Comparison of the optimum number of quantum wells in GaN-based blue light-emitting diodes grown on sapphire and Si(111) substrates

We experimentally investigate the internal polarization field and the optimum number of quantum wells (QWs) in InGaN/GaN-based blue light-emitting diodes (LEDs) grown on sapphire and Si(111) substrates. The internal polarization field is measured using the reverse-bias electro-reflectance spectroscopy, which reveals that LEDs grown on sapphire substrates has >50% higher strain than those grown on Si substrates. When the electroluminescent (EL) characteristics of LED samples with different number of QWs are compared, the optimum number of QWs that results in the highest EL intensity is found to be five for LEDs on sapphire and seven for LEDs on Si. The different optimum number of QWs can be interpreted by the different strain states in multiple QWs (MQWs) of LEDs on sapphire and Si substrates. As the internal polarization field increases, carrier distribution in MQWs becomes more inhomogeneous and accumulated strain in MQWs could generate more dislocations, which limits the optimum number of QWs for LEDs having large internal polarization fields.