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Lasing characteristics of randomly assembled ZnO nanowires (NWs) coated with different thickness of MgO layers are investigated. It is found that the MgO coated randomly assembled ZnO NWs demonstrate random lasing action and the formation of coherent optical feedback is dependent on the thickness of MgO coating. Pump threshold of the MgO coated randomly assembled ZnO NWs increases with the increase of MgO thickness. Nevertheless, the appropriate use of MgO coating can reduce the pump threshold by ~30% and the corresponding characteristic temperature can be improved by 28 K.
©2010 Optical Society of America
1. Introduction
ZnO, which has a wide band gap (~3.37 eV) and a large exciton binding energy (~60 meV), has potential to be used for the development of high-temperature ultraviolet (UV) optoelectronic devices [1]. Recently, extensive investigations have been concentrated on the realization of UV light-emitting devices and lasers by using ZnO nanowires (NWs) [2]. Furthermore, randomly assembled ZnO NWs has been applied to realize UV random lasers. Lasing characteristics of the UV random lasers can also be improved by embedding the randomly assembled ZnO NWs inside SiO2. The corresponding characteristic temperature, T C, has found to be improved by 10 K [3]. Nevertheless, optical quality of the randomly assembled ZnO NWs should be further improved in order to be used as the active media of UV random lasers.
Passivation techniques have been proposed to improve the optical characteristics of ZnO nanostructures [4]. Large enhancement of UV emission intensity has been obtained from ZnO films capped by 200 nm thick Ag film [5]. In addition, ZnO quantum dots capped with poly(vinylpyrrolidone) molecules have shown enhancement of UV emission [6]. However, the use of randomly assembled ZnO NWs with passivation as an active medium of UV random lasers has not been considered. In this paper, lasing characteristics of randomly assembled ZnO NWs coated with different thickness of MgO coating are investigated. It is found that a thin layer of MgO coating (≤ 30 nm) is sufficient to improve the lasing characteristics of the randomly assembled ZnO NWs. However, a thick MgO coating (≥ 150 nm) reduces the scattering strength of the randomly assembled ZnO NWs so that the corresponding lasing performance is deteriorated.
2. Experiment
ZnO NWs were fabricated on Si substrate by vapor phase transport technique [3]. During the fabrication, temperature at the centre of tube furnace was increased from room temperature to 960 °C at a constant rate of 50°C/min. Inside the large quartz tube, flow rate and pressure were maintained at 50 sccm and 20 mbar respectively. Fabrication time was set for 1 hr, and the furnace was cooled down to room temperature for 3 hrs. Figure 1(a) shows the scanning electron microscopy (SEM) image of the ZnO NWs. Average length and diameter of the ZnO NWs were found to be ~10 µm and ~100 nm respectively. Figure 1(b) gives the x-ray diffraction (XRD) spectrum of the entire nanostructures. It is observed that the peaks of the XRD spectrum index wurtzite ZnO. Figures 1(c) and 1(d) show the high resolution transmission electron microscopy (HRTEM) images of the ZnO NWs. It is noted that the NW has a good crystallinity of wurtzite ZnO structure with lattice constants of a and c equal to 0.32 and 0.52 nm respectively. As indicated in Fig. 1(d), the surface of the NW is rough and some lattice distortions are clearly observed. In addition, the surface layer appears to have a distorted lattice structure, see also the insert of Fig. 1(d).
Fig. 1 (a) SEM images of the ZnO NWs, (b) XRD pattern of the sample, (c) TEM image of ZnO NWs and (d) HRTEM image of a single ZnO NW. The arrow indicates the lattice distortion on the surface of NW and the insert is the enlarged HRTEM image of the NW.
