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ZnO thin films with three different types of surface structures, smooth, large nanowall-networks, and small nanowall-networks, were grown by a vapor phase transport process. The XRD patterns of these samples showed a high c-axis orientation. Photoluminescence spectra of these samples showed that the intensities of ultraviolet (UV) emission from ZnO thin films having a surface with nanowall-network structures were enhanced by 1-2 orders of magnitude, the linewidths of UV emission were reduced, and the peak positions blueshifted significantly compared with the ZnO film having a smooth surface. The greatly enhanced UV emission was attributed to high crystalline quality with the oriented growth, and whispering gallery mode resonance in the nanowall-formed cavities.
©2008 Optical Society of America
1. Introduction
The needs for white light-emitting diodes with high efficiency and short wavelength laser sources in optical data storage systems have led to the development of violet and blue emitters such as GaN or ZnO based semiconductor diodes [1]. The exciton binding energy of ZnO (60 meV) is much larger than that of GaN (25 meV), which enables efficient excitonic emission at room temperature. The optical properties of various nanostructures based on ZnO materials have been widely studied. In most cases, visible light emissions related to defects or impurities dominate the luminescence spectra, which impedes the applications in ultraviolet (UV) light-emitting devices. To realize exciton-based photonic devices, it is necessary to get strong UV luminescence. Therefore, a few attempts have been made to enhance the exciton emission from ZnO by improving the crystalline quality [2],fabricating highly oriented nanostructures [3,4], surface plasmon resonance [5,6], and nanocomposites [7,8]. The microcavity can enhance the rate of emission at the resonance wavelengths of the microcavity. The spontaneous emission of an optical emitter can be modified by the density of photon states in microcavities [9]. Therefore, the photoluminescence (PL) will depend strongly on the crystallographic growth direction [10] and the nanostructure. In the present work, we report the giant enhancement of UV emission in ZnO films having surfaces with nanowall-network structures compared with the ZnO film having a smooth surface.
2. Experiment
ZnO films with smooth, large nanowall-network, and small nanowall-network surface structures were grown on Si (100) substrates in a tube furnace by a vapor transport process. Zn powder (0.1 g, purity 99.0%) was placed at the sealed end of a small quartz tube, which was loaded to the center region of a furnace. Three pieces of pre-cleaned Si (100) wafers were placed 8.5 cm, 6.5 cm, and 4.5 cm downstream from the evaporation source. The flow rate of the carrier gas (Ar) was kept at 100 SCCM (standard cubic centimeter per minute). The furnace was heated up from room temperature to 600 °C at 28 °C/min, and maintained at 600 °C for 30 min. The working pressure of the system was about 104 Pa. When the temperature stayed at 600 °C, oxygen (O2) was introduced into the tube at a flow rate of 40 SCCM. After the growth, the system was slowly cooled down to room temperature at 8 °C/min. The morphology was studied by using scanning electron microscopy (SEM) (JSM-5600LV JEOL). PL spectra were taken 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
Figure 1 shows SEM images of the three samples grown at different distances of 8.5 cm, 6.5 cm, and 4.5 cm downstream from the Zn source. The sample grown at a distance of 8.5 cm from the source, referred to as sample A hereafter, is shown in Fig. 1(a). It has a smooth surface. The sample B grown at 6.5 cm from the source has a surface with nanowall networks, as shown in Fig. 1(b). The average thickness of the nanowalls is about 200 nm and their diameter ranges from 300 nm to 2 µm. Sample C, grown at distance of 4.5 cm from the source, is shown in Fig. 1(c). It has a surface with smaller nanowall networks compared to sample B. The average thickness of nanowalls is about 100 nm and their diameter ranges from 200 to 500 nm. This nanowall-network structure is similar to the reported [11,12]. However, in our cases, the distance from the Zn source plays a critical role in the surface structure of ZnO thin films. At different distances from the Zn source, the substrate temperatures are different, thus the growth temperatures are different. Meanwhile, the concentrations of the evaporated Zn vapor are also different due to the different distances. Therefore, both the growth temperature and the concentration of the Zn vapor affect the surface structure of the grown ZnO thin films. The similar influences of the location of substrate and temperature on the morphology were reported recently by Ramgir et al. [13].
Fig. 1. SEM images of the samples grown at different distances from the Zn source. (a) ZnO thin film grown at 8.5 cm, called sample A, has a smooth surface. (b) ZnO thin film grown at 6.5 cm, called sample B, has a surface with large nanowall-networks. (c) ZnO film grown at 4.5 cm, called sample C, has a surface with small nanowall networks.
Fig. 2. X-ray diffraction patterns of ZnO thin films with three types of surface structures. (a) a smooth surface, (b) a large nanowall-network surface, and (c) a small nanowall-network surface..
Figure 2 shows the XRD spectra of the as-grown ZnO samples at different distances from the Zn source. As shown by spectra A, B, and C, ZnO (002) peak and its second order (004) peak appear, which indicating that all of the samples are c-axis oriented. The peaks for ZnO other planes are hardly observed. Such result indicates that all samples are of good crystalline quality and highly c-axis orientation. Nevertheless, the intensities of the (002) peaks of two types of nanowalled structures of ZnO are stronger than that of the ZnO film with a smooth surface. Moreover the (002) peak of the sample C is stronger than that of the sample B. The result indicates that ZnO thin film with a small nanowall-network surface is of the highest orientation, and the c-axis orientation of ZnO thin film with a large nanowall-network surface is higher than the smooth ZnO thin films.
