1.
Introduction

GaN-based light-emitting diode (LED) growth on large size Si substrates is deemed as one of the most effective approaches to significantly reduce the cost of solid-state lighting^[1], despite the two major technical challenges for the epitaxial growth. The large lattice mismatch between GaN and Si (~17%) usually induces a high density of dislocation defects (10⁹–10¹⁰ cm^?2), affecting the LED efficacy^{[2, 3]}. Because of the huge misfit in the coefficient of thermal expansion (CTE, ~54%), GaN shrinks twice as fast as Si substrate during the cool-down from high temperature after the epitaxy, which often results in tensile stress and crack network formation in the GaN film^[4]. In recent years, tremendous progress has been made in the epitaxial growth of GaN on Si substrates. By employing AlN/AlGaN multiple layer or step graded AlGaN layers^[5–10], and/or SiN_x interlayer^{[11, 12]}, high-quality, crack-free GaN-on-Si with a comparable threading dislocation density (TDD) to GaN grown on sapphire substrate has been successfully realized, which greatly improved the internal quantum efficiency (IQE) of the LEDs grown on Si. In recent years, GaN-on-Si LEDs have been mass produced by LatticePower^[13], Toshiba^{[11, 12]}, Samsung^{[14, 15]}, and Plessey^[16], showing performances comparable with those of LEDs grown on sapphire and SiC substrates.



However, it is still essential to further improve the light output power (LOP) of LEDs grown on Si. Apart from improving the IQE, enhancing the light extraction efficiency (LEE) is indispensible to improve the LED LOP. In fact, the LEE of LED grown on sapphire substrates has been widely studied, and great improvements have been achieved through the optimization of surface texturing or roughening^[17–20], flip-chip packaging^[21], combination of thin-film LED concept with flip-chip technology (TFFC)^[22], photonic crystals^{[23, 24]}, and so on. In the field of high power LEDs for high injection current applications, the vertical thin-film LED (VTF-LED) and the flip-chip LED (FC-LED) configurations have been widely used as their advantages in light extraction and heat dissipation.



In this work, we report the fabrication of via-thin-film LEDs (via-TF-LEDs) to enhance the LOP of blue/white GaN-based LEDs grown on Si (111) substrates. The comparison study of the device performance indicates that the via-TF-LED can give a higher LOP and is more suitable for high-power application. What is more, via-TF-LED technology based on Si substrate is promising in improving the yield of chip fabrication and further cutting down the cost of high-power LED chips, because the fabrication processes of via-TF-LED on Si substrate are much simpler than those on sapphire substrates, as Si substrates can be easily removed by matured chemical wet etching while the complex laser-lift-off (LLO) technology is indispensable to detach sapphire substrates.

2.
Experiment

The LED epitaxial wafers with a dominated wavelength of around 455 nm were grown on 2-inch Si (111) substrates using a metalorganic chemical vapor deposition (MOCVD) system. Three kinds of LED structures were examined: the conventional lateral structure LED (LS-LED) featured with a textured indium tin oxide (ITO) surface and without removing the Si substrate, the vertical TF-LED featured with a roughened N-polar n-GaN surface and the p-GaN surface bonded to a wafer carrier with an Ag-based reflective electrode, and the via-TF-LED. Different from the vertical TF-LED with the finger-like n-electrodes fabricated on the emitting surface of the roughened N-polar n-GaN, an array of embedded n-type via pillar metal contact from the p-GaN surface etched through the multiple-quantum-wells (MQWs) into the n-GaN layer was made to serve as n-electrode in the via-TF-LED. All the LED devices had the same chip size of 1.1 × 1.1 mm². The schematic diagrams of the device structures are shown in Fig. 1.






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Figure1.
(Color online) Schematic diagrams of three kinds of LED structure. (a) LS-LED. (b) TF-LED. (c) Via-TF-LED.