A facile hydrothermal technique, which utilized a solution of MgCl2.6H2O with 50 mmol of Mg2+ concentration and urea, was used to coat MgO layer on the ZnO NWs at 92°C. Details of the growth process were described elsewhere [7]. It is noted that uniform coating of MgO can only be realized if the concentration of Mg2+ is more than 50 mmol. After the reaction, the sample was rinsed with deionized water, dried and annealed at 400 °C for 1 hr in order to remove the defect from the MgO amorphous layer. Figure 2(a) shows a SEM image of the ZnO NW coated with MgO layer. Compared with the bare ZnO NWs, the diameter of the NWs has increased from ~100 to ~160 nm. Figure 2(b) gives the energy-dispersive X-ray spectroscopy (EDS) spectrum of the sample. It is verified that the presence of Mg element on the ZnO NWs. Figure 2(c) shows the low-magnification TEM image of the MgO coated ZnO NW. The insert of Fig. 2(c) displays the local composition distribution of the passivated NW by elemental mappings. It is revealed that the ZnO NW has been uniformly coated with MgO layer. Figure 2 (d) shows the HRTEM image of the MgO layer. It is clearly observed that the ZnO NWs are covered by a ~30 nm MgO layer. A HRTEM image of the interface between ZnO NW and MgO layer is shown in Fig. 2(e). A distinct boundary is visible and no transitional layer is found. This indicated that the diffusion of MgO atom into ZnO lattice was prevented during the annealing process. It is also observed that the MgO coated ZnO NWs have preserved a preferred orientation of (002) with lattice constants a and c kept at the same values as those of the bare ZnO NWs.
Fig. 2 (a) SEM image and (b) EDS spectrum of MgO coated ZnO NWs. (c) TEM image of a single ZnO NW coated with amorphous MgO thin layer and the insert shows the composition distribution of Zn and Mg. (d) HRTEM image of the MgO layer and (e) interface of ZnO NW and amorphous MgO layer.
Thickness of the amorphous MgO layer can be changed by altering the concentration of Mg2+ during the hydrothermal process. The above experiment was repeated by varying the concentration of Mg2+ between 50 and 250 mmol. Figure 3 shows the ZnO NWs coated with different thickness of amorphous MgO layer (from ~75 to ~150 nm). It is verified that the structural and chemical properties of the MgO coated ZnO NWs are identical to that shown in Fig. 2 except the thickness of MgO is different. Using these samples, the influence of MgO coating on the lasing characteristics of randomly assembled ZnO NWs can be investigated.
Fig. 3 SEM images of the ZnO NWs coated with different thickness of MgO layers. (a) ~75, (b) ~95 and (c) 150 nm thick of MgO layers.
Figure 4 plots the room-temperature photoluminance (PL) emission spectra of the ZnO NWs with and without MgO coating. The excitation intensity was maintained constantly in the experiment. PL spectrum of the bare ZnO NWs consists of two bands: a broad green emission at ~520 nm and a near-band-edge UV emission peak at ~380 nm. The visible emission can be attributed to surface defect states recombination of the ZnO NWs [8]. As chemisorption sites (primarily oxygen vacancy sites) [9] is easily formed on the surface of ZnO NWs, the visible emission observed from the ZnO NWs can be related to oxygen vacancies [10]. It is noted that the UV emission intensity of MgO coated ZnO NW is significantly improved when compared to that of the bare ZnO NWs. This is because the MgO layer suppresses the nonradiative recombination centers on the surface of the ZnO NWs so that the excitonic radiative recombination at UV wavelength is enhanced. Similar improvement in UV emission intensity has also been reported in ZnO NWs covered with dielectric shells [11]. Furthermore, it is noted that the UV emission intensities of the passivated ZnO NWs are less dependent on the thickness of MgO layers as MgO has larger bandgap energy than that of ZnO.
Lasing characteristics of the samples were studied by using a frequency-triple 355 nm pulsed Nd:YAG laser (120 ps pulsewidth and 10 Hz repetition rate) as the excitation source [12]. A cylindrical lens was used to focus a pump stripe of ~500 µm wide onto surface of the samples. Emission was collected by an objective lens from the edge of the samples. As the NWs were roughly aligned perpendicular to the substrate’s surface, only light scattered from the sides of the NWs would be detected. In the experiment, the measured results had been averaged over different areas of the samples in order to avoid the localization effect of the random medium. Figure 5 shows the light-light curves of the randomly assembled ZnO NWs with and without MgO coating measured at room temperature. It is noted that the pump threshold, P th, of the bare ZnO NWs is ~0.42 MW/cm2. On the other hand, P th, of the MgO coated ZnO NWs increases monotonically from ~0.3 to ~0.48 MW/cm2 for the increase of MgO thickness. P th versus MgO thickness is also plotted in the insert of Fig. 5. This is because the use of thick MgO coating decreases the effective refractive index of the ZnO NWs as well as reduces the scattering strength of the randomly assembled ZnO NWs. As a result, cavity loss of the randomly assembled ZnO NWs increases with the thickness of MgO. Hence, there is no guarantee that the use of MgO passivation can improve the lasing characteristics of randomly assembled ZnO NWs.
