Energy dissipation channels affecting photoluminescence from resonantly excited Er3+ ions doped in epitaxial ZnO host films

Abstract

Investigation of ZnO thin films deposited on ferromagnetic metallic buffer layer by molecular beam epitaxy toward realization of ZnO-based magnetic tunneling junctions J. Appl. Phys. 113, 17C106 (2013); 10.1063/1.4794875Microstructural compositional, and optical characterization of GaN grown by metal organic vapor phase epitaxy on ZnO epilayersWe identified prerequisite conditions to obtain intense photoluminescence at 1.54 lm from Er 3þ ions doped in ZnO host crystals. The epitaxial ZnO:Er films were grown on sapphire C-plane substrates by sputtering, and Er 3þ ions were resonantly excited at a wavelength of 532 nm between energy levels of 4 I 15/2 and 2 H 11/2 . There is a threshold deposition temperature between 500 and 550 C, above which epitaxial ZnO films become free of miss-oriented domains. In this case, Er 3þ ions are outside ZnO crystallites, having the same c-axis lattice parameters as those of undoped ZnO crystals. The improved crystallinity was correlated with enhanced emissions peaking at 1538 nm. Further elevating the deposition temperature up to 650 C generated cracks in ZnO crystals to relax the lattice mismatch strains, and the emission intensities from cracked regions were three times as large as those from smooth regions. These results can be consistently explained if we assume that emission-active Er 3þ ions are those existing at grain boundaries and bonded to singlecrystalline ZnO crystallites. In contrast, ZnO:Er films deposited on a ZnO buffer layer exhibited very weak emissions because of their degraded crystallinity when most Er 3þ ions were accommodated into ZnO crystals. Optimizing the degree of oxidization of ZnO crystals is another important factor because reduced films suffer from non-radiative decay of excited states. The optimum Er content to obtain intense emissions was between 2 and 4 at. %. When 4 at. % was exceeded, the emission intensity was severely attenuated because of concentration quenching as well as the degradation in crystallinity. Precipitation of Er 2 O 3 crystals was clearly observed at 22 at. % for films deposited above 650 C. Minimizing the number of defects and impurities in ZnO crystals prevents energy dissipation, thus exclusively utilizing the excitation energy to emissions from Er 3þ ions. V C 2015 AIP Publishing LLC. [http://dx.

Atmosphere-Dependent Photoconductivity of ZnO in the Urbach Tail

“…The crystallinity of epitaxial films provides a near perfect crystal, but the maintenance of stoichiometry is still a challenge. At higher deposition rates, sputtering also provides suitable thin films, mainly polycrystalline, which the management of stoichiometry can be achieved by atmosphere control including oxygen [12,13]. Alternative techniques like sol-gel associated to dip-coating and spray pyrolysis can also be employed to thin film production avoiding the requirement of vacuum control and also producing polycrystalline films.…”

Electroluminescence from Silicon-Based Light-Emitting Devices with Erbium-Doped ZnO Films: Strong Enhancement Effect of Titanium Codoping

“…For the LED with the ZnO:Er film, as shown in Figure e,g, the Er 3+ ions are seriously segregated in the region (∼5 nm thick) near the ZnO/SiO 2 interface, with a peak concentration of ∼7%. Then, the luminescence from such segregated Er 3+ ions with a considerably high concentration is believed to suffer from a serious concentration quenching effect, according to the results of previous reports. , Moreover, the hot electrons lose their energies significantly due to the impact on numerous Er 3+ ions in the segregated region near the ZnO/SiO 2 interface. In other words, the hot electrons which can be transported to the interior of the ZnO:Er film are dramatically decreased, resulting in much less excited Er 3+ ions.…”

“…Herein, the interior of the ZnO:Er film primarily refers to the region with the width equivalent to the mean free path of hot electrons in ZnO, which may be tentatively estimated to be ∼30 nm, referring to the Monte Carlo calculation results . Regarding the LED with the ZnO:(Ti, Er) film, as displayed in Figure f,h, the Er-segregation near the ZnO/SiO 2 interface is quite small with a peak concentration of ∼2%, which will not result in the concentration quenching effect. , Moreover, as elucidated above, a little bit more Er 3+ ions are incorporated into the ZnO matrix. Under the circumstances, the quenching of Er 3+ -related emissions and the significant loss of energy of the hot electrons hardly occur near the ZnO/SiO 2 interface within the LED with the ZnO:(Ti, Er) film.…”

Photoluminescence of Erbium-Doped ZnO Nanostructures

“…The DLE band was found to be a broad and asymmetric photoluminescence band. H. Akazawa et al [31] identified the conditions to obtain strong photoluminescence intensity in a matrix of ZnO crystals doped with Er ions. The samples were synthesized using the sputtering method and heat treated.…”