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Indium tin oxide (ITO)/ indium oxide (InxO) double layer structure was adopted as the transparent conduction and light scattering function layer to improve the light extraction efficiency of the GaN-based blue LEDs. The double layer structure was first deposited in one run by electron beam evaporation using ITO and Indium as the source respectively, and then annealed in an oxygen environment. This method can fabricate transparent electrode with microstructure and low specific contact resistivity one time free from lithography and etching, which makes the fabrication process simple and at a lower cost. For the 220 nm ITO/ 170 nm InxO double layer sample annealed at 600°C for 15 min in oxygen, measurement results show that its root mean square of roughness of the surface microstructure can be as high as 85.2 nm which introduces the strongest light scattering. Its light transmittance at 450 nm can maintain 92.4%. At the same time, it can realize lower specific contact resistivity with p-InGaN. Compared with the GaN-based blue LEDs with only 220 nm ITO electrode, the light output power of the LEDs with 220 nm ITO/ 170 nm InxO double layer structure can be increased about 58.8%, and working voltage at 20 mA injection current is decreased about 0.23 V due to the enhanced current spreading capability. The light output power improvement is also theoretically convinced by finite difference time domain simulations.
© 2016 Optical Society of America
1. Introduction
GaN-based high efficiency white light emitting diodes (LEDs) are considered as the third generation lighting sources [1–4 ]. The distinct difference of refractive index between the GaN layer and packaging material, which will decrease the light extraction efficiency due to the total internal reflection (TIR), is one of the key obstacles to enhance the performance of the blue LEDs [5–8 ]. To improve the LED light extraction efficiency, many methods have been developed, including photonic crystals [9, 10 ], surface texturing [11,12 ], flip-chip LEDs [13,14 ], graded-index materials [15] and patterned sapphire substrates [16, 17 ]. For example, hybrid ZnO micro-mesh and nanorod arrays were fabricated based on photolithography techniques and a two-step wet chemical growth process [18–20 ]. By using the laser-lift-off technique followed by an anisotropic etching process, n-side-up GaN-based LEDs with a hexagonal “cone-like” surface have been fabricated [21–24 ]. Surface textured indium tin oxide window layers were investigated using natural lithography with polystyrene spheres as the etching mask [25–27 ]. It can be seen that the above methods need additional lithography or etching process which increased the complexity of the production process and the costs.
In order to shorten the processing time, decrease the fabrication steps, and reduce the cost, it is desired to enhance the light extraction efficiency of the LEDs without additional lithography and etching process. A typical work is the in situ epitaxy of the p-type GaN layer with surface having truncated pyramids [28]. Furthermore, nanometer technology has played a key role. For instance, Indium-tin-oxide (ITO) nanorods prepared by oblique electron-beam evaporation in a nitrogen ambient was demonstrated to increase the light output power of the GaN-based LEDs by 35.1% [29]. NiCoO nanoparticles formed on the p-GaN layer by solution-based synthesis have been developed [30]. These methods may need a longer processing time or special requirements for equipment.
On the other hand, for indium material, it is easy to obtain simultaneously reasonable conductivity, transmissivity and surface roughness only by annealing in oxygen. As anti-reflection layer, InxO nanomaterials have received considerable attentions in the fabrication of Si solar cells [31–33 ].The refractive index of InxO at wavelength of 450 nm is about 1.94, which is lower than that of the GaN layer (nGaN = 2.54). The total internal reflection critical angle on the interface of GaN layer and InxO layer can be as high as 49.8° [34,35 ]. In addition, the roughness of the InxO can be used to enhance the light extraction efficiency of the LEDs. However, there seems no report that transparent, conductive InxO layer with surface roughness fabricated by indium materials annealing in oxygen have been used to enhance the light extraction efficiency of GaN-based blue LEDs.
