1. Introduction

Currently, there is a growing interest to study high-performance nitride device on bulk GaN substrate, such as light-emitting diodes, laser diodes and heterostructure field-effect transistors. High-performance LEDs in the green [1] and ultraviolet [2,3] region have already been achieved on several semipolar planes due to their reduced polarization-related electric fields. However, for the potential application in solid-state lighting using these nitride-based LEDs, it is still necessary to further improve output efficiency. It has been found that light extraction efficiency of GaN-based LEDs is mainly limited by the large difference in refractive index between GaN film and the surrounding air. Only a few photons can escape from the GaN film to air, since the refractive indexes of GaN and air are 2.5 and 1. Therefore, any successful approach to improve light extraction efficiency needs to decrease the total internal reflection and increase the possibility of photon scattering at the interface from semiconductor to air. Over the past decade, the improvement of light-extraction efficiency in InGaN-based LEDs has been realized through ZnO nanorod array [4], photonic crystal structures [5–7], graded-reflective-index antireflection coating [8], nanorod arrays using SiO₂ nanosphere [9,10], and random surface texture formed by a photoelectrochemical (PEC) process [11–13]. Among these methods, PEC etching has some advantages in terms of easy scalability to large wafers and the processing cost. However it has been found that the cone density [14] is the main factor to determine the extraction efficiency of LEDs, and there is a limit to enhance the extraction efficiency if the surface textures are formed on a two-dimensional planar surface. Lee [15] et al. and Kim [16] et al. reported that LEDs fabricated with integrated texture surface can increase the angular randomization of photons at the emission surface, which showed a large enhancement of light output power.

Compared with vertical-injection thin-film LEDs, thin-film flip-chip LEDs have advantages in light extraction for avoiding absorption from the top-contact and wire bond pads. Horng [17] et al. have reported that the light output power of thin-film flip-chip LED can be greatly enhanced by using patterned sapphire together with chemical wet-etching. However, the application of thin-film flip-chip LEDs is still hindered by the laser lift off (LLO) process [18] and its lambert-like radiation profile [19]. Flip-chip LEDs on bulk GaN substrate can neglect this effect caused by LLO process, and with this structure, various nano-structures can also be easily transferred to n-GaN surface through a variety of nanotechnology. Furthermore, there is more flexibility for the relatively thick light-output window layer to change the lambert-like radiation profile. Recently, special chip shaping [20] designs have been used to improve the light extraction efficiency of bulk c-plane light-emitting diodes, and output power of LEDs with a roughened backside GaN substrate through a chemical wet-etching process have also been reported [21]. But the output power of the device on bulk GaN substrate still need to be further improved. In this paper, the flip-chip light-emitting diodes were fabricated on freestanding GaN substrate (FS-FCLEDs) with hemisphere-cones-hybrid surface using self-assembled CsCl nano-islands and chemical wet etching. The morphology of surface textures and its influence on the optical properties of the FS-FCLEDs were also investigated. Compared with the corresponding flat LEDs, FS-FCLEDs with hemisphere-cones-hybrid surface show a great enhancement of light extraction efficiency. This method can also help improve the light extraction efficiency of nonpolar- and semipolar- oriented devices [22].

2. Fabrication

The FCLEDs were grown on 2-in. commercially available low-defect-density freestanding GaN substrates via Aixtron metal organic chemical vapor deposition system (MOCVD). The epitaxial structure consists of 2 µm thick undoped GaN, 4 µm thick Si-doped GaN, five-periods InGaN/GaN multiple quantum wells (MQWs), a 20 nm-thick Al_0.15Ga_0.85N electron blocking layer and 150nm thick Mg-doped p-type GaN. Next, the LED mesa (with 1 × 1mm²) was obtained by standard photolithography patterning. After cleaning, a passivation (SiO₂) layer was deposited onto the cutting wall face followed by p- and n- electrode deposition process, We deposited Ni–Ag–Pt–Au (5Å/3000Å/500Å/5000Å) onto the p-GaN top layer as p-ohmic contact layer and reflective mirror using an electron-beam evaporator followed by a rapid thermal annealing at 500°C for 1min in air ambient. The n-contact metal Ti/Al/Ti/Au (200Å/800Å/300Å/10000Å) was then deposited with appropriate photolithography and lift-off process. Finally, after polishing the layer thickness to 300µm by mechanical polishing, a nanotextured process was employed as shown in Fig. 1 . Firstly, a CsCl thin film was deposited on the top surface of FS-FCLEDs using our homemade thermal evaporation system at room temperature. After exposing thin film in water vapor, self-assembled CsCl nanoislands formed on the n-GaN surface. Next, CsCl nanoislands were transferred by ICP etching at excitation power 450W, bias power 75, and chamber pressure 4 mTorr with a Cl₂/Ar₂/BCl₃ flow rate of 45/5/5 SCCM. After the remaining CsCl islands removed by deionized water, we divided them into five different subgroups and further etched them using 2M KOH solution at temperature 60°C for 5, 10, 15, 20, 25 minutes, respectively.

