GaN Heterojunction LEDs Achieved by Coupling

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Functional Inorganic Materials and Devices

Color-Tunable ZnO/GaN Heterojunction LEDs Achieved by Coupling with Ag Nanowires Surface Plasmons Liu Yang, Yue Wang, Haiyang Xu, Weizhen Liu, Cen Zhang, Chunliang Wang, Zhongqiang Wang, Jiangang Ma, and Yichun Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00940 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Materials & Interfaces

Color-Tunable ZnO/GaN Heterojunction LEDs Achieved by Coupling with Ag Nanowires Surface Plasmons Liu Yang,a Yue Wang, a Haiyang Xu,* a,b Weizhen Liu,* a,b Cen Zhang, a,b Chunliang Wang, a,b Zhongqiang Wang, a,b Jiangang Ma a,b and Yichun Liu* a,b a

Centre for Advanced Optoelectronic Functional Materials Research and Key Laboratory of UV-Emitting Materials and Technology (Northeast Normal University), Ministry of Education, Changchun 130024, China. b National Demonstration Centre for Experimental Physics Education (Northeast Normal University), Changchun 130024, China *E-mail: [email protected], [email protected] and [email protected]

Abstract Color-tunable light-emitting devices have great impact on our daily life. Herein, LEDs with tunable electroluminescence (EL) color was achieved via introducing Ag nanowires surface plasmons into p-GaN/n-ZnO film heterostructures. By optimizing the surface coverage density of coated Ag nanowires, the EL color was changed continuously

from

temperature-variable

yellow-green fluorescence

to emission

blue-violet.

Transient-state

characterzations

uncovered

and that

spontaneous emission rate and internal quantum efficiency of the near-UV emission were increased as a consequence of the resonance coupling interaction between Ag nanowires surface plasmons and ZnO excitons. This effect induces the selective enhancement of blue-violet EL component but the suppression of defect-related yellow-green emission, leading to the observed tunable EL color. The proposed strategy of introducing surface plasmons can be further applied to many other kinds of LEDs for their selective enhancement of EL intensity and effective adjustment of emission color. Keywords: color-tunable electroluminescence, p-GaN/n-ZnO heterostructures, Ag nanowire surface plasmons, exciton-plasmon resonance coupling, defect-related emission

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1. Introduction Wavelength-tunable light-emitting devices are very important to people’s daily life. For example, realization of full-color display relies on high-efficiency semiconductor LED array with tunable emission wavelength in the visible range.1-3 Indoor illumination requires electrically-driven light emitters with mild color temperature and good color rendering property.4,5 Therefore, electroluminescence (EL) devices with tunable emission wavelength and improved EL intensity are highly desired and worthy of further development. Wide-band-gap semiconductor ZnO (direct band gap: 3.37 eV, exciton binding energy: 60 meV) is usually taken as the active emissive layer for short-wavelength UV LEDs.6-8 To extend the EL wavelength range of ZnO-based LEDs to visible region, colloidal nanocrystals such as CdTe and CdSe are utilized to decorate ZnO nanostructures, by which broad-band visible emissions appear in the EL spectra of these inorganic nanocrystal decorated LEDs.9,10 Besides, element doping or defect engineering is also used to shrink the effective optical-band-gap of ZnO via producing intermediate state levels, which enables the ZnO active layer to emit visible light.11,12 However, these methods will either complicate the device structure or degrade the device performance as a result of the impurity/defect induced non-radiative process. Surface plasmon (SP) is a kind of novel elementary excitation mode. At the metal/semiconductor (or metal/dielectric) heterointerface, plenty of free electrons will form a collective oscillation mode when they are stimulated by incident electromagnetic waves.13-15 With suitable resonance energy, metal SPs can be employed as optical antenna to selectively enhance the luminescence intensity of partial wavelength band, achieving the emission wavelength adjustment of a light emitter.16-18 However, such an effective strategy has rarely been adopted to fabricate color-tunable LED device. In this work, wavelength-tunable LEDs were demonstrated based on Ag-SPs enhanced p-GaN/n-ZnO film heterostructure. By introducing different densities of Ag nanowires (NWs) into the LED devices, the EL color can be continuously tuned from yellow-green to blue-violet. The reason for the EL wavelength tunability is attributed ACS Paragon Plus Environment

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to the effective enhancement of blue-violet emission, which is caused by the resonance coupling effect between Ag-NWs SPs and ZnO excitons. Time-resolved and temperature-dependent fluorescence studies support the argument for this physical mechanism.

