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GaN-based light-emitting diodes (LEDs) on patterned sapphire substrate (PSS) with patterned composite SiO2/Al2O3 passivation layers and TiO2/Al2O3 distributed Bragg reflector (DBR) backside reflector have been proposed and fabricated. Highly passivated Al2O3 layer deposited on indium tin oxide (ITO) layer with excellent uniformity and quality has been achieved with atomic layer deposition (ALD) technology. With a 60 mA current injection, an enhancement of 21.6%, 59.7%, and 63.4% in the light output power (LOP) at 460 nm wavelength was realized for the LED with the patterned composite SiO2/Al2O3 passivation layers, the LED with the patterned composite SiO2/Al2O3 passivation layers and Ag mirror + 3-pair TiO2/SiO2 DBR backside reflector, and the LED with the patterned composite SiO2/Al2O3 passivation layer and Ag mirror + 3-pair ALD-grown TiO2/Al2O3 DBR backside reflector as compared with the conventional LED only with a single SiO2 passivation layer, respectively.
© 2013 Optical Society of America
1. Introduction
High-efficiency GaN-based light-emitting diodes (LEDs) with emission wavelength varied from ultraviolet (UV) to visible range, have received considerable attention as a potential light source in solid-state lightings for general illumination [1]. However, the light extraction efficiency (LEE) of GaN-based LEDs remains low because of the so-called total internal reflection (TIR) that takes place at the hetero-interface between the LED surface and air [2]. Considering the refractive indices of GaN and air, the critical angle θc for TIR is as small as 24.6° [3]. This small θc hinders the light extraction seriously, as most of the light beams with an incident angle larger than θc will be reflected back to the LED and eventually absorbed by the GaN-based epitaxial layers. Therefore, several recent studies have focused on improving the LEE by changing light beam trajectories using various kinds of approaches, such as integration of two-dimensional photonic crystal structures [4–6], preparation of patterned sapphire substrate (PSS) [7–12], and control of surface textures [13–17].
In recent years, to improve the optical and electrical performances of the LEDs, some innovative designs of the structural parameters for the surface and backside of the LEDs have been reported [18–38]. As some examples, GaN-based LEDs with SiO2 [18, 19], SiNx [20] and SiONx [21, 22] passivation layers have been investigated to enhance the light output power (LOP). In order to increase luminous intensity, ammonia sulfide was used to passivate the perimeter of AlGaInP LEDs by Su et al. [23]. So and his associates [24] proposed to deposit an Al2O3 passivation layer on the surface of InGaN/GaN-based LED chips to enhance the brightness of LED lamps. On the other hand, adding a backside reñector beneath the substrate had been demonstrated to be an effective method to reflect those originally downward-going photons going upward and thus to increase the LOP [25–38]. In particular, GaN-based LEDs with TiO2/SiO2 distributed Bragg reflector (DBR) and metallic mirrors have been reported [25–29]. The air-gap DBR structure in LEDs, which acts as a light reflector to redirect light into the escape cones on both front and back sides of the LEDs, has also been developed [30–34]. By using DBR reflectors made of AlN, GaN, and AlGaN, high-efficiency LEDs with emission wavelength varied from UV to visible range have been realized [35–38].
In this work, we report high performance GaN-based LEDs grown on PSS with the patterned composite SiO2/Al2O3 passivation layers and TiO2/Al2O3 DBR backside reflector. With the help of atomic layer deposition (ALD) technology, highly passivated Al2O3 layer with excellent uniformity and quality has been achieved. Due to the selective etching characteristic of SiO2 and Al2O3, the etching process of SiO2 layer with the fluorine-based etching gases would end on the surface of Al2O3 layer underneath and would not cause any damage to indium tin oxide (ITO) or p-GaN layer. The SiO2 layer with patterned hemisphere arrays creates an ensemble of extra light-exiting angles that is powerful in decreasing the TIR effect and hence increasing the LOP. The DBR made by ALD process has been shown to have better quality than that made by electron beam evaporation (EBE), and the ALD process time is nearly the same as the EBE process or even less. Moreover, to grow a DBR composed of at least two or more kinds of materials, it is generally necessary to raise and lower the temperature frequently in the EBE process whereas the temperature can be maintained the same in the ALD process for depositing different materials. Furthermore, due to the good adhesion between the TiO2/Al2O3 DBR and the metal mirror, the fabrication process was simplified and a more reliable backside reflector was achieved.
