(379d) Nanopatterning, Growth, and Defect Control in Semiconductor Structures

There has been a tremendous body of research into the development of nanoscale objects and materials. While these materials exhibit unique properties on their own, the technological development of these materials requires their integration into existing and evolving device and materials platforms. A self-assembled block co-polymer (BCP) approach to nanoscale patterning, which offers rapid and cost-effective full wafer patterning at the 20-nm length scale, is finding applications in the wafer-scale development of nanoscale structures. This talk will deal with several new applications of this approach used to achieve improvements in heteroepitaxial growth of large lattice mismatched materials and the formation of uniform nanostructured device structures, such as Quantum Dots for laser applications. The BCP and its controlled conformations in thin film form allow for the development of a wafer-scale nanoscale ordered pattern. This pattern can be transferred to a substrate through a variety of means and used to control the growth of semiconductor materials. The use of these patterns in the growth of lattice mismatched films can lead to changes in the development of the defect structure associated semiconductor growth. We have used the self-assembling block co-polymer (PS-b-PMMA) and reactive ion etching to generate nanoscale holes arranged in a quasi-hexagonal pattern in a SiO2 mask layer covering an entire wafer. The patterned wafer is then used as a substrate for epitaxial growth. Growth is initiated in the nanoscale holes and proceeds laterally over the SiO2 layer. Because of small separations (~20 nm) between growth windows, film coalescence occurs rapidly, allowing nearly full relaxation of lattice mismatch strain. This early, island-based relaxation leads to significant reduction in threading dislocation density, as observed by x-ray diffraction and TEM. The reduction in the defect density occurs within the early stages of growth, specifically within the individual island structures and this improvement is realized soon after the coalescence of the film, suggesting that a buffer layer much thinner than 250 nm would be as effective in defect reduction. This improvement in materials properties through nano-patterned growth should be applicable to a wide range of materials and lattice mismatch situations.

The theoretical advantages of ideal Quantum Dots (QDs) as the active region for diodes lasers are well established, including ultra-low transparency current density, low linewidth enhancement factors (i.e. low chirp), and temperature insensitive device performance. Self-assembled QDs have been widely studied as a means to achieve the theoretical advantages of QDs. Although with self-assembled QDs there are many issues, such QD size variations and the existence of a continuous semiconductor layer between the QDs often referred to as a wetting layer, which have inhibited reaching ?ideal QD? device performance. The wetting layers are inevitably produced in the fabrication process of self-assembled QDs. Furthermore, self assembled QDs generally require growth temperatures significantly lower than typically used for obtaining high optical quality material, leading to degradation of device performance. An alternate approach for QD formation is the use of nanopatterning combined with selective MOCVD growth. Using the technique of diblock copolymer lithography followed by selective MOCVD growth of the QDs, a higher degree of control of QD shape, size uniformity, and composition is expected, compared with self-assembled growth. Higher growth temperatures compared with self-assembled growth are possible, resulting in improved QD material quality. In addition, the problematic wetting layer states can be eliminated, and improved optical gain is expected. Control of the QD height, shape, and strain, also allows for the design of increased energy spacing between ground and excited QD states and control of the emission wavelength. This talk will review these specific applications and discuss future applications in the area of heterogeneous materials integration.