NANO LETTERS
High-Brightness Light Emitting Diodes Using Dislocation-Free Indium Gallium Nitride/Gallium Nitride Multiquantum-Well Nanorod Arrays
2004 Vol. 4, No. 6 1059-1062
Hwa-Mok Kim,*,† Yong-Hoon Cho,‡ Hosang Lee,† Suk Il Kim,† Sung Ryong Ryu,† Deuk Young Kim,† Tae Won Kang,† and Kwan Soo Chung§ Quantum-functional Semiconductor Research Center, Dongguk UniVersity, Seoul 100-715, Department of Physics, Chungbuk National UniVersity, Cheongju 361-763, Department of Electronic Engineering, Kyunghee UniVersity, Yongin 449-701, Korea Received March 10, 2004; Revised Manuscript Received May 4, 2004
ABSTRACT We demonstrate the realization of the high-brightness and high-efficiency light emitting diodes (LEDs) using dislocation-free indium gallium nitride (InGaN)/gallium nitride (GaN) multiquantum-well (MQW) nanorod (NR) arrays by metal organic-hydride vapor phase epitaxy (MO−HVPE). MQW NR arrays (NRAs) on sapphire substrate are buried in spin-on glass (SOG) to isolating individual NRs and to bring p-type NRs in contact with p-type electrodes. The MQW NRA LEDs have similar electrical characteristics to conventional broad area (BA) LEDs. However, due to the lack of dislocations and the large surface areas provided by the sidewalls of NRs, both internal and extraction efficiencies are significantly enhanced. At 20 mA dc current, the MQW NRA LEDs emit about 4.3 times more light than the conventional BA LEDs, even though overall active volume of the MQW NRA LEDs is much smaller than conventional LEDs. Moreover, the fabrication processes involved in producing MQW NRA LEDs are almost the same for conventional BA LEDs. It is, thus, not surprising that the total yield of these MQW NRA LEDs is essentially the same as that of conventional BA LEDs. The present method of utilizing dislocation-free MQW NRA LEDs is applicable to super-bright white LEDs as well as other semiconductor LEDs for improving total external efficiency and brightness of LEDs.
Group III nitride semiconductors have attracted much attention especially for their light-emitting device applications, such as highly efficient light emitting diodes (LEDs) and cw laser diodes (LDs).1-3 Although, conventional broad area (BA) LEDs have many advantages, including simplicity design, low-temperature sensitivity, and operation without threshold, they also suffer from some limitations, such as poor light extraction, wide spectral width, and large output divergence. Recently, studies have shown that micron-sized array LEDs fabricated by dry etching offer higher light output efficiencies than conventional BA LEDs.4-7 Unfortunately, as in the conventional BA LEDs, many threading dislocations are produced in these micro-sized LEDs. These threading dislocations, which act as nonradiative centers and degrade performance significantly, are due to the large lattice mismatch to the substrate. On the other hand, threading dislocations do not occur during the growth of nanorods (NRs), since the growth mechanism of NRs is completely different, as follows. First, nanometer-sized seeds are grown, † ‡ §
Dongguk University. Chungbuk National University. Kyunghee University.
10.1021/nl049615a CCC: $27.50 Published on Web 05/19/2004
© 2004 American Chemical Society
without catalysts, on the substrate. Second, NRs are grown on the seeds, and NRs normal to the substrate are obtained.8 Therefore, the formation of dislocation-free InGaN/GaN multiquantum wells (MQWs) in the NR have the potential for negligible nonradiative recombination loss, and thus the efficiency of light output power is much higher than in InGaN/GaN MQW layers. Moreover, the sidewalls of the NR play an important role in extracting light efficiently. A larger surface area is desirable, since it provides more pathways by which generated photons can escape. Therefore, adoption of NR array (NRA) geometry holds promise for the fabrication of highly efficient LEDs. Despite these great advantages, LEDs and LDs composed of NR (or nanowire (NW)) arrays (NRAs or NWAs) have not yet been fabricated and studied. Only a few researchers and we have reported on the single-wire (rod) scale devices.8-14 Here, we demonstrate the realization of high-brightness and high-efficiency LEDs using dislocation-free InGaN/GaN MQW NRAs by metalorganic-hydride vapor phase epitaxy (MO-HVPE). The benefits of the MQW NRA LEDs are examined in this study, and their characteristics are compared to those of conventional BA LEDs.
