GaN-based microdisk lasers on Si

Abstract

III-nitride compound semiconductors have emerged as indispensable materials for a wide range of optoelectronic devices. With wide and direct energy bandgaps, emissions from III-nitride semiconductors can cover the wavelength range from deep-ultraviolet to the near-infrared, making them ideal candidates for high-efficient light sources. Meanwhile, optical microcavities as significant building blocks of various optoelectronic devices have emerged as a distinct subject as well. High-quality microcavities are essential components for highly efficient micro-lasers via excellent optical confinement. Among them, microdisk lasers have drawn much attention owing to their high Q-factors and small mode-volumes as well as simple architectures. However, low-temperature operation, optically-pumping, and high thresholds are the existing obstacles ahead for the “state-of-art” nitride-based microdisk lasers stepping into real-life applications.

Therefore, in this thesis, riding on the advantages of microsphere lithography (MSL) techniques, 2-μm GaN-based undercut microdisk cavities on Si with smooth sidewalls and excellent circularity have been demonstrated, which enables low-threshold lasing actions via whispering-gallery modes (WGMs) achieved at room temperature by reducing the energy loss at the disk edge, with Q-factors exceeding 1500 and a lasing threshold (Eth) as low as 8.43mJ/〖cm〗^2. These undercut GaN-on-Si microdisks are also providing a platform of research on the strain and optical coupling effects to the emission properties from these disk cavities. Strain-relaxation effects in 7-μm undercut GaN/InGaN microdisk cavities on Si have been further investigated in detail. The mitigation of biaxial tensile stress is found to be dependent on the contact areas between the Si posts and GaN microdisks. Quantum-confined Stark effect (QCSE) in the quantum wells (QWs) is reduced, leading to an enhancement of ~18.3% in its internal quantum efficiency (IQE). Light extraction is significantly enhanced in the suspended regions owing to reduced optical absorption at AlN/Si interface. Meanwhile, spectral blue-shifts of ~45.6 meV are observed at the microdisk edge. Such localization of strain relaxation can be exploited for precise strain-engineering in the microdisks. Further moving forwards to achieve electrically-pumped microdisk lasing on Si requires better control over the thread-dislocation densities and optimal thickness of the microdisk cavities for obtaining efficient optical gains and optical coupling with WGMs. Optically-pumped lasing properties of microdisk cavities with different thicknesses reveal a significant increase in spontaneous emission factors and an obvious reduction in Eth by reducing the disk thickness, which are meaningful to the cavity-design and epi-growth for demonstrating electrically-pumped microdisk lasing in the next step. GaN-based micro-dome cavities on Si have been demonstrated using a modified MSL technique, in which Fabry-Pérot resonances are totally ruled out. The WGMs blue-shift monotonously as excitations approaching Eth. Concurrently, modehopping effect is observed as the gain red-shifts under higher excitations. As the excitation exceeds ~15.1 mJ/〖cm〗^2, amplified spontaneous emission followed by optical lasing is attained at room temperature, evident from a super-linear increase in emission intensity together with linewidth reduction to ~0.7 nm for the dominant WGM. Optical behaviors within these WGM microcavities are also investigated using numerical computations and verified by 2D/3D finite-difference time-domain (FDTD) simulations.