Nonpolar ZnO Films Research Articles

Schottky contact on hydrothermally grown a-plane ZnO for hydrogen sensing and UV detection

Pt Schottky contact on nonpolar a-plane ZnO film grown by a simple hydrothermal method on a-plane GaN was investigated. The Schottky barrier height was measured to be 0.64 eV at room temperature and atmospheric pressure. The effect of ZnO crystal polarity on Schottky barrier height was studied, and the barrier height of a-plane ZnO was compared to the values of c-plane in the literature. Also, hydrogen sensing and UV detection characteristics of a-plane ZnO Schottky diode were evaluated. After exposure of 4% hydrogen in nitrogen, the diode showed current increase due to Schottky barrier height reduction, and 2006% of maximum sensitivity to hydrogen gas was observed. For repeated UV illumination, the Schottky contact presented stable and recoverable current response.

Investigation on non-polar m-plane ZnO and Na-doped p-type ZnO films grown by plasma-assisted molecular beam epitaxy

Non-polar ZnO and Na-doped ZnO films were epitaxially grown on m-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The films exhibit dominant ( $$ 10\overline{1} 0 $$ ) (m-plane) orientation as identified by the X-ray diffraction pattern. The quality of the obtained m-plane ZnO film is evidenced by X-ray diffraction rocking curves full width at half maximum of 1125 arcsec for the ( $$ 10\overline{1} 0 $$ ) reflection and 1427 arcsec for the ( $$ 10\overline{1} 1 $$ ) reflection, respectively. Hall-effect measurements show that the m-plane Na-doped ZnO film exhibits p-type conductivity with a hole concentration of 2.50 × 1017 cm−3, while the m-plane ZnO film exhibits compensatory conductivity. Na atoms substituting for Zn atoms are believed to be the origin of p-type conductivity. The Na-related acceptor level is deduced to be ~120 meV by temperature-dependent photoluminescence, indicating the superiority of m-plane ZnO film in p-type doping compared with the polar ZnO film.

Heteroepitaxial growth of nonpolar Cu-doped ZnO thin film on MnS-buffered (100) Si substrate

The preparation of nonpolar ZnO and Cu-doped ZnO thin films on Si substrates was studied for the application to the fabrication of green-light-emitting diodes. The use of rocksalt MnS and wurtzite AlN as buffer layers is a key technology for achieving the heteroepitaxial growth of nonpolar ZnO thin film on a (100) Si substrate. X-ray diffraction and photoluminescence measurements revealed that deposition under a high oxygen partial pressure (∼1 Torr) can enhance the nonpolar crystallization of undoped ZnO, and can simultaneously suppress the formation of defects such as oxygen vacancies. These techniques can be also applied to the growth of Cu-doped ZnO. A room-temperature photoluminescence study revealed that nonpolar -oriented Cu-doped ZnO film exhibits enhanced green emission owing to the doped Cu ions.

Enhanced photoluminescence of nonpolar p-type ZnO film by surface plasmon resonance and electron transfer.

Nonpolar oriented Na-doped ZnO films were grown on m-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The films show repeatable p-type conductivity with a hole concentration of about 3.0×10(16) cm(-3) as identified by the Hall-effect measurements. 10-fold enhancement in the near-band-edge (NBE) emission of the nonpolar p-type ZnO by employing Pt nanoparticle surface plasmons has been observed. In addition, the deep level emission has been entirely suppressed. The underlying mechanism behind the enhancement of NBE emission and the quenching of defect emission is a combination of the electron transfer and the resonant coupling between NBE emission and Pt nanoparticle surface plasmons.

Epitaxial properties of ZnO thin films on LaAlO3 substrates by pulsed laser deposition

Wurtzite ZnO thin films with different epitaxial relationships have been grown on (001)-, (011)-, and (111)LaAlO3 (LAO) single-crystal substrates by pulsed laser deposition. Nonpolar (112¯0)ZnO films with two orthogonal domains were obtained on (001) LAO, in which the in-plane orientation relationship is demonstrated to be 〈0001〉ZnO//〈110〉LAO. For ZnO on (011)- and (111)LAO substrates, a single-domain epitaxy with c axial orientation is observed, in which the in-plane relationships were 〈11¯00〉ZnO//〈011¯〉LAO irrespective of the substrate orientations. Based on the in-plane orientation relationship, the lattice mismatch has been obtained for these three oriented ZnO/LAO heterointerfaces.

