Chapter 1 Introduction
3.4 Effects of different n-electrode patterns on characteristics of the large
large-area LEDs
In the p-side-down LLO-LED configuration, the n-GaN layer serves as a better current spreading layer than the p-GaN layer in the p-side-up configuration on a sapphire substrate due to the higher electron mobility and greater thickness of the n-GaN layer. Since the light emission intensity is directly proportional to the current density, a uniform current distribution in a n-GaN layer to provide a uniform light emission pattern is necessary. In this section, four different geometric n-electrode patterns were deposited on large-area p-side-down LLO-LEDs of 1000×1000 μm2 in dimensions. The current crowding effect in the p-side-down GaN LLO-LEDs under high current injection level was observed and studied. The light emission patterns of LEDs with different n-electrode patterns are compared. The
electrode-pattern-dependent light output power is also discussed.
The fabrication process of the large-area p-down LEDs was same as mentioned in previous section. In the final step of deposition of n-electrode pad, Ti/Al layers with different patterns were deposited on the n-GaN layers as an n-type contact without additional transparent contact layers. The schematic top view of the LED devices with four different n-electrode patterns is shown in Fig. 3.8. The detailed dimensions of each electrode pattern are shown in Table 3.2.
In Fig. 3.9 the light output-current (L-I) characteristics of LED a under continuous-wave (cw) and pulse operations with various duty cycles are compared.
Under the same driving current operation, the light output power increased as the operation duty cycle was decreased from cw to 0.01% as shown in the figure. The output power saturation is also less pronounced as the duty cycle is decreased. The inset shows the micrographic surface light emission pattern of LED a driven at 100 mA. The injected current crowded near the electrode, resulting in an area with a higher current density corresponding to the brighter area around the circular contact indicated by the surface light emission pattern of LED a. The crowded current could induce thermal and carrier over flow effects [3.4-3.7] which saturate and decrease the output power of the LEDs. The thermal effect induced by current crowded around the electrode is one reason for the power saturation of LED a as suggested by the L-I
characteristics for different operation duty cycles.
In order to reduce the current crowding effect and investigate the influence of the n-electrode pattern on optical characteristics, different n-electrode patterns were designed for the LLO-LEDs. Surface light emission patterns of the four LEDs with different n-electrode are shown in Fig. 3.10. The light emission patterns were obtained and analyzed at a driving current of 450 mA using a near-field microscope with a charge-couple device and a video analyzer (BeamView Analyzer, Coherent Inc.) linked to a computer. The solid curves at the bottom of the images stand for the relative light output intensity measured along the dashed lines. The light intensity was normalized with the peak values, normally appearing at the edges of the circular electrodes in the four LEDs. In Fig. 3.10(a), the light emission close to the circular electrode shows great intense intensity, which reveals that the injection current crowded around the electrode pad. In the absence of a transparent contact layer for current spreading, the n-electrode of LED a is insufficient to provide uniform current spreading in the large-area p-side-down LEDs configuration. In Fig. 3.10(b), more intense light emission around the extended cross-shaped electrode is observed. With the cross-shaped electrode for enhancement of current spreading, the distribution of light emission was more uniform compared with that of LED a, as shown by the relative intensity curve. The extended cross-shaped electrode improved the current
spreading over the large-area mesa and consequently provided a more uniform light emission pattern. The light emission patterns of the LEDs with two other electrodes are shown in Figs. 3.10(c) and 3.10(d). The light intensity of LED d showed a uniform distribution from the center to the edge of the mesa. In contrast, the light intensity of
LED c decreased near the edge of the mesa. The light emission pattern of LED d is
more uniform than that of LED c, in which a more intense emission appeared inside the square electrode. Figure 3.11 shows the 3-D isometric plot of the spatial intensity distribution of the LEDs with different electrode patterns. The isometric plots provide an intuitional observation of emission intensity distribution for these four LEDs.
The L-I characteristics of the LEDs with four different n-electrode patterns under cw operation are shown in Fig. 3.12. The insets show the micrographic top view of the four LEDs driven at 100 mA in the sequence LED d, LED c, LED b, and LED a from top to bottom. The output power measurement was performed from the upper side of the chip using a large-area Si photodiode placed 5 mm above the samples. The light output power of the LEDs showed a linear increase as the driving current was increased to 250 mA. The light output power of the four LEDs is also approximately equal when the driving current was below 250 mA. The injection current is supposed to spread uniformly over the mesas on the four LEDs, which results in equal light output power when the driving current was below 250 mA. As the driving current was
increased above 250 mA, the light output power of LED a began started to saturate and decrease due to the carrier overflow effect and the thermal effect caused by the high current density distributed around the circular electrode corresponding to the previous discussion for Fig. 3.9. In LED b, the output power saturation was also observed under a current injection level above 600 mA. LED d showed superior L-I characteristics compared with the other LEDs due to its well-designed electrode pattern for providing uniform current spreading as indicated in Fig. 3.10(d), which consequently reduced the thermal and carrier overflow effects caused by localized high injection current density. As the injection current was driven at 1000 mA, the light output power of LED d was 1.15, 1.30 and 3.15 times larger than that of LED c,
LED b and LED a, respectively. The different light output powers among the four
LEDs as the driving current increased to 1000 mA is caused by different current densities in the active region of each LED, which depend on the distribution of the injected current over the LEDs, resulting in carrier over flow and thermal effects at different levels and consequently different external quantum efficiencies [3.8]. The results indicate that the patterns of n-electrodes have a marked influence on the light output power.
