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The metallic-structure dependent localized surface plasmons (LSPs) coupling behaviors with InGaN QWs in a green LED epitaxial wafer are investigated by optical transmission, scanning electron microscopy (SEM) and photoluminescence (PL) measurements. Ag nanoparticles (NPs) are formed by thermal annealing Ag layer on the green LED wafer. SEM images show that for higher annealing temperature and/or thicker deposited Ag layer, larger Ag NPs can be produced, leading to the redshift of absorption peaks in the transmission spectra. Time resolved PL (TRPL) measurements indicate when LSP-MQW coupling occurs, PL decay rate is greatly enhanced especially at the resonant wavelength 560 nm. However, the PL intensity is suppressed by 3.5 folds compared to the bare LED. The resonant absorption and PL suppression are simulated by three dimension finite-difference-time-domain (FDTD), which suggests that Ag particle with smaller size and lower height lead to the larger dissipation of LSP.
©2013 Optical Society of America
Introduction
Although many progresses have been made for GaN-based light emitting diodes (LEDs), an intense efficiency improvement is still essential to high injection level and/or emission under metal contact. Recent works have been used to address the charge separation and low internal quantum efficiency in InGaN-based QWs, namely by using: 1) non/semi-polar quantum wells (QWs) [1, 2], 2) novel QWs with large overlap design [3, 4], and 3) novel materials with reduced interband Auger process [5]. Surface plasmon (SP) coupling with multiple quantum wells (MQWs) for emission enhancement is of great interest for many researchers [6–15]. Since the density of states (DOS) of SP mode is much larger, the SP-MQW coupling rate should be very fast. And this new path of a recombination increases the spontaneous emission rate and therefore the optical output efficiency [6,7]. An enhancement of light emission intensity from InGaN MQW by 17 folds via controlling the energy transfer between QW and surface plasmons has been reported [6]. But such a great enhancement brings some questions as whether the coated metal layer influences the intensity of the pumped laser or not, and what’s the initial quantum efficiency of the sample without metal coating. The localized surface plasmons (LSPs)-enhanced GaN-based blue LED with Ag nanoparticles (NPs) layer embedded in the GaN layer has also been fabricated [8–11]. However, in these work whether the LSPs resonance energy matches the QWs energy remains unresolved and the p-GaN regrowth may be difficult on Ag NPs. LSPs-enhanced GaN-based LED with the deposition of Ag NPs on p-GaN are widely studied, such as the increase of EL intensity and reduction of efficiency droop effect [12–15], which is significant for the application of LSPs-enhanced LED.
LSP can effectively radiate out without momenta matching to photons. It is expected to create an alternative emission channel by coupling LSP with a light emitter in high indium InGaN QWs, where the radiative recombination rate is very low because of low crystal quality and band bending effect. However, LSPs-MQWs coupling does not mean the improvement of luminous efficiency of LED [16–19]. When SP is coupled with dipole in active layer, the radiation rate becomes high while the emission fraction may be low [16]. Emission enhancement via SP-dipole coupling is determined by proportion of non-radiative recombination, SP strength and phase retardation of dipole to SP [17]. The non-radiative energy will finally be dissipated by free electron oscillation and the creation of electron-hole pairs via either intraband excitations within the conduction band or interband transitions from lower-lying d-bands to the s-p conduction band [18]. Although the feedback effect of SP on the dipole behavior has been considered in the coupling, SPs-multiple dipoles coupling in InGaN QWs is not clear yet. The multiple SP-dipoles coupling is expected to cause coherent emission [17]. Moreover, the coupling of LSP to localized and delocalized carriers in InGaN QWs is also under investigated [20]. So it is reasonable to suppose that most carriers’ energies in InGaN QWs are transferred into LSP by LSP coupling with MQW. And then the energies coupled to LSP are scattered out and/or dissipated in Ag NPs. The proportion of scattered energy to dissipated energy of LSP determines emission enhancement or suppression. Therefore, the scattering suppression mechanism of LSPs deserves intensive study to effectively extract the coupled energy of LSPs to light emission.
