Mr. Richard Liu Profile

Richard Liu

at Univ of Illinois

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Proceedings Article | 1 March 2019 Paper

Suppression of indium clustering and quantum confined stark effect of InGaN LED on silicon (111)

Richard Liu, Callan McCormick, Can Bayram

Proceedings Volume 10918, 1091822 (2019) https://doi.org/10.1117/12.2506426

KEYWORDS: Silicon, Gallium nitride, Indium gallium nitride, Light emitting diodes, Raman spectroscopy, Sapphire, Quantum wells, Indium gallium aluminum nitride

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Optical properties of InGaN/GaN multi-quantum-well (MQWs) grown on sapphire and on Si(111) are reported. The tensile strain in the MQW on Si is shown to be beneficial for indium incorporation and Quantum-confined Stark Effect reduction in the multi-quantum wells. Raman spectroscopy reveals compressive strains of -0.107% in MQW on sapphire and tensile strain of +0.088% in MQW on Si. Temperature-dependent photoluminescence shows in MQW on sapphire a strong (30 meV peak-to-peak) S-shaped wavelength shift with decreasing temperature (6 K to 300K), whereas MQW on Si luminescence wavelength is stable and red-shifts monotonically. Micro-photoluminescence mapping over 200 by 200 μm² shows the emission wavelength spatial uniformity of MQW on Si is 2.6 times higher than MQW on sapphire, possibly due to a more uniform indium incorporation in the multi-quantum-wells as a result of the tensile strain in MQW on Si. A positive correlation between emission energy and intensity is observed in MQW on sapphire but not in those on Si. Despite the lower crystal quality of MQW on Si revealed by atomic force microscopy, it exhibits a higher internal quantum efficiency (IQE) than MQW on sapphire from 6 K to 250 K, and equalizes at 300 K. Overall, MQW on Si exhibits a high IQE, higher wavelength spatial uniformity and temperature stability, while providing a much more scalable platform than MQW on sapphire for next generation integrated photonics.

Proceedings Article | 27 January 2017 Presentation + Paper

Polarization-free integrated gallium-nitride photonics

C. Bayram, R. Liu

Proceedings Volume 10111, 101110Y (2017) https://doi.org/10.1117/12.2251607

KEYWORDS: Gallium nitride, Silicon, Light emitting diodes, Photonics, Diffraction, Crystals, Indium, Green light emitting diodes, Light sources and illumination, Indium gallium nitride

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Gallium Nitride (GaN) materials are the backbone of emerging solid state lighting. To date, GaN research has been primarily focused on hexagonal phase devices due to the natural crystallization. This approach limits the output power and efficiency of LEDs, particularly in the green spectrum. However, GaN can also be engineered to be in cubic phase. Cubic GaN has a lower bandgap (~200 meV) than hexagonal GaN that enables green LEDs much easily. Besides, cubic GaN has more isotropic properties (smaller effective masses, higher carrier mobility, higher doping efficiency, and higher optical gain than hexagonal GaN), and cleavage planes. Due to phase instability, however, cubic phase materials and devices have remained mostly unexplored. Here we review a new method of cubic phase GaN generation: Hexagonal-to-cubic phase transition, based on novel nano-patterning. We report a new crystallographic modelling of this hexagonal-to-cubic phase transition and systematically study the effects of nano-patterning on the GaN phase transition via transmission electron microscopy and electron backscatter diffraction experiments. In summary, silicon-integrated cubic phase GaN light emitters offer a unique opportunity for exploration in next generation photonics.

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