Effect of surface treatment on the performances of vertical-structured GaN-based LEDs with electroplating metallic substrate

Vertical-structured GaN-based light-emitting diodes with metallic substrate (VM-LEDs) were fabricated employing nickel electroplating and patterned laser lift-off (LLO) techniques, as well as the effect of the cathode surface-treatment on the performance characteristics of the LEDs was investigated. Experimental results show that the light extraction efficiency of VM-LEDs could be significantly enhanced by surface roughening using Cl ₂/He/CH₄ plasma and KOH etching; in addition, the n-pad contact characteristics could also be improved by chemical treatment using HF and HCl solutions. As compared to that of conventional blue-LEDs with lateral structure, a light output power (Lop) of 2.12 (2.32) times at 20 (80) mA with a forward operating voltage of 3.2 (3.65) V has been achieved Recently, many attempts have been made to develop high-efficiency LEDs for solid state lighting [1-2]. Efforts to enhance the external efficiency of the conventional GaN-based LEDs, by means of vertical-conducting structure, surface texturing, or transparent conduction layer, have been made [3-5]. In this study, a patterned LLO process to define the device area and separate the GaN epilayer from sapphire substrate simultaneously, as well as an electroplated nickel layer to serve as the metallic substrate was proposed for the fabrication of an n-side-up vertical-structured GaN-based LED (abbreviated as VM-LEDs). In addition, the effect of the cathode processing, such as plasma and chemical treatment of top device surface, on the performance of the VM-LEDs was reported. In experiments, the LEDs used in this study were grown on a c-plane sapphire substrate by metalorganic chemical vapor deposition (MOCVD). The GaN epilayer transfer process begins with the E-beam-evaporated deposition of a highly reflective metal system comprising Ni/Au/Ti/Al/Ti/Au film as an ohmic contact to p-GaN, and also as an adhesive layer to the subsequent electroplated nickel layer. The electroplating process was conducted under a current of around 1.7 A with the plating solution kept at around 55°C before the patterned LLO process. A 248 nm KrF excimer laser was directed through a copper mask to the backside of the sapphire substrate. A patterned device region could be formed simultaneously, while the sapphire substrate was separated from the GaN device, by adjusting the spot size and interval between neighbor laser beams. The surface of the separated GaN device was then etched using Cl₂/He/CH₄ inductively coupled plasmas (ICP). To improve the contact characteristics and recovered the possible damage during the ICP process, chemical treatment (KOH, HF and HCl solutions) was conducted, before formatting the Ti/Al/Ti/Au contact pad. Figure 1(a) shows the top view of LED sample after the patterned LLO process, in which all samples are with a pitch of 250 μm and have a chip size of 300×300 μm². The schematic device structure of the fabricated VM-LEDs was presented in Fig. 1(b). Noted that an n-side-up vertical-conducting GaN-based LED structure with rough surface has been obtained. Figure 2 shows the scanning electron micrographs of the LLO GaN surface etched by ICP with Cl₂(10 sccm)/He(10 sccm)/CH ₄(2.7 seem) at LF (400W) and RF (120W) for 380 sec, and treated with various chemical solutions. It is seen that a rough surface with ball-features of 0.3-0.8 μm in diameter was produced by only the dry etching process. It is believed that the roughened surface would increase the light extraction efficiency. To evaluate the contact performance, the I-V characteristics of the cathode metal scheme (Ti/Al/Ti/Au) on HCl/HF-treated n-GaN contacts for various ICP- and previous KOH- etching times was measured and results were shown in Fig. 3. Notice that the n-pad contacts exhibit excellent ohmic behavior even for all KOH chemical-treated samples; though the contact resistivity might be related to different wet etching times. Comparison of the measured I-V characteristics of VM-LEDs and lateral LEDs was shown in Fig. 4. It is seen that the forward voltage drop (V_F) of the VM-LEDs (without KOH-treatment) at 20 (80) mA is 3.2 (3.65) V, which is lower than that of the lateral LEDs, no needing an additional current spreading transparent contact layer on the top of device for decreasing the V_F [5]. Moreover, as compared to lateral LEDs, the reduction in conductance resistance of VM-LEDs is mainly attributed to the use of metallic substrate which enables a relatively much less current crowding effect as well as a shorter and higher conductivity of the current conduction path. Figure 5 shows the measured upward EL power from the surface of the chip versus dc injection current (L-I) characteristics for the VM- and lateral LEDs for various etching times. Our results show that the ICP- and 90-sec-KOH-treated VM-LEDs at 20 (80) mA revealed an enhancement of Lop by a factor of 1.74 (1.9) and 2.12 (2.32) as compared to that of lateral LEDs, respectively. Based on these results, the presented in conclusion, the use of a patterned LLO process and an electroplating-Ni process as well as a surface roughening employing plasma and wet etching were proposed for the fabrication of VM-LEDs with a low forward operating voltage arid high optical output power. As compared to the conventional blue-LEDs, a light output power of 2.12 (2.32) times at 20 (80) mA has been obtained from the VM-LEDs. It is expected that the proposed VM-LEDs would be very advantageous for She applications of high-power and cost-effective solid-state lighting.