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Uncommon photoluminescence (PL) with fine structure from amorphous indium zinc oxide (IZO) without and with silver (Ag) nano-particles embedment is observed. Significant enhancement in light emission is found for the sample when the embedded Ag nominal layer thickness is increased to 2 nm. The amorphous IZO (a-IZO) samples with and without Ag layer embedment exhibit two-peak PL at ~398nm (~3.12eV, violet light) and ~450nm (~2.76 eV, deep blue light), originating from the optical gap emission and the radiative recombination at the defect level. Interestingly, the intensity of the peak at ~398nm does not show any obvious change while the intensity of the deep blue light emission at ~450nm is significantly increased first and then decreased with the increment of Ag embedment. The unexpectedly enhanced PL at 450nm from IZO with a small amount of Ag embedment is attributed to the coupling between excited electron-hole pairs in IZO films and the surface plasmon of Ag particles. A careful analysis and discussion reveals an interesting mechanism of a-IZO and Ag interaction.
© 2015 Optical Society of America
1. Introduction
Light emission from semiconductors has led to a lot of applications including light emitting diodes (LEDs) and lasers. However, there are strict requirements for a semiconductor to emit light: direct gap and minimum defects. It is known that almost no light is emitted from a polycrystalline sample since photoluminescence can be easily quenched due to grain boundary states, even if the single crystal sample of this semiconductor can emit strong light. An amorphous semiconductor, different from either single crystal or polycrystal materials, is composed of randomly arranged atoms. It is interesting and important to find if amorphous semiconductors can emit light, due to scientific curiosity and potential applications. For example, flexible light emitting devices still require light to emit from the devices; however, it is impossible to grow single crystal semiconductors due to the low processing temperature of flexible electronics.
Although it is not new to observe photoluminescence emitting from some amorphous semiconductors, including amorphous hydrogen doped silicon (a-Si:H), amorphous PbTiO3, amorphous SrTiO3 and amorphous IZO, it is a challenge to achieve strong light emission and understand the mechanisms [1]. For these materials, amorphous IZO is especially interesting because it is a semiconductor even though it is optically transparent. It has a wide-gap of about 3 eV that is above the blue light energy; therefore, IZO is transparent to visible light. If IZO can emit strong light, it is possible to develop invisible electronics that can emit light. Our amorphous semiconductor is processed at low temperature; therefore, such invisible electronics can even be prepared on flexible polymer substrate that can only be processed and used at low temperatures.
IZO is a special semiconductor that is transparent like a glass and is easy to be fabricated into amorphous. It has been employed as transparent conducting oxide (TCO) [2–4] and also as active semiconductor material in thin film transistors devices for advanced flat panel screens [5]. However, the weak light-emission from IZO makes it almost of no practical application as a photon-emission transparent material [6, 7]. Therefore, it is desirable and necessary to explore ways in the enhancement of light emission from a-IZO for approaching to light-emitting device applications.
In this paper, we will report uncommon emission and significant enhancement of light from amorphous indium zinc oxide (a-IZO) embedded with silver nano-grains. The PL spectra of a-IZO without and with Ag embedment are obtained, compared and discussed. The phenomenon of an emission of deep blue light at 450nm wavelength that is strangely deviating from the optical gap of a-IZO is also found and analyzed.
2. Experimental
The Ag embedded a-IZO samples were deposited, at room temperature (RT), by radio-frequency sputtering of IZO target (90 wt% In2O3–10 wt% ZnO) and 99.999% pure Ag target accordingly on borosilicate glass substrates. The IZO target was used for the oxide layer and the Ag target was used for the metal layer of the IZO-Ag-IZO sandwich configuration. The base pressure of the reaction chamber was 4 × 10−7 Torr and the working pressure of Ar gas (99.999%) was 1.1 × 10−2 Torr. The glass substrates were sequentially cleaned in ultrasonic baths of acetone, ethanol and deionized water and then dried with blowing N2 before introducing into the reaction chamber. The top and bottom IZO layers were deposited with a power of 60 W for 30min, resulting in a constant thickness of 120 nm for each IZO layer. The middle metal layer was deposited with a nominal thickness of 0nm, 1nm, 2nm, 8nm, and 16nm respectively, which was controlled by sputtering time after the sputtering rate calibration, and the Ag embedded a-IZO samples are designated as Ag0 (IZO only), Ag1, Ag2, Ag8, and Ag16, respectively. The nominal thickness of Ag was determined from the deposition time with a calibrated and controlled Ag deposition rate.
