Nonpolar ZnO Films Research Articles

Preparation and growth mechanism of nonpolar, semipolar and polar ZnO films: Correlation of preferred orientation, defects and optical properties

In the study, ZnO films with different preferred orientations were controllably prepared by using NH3·H2O to modulate the pH0 of chemical bath deposition. The samples were all characterised by field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray diffraction (XRD), ultraviolet-visible spectrophotometry (UV–VIS), X-ray photoemission spectroscopy (XPS) and photoluminescence (PL). The results show that ZnO films with polar, semipolar and nonpolar preferred orientations can be synthesised in a controlled manner by varying the pH0. Meanwhile, the influence of NH3·H2O concentration on the growth process of ZnO films is investigated in detail, and the relationship between the preferred orientation and optical properties is preliminarily discussed. At pH0= 7.32, the films exhibited semipolar (101) preferred orientation, the highest optical transmittance (89.15 %), and the highest concentration of Zni in the films. This study provides a method for the controlled synthesis of ZnO films with polar, semipolar and nonpolar preferred orientations and offers new possibilities for ZnO-based transparent semiconductor devices.

Cu, Ni and Ag ions assisted preparation of nonpolar preferential oriented ZnO films with controlled morphology and optical properties

ZnO nanodevices have been widely used in practice, but one-dimensional ZnO grown in a polar preferred orientation is still partially localized. In this work, copper nitrate, nickel nitrate and silver nitrate were assisted with zinc acetate and hexamethylenetetetramine for the synthesis of ZnO films by conventional chemical bath deposition method. All samples were characterised by X-ray diffraction, field emission scanning electron microscopy, atomic force microscopy, photoluminescence, and UV–visible spectrophotometer. The results show that non-polar (110), (100) and coexisting preferred orientation ZnO films were controllably prepared by controlling the type and concentration of nitrate. On this basis, the growth process and mechanism of ZnO films with different nonpolar preferred orientations were characterized. It was also found that the ZnO films with 5 % Cu(NO3)2·3 H2O had the minimum surface roughness. The PL spectra confirmed the type and number of defects were also related to the preferred orientations. In addition, the non-polar (110) preferred orientation ZnO films had the highest optical transmittance of 82.26 %. This work enriches the method of preparing ZnO films with various nonpolar preferred orientations and provides new ideas for the design of ZnO-based transparent nanodevices.

Domain boundaries and growth manner of a-plane ZnO film on symmetric (0 0 1)LaAlO3 substrate

Non-polar a-plane ZnO film grown on (001)LaAlO3 (LAO) substrate has been systematically characterized by employing transmission electron microscope. On (001)LAO substrate, a-plane ZnO film comprises two main orientation types of domains perpendicular to each other. In one orientation type of a-plane ZnO domains, stacking mismatch boundary (SMB) and inversion domain boundary (IDB) form in the coalescence of two domain islands parallel to each other. SMB is composed of m- and r-planes. IDB extends along m-plane. The in-plane growth of a-plane ZnO domains extends along the direction with ∼45°-rotation to [0001]ZnO. The boundaries of two perpendicular a-plane ZnO domains consist of 45°-boundaries and r-plane twin boundaries. The profile of a-plane ZnO domain appears as L-shape.

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Nonpolar ZnO films were grown on r-plane sapphire substrates through the reaction between dimethylzinc (DMZn) and high-temperature H2O, the latter produced by the Pt-catalyzed reaction between H2 and O2. The resulting ZnO films were characterized using atomic force microscopy, X-ray diffraction, and photoluminescence (PL) measurements. They were found to have an anisotropic surface morphology with stripe arrays, and exhibited a diffraction peak associated with ZnO (11-20) index planes. The PL spectra indicated anisotropy in polarization between the directions parallel and perpendicular to the c-axis. The band-edge luminescence at 3.3 eV exhibited a maximum when the electric field vector E was perpendicular to the c-axis (parallel to the [1-100] direction) and another minimum when E was parallel to the c-axis. The angular dependence of the linear polarization of the band-edge luminescence was large for ZnO films grown at low temperatures. The large degree of polarization observed for low-temperature growth is thought to be due to the film geometry.

