Epitaxial ZnO Films Research Articles

Structural and optical properties of Eu-doped ZnO epitaxial thin films grown by pulsed-laser deposition

Pulsed-laser deposition was utilized to fabricate Eu-doped ZnO epitaxial films on c-plane sapphire substrates with Eu concentrations ranging from 0.5 to 4.0 at. %. The structural properties were analyzed using x-ray diffraction surface normal radial scans and azimuthal cone scans, which confirmed the epitaxy of the film samples. Reciprocal space mapping was performed on ZnO(101̄1) to visualize the effect of Eu incorporation. X-ray fluorescence mapping confirmed the homogeneous distribution of Zn and Eu, and x-ray absorption near-edge structure spectra directly confirmed the trivalent state of Eu ions. The optical properties were assessed using temperature-dependent photoluminescence (PL). Various defects were identified. With increasing Eu dopant concentration, PL emissions from defects and the Eu 4f-intraband transitions gradually became the predominant features in the PL spectra at low temperatures. Furthermore, PL analysis suggested that Eu ions substituted Zn, occupying sites with lower C3v symmetry due to the distortion caused by Eu incorporation.

Epitaxial (0001) ZnO Films obtained by Magnetron Sputtering on C-Sapphire Substrates with a WO2 + x Buffer Nanolayer

Epitaxial (0001) ZnO Films obtained by Magnetron Sputtering on C-Sapphire Substrates with a WO2 + x Buffer Nanolayer

X-Ray- and Synchrotron-Radiation Study of the Defect Structure of Epitaxial ZnO Films Grown by Magnetron Deposition on Al2O3 and LaMgAl11O19 Substrates with (0001) Orientation

The results of studying the structural features of samples of zinc-oxide films obtained by magnetron deposition on chips of lanthanum-magnesium hexaaluminate and the surface of sapphire substrates with a gold buffer layer are presented. Analysis of the structure and morphology of the films is carried out using a set of methods, including high-resolution X-ray diffractometry, the method of constructing pole figures, and transmission electron microscopy. It is shown that when using cleavages of lanthanum-magnesium hexaaluminate, an epitaxial ZnO film is formed without signs of growth rotating domains. The use of a gold buffer layer during growth on sapphire substrates improves the crystalline quality of ZnO films, but does not completely suppress domain growth.

Compliant substrate epitaxial MgZnO films using fluorphlogopite mica approaching homoepitaxy quality

Based on the success of high-quality epitaxial grown ZnO film on the compliant Fluorphlogopite Mica (F-Mica) substrate, the growth of MgZnO films on F-Mica substrate is studied to promote the development of flexible ZnO quantum well structures and devices. These flexible epitaxial MgZnO films show high luminous quality approaching homoepitaxial films. The phenomenon of Mg compositional fluctuations is confirmed by the research on band tail state. The effect of the compositional fluctuations on epitaxial growth dynamical and thermodynamic processes is also revealed by the comprehensively investigation of the crystal quality, optical properties, morphology, and growth rate. The study provides guidance for energy band engineering of ZnO on flexible substrates, and lays the foundation for the bendable optoelectronic devices on F-Mica substrate.

Dependence of Resonant Light Wavefront Reversal on Polaritons on the Optical Pump Intensity in Zinc Oxide Films

The possibility of resonant light wavefront reversal in an excited semiconductor medium has been demonstrated theoretically and experimentally. Induced light wavefront reversal in the infrared spectral range equal to half the energy of the radiative recombination of polaritons has been detected on epitaxial ZnO films under nitrogen laser pumping at room temperature. Dependences of the intensity of the wavefront reversal signal on the energy of the incident photon and the laser pump intensity have been examined.