 

Fig. 1 Schematic diagrams of fabrication process for nanotextured FS-FCLEDs.

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3. Results and discussion

Figure 2 shows the top-view SEM images of CsCl nanoislands on n-GaN surface before and after ICP etching. We can see that different size (800, 600 and 300 nm) of CsCl nanoislands were successfully transferred to n-GaN surface by ICP etching. The CsCl nanoislands were developed at room temperature with a relative humidity of 40-50% for 20-50 min. The coverage ratio of the above different size CsCl nanoislands is 30% (the ratio of area covered by CsCl nanoislands to the GaN surface). The average diameter and the coverage ratio of CsCl nanoislands can be controlled by changing the CsCl film thickness, relative humidity, exposed time and temperature. We also found that with the diameter of CsCl nanoisland decreasing, it will lead to the asymmetry of nanoisland increasing.

 

Fig. 2 SEM images of self-assembled CsCl nanoislands on GaN surface before ((a), (b), (c)) and after ((d), (e), (f)) ICP etching with average size of 800nm, 600nm, 300nm respectively.

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Figure 3(a) shows 45 tilt-view SEM images of FS-FCLEDs after CsCl nanoislands with an average diameter 800nm successfully transferred to n-GaN surface. From the magnified view of the top surface of n-type GaN shown in Fig. 3(b), we can observe a great number of self-assembled nano-cones standing amid the hemispheres. This may be because that the relatively small CsCl nano-islands can be formed among the big CsCl nano-islands on the n-GaN surface during the self-assembly process, as shown in the insert of Fig. 3(c). the details of the individual hemisphere can be clearly seen in cross-sectional SEM image, as shown in Fig. 3(d). The average height, diameter and density of hemisphere of n-GaN are 400-500nm, 800-900nm and 10⁸cm⁻², respectively.

 

Fig. 3 (a) SEM images of the nanotextured n-GaN surface; (b) morphology of hemispheres; (c) morphology of nano-cones; (d) cross-sectional view of nanotextured n-GaN.

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Since the average diameter of CsCl nanoislands can be easily controlled by the growth conditions, it will be easier to transfer different size of nanostructure to n-GaN surface by adjusting the ICP etching conditions. Herein, it is very important to find the influence of morphology of nonatextured surface on the optical properties of the FS-FCLEDs. To optimize nonatextured surface for light extraction, a 2D finite-difference time-domain simulation (FDTD) method was employed in this study, as shown in Fig. 4 . The structure used in this simulation model was a 300um thick LED chip with a 400um side dimension, and 100nm silver reflector placed at the bottom side. The refraction index of GaN is about 2.5. A point dipole polarized along the x, y, and z direction was used as a radiation source. The wavelength of the light source is 460 nm. We use the perfectly matched layer (PML) boundary condition for the simulation except for the silver mirror treated with metal boundary condition. The output power of the FS-FCLEDs was detected by a power monitor above the LED structure. Statistical average output power was obtained by the sum of the result of each simulation. Meanwhile box monitors was used to receive the output power of overall emitted power from the source. The extraction efficiency of hemisphere-shaped FS-FCLEDs was calculated from the power flux received by power monitors with respect to the overall emitted power from the source. In order to agree with experimental structure, the ratio of the hemisphere diameter and height was 2 with a coverage ratio 30%. Figure 4(b) shows the extraction efficiency as a function of hemisphere sizes. Compared to flat FCLED, the FS-FCLEDs with an average hemisphere size 800nm have the extraction efficiency 45.2%. Small or large hemisphere is not suitable for enhancement of light extraction.

 

Fig. 4 (a) Schematic FS-FCLEDs with hemisphere-shaped arrays for numerical analysis. (b) light extraction efficiency as a function of hemisphere sizes for FS-FCLEDs.

