Vertical GaN-based light-emitting diodes (LEDs) were fabricated with a Si substrate using the wafer-bonding technique. Lapping and dry-etching processes were performed for thinning the sapphire substrate instead of removing this substrate using the laser lift-off technique and the thinning process associated with the wafer-bonding technique to feature LEDs with a sapphire-face-up structure and vertical conduction property. Compared with conventional lateral GaN/sapphire-based LEDs, GaN/Si-based vertical LEDs exhibit higher light output power and less power degradation at a high driving current, which could be attributed to the fact that vertical LEDs behave in a manner similar to flip-chip GaN/sapphire LEDs with excellent heat conduction. In addition, with an injection current of 350 mA, the output power (or forward voltage) of fabricated vertical LEDs can be enhanced (or reduced) by a magnitude of 60% (or 5%) compared with conventional GaN/sapphire-based LEDs.
© 2011 OSA
High-power GaN-based light-emitting diodes (LEDs) have been recognized as a promising next-generation general lighting source . Recently, the vertical thin-GaN LED have been identified as the most promising structure for improving the efficiency of high-power LEDs over other traditional LED structures with sapphire substrates [2–4]. Unlike conventional LED structures, a conductive substrate and a metal reflector are implanted into thin-GaN LEDs to improve light-extraction efficiency (LEE) and to reduce thermal resistance [2–4]. To achieve this goal, the well-known techniques of laser lift-off and wafer bonding processes are used to remove the epitaxial substrates (i.e., sapphires) and to support the thin-GaN films through an electrical and thermal conductive carrier substrate (such as a Si substrate), respectively. To separate GaN thin films from the sapphire substrate, laser pulses are directed through the transparent sapphire substrate and are absorbed throughout the sapphire/GaN interface, thereby resulting in the decomposition of the GaN layer, and hence, the peeling of epitaxial layers from the sapphire substrate . However, the transfer of GaN films to receptor substrates results in a considerable release in stresses, which originate from the large lattice and thermal mismatch between the GaN-based layers and sapphire substrate. The released stresses may result in structure defects and even cracks in the epitaxial layers that degrade device performance . In addition to the inherent mismatch, laser-induced damage also results in residual stresses after the laser lift-off (LLO) process, thereby degrading the electrical and optical properties of LEDs . In the present study, a vertical-type GaN-based LED (VLED) without the removal of the sapphire substrate is demonstrated. That is, thinning and etching were performed on sapphire substrate instead of the LLO process to expose the n-GaN layer for the fabrication of VLEDs. The detailed process and the properties of VLEDs are presented in the present paper.
The GaN-based LEDs used in the present study were grown using metal-organic vapor-phase epitaxy. The layer structure of the LEDs was similar to our previous reports . After epitaxial growth, multiple metal layers of Ni/Ag/Ni/Au (1/200/50/800 nm) were deposited onto the p-GaN top layer of GaN-based LED wafers to serve as the reflector/ohmic contact layer. After the formation of the reflector/ohmic contact layer, a 1.5 μm thick gold layer was used to serve as the bonding layer. In addition, a bilayer Ti/Au (20 nm/1500 nm) metal was deposited onto the surfaces of Si substrates to serve as ohmic contacts, and these wafers served as receptors for the wafer bonding process. The GaN-based LED wafers were then bonded onto the Si substrates at 350 °C for 60 min. After the bonding process, the sapphire substrates were lapped and polished to 7 μm. To expose the n+-GaN layer, the remaining sapphire layer and the undoped GaN(u-GaN) layer were selectively etched away using the dry-etching technique. BCl3 of 30 sccm was used as the etching gas associated with RF (radio frequency) power of 300 W and ICP (inductively coupled plasma) power of 100 W. The process pressure was 5 × 10−3 torr. The Ti/Al/Ti/Au (2/20/100/2000 nm) metal layers were then deposited onto the exposed n+-GaN layer to form the n-type ohmic contacts (cathode electrodes) on the LED wafers. Finally, the Si substrates were thinned to 150 μm and coated with Ti/Au (20 nm/200 nm) metals to serve as the backside ohmic contact layer. LEDs fabricated using the aforementioned process were of the vertical type, similar to the conventional vertical LEDs fabricated using the LLO and wafer bonding processes. These LEDs were labeled as LED-II. Figure 1(a) shows a schematic diagram of the vertical LED structure with a Si substrate. For comparison, lateral-type LEDs without a Si substrate were also prepared and labeled as LED-I. Figure 1(b) shows the schematic LED structure with electrodes on the same surface as the sapphire substrate. All the experimental LEDs used in the present study had an area of 1 mm × 1 mm. Room-temperature current-voltage (I-V) characteristics of experimental LEDs were measured using the HP-4156C semiconductor parameter analyzer. The light output power-current (L-I) of the LEDs were measured using a calibrated integrating sphere.
