1 Pinning effect on the band gap modulation of crystalline Be x Zn 1-x O ternary films grown on Al 2 O 3 (0001) Dae-Sung Park † , James. J Mudd † , Marc Walker † , Djelloul Seghier †,§ , Aleksander Krupski † , Nessa Fereshteh Saniee † , Chel-Jong Choi ‡ , Chang-Ju Youn ‡ , Sean Robert Craig McMitchell † , and Chris F. McConville *,† † Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom ‡ School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, South Korea ABSTRACT: We have investigated the influence of Be concentration on the microstructure of BexZn1-xO films (from x = 0 to 0.77), grown on Al2O3 (0001) substrates using radio-frequency co-sputtering. With increasing Be concentration, the BexZn1-xO (0002) X-ray diffraction peak shows a sys- tematic shift from 33.86 o to 39.39 o , and optical spectroscopy shows a blue- shift of the band gap from 3.24 to beyond 4.62 eV towards the deep UV regime, indicating that Be atoms are incorporated into the host ZnO lattice. During the band gap modulation, structural variations (e.g. phase separa- tion and compositional gradients of Be) in the films were observed along with a significant change in the mean grain size. X-ray photoelectron spec- troscopy indicates higher concentrations of metallic Be states in films with smaller grain size. Correlation between these two observations indicates that Be segregates to near grain boundaries. A structural model is proposed through simulation, where an increase in grain growth driving force dominates over the Be particle pinning effect. This leads to further coalescence of grains, reactivation of grain growth, and the uniform distribution of Be composition in the BexZn1-xO ternary alloy films. KEYWORDS: Oxide alloys, Band gap engineering, BexZn1-xO, Grain growth, Heterostructures, Thin films  INTORDUCTION Wide band-gap oxides have received a great deal of interest due to their potential use in optoelectronic applications, including ultraviolet (UV) laser diodes (LDs), light-emitting diodes (LEDs), high-mobility transistors and gas sensors. 1-6 Quantum confinement (QC) and the fractional quantum Hall effect (FQHE) are phenomena that have been observed in well-de- signed oxide heterostructures and can add to the wealth of po- tential applications. 7 In order to design such quantum structures, it is essential to understand the band-gap engineering of oxides and the nature of oxide-based heterointerfaces. In addition, var- ious charge states can occur at such surfaces and interfaces (ei- ther charge accommodation or depletion), as a result of the sur- face termination, the formation of surface defects (e. g. oxygen vacancies or cation interstitials), the presence of impurities (e. g. hydrogen - Hi) or other chemisorbed species. 8-10 ZnO-based materials have many applications and exhibit many interesting physical and electronic properties, including a large exciton binding energy (~ 60 meV) at room temperature, and high transparency in both ultra-violet (UV) and visible spectral ranges. 11 In a wider band-gap engineering process of ZnO-based materials, A. Ohtomo et al. 12 first proposed that the band gap energy (Eg) of Zn1-xMgxO could be tuned from 3.3 eV (ZnO) to 7.8 eV (MgO) by substitution of Mg into Zn lattice sites in the wurtzite structure. The alloying process, however, was limited to x ≤ 0.36 to a structural transition from the ZnO hexagonal phase to the rocksalt phase of MgO at high concen- tration. Ryu et al. 13 suggested that the Eg for ZnO could be fully modulated by alloying with BeO (Eg = 10.6 eV) without a struc- tural phase transition due to the two materials having the same hexagonal symmetry. Subsequently, a BexZn1-xO-based multi- quantum-well structure was designed by periodically stacking ZnO wells with Be0.2Zn0.8O barriers, resulting in UV light emis- sion generated from a conventional LED structure. 5 After this demonstration, Klingshirn et al. 14 suggested that one possible explanation of compositional fluctuations and local segrega- tion, was diffusion of Be to the ZnO interfaces in the quantum well device structures. Therefore, understanding the growth mechanisms of BexZn1-xO films on large-mismatched substrates is key to improving device performance. In previous studies, growth of BexZn1-xO ternary thin films using various growth methods 15-18 resulted in poor quality single crystals due to the