A green and low-cost method to prepare high-quality GaN (gallium nitride) nanowires is important for the applications of GaN-based devices on a large scale. In this work, high-quality GaN nanowires are successfully prepared by a green plasma enhanced chemical vapor deposition method without catalyst, with Al<sub>2</sub>O<sub>3</sub> used as a substrate, metal Ga as a gallium source and N<sub>2</sub> as a nitrogen source. The obtained GaN nanomaterials are investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and photoluminescence (PL) spectroscopy. The XRD results demonstrate that hexagonal-wurtzite GaN is obtained and no other phases exist. The SEM results show that GaN nanowires and hexagonal GaN microsheets are obtained at different temperatures. When the growth temperature is at 950 ℃ (reaction time for 2 h), the hexagonal GaN microsheets each with a size of 15 μm are obtained. When the growth temperature is at 1000 ℃(reaction time for 2 h), the GaN nanowires with the lengths in a range of 10–20 μm are obtained. With the reaction temperature increasing from 0.5 h to 2 h, the lengths of GaN nanowires increase. The TEM results suggest that the GaN nanowires are of high crystallinity and the growth direction of GaN nanowires is in the [0001] direction. The Raman results indicate that there exists a compressive stress in the GaN nanowires and its value is 0.84 GPa. Meanwhile, the growth mechanism of GaN nanowires is also proposed. The morphologies of GaN nanomaterials are tailed by the growth temperature, which may be caused by Ga atomic surface diffusion. Ga atoms have low diffusion energy and small diffusion length at 950 ℃. They gather in the non-polar m-plane. The (0001) plane with the lowest energy begins to grow. Then, hexagonal GaN microsheets are obtained. When reaction temperature is at 1000 ℃, the diffusion length of Ga atoms increases. Ga atoms can diffuse into (0001) plane. In order to maintain the lowest surface energy, the GaN nanowires grow along the [0001] direction. The PL results indicate that the obtained GaN nanowires have just an intrinsic and sharp luminescence peak at 360 nm, which possesses promising applications in photoelectric devices such as ultraviolet laser emitter. Our research will also provide a low-cost and green technical method of fabricating the new photoelectric devices.
In this study, the high quality GaN films have been prepared by a simple, green and low-cost plasma enhanced chemical vapor deposition (PECVD)method at 950 ℃, with Ga2O3 and N2 as gallium source and nitrogen source, respectively. In order to improve the crystal quality of GaN films and figure out the photorespone mechanism of GaN films, the effect of the preparation temperature of GaN buffer layer on the crystal quality and photoelectric properties of GaN thin films was investigated. It is indicated that with the increase of the buffer temperature of GaN films, the crystal quality of GaN films increases first and then decreases, and the highest crystal quality is obtained at 875 ℃. When buffer layer temperature is 875 ℃, the calculated total dislocation density is 9.74?109 cm-2,and the carrier mobility is 0.713 cm2/V·s. After annealing, the crystal quality of GaN film have been improved. The total dislocation density of GaN film decreases to 7.38?109 cm-2,and the carrier mobility increases to 43.5 cm2/V·s. The SEM results indicate that GaN film (buffer layer temperature at 875 ℃) has smooth surface and compact structure. The Hall and XPS results indicate that there are N vacancy, Ga vacancy or O doping in the GaN film, which act as deep level to capture photogenerated electrons and holes. With increasing the bias, the photoresponsivity of the GaN film photodetector gradually increases and then reaches saturation. This is due to the deep levels produced by vacancy or O doping. In addition, photocurrent response and recovery of GaN film are slow, which also due to the deep levels formed by vacancy or O doping. At 5V bias, the photoresponsivity of GaN film is 0.2 A/W, and rise time is 15.4 s, and fall time is 24 s. Therefore, the high quality GaN film prepared by the proposed green and low-cost PECVD method present a strong potential of application in ultraviolet photodetector. The PECVD method developed by us provides a feasible way for the preparation of high quality GaN films, and the understanding of the photoresponse mechanism of GaN films provides a theoretical basis for the wide application of GaN films.
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