On the other hand, the energy matching between bandgap energy of the QWs and the resonance energy of LSP is essential to realize the SP-enhanced LED. Several recent works have reported on various methods to engineer surface plasmon resonant frequency for optimizing the Purcell Factor in the visible wavelength spectrum. The engineering of surface plasmon frequency can be achieved by engineering the metallic NP sizes [21], by varying the thicknesses of double metallic layers [22], and by varying the periodicity in 2-D lattices [23]. The use of double metallic layers has also been used for achieving improvement in Purcell factor in green-emitting InGaN QW LEDs [22]. Apart from these methods, the morphology of metallic NPs also determines the resonance energy of a generated LSPs [24, 25]. It is worth studying the morphology effect of Ag NPs on LSPs coupling [12, 14, 19]. In this paper, the LSPs phenomena for annealed Ag layer on InGaN/GaN MQW structure are investigated by scanning electron microscopy (SEM), optical transmission, photoluminescence (PL) and time-resolved PL (TRPL) measurements. To illustrate the phenomena of LSPs-MQW coupling, the LSPs resonance of Ag NPs with light incidence is analyzed by the 3D FDTD simulation. The effects of diameter, height, Ag2O coating of Ag NPs and Ag residual layer are studied on the LSP absorption and scattering.
Experimental
The LED epitaxial structure with 40 nm-thick p-GaN layer was grown by metal–organic chemical vapor deposition (MOCVD) on the double polished c-plane sapphire substrate. 5, 8 and 10 nm thick Ag layer were deposited on the LED epitaxial wafers by e-beam evaporator, respectively. Then these samples were thermal annealed with different conditions: 200 °C annealing for 30 min, and 300/400/500/600 °C annealing for 10 min, respectively. The SEM images were taken for the annealed Ag layers on p-GaN by FEI NanoSEM 430. The optical transmission spectra were measured, with grating spectromete resolution by 1 nm, for these LED samples. In addition, the room temperature PL measurements were performed by a 405 nm laser, with the configuration of bottom (sapphire) excitation and bottom PL signal collecting. Since the laser excitation power density was 50 W/cm2, the carrier density in the active region was estimated as 1 × 1018/cm3. TRPL measurements were performed by LifeSpec-Red Picosecond Lifetime Spectrometer equipped with a pulsed 372 nm laser, with a pulse duration by 69 ps. And since the laser excitation power density was 155 W/cm2, the carrier density in the active region was estimated as 2.9 × 1018/cm3. The experiment configuration was similar to that of room temperature PL measurements. This instrument was made use of the time-correlated single photon counting (TCSPC) techniques with a resolution of 19.531 ps within a range of 20 ns, and the peak photon number was set as 3000.
On the other hand, the full 3D FDTD simulation was performed to illustrate the electric field distribution of Ag NPs. PML boundary condition was used. The simulation area was 2*2*2 um. Since the minimum spatial size (grid size) should be small enough to resolve the field distribution, the spatial resolution was set to be 1 nm, which was confirmed by the shift of the absorption peak no more than 5 nm. The single Ag spherical cap was placed on top of p-GaN (n = 2.55). And the height of the spherical cap was changed from 1 to 2 times the length of radius. The enveloped Ag2O film and the residual Ag film were introduced to the system, respectively. The source (λ = 300~800 nm) was a total-field scatter-field (TFSF) source which could give out plane wave in a region without diffraction. The absorption, dissipation and scattering energy through the LSP of Ag NP on p-GaN were calculated.