Figure 1. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images of vertically well-aligned InGaN/GaN MQW NRAs grown on c-plane sapphire substrate. (A) Cross-section-view SEM image of InGaN/GaN MQW NRAs. Scale bar is 500 nm. (B) 30° tilt-view high-resolution scanning electron microscope (HR-SEM) image of InGaN/GaN MQW NRAs. InGaN/GaN MQW NRs were vertically aligned on the sapphire substrate. Scale bar is 500 nm. (C) TEM image of In0.25Ga0.75N/ GaN MQWs in a NR. The length of NR was about 1 µm and the diameter was about 70 nm around the middle point of the NR. Scale bar is 30 nm. Inset shows the corresponding selective area electron diffraction (SAED) pattern.
Single-crystal InGaN/GaN MQW NRAs were grown directly on the c-plane sapphire substrate without catalysts or template layers by the MO-HVPE method. Gallium (Ga) and trimethylindium (TMIn) were used as Ga and indium (In) precursors (group III), respectively. NH3 was used as a nitrogen precursor (group V). The n- and p-type NR arrays were doped with SiH4 and Cp2Mg, respectively. In this study, the MQW NR structure consists of about 1.5 µm unintentionally n-type doped GaN buffer layers (n-type contact regions), about 0.5 µm of Si-doped GaN NRs, and a sixperiod InGaN/GaN MQW targeted for emission at ∼ 470 nm, topped with about 0.4 µm Mg-doped GaN NRs, grown directly on the c-plane of a sapphire substrate without catalysts or template layers. The growth time for the GaN barrier layers in the MQW structure was fixed at about 25 s, resulting in an average barrier thickness of about 12 nm. Growth time for InGaN well layers was fixed at about 10 s, resulting in an average well width of about 4.8 nm, as determined by transmission electron microscope (TEM). The composition of the In1-xGaxN layers in the MQW NRs was determined by energy-dispersive X-ray spectroscopy (EDX), and for all samples x was approximately 0.25. The MQW NRs prepared in this study consist of a sixperiod In0.25Ga0.75N/GaN (4.8 nm/12 nm) on n-type GaN NRs. A cross-sectional-view field emission scanning electron microscope (FE-SEM) image of the MQW NRAs is shown in Figure 1A. Figure 1B is a 30° tilt-view high-resolution scanning electron microscope (HR-SEM) image, revealing that the NRs are well-aligned vertically, with homogeneous lengths and diameter (around middle point) distributions. 1060
Typical mean diameters and lengths were in the range of 70-90 nm around the middle point (p-n junction with InGaN/GaN MQWs) of NR and 1-1.2 µm, respectively. Intervals between NRs were sufficiently wide, as shown in Figure 1B. Figure 1C and inset show the TEM image and the corresponding selective area electron diffraction (SAED) pattern of the middle area of the In0.25Ga0.75N/GaN MQW NR. Figure 1C exhibits dark and bright layers in the MQWs. Dark regions designate In0.25Ga0.75N layers, and bright regions correspond to GaN layers. The interfaces between the different layers are clearly visible. The MQW NRs grown along the c-axis of the hexagonal crystal structure are dislocation-free. The interfaces between In0.25Ga0.75N and GaN are as clean as the GaN layer, and interface dislocations are not observed. In the inset of Figure 1C, the [0001] direction was parallel to the long axis of rod, indicating that the [0001] direction is a common growth direction of NR. The SAED pattern of the same NR from the [2110] direction confirmed that it was a single crystal. After the MQW NRA growth processes, NRAs were buried in spin-on glass (SOG) for the isolation of individual NRs, and for the purpose of bringing the p-type NRs into contact with the p-type electrodes. Intervals between the NRs must be at least 80 nm so that the SOG can fill the interstices between NRs. Before metal deposition, the n-type contact area is exposed by a lift-off process. This process includes elements of photolithography and dry etching. A conventional Ti/Al (20/200 nm) electrode is deposited by electronbeam evaporation to create the ohmic contact on the GaN buffer layer (n-type region). A thin transparent film of Ni/ Au (20/40 nm for current spreading and light emission) is deposited on the entire surface of the SOG-coated NRA sample, to form a connection with the p-type ohmic contacts of each individual nanosized LED. These contacts must form, in a network-like configuration, before the p-type probe-pad deposition. A Ni/Au (20/200 nm for pad) electrode is used to create ohmic contact on top of p-type NRs, by electronbeam evaporation. Figure 2A shows a cross-sectional schematic diagram (left) and SEM image (right) of the fabricated InGaN/GaN MQW NRA LED structure. For SEM measurement, this sample was coated by platinum metal. The fabrication processes are almost identical to those of conventional