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trench is implemented, which leads to a free-standing GaN film. If there is cracking in one post, it will not propagate to next.
With this approach it is possible to realize a manufacturing yield of more than 95 percent of chips without cracking, for a chip size is 1mm or smaller and a GaN thickness exceeding 4.5 µm. A dislocation density as low as 5x108
film on a silicon substrate.
Figure 2 demonstrates low dislocation density from a GaN wafer grown on a silicon substrate. Figure 3 shows a high quality GaN-on-silicon chip without cracking.
Figure 2: Low dislocation density from a GaN wafer grown on silicon substrate
If sapphire substrates are used, compressive strain in GaN film results from the lattice constant and thermal mismatch, but this does not lead to cracking.
In other words, this highly tensile strained GaN film on silicon substrate presents the biggest challenge for making high-performance, high-reliability LEDs. Figure 1 illustrates a GaN-on-silicon wafer with cracking everywhere.
Managing the stress Successful high performance LED manufacture demands low-dislocation, crack-free films. Managing the stress is the first hurdle to overcome. Large lattice-constant mismatch between GaN and silicon leads to high dislocation densities, which can be as high as 109
-1010 cm-2 .
Even though the GaN-based LED is far less sensitive to high dislocation densities than its GaAs or InP cousins, such a high dislocation density still causes low internal quantum efficiencies. This prevents the realization of high- power, high-reliability LEDs. But we can address this issue by introducing patterned substrates that isolate the stress caused by cracking, and a special AlGaN/AlGaN multilayer buffer structure that manages internal strain. We have found that patterned substrates are effective in containing the strain and limiting cracking propagation. The width, depth and shape of the trench are crucial levers to minimizing the stress and limit the cracking. In order to contain the stress and eliminate cracks, a deep
High performance LEDs must also have high internal quantum efficiency. For GaN-on-silicon growth, thick n- GaN layers have always been a problem, because as the epilayer gets thicker, stress increases and the wafer tends to bend and crack. We address this with a multiple special buffer layer.
The AlN layer tends to provide a good buffer for controlling the stress. We are being able to design and grow our epistructure with a 4-5 µm n-GaN layer. Figure 4 shows SEM pictures of the epi layer and quantum well design.
Preventing silicon absorption Silicon absorbs visible light. So in order to have efficient LED operation, the silicon substrate must be liberated from the device, a thin-film, vertical structure. Figure 5 illustrates the silicon-based thin film process. GaN is grown on a prepared, patterned (111) silicon substrate. A metal contact is deposited on the p-side of the GaN film. This acts as a light reflector adding further benefits to the device. A bonding metal is then deposited on top of the p-metal, subsequently bonded on a carrier substrate (pre- deposited with a bonding metal). The substrate can be silicon, germanium or a metal plate. The bonding metal can be gold or one of its alloys, depending on different process designs for different applications.
/cm2 can be routinely achieved for a GaN
Figure 3: high quality GaN on silicon chips without cracking
June 2010
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