High-quality UV optoelectrical and high-mobility T-ODL/Ti:ZnO epilayers on an amorphous substrate through tailoring the interfacial nucleation process by atomic layer deposition

Abstract

Recent efforts on extending the applications of ALD films as the main functional layer in electronic and photonic devices, beyond insertion and encapsulation, have been hindered by their poor crystallinity and the resulting inferior electrical/optical performances, which could be largely due to misoriented initial nucleation during the ALD growth. In this study, we have developed a facile and substrate-independent approach for crystalline improvement in ZnO films by ALD at low temperatures through tailoring the initial nucleation orientation process using an ultra-thin oriented doped layer (ODL), which is found to induce strong orientation and significant improvement in the crystal quality, largely decreasing the grain boundary (GB) area. Strong and complete a-axis- and c-axis-oriented films are achieved by tuning the thickness and composition of ODL, respectively, with highly enhanced UV luminescence and electrical properties owing to the decrease in surface states at GB. In particular, the optimized 15 nm-ODL/Ti:ZnO (1t39z) film with c-axis orientation shows minimum resistivity of 4.5 × 10⁻⁴ Ω cm, and the intensity of its UV emission is 15.3 times stronger than that of the film without ODL. Moreover, the optimum ODL/Ti:ZnO film is found to have excellent heat treatment performance due to good initial crystallinity and high in-plane symmetry of the c-axis, and it exhibits quasi-single crystal structure under TEM after annealing at 650 °C with mobility as high as 46.8 cm² V⁻¹ s⁻¹ and minimum resistivity of 2 × 10⁻⁴ Ω cm. This study sheds light on the growth mechanism of ALD and provides a promising approach for achieving high-quality ALD films, which is of significance for fabricating high-quality devices on the nanoscale.