transport measurements. The in-plane magnetization of the ZFO/STO films is found to increase as the in-plane compressive strain develops, which is further proved in the (001)-ZFO/PMN-PT film where the film strain state can be in situ controlled with applied electric field. This compressive-strain-enhanced magnetization can be attributed to the strain-mediated electric-field-induced in-plane magnetic anisotropy field enhancement. The above results indicate that strain engineering on magnetic oxide semiconductor ZFO films is promising for novel oxide-electronic devices.

Influence of Y doping concentration on the properties of nanostructured MxZn1−xO (M=Y) thin film deposited by nebulizer spray pyrolysis technique

Yttrium doped Zinc Oxide (YxZn1−xO) thin films deposited at a substrate temperature 400°C. The effect of substrate temperature on the structural, surface morphology, compositional, optical and electrical properties of YxZn1−xO thin films was studied. X-ray diffraction studies show that all films are polycrystalline in nature with hexagonal crystal structure having highly textured (002) plane parallel to the surface of the substrate. The structural parameters, such as lattice constants (a and c), crystallite size (D), dislocation density (δ), microstrain (σ) and texture coefficient were calculated for different yttrium doping concentrations (x). High resolution scanning electron microscopy measurements reveal that the surface morphology of the films change from platelet like grains to hexagonal structure with grain size increase due to the yttrium doping. Energy dispersive spectroscopy confirms the presence of Y, Zn and O elements in the films prepared. Optical studies showed that all samples have a strong optical transmittance higher than 70% in the visible range. A slight shift of the absorption edge towards the large wavelengths was observed as the Y doping concentration increased. This result shows that the band gap is slightly decreased from 3.10 to 2.05eV with increase of the yttrium doping concentrations (up to 7.5%) and then slightly increased. Room temperature PL measurements were done and the band-to-band emission energies of films were determined and reported. The complex impedance of the 10%Y doped ZnO film shows two distinguished semicircles and the diameter of the arcs got decreased in diameter as the temperature increases from 70 to 175°C.

Preparation and photocatalytic activity of MgxZn1−xO thin films on silicon substrate through sol–gel process

Magnesium doped zinc oxide (MgxZn1−xO) thin films were synthesized on silicon substrate through sol–gel process. Mg0.15Zn0.85O thin films were annealed at 500–800 °C and ZnO, Mg0.1Zn0.9O, Mg0.05Zn0.95O thin films were annealed at 600 °C for 60 min, respectively. The results show that all the samples are of a hexagonal wurtzite structure of ZnO. The surface morphology is strongly dependent on mean grain size and surface fluctuation. Fourier transform infrared spectra reveal that the vibration peak at 420 cm−1 is of the intrinsic lattice absorption of ZnO. The peak at 1083 cm−1 belongs to SiOSi asymmetric stretching vibration. Photoluminescence spectra show that the ultraviolet emission (365–400 nm) and the broad visible emission (469–569 nm) are observed. In particular, Mg0.05Zn0.95O thin film annealed at 600 °C exhibits the highest photocatalytic activity, degrading MO by almost 85.8% after 180 min illumination. The photocatalytic activity of the thin film is a synergistic effect defined by grain size, roughness factor, oxygen defects and amorphous MgO.

Optical characterization by variable angle spectroscopic ellipsometry of nitrogen-doped MgxZn1 − xO thin films prepared by the plasma-assisted reactive evaporation method

Non-doped MgxZn1−xO films (MgxZn1−xO films) and nitrogen-doped MgxZn1−xO films (MgxZn1−xO:N films) were grown epitaxially on Zn faces of ZnO single crystal substrates by the plasma-assisted reactive evaporation (PARE) method using ZnMg alloys. Optical parameters, refractive index n, extinction coefficient k, and absorption coefficient α of these films and ZnO substrate were easily estimated by variable angle spectroscopic ellipsometry. The k values of these films abruptly increased with increase in photon energy near the absorption edge region. Dispersions of n of these films were sharper with larger peak values than that of a ZnO substrate. MgxZn1−xO films and MgxZn1−xO:N films grown on a ZnO substrate by the PARE method are of high quality with less defects than those of a ZnO substrate. Maximum value of α of these films calculated from k was about 2×105cm−1. The values of intercepts on photon energy hν axis of (αhν)2‐hν plots for these films and the ZnO substrate agree with peak energy of PL spectra, the origin of which is free excitons. These results indicate that absorption in those materials is dominated by exciton absorption.

Electrical, optical, and microstructural properties of sol–gel derived HfZnO thin films

HfxZn1−xO thin films are deposited on glass substrates by the sol–gel method. The incorporation of Hf increases the crystallinity of the as-deposited films. The bandgap increases with the Hf content but reduces after thermal annealing because of the relaxation of built-in stress, atomic rearrangement, and precipitation of HfO2. The resistivity of ZnO decreases as the annealing temperature increases owing to the improvement of crystallinity and the reduction of defect densities. On the contrary, the resistivity of HfZnO thin films increases with the annealing temperature owing to the precipitation of HfO2 and reduction of oxygen vacancies. The incorporation of Hf in the films improves the Zn–O bonding state, and thermal annealing enhances metal–oxygen bonding.