In this paper, indium oxide formed by annealing indium layer in oxygen was adopted as the transparent/conduction layer with rough surface to enhance the light extraction efficiency of the GaN-based blue LEDs. In order to alleviate the lower conductivity of the InxO material, ITO was deposited between p-InGaN and InxO. In this process, ITO and indium were first deposited by electron beam evaporation at the same run, then annealed in oxygen to achieve lower specific contact resistivity, higher optical transmittance and stronger light scattering. Therefore, the fabrication process is just the same as the fabrication of general ohmic contact, and the additional lithography and etching are eliminated. Theoretical simulations and experiments are performed to ascertain the optimal fabrication condition of the InxO microstructure. Our results show that both the optical and electrical properties of the GaN-based blue LEDs can be improved by this one-step fabrication technology. This simple, low-cost fabrication process can be easily transferred to the LED chip fabrication industry.
2. Fabrication and characterization of ITO/ InxO transparent ohmic contact with microstructure formed by annealing in oxygen
2.1 Fabrication of ITO/ InxO samples
The ITO/ InxO samples were prepared on the quartz glass and their fabrication process is illustrated in Fig. 1 . Firstly, a ITO film with thickness of 220 nm was evaporated by using home-made electron beam vacuum deposition system and ITO with 90% InO: 10% SnO as the evaporation source material. Subsequently, the indium film was deposited on the 220 nm ITO layer at the same evaporation run. Finally, ITO/ InxO samples were annealed at 600°C in oxygen for 5 min, 10 min, 15 min and 20 min to form the microstructure and increase the optical transmittance by using a high temperature tube furnace, respectively. The sample of 220 nm ITO was named as sample A, while other ITO/ InxO double layer samples were named as sample B, C, D, and E, and their InxO layer thickness was 80 nm, 130 nm, 170 nm and 210 nm, respectively. Veeco Dektak 150 system was used to measure the ITO and Indium film thickness.
2.2 light transmittance measurement
Fourier infrared spectroscopy (VERTEX 80) operated under integrated sphere mode was used to measure the light transmittance of all samples. Before measurement of the samples, the spectroscopy was calibrated according to the operation manual. The light transmittance of the ITO/ InxO layer was obtained by dividing the measured light transmittance from that of the quartz glass at arbitrary wavelength.
First, the light transmittance at 430 nm of all samples annealed at 600°C and in oxygen, as a function of annealing time was shown in Fig. 2(a) . It is found that, except for sample E, the light transmittance first increases as the annealing time elongates from 5 min to 10min, then saturates at 15 min. At annealing time of 15 min, the light transmittance of sample A -D remains above 90%. While the light transmittance of sample E is lower due to the uncompleted oxidation of indium resulted from the high thickness. From this view, annealing time was optimized as 15 min.
Fig. 2 (a) Calculated light transmittance at 430 nm of all samples as a function of the annealing time; (b) Calculated light transmittance of all samples as a function of the wavelength; (c) Calculated light transmittance at 430 nm as a function of the indium thickness.
After annealing at 600°C for 15 minutes in oxygen, the obtained light transmittance of all samples as a function of wavelength were shown in Fig. 2(b). It can be seen that the light transmittance of sample B-E remains saturation at wavelength above 450 – 460 nm, while that of sample A reaches the maximum at 450 nm. At 430 nm, which is the wavelength of our LED samples, the light transmittance decreases slightly as the thickness of the indium layer increases from 0 nm to 210 nm due to the weak absorption of the InxO material, as shown in Fig. 2(c).
2.3 Surface morphology
Veeco Nanoscope V Atomic Force Microscopy (AFM) was used to characterize the surface morphology (5 × 5 μm2) of the samples, and the measured results were shown in Fig. 3 , and the calculated root mean square (RMS) of the roughness, the characteristic height and width of the microstructure were summarized in Table 1 . For sample A, the RMS of the surface morphology is about 16.1 nm. When the thickness of the InxO layer on ITO increases from 80 nm to 210 nm, the RMS increases from 35.2 to 96.9 nm monotonously. It is obviously that the deposition of the indium layer followed by annealing in oxygen enhances the roughness of the surface dramatically. Almost the same trend has been found in the evaluation of the characteristic height and width of the surface microstructure. This phenomenon may be related to the In grain formation in evaporation process, InxO large grain formation and strain release in high temperature oxidation process [36,37 ].
