interview industry
We can produce LED chips with various sizes on silicon substrates. Low operating voltage and high output power are demonstrated,and
we have launched reliable products for display applications (small die), LCD backlighting (mid size die) and general lighting (large die)
The original (111) silicon substrate is then removed by wet chemical etching, followed by n-contact metal deposition. For high power devices, we will implement a surface roughening process. The LED emits through the n-GaN side. Figure 6 are actual 500 µm and 1 mm LED chips made from GaN on silicon.
We can produce LED chips with various sizes on silicon substrates. Low operating voltage and high output power are demonstrated, and we have launched reliable products for display applications (small die), LCD backlighting (mid size die) and general lighting (large die). Figure 7a is an example of a 450 nm, 1 mm chip emitting 480 mW when driven at 350mA.
Figure 4: SEM pictures of the epi layer and quantum well design
Output corresponds to 100-110 lm cool-white flux at 350mA. The typical quantum efficiency as a function of current for a GaN device on a silicon substrate is shown in Figure 7b. Characteristics are no different from GaN LEDs on sapphire or SiC substrates.
This shows the GaN-on-silicon material quality is as good as that on sapphire or on SiC, and demonstrates that this class of LED is a very promising candidate for large-scale, low-cost production.
Reliability concerns? The GaN-on-silicon substrate is highly stressed, so there are genuine concerns regarding its long-term reliability. We have conducted a long-term reliability test at three times normal operating current and a 75 degrees Celcius board temperature (approximately 110-120 degrees Celcius junction temperature).
Figure 8 details the results of 0.5mm (at 200mA) and 1mm (at 900mA) chips under a highly stressed reliability test. They show no or very little power degradation. Similar reliability is seen in GaN material grown on sapphire.
Up until now, more than 95 percent of GaN-based LEDs are made on sapphire substrates, with the majority of the remainder grown on SiC. We have been marketing silicon- based LED products for close to two years. Initial production had many issues, mainly related to production line yields.
Customers had many issues too, such as handling and packaging, which resulted from the brittle carrier material of the thin film structure. However, thanks to process
Figure 6: High-performance 500 µm and 1 mm chips made from GaN-on-silicon substrates
June 2010
www.compoundsemiconductor.net 33
After EPI growth, GaN LED structure on (111) Si substrate
P-contact Reflector metal and bonding metal deposition and wafer bonding
Wet etch removal of (111) Si substrate
N-side ohmic contact and top surface roughening
Figure 5: Lattice Power’s silicon based thin film process
carrier
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