Sapphire n-GaN MQWs
p-GaN
SiN SiN mask
SiN
SiN Bonding metals
(1) (2)
(3) (4)
(5) (6)
Figure 3.1 Schematic fabrication steps of the large emission-area GaN LEDs on Cu substrate.
(7) (8)
(9)
Figure 3.1 Schematic fabrication steps of the large emission-area GaN LEDs on Cu substrate.
KrF: excimer laser
n-contact
Graphite plate Mo screw
Stainless steel plate Mo nut
Pre-bonded sample
Figure 3.2 Schematic diagram of the fixture for metal bonding process.
Figure 3.3 SEM image of the n-GaN surface after LLO process.
Arrows indicate some Ga droplets left by the decomposition of GaN.
Table 3.1 Electrical and thermal properties for several materials used in this
n-contact n-GaN
p-GaN
InGaN/GaN MQWs
Cu substrate Bonding metals
SiN passivation
Figure 3.4 Schematic structure of the large-area emission LED on Cu.
Figure 3.5 SEM image of the LED on Cu substrate.
GaN LED film
Cu substrate n-contact
Figure 3.6 (a) L-I-V characteristics of the large-area-emission GaN LEDs on Cu substrate under continuous-wave operation. (b) Electroluminescence of the LLO-LED under a driving current of 200 mA.
380 400 420 440 460 480 500 520 540 0
0 100 200 300 400 500 600 700 0.0
0.2 0.4 0.6 0.8 1.0 1.2
Ls=400 μm
Emission intensity (arb. unit)
Distance from n-electrode edge (μm)
x=0
Figure 3.7 Normalized emission intensity as a function of the distance from the electrode edge. Inset shows the micrograph of the optical emission from the LED driven at 200 mA.
LED a LED b LED c LED d
Length of cross: 700 μm
Square width: 520 μm Length of cross: 700 μm
Square width: 350 & 700 μm Cross width: 700 μm Diameter: 120 μm
* Circular diameter and linear linewidth are 120 mm and 20 mm, respectively in the four patterns.
Pattern Dimension Figure 3.8 The schematic diagrams top view of the LED devices with four different n-electrode patterns.
Table 3.2 Detailed dimensions of the four electrode patterns.
0 100 200 300 400 500 600 0
2 4 6 8 10 12 14 16
Driving current (mA)
Li ght out p ut pow e r ( a rb . uni t) 0.01%
10%
CW
LED a
Figure 3.9 L-I characteristics of LED a under cw and pulse operations with various duty cycles.
Cu substrate
(c) (d) (a) (b)
Figure 3.10 Light emission patterns and intensity distributions of the LEDs with different n-electrode patterns.
Relative intensity
0 1
Figure 3.11 3-D isometric plot of the spatial intensity distribution of the LED a, LED b, LED c and LED d with different electrode patterns respectively.
0.5
LED a LED b
LED c LED d
0 200 400 600 800 1000 1200 0
50 100 150 200 250 300 350 400
Light output power (mW)
Driving current (mA) LED d
LED c LED b LED a
Figure 3.12 L-I characteristics of the LEDs with four different n-electrode patterns under cw operation.
Reference
[3.1] Y. S. Wu, R.S. Feigelson, R. K. Route, D. Zheng, L.A. Gordon, M. M. Fejer, and R. L. Byer: I. Electrochem. Soc. 145, 366 (1998)
[3.2] N.A. Lange: Handbook of Chemistry, Handbook Publishers, Sandusky, Ohio, (1956)
[3.3] S. Nakamura and G. Fahsol: The Blue Laser Diode (Springer, Berlin, 1997) [3.4] X. Guo and E. F. Schubert: Appl. Phys. Lett. 78, 3337 (2001)
[3.5] X. Guo and E. F. Schubert: J. of Appl. Phys., 90, 4191 (2001)
[3.6] Hyunsoo Kim, Seong-Ju Park, and Hyunsang Hwang and Nae-Man Park:
Appl. Phys. Lett. 81, 1326 (2002)
[3.7] Hyunsoo Kim,Jaehee Cho, Jeong Wook Lee, Sukho Yoon, Hyungkun Kim, Cheolsoo Sone, Yongjo Park and Tae-Yeon Seong: Appl. Phys. Lett. 90, 063510 (2007)
[3.8] M. Yamada, T. Mitani, Y. Narukawa, S. Shioji, I. Niki, S. Sonobe, K. Deguchi, M. Sano and T. Mukai: Jpn. J. Appl. Phys. 41, L1431 (2002)