2. Experimental Section In the current work, n-ZnO films (Hall mobility: 20 cm2V-1s-1, electron concentration: 9.54×1017 cm-3) with a thickness of ~30 nm were deposited on the p-GaN: Mg wafers (Hall mobility: 13 cm2V-1s-1, hole concentration: 2.7×1017 cm-3) by plasma-assisted pulsed laser deposition. Please refer to our previous reports for the experimental details on preparing ZnO films.19 Herein, the layer thickness of ZnO film is intentionally designed as 30 nm, because it also serves as a coupling-spacer layer for the subsequent SP enhancement effect (more discussion can be found in Supporting Information Note 1). For the SP-decorated devices, an ethanol solution containing 0.02 mg/mL Ag-NWs (purchased from Sigma-Aldrich) was spin-coated (rotation speed: 800 rpm, duration time: 5 minutes) onto the ZnO film surfaces. By controlling the volume of spin-coating droplets, the amount of the coated Ag-NWs can be easily adjusted. Herein, four groups of Ag-NWs-covered ZnO film with surface coverage density of S1=11.3×106 cm-2, S2=15.2×106 cm-2, S3=23.8×106 cm-2 and S4=40.5×106 cm-2 were prepared. Next, metals Au/Ni and In are respectively evaporated onto the n-ZnO films and p-GaN wafers as contact electrodes for current injection. Meanwhile, a bare p-GaN/n-ZnO LED without Ag-NWs covering was also fabricated in parallel for comparison (see Figure 1a for schematic structure diagram of the bare PN junction diode). It should be emphasized that all spectral measurements are carried out under the same conditions.

3. Results and Discussion Figure 1b exhibits the current-versus-voltage (I-V) plots of the bare PN heterojunction, which exhibits a typical rectification transport behavior. The turn-on voltage of the device is around 6.5 V, which is a little bit higher than the band-gap of ACS Paragon Plus Environment


GaN (3.40 eV) and ZnO (3.37 eV). This may be because of the relatively large electrode contact resistances, which shall share a considerable amount of applied voltage. The contact resistance values for In/n-ZnO (~350 Ω) and Au/Ni/p-GaN (~1900 Ω) can be respectively calculated according to the inset of Figure 1b. Thus, further reducing the electrode contact resistances by adopting other types of metal electrode or suitable thermal annealing process may be beneficial to reducing the apparently large turn-on voltage. Besides, the relatively large bulk resistivity (10 Ω·cm) of p-GaN: Mg substrates due to their low hole concentration, will also share a certain amount of the applied voltage and increase the measured turn-on voltage value. Hence, searching for p-GaN wafers with higher conductivity is also helpful for lowering the device turn-on voltage. The inset in Figure 1b displays the near-linear I-V feature between two In electrodes or Ni/Au electrodes, suggesting that these metal electrodes have generated ohmic contacts with the n-ZnO films or p-GaN wafers. Photoluminescence (PL) emission spectra of the n-ZnO film and p-GaN wafer, measured at room temperature (RT), are displayed in Figure 1c. As is shown, two emission bands can be distinguished in the n-ZnO film PL spectrum. One is a UV emission band peaked at ~380 nm, originating from the radiative recombination of ZnO near-band-edge (NBE) excitons.20 The other is a visible emission band centered at ~560 nm, resulting from the defect-related radiative transitions.21,22 The p-GaN wafer exhibits a blue emission PL spectrum with a peak value located at ~426 nm. This blue light emission should be attributed to the Mg-acceptor-related radiative transitions in p-GaN: Mg.23,24 To study the electrically-driven luminescence properties of this p-GaN/n-ZnO LED device, EL measurements are conducted and relevant spectra data are illustrated in Figure 1d. As we can see, although the overall EL intensity is improved with increasing injection current, the EL profile doesn’t change significantly. By comparing with the above PL spectra, the luminescence origins of these EL spectra can be assigned as the ZnO NBE recombination at ~382 nm, GaN Mg-acceptor transition at ~425 nm and a wide ZnO defect-related radiation band at ~560 nm (see Supporting Information Figure S1 for more discussion). As can be seen, all EL spectra from this bare PN junction LED are ACS Paragon Plus Environment

