Physics Alum, Professor Explore Optical Constraints of Semiconductor Zinc Oxide for First Time

“Recently, an extraordinary effort has been directed toward group II–VI semiconductors such as zinc oxide (ZnO) material … because of its potential applications in the fields of electronic and optoelectronics,” including solar cells and renewable energy storage, write an Ohio University alum and Physics & Astronomy professor who co-authored an Applied Surface Science journal article on “Multiple oscillator models for the optical constants of polycrystalline zinc oxide thin films over a wide wavelength range.”

AFM image of 600 nm ZnO thin film deposited on Si(1 1 1) substrate.

But the experimental and theoretical data on the optical constraints of ZnO thin films are “surprisingly limited” and usually done over a limited wavelength range, they write. Their paper presents, for the first time, the optical constants and the thicknesses of the zinc oxide films over a wide wavelength range.

“Determination of the optical bandgap energy (Eg) of a thin film material is often essential to develop its electronic band structure and hence adapted its application in the filed of optoelectronic devices,” write the co-authors, Dr. Jebreel Mohammad Khoshman ’03M, ’05 Ph.D., who earned two degrees in Physics from the College of Arts & Sciences at Ohio University, and Dr. Martin Kordesch, Professor of Physics & Astronomy. Khoshman is now Associate Professor in the Department of Physics at Al-Hussein Bin Talal University in Jordan. Additional co-authors were J.N. Hilfiker of J.A. Woollam Company and N. Tabelt of the Physics Department at the Center of Research Excellence in Renewable Energy at King Fahd University of Petroleum & Minerals in Saudi Arabia.

In their paper, “ZnO thin films were obtained by reactive sputtering of Zn in pure N2 plasma at room temperature. The optical constants and the thicknesses of the films were mainly investigated, for the first time, by means of variable angle spectroscopic ellipsometry (VASE) over a wide wavelength range (190–1400 nm) through the Genosc™ parameterized semiconductor oscillator functions and Gaussian oscillator models. X-ray diffraction (XRD) was employed to examine the structure properties of the films while X ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition of the studied samples. The value of the bandgap energy of the films was investigated by the analysis of the ellipsometric data, photometric spectra, and low temperature photoluminescence (PL) spectrum.”

Highlights

• ZnO thin films were grown by RF sputtering system in a pure N2.
• The films exhibited a polycrystalline structure with a preferred orientation of (1 0 1).
• The optical constants of the ZnO films in the range 190–1400 nm had been described using VASE through the multiple oscillator models.
• The equality of the films was achieved through the depolarization measurements with the degree of polarization of 98%.
• The low PL spectrum of the films showed one emission band centered at 3.40 eV.

Conclusions: “ZnO thin films were deposited by a reactive RF-magnetron sputtering on Si(1 1 1) and quartz substrates in a pure N2 atmosphere at room temperature. All films revealed a polycrystalline structure with a preferred orientation of (1 0 1). For the first time, a systematic procedure for acquiring the thicknesses and optical constants of the ZnO films in the ultraviolet–visible–near infrared range (190–1400 nm) had been described using variable angle spectroscopic ellipsometry through the multiple oscillator models. Combining multiple oscillator types provided a very flexible approach to fitting optical constants while simultaneously enforcing Kramers–Kronig consistency in the fitted optical constants. The equality of the films was also achieved through the depolarization measurements with the degree of polarization of 98% for the most desired spectral range. In addition to, the optical bandgap values were also calculated using the VASE (3.38 ± 0.03 eV) and SP (3.40 ± 0.03 eV) data. These values were found to be in an excellent agreement with the low temperature (5 K) PL studies which exhibited one prominent peak at 3.41 eV.”

Acknowledgments: “The authors would like to thank Andrew Wright and the ThermoFisher Scientific Company for assistance with the X-ray Photoelectron Spectrometer (XPS) measurement. This work was funded by the Deanship of Scientific Research at the University of Dammam (Project No. 2012155).”