Results and discussion
The transmission spectra for the six Ag structures on the LED epitaxial wafer are shown in Fig. 1. The Ag structures are formed by annealed 10 nm-thick Ag film under different temperature. And the incident light ranges from 300 to 800 nm. The baseline is obtained from the transmission spectrum of the bare LED wafer on polished sapphire substrate. And the interference effect is modified by the envelope method [26]. The main resonant absorption peaks can be obviously observed in most curves except the one after 600°C annealing in Fig. 1(a). In the case of 600 °C annealing, the absorption peak maybe locates in the infrared region above 800 nm. The absorption of as deposited Ag layer sample across the whole spectra range is presented, and a relative small dip in the transmission curve can be observed as well. The second dips in Fig. 1(a) can also be observed by some Ag structured samples annealed below 400 °C. The transmissions are recovered to 1, even above 1 beyond the absorption regions for the annealed samples. In the absorption region, the interference peaks become very weak since the high absorption coefficient makes interference in thin GaN film neglected [27]. For convenience, the absorption peak, full width of half maximum (FWHM) of the peak, and peak intensity are drawn from Fig. 1(a), and dependences of them on the annealing temperature are plotted in Fig. 1(b). The absorption peak is blue shifted with the increase of thermal annealing temperature below 300 °C, while it is red shifted as the annealing temperature is above 300 °C. The dependence of FWHM on the annealing temperature shows the similar trend. And the peak intensity is increased monotonously with annealing temperature.
Fig. 1 (a) Transmission spectra of Ag structures on the LED epitaxial wafer formed by annealed 10 nm Ag film under different temperature. (b) Absorption peak, FWHM and peak intensity in transmission spectra of Ag structured samples as a function of temperature.
Figure 2 shows the top-view SEM images of the as deposited and annealed 10 nm-thick Ag layer. As deposited Ag layer is a kind of thin film which includes many flat large particles. The process of deposition of thin metal film is always accompanied with the discontinuous distribution of crystal nucleus and nanoparticles, whose sizes are related to the deposited method and evaporation condition. The size of Ag particles is about 60-70 nm, and the height is nearly the same as that of the Ag layer. So the absorption peaks at 559 and 425 nm correspond to LSP resonance at the interface of flat Ag particles and GaN, and SP resonance at the interface of Ag film and GaN [14], respectively. The absorption coefficient of the continuous Ag film is also large in the range of 300-800 nm. When the Ag layer is annealed at 200 °C, small Ag semi-ellipsoid particles appear, with the average diameter of 60.1 nm and height of 18.4 nm (measured by cross-sectional SEM). Such spontaneous process results from thermal annealing is called Ostwald ripening [28], which allows the mass transport of small particles across the surface to form large ones to lower the total surface energy. When the temperature is increased to 300 °C, the Ag NPs become hemispheroid, with the average diameter decreased a bit to 57.0 nm, while the height sharply increased to 36.9 nm. When the temperature is above 300 °C, the average diameter and height of Ag hemispheroid particles become larger with increasing thermal annealing temperature. The shape is irregular as the annealing temperature is above 500 °C. It is obvious that the average size of Ag particle is increased when the annealing temperature is higher. It is beneficial to reduce the surface energy by the shape change from flat to semi-ellipsoid and hemispheroid with the increase of annealing temperature. Some researchers have reported that the absorption peak redshifts and its intensity and FWHM are increasing with diameter of Ag NPs through LSP resonance and scattering [18, 19, 29]. The lower peak intensity of the as deposited Ag layer is due to small LSP resonance for the small aspect ratio of Ag particles. The second dips at shorter wavelength in Fig. 1(a) mean high order SP mode resonance occurring. For Ag structured samples annealed above 400 °C, second dips maybe exist with a wavelength shorter than that of band edge of GaN. It is notable that the transmission is above 1 beyond the absorption region for the samples annealed under 300°C. It is believed to the larger ratio of the height to the diameter of Ag particles, which will be discussed in the following.
Fig. 2 Top-view SEM images of six Ag structures on the blue LED epitaxial wafer, including (a) a 10 nm-thick as deposited Ag film, thermally annealed nanostructures of the Ag film under (b) 200°C for 30 min, (c) 300°C for 10 min, (d) 400°C for 10 min, (e) 500°C for 10 min, and (f) 600°C for 10 min.
Using FDTD method, the resonant absorption by LSPs in the Ag NPs is simulated. According to the cross-sectional SEM images of Ag structures and discussion above, the shape of these annealed Ag NPs is spherical cap. An aspect parameter α = h/r is used to characterize the shape of particles, where h represents the height of spherical cap and r represents the radius. In the images of cross-view SEM, α is around 0.3~1.66. The schematic structure of the Ag-particles on GaN is shown in Fig. 3. The red frame is TFSF source with a direction vertical to the GaN/vacuum interface, and the two yellow frames are energy receiver monitors. The monitor outside the source can gather scattering energy, and the one between the source and Ag particle can receive thermal dissipation energy in metal material.
