Graphene, a 2D material with exceptional properties like high carrier mobility, high transmittance, and excellent thermal conductivity, is a promising replacement for indium tin oxide (ITO) as transparent electrodes in GaN light-emitting diodes (LEDs). However, traditional graphene growth methods often involve complex transfer processes that hinder large-scale industrial production. Two main approaches for direct graphene growth on GaN exist: high-temperature non-catalytic CVD and metal-catalyzed CVD. High-temperature CVD methods, while explored previously, suffer from challenges in achieving high-quality graphene due to the lack of metal catalysts and the lattice mismatch between GaN and graphene. The high temperatures also risk buckling and decomposition of the GaN substrate, negatively impacting LED performance. Metal-catalyzed CVD, while promising, often results in the metal catalyst being difficult to remove completely, leading to opacity and requiring manual transfer processes, thus negating the intended transfer-free benefits. Previous methods also rarely address the self-heating issue in GaN substrates by leveraging graphene's high thermal conductivity. This work presents a novel method to overcome these limitations, enabling transfer-free growth of high-quality, patterned graphene directly on GaN LEDs, addressing both the electrical and thermal challenges of these devices. The key innovation lies in the use of Co as both a catalyst and an etching mask, allowing for patterned graphene growth without lithography, combined with a penetration etching method to remove the Co cleanly, resulting in in-situ Ohmic contact and achieving a transfer-free process suitable for industrial scalability.

Numerous studies have explored the use of graphene as a transparent conductive electrode in various optoelectronic devices. Early work on graphene focused on its fundamental properties, demonstrating high carrier mobility [1, 2], optical transparency [3], and thermal conductivity [4]. The potential of graphene for replacing ITO in photonics and optoelectronics has been widely discussed [5]. Efforts to grow graphene directly on GaN have yielded varying results. Some research has investigated catalyst-free methods [6, 7, 8], but these frequently struggle to produce high-quality graphene. The authors' group previously demonstrated a pioneering effort in directly growing graphene on GaN wafers [7]. However, achieving high-quality graphene directly on GaN without a catalyst remains challenging. The use of metal catalysts [10, 11, 12] has also been explored, but the subsequent removal of these catalysts has presented significant challenges, often reverting to transfer methods [13, 14]. The significant self-heating issue in GaN substrates, which has a thermal conductivity ranging from 125 to 225 Wm⁻¹K⁻¹, highlights the potential of high-thermal conductivity graphene (up to 5300 W m⁻¹K⁻¹ at room temperature) to improve device performance [15, 16]. This paper addresses these existing shortcomings by introducing a novel approach to achieve high-quality, transfer-free graphene growth on GaN LEDs.

The process begins with cleaning a 2-inch wafer of epitaxial GaN on a sapphire substrate. A 300 nm layer of Co is then deposited and patterned onto the GaN using lift-off lithography and sputtering. The patterned Co serves as a mask for inductively coupled plasma (ICP) dry etching of the GaN, exposing the heavily doped n-GaN layer. This patterned Co layer then acts as the catalyst for graphene growth. Graphene is grown at 600 °C for 3 minutes using plasma-enhanced chemical vapor deposition (PECVD) with a CH₄/H₂/Ar (5/20/960 sccm) gas mixture and 20 W (15 kHz AC) plasma. This lower growth temperature, facilitated by plasma assistance, prevents indium segregation in the InGaN layers. After graphene growth, a thin layer of poly(methyl methacrylate) (PMMA) is spin-coated onto the sample and dried. A CuSO₄:HCl:H₂O etching solution is then used to remove the Co. Crucially, this solution penetrates both the PMMA and the graphene layers to etch the underlying Co, a technique termed 'penetration etching'. The PMMA is subsequently removed with acetone, leaving the graphene directly on the GaN. Finally, p and n electrodes (Ti: 20 nm, Au: 500 nm) are deposited using lift-off lithography and sputtering. The method eliminates the need for graphene lithography and avoids direct contact between photoresist and graphene, improving efficiency and preventing undesirable graphene doping.