Table 1. Characteristics of the Surface Microstructure Measured by AFM
2.4 Light scattering characteristics of ITO/ InxO samples
To measure the capability of the light scattering of the ITO/ InxO samples, Techno Team RIGO 801 system was used. The schematic diagram of the measurement system was shown in Fig. 4 . ITO/ InxO samples were fixed horizontally at the center of the measurement system. Collimated Nd: YAG laser pumped by 808 nm diode-laser array with output power of 20 mW and wavelength of 420 nm was used as the incident light, and its emission direction was adjusted to normal of the samples’ surface. CCD camera is rotated in the hemisphere space around the sample to detect the light intensity scattered by the samples. Based on the above configuration, we can obtain accurately the three dimensional scattered light intensity distribution.
The measured scattered light intensity distribution of sample A, B, C, D and E annealed at 600°C and oxygen for 15 min were shown in Fig. 5(a) , respectively. It is obviously that all the distribution shows the axial symmetry, and the symmetry axis is just as the normal of the sample’s surface. The scattered light intensity distribution along any azimuth for all the samples was shown in Fig. 5(b). Based on these results, it is easy to calculate the flux of the scattered light with scattering angel above 5° or 30° and the ratio of the relative flux to that of the total flux passing through the samples, as shown in Table 2 . From Fig. 5 and Table 2, we can see that sample A shows the minimum light scattering capability, and the flux ratio of the light with scattering angel above 5° to that of the total scattering light is only about 6.42%,while for scattering angel above 30°, this value decreases to 3.72%. When the thickness of the InxO increases from 80 nm to 130 nm, the flux ratio with scattering angel above 5° first increases dramatically, then saturates at about 72.67% when the InxO thickness is above 170 nm. However, the flux ratio with scattering angel above 30° shows significantly different trend. It first increases dramatically when the thickness of the InxO increases from 80 nm to 170 nm, then decreases when the thickness of the InxO exceeds 210 nm, and it reaches the maximum of 29.33% with the InxO thickness of 170 nm.
Fig. 5 (a) The measured scattered light intensity distribution of sample A, B, C, D and E; (b) The scattered light intensity distribution along any azimuth for all the samples.
Table 2. Scattering Flux with Different Scattering Angle Range
Comparing above results with surface morphology of sample A - E, it can be supposed that both the RMS and the characteristic width of the microstructure play a decisive role in the light scattering ability of the film. If the thickness of the InxO is below 170 nm, the RMS of the microstructure is small. If the thickness of the InxO exceeds 210 nm, the characteristic width is very large. These two conditions are not conducive to improving the ability of light scattering of large scattering angle. For the 220 nm ITO /170 nm InxO film, the RMS and the characteristic width reaches the best balance, and the light scattering capability is the strongest, therefore, 220nm ITO /170nm InxO is selected as the optimum structure.
3. GaN-based blue LED fabrication with ITO/ InxO transparent ohmic contact with microstructure formed by annealing in oxygen
GaN-based blue LED structure was grown on (0001) sapphire substrates by AIXTRON 2000HT metal organic vapor phase epitaxy (MOVPE) reactor. A 20 nm low-temperature grown GaN material was deposited directly on the sapphire as the buffer layer, then 4 μm Si-doping GaN layer and n-type 20-pair of In0.03Ga0.97N/GaN superlattices layer were grown successively. The multiple quantum well active region was consisted by 8 pairs of In0.17Ga0.83N/GaN quantum well, followed by 30-nm-thick Mg-doped AlGaN layer and a 150-nm-thick Mg-doped GaN layer. In order to decrease the ohmic contact resistivity, 3 nm Mg-doped InGaN layer was epitaxy grown finally [38]. Before the next procedure, all of the LED structure was annealed in 550°C for 30 minutes in oxygen to activate the Mg acceptor.