Fig. 3 The schematic structure of the Ag-particle on GaN in 3D FDTD simulation. The red frame corresponds to the TFSF source. The yellow ones are the monitors for scattering and dissipation of the electromagnetic wave.
The dependence of absorption spectra on α of Ag particle with a diameter of 60 nm on 3 nm-thick residual Ag film is presented in Fig. 4(a). There are 2 series absorption peaks. The left peak is around 370 nm which corresponds to the high order LSP mode. And the right peak is red shifted as increasing of α. Besides, as the diameter of Ag NP increases, the absorption peaks red shift and become wider and stronger at the same α, which conforms to the experimental results. However, for the Ag NP with a diameter of 60 nm without Ag2O coating or residual Ag layer in Fig. 4(b), the wavelength of absorption peak is longer than 550 nm at α = 1.5, while the experimental result shows that it would be around 500 nm. Tominaga et al have reported that the silver oxide plays an important role in optical near-field and surface plasmon [30]. They believe the Ag2O phase is the most stable phase in all the silver oxide. In this work, small amount of O2 leaking into the thermal furnace will lead to Ag2O covering on Ag particles under high temperature. Therefore, 5 nm thick Ag2O cover layer is introduced into the system where the Ag particle’s diameter is still 60 nm. The left absorption peaks sharply red shift to about 460 nm. However, there is only a slight shift for the right absorption peaks in Fig. 4(b). Since the experimental transmission results just indicate the second peaks locates around 370 nm, the influence of Ag2O on peak wavelength can be neglected after thermal annealing.
Fig. 4 (a) The dependence of absorption spectra on α of Ag NP with a diameter of 60 nm on 3 nm-thick residual Ag film. (b) The right absorption peaks of 60 nm-diameter Ag NP with different accessorial structures are plotted against α.
On the other hand, a raw calculation suggests that 3 to 5 nm thick residual Ag film still exists on the p-GaN after annealing according to the size and density of Ag NPs. Figure 4(b) shows the right absorption peaks against α with different thickness of residual Ag layer. Figure 4(a) indicates that when α is 1.66, the right absorption peak locates at 520 nm. As the residual Ag film is increased to 5 nm-thick, the right absorption peaks are blue shifted to 503 nm. It is well conformed to the experimental results. The as deposited Ag film with thicker residual layer shows redshift of the right absorption peak, which can be attributed to the smaller α.
By now, it can be concluded that the diameter of Ag particle, the height of spherical cap and the residual thickness of Ag film determine the location of the absorption peaks. Transmission spectra of Ag structures formed by 5 and 8 nm-thick Ag film on the LED epitaxial wafer under different annealing temperature present the same regularity. And with the increase of deposited Ag film thickness at the same annealing condition, the formed Ag NPs become larger and the density of Ag NPs increases as well. Reflected in the transmission spectra, a redshift of absorption peak can be observed, and the absorption becomes stronger as more Ag NPs are formed. However, the residual Ag film will be thickened, which will affect the oscillation strength in LSP reversely.
The location of absorption peak is corresponding to the LSPs resonance absorption wavelength. There is an optimum thermal annealing condition that LSPs resonance energy matches the MQW emission energy. LSPs-MQW coupling will be more effective for green LED due to its low internal quantum efficiency [19]. Therefore, a green LED with emitting peak around 543 nm is adopted. A 40 nm-thick p-GaN layer is capped on InGaN/GaN MQWs. Then an 8 nm-thick Ag film is deposited on the LED wafer, and annealed at 500 °C for 10 minutes in N2 ambient. The absorption peak for this Ag-coated LED is measured as 560 nm, which is reasonable compared to 628 nm for 10 nm-thick Ag layer annealed at the same condition.