NaCl flux growth of non-polar m-plane ZnO epitaxial thin film on c-plane sapphire substrate

We have developed a technique to fabricate non-polar m-plane ZnO thin films: the post NaCl flux treatment of Zn-containing precursors, such as amorphous zinc carbonate hydroxide, Zn5(CO3)2(OH)6. Conventionally, single-crystal m-plane sapphire or MgO (001) substrates have been utilized to fabricate m-plane ZnO thin films. In contrast, this method facilitates the growth of m-plane ZnO thin films on various substrates, such as sapphire and quartz glass, despite its c-plane growth habit. Furthermore, the m-plane ZnO thin film was demonstrated to epitaxially grow even on a c-plane sapphire substrate. Owing to the good lattice matching with the substrate, the m-plane ZnO epitaxial film had high crystallinity comparable to a c-plane ZnO epitaxial film that was created using a conventional pulsed laser deposition method, and exhibited similar photocatalytic activity to the c-plane ZnO film. This research offers valuable insights into the fundamental mechanisms of flux growth orientation control, which can be beneficial in designing ZnO-based electronic devices and photocatalysts.

The microstructural evolution of sputtered ZnO epitaxial films to stress-relaxed nanorods

Epitaxial ZnO thin films and nanorods were grown on c-sapphire substrate by reactive sputtering of Zn in Ar-O2 atmosphere at substrate temperatures ranging from near room temperature to 700 °C. Scanning electron microscopy showed that with increase in substrate temperature, the morphology transformed from columnar films to vertically aligned ZnO nanorods. High resolution x-ray diffraction revealed improvement in crystalline and epitaxial quality with increase in substrate temperature, along with the decrease of edge dislocation density from 5 × 1012 cm−2 to 8 × 1010 cm−2. The films grown near room temperature showed large hydrostatic strain (∼6 × 10−3) and compressive intrinsic biaxial stress (-0.8 GPa), which decreased substantially with increase in substrate temperature, due to the desorption of excess oxygen and reduction in edge dislocation density. Above the substrate temperature of 500 °C, the intrinsic biaxial stress reversed from compressive to mildly tensile in the case of nanorods, due to the intrinsic tensile stress, originating from crystallite coalescence. Raman measurements correlate well with the changes in biaxial stress and confirm the improvements in crystallinity and epitaxial quality of nanorods grown at higher temperatures. Room temperature photoluminescence of the ZnO films showed weak near-band-edge emission, which enhanced drastically in the case of nanorods due to their superior microstructure and epitaxial quality.

Comparative X-ray Diffractometry of the Defect Structure of ZnO Epitaxial Films Deposited by Magnetron Sputtering on C-Plane Al2O3 Substrates in Inhomogeneous Electric Field

Comparative X-ray Diffractometry of the Defect Structure of ZnO Epitaxial Films Deposited by Magnetron Sputtering on C-Plane Al2O3 Substrates in Inhomogeneous Electric Field

Suppression of grain boundary contributions on carrier mobility in thin Al-doped ZnO epitaxial films

Realizing very thin transparent conductive (TCO) films is a key aspect of many applications, but often the film quality tends to worsen at very small thicknesses. In this work we demonstrate that Al-doped ZnO (AZO) thin films grown epitaxially on SrTiO3 retain their optimal properties even at thicknesses as low as 30 nm. We deposit by radio frequency magnetron sputtering and investigate the film morphology, structure, crystallinity, electrical and optical properties. We prove that the Hall mobility of epitaxial films is limited mostly by ionized impurity scattering, with a negligible contribution of grain boundary scattering, leading to comparably high mobility in thin epitaxial films. On the contrary, in polycrystalline films the properties of AZO films strongly deteriorate at reduced film thickness, due to grain boundary contributions. The optical carrier density and mobility do not change significantly between epitaxial and polycrystalline films, suggesting that the different electrical properties are indeed mainly due to grain boundary scattering and not to a reduction of carriers and intrinsic mobility. In this way we obtain an epitaxial thin TCO film with the optimal properties of a bulk by a scalable technique, paving the way to applications in plasmonic devices and in thin films solar cells.

Is all epitaxy on mica van der Waals epitaxy?

