Figure 2-10 presents the current versus voltage (I-V) of the SiC-substrate LED bonded at 250°C for 60 min. The device exhibited normal p-n diode behavior with a forward voltage of 2.1V at 20 mA with the chip size of 300 × 300 µm2. This indicated that the wafer-bonding process did not degrade the performance of LED. Figure 2-11 shows the effects of injection current on the luminous intensity of the conventional GaAs-substrate LED and the SiC-substrate-bonded LED. When the injection current was 20 mA, the luminous intensities of SiC-substrate LED (248 mcd) was higher than that of conventional LED (77 mcd), indicating that the wafer-bonding process did not degrade the performance of SiC-substrate LED and the Au/BeAu/Au mirror provide a high reflectivity to enhance the light intensity as shown in Fig. 2-9. When the injection current reached 90 mA, the GaAs-substrate LED were saturated at the luminous intensity of 190 mcd. However, the intensity of the SiC-substrate LED did not saturated at 100 mA. The maximum luminous intensity of SiC-substrate LED could reach as high as about 4300 mcd at 650 mA, which was about twenty times higher than the saturation intensity of the GaAs-substrate LED at 100 mA. It was obvious that the luminous intensity of SiC-substrate LED was much higher than that of GaAs-substrate LED.
Figure 2-12 shows the peak spectral wavelength as a function of the DC drive current.
The data shows that the emission peak wavelengths shift toward longer wavelengths with increasing DC drive current, which is caused by the joule heating. Unlike the 13nm red shift exhibited at 90 mA in GaAs-substrate LED, the Cu-substrate LED devices exhibits a more
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favorable 1 nm red shift at 90 mA. The red shift of the emission wavelength occurred as the temperature increased. From the shift in emission wavelength and a wavelength shift with temperature of 0.096 nm/°C for 620 nm AlGaInP LED [10], it could be estimated the temperature of GaAs-substrate LED was about 125°C higher than that of the SiC-substrate LED at the forward current of 90 mA. This difference might also be a result of the thermal conductivities; the thermal conductivity of 4H-SiC (370 Wm-1K-1) is eight times higher than that of GaAs (46 Wm-1K-1). Therefore, by using these wafer bonding techniques to bond LED to SiC substrates, the joule-heating problem in conventional LED can be also significantly reduced.
2-4 Summary
In summary, high-power Cu-substrate and SiC-substrate LED fabricated by bonding AlGaInP LED structure to Cu and SiC substrate were investigated in this study. An ITO film was used as an intermediate layer to bonded wafers for the Cu-substrate LED. It was found that the sample did not bond at temperatures below 400°C. When bonding temperature reached 600°C, the Cu element diffused through the ITO layer and destroyed the LED structure. Fortunately, Cu did not penetrate the ITO layer when samples were bonded at 500°C for 30 minutes, and the high-power Cu-substrate LED were successfully fabricated. In Cu-substrate LED, the joule heating exhibited in conventional GaAs-substrate LED was significantly reduced because the Cu substrate has larger thermal conductivity and lower thermal resistance as compared with GaAs substrate. It was found that Cu-substrate LED could be operated in a much higher injection forward current, 800 mA, which was eight times higher than that used in GaAs-substrate LED. The luminous intensity of the Cu-substrate LED could reach as high as 1230 mcd, which was three times higher than that of the
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GaAs-substrate LED. For the SiC-substrate-bonded LED, the luminous intensity of SiC-substrate LED was 3.2 times higher than that of conventional LED at the injection current of 20 mA. The improvement of the emission of light could be reflected downward using a mirror with reflectivity of 87 % at 620 nm. The saturation current could reach at 650 mA, which was better than GaAs-substrate LED.
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Fig. 2-1. A successful Cu-substrate-bonded LED sample.
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Fig. 2-2. A scanning electron micrograph of the cross section of the LED structure.
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Fig. 2-3. Auger depth profiles of AlGaInP LED/ ITO-Cu sample bonded at 500°C for 30 minutes.