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Figure 5 shows the L-I characteristics of flat LEDs and nanotextured LEDs. Compared to the flat-LEDs, the nanotextured-LED shows a larger enhancement of light extraction efficiency. The light output power of nanotextured LEDs with average diameters of 800 and 1000nm hemisphere were increased with enhancement factor of 1.5 and 1.32 under an injection current of 350 mA, respectively. The inset of Fig. 5 shows optical microscopic images of (a) flat LED, and (b) nano-textured LED with 800nm hemisphere operated at an injection current of 10mA, respectively. In comparison with a flat LED, stronger optical emission with better uniform distribution was observed in nanotextured FS-FCLEDs. This result shows that the light-extraction efficiency of InGaN LED can be significantly improved when the n-GaN layer is textured with self-assembled CsCl nano-islands.

 

Fig. 5 L-I characteristics of flat-LEDs and nanotextured FS-FCLEDs and the optical micrographs of flat-LEDs (a) and nanotextured FS-FCLEDs(b) at 10mA injection current respectively.

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To further improve the extraction efficiency of FS-FCLEDs, the chemical wet etching technology was also employed to roughen the hemisphere-shaped n-GaN surface. The output power of mean performing devices of different chemical etching time from 5 to 25 minutes was compared. The output power of LEDs at 350mA drive current was chosen to measure the device performance. As shown in Fig. 6(a) , with the chemical etching time increasing from 10 to 25 minutes, the output power decreases sharply. This behavior is attributed to the decreased size and density of the nano-cones reduces the randomization of photons with increasing chemical etching time. Compared to the non-wet etching and flat FS-FCLEDs, the optical power of FS-FCLEDs with further roughening under conditions of 2M KOH solution, etching temperature 60°C, 10min etching time shows an enhancement factor of 1.27 and 1.9 respectively. Such an enhancement can be explained by that the hemisphere-cones-hybrid surface can reduce the internal total reflection and increase the light scattering probability.

 

Fig. 6 (a) Light-output power of the nanotextured FS-FCLEDs as a function of chemical etching times, the corresponding SEM images of nanotextured n-GaN surface are shown in the background; FDTD simulation results of wave propagating through (b) individual hemisphere surface, (c) individual hemisphere-cones-hybrid surface (color bar represents log scale of electric intensity).

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To understand this phenomenon, we use FDTD to simulate the FS-FCLEDs with hemisphere-shaped surface and hemisphere-cones-hybrid surface at the same boundary conditions, as shown in Fig. 4(a). A field profile monitor was used to receive the electronic field around the nanotextured surface. In order to accurately depict light scattering at the nanotextured surface, a mesh range from an individual hemisphere with 5nm step is employed. We compared the propagation of electromagnetic waves passing through at individual hemisphere and hybrid surface, as shown in Figs. 6(b) and 6(c). It can be seen that the hybrid surface exhibits the more distinct light scattering. To analyze the hybrid surface, wave like features such as interference and diffraction must be taken into consideration. The distinct light scattering behavior of integrated surface with hemisphere and nano-cones is due to a combination of diffraction of waves at the nano-cones edges, and complicated interference of waves within the hemisphere and nano-cones [23].

4. Conclusion

In summary, the enhancement of the light extraction efficiency of FCLEDs on freestanding GaN substrate with hemisphere-cones-hybrid surface fabricated by both dry and wet etching method was observed. The hemisphere arrays were transferred from self-assembled Cesium Chloride (CsCl) nanoislands by ICP dry etching. Compared to flat LEDs, the light output power of the LEDs with hemisphere surface shows an efficiency enhancement factor of 1.5 at the injection current 350mA. The LEDs with hemisphere array surface have been treated with chemical wet etching for different etching time to further improve the light extraction efficiency. Compared with flat FS-FCLEDs, an enhancement factor of up to 1.9 on optical output power from FS-FCLEDs with hemisphere-cones-hybrid surface has been observed. FDTD simulation results show that the enhancement of the output power is mainly attributed to the increased light scattering by using the hybrid surface due to a combination of diffraction of waves at the nano-cones edges, and complicated interference of waves within the hemisphere and nano-cones. With the rapid development of the nitride-based LED and the solid-state lighting, the bulk crystal homoepitaxy will become more common. The results indicate that the method employed in this study will be a very promising way to significantly enhance the light extraction of the bulk nitride-based devices.