The innovation engine for new materials

Hazel Gardner


Trinity College, University of Oxford


Materials Science


Christopher D. Pynn

Faculty Sponsor(s): 

Steven P. DenBaars

Project Title: 

Improved Efficiency of Green InGaN/GaN LEDs Grown on Semipolar Bulk GaN Substrates

Project Description: 

To make RGB white lighting competitive with phosphor-based technology, LEDs with high efficiency at all values of drive current (low efficiency-droop) are required. Current state-of-the-art green LEDs grown on semipolar substrates (substrates inclined to the c-axis of the GaN crystal structure) contain single quantum wells because transport across the GaN barriers between wells is difficult. This work explores the use of multiple quantum wells (MQWs), thinner GaN barriers and limited area epitaxy (LAE) to reduce efficiency-droop and increase peak external quantum efficiency (EQE). MQW LEDs, with thin GaN barriers to improve transport between wells, were grown by metal organic chemical vapour deposition. LAE was employed to increase efficiency by selectively preventing relaxation of strain developed during growth. Misfit dislocations, which are present when the sample has relaxed, were observed using fluorescence microscopy, confirming that decreasing the barrier thickness decreases the critical thickness at which relaxation occurs. Roughness was observed on the sample containing a 3nm barrier, suggesting that surface morphology limits minimum barrier thickness. Preliminary electroluminescence results have shown that decreasing barrier thickness from 12nm to 4nm decreases the forward voltage from 6V to 4V, and decreases the efficiency-droop of the device by a factor of 5. Once the device has been processed and packaged, peak EQE and forward voltage will be measured. EQE values will be measured as a function of driving current in order to study efficiency-droop. We hope to show that LAE and thin GaN barriers increase peak EQE and decrease efficiency-droop of green semipolar LEDs.