Epitaxial Growth of Nonpolar ZnO Films on Sapphire Substrates with a Terrace-Step Nanorelief

The processes of fabrication of nonpolar (11 $$\bar {2}$$ 0) ZnO films with a thickness of up to 7.25 µm on (1 $$\bar {1}$$ 02) sapphire (Al2O3) substrates with a terrace-step surface nanostructure by magnetron sputtering at a growth rate of up to 4.5 nm/s has been studied. The effect of steps in the initial growth stages has been found, which leads to the formation of isolated ZnO nanostructures decorating the steps. In all next growth stages, a continuous (11 $$\bar {2}$$ 0) epitaxial ZnO film with the longitudinal [0001] axis of crystallites, oriented parallel to the step edge on the sapphire surface has been observed. It is established that the interplanar spacing in the ZnO films in the [11 $$\bar {2}$$ 0] direction, determined from the X-ray diffraction data, depends on the thickness and deposition rate and approaches the value of X-ray diffraction references for ZnO with an increase in thickness to 7.25 µm and a decrease in the growth rate to 2.1–2.3 nm/s.

A facile and reproducible synthesis of non-polar ZnO homojunction with enlarged rectification rate and colorful light emission

A novel fabrication of non-polar Sb-doped ZnO homojunction was realized via a facile and reproducible low-temperature aqueous solution deposition in the presence of Sb dopant dissolved in ethylene glycol (EG). The undoped non-polar ZnO films were introduced as the non-polar substrates for the non-polar growth of Sb-doped ZnO film as well as the SZO/ZnO homojunction. The parameters including the reaction time and dopant concentration for the synthesis of non-polar SZO/ZnO homojunction were identified. The existence and distribution of Sb dopant in SZO layer were confirmed by X-ray photoelectron spectroscopy (XPS) and elemental mapping analysis. The I–V measurement result indicated the p-type conduction of SZO layer and the synthesized p-SZO/n-ZnO homojunction with non-polar preferred orientations possessed both ultra-low turn-on voltage (1.5 V) and large rectification rate (>105). The excitation of orange-red light emissions from deep-level energy band gap demonstrated the (SbZn-xVZn) complex defect pattern. This novel non-polar SZO/ZnO homojunction could be an excellent candidate for the applications of one-dimensional ZnO nanomaterials in the electronic/photoelectronic field.

Post-annealing induced oxygen vacancy mediated non-polar ZnO films with excellent opto-electronic performance

The considerably enhanced opto-electronic properties of ZnO films with non-polar orientations have been realized by introducing oxygen vacancies. Excitation of oxygen vacancies has been achieved by oxygen-deficient post-annealing treatment. The existence of oxygen vacancies is directly confirmed by cross-sectional XPS and positron annihilation Doppler broadening measurements. An enhanced UV-emission, more simplex green-light emission and sharply decreased electrical resistivity of post-annealed non-polar oriented ZnO films have been acquired in oxygen-deficient atmosphere. I-V measurement indicates that non-polar oriented ZnO/p-Si heterojunction with optimized post-annealing treatment owns an excellent forward electrical transportation and extremely low threshold voltage, have great competitiveness for the commonly used one-dimensional ZnO nanomaterials in photo-electronic field.

Effect of LaNiO<sub>3</sub> Buffer Layer on the Electrical and Optical Properties of Nonpolar ZnO Film Deposited on (100) Si Substrate

Nonpolar ZnO films are deposited on (100) Si substrate using LaNiO3 conducting buffer layer by radio frequency sputtering. X-ray diffraction results show that ZnO films are (110) and (002) orientation with and without LaNiO3 buffer layer. The current behavior of ZnO/LaNiO3 heterojunction exhibits ohmic conduction which is different from the diode-like rectification current behavior of ZnO film using insulated buffer layers. The photoluminescence properties indicate that the (110)-oriented nonpolar ZnO film has better band-edge emission than that of (002)-oriented polar ZnO film. It is suggested that LaNiO3 buffer layer can be used to deposit silicon-based ZnO film with well ohmic contact electrode in optoelectronic devices.

Microstructure of oxygen-deficient annealing-induced non-polar ZnO films with extremely low electrical resistivity

The influence of oxygen vacancy mediation on the microstructure of post-annealed non-polar ZnO films has been investigated.

Growth modulation of simultaneous epitaxy of ZnO obliquely aligned nanowire arrays and film on r-plane sapphire substrate

Simultaneous epitaxial growth of film and nanowire array on a substrate is of both scientific significance and practical importance for nanoscale optoelectronics. Nevertheless, in situ building conducting connection between individually isolated nanowires grown on insulating substrates is still challenging. Herein, we demonstrate a novel and facile strategy for the simultaneous epitaxial growth of nonpolar a-plane ZnO film and obliquely aligned nanowire array on Au-coated r-plane sapphire substrate. The morphology, structure, components, and optical properties of the as-synthesized ZnO nanostructures were investigated using field-emission scanning electron microscopy, X-ray diffraction, field-emission transmission electron microscopy, energy-dispersive spectroscopy, X-ray photo-electron spectroscopy, and photoluminescence spectroscopy. A cooperative growth mechanism is proposed: Au-catalyzed vapor transport initiates the co-occurrence of nonpolar a-plane and polar c-plane ZnO nuclei, and subsequently, the non-upward directed Au catalyst helps the nonpolar a-plane ZnO nuclei develop into a ZnO conductive film at the bottom and zinc self-catalyzed vapor–liquid–solid growth helps the polar c-plane ZnO nuclei develop simultaneously into obliquely aligned nanowire arrays. The proposed strategy realized in situ synthesis of nanowires with conductive connection and it can benefit the application of ZnO nanowires in optoelectronics.

Photoluminescence enhancement in non-polar ZnO films through metallodielectric mediated Al surface plasmons.

Non-polar ZnO thin films are grown on m-plane sapphire substrates by plasma-assisted molecular beam epitaxy. Emission enhancement from non-polar ZnO thin films coated with Al/AlOx has been studied by photoluminescence spectroscopy. AlOx has been used to mediate the surface plasmon (SP) energy of Al nanoparticles. Taking advantage of the resonant coupling between the UV emission of non-polar ZnO film and Al nanoparticle SPs, an 84-fold enhancement of the UV emission and an 8.3-fold enhancement of internal quantum efficiency (ηint) have been achieved under the optimized sputtering time and energy of SPs.

Preparation of a Non-Polar ZnO Film on a Single-Crystal NdGaO3 Substrate by the RF Sputtering Method

Preparation of non-polar ZnO ( $$ 11\overline{2} 0 $$ ) films on single-crystal NdGaO3 (NGO) (001) substrates was successfully achieved by the radio frequency (RF) sputtering method. Orientation, deposition rate, and surface roughness of ZnO films strongly depend on the working pressure. Characteristics of ZnO films deposited on single-crystal NGO (001) substrates were compared with those of ZnO films deposited on single-crystal sapphire ( $$ 01\overline{1} 2 $$ ) substrates. An x-ray diffraction peak of the ZnO ( $$ 11\overline{2} 0 $$ ) plane was observed on ZnO films deposited on single-crystal NGO (001) substrates under working pressure of less than 0.5 Pa. On the other hand, uniaxially oriented ZnO ( $$ 11\overline{2} 0 $$ ) films on single-crystal sapphire ( $$ 01\overline{1} 2 $$ ) substrates were observed under working pressure of 0.1 Pa. The mechanism by which the diffraction angle of the ZnO ( $$ 11\overline{2} 0 $$ ) plane on single-crystal NGO (001) substrates was shifted is discussed on the basis of anisotropic stress of lattice mismatch. The deposition rate of ZnO films decreased with an increase in working pressure, and the deposition rate on single-crystal NGO (001) substrates was larger than that on single-crystal sapphire ( $$ 01\overline{1} 2 $$ ) substrates. Root mean square (RMS) roughness of ZnO films increased with an increase in working pressure, and RMS roughness of ZnO films on single-crystal NGO (001) substrates was smaller than that of ZnO films on single-crystal sapphire ( $$ 01\overline{1} 2 $$ ) substrates even though the film thickness on single-crystal NGO (001) substrates was greater than that on sapphire substrates. It is thought that a single-crystal NGO (001) substrate is useful for deposition of non-polar ZnO ( $$ 11\overline{2} 0 $$ ) films.

