Band gap engineering based on BeZnO alloy films prepared by RF magnetron co-sputtering

Zinc oxide (ZnO) is an attractive material because of its applicability for blue and ultraviolet (UV) light-emitting diodes (LED) and laser diodes (LD). However, in order to be used in these applications, the quantum well structure with ZnO acting as an active layer is inevitably required. Beryllium zinc oxide (BeZnO) is presently receiving attention as a new material satisfying such a requirement. For the quantum-well structure using BeZnO and ZnO, BeZnO can be utilized as the barrier layer while ZnO combined as a well layer. Such application also requires a technique of bandgap control on BeZnO. Fortunately, BeO is an oxide material with a hexagonal structure, similar to ZnO, and unlike MgO, which has a cubic structure. By modulating its Be composition, the bandgap energy can be tuned from 3.37 (ZnO) to 10.6 eV (BeO). There have been, however, few experimental studies on the bandgap engineering of this alloyed film. In this work, the alloyed BeZnO films with various Be concentration were grown on the c-Al₂O₃ substrate at 400 ^oC by using RF magnetron co-sputtering technique. Thus, the properties on the grown BeZnO films were investigated through X-ray diffraction (XRD), atomic force microscopy (AFM), and transmittance measurements. Based on these properties, we discussed bandgap engineering on the BeZnO. According to the X-ray diffraction (XRD) patterns results the peak position of the (0 0 2) plane is linearly shifted to the higher 2è value and the crystal quality of the films decrease with the increasing of Be concentration. With increasing Be concentration, the AFM analysis shows that the grain size as well as the roughness of the films constantly increases and the optical band gap of the films is in the range from 3.07 to 4.54 eV. These results show that the energy band gap of ZnO can be tuned to higher values by using BeZnO alloys, and that ZnO and BeZnO alloys are useful for designing and developing quantum wells and superlattices. ZnO/BeZnO heterostructures may be useful in developing new ZnO-based optoelectronic devices. This work supported by the Korea Research Foundation Grant funded by the Korean Government. (KRF-2008-005-J00302)

From the X-ray measurement, the intensity gradually decreases with increasing Be concentration and the (002) diffraction peaks shifted to higher angle. This indicates the quality of films decrease with increasing Be concentration due to increase mismatch between the film and the substrate. The peaks at 2è = 34.35^o, 34.47^o, 34.71^o, 35.03^o and 35.27^o correspond to the diffraction from the (0 0 0 2) plane of Be_xZn_1-xO films, depending on the Be composition. With increasing x, the optical bandgap energy ranged from 3.07 to 4.54 eV and a large bowing parameter of 6.32 eV was measured. Therefore, this finding may open up new possibilities for bandgap engineering using bandgap bowing. This indicates that the Be_xZn_1-xO layers can be used as potential barrier layers in active layers consisting of the ZnO/Be_xZn_1-xO quantum well structure through the bandgap engineering by alloying ZnO with Be.

Acknowledgements

This work supported by the Korea Research Foundation Grant funded by the Korean Government. (KRF-2008-005-J00302)