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Abstract
The low temperature photoluminescence (PL) of ZnO and Al-capped ZnO have been investigated to reveal the influence of Al capping on optical properties of ZnO. Near band edge (NBE) emission of Al-capped ZnO presents an enhancement and negative thermal quenching temperature dependence compared with bare ZnO due to the resonant coupling between excitons in ZnO and surface plasmons (SPs) of Al nanoparticles. SPs have a prominent influence on less localized excitons. This feature of SPs together, with the decreased non-radiative recombination rate, gives rise to the increasing enhancement ratio of NBE emission after Al capping with increasing temperature.
© 2017 Optical Society of America
Introduction
Since zinc oxide (ZnO) is a wide band-gap (3.37eV) semiconductor with large exciton binding energy of 60meV at room temperature, it has great advantages on short wavelength optoelectronic applications, such as ultraviolet (UV) light emitting diodes (LEDs) and low-threshold UV lasers [1–3]. In order to obtain better performance of the devices, high efficient UV emission from ZnO is required. Surface plasmons (SPs), i.e. electron plasmon waves excited by light at metal surface, have widely been reported to enhance the near band edge (NBE) emission of ZnO [4–7]. It is known that when SPs energy is comparable to the band gap of semiconductor, enhanced emission will be observed due to resonant coupling of SPs with spontaneous recombination. A wide range of metals e.g. Au, Ag, Pt, Al etc. were used to provide surface Plasmon resonance (SPR) to improve NBE emission of ZnO. Among all the metals that can generate SPR, Al is the most ideal one for SPs coupling with NBE emission of ZnO, given its abundance, low cost and most importantly its SPR frequency in the UV spectral region [7–9]. Fang et al. [10] reported enhancement in both NBE and visible emission in Al-capped ZnO, which was ascribed to electron transfer due to Ohmic contact between Al and ZnO. While Lu et al. [7] observed enhanced NBE emission and suppressed visible emission in Al-capped ZnO, and a faster decay time measured by time-resolved PL were presented to prove the coupling effect of Al SPs. Therefore, contradictory mechanisms of metal coating effect on ZnO have been deduced from different experimental results. The detailed mechanism of the enhancement is still vague on account of the complicated interaction process between ZnO and Al nanoparticles. It is therefore of high importance to study the influence of Al coating on optical properties of ZnO in detail to figure out the mechanism of the NBE enhancement.
Herein, the temperature-dependent photoluminescence (PL) was investigated to study the Al capping effect on the optical property of ZnO. Different thermal quenching behaviors have been observed in bare and Al-coated ZnO samples. Low temperature PL spectra together with the calculated variation of non-radiative activation energy reveal the NBE enhancement mechanism of Al-capped ZnO.
Experimental section
ZnO nanorod arrays were synthesized by hydrothermal method on a n-type (001) silicon wafer [11]. A thin layer of ZnO acting as seed layer was deposited on the Si substrate by magnetron sputtering followed by a post annealing process at 550 °C for 1 h. The Si substrate with ZnO seed layer was then put in a container filled with 0.1 M Zinc acetate dehydrate (Zn(CH3COO)2·2H2O) and 0.1 M Methenamine (C6H12N4). After stirring, the container was kept in an oven for 2 h at 95 °C. The as-grown samples were then annealed in air at 900 °C for 1 h. Afterwards, an Al layer of 5 nm was deposited on the post-annealed ZnO by radio frequency magnetron sputtering under the power of 50W in Ar atmosphere of 1 Pa for 30s. ZnO with and without decorated Al are denoted as bare ZnO and Al-capped ZnO, respectively. The morphology and composition of the samples were characterized by a field-emission scanning electron microscope (SEM, Hitachi S-4800) with energy dispersive spectrometer (EDS). X-ray diffraction (XRD) data of the samples were collected from a PANalytical X’Pert PRO diffractometer using Cu Kα radiation. PL measurement was performed on a luminescence spectrometer (Edinburgh Instruments FLS 920) with He-Cd laser at 325nm as the excitation source. Temperature-dependent PL was measured from 16K to 300K using a closed cycle helium cryostat and a temperature controller.
Results and discussions
SEM images of bare and Al-capped ZnO are shown in Figs. 1(a) and 1(b), respectively. The ZnO nanorods possess an average diameter of 150 nm and merge with each other forming the columnar arrays. Since the deposition of Al is through sputtering process, Al atoms turn to form a semi continuous film, consisting of nanoparticles. Therefore, ZnO nanorods present a granular surface texture after Al capping, as shown in Fig. 1(b). Element mapping images of Al-capped ZnO are shown in Figs. 1(c), 1(d), and 1(e), further confirming the presence of Al. XRD pattern of bare ZnO is shown in Fig. 1(f). The (0002) Bragg peak at 34.69° indicates that the obtained ZnO has a wurtzite structure and grows preferentially along the c-axis orientation.
Fig. 1 SEM images of (a) bare ZnO, (b) Al-capped ZnO. (c), (d), (e) Zn, O, Al EDS element mapping images of Al-capped ZnO, respectively. (f) XRD patterns of bare ZnO.
Typical PL spectra of ZnO with and without Al coating are shown in Fig. 2. A near-band-edge (NBE) emission peaked at 380 nm in UV spectral region and a wide visible band can be observed in bare ZnO. The NBE emission is usually attributed to the band-edge transition or exciton recombination [12], and the visible band is related with intrinsic defects in ZnO [11,12]. After Al coating, NBE emission enormously enhances while the visible band is slightly suppressed. The pronounced NBE enhancement is generally attributed to the coupling between SPs of Al and the spontaneous radiative recombination in ZnO [7], since the SPs of Al matches well with the band gap of ZnO [13].
To figure out the enhancement mechanism in NBE region, we systematically investigated the PL property of ZnO with and without Al capping at low temperatures. Figure 3(a) shows PL spectra of bare ZnO measured at low temperatures from 16 K to 300 K. It clearly shows a typical thermal quenching behavior, in which all peaks decrease monotonically and shift to lower energy site with the increasing temperature due to temperature-induced increase in non-radiative recombinations [14]. The strongest peak at the highest energy site of bare ZnO at 16 K originates from two components: donor-bound exciton (D0X) at 369.2nm (3.36eV) [15–17] and defect-bound exciton (DBX) at 373.7nm (3.32eV). The DBX is known as A-line originated from excitons bound at structural defects such as crystal irregularities [16,18,19]. The peaks centered at 377.3 nm, 382.1 nm, 385.8 nm and 391.1 nm at 16K can be attributed to the first and second orders of LO phonon replicas of D0X and DBX, denoted as D0X-1LO, DBX-1LO, D0X-2LO and DBX-2LO, respectively, because the energy difference between D0X (DBX) and the corresponding peaks at lower energy site is about an integer multiple of 72meV, which is in consistent with the characteristic energy of longitudinal optical (LO)-phonon replicas in ZnO [20–23]. These pronounced replicas originate from the strong exciton-phonon coupling effect due to the high ionicity and polarity of ZnO [24]. As the temperature increases, the structural defects related DBX emission and its phonon replicas remain at RT, while the D0X emission and its phonon replicas thoroughly decrease resulting from thermal dissociation of donor-bound excitons turning to free excitons (FX) at RT [24]. Therefore, D0X has smaller binding energy than DBX, and is easier to be thermal dissociated to less localized states.
Fig. 3 (a) Temperature-dependent PL spectra of bare ZnO (b) Temperature-dependent PL spectra of Al-capped ZnO versus bare ZnO (c) PL spectra of bare and Al-capped-ZnO measured at 16K.
The temperature-dependent PL spectra of Al-capped ZnO are plotted in comparison with that of the bare ZnO at the same measuring condition in Fig. 3(b). It clearly shows that the entire intensity of the emission strongly enhanced after Al capping. Moreover, the NBE emission of Al-capped ZnO seems to possess different temperature dependence instead of decreasing monotonically with increasing temperature as thermal quenching behavior in bare ZnO. To figure out the change of optical properties caused by Al capping, the PL spectra with and without Al capping measured at 16K is shown in Fig. 3(c), representatively. The enhanced NBE emission maximizes at the first order LO phonon replica of D0X. The dominated phonon replica indicates a strong electron-phonon interaction, which results from the enhanced nearby electromagnetic field due to the confined oscillation of SPs mode in Al nanoparticles [25]. Blue shift of NBE peaks after Al capping can be observed due to stronger SP-coupling effect on higher energy site of the emission since DOS (density of state) of Plasmon increases with photon energy [26]. In Fig. 3(c), peak 1 centered at 370 nm is the enhancement and blue-shift of the peak right beneath it, which centered at 371.5 nm, originated from the superposition of D0X and DBX, as denoted in Fig. 3(a). The dominated peak 2 centered at 376.5 nm is the enhanced and blue-shifted D0X-1LO. Besides, it is noteworthy that peak 3 in Fig. 3(b) is mainly composed of the enhanced and blue shifted D0X-2LO. Hence, the SPs of Al seems to have much larger influence on D0X than DBX, indicating a larger SPR coupling rate with less localized excitons.
The temperature-dependent integrated NBE intensities of bare and Al-capped ZnO and the corresponding enhancement ratios after Al capping are demonstrated in Figs. 4(a) and 4(b), respectively. Al-capped ZnO always presents higher emission intensity than bare ZnO throughout the whole temperature range due to the enhancement brought by SPs coupling effect. The integrated NBE intensity of bare ZnO exhibits a thermal quenching behavior as discussed above; while an anomalous negative thermal-quenching (NTQ) can be observed in Al-capped ZnO, where the emission intensity increases with increasing temperature within a certain temperature range.