X-ray diffraction (XRD) patterns were obtained by BRUKER AXS (model D8 ADVANCE, λCuKα = 0.154056 nm). Elemental analyses of the samples were characterized by secondary ion mass spectroscopy (SIMS). The absorption spectra of the Ag embedded a-IZO samples were measured with a SHAIMADZU UV-3101PC UV-VIS-NIR spectrophotometer. Photoluminescence (PL) spectra were recorded using a model of J-Y Horiba, Labram HR800. The PL quantum yields of the samples were tested at RT.
3. Results and discussion
X-ray diffraction (XRD) results for all samples are shown in Fig. 1(a). No sharp peaks but humps appear in all the XRD patterns, indicating the amorphous status of these IZO films, in agreement with literature for IZO samples deposited at similar conditions [8]. The lack of Ag peaks in the XRD patterns may be due to that Ag layers are too thin. A cross-sectional FESEM image of sample Ag8 demonstrates a sandwich configuration of IZO/Ag/IZO and indicates a very thin Ag thickness seen as a dark line in the middle of this sample configuration. The Ag existence and influence can be confirmed by SIMS and PL results.
Fig. 1 (a) XRD patterns of the Ag embedded a-IZO samples in the 2θ range of 20°-80°; (b) A cross-sectional FESEM image of sample Ag8.
From SIMS results in Fig. 2, the secondary ion intensities of O, In, and Zn in samples Ag1-Ag16 all show that both the bottom and top IZO layers, relevant to the Ag layer, are having similar thicknesses as expected. For any of the samples of Ag1-Ag16, a Ag SIMS peak exists. We notice that Ag peak width is almost the same for all the samples, indicating that Ag thickness does not change much. Such a similar Ag thickness result is due to island, rather than layer-by-layer, growth of Ag, as evidenced from our microscopy image of an IZO-Ag-IZO system reported previously [9]. This argument is also supported by the non-zero signals of Zn, In and O at the Ag peak position, because IZO in top IZO layer may fill in between Ag islands deposited on the bottom IZO layer. The increasing downwards tips of In, Zn and O curves at the position of the Ag peak with an increase in Ag peak height are caused by an increase lateral area of Ag islands during the Ag deposition.
Now let us study the light emitting properties of the samples. Figure 3(a) shows the PL spectra of all the Ag embedded a-IZO samples with various Ag nominal thicknesses, indicating a fine structure of two single peaks located at ~398nm (~3.12eV) and ~450nm (~2.76 eV). The PL intensity increases from the bare IZO sample (a-IZO) to Ag2 and then decreases from Ag2 to Ag16, shown in Fig. 3(b). In order to clarify the mechanism of the photoluminescence and variation, each spectrum can be deconvoluted into two resolved peaks at ~398nm (~3.12eV) and ~450nm (~2.76 eV), shown in Figs. 3(c)–3(g), respectively. In order to first understand the origin of the two resolved peaks of the bare IZO sample in Fig. 3(c), the optical direct gap of the bare IZO sample is determined by the Tauc plot following Eq. (1) [10]:
Fig. 3 (a) PL spectra of the Ag embedded a-IZO samples; (b) The variation of the PL peak intensity as a function of Ag nominal thickness; PL spectra of sample IZO (Ag0), Ag1, Ag2, Ag8 and Ag16 can be deconvoluted into two resolved peaks at ~398nm and ~450nm, shown in Figs. 3(c)–(g), respectively; (h) The peak intensity of the two resolved peaks at ~398nm and ~450nm as a function of Ag nominal thickness.
