The ultraviolet (UV) emission from the Au-coated ZnO films was greatly enhanced and the visible emission was significantly suppressed compared with the un-coated ZnO films. Great changes in photoluminescence of ZnO films are attributed to the electron transfer between conduction band and defect levels through the localized surface plasmons. The increase of electron density in conduction band causes enhanced UV emission, while the decrease in electron density in defect level leads to the suppression of the visible emission. Such ZnO films with enhanced UV emission have potential applications in the highly efficient solid state emitters.
©2009 Optical Society of America
Over the last few years, much attention has been devoted to the investigation of wide band gap semiconductors for applications in high density optical storage and super bright LEDs . Zinc oxide (ZnO) is one of the most important wide band gap semiconductor materials for optoelectronics due to a band gap of ~3.37 eV and an exciton binding energy of 60 meV. It has considerable potential applications in the short-wavelength light sources such as laser diodes . However, in most cases, visible light emissions related to defects or impurities dominate its luminescence spectra. Hence, it is important to obtain highly efficient ultraviolet (UV) emission from the near band edge for application in highly efficient short wavelength light sources and optoelectronic devices. Numerous studies have been conducted to improve the band edge emission and to suppress the visible emission [3, 4], but only small progress has been achieved [5, 6].
Localized surface plasmons are the oscillation of charge density at the interface between metal and dielectric. Nanostructures of noble metals, such as platinum, gold and silver, have strong localized surface plasmons effects, like strong absorption and luminescence [7, 8]. The enhancement of spontaneous emission from ZnO by surface plasmons mediated by aluminium , platinum , and silver  had been reported. Moreover, the enhancement of UV lasing emission from ZnO films was obtained by surface plasmons resonance . Recently, modifying optical properties of ZnO with metals has attracted much attention. Chen et al. found that the UV and visible emission intensities could be tailored by Au nanoparticles . Cheng et al. studied the enhancement of the light emission from ZnO films by sputtering Ag islands on their surfaces . The significant changes in photoluminescence (PL) of Au-ZnO composite nanocrystals [15, 16] were also reported. Although they pointed out that the metals surrounding ZnO could significantly modify its optical properties, the correlation between the visible emission quenching and the UV enhancement was not explained in detail. In this paper, we develop the model of the transfer of electrons among different states to explain the UV enhancement and the visible suppression. The change of the intensity of different deep-level emission demonstrated that the electron transfer from the different defect states to the conduction band of ZnO through Au nanoparticles. These results are useful for the development of high efficiency ZnO-based LEDs, and helpful in understanding the mechanism of surface plasmons coupling.
ZnO films were grown on sapphire substrates in a tube furnace by chemical vapour deposition. The Zn powder (purity 99.0%) was placed in a quartz boat, which was positioned at the sealed end of a small quartz tube with a diameter of 2 cm. The pre-cleaned sapphire substrate was placed at 2 cm downstream from the evaporation source and the substrates from each other are about 2 cm. The rotary vacuum pump was used to realize the vacuum. The tube furnace was heated up from room temperature to 650 °C at a rate of 10 °C/min. Argon as the carrier gas was introduced into the tube furnace with a flow rate of 200 sccm. When the temperature was raised to 300 °C, oxygen was introduced into the system at a flow rate of 40 sccm. The flow rates of both gases were controlled by the mass flow controller and the mass flow meter (MFM D08 Series). Two experiments were performed at different conditions. For a typical experiment, the temperature was maintained at 650 °C for 30 minutes, and then raised from 650 °C to 900 °C. The as-prepared sample was named sample A. For another typical experiment, the temperature was raised to 1000 °C. Sample B was assigned. The final temperatures of 900 °C and 1000 °C were maintained for 20 min for samples A and B, respectively. After the growth, the system was slowly cooled down to the room temperature. Au nanoparticles were sputtered on the surfaces of both ZnO films by DC sputtering. A current of 8 mA was used at the pressure of ~7 Pa. In this case, the rate of deposition of the Au nanoparticles is ~0.056 nm/sec. The deposition time was 80 sec. The crystal structures of un-coated ZnO films were analyzed by X-ray diffraction (XRD) (Dandong, DX 2500 diffractometer with Cu Ka radiation). The surface morphologies of Au-coated ZnO films were characterized by scanning electron microscopy (SEM, JSM-5600LV JEOL). The surface roughness was studied by Atomic Force Microscope (AFM) with tap-ping mode (NT-MDT Zelenograd, Moscow, Russia) at room temperature. PL spectra of ZnO films with and without Au coating were measured at room temperature with a Fluorolog Tau-3 spetrofluorometer (Jobin Yvon/SPEX Horiba). The 320 nm line of a 450-W Xe lamp was used as the excitation source.
3. Results and discussion
The crystal quality of the as-prepared ZnO films was analyzed by XRD. Figure 1 shows XRD patterns of un-coated ZnO films grown on the sapphire substrate. Besides the peak at 42.0° from the sapphire substrate, only two peaks at 34.7° and 36.3° were observed for each sample, corresponding to (002) and (101) planes of ZnO, respectively. This indicates that the as-prepared ZnO films belong to the hexagonal structure. The relative intensity of (002) peak stronger than (101) peak suggests that ZnO films are highly c-axis oriented. No peaks due to Zn or other impurities can be observed.
