Biography

Dr. Stefan Zollner received his Ph.D. in Semiconductor Physics in 1991 from Universität Stuttgart (Germany) with research performed at the Max-Planck-Institute for Solid State Research. After a postdoctoral position at IBM Yorktown Heights, he accepted a tenure-track faculty position at Iowa State University in 1992. From 1997 to 2010, he held various engineering and management positions at Motorola, Freescale Semiconductor, and IBM related to semiconductor process development (CMOS, BiCMOS, GaAs). Since 2010, he has been Professor and Physics Department Head at New Mexico State University in Las Cruces. Dr. Zollner has published about 180 journal articles and chapters and given nearly 300 conference presentations. Since 2010, his external research expenditures at NMSU have been over 1.6 million dollars. He is an IEEE Senior Member and a Fellow of the American Physical Society and the American Vacuum Society. During the 2018/19 academic year, he was a visiting researcher at the Air Force Research Laboratory (Albuquerque, NM, and Dayton, OH) and the Institute of Physics at the Czech Academy of Sciences in Prague, Czech Republic. His research investigates the optical constants of solids using spectroscopic ellipsometry.

Recent Publications

Shirly Espinoza, Steffen Richter, Mateusz Rebarz, Oliver Herrfurth, Rüdiger SchmidtGrund, Jakob Andreasson, and Stefan Zollner, Transient dielectric functions of Ge, Si, and InP from femtosecond pump-probe ellipsometry, Appl. Phys. Lett. 115, 052105 (2019). 

Abstract: Transient dielectric functions with a 120 fs time resolution of Ge, Si, and InP were acquired from 1.7 to 3.5 eV with a femtosecond pump-probe rotating-compensator ellipsometer. The intensity of the pump laser (with 1.55, 3.10, or 4.65 eV photon energy) was adjusted to create an initial near-surface carrier density of 1020 cm-3. In Ge, there is a significant (~15%) decrease in the E1 and E1 + Δ1 critical point absorption and a Kramer’s–Kronig consistent change in the refractive index because photoexcited electrons at L block these transitions and reduce their amplitudes. Only a small redshift of the E1 critical point is observed, which we attribute to lattice heating and exchange-correlation effects. Minimal changes were found for Si and InP, where electrons near Δ and Γ do not participate in interband transitions between 1.7 and 3.5 eV.  

Stefan Zollner, Pablo P. Paradis, Farzin Abadizaman, and Nuwanjula S. Samarasingha, Drude and Kukharskii mobility of doped semiconductors extracted from Fourier transform infrared ellipsometry spectra, J. Vac. Sci. Technol. B 37, 012904 (2019). 

Abstract: The factorized plasmon-phonon polariton description of the infrared dielectric function is generalized to include an additional factor to account for the effects of interband electronic transitions. This new formalism is superior to the usual Drude–Lorentz summation of independent oscillators, especially in materials with large transverse-longitudinal optical phonon splittings, multiple infrared-active phonon modes, or high concentrations of free carriers, if a broadband description of the dielectric function from the far-infrared to the vacuum-ultraviolet spectral region is desired. After a careful comparison of both approaches, the factorized description is applied to the dielectric function of undoped and doped semiconductors (GaAs, GaSb, and InAs) and metal oxides from 0.03 to 9.0 eV. Specifically, the authors find that both descriptions of the far-infrared dielectric function yield the same carrier density and mobility, at least for a single species of carriers. To achieve valid results for moderately high doping concentrations, measurements to lower energies would be helpful. 

Rigo A. Carrasco, Stefan Zollner, Stephanie A. Chastang, Jinsong Duan, Gordon J. Grzybowski, Bruce B. Claflin, and Arnold M. Kiefer, Dielectric function and band structure of Sn1−xGex (x<0.06) alloys on InSb, Appl. Phys. Lett. 114, 062102 (2019). 

Abstract: Tin-rich Sn1-xGex alloys with Ge contents up to 6% were grown pseudomorphically on InSb (001) substrates by molecular beam epitaxy at room temperature. The alloys show a germanium-like lattice and electronic structure and respond to the biaxial stress within continuum elasticity theory, which influences bands and interband optical transitions. The dielectric function of these alloys was determined from 0.16 to 4.7 eV using Fourier-transform infrared and spectroscopic ellipsometry. The E1 and E1 + Δ1 critical points decrease with the increasing Ge content with a bowing parameter similar to the one established for Ge-rich Sn1-xGex alloys. On the other hand, the inverted direct bandgap Ē0 is nearly independent of the Ge content, which requires a bowing parameter of about 0.8 eV, much lower than what has been established using photoluminescence experiments of Ge-rich relaxed Sn1-xGex alloys. 

Rigo A. Carrasco, Cesy M. Zamarripa, Stefan Zollner, José Menéndez, Stephanie A.Chastang, Jinsong Duan, Gordon J. Grzybowski, Bruce B. Claflin, and Arnold M. Kiefer, The direct band gap of gray α-tin investigated by infrared ellipsometry, Appl. Phys. Lett.113, 232104 (2018). 

Abstract: Using Fourier-transform infrared ellipsometry, the authors provide spectroscopic evidence about the valence band (VB) structure of diamond-like α-tin. The mid-infrared dielectric function of α-tin grown pseudomorphically on InSb or CdTe by molecular beam epitaxy shows a strong Ē0 peak near 0.41 eV. This peak is assigned to allowed intravalence band transitions from the Γ–7 (electron-like) VB to the Γ+v8 heavy hole VB and/or interband transitions from Γ–7 to the Γ+c8 light “hole” conduction band. The strength of this peak requires a hole density in the mid-1018 cm-3 range at room temperature, which might be caused by unintentional doping, by thermal electron-hole pair generation, or by the possibility that the L+6 conduction band might have an energy slightly lower than the Γ+v8 VB maximum. Alternatively, this Ē0 peak might be enhanced by the M-shape of the Γ–7 VB caused by interactions with the Γ+7   split-off hole VB. A sum-rule analysis of the dielectric function between 0.16 and 6.5 eV is consistent with a high-frequency dielectric constant of 24, which has at most a weak temperature dependence between 100 and 300 K. 

Dominic Imbrenda, Ryan Hickey, Rigo Carrasco, Nalin S. Fernando, Jeremy VanDerslice, Stefan Zollner, and James Kolodzey, Infrared dielectric response, index of refraction, and absorption of germanium-tin alloys with tin contents up to 27% deposited by molecular beam epitaxy, Appl. Phys. Lett. 113, 122104 (2018). 

Abstract: The dielectric spectral response of Ge1-xSnx thin film alloys with relatively high Sn contents (0.15 ≤ x ≤ 0.27) and thickness from 42 to 132 nm was characterized by variable angle spectroscopic ellipsometry over the wavelength range from 0.190 to 6µm. The Ge1-xSnx thin films were deposited on Ge substrates by molecular beam epitaxy using an electron-beam source for Ge to achieve a substrate temperature below 150 C to prevent the surface segregation of Sn. From the measured dielectric function, the complex refractive index was calculated indicating an increase in the real index with the Sn content at mid-infrared wavelengths. The ellipsometry revealed that the band structure critical point energies red-shifted with the increasing Sn content. The optical absorption coefficient was calculated from the imaginary index and showed a strong absorption into, and beyond, the mid-infrared with the increasing Sn content. 

More articles and publications can be found here.