BA LEDs, except for the SOG coating process. Conventional BA LEDs with the same area were also fabricated by conventional MOCVD for comparison studies. In Figure 2B, C, and D, we show the electroluminescence (EL) properties (Figure 2B) for the InGaN/GaN MQW NRA LEDs as a function of injected dc current ranging from 20 to 100 mA. It can be seen that a blue-shift occurs, directly with increasing current (Figure 2C). This is attributed to screening of the built-in internal polarization field in the QWs by the injected carriers.15 Carrier transportation in the NRA LEDs is physically confined to the vertical direction due to the geometry of the one-dimensional structure. In turn, this structure of carrier movement ensures efficient use of the injected carriers. The turn-on voltages of MQW NRA LEDs are slightly higher than that of conventional BA LEDs, as seen from the I-V Nano Lett., Vol. 4, No. 6, 2004
Figure 3. Electrical properties of InGaN/GaN MQW NRA LEDs (2) compared to conventional BA LEDs (9). (A) Current-voltage (I-V) characteristics of the MQW NRA LEDs and conventional BA LEDs. Insets show the I-V characteristics of a single GaN NR p-n junction with a six-period InGaN/GaN MQW and a schematic diagram of an InGaN/GaN NRA LED. (B) Light output power-forward current (L-I) characteristics of a six-period InGaN/ GaN MQW NRA LED using an on-wafer testing configuration, as compared to a conventional BA LED. Insets show the top-view photograph image (above) and schematic diagram (below) of a blue emission from In0.25Ga0.75N/GaN MQW NRA LEDs at 20 mA dc current.
Figure 2. Schematic diagram and SEM image of cross-sectional MQW NRA LED structures and electroluminescence (EL) characteristics of InGaN/GaN MQW NRAs LEDs. (A) Schematic diagram (left) and SEM image (right) of cross-sectional InGaN/ GaN MQW NRA LEDs. As shown in the right image, a crosssection of this sample was coated by platinum metal for SEM measurement. Scale bar is 1 µm. (B,C,D) Room temperature electroluminescence (EL) characteristics of InGaN/GaN MQW NRA LEDs at various applied dc currents. DC current was applied using a Keithley 228A voltage/current source. EL peak position at 20 mA was 466 nm (Figure 2B). A significant blue-shift is observed with increasing current (Figure 2C). Also, EL peak intensity increased directly with injection current (Figure 2D).
characteristics shown in Figure 3A. The higher turn-on voltages can be attributed to the effective contact area of the p-type GaN NR in the devices. The MQW NRA LEDs are essentially an interconnected array of individual nanosized LEDs in the SOG, each with their own ohmic contact, so that a larger resistive drop may be expected across the contact. The slightly higher threshold voltages in MQW NRA LEDs can also be attributed to the effective contact area of the p-type GaN NR in the devices. Similar factors apply in case of the microring LEDs,7 which have larger contact areas than do the MQW NRA LEDs. The inset of Figure 3A depicts the current-voltage (I-V) behavior of a six-period InGaN/GaN MQW NR LED measured at room temperature. The carrier concentrations of the n- and p-type NR regions were about 1 × 1018 cm-3, and about 5 × 1017 cm-3, respectively. The length of this NR was about 1 µm, and its diameter was about 70 nm. The n- and p-type electrodes of single NR p-n junctions were fabricated using focused ion beams. Details of the NR LED fabrication process have been Nano Lett., Vol. 4, No. 6, 2004
previously published.14 A green curve represents the I-V behavior of the single InGaN/GaN MQW NR p-n junction. I-V measurements showed rectifying behavior consistent with the presence of the NR p-n junction with a turn-on voltage of about 1.5 V in forward bias. I-V measurements of NR p-n structures exhibit a clear current rectification of the single materials NR transport. Our single InGaN/GaN MQW NR p-n junction, composed of a single epitaxial growth, can control one-dimensional current transport through a single NR. Figure 3B shows the light output power versus forward current (L-I) for the InGaN/GaN MQW NRA LEDs and for the conventional BA LEDs measured from the top of the samples. The inset of Figure 3B shows a photograph image taken from the top of the MQW NRA LEDs on sapphire substrate under a biased condition of about 20 mA. The actual total output power of this LED should be much higher than the measured value shown here since most of the light emitted from these LEDs was not collected. This was unavoidable, due to the extremely small (1 × 1 mm2) area of the detector. An important result, shown in Figure 3B, is that the light power output of MQW NRA LEDs is significantly higher than the output of conventional BA LEDs. At an injection dc current of 20 mA, the MQW NRA LEDs emit about 4.3 times more light than the conventional BA LED, providing solid evidence that the sidewalls facilitate light extraction. This can be attributed to the presence of a large sidewall surface area for light extraction, as in microdisk and microring LEDs.6,7 It is well recognized that extraction efficiency is only a few percent in conventional BA LEDs owing to the total internal reflection occurring at the LED/air interface. It is, then, quite clear that it is much easier for generated light to leak out of MQW NRA LEDs than in conventional BA LEDs. It is thus 1061
