Synthesis and Characterization of Zinc Tin NitrideIan Curtin, Paul Quayle, Kathleen Kash

Dept. of Physics, Case Western Reserve University, Cleveland, Oh 44106

IntroductionGroup 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.

Methods

Results and Discussion

Conclusions

AcknowledgmentsI 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.

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.

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]

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.

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

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.

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.

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 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.

Abstract

[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.

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.