MgZnO/MgO strained multiple-quantum-well nanocolumnar films: Stress-induced structural transition and deep ultraviolet emission

MgZnO/MgO strained multiple-quantum-wells (S-MQWs) with different structures are grown by pulsed laser deposition. The S-MQW films are composed of closely arranged, vertically oriented nanocolumns. Z-contrast scanning transmission electron microscopy observations and line-scan compositional analysis reveal that the nanocolumns are compositionally modulated along their length and have a MQW structure. A coherent epitaxial relationship with a sharp interface is established between MgZnO and MgO layers with large lattice mismatch. It is worth noting that the MgZnO layer undergoes a structural transition from hexagonal to cubic phase with its thickness decreasing. The calculations reveal that a large in-plane compressive stress dominates such an interesting phase transition process, stabilizes the low Mg-content MgZnO alloy in the anomalous cubic phase, and also leads to a broadening of the band gap. As a result, the wavelength-tunable deep-ultraviolet emission in the range of 261-314 nm is obtained from these S-MQW nanocolumnar films. The 261 nm is known to be the shortest emission wavelength ever reported for MgZnO material. (C) 2011 Elsevier B. V. All rights reserved.