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dominated by the yellow-green light emission. This phenomenon has also been observed by other groups, and is presumably because of the stronger defect/impurity-related transitions occurring in the ZnO side (see Supporting Information Note 2 for more discussion).22,25,26 To better comprehend the transport and recombination dynamics of charge carriers, Figure 1e plots the energy band alignment of this p-GaN/n-ZnO heterojunction under thermal equilibrium conditions. To effectively tune the EL wavelength/color of the LED, the Ag-NWs decoration is introduced into the device design. Figure 2a to 2d show typical scanning electron microscopy (SEM) images of the covered Ag-NWs with four varied surface densities. One can see that these Ag-NWs are uniformly distributed on the ZnO film. Their mean diameter is 65 nm and average length is 7 µm. As the volume of spin-coating droplet varies from 8 to 32 µL, the surface density of the Ag-NWs is gradually increased from S1=11.3×106 cm-2 to S4=40.5×106 cm-2. To explore the optical properties

of

these

Ag-NWs,

Figure

2e

presents

the

measured

optical

absorption/extinction spectra of these Ag-NWs. As the surface coverage increases, the SP resonance intensity of these Ag-NWs increases, and no notable change is observed in the spectral width and peak position, suggesting that the coupling among Ag-NWs SPs is negligible even at relatively high surface density of S4=40.5×106 cm-2.27,28 In addition, only the radial mode of Ag-NWs at ~385 nm is observed, whereas the longitudinal extinction mode at longer wavelength does not show up. This is because the metal NWs employed here are relatively long. Along the NW length direction, the longitudinal SP mode propagates and is eventually dissipated as Joule heat.14,27 As can be seen, the radial SP extinction mode covers a broad wavelength region from blue to UV with a maximum value peaked at ~385 nm (see Supporting Information Figure S2 for more discussion on the Ag NW radial SP mode). This feature indicates that the EL emission in the blue-violet region can be selectively and effectively enhanced if these Ag-NWs are employed as the SP resonance platform. Figure 2f demonstrates the fluorescence emission spectra of the Ag-NWs-covered ZnO films (denoted as ZnO@Si, i = 1, 2, 3 and 4) at RT. In comparison with the bare ZnO film, Ag-NWs-covered ZnO films show similar visible emission intensity but markedly ACS Paragon Plus Environment