Epitaxy on layered substrates, for example, mica, manifests different characteristic from that on non-layered substrates, which is demonstrated to be the fantastic road towards high-quality functional films. To date, van der Waals epitaxy has been generally accepted as the mechanism to describe the interaction between layered substrate and epilayer. However, some experimental evidences also revealed that pure van der Waals interaction was not sufficient for long-range ordering nucleation. In this work, we provide unambiguous evidence demonstrating that the epitaxy on mica is not always the van der Waals type. Detailed studies on the interface microscopic structure and strain release process of ZnO epitaxial films indicate a new epitaxy mode on fluorphlogopite mica substrate. ZnO epitaxial films on fluorphlogopite mica substrates with superior crystal quality and excellent optical property are obtained. The improved epitaxy of ZnO film is ascribed to compliant substrate epitaxy rather than van der Waals epitaxy. It is anticipated that this epitaxy mechanism provides a low-cost reusable-substrate method for high-quality functional films and flexible device application.

Polarized dependence of soft X-ray absorption near edge structure of ZnO films implanted by Yb

Virgin and Yb-implanted epitaxial ZnO films grown using atomic layer deposition (ALD) were investigated by X-ray absorption spectroscopy (XAS). XAS study revealed a strong polarization dependence of films determined by the orientation of the polarization vector of the synchrotron radiation to the sample surface. It also indicated that the implantation and subsequent annealing have an important influence on the native point defect complexes in the ZnO. Comparison of experimental spectra with the modelled ones, which are computed based on the linear combination of model spectra corresponding to the selected point defects and their complexes, confirmed the presence of donor-acceptor complexes (mVZn - nVO, m = 1,4; n = 1,2) in the samples under study. The mechanism of vacancy complexes formation is unclear as it takes place under non-equilibrium conditions, for which any theoretical method has not been well established. Exploring the 3 d → 4 f absorption, it was found that oxidation state of Yb in ZnO is 3+, which is consistent with the XPS findings and previously conducted Resonant Photoemission Spectroscopy (RPES) investigations. The inversion of the polarization dependence for samples with different Yb fluences visible in Yb M5 spectra can be associated with a tilt of the oxygen pseudo octahedra or/and with their distortion. The analysis of the presented data suggests that the donor-acceptor complexes are present both in as grown and implanted films and may influence their electrical properties. This suggestion was confirmed by previous Hall measurements showing that the resistivity of annealed ZnO:Yb film with a fluence of 5e15 ions/cm2 decreases by about one order compared to the one with a fluence of 5e14 ions/cm2.

Tailorable properties of Nd-doped ZnO epitaxial thin films for optoelectronic and plasmonic devices

Transparent conductive oxides with both low resistivity and high transparency in the visible and near-infrared wavelengths are required for solar cells and plasmonic applications. In this work we demonstrate that highly conducting and transparent Nd:ZnO thin films have been grown on c-cut sapphire single crystal substrates by pulsed electron beam deposition under argon (10−2 mbar). At 2% at. Nd doping, these films crystallize in the wurtzite structure at T > 300 °C, with well-defined epitaxial relationships with the c-cut sapphire as a function of the substrate temperature. The improvement of the film structural quality with increasing growth temperature led to the lowest resistivity value of 5.12 × 10−4 Ωcm and highest transparency that were maintained more than 36 months under ambient conditions. X-ray photoelectron valence band measurements revealed that the dominant Zn-polar face of Nd:ZnO thin films explains the improved electrical properties. The dielectric functions of films were determined and show that these films are suitable as transparent contact electrodes for solar cell devices or tunable epsilon-near-zero materials for plasmonics.

