IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 5, MAY 2010
455
Enhanced Output Power of InGaN-Based Light-Emitting Diodes With AlGaN/GaN Two-Dimensional Electron Gas Structure Jae-Hoon Lee, Senior Member, IEEE, and Jung-Hee Lee, Senior Member, IEEE
Abstract—We demonstrate high-performance InGaN-based light-emitting diodes (LEDs) with tunneling-junction-induced 2-D electron gas (2DEG) at an AlGaN/GaN heterostructure, which is inserted in the middle of the P+ -GaN contact layer of a conventional LED structure. The output power of a LED with a 2DEG insertion layer shows 17% enhancement compared to that of a conventional LED at 20 mA. This enhancement in output power for the LED with a 2DEG insertion layer could be attributed to both enhanced hole-injection efficiency and lateral current spreading by the presence of 2DEG at the AlGaN/GaN heterostructure. Index Terms—AlGaN/GaN, heterostructure, InGaN, lightemitting diode (LED), tunneling junction, 2-D electron gas (2DEG).
I. I NTRODUCTION
G
ROUP-III-NITRIDE semiconductors and their ternary solid solutions have attracted much attention because of their wide applications such as light-emitting diodes (LEDs), laser diodes, and high-power and high-temperature electronics [1]. In particular, white-light LEDs are the most promising solid-state lighting devices to replace conventional incandescent and fluorescent lamps. Although the technology has matured, there still exist challenges in growing a highly p-type doped layer and in optimizing the related device fabrication process. Lateral LEDs use lateral carrier injection, which can lead to nonuniform current spreading due to high p-GaN resistivity [2]. The current-crowding effect is not only responsible for the local light emission but also strongly affects the device reliability. It is therefore essential that the highly resistive pGaN contact layer must be combined with transparent metal surface coverage for better current spreading in lateral LEDs. There have been intensive research works for improving the light extraction efficiency through applying various techniques
Manuscript received December 24, 2009; revised January 23, 2010. Date of publication March 22, 2010; date of current version April 23, 2010. This work was supported in part by the Korea Science and Engineering Foundation through the National Research Laboratory Program funded by the Ministry of Science and Technology under Grant M10600000273-0650000-27310, by Brain Korea 21, and by Korea Electronics Technology Institute. The review of this letter was arranged by Editor P. K.-L. Yu. Jae-Hoon Lee is with the Manufacturing Technology Group, Samsung LED Company, Ltd., Suwon 443-743, Korea (e-mail: jaehoon03.lee@ samsung.com). Jung-Hee Lee is with the School of Electronic Engineering and Computer Science, Kyungpook National University, Daegu 702-701, Korea (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2010.2042274
for better current spreading in LEDs [3], [4]. The tunneling junction is shown to be an effective method to reduce the operation voltage. Recently, n+ /p+ -GaN tunnel junctions have been applied to achieve uniform luminescence and exhibited an improved radiative efficiency [5]. Sheu et al. reported that an n+ InGaN/GaN short-period superlattice tunneling contact layer instead of high-resistivity p-GaN as a top contact layer could be a viable solution [6]. A low-operation-voltage GaN-based LED, which utilizes a Mg-doped AlGaN/GaN strained-layer superlattice (SLS) contact layer, has also been demonstrated [7]. The p-type AlGaN/GaN SLS can increase the ionization efficiency of acceptors due to two-dimensional (2-D) hole gas (2-DHG), which results in a much smaller lateral resistivity in the in-plane lateral direction. However, the mobility of 2-DHG is lower compared to that of 2-D electron gas (2DEG) formed at the n-type AlGaN/GaN heterostructure [8]. In this letter, the n-AlGaN/GaN tunnel junction, which is inserted in the middle of the p+ -GaN contact layer, is proposed for the first time to improve the hole-injection efficiency and lateral current spreading, accomplished by optimizing the thickness and the doping concentration for the AlGaN/GaN heterostructure. II. E XPERIMENTS The LED sample proposed in this letter was grown on (0001) cone-shape-patterned sapphire substrates using metal–organic chemical vapor deposition [9]. The layer structure for the LED with a total thickness of about 6 μm consists of undoped GaN/Si-doped n-GaN, five pairs of InGaN–GaN multiple quantum wells (MQWs), Mg-doped p-AlGaN/GaN SLS electron blocking layer, Mg-doped 80-nm-thick p+ -GaN layer, the proposed tunnel-junction current-spreading layer of 10-nm-thick undoped-GaN and 25-nm-thick Si-doped AlGaN, and, finally, Mg-doped 40-nm-thick p+ -GaN top contact layer, as shown in Fig. 1. For comparison, two additional LED samples, which do not have a 2DEG current-spreading layer, were also grown; the one was grown with the 10-nm-thick undoped GaN layer only without growing the 25-nm-thick Si-doped AlGaN layer, and the other was grown without both layers (corresponding to a conventional LED structure). The Mg doping in the p+ -GaN layer is approximately 9 × 1019 cm−3 , which was measured by a secondary ion mass spectroscope. The AlGaN layer in the current-spreading layer was doped with a Si concentration of approximately 6 × 1018 cm−3 . The Al content of AlGaN is 20%, as determined by high-resolution X-ray diffraction. To
0741-3106/$26.00 © 2010 IEEE
456
Fig. 1. layer.
IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 5, MAY 2010
Schematic view of the LED structure with an AlGaN/GaN insertion
measure the 2DEG mobility and sheet carrier density of the AlGaN/GaN heterostructure, a control sample was prepared on the LED structure without growing the top p+ -GaN contact layer. The Hall mobility and sheet carrier density of the sample were ∼ 312 cm2 /V · s and ∼ 7 × 1013 cm−2 , respectively. Devices of 260 μm × 670 μm dimensions were fabricated by a normal side-view LED chip process using ITO transmittance for the p-contact and Cr/Au metals for the n-contact. After that, the sapphire substrate was thinned to about 80-μm thickness by using backside lapping and polishing [10]. The current–voltage (I–V ) characteristics were measured at room temperature using an HP4156 semiconductor analyzer. The output power of the fabricated LEDs was measured by using an integrating sphere to collect the light emitted in all directions from the LEDs at room temperature. III. R ESULTS AND D ISCUSSION The I–V characteristics of all fabricated LEDs are shown in Fig. 2. The forward voltages of the LEDs at 20 mA were approximately 3.20, 3.24, and 3.28 for the LED without an AlGaN/GaN current-spreading layer (Ref-LED; conventional LED), the LED with an undoped GaN interlayer only (GaNLED), and the LED with an AlGaN/GaN current-spreading layer (2DEG-LED), respectively. The forward voltage of the 2DEG-LED is slightly higher than that of the Ref-LED. This is because the inserted n-AlGaN/GaN current-spreading layer increases the series resistance of the device, which acts as a voltage drop, as shown in the inset of Fig. 2. However, the leakage currents of the Ref-LED, GaN-LED, and 2DEG-LED were −18, −20, and −17 nA at a reverse voltage of −10 V, respectively, showing negligible difference between samples, which demonstrates that the insertion of the current-spreading layer does not degrade the electrical characteristics of the LEDs. Fig. 3 shows the light output power and external quantum efficiency (EQE) as a function of operation current. The total output power of the devices was measured by using a 0.6t sideview package with a horizontal thickness of 6 mm without
Fig. 2. I–V characteristics of the Ref-LED, GaN-LED, and 2DEG-LED. The inset shows the schematic view of the band structure for the 2DEG-LED at forward bias.
phosphor to collect the light emitted in all directions from the LEDs. The output powers of the Ref-LED, GaN-LED, and 2DEG-LED at 20 mA were measured as 9.7, 9.2, and 11.4 mW (with corresponding EQE values of 17.3%, 16.2%, and 20.3%), respectively, showing ∼17% improvement in output power for the 2DEG-LED compared to the conventional Ref-LED. The improvement in output power for the 2DEGLED is because the lateral distribution of the injected carriers becomes very uniform by the presence of AlGaN/GaN currentspreading layer [8]. As shown in the inset of Fig. 2, the tunneling junction is formed between the quantum well with high 2DEG density, formed at the AlGaN/GaN heterointerface, and the bottom P+ -GaN layer [5], [6]. With application of a forward bias on the LED, the electrons in the valance band of the bottom p+ -GaN layer are supposed to tunnel into the empty subbands of the 2DEG valley at the AlGaN/GaN heterostructure, which, in turn, increases the hole concentration in the bottom p+ -GaN layer and hence increases the hole-injection efficiency into the active MQW region. On the other hand, the light output power of the GaN-LED at 20 mA is slightly decreased, which is 5% lower than that of the conventional Ref-LED, because the 10-nm-thick inserted undoped GaN layer alone does not play a role of current-spreading layer but rather increases the series resistance of the P+ -GaN layer. The improvement in output power for the 2DEG-LED is further enhanced at a higher current level, as shown in Fig. 3. For instance, at 200 mA, the output power of the 2DEG-LED is 20% higher than that of the Ref-LED. We believe that the improvement in output power is because the high 2DEG density formed at the AlGaN/GaN interface effectively spreads the current and also well dissipates the heat generated during high-current operation. However, the output power of the GaN-LED is 21% smaller than that of the Ref-LED. This is because Joule heating becomes very significant when high current flows through the device with increased series resistance caused by inserting the undoped GaN layer. The less Joule heating in the 2DEG-LED was also
LEE AND LEE: ENHANCED OUTPUT POWER OF InGaN-BASED LEDs WITH AlGaN/GaN 2DEG STRUCTURE
457
IV. C ONCLUSION We have investigated the electrical and optical properties of the LED with a 2DEG current-spreading layer formed by tunneling of electrons at the n-AlGaN/GaN heterostructure. The 2DEG-LED has exhibited 17% higher output power at 20 mA compared to that of the conventional Ref-LED. The improvement is more prominent at high-current-level operation because the high 2DEG density is very effective in both spreading the current in the lateral direction and in dissipating the heat generated in the device during high-current operation. R EFERENCES
Fig. 3. Light output power and EQE of the Ref-LED, GaN-LED, and 2DEGLED as a function of operation current.
