All ETDs from UAB

Advisory Committee Chair

Renato P Camata

Advisory Committee Members

Mary Ellen Zvanut

David J Hilton

Sergey B Mirov

Patrick A Kung

Sergey Vyazovkin

Document Type

Dissertation

Date of Award

2018

Degree Name by School

Doctor of Philosophy (PhD) College of Arts and Sciences

Abstract

Transition metal (TM)-doped II-VI semiconductor thin films have been shown to be attractive materials for mid-infrared (mid-IR) laser sources due to broad applicability in the sensing and detection of a wide range of organic molecules. Specifically, when ZnSe is doped with Cr2+ ions, the resulting broadband emission characteristics in the 2-3 µm spectral range create the potential for tunable lasing in the mid-IR. This dissertation research is motivated by the need for low cost, efficient, portable materials to be incorporated into multilayered mid-IR optoelectronic devices providing appropriate optical confinement, efficient quantum confinement of carriers, and optical emission in the mid-IR spectral range. The first primary objective of this dissertation research is to investigate the crystal quality of ZnSe-based thin film structures and the factors controlling the growth dynamics. This objective was pursued through X-ray Diffraction (XRD) analysis of numerous thin film structures, used to illuminate material properties associated with the crystalline quality and defect density of deposited layers. A significant focus was the pursuit of conditions for epitaxial growth of ZnSe-based materials by pulsed laser deposition (PLD). Conditions of epitaxy were researched in the growth of ZnSxSe1-x thin films to be used as lattice-matching layers in a multilayered structure deposited on cubic (100) GaAs substrates. Certain combinations of sulfur concentration and deposition temperature were found to result in epitaxially-oriented films, but with dislocation densities on the order of ~1010 cm-2. This was the first instance of growth of this material by PLD to our knowledge, and further progress was made in reducing dislocation densities with the addition of a Cr2+:ZnSe layer at the GaAs interface. The effects of increasing thickness of the Cr2+:ZnSe layer were studied in efforts to determine limits to epitaxy on this system. Epitaxially-oriented structures were achieved with the implementation of a Cr2+:ZnSe layer (150 nm) at the GaAs interface, whereas polycrystalline structures were obtained at thicknesses above this critical value. The second objective of this dissertation research is to explore the effects of crystallinity of thin film structures on the electrical properties of deposited ZnSe-materials. This objective was pursue via electrochemical impedance spectroscopy (EIS) studies on various thin films structures for which the crystallographic properties were also determined. After annealing all samples in forming gas, impedance measurements were used to investigate electronic structure, and current-voltage measurements were utilized in extracting effective barrier heights in complex polycrystalline samples. A series of Cr2+:ZnSe/ZnS0.1Se0.9 multilayered structures were deposited on GaAs substrates with deposition conditions known to produce polycrystalline films of ZnS0.1Se0.9 on GaAs in order to explore the resulting crystallographic and electrical properties as a function of thickness of the Cr2+:ZnSe layer. Measured values for effective barrier heights of Schottky-like potential barriers contributed by the collection of grain boundaries throughout the samples showed an increase in effective barrier height with increasing thickness of the Cr2+:ZnSe layer. This increase in effective barrier height also aligns with the relative increase of secondary diffraction peaks observed in the XRD glancing angle 2θ data, which indicates an increase in the number of grain boundaries throughout the structures with increasing thickness. Measured values of specific contact resistance, indicative of the net resistance contributed from junction barriers primarily due to grain boundaries, were also observed to increase with thickness and polycrystalline features. The third objective of this dissertation was the realization of a ZnSe-based electroluminescent device grown by PLD. This objective was achieved by growing a Cr2+:ZnSe layer on top of conductive ITO/Glass substrates, utilizing a silicon mask to leave a portion of exposed ITO that would be used as the lower electrode. Ni contacts were deposited on the top surface of the ZnSe film, and electrical measurements were made utilizing a mesa geometry. Current densities as high as 30 mA/cm2 were attained with an applied DC voltage of 16.7 V, and electroluminescence spectra were obtained for applied voltages with the range 10-20 V. The intensity of luminescence increased with increasing applied voltage, and the broad peak of emission was centered around ~650 nm. The broad nature of the peak suggests emission from multiple deep centers within ZnSe, and a side peak of low intensity at ~460 nm was observed and is likely the band-edge emission of ZnSe at room temperature. Absorption by chromium ions could partially contribute to the low measured intensities of emission in this 400-900 nm spectral region, and if due to sub-band excitation of chromium ions, would possibly result in emission in the mid-IR spectral range. An electroluminescent signal was also detected within the 2-3 μm spectral range with a similar experimental setup utilizing low and high pass optical filters. The intensity was too weak to obtain a spectrum of the detected luminescence, but this result is promising for future investigation of mid-IR emission from similar Cr2+:ZnSe thin film structures.

Share

COinS