3. Results and discussions
Figure 1(c) shows the typical cross-sectional view scanning electron microscopy (SEM) image of LED-II, which was taken to inspect the etching profile of sapphire and the bonding interface between the Si substrate and GaN layer. The void-free bonding interface is displayed in the typical SEM images, indicating that the bonding interface was reasonably good. Vertical thin-GaN LEDs with a smooth bonding interface facilitates heat dissipation, mechanical support, and current conduction. The presence of voids around the bonding interface reduces the reliability and performance of the LEDs . Figure 2(a) shows the typical forward I-V characteristics of LED-I and LED-II. The forward voltages at a driving current of 350 mA (Vf) were 3.45 and 3.28 V for LED-I and LED-II, respectively. The slightly higher Vf observed in LED-I can be attributed to the relatively its higher series resistance (Rs) compared with LED-II. In fact, the Rs extracted from the I-V characteristics were approximately 1.6 and 1.1 Ω for LED-I and LED-II, respectively. As shown in Fig. 1(b), the current path between the anode and cathode electrodes in LED-I could be expected to be longer than those in LED-II. Although LED-II consists of a greater number of metal/semiconductor interfaces and an additional Si substrate, these factors in the vertical LEDs with adequate bonding and ohmic contact layers contribute limited magnitude in Rs. Therefore, the higher Rs of LED-I was attributed to the relatively larger distance between anode and cathode electrodes. In other words, the low Rs in the vertical LEDs can be attributed to a relatively better current-spreading effect compared with that of the lateral LEDs. In addition to the superior forward I-V characteristics, typical reverse leakage current (Ir) of the proposed vertical LEDs (LED-II) was comparable with the conventional lateral LEDs (LED-I) but markedly lower than the conventional vertical LEDs, as shown in Fig. 2(b). The relative higher Ir could be attributed to the fact that strain release and laser-induced damage on the GaN-based epitaxial layers led to the degradation of electrical properties of the conventional vertical LEDs. Figure 2(a) also shows the typical L-I characteristics of LED-I and LED-II. With an injection current of 350 mA, the light output powers were 205 and 243 mW for LED-I and LED-II, respectively. This significant improvement in the output power of LED-II was attributable to the increase in its LEE compared with that of LED-I. That is, LED-II behaved in a manner similar to flip-chip LEDs . For LED-II, photons were emitted through sapphire with a relatively lower refractive index compared with that of LEDs with a GaN-emitting surface. Compared with the LED-I with emitting surface, the indium tin oxide (ITO) layer possesses a relatively lower critical angle for light escaping into the air because the refractive index of ITO is larger than that of sapphire. In addition, the remaining top sapphire layer with a thickness of 7 μm was significantly thicker than that of the ITO layer. Therefore, the top sapphire layer served as a window layer to facilitate light extraction. Also, LED-II possesses the nature of thin-GaN LEDs [2–4], that is, downward-propagating light is reflected up by a metal reflector sandwiched between the p-GaN layer and Si substrate. On the other hand, the light-shadowing effect by opaque electrodes in LED-II is weaker than that in LED-I, as shown in the insets of Fig. 1 (a) and (b).
