Epitaxial growth of ZnO layers using nanorods with high crystalline quality

Epitaxial ZnO films were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition using a ZnO multi-dimensional structure having the sequence of ZnO film/ZnO nanorods/sapphire. The vertically well-aligned one-dimensional ZnO nanorods were grown epitaxially on the sapphire substrate with in-plane alignment under suitable growth conditions and then used as seeds for the subsequent epitaxial ZnO layer. For the transition of the ZnO structures from the nanorods to the film, the growth temperature and working pressure were controlled, while keeping the other conditions fixed. The growth of the ZnO films on the well-aligned ZnO nanorods results in homoepitaxial growth with the identical orientation relationship along the in-plane direction as well as the same c-axis orientation. The microstructural analysis of the multi-dimensional structure and analysis of the microstructural evolution from the one-dimensional nanorods to the two-dimensional film were conducted using transmission electron microscopy.     

Growth and characterization of ZnO films on (11-20) sapphire substrates by atomic layer deposition using DEZn and N2O

Zinc oxide (ZnO) films were grown on (11-20) sapphire substrates at 600 °C by atomic layer deposition (ALD) using diethylzinc (DEZn) and nitrous oxide (N₂O). A ZnO buffer layer was deposited at low temperature (LT) prior to the growth of a bulk ZnO film for a typical growth run. In some cases, buffer-layer annealing or post-annealing treatments were employed to optimize ZnO growth. Based on the experimental results of X-ray diffractometry (XRD) and transmission electron microscopy (TEM), all the as-grown ZnO films were found to show c-axis preferred orientation with co-existence of <1-100>_ZnO∥<1-100>_sapphire and <11-20>_ZnO∥<1-100>_sapphire relationships in the (0001)ZnO/(11-20)sapphire hetero-interface. Typical room temperature (RT) photoluminescence (PL) spectrum of the as-grown ZnO film shows only near band edge (NBE) emissions without defect luminescence. ZnO films with improved quality were achieved by post-annealing or buffer-layer annealing treatments. In particular, buffer-layer annealing was found to improve the crystalline and optical properties of a ZnO film substantially.     

Epitaxial growth of WOx nanorod array on W(001)

Nanorods of substoichiometric tungsten oxide (WO_x) were grown on W(001) substrates. Two methods for the growth of nanorods were used: oxidation of the substrate under appropriate conditions and the deposition of tungsten oxide from a tungsten foil heated in the presence of oxygen. The grown nanorods were observed using a scanning electron microscope and an atomic force microscope. The diameters of the nanorods were 5–20 nm. The nanorods were slightly inclined from the directions parallel or normal to the surface. The inclination of nanorods was explained in terms of the epitaxial relationship between WO₃ crystals and the W(001) substrate. The WO₃ crystals formed at the initial stage of growth act as the nuclei of WO_x nanorods. We observed selective enhancement of the growth in a certain epitaxial direction depending on the method of growth, and an array of WO_x nanorods was produced on the W(001) substrate.     

Epitaxial growth of WOx nanorod array on W(001)

Nanorods of substoichiometric tungsten oxide (WO_x) were grown on W(001) substrates. Two methods for the growth of nanorods were used: oxidation of the substrate under appropriate conditions and the deposition of tungsten oxide from a tungsten foil heated in the presence of oxygen. The grown nanorods were observed using a scanning electron microscope and an atomic force microscope. The diameters of the nanorods were 5–20 nm. The nanorods were slightly inclined from the directions parallel or normal to the surface. The inclination of nanorods was explained in terms of the epitaxial relationship between WO₃ crystals and the W(001) substrate. The WO₃ crystals formed at the initial stage of growth act as the nuclei of WO_x nanorods. We observed selective enhancement of the growth in a certain epitaxial direction depending on the method of growth, and an array of WO_x nanorods was produced on the W(001) substrate.     

Substrate engineering of LaAlO3 for non-polar ZnO growth

We demonstrate that growth of non-polar ZnO in a-plane and m-plane can be achieved through substrate engineering of LaAlO₃ with (001) and (112) surface. X-ray diffraction, reflection high energy electron diffraction and cross-sectional transmission electron microscopy with selected area diffraction reveal that a-plane ZnO on LaAlO₃ (001) consists of two types of domains perpendicular to each other with in-plane orientation relationships of [0001]_ZnO // [11?0]_LAO and [11?00]_ZnO // [11?0]_LAO. Single domain epitaxy of m-plane ZnO on LaAlO₃ (112) can be obtained with in-plane orientation relationships of (101?0)_ZnO//(112)_LAO, [0001]_ZnO // [1?10]_LAO and [12?10]_ZnO // [1?1?1]_LAO.     

