High power vertical-structure GaN-based LEDs with improved current spreading and blocking designs

Based on experimental results which reveal that the contact of Indium-Zinc-Oxide (IZO) and IZO/Ti to n-GaN layer is Schottky and ohmic, respectively, localized Ti deposition associated with a transparent IZO layer is proposed to serve as both current blocking and current spreading layer. In addition, an anisotropic mesa etching on the surface layer (n-GaN) of regular Vertical-conducting Metal-substrate GaN-based Light-Emitting Diodes (VM-LEDs) is also proposed to further decrease the resistance difference between the outside path and the inner one. The effectiveness of the proposed schemes were verified by a two-dimensional device simulator (ISE-TCAD), which indicates that significant immune of current crowding under cathode contact pad would be possible once an optimal combination of the n-GaN layer etching depth and width, IZO thickness, and Schottky blocking width has been achieved. In experiments, 40-mil LEDs with an anisotropic mesa etching of 400 μm in width and 2 um in depth, 200 μm in Schottky blocking width, and a 300-nm-thick IZO layer have been successfully fabricated. Typical improvement in light output power by about 25% at an injection current of 350 mA as compared to the regular VM-LEDs 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 release the problems of regular lateral structure LEDs mentioned above by means of vertical-conducting structure, transparent conduction layer (TCL), and surface texturing, etc., have been reported and encouraging results have been achieved [3-4]. The authors' group has developed a Vertical-conducting Metal-substrate GaN-based Light-Emitting Diodes named as VM-LEDs [5-6], as shown in Fig. 1(a). To effectively solve the current crowding effect (CCE) in regular VM-LEDs, we further developed a graded TCL/n-GaN scheme [7-8]. Though LEDs based on the graded TCL/n-GaN scheme has been shown providing a significant improvement in light output power (Lop) and much more uniform in current and light emission distribution as compared to those of regular VM-LEDs, the use of Excimer laser etching is still a challenge issue of expensive cost and unsuitable for batch process. A novel scheme using patterned Ti deposition associated with a transparent IZO layer is proposed for current blocking and spreading of VM-LED. An anisotropic mesa etching on the surface layer (n-GaN) of the VM-LED associated with the IZO TCL is also proposed to further release CCE of the device. Figure 1(b) illustrates the basic concept behind the proposed device structure. In addition to current blocking via a Schottky IZO/n-GaN contact, once the top n-GaN layer was properly mesa-etched and the thickness and resistivity of the TCL layer has been optimized, the overall difference in series resistances along any two possible conducting paths could be minimized. Based on the proposed 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 3 μm and ∼5×10 ¹⁸ cm^-3 respectively. Although the optimum structure (including etching width, depth, and TCL thickness, etc.) design is still underway, our results reveals that, as compared to regular VM-LED, fairly good uniformities in current and light emission distributions in the active region have been obtained, indicating that the proposed device confronts a much less impact from CCE. In experiments, an 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 proposed device. It is noted that steps (a)-(c) 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 [5-6]. Step (d) was the mesa etching process using an inductively coupled plasma (ICP) system. The nanofocus OM photograph shown in Fig. 4(a) reveals that the n-GaN layer has a smooth mesa surface with an etching width of around 400 um and depth of around 2 μm [Fig. 4(b)]. Step (e) was the patterned Ti deposition process in which a layer Ti of 2.5 nm was deposited on the top n-GaN layer to form ohmic contact. Step (f) was IZO deposition process which a transparent IZO layer with thickness of 300 nm was deposited to serve as current spreading and current blocking. Finally, an ohmic electrode was deposited onto the top of fabricated device. Note that devices without anisotropic etching and without current blocking (i.e., regular VM-LEDs) were also fabricated for comparison.Based on controlled samples, sheet resistance and resistivity of the anisotropic etching n-GaN/TCL structure are of around 10 Ω/□ and 8×10^-5 Ω-cm, respectively. Based on an LED tester, typical measured light output power-current (Lop-I) and current-voltage (I-V) characteristics of proposed LED and VM-LED were shown in Fig. 5. The insets of the figure also show the photos of light emission from each device at 350 mA. The corresponding light emission patterns were obtained and analyzed by using software of Process Diffraction [10]. The solid curves at the bottom of the images denote the relative light output intensity along the dashed lines. Our preliminary results from more than 50 samples show that an average improvement in Lop by about 25% @ 350 mA has been obtained from proposed devices as compared to that of regular VM-LEDs. Slightly large forward voltage of proposed device (3.549 V @350 mA) compared to that of VM-LED (3.462 V @350 mA), which might come from the Schottky blocking. In conclusion, a vertical GaN-based LED device with a Schottky current blocking and a mesa etching employed to improve current crowding effect and light emission distribution has been proposed and demonstrated. Theoretical calculations show that the uniformities of current and light emission distribution strongly depend on the structure and material parameters of both n-GaN and TCL layers as well as the geometries of the mesa etching and Schottky blocking. In experiments, VM-LEDs with the proposed structure have been successfully fabricated and an average improvement in light output power by about 25% at an injection current of 350 mA as compared to that of regular VM-LEDs has been obtained. It is expected that the proposed device structure using ICP mesa etching associated with TCL and Schottky current blocking would provide an efficient way in avoiding CCE in larger-area high-power LEDs and would be very advantageous for the applications of LEDs on solid-state lighting.