enhanced UV emission intensity, due to the resonance interaction between metal SPs and semiconductor excitons. As the surface density of Ag-NWs increases, the UV PL intensity is gradually enhanced, because more and more Ag-NWs SPs participate in the resonance interactions with ZnO excitons. It is found that the optimized surface density of Ag-NWs is S3. Further increasing the surface coverage to S4 only results in the reduced PL intensity. This is because excessive Ag-NWs overloaded on the ZnO film decrease the overall light-extraction-efficiency.29,30 Such a selective enhancement promises an efficient way for adjusting the EL spectral profile and LED color. Figure 3a shows the structure diagram of the Ag-NWs-decorated LEDs, in which the ZnO film are covered with the Ag-NWs of varied surface coverage. To investigate the electrical transport properties of these Ag-NWs-decorated devices, I-V curves are measured and depicted in Figure 3b. It is noted that injection current of the devices with Ag-NWs is larger than that of the one without Ag-NWs under the same bias voltage. Further, the injection current gradually rises when the coverage density of Ag-NWs increases from S1 to S4. This is because the introduction of metal NWs network can form lateral conductive paths and effectively improve the device conductivity.27,31 When the metal NW network is introduced into the device, more leakage passages will be established and the device would be more conductive.32,33 For a better and fairer comparison, the EL spectra of all studied LEDs are collected under the same level of injection current value. Figure 3c exhibits the RT EL spectra of the Ag-NWs-decorated LEDs (denoted as PN@Si, i = 1, 2, 3 and 4) at a 6 mA injection current. Different from the bare device dominated by an obvious yellow-green light emission (see the dashed line in Figure 3c), Ag-NWs coated LEDs show a gradual increment in the blue-violet EL intensity but a decrement in the yellow-green emission intensity with increasing coverage density of Ag-NWs. As mentioned in the beginning, the p-GaN/n-ZnO EL spectrum can be decomposed into several emission sub-bands. To figure out the specific change of every EL sub-band after the introduction of Ag-NWs SPs, Gaussian deconvolution fitting is performed for these spectra. Figure 4a shows the detailed spectral fitting results. For the bare PN LED, the whole EL band is predominated by the ACS Paragon Plus Environment

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defect-induced yellow-green emission, while the blue-violet emission intensity is very weak. As the Ag-NWs are coated onto the device, the EL intensity of ZnO NBE emission, as well as that of GaN Mg-acceptor emission, is gradually increased and a maximal blue-violet light intensity is obtained at the optimized Ag-NWs density of S3. Further elevating the surface coverage of Ag-NWs only leads to the weakening of EL intensity, as a result of the above-mentioned reduction effect of light extraction efficiency (see Supporting Information Figure S3 for more discussion). On the contrary, the emission intensity of defect-related yellow-green light is decreased remarkably after the introduction of Ag-NWs. This “wane and wax” phenomenon of different EL components can be understood as follows. Internal quantum efficiency and spontaneous emission rate of ZnO NBE emission are obviously improved thanks to the SPs-excitons coupling, which will be proved and discussed in detail later. In this case, plenty of band-edge states could be emptied out rapidly, and could further be utilized to populate injected charge carriers. Therefore, more and more injected carriers occupy the vacant band-edge states and participate in the process of NBE radiative recombination, giving rise to the observed ZnO NBE emission enhancement. In addition, the GaN Mg-acceptor related blue emission is also improved owing to the partial spectrum overlapping between Ag-NWs SPs extinction and the broad GaN emission band. Taken together, the effective improvements of ZnO NBE emission and GaN Mg-acceptor emission contribute to the selective improvement of the blue-violet EL intensity. On the other hand, because most injected carriers are involved in the process of band-edge recombination, less and less carriers will take part in the defect-related deep-level emission from the viewpoint of conservation of matter. Hence, the yellow-green EL intensity becomes weaker and weaker. In fact, the opposite variation trends between long- and short-wavelength emission intensities just make the observed EL color change dramatically from yellow-green to blue-violet (see the EL photographs of these LEDs in Figure 4a. More importantly, the simultaneous occurrence of ZnO near-UV EL enhancement and defect-related yellow-green emission weakening confirms that the resonant coupling effect mainly takes place between ZnO excitons and Ag-NWs SPs, which makes a primary ACS Paragon Plus Environment


