Design and Fabrication of Highly Efficient GaN-Based Light-Emitting Diodes

Transcription

1 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 10, OCTOBER Design and Fabrication of Highly Efficient GaN-Based Light-Emitting Diodes Hyunsoo Kim, Seong-Ju Park, and Hyunsang Hwang, Member, IEEE Abstract A promising fabrication method and an innovative geometrical design for highly efficient GaN-based light-emitting diodes (LEDs) were investigated based on current spreading phenomenon. Based on theoretical considerations, it was possible to determine the critical transparent-electrode thickness, which resulted in significant improvements in the electrical and optical characteristics of LEDs. In addition, we were able to define conditions for an ideal geometrical design and the resulting product exhibited significant improvements in electrical and optical characteristics in spite of the fact that a transparent electrode, acting as a p-type current spreader, was not used. Considering the simple fabrication process and high device performance, the proposed fabrication methods, as well as the innovative geometrical design, have considerable promise for use in practical applications. Index Terms Current spreading, GaN, geometrical design, light-emitting diode, model. I. INTRODUCTION GaN-BASED light-emitting diodes (LEDs) have been the subject of extensive investigations, and have been developed because of their potential applications in areas such as fullcolor displays, full-color indicators, and high-efficiency lamps [1] [8]. However, the GaN-based LED has a critical weakness in that its fabrication mainly involves the use of the lateral carrier injection type due to the absence of an appropriate conducting substrate. In this case, a current crowding problem is often encountered and impedes the development of the efficient GaNbased LEDs. Several groups have reported on studies relating to this issue, both theoretically and experimentally [9] [11]. Based on the assumption that the transparent electrode represents a perfect current spreader, Eliashevich et al. reported that the conductivity of an n-type GaN layer has a profound effect on uniform current spreading [9]. It has been shown that the uniform current spreading is essentially attained above the critical n-type carrier concentration. However, a thin metal film has a much higher sheet resistance than its corresponding bulk material due to the reflection of conduction-electrons from defects that are trapped in the film during the deposition and from internal surfaces [12]. Therefore, the resistivity of the transparent electrode should not be ignored in the development of a more accurate model. Based on this consideration, more highly developed theoretical model has recently been proposed and important parameters such as the current density, the resistivities Manuscript received February 1, This work was supported in part by the Korea Energy Management Cooperation and the Brain Korea 21 Project. The review of this paper was arranged by Editor P. Bhattacharya. The authors are with the Department of Materials Science and Engineering, Kwangju Institute of Science and Technology, Kwangju, , Korea ( hwanghs@kjist.ac.kr). Publisher Item Identifier /TED of the transparent electrode and n-type layers, and the effective length for the lateral current path were found to be important factors in uniform current spreading [10], [11]. From the standpoint of both uniform current spreading and high extraction efficiency of a generated light, the determination of the proper thickness of the transparent electrode becomes very important. However, no systematic study on the transparent electrode has yet been reported because of a lack of understanding of the relationships between the transparent electrode and the n-type layer with respect to current spreading. In this study, we report on a method for determining the critical transparent-electrode thickness for the realization of highly efficient LEDs. Based on the effective length factor, which involves device geometry, significant improvements in LED characteristics have also been demonstrated by local modification of the p-type pad (electrode) geometry [10], [11]. In addition, although it does not mainly concern the current spreading problem, the geometrical design such as the interconnected microdisk LED also showed a 60% increase in optical emission efficiency compared to the conventional broad-area LED [13]. These results indicate the importance of geometrical design on device efficiency. In this regard, in terms of the ideal geometrical design, which gives perfectly uniform current spreading conditions, we report on an attempt to realize highly efficient