ZnO, with a large exciton binding energy (60 meV) and a wide band gap (3.37 eV) at room temperature, has aroused renewed interest for possible applications in blue/UV optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes (LDs), spin-LEDs, solar-blind UV photodetectors, and transparent electronic devices. To realize these applications, one of the key issues is the growth of high quality p-type ZnO, which is difficult as ZnO is naturally n-type and has low solubility in p-type dopants. p-type ZnO has been realized by doping group V and group I species such as N, [1][2][3][4] P, [5] As, [6] and Li. [7] Techniques used for ZnO film growth include molecular beam epitaxy (MBE), [1] pulsed laser deposition (PLD), [8] metal-organic (MO)CVD, [2] and RF/DC magnetron sputtering.[3] Several groups have even observed electroluminescence at room temperature from ZnO p-n homojunctions, [9][10][11] which is a great step towards the practical application of ZnO optoelectronic devices. It is known that device fabrication requires high-quality crystallinity and abrupt interfaces between epitaxial layers. However, previously reported p-type ZnOs were grown at relatively high temperatures, such as 400-600°C, [6,9,10] and some were annealed at even higher temperatures. [4,6,9] .These approaches could induce thermal atomic diffusion at the interfaces, and so are not suitable for industrial production. In this communication, p-type ZnO thin films were grown at the low temperature of 250°C using plasma-assisted MOCVD. In the process of growth, NO or N 2 O plasma was used as both the O source and the N-doping source, which not only increased the activity of the N atoms but also improved the reactivity of the O atoms. In this way, high quality p-type ZnO films can be obtained at low temperature. We prepared two samples, one of ZnO:NO and the other ZnO:N 2 O. Both films show a flat and compact surface, and the surface morphology of the ZnO:NO thin film is shown in Figure 1. The film is composed of homogenous grains with an average grain size of 30 nm. In order to investigate the crystalline properties of these films, X-ray diffraction (XRD) spectra were collected and are shown in Figure 2. Both samples give almost the same spectrum in which only one peak appears, indicating a high (0002) preferential orientation. The diffraction angles of both films are in agreement with bulk ZnO. It seems that N-doping induces no distinct distortion in the lattices. The intensity of the (0002) diffraction peak is relatively stronger for ZnO:NO film. This may be due to the better crystalline quality of ZnO:NO film, whilst more defects such as (N 2 ) O or other structure defects could be contained in ZnO:N 2 O film. [12] Details of the mechanism will be discussed in the Hall measurements shown below.
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