2. Experiments
The GaN-based LEDs in this study were grown by metal organic chemical vapor deposition (MOCVD) on the C-plane (0001) 2-inch-diameter wafer with the specially designed PSS. The MOCVD growth process for the GaN-based LEDs on PSS is as follows. First of all, a 3.2 μm-thick photo-resist film was coated on the flat sapphire substrate. Then, standard photolithography was used to create patterns on the photo-resist film. The sapphire substrate was subsequently etched by using inductively coupled plasma (ICP) with Cl2 as the reactive gas. The base diameter for each pattern and the interval between patterns are 3.6 μm and 6.4 μm, respectively. The height of the pattern is approximately 1.1 μm. The GaN-based LED structure grown on PSS consists of a 30 nm-thick GaN buffer layer, a 4.5 μm thick n-type GaN:Si layer, six pairs of InGaN/GaN multiple quantum wells (MQWs) active layers, and a 0.9 μm-thick p-type GaN:Mg layer. A 240 nm-thick ITO layer was deposited on the LED top surface to serve as the transparent contact layer. Cr/Pt/Au contact was evaporated onto the surface of ITO layer as the p electrode and onto the surface of the exposed n-type GaN:Si layer as the n electrode.
Firstly, to evaluate the influence of the passivation layers on the electrical and optical performances of the GaN-based LEDs, four LED samples passivated respectively with an ALD-grown Al2O3 layer, an EBE-grown Al2O3 layer, a plasma-enhanced chemical vapor deposition (PECVD)-grown SiNx layer, and a PECVD-grown SiO2 layer were fabricated. The ALD-grown Al2O3 layer could be made to cover the whole surface of the LED except for the n- and p-type contact areas, including the side-wall surface of the MQWs active region exposed after ICP etching.
Secondly, in order to improve the electrical and optical performances of GaN-based LEDs, novel patterned composite SiO2/Al2O3 passivation layers consisting of two dielectric stack layers, SiO2 and Al2O3, were proposed and developed in this study. As shown in Fig. 1, the Al2O3 layer acts as an effective surface passivation layer and the SiO2 layer with patterned hemisphere arrays creates an ensemble of extra light-exiting angles to decrease the TIR and hence to increase the LOP. Six GaN-based LED samples with different passivation structures (but without DBR backside reflector) were fabricated and analyzed. For fabricating the patterned composite SiO2/Al2O3 passivation layers, at first a modified cleaning technology was used to remove residual contaminants on both sides of the wafers with HCl and HF acids to obtain clean surfaces with reduced roughness. Then, the double dielectric stack layers that are consisted of a 10 nm-thick Al2O3 layer deposited by ALD and a 500 nm-thick SiO2 layer deposited by PECVD, were sequentially grown on the surface of the LED to complete the preliminary fabrication process of the passivation layers. Next, a positive photo-resist film with a thickness of 1 μm was coated on the SiO2 film and exposed with a Nikon stepper 1755i7A to form the triangularly patterned array which was used as the soft etching mask. Here the diameter of the patterns is 2 μm and the spacing between the patterns is 1 μm. After development, the so-called photoresist reflow method was adopted to form the hemispherical photo-resist pattern. The photoresist reflow process is associated with the melting characteristic of the photo-resist islands. When the photoresist islands are melted, the fluid photoresist surfaces are pulled into such a shape that the surface energy of the fluid is minimized. It was found that the reflow temperature and time played the key roles in determining the shape of photoresist island pattern. When the photoresist film was baked for longer reflow time or at higher reflow temperature, the base diameter of hemisphere is larger, and the height of hemisphere is lower. ICP etching technology was utilized to transfer the hemispherical patterns to the SiO2 layer. As the CHF3/O2 gas ratio was 40:15, the etching selectivity was 1:1.1, or the etching rates for both SiO2 and O2 were nearly the same. At such an etching condition, the patterns on the photo-resist soft mask were successfully transferred to the SiO2 hard mask layer without distorting the original surface profile.
