Enhanced light extraction of nonpolar a-plane (11-20) GaN light emitting diodes on sapphire substrates by photo-enhanced chemical wet etching

1. Introduction

Nitride-based light-emitting diodes (LEDs) on nonpolar orientations have received increasing attention over the past several years due to their promise of removing the quantum-confined Stark effect. The polarization-related electric field induces electron and hole wavefunctions to be spatially separated in strained quantum well (QW) structure, leading to a reduced optical transition probability [1–3]. The absence of internal electric field in nonpolar gallium nitride (GaN) LEDs is advantageous to achieve not only highly-efficient light emitters with thicker QWs, but also improved wavelength stability with injection currents. Many research groups have successfully demonstrated nonpolar a-plane (11-20) and m-plane (1100) GaN LEDs as well as semipolar (10-1-1) and (11-22) GaN LEDs [4–12]. Masui et al. had recently reported comprehensive review on achievements and challenges on nonpolar and semipolar nitride LEDs [13].

There have been intensive efforts to increase the extraction efficiency in GaN LEDs to improve the external quantum efficiency. It is well known that a significant fraction of generated lights are trapped inside the cavity due to total internal reflection. Thus, a variety of extraction methods are currently employed such as highly reflective p-electrode, chip geometry deformation, patterned sapphire substrate, photonic crystals, and surface roughening. Since Minsky et al. had demonstrated photoenhanced chemical (PEC) wet etching on GaN, the surface texturing with hexagonal pyramidal structures on chemically active nitrogen-face GaN is now commonly used in vertical thin-film GaN LEDs due to its simple and damage-free etching process [14,15]. It is obvious that PEC wet etching can be also implemented in nonpolar and semipolar GaN films by selectively etching different crystallographic planes [16–18]. Hardy et al. recently demonstrated a nonpolar m-plane superluminescent diode by forming hexagonal pyramids on –c (000-1) face via PEC etching [19]. However, there are few reports available on PEC wet etching on heteroepitaxial a-plane GaN LEDs. In this work, we investigated how selective wet etching changes the morphology of a-plane GaN surfaces as well as mesa sidewall faces in nonpolar a-plane GaN LEDs on r-plane sapphire substrates.

2. Experimental details

High crystalline a-plane (11-20) GaN film was directly grown on r-plane (1-102) sapphire substrates by metalorganic chemical vapor deposition, utilizing a two-step growth method with high-temperature GaN buffer. Nonpolar a-plane InGaN/ GaN LEDs were subsequently grown on 4.5 μm thick a-plane GaN films. The doping concentration of the Si-doped n-GaN layer was 3 × 10¹⁸ cm⁻³, and four periods of 5 nm thick InGaN QW layer and 9 nm thick GaN barrier layer were grown for the active region, followed by 150 nm thick Mg-doped p-GaN layer with doping of 1.4 × 10¹⁸ cm⁻³. For a p-electrode, Ni/Au (3 nm/3 nm) was deposited on the p-GaN layer using an e-beam evaporator, followed by annealing in air ambient to form Ohmic contacts. The mesa pattern with dimension of 200 μm × 500 μm was transferred into the n-GaN layer down to 1 μm by inductively coupled plasma etching. Cr/Au (50 nm/400 nm) was deposited as the n-electrode.

A schematic diagram of heteroepitaxial a-plane InGaN/GaN LED structure is presented in Fig. 1(A) . Note that our a-plane non-polar InGaN/GaN LED devices are aligned with c-axis [Fig. 1(B) and 1(C)]. Figure 1(C) is a plan-view optical microscope image of LED chip. Fabricated LED devices were then immersed in a potassium hydroxide (KOH) solution for different periods of time. PEC etchings were performed at stirring rate of 300 rpm at 60°C with ultraviolet illuminations. Electrical and optical properties of LEDs were characterized by an on-wafer measurement system.

 

Fig. 1 (A). Schematic diagram of heteroepitaxial a-plane InGaN/GaN LED structure (B). Schematic diagram of GaN planes showing c-axis, c-plane and a-plane. (C) Microscope image of the processed a-plane InGaN/GaN LED.

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3. Results and discussion

Plan-view scanning electron microscope (SEM) images of the mesa sidewall face in the + c-axis [0001] direction are shown Fig. 2(A) before and Fig. 2(B) after KOH etching in KOH solution. Figure 2(B) shows a clear evidence of PEC etching on the mesa sidewall in the + c-axis direction. A bundle of trigonal prisms having various widths from hundreds of nanometer to 1 micron were observed. These submicron-sized trigonal prisms with prismatic planes of {1-100} also indicate that the normal plane is a-plane (11-20). It is notable that the p-GaN layer was roughened in this case with relatively low temperature, and the p-GaN layer was intact after PEC etching due to Ni/Au contact layer.

 

Fig. 2 SEM images in the direction of + c-axis [0001] of the mesa sidewall of LED samples (A) before and (B) after 5 min etching.

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Figure 3(A) , 3(B), and 3(C) show SEM images of the sidewall faces after 5 min etching in the direction of the + c-axis, the + m-axis [1-100], and the –c-axis [000-1], respectively. The sidewall face in the + m-axis direction turned into rough surface morphology, which was the same case for the sidewall face in the –m-axis direction. Figure 3(C) is the sidewall face in the –c-axis direction, showing different surface morphology with the one in the + c-axis direction. There appeared a specific crystallographic plane as an etch-stop plane at the bottom of the sidewall, which was previously reported to be (−1-12-2) plane [20]. Typical cone-shaped morphology can be seen on top of the sidewall in the –c-axis direction. It is important to note that the surface of n-type GaN between mesa sidewalls and n-contact is also textured simultaneously, showing a high density of trigonal prisms with a few hundred nanometer scale aligned along the + c-axis. As can be seen Fig. 2 and 3, PEC wet etching produced unique feature of etching morphology on the mesa sidewall faces and the n-type GaN surface, which is beneficial to increase the light extraction efficiency of a-plane GaN LEDs.

 

Fig. 3 SEM images of PEC textured LED samples in the directions of (A) + c-axis [0001], (B) + m-direction [1-100], and (C) –c-axis [000-1].

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In order to investigate the effects of PEC wet etching, the output powers of etched LED samples were measured at the current injection of 20mA from on-wafer measurements (Fig. 4 ). The light output powers of 5 min etched LEDs were 25% higher in average than those of control samples. The increase of output power can be attributed to the roughened n-type GaN surface as well as the sidewall faces. As shown in Fig. 5 , the radiation patterns confirm that more lights are extracted in all directions for the c-axis and m-axis rotations at 20 mA. PEC wet etching produces unique feature of etching morphology on the sidewall faces and the n-type GaN surfaces in a-plane GaN LEDs and significantly improves the extraction efficiency of heteroepitaxial a-plane GaN LEDs.

 

Fig. 4 The optical output powers of control and PEC textured LED samples for 5 min, measured on-wafer at the current injection of 20mA.

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Fig. 5 The comparison of radiation patterns of control and PEC textured a-plane LEDs, measured in m-axis and c-axis rotations.

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4. Conclusion

We report on the enhanced light extraction of heteroepitaxial a-plane GaN LEDs by selectively etching the sidewall faces and the n-type GaN surface via PEC wet etching. The light output powers of PEC etched LED samples have increased by 25% at 20mA current injection. The radiation patterns have also shown that lights are extracted at all directions, indicating that PEC wet etching is also quite effective to improve the extraction efficiency of heteroepitaxial a-plane GaN LEDs.