1. Introduction

Since its discovery in 2004, graphene, an ultra-thin two-dimensional form of covalently bonded carbon atoms with a hexagonal lattice structure, has been attracting much interest due to its excellent physical properties such as quantum electronic transport, extremely high intrinsic mobility, superior thermal conductivity, and high elasticity and optical transmittance [1,2]. Accordingly, much effort has been dedicated to the study and application of graphene-based materials. In particular, graphene with good transport properties due to low electron-phonon scattering and excellent optical transmittance in the ultra-violet (UV) region has been used as a transparent current spreading electrode (TCSE) in UV light-emitting diodes (LEDs) [3–6]. This is because the conventionally used indium tin oxide (ITO) in visible LEDs can no longer be used due to its poor optical transmittance in the UV range. However, the adoption of bare graphene electrode on p-GaN in UV LEDs gives rise to several problems such as large turn-on voltage, low hole injection efficiency towards the active region, severe current crowding near the p-electrode, and heat generation due to high sheet resistance caused by grain boundaries in chemical vapor deposition (CVD)-grown graphene [3–6]. A high electrical resistivity of CVD-grown graphene electrode can arise from other defects formed on grain boundaries, such as point defects, wrinkles, folds, tears and cracks. These defects could disrupt the sp² delocalization of π-electrons in graphene and scatter the charge carriers, resulting in decreased ballistic transport path length and carrier mobility [7,8].

Several pioneering works have been reported on the improvement of sheet resistance and work function of graphene electrode by aqueous gold chloride (AuCl₃) chemical doping [4,9,10]. Also, the use of thin metal film on graphene to decrease the sheet resistance has been reported [5,11]. However, these approaches have some limitations such that the chemical doping is unstable with time and the use of metal films diminishes the transmittance.

Recently, silver nanowires (AgNWs) have attracted significant attention owing to their advantages of material properties such as high flexibility, electrical conductivity, and optical transmittance [12–14]. However, the random arrangement of nanowires gives rise to high resistance for a two-dimensional network and the holes within the network are detrimental for devices that depend on vertical current transport. Combining graphene and Ag nanowires is therefore considered as a promising strategy to overcome the drawbacks of individual counterparts [15–18]. Kholmanov et al. [15] experimentally demonstrated that the influence of line defects and line disruptions on the transport properties of graphene can be minimized by using the metal nanowires. A hybrid structure with graphene on subpercolation network of Ag nanowires offered a low sheet resistance of 64 ± 6.1 Ω/sq. As well, in a recent study reported by Choi et al. [16], new insights into the mechanism of conductivity improvements in graphene covered by low-density Ag NWs have been provided. According to their results, at concentrations below the percolation threshold, the enhancement in the electrical properties of graphene is due to bridging among the polycrystalline domains rather than doping. Chen et al. [17] introduced a method for producing a free-standing highly conductive graphene paper with the addition of various amount of Ag nanowires by vacuum filtration. According to their results, the conductivity of the Ag nanowire/graphene composite paper increases linearly with increasing Ag nanowire concentration.

In this work, a GaN-based UV-LED with emission maximum at 375 nm is fabricated using AgNWs decorated graphene electrode (ADGE) as a TCSE. By controlling the density of the Ag NWs, we achieved a sheet resistance of 50 ± 5 Ω/sq and a transmittance of 87.7% at 375 nm for the ADGE. The performance of the UV-LED using the ADGE is systematically studied and the results are presented.

2. Experimental

The AlInGaN-based UV-LEDs were grown on sapphire substrate by metal-organic chemical vapor deposition. A 25 nm GaN buffer layer was deposited onto sapphire at 550 °C. This was followed by the growth of a 1.5 μm un-doped GaN layer and a 2 μm Si-doped n-GaN layer at 1040 °C. Then, five pairs of In_0.03Ga_0.97N quantum wells and Al_0.08Ga_0.92N barrier layers with thicknesses of 3 and 12 nm, respectively, were grown at 800 °C. Finally, a 25 nm Mg-doped p-Al_0.25Ga_0.75N electron blocking layer and a 100 nm p-GaN contact layer were grown at 1040 °C. Rapid thermal annealing (RTA) was performed at 940 °C for 40 s under N₂ ambient to activate the Mg dopants. The hole carrier concentrations in p-GaN were estimated to be ~10¹⁶ cm⁻³.