ZnO nanorod arrays grown on glass substrates below glass transition temperature by metalorganic chemical vapor deposition

High density, well-aligned ZnO nanorods with uniform distributions in their diameters and lengths are successfully prepared on amorphous substrates by metalorganic chemical vapor deposition. The X-ray diffraction measurements indicate that the ZnO nanorods are of wurtzite crystal structure, and are grown preferentially on glass substrates along the [0001]_ZnO direction. The degree of the preferred orientation of the ZnO nanorods is enhanced by increasing the growth temperature, as confirmed by the X-ray diffraction and selected area electron diffraction patterns. Photoluminescence investigations revealed the enhancement of the band edge emission with increasing growth temperature, suggesting the improvement in the optical quality of the ZnO nanorods with increasing temperature.     

Effects of pre-annealing conditions on the characteristics of ZnO nanorods and ZnO/p-Si heterojunction diodes grown through hydrothermal method

Pre-annealing of seed layers before the growth of ZnO nanorods (NRs), at various temperatures (non-annealing ~ 800 °C) and in various atmospheres (vacuum, N₂, or O₂), was systematically studied to investigate the effects of pre-annealing on the material properties of ZnO NRs as well as the rectifying behaviour of ZnO NRs/p-Si heterojunction diodes (HJDs). A seed layer was initially prepared on the Si substrate through hydrothermal (HT) method and subsequently pre-annealed; finally, the ZnO NRs were grown through the same HT method. We found that without the annealed seed layer, the ZnO NRs cannot be grown on the Si template and increase in the pre-annealing temperature led to better crystallization and fewer defect-centres in ZnO NRs. However, at a high pre-annealing temperature, the characteristics of ZnO NRs degraded due to the evaporation of oxygen atoms, resulting in more oxygen-vacancy-related defects. The smallest diameter and shortest length of ZnO NRs were observed on the samples pre-annealed at 450 °C. The short length of ZnO NRs implies a slow growth rate, because of which the NRs have sufficient time to align normal to the surface of the substrate. When the seed layer is pre-annealed in an O₂ atmosphere, the oxygen atoms fill the oxygen-vacancy-related defects, which lead to a higher nucleation density and improved characteristics of ZnO NRs. This leads to an extremely high rectification ratio of 1.8 × 10⁵ in ZnO NR/p-Si HJDs. The related mechanisms were explored in this study.     

Controlled growth of quasibicrystal zinc oxide structures

The first results on the controlled growth of quasibicrystal structures containing interblock boundaries in epitaxial zinc oxide layers on sapphire (α-Al₂O₃) are reported. The structures with boundaries oriented in a preset direction can be used as a base for submicron microelectronic devices. Using the method of buffer layers, it is possible to obtain highly oriented layers of (11[`2]0)ZnO(11\bar 20)ZnO and (0001)ZnO with clear boundaries between blocks on the same (10[`1]2)Al₂ O₃(10\bar 12)Al_2 O_3 substrate surface. Data on the features of structure and morphology of these layers are presented.     

Low-temperature synthesis of ZnO nanorods using a seed layer of zinc acetate/sodium dodecyle sulfate nanocomposite

Aligned ZnO nanorods were synthesized by a simple hydrothermal method without calcination. A seed layer of zinc acetate (ZnAc₂)/sodium dodecyle sulfate (SDS) nanocomposite was used for nucleation of ZnO nanorods. First, a ZnAc₂/SDS composite was deposited on a Si substrate by spin-coating. And then, ZnO nanorods were grown under hydrothermal conditions at 90 °C. ZnO crystals were grown in the direction of c-axis perpendicular to the surface of the Si substrate. However, nucleation did not occur on the substrate of a ZnAc₂ seed layer without SDS, indicating that the presence of the ZnAc₂/SDS seed enhanced the nucleation of ZnO crystals. These results show that high dispersion of ZnAc₂ in the nanocomposite effectively assists a nucleation of ZnO crystals.     

Dendritic and epitaxial growths of ZnO particle-rod and rod-rod nanostructures with quasi-one-dimensional and two-dimensional configurations

Quasi-one-dimensional and two-dimensional ZnO nanostructures have been fabricated through thermal evaporation approach. The microstructures of the ZnO nanostructures have been studied using scanning electron microscopy and high-resolution electron microscopy. Quasi-one-dimensional ZnO nanostructures are formed by dendritic growths of ZnO nanoparticles from the stem nanorods surfaces, forming particle-rod nanostructures. While epitaxial growths of branch nanorods from the stem nanorods configure two-dimensional ZnO nanostructures. The epitaxial growth orientation relationship can be described as [2? 110]_R1 || [2? 110]_R2 and (0001) _R1 || (011?0)_R2. The growth mechanism of the quasi-one-dimensional and two-dimensional ZnO nanostructures has been discussed.     

