Role of beam divergence and ion-to-molecule flux ratio in ion-beam-assisted deposition texturing of MgO
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The effect of process conditions on the biaxial texture of MgO films grown by ion-beam-assisted deposition (IBAD) was studied. The texture showed a strong dependence on the Ar+/MgO flux ratio, but a weak dependence on the divergence of Ar+ beam. One hundred-nanometer-thick epi-MgO on less than 10-nm-thick textured IBAD-MgO films that were grown on 7-nm-thick Y2O3 layers on fused silica, metal alloy tape, and polished Si substrates showed biaxial texture with in- and out-of-plane orientation distributions of less than 4° and 2°, respectively. These results strengthen the notion that the IBAD technique could serve as a universal technological process to integrate amorphous and polycrystalline substrates with various oxide and semiconductor films that need to be grown with good biaxial texture.
I. INTRODUCTION
The ion-beam-assisted deposition (IBAD) technique has successfully been used for preparing biaxially textured YBa2Cu3O7-␦ (YBCO) superconducting films on polycrystalline metal-alloy tapes.1,2 Early experiments used yttria-stabilized zirconia (YSZ) in the IBAD process to achieve a biaxially textured template for the subsequent growth of epitaxial YBCO films on inexpensive and flexible metal-alloy tapes for electric power applications.3,4 An alternative IBAD process that uses MgO instead of YSZ evolves 100 times more quickly,5,6 promising a faster and cheaper texturing method.7,8 For electric power applications of superconducting YBCO, good in-plane texture is necessary because current carrying capacity strongly depends on absence of high-angle grain boundaries.4,9 There are many other technological applications of films where the electronic, mechanical, or electromechanical performance depends on film grain texture.10 For example, biaxially oriented piezoelectric films are expected to have electromechanical properties similar to those of single-crystalline films, whereas polycrystalline piezoelectric films exhibit degraded translational range of motion.11 Other potential applications include silicon-on-insulator structures and solar cells. The IBAD-MgO process has recently been studied both analytically and experimentally.11–16 The current understanding is that the MgO film texture is largely
a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 19, No. 2, Feb 2004
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established at the nucleation stage (∼2-nm nominal film thickness) of the IBAD process before the MgO islands coalesce into a continuous film.5,14 Thus, it is critical to control the substrate condition, film–substrate interactions, and initial phases of film growth. Early work on IBAD-MgO was done using a-Si3N4 templates.5 Subsequent work by Arendt and co-workers showed the applicability of very thin nanocrystalline Y2O3 as seed for IBAD.17
II. EXPERIMENTAL
In this report, we present a systematic study of the combined effect of ion beam divergence and ion/MgO flux ratio on the MgO film texture. We have measured the Ar+ flux from our 22 × 6-cm radio-freq
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