Achieving highly-enhanced UV photoluminescence and its origin in ZnO nanocrystalline films

Abstract

a b s t r a c tZnO is an efficient luminescent material in the UV-range~3.4 eV with a wide range of applications in optical technologies. Sputtering is a cost-effective and relatively straightforward growth technique for ZnO films; however, most as-grown films are observed to contain intrinsic defects which can significantly diminish the desirable UV-emission. In this research the defect dynamics and optical properties of ZnO sputtered films were studied via post-growth annealing in Ar or O 2 ambient, with X-ray diffraction (XRD), imaging, transmission and Urbach analysis, Raman scattering, and photoluminescence (PL). The imaging, XRD, Raman and Urbach analyses indicate significant improvement in crystal morphology and band-edge characteristics upon annealing, which is nearly independent of the annealing environment. The native defects specific to the as-grown films, which were analyzed via PL, are assigned to Zn i related centers that luminesce at 2.8 eV. Their presence is attributed to the nature of the sputtering growth technique, which supports Zn-rich growth conditions. After annealing, in either environment the 2.8 eV center diminished accompanied by morphology improvement, and the desirable UV-PL significantly increased. The O 2 ambient was found to introduce nominal O i centers while the Ar ambient was found to be the ideal environment for the enhancement of the UV-light emission: an enhancement of~40 times was achieved. The increase in the UV-PL is attributed to the reduction of Zn i -related defects, the presence of which in ZnO provides a competing route to the UV emission. Also, the effect of the annealing was to decrease the compressive stress in the films. Finally, the dominant UV-PL at the cold temperature regime is attributed to luminescent centers not associated with the usual excitons of ZnO, but rather to structural defects.

UV-luminescent MgZnO semiconductor alloys: nanostructure and optical properties

“…Both samples have similar granular morphology. The UV-PL of the ZnO shows the expected emissions of the excitons and the e-emission which is characteristic to nanocrystalline-type ZnO studied previously in our papers [4,39]. The key issue is that in contrast to the ZnO, the PL of the MgZnO is featureless and broad with a linewidth *88 meV, This implies that even at the relatively low Mg composition *17 % the impact of the inherent alloy random distribution is quite significant.…”

“…8 The 77 K Raman spectra of the LO mode of the as-grown alloy and that of the 900°C annealed sample. The *9 cm -1 Raman shift between the two spectra is attributed to stress relaxation initiated by the high-temperature annealing ZnO nanocrystalline films grown via the sputtering technique, Raman analysis has indicated a similar relaxation upon annealing of *1 GPa [39]. Thus, the compressive stress in our as-grown MgZnO sample can be attributed mainly to morphology, and to some extent to defect aggregations, both of which were significantly reduced during the annealing process.…”

“…The UV-PL properties of the 900°C is significantly improved: it has enhanced intensity *4 times that of the 750°C annealed film, and a narrower linewidth *163 meV to be relatively low [35]. Alternatively, the mobility temperature of these defects, which was previously found to be *170 K [38], is significantly lower than our annealing temperature range at 1173 and 1023 K (for further discussion on the stability of Zn i in our samples see [39]). In light of these properties, it is reasonable to expect that Zn i related defects should be eliminated by out-diffusion as well as via incorporation into substitution lattice sites, thus diminishing the 2.3 eV PL emission.…”

“…In has been previously demonstrated that the UV-PL of ZnO can be profoundly enhanced via annealing [39]. Less work has been done regarding this topic for MgZnO alloys, specifically for sputtered alloys [40][41][42].…”

Photoluminescence enhancement of ZnO via coupling with surface plasmons on Al thin films

“…9 A major asset of ZnO is that its Photoluminescence (PL) spectra consist of two emission components: a narrow UltraViolet (UV) peak at around 380 nm, which is correlated with the near band-edge (NBE) exciton emission, 10 and a broad visible (VIS) peak spanning from 450 to 750 nm, which is associated with deep level emission (DLE) from intrinsic and extrinsic defects. [11][12][13][14] The fabrication of high-quality ZnO films with strong UV or VIS emission is a challenging task; a high-temperature deposition or/and a post-deposition annealing scheme is usually required 15,16 with thermal annealing 17 and rapid thermal annealing, 18 which however are not compatible with temperature sensitive underlayers, such as thin metal layers. Consequently, laser annealing 19,20 (LA), being a cold process, is a favourable alternative for processing such structures.…”