Growth of a-axis ZnO films on the defective substrate with different O/Zn ratios: A reactive force field based molecular dynamics study

The understanding of the growth process and formation mechanism of non-polar ZnO films in atomic-scale is crucial in adjusting and controlling the film deposition conditions. Using the advanced reactive force field based molecular dynamics method, we theoretically studied the effect of O/Zn ratios (8/10–10/8) on the quality of ZnO films. The comprehensive investigation of energy and temperature fluctuation profile, radial distribution function, the sputtering and injecting phenomenon, and layer coverage indicated that the film grown under stoichiometric conditions possesses the optimized quality. Furthermore, the auto-transformation ability of the substrate from defective to perfect stacking was presented and discussed by comparing to the perfect structure. The instant film growth configurations, atomic layer snapshots, and the interfacial morphology evolution were provided step-by-step to reveal the defect type and initial film nucleation and growth mechanism.

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.

Anisotropic microstructure of hydrothermally-grown non-polar a-plane ZnO on a-plane GaN film

Nonpolar a-plane ZnO (a-ZnO) films with good crystalline quality have been achieved by a hydrothermal growth method on nonpolar a-plane GaN (a-GaN) films without any nucleation steps. The X-ray diffraction θ–2θ scan of the a-ZnO film grown on a-GaN film showed that the film was uniformly a-plane 112¯0 oriented. The a-ZnO films exhibited anisotropic peak broadenings of the on-axis rocking curves with X-ray in-beam orientations. The full width at half maximum of the on-axis rocking curve of nonpolar a-ZnO film along the c-axis direction was measured to be 2092arcsec, which is almost two times larger than the one along the m-axis direction (1219arcsec). The Williamson–Hall plot analysis of the symmetric 112¯0 and 224¯0 Bragg reflections showed that the mosaic tilt values were 0.647° and 0.407° along the c-axis and m-axis directions, respectively. It clearly indicates that large mosaic tilt along the c-axis in a-ZnO film is mainly responsible for the anisotropic peak broadenings of the on-axis rocking curves with respect to X-ray in-beam orientations.

Comparative studies of nonpolar (10-10) ZnO films grown by using atomic layer deposition and radio-frequency magnetron sputtering

We comparatively investigated the crystal and the optical properties of nonpolar (10-10) ZnO films grown on m-plane sapphire substrates by using atomic layer deposition (ALD) and radio frequency (RF) magnetron sputtering. From high-resolution X-ray ω/2θ scans, the (100) peak of the ALD-grown ZnO film was clearly developed at ~ 15.9 ° while that of the RF sputter-grown ZnO was broadly observed at 15.6 ~ 15.9 °, indicating that a nonpolar (10-10) ZnO film would be preferentially grown on an m-plane sapphire substrate. The photoluminescence bandedge emission intensity of the ALD-grown (10-10) ZnO film was ten times higher than that of the RF sputtergrown ZnO film. In addition, the electroluminescence intensity of a semipolar (11–22) GaN-based light-emitting diode (LED) with an ALD-grown (10-10) ZnO film as a transparent conductive oxide material was much higher than that of a semipolar (11–22) GaN-based LED with RF sputter-grown (10-10) ZnO film.

Homobuffer thickness effect on the conduction type of non-polar ZnO thin films

Non-polar (101¯0) ZnO thin films were epitaxially grown on m-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The homobuffer thickness effect on the conduction type of undoped ZnO thin films is carefully investigated. With a relatively thicker buffer layer, weak p-type conductivity with a hole concentration of 1.6×1016cm−3, a Hall mobility of 0.33cm2V−1s−1, and a resistivity of 1.2×103Ωcm are achieved for the film. By careful analysis of results from low temperature photoluminescence and transmission electron microscopy measurements, a correlation of the 3.32-eV emission to the p-type conductivity in the undoped non-polar ZnO films is revealed and discussed. The results are important to help deepen understanding of the origin of p-type behavior in ZnO-based materials.

Effect of Na contents on fabrication of p-type non-polar m-plane ZnO films

We report on the growth and characterization of Na-doped non-polar ZnO thin films, which have been prepared on m-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The effects of Na contents on structural, morphological, electrical, and optical properties of Na-doped non-polar m-plane ZnO films are investigated. All the doped thin films have uniform m-plane orientation, which benefit from Na-doping. Na content plays a key role in determining the conduction of the ZnO films. An optimized result with a hole concentration of 5.3×1016cm−3, a Hall mobility of 0.22cm2V−1s−1, and a resistivity of 530Ωcm is achieved at beam equivalent pressure of the elemental Na source of 8.7×10−9Torr, and the films are electrically stable over several months.

