
NSF Org: |
DMR Division Of Materials Research |
Recipient: |
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Initial Amendment Date: | April 10, 2020 |
Latest Amendment Date: | April 10, 2020 |
Award Number: | 2003266 |
Award Instrument: | Standard Grant |
Program Manager: |
Yaroslav Koshka
ykoshka@nsf.gov (703)292-4986 DMR Division Of Materials Research MPS Directorate for Mathematical and Physical Sciences |
Start Date: | July 1, 2020 |
End Date: | June 30, 2024 (Estimated) |
Total Intended Award Amount: | $481,231.00 |
Total Awarded Amount to Date: | $481,231.00 |
Funds Obligated to Date: |
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History of Investigator: |
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Recipient Sponsored Research Office: |
450 JANE STANFORD WAY STANFORD CA US 94305-2004 (650)723-2300 |
Sponsor Congressional District: |
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Primary Place of Performance: |
476 Lomita Mall Palo Alto CA US 94305-4008 |
Primary Place of
Performance Congressional District: |
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Unique Entity Identifier (UEI): |
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Parent UEI: |
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NSF Program(s): | ELECTRONIC/PHOTONIC MATERIALS |
Primary Program Source: |
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Program Reference Code(s): |
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Program Element Code(s): |
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Award Agency Code: | 4900 |
Fund Agency Code: | 4900 |
Assistance Listing Number(s): | 47.049 |
ABSTRACT
Nontechnical Description
An important area of application for semiconductor materials is in optoelectronic devices; for example, in devices such as lasers or light-emitting-diodes (LED?s) that use electrons to stimulate the emission of light. There is great interest, both scientifically and technologically, in semiconductors that can emit light with wavelengths somewhat longer than visible light, i.e. in the mid-infrared part of the spectrum. Such light sources, when fabricated on silicon chips, could become key components in future ubiquitous chemical sensor networks, in speeding up data transfer between and on silicon chips, and in motion sensors required by autonomous vehicles. This research project focuses on a semiconductor material system, germanium-tin, that holds great promise for mid-infrared light emission on silicon chips. The efficiency of light emission by germanium-tin is limited by the presence of atomic scale defects that grow into the material when it is synthesized. This project characterizes the nature and number of such defects, and investigates methods for annihilating or altering them to minimize their effects on germanium-tin. Undergraduates are involved in these research activities, with special efforts made to recruit highly competitive undergraduate researchers from groups that are under-represented in the US science and engineering workforce. The project includes a partnership with Stanford?s RISE outreach program, to inspire high school students to consider further education and careers in STEM fields.
Technical Description
Exhibiting a direct bandgap at sufficiently large (x ~ 10 atomic %) tin composition, Ge(1-x)Sn(x) alloys hold great promise for mid-infrared (IR) light emitters and absorbers, while also being monolithically compatible with silicon electronic and photonic technologies. Previous research on germanium-tin epitaxial films grown on silicon has demonstrated mid-IR optically-pumped lasing, and there has been a gradual trend of increasing Sn content to access longer wavelength operation. The light emission characteristics of Ge(1-x)Sn(x) are still far from optimal. Low growth temperatures (< 300°C) used to promote high Sn content alloys cause large concentrations of acceptor-type vacancy defects to form. Strong pairing of Sn atoms with these vacancies is predicted theoretically and will result in enhanced non-radiative carrier recombination, reducing the efficiency of light emission and absorption. This project uses strain-engineered core-shell nanowire structures as a platform to study post-growth annealing to dissociate Sn-vacancy pairs and to annihilate vacancies incorporated in the Ge(1-x)Sn(x) shells during their growth. Shells of thickness up to 500 nm are of particular interest, to achieve wire structures capable of efficiently guiding mid-IR light. Synchrotron diffuse x-ray scattering is used to characterize trends in the relative concentration of vacancies bound to Sn atoms, divacancies, clusters and monovacancies in the alloys versus annealing time and temperature. A key goal is to understand the rates and mechanisms governing the approach to vacancy equilibrium in these alloys. Extended x-ray absorption fine structure analysis provides an additional probe of local bonding around Sn atoms and the stability of Sn-vacancy pairs. The project also examines atomic fluorine as a chemical vacancy passivant, building on prior experience with F passivation of Si surface states and vacancies in Ge. Coupling between x-ray and optoelectronic characterization of the core-shell wires can reveal fundamental insights into the connection between point defects and device-relevant properties. Temperature-dependent photoluminescence, photoconductivity and ultra-fast pump-probe measurements are used to probe Ge(1-x)Sn(x) band structure and the effects of different vacancy defect populations on carrier recombination dynamics.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Intellectual Merit:
Semiconductor materials are widely used in devices for light absorption (e.g. photodetectors) and light emission (e.g. lasers). There are important applications for semiconductors that absorb or emit light at mid-infrared wavelengths. However, the set of known mid-IR active materials is limited and these materials are almost all chemically incompatible with silicon, making them difficult to integrate with electronic circuitry. Thin film alloys of germanium (Ge) and tin (Sn) are an exception to the rule, and they have been increasingly studied for mid-IR optoelectronic devices. The major challenge for using Ge-Sn alloys in such devices is the need to produce thin films with tin composition exceeding 10 atomic % to achieve the most efficient light emission and absorption properties, while minimizing the crystalline defects that compromise their performance. In this project, we used Ge-Sn thin films grown by chemical vapor deposition around small (~ 50 nm) diameter nanowires so that they are nearly strain-free and had no crystalline line defects (dislocations) in studies using advanced X-ray and optical methods to probe local disorder (including vacancies – atoms missing from normally occupied atomic sites in the crystal) and its effects on the thin films’ light emission characteristics. Furthermore, by investigating coatings that block Sn atoms from segregating to their surfaces, we were able to heat Ge-Sn alloy layers to higher temperatures than previously demonstrated and use this to enhance their semiconducting properties. This approach should be generally applicable to fabrication of Ge-Sn lower defect densities and superior mid-IR light absorption and emission performance.
Broader Impacts:
Lecture and homework materials on nucleation and mass transport during chemical vapor deposition and nanowire growth have been derived from research activities performed in this project. These have been featured in the PI's core graduate kinetics course, MatSci 212 Rate Processes in Materials. In October 2022, the PI traveled to Washington, D.C. As part of this trip, he visited Howard University to present a seminar that included results arising from this research project. The purpose of the visit was to meet undergraduate students at Howard, an HBCU, and describe graduate materials research opportunities at Stanford and research internships at the SLAC National Accelerator Laboratory, and also to develop connections with Howard University faculty members engaged in materials research, for possible future collaborations. The project supported research by two undergraduates, one M.S. student, two Ph.D. students and two postdoctoral researchers. Three of these seven students/trainees have been female scientists/engineers, and all but one of the seven have graduated or completed their training.
Last Modified: 10/29/2024
Modified by: Paul C Mcintyre
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