Dario Schiavon

Optically pumped GaInN/GaN multiple quantum wells for the realization of efficient green light-emitting devices

We report on a green light-emitting device, in which the light of an efficient blue 1 mm2 GaInN/GaN light-emitting diode (LED) is converted into green light by an optically pumped GaInN/GaN multiple quantum well structure. This solution reached an efficacy of 127 lm/W, i.e., higher than that of state-of-the-art 1 mm2 GaInN/GaN LEDs emitting directly at the target wavelength, at 350 mA current and 535 nm peak wavelength. Optically pumped converters overcome the design limitations of typical multiple quantum well LEDs, where carrier transport issues limit the maximum number of functioning wells and might help to solve the problem of the green gap.

Elimination of trench defects and V-pits from InGaN/GaN structures

The microstructural evolution of InGaN/GaN multiple quantum wells grown by metalorganic chemical vapor phase epitaxy was studied as a function of the growth temperature of the GaN quantum barriers (QBs). We observed the formation of basal stacking faults (BSFs) in GaN QBs grown at low temperature. The presence of BSFs terminated by stacking mismatch boundaries (SMBs) leads to the opening of the structure at the surface into a V-shaped trench loop. This trench may form above an SMB, thereby terminating the BSF, or above a junction between the SMB and a subsequent BSF. Fewer BSFs and thus fewer trench defects were observed in GaN QBs grown at temperatures higher than 830 °C. Further increase in the growth temperature of the GaN QBs led to the suppression of the threading dislocation opening into V-pits.

Suppression of extended defects propagation in a laser diodes structure grown on (20-21) GaN

In this work, we fabricated green-light-emitting laser structures on a (20-21) semipolar GaN substrate. Using cathodoluminescence mapping, x-ray diffraction, and transmission electron microscopy, we revealed the formation of relaxation defects within InGaN waveguides and AlGaN claddings. The observed defects in the AlGaN layers are stripe-like and extend along the a axis, but in the InGaN layers they form a characteristic checkered pattern. We demonstrate that using the selective area growth method we can effectively suppress the formation of both types of defects, thus enabling the fabrication of defect-free green laser structures on semipolar GaN substrates.

Direct evidence of photoluminescence broadening enhancement by local electric field fluctuations in polar InGaN/GaN quantum wells

In this work the effect of external electric field on the broadening of optical transitions in a triple polar InGaN/GaN quantum well is studied. Experimental investigation using photoluminescence and electroreflectance show that the reduction of the internal electric field by an external voltage reduces the broadening of the transitions. This is direct evidence that the broadening of photoluminescence in InGaN/GaN quantum wells is enhanced by the built-in electric field. This conclusion is supported by theoretical modelling within the random quantum well model. Additionally, we show that the exciton–phonon coupling can be controlled by an external electric field.