Date of Graduation

12-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Gregory J. Salamo

Committee Member

Shui-Qing (Fisher) Yu

Second Committee Member

Huaxiang Fu

Third Committee Member

Hugh Churchill

Keywords

MBE, Integrated Microwave Photonics, Thin film growth, XRD, Twinning

Abstract

The microwave signal processing in the optical domain creates new opportunities for information and communication technology (ICT) and networks by increasing speed, bandwidth, and processing capability. IMWP incorporates the functions of microwave photonics components/subsystems in monolithic or hybrid photonic circuits to meet future needs. Sapphire platforms have the potential to integrate all-in-one, for instance, light source, analog signal processing, light detection, CMOS control circuit, silicon on sapphire to achieve high-performance, low-cost mixed-signal optical links etc. Molecular beam epitaxy (MBE) has been used to grow GaAs on sapphire substrates to integrate optoelectronic devices in the same platform.

The initial stage of GaAs thin film growth has been investigated extensively in both c and r plane sapphire substrates. Direct growth of GaAs on both sapphires results in three-dimensional (3D) islands. In c-plane sapphire, 50% twin volume of GaAs islands, and in r-plane sapphire, two primary domains of GaAs islands are observed. A strong interaction between the growth of GaAs on r-plane sapphire is observed than the growth of GaAs on c-plane sapphire. A thin AlAs nucleation layer improves the wetting of both substrates. After inserting the AlAs nucleation layer, in c- plane sapphire, twin volume is reduced to 16%, and in r-plane sapphire twin is introduced. Further, we investigated the effect of growth temperature, pre-growth sapphire substrate surface treatment, and in-situ annealing on the quality of GaAs epilayer on c-plane sapphire substrates. We have been able to reduce the twin volume to below 2% and an XRD rocking curve linewidth to 223 arcsec in a c-plane sapphire substrate.

To grow high quality GaAs (111)A buffers on an atomically flat sapphire substrate, a two-step growth method has been introduced where, at an early stage, a GaAs layer has been grown at low temperature, followed by a second high-temperature GaAs growth layer. In addition to the two-step process, an AlAs nucleation layer and multiple annealing steps have been employed. Relaxed, smooth surface morphology and high-quality GaAs is achieved with the presence of the LT GaAs layer. A two-dimensional InGaAs quantum well (QW) was epitaxially grown on the 70 nm GaAs buffer and compared with a reference to two-dimensional InGaAs QW grown on GaAs (111)A substrate. Along with X-ray and high-resolution cross-section transmission electron microscopy, comparable QW photoluminescence intensity and linewidth confirmed our growth strategies effectiveness to produce high-quality GaAs on sapphire.

The film thickness was improved up to almost 500 nm by changing the growth parameters such as growth temperature and annealing temperature. We have achieved the room temperature (RT) photoluminescence. The 16 μm microdisk laser was fabricated on GaAs/sapphire system and characterize by micro-PL measurements. The lasing was not successful. The higher surface roughness needs to be optimized to get lasing. We will fabricate GaAs/sapphire QW for electrically pumped laser and realize the photonic chip on the sapphire platform in a long-term goal.

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