A clean Si substrate was placed on top of the
Al2O3 boat to collect samples. The furnace was heated to 1,050°C at a rate of 20°C/min and kept at that temperature for 60 min. After the furnace had naturally cooled down to room temperature, the ZnO MRs were deposited on the Si substrate. To construct the LED, a p-type GaN layer was grown on a (0001) sapphire substrate with hole concentration and mobility of 1017 cm−3 and 10 cm2/V-s, respectively, was used as the hole injection layer. A thin layer of PMMA was partly coated on the p-type GaN film to serve as an insulating layer. After the substrate was heated at check details 50°C for 20 min to improve the quality of the PMMA, a single ZnO MR was transferred to the prepared p-GaN substrate and crossed the boundary with the p-GaN and PMMA. Finally, the ZnO MR was fixed by Ag paste which served as the cathode, while another Ag electrode on the GaN film worked as the anode. The sample morphology was examined with a high-resolution Zeiss FEG scanning electron microscope (SUPRA 55, Carl Zeiss, Oberkochen, Germany). The polarized micro-Raman spectra of the individual ZnO MR were measured using a Horiba Jobin-Yvon iHR320 spectrometer (Horiba, Kyoto, Japan) in a backscattering configuration. The 532-nm line of a frequency-doubled
Nd:YAG laser with 4.2-mW power was used for off-resonance excitation. The I-V measurements were carried out P505-15 ic50 with a Keithley 2400 source meter (Cleveland, OH, USA). Micro-photoluminescence (μ-PL) and EL measurements were conducted by the above spectrometer. The optical source was GF120918 provided by a 0.3-mW He-Cd laser with the wavelength of 325 nm. All measurements were performed at room temperature. Results and discussion Figure 1a shows
uniform size of 700 μm in length of the individual ZnO microrod. The inset of the SEM image in Figure 1b reveals that many the MR has a hexagonal cross-section and smooth side facets that are 6 μm in diameter. The upper trace of the Figure 1a shows the polarized Raman spectra results. Three distinct peaks at 380, 410, and 437 cm −1 were observed, which can be identified to A1(TO), E1 (TO), and E2 (high) modes, respectively. The peak at 331 cm−1 can be assigned to the second-order Raman scattering arising from zone-boundary phonons 2-E2(M) of ZnO. A strong A1 (TO) mode in the parallel polarization configuration and a predominant E2 (high) mode in the perpendicular polarization configuration indicate that the MR has a c-axis single crystalline wurtzite structure [23, 24]. The schematic diagram of the n-ZnO MR/p-GaN heterostructure LED is shown in Figure 1c. Figure 1d displays a current–voltage (I-V) curve for the device and presents a typical rectifying curve of the heterostructured diode device, suggesting the formation p-n junctions at the interface. The reverse turn-on voltage is 6 V. Figure 1 SEM image, polarized μ-Raman spectra, schematic, and I-V characteristics. (a) SEM image of an individual ZnO MR. The inset shows the enlarged SEM image.