Improved Crystal Quality of Transparent Conductive Ga‐doped ZnO Films by Magnesium Doping Through Radio‐Frequency Magnetron Sputtering Preparation

This study investigates the enhanced structural, and optoelectronic properties of transparent conductive Ga‐doped MgxZn1−xO (GMZO) thin films with a varied magnesium (Mg) composition of 2% and 8%, respectively. The X‐ray diffraction (XRD) measurements revealed that GMZO with an 8% Mg composition shows a stronger (002) diffraction intensity and narrower linewidth than that with a 2% Mg composition. Improved crystallinity and enlarged grain size in the postgrowth thermal annealed GMZO thin films were also observed in XRD and morphological measurements by atomic force microscopy. Photoluminescence measurements were conducted to investigate the improved GMZO thin‐film quality, and the oxygen vacancy signal was found to decrease with increased Mg content, consistent with X‐ray photoelectron spectroscopy measurements. This study also shows high optical transmittance over 98%, and a low resistivity of 5.7 × 10−4 Ω·cm in Ga‐doped MgxZn1−xO (x = 0.02) thin film, which indicates the highly promising candidate for use in optoelectronic devices.

Growth and characterization of Mg x Zn1−x O thin films by aerosol-assisted chemical vapor deposition (AACVD)

The growth and characterization of MgxZn1−xO thin films by aerosol-assisted chemical vapor deposition (AACVD) technique is reported in this paper. We have grown the thin films of ZnO by adding varying concentrations of magnesium (Mg) on a glass substrate. The precursor from which the MgxZn1−xO thin films were grown was made up of a mixture of zinc acethylacetonate and magnesium acetate tetrahydrate in boiled isopropyl alcohol. Oxygen gas was used as a carrier gas and substrate temperature was maintained at 400°C. MgxZn1−xO thin films were finally characterized by x-ray diffraction (XRD), atomic force microscopy (AFM) and UV-VIS-NIR spectroscopy. XRD results show that MgxZn1−xO thin films displayed a wurtzite structure and addition of Mg leads to a slight shift towards higher 2-theta values. AFM results show that MgZnO thin films were uniformly covered with nano flakes and their size decreases with an increase in Mg content. Optical studies show that with the increase of Mg content, transparency as well energy band gap of the MgxZn1−xO thin films increases, which also agrees with the reported values.

Effects of tetrahedral Fe2+ on the structural, magnetic and electronic properties of solution-based titanomagnetite thin films

Polycrystalline titanomagnetite (Ti x Fe3−x O4) thin films prepared by using a sol-gel process exhibited a phase-pure spinel $$\left( {Fd\bar 3m} \right)$$ structure for Ti compositions up to x = 0.6. An X-ray photoelectron spectroscopy (XPS) investigation disclosed an increase of the Fe2+ concentration with increasing x, indicating a reduction in ionic valence, Fe3+ → Fe2+, induced by Ti4+ occupation of the cationic sublattice. Analyses on XPS and X-ray diffraction spectra of the Ti x Fe3−x O4 samples suggest that the Fe2+ ions prefer the tetrahedral sites while the Ti4+ ions prefer the octahedral sites of the sublattice. Magnetic hysteresis measurements on the Ti x Fe3−x O4 films revealed significant loss of the saturation magnetization (M s ) with increasing x: M s is reduced to 50% that of Fe3O4 for x = 0.10 and to 10% for x = 0.60. The big loss of M s caused by small Ti doping suggests a significant disruption of the inter-site Fe3+-Fe3+ super-exchange interaction in thin-film titanomagnetites.

Nanostructured CexZn1−xO thin films: Influence of Ce doping on the structural, optical and electrical properties

Thin films of CexZn1−xO thin films were deposited on glass substrates at 400°C by nebulizer spray pyrolysis technique. Ce doping concentration (x) was varied from 0 to 10%, in steps of 2.5%. X-ray diffraction reveals that all the films have polycrystalline nature with hexagonal crystal structure and high preferential orientation along (002) plane. Optical parameters such as; transmittance, band gap energy, refractive index (n), extinction coefficient (k), complex dielectric constants (εr, εi) and optical conductivity (σr, σi) have been determined and discussed with respect to Ce concentration. All the films exhibit transmittance above 80% in the wavelength range from 330 to 2500nm. Optical transmission measurements indicate the decrease of direct band gap energy from 3.26 to 3.12eV with the increase of Ce concentration. Photoluminescence spectra show strong near band edge emission centered ∼398nm and green emission centered ∼528nm with excitation wavelength ∼350nm. High resolution scanning electron micrographs indicate the formation of vertical nano-rod like structures on the film surface with average diameter ∼41nm. Electrical properties of the Ce doped ZnO film have been studied using ac impedance spectroscopy in the frequency range from 100Hz–1MHz at different temperatures.