Final PhD Seminar

Selective-area Epitaxial Growth of InGaAs/InP Quantum Well Nanowires and their Application to Light Emitting Diodes

Mr Inseok Yang

The fourth industrial revolution features hyper-connectivity, which means the creation and transport of a huge amount of data that have never been dealt with previously. For a smooth transport of massive data, optical interconnect technology for fast broadband communications between chips as well as boards are required. Especially, Si-based integrated photonics has been actively investigated due to advantages such as advanced processing technology, cost effectiveness and, most importantly, the possibility for monolithic integrations with Si-based electronic chips. To realise low power consumption and highly compact Si-based photonic integrated circuits, III-V semiconductor nanowires are considered as a promising candidate for the integrated light emitting devices owing to their promise for high-quality growth on Si as well as superior optical/electrical properties. In particular, III-V nanowires grown by selective area epitaxy (SAE) technique exhibits distinct advantages such as controllability of their size and position, high uniformity, and free of metal contamination issue, and thus, facilitates the integration with other Si-based electronic devices. In this work, the SAE growth of InGaAs/InP quantum well nanowires using metal organic chemical vapor deposition (MOCVD) and their applications to both single and array nanowire light emitting diodes (LEDs) have been explored.

Firstly, the overall growth conditions for uniform InGaAs/InP multiple quantum well nanowires by SAE using MOCVD technique are studied. The optimum growth condition and dimension of quantum well nanowires are identified and doping effect is also studied for the subsequent device application.

To understand the quantum well nanowire growth mechanism and its distinctive faceting morphology, the quantum well nanowires are studied via spherical-aberration corrected scanning transmission electron microscopy (AC-STEM) analyses and it is revealed that the zinc blende disc induced by the axial quantum well in the zinc blende structure drives the overall radial growth evolution. The multiple quantum well nanowires exhibit good uniformity and desirable quantum well spatial distribution for p-n junction-based LEDs. Furthermore, bright photoluminescence at two different wavelengths owing to the axial and radial quantum wells in the nanowire is observed, which could be employed for the realization of the future devices such as multiple-wavelength light sources or integrated self-switchable wavelength selectors.

Furthermore, the multi-wavelength single nanowire LEDs based on the single InGaAs/InP quantum well nanowires are also demonstrated. It is found that the quantum well consists of three components with different thickness and chemical composition, leading to multi-wavelength luminescence spectra. The electroluminescence (EL) spectra exhibit significant dependence on current injection levels where the EL from the radial quantum well increases with increasing current injection. By examining the equivalent circuits of the single nanowire LED, the dependence on the current injection levels is found to be closely related to the distinctive morphology of the quantum well nanowires. Through FDTD simulation, it is also revealed that the optical mode at ~1.27 µm is confined better in the triangular facet rather than in the hexagonal facet of the quantum well nanowire LED.

Finally, more uniform nanowire array, which is essential for the fabrication of nanowire array LEDs with a large area, is achieved via 2-step growth technique. The 2-step growth also inhibits the inclined lateral facets which induces a loss of EL. The array quantum well nanowire LEDs based on the improved nanowire arrays are successfully demonstrated. In addition, the effect of the side contact coverage on the EL characteristics is also observed. From FDTD simulation, it is found that the light extraction to the top is enhanced by reducing the polymer thickness which can be readily achieved by adjusting the oxygen plasma strip process. Moreover, the light leakage through the substrate can also be decreased by increasing the mask oxide thickness.

Our detailed morphological structural and optical study of the InGaAs/InP quantum well nanowire structures provides important insights and guidance for further improvement of material quality and design/demonstration of quantum well nanowire-based nanoscale devices for a wide range of optoelectronic applications. Moreover, the multi-wavelength single/array nanowire InGaAs/InP quantum well LEDs demonstrated in this work also show great potential for achieving integrated wavelength-selective nanoscale LEDs.

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