1. Introduction

According to a recent report, a GaN-based LED can output 1250 lm white light with power consumption of 7.3 W, indicating a luminous efficacy of 171 lm/W, which is ten times efficacy of incandescent lamp^[1]. In addition to high light output, reduction in the LED manufacturing cost has also attracted a great deal of attention in academic and industrial world. As a substrate material, silicon has many advantages in terms of lower costs than the sapphire and SiC substrate, mature process, large size, and wet etching. Silicon substrate has been considered an alternative substrate for GaN growth and low cost LED manufacturing. The difficulty of GaN growth on silicon is caused by 17% lattice constant mismatch and 59% thermal expansion coefficient mismatch. The thermal expansion mismatch between GaN and silicon leads to tensile stress in GaN and often cracks in the GaN epitaxial film. It is possible to grow crack-free GaN on silicon with a similar quality to that of GaN grown on sapphire or SiC substrate by optimizing the buffer layer. Based on Si substrate GaN epi breakthrough, a new technology platform has been developed to manufacture high performance LEDs. This new technology can significantly reduce the cost of LEDs while improving the performance. Currently the typical performance for 45 mil silicon substrate LEDs is 120 lm/W in cool white at 350 mA. Also chips made from silicon substrate have a lower reverse leakage and better resistance to ESD.

On the other side, traditional current injection to the vertical thin film LED chips can improve current dispersion and enhance light extraction efficiency^{[2, 3]}. However, the metallization and the electrode pattern will inevitably cause light absorption. There is a conflict between the light extraction efficiency and forward current^[4]: more light output needs large metallization and the electrode pattern to prevent the current-crowding effect, but larger metallization and electrode pattern can create more light absorption. In order to solve this problem, Si substrate thin-film and flip-chip LED process was proposed, which can take advantage of traditional thin-film LED and reduce the conflict as described above^[5]. Therefore, GaN-on-Si thin-film flip-chip LED can provide a new route for LEDs with low cost and high luminous efficacy.

2. Chip preparation

GaN are grown by metal-organic chemical vapor deposition (MOCVD). A fabrication process of GaN-based LEDs on Si (111) substrate is introduced using AlN buffer layer. During fabrication, the TMIn, TMGa, TMAl and NH$_{3}$ are used as In source, Ga source, Al source and N source respectively. The CP$_{2}$Mg and SiH$_{4}$ are used as Mg source and Si source for P-doped-GaN and N-doped-GaN respectively. GaN-based thin film includes: AlN buffer layer, Si doped N-GaN layer about 2.5 $\mu$m thick, InGaN/GaN active layer, P-AlGaN electronic diffusion barrier layer about 25 nm, and Mg doped P-GaN layer about 200 nm thick. The MQW active layer has nine InGaN/GaN blue light quantum wells, with thickness of the InGaN well 3 nm, and the GaN barrier 10 nm. The cross-sectional view of GaN-based MQW thin film epitaxy is shown in Fig. 1. By optimizing growth condition, a flat and crack-free GaN layer is obtained, and the FWHM of DCXRD (002) and (102) are 420 arcsec and 450 arcsec respectively, indicating a high quality GaN-based thin film. After growth, a thermal annealing process is performed on the epitaxial wafers to activate the P-type at a temperature of 600 ℃ for 20 min, with the gas composition of N$_{2}$ : O$_{2}$ $=$ 4 : 1.

The preparation processes of the LED chip are described as follows: (1) Photoetching and ICP etching the epitaxial wafer to the N-GaN layer. (2) Evaporating a layer of highly reflective Ag alloy as the P-GaN ohmic contact layer by the electron beam evaporation equipment, through photoetching and etching the Ag alloy in isolation from the N-GaN. (3) Growing a passivation layer on the Ag alloy by the PECVD equipment, through photoetching and etching, removing the passivation part which is on the N-GaN layer. (4) Evaporating a layer of N-GaN ohmic contact metal for the current dispersion and a bonding metal layer by the electron beam evaporation equipment. (5) Using the metal-bonding technique to place the wafer onto a silicon substrate under the heating and pressuring condition. (6) By using nitric acid and hydrofluoric acid mixture, removing the original growth Si (111) substrate. (7) Removing the AlN buffer layer by completely submerging the sample, from which the substrate has been already transferred, in the potassium hydroxide solution to roughen the N-GaN surface^[6]. (8) Photoetching and etching the sample to obtain the chip array. (9) Photoetching, evaporating the pad metal, and removing excess metal by lift-off. (10) Sticking the sample onto blue tape, and separating the chips. The cross-sectional view of the thin film chip is as shown in Fig. 2(a), and the optical top view under the microscope is as shown in Fig. 2(b).

The chip size prepared in the experiment is 1.1 $\times$ 1.1 mm$^2$. We can obtain blue LED lamps encapsulated with silicone and white LED lamps packaged with standard YAG phosphor.

3. Chip testing

We have tested the blue and white lamps respectively by the integral sphere tester. Figure 3 illustrates the radiometric light output and external quantum efficiency versus forward current for a silicone encapsulated blue LED lamp. The optical powers and external quantum efficiency for a silicone encapsulated blue lamp are 546 mW @ 453 nm and 50.3% at forward current of 350 mA, 956 mW @ 451.5 nm and 44.3% at forward current of 700 mA. Figure 4 illustrates the photometric light output versus forward current for a white LED lamp packaged with standard YAG phosphor. At forward current of 350 mA, the photometric light output can reach 120.1 lm, with corresponding color temperature of 5700 K and the color rendering index of 70. These results show that the LED performance is as good as that of LEDs from sapphire and SiC substrates.

4. Summary

GaN based MQW epitaxial layers are grown on Si (111) substrate using AlN as the buffer layer. Si substrate thin-film and flip-chip LEDs are prepared by substrate transferring technology in combination with thin-film and flip-chip design, to enhance light extraction efficiency and prevent current-crowding effect. The performances of blue and white LED lamps show that they are as good as those from sapphire and SiC substrate. So, GaN-on-Si based thin-film flip-chip LED technology has the potential to be future generation LED technology with low cost and high luminous efficacy.