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IP: 93.91.26.29 On: Thu, 30 Jul 2015 17:15:34 Copyright: American Scientific Publishers Copyright © 2014 American Scientific Publishers All rights reserved Printed in the United States of America Article Journal of Nanoscience and Nanotechnology Vol. 14, 6124–6127, 2014 www.aspbs.com/jnn Investigation of Mg Doping Profile in the p-Cladding Layer for High-Brightness AlGaInP-Based Light Emitting Diodes Hwa Sub Oh 1 , Ho Soung Ryu 1 3 , Joon Mo Park 1 , Hyung Joo Lee 2 , Young Jin Kim 2 , In Kyu Jang 2 , Ji Hoon Park 2 , Joon Seop Kwak 3 , and Jong Hyeob Baek 1 ∗ 1 LED Device Research Center, Korea Photonics Technology Institute, 971-35 Wolchul-dong, Buk-gu, Gwangju, Korea 2 Process Engineering Department, Kodenshi Auk Incorporation, 513-37 Eoyang-doing, Iksan city, Chonbuk, Korea 3 Department of Printed Electronics Engineering, Sunchon National University, Chonnam 540-742, Korea We investigated 590 nm light-emitting diodes appropriate for full-color display applications in terms of their electrical and optical behaviors during operation according to their Mg doping profile in the p-cladding layer. As the hole concentration in the “b” zone of the p-cladding layer is increased from 34 × 10 17 to 67 × 10 17 , the light output power increases by 41% due to the enhancement of the hole injection into the active region and also due to the minimization of the carr ier overflow problem. However, at an oversaturation of Mg doping with excess [Cp 2 Mg]/[III] in the “b” zone, the internal quantum efficiency degrades because of the decrease in hole concentration because of the oversaturated material problem. Keywords: Mg Doping Profile, p-Cladding Layer, AlGaInP-Based LED. 1. INTRODUCTION Recently, AlGaInP-based light-emitting diodes (LEDs) have experienced an impressive evolution in both device performance and market volume. In particular, high- brightness LEDs are gaining interest for use in commercial applications such as automotive lighting, full-color dis- plays, and general illumination. To increase their util- ity in these applications, improved performance such as shorter wavelengths and high-powered devices have been pursued. 1–5 However, (Al x Ga 1−x  05 In 05 P heterostruc- tures have a small conduction band offset that limits their electron confining potential. 6 This weaker electron confinement subsequently leads to electron heterobarrier leakage in AlGaInP heterostructure LEDs, especially in short-wavelength devices, where a fraction of the electrons injected into the active region have a sufficient thermal energy to escape into the p-cladding layer. To overcome this problem, AlGaInP-based LED structures require an ∗ Author to whom correspondence should be addressed. optimized doping profile in the p-cladding layer in order to prevent carrier overflow and to gain a higher light output power (P out . In the field of AlGaInP-based LEDs that emit a short peak wavelength at around 590 nm, how- ever, the effects of the doping profile in the p-cladding layer on LED performance has yet to be systematically studied. In this study, we investigate the behaviors of electri- cal and optical characteristics according to their Mg dop- ing profile in the p-cladding layer by analyzing device performances. 2. EXPERIMENTAL DETAILS We conducted metal-organic vapor phase epitaxy (MOVPE) to grow LED structures on a 2-in (100) GaAs substrate that was tilted 10  toward 011 to sup- press spontaneous ordering in the GaInP and AlGaInP epilayers. 