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Optical constants of evaporated amorphous zinc arsenide (Zn₃As₂) via spectroscopic ellipsometry

BYU Authors: J. Colter Stewart, Micah N. Shelley, Nathan R. Schwartz, Spencer K. King, Daniel W. Boyce, James W. Erikson, David D. Allred, and John S. Colton, published in Opt. Mater. Express

We have used spectroscopic ellipsometry to measure the optical constants of evaporated amorphous zinc arsenide (Zn3As2). A five parameter model using a Tauc-Lorentz oscillator was found to fit well each of six amorphous samples deposited on Si3N4/silicon, allowing the layer thicknesses and optical constants to be deduced. Layer thicknesses varied from 20 to 70 nm. The fitted value of the optical gap (Tauc gap) is 0.95 eV, close to the 1.0 eV band gap for crystalline bulk zinc arsenide. A single set of parameters from an ensemble Tauc-Lorentz model can be used to determine the thicknesses of amorphous Zn3As2 layers as long as the layers are \&\#x2273; 25 nm thick. Measured film thicknesses do not correlate with targeted thicknesses, likely due to low sticking coefficients of evaporated zinc arsenide.

Using ellipsometry and x-ray photoelectron spectroscopy for real-time monitoring of the oxidation of aluminum mirrors protected by ultrathin MgF₂ layers

BYU Authors: Brian I. Johnson, Tahereh G. Avval, Grant T. Hodges, Victoria Carver, Karen Membreno, David D. Allred, and Matthew R. Linford, published in Proc. SPIE

To maintain high, broad-band reflectance, thin transparent fluoride layers, such as MgF₂, are used to protect the of aluminum mirrors against oxidation since aluminum oxide absorbs short wavelength light. In this study, we present, for the first time, combined X-ray photoelectron spectroscopy (XPS) and ellipsometric (SE) studies of aluminum oxidation as a function of MgF₂ over a range of layer thickness (0-6 nm). We also show for the first time, dynamic SE data which, with appropriate modeling, tracks the extent of oxide growth every few seconds over a period of several hours after the evaporated Al + MgF₂ bilayer is removed from the deposition chamber, exposing it to the air. For each SE data set, because the optical constants of ultrathin metals films depend strongly on deposition conditions and their thickness, the optical constants for Al, as well as the Al and Al₂O₃ thicknesses, were fit. SE trends were confirmed by X-ray photoelectron spectroscopy. There is a chemical shift in the Al 2s electron emission peak toward higher binding energy as the metal oxidizes to Al⁺³. The extent of oxide growth can be modeled from the relative area of each peak once they are corrected for the attenuation through MgF₂ layer. This generates an empirical formula: oxide thickness= k*log(t) +b, for the time-dependent aluminum-oxide thickness on aluminum surfaces protected by MgF₂ as a function of MgF₂ layer thickness. Here, k is a factor which depends only on MgF₂ thickness, and decreases with increasing MgF₂ thickness. The techniques developed can illuminate other protected mirror systems.

Oxidation of Aluminum Protected by Wide Band Gap MgF₂ Layers as Followed by X-ray Photoelectron Spectroscopy

BYU Authors: David D. Allred and Matthew R. Linford, published in Society of Vacuum Coaters, 2019 Technical Conference Proceedings, Optical Coatings (August 22, 2019).

Aluminum enjoys broad band reflectivity and is widely used as an astronomical reflector. However, it oxides rapidly, and this oxide absorbs very short wavelength light, which limits the performance of aluminum mirrors. Accordingly, thin transparent layers, such as films of MgF2, are used to protect aluminum. In this study, we present an X-ray photoelectron pectroscopy (XPS) study of the chemical changes in MgF2 - protected aluminum that take place as it oxidizes (is exposed to the air). XPS reveals the rate of Al oxidation for different MgF2 thicknesses as determined from measurements obtained from 5 min to 8 months of air exposure. The degree of Al oxidation depends on the MgF2 over layer thickness.

Expanding the Far UV Range of Aluminum-Coated Mirrors for Space-Based Observations to Reflect Hydrogen Lyman Lines via Fluoride Multilayers

BYU Authors: David D. Allred, J. Gabriel Richardson, and R. Steven Turley, published in Optical Interference Coatings 2019, (Santa Ana Pueblo, NM, June 2019).

While no solid barrier layer is transparent below ~103nm, simulations show that ~9.5nm LiF on 8.5nm MgF2 on Al could reflect some hydrogen Lyman lines better than a single fluoride layer does. Experiments are promising.

Y₂O₃ optical constants between 5 nm and 50 nm

BYU Authors: Joseph B. Muhlestein, Benjamin D. Smith, Margaret Miles, Stephanie M. Thomas, Anthony Willey, David D. Allred, and R. Steven Turley, published in Opt. Express

We report optical constants of e-beam evaporated yttrium oxide Y2O3 thin films as determined from angle-dependent reflectance measurements at wavelengths from 5 to 50 nm. Samples were measured using synchrotron radiation at the Advanced Light Source. The experimental reflectance data were fit to obtain values for the index of refraction and thin film roughness. We compare our computed constants with those of previous researchers and those computed using the independent atom approximation from the CXRO website. We found that the index of refraction near 36 nm is much lower than previous data from Tomiki as reported by Palik. The real part of the optical constants is about 10% to 15% below CXRO values for wavelengths between 17 nm and 30 nm. Films were also characterized chemically, structurally, and optically by ellipsometry and atomic force microscopy.

Think First, Act Later - A Course Structure for Improving Student Designed Experiments

BYU Authors: Nathan D. Powers, Dallin S. Durfee, and David D. Allred, published in 2018 Conference on Laboratory Instruction Beyond the First Year of College, Part of the BFY Conference series, (Baltimore, MD, July, 2018).

There is a natural tendency for students to act first (e.g. - build and conduct experiments) and think later (e.g. - outline goals, identify challenges, predict outcomes, etc.). This is often apparent in labs that include student design components. We have developed a lab course structure that teaches students how to develop their ideas and make plans before beginning an experiment by providing multiple opportunities for peer and instructor feedback. As a result, we have seen significant improvements in the success rate and quality of student-designed experiments and presentations. We provide a detailed explanation of the course structure and rubrics and evidence of the impacts of this course structure.

BYU Physics and Astronomy

Selected Publications