Invited Paper TF-TuE1
Sputtering Growth of ZnO-based Semiconductors with Band Gap Tunability Over the Entire Visible Spectrum

Tuesday, December 9, 2014, 5:40 pm, Room Makai

ZnO is a remarkable multi-functional material with a distinctive property set and a huge range of existing applications. Recently, some research has been directed towards development of ZnO based materials with smaller band gap (Eg), which allow light emission/absorption over a broad spectrum from the UV to the visible region. One of the most promising means to narrow the band gap is to alloy ZnO with CdO that has small direct band gap of 2.3 eV. However, there are technological challenges for the growth of high-cadmium-content CdZnO because of the crystal phase segregation between wurtzite ZnO and rocksalt CdO.

In this context, we have developed a new compound, ZnInON (ZION), which is a pseudo-binary alloy of ZnO with InN. Since both ZnO and InN have the same wurtzite crystal structure and different band gaps (ZnO: 3.4 eV; InN: 0.7 eV), the alloy may possess band-gap tunability in a wide range. Here, we first demonstrate the sputtering synthesis of ZION and the band gap tuning by changing the chemical composition ratio. The detailed analysis on the crystal structure as well as the photo-electric properties of this new material are then conducted. Finally, we discuss the advantages of ZION as light-absorbing layers of multi-quantum well (MQW) solar cells, the theoretical efficiency of which exceeds 50%.

From the analyses of wide-range reciprocal space mapping of XRD spectra, we found that ZION has the same crystal structure as wurtzite ZnO and InN. The lattice constant of ZION is in between ZnO and InN, and decreases continuously with decreasing [Zn]/([Zn]+[In]), which indicate that ZION is a pseudo-binary system of ZnO and InN. The transmittance spectra reveal that the optical absorption edge is shifted continuously toward longer wavelengths with decreasing [Zn]/([In]+[Zn]). The band gap of ZION films, being determined by Tauc's plot, changes from 3.4 eV to 1.5 eV, which covers entire visible spectrum. The analyses of transmittance spectra indicate that ZION is of direct band gap, and thus it has high absorption coefficient around 10⁵ cm^-1. Furthermore, we found significant advantages of ZION in terms of applications in multi-quantum-well (MQW) solar cells. The lifetime of photo-generated carriers in QWs is increased by a few orders of magnitude when ZION is used as a MQW material instead of conventional GaAs-based materials, owing to the large piezoelectric field (several MV×cm^-1) that separate electrons and holes. These results indicate that ZION is a promising material that opens new pathways for realizing new generation solar cells with very high efficiency.