This project seeks to achieve the optimized growth of InGaN quantum dots within integrated heterostructures that can be structured to form high quality optical cavities for the quantum dots. Ultimately, such integrated structures promise ultra-efficient optical sources in the visible range of the spectrum, controlled single photon sources, and new testbeds for the exploration of quantum information concepts, all operating at room temperature. The III-nitride materials have exceptional optical properties, but considerable challenges remain in the growth of low-defect materials in this system; this is particularly true for quantum dots.
The fundamental optical performance of InGaN quantum dots could be enormously enhanced through their appropriate integration into high quality optical cavities, such as microdisks or photonic crystal structures. In addition, such cavities could serve as highly sensitive probes of the performance of the dots, monitoring the effects of local defects, charge traps or electric fields. There are numerous material challenges associated with the fabrication of such structures, and care must be taken to design the entire material structure in an integrative manner. This program will bring together the critical complementary expertise in the growth of InGaN quantum dots(Cambridge University), in the fabrication of cavities and structures in the GaN material system(Harvard), and in the optical characterization of those structures (Oxford). The program incorporates opportunities for all researchers, including postdocs, graduate students and undergraduates to participate in exchange visits, laboratory experiences and workshops among the partner institutions. The team will also communicate and exchange information through a web presence and video conferencing.
The overall goal of this program was to create nanometer-scale environments for quantum dots formed in the gallium nitride materials, to better understand how to elicit the highest efficiency optical performance of these materials. The program involved laboratories in the U.S. and U.K., sharing complementary knowledge, expertise and personnel. Gallium nitride and its ‘family’ of materials (indium nitride, aluminum nitride) possess an interesting set of outstanding optical qualities that are nevertheless not well understood, rendering these materials rather ‘magical’ in their capabilities. It was only in the early 1990’s that the first light emitting diodes in these materials emerged from research laboratories, and yet today these materials form the basis for large-scale displays played across the sides of large buildings, as well as being the major players in solid state lighting: an energy efficient alternative to incandescent or even fluorescent bulbs. Yet despite the rapid transition of these materials into the marketplace, there is still much we don’t understand about the basic mechanisms of light or photon generation, particularly taken in the context of the very high density of dislocations that still plague these materials. So-called threading dislocation densities are typically around 109 cm-2 – whereas in other semiconductors the defect density is usually six or seven orders of magnitude lower. The high number of dislocations is a result of the way these materials are formed, without any ‘natural’ substrate of the same lattice size that could serve as template for the well-controlled growth of the materials. These issues have similarly impeded researchers from pushing these materials into new, more energy-efficient and visionary directions, such as the well-controlled formation of ‘quantum dots’ in these materials. The quantum dots comprise perhaps several hundred atoms, yet behave in many ways like ideal, single atoms, with well-behaved electronic levels that in turn give rise to well-behaved, distinctive photon energies. By forming, predicting and understanding the properties of these quantum-dot atoms, it is possible for us to sculpt a unique photonic environment, matched to those atoms, an environment that minimizes ‘dissipative’ mechanisms that lead to energy loss. These unique photonic environments are also referred to as ‘optical cavities’. This has been done for a variety of other semiconductors, such as gallium arsenide, materials with a higher quality of crystalline perfection, and much lower defect densities. Our goal was to improve the formation of the quantum dots in an integrated fashion with the formation of the nanoscale environments that surround those quantum dots, to probe the optical performance, evaluate sources of photon energy loss, and to try to correlate such loss with the defects in the material. The optical cavities we chose to work with are called ‘microdisks’: mushroom-like in shape, with diameters ranging from 1 to 3 microns. We developed techniques of forming these structures so that we could readily and rapidly generate many cavities on a single sample, and thus carry out substantial statistical correlations to clearly reveal important trends to us. We learned that there are inevitable trade-offs in structuring a material both to facilitate the fabrication of a desired final device-form, and still demand the highest optical performance from the quantum dots. This is illustrated in the first figure, showing the dramatic drop in the quality factor of our microdisks when we sought to introduce a ‘simple’ change in the growth procedure, allowing us to form the mushroom shapes of our microdisks. We found a solution for this dilemma, and proceeded to the next stage of optimizing materials and structures. Ultimately, through the rapid feedback between analysis of the cavities and change in the materials growth, we were able to progress to see quality factors for our microdisk cavities that were substantially higher than any that had been previously reported. We were able to pump these microdisk structures to achieve lasing, finding record low lasing thresholds for quantum dot lasers (see figure 2). As well, we were able to sample a variety of microdisks, measure lasing thresholds, and then trying to make correlations between lasing threshold and materials quality. We have made much progress through the course of this research program: understanding how to best match material to cavity structure to optimize the entire structure performance, to realize the highest quality factors in this material system, belying an intrinsic limit to photonic performance, and also in achieving low threshold lasing in our structures. All of this points to a pathway for better understanding of this elusive material, as well as improvement of the efficiency of devices formed from this material. At the same time, we believe that we are just at the beginning of very intriguing studies and understanding of the true role of materials defects in photonic performance (less than initially thought!), and how we might further progress to truly bring the optical potential of these nitride materials into reality.
