Epitaxial ZnO Films Research Articles

Off-axis sputter deposition of ZnO films on c-sapphire substrates by utilizing nitrogen-mediated crystallization method

High-quality epitaxial ZnO films on c-plane sapphire substrates have been obtained by utilizing an off-axis sputtering configuration together with buffer layers prepared via nitrogen-mediated crystallization (NMC). The role of NMC buffer layers is to provide high density of nucleation site and, thus, to reduce the strain energy caused by the large lattice mismatch (18%) between ZnO and sapphire. The NMC buffer layers allow two-dimensional growth of subsequently grown ZnO films, being particularly enhanced by employing an off-axis sputtering configuration in which the substrate is positioned out of the high-energy particles, such as negative oxygen ions originating from the targets. As a result, ZnO films with smooth surfaces (root-mean-square roughness: 0.76 nm) and a high electron mobility of 88 cm2/V·s are fabricated. Photoluminescence spectra of the ZnO films show strong near-band-edge emission, and the intensity of the orange-red defect emission significantly decreases with increasing horizontal distance between the target and the substrate. From these results, we conclude that off-axis sputtering together with NMC buffer layers is a promising method for obtaining high-quality epitaxial ZnO films.

Growth of nonpolar ZnO Films on (100) β-LiGaO2 substrate by molecular beam epitaxy

Nonpolar m-plane (11̄00) ZnO epitaxial films were grown on (100) β-LiGaO2 (LGO) substrates by plasma assisted molecular beam epitaxy (PAMBE). The dependence of growth characteristics on the growth temperatures was investigated. The surface morphologies of ZnO films were characterized by scanning electron microscopy and atomic force microscopy. Furthermore, the structural properties characterized by high resolution X-ray diffraction (HRXRD) indicated that the ZnO epilayers were grown in the nonpolar [11̄00] orientation. Detailed structural characterization and defect analysis of nonpolar ZnO epilayer on β-LiGaO2 substrate were studied by transmission electron microscope (TEM). Optical properties of m-plane ZnO films were investigated by Raman spectroscopy and photoluminescence analyses.

Recombination dynamics of a localized exciton bound at basal stacking faults within the m-plane ZnO film

We investigated the carrier dynamics near basal stacking faults (BSFs) in m-plane ZnO epitaxial film. The behaviors of the type-II quantum wells related to the BSFs are verified through time-resolved and time-integrated photoluminescence. The decay time of the emission of BSFs is observed to have a higher power law value and longer decay time than the emission of the donor-bound excitons. The spectral-dependent decay times reveal a phenomenon of carriers migrating among band tail states, which are related to the spatial distribution of the type-II quantum wells formed by the BSFs. A high density of excited carriers leads to a band bending effect, which in turn causes a blue-shift of the emission peak of BSFs with a broadened distribution of band tail states.

Local vibrational modes competitions in Mn-doped ZnO epitaxial films with tunable ferromagnetism

We reported spectroscopic investigations of high quality Mn-doped ZnO (ZnMnO) films grown by oxygen plasma-assisted molecular beam epitaxy. Raman scattering spectra revealed two local vibrational modes (LVMs) associated with Mn dopants at 523 and 712 cm−1. The LVMs and magnetic properties of ZnMnO films can be synchronously modulated by post annealing processing or by introducing tiny Co. The relative intensity of two LVMs clearly shows competitions arising from uncompensated acceptor and donor defects competition for ferromagnetic and nonmagnetic films. The experimental results indicated that LVM at 523 cm−1 is attributed to Mn—(Zinc-vacancy) complexes, while LVM at 712 cm−1 is attributed to Mn—(Oxygen-vacancy) complexes.

Effect of microwave remote plasma and radiofrequency plasma on the photoluminescence of (0001) epitaxial ZnO films

Photoluminescence of (0001) epitaxial ZnO films with thicknesses of 10, 30 and 100 nm on C-sapphire substrates have been studied at room temperature and after exposure to Ar, Ar–O2, Ar–N2 and Ar–H by remote microwave and radiofrequency plasmas. The photoluminescence are not modified by remote plasma treatments where only neutral species were involved. On the contrary, the photoluminescence signal is enhanced or quenched after radiofrequency plasma treatments when energetic ion species are involved in the surface treatment processes. Little changes of electric properties are observed, however, the optical transmission indicates that the absorption edge and probably also the index of refraction are affected. Photoluminescence peak shifts, widths and intensities changes show very strong similarities with polarized emission of ZnO single crystal where it exists a strong dichroism. The photoluminescence emission properties may then result from this optical modification. However, the plasma treatments on the different samples show very low stability in time, except, for the treatment in argon plasma alone. In this later case, in-situ monitoring of photoluminescence as a function of temperature revealed a partial recovery of the photoluminescence properties after a heat treatment at 400 °C for few minutes. These results indicate that photoluminescence of (0001) ZnO thin film, related to σ-emission polarized emission from c-axis polar surfaces, is highly affected by surface and implanted charged species.