GaN-based LEDs in the absence of a transparent electrode. II. EXPERIMENT Metalorganic chemical vapor deposition was used to grow a 1.5- m-thick n-gan : Si layer on a (0001) sapphire substrate. This was followed by the growth of m-thick InGaN/GaN multiple quantum well (MQW) layers with five periods, followed by the deposition of a m-thick p-gan : Mg layer. The procedure for the growth and epilayer structure of the MQW LED have been described elsewhere [14]. In terms of fabrication of the LED device, the p-type layer was selectively etched to expose the n-type layer using an inductively-coupled plasma (ICP) etching system. A Ni/Au transparent layer was then deposited on the surface of the p-gan layer. This was followed by the deposition of a Ni/Au (30 nm/100 nm) layer in order to achieve a p-ohmic pad. For an n-ohmic pad, a Ti/Al (30 nm/100 nm) layer was deposited on the n-gan, and the metal-deposited samples were then annealed at 450 C for 30 s in a rapid thermal annealing system. All electrical and optical properties of the LED were evaluated via on-wafer probing of the devices. The current voltage ( ) characteristics were measured using a parameter analyzer (HP 4155A). The light output power of the LED was measured using a UV/VIS 818 photodiode /02$ IEEE

2 1716 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 10, OCTOBER 2002 III. RESULTS AND DISCUSSIONS A. Critical-Transparent Electrode Thickness Fig. 1(a) shows a three dimensional schematic view of the GaN-based LED, which was designed using earlier device modeling results [10], [11]. In previous modeling studies, it was shown that perfectly uniform current spreading is possible when the following equation is satisfied where is the current density and ( ) the resistivity of the transparent electrode (n-type layer). The geometrical parameters,, and are also defined in Fig. 1(b), which shows a top view of the fabricated LED for the device model. In order to derive (1), the voltage drop was expressed as, assuming a constant current density for the series mode of the device [9] [11]. However, for developing a more accurate model, the current should be constant, since the cross-sectional area is significantly different for each component. Therefore, the basic equation for the voltage drop can be expressed as (1) (2) where is the resistance. Based on this expression, the resulting equation for a perfectly uniform current spreading can be rewritten as Fig. 1. (a) Schematic view of the GaN-based LED. (b) Top view of the fabricated LED with an l=w ratio of 1.50, as measured by optical microscope. This geometry was designed for a device model with respect to current spreading. where and represent the thickness of the transparent electrode and the n-type layer, respectively, and and are assumed to be equal, for the sake of simplification. Since is defined as the sheet resistance, (3) can be presented as (3) (4) or (5) (6) Fig. 2. Sheet resistance of the transparent electrode ( ) as a function of film thickness (t ). The inset shows the geometry used for measuring the light output power of the LED. Equation (5) indicates that perfectly uniform current spreading is possible when the sheet resistances of the transparent and n-type layer are identical. Regarding this condition, it is possible to determine the critical transparent-electrode thickness for the intentionally designed LED of Fig. 1. Fig. 2 shows the sheet resistance variation of the transparent electrode as a function of film thickness. The thickness of the transparent electrode (Ni/Au) was controlled by a thickness monitor in the e-beam evaporator and corrected via the use of X-ray reflectivity measurements. For measurement of the sheet resistance, a Ni/Au film with a 1 : 1 thickness ratio was evaporated onto the nonactivated mm p-type GaN sample and then annealed at 450 C for 30 s in an N ambient. The sheet resistance was then measured using a four-point probe system. In Fig. 2, it can be seen that the sheet resistance of the transparent electrode significantly increases as the thickness is reduced. This is due to the enhanced reflection of conduction-electrons caused by a relative increase in dislocation, defect, and internal surface scattering with decreasing film thickness, which decreases the mean-free-path of electrons and the conductivity of the metal film [12]. Based on this experimental behavior and (5), it is possible to determine the critical transparent-electrode thickness for LED samples with an arbitrary sheet resistance of the n-type layer. To ensure