Chemistry and Kinetics of ZnO Growth from Alkaline Hydrothermal Solutions

Special features (polar growth and nonstoichiometry) of ZnO single crystals grown on seed plates of various crystallographic orientations in the ZnO–KOH–H₂O and ZnO–KOH–LiOH–H₂O hydrothermal systems are analyzed. The growth proceeds via the interaction of ZnO^2- ₂ anions with crystal surfaces, and its rate depends on the atomic structure and electric charge of the surface. The mechanism underlying the influence of Li⁺ ions on polar ZnO growth is considered. Partial replacement of Zn²⁺ by Li⁺ decreases the positive charge on the (0001) face and hinders the attachment of ZnO^2- ₂ anions. The incorporation of Li ions into the (0001¯) face decreases its negative charge and accelerates growth in the [0001¯] direction.     

A structural investigation of highly ordered catalyst- and mask-free GaN nanorods

GaN nanorods were grown on r-plane sapphire substrates by a two-step approach. Nucleation sites for the nanorods were provided by the formation of AlN islands during nitridation in a metal organic vapor phase system. These islands are a-plane oriented as expected for nitride growth on r-plane sapphire. The nanorods themselves were grown by plasma assisted molecular beam epitaxy. The nanorods show an inclination towards the surface normal of 28.3° and are highly ordered. Studies with high resolution x-ray diffraction polar plots reveal the epitaxial relationship between the substrate and nanorods as a c-direction growth on inclined m-plane facets of the nitridated islands. The determined lattice constants show nanorods which are strain free. The growth direction of the nanorods has been confirmed in a transmission electron microscope by convergent beam electron diffraction patterns to be in the N-polar [Formula: see text] direction.     

Synthesis, characterization and photoluminescence studies of ZnOrod@SnO2 nanocomposites

The synthesis of II-VI semiconductor (ZnO_rod@SnO₂) nanocomposite materials with core-shell morphology has been reported. ZnO nanorods were grown by hydrothermal technique using zinc acetate as the reactant. SnO₂ was coated on the nanorods by a simple technique of colloid chemistry. The formation of tin dioxide shell on the ZnO nanorods was confirmed by the TEM images of the resultant materials. The formation of the nanocomposite was also supported by XRD pattern. The effect of tin dioxide shell on the optical properties of ZnO was investigated by photoluminescence spectroscopy and Raman spectroscopy.     

Growth of zinc oxide nanorod structures: pressure controlled hydrothermal process and growth mechanism

Zinc oxide (ZnO) nanorods of various morphologies are grown on zinc substrate by pressure-assisted hydrothermal process and the growth mechanism is investigated with the help of molecular dynamics (MD) simulation results. Hydrothermally reacted ZnO₂ nanostructure bottom-up formation from Zn substrate is a useful process employed here. A systematic study on the role of process control parameters such as pressure and temperature on nanorod growth has been carried out. Correlation among the process parameters to form ordered nanostructures is established. The effect of pressure on the diameter and length of the grown ZnO nanorod structures is studied, which is precisely tunable. With a decrease in pressure from 500 to 400 kPa, the nanorod diameter is reduced by 22.2 %, while its length is increased by 24.8 %. At lower vapor pressure, the nanorod tips are sharper, whereas at higher vapor pressure they are flat. These variations along with a detailed analysis of MD simulations helps us hypothesize that pressure plays an important role in governing the diffusion of oxygen atom onto zinc surface and generating wurtzite phase. Simulation results clearly show that ZnO nanorods lift off due to their interaction with the Zn atoms on the substrate and the resulting forces.     

Fabrication of GaN epitaxial thin film on InGaZnO4 single-crystalline buffer layer

Epitaxial (0001) films of GaN were grown on (111) YSZ substrates using single-crystalline InGaZnO₄ (sc-IGZO) lattice-matched buffer layers by molecular beam epitaxy with a NH₃ source. The epitaxial relationships are (0001)_GaN//(0001)_IGZO//(111)_YSZ in out-of-plane and [112¯0]_GaN//[112¯0]_IGZO//[11¯0]_YSZ in in-plane. This is different from those reported for GaN on many oxide crystals; the in-plane orientation of GaN crystal lattice is rotated by 30° with respect to those of oxide substrates except for ZnO. Although these GaN films showed relatively large tilting and twisting angles, which would be due to the reaction between GaN and IGZO, the GaN films grown on the sc-IGZO buffer layers exhibited stronger band-edge photoluminescence than GaN grown on a low-temperature GaN buffer layer.     

