Zinc oxide (ZnO) nanostructures are generating significant interest due to unique characteristics that make them good candidates for UV optoelectronic applications such as biosensors and resonators. These properties are due to the wide bandgap of ZnO (3.37eV at room temperature) and to its large exciton energy (60meV), which makes it possible to employ excitonic recombination as a UV-lasing mechanism. ZnO is also a piezoelectric and biosafe material that has probably spawned the richest family of nanostructures to date. Moreover, the ferromagnetic properties of ZnO doped with rare earth metals are also of interest for the design of novel devices that store information as a particular spin orientation (spintronics).
Of the techniques for growing ZnO nanostructures with controlled dimensions, we have been using two of the most common and cost-effective, namely, the catalytic vapor-liquid-solid (VLS) method and a low-temperature technique based on chemical engineering. When optimized, both approaches can be used to produce large-scale wafers and are suitable for commercial production. Figure 1 shows the schematics of the oven used in our VLS growth experiments.1 We have generated a wide family of different ZnO nanostructures, including wires (both vertically aligned and randomly oriented), ribbons, dots, flowers, branched structures, and leaves, on a variety of substrates with crystalline or amorphous surfaces.2
Figure 1. Fabrication of zinc oxide (ZnO) nanowires using the catalytic VLS growth method. Insert: Transmission electron microscope image of a nanowire with a gold (Au) particle at the tip. Ar: Argon.
The room temperature photoluminescence (PL) spectra of typical ZnO nanowire samples are characterized by two main emission bands. The first is a sharp free-exciton UV band that usually centers on (∼)380nm, and the second is a wider broad band observed between 420 and 700nm, historically referred to as the green luminescence or deep-level emission band. Although all ZnO nanostructures display both bands, their relative intensity varies depending on different growth methods and parameters. Deep-level emission is the source of observed white light emission. The assignment of this band is still controversial, and we have attempted to elucidate its origin. In a recent experiment, different ZnO bulk samples were annealed for 1h at 50–1050∘C in the presence of ZnO powder, metallic zinc, pure oxygen, or air. All the experiments, except annealing in air, were performed in quartz-encapsulated samples filled with the corresponding gas or powder. After annealing, PL spectra were recorded in the 27–300K temperature range using the 350nm line from an argon ion laser as the excitation source. Figure 2 shows typical spectra recorded for samples annealed in zinc-rich environments.
Figure 2. Typical emission spectra of samples annealed in a zinc-rich (a and b) and an oxygen-rich environment (c and d). PL: Photoluminescence. VZn: Zinc vacancy. Vo: Oxygen vacancy.
We concluded that the source of the deep-level emission band was in fact the result of superposing different PL emissions, specifically, including bands due to zinc and oxygen vacancies (labeled VZn and Vo in Figure 2).3
Using some of our ZnO nanowire samples, we developed a process to fabricate high-brightness light-emitting diodes (HBLEDs) with emission in the white-light wavelength region. Figure 3 shows the device schematics, together with digital micrographs of the operating diode. The sample that is emitting the white light in Figure 3 has dimensions of only 0.2(×)0.5mm.
Figure 3. Schematic diagram of an HBLED and digital micrographs of the packaged device and emitting white light.
We also observed stimulated emission from our ZnO nanowires on silicon (Si). Figure 4(a) shows a scanning electron microscope image of well-aligned ZnO nanowires grown on Si substrates using the VLS catalytic approach. In the stimulated emission experiment, this sample was excited at 270nm with a pulsed laser at different excitation powers. The spectra are shown in Figure 4(b). "The spontaneous emission at low power becomes visible at a threshold excitation value. Above this threshold, the stimulated emission finally becomes dominant.
Figure 4. (a) Scanning electron microscope image of the ZnO nanowires used for a stimulated emission experiment. (b) Emission spectra recorded at room temperature using three different laser excitation powers: 14.4, 32, and 56kW/cm(2). Insert: Integrated emission intensity of the ZnO nanowires versus excitation power.
In conclusion, we have demonstrated the possibility of developing HBLEDs using ZnO nanostructures. We also established that the observation of white light emission is actually an intrinsic ZnO property. We used the material in its nanostructural form to allow use of a variety of substrates, which would not be possible with thin films. The fact that our ZnO nanowire HBLEDs can be grown in this way makes them more attractive for commercialization when compared with similar wide bandgap semiconductors.
Department of Science and Technology
Magnus Willander holds full professorships in the Physics Department at Göteborg University and the Department of Science and Technology at Linköping University. His research is focused on photonic materials and devices, and he combines experimental and theoretical research in these areas. He has published approximately 800 papers and seven books.
QingXiang Zhao, Omer Nur
Department of Science and Technology
Qing Xiang Zhao holds a senior lecturer position in the Institute of Science and Technology at Linköping University. His research involves both experimental and theoretical work on semiconductor materials and fundamental physics. He has published approximately 170 papers and one book.
Omer Nur holds a senior lecturer position in the Department of Science and Technology at Linköping University. His research interests focus on device physics and technology. He has published approximately 100 articles.
1. M. Willander, O. Nur, Y. E. Lozovik, S. M. Al-Hilli, Z. Chiragwandi, Q.-H. Hu, Q. X. Zhao, P. Klason, Solid and soft nano-structured materials: fundamentals and applications, Microelectron. J. 36, no. 11, pp. 940-949, 2005.doi:10.1016/j.mejo.2005.04.020