6 Here, trimethylaluminum (TMAl), trimethyl- gallium (TMGa), and trimethylindium (TMIn) were used 6124 J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 8 1533-4880/2014/14/6124/004 doi:10.1166/jnn.2014.8320 Delivered by Publishing Technology to: ? IP: 93.91.26.29 On: Thu, 30 Jul 2015 17:15:34 Copyright: American Scientific Publishers Oh et al. Investigation of Mg Doping Profile in the p-Cladding Layer for High-Brightness AlGaInP-Based Light Emitting Diodes Figure 1. Schematic of LED structures having different Mg doping pro- files in the p-cladding layer. as the group-III sources; phosphine (PH 3  and arsine (AsH 3  were the group-sources, and disilane (Si 2 H 6  and biscyclopentadienylmagnesium (Cp 2 Mg) were the n- and p-type doping sources, respectively. The growth temper- ature was set at 700  C, for a growth rate of about 1.5 m/h. The multi-quantum well (MQW) structures for emitting a 590 nm peak wavelength consisted of 4.5 nm (Al 02 Ga 08  05 In 05 P quantum wells separated by 30 nm (Al 05 Ga 05  05 In 05 P barrier layers, which were sandwiched between two Al 05 In 05 P cladding layers. To improve the LED performance relative to the Mg doping concentra- tion, the p-cladding layer was divided into three zones: “a”, “b”, and “c” zones. In addition, a 10-m-thick p-GaP window layer capped the p-cladding layer in order to eval- uate the actual device performance. Schematics of the LED structures are depicted in Figure 1. Table I presents the detailed Mg doping profiles for the three zones of the p-cladding layer in the LED struc- tures. Note that the “a” zone in the p-cladding layer is intentionally left undoped and that the “c” zone is rela- tively lightly doped with 10 ×10 −4 [Cp 2 Mg]/[III] in order to prevent Mg diffusion into the active region. To inves- tigate and optimize the hole injection into the active region and to prevent electron overflow, the “b” zone in the p-cladding layer is doped with [Cp 2 Mg]/[III] values of 10 × 10 −4 ,20 × 10 −4 ,30 × 10 −4 , and 50 × 10 −4 in the Test 1, Test 2, Test 3, and Test 4 LED structures, respectively. To determine the hole concentration in the p-cladding layer according to the [Cp 2 Mg]/[III] value, electrochemical capacitance–voltage (ECV) measurements at 300 K were performed. To evaluate the devices, the Table I. Detailed Mg doping profiles for “a,” “b,” and “c” zones in the p-cladding layer of LED structures. Mg Doping profile at p-cladding layer “a” zone “b” zone “c” zone No. [Cp 2 Mg]/[III] Hole conc. [cm −3 ] [Cp 2 Mg]/[III] Hole conc. [cm −3 ] [Cp 2 Mg]/[III] Hole conc. [cm −3 ] Test 1 0 − 10 × 10 −4 34 × 10 17 10 × 10 −4 34 × 10 17 Test 2 0 − 20 × 10 −4 51 × 10 17 10 × 10 −4 34 × 10 17 Test 3 0 − 30 × 10 −4 67 × 10 17 10 × 10 −4 34 × 10 17 Test 4 0 − 50 × 10 −4 53 × 10 17 10 × 10 −4 34 × 10 17 wafers were sectioned into 300 m × 300 m chips, with 80-m-diameter metal contacts located on the top p + -contact layer. After dicing, the chips were mounted onto TO-18 headers with no epoxy encapsulation before being measured using a large-area Si photodiode that was placed on top of the device. 3. RESULTS AND DISCUSSION Figure 2 presents the typical current–voltage (I–V ) char- acteristics and light output power (P out  of LEDs having different Mg doping profiles in the p-cladding layer. The I–V characteristics in the “b” zone in the p-cladding layer show similar behaviors regardless of the p-doping concen- tration. These data indicate that the Mg doping level in the “b” zone is not significantly affected by the operating voltage, in terms of device performance. Figure 2(b) then shows the light output-current charac- teristics of LEDs having different Mg doping levels