After the growth, discrete LED chips were fabricated with a chip size of 350 × 350 µm² in which the mesa region was defined by an inductively coupled plasma (ICP) etcher using Cl₂/ BCl₃ gases until n-GaN layer was exposed for n-electrode contact. Large scale graphene layers were synthesized on ~35 µm thick Cu-foil by CVD method. Details can be found in Ref.19. CVD-grown graphene films with poly methyl methacrylate (PMMA) were transferred onto p-GaN layer of the device. After removing the PMMA using hot acetone, graphene electrode was patterned by an ICP-reactive ion etcher using O₂ plasma. In order to form ADGE, the TCSE area was selectively opened using a photoresist lift-off process. An aqueous solution containing AgNWs was spin-coated at a speed of 1000 rpm onto the graphene surface. Thereafter, Cr (50 nm)/Au (250 nm) metals for the p- as well as the n-electrode were deposited onto both ADGE and n-GaN layer using electron beam evaporator. A schematic diagram of the fabricated UV-LED with ADGE as a TCSE is shown in Fig. 1.

 

Fig. 1 Schematic of a UV-LED with AgNWs decorated graphene electrode (ADGE) as a transparent and current spreading electrode.

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3. Results and discussion

Prior to applying the Ag nanowires on graphene, the deposition and opto-electrical properties of the AgNW networks are optimized. Figures 2(a)-2(e) show scanning electron microscopy (SEM) images of AgNW networks on sapphire substrate coated at various spin speed. One can notice uniform distribution of nanowires on the substrate without severe aggregation, analogous to a recent study which emphasized the advantages of spin coating over other methods in the deposition of AgNW networks [14]. The average diameter and length of AgNWs were approximately 100 nm and few tens of micrometers, respectively. Also, the AgNW density decreases with spin speed, as shown in SEM images. Considering the fact that nanowires with larger diameter and length could lead to percolation at low number density of nanowires and a low sheet resistance, relatively large size nanowires were employed in this study, and detailed studies on the effect of nanowire length and diameter on the optoelectrical properties of Ag nanowire networks can be found elsewhere [20,21].

 

Fig. 2 SEM images of AgNW networks on polished sapphire substrate deposited by spin coating at (a) 100, (b) 500, (c) 1000, (d) 1500, and (e) 2000 rpm. (f) Optical transmittance and (g) sheet resistance of respective samples shown in (a) to (e).

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The optical transmittance spectra of AgNW networks deposited at various spin speed are shown in Fig. 2(f). As the spin speed increases from 100 to 2000 rpm, the transmittance of the AgNW network increases from 71.7 to 92.1% at the target wavelength of 375 nm. This is attributed to the enhanced vacant spaces in AgNW networks and diffusive reflection from AgNWs. In addition, the AgNWs show relatively strong absorption around 390 nm due to the excitation of the localized surface plasmon resonance of AgNWs [12,14,21]. The Hall measurements were carried out in order to understand the sheet resistance behavior of the AgNW networks as a function of spin speed. One can notice from Fig. 2(c) that when spin speed increases from 100 to 2000 rpm, the sheet resistance also increases from 85 ± 3 to 170 ± 10 Ω/sq. This result can be ascribed to the reduced effective current paths in the AgNW network caused by decreased AgNW density. In other words, a random conducting network of AgNWs provides carrier transporting channels, leading to low sheet resistance. The optical transmittance and sheet resistance are equally important for a TCSE in LEDs, because the device performance is strongly dependent on both parameters. Thus, Ag NW network with a transmittance and sheet resistance of 90.8% at 375 nm and 98 ± 5 Ω/sq, respectively, obtained at 1000 rpm, is considered optimum to apply on graphene.

Figures 3(a) and 3(b) show SEM images of CVD-grown graphene electrode and ADGE, respectively, on p-GaN layer in a LED structure. Figure 3(a) shows that CVD-synthesized graphene electrode is clean without noticeable PMMA residues and Cu catalysis particles, but wrinkles were formed for thermal stress minimization. It could originate from the nucleation of defect lines on the step edges of Cu terraces during the CVD growth of graphene on Cu foil [22]. The electron diffraction image in the inset of Fig. 3(a) exhibits typical hexagonal pattern expected for graphene. SEM image of ADGE shown in Fig. 3(b) reveals that AgNWs are uniformly coated on the graphene electrode.