Fig. 5 Light-light curves of the randomly assembled ZnO NWs with and without MgO coating. The inset plots P th versus thickness of MgO layer.
Room-temperature emission spectra of the ZnO NWs are shown in Fig. 6 . As the profile of the emission spectra of 95 nm MgO coated sample is similar to that of 75 nm MgO coated sample, they are not shown in Fig. 6. It is observed that sharp peaks with linewidth of ~0.4 nm are emerged from the emission spectra for all the samples at ~1.02 × P th pump intensity. As the pump intensity further increases, more shape peaks are excited. Furthermore, it is found that the lasing spectra observed from different observation angle of the samples are different and the value of P th is dependent on the excitation area. Hence, it is verified that the lasing characteristics of the randomly assembled ZnO NWs can be attributed to random lasing action [3,13].
Fig. 6 Room-temperature emission spectra of the randomly assembled ZnO NWs with and without MgO coating. (a) no MgO coating, (b) 30, (c) 75 and (d) 150 nm thick MgO coating. The samples were optically pumped at ~1.02, ~1.2 and ~1.5 × P th.
Furthermore, the profile of emission spectra of the randomly assembled ZnO NWs is affected by the thickness of MgO coating. It is noted that the increase of MgO thickness reduces the number of sharp peaks as well as broadens the emission spectra. The reduction of sharp peaks implies the increase of cavity loss of the random cavities. In addition, the broadening of emission spectra indicates the suppression of the formation of coherent optical feedback inside the random cavities. This is due to the reduction of effective refractive index of the MgO coated ZnO NWs reduces the corresponding scattering strength. Hence, it is verified that thick MgO coating deteriorates the lasing performance of the MgO coated randomly assembled ZnO NWs.
As the samples support random lasing action, cavity length of the lasing modes, L, can be deduced by Fourier transform (FT) from the corresponding lasing spectra [13]. Figure 7 plots the FT profile of the randomly assembled ZnO NWs with and without MgO coating. In the measurement, the emission intensities of the samples were roughly kept at the same value. It may be difficult to identify the value of L from Fig. 7 as the laser cavities are randomly distributed over the random medium. Nevertheless, it is estimated that the value of L reduces (i.e., from ~23 to ~5 μm) with the increase of MgO thickness. In our previous analysis of highly disordered ZnO films, we have shown that the small value of L represents the high cavity loss of the random cavities [12]. This is because the Gaussian profile of the pump stripe can only provide sufficient excitation energy over a small area so that only a small laser cavity can be formed inside a random medium with high cavity loss. Hence, this observation is in consistent with our finding that the values of P th increase with the thickness of MgO coating.
Fig. 7 Fourier transform profile of the lasing spectra of the randomly assembled ZnO NWs with and without MgO coating. The insert schematic shows the possibility on the formation of random cavity inside the randomly assembled ZnO NWs.