The manufacturing process of via-TF-LED is as follows. Firstly, a layer of highly reflective Ag-based alloy was deposited onto the p-GaN as an ohmic contact, and also as a reflector. Subsequently, standard photolithography and etching were used to form the pattern of the reflector. Next, within the patterned reflector, the wafer was partially etched until the n-GaN was exposed. Then, a passivation layer was deposited onto the wafer by plasma-enhanced chemical vapor deposition (PECVD), followed by the partial removal of passivation layer on the n-GaN surface through standard photolithography and etching. Then, via pillar ohmic contacts with the n-GaN and bonding metal were deposited sequentially on the wafer by E-beam evaporation. After that, the wafer was flipped and bonded to a wafer carrier Si (100). The Si (111) substrate was then removed by chemical wet etching, and the AlN/AlGaN buffer layers were dry etched until the N-polar n-GaN was exposed. The N-polar n-GaN surface was roughened using KOH solution, followed by partial etching of the n-GaN to pattern the emitting area, and to expose a part of p-GaN ohmic contact where the p-electrodes were deposited.

3.
Results and discussion

Crack-free GaN-on-Si LED epitaxial wafers were obtained with a mirror-like surface after cooling down to room temperature. High-resolution X-ray diffraction (HRXRD) measurements were employed to evaluate the crystalline quality of the GaN epitaxial layers. Fig. 2 shows the X-ray rocking curves (XRC) around (0002) and (10$bar 1$2), with full width at half maximums (FWHMs) of 259 and 262 arcsec, respectively, which are comparable with that of GaN grown on sapphire substrates. From the XRC FWHMs, the threading dislocation density is estimated to be around 2.4 × 10⁸ cm^?2[25]. The high-crystalline-quality GaN layers were realized by employing an Al-composition step-graded AlN/AlGaN multi-layer buffer, including a 280-nm-thick AlN layer, a 180-nm-thick Al_0.35Ga_0.65N layer, and a 320-nm-thick Al_0.17Ga_0.83N layer, which was deposited before the thick GaN layer to build up compressive strain. The as-accumulated compressive strain can not only compensate the tensile stress due to CTE mismatch during the cool down, but also induce the inclination and annihilation of threading dislocations (TDs) at the interfaces. The detailed study about the strain relaxation and the dislocation filtering in the GaN grown on Si (111) with an AlN/Al(Ga)N multi-layer buffer can be found in our previous reports^[7–9].






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Figure2.
(Color online) X-ray rocking curves around (0002) and (10$bar 1$2) for GaN grown on Si (111) substrate.




Figs. 3 and 4 show the optical microscope images and scanning electron microscope (SEM) images of the top surface of these LED devices, respectively. Both the n-electrodes and the p-electrodes were located on the front side of the LS-LED with a cross-finger-like shape, as can be seen in the Figs. 3(a) and 4(a). For the TF-LED, however, while the n-electrodes remained on the front side of the device, the p-electrodes were at the backside, as shown in Figs. 3(b) and 4(b). For the via-TF-LED, the appearance of the optical image and the SEM image turned out a bit different. Dark spots can be distinguished in the optical image [Fig. 3(c)] while no distinct feature can be observed in the SEM image [Fig. 4(c)] except for the two p-pads at the corners. It should be noted that the dark spots in Fig. 3(c) were actually the n-type via-like pillar metal contacts embedded in the epitaxial material. The n-type via-like metal contacts could be distinguished under optical microscope because n-GaN layers are optically transparent.






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Figure3.
(Color online) Optical microscope images of the LED devices. (a) LS-LED. (b) TF-LED. (c) Via-TF-LED.






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Figure4.
(Color online) SEM images of the LED devices. (a) LS-LED. (b) TF-LED. (c) Via-TF-LED.




