Na-doped ZnO nanorods fabricated by chemical vapor deposition and their optoelectrical properties
Introduction
ZnO has drawn tremendous attention due to its wide direct band gap (∼3.37 eV at room temperature), large exciton binding energy (60 meV), and promising applications in short-wavelength optoelectronic devices [1]. It is essential to develop high-quality n- and p-type ZnO to realize these applications. Generally, ZnO is an n-type semiconductor due to its native defects, such as oxygen vacancies and zinc interstitials [1]. The electron concentration can be further increased by doping group III elements, such as Al [2], Ga [3] and In Ref. [4]. Furthermore, extensive efforts have been made in the preparation of ZnO-based heterojunctions by using p-GaN [5], p-Si [6] and p-SiC [7] for optoelectronic applications. However, heterostructured devices have poorer performance than those of homojunctions due to the reduced lattice mismatching, which could avoid introducing more defects in the fabrication of devices. Therefore, the ZnO homojunction is necessary for the optoelectronic application of ZnO, and the realization of stable p-type ZnO is particularly essential [8], [9]. Recently, research on p-type ZnO has been carried out by various doping techniques, including chemical vapor deposition (CVD) [10], the hydrothermal method [11], metal organic chemical vapor deposition (MOCVD) [12], pulsed laser deposition (PLD) [13] and sol-gel deposition [14]. Currently, group V (N, P, As, and Sb) and IA (Li, Na and K) elements have been considered as p-type dopants of ZnO by many research groups [8], [9]. Group IA elements, especially, have been regarded as ideal dopants due to their shallow acceptor levels according to theoretical prediction [15]. Also, Na has a shallow acceptor level of 170 meV in ZnO when Na substitutes for Zn [15]. Recently, much research has been conducted regarding p-type Na-doped ZnO films fabricated by using different growth techniques [16], [17], [18], [19]. However, the studies about Na-doped ZnO nanostructures are limited [13], [20], [21], [22], and further discussion is needed on the electrical and optical properties of Na-doped ZnO nanostructures. Liu et al. reported on Na-doped p-type ZnO microwires created by a vapor-liquid-solid process with a hole concentration of 1.3 × 1016 cm−3 and a carrier mobility of 2.1 cm2 V−1 S−1 [20]. Huang et al. created Na-doped p-type ZnO nanorods on a molecular beam epitaxy (MBE) ZnO layer seeded Si through a quartz tube furnace system [21]. Lei et al. demonstrated the successful growth of Na-doped ZnO nanorod arrays by MOCVD [22]. Qiu et al. reported on Na-doped ZnO nanowires grown on gold catalytic Si substrates by the high pressure PLD process [13]. However, among these fabrication techniques, CVD is one of the most powerful and economical methods for oxide nanorod growth. In this work, Na-doped ZnO nanorods on ZnO layer seeded Si substrates were prepared using the CVD method. Detailed influence of Na concentration on morphological and electrical properties of the Na-doped ZnO nanorods have been investigated by SEM, HRTEM, XPS and Hall measurement. Low temperature-dependent photoluminescence (PL) properties of Na-doped ZnO nanorods have been studied carefully. The p-type conductivity of Na-doped ZnO nanorods was demonstrated by the Hall measurement and I-V characteristics of ZnO:Na nanorods/ZnO/n-Si structure.
Section snippets
Experimental
Na-doped ZnO nanorods were synthesized on ZnO layer seeded Si substrates through a simple vapor-phase transport process in a conventional horizontal tube furnace. The detailed growth process has been reported in our previous work [23]. In brief, ∼200 nm-thick ZnO seed layers were fabricated by a sputtering technique. A mixture of ZnO (Alfa Aesar, 99.99%), graphite powder (Alfa Aesar, 99.995%) (0.1 g, molar ratio 1:1), and sodium pyrophosphate powder (Amethyst Chemicals, 99.9%) (0, 0.01, 0.02,
Results and discussion
Fig. 1(a)–(c) illustrate the SEM images of the as-prepared undoped and the representative Na-doped ZnO nanorods, where the concentrations of Na determined by XPS are 0%, 0.5 at.% and 2.1 at.%, respectively. As shown in Fig. 1(a), the average diameter and the length of undoped ZnO nanorods are about ∼120 nm and several micrometers, respectively. ZnO nanorods gradually shrink at the top and become tapered at the tip with an increasing Na concentration, showing sharpened-pencil-like morphology [
Conclusions
Na-doped ZnO nanorods were successfully grown on Si substrates by chemical vapor deposition. In comparison with undoped ZnO nanorods, Na-doped ZnO nanorods with Na concentrations in the range of 1.2–2.1 at.% exhibit p-conductivity. The origin of p-type conductivity is believed to be the NaZn acceptor. The acceptor level of ∼132 meV of NaZn is identified by temperature-dependent PL analysis. The rectifying behavior of the I-V characteristic of p-ZnO:Na nanorods/ZnO/n-Si structure confirms the
Acknowledgements
This work is supported by National Natural Science Foundation of China 51572092.
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