Fig. 4 (a) Temperature-dependent NBE integrated intensity for bare ZnO and Al-capped ZnO (b) temperature dependent enhancement ratio of integrated NBE emission intensity after Al capping
NTQ behavior of ZnO and other semiconductors have previously been reported by other researchers and attributed to the release of carriers/excitons from trap states [14,27–29]. Chen et al. reported the NTQ behavior of green emission originated from surface states which can be eliminated by passivation of Al2O3 [30]. NTQ behavior of exciton emission in the temperature range of 0.06<1/T<0.2 (5-16.7 K) was attributed to intermediate states formed by lattice defects in ZnO samples with relatively low quality [31]. However, in our study, the anomalous NTQ behavior of the NBE emission happened within a wide temperature range (60-260 K) solely after the deposition of Al which thus cannot be attributed to either of afore mentioned origins. Moreover, the NBE enhancement ratio induced by Al capping increases from approximately 3 folds at 16 K to 13 folds at 300 K, which indicates a strong relationship between SPs coupling rate and the temperature.
To illustrate the temperature dependence of NBE emission, the integrated NBE intensity is plotted with respect to the reciprocal of temperature in an Arrhenius form. The experimental data and the corresponding fitting curves of bare and Al-capped ZnO are shown in Figs. 5(a) and 5(b), respectively.
Fig. 5 Integrated PL intensity of NBE as a function of the reciprocal of temperature and the corresponding fitting curves of (a) bare ZnO and (b) Al-capped ZnO. The fitting value of activation energy and their error range are shown in the figure.
The typical thermal quenching of bare ZnO shown in Fig. 5(a) can be described by the formula below [32]:
,where is the emission intensity at 0 K, is Boltzmann’s constant, is the temperature, and are constants describing the relative contributions of different activation processes,andrepresent the activation energy of two non-radiative recombination channels at lower (T < 90 K) and higher (90 < T < 300) temperatures, respectively. For bare ZnO, is 6.57 ± 0.80 meV and is 38.30 ± 1.33 meV. The activation energy at lower temperature describes the thermal escaping of localized carriers and can be deemed as localization potential well depth [33], while describes non-radiative recombination centers activated at higher energy which may be related to defect clusters. As the temperature increases, the non-radiative recombination channels are successively activated and localized excitons in bare ZnO will be excited into less localized or free states. These excitons may then be captured by non-radiative recombination centers due to an increased diffusion length, leading to the decrease of photoluminescence emission [29].The typical thermal quenching behavior of bare ZnO can be well fitted by the bi-exponential decay function (1), while for Al-capped ZnO, an extra term are introduced to describe the NTQ behavior i.e. increasing radiative emission intensity with increasing temperature. Thus the overall temperature-dependence of the integrated NBE intensity after Al capping can be fitted by the formula below [27]:
Compared with the spontaneous radiative and non-radiative recombination channels in bare ZnO, excitons in Al-capped ZnO possess a new pathway of radiative recombination induced by SPs coupling process [34]. With Al capping, exciton recombination will couple with surface plasmon modes instead of emitting photons directly to free space and the resonant SPs will be scattered into photons again with enhanced intensity. The coupling and scattering processes induced by SPs make the spontaneous emission more readily extracted from ZnO [35], and can be studied by modeling methods in detail [36].Since the out coupling effect of SPs is more prominent on less localized excitons, the enhancement ratio of NBE emission increases with increasing temperature, and the calculated can be deemed as the activation energy of the new radiative pathway induced by SPs of Al nanoparticles. Binding energy of excitons to defects range from 10 - 20 meV, as the temperature increases, more and more bounded excitons thermally dissociate, benefiting the SPs induced NBE emission enhancement. Meanwhile, non-radiative recombination tends to weaken the emission intensity. These two processes are in competition with each other, resulting in the variation of the PL intensity of Al-capped ZnO. Besides, the plasmonic DOS (density of state) also decreases a bit with the increasing temperature [37]. Therefore, the enhanced NBE emission of Al-capped ZnO begins to drop above 260 K, as seen in Fig. 4(a). Besides, the obtained activation energy of non-radiative recombination at low temperature is 5.23 ± 0.85 meV with no discernable change from. Whereas is calculated to be 42.30 ± 6.18 meV, which enhances after Al capping, indicating a decreasing rate of non-radiative recombination. The decrease of non-radiative recombination rate is also induced by the SPs coupling effect. Excitons in ZnO either recombine radiatively or being captured by non-radiative recombination centers and decay non-radiatively. The deposition of Al provide a new path way for excitons to couple with SPs of Al, therefore, instead of being captured by the non-radiative recombination centers, thermally excited excitons in Al-capped ZnO will preferentially couple with the SPs of Al nanoparticles. Hence, as many of these excitons as possible are allowed to recombine and produce photons, resulting in enhanced NBE emission with the increasing temperature.Conclusions
In this study, PL property of ZnO with and without Al capping were investigated at low temperatures from 16K to 300K to figure out the enhancement mechanism of NBE emission after the decoration of Al. Thermal quenching and negative thermal quenching with the increase of the measurement temperature for integrated NBE emission were observed in bare ZnO and Al-capped ZnO, respectively. The negative thermal quenching of integrated NBE emission of Al-capped ZnO and its enhancement ratio increasing from 3 folds at 16K to 13 folds at 300K, we thought, are attributed to an additional radiative recombination channel caused by SPs and the decrease of non-radiative recombination rate.
Funding
National Natural Science Foundation of China (Nos. 51672246 and 51272232); and the Fundamental Research Funds for the Central Universities (2015QNA3004, 2016XZZX002-01).
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