Study of pulsed-DC sputtering induced Ge2Sb2Te5 thin films using facile thermoelectric measurement

Significance Statement

Thermoelectric materials can serve as an alternative potential source of power by the simple use of the interconvert gradients in electric potential and temperature. Its performance given by the dimensionless figure of merit ZT depends on the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature.

When determining the Seebeck coefficients, different configurations of 2-probe or 4-probe have been used. The hot-strip method used for thin films measurement usually employs 2-probe geometry in an axial flow arrangement. However, they face complexity from point of sample preparation, non-effective consideration of electron-transport through the supporting membrane and also generates anisotropic voltage signals across the normal to hot-cold junction line when films deposition on real materials is considered.

The pseudo-binary alloys of (GeTe)m-(Sb2Te3)n have been used in data storage for couple of years because of their ultrafast phase switching between metastable crystalline and amorphous phases. However, despite theoretical works on different compositions of GeSbTe alloy films, experimental reports on thin films are still in starting phase and a proper exploration of its potential is needed for industrial applications.

A collaborative research team of Dr. Manish Kumar, Mr. Athorn Vora-ud and Professor Jeon Geon Han from Center for Advanced Plasma Surface Technology at Sungkyunkwan University (Republic of Korea) and Dr. Tosawat Seetawanb from Sakon Nakhon Rajabhat University (Thailand) performed this work. Dr. Kumar developed the facile thin films thermoelectric measurement system to be used in the temperature range of 300K to 600K. Mr. Vora-ud deposited the films and all authors contributed in analysis of the results. The work published in journal, Materials and Design uses the same system to investigate the thermoelectric properties of Ge2Sb2Te5 thin films synthesized by pulsed DC magnetron sputtering method.

In their study, the researchers used a variety of techniques in their experiments such as an optical emission microscopy to perform plasma diagnostic. Thickness of the deposited film was measured using surface profiler to evaluate the film growth rate. The author also made use of x-ray diffraction technique and field emission scanning electron microscopy for phase identification and surface morphology coupled with atomic composition of Ge2Sb2Te5 thin films respectively. Electrical properties of as-deposited thin films were measured by a two-probe method and annealed films by a four-point probe of the van der Pauw method using a Hall measurement system.

Seebeck coefficients were measured using the authors developed measurement set-up which consists of two copper blocks (for hot and cold junction) and a 6-probe arrangement mounted on an aluminum base plate with the support of aluminum shafts.

The researchers found that intensities of emission lines of germanium, antimony and tellurium slightly enhanced when increasing the working pressure which is an indication of plasma density enhancement but their intensity excitations were relatively weak in comparison of argon excitations. Beyond 6mTorr, the discharge power remained more or less similar up to 8mTorr and deposition increased linearly from 400nm/min to 440nm/min which confirmed enhancement of plasma density with increasing working pressure.

It is worthy to mention that all films were deposited with high uniformity and compactness without any signature of surface defects. As-deposited films have surface features which correspond to amorphous nature after annealing.

Resistivity value decreased from 1.55Ωcm (as-deposited) to 4.76 x 10-2Ωcm (annealed at 673K). Resistivity also decreased on increasing thickness of films (from 25nm to 200nm) while there was an increase in resistivity with increase of working pressure from 5mTorr to 8mTorr.

Seebeck coefficient for fixed thickness 200nm and working pressure of 6mTorr was highest at 271.50μV/K at annealing temperature of 473K. From the results also, appropriate resistivity needs optimum thickness equal to 200nm for good value of power factor and optimum pressure condition of 6mTorr is suggested.

It can be concluded from the authors findings that the throughout process greater than 400nm/min and obtained Seebeck coefficient value of 271.50μV/K are highly promising for industrial utilization.  According to Dr. Kumar “This is the first step in development of low cost thermoelectic measurement systems ready for commercialization”.