where α is the absorption coefficient, hν is the photon energy, B is a constant, and Eg is the optical direct gap. As shown in Fig. 4, the bare IZO sample has an optical gap of 3.13 ± 0.01 eV, in agreement with Shin et al’s reported value (~3.14 eV) [6] and similar to Leenheer et al’s result (~3.1 eV) [11]. There are also other studies reporting optical gaps ranging from 3.5 to 3.9 eV for highly conductive IZO [12–14]. Therefore, for the bare IZO sample, the emission at ~398nm (~3.12eV) can be attributed to an optical gap emission [15,16]. For the stronger peak at ~450nm (~2.76eV) of the PL spectrum, it is probably due to the radiative recombination of a photocarrier that is localized in the tail states with an oppositely charged photocarrier trapped at the defect level [17] since the IZO sample is amorphous and full of disorder induced dangling bonds/defect levels.
Fig. 4 (a) Plot of the variation of (αhν)2 with photon energy hν for the bare IZO sample. The direct optical gap is obtained by extrapolating to the energy axis as shown. (b) Absorption spectrum of sample Ag1.
After embedding Ag into the IZO films, there still appear two peaks located at ~398nm and ~450nm for each PL spectrum, shown in Figs. 3(d)–3(g). These peaks fall in the wavelength range of violet (380-450 nm). Interestingly, the intensity of the peak at ~398nm (~3.12eV) does not show obvious change while the intensity of the peak at ~450nm (~2.76 eV) is significantly increased from the bare IZO sample to Ag2 and then decreased from Ag2 to Ag16, shown in Fig. 3(h). It should be noted that the intensity changing trend of the ~450nm peak is the same as the trend of the PL intensity variation as the Ag nominal thickness increases in Fig. 3(b). Ag nanoparticles have been reported to be able to assist emission enhancement with a wavelength ranging in 330~710nm depending on different emitting materials/configurations by tuning the type, size, geometry, and interparticle distance of Ag nanoparticles among dielectrics [6, 18]. As demonstrated in Fig. 4 (b), a Ag nanoparticles surface plasmon resonance peak appears in the range of 428nm~680nm. For our Ag/IZO system, when the resonant frequency of localized surface plasmon (LSP) resonances induced by Ag nanoparticles overlaps the IZO emission frequency (450nm in our case), the energy coupled to the LSP mode is significantly increased and thus the emission is enhanced. Both peaks at 398nm and 450nm cannot be attributed to emission of Ag itself since peaks of Ag luminescence locates less than 350nm [19]. Therefore the enhanced photoluminescence at ~450nm (~2.76 eV) as well as for the whole spectrum from sample Ag0 (IZO) to Ag2 can be considered resulting from the coupling between LSP of Ag particles and excitons in IZO films. Based on our knowledge gained from the study of Au capped ZnCdO and Pt capped MgZnO [20, 21], we propose a possible mechanism for the first time to gain a clearer understanding of the PL enhancement at ~450nm and of the whole spectrum for the Ag embedded IZO films. The increased PL intensity demonstrates an improvement in the external quantum efficiency (ηext) of IZO film with Ag particles. The ηext is related by the internal quantum efficiency (ηint) and light extraction efficiency (ηextraction) through Eq. (2):
In this equation, the internal quantum efficiency of bare IZO (ηint(IZO)) can be expressed as Eq. (3)
where τR and τNR are the radiative and nonradiative decay time(s) of IZO. After Ag embedment, the internal quantum efficiency (ηint(Ag-IZO)) follows Eq. (4):
where τ*R and τ*NR are the radiative and nonradiative decay time(s) of IZO embedded with Ag particles, and 1/τLSP is the electron-hole-LSP coupling rate. The coupled LSP can be converted to free space radiation via scattering by rough surface of Ag particles, which will enormously increase the light ηextraction. Therefore, the ηext of the bare IZO film can be immensely enhanced after Ag embedment and reaches the maximum with a nominal Ag thickness of 2nm. Samples Ag1 and Ag2 are found with Ag nanoparticles embedded between IZO layers while samples Ag8 and Ag16 have semi-continuous Ag layers inside the top and bottom IZO layers. When the Ag particles gradually merge together, the attenuation effect from the reflection and absorption dominates and the ηextraction decreases significantly, leading to the immense decrease in PL emission.