3 KIM et al.: DESIGN AND FABRICATION OF LIGHT-EMITTING DIODES 1717 TABLE I SHEET RESISTANCE ( ) AND THE LIGHT TRANSMITTANCE (T ) OF THE TRANSPARENT ELECTRODE AS A FUNCTION OF FILM THICKNESS (t ). THE LIGHT TRANSMITTANCE (T ) OF THE TRANSPARENT ELECTRODE WAS MEASURED AT 470 NM WAVELENGTH USING A UV-VIS SPECTROMETER. LED1 AND 2REPRESENT THE FABRICATED LEDS WITH A VALUE OF 90 AND 55 =,RESPECTIVELY. THE SERIES RESISTANCE (R ) WAS EXTRACTED VIA LINEAR FIT FROM THE MEASURED I V CURVE. THE LIGHT OUTPUT POWER OF ALL LEDs WAS MEASURED AT A PHOTODIODE MEASUREMENT ANGLE OF 90 AS SHOWN IN THE INSET OF Fig. 2. IN ADDITION, FOR AN ACCURATE COMPARISON OF THE MEASURED OUTPUT POWER FOR ALL LEDs, THE POSITIONS FOR THE LED AND THE PHOTODIODE WERE FIXED. THE INTEGRATED LIGHT OUTPUT POWER (L ) WAS OBTAINED FROM THE INTEGRATION OF THE LIGHT OUTPUT POWER (L) MEASURED AT CURRENTS RANGING FROM 0 TO 100 ma the relation (5), various thickness schemes of the transparent electrode were adopted for the fabrication of two LED samples, which have values of about 90 (corresponding to a carrier concentration of 1 10 cm ) and cm as shown in Table I. For LED samples with values of 90 and 55, it is predicted that the theoretical critical thicknesses of the transparent electrode are near 40 and 60 Å, respectively. Considering the optical efficiency of LEDs, the light transmittance of the transparent electrode must also be taken into consideration because it is strongly dependent on the thickness. Table I shows the light transmittance of the transparent electrode at the wavelength of 470 nm as a function of electrode thickness. For measurement of the light transmittance, the samples, which were prepared for measurement of the sheet resistance, were used. It is clear that the light transmittance begins to significantly degrade above a thickness of 60 Å. In Table I, it can be seen that the series resistances of the 90 -LED1s are heavily dependent on the transparent-electrode thickness. This is an essential proof that the sheet resistance of the transparent electrode (i.e., the transparent-electrode thickness) is a very important factor in a device performance. The series resistance was calculated from the diode current of a pn junction. When the series resistance contributes to device behavior, the diode equation can be written as [15] where is the prefactor, the measured voltage from the curve, the series resistance, and the ideality factor. Equation (7) can be rewritten as Therefore, according to (8), the series resistance can be extracted via a linear fit from the measured current voltage ( ) curve. For an accurate determination, a calculation of the series resistance was made based on the high current injection ( 1 (7) (8) ma) in the curves, which is dominated by the series resistance [16]. For LED1 with a value of 90, the series resistance of LEDs was minimized at a thickness of 60 Å, indicating that the experimentally determined critical thickness of the transparent electrode for 90 -LED is 60 Å as a result of the largely optimized current injection. In addition, the highest light output power was observed at a thickness of 60 Å, which is in good agreement with the electrical characteristics of LED. However, since the light output efficiency is also strongly dependent on the light transmittance, the highest light output power at a thickness of 60 Å, can be attributed to the combined effects of the most uniform current spreading and the proper light transmittance. Therefore, it is more desirable to determine the experimental critical transparent-electrode thickness based on electrical characteristics. According to above results, it should be noted that the experimental critical thickness for 90 -LED1 was shifted by 20 Å compared to the theoretical prediction (40 Å). This deviation will be discussed later. For LED2 with a value of 55, the series resistance was gradually decreased with increasing transparent-electrode thickness, indicating that the experimental critical thickness is larger than 60 Å. On the other hand, the integrated light output power was highest at a thickness of 60 Å, and was significantly degraded at the 120-Å thickness. This relates to the significant deterioration of the light transmittance of the transparent electrode. According to the electrical characteristics, it is evident that the experimental critical thickness of the transparent electrode was shifted toward greater thickness from the theoretical prediction (60 Å). However, considering the relatively small difference in series resistance between 60 and 120 Å-thickness as well as significant discrepancies between electrical and optical performance, it is difficult to conclude that the experimental critical thickness for 55 -LED is 120 Å. The systematic discrepancy between the experimental and the theoretical predictions can be explained in several ways. First, for simplification, it was assumed that the device widths for an n-type layer and a transparent electrode are equal in (4). However, the actual values of and are 220 and

