III-Nitride semiconductor materials have been widely used for light emitting didoes (LEDs) for a wide range of lighting applications due to their excellent chemical stability and direct wide band gap energy nature. InGaN/GaN LEDs are conventionally grown on sapphire substrates with lateral contact device structure. However, LEDs fabricated with lateral contact device structure suffer from current crowding issue at the mesa boundary and contact edge when current flowing from the p-type GaN to n-type GaN, resulting in localized heating which reduces the internal quantum efficiency. In addition, sapphire substrates are relatively expensive and have a low thermal conductivity of 35 W/mK. Therefore, domestic LED lighting manufacturers have recently changed their interests to growing InGaN/GaN LEDs on Silicon (Si) substrates rather than sapphire substrates. Si substrates not only have higher thermal conductivity (i.e. 149 W/mK), but are also cheaper than sapphire substrates. However, Si substrates are light absorbing; if LEDs fabricated with conventional lateral contact structure the emitted light, which travels toward the Si growth substrate, is absorbed and wasted. To resolve this issue, the vertical LED (VLED) structure is adopted for InGaN/GaN LED grown on Si substrates. The original Si growth substrate is removed and the InGaN/GaN LED epitaxy is bonded on to a Si carrier substrate with a reflectivity p-type ohmic contact. In this thesis, a complete fabrication process of InGaN/GaN grown on Si VLEDs with a novel reflective p-contact was developed and characterized. A reflective p-type ohmic contact of Ni/Ag/Ni was developed to act as a mirror, which prevents the light absorption by the Si carrier substrate. This contact has a high reflectivity of 75% and a low contact resistivity of 6.3x10-5 Ωcm2 after annealing in an oxygen environment for 2 minutes at 450 ℃ to form an ohmic contact to p-GaN. The light output power of our VLED using Ni/Ag/Ni contact is approximately 20 mW higher than the VLEDs without any reflectors fabricated by Xiong et. al.. Although the forward voltage of our VLEDs is 0.5 V higher than VLEDs made by Xiong et. al., this can be attributed to our Si carrier substrates do not have any thickens reduction process. The impact of the thickness of Si carrier substrate on the electrical and optical performance of the VLED was also characterized. By using a Si carrier substrate with a thickness of 475 μm rather than 675 μm, the quantum confined stark effect was reduced and the external quantum efficiency was improved by 4 %. Also, the operational voltage was reduced to 5.7 V from 6.7 V at the drive current of 300 mA. Finally, a KOH surface roughen process for the N face n-GaN was devised to resolve the total internal reflection issue at the interface between the n-GaN and air. The light output power of the VLED increased from 53 mW to 97 mW after etching in 4M KOH solution at 100 ℃ for 3 minutes.
|Date of Award||11 Jan 2017|
|Supervisor||Duncan Allsopp (Supervisor)|