Figure 5 shows three PL spectra measured from the uncoated region of the LED wafer, the region coated with Ag NPs and the region coated with 8 nm-thick Ag layer. It is found that the as deposited Ag film on LED wafer intensely suppresses the PL intensity by 5.1 folds of the bare LED sample, and shows a blue-shift of 1 nm compared to the bare one. A slight resonant peak at 548 nm for this sample can be obtained from the transmission spectrum. It is also observed that the PL intensity of Ag NPs sample fabricated by 500 °C annealing for 10 min is suppressed by 3.5 folds of the bare LED sample, and the peak wavelength red shifts to 546 nm from 543 nm. Since most carriers’ energies in InGaN QWs are transferred into LSP by LSP coupling with MQW, and the energies coupled to LSP would be finally scattered out and dissipated in Ag NPs. The proportion of dissipated energy to scattered energy of LSP determines the emission enhancement or suppression. Therefore, the PL intensity suppression is attributed to the large proportion of dissipation energy of LSP caused by free electron oscillation and the creation of electron-hole pairs [18]. As to the as deposited Ag film sample, the larger PL suppression is due to the matched resonant wavelength (543 nm vs. 548 nm) and weak scattering ability of the flat particles. The blue-shift is caused by the resonant wavelength of 548 nm located in the left of the PL spectrum. For 500°C annealing sample, although the resonant wavelength is not well matched and located at the right side of the PL spectrum, the diameter and height of Ag NPs are larger than those of as deposited one. Then the resonant absorption and scattering will be enhanced simultaneously. However, the scattering is not large enough to eliminate the effect of dissipation in LSP. So the suppression of PL intensity is still as large as 3.5 folds. Although LSPs resonance absorption energy matches the MQW energy, and the MQW energy could rapidly transfers to LSPs, such LSPs energy fails to be extracted into air effectively and loses, which lead to the PL suppression. The absorption, scattering and dissipation mechanism by Ag particles will be further discussed with 3-FDTD simulation in the following.
Fig. 5 PL spectra for the bare LED wafer, LED wafers coated with Ag NPs and 8 nm-thick as deposited Ag film, respectively.
Figures 6 show the results of TRPL measurements on the bare and annealed Ag-coated LED samples. The decay curves can be fitted to double exponential function, as shown in Fig. 6(a). The fast decay stage is attributed to the rapid carrier recombination in InGaN wells. The rapid processes include recombination through more overlapped electron and hole wave functions and/or LSPs-MQW coupling. The slow decay stage corresponds to localized carrier recombination which is dominated by quantum confined Stark effect [19, 31, 32]. The fast decay times (τPL) at 560 nm are obtained as 0.19 and 1.03 ns for the Ag-coated LED and the bare LED, respectively, which indicates a PL decay rate enhancement by 6.2 folds for the Ag-coated LED through LSPs-MQW coupling. Figure 6(b) depicts the PL fast decay time for both LED samples against wavelength. And it apparently shows the enhancement of decay rates at longer wavelengths for the annealed Ag-coated LED. It indicates that carrier recombination through LSPs-MQW resonant coupling is much faster than other recombination processes. Therefore, the emission suppression and recombination rates enhancement of the annealed Ag-coated LED are attributed to the MQW- LSP coupling effect.
Fig. 6 (a) Time dependent PL intensity at 560 nm of bare GaN and sample coated with annealed Ag layer. (b) The fast decay times, deduced from time-resolved PL, of the bare sample and the Ag-coated sample are plotted against wavelength.
In order to explore the PL suppression mechanism, the electric field distribution after excited by the TFSF source (λ:300-800 nm) is also simulated with FDTD method, as shown in Figs. 7. Two different diameter of the Ag NP are chosen, which are set as 60 and 150 nm, respectively. And the residual Ag film is set as 3 nm. After excited, the absorbed energy by LSP in Ag NP will be scattered out or dissipated. Figure 7(a) shows the dissipation and scattering electric field intensities against wavelength after excited by the TFSF source for 60 nm-diameter Ag NP with different α, and 3 nm thick residual Ag film is added to the system. With the increase of α, the blueshift of main dissipation and scattering peaks can be observed, while the high order peaks are almost fixed with a slight redshift. It is notable that the scattering electric field intensity by Ag NP is much smaller than that of dissipation. The small size causes strong resonant absorption and less light extraction, which leads to the suppression of PL. The images of electric field distribution in the 60 nm-diameter Ag NP are also acquired by FDTD, as shown in Fig. 7(b). It is observed that most energy is confined in the core of the Ag NP. With the increase of α, the electric field is strengthened on the top surface of the spherical cap, which can be radiated out more easily.
