Use of Anisotropic Laser Etching and Transparent Conducting Layer to Alleviate Current Crowding Effect in Vertical-Structured GaN-Based Light-Emitting Diodes

摘要

An anisotropic laser etching to the surface layer (n-GaN) of vertical-structured GaN-based light-emitting diodes (LEDs) associated with a transparent conducting layer (TCL) to release current crowding effect (CCE) for better light emission uniformity and higher optical efficiency is proposed and demonstrated. The theory behind the proposed scheme was verified by a two-dimensional device simulator (ISE-TCAD), which indicates that immune of CCE would be possible once an optimal combination of the concave-structured n-GaN layer and TCL has been achieved. In experiments, 40-mil LEDs with an anisotropic etching area of 800μm in diameter, an etching depth of 1.75μm at center, and a 300-nm-thick Indium-Zinc-Oxide (IZO) layer have been successfully prepared. Typical improvement in light output power by 26% at an injection current of 350 mA as compared to the one without anisotropic etching has been obtained. Recently, many attempts have been made to develop high-efficiency high-power GaN-based LEDs for solid-state lighting [1-2]. Efforts to enhance the external efficiency of conventional GaN-based LEDs, by means of vertical-conducting structure, surface texturing, or transparent conduction layer, etc., have been proposed [3-4]. To effectively improve current and light emission distributions of LEDs, a vertical-structured metallic substrate GaN-based LED (abbreviated as VM-LED), as shown in Fig. 1(a), has been developed and demonstrated [5-8]. Though the VM-LED has been shown providing a significant improvement in light output power (Lop) by about 97.1% and much less forward voltage drop (V{sub}f) as compared to those of conventional lateral-structured GaN-based LEDs, severe current crowding under contact pad region is still a challenge issue. A novel scheme using anisotropic laser etching to the top n-GaN layer of the VM-LED accompanying an IZO TCL is proposed to release current crowding effect and further improve light emission uniformity and efficiency of the VM-LEDs in this work. Figure 1(b) illustrates the basic concept behind the proposed device structure (named as AE-LED). Through a suitable design of etching curvature on the top n-GaN layer and both the thickness and resistivity of the TCL layer, a good balance of series resistances along any possible conducting paths could be realized (e.g., R{sub}1=R{sub}2+R{sub}(TCL)). Based on the proposed device structure, distributions of current across the active region and the corresponding light emission were simulated by a device simulator ISE-TCAD [9] and results were shown in Fig. 2. Note that the thickness and doping concentration of the n-GaN layer are 3μm and ～5×10{sup}18cm{sup}(-3) respectively. Although the optimum concaved structure (including etching curvature, depth, and size, etc.) design is still underway, our results show that, as compared to VM-LED, fairly good uniformities in current and light emission distributions in the active region of the AE-LED have been obtained, indicating that the proposed AE-LED suffers a much less impact from current crowding effect. In experiments, a LED structure comprises a sapphire substrate, a buffer layer, a 0.5-μm-thick undoped GaN layer, a 3-μm-thick Si-doped n-GaN cladding layer, an undoped 5-period GaN/InGaN multiple quantum well (MQW), a Mg-doped p-cladding layer, and a 0.15- μm-thick Mg-doped GaN layer was used. Figure 3 illustrates the key fabrication processes flow of the AE-LED. It is noted that steps (a)-(d) were employed to transfer the GaN epilayer structure from the sapphire substrate onto a metal substrate comprising a (Au/Ti/Al/Ti)/electroplated-Ni metal system. And step (e) was the anisotropic etching process which employed a KrF Excimer laser (248 nm). Note that the laser beam was directed through a special design copper mask onto the surface of n-GaN epilayer and the sample was rotated during laser beam irradiation. Accordingly, the amount and depth of laser beam irradiation on the n-GaN layer was different radically,