Optical characteristics of nonpolar a-plane ZnO thin film on (010) LiGaO2 substrate

We investigate the optical properties of non-polar a-plane ZnO film grown on (010) LiGaO2 substrate by chemical vapor deposition. X-ray diffraction indicates the a-plane orientation and Raman spectroscopy reveals the phase purity. Four distinct features in the near band-edge range are identified as neutral-donor-bound-exciton (D°X), free-exciton (FX), free-to-bound, and donor-acceptor pair transitions. The thermal activation energy of D°X is 12 meV while an acceptor energy level of 124 meV is estimated. Temperature evolution of photoluminescence (PL) shows that the room-temperature luminescence is a mixture of free-to-bound and FX transitions. The polarization dependence of the near-band-edge transitions were investigated. The largest degree of polarization of 95.5% occurs in FX transition. It can be attributed to anisotropic in-plane strain and the nonpolar a-plane orientation.

Growth and characterization of nonpolar, heavily Mn-substituted ZnO films

Eight percent of Mn was successfully diluted into nonpolar ZnO films deposited by pulsed laser deposition on single crystal (100) SrTiO3 substrates. X-ray diffraction patterns and energy-dispersive X-ray spectroscopy confirmed high crystallinity of the films and excluded unintentional magnetic doping. A unique surface domain structure was observed by scanning electron microscope and atomic force microscope, which might play a vital role to strain release induced by lattice mismatch between nonpolar (11–20) ZnO film and (100) SrTiO3 substrate. In addition, the films showed strong ferromagnetism with a large coercivity HC ∼ 180 Oe at room temperature. The large magnetic moment is ascribed to carrier-mediated exchange interaction between the Mn ions, where the majority of the carriers are oxygen vacancies.

Epitaxial growth of nonpolar and polar ZnO on γ-LiAlO2 (100) substrate by plasma-assisted molecular beam epitaxy

Low lattice mismatched γ-LiAlO2 (100) substrates were employed for the epitaxial growth of ZnO by plasma-assisted molecular beam epitaxy. The crystallographic orientations of the samples were determined by the X-ray diffraction (XRD) patterns. The surface morphology and roughness of ZnO films were investigated by scanning electronic microscopy (SEM) and atomic force microscopy (AFM). Results confirmed that both nonpolar (101¯0)- and polar (0001)-oriented ZnO epilayers can be grown, demonstrating the dual orientation-selection characteristic of the γ-LiAlO2 substrate. The growth temperature and grow rate are two major factor which affects the orientation competition. A two-step growth method was therefore designed to improve the quality of the polar ZnO film grown at high temperature. Room temperature photoluminescence spectra of both polar and nonpolar ZnO films showed a near band edge emission peak at around 377nm and a negligible green band emission.

Nonpolar light emitting diodes of m-plane ZnO on c-plane GaN with the Al2O3 interlayer

Nonpolar m-plane ZnO films are deposited on GaN (0002) with a 10 nm Al2O3 interlayer by atomic layer deposition. The growth direction of the ZnO films directly on GaN (0002) is [707¯4] (perpendicular to (101¯1) plane), whereas with the Al2O3 interlayer it changes into [101¯0]. With the Al2O3 interlayer, the m-plane ZnO films are presented and the leakage current of the heterojunctions dramatically reduces. The electroluminescence spectra of the n-ZnO/Al2O3/p-GaN heterojunctions are dominated by a blue emission under forward biases, whereas it is violet under reverse biases.

Growth and characterization of polar and nonpolar ZnO film grown on sapphire substrates by using atomic layer deposition

We investigated the electrical and the optical properties of polar and nonpolar ZnO films grown on sapphire substrates with different crystallographic planes. High resolution X-ray results revealed that polar c-plane (0001), nonpolar m-plane (10-10) and a-plane (11-20) ZnO thin films were grown on c-plane, m- and r-sapphire substrates by atomic layer deposition, respectively. Compared with the c-plane ZnO film, nonpolar m-plane and a-plane ZnO films showed smaller surface roughness and anisotropic surface structures. Regardless of ZnO crystal planes, room temperature photoluminescence spectra represented two emissions which consisted of the near bandedge (~380nm) and the deep level emission (~500nm). The a-plane ZnO films represented better optical and electrical properties than c-plane ZnO, while m-plane ZnO films exhibited poorer optical and electrical properties than c-plane ZnO.

