Electrical Engineering Grad Student Racks Up Awards
San Diego, CA, May 14, 2007 -- For his work on how semiconducting nanowires grow and behave, Shadi A. Dayeh, a graduate student in the Department of Electrical and Computer Engineering (ECE) at the UCSD Jacobs School of Engineering, has recently earned a series of awards.
FE-SEM of InAs NWs grown by the VLS technique on InAs(111)B substrate. Scale bar is 2μm.
In March 2007, Dayeh learned that he won one of three best paper awards at the 2006 Electronic Materials Conference (EMC). Then, at the Materials Research Society (MRS) Spring meeting this April, Dayeh took home two more awards: the graduate student “silver medal” award for his body of research on the synthesis and fabrication of compound semiconducting nanowires and devices for novel electronics; and a best poster award for research on field-, diameter-, and surface state- dependent transport behavior in semiconducting nanowires. In January 2007, Dayeh also received a Young Scientist Award at the 34th Conference on the Physics & Chemistry of Semiconductor Interfaces (PCSI-34).
In his award winning paper at EMC 2006, Dayeh helped to resolve a debate regarding which mechanism governs the growth of an important class of nanowires – the III-V compound semiconducting nanowires.
Dayeh and a team of electrical engineers from the Jacobs School demonstrated that the vapor-liquid-solid (VLS) growth mechanism that was proposed in 1964 does, in fact, extend to III-V compound semiconducting nanowires – a group that includes indium arsenide (InAs) nanowires. This is noteworthy because, recently, researchers have claimed that III-V compound semiconducting nanowires grow via the vapor-solid-solid (VSS) growth mechanism.
Schematic illustration of the Vapor-Liquid-Solid (VLS) nanowire growth mechanism. Click here for a description of the VLS growth mechanism from the journal Science (subscription required).
“Shadi’s work not only improves our understanding of three-five nanowire growth, but it also opens the door to a wider tuning range of temperature and precursor flow rates to control nanowire growth rates, morphology, and material properties,” said Deli Wang.
The recent reports that the vapor-liquid-solid growth mechanism did not extend to III-V compound semiconducting nanowires left Dayeh unconvinced. After replicating the findings, he and the rest of the Jacobs School team tapped their understanding of the fundamental science of how nanowires grow and began experimenting.
Shadi A. Dayeh, a graduate student in the Department of Electrical and Computer Engineering (ECE) at the UCSD Jacobs School of Engineering
“As the temperature increases, we know that the molar fraction of the group five element increases with respect to that of the group three element. This enhances thin-film growth and minimizes nanowire growth,” said Dayeh, who wondered if this could explain the reports that nanowire lengthening stopped when the growth temperature reached a certain critical value. This temperature was thought to be the melting temperature of the group III-Au alloy, typically employed in III-V compound nanowire growth.
Following this train of thought, Dayeh began tinkering with the initial molar ratios of the group III and group V precursor materials for InAs nanowires – tri-methyl-indium [In(CH3)3]and arsine [AsH3], respectively.
By starting with lower arsine concentrations, Dayeh demonstrated that III-V compound semiconducting nanowire growth prevails at higher temperatures according to the vapor-liquid-solid growth mechanism. He confirmed that the cessation of nanowire growth at certain high temperatures was tied to an imbalance in the molar ratios of the starting ingredients and not to a halted vapor-solid-solid growth mechanism.
Dayeh’s work on nanwires is broad in scope.
“My very recent work has shown that nanowires grown at different temperatures and in different crystallographic orientations exhibit notably different electronic properties,” said Dayeh.
In the work presented at the MRS 2007 spring meeting, Dayeh and colleagues at the Jacobs School performed a systemic study of carrier transport properties in InAs nanowire field effect transistors. This kind of research enables the understanding of important physical phenomena at the nanoscale and provides basic parameters for the design and fabrication of functional devices and integrated systems. The experimental studies include device scaling and miniaturization effects on performance as well as transport behavior and morphological changes when exposed to high applied electric fields. Among other implications, this line of research has highlighted thermal management as an important issue in realizing the full potential of nanowire-based devices
FE-SEM image of an InAs nanowire field effect transistor. Insets are the DC equivalent circuits with parasitic components.
In an earlier publication in the Journal of Vacuum Science and Technology B, Dayeh and Xiaotian Zhou, a graduate student in the UCSD materials science program who is advised by Edward Yu, observed room temperature ballistic transport in InAs nanowires over lengths up to ~200nm, another indication of the promise of this material.
“Shadi's studies of transport phenomena in InAs nanowires are helping to establish the foundation for the application of these materials in future high-performance electronic devices and circuits,” said Edward Yu.
In addition, the journal Small recently published research by Dayeh and colleagues at the Jacobs School demonstrating the promising potential of using InAs nanowires for high-speed nanoelectronics and providing analysis that enables accurate parameter extraction from such devices. In particular, the electrical engineers fabricated and characterized underlap top-gate and global back-gate InAs NWFETs, and demonstrated the highest semiconductor nanowire electron mobility reported to date.
“As a co-author on this paper, I experienced Shadi’s sharp intellect, creativity and sensitivity to detail. Shadi certainly exemplifies one of the great aspects of our ECE department. This is a place where students are challenged to solve some of the most pressing engineering questions of our time,” said Paul Yu, chair and professor, Department of Electrical and Computer Engineering, Jacobs School of Engineering .
Dayeh is also part of a broad collaboration of researchers at the Jacobs School that has recently reported breakthroughs in p-type ZnO nanowire synthesis and high sensitivity ZnO nanowire photodetectors, both published in Nano Letters and highlighted in the technology press.
Further information can be found at the Nano-Electronics, Photonics and Medicine and Nanoscale Characterization and Devices Laboratory websites.
Nano-Electronics, Photonics and Medicine (Deli Wang lab)
Nanoscale Characterization and Devices Laboratory (Edward Yu lab)
Funders: Office of Naval Research (ONR-Nanoelectronics), the National Science Foundation (NSF), Sharp Labs of America.
Dayeh and colleagues also recognize the staff of Calit2’s Nano3 Facility for maintenance of the nanofabrication environment.
Shadi Dayeh’s Recent Awards:
EMC 2006 best paper award: “Growth Mechanism and Optimization of InAs Nanowires Synthesized by OMVPE,” by Shadi A. Dayeh, David Aplin, Edward T. Yu, Paul K.L. Yu, and Deli Wang. All authors are from the Department of Electrical and Computer Engineering, UC San Diego Jacobs School of Engineering.
Spring MRS 2007 best poster award: “Field-, Diameter-, and Surface State- Dependent Transport Behavior in Semiconductor Nanowires,” by Shadi A. Dayeh, Paul K. L. Yu, Edward T. Yu, and Deli Wang. All authors are from the Department of Electrical and Computer Engineering, UC San Diego Jacobs School of Engineering.
Spring MRS 2007 Graduate Student Award: Silver Medal.
PCSI-34 2007 Young Scientist Award.