Enhanced performance of GaN-based long-wavelength blue light-emitting diodes with graphene-ITO composite film as transparent conductive electrode

We present enhanced performance of GaN-based long-wavelength blue light emitting diodes (LEDs) with hybrid transparent conductive electrodes (TCE) made of graphene and indium tin oxide (ITO) composite. The nearly 100% transmittance TCE were gained when graphene grown by chemical-vapor-deposition was annealed with ITO on the top of it. Compared to conventional LEDs, the work voltage was reduced to 3.5 V at 5 mA forward current. The light emission power was improved about 190%. The good peformance can be attributed to the improved lateral current spreading over the composite graphene-ITO TCE before p-GaN layer injected by carriers.


Introduction
The GaN-based light-emitting diodes (LEDs) have been of great interest in broad range of applications, such as LED backlights, full-color displays, general lighting, etc [1][2][3]. However, high light extraction efficiency is critical to realize high-brightness GaN LEDs. Generally, light extraction efficiency is greatly limited by low carrier mobility and high resistivity of p-GaN, leading to the crowding effect of current under the p-electrode metal contact area [4,5]. In order to overcome the problem, transparent conducting materials such as indium tin oxide (ITO), ZnO with Ga-doped and Al-doped (GZO and AZO) have been widely studied as the transparent conducting electrodes [6][7][8][9]. In particular, ITO has been widely used because of its high transparency and low resistivity [10,11]. As known, ITO is costly and chemical instable, therefore graphene as a carbon-based material and a new type of transparent conductive material is widely studied to replace ITO [12,13].
It is important to note that the field of GaN-based LEDs, transparent conducting electrodes, and the current crowding effect has witnessed significant advancements in recent years. For instance, a comprehensive overview of the latest developments in this field was provided by Fan et al in Laser & Photonics Reviews (2023) , which highlights new strategies for improving the efficiency and performance of GaN-based LEDs [14]. Additionally, Hu, H., et al discussed advancements in transparent conducting electrodes for LEDs in Nano Energy (2020) [15], covering the synthesis and characterization of novel materials. Furthermore, notable contributions to the field were made by Zhou et al in Optics Express (2019) [16] and in Optics Express (2017) [17]. Zhou et al investigated the current crowding effect in GaN-based LEDs and proposed innovative designs to mitigate the issue, while Zhang et al presented research on the same topic, focusing on addressing the current crowding effect. The development of efficient and environmentally friendly techniques for fabricating high-performance optoelectronic devices remains a crucial research area. Hemasiri et al (2017) focused on synthesizing a highly conductive graphene/indium tin oxide (ITO) transparent bi-film through a two-step process [18]. Fernández et al (2020) developed hybrid transparent contacts by combining transparent conductive oxides with graphene monolayers [19].
In fact, two-dimensional graphene has been of great interest worldwide due to the excellent properties such as optical transmission and high electrical conductivity [20,21]. Graphene as the sp2-hybridized carbon material has superior good electrical conductivity, mechanical robustness, and thermal conductivity. Moreover, the truth that it is highly transparent due to the only several atomic layers. Because of superb electrical and optical merits, the graphene draws attention for applications as TCE materials in GaN-based LEDs [12,13,22]. Although, several pioneering work have reported the applications of graphene films as TCEs in GaN-based LEDs, the lower current spreading is still an important problem because of the non-uniformity and low quality of graphene films. Consequently, the light extraction efficiency is still poor compared to LEDs with ITO TCEs [22][23][24]. Therefore, it is still urgent to see that graphene as TCE will enhance the GaN-based LEDs performance.

Experiments
The samples grown by vertical metal organic chemical vapor deposition (MOCVD) on 2-inch (0001) patterned sapphire substrates (PSS) were used in this study. In this experiment, the epitaxial wafer was with a commercial structure consisting of p-GaN, InGaN multiple quantum wells n-GaN, and undoped (u)-GaN, consequently. To start the processing, the GaN wafer was firstly emerged in hydrochloric acid to remove metal contaminations and native oxides and then cleaned in sulfuric acid and hydrogen peroxide to remove organic contaminations. Afterwards, a layer of graphene film grown by CVD was transferred onto the GaN-based long-wavelength blue LEDs to get the electrical contact. After transferring graphene on the p-GaN surface, the sample was then loaded to e-beam evaporator for 10 nm thick ITO deposition. Then defined mesa areas were gained by photolithography, where oxygen plasma and/or ITO etchant were used to remove the graphene and/or ITO nanolayers outside the mesa areas. Then, the wafer was loaded to the inductively coupled plasma etcher for mesa isolation. The etching depth was 1.1-1.2 μm, which was deep enough to reach n+-GaN. Finally, patterned metal electrodes (1.5μm Cr/Pt/Au) were deposited onto n+-GaN and ITO as the n-type electrodes (cathode) and the p-type (anode), respectively. In addition, LEDs with bare graphene and ITO transparent contact (sample 1#, sample 2#), as shown in figures 1(a)-(b), were also prepared as reference samples to LEDs with graphene/ITO transparent contact (sample 3#)( figure 1(c)). The optical image from the top side of samples is shown in figure  1(d). All fabricated LEDs as shown in figure 1(d) have a dominant emission wavelength of 468 nm and a mesa area of 150 × 200 μm2. The experimental current-voltage (I-V) curves of LEDs were gained using the Keithley semiconductor parameter analyzer, and the output powers of the LEDs were gained with a calibrated integrating sphere without epoxy encapsulation at room temperature. The calibrated charge-coupled device (CCD) camera fixed on the microscope was used to take the output light intensity images of the LEDs.

