Technical: This project focuses on the development of a fundamental understanding of strain reduction mechanisms in overgrown gallium nitride (GaN) layers on silicon. The central idea is to use ion implantation through an AlN buffer layer to create a highly defective layer in silicon substrate, in the vicinity of the interface between the AlN buffer and silicon substrate. Reduction in strain due to the formation of the defected layer is expected to greatly reduce GaN film fracturing as well as the formation of threading dislocation defects. Preliminary results suggest that a large reduction in strain in GaN is possible using this approach. However, the fundamental mechanism responsible for such improvements is not yet understood. In this project, various ex-situ and in-situ materials characterization techniques will be used for the formulation of an understanding of the relationship between ion implantation conditions and the morphology and thickness of the epitaxially grown AlN buffer layer as well as the type and density of dislocations and film fracturing observed in the overgrown GaN layer.
This project addresses basic research issues in a topical area of materials science with high technological relevance. The success of the project would help to the development of high performance optoelectronic devices on large area dislocation reduced III-nitrides on Si(111) and Si(100) substrates. The project cultivate a sense of leadership in graduate students while offering undergraduate students an opportunity to learn scientific research and problem-solving skills via mentoring from graduate students and faculty in a collaborative environment. The project involves faculty and students at the College of Nanoscale Science and Engineering of the University at Albany - State University of New York and at the Department of Materials Science and Engineering at Pennsylvania State University. In addition, a public outreach program, Science and Technology exhibit in the Mall, is expected to expose over 1000 families and their children to science and technology each year in a fun and informal environment.
The intellectual merit of the our NSF funded research has been to establish a fundamental understanding of the process of stress modification in overgrown III-Nitride film grown epitaxially on engineered AlN/Si substrate. To date, the development of III-nitride devices has been performed by growing the layers on lattice-mismatched substrates, the main drawback of which is the high density of defects in the material. These can act as non-radiative recombination and/or scattering centers, limiting their use in devices, particularly those that require high carrier injection and high breakdown voltages. This is even more serious for such devices grown on a technologically important Si substrate due to the high density of epi-layer cracking and defect formation, caused by the high CTE mismatch with GaN (56%) and lattice mismatch (17%). To date we have reported a reduction in strain in GaN of over 80% as observed by cracking reduction and dislocation density reduction, using ion implantation assisted stress modified AlN/Si substrate. Recent results indicate that the fundamental mechanism at work may in large part be related to AlN grain re-orientation facilitated by the implantation and annealing process. Expansion of our work into theory paved the way for two publications on polar and non-polar AlN epitaxy via Molecular Dynamics and development of an appropriate pseud-potential for heteroepitaxy. In a nutshell, the significant achievements of our work funded by NSF under this grant are: • Experimental evidence pointing to AlN island rotation for AlN buffer grown on Si substrate, upon ion implantation and post-implantation annealing. • The role of ion implantation angle, dose, energy and AlN buffer thickness and morphology in stress modification in overgrown GaN is elucidated. Results of high resolution TEM, Raman spectroscopy, and infrared spectroscopic elipsometry (IRSE) show that: o the presence of a highly defective Si under the AlN/Si interface is crucial, o the defective Si layer is amorphous upon implantation and changes to a combination of polycrystalline and amorphous upon post-implantation annealing, o AlN buffer morphology is crucial, o the final stress in the overgrown GaN does not show strong dependency on annealing time (of longer than 10 min) or the annealing ambient (nitrogen) for the implanted AlN/Si; • Establishment of a non-destructive characterization model to successfully predict the thickness of various layers (i.e. defected layer in Si, AlN thickness, etc) for unimplanted and implanted samples using spectroscopic elipsometry (SE). Results obtained from SE models were confirmed by direct observation using TEM. • First report of homoepitaxy of polar and non-polar AlN and heteroepitaxy of AlN on Si, using MD simulation. Development of a model for large grain re-orientation via the process of implantation and annealing at moderate temperatures and the impact on stress modification in overgrown layers, in a highly mismatched system, could have a transformative impact on further compound semiconductor (heterojunction) device development and improvement. The broader impact of the proposed work has several facets. The overall goal of this proposed research utilized active participation from undergraduate and graduate students in performing research on the novel concept proposed. Three graduate students, two undergraduates and one high school student were involved in this project, in addition to the two female faculty PIs. Cultivation of a sense of leadership in graduate students while offering undergraduate and highschool students the opportunity to learn scientific research and problem-solving skills via mentoring from graduate students and faculty has been at the heart of this program. Our graduate students successfully presented their work at multiple national and international conferences, contributed to reporting significant scientific results in multiple published work in peer-reviewed journals, participated in outreach programs, and in one case successfully defended a PhD (and since joined the ranks of our society's highly trained workforce). This funded project paved the way for a female undergraduate student from CNSE-SUNY to be amongst the 300 recipients of the Berry Goldwater in 2013, and our high school intern to be amongst the INTEL semi-finalist in 2012. In temrs of the economic impact, development of a "semi-compliant substrate" for III-Nitride growth on Si is highly important, due to the technological relevency of wide bandgap III-nitrides for applications from UV emitters and detectors to power electronics and sensing; as well as the tremenous cost saving if these devices can be successfully and reliably realized on and integrated with Si microelectronics. Our research and its findings have helped us to be even closer to such outcome. Moreover a public outreach program developed by the PI, "Nano/Science in the Mall", will continue to expose public to important topics in science and technology each year in a fun and informal environment. This will further enhance the transformative aspect of our research.