Fig. 5(a) shows the LOP–current–voltage (L–I–V) curves of the three types of blue LEDs: LS-LED, TF-LED and via-TF-LEDs. For all three LED structures, the LOPs increased sublinearly as the current increased. At a very low current, the LOP of the via-TF-LED was a little lower than that of the TF-LED. However, as current increased, the LOP of the via-TF-LED surpassed the other LEDs, and the differences in LOP became larger as the current further increased. The LOP of the via-TF-LED at a forward current of 350 mA was 568 mW, which was 7.8% higher than the 527 mW of the TF-LED, and 3.5 times higher than the 124 mW of the LS-LED. The significant improvement in LOP of the via-TF-LED could be attributed to three main factors. Firstly, the removal of the epitaxial Si (111) substrate can avoid the Si substrate absorption which greatly affected the LOP of the LS-LED. Secondly, the replacement of the finger-like n-electrodes on the n-GaN surface by the embedded via-like n-electrodes in the epitaxial film eliminated the light blocking by the n-electrodes on the emitting surface which affected the LOP of the TF-LED. Lastly, the light extraction was enhanced by the roughened light-escaping surface. The forward voltages at 350 mA were 3.02, 2.91, and 3.16 V for the via-TF-LED, the TF-LED and the LS-LED, respectively. The voltages of the via-TF-LED and the TF-LED were much lower than that of the LS-LED, which were due to the much better current spreading for the p-side with the Ag-based blanket ohmic contact with the p-GaN surface. In contrast, the thin textured ITO film deposited on the p-GaN surface of the LS-LED was much less capable in current spreading. The ITO film thickness was limited to avoid severe light absorption. It was noticed that the voltage of the via-TF-LED was still a little higher than that of the TF-LED, which could be reduced by increasing the thickness and the Si doping level of the n-GaN layer in the LED for improving the current spreading of the n-side.






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Figure5.
(Color online) (a) LOP–current–voltage curves and (b) white luminous flux-current-voltage curves of the three kinds of LEDs: the LS-LED, the TF-LED and the via-TF-LED.




After covering with yellow phosphor, the blue LEDs converted some blue photons into yellow light which, mixed with the transmitted blue photons, gave white light. The white luminous flux-current-voltage curves of the white LS-LED, the white TF-LED and the white via-TF-LED were measured and shown in Fig. 5(b). The white luminous flux curves of the three kinds of LEDs showed a similar trend to that of the blue LOP curves. The luminous flux of the white via-TF-LED was much higher than those of the other LEDs at high current, and the difference in luminous flux became larger as the current increased. The luminous flux of the white via-TF-LED at 350 mA was 149.9 lm, while those for the white TF-LED and the white LS-LED were 132.1 and 23.6 lm, respectively. The white via-TF-LED emitted an enhanced white luminous flux by 13.5% and over 5 times than the white TF-LED and the white LS-LED, respectively.



The external quantum efficiency (EQE) of the three kinds of blue LEDs was measured as a function of the current density (Fig. 6). For all three blue LEDs, the EQE peaked at a low current density (below 10 A/cm²), and then gradually dropped with the current density. The via-TF-LED has the highest EQE when the current density reached 1 A/cm², and the peak current density (J_max) of the via-TF-LED was larger than the other LEDs. The maximum EQE (62.7%) of the via-TF-LED appeared at about 6 A/cm², and was 4% higher than that of the TF-LED and 3 times higher than that of the LS-LED. The results indicate that the via-TF-LED outperformed the TF-LED and the LS-LED under a high current operation, and hence is more suitable for high power applications.






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Figure6.
(Color online) EQE of the three types of blue LEDs as a function of current density.

4.
Conclusion

In summary, GaN-on-Si via-TF-LED was fabricated with a roughened n-GaN surface and the p-GaN surface bonded to a wafer carrier with a Ag-based reflective electrode, together with an array of embedded n-type via pillar metal contact from the p-GaN surface etched through the MQWs into the n-GaN layer. The as-fabricated via-TF-LED showed an improved LOP than the TF-LED and the LS-LED because of the eliminated light absorption by the Si (111) epitaxial substrate and the finger-like n-electrodes on the emitting surface. When operated at 350 mA, the via-TF-LED gave an enhanced blue LOP by 7.8% and 3.5 times, and an enhanced white luminous flux by 13.5% and over 5 times, as compared to the vertical TF-LED and the conventional LS-LED. With a further reduction of the n-GaN sheet resistance, the via-TF-LED with a high EQE and a high J_max, is very suitable for high-power applications.

Acknowledgements

We are thankful for the technical support from Nano Fabrication Facility, Platform for Characterization & Test, and Nano-X of SINANO, CAS.