Surface and interface engineering of ZnO based heterostructures fabricated by pulsed-laser deposition

Abstract

ZnO and related alloys are an important class of materials to realize transparent electronics because of their characteristic wide band-gap and high mobility, and also because of their practical advantages, such as: available n-type materials and bulk single crystals, low-cost production, and absence of toxicity. Our studies have been conducted for more than a decade and they have enabled surface and interface engineering on an atomic scale, presenting a promising technology for developing electrical devices of various kinds. The quality of the epitaxial films was improved drastically when grown on high-temperature annealed buffer layers prepared on lattice-matched ScAlMgO4 substrates using pulsed-laser deposition. We carefully investigated the growth temperature dependence of surface morphology and electrical properties. Electron mobility was recorded as 440 cm2 V−1 s−1 at room temperature and 5500 cm2 V−1 s−1 at 1 K, leading to observation of the integer quantum Hall-effect (QHE) in abrupt ZnO/MgxZn1−xO interfaces. Two-dimensional electron gas (2DEG) was formed spontaneously in the interface because of the polarization mismatch between the layers. The observation of QHE enables us to access the direct determination of the interfacial electronic structure. In addition, the field-effect control of 2DEG has been demonstrated using lattice-matched interfaces as high-mobility channels.