in the “b” zone. In addition, the relative increase of P out (RIP) of the same LEDs at a 200 mA operating current is shown in the figure inset. As the [Cp 2 Mg]/[III] value in the “b” zone is increased from 10 × 10 −4 to 30 × 10 −4 , the RIP increases from 1.0 to 1.4, though at a further increase of [Cp 2 Mg]/[III] to 50 × 10 −4 , the RIP degrades to 1.3. The improvement of internal quantum efficiency is due to the fact that the highly p-doped “b” zone increases the potential barrier in the p-cladding layer, which minimizes the electron overflow problem, and the improved hole con- ductivity helps to enhance the hole injection into the active region. 7 The RIP decrease in the Test 4 LED structure indi- cates that the excess [Cp 2 Mg]/[III] deteriorates the internal quantum efficiency. As such, it is important to optimize the [Cp 2 Mg]/[III] value in the p-cladding layer in order to improve the light output power. One possible explanation for the decreased light out- put of the LEDs under an excess [Cp 2 Mg]/[III] value is the degradation of the active region by Mg diffu- sion. Meneghesso et al. 8 reported that the degradation of optical power is accompanied by both an increase of a generation-recombination current at a low forward bias and an increase of the device ideality factor. To exam- ine the influence of Mg diffusion into the active region, the I–V characteristics plotted on a log–log scale and the ideality factor of LEDs with different Mg doping profiles J. Nanosci. Nanotechnol. 14, 6124–6127, 2014 6125 Delivered by Publishing Technology to: ? IP: 93.91.26.29 On: Thu, 30 Jul 2015 17:15:34 Copyright: American Scientific Publishers Investigation of Mg Doping Profile in the p-Cladding Layer for High-Brightness AlGaInP-Based Light Emitting Diodes Oh et al. Current [mA] Voltage [V] Test 1 Test 2 Test 3 Test 4 0 50 100 150 200 250 300 0 200 400 600 800 1000 1200 1400 Test 1 Test 2 Test 3 Test 4 0.8 1.0 1.2 1.4 1.6 Relative increase of P out P out [a.u.] Current [mA] Test 1 Test 2 Test 3 Test 4 0 20 40 60 80 100 0.0 0.4 0.8 1.2 1.6 2.0 2.4 (a) (b) Figure 2. (a) Forward I–V characteristics and (b) light output power of LEDs having different Mg doping profiles in the p-cladding layer. The inset in (b) shows the relative increase of brightness for the same LEDs. at p-cladding layer are shown in Figure 3. However, it should be noted that no leakage current increase at a low bias was observed. In addition, the LEDs with dif- ferent Mg doping levels in the “b” zone did not induce 0.0 0.4 0.8 1.2 1.6 2.0 2.4 Test 1 Test 2 Test 3 Test 4 1.6 1.7 1.8 1.9 2.0 Ideality factor Current [mA] Voltage [V] Test 1 Test 2 Test 3 Test 4 1E-7 1E-6 1E-5 1E-4 1E-3 0.01 0.1 1 10 100 Figure 3. Forward I–V characteristics plotted on a log–log scale of LEDs having different Mg doping profiles in the p-cladding layer. The inset shows the ideality factors of the same LEDs. significant changes in ideality factors. Hence, these results imply that the decrease in the light output of LEDs with an excess [Cp 2 Mg]/[III] value cannot be explained sim- ply based on degradation caused by Mg diffusion into the active region. Indeed, these results indicate that the intentionally undoped “a” zone and relatively low-doped “c” zone efficiently prevent Mg diffusion into the active region. Another possible explanation for the decreased light out- put of the LEDs with excess [Cp 2 Mg]/[III] in the “b” zone is directly related to the decreased hole concentra- tion. The hole concentration of the p-type Al 05 In 05 P layer measured using the ECV shows that as the [Cp 2 Mg]/[III] value is increased from 10 × 10 −4 to 30 × 10 −4 , the hole concentration increases from 34 × 10 17 to 67 × 10 17 , and that at a further increase to 50 × 10 −4 , the hole concen- tration decreases to 53 × 10 17 due to the oversaturated material problem. 