Lateral epitaxial overgrowth (LEO) of ZnO has been demonstrated in water at 90 °C. The process starts with hydrothermal epitaxial growth of ZnO(0001) on MgAl2O4(111), followed by channel stamping of photoresist to define “growth windows”. LEO films grow in zinc‐precursor solutions at pH 10.9; sodium citrate addition controls out‐of‐plane growth. Transmission electron microscopy indicates threading dislocation reductions from ∼ 2 × 1010 to < 2 × 108 cm–2 from the window to the wing regions. Microphotoluminescence and Hall‐effect measurements indicate improved material quality. Wing tilt was observed. Double LEO demonstrates the possibility of complete dislocation reduction.

Heteroepitaxial ZnO films are successfully grown on nondoped GaN‐buffered Al2O3 (0001) substrates in water at 90 °C using a two‐step process. In the first step, a discontinuous ZnO thin film (ca. 200 nm in thickness) consisting of hexagonal ZnO crystallites is grown in a solution containing Zn(NO3)·6 H2O and NH4NO3 at ca. pH 7.5 for 24 h. In the second step, a dense and continuous ZnO film (ca. 2.5 μm) is grown on the first ZnO thin film in a solution containing Zn(NO3)·6 H2O and sodium citrate at ca. pH 10.9 for 8 h. Scanning electron microscopy, X‐ray diffraction, UV‐vis absorption spectroscopy, photoluminescence spectroscopy, and Hall‐effect measurement are used to investigate the structural, optical, and electrical properties of the ZnO films. X‐ray diffraction analysis shows that ZnO is a monocrystalline wurtzite structure with an epitaxial orientation relationship of (0001)[11$ \bar 2 $0]ZnO∥(0001)[11$ \bar 2 $0]GaN. Optical transmission spectroscopy of the two‐step grown ZnO film shows a bandgap energy of 3.26 eV at room temperature. A room‐temperature photoluminescence spectrum of the ZnO film reveals only a main peak at ca. 380 nm without any significant defect‐related deep‐level emissions. The electrical property of ZnO film showed n‐type behavior with a carrier concentration of 3.5 × 1018 cm–3 and a mobility of 10.3 cm2 V–1 s–1.

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Controlled and extended growth of micro-and nanostructured material systems is becoming increasingly important because of the ever-decreasing dimensions of a variety of devices, including those used for chemical and biological sensing and diagnosis, catalysis, energy conversion and storage, lightemitting displays, and optical storage. There is also significant academic interest in these systems because their properties can be remarkably improved over those of the bulk material due to quantum-sized effects. Beginning with the preparation of oriented carbon nanotubes, [1,2] there has been a dramatic increase in the volume of research into tubular and rodlike nano-and microscale structures for a variety of materials, such as In 1-x Ga x P [3] and MnOOH [4] Zinc oxide has been considered as the most promising candidate material for a wide variety of applications, such as room-temperature UV lasing, [10] transparent conduction electrodes, [11] surface-acoustic-wave devices, [12] and gas sensors, [13,14] because of its unique properties of a wide bandgap (E g = 3.37 eV) [15] with a large exciton binding energy (60 meV) and a large piezoelectric constant.[16] Recent progress in processing and properties of ZnO has been reviewed. [17][18][19] Therefore, there has also been significant research into the growth and properties of ZnO nano/microstructures of wires, tubes, belts, and rods. [20,21] The synthesis of nano/microstructures of ZnO has been carried out using various methods, such as evaporation and condensation, [22][23][24][25] physical vapor deposition, [26][27][28] chemical vapor deposition, [29,30] and solvothermal [31] and hydrothermal methods. [32][33][34][35][36] Among them, the hydrothermal method is one of the most attractive candidates for industrial use because industrial processes generally require rapid, low-cost techniques, which are the advantages of the hydrothermal method. The morphology of ZnO crystals can be controlled by varying growth conditions such as pH, [37] temperature, [38] and adsorbing molecules. [32,39] Nano/micrometer-scale ZnO structures that only have out-of-plane orientation without in-plane orientation with a substrate have been successfully grown.[ [40][41][42] However, the direct fabrication of periodic arrays of nano/micrometer-scale ZnO morphologies as single crystals with inplane and out-of plane epitaxial relations with a substrate have not been achieved yet.It is common to use photolithographic techniques to fabricate patterned periodic arrays of micrometer to sub-micrometer scale features of interesting materials on a substrate. A novel technique called "channel stamping" has also been developed as a soft lithographic alternative to photolithography, where either a metallo-organic precursor solution or a polymer solution is stamped from the channels of a polydimethylsiloxane (PDMS) stamp. Channel stamping has been used to produce micrometer and sub-micrometer scale features. [43,44] Several years ago, Andeen et al. [45] demonstrated that epitaxial ZnO films could be grown ...

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