Abstract
In this dissertation, an artificial structure “Quantum Structure Lattice (QSL)” fabricated on InGaN/GaN quantum well (QW) heterostructure has been demonstrated. At first, the numerical analysis method “finite-difference time-domain (FDTD)” is employed to simulate the radiation pattern of bulk single quantum well (SQW) and QSL arrays embedded in SQW. The simulation results illustrate that the QSL arrays with appropriate design has potential to control the surface radiation pattern of the structure. Furthermore, collimated radiation pattern can be observed clearly in QSL array that fulfills lower orders of Bragg diffraction condition. Second, QSL array fabrication process over whole wafer has been achieved using soft nanoimprint lithography (soft-NIL). The soft-NIL process was realized by employing a transparent polydimethylsiloxane (PDMS) mold to transfer nano-patterns into the substrate by deforming the photoresist coated substrate surface and cured by employing an ultra-violate light. The experiment results show that the soft-NIL works well to transfer ultra-uniform nano-pattern onto 2” substrates. The well-developed soft-NIL is used to fabricate QSL arrays from the active region of the GaN/InGaN heterostructure. The photoluminescence (PL) measurements illustrate that the QSL array has the ability to control the radiation pattern of the spontaneous emission, and the experimental results are verified by the FDTD simulations. In PL measurements of GaN-on-sapphire structures, multiple peaks in PL spectrum are commonly observed, which causes ambiguity in interpreting the real PL peak position. This is caused by multiple light reflections in air/(In)GaN/Sapphire thin layer structures. In order to extract the correct emission peak from the measured PL spectrum, a semi-empirical method has been developed to eliminate the interference (Fabry-Perot) effect. Using three selected interference peaks in the measured PL spectrum to deduce the interference function (IF) of the thin layer structure, the corrected spectrum can be derived. The experiment results illustrate that the semi-empirical method is a fast and simple way to correct the distorted PL spectrum caused by multilayer interference. Finally, QSL arrays are integrated into the active region of multiple quantum wells light emitting diode (LED) structures. Three different QSL fabrication methods are proposed, which include GaN regrowth after reactive-ion etching (RIE) etching, GaN regrowth after ion-implantation, and direct ion-implantation techniques. Electroluminescence (EL) spectra illustrate that QSL MQW-LEDs show collimated characteristic radiation patterns similar to those from a single QSL array heterostructure.