III-V Nanowires and Nanoneedles on Lattice Mismatched Substrates for Optoelectronic Device Applications
We believe that high-speed, low power consumption diode lasers and photodetectors directly integrated onto Si CMOS devices are key elements to high speed optical interconnects. Despite many years of research, integration of direct bandgap III-V compounds onto Si CMOS remains challenging. The bottleneck has been the process incompatibility of the two types of material systems. It is now widely accepted that optoelectronic devices should be fabricated on finished CMOS ICs to avoid these issues. One critical parameter is temperature - the temperature at which high-quality III-V materials can be grown needs to be low enough to sustain CMOS ICs. Another major challenge for the integration is the large lattice mismatch between III-Vs and Si. The large lattice mismatch results in a high misfit dislocation density for III-V thin films grown onto Si which largely degrades the crystal quality.
In this dissertation, I will present III-V nanowires and nanoneedles which we successfully grew dislocation-free on Si and other kinds of lattice mismatched substrates with CMOS-compatible growth temperatures. The strain energy due to the lattice mismatch is relieved via elastic relaxation for these one-dimensional materials.
For the Au-catalytic vapor-liquid-solid III-V nanowires on Si, the nanowires were grown at 430-470°C in a metal-organic chemical vapor deposition system. We observed that there existed a critical diameter for epitaxial nanowires grown on lattice-mismatched substrates, up to as large as 11.6% mismatch for InAs nanowires on Si. Below the critical diameter, well aligned nanowires with bright photoluminescence can grow, while above the critical diameter, spiky structures form. We report well aligned InP nanowires on Si with a very narrow photoluminescence linewidth of 1.4 meV, indicating excellent crystal quality. Regarding the growth parameter study, the precursor V/III ratio could be used to tailor the InP nanowire shape and the optical properties.
For the catalyst-free GaAs-based nanoneedles, including InGaAs and AlGaAs materials, needles were successfully grown on GaAs, Si and sapphire substrates at 400°C. A typical nanoneedle has a hexagonal cross section with a 6-9° taper angle, which results in a high aspect ratio. The nanoneedle tip has only a few atoms in diameter but the base can be sub-micron wide which allows the typical microfabrication processes, such as optical lithography, to be applied. Core-shell GaAs/AlGaAs and InGaAs/GaAs quantum-well nanoneedles are demonstrated with very bright photoluminescence indicating superior material quality. Nanoneedles doped n-type and p-type are also demonstrated with Te and Zn dopants.
Regarding device level work, a GaAs-nanoneedle based photodetector on Si and an InGaAs/GaAs quantum-well nanoneedle-based light emitting diode on Si will also be presented in the dissertation. These III-V optoelectronic devices monolithically integrated on Si demonstrate that high performance optical interconnects for Si CMOS devices could be realized with these novel one dimensional materials.