High Brightness Light Emitting Diodes Chapter 7~8

1. High Brightness Light Emitting Diodes Chapter 7~8 Reporter：陳秀芬 Adviser：郭艷光 教授 Date：2003/5/5(Study meeting)

2. Chapter 7 OMVPE of GaN for High-Brightness LED • Hydride vapor-phase epitaxy (氫化物氣相磊晶法) • Molecular-beam epitaxy (分子束磊晶法) • Organometallic vapor-phase epitaxy (有機金屬氣相磊晶法) • Laser diodes

3. Hydride vapor-phase epitaxy (HVPE) • In 1932, GaN was first synthesized by Johnson et al. They used the direct reaction between metallic Ga and ammonia (NH3) • In 1969, the first single-crystalline GaN was grown by Maruska and Tietjen using HVPEon sapphire substrates. GaCl and NH3 were used as source gases, which were reacted at about 1000℃. • In the 1970s and early 1980s, HVPEwas commonly used for the fabrication(製造)of MIS-type GaN LEDs.

4. The Major Problems • (1)generation of many cracks and pits. • In order to reduce the concentration of residual donor, thick films are required. • However, many cracks occur when the films are too thick. Because there are large differences of the lattice constant and thermal expansion coefficients between GaN and sapphire. • (2)poor controllability of the thickness of the insulating layer that governs the operating voltage. • The operating voltage was about 10V.

5. Molecular-beam epitaxy(MBE) • MBE is another important technique for the fabrication of LED, because of the more precise controllability of thickness and alloy composition. • In 1968,the use of a nitrogen(N) plasma decomposition(分解)technique for the growth of group Ⅲ nitrides was demonstrated by Wauk and Winslow. In this way, the residual donor concentration is reduced. • In 1995, Molnar et al. showed the first p-n-junction type GaN LED grown by MBE.

6. Organometallic vapor-phase epitaxy(OMVPE) • In 1984, the first blue LED on GaN grown by MOVPE was fabricated by Kawabata et al. The performance of their MIS-type LEDs was inferior to those fabricated by HVPE. • In 1986, a great advance in crystal growth by MOVPE was achieved by Amano et al. They proposed the use of a low-temperature-deposited(LT,低溫沉積) buffer layer. • In 1991, The buffer layer method was followed by Nakamura, who used an LT-GaN buffer layer. Today, these LT-AlN or LT-GaN buffer layers are standard for the growth of GaN and nitride alloys on sapphire substrate by OMVPE.

7. Timing chart and the model of GaN LED with the LT-buffer layer The crystalline quality,the luminescence and the electrical properties have been evidently improved simultaneously using the LT-buffer layer method.

8. In order to achieve visible emission, smaller bandgap materials were used, like GaInN. In 1993, Nakamura first used GaInN as an active layer for violet LEDs. Control of thickness and alloy composition are important, because ternary(三元的) compounds such as AlGaN and GaInN are lattice-mismatched systems. • In 1995, Nakamura showed efficient blue, green, and yellow LEDs fabricated using very thin GaInNquantum well active layers.

9. Critical layer thickness of AlxGa1-xN and Ga1-yInyN on GaN The critical layer thickness for GaInN on GaN is much lower than that for AlGaN on GaN.

10. About conductivity control • n-type: In 1990, Akasaki showed that SiH4 is suitable for Si doping during OMVPE growth of GaN and AlGaN using the LT buffer method, and Si behaves as a donor. • p-type: It had not succeed to produce p-type GaN until 1989. Good controllability of Mg concentration during OMVPE growth of GaN was reported in 1990. In 1992, Nakamura achieved p-type GaN by thermal annealing. They showed that Mg is passivated by hydrogen in the as-grown state. • By using the LT-buffer layer and conductivity control, it’s possible to fabricate high-performance blue LEDs and short-wavelength LDs.

11. Chronological table of the external quantum efficiency of GaN-based LED The efficiency began to increase greatly soon after the successes in producing high-quality crystals by using the LT-buffer layer method, which resulted in p-n junction LEDs.

12. Laser Diods • In 1971, the first stimulated emission and laser action by optical pumping was reported by Dingle et al. at a temperature 2K. • In 1990, Amano et al. reported the first stimulated emission by optical pumping at room temperature. They used high-quality GaN epitaxial layers grown with the LT-buffer layer. • In 1996, Nakamura et al. fabricated the first N-based LD.

13. Chapter 8 Group Ⅲ-V N-based ultraviolet Blue-green-yellow LEDs and LDs • GaN growth • Undoped GaN, n-type GaN, and p-type GaN • GaN p-n junction LED • InGaN growth • Undoped InGaN • Impurity doping of InGaN • LEDs • LDs

14. GaN growth • Undoped GaN : • Usually, GaN films are grown on a sapphire substrate at temperature about 1000℃.(Because of “money”) • The lattice constant along the a-axis of the 6H-SiC, sappire, and GaN are respectively : 3.08Å, 4.758Å, 3.189Å. • GaN layers usually show n-type conduction. • n-type GaN : • Si is considered to be a good donor impurity for GaN in order to control the carrier concentration. So is Ge.

15. p-type GaN : • It was hard to obtain p-type GaN films. • In 1992, low-resistivity Mg-doped p-type GaN films were obtained by N2-ambientthermal annealing at temperature above 400℃. • GaN p-n junction LED : • In 1991, GaN p-n junction blue LEDs were fabricated.

16. InGaN growth • Undoped InGaN : • To prevent InN dissociation during growth, relatively high-quality InGaN films were obtained on sapphire substrate. It used a high growth temperature (800℃), and a high In mole fraction in the vapor.(1991) • The energy bandgap of InGaN : Eg(x) = x Eg,InN + (1-x) Eg,GaN – b x (1-x) ( Eg,InN= 1.95 eV ; Eg,GaN= 3.40 eV ; b= 1.00eV )

17. PL spectra of InGaN(273 K)

18. Impurity doping of InGaN • Comparing Si-doped InGaN with undoped InGaN, the peak wavelength of the emission is the same. However, the intensity becomes much stronger than for undoped InGaN films. • In 1994, Si and Zn were codoped into the InGaN films to obtain larger emission center, giving a luminous intensity high enough to be detected by the human eye.

19. Light-Emitting Diodes • InGaN/Galliun Nitride DH LEDs were fabricated for the first time in 1993. • In 1994, InGaN/AlGaN DH LEDs were fabricated. • In 1993, two kinds of InGaN/InGaN MQW structures were grown on GaN films. (one’s well layer = 100 Å ; the other’s well layer = 30 Å) • After then, the blue, green, yellow SQW LEDs with different In mole fraction of InGaN were fabricated.

20. PL spectra of the two kinds of InGaN/InGaN MQW structures

21. EL of the blue, green, yellow SQW LEDs

22. Comparison of various green and blue LEDs

23. Laser Diodes • Optically pumped stimulated emission from GaN was first observed in 1971. • Since then, many researchers have been working toward the optimization of the design for VCSELs and LDs. • However, stimulated emission has been observed only by optical pumping, and not by current injection. (Before 1991)

24. Thanks for your attention!