The LED chip size was 300 μm × 300 μm, and the fabrication process was shown in Fig. 6 . First, the n-GaN was exposed with inductively coupled Cl2/BCl3/Ar plasma etching by Oxford ICP 180 system [39]. Then, 220 nm ITO/Indium ohmic contact was deposited by home-made electron beam evaporator. Five kinds of LED samples named as sample LA, LB, LC, LD and LE were prepared. The thickness of indium in the five samples are 0 nm, 80 nm, 130 nm, 170 nm and 210 nm, respectively. Then, all the samples were annealed at 600° C in oxygen ambient to obtain the transparent, conductive ohmic contact with microstructure. Finally, the Cr/Au was deposited on the n-GaN layer and p-GaN layer as n- and p-electrodes. The top-view of GaN-based blue LED chip was shown in Fig. 7 .
Fig. 6 Schematic diagram of fabrication process of GaN blue LED structures with ITO/ InxO ohmic contact.
On the other hand, in order to optimize the annealing time to form low specific contact resistivity, the dot circular transmission line (DCTL) model was adopted [40], and the patterned 220 nm ITO / 170 nm InxO layer was fabricated on the p-InGaN layer by lift-off.
The I-V curves of the LED and dot circular transmission line model were measured by Agilent 4155C, and the light ouput power of the LED were measured by Lakeshore cryogenic probe station (CRX-4K) at 20 mA.
4. Results and discussion
4.1 I-V characteristics between220 nm ITO/ 170 nm InxO and p-InGaN
To study the effect of the annealing time on the contact behavior between 220 nm ITO/ 170 nm InxO and p-InGaN, the 6 μm gap in the DCTL was selected to measure the I-V characteristics. The I-V curves as a function of annealing time in oxygen and 600°C was shown in Fig. 8 . It can be seen that there exists very high resistance between p-InGaN and the non-annealed ITO/Indium contact. When the annealing time prolonged from 5 min to 10 min, the current increases at the same voltage, and almost linear I-V curve was obtained for 15 min annealing time. However, when the annealing time exceeds 20 min, the I-V characteristics deteriorated gradually. Therefore, the optimum annealing time is 15 min, and the 7.4 × 10−3 Ω•cm−2 specific contact resistivity can be achieved. It is worth noting that this optimized annealing condition exactly coincids with that of the light transmittance and scattering capability of 220 nm ITO/ 170 nm InxO.
Fig. 8 I-V characteristics between220 nm ITO/ 170 nm InxO and p-InGaN as a function of annealing time in 600°C.
4.2 Performance of GaN-based LEDs with ITO/ InxO contact
The measured I-V characteristics of the all samples were depicted in Fig. 9(a) . The working voltage at a forward current of 20 mA is 3.26V, 3.18V, 3.07V, 3.03V and 3.01 for sample of LA, LB, LC, LD and LE, respectively. It is found that the introduction of the InxO layer improves the I-V Characteristics due to the enhancement of the current spreading resulting from the conduction and the bottom connection of the InxO microstructures, and the LED working voltage decreased monotonically with the increase of the InxO thickness.
Fig. 9 (a) The measured I-V characteristics for all LED samples; (b) Light output power of all samples as a function of the injection current ;(c) Electroluminescence spectrum of all LED samples at 20 mA injection current.
Figure 9(b) showed the light output power of all samples as a function of the injection current. We can see that, at the same injection current, the light output power of the LED enhances dramatically when the thickness of the InxO increases from 80 to 130 nm, then reaches the maximum when the thickness of the InxO layer becomes 170 nm. It is reasonable that the gradual enhancement of light scattering ability is the main reason. When the thickness of the InxO exceeds 210 nm, due to the higher light absorption and weaker light scattering, the light output power decreases to some extent. It is also convinced by the electroluminescence spectrum of all the LED samples at 20 mA injection current, as displayed in Fig. 9(c). Compared with the LEDs with only 220 nm ITO electrode, the light output power of the LEDs with 220 nm ITO/ 170 nm InxO double layer structure can be increased about 58.8%.