Fig. 1 (a) Schematic diagram of LED layer structure grown on PSS with the patterned composite SiO2/Al2O3 passivation layers and TiO2/Al2O3 DBR backside reflector. (b) The cross-sectional view scanning electron microscopy (SEM), images for the fabricated LED. The domain view of the patterned composite SiO2/Al2O3 passivation layers and PSS is on the upper right, and the domain view of the DBR is on the lower right.
Finally, to further enhance the LOP of the LEDs, a TiO2/Al2O3 DBR reflector was added to the backside surfaces of the lapped and polished PSS by ALD technology. Trimethylaluminium (TMA) and water vapor (H2O) were used as the chemical precursors to grow Al2O3 film. On the other hand, TiCl4 and H2O were used as precursors to grow TiO2 film. The precursors were alternatively fed into the reactor using pure N2 as the carrier gas. Both Al2O3 and TiO2 films were grown at a fixed temperature of 250 °C so that the fabricating time was saved and high quality TiO2/Al2O3 DBR reflector was achieved. In order to avoid gas phase pre-reactions caused by intermixing of the precursors, the reactor was purged with pure N2 gas after each precursor-feeding pulse. A complete growth cycle consists of 100 ms pulse of TMA or TiCl4, 1.5 s purge pulse of N2, 100 ms pulse of water vapor, and 1.5 s purge pulse of N2. After the DBR growth, a 100 nm-thick Al or Ag mirror was deposited onto the DBR surface by EB deposition process to further enhance the reflectivity of the backside reflector. 10 nm-thick Ni film was also deposited onto the surface of Ag mirror to prevent the formation of silver sulfide. The thickness for each TiO2 (n = 2.5) layer and each Al2O3 (n = 1.6) layer in the TiO2/Al2O3 DBR reflector was kept at 49 and 67 nm, respectively.
Current-voltage (I-V) and LOP measurements of the LED chips were performed by FitTech IPT6000 LED chip/wafer probing and testing system. And a spectroscopic ellipsometry (SE) system [HORIBA-JY UVISEL] was employed to measure the reflectivity of the fabricated backside reflectors. MATLAB software was used to simulate the reflectivity of DBR. The measured refractive index of TiO2 and Al2O3 were used to improve the accuracy of simulation. And a microwave probe station was employed to measure the reverse leakage current, IR for the fabricated LEDs. To characterize the electro-static discharge (ESD) endurance of the LEDs, we applied negative ESD bias onto the LED chips up to 2000 V using the human body mode (HBM). After each test, a reverse bias of −5 V was applied to the LED chips and if the leakage current was larger than 5 μA, the LED chip was considered to be failed. The ESD yield is defined as the number of LED chips passing the test divided by the total number of LED chips tested.
3. Results and discussion
As can be seen in Table 1, the GaN-based LED with a PECVD-grown SiO2 passivation layer has the highest LOP, meanwhile it has the largest leakage current or the worst passivation effect. In contrast, the LED with an ALD-grown Al2O3 passivation layer exhibits the best passivation effect and decent LOP. The reason for this result is because the ALD technology offers excellent uniformity, great control capability, and high accuracy in layer thickness. In other words, the quality of ALD-made Al2O3 film is the best within the four LED samples with various kinds of single passivation layer whereas the ALD process time is nearly the same as EBE process or even less.
Table 1. Summary of the electrical and optical performances for the LEDs with an ALD-grown Al2O3 passivation layer, an EBE-grown Al2O3 passivation layer, a PECVD-grown SiNx passivation layer, and a PECVD-grown SiO2 passivation layer, respectively.
Moreover, as shown in Fig. 1(a), since the side-wall surface of the LED chip could be covered with the Al2O3 film by ALD, the leakage current and the probability of nonradiative recombination through the surface trap could be significantly reduced, and the radiative recombination efðciency in the MQWs active region near the side-wall edge could be effectively increased. The cross-sectional view SEM image for a typical fabricated GaN-based LED with patterned composite SiO2/Al2O3 passivation layers and TiO2/Al2O3 DBR backside reflector is shown in Fig. 1(b). After scribing and breaking the wafers into the LED chips, Ag or Al film was found to be partially lifted off from the conventional Ag + TiO2/SiO2 DBR or Al + TiO2/SiO2 DBR reflector, whereas the Ag or Al film on the Ag + TiO2/Al2O3 DBR or Al + TiO2/Al2O3 DBR reflector was not lifted off.