4 1718 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 10, OCTOBER m, respectively. Therefore, for an accurate determination of the critical thickness, (5) must be modified so as (9) which leads to a correction in sheet resistance by a factor of 15%. In addition, for the basic modeling in terms of the uniform current spreading, the lateral voltage drop through the p-type layer was neglected, on the assumption that the electrical resistivity of the p-type layer is much larger than that of the transparent electrode [10], [11]. Although this assumption is valid in terms of the practical thickness of the transparent electrode, it is certain that this assumption might contribute to the shift in the experimental critical thickness toward thicker ranges. Furthermore, differences in measurement techniques should be also noted. As described previously, the sheet resistance of the transparent electrode was measured using a four-point probe system. However, the sheet resistance of a 5 5mm n-type GaN layer was measured using the In/Zn contact from a Hall measurement system. Therefore, an absolute comparison of the measured sheet resistance is difficult due to the existence of the In/Zn contact resistance for an n-type layer as well as the differences in the measurement systems. Based on the previous considerations, it can be concluded that the determination of the critical transparent-electrode thickness based on (5) must be corrected by about 46%, which was roughly estimated from the experimental data on the 90 -LED. Therefore, for the 55 -LED, it can be estimated that a more accurate critical thickness of the transparent electrode will be near 100 Å. This estimation is plausible considering that the deterioration of the light transmittance by 17.4% from 60 to 120 Å-thickness does not sufficiently explain the degradation of the integrated light output power of 55 -LED by 85% with an increase in the transparent-electrode thickness from 60 to 120 Å. It is noteworthy that the electrical and optical characteristics of the 55 -LEDs were much superior to those of the 90 -LEDs irrespective of the determined critical transparent-electrode thickness. This result suggests, for the highest device efficiency, first, the resistance of the n-type layer should be minimized in order to maximize the carrier injection into the n-type layer. The critical transparent-electrode thickness should then be determined according to the proposed method, resulting in the maximization of the carrier injection into the p-type layer through the transparent electrode. B. Innovative Geometrical Device Design Equation (6) predicts that perfectly uniform current spreading is possible when the geometrical factor of ratio approaches zero. In order to confirm this condition, LED samples with various ratios were prepared as shown in the inset of Fig. 3(a). Fig. 3(a) shows the characteristics of LEDs with various ratios. For a reasonable comparison, we attempted to minimize the effect of the material-related factor by adopting the most efficient LED system, for both aspects of the electrical and optical performance, which has an n-layer sheet resistance of 55 and a transparent-electrode thickness of 60 Å. Fig. 3(a) Fig. 3. (a) Current density voltage (J V ) and (b) the differential quantum efficiency-current density ( J) characteristics of LEDs with various l=w ratios. The inset of (a) shows a top view of the fabricated LED with two extreme l=w ratios, as measured by optical microscope. The inset of (b) shows the geometry used for measuring the light output power of the LED. It should be noted that an absolute comparison of the differential quantum efficiency () for each measurement angle of 0 and 90 is impossible due to the limitation of the measurement system (x 6= y). It is only possible to compare the relative differential quantum efficiency of LEDs with various l=w ratios under the same measurement angle. clearly shows that the electrical characteristics are gradually improved with decreasing ratios. In order to investigate the relation between electrical and optical performance, we plotted the differential quantum efficiency from the measured light output power as shown in Fig. 3(b), calculated using the following equation: number of emitted photons number of injected electrons junction area (10) where is the measured light output power (in Watts), the injection current (A), the photon energy (ev) radiated from the LED, and the junction area (or p-mesa area). Fig. 3(b) clearly shows that the differential quantum efficiency is greatly improved with decreasing ratios. Interestingly, we were able to observe more prominent improvements of the differential quantum efficiency with decreasing ratios when measured at a photodiode measurement angle of 0. This can be attributed to the relative change of the dominant light-emitting area from through the transparent electrode

