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Synthesis and Characterization of Zinc Tin Nitride
Ian Curtin, Paul Quayle, Kathleen Kash
Dept. of Physics, Case Western Reserve University, Cleveland, Oh 44106
Abstract
Zinc Tin Nitride is a semiconducting material that
to date has not been synthesized, but is predicted to have
useful applications in optoelectric devices. The goal of this
experiment was to conduct the first reported growth of
ZnSnN2, determine it’s optimal growing conditions, and
aid in design development of the experimental package.
Although we didn’t conclusively grow ZnSnN2 we did
gain useful insight into phase separation and are able to
provide a larger platform of knowledge for future research.
Methods
ZnSnN2 growths were performed inside a high
vacuum plasma system. A Zn-Sn liquid alloy was created
inside of a crucible and was then exposed to a 290 W
nitrogen plasma at 400º C and held at a pressure of 7 mtorr
for 3 hours. The sample was then allowed to cool with the
nitrogen plasma still on.
Grwoth conditions were chosen based on previous
successful growths of InN, as history has shown materials
with similar band gaps form at similar temperatures.
Results and Discussion
Introduction
Group III nitride semiconductors (GaN, InN,
and AlN) are a widely studied group of materials that
have many applications in optoelectronic devices.
Zn-IV nitride semiconductors (ZnGeN2,
ZnSnN2, and ZnSiN2) have had very little experimental
work done on them and are constructed by replacing
half of the atoms from a group III nitride with Zn and
the given element’s neighbor to the right in the fourth
row of the periodic table. This makes Zn-IV nitrides
analogous to the group III nitrides both in their
bandgaps and crystalline lattice structure, but have
distinct predicted properties which could make them
superior to their predecessors. To date, ZnSnN2 has yet
to be synthesized or characterized, but could serve as a
stable equivalent to InN.
It was also observed in these growths that in order to saturate
the melt with a sufficient amount of nitrogen, the pressure had
to be lowered a couple of orders of magnitude from previous
attempts into the militorr range. This was able to be observed
by lowering the crucible height to be able to see the melt
during growth. Further saturation will be possible in the near
future with the installation of a longer quartz tube,
shortening the diffusion length of the plasma reaching the
melt.
The samples grown were inspected under an optical
microscope and a scanning electron microscope for sign of
crystalline morphology. Elementary chemical analysis was
also performed by energy dispersive X-ray spectroscopy
(EDX).
Sample 1: 9 at% Zn to 91 at % Sn
Phase Separation: One question we were faced with was what
happens to the ZnSn alloy as the sample changes from a liquid to
a solid. Thermodynamic theory suggests that as a eutectic
mixture of a certain composition cools, it will separate from a
homogenous liquid into distinct states of different compositions,
all in equilibrium, that minimize the Gibbs free energy.
Sample 2: 22 at % Zn to 78 at% Sn
Sample 3: 29 at% Zn to 71 at% Sn
During the growth process, upon exposure to the nitrogen
plasma all samples changed from a shiny metallic surface
to a darker textured surface. Upon further inspection
Sample 1 is the most likely to have developed trace
ammounts of ZnSnN2. Optical and SEM images showed
signs of crystalline morphology and elementary chemical
analysis showed the presence of Zn, Sn, and N in the
sample.
Conclusions
Unfortunately we weren’t able to conclusively
grow large ammounts of ZnSnN2. Initial results did
reveal the presence of Zn, Sn, and N in sample 1and
exhibited signs of crystalline morphology. More in depth
analysis is needed to determine the exact compositions
and structures of the material.
We were able to learn a lot about phase
seperation and the ZnSn alloy as it cools from a
homogenous liquid into several distinct states.
Significant advances were also made in the
design of the system allowing for observation of the
melt during the growth period. This will prove to be
important in future growth runs allowing the
experimentalist to vary parameters while being able to
see if a film is forming. Soon we will be able to install
a longer quartz tube increasing the ammount of ionized
reaching the melt.
Due to time constraints I was not able to fully
investigate the optimal growing conditions. However,
this research will provide a larger platform of
knowledge for future experiments.
Acknowledgments
I would like to thank Dr. Kash, Paul Quayle, Eric
Blanton, and Jermey Trombley for their guidance
and support during this project and the NSF REU
grant DMR-0850037 grant for providing
funding. Also, I’d like to thank Betty Gaffney for
making the program run so smoothly.
References
Plot of lattice constants vs.band gap energy for group III
nitrides and Zn-IV nitrides. Values for ZnSnN2 and ZnSiN2
predicted by theory. Values for ZnGeN2 determined
experimentally. [1]
Sample 1: Image from optical
microscope at 500x. Displays layering
typical of polycrystalline growth.
Sample 1: EDX averaged over the surface
of sample showing large amounts of Zinc
and Tin and trace amounts of nitrogen. It’s
important to note the layers grown are too
small for EDX to accurately characterize.
Sample 3: SEM imaging and EDX analysis clearly
show how the ZnSn alloy solidifies into two distinctly
different compositions predicted by the ZnSn binary
phase diagram.
[1] Paudel TR and Lambrechet WRL. 2008.
First-principles study of phonons and related
ground-state properties and spectra in Zn-IV-N2
compounds. Phys. Rev. B 78:115204.