expected that extraction efficiency should be significantly higher in these MQW NRA LEDs. Furthermore, from the temperature-dependent PL experiments, we observed better internal quantum efficiency for dislocation-free MQW NRAs than has been evidenced in conventional MQW layers. Therefore, we conclude that extraction efficiency and internal quantum efficiency are increased significantly, due to the large sidewall surface area and dislocation-free characteristics of our MQW NRA LEDs, respectively. Although the total active volume is reduced, the enhancement in both extraction and internal quantum efficiencies gives the MQW NRA LEDs an advantage in overall external quantum efficiency over the conventional BA LED. We have fabricated and characterized high-brightness and high-efficiency LEDs, by using dislocation-free InGaN/GaN MQW NRAs. Device characteristics have been measured and compared with those of conventional BA LEDs. The extraction efficiency was found to be much higher in MQW NRA LEDs than the conventional BA LED. Light output measurements indicated that light output from MQW NRA LEDs was about 4.3 times that of conventional BA LEDs. Enhanced light emission efficiency makes MQW NRA LEDs clearly superior in many applications, most notably superbright LEDs for lighting, full color displays, etc. The present method of utilizing MQW NRA LEDs to improve LED brightness is also applicable to other types of semiconductor LEDs. LEDs based on III-nitride MQW NRAs are currently being used to generate white light, using a coating of SOG, mixed with yellow phosphor. Another promising new technique involves controlling the quantum well thickness of InGaN/GaN MQWs in the NR and In compositions in the MQWs. We believe that this work opens up exciting opportunities for the realization of high-brightness and highefficiency NRA LEDs and LDs, in that our method conjoins, bottom-up (i.e., InGaN/GaN MQW NRA growth) and topdown (i.e., conventional LED patterning by photolithography and dry etching) manufacturing. Acknowledgment. We thank Prof. S. J. Lee and Prof. N. M. Kim for helpful discussions; C. S. Ahn at AP Tech
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Co. for TEM measurement; and AVS Co., VTS Co., and KVT Co. for the use of their facilities for LED fabrication and electrical measurements. This work was supported by the Korea Science and Engineering Foundation through the Quantum-functional Semiconductor Research Center at Dongguk University in 2004. Supporting Information Available: Large scale images of Figure 1, Figure 2, and Figure 3 shown in text; CL spectra of MQW NRA LEDs and BA LEDs at room temperature (Fig. S4); detailed explanations of the SOG coating process, MQW NRA LED fabrication, and the electrical characterization at room temperature. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Morkoc¸ , H.; Strite, S.; Gao, G. B.; Lin, M. E.; Sverdlov, B.; Burns, M. J. Appl. Phys. 1994, 76, 1363-1398. (2) Nakamura, S. Science 1998, 281, 956-961. (3) Nakamura, S.; Fasol, G. The Blue Laser Diode; Springer: New York, 1997. (4) Jin, S. X.; Li, J.; Li, Z.; Lin, J. Y.; Jiang, H. X. Appl. Phys. Lett. 2000, 76, 631-633. (5) Dai, L.; Zhang, B.; Lin, J. Y.; Jiang, H. X. J. Appl. Phys. 2001, 89, 4951-4954. (6) Jin, S. X.; Li, J.; Lin, J. Y.; Jiang, H. X. Appl. Phys. Lett. 2000, 77, 3236-3238. (7) Choi, H. W.; Dawson, M. D.; Edwards, P. R.; Martin, R. W. Appl. Phys. Lett. 2003, 83, 4483-4485. (8) Kim, H.-M.; Kang, T. W.; Chung, K. S. AdV. Mater. 2003, 15, 567569. (9) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66-69. (10) Wang, J.; Gudiksen, M. S.; Duan, X.; Cui, Y.; Lieber, C. M. Science 2001, 293, 1455-1457. (11) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897-1899. (12) Huang, Y.; Duan, X.; Cui, Y.; Lieber, C. M. Nano Lett. 2002, 2, 101-104. (13) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617-620. (14) Kim, H.-M.; Cho, Y. H.; Kang, T. W. AdV. Mater. 2003, 15, 232235. (15) Choi, H. W.; Jeon, C. W.; Dawson, M. D.; Edwards, P. R.; Martin, R. W.; Tripathy, S. J. Appl. Phys. 2003, 93, 5978-5982.
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