5 KIM et al.: DESIGN AND FABRICATION OF LIGHT-EMITTING DIODES 1719 Fig. 4. Photographs of the fabricated (a) conventional LED with a l=w ratio of 1.00, (b) newly designed LED (new1), and (c) another newly designed LED (new2). (vertical direction) to the sidewalls (lateral direction) with decreasing ratios. Based on these experimental behaviors, the highest device efficiency is predicted when the ratio approaches zero. This fact is very attractive because it provides the possibility of constructing highly efficient LEDs without the need for a transparent electrode. Fig. 4 shows the conventional [Fig. 4(a)] and the novel LEDs [Fig. 4(b) and (c)], designed based on this consideration. It should be noted that the LED (c) (which we will call new2 ) has larger emitting area than the LED (b) (which we will call new1 ) by a factor of 13%. Also, it is noteworthy that new1 has slightly larger p-pad area than new2. For an exact comparison of the device performance between the conventional and the newly designed LEDs, the same overall dimensions of m were adopted for all LEDs and the same wafer was used for LED fabrication. Fig. 5 shows the characteristics of the conventional and the newly designed LEDs with a value of 90. For the newly designed LEDs, a clearly improved characteristic was observed compared to the conventional LED, which results in 50.6 and 26.8% reduction in series resistance of the diodes for new1 and new2, respectively. This result is very surprising, considering the fact that the junction area of the newly designed LED is much smaller than the conventional LED by about 30%. Therefore, it can be concluded that the newly designed LEDs are very effective in enhancing the current injection efficiency. As expected, the characteristics show more clear difference between the conventional and the newly designed LED as shown in the inset of Fig. 5. However, it should be noted that the conventional LED shows abnormal curve with a high series resistance, which is attributed to the high sheet resistance of the n-type layer. Therefore, it is important to address the issue of whether the improvement of the electrical characteristics for the newly designed LEDs is sample-dependent or not. Fig. 6 shows the characteristics of the conventional and the newly designed LED with a value of 55.Itis noteworthy that the newly designed LEDs also show improved Fig. 5. I V characteristics of the conventional and the newly designed LEDs with a value of 90 =. For the fabrication of the conventional LED, 60 Å-thickness of the transparent electrode was adopted. For an n- and p-type electrode pad, a Ti/Al (30 nm/100 nm) and a Ni/Au (30 nm/100 nm) were commonly adopted for the fabrication of all LEDs. The inset shows the current density voltage (J V ) characteristics of the conventional and the newly designed LEDs. Fig. 6. I V and the current J V characteristics (inset) of the conventional and the newly designed LEDs with a value of 55 =. For the fabrication of the conventional LED, 60-Å thickness of the transparent electrodes was adopted. electrical characteristics compared to the conventional LED, which results in a 56.2% and 43.7% reduction in series resistance for new1 and new2, respectively. According to these results, we conclude that the proposed geometrical design holds considerable promise in terms of electrical characteristics and that sample dependence is not a significant factor. In addition, it should be also noted that the electrical characteristics of new1 are superior to those of new2, which is indicative of a sensitive pattern dependence. Fig. 7 shows the power current ( ) characteristics of the conventional and newly designed LEDs with a value of 90. These data clearly show that the newly designed LEDs exhibit improved light output power for the same injected current, indicating that the newly designed LEDs are very effective in extracting generated light. Compared to the curves measured at 90, those of the newly designed LEDs measured at 0 show much greater improvement in light output power. As discussed above, this is related to the variation of

