Selective area growth of III-nitride based micro-structures on three dimensional patterned sapphire substrates and related device fabrication

Abstract

GaN based light-emitting diodes (LEDs) can cover the entire range of visible light spectrum, thereby showing great potential in various applications. However, III-V nitrides planar films grown on sapphire substrates still suffer from large dislocation density due to the large lattice mismatch, large spontaneous polarization and piezoelectric fields induced by the residual strain, and low light extraction efficiency resulting from the high refractive index. In order to overcome these issues, alternative approaches have been employed by achieving semi-polar or non-polar GaN facets. The selective area growth (SAG) is a promising epitaxy technique for creating three dimensional (3D) GaN structures with semi-polar facets. However, the SAG technique typically requires complicated processes to obtain well designed substrates. In this thesis, a simple, flexible and low-cost method for micro patterned substrate fabrication by means of laser drilling is presented. Single pyramid arrays, asymmetric pyramid arrays and trapezoidal stripes were grown on different patterned substrates by SAG, for applications of flexible LEDs, phosphor free broadband emission and color tunable devices, respectively. Totally separated single pyramids were achieved by carefully adjusting the drilling parameters. The material quality of a single pyramid can be improved by increasing the depth of drilling holes, since the threading dislocations (TDs) can be effectively annihilated by crystal nucleus coalescence in a deep hole. The internal quantum efficiency was estimated to be improved by a factor of 3 for the pyramid structure compared with planar LEDs. The light extraction efficiency of the pyramid arrays is proven to be higher than planar LEDs through simulation and calculation. In addition, the pyramid arrays have a larger viewing angle with more uniform light emission. The mechanical simulation results also indicate that the pyramid structure can bear an extremely large bending without cracking, which is suitable for fabrication of superior flexible devices. Flexible LEDs were obtained based on single pyramid arrays. The pitch size of the pyramid array was 30 μm, and the emitting area was 5x5 mm² for green LEDs and 7x5 mm² for blue LEDs. The emitting area can even be larger. Laser lift-off (LLO) and dual transfer processes were applied to transfer pyramid arrays face up onto the flexible substrates. Dual transfer processes ensure the front surface of the pyramid is the light emitting surface, which is more efficient than back side emission. No significant reduction appeared until it reached a curvature radius of 0.5 mm. The results prove that the separated pyramid arrays could prevent the occurrence of fracture during the LLO and the bending processes. This method is applicable for LEDs of any shape, size and color with no limitation in the choice of substrates. Substrate replacement and size shaping can be performed even after the accomplishment of the device fabrication. The asymmetric pyramids were grown on a sapphire substrate with patterns of two closely designed concave holes. Indium segregation and QWs thickness variation occurs at different locations due to the different growth rates during the coalescence process, resulting in a broadband emission. 500 μm long stripes were also fabricated on patterned sapphire substrates to enhance the polarization effect by significantly increasing the structure size in one direction. An obvious polarized light emission appears for stripe LEDs compared with single pyramid or asymmetric pyramid LEDs. However, there is only one peak for stripe structures. To achieve a color tunable device, multiple wavelengths should be contained, which can be achieved by designing various sized trapezoidal stripes with (1-101) and (0001) facets. The methods to grow 3D micro structures on patterned substrates and fabricate flexible device based on the micro structures described in this thesis have broad application prospects in the field of optoelectronic devices.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology