SINOPLE
Surface engineered InGaN heterostructures on N-polar and nonpolar substrates for green light emitters
Project description
While InGaN based light emitters in the violet blue range are currently commercially available and exhibit high external and internal quantum efficiencies despite high threading dislocation densities. However, it has been found that the internal quantum efficiency (IQE) decreases severely with increasing In-content (and concomitant increasing wavelength). For LEDs emitting in the green spectral range, values ranging between 25% and 10% are obtained for emission wavelength between 525 and 540 nm, while blue and violet emitters reach values higher than 50%. Stimulated emission has been observed for InGaN structures down to 485 nm on c-plane and m-plane substrates. Despite intense research efforts there are no reports of InGaN based lasers emitting in the green spectral range. The origin of the reduced IQE of In-rich InGaN quantum wells is currently not well understood. Most researchers agree, that there are a number of challenges that must be overcome. These can be split into materials-related challenges and device-related challenges. As regards materials related challenges, according to common belief the following are most crucial:
- Deterioration of the material due to the low growth temperatures required in MOCVD to obtain high In-concentrations.
- The high lattice mismatch between InGaN and GaN that is believed to lead to formation of additional misfit dislocations at the interface between the active region and the GaN buffer acting as nonradiative recombination centres.
- In-segregation, spinodal decomposition and phase separation expected for In contents around 30 %.
- Piezoelectric and pyroelectric fields due to the large lattice mismatch between GaN and InGaN
- As regards device related challenges the most important are:
- Carrier transport through the quantum wells, which are hampered by the high internal barriers caused by the band offsets.
- Carrier injection, which are influenced by huge pyro- and piezoelectric fields
- proper design of device especially with respect to gain saturation
The goal of this project is to develop the potential of molecular beam epitaxy on nearly dislocation free GaN single crystals for semiconductor lasers in the green spectral range (520-550nm). The active structure will consist of In-rich InGaN layers. Our goal is to push the internal quantum efficiencies of green emitting InGaN devices beyond 30% and to obtain stimulated emission beyond 500 nm. This will be done by (i) engineering the active structure of the device to reduce the effect of piezoelectric fields on carrier-recombination (ii) exploring molecular beam epitaxy on non-polar, semipolar and N-polar surfaces to obtain maximum In incorporation and by (iii) improving the structural perfection of the active layers by surface engineering. We will combine theory, i.e. modern ab-initio based thermodynamics and surface kinetic calculations, with state of the art characterisation techniques. The project will take full advantage of the know-how acquired at TopGaN in the growth of UV lasers by MBE and by using unique dislocation free substrates. The progress in substrates made at TopGaN is especially important, since they enable growth to be performed on any defined surface orientation required.
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