36 LED chip size: 250 × 250 µm2 Metal contact diameter: 100 µm
0 50 100 150 200 250 300 350 400
0 0.5 1 1.5 2 2.5 3
CURRENT (mA)
VOLTAGE (V)
Fig. 2-4. Current-voltage characteristic of the Cu-substrate-bonded LED devices fabricated by wafer bonding technology.
37 620
621 622 623 624 625 626 627
0 50 100 150 200 250 300 350 400
GaAs substrate LED Cu substrate LED
PEAK WAVELENGTH (nm)
CURRENT (mA)
Fig. 2-5. Peak spectral wavelength against DC injection current for the LED with a GaAs substrate and a Cu substrate.
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(a) GaAs Substrate LED (b) Cu Substrate LED
(a)
(b)
0 200 400 600 800 1000 1200 1400
0 200 400 600 800 1000
LUMINANCE INTENSITY (mcd)
CURRENT (mA)
Fig.2-6. L-I curves for conventional GaAs-substrate LED and Cu-substrate LED.
39 -15
-10 -5 0 5 10 15
1 10 100 1000
LIGHT OUTPUT VARIATION (%)
TIME (HOUR)
20 mA, 25°C
Fig. 2-7. Luminescence-output intensity variation as functions of time.
During this life test measurement, a 20 mA current was injected into the Cu-substrate LED at room temperature.
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Fig. 2-8. Schematic diagram of the LED/ metal/ SiC wafer fusion process: (a) SiC substrate and LED wafer with In and BeAu metal; (b) wafer bonding process; (c) removal of GaAs substrate and etch stop layer; and (d) mesa-etched and electrodes deposition.
SiC sub.
Electrodes LED structure BeAu layer In metal SiC sub.
GaAs sub.
( a ) ( b )
( c ) ( d )
SiC sub.
GaAs sub.
SiC sub.
41 0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
400 500 600 700
W avelength (nm)
Reflectivity
Fig. 2-9. The reflectivity of the BeAu/Au mirror.
42 0
20 40 60 80 100
0 0.5 1 1.5 2 2.5 3
Voltage (V)
Current (mA)
Chip size: 300X300µm2
Fig. 2-10. The I-V characteristics of SiC-substrate LED.
43 0
500 1000 1500 2000 2500 3000 3500 4000 4500
0 200 400 600 800
Current (mA)
Intensity (mcd)
SiC sub. LED GaAs sub. LED
Fig. 2-11. The L-I curves of conventional GaAs-substrate LED and SiC-substrate LED.
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620 625 630 635 640 645
0 200 400 600 800
Current (mA)
Peak wavelength (nm)
GaAs sub. LED SiC sub. LED
Fig. 2-12. Peak spectral wavelength against DC injection current for the LED with a GaAs substrate and a SiC substrate.
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Chapter 3 Performance of InGaN-GaN light emitting diode fabricated using glue bonding on 50mm Si substrate
3-1 Introduction
High-brightness GaN-based light-emitting diode (LED) has attracted considerable attention for their versatile applications in mobile phones, full-color displays and lighting [1]. Although the development of these GaN-based LED is very successful, the poor conductivity of p-GaN limits the performance of LED because of current crowding [2].
This problem can be solved using a thin Ni/Au layer or a highly transparent (> 80%) indium tin oxide (ITO) layer as a current spreading layer [3]-[4]. However, the poor electrical characteristics (electrical resistivity = 1011 –1016 ohm-cm) of the nonconducting sapphire substrate necessitate the need for p- and n- metal electrodes on the top surface of the devices. Hence, some of the active layer in the n-contact region is sacrificed. Moreover, the heat dissipation of the sapphire substrate is also poor, so the GaN-based LED are generally operated at low injection current. These problems can be solved by transferring GaN LED onto Si [5]-[6] or Cu substrates [7]. Much of this investigation is focused on the intermetallic bonding. In this work, an n-side-up InGaN LED with vertical electrodes was fabricated by glue bonding. A high-temperature stable organic film is utilized as the bonding agent to prevent any possible reaction with the metal reflector.
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