The coupling efficiencies of sputtered, c-axis-orientated zinc oxide (ZnO) films in guided wave resonance optical modulators have been measured. The ZnO planar waveguide is sandwiched between two 3 nm thick chromium layers on top of a Si02p(100) substrate. This novel design facilitates application of a modulation voltage directly across the dielectric film in a prism coupler set-up and thus avoids voltage losses across the silicon dioxide (SO,) optical isolation layer.We present a comparison of four differently prepared ZnO waveguideslmodulators to investigate the influence of film crystallinity on the coupling efficiency and electro-optical parameters. The coupling efficiency of a ZnO waveguide is found to be dependent on both film thickness and average grain diameter measured in the plane of the film. A high efficiency (0.76) can be achieved for film thicknesses below 300 nm when the average grain diameter is 26 nm. This value is comparable to the maximum value of tvDicallv around 0.

X-ray absorption spectroscopy has been used to investigate the self-texture of ZnO films grown on Si(100) (lattice-mismatched substrate) by single-source chemical vapor deposition (SS-CVD) using Zn 4 O(acetate) 6 as precursor. For this system nonepitaxial growth of polycrystalline, c-axis oriented films can be controlled by addition of a water ambient during the deposition for a large variety of substrates. Angle dependent near edge X-ray absorption fine structure spectroscopy (NEXAFS) was used to study the orientation of the adsorbent molecules as a function of both water ambient used during the deposition and film thickness. The onset of self-structuring under optimum growth conditions (highest water pressure, P H 2 O ) 5 × 10 -4 mbar) is found to occur at film thicknesses on the order of 50 Å. The role of the water ambient during the film deposition is discussed using a kinetic model in which lattice stabilization due to filling of oxygen vacancies promotes the film crystallization. The model is used to explain the measured changes in the atomic environment of the oxygen atoms in the deposited ZnO films with variation in ambient water used. The interfacial restrictions in nonepitaxial CVD film growth are discussed. Our results indicate that in addition to structural restrictions due to lattice mismatch the chemical reactivity of the substrate material clearly must be considered.

In situ X-ray absorption spectroscopy analysis of early states of ZnO film growth on Si(100) by single source chemical vapor deposition (CVD) has been performed using basic zinc acetate as precursor. A high concentration of carbon is detected at the interface, which decreases with increasing film thickness (~2 nm thickness), and as expected there is some oxidization of the Si surface. This is explained by the chemical nature of the immediate surface upon which the deposition takes place, varying from a reactive, clean Si surface to a less reactive, mixed oxide layer after successive deposition steps. On the latter surface, the decomposition fragments are believed to be more volatile and thus the resulting film contains less carbon contamination. The results confirm that the tetrahedral core of the central oxygen atom with four neighboring zinc atoms, which reassembles the structure in solid ZnO , is kept intact upon decomposition of the precursor on the heated Si (100) (400°C) substrate. However, no long range orientation of the ZnO tetrahedrons was found, indicating that the resulting ZnO film has no preferred crystalline structure for the film thickness investigated here (~2 nm).

Valence-band and core-level XPS have been used to characterize the structure and growth of aluminium oxidized in a low-radiofrequency plasma. A change in the oxygen/aluminium ratio as a function of the metal substrate temperature during the plasma treatment correlated with changes in the valence band. Valence-band width and shape indicated that the films were characteristic of g-alumina. The change in shape of the high-binding-energy component of the band suggested that structural changes occurred to the film as a function of temperature. Plasma film growth could be described in terms of the Cabrera-Mott theory of field-assisted diffusion, with reaction velocities being higher than thermal oxidation rates reported in the literature. The reduction in the activation energy for plasma film growth was consistent with anion migration in a network-forming oxide.  For and on behalf of the Commonwealth of Australia

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