Table 2 summarizes the detailed reflow conditions and etched surface profile parameters for the patterned composite SiO2/Al2O3 passivation layers.
Table 2. The detailed reflow conditions and etched surface profile parameters for the patterned composite SiO2/Al2O3 passivation layers.
Figures 2(a)-2(f) show the cross-sectional (upper) and top-view (below) SEM images for the six LED samples with different types of patterned composite SiO2/Al2O3 passivation layers fabricated under different reflow conditions. The LEDs made with 0, 3, 5, 7, 9, and 11 min-reflow process are denoted as samples A, B, C, D, E, and F, respectively. It was noted that at a constant reflow temperature, the base diameter of hemisphere increased and the height of hemisphere decreased when the reflow time was increased. Figure 3(a) shows the photograph taken with a microscope for a typical LED chip fabricated with a size of 300 μm × 700 μm in our study. By means wet-etching technology, the patterned composite SiO2/Al2O3 passivation layers on the surfaces of both p-electrode and n-electrode were then removed with HF acid without any damage to the electrodes, as shown in Fig. 3(b).
Fig. 2 The cross-sectional (upper) and top view (below) SEM images for the six LED samples with different patterned composite SiO2/Al2O3 passivation layers, denoted as sample A (a), B (b), C (c), D (d), E (e), and F (f), respectively.
Fig. 3 (a) The microscope image for a typical GaN-based LED chip in our study with a size of 300 μm × 700 μm; (b) the microscope images for both p-electrode and n-electrode before and after wet etching.
Table 3 shows the electrical and optical performances of the LED samples A, B, C, D, E, F, and the reference sample only with a single SiO2 passivation layer. It is clear from Table 3 that the highest LOP was achieved with sample E in which the height and base diameter of the SiO2 hemispheres is 350 nm and 1.7 μm, respectively. On the other hand, the leakage current IR was identical (~1.1 × 10−9 A at −5 V reverse bias) for all the six LED samples with the patterned composite SiO2/Al2O3 passivation layers, which is, however, at least two orders lower in magnitude as compared with that of the reference sample (1.8 × 10−7 A at −5 V reverse bias). Therefore, sample E was identified as the baseline sample with the patterned composite SiO2/Al2O3 passivation layers to fabricate the LEDs with the backside reflector added.
Table 3. The electrical and optical performances of the LED samples A, B, C, D, E, F, and the reference sample only with a single SiO2 passivation layer.
Figure 4(a) shows I-V characteristics for the LED only with 9 min-reflow patterned composite SiO2/Al2O3 passivation layers (sample E), the LED with 9 min-reflow patterned composite SiO2/Al2O3 passivation layers and Al mirror + 3-pair TiO2/SiO2 DBR backside reflector (sample G), the LED with 9 min-reflow patterned composite SiO2/Al2O3 passivation layers and Al mirror + 3-pair ALD-grown TiO2/Al2O3 DBR (sample H), the LED with 9 min-reflow patterned composite SiO2/Al2O3 passivation layers and Ag mirror + 3-pair TiO2/SiO2 DBR (sample I), the LED with 9 min-reflow patterned composite SiO2/Al2O3 passivation layers and Ag mirror + 3-pair ALD-grown TiO2/Al2O3 DBR (sample J), and the reference LED sample only with a single SiO2 passivation layer. As shown clearly in Fig. 4(a), the I-V curves for these six LED samples were almost identical because they had exactly the same epitaxial layer structure and passivation layers. In fact, with a 60 mA current injection, it was found that the forward voltages were 3.1 V for all these six LED samples. Figure 4(b) shows the electroluminescence (EL) spectra for the six LED samples. Even if the dominant EL emission peak is identical at 460 nm for all the six LED samples, the EL intensity of sample J was the highest.
