In this thesis, we employed the MOCVD ( Metalorganic Chemical Vapor Deposition ) to demonstrate the experiments. The MOCVD is the major technique for fabricate GaN base LED. The MO sources include TMGa ( Trimethylgallium )、TMAl ( Trimethylaluminum )、TMIn ( Trimethylindium )、Cp2Mg (Biscyclopentadienyl-Magnesium ) and NH3
( Ammonia )、 SiH4 ( Silane ) for hydride sources. The Wet-etching PSS was fabricated by the mixture solution of H3PO4 and H2SO4 with the ratio of 5 to 2 in 270℃ for 12 mins. The shape and dimension of the patterns are popular specifications as 2 x 1.5 x 1.5 ( µm ) as indicating in figure 17.
Figure 17. The dimension of the patterns on Wet-etching PSS
Shape：：：： Triangle pyramidal Diameter：：：：2µm
Pitch：：：：1.5µm Height：：：：1.5μμμm μ
2.1 The Experiment Principle
There were few reports or discussions relate the GaN growth condition of Wet-etching PSS by epitaxy before. And not to mention about how to avoid the GaN epilayer growth on r-plane. Therefore the past reports had no much reference in public information.
In 1991, the K. Hiromatsu defined the procedure of the GaN growth on sapphire for the purpose of AlN buffer[44]. The procedure include :
( 1 ) AlN growth formed Columnar in low temperature ( 2 ) GaN growth nucleated in high temperature
( 3 ) GaN film growth continued
( 4 ) GaN growth formed the island shape ( 5 ) GaN lateral growth
( 6 ) Coalescence
The procedure showed in figure 18. The GaN would be formed the columns in low growth temperature because the GaN didn’t has enough crystallize energy. The GaN columns only grew up vertically but not coalescent. Therefore the GaN won’t form a large superficial of film and the flat surface. In the same year, S. Nakamura employed the GaN buffer to replace AlN buffer and proved the GaN buffer possessed the higher performace than AlN buffer[45].
The other side, the study of GaN grown on Si(111) substrate, that employed a low growth temperature AlN inserted the interface between the GaN and Si substrate[46]. Because the LT-AlN had lots pits that will lead the
Therefore, we supposed the same mechanisms should also suit the GaN growth on Wet-etching PSS. The critical point is to prevent the GaN formed the large dimension grains by low growth temperature. The difference compare to conventional epitaxy is persisting the low growth temperature to grow GaN columns until cover the pattern completely. Than increase the growth temperature to cause the GaN lateral growth over the patterns and form a flat film finally. The GaN growth in low temperature perhaps grew become the polymer even amorphous. There are most exist a lot of defects in GaN. The defects will release the stress between GaN and sapphire. Then increases the growth temperature, the LT-GaN will be recovered by lateral growth in high temperature. Though the LT-GaN was poor but that would improve during heating to high temperature[47]. The recover temperature demonstrated about 1000℃[48]. Finally, grows n-GaN in the high temperature and accomplishes the LED structure. The LT-GaN growth mechanism lists below and sketches as figure 19.
(1) Buffer growth
(2) GaN growth in Low Temperature
(3) GaN Columnar growth
(4) GaN island growth in High Temperature
(5) GaN Lateral growth
(6) Coalescence
Figure 18. The procedure of the GaN growth on sapphire substrate[45]
1.
(1)Buffer growth
2.
(2)GaN growth in Low Temperature
3.
(3)GaN Column growth
4.
(4)GaN island growth in High Temperature
5.
(5)GaN Lateral growth
6.
(6)Coalescence
Figure 19. The procedure of the GaN growth on Wet-etching PSS
2.2 The Normal GaN LED Structure
The generally, GaN LED has the basic structures : (1)Buffer layer (2) u-GaN (3) n-GaN (4) MQW (5) p-AlGaN (6) p-GaN. Recently, most research groups will insert a CART ( Charge Asymmetric Resonance Tunneling ) that composed by InxGa1-xN or GaN/InxGa1-xN Super lattice before the MQW[49][50]. The CART structure could reduce the forward voltage, relaxed the QW stress, and enhance output power. The general sketch of CART GaN LED structure showed in figure 20. In this thesis, the purpose is grow the GaN on Wet-etching PSS. Therefore we employed the same structure of GaN LED for all the samples. The sketch and growth conditions as showed in figure 21. and table 4. The u-GaN divided 2 stages of roughing and recover layer. The roughing layer is low temperature grow and the recover layer is high temperature grow in definition. The durations of roughing and recover are 60 minutes and 30 minutes respectively and illustrated by reflectance of surface as figure 22. The reflectance was provided by MOCVD for mornited the epilayer thickness during epitaxy.
Figure 21. The sketch of GaN LED structure
Table 4. The sketch and growth conditions of the normal GaN LED
2.3 The Experiment Design
Base on the normal GaN LED, we try to employ low temperature to grow the GaN recover layer. Therefore we fixed the growth parameters but the growth temperature of roughing layer. Adjust the growth temperature to 1030℃℃ 、℃℃、、、1000℃、、、、970℃、、、、and 940℃respectively for 4 experiments.
Furthermore, increased the recover layer thickness to 3 µm for fully cover the patterns. The other growth parameters would fix as possible as the normal GaN LED(Table 4.) and finish the GaN LED epitaxy. The growth parameters of 4 experiments were list in table 5.
• Buffer Layer
Temperature：530℃
Thickness ：200Å
• Roughing / Recover / u-GaN Layer Temperature：1030 =>1060 ℃
Figure 22. The reflectance of surface during GaN growth
• Buffer Layer
Temperature：530℃
Thickness ：200Å
• Roughing / Recover /u-GaN Layer Temperature：1030 =>1060 ℃
Table 5. The Experiment parameters
Normal Recover layer and 4 samples Conditions：