Effect of high temperature thermal treatment of (100) γ-LiAlO2 substrate on epitaxial growth of ZnO films by plasma-assisted molecular beam epitaxy

Non-polar ZnO thin films were fabricated on (100) γ-LiAlO2 substrates by plasma-assisted molecular beam epitaxy. The effect of high temperature thermal treatment of substrate on the crystalline orientation and quality of ZnO thin film was investigated. The film grown on (100) γ-LiAlO2 substrate without high temperature thermal treatment consists of domains of both polar and non-polar orientations as identified by the X-ray diffraction pattern. Using high temperature thermal treatment of substrate, the growth of polar ZnO has been suppressed effectively. Besides, high temperature thermal treatment of substrate improves the crystalline quality of epitaxial ZnO thin film, which exhibits a smaller full width at half maximum value of ZnO (101−0) diffraction peak and a weaker deep level emission of photoluminescence. The suppression of polar ZnO growth and the quality improvement of the epitaxial ZnO films are due to the improvement of surface morphology and roughness of the substrate upon high temperature treatment.

Suppression of growth domains in epitaxial ZnO films on structured (0001) sapphire surface

Zinc oxide layers have been grown by magnetron sputtering in an oxygen atmosphere on structured sapphire surfaces. The formation of ZnO islands oriented in two directions (the so-called domains) was observed on (0001) Al2O3 surfaces with steps spaced by a distance from several thousands to several tens of thousands of nanometers. The islands formed along steps on (0001) Al2O3 surfaces with an ordered terrace-step structure (and, subsequently, ZnO films) have only one orientation. Another method is proposed for suppressing domains during ZnO growth on (0001) Al2O3 d.

Specific features of the growth of A II B VI films on (0001)Al2O3 substrates

The structure and orientation of CdTe and ZnO films on sapphire have been investigated for different techniques of pregrowth substrate treatment. Polycrystalline CdTe films are found to grow of substrates unannealed or annealed in vacuum at a residual pressure P < 0.13 Pa. Epitaxial CdTe films with the sphalerite cubic structure, oriented parallel to the substrate by the (111) plane, grow on substrates annealed in air at a temperature of 1000°C or more and having a system of smooth terraces and steps on the surface. For ZnO films with a wurtzite hexagonal structure obtained by magnetron sputtering, a similar correlation between the structural quality and the regime of treatment of sapphire substrates is observed. It is shown that thermal annealing of (0001) sapphire plates in air is the optimal way of substrate preparation for growing epitaxial ZnO films with the base orientation. The obtained epitaxial CdTe films contain a certain amount of structural defects (mosaicity and twins), while the epitaixal ZnO films treated in the same way are close to perfect.

Raman scattering investigations on Co-doped ZnO epitaxial films: Local vibration modes and defect associated ferromagnetism

Raman scattering spectroscopy has been performed on high quality Co-doped ZnO epitaxial films, which were grown on Al2O3 (0001) by oxygen-plasma assisted molecular beam epitaxy. Raman measurements revealed two local vibration modes (LVMs) at 723 and 699 cm−1 due to the substitution of Co2+ in wurtzite ZnO lattice. The LVM at 723 cm−1 is found to be an elemental sensitive vibration mode for Co substitution. The LVM at 699 cm−1 can be attributed to enrichment of Co2+ bound with oxygen vacancy, the cobalt–oxygen vacancy–cobalt complexes, in Zn1−xCoxO films associated with ferromagnetism. The intensity of LVM at 699 cm−1, as well as saturated magnetization, enhanced after the vacuum annealing and depressed after oxygen annealing.

Growth and characterization of n-ZnO/p-GaN nanorods on silicon for the fabrication of heterojunction diodes

A heterojunction n-ZnO/p-GaN diode device was fabricated and characterized on Si (111) substrate. Vertically-aligned Mg-doped GaN nanorods (NRs) were grown on Si (111) by plasma assisted chemical vapor deposition. Intrinsic n-type ZnO was subsequently grown on top of p-GaN nanorods by hydrothermal method at low temperature. The effects of precursor concentrations on the morphology and optical properties of ZnO nanostructures were investigated. Various ZnO nanostructures could be synthesized to obtain different heterojunction nanostructures. The high resolution transmission electron microscopy and selected area electron diffraction results further verified that the GaN NRs were single crystals with the growth orientation along [0001], and the epitaxial wurtzite ZnO films were grown on GaN NRs. The n-ZnO film/p-GaN NR heterojunction diodes were thus fabricated. Diode-like rectifying behavior was actually observed with a leakage current of less than 2.0×10−4A at −20V bias, a forward current of 7.2×10−3A at 20V bias, and the turn-on voltage at around 5.6V.

