Growth and properties of well-aligned ZnO hexagonal cones prepared by carbonthermal reaction

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Abstract

Well-aligned ZnO double-cones were successfully synthesized on Si (1 0 0) substrates by carbonthermal reaction. The double-cone consists of a reverse-cone and a bottom-cone connected by a tenuous neck. Scanning electron microscopy, X-ray diffraction, Raman spectroscopy and low temperature photoluminescence measurements were employed to study crystal structure and optical properties of the cones. It is shown that the double-cones have hexagonal crystal structure, uniform sizes and high crystal quality. A growth mechanism was proposed to understand the formation of this novel structure, which involves the variation of chemical composition of the vapor phase and the temperature gradient in the tube.

Introduction

Zinc oxide is II–VI compound semiconductor with a wide direct band gap of 3.437 eV at 2 K [1]. One of the outstanding features of ZnO is its large exciton binding energy (60 meV), which leads to exciton stability at room temperature (thermal energy 26 meV) [2], [3], and enables optical devices synthesized by ZnO to function at a low threshold voltage. Because of its excellent electrical and optical properties, ZnO has been widely used as transparent electrodes [4], gas sensors [5], [6], and bulk acoustic waves devices [7]. ZnO has attracted large attention, especially for its potential applications in short-wavelength optical devices such as light emitting diodes (LEDs) and laser diodes (LDs), and ultraviolet lasing action has been observed in ZnO thin films and nanostructures [3], [8], [9]. For applications in practical devices, high crystal quality ZnO is necessary and many techniques have been used for preparation, such as molecular-beam epitaxy (MBE) [10], metalorganic chemical vapor deposition (MOCVD) [11], and pulsed laser deposition (PLD) [12]. Vapor-phase evaporation and transportation, another simple but effective approach, has also been employed and many semiconductor materials with interesting morphologies, such as rod, needle, tube [13], [14], wires [15], and belts [16] have been obtained. Here we present a new ZnO morphology of a double-cone structure prepared on Si (1 0 0) substrates by evaporating ZnO and graphite mixture in a quartz tube in atmosphere. The structure and optical properties of the double-cones were characterized and the growth mechanism was discussed.

Section snippets

Experiment

A double temperature zone horizontal tube furnace was used as the evaporation apparatus. The source material, ZnO (99.99%) powder mixed with graphite powder (molar ratio 1:1), was placed in the closed end of a quartz tube (∅0.7×30 cm). Several n-type silicon plates used as substrates were placed at the downstream positions. The quartz tube was inserted into the tube furnace and heated to 1200°C at atmospheric pressure. At this temperature, the following carbonthermal reactions occurred [17]:C+12O

Morphology and growth mechanism

Fig. 1 shows the typical SEM images of a sample prepared for 30 min. The product consists of hexagonal double-cone structured crystallites. Each double-cone has a big perpendicular hexagonal cone (height is about 15 μm, estimated diameter >4 μm) on the bottom and a small reverse hexagonal cone (diameter 3 μm and height 2.5 μm) on top (in the discussion below, we call them bottom cone and reverse cone, respectively). The reverse cone and bottom cone are connected by tenuous neck (diameter 1 μm, and

Conclusion

A new morphology of ZnO double-cones was synthesized on Si (1 0 0) substrates by carbonthermal reaction. SEM measurements demonstrate that these double-cones have uniform sizes and are well-aligned vertical to the substrate. Growth process of double-cones is discussed according to the microstructures of different prepared time samples. A competitive growth mechanism is employed to explain why the cones grow vertically on a substrate with large lattice mismatch. In addition, chemical composition

Acknowledgements

This work was supported by Natural Science Foundation of China (Grant No. 50121202, 60376008).

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