It is observed in Fig. 6 that the peak emission wavelength of the bare ZnO NWs (i.e., ~379 nm) is different to that of the MgO coated ZnO NWs (i.e., ~383 nm). This may be due to the change of radiative recombination mechanism of the ZnO NWs under the influence of MgO coating. Hence, lasing characteristics of the randomly assembled ZnO NWs operating at different operating temperature, T, were investigated. This was done by mounting the samples onto an electrical ceramic heater of size 1 × 1 cm2 so that T can be varied between 300 and 500 K. The insert of Fig. 8 shows the energy of dominant lasing peaks versus T for the bare and 30 nm MgO coated randomly assembled ZnO NWs. For the MgO coated ZnO NWs, the dominant lasing peaks red-shift from 3.24 to 3.13 eV. This indicated that the radiative recombination process is mainly due to exciton-exciton scattering (EES) [13,14]. Energy of the dominant peaks red-shift at a constant rate of –0.45 meV/K with the increase of T. On the other hand, the ZnO NWs shows redshift of dominant peaks from 3.28 to 3.23 eV. This implied that the radiative recombination process is related to free-excition (FE) [14]. The variation of emission peak energies versus T has a linear relationship, implying that the change of recombination mechanism of MgO coated ZnO NWs is mainly due to the external effect – MgO coating. Randomly assembled ZnO NWs with other thickness of MgO coating also show almost identical redshift of dominant lasing peaks versus T so that they are not repeated in Fig. 8. Hence, the lasing characteristics of MgO coated ZnO NWs over a range of T is less dependent on the thickness of MgO passivation.
Fig. 8 Plots of P th versus T of randomly assembled ZnO NWs with and without MgO coating. The inset shows the corresponding peak wavelength energy versus T.
Figure 8 also plots the value of P th of the bare and 30 nm MgO coated ZnO NWs versus T. It is noted that the MgO coated ZnO NWs can support lasing up to T = 500 K; however, the bare ZnO NWs ceases to lase at 450 K. Solid lines represent the best fit using least-squares fitting of the experimental data for the empirical formula: P th(T) = P 0exp(T/T C), where P 0 is the threshold pump intensity at T = 0 K. T C is an indicator on the temperature stability level (temperature insensitivity) of the lasers [13]. The higher the value of T C can be obtained, the better the high-T laser performance will be achieved. From Fig. 8, it is found that T C of the bare (30 nm MgO coated) randomly assembled ZnO NWs is 93 (121) K. It can also be shown that T C of other MgO coated randomly assembled ZnO NWs remains roughly the same.
3. Discussion and Conclusion
From the about experiment, we have discovered that there are two effects of MgO coating on the lasing performance of the randomly assembled ZnO NWs:
• The presence of MgO improves the optical quality of ZnO NWs. This is because the MgO layer suppresses the nonradiative recombination centers from the surface of the ZnO NWs and preserves the EES radiative recombination process. Hence, enhancement of UV radiation intensity is obtained from the MgO coated ZnO NWs and the UV emission can also be maintained at T = 500 K. It must be noted that optical quality of the ZnO NWs is less dependent on the thickness of MgO coating. This is because the bandgap energy of MgO is larger than that of ZnO so that MgO coating do not absorb UV emission from the ZnO NWs.
• The presence of thick MgO layer deteriorates the lasing performance of the randomly assembled ZnO NWs. As the MgO coated randomly assembled ZnO NWs supporting random lasing action, the formation of coherent optical feedback is determined by the scattering strength of the ZnO NWs. It is noted that the increase of MgO coating reduces the effective refractive index of the MgO coated ZnO NWs. Hence, the scattering strength can also be reduced with the increase of thickness of the MgO coating. Furthermore, the value of P th (the number of lasing peaks) increases (decreases) with the increase of MgO thickness. This is because the cavity loss is inversely proportional to the scattering strength of the MgO coated ZnO NWs. Therefore, the lasing performance of MgO coated randomly assembled ZnO NWs is dependent on the thickness of MgO coating.
In conclusion, the dependence of MgO thickness on the lasing characteristics of randomly assembled ZnO NWs is studied. It is shown that the lasing mechanism of the randomly assembled MgO coated ZnO NWs is due to random lasing action. Furthermore, the value P th of the randomly assembled ZnO NWs increases with the thickness of MgO coating. This is because the presence of MgO layer deteriorates the formation of coherent optical feedback as well as increases the cavity loss of the random cavities. Hence, a thin layer of MgO coating is sufficient to improve the lasing characteristics of the randomly assembled ZnO NWs – to recover the surface defect states of ZnO NWs and to preserve the conditions to form coherent optical feedback. It is shown that P th (T C) of the random assembled ZnO NWs can be reduced (increased) from ~0.42 to ~0.3 MW/cm2 (92 to 121 K) by the presence of 30 nm thick MgO coating.
Acknowledgment
This work was supported by a LKY PDF 2/08 startup grant.
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