Results and discussion
The current spreading of long-wavelength blue LEDs can be highly enhanced with low sheet resistance of graphene as TCE. Meanwhile, defects with graphene will reduce its conductivity and thus decrease the light extraction of long-wavelength blue LEDs. Therefore, graphene quality is very important for its use in LEDs. The uniform of graphene on GaN-based long-wavelength blue LEDs is confirmed by Raman spectra with 633 nm laser as an excitation source. Figure 2 shows the Raman spectra of graphene at the different parts of GaN. The weak D peak means the high quality of the graphene film [25]. Moreover, Raman spectra are almost the same at different locations, which confirms the good uniformity of graphene.
Next, a customized prober was used to characterize the fabricated long-wavelength blue LED chips that was connected to a source meter of Keithley 2400 and CAS 140 CT optical power meter. The current-voltage (I-V) characteristics of the long-wavelength blue LEDs are shown in figure 3(a) with direct current (DC) injection for the different TCEs, i.e., graphene, ITO, and graphene-ITO. At a typical 5 mA current injection, high operating volt-age (V f = 8 V) was observed in GaN based long-wavelength blue LEDs with graphene-only TCE. It has been reported that this high V f is mainly due to the big difference of Schottky barrier height/work function mismatch between p-GaN (Φp-GaN = 7.5 eV) and graphene (ΦG = 4.5 eV) [12,14,[24][25][26][27][28]. In addition, unfavorable charge scattering and conductivity across grain boundaries of polycrystalline graphene film also had bad influence on the lateral current spreading performance of bare graphene TCE [23]. On the other hand, when 10 nm thick ITO film was used as TCE, the V f was cut down to 6.12 V at 5 mA. This value is relatively high than conventional LEDs, because we use ITO with thickness of only around 10 nm. The decrease in the turn-on voltage compared to the bare-graphene LEDs may be largely due to the good ohmic contact of ITO and the p-GaN. As we mentioned, ITO is an ideal transparent conductive material for LEDs, as it has low optical absorption coefficient and good electrical conductivity, such as low resistivity (5 × 10−4 Ω·cm) and high transmission in visible range (92%) [29][30][31][32]. However, the drawback of ITO is very obvious. ITO is always criticized for its low mobility, fragility and the scarce of indium [33,34]. In order to overcome this problem, an interlayer of graphene was inserted between p-GaN and ITO film. As a consequence, the fabricated graphene-ITO composite transparent conductive layer demonstrated a well improved V f of 3.7 V at 5 mA. The injection current (I) of long-wavelength blue LED can be divided into two components by the equation: Where, Iv and Is are the current flow under the p-electrode and the spreading current. As it is known, lateral current spreading is very important for LED to achieve lighting uniformity. Therefore, a 40% reduction in V f for graphene-ITO LEDs is attributed to the smaller lateral spreading current (Is) using graphene interlayer. By using composite films made by graphene-ITO as the combined strategies , the obtained V f at 5 mA on GaN-based LEDs is equivalent with other reported TCE systems such as 3 nm ITO + 3 layer graphene (V f = 5.6 V), graphene/Ag NWs (V f = 6.6 V), HNO3-doped graphene (V f = 5.35 V), and Au/graphene (V f = 4.63 V) [12,24,35]. Figure 3(b) shows characteristics of the light output power with injection current for three types of LEDs with different TCEs, i.e., graphene, ITO, and graphene-ITO. Obviously, the light output power of graphene-ITO as TCE is very high because of the good current spreading. Compared with only ITO based long-wavelength blue LEDs, an 190% improvement in the light output power is achieved. However, we also see that the light output power of graphene based long-wavelength blue LED is slightly higher than the ITO based LED. It may be that the current of the ITO based LED is mainly crowded near the p electrode because of its good contact with p-GaN [29,36].
We also made a study on the near-field intensity profile of the three long-wavelength blue LED samples at 5 mA shown in figure 4 to comprehend the effect of the composite graphene-ITO transparent conductive layer. The red color and the green of the images indicate high intensity profile and low intensity profile, respectively.  The 5 mA near-field images of both samples were made at the same conditions and are all normalized to the same range. The 5 mA near-field intensity profile image of sample 2# shows a bigger high intensity profile region than that of sample 1#. The enlarged near-field high intensity profile region of sample 2# can be attributed to the better contact of ITO transparent layer and p-GaN. Generally, inevitable defections could be made during the processing of large scale CVD-grown graphene, and also CVD grown graphene has a larger square resistance than ITO TCL. From figure 4(c), we can see that sample 3# has the better light intensity than sample 1# and sample 2#. In theory, the high intrinsic carrier mobility of graphene is more than 21,000 cm2/ V·s at room temperature [37]. Like that, the carrier injected into the graphene layer moves laterally and the spreading current (Is) of sample 3# is enhanced. Apart from that, graphene has a similar work function. The work function of graphene is 4.5 eV and that of ITO is 4.4 eV to 4.5 eV. Consequently, a low contact resistance was achieved by their combination, which is to the ohmic junction of LEDs.

Conclusions
In conclusion, graphene films with ITO synthesized by CVD as transparent electrodes of GaN-based longwavelength blue LEDs are applied in this work. The results show that a transparent current spreading layer was successfully made by the graphene with ITO electrodes on the GaN-based long-wavelength blue LED devices. In particular, we have achieved a 90% improvement of light output power and a well improved V f of 3.7 V at 5 mA compared to LED with graphene as TCE. This work proves the possibility that the combination of graphene film synthesized by CVD and a layer of ITO film can be used as potential substitutes for ITO film in inorganic longwavelength blue LEDs, which will promote the practical application of graphene film as transparent electrode of various optoelectronic devices.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).