Zinc aluminogallate, Zn(AlxGa1−x)2O4 (ZAGO), a single-phase spinel structure, offers considerable potential for high-performance electronic devices due to its expansive compositional miscibility range between aluminum (Al) and gallium (Ga). Direct growth of high-quality ZAGO epilayers however remains problematic due to the high volatility of zinc (Zn). This work highlights a novel synthesis process for high-quality epitaxial quaternary ZAGO thin films on sapphire substrates, achieved through thermal annealing of a ZnGa2O4 (ZGO) epilayer on sapphire in an ambient air setting. In-situ annealing x-ray diffraction measurements show that the incorporation of Al in the ZGO epilayer commenced at 850 °C. The Al content (x) in ZAGO epilayer gradually increased up to around 0.45 as the annealing temperature was raised to 1100 °C, which was confirmed by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy. X-ray rocking curve measurement revealed a small full width at half maximum value of 0.72 °, indicating the crystal quality preservation of the ZAGO epilayer with a high Al content. However, an epitaxial intermediate �–(AlxGa1−x)2O3 layer (� - AGO) was formed between the ZAGO and sapphire substrate. This is believed to be a consequence of the interdiffusion of Al and Ga between the ZGO thin film and sapphire substrate. Using density functional theory, the substitution cost of Ga in sapphire was determined to be about 0.5 eV lower than substitution cost of Al in ZGO. Motivated by this energetically favorable substitution, a formation mechanism of the ZAGO and AGO layers was proposed. Spectroscopic ellipsometry studies revealed an increase in total thickness of the film from 105.07 nm (ZGO) to 147.97 nm (ZAGO/AGO) after annealing to 1100 °C, which were corroborated using TEM. Furthermore, an observed increase in the direct (indirect) optical bandgap from 5.06 eV (4.7 eV) to 5.72 eV (5.45 eV) with an increasing Al content in the ZAGO layer further underpins the formation of a quaternary ZAGO alloy with a tunable composition.

NiO thin films with varied oxygen contents are grown on Si(100) and c-Al2O3 at a substrate temperature of 300 degrees C using pulsed dc reactive magnetron sputtering. We characterize the structure and optical properties of NiO changes as functions of the oxygen content. NiO with the cubic structure, single phase, and predominant orientation along (111) is found on both substrates. X-ray diffraction and pole figure analysis further show that NiO on the Si(100) substrate exhibits fiber-textured growth, while twin domain epitaxy was achieved on c-Al2O3, with NiO(111) k Al2O3(0001) and NiO[1 (1) over bar0]k Al2O3[10 (1) over bar0] or NiO[(1) over bar 10]k Al2O3[2 (1) over bar(1) over bar0] epitaxial relationship. The oxygen content in NiO films did not have a significant effect on the refractive index, extinction coefficient, and absorption coefficient. This suggests that the optical properties of NiO films remained unaffected by changes in the oxygen content.

Detta arbete utforskar optiska egenskaper som manifesteras när ljus interagerar med olika material. Avhandlingens tonvikt ligger på generation av cirkulärpolariserat ljus samt bandgapsrelaterade egenskaper. Studierna är centrerade kring spektroskopisk Mueller-matrisellipsometri, med målet att syntetisera och karakterisera nanostrukturerade och högkvalitativa tunna filmer. Detta för att utöka förståelsen av de optiska fenomen som uppstår från deras underliggande struktur respektive elektroniska övergångar.

Artikel I, II och III i avhandlingen är relaterade till optiska polarisationsstudier och belyser betydelsen av morfologi och struktur hos optiska och fotoniska material. För att studera detta syntetiserar vi AlN baserade kirala skulpterade tunna filmer med hjälp av magnetronsputtring vid små infallsvinklar (glancing angle deposition) och diskuterar inverkan av olika tillväxtparametrar på filmernas morfologi och kristallstruktur. Genom att studera detta visar vi hur struktur och kristallografiska orientering kan skräddarsys för att reflektera smala spektrala band av där ljuset har hög grad av cirkulär polarisation. Vi visar även hur de tunna filmerna selektivt reflekterar den ena polarisationsriktningen (vänster/höger) beroende på det inkommande ljusets infallsvinkel och våglängd. Genom en serie teoretiska och experimentella studier ger vi detaljerad insikt i hur de studerade nanostrukturerna samverkar med ljus.

Artikel IV och V undersöker de optiska egenskaperna som härrör från elektroniska övergångar i tunna filmer, med fokus specifikt på dielektricitetsfunktionen och optiska bandgapsfenomen. Dessa egenskaper studeras med för högkvalitativa enkristallina och homogena tunna filmer av ZnGaO, syntetiserade med metallorganisk CVD-teknik (Chemical Vapour Deposition). Vi utvärderar olika formalismer för att beräkna bandgapsvärden och bedömer deras noggrannhet och tillämpbarhet. Den modifierade Cody formalismen framhålls som bästa val på grund av dess egenskap att tillhandahålla den mest linjära regionen för att extrapolera fram värden på bandgapsenergierna. Med hjälp av teoretiska beräkningar och experiment analyserar vi även utvecklingen de tunna filmernas kristallstruktur och optiska egenskaper när de värms upp till höga temperaturer. Resultaten förklarar samspelet mellan de strukturella egenskaperna och kopplingen till bandgapsvärdena.

Sammantaget ger denna avhandling en grundläggande förståelse för strukturella och inneboende egenskaper hos material som styr samverkan mellan ljus och materia. Denna forskning banar väg för vidareutveckling av tunnfilmsbaserade polarisationsfilter och avancerade optoelektroniska komponenter.

Electronic grade ZnGa2O4 epitaxial thin films were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition and investigated using spectroscopic ellipsometry. Their thickness, roughness and optical properties were determined using a Multiple Sample Analysis based approach by the regression analysis of optical model and measured data. These samples were then compared to samples which had undergone ion etching, and it was observed that etching time up to four minutes had no discernible impact on its optical properties. Line shape analysis of resulting absorption coefficient dispersion revealed that ZnGa(2)O(4 )exhibited both direct and indirect interband transitions. The modified Cody formalism was employed to determine their optical bandgaps. These values were found to be in good agreement with values obtained using other popular bandgap extrapolation procedures. Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published articles title, journal citation, and DOI.

Glancing angle deposition (GLAD) of AlN nanostructures was performed at room temperature by reactive magnetron sputtering in a mixed gas atmosphere of Ar and N-2. The growth behavior of nanostructures shows strong dependence on the total working pressure and angle of incoming flux. In GLAD configuration, the morphology changed from coalesced, vertical nanocolumns with faceted terminations to highly inclined, fan-like, layered nanostructures (up to 38 degrees); while column lengths decreased from around 1743 to 1068 nm with decreasing pressure from 10 to 1.5 mTorr, respectively. This indicates a change in the dominant growth mechanism from ambient flux dependent deposition to directional ballistic shadowing deposition with decreasing working pressures, which is associated with the change of energy and incident angle of incoming reactive species. These results were corroborated using simulation of metal transport (SiMTra) simulations performed at similar working pressures using Ar and N separately, which showed the average particle energy and average angle of incidence decreased while the total average scattering angle of the metal flux arriving at substrate increased with increasing working pressures. Observing the crystalline orientation of GLAD deposited wurtzite AlN nanocolumns using X-ray diffraction (XRD), pole-figure measurements revealedc-axis growth towards the direction of incoming flux and a transition from fiber-like to biaxial texture took place with increasing working pressures. Under normal deposition conditions, AlN layer morphology changed from {0001} to {10 (1) over bar1} with increasing working pressure because of kinetic energy-driven growth.