6 1720 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 10, OCTOBER 2002 Fig. 7. Light output L I characteristics of the conventional and newly designed LEDs with a value of 90 =. As described in the inset of Fig. 3(b), it is not possible to compare the absolute light output power of LEDs between measurement angle of 90 and 0. the dominant light-emitting area. That is, it was shown that the dominant light-emitting area changes from arising through the transparent electrode (vertical direction) to the sidewalls (lateral direction) with decreasing ratios. Therefore, based on this consideration, it is clear that while vertical light extraction is most favorable for the conventional LED, lateral extraction is favored for the newly designed LED. However, because of the high sheet resistance of the n-type layer, the optical efficiency of the conventional LED might be significantly degraded. Therefore, it is also important to consider the sample dependence of optical efficiency in LEDs with the innovative geometrical design. Fig. 8(a) shows the characteristics of the conventional and newly designed LEDs with a value of 55. Compared to the data shown in Fig. 7 at a measurement angle of 90, the newly designed LED with a value of 55 did not show a drastic improvement of output power compared to the conventional LED and even the output power for was slightly degraded. Considering this observed sample dependence, it would be expectable that, if a higher-quality LED sample is prepared, compared to those investigated in this study, the conventional LED may show a superior light output power at a measurement angle of 90. However, we also believe that the difference will not be large because our conventional LED samples possess sufficient electrical and optical performance, in that they showed ideal light extraction through all areas of the transparent electrode. Moreover, considering the prominent improvement in light output power in the lateral direction, it is likely that the newly designed LEDs still have great potential for use in practical applications. In order to investigate optical performance in detail, the characteristics were plotted and are shown Fig. 8(b). In the vertical direction 90, the light output power of the newly designed LED is lower than that of the conventional LED. Considering the same current density, this is due to the reduced junction area for the newly designed LED. However, in spite of the reduced junction area, the newly designed LED (new2) showed a higher output power in the lateral direction, indicating an extremely improved device efficiency. Fig. 8(c) shows the differential quantum efficiency versus current den- Fig. 8. (a) Light output L I, (b) the light output power current density (L J), and (c) the differential quantum efficiency-current density (0J) characteristics of the conventional and newly designed LEDs with a value of 55 =. sity characteristics for all LEDs. According to this plot, it is clear that the newly designed LEDs dramatically maximize the optical efficiency of the devices. It is also noteworthy that, although the newly designed LEDs (new1 and new2) involve the common concept of perfect current injection, a clearly different behavior was observed in optical as well as in electrical characteristics. That is, while the new1 showed electrical characteristics superior to new2, new2 showed superior optical characteristics compared to new1. This significant pattern influence on device performance suggests that, for geometrical LED design as a practical application, more consideration should be shown for design and additional testing must be taken into consideration.

7 KIM et al.: DESIGN AND FABRICATION OF LIGHT-EMITTING DIODES 1721 performance from the determination of the critical transparent-electrode thickness during the fabrication process. For this work, first, the resistance of the n-type layer should be minimized in order to maximize the carrier injection into the n-type layer. The critical transparent-electrode thickness should then be determined according to the relation of, resulting in the maximization of the carrier injection into the p-type layer through the transparent electrode. This result will be very useful for the fabrication of highly efficient devices in the conventional LED process. In addition, we also investigated the geometrical design rule for the highly efficient LED in terms of a perfect current spreading. Based on this consideration, it was even possible to realize the ideal LED geometry without the need for a transparent electrode, which resulted in improvement in LED performances, compared with a conventional LED. Finally, it is concluded that, although the determination of the critical transparent-electrode thickness has a great influence on the device performance, the design of ideal geometry is a better method to fabricate the highly efficient LED considering the extremely improved device performance as well as the simple fabrication process. ACKNOWLEDGMENT The authors would like to thank N.