Carrier transport mechanism on ZnO nanorods/p-Si heterojunction diodes with various atmospheres annealing hydrothermal seed-layer

Annealing in various atmospheres (vacuum, N₂, and O₂) was employed for a hydrothermal seed-layer. The influence on ZnO nanorods (NRs) and carrier transport of ZnO NRs/p-Si heterojunction diodes (HJDs) was investigated. In this work, a hydrothermal method was employed to prepare a seed-layer on a Si substrate, and then annealing at 450 °C in various atmospheres was carried out to improve the subsequent growth of ZnO NRs according to the same method. Observations indicated that ZnO NRs with an O₂-annealed seed-layer have a higher nucleation density and absorb fewer OH groups or O₂⁻ ions, and hence they have fewer defect-level centres. This leads to a very large rectification ratio of 1.9 × 10⁵ in the ZnO NRs/p-Si HJDs because oxygen atoms compensate for the oxygen vacancy-related defects. More band-gap states are present at the ZnO/p-Si interface for the vacuum annealing sample, and this enables recombination-tunnelling transport with a rather large ideality factor of 7 at forward voltage less than 0.7 V. In contrast, diffusion-recombination transport was obtained in the N₂- and O₂-annealed samples with ideality factors as low as 2.4 and 2.2, respectively.     

Growth, structural and electrical properties of polar ZnO thin films on MgO (100) substrates

ZnO films have been grown on (100) oriented MgO substrates by pulsed-electron beam deposition in the room temperature to 500 °C range. Highly (00·2) textured films are obtained for a growth temperature higher than 200 °C, and epitaxial films are formed at 500 °C with the following epitaxial relationships: (1-1·0)_ZnO // (110)_MgO and (11·0)_ZnO // (110)_MgO, despite the difference in symmetry between film and substrate. The low temperature resistivity curves evidenced a metal-semiconductor transition for the ZnO films grown in the 300 to 500 °C range which has been interpreted in the frame of the model of conductivity in disordered oxides.     

The growth and the ultraviolet photoresponse properties of the horizontal growth ZnO nanorods

The horizontal ZnO nanorods (NRs) were grown by using a low temperature hydrothermal method between the lithographic ZnO interdigital electrodes. In order to horizontally grow the ZnO nanorods, the vertical growth was restrained by coating with the photoresist on the surface nucleation sites. By controlling the distance between the electrodes, only the electrodes for an interval of 7 μm can be connected by the horizontal nanorods to form device. The electrical property of the device was measured. The detector showed a narrow ultraviolet photoresponse with a response peak at 379 nm, which was according with the peak of the photoluminescence. The mechanism of photoresponse was discussed.     

Growth of aligned ZnO nanorods on transparent electrodes by hybrid methods

The fabrication of ZnO (80 nm) thin film was achieved by hybrid atomic layer deposition (ALD) to prevent the reaction between the reactants and conductive layer of the substrates. ZnO nanorods (ZnO-NRs) growth over the substrates was performed by wet chemical procedure in which Zn(NO₃)₂ and hexamethylenetetramine were used as the precursors. HR-TEM, SAED, FE-SEM, X-ray diffraction (XRD), and UV–Vis spectroscopy were employed to characterize the ZnO-NRs samples on the substrates. XRD and HR-TEM analyses confirmed that the ZnO nanorod structure is hexagonal wurtzite type with growth in the [0001] direction. Length and thickness of the ZnO-NRs ranged between 45  and 90 nm and 480  and 600 nm, respectively. It was observed that the growth rate of NRs in [0001] direction is 10 times higher than in [1000] direction. The growth mechanism and resulted dimensions of nanorods are function of the synthesis parameters (in hybrid ALD process) such as reaction time, temperature, precursor molar ratio, and thickness of ZnO film.     

Low temperature epitaxial growth of ZnO layer by plasma-assisted epitaxy

Plasma-assisted epitaxial growth of ZnO layers were achieved on C- and R-plane sapphire substrates in oxygen plasma excited by radio frequency power at 13.56 MHz with evaporation of pure elemental Zn. The ZnO layers were grown at 300–400°C with high growth rate around 1.7 μm/h. Surface cleaning of sapphire substrates using Ar-plasma was crucial for good quality ZnO growth. Photoluminescence spectra at 10 K were dominated by band-edge emission due to bound excitons without deep level emission in green-light region. The intensity of band-edge emission was strongly dependent on applied radio frequency power to excite Ar- and O₂-plasma for sapphire surface cleaning and ZnO growth, respectively, and was about 50 times larger on the layer grown in oxygen plasma than that grown in non-excited oxygen gas. The ZnO layer grown on R-plane sapphire was epitaxially grown above 300°C in oxygen plasma, however, on C-plane sapphire the ZnO layer was easily polycrystallized for thick films even at 400°C. Growth mode and surface morphology of ZnO layers were drastically changed with the substrate orientation.