contribution to the observed color-tunable EL spectra. The variation tendency of each EL component is respectively illustrated in Figure 4b for clarity. It is clear that the ZnO defect-related yellow-green light emission demonstrates a monotonically decreasing trend, but the ZnO NBE UV and GaN Mg-acceptor blue emission bands show maximum spectral weights at the optimized Ag-NWs density of S3=23.8×106 cm-2. Figure 4c exhibits the enhancement ratios of every EL sub-band as a function of Ag-NWs surface density. Obviously, the enhancement ratio of ZnO NBE emission is the highest among all three EL components/sub-bands. Detailed reasons are discussed as follows. As shown in Figure 5a, the ZnO NBE emission is located at the near-UV region, very close to the Ag SPs extinction wavelength at ~385 nm. Thus, the spectrum superposition between Ag SPs extinction and ZnO NBE emission is the maximal, ensuring the most effective coupling interaction of ZnO and Ag. By comparison, GaN Mg-acceptor blue emission is farther away from the Ag SPs peak, leading to only partial spectral overlap. Hence, the resonant coupling between GaN and Ag is weaker than that between ZnO and Ag, giving birth to relatively weak GaN emission enhancement. As for the ZnO yellow-green EL band, it is too far away from the Ag SPs resonance peak and can hardly be enhanced. To further prove these arguments, simulated electric field intensity distributions between a single Ag-NW and ZnO/GaN heterojunction are demonstrated in Figure 5b to 5d. Herein, the simulation strategy is finite-difference time-domain (FDTD) approach. The analogue model was built according to the practical PN junction device configuration. A dipole light source was put at the ZnO/GaN heterojunction area, corresponding to the active emissive layer of charge carriers. As can be seen, field intensity distribution of Ag-NW SPs reaches the strongest at a excitation wavelength of 382 nm (see Figure 5b), corresponding to the ZnO NBE exciton emission. As the excitation wavelength shifts to 425 nm (GaN Mg-acceptor emission) and 560 nm (ZnO defect-related emission), the observed field intensity becomes weaker and weaker, as shown in Figure 5c and 5d. The simulation results are in good consistency with the former experimental analyses, which unambiguously proves that coupling effect of Ag-NWs SPs and ZnO excitons is the ACS Paragon Plus Environment

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most effective. Due to this efficient excitons-SPs resonant coupling, the near-UV EL emission is notably improved while the yellow-green broad-band emission is simultaneously suppressed, achieving the tunable EL color from yellow-green to blue-violet. To further study the chromatic properties of these PN junction LEDs, chromaticity coordinates of the above EL spectra are shown in Figure 6a according to the Commission Internationale de L’Eclairage (CIE) 1931 standard. As is shown, the chromaticity coordinate almost spans across the color space map from yellow-green to blue-violet region with increasing surface coverage of Ag-NWs. Especially, when the surface density of Ag-NWs is at S2=15.2×106 cm-2, the LED device demonstrates a near white light emission with chromaticity coordinate of (0.317, 0.324), very close to the standard white light of (0.330, 0.330). Figure 6b illustrates the correlated color temperature (CCT) of these Ag-NWs-decorated devices, which exhibits a broad CCT range from 4.3×103 to 4.2×105 K. Such a wide CCT range, as well as the easily tunable EL color from yellow-green to blue-violet, enables these Ag-NWs-decorated LEDs to have potential applications in a variety of occasions. As discussed before, the tunable EL color of these Ag-NWs-decorated LEDs stems from the selective improvement of the blue-violet light emission, which is primarily caused by the resonance coupling effect of ZnO excitons and Ag-NWs SPs. Time-resolved PL (TR-PL) and temperature-dependent PL (TD-PL) measurements, which are expected to provide direct evidence for this resonance coupling effect, are carried out and correspoding results are presented in Figure 7a and 7b. Here, a PTI transient-state fluorescence spectrometer is employed to characterize the TR-PL spectra of Ag-NWs decorated ZnO film with surface density of S3 (denoted as ZnO@S3) and the bare ZnO film, respectively. The specific parameters of the PTI instrument can be found in our previous work.34 In Figure 7a, one can see that the Ag modified ZnO film exhibits a shorter lifetime in comparison with the bare ZnO film. Through a bi-exponential fitting,34,35 the effective lifetime of the ZnO@S3 film is deduced as τ* = 1060 ps and that of the bare ZnO film is τ = 1590 ps (see Supporting Information Figure S4 for more PL lifetime data). Usually, there are two kinds of ACS Paragon Plus Environment


