SAMPLE PROJECTS

I. AlGaInN blue and ultraviolet light emitting diodes (LEDs) and laser diodes (LDs)

The intense attention in III-nitride research towards emitters with wavelengths shorter than, say 360nm, is fuelled by a plethora of potential applications. These include chemical/biochemical analysis and solid-state lighting initiatives (with wavelength-conversion phosphor coatings for white light). It is possible that future high density optical storage may exploit such UV sources, although this requires re-tooling the cirrent recording media. Most biological molecules contain chemical bonds which have strong optical resonances within the UV range of approximately (270~350 nm).  A compact and efficient UV light source is expected to provide a new application link between optoelectronics and biological sensings, enabling biophotonic applications such as fluorescence-based bioagent identification, UV-based curing and sterilizing. Lastly, an increased Rayleigh scattering (~l-4) by the atmosphere in the UV makes these emitters appealing for possible short-range, non-line-of-sight (NLOS) covert communication in the battlefield.

II. III-Nitride vertical cavity surface emitting lasers and LEDs

Vertical cavity lasers from the infrared to the red have frog-leaped from a laboratory curiosity to a position of technological importance, if not dominance, in many areas of contemporary semiconductor optoelectronics. There are ample reasons to pursue the extension of VCSELs, resonant cavity LEDs (RCLEDs), and related planar devices to the short wavelength edge of the visible visible spectrum and on to the ultraviolet, with significant application potential for optical storage, projection-based displays, chip-scale biochemical/biological analysis, lithography and printing, and semiconductor-based solid state lighting. The primary technical challenge is two-fold. First, the implementation of a high quality (Q-) factor optical resonator places demands on epitaxial growth and component processing. Second, dictated mainly by the low conductivity of the distributed Bragg reflectors (DBR), and of p-GaN and its alloys in general, a practical diode device must incorporate innovative current injection schemes and architectures.

III. GaN and AlGaN Quantum Dots through Droplet Epitaxy

Compound semiconductor quantum dots (QDs) are of interest as a potential active medium component in optoelectronic devices, as well as an object of study of physical phenomena related to zero-dimensional carrier confinement. Formation of GaN islands on AlN templates due to the mismatch-induced compressive strain has been reported.  However, the use of AlN as a template presents a major hindrance to the incorporation of GaN dots into electrically injected structures due to its limited conductivity.  Alternative approaches to GaN island formation include the use of silicon as anti-surfactant on AlGaN templates and nitridation of Ga droplets on SiC by nitrogen-seeded gas source in vacuum. Contrary to the modern epitaxial approach in making solid compound semiconductors directly from vapor species (as in MBE and MOCVD), we propose a deliberate synthesis of an intermediate, liquid phase (in our case gallium and/or aluminum metals) as stepping stones toward the formation of crystalline GaN and/or AlGaN QDs.

This new synthesis technique is made possible by observing two thermodynamic principles in surface energy and phase transition: 1) the tendency of liquid metal to "ball up" at the inception of a new phase for surface minimization is a self-assembled pathway to nanoscale droplets, and 2) the metallic liquid droplets can be converted to quantum dots by reacting with nitrogen atoms, a process mimicking liquid-phase epitaxy (LPE) which is much closer to equilibrium and is known for the preparation of stoichiometric GaAs with the lowest amount of point defects. 

IV. GaN and AlGaN Nanowires and Nanostructures

In the past five years there has been an increasing effort, pioneered largely by the community of inorganic synthesis, in preparing semiconductor-based nanowires using catalyst-mediated growth process.  While the proof-of-concept work of synthesizing nanostructures by vapor-liquid-solid (VLS) growth mechanism had been unambiguously established in semiconductors such as Si, Ge, GaAs, GaP, and GaN, the majority of the published work employed inorganic synthesis techniques such as flow-tube-based vapor transport and laser ablation that offer only limited flexibility and control. The synthesis of nanostructures by modern epitaxial process such as MOCVD or MBE would have clear advantages including a better controlled process environment (in terms of temperature, reactant flows, source purity) that will enable microscopic understanding of synthesis mechanisms and the possibility of modulating the chemical composition along the wire direction on the atomic scale, thus enabling the realization of novel quantum structures and metastable configurations. Additionally, due to the daunting challenges in directed manipulation and assembly of individual nanowires into complicated circuits there is a pressing need to enhance the single-wire functionality in-situ through heteroepitaxy. The employment of atomistic epitaxial tools such as MOCVD will inject exciting degrees of freedom into nanowire device synthesis.