9 These results indicate that the hole concentration at the “b” zone in the p-cladding layer is directly influenced by improvement in the spontaneous light-emitting efficiency and that carefully optimizing the [Cp 2 Mg]/[III] value is important for attaining the high- est hole concentration—and thereby improving the device performance. 4. CONCLUSION To make 590 nm high-brightness LEDs that are appro- priate for full-color display applications, we studied the electrical and optical behaviors of device performances according to their Mg doping profile in the p-cladding layer. As the [Cp 2 Mg]/[III] value in the “b” zone was increased from 10 × 10 −4 to 30 × 10 −4 , the light output power increased by 41% due to the enhanced hole injec- tion into the active region and also by minimization of the carrier overflow problem. In addition, the oversaturation of Mg doping in the “b” zone with excess [Cp 2 Mg]/[III] deteriorated the internal quantum efficiency by decreas- ing the hole concentration. As a result, by evaluating the I–V characteristics and ideality factors, we found that the intentionally undoped “a” zone and relatively low-doped “c” zone efficiently acted to prevent Mg diffusion into the active region. Acknowledgment: This work was supported by a Korea Research Foundation Grant from the ATC project (No. 10035863) provided by the Ministry of Knowledge Economy, Korea. References and Notes 1. J. W. Seo, H. S. Oh, J. S. Kwak, H. D. Song, K. W. Park, D. H. Park, S. W. Ryu, and Y. H. Park, J. Korean Phys. Soc. 55, 314 (2009). 2. P. Blood, Mat. Sci. Eng. B 66, 174 (1999). 3. C. M. Reaves, R. I. Pelzel, G. C. Hsueh, W. H. Weinberg, and S. P. DenBaars, Appl. Phys. Lett. 69, 3878 (1996). 6126 J. Nanosci. Nanotechnol. 14, 6124–6127, 2014 Delivered by Publishing Technology to: ? IP: 93.91.26.29 On: Thu, 30 Jul 2015 17:15:34 Copyright: American Scientific Publishers Oh et al. Investigation of Mg Doping Profile in the p-Cladding Layer for High-Brightness AlGaInP-Based Light Emitting Diodes 4. M. Eichfelder, W. M. Schulz, M. Reischle, M. Wiesner, R. Robach, M. Jetter, and P. Michler, Appl, Phys. Lett. 95, 131107 (2009). 5. H. S. Oh, S. M. Kim, J. H. Joo, J. H. Baek, and J. S. Kwak, J. Nanosci. Nanotechnol. 11, 1503 (2011). 6. J. Johansson, W. Seifert, T. Junno, and L. Samuelson, J. Cryst. Growth 195, 546 (1998). 7. P. Blood, Mat. Sci. Eng. B66, 174 (1999). 8. G. Meneghesso, S. Levada, R. Pierobon, F. rampazzo, E. Zanoni, A. Cavallini, A. Castaldini, G. Scamarcio, S. Du, and I. Eliashevich, IEDM Technical Digest, IEEE, San Francisco (2002), p. 103. 9. G. J. Bauhuis, P. R. Hageman, and P. K. Larsen, J. Crystal Growth 191, 313 (1998). Received: 17 March 2013. Accepted: 17 April 2013 J. Nanosci. Nanotechnol. 14, 6124–6127, 2014 6127 . al. Investigation of Mg Doping Profile in the p-Cladding Layer for High-Brightness AlGaInP-Based Light Emitting Diodes Figure 1. Schematic of LED structures having different Mg doping pro- files in. (P out  of LEDs having different Mg doping profiles in the p-cladding layer. The I–V characteristics in the “b” zone in the p-cladding layer show similar behaviors regardless of the p -doping concen- tration output power of LEDs having different Mg doping profiles in the p-cladding layer. The inset in (b) shows the relative increase of brightness for the same LEDs. at p-cladding layer are shown in Figure