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decay channels for excited state carriers in luminescent materials, radiative recombination channel and non-radiative recombination channel.36,37 By coating the Ag-NWs SPs onto the ZnO film, a new decay route—resonant coupling between excitons and SPs, can be formed. In this way, the spontaneous recombination rate of the ZnO@S3 film is increased, while its effective PL lifetime is simultanously decreased. At the meanwhile, internal quantum efficiency (IQE) of the light emitters is also improved because of the elevated spontaneous recombination rate. Theoretically, the IQE equations for the bare ZnO film and ZnO@S3 film are respectively expressed as:35

η IQE =

η

∗ IQE

=

k rad k rad + k non

(1)

krad + Cext' ksp

(2)

krad + knon + ksp

∗ where the η IQE and η IQE are IQE values of the bare ZnO film and ZnO@S3 film,

krad is the radiative transition rate, while knon is the non-radiative transition rate, ksp is the energy coupling rate from exciton to SP. C ' ext represents the conversion ratio of ∗ SPs to photons, and is roughly viewed as 100% herein.34,35 Obviously, the η IQE is

* higher than the η IQE due to η IQE − ηIQE =

knon × ksp (krad + knon + ksp ) × (krad + knon )

> 0 . To

prove that the IQE of the ZnO@S3 film is indeed increased, it is necessary to measure the TD-PL spectra for IQE estimation. The measurement temperature range is 77 to 300 K. PL integrated intensity from the two samples measured at different temperature are drawn as Arrhenius curves in Figure 7b for IQE comparison. The ∗ estimated IQE values are η IQE = 12.8% and η IQE = 19.7%, respectively. Therefore,

it is experimentally confirmed that the IQE of the ZnO@S3 film is improved thanks to the SP resonant coupling effect of the introduced silver NWs. It is noteworthy that the EL spectra from bare PN junction and the


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silver-decorated devices were tested at 6 mA injection current for comparative study (see Supporting Information Note 3 for more discussion). Thus, the applied bias voltage on Ag decorated LEDs is smaller than that on the bare PN LED (see Figure 3b). That is, input electrical power P (P = U×I, U is the externally applied voltage while I is the corresponding injection current) for the former is smaller than that for the latter. So, the improvement of blue-violet EL emission is indeed coming from the exciton-SP resonance coupling effect, rather than the seemingly increased electrical injection level. Finally, it is worth emphasizing that the topic of the present work is different from the previous reports.32,34 Herein, a simple PN junction LED (p-GaN/n-ZnO heterojunction) is fabricated. A strong yellow-green EL emission is obtained, and is also needed in the current work. Our experimental design is to utilize and modulate this defect-related visible emission to realize a color-tunable LED. The influence of Ag nanowires surface density on the device EL performance is carefully studied, and an optimized Ag nanowires surface density is obtained. In fact, just because of the change of Ag nanowire covering density, the EL emission color could be flexibly and continuously tuned from yellow-green to blue-violet. To our best knowledge, the combination of both defect-emission modulation and metal SPs decoration has rarely been adopted to fabricate color-tunable LED device.

4. Conclusion In summary, color tunable LED was constructed by decorating p-GaN/n-ZnO LED with Ag-NWs SPs. Through adjusting the surface density of Ag-NWs, the EL emission color can be easily switched from yellow-green to blue-violet. Especially, a near-white EL spectrum with chromaticity coordinate of (0.317, 0.324) is realized when the surface density is S2=15.2×106 cm-2. The underlying mechanism for this tunable EL color is due to different degrees of exciton-SP resonance coupling, which is verified by the TR-PL and TD-PL measurements. The facilely tunable EL color from yellow-green to blue-violet, as well as the wide CCT covering range, indicates that these Ag-NWs-decorated p-GaN/n-ZnO LEDs show application prospects in ACS Paragon Plus Environment


optoelectronics field (e.g. indoor illumination and panel display). The proposed strategy of introducing SPs provides a feasible way for adjusting the EL spectral profile and LED color.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: energy level distributions in the p-GaN: Mg wafer, Ag NW radial SP mode, EL integrated intensity and intensity ratio, RT TR-PL spectra of the ZnO@S2 sample and the bare ZnO film, the layer thickness of ZnO film, EL emission from bare PN, injection current density