-M. Park and J.-S. Jang, Kwangju Institute of Science and Technology, Kwangju, Korea, for many useful discussions. Fig. 9. I V characteristics for both conventional Nichia-type (right side) and the advanced type-leds (left side). (b) The light output power current (L I) characteristics for both LED types. The overall dimension for both LEDs is m. Finally, we attempted to employ the innovative geometrical design for a commercial Nichia type LED. For this work, conventional Nichia type and the advanced Nichia type LED were fabricated on the same wafer with a value of 55 as shown in the inset of Fig. 9(a). It is clear that the advanced Nichia type leads to improved electrical characteristics, resulting in a reduction in series resistance by 43.2%. In addition, improved optical characteristics were also observed for the advanced type as shown in Fig. 9(b). These results indicate that the proposed innovative design has great potential for use in the practical applications. In order to explain the improved device performance of the newly designed LEDs, we mainly focused on maximized current injection from the viewpoint of current spreading. However, it is also noteworthy that the enhanced recombination and extraction efficiency might constitute an improved device performance as the result of the microsizing effect [13]. This explanation is plausible considering that the conventional line width of the active area discussed in this study is below 45 m. IV. 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8 1722 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 10, OCTOBER 2002 [12] J. E. Siewenie and L. He, Characterization of thin metal films processed at different temperatures, J. Vac. Sci. Technol. A, vol. 17, pp , [13] S. X. Jin, J. Li, J. Y. Lin, and H. X. Jiang, InGaN/GaN quantum well interconnected microdisk light-emitting diodes, Appl. Phys. Lett., vol. 77, pp , [14] D.-J. Kim, Y. T. Moon, K. M. Song, I.-H. Lee, and S.-J. Park, Effect of growth pressure on indium incorporation during the growth of InGaN by MOCVD, J. Electron. Mater., vol. 30, pp , [15] D. K. Schroder, Semiconductor Material and Device Characterization. New York: Wiley, [16] P. G. Eliseev, P. Perlin, J. Furioli, P. Sartori, J. Mu, and M. Osinski, Tunneling current and electroluminescence in InGaN:Zn,Si, AlGaN/GaN blue light-emitting diodes, J. Electron. Mater., vol. 26, pp , Seong-Ju Park was born in Korea in He received the B.S. degree in chemistry from Seoul National University, Seoul, Korea, in 1976, and the M.S. and Ph.D. degrees in physical chemistry from Seoul National University and Cornell University, Ithaca, NY, in 1979 and 1985, respectively. From 1985 to 1987, he worked as a Postdoctor with the IBM T. J. Watson Research Center, Yorktown Heights, NY. From 1987 to 1995, he was with the Electronics and Telecommunications Research Institute, Daejon, Korea, as a Principal Researcher. In 1995, he joined the Faculty at Kwangju Institute of Science and Technology, Kwangju, Korea, as a Professor in the Department of Materials Science and Engineering. Presently, he is a Professor with the department and the Director of the Center for Photonic Materials Research. He has engaged in researches on growth and characterization of semiconductor epitaxial structures, characterization of nanoelectronic and photonic materials, atomic and electronic structures of semiconductor surfaces, and plasma etching and reaction mechanisms. Hyunsoo Kim was born in Ulsan, Korea, on October 6, He received the B.S. degree in metallurgical engineering from Pusan National University, Pusan, Korea, in 1997, and the M.S. degree in materials science from Kwangju Institute of Science and Technology, Kwangju, Korea, in 1999, where he is currently pursuing the Ph.D. degree in materials science and engineering. His current research interests are device design, modeling, and reliability of GaN-based light-emitting diodes. Hyunsang Hwang (M 93) was born in Korea in He received the B.S. degree in metallurgical engineering from Seoul National University, Seoul, Korea, in 1988, and the Ph.D. degree in materials science from the University of Texas, Austin, in From 1992 to 1997, he was with the LG Semiconductor Corporation, Korea, as a Principal Researcher. In 1997, he joined the Faculty at Kwangju Institute of Science and Technology, Kwangju, Korea, as a Professor in the Department of Materials Science and Engineering. Presently, he is a Professor with the department. His research interests are process and device design of deep submicron MOSFET, MOSFET device reliability, ultrathin dielectric, and optoelectronic devices.