In Fig. 3, we compared the emission characteristics of the three types of ZnO films with different surfaces from PL spectra. PL measurements were performed at room temperature with an excitation wavelength of 320 nm. In Fig. 3(a), all samples show a strong UV emission, which is attributed to the radiative recombination of free excitons.
Fig. 3. (a). Room-temperature PL spectra of three types of ZnO thin films having different surface structures. The PL signals of samples A and B are amplified by a factor of 175 and 7.8, respectively. (b) The enlarged view of UV emission bands.
For comparison, UV emission intensity is normalized. The PL signals of samples A and B are amplified by factors of 175 and 7.8, respectively. It is obvious that the UV emission intensities of the ZnO films having a surface with nanowall networks are greatly increased compared to the smooth ZnO film. The UV emission intensity of sample C is increased by a factor of 175 compared to the sample A, and by a factor of 7.8 compared to sample B. The visible emission from sample A centered at 2.48 eV (500 nm) is relatively remarkable. Samples B and C show a relatively weak visible emission centered at 2.28 eV (544 nm). These visible emissions are usually considered to be related to intrinsic defects in ZnO crystal. Thus, the crystalline quality of sample A is poor compared to samples B and C. The oriented growth of nanowalls improves the crystalline quality, which contributes to the strong UV emission of the free exciton radiative recombination at near band edge. Nevertheless, the increase in the UV emission is also related to the presence of the nanowall-network structures at the film surface. In addition to the change in the emission intensity, Fig. 3(a) also exhibits a significant change in the spectral purity, which is the narrowed full width at half-maximum (FWHM) of free exciton emission band. Sample A has the broadest FWHM (197 meV) of the UV emission band, and sample C has the narrowest FWHM (131 meV), which is reduced by a factor of 0.4 compared to the sample A. The FWHM for sample B is in the middle (148 meV). Therefore, FWHM of the UV emission can be influenced by the surface structure. FWHM of the UV emission decreases with reduced size of the nanowall networks at the surface. The reduction of the linewidth indicates that the ZnO films having a surface with nanowall networks have higher optical quality. Moreover, the smaller the size of the nanowall networks, the higher optical quality.
Figure 3(b) shows the enlarged view of the UV emission bands for all samples. For samples B and C, the peaks of the UV emissions have blueshifted significantly compared to sample A. Their UV peaks have blueshifted by 11 meV and 26 meV to 3.287 eV and 3.302 eV, respectively, compared to that of sample A (3.276 eV). It is obvious that the UV emission blueshift is closely related to the nanowalls at the surface. For the ZnO films having a surface with nanowall networks, the circular optical modes in such nanowall micoresonators can be understood as closed circular beams supported by total internal reflections from boundaries of the microresonators. Different resonant reflected wavelengths in circular nanowall microcavities were tuned by their diameters [14]. The circular nanowall microcavities with smaller diameters allow the resonant reflection of short wavelength light. For larger diameters, the reflection of long wavelength light causes resonance [15]. Therefore, compared to the smooth ZnO films, the blueshift of UV emission from the ZnO films having a surface with nanowall networks arises from different resonantre reflected wavelengths of circular nanowall microcavities.
It should be noted that although the crystalline quality of ZnO films having a nanowall-network surface was improved by the oriented growth, these pronounced changes in the UV emission characteristics depended on the presence and the size of nanowall networks at the surface of the ZnO films. For the ZnO films having a surface with nanowall networks, the rough surface can improve the efficiency of light, and the surrounded nanowalls form resonant cavities. The confined light due to total internal reflection in these cavities can support whispering gallery modes (WGMs), as shown in Fig. 4. The WGM resonances are responsible for the giant enhancement of the spontaneous emission observed in PL spectra [16,17]. Since the enhancement is related to the effective volume of the optical mode [18], smaller cavities resulting in small mode volumes lead to stronger enhancement in spontaneous emission [19]. The similar WGM enhanced emission from ZnO nanostructrues was also recently observed [20]. But we believe that the WGM enhanced emission presented here is attributed to the collective contribution of the interaction between all microcavities on the surface of ZnO film and ultraviolet luminescence emitted from ZnO films with microcavity structures. Such microcavity enhancement effect on PL will be of great significance in the emitting light sources having surfaces with microcavities. Surely the collective effect is closely related to the interaction between individual microcavities and ultraviolet luminescence. For further studying the single microcavity effect on the optical properties, the experiments will be performed with Near-field Scanning Optical Microscopy (NSOM). The results of the investigation will be reported in detail in the near future.
4. Conclusions
In conclusion, we investigated the PL properties of ZnO thin films having different surface structures. It was found that the uv emissions from the ZnO films having a surface with nanowall networks were enhanced by 1-2 orders of magnitude compared to the smooth ZnO film. Meanwhile, their UV band linewidths were reduced, and the peaks blueshifted significantly. These pronounced changes depend on the sizes of nanowall networks at the surface of the ZnO films. The remarkable increase in the intensity of UV emission was attributed to high crystalline quality with the oriented growth and WGM optical resonance in the nanowall-formed cavities. The strong UV emission opens up new possibilities for low-threshold lasing and other highly efficient solid state emitters.
References and links
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