Fig. 7 (a) Dissipation and scattering intensities and (b) electric field distribution for Ag NP with 60 nm-diameter, α = 1, 1.33, 1.66. (c) Dissipation and scattering intensities and (d) electric field distribution for Ag NP with 150 nm-diameter, α = 1, 1.46, 1.93. The residual Ag films for both models are 3 nm-thick.
Contrarily to the interaction between the light and LSP in 60 nm-diameter Ag NP, the scattering intensity is much larger than the dissipation intensity for 150 nm-diameter Ag NP, as shown in Fig. 7(c). It is also found the blue shift of the main peaks and the red shift of the high order peaks with the increase of α. As a full Ag spheroid (α ≈2), the main peak shifts to 580 nm and high order peak shifts to 420 nm. When α is smaller than 1.4, the main peaks locate in the range above 760 nm. Figure 7(d) shows the images of electric field distribution in the 150 nm-diameter Ag NP. Most electric field distributes on the top surface of the spherical cap, which can be extracted as photons to air easily. This result means the energy absorbed by Ag NP can be easily scattered out when the diameter of the particle is as large as 150 nm. However, the scattering ratio to the whole absorption energy is determined by the diameter and height of Ag particle. It is reasonable that Ag NP with larger diameter and α may draw the absorbed energy and scatter out in the same emission region easily.
On the other hand, the FDTD simulation on the Ag NP without residual Ag film is also performed. The ratio of the scattered energy to the dissipation one is increased compared to the samples with residual Ag film. The simulated results show that the high order peak reaches to 470 nm when D = 150 nm and α = 1.5. These results mean that the high order emission wavelength of Ag NP can be tuned to the blue band for white LED packaging. The efficiency enhancement of green LED by Ag NPs has been reported by many groups [14, 24, 29]. Since large Ag NPs lead to high scattering intensity, and the high order peak can be tuned to the blue band, it is promising to utilize the high order peak to realize the efficiency enhancement of blue LED. These results are deduced from the coupled LSP without considering SP-QW coupling process. When the emission of dipole is affected by LSP resonance, the resonant wavelength will be blue shifted more than 10 nm [17]. And the coherent emission shows more light output when considering the coupling of LSP and multiple dipoles.
In this paper, the Ag-based nanostructures discussed are realized by using rapid thermal annealing method, which could not control the morphology of Ag NPs precisely, however. It is important to note that recent works have reported the fabrication of 2-D nanostructures via diblock copolymer lithography [33, 34] and colloidal-based self-assembled rapid convection deposition [35], resulting in highly-ordered nanostructures arrays. And such application to fabricate the metal nanostructures arrays deserves further investigation.
Conclusion
To summarize, annealing method has been used to fabricate Ag NPs with different diameters and heights. And the coupling of LSPs and MQW are studied by transmission and PL measurements. Different light resonant absorption peaks for the Ag NPs are observed under different thermal annealing temperatures. The PL decay rate is enhanced by 6.2 folds for the resonant structures at 560 nm through the coupling of LSPs and MQW. FDTD simulation proves the coupling of the LSPs and MQW when appropriate size and height of Ag NPs are chosen. The scattering suppression is due to smaller size and lower height of spheroid Ag NPs. Ag NPs with larger diameter and height can engineer the plasmonic resonance wavelength and enhance the light scattering out.
Acknowledgment
This work was supported by projects of National Key Basic Research Special Foundation of China under Nos. TG2011CB301905, TG2012CB619304 and Natural Science Foundation of China under Nos. 60876063, 61076012. The authors are grateful for the advices of professor Ying Gu.
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