A-axis oriented Zn0.72Mg0.28O epitaxial thin films with large second-order nonlinear susceptibility

Large nonlinear susceptibility that originates from strong electronic polarization enables ultrafast nonlinear optical devices. This work discovers that Mg incorporation has important contribution for enhancing second-order nonlinear susceptibility (χ (2)) and laser-induced surface-damage threshold of wurtzite ZnO epitaxial thin films deposited by radio-frequency magnetron sputtering method. Second-harmonic generation measurements derive that as-deposited Zn0.72Mg0.28O shows a gain of 48%, 77% and 33% in χ 33, χ 31 and χ 15 with respect to as-deposited ZnO. Specially, the annealed Zn0.72Mg0.28O has a χ 33 value of −57.0 ± 1.8 pm V−1, which gets comparable to that of LiNbO3 crystals. Triple-axis x-ray diffraction measurements conclude that the Mg incorporation should increase the χ (2) under optical frequency electric field by strengthening electronic polarization rather than increasing the residual strain in the film. Furthermore, the annealed Zn0.72Mg0.28O exhibits an increase of 48% in laser-induced surface-damage threshold relative to [11–20] ZnO crystals. These findings open the way of the Zn0.72Mg0.28O thin films to ultrafast nonlinear optical devices.

Impact of band-gap graded structures artificially implemented in Mg–ZnO epitaxial films on photoelectrochemical properties

The impact of band gap-graded structures artificially implemented in a photocatalyst on the photoelectrochemical properties was investigated in a model system of epitaxial thin film Mg–ZnO.

Lattice Orientation Heredity in the Transformation of 2D Epitaxial Films.

The ability to control lattice orientation is often an essential requirement in the growth of both 2D van der Waals (vdW) layered and nonlayered thin films. Here, a unique and universal phenomenon termed "lattice orientation heredity" (LOH) is reported. LOH enables product films (including 2D-layered materials) to inherit the lattice orientation from reactant films in a chemical conversion process, excluding the requirement on the substrate lattice order. The process universality is demonstrated by investigating the lattice transformations in the carbonization, nitridation, and sulfurization of epitaxial MoO2 , ZnO, and In2 O3 thin films. Their resultant compounds all inherit the mono-oriented crystal feature from their precursor oxides, including 2D vdW-layered semiconductors (e.g., MoS2 ), metallic films (e.g., MXene-like Mo2 C and MoN), wide-bandgap semiconductors (e.g., hexagonal ZnS), and ferroelectric semiconductors (e.g., In2 S3 ). Using LOH-grown MoN as a seeding layer, mono-oriented GaN is achieved on an amorphous quartz substrate. The LOH process presents a universal strategy capable of growing epitaxial thin films (including 2D vdW-layered materials) not only on single-crystalline but also on noncrystalline substrates.

Ultrananocrystalline diamond‐like carbon (UN‐DLC) assembled on epitaxial ZnO film by PLD technique and SIMS Raman Rutherford spectroscopic fingerprint investigation

AbstractNanocomposite materials coating opens up tremendous opportunities in the development of commercial products for household to industrial applications. In this present article, we are reporting the formation of 10‐nm carbon (C) layer underneath the 6‐nm ZnO epitaxial thin‐film grown on simple quarts or Si(100) substrate by pulsed laser deposition. As‐obtained nanocrystalline carbon layers between the Si(100) and ZnO and above ZnO epitaxial film were studied by micro‐Raman, secondary ion mass (SIMS), and resonant Rutherford backscattering (RRBS) spectroscopic methods. The carbon diffusivity analysis revealed that the diffusion at nanoscale is governed predominantly by a nonlinear Fick's law rather than the Soret‐Ludwig law. Quantum diffusion dominates the classical diffusion and carbon diffusivity into ZnO layer and was found to be ~7.8 × 10−12 cm2 s−1. This diffusion explains the doping profiles of ZnO layer up to its surface where carbon packing suggests presence of ultrananocrystalline‐diamond‐like‐carbon (UN‐DLC) thin films revealed by Raman spectroscopy.

