Chee‐Kin Kwok

Near‐band gap optical behavior of sputter deposited α‐ and α+β‐ZrO2 films

The functional dependence of the optical absorption coefficient on photon energy in the 4.9–6.5 eV range was determined for α‐ and α+β‐ZrO2 films grown by reactive sputter deposition on fused silica. Two allowed direct interband transitions in α‐ZrO2 were identified, with energies equal to 5.20 and 5.79 eV. Modification of these transitions in α+β‐ZrO2 is reported.

The transition from αZr to αZrO2 growth in sputter‐deposited films as a function of gas O2 content, rare‐gas type, and cathode voltage

In this study, a Zr target was sputtered in rf‐excited, rare gas discharges (Kr, Ar, Ne) containing O–4% O2, operated at cathode voltages from −1.3 to −1.9 kV. In situ optical emission spectrometry was used to monitor three transitions of the neutral excited Zr atom in the discharge (λ3520, 3548, and 3601 A). Films were deposited on fused SiO2 and their crystallography, optical behavior, and electrical resistivity were determined, postdeposition. The sequence of phases that evolved in the Zr–O system was αZr→transition phases→α+βZrO2→αZrO2. The sequence was shifted towards αZrO2 with increasing gas O2 content, changing rare gas from Kr→Ar→Ne, and decreasing cathode voltage. The results are discussed in terms of a two‐step oxidation process involving both electrodes.

Indirect band gap in α‐ZrO2

Measurements of the absorption coefficient on the fundamental optical absorption edge of α‐ZrO2 show that an indirect interband transition at 4.70 eV precedes two previously reported direct transitions. This result is in agreement with recent theoretical calculations of the α‐ZrO2 band structure.

Cathode voltage-gas composition-film crystallography relationships for sputter-deposited Vanadia (V2O5)

Abstract Films were grown by r.f.-excited reactive sputter deposition on -cutsilicon substrates using a vanadium target and O2-bearing discharges containing 0%–98% argon operated at four values of cathode voltage. The 4111.6 A emission line intensity from neutral excited vanadium atoms in the discharge was monitored by optical spectrometry, and these data were used to detect target surface oxidation. The region of cathode voltage-gas O2 content space over which crystalline vanadia was produced was defined. All crystalline vanadia grew with vanadyl oxygen layers oriented parallel to the substrate (b axis normal). The interlayer spacing b varied from less than to greater than b0, the value for bulk vanadia. An oxidized target surface was a requirement for an interlayer spacing b > b0 but not a guarantee of it. As the cathode voltage decreased, the critical gas O2 content at which the target surface became oxidized was shifted to lower values and the critical gas O2 content at which b > b0 was shifted to higher values, opening up a region in which crystalline vanadia with b ⪕ b0 and short-range order vanadia were produced from an oxidized target surface.

Near-ultraviolet optical absorption in sputter-deposited cubic yttria

Cubic yttria films were sputter deposited on unheated fused silica substrates. In situ optical spectrometry was used to monitor emission from six YI transitions in the discharge. A simple formalism was developed for estimating the ratio of yttrium arriving at the substrate in atomic form to yttrium bonded to oxygen in an unspecified molecular form. The optical absorption coefficient (α=∼103–105 cm−1) of the films was determined, post‐deposition, over the energy range 5.0–6.2 eV. The results show that a large atomic yttrium flux to the substrate during deposition resulted in a ‘‘disordered’’ optical absorption edge. A large flux of yttrium bonded to oxygen resulted in an edge with two direct interband transitions separated by 0.66 eV, in good agreement with the bulk single crystal e2 data and a recent theoretical energy band calculation for cubic yttria.

Phase Mapping Sputter Deposited Wide Band-Gap Metal Oxides

Reactive sputter deposition is a widely-used process for growing films of high melting point materials near room temperature and desirable metastable structures not attainable in material grown under conditions of thermodynamic equilibrium. Both categories include wide band-gap metal oxides. A first step towards reproducible growth is to develop a “phase map” for the metal-oxygen system of interest. The map graphically relates independent sputter deposition process parameters, the growth environment, and the metallurgical phase(s) formed in the film. This paper shows how phase maps are constructed and used to observe general trends in oxide phase formation sequence, with examples from the Nb-O, Y-O, and Zr-O systems.