In order to gain more insights of the coupling mechanism, the PL quantum yields have been tested at RT. Figure 5 shows PL relaxations of samples IZO, Ag2 and Ag8. In general, the PL relaxations of the samples are not single-exponential. After fitting the relaxation plot using Eq. (5) (the biexponetial equation), shown in Fig. 5, the PL relaxations could be decomposed into fast and slow decay components.
Fig. 5 The PL relaxations of sample IZO, Ag2 and Ag8. The three solid lines denote the fitting of each decay curve.
During the fitting process, R-square (the most important parameter in evaluating the goodness of fit) of the sample IZO, Ag2, and Ag8 is 98.84%, 98.97%, 99.01%, respectively, indicating that our biexponetial model fits the experimental results very well. As shown in Table 1, the fast decay component of the bare IZO sample is with a constant τ1 of 3 ns and has a contribution of 99.9% to the whole emission profile; its slow decay component has a constant 542 ns and makes a contribution of 0.1% to the emission. It is believed that the fast decay component is related with the structural imperfections [22]. A high fast decay component indicates that there exists a large number of structural imperfections in the bare IZO sample, well agreed with its amorphous structure. For sample Ag2, the contribution of the fast decay component to the emission profile is decreased, which indicates that the fast decay component becomes suppressed, suggesting that the amount of structural imperfections is reduced [22] for sample Ag2. Ag may take an active role in suppressing the structural defects and enhancing photoluminescence for sample Ag2. However, for sample Ag8, the contribution of the fast decay component to the emission profile is increased, indicating that a large amount of Ag will induce more imperfections and lead to a weaker luminescence.
Table 1. The fit parameters of the relaxation plots. The parameters are derived from the equation I(t) = A1exp(-t/τ1) + A2exp(-t/τ2).
4. Conclusion
In this paper, the light-emitting properties of a-IZO without and with Ag embedment are systematically studied. Significant enhancement in light emission is found when the embedded Ag nominal thickness is increased to 2 nm. The bare IZO sample exhibits a two-peak photoluminescence at ~398nm (~3.12eV) and at ~450nm (~2.76 eV), which originates from the optical gap emission (3.13 ± 0.01eV) and the radiative recombination (~2.76 eV) of a photocarrier in the tail states with an oppositely charged photocarrier trapped at the defect level. PL spectra of the Ag embedded samples are also deconvoluted into two resolved peaks at ~398nm and ~450nm. Interestingly, the intensity of the peak at ~398nm does not show obvious change while the intensity of the peak at ~450nm is significantly increased from the bare IZO sample to Ag2 and then decreased from Ag2 to Ag16. The unexpected PL enhancement at ~450nm from IZO to Ag2 is attributed to the coupling between excited electron-hole pairs in IZO films and the LSP of Ag particles; the PL decrease from Ag2 to Ag16 is due to the domination of the attenuation effect from the reflection and absorption since Ag particles gradually merge together for a nominal Ag thickness of 8nm or over.
Acknowledgments
Funding supports from Singapore MoE T2 grant R-284-000-125-112, the Fundamental Research Funds for the Central Universities, China University of Geosciences(Wuhan)(No. CUG140613), the Natural Science Foundation of Hubei Province of China (No. 2014CFB259) and the National Natural Science Foundation of China (No. 61404116) are appreciated.
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