Figure 5(a) shows the LOP for the fabricated LEDs as a function of the injection current. With a 60 mA current injection, the light output powers were 55.2, 68.4, 70.6, 72.5, 74.2 and 45.4 mW for the samples E, G, H, I, J, and the reference sample, respectively. It is remarkable in Fig. 5(a) that the LOP of the sample J is 34.4%, 8.5%, 5.1%, 2.3%, and 63.4% higher than the samples E, G, H, I, and the reference, respectively. In other words, the optical performance of the GaN-based LEDs can be greatly improved by the introduction of the patterned composite SiO2/Al2O3 passivation layers and Ag mirror + 3-pair ALD-TiO2/Al2O3 DBR backside reflector. The reverse leakage current is an important guideline to characterize the electrical performance of the GaN-based LEDs. Figure 5(b) shows the reverse leakage current IR as a function of the reverse bias voltage for sample E, sample J, and the reference sample. As shown in the figure, the leakage current at −5 V bias for sample E (−1.1 × 10−9 A) and sample J (−1.8 × 10−9 A) is of the same order in magnitude, and at least two orders lower in magnitude compared to that of the reference (−1.8 × 10−7 A). It also seems that the fabrication of DBR backside reflector has little negative impact on the passivation effect, which could be reduced by the employment of the ALD-grown Al2O3 layer. This fact indicates once again that the ALD-grown Al2O3 layer is very effective to passivate the exposed surfaces of ITO and n-GaN layers after ICP etching and hence to decrease the trap density near the surface, minimizing the leakage current through the surface of the LED. The decrease in trap density with the use of ALD-grown Al2O3 passivation layer on the surfaces of ITO and n-GaN layers leads to a decrease in nonradiative surface recombination, which in turn increases the radiative efficiency and enhances the LOP of the LED. ESD endurance or pass yield is an important parameter reflecting the passivation effect of the LED. Figures 6(a) and 6(b) show the ESD pass yield mapping under 2000V reverse voltage for the sample E and the sample J. The ESD yield for both sample E (93.67%) and sample J (93.15%) stay at a similar level higher than 93%, implying that the GaN-based LEDs with the patterned composite SiO2/Al2O3 passivation layers have excellent passivation effect regardless of the addition of the DBR backside reflectors.
Fig. 5 (a) The LOP of the six fabricated LEDs as a function of the injection current; (b) the reverse leakage current IR as a function of reverse bias voltage for sample E, sample J, and the reference sample. Inset in (b) shows the enlarged IR curves for sample E and sample J.
Fig. 6 The ESD pass yield mapping under −2000 V reverse bias voltage for (a) sample E and (b) sample J.
4. Conclusion
GaN-based LEDs grown on the PSS with the patterned composite SiO2/Al2O3 passivation layers and TiO2/Al2O3 DBR backside reflector have been proposed and fabricated. Highly passivated Al2O3 layer deposited on ITO layer with excellent uniformity and quality has been achieved with ALD technology. The SiO2 layer deposited on the surface of Al2O3 layer and patterned with hemisphere arrays creates an ensemble of extra light-exiting angles that is powerful in decreasing the TIR effect and hence increasing the LOP of the GaN-based LED. The leakage current at a reverse bias of −5 V for the LED passivated with the patterned composite SiO2/Al2O3 layers was found to be at least two orders lower in magnitude compared to the LED passivated with a conventional single SiO2 layer. Moreover, to further enhance the LOP of the GaN-based LEDs, an ALD-grown TiO2/Al2O3 DBR reflector was added to the backside of the devices. In addition, it was demonstrated that the adhesion problem between the metal mirror and conventional TiO2/SiO2 DBR could be solved by utilizing the ALD-grown TiO2/Al2O3 DBR. These results show that both the patterned composite SiO2/Al2O3 passivation layers and the ALD-grown TiO2/Al2O3 DBR backside reflector are extremely useful to greatly improve the optical and electrical characteristics of the GaN-based LEDs, and are thus very promising for fabricating high performance and high power GaN-based LEDs.
Acknowledgments
The authors would like to thank Dr. L. Han, Dr. Q. Ge, and Dr. B. Sun for their fruitful discussion and technical support throughout this work.
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