Two shallow donors related to Zn interstitial in S-ion implanted ZnO epitaxial film

Two shallow donors are observed in sulfur-ion implanted ZnO epitaxial films on sapphire substrates. The resistivity varies from ~103Ωcm for un-implanted samples to 1.7×10−2Ωcm for as-implanted ones. This low resistivity is attributed to zinc interstitial (Zni), corresponding to the activation energy (26meV) estimated from the temperature dependence of electron concentration. The presence of Zni is evaluated from ion channeling by Rutherford backscattering spectroscopy. The disordered concentration is ~40% of Zn atoms at a maximum point (300nm) from the surface in as-implanted samples and recovers to ~80% after 1000°C annealing. In 1000°C annealed samples, a shallower donor level of 12meV is observed, suggesting the presence of complex defects consisting of remaining Zni and sulfur atoms replaced to the oxygen atom.

Ultrafast dynamics of the dielectric functions of ZnO and BaTiO3 thin films after intense femtosecond laser excitation

The ultrafast carrier dynamics of epitaxial ZnO and BaTiO3 thin films after intense excitation at 3.10 eV and 4.66 eV photon energy has been studied by femtosecond absorption spectroscopy. Modelling the transient transmission changes on the basis of spectroscopic ellipsometry data and pertinent equilibrium model dielectric functions extended by additional terms for the effects at high carrier density (P-band luminescence and stimulated emission from electron-hole-plasma), a self-consistent parameterized description was obtained for both materials. Excited carrier lifetimes in the range of ≈2 to ≈60 ps and long-lived thermal effects after several hundred ps have been identified in both materials. These findings form a reliable basis to quantitatively describe future femtosecond studies on ZnO/BaTiO3 heterolayer systems.

Polarity characterization by anomalous x-ray dispersion of ZnO films and GaN lateral polar structures

We demonstrate the use of anomalous x-ray scattering of constituent cations at their absorption edge, in a conventional Bragg-Brentano diffractometer, to measure absolutely and quantitatively the polar orientation and polarity fraction of unipolar and mixed polar wurtzitic crystals. In one set of experiments, the gradual transition between c+ and c− polarity of epitaxial ZnO films on sapphire as a function of MgO buffer layer thickness is monitored quantitatively, while in a second experiment, we map the polarity of a lateral polar homojunction in GaN. The dispersion measurements are compared with piezoforce microscopy images, and we demonstrate how x-ray dispersion and scanning probe methods can provide complementary information that can discriminate between polarity fractions at a material surface and polarity fractions averaged over the film bulk.

Hydrothermal epitaxial growth of ZnO films on sapphire substrates presenting epitaxial ZnAl2O4 buffer layers

This article describes our investigation of the hydrothermal epitaxial growth of c-plane ZnO films on Al2O3 substrates presenting ZnAl2O4 buffer layers. We obtained (111) ZnAl2O4 epitaxial layers on a-plane Al2O3 substrates readily through solid phase epitaxy. Although the ZnAl2O4 buffer layers grew epitaxially with a (111) out-of-plane orientation and comprised two coexisting equivalent azimuthal variants with relative 180° in-plane rotation, the ZnO epitaxial films grown upon them exhibited a c-plane orientation with unitary in-plane epitaxial orientation of <11¯00>ZnO∥<11¯0>ZnAl2O4 on the two different ZnAl2O4 variants. Taking the coincidence of the site lattices between the (0001) plane of ZnO and the (111) plane of ZnAl2O4 into account, a reduction in lattice misfit was achieved through a 30° rotation between the lattices of the ZnO and the ZnAl2O4. We used X-ray diffraction and transmission electron microscopy to obtain detailed microstructural views of the hydrothermally grown ZnO epitaxial films on the ZnAl2O4 buffer layers.

Preparation of Low-Resistivity Ga-Doped ZnO Epitaxial Films from Aqueous Solution Using Flow Reactor

An aqueous process based on a unique flow-reactor design was developed for the preparation of gallium-doped ZnO (ZnO:Ga) epitaxial films with a low electrical resistivity. In this process, a ZnO:Ga film was grown on a ZnO-seeded sapphire substrate heated at 80°C under a constant flow of a reaction solution. The Ga content of the resulting films was found to increase in relation to the concentration of GaCl3 used–0 to 9 mM GaCl3–resulting in epitaxial growth of ZnO containing 0–5% Ga, whereas a polycrystalline ZnO film was produced with 10 mM GaCl3. The electrical resistivity of the as-grown ZnO:Ga films varied from 0.2 to 2 Ω cm, but was reduced by two to three orders of magnitude after the films were annealed in air at 300°C. Thus, the lowest resistivity of 7 × 10–4 Ω cm was obtained with an annealed film containing 2.5% Ga, whose carrier concentration and mobility were 7 × 1020 cm–3 and 13 cm2 V–1 s–1, respectively. Furthermore, even though the non-doped ZnO film was rendered translucent by annealing, ZnO:Ga films with 1.8–4.0% Ga still exhibited transmittance as high as ∼80% in the visible spectrum.