Fig. 4. EL spectrum of the Ref-LED and 2DEG-LED as a function of operation current.
confirmed from the electroluminescence (EL)-spectrum shift measurement with increasing injection currents, as shown in Fig. 4. The dashed lines in the figure indicate the amount of the red shift. The EL peak positions for both the Ref-LED and 2DEG-LED were red shifted [11]. However, the amount of the shift is quite less for the 2DEG-LED as the current level increases from 20 to 200 mA (red shift from 438 to 452 nm for the Ref-LED and from 443 to 453 nm for the 2DEG-LED).
[1] S. M. Pan, R. C. Tu, Y. M. Fan, R. C. Yeh, and J. T. Hsu, “Enhanced output power of InGaN–GaN light-emitting diodes with high-transparency nickel-oxide–indium–tin-oxide ohmic contacts,” IEEE Photon. Technol. Lett., vol. 15, no. 5, pp. 646–648, May 2003. [2] X. Guo and E. F. Schubert, “Current crowding in GaN/InGaN light emitting diodes on insulating substrates,” J. Appl. Phys., vol. 90, no. 8, pp. 4191–4195, Oct. 2001. [3] J. S. Jang, “High output power GaN-based light-emitting diodes using an electrically reverse-connected p-Schottky diode and p-InGaN–GaN superlattice,” Appl. Phys. Lett., vol. 93, no. 8, pp. 081118–081120, Aug. 2008. [4] Y. J. Liu, C. H. Yen, L. Y. Chen, T. H. Tsai, T. Y. Tsai, and W. C. Liu, “On a GaN-based light-emitting diode with a p-GaN/i-InGaN superlattice structure,” IEEE Electron Device Lett., vol. 30, no. 11, pp. 1149–1151, Nov. 2009. [5] S. R. Jeon, Y. H. Song, H. J. Jang, G. M. Yang, S. W. Hwang, and S. J. Son, “Lateral current spreading in GaN-based light-emitting diodes utilizing tunnel contact junctions,” Appl. Phys. Lett., vol. 78, no. 21, pp. 3265–3267, May 2001. [6] J. K. Sheu, J. M. Tsai, S. C. Shei, W. C. Lai, T. C. Wen, C. H. Kou, Y. K. Su, S. J. Chang, and G. C. Chi, “Low-operation voltage of InGaN/ GaN light-emitting diodes with Si-doped In0.3 Ga0.7 N/GaN short-period superlattice tunneling contact layer,” IEEE Electron Device Lett., vol. 22, no. 10, pp. 460–462, Oct. 2001. [7] J. K. Sheu, G. C. Chi, and M. J. Jou, “Low-operation voltage of InGaN/ GaN light-emitting diodes by using a Mg-doped Al0.15 Ga0.85 N/GaN superlattice,” IEEE Electron Device Lett., vol. 22, no. 4, pp. 160–162, Apr. 2001. [8] D. H. Youn, J. H. Lee, V. Kumar, K. S. Lee, J. H. Lee, and I. Adesida, “The effects of isoelectronic Al doping and process optimization for the fabrication of high-power AlGaN–GaN HEMTs,” IEEE Trans. Electron Devices, vol. 51, no. 5, pp. 785–789, May 2004. [9] J. H. Lee, J. T. Oh, Y. C. Kim, and J. H. Lee, “Stress reduction and enhanced extraction efficiency of GaN-based LED grown on cone shape-patterned sapphire,” IEEE Photon. Technol. Lett., vol. 20, no. 18, pp. 1563–1565, Sep. 15, 2008. [10] J. H. Lee, N. S. Kim, D. Y. Lee, and J. H. Lee, “Effect of residual stress and sidewall emission of InGaN-based LED by varying sapphire substrate thickness,” IEEE Photon. Technol. Lett., vol. 21, no. 16, pp. 1151–1153, Aug. 15, 2009. [11] S. Y. Huang, R. H. Horng, P. L. Liu, J. Y. Wu, H. W. Wu, and D. S. Wuu, “Thermal stability improvement of vertical conducting green resonantcavity light-emitting diodes on copper substrates,” IEEE Photon. Technol. Lett., vol. 20, no. 10, pp. 797–799, May 15, 2008.