Epitaxial growth of non-polar ZnO films on MgO substrate

The pulsed laser deposition of pure and doped (Nd or Fe) ZnO films on (001) oriented MgO substrates was investigated. Polar (alternate Zn and O planes along the main axis growth), or non-polar ZnO films were obtained according to the growth conditions, and epitaxial relationships were observed in both cases. For the non-polar films, different in-plane orientations were observed, and the domain matching epitaxy was used in order to describe the in-plane relationships of the film with respect to the MgO substrate. These relationships correspond to different angles between the in-plane [001]ZnO and [100]MgO directions and are different from those previously proposed for the epitaxy of non-polar ZnO on MgO. In addition to the lattice mismatch between film and substrate, an important parameter governing the in-plane orientation is the minimum size of the epitaxial domain which mainly determines the in-plane alignment. Indeed, for doped-ZnO films, the coherence length, i.e. the spatial extension of the crystallites is limited by the presence of the dopants. This leads to different epitaxial relationships with respect to undoped films, and which are related to the epitaxial domain size.

Polarization properties of nonpolar ZnO films grown on R-sapphire substrates using high-temperature H2O generated by a catalytic reaction

Nonpolar ZnO films were grown on r-plane sapphire substrates through the reaction between dimethylzinc (DMZn) and high-temperature H2O, the latter produced by the Pt-catalyzed reaction between H2 and O2. The resulting ZnO films were characterized using atomic force microscopy, X-ray diffraction, and photoluminescence (PL) measurements. They were found to have an anisotropic surface morphology with stripe arrays, and exhibited a diffraction peak associated with ZnO (11-20) index planes. The PL spectra indicated anisotropy in polarization between the directions parallel and perpendicular to the c-axis. The band-edge luminescence at 3.3eV exhibited a maximum when the electric field vector E was perpendicular to the c-axis (parallel to the [1-100] direction) and another minimum when E was parallel to the c-axis. The angular dependence of the linear polarization of the band-edge luminescence was large for ZnO films grown at low temperatures. The large degree of polarization observed for low-temperature growth is thought to be due to the film geometry.

Polar and nonpolar self-assembled Co-doped ZnO thin films: Structural and magnetic study

Novel nanostructures of self-assembled Co-doped ZnO films were successfully synthesized by hydrothermal synthesis process. Our investigation indicates that the growth reaction rate is the key factor that controls the morphology evolution and crystallographic orientation of self-assembled Co-doped ZnO films via changing the reaction solution concentration. The preferential growth direction of Co-doped ZnO films alters from polar [0001] to non-polar [10‐10] lattice direction by increasing the concentration, which facilitates the transformation from 1D nanorods, assembled in straw-like structure, to 2D nanosheets with dendrite-like structure. Interestingly, room-temperature ferromagnetism is observed in both of polar (0001) c-plane and non-polar (10−10) m-plane Co-doped ZnO films. As the most promising nanostructures, the assembled nanostructured Co-doped ZnO films may find future applications in spintronics devices.