Nanorod assisted lateral epitaxial overgrowth of ZnO films in water at 90 °C

A novel method was used to grow epitaxial ZnO films by employing a nanorod assisted lateral epitaxial overgrowth process at a low growth temperature of 90 °C in water utilizing a continuous circulation reactor. The relatively smooth films had an epitaxial relationship with the sapphire substrate, as confirmed by off-axis φ scans. Films grown for 72 hours had a high carrier concentration of 3.12 × 1018 cm−3 and mobility of 9.75 cm2 V−1 s−1 after thermal treatment at 300 °C. The threading dislocation density of the thickest film was 5 × 108 cm−2, 2 orders of magnitude lower than that of normal solution grown ZnO films.

Room-temperature ferromagnetism in epitaxial p-type K-doped ZnO films

Room-temperature ferromagnetism with p-type conductivity was observed in epitaxial K-doped ZnO films prepared by RF-magnetron sputtering. The coincident changes in the electrical, optical and magnetic properties indicate that the cation holes play important roles in mediating the ferromagnetism in K-doped ZnO films. The maximum saturation magnetization of 8emu/cm3 was obtained in the 8% K-doped film and the thermal annealing in air could stabilize the ferromagnetic signature. Finally, first-principle calculations reveal that the magnetic properties in K-doped ZnO films are attributed to the strong p–p interaction between the unpaired 2p electrons at O sites.

Low-voltage blue light emission from n-ZnO/p-GaN heterojunction formed by RF magnetron sputtering method

High quality n-ZnO/p-GaN heterojunction was fabricated by growing highly crystalline ZnO epitaxial films on commercial p-type GaN substrates via radio frequency (RF) magnetron sputtering. Low-voltage blue light emitting diode with a turn-on voltage of ∼2.5 V from the n-ZnO/p-GaN heterojunction was demonstrated. The diode gives a bright blue light emission located at ∼460 nm and a low threshold voltage of 2.7 V for emission. Based on the results of the photoluminescence (PL) and electroluminescence (EL) spectra, the origins of the EL emissions were studied in the light of energy band diagrams of ZnO–GaN heterojunction, and may attribute to the radiative recombination of the holes in p-GaN and the electrons injected from n-ZnO, which almost happened on the side of p-GaN layer. These results may have important implications for developing short wavelength optoelectronic devices.

Defects in nonpolar (134¯) ZnO epitaxial film grown on (114) LaAlO3 substrate

The defects in (134¯0)ZnO epitaxial film grown on (114)LaAlO3 (LAO) have been systematically investigated by using transmission electron microscopy. At the ZnO/LAO interface, the Burgers vectors of misfit dislocations are identified to be 1/3[1¯21¯0] and 1/2[0001]. Threading dislocations with the Burgers vectors of 1/3⟨112¯0⟩ and ⟨0001⟩ are distributed on the basal plane. In (134¯0)ZnO film, the predominant planar defects are basal stacking faults (BSFs) with 1/6⟨202¯3⟩ displacement vectors. The densities of dislocations and BSFs are about 3.8 × 1010 cm−2 and 3.1 × 105 cm−1, respectively.

Linear facing target sputtering of the epitaxial Ga-doped ZnO transparent contact layer on GaN-based light-emitting diodes

In this study, we reported on the plasma damage-free sputtering of epitaxial Ga-doped ZnO (GZO) films on the p-GaN layer for use as a transparent contact layer (TCL) for GaN-based light-emitting diodes (LEDs) using linear facing target sputtering (LFTS). Effective confinement of high-density plasma between faced GZO targets and the substrate position located outside of the plasma region led to the deposition of the epitaxial GZO TCL with a low sheet resistance of 25.7 Ω/s and a high transmittance of 84.6% on a p-GaN layer without severe plasma damage, which was found using the conventional dc sputtering process. The low turn-on voltage of the GaN-based LEDs with an LFTS-grown GZO TCL layer that was grown at a longer target-to-substrate distance (TSD) indicates that the plasma damage of the GaN-LED could be effectively reduced by adjusting the TSD during the LFTS process.