Another benefit of the design scheme of LED-II is that the heat generated in LED flows directly from the p-n junction to the Si substrate. However, heat extraction through the thermally resistive sapphire substrate is inevitable for conventional GaN-based LEDs, i.e., LED-I. The degradation of light output power and redshift of emission wavelength of LED-I attributable to junction heating are expected when the LEDs are operated under a high driving current because of the low thermal conductivity of sapphire. As shown in Fig. 2, LED-II can clearly operate up to 1.0 A without significant power degradation. However, LED-I exhibits substantial power degradation when the injection current exceeds 600 mA. Furthermore, the heating effect on the redshift of the emission wavelength was also observed. Figure 3 shows the typical curves of emission wavelengths against injection current for LED-I and LED-II. At a low injection current, the quantum-confined Stark effect (QCSE) attributable to the polarization field in the QWs reflects on the blueshift of the emission wavelengths . The redshift of the emission wavelength resulting from band-gap shrinkage at a high injection current is attributed to the junction heating effect. LED-II exhibited low Rs and excellent heat conduction, resulting in low junction heating. As shown in Fig. 3, the redshift of the emission wavelength was not observed in LED-II even when the injection current reached 1000 mA. In contrast, LED-I exhibited a significant redshift as the injection current exceeded 200 mA. The redshift occurring in the relatively lower current levels revealed that junction temperature increased rapidly with increasing injection current and that the heat extraction rate of LED-I through sapphire was lower than the heat generation rate. Thus, the band-gap shrinkage rate resulting from junction heating versus injection current was higher than that of the band-gap widening rate.
To further enhance the light output power, surface texturing was attempted to be performed on the sapphire surface of LED-II. The texturing process was conducted after the thinning of sapphire and before exposure of the n+-GaN layer. In this process, the Ag layer with a thickness of 100 nm was initially deposited on the sapphire surface and was then thermally annealed at 350 °C for 10 min under nitrogen ambient produced self-assembled Ag metal islands to serve as etching nanomask. Next, plasma etching using chlorine-based gas sources was performed on the wafers with the Ag masked sapphire surface. Figure 4 shows the typical atomic force microscopy images of the sapphire surface. After the etching process, the textured sapphire surface had a root-mean-square (RMS) roughness of approximately 84 nm. The RMS roughness of sapphire without texture was approximately 6 nm. The fabricated vertical LEDs with textured sapphire were labeled LED-III. Figure 2 also shows that the I-V characteristics of LED-III are almost identical to those of LED-II. In addition, a significant improvement in light output power was achieved. With an injection current of 350 mA, LED-III exhibited output power of approximately 330 mW, corresponding to enhancements of 60% and 35% compared with LED-I and the LED-II, respectively. These results indicated that the texturing process performed on the wafers could significantly enhance the LEE of LEDs and did not alter their electrical properties.
GaN/Si-based vertical LEDs emitted from a sapphire surface were demonstrated to have higher light output power and lower forward voltage. The sapphire layer was retained in the vertical LEDs, so these LEDs behaved in a manner similar to flip-chip GaN/sapphire LEDs with excellent heat conduction and exhibited low power degradation at high current driving. The enhanced heat conduction in GaN/Si-based vertical LEDs was indirectly evidenced by the minor shifts in emission wavelengths at high driving currents. Although the proposed vertical GaN LEDs have potential for applying to future solid-state lighting, the sapphire thinning process should be optimized to improve the uniformity and throughput of devices. For this issue, two-step thinning process using wet and dry etching is expected to be a potential solution. Related experiments are underway and the detailed results will be published elsewhere.
Financial support from the Bureau of Energy, Ministry of Economic Affairs of Taiwan, ROC, through grant No.100-D0204-6 and the LED Lighting Research Center of NCKU are appreciated. The authors would also like to acknowledge the National Science Council for the financial support of the research Grant Nos. NSC 98-2221-E-218-005-MY3, 100-2112-M-006-011-MY3 and 100-3113-E-006-015.
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