4.3 Finite difference time domain (FDTD) simulations
In order to confirm the above results theoretically, FDTD simulation was carried out. Figure 10 (a) and (b) illustrate the physical model that used to depict the photon travelling path in GaN-based LEDs with 220 nm ITO as transparent electrode and ITO/ InxO double layer as transparent electrode, respectively. In this model, the structure of the GaN-based LED was simplified reasonably according to the slightly refractive index difference, and the validity of the results will not be degraded.
Fig. 10 Schematic diagrams of the possible photon travelling path (a) without InxO microstructure, and (b) with InxO microstructure.
In the simulation, the chip scale was set as 300μm × 300μm, while the thicknesses and the characteristic size of InxO microstructure is variable, and its value was set according to the measurement by AFM given in Table 1 and Fig. 3. It is worth noting that the microstructure was arranged periodically.
The simulated LED light emitting intensity distributions for different InxO thickness in polar coordinates are shown in Fig. 11 . It is clear that the light emitting intensity increases monotonically when the InxO thickness increases from 80 nm to 170 nm. This simulation verifies that the photon escaping probability can be enhanced by the proper surface microstructure. However, for the InxO thickness of 210 nm, not only the light emitting intensity pattern is changed greatly, but also the relative intensity for most emitting direction decreases to some extent. This result is consistent with the lower light transmittance and light scattering capability for the 210 nm InxO layer.
Fig. 11 The simulated LED light emitting intensity distributions for different InxO thickness in polar coordinates.
The simulated LED light output power as a function of the InxO thicknesses is shown in Fig. 12 , where the LED light output power without the InxO microstructure is designated as 1.0. It is clear that there exists almost same trend between Fig. 12 and 9(c) . From Fig. 12, we can find that the light output power of the LED with 170nm InxO layer is enhanced about 41.9% compared with that of the LED without InxO layer. This value is slightly lower than 58.8% obtained in the experiment, perhaps due to the irregular arrangement of the practical InxO microstructure.
5. Conclusions
ITO/ InxO double layer structure was adopted as the transparent conduction and light scattering function layer to improve the light extraction efficiency of the GaN-based blue LEDs. The double layer structure was first deposited in one run by electron beam evaporation using ITO and indium as the source respectively, then annealed in oxygen environment. This method can fabricate transparent electrode with microstructure and low specific contact resistivity one time free from lithography and etching, which makes the fabrication process simple and lower cost. For the 220 nm ITO/ 170 nm InxO double layer sample annealed at 600°C for 15 min in oxygen, measurement results show that its root mean square of roughness of the surface microstructure can be as high as 85.2 nm which introduces the strongest light scattering, its light transmittance at 450 nm can maintain 92.4%, at the same time it can realize lower specific contact resistivity with p-InGaN. Compared with the GaN-based blue LEDs with only 220 nm ITO electrode, the light output power of the LEDs with 220 nm ITO/ 170 nm InxO double layer structure can be increased about 58.8%, and working voltage at 20 mA injection current is decreased about 0.23 V due to the enhanced current spreading capability. The light output power improvement is also theoretically convinced by FDTD simulations.
Acknowledgments
This work was supported by the National Basic Research Program of China (Grant No. 2015CB351900), the Guangdong Province Science and Technology Program (Grant No. 2014B010121004), the National Natural Science Foundation of China (NSFC) (Grant Nos. 61210014, 61321004, 61307024, 61307024, and 51561165012), the High Technology Research and Development Program of China (Grant No. 2015AA017101), Tsinghua University Initiative Scientific Research Program (Grant No. 20131089364 and 20161080068) and the Independent Research Program of Tsinghua University (Grant No. 2013023Z09N).
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