The grown graphene films exhibit excellent uniformity, as observed by scanning electron microscopy (SEM). Raman spectroscopy confirms the presence of sp² carbon atoms, with G and 2D peaks at 1573 cm⁻¹ and 2692 cm⁻¹, respectively. D/G and G/2D ratios are 0.28 and 1.99, indicating good crystalline quality. The sheet resistance of the graphene is in the range of 631.2–868 Ω sq⁻¹. Optical transmittance measurements reveal an average transparency of 89.84% in the 400–1000 nm wavelength range. Atomic force microscopy (AFM) analysis shows that the graphene growth process does not significantly increase the surface roughness of the GaN, maintaining a roughness of 0.604 nm. Raman mapping further supports the high crystallinity and flatness of the graphene. Nearly 100% integrity of the patterned graphene array on GaN is achieved, demonstrating the effectiveness of the penetration etching method. Circular transfer length method (CTLM) measurements determine a contact resistivity of 0.421 Ωcm² between the graphene and p-GaN. This value, while slightly higher than that of GaN-ITO contacts, is superior to GaN-transferred graphene contacts. The graphene-coated GaN LED exhibits improved electrical performance compared to a pristine GaN LED, requiring a lower voltage (5.54 V vs. 6.59 V) to reach 20 mA. The luminous flux is also higher for the graphene-coated LED (6% at 20 mA, 18% at 100 mA), along with increased radiation flux and a narrower full width at half maximum (FWHM) in the electroluminescence (EL) spectra. Thermal measurements reveal significantly lower junction temperature (40.25 °C vs. 63.61 °C at 2 mA) and thermal resistance (42.25 °C W⁻¹ vs. 85.8 °C W⁻¹) for the graphene-coated LED compared to the pristine LED, showcasing the excellent heat spreading capabilities of the graphene layer. At higher current (100 mA), the difference in temperature increase between the two LEDs is even more pronounced, with the graphene-coated LED showing significantly lower temperature increase.

The results demonstrate the successful fabrication of high-quality, wafer-scale, patterned graphene directly on GaN LEDs using a novel, transfer-free method. The use of Co as both a catalyst and etching mask eliminates the need for lithography and simplifies the fabrication process. The penetration etching technique ensures the complete removal of the Co, resulting in excellent in-situ Ohmic contact and avoiding the issues associated with traditional transfer methods. The superior electrical and thermal performance of the graphene-coated LEDs compared to their pristine counterparts confirm the benefits of the approach. The improved light emission efficiency, lower junction temperature, and reduced thermal resistance highlight the potential for this method to enhance the performance and reliability of GaN-based LEDs, particularly at higher operating currents. The transfer-free nature of this technique enhances scalability and compatibility with standard semiconductor manufacturing processes, paving the way for its industrial adoption.

This research presents a significant advancement in the fabrication of graphene-based LEDs, offering a transfer-free, scalable, and high-performance approach. The use of a patterned Co catalyst and a novel penetration etching method has successfully eliminated the challenges associated with traditional graphene transfer processes. The resulting graphene-coated GaN LEDs demonstrate improved electrical and thermal properties, leading to enhanced light emission efficiency and device reliability. This method represents a crucial step towards the industrial application of graphene and other 2D materials in high-performance electronic and optoelectronic devices. Future work may focus on optimizing the graphene growth parameters to further reduce sheet resistance, exploring other catalyst materials, and extending this technique to other semiconductor materials and devices.

While the current method demonstrates significant improvements, there are still limitations. The success rate of the lift-off process for the patterned metal electrodes is greater than 90%, but some graphene detachment and damage can occur during this step. Further optimization of the PMMA removal process could potentially improve this yield. Additionally, the contact resistivity between the graphene and p-GaN, while better than with transferred graphene, is still higher than for ITO. Future research could explore strategies to further reduce this contact resistance, such as doping or surface treatment. The current study was limited by the size of the PECVD equipment used. Scaling up the equipment would allow for the growth of larger-area graphene films, further enhancing the industrial applicability of this method.