Growth of large-area non-polar ZnO film without constraint to substrate using oblique-angle sputtering deposition

Abstract Non-polar ZnO thin film with high crystal quality is grown on a glass substrate using one-step oblique-angle deposition. Cross-sectional transmission electron microscopy images and selected area electron diffraction patterns reveal that the film is constructed as a stack of grains from the bottom to the top with the [0 0 0 2] axis gradually titled from a vertical to a nearly horizontal orientation with respect to the substrate. The (0 0 0 2) pole figure exhibits a continuous angle distribution in the ψ direction with the most concentration at approximately ψ = 18° and ϕ = 0°. Strong anisotropic effects in local electronic structure were observed for the highly oriented ZnO surface rod by angle-dependent X-ray absorption near-edge structure measurements. The structure also exhibits polarization that depends on Raman scattering.

On the interplay of point defects and Cd in non-polar ZnCdO films

Non-polar ZnCdO films, grown over m- and r-sapphire with a Cd concentration ranging between 0.8% and 5%, have been studied by means of slow positron annihilation spectroscopy (PAS) combined with chemical depth profiling by secondary ion mass spectroscopy and Rutherford back-scattering. Vacancy clusters and Zn vacancies with concentrations up to 1017 cm−3 and 1018 cm−3, respectively, have been measured inside the films. Secondary ion mass spectroscopy results show that most Cd stays inside the ZnCdO film but the diffused atoms can penetrate up to 1.3 μm inside the ZnO buffer. PAS results give an insight to the structure of the meta-stable ZnCdO above the thermodynamical solubility limit of 2%. A correlation between the concentration of vacancy clusters and Cd has been measured. The concentration of Zn vacancies is one order of magnitude larger than in as-grown non-polar ZnO films and the vacancy cluster are, at least partly, created by the aggregation of smaller Zn vacancy related defects. The Zn vacancy related defects and the vacancy clusters accumulate around the Cd atoms as a way to release the strain induced by the substitutional CdZn in the ZnO crystal.

Control of epitaxial relationships of ZnO/SrTiO3 heterointerfaces by etching the substrate surface

Wurtzite ZnO thin films with different epitaxial relationships are obtained on as-received and etched (001), (011), and (111) SrTiO3 (STO) by metal-organic chemical vapor deposition (MOCVD). ZnO films exhibit nonpolar (1120) orientation with in-plane orientation relationship of <0001>ZnO//<110>STO on as-received (001) STO, and polar c-axis growth with <1100>ZnO//<110>STO on etched (001) STO substrates. ZnO films change from polar (0001) to semipolar (1012) oriented on as-received and etched (011) STO. On as-received and etched (111) STO, ZnO films show the same growing direction of polar (0001), but different in-plane orientations with 30° rotation. The change of epitaxial relationship of ZnO films on as-received and etched (001), (011), and (111) STO substrates is accompanied with the increase of lattice mismatch, decrease of bond density, and increase of substrate surface roughness. In other words, the epitaxial relationships of ZnO/STO heterointerfaces can be controlled by etching the substrates. These results show that polar, nonpolar, and semipolar ZnO films for different applications can be grown epitaxially on STO substrates by MOCVD.

Realization of p-type non-polar a-plane ZnO films via doping of Na acceptor

p-Type non-polar ZnO films have been realized via doping of Na acceptor on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The film is (112¯0) oriented (a-plane) as identified by the X-ray diffraction pattern. Structural properties are anisotropic and surfaces of films show stripes running along the ZnO [0001] direction. Secondary ion mass spectroscopy confirmed that Na has been doped into ZnO films. The chemical states of Na were analyzed by x-ray photoelectron spectroscopy. Deep level emission is dominant in room-temperature photoluminescence, which is mainly due to oxygen vacancy. With an effective incorporation of Na, Na-doped non-polar ZnO film exhibits p-type conductivity with a hole concentration of 1.81×1015cm−3, a Hall mobility of 0.402cm2V−1s−1, and a resistivity of 8575Ωcm, respectively. The origin of weak p-type behavior is most likely a certain amount of oxygen vacancies existed in the film, which partly compensates Na acceptor.