Band Gap Engineering of ZnO by Alloying of MgO, BeO and TiO2 for UV Sensor Applications
Liriano Rosario, José M.
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ZnO-based alloys from II-VI semiconductors are considered to be a promising alternative to III-V nitrides for optoelectronic devices in the ultraviolet range as an alternative to the GaN material system. ZnO is a hard material, has a large exciton binding energy (60meV), is abundant and of low cost. The band gap engineering of ZnO films has been achieved by alloying it with much wider band-gap semiconductors to eliminate interference in the visible range and improve its electrical properties. The goal in the present dissertation was to synthesize ZnO, ZnMgO, ZnBeMgO and ZnTiMgO alloys in thin-film form using RF sputtering, chemical solution deposition and pulsed laser deposition techniques and to then study the structural, optical and electrical properties of these thin films for potential applications in new optoelectronic devices. The ZnO, ZnMgO, ZnBeMgO and ZnTiMgO thin films deposited are highly oriented along the c-axis, in the (0002) direction, without any segregation phase corresponding to MgO, BeO or TiO2. We also observed a shift to higher (Be+2/Mg+2) or lower (Ti+4) 2𝜃 values in the XRD data, suggesting the substitution of Be+2, Mg+2 and Ti+4 ions at the Zn+2 sites of the ZnO host lattice. The photoluminescence (PL) measurements carried out on ZnMgO suggested that the peaks occurring in the UV region are due to band to band emission whereas the peaks in the visible region can be attributed to oxygen related defects. Increasing the Mg concentration, the band-to-band transition emission peak intensity decreases systematically and almost disappears, indicating that non-radiative iii recombination processes are dominating. From Raman, an additional mode around 556cm−1 in the Be/Mg co-doped films can be assigned to the quasi-longitudinal-optical (LO) phonon mode, due to abundant shallow donor defects, such as zinc interstitials (Zni) and/or oxygen vacancies (VO), bound at the tetrahedral Mg/Be sites. The existence of the quasi-LO phonon mode manifests in the incorporation of Mg/Be in the ZnO lattice as its intensity increases with an increase in dopant concentration. The band-gap energy of Mg, Be and Ti co-doped ZnO thin films was enlarged with increasing dopant concentration. By changing Mg, Be and Ti content and utilizing different deposition techniques, the band gap of ZnMgO, ZnBeMgO and ZnTiMgO thin films can be tuned from 3.37 eV to 4.51 (Zn0.70Be0.10Mg0.20O by PLD), 3.84 eV (Zn0.60Be0.10Mg0.30O by spin coating) and 4.08 eV (Zn0.76Ti0.04Mg0.20O by PLD). From the I-V photoresponse measurements performed on ZnO, ZnMgO, ZnBeMgO and ZnTiMgO thin films under dark conditions and UV illumination, a larger response was shown under UV light at three orders of magnitude higher than those under dark conditions. These increases in band gap, to the Ultraviolet B region and the larger photocurrent under UV light, suggest that these ZnO alloys compounds are suitable for the fabrication of UV detectors.