Towards high-performance linear piezoelectrics: Enhancing the piezoelectric response of zinc oxide thin films through epitaxial growth on flexible substrates

Ferroelectrics are popular for sensors, actuators and transducers. However, the energy barrier for reversing the polarization leads to extra energy losses, while the hysteresis causes phase-lags and non-linearities between input and output signals. The high piezoelectric constants of ferroelectrics dictate the figure-of-merit of piezoelectric devices. Enhancing the piezoelectric constant of linear-piezoelectrics potentially leads to next-generation piezoelectric devices free from issues associated with ferroelectrics. By epitaxial growth of undoped ZnO films on flexible Hastelloy substrates, we demonstrated an ~85% improvement in the piezoelectric constant, and more importantly maintained the linearity of the piezoelectric response. Strong out-of-plane and in-plane crystallographic alignments are detected in the epitaxial sample, yet modeling of the effective thin-film piezoelectric constant shows only a limited contribution of the optimized textures. The improvement in the piezoelectric response is ascribed mostly to the in-plane epitaxial strain resulted from the lattice mismatch of ZnO to the substrate. The epitaxial ZnO film on Hastelloy promisingly offer high-precision positioners with nano-/pico-meter resolutions. Enhancing the piezoelectric constant by strain engineering potentially removes the barrier for applications of linear-piezoelectrics and enriches the piezoelectric research community.

Metal–insulator transition in epitaxial Ga-doped ZnO films via controlled thickness

Understanding and tuning of metal–insulator transition (MIT) in oxide systems is an interesting and active research topics of condensed matter physics. We report thickness dependent MIT in Ga-doped ZnO (Ga:ZnO) thin films grown by pulsed laser deposition technique. From the electrical transport measurements, we find that while the thinnest film (6 nm) exhibits a resistivity of 0.05 Ω cm, lying in the insulating regime, the thickest (51 nm) has resistivity of 6.6 × 10−4 Ω cm which shows metallic type of conduction. Our analysis reveals that the Mott’s variable range hopping model governs the insulating behavior in the 6 nm film whereas the 2D weak localization (WL) phenomena is appropriate to explain the electron transport in the thicker Ga:ZnO films. Magnetoresistance study further confirms the presence of strong localization in 6 nm film while WL is observed in 20 nm and above thicker films. From the density functional calculations, it is found that due to surface reconstruction and Ga doping, strong crystalline disorder sets in very thin films to introduce localized states and thereby, restricts the donor electron mobility.

Nonresonance Phase Conjugation of Light in Optically Pumped ZnO Films

The possibility of the nonresonance phase conjugation of light occurring in an excited semiconductor medium is shown theoretically and experimentally. In epitaxial ZnO films pumped with a nitrogen laser at room temperature, the induced phase conjugation of light in the visible and infrared spectral regions is detected. The dependences of the phase-conjugation signal intensity on the incident photon energy and laser-pumping intensity are studied. Interpretation of the effect as a result of the absorption and refraction of light at laser-induced free charge carriers in the semiconductor medium is proposed.

Optical Response of Epitaxial ZnO Films Grown by Atomic Layer Deposition and Coimplanted with Dy and Yb

In this study, structural and optical properties of ZnO:Dy, ZnO:Yb, and ZnO:(Dy,Yb) films are investigated. Epitaxial ZnO films are grown by atomic layer deposition at 300 °C and implanted with Yb and Dy ions to the fluence of 5 × 1014 cm−2. The effects of Dy and Yb implantation, Dy/Yb coimplantation, and postgrowth annealing are studied by channeling Rutherford backscattering spectrometry (RBS/c) and room‐temperature photoluminescence (RT PL). The damage built up after implantation is found to be similar for Dy and Yb ions, but crystal structure recovery after annealing is more efficient for ZnO:Yb than for ZnO:Dy, when similar annealing conditions are applied. Colorimetric analysis of the RT PL spectra shows that (Dy,Yb) coimplantation allows shifting the optical response of ZnO:Dy films toward white color light. The correlated color temperature can be tuned in this way from 2500 to 3200 K, which makes these films attractive for indoor applications.