  

Study of pulsed-DC sputtering induced Ge2Sb2Te5 thin films using facile thermoelectric measurement. Advances in Engineering

 

About the author

Dr. Manish Kumar is Senior Researcher at Center for Advance Plasma Surface Technology (CAPST), Sungkyunkwan University in South Korea.

He received his Ph.D. in Physics from Indian Institute of Technology Delhi, India in 2010, which involved study of mesoporous oxide and plasmonic nanocomposite thin films. During 2009-12, he worked as Research Associate at Inter University Accelerator Centre on Ion-Matter interactions. During 2012-14, he worked as Assistant Professor at Central University of Rajasthan, and developed Thin Films Synthesis Facilities at the newly developing department of Physics, and supervised 6 M. Sc. research projects.

Since 2014, he has been working at CAPST. He leads a group on plasma sputtering induced coatings for Energy, Optoelectronic- and Bio-applications. He is also involved in development of advanced plasma processing and measurement set-ups. His research interests involve Thin films, Surface-interface effects, Photon-Matter interactions, Thermoelectrics, Plasmonics, Ion-Matter interactions, Plasma processes and Optical/ Energy/ Bio-applications.

He has authored and co-authored more than 50 research articles in leading peer reviewed International journals and conferences. He is member of Material Research Society and life member of Materials Research Society of India. He is an editorial board member of Journal of Nanotechnology and Materials Science and also an active reviewer for major journals series i.e. Nature, Wiley, RSC, Elsevier, Springer etc.  

 

About the author

Prof. Jeon Geon Han is the director of the Center for Advanced Plasma Surface Technology and a Professor at the School of Advanced Materials Science and Engineering of the Sungkyunkwan University in South Korea.

He received his Ph.D. in Materials Engineering in 1985 from the Georgia Institute of Technology, U.S.A. During 1985-1986, he was a Research Associate of the Georgia Institute of Technology. In 1987, he was appointed an Assistant Professor, and later promoted to Associate professor and subsequently, became a professor in the School of Advanced Materials Science and Engineering in 1996, at SKKU. His main interests are fundamental design and synthesis of next-generation multifunctional film materials, development of advanced plasma surface and film processes using novel plasmas, biomedical, and engineering applications of nanomaterials in the industry, development of novel plasma sources, studies on plasma discharges, development of plasma diagnostics especially for plasma processing, etc.

He has authored and co-authored over 300 papers in peer-reviewed international and national journals and is the co-inventor of > 60 patents. He was the Editor in the special proceedings of international conferences to Surface Coatings and Technology and Thin Solid Films (1998, 2002, 2003). He has also served as a guest editor for the special issues of the AEPSE 2015 conference for the Journal Surface and Coatings technology. Also, he is an active reviewer of several Journal series i.e. AIP, IOP, IEEE, RSC, Elsevier etc.

He is a committee member of KVS, and Surface Technology Division of National Projects of Korean Government and has served as a President of Korea Institute of Surface Engineering in 2006-2008, as a Head of Institute of Industrial Vacuum Technology in 1998-2000, and as a Director of Korea-Germany Cooperation Project on Vacuum and Plasma Technology in 1999-2001. He has been serving as an executive committee member of surface engineering division, IUVSTA since 2010. He is also the recipient of Sungkyunkwan University’s “Best Professor Fellowship”, the “President award of Republic of Korea-2006”, and “Honorary doctor-2015, Chiangmai University, Thailand”. 

 

Journal Reference

Manish Kumar1 , Athorn Vora-ud1,2, Tosawat Seetawan2, Jeon Geon Han1 . Study of pulsed-DC sputtering induced Ge2Sb2Te5 thin films using facile thermoelectric measurement,  Materials & Design, Volume 98, 2016, Pages 254–261.

Show Affiliations
  1.  Center for Advanced Plasma Surface Technology, NU-SKKU Joint Institute for Plasma-Nano Materials, School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
  2.  Program of Physics, Faculty of Science and Technology, Sakon Nakhon Rajabhat University, Sakon Nakhon 47000, Thailand

 

 

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