Presentation: Bulk Single Crystal Gallium Nitride Growth
Prof. S. Pimputkar, Materials Science and Engineering Department and Center for Photonics and Nanoelectronics, Lehigh University, Bethlehem, PA
Abstract
Gallium nitride (GaN) is a wide bandgap semiconductor which has received
widespread attention as it has enabled the development of short
wavelength (UV, blue) optical emitters and promises highly efficient,
high power electronics. Common to these fields is the need for a high
quality, single crystal bulk GaN substrate to minimize efficiency
losses caused by dislocations, permit growth of GaN epitaxial layers in
arbitrary crystallographic orientations and enable vertical device
structures to further increase the operating breakdown voltages for
GaN power electronics.
Bulk GaN growth is challenging due to the inability to readily melt the
material (Tmelt > 2200 K, Pmelt > 6 GPa). The ammonothermal (AT) method
circumvents this challenge by dissolving GaN into a supercritical ammonia
solution under elevated temperatures (~ 600 degC) and pressures
(~ 2000 - 3000 atm). Mineralizers, such as alkali metals or halides, are
added to the solution to increase GaN solubility leading to basic or
acidic solutions, respectively. Basic solutions are less corrosive than
acidic solutions permitting use of unlined Ni-Cr superalloy autoclaves,
whereas acidic autoclaves typically require precious metal lined autoclaves.
Recently, molybdenum-based autoclaves have for the first time demonstrated
their suitability as an unlined autoclave for use in both acidic and basic
growth environments. Non-hermetically sealed capsules have been developed
enabling higher purity growths of GaN in basic environments.
Important advances in the field which will be highlighted include improvement
of growth rates leading to demonstrated peak growth rates of 60 - 340 um/day
for basic systems and 250 - 960 um/day for acidic systems, development of
an equation of state to thermodynamically describe the decomposition of
ammonia at typical operating conditions, and a reinvestigation of the
observed retrograde behavior of GaN solubility in basic ammonothermal
systems. Properties of ammonothermally grown GaN crystals will be presented,
including improvements in crystal purity, explanations for their observed
sub-bandgap optical absorption and coloration due to the presence of point
defects, including most notably hydrogenated gallium vacancies.
Biography (from Department of Material Science and Engineering, Center for Photonics and Nanoelectronics, Lehigh University website)
- Education:
- Ph.D. in Materials, University of California, Santa Barbara, USA, 2012
- B.S. in Mechanical Engineering & Minor in Materials Science and Engineering, Illinois Institute of Technology, USA, 2006
- Matura in Mathematics and Natural Sciences, Kantonschule Limmattal, Switzerland, 2002
- Research interests:
- Develop innovative methods for growth of advanced electronic, magnetic and optical materials
- Explore materials with unique intrinsic properties and grow them as large single crystal materials
- Leverage big-data and computational methods to identify promising energy materials
- Investigate and invent suitable systems to grow extremely high aspect ratio 1D & 2D materials
- Seek methods to effectively communicate research and enhance collaborations
Talk Details
Slides from talk