Acknowledgements This work was supported by the Program of the NSFC (No. 61574031, 21501167, 61505026, 51732003 and 61604037), the ‘‘111’’ Project (No. B13013), the Fund from Jilin Province (No. 20160101324JC, 20160520115JH and 20160520009JH), the Fundamental Research Funds for the Central Universities (No. 2412017FZ010).

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Figure 1 (a) Structure diagram of a bare p-GaN/n-ZnO film LED. (b) I-V rectifying curve of this PN junction device; near-linear I-V curves of p-GaN/Ni/Au and n-ZnO/In are presented in the inset. (c) PL spectra of the p-GaN wafer and n-ZnO film layer measured at RT; the light excitation source is a He-Cd laser with 325 nm emission wavelength. (d) EL spectra of the p-GaN/n-ZnO LED obtained at different injection currents; the inset is an EL photograph of the LED with yellow-green light emission at a 6 mA injection current. (e) Energy-band-alignment and carrier recombination dynamics of the p-GaN/n-ZnO heterojunction LED device under thermal equilibrium conditions.



Figure 2 Typical SEM images of different surface densities of spin-coated Ag-NWs prepared from different volume of Ag-NWs solution (a) 8 µL, (b) 16 µL, (c) 24 µL, (d) 32 µL. (e) Optical extinction spectra of these Ag-NWs with varied surface coverage. (f) RT PL spectra of the ZnO films (prepared on transparent c-sapphire wafers) with and without Ag-NWs.


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Figure 3 (a) Structure diagram of the Ag-NWs-decorated LED. (b) I-V curves of the Ag-NWs-decorated LEDs with different surface coverage. (c) RT EL spectra of the Ag-NWs-decorated devices at a 6 mA injection current.



Figure 4 (a) Gaussian deconvolution fittings of the p-GaN/n-ZnO LEDs with different surface densities of Ag-NWs. Right panel shows the EL photograph of these LEDs recorded at a 6 mA injection current. (b) Relative spectral weight of three EL components (ZnO UV exciton emission, GaN blue emission and ZnO deep-level visible emission). (c) Variation trend of EL enhancement ratio for each luminescence component at different Ag-NWs surface coverage density.


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Figure 5 (a) The Ag-NWs SPs extinction curve (purple solid line) and a prototypical EL spectrum of the Ag-NWs-decorated p-GaN/n-ZnO LED (red solid line). Dashed lines are Gaussian deconvolution fittings of this EL spectrum, corresponding to the ZnO NBE emission (blue dashed line) and GaN Mg-acceptor emission (megenta dashed line), respectively. (b) to (d) FDTD simulations of the electric field intensity distributions of a single Ag-NW decorated ZnO/GaN heterojunction. The excitation wavelength employed for simulation is 382 nm in (b), 425 nm in (c) and 560 nm in (d), respectively.



Figure 6 (a) CIE 1931 chromaticity diagram of the Ag-NWs-decorated LEDs operated at 6 mA injection current. (b) CCT of the Ag-NWs-decorated LEDs as a function of the Ag-NWs surface density.


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Figure 7 (a) RT TR-PL spectra of the bare ZnO film (black squares) and the Ag-NWs-decorated ZnO films with a surface density of S3=23.8×106 cm-2 (ZnO@S3, red circles); the detection wavelength is 380 nm, and the data fittings were conducted according to a bi-exponential attenuation function. (b) Arrhenius curves of the ultraviolet emission integrated intensity (normalized) of the bare ZnO film (black squares) and the ZnO@S3 film (red balls). These ZnO films were deposited on transparent c-sapphire wafers for measurements.



Graphical abstract:


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GaN Heterojunction LEDs Achieved by Coupling

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