 

Fig. 3 SEM images of (a) graphene electrode and (b) ADGE on p-GaN epilayer in a UV-LED structure, and (c) Raman spectra of respective samples. The inset to Fig. 3 (a) is the electron diffraction image of graphene.

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To study the structural properties, Raman spectrum of graphene electrode and ADGE were recorded after they were formed on 300 nm-SiO₂/Si substrate. Figure 3(c) shows Raman spectra of the samples obtained by using a 514 nm-line of an Ar ion laser as an excitation source. Two major peaks, i.e., the G- and 2D-band peaks of graphene are observed from graphene electrode and ADGE. In the case of ADGE, a decrease of 2D/G intensity ratio and a blue shift in the G- and 2D-band peak positions are observed in comparison to graphene only electrode. This can be attributed to the compressive strain induced by lattice mismatch and p-type doping effect associated with the direct charge transfer between AgNWs and graphene [23,24]. In addition, the D band peak is seen around 1360 cm⁻¹ likely due to the surface enhanced Raman scattering effect (SERS) from AgNWs and formation of disorders or defects caused by the AgNW incorporation on the graphene electrode [25,26]. Similar D-band peak enhancement was observed in Au nanoparticles covered graphene by Shin et al. [11], and the D-band peak enhancement induced by SERS was observed from single layered graphene.

The optical and electrical properties of graphene electrode and ADGE are also investigated. To evaluate the optical properties, ADGE was formed on a polished sapphire substrate, and the optical transmittance was compared to graphene electrode. As shown in Fig. 4(a), at 375 nm, the transmittance values of graphene electrode and ADGE are found to be 92.8 and 87.7%, respectively. The graphene electrode reveals good transmittance through the entire range of wavelength investigated including the UV region, whereas the ADGE shows relatively high absorption at near-UV wavelengths due to LSPR. Despite this fact, the transmittance of the ADGE is much better than the conventionally used ITO in UV region. A comparison of the optoelectrical characterisics of single and hybrid electrodes based on graphene and Ag nanowire networs reported in the literature and achieved in this study can be found in Table 1.

 

Fig. 4 (a) Transmittance spectra of graphene and ADGE. (b) I-V characteristics of graphene and ADGE contacts on p-GaN, measured between adjacent pads with a gap spacing of 5 µm. The inset shows the sheet resistance of graphene electrode and ADGE.

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Table 1. Transmittance and sheet resistance (R_sh) of similar electrode materials reported in the literature.

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To study the changes in electrical conductivity of graphene after applying AgNWs, the sheet resistance is measured by Hall measurement and the results are given in the inset to Fig. 4(a); the sheet resistances are measured to be 550 ± 20 Ω/sq and 50 ± 5 Ω/sq for the graphene electrode and ADGE, respectively. The grain size of crystalline copper, on which the graphene layer is synthesized, is typically a few µm [27] and defects have been found on the grain boundaries, giving the high sheet resistance for the CVD-synthesized graphene electrode. After applying AgNWs on graphene electrode, influences of those defects are reduced owing to the opening up of new conduction channels by bridging grain boundaries and line disruption in graphene electrode [15,16]. They decrease the chance of current encountering the graphene defects and hence reduce the sheet resistance. The effect of AgNWs on the specific contact resistance (ρ_sc) of graphene/p-GaN contact is studied using circular transmission line method (CTLM). Figure 4(b) presents current-voltage (I-V) characteristics of graphene electrode and ADGE contacted onto p-GaN epilayer, measured between adjacent pads with a gap spacing of 5 µm. The I-V curve of a direct contact of graphene electrode on the p-GaN layer exhibits non-linear feature, whereas good ohmic characteristic is achieved after applying Ag nanowires onto graphene electrode. The ohmic contact formation is attributed to the reduced sheet resistance and Schottky barrier height. The value of specific contact resistance (ρ_sc) obtained for AgNWs on graphene electrode is 1.3 × 10⁻¹ Ωcm², which is lower than the value obtained for a chemically modified multilayer graphene on p-GaN [10].