The SEM images of Au-coated ZnO films are shown in Fig. 2. The surfaces of both samples are considerably rough, and consist of irregular walls. It can be clearly seen that the surface of Au-coated ZnO film (sample A) grown at 900 °C is more rough than that of sample B grown at 1000 °C. The difference in surface structure would lead to different resonant effect of surface plasmons at the interface between Au nanoparticles and ZnO films.
In Fig. 3, we compared the emission characteristics of the Au-coated and un-coated ZnO films from PL spectra. PL measurements were performed at room temperature with an excitation wavelength of 320 nm. In Fig. 3(a), PL spectra of the un-coated ZnO film consist of a weak UV peak and a relatively strong visible peak. However, for the Au-coated ZnO films we can observe a very strong UV emission at the wavelength of 380 nm and an almost complete quenching of the visible emission. For the sample A, the UV emission intensity of the Au coated ZnO film is increased by a factor of ~18 compared with the un-coated ZnO film, while the visible emission from the Au coated ZnO is quenched completely compared with the remarkable peak centered at 596 nm in PL of the un-coated ZnO films. For the sample B, the UV emission intensity of the Au coated ZnO films is increased by a factor of ~11.5 compared with the un-coated ZnO film as shown in Fig. 3(b), while the visible emission from the Au coated ZnO film is not quenched completely compared with the remarkable peak centered at 525 nm in PL of the un-coated ZnO films. Regarding the origin of the deep-level emissions from ZnO, it is still controversial due to complicated crystal defects [17, 18]. It is generally accepted that the deep-level emissions are closely related with oxygen vacancies. Different peak positions of visible emissions may be related to different surface statuses of the nanostructures.
It should be noted that the pronounced changes in PL of ZnO films is attributed to the Au nanoparticles. Au nanoparticles support localized surface plasmons . At the interface between Au nanoparticles and the rough surface of ZnO films, localized surface plasmons can be created due to the resonant interaction between the electron-charge near the surface of the Au nanoparticles and the electromagnetic field of the incident light of 320 nm. From AFM images shown in Figs. 4(a) and 4(b), it can be seen that the average roughness for samples A is larger than for B. The density of sample B is larger than that of sample A due to the difference in the considerable roughness. A larger roughness can more efficiently enhance emission by surface plasmon coupling. The resonant oscillation of electrons in Au nanoparticles creates a local light field close to the particle surface that may strongly exceed the strength of the incident light field . In the presence of Au nanoparticles, the giant enhancement of energy density of excitation source resulted in the improvement of excitation rate in excitation process and the decay rate in emission process. Figure 5 shows that the energy band diagrams of the un-coated and Au-coated ZnO films. The energy level of the defect state in sample A is denoted D, corresponding to 2.081 eV (the wavelength of 596 nm), while the level energy of the defect state E is 2.375 eV corresponding to the wavelength of 522 nm. Obviously, the energy of the defect state D is lower than that of the defect state E, as shown in Fig. 5(a). Visible bands of two kinds of samples with different surface roughness are attributed to the recombination of electrons in different energy levels and holes in valence band. The electron transfer from the defect states to the Au nanoparticles not only results in the increase of the resonant electron density, but also creates energetic electrons in higher energy state . These resonant electrons are so active that they can escape from the surface of Au nanoparticles to the conduction band of the ZnO film . Thus, the electron density in the conduction band of the ZnO is significantly increased, which leads to a significant increase of the intensity of UV emission from ZnO. In our experiments, we obtained the UV enhancement factor of ~18 and ~11.5, for sample A and sample B, respectively. Electron transfer processes via the coupling localized surface plasmons at the interface between Au nanoparticles and ZnO films are shown in Fig. 5(b). The difference in the suppression of the visible emission between samples A and B could possibly result from the difference between the defect state and Fermi level of Au. The energy of the defect state D (2.081eV) is closer to the Fermi level of Au compared with sample B. Much more electrons in defect state D of sample A are transferred to the conduction band of ZnO via the localized surface plasmons. As a result, the visible emission from sample A is suppressed completely while the visible emission from the sample B is suppressed incompletely.
We have observed 18-fold, 11.5-fold enhancements of the UV emission from the Au coated ZnO films and complete, nearly complete quenching of the visible emission compared with the un-coated samples. We introduce a simple electron transfer model for this phenomenon: the electrons in the defect state can be transferred to the conduction band of ZnO via its coupling with the localized surface plasmons excited by the incident light. The electrons in the defect state are transferred to the localized surface plasmons excited by the incident light. This process results in the increase of electron density in conduction band and the decrease of electron density in defect state. The PL properties of ZnO films can thus be modified by the localized surface plasmons. The ZnO films with enhanced UV emission may have potential applications in low-threshold lasers and other highly efficient solid state emitters.
This work is supported by the Department of Science and Technology of Henan Province, China, under grant No. 084300510054 and the Department of Education of Henan Province, China, under grant No. 2009A140002.
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