Photoconduction properties and anomalous power-dependent quantum efficiency in non-polar ZnO epitaxial films grown by chemical vapor deposition

Photoconduction (PC) properties in the ZnO films with the (110) nonpolar surface (a-plane) epitaxially grown by chemical vapor deposition on the LiGaO2 (010) substrates with low lattice mismatches (4.0% along the c-axis and 3.8% along the m-axis) have been studied. The structural and optical qualities of the epitaxial films have been characterized using theta-two theta and phi scans, X-ray diffraction, rocking curve, and photoluminescence measurements. The nonpolar ZnO film exhibits a near visible-blind ultraviolet photoresponse. The optimal photocurrent to dark current ratio (i.e., sensitivity) can reach 13360%. The responsivity of the a-plane ZnO photoconductor-type detector can also reach 17 AW−1, which is two to four orders of magnitude higher than those of the m-plane, a-plane, and r-plane photodiodes based on ZnO/ZnMgO quantum wells. The normalized gain at 2.9 cm2V−1 of the nonpolar film is also comparable with the optimal recorded value of the ZnO nanowires. In addition, the PC mechanism has also been investigated by the power-dependent and time-resolved photoconductivity measurements. The power-sensitive responsivity can be attributed to the effect of light intensity on carrier lifetime and quantum efficiency. The photovoltaic effect of the surface depletion region is inferred to be the reason resulting in the anomalous power-dependent quantum efficiency.

Effect of temperature on the growth ofa-axis ZnO films: a reactive force field-based molecular dynamics study

Abstract

Non-polar a-plane ZnO films grown on r-Al2O3 substrates using GaN buffer layers

In this work, GaN buffer layer has been used to grow non-polar a-plane ZnO films by laser-assisted and plasma-assisted molecular beam epitaxy. The thickness of GaN buffer layer ranges from ∼3 to 12nm. The GaN buffer thickness effect on the properties of a-plane ZnO thin films is carefully investigated. The results show that the surface morphology, crystal quality and optical properties of a-plane ZnO films are strongly correlated with the thickness of GaN buffer layer. It was found that with 6nm GaN buffer layer, a-plane ZnO films display the best crystal quality with X-ray diffraction rocking curve full-width at half-maximum of only 161 arcsec for the (101) reflection.

Single domain non-polar (12[formula omitted]0)ZnO epitaxy on (118)LaAlO3 by pulsed laser deposition

An unconventional non-polar (123¯0)ZnO epitaxial film is grown on (118)LaAlO3(LAO) substrate by pulsed laser deposition. X-ray diffraction pole figures and transmission electron microscopy show that the crystal orientation relationships between non-polar (123̄0)ZnO and (118)LAO substrate are (0001)ZnO//(1̄10)LAO and (112̄0)ZnO//(001)LAO. The Raman spectrum with a significant E2 peaked at 439cm−1 clearly indicates the formation of non-polar ZnO film. Photoluminescence measurement of the non-polar ZnO film shows a strong near-band-edge emission located at 3.3eV with negligible green-yellow emission.

Growth and characterization of nonpolar (10-10) ZnO transparent conductive oxide on semipolar (11–22) GaN-based light-emitting diodes

We have grown thin films of nonpolar m-plane (10-10) ZnO on a semipolar (11–22) GaN template by atomic layer deposition (ALD) at low growth temperatures (<200 °C). The surface morphology of the ZnO film is found to be an arrowhead-like structure, which is a typical surface structure of the semipolar (11–22) GaN films. On increasing the growth temperature of the ZnO films, the concentration and mobility of the charge carriers in the ZnO film are increased. However, the optical transmittance decreases with an increase in the growth temperature. Based on these results, we have fabricated semipolar (11–22) GaN-based light-emitting diodes (LEDs) with nonpolar m-plane ZnO film as a transparent conductive oxide (TCO) to improve the light extraction efficiency. In spite of a decrease in the optical transmittance, the operation voltage of semipolar (11–22) GaN-based LEDs is found to decrease with an increase in the growth temperature, which might be due to the improvements in the electrical properties and current spreading effect, resulting in an increase in the optical output power.