Figures 5(a) and 5(b) show I-V characteristics and electroluminescence (EL) spectra of the UV-LEDs fabricated with graphene, AgNWs, and ADGE, respectively. The forward voltages at an input current of 20 mA are found to be 10.3, 8.7 and 7.8 V for devices with graphene electrode, AgNWs electrode and ADGE, respectively. The forward voltage of the UV-LED with ADGE is remarkably reduced compared to that of the UV-LEDs with graphene and AgNWs electrode. The high operation voltage of LED with graphene electrode is attributed to high sheet resistance and non-ohmic contact nature caused by the work function difference between p-GaN and graphene film [6,10]. The combination of graphene and AgNWs yields comparatively low forward voltage caused due to effective reduction in the sheet and contact resistances and by the Ohmic junction. That is, AgNWs can provide efficient current diffusion pathways for graphene electrode which then deliver current to the active junction of the LED through p-GaN layer.

 

Fig. 5 (a) I-V characteristics and (b) EL spectra of UV-LEDs with graphene, AgNWs, and AgNWs-coated-graphene electrodes. (c) EL images of respective LEDs at an injection current of 20 mA.

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The electroluminescence (EL) spectra of the UV-LEDs with graphene, AgNW network, and ADGE are shown in Fig. 5(b). Even though the transmittance of graphene electrode is found higher than that of ADGE, the EL intensity is significantly low for the former because of poor current spreading and associated non-uniform carrier injection. Furthermore, no EL was observed when current injection exceeded above 20 mA, because of the burning of graphene as reported earlier [5,6]. On the other hand, the EL of UV-LED with ADGE is significantly enhanced compared to that of the devices with graphene and AgNWs, due to the effective current spreading and enhanced injection efficiency toward p-GaN surface caused by the low sheet and contact resistances. Furthermore, the devices with ADGE showed stable light emission even at injection currents above 20mA. The main advantage of applying AgNWs can be found in EL images shown in Fig. 5(c). The light emission from the UV-LED with graphene electrode is non-uniform and only bright near the p-electrode area due to high sheet resistance of the CVD-grown graphene electrode. Similarly, the light emission in the UV-LED with AgNWs is strong only near the p-electrode due to the inadequate current spreading by the AgNW networks. After coating AgNWs on graphene electrode, nearly uniform light emission is observed over the whole emission area of the device. This is because current spreads easily over the entire graphene area by the presence of conducting AgNW network, and then get injected vertically across the graphene/p-GaN ohmic contact with low voltage drop.

The effect of surface plasmon needs to be considered here, as the emission wavelength overlaps with the LSPR of Ag NWs. Despite some contradictory reports on the surface plasmon based GaN LEDs, we believe that the resonant coupling of quantum-well emission into LSPR is absent in our device geometry, because the nanowire to quantum-well separation distance is far higher than the surface plasmon fringing field penetration depth into the semiconductor. This conclusion is based on the fact that the EL spectra (Fig. 5b) show no obvious blue shift in the emission maximum before and after applying the Ag NWs on graphene. However, a few studies disclosed that the light extraction can be enhanced even when the distance of separation between the metal and the quantum-well is above 100 nm, essentially by the coupling of surface plasmon modes into the internally trapped light modes [28] or the transverse magnetic modes [29] in the LED. This is rather a complex case in our structure, and further studies are needed to address the definite role of surface plasmons. Despite that the opto-electrical performances of ADGE can be further improved by optimizing the contact properties between graphene and AgNWs by methods such as thermal annealing, and this combination of graphene and Ag nanowires could be the potential TCSE for GaN-based UV LEDs due to its high transmittance in the UV region where the transmittance of ITO decreases sharply.

4. Conclusions

In summary, the performance of 375 nm UV-LED using graphene, AgNW network, and Ag nanowires decorated graphene as transparent current spreading electrodes has been studied. The introduction of Ag nanowires on graphene surface modifies the graphene/p-GaN junction from rectifying to ohmic due to the reduction in sheet resistance. Consequently, a uniform light emission from the entire device area is realized with a comparatively low forward voltage and significantly enhanced electroluminescence intensity. Thus, a combination of graphene and Ag nanowires can be a potential transparent current spreading electrode for UV-LEDs.