Er-doped ZnO Films Research Articles

Luminescence of Er-doped ZnO Films: Effects of Thermal Annealing and Doping Concentration

AbstractPhotoluminescence (PL) of erbium-doped zinc oxide films with nano-sized grains was studied. The films were grown on silicon (100) and fused silica substrates using e-beam evaporation. The evaporating targets used were sintered pellets of ZnO and Er2O3 mixtures with two different Er concentrations. The films were subsequently annealed at 700 °C in air for an hour. PL was measured at two excitation wavelengths, 325 and 488 nm. The 325 nm is used for exciting the host semiconductor ZnO and 488 nm is used for directly exciting Er3+ ions in the ZnO host. Strong Er3+ luminescence of 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions was observed from annealed film with 4.0 % Er2O3 concentration using either 325 or 488 nm excitation. With 325 nm excitation, the Er3+ luminescence observed is attributed to energy transfer from the excitons in ZnO host to the Er3+ ions doped. The effective energy transfer from ZnO host to the doped Er3+ ions is an essential property for the realization of actual current-injection opto-electronic devices operating at wavelengths of Er3+ emission, for example, at 1.54 μm for the erbium-doped fiber amplifier (EDFA). Our PL results indicate that thermal annealing plays an important role for optically activating the doped Er3+ ions in ZnO nano-crystalline grains of the film.

Photoinduced piezo-optical effect in Er doped ZnO films

A large increase of diagonal piezo-optic coefficient (up to 3.7×10−13m2∕N at λ=633nm) was observed in Er doped ZnO films under photoinduced treatment by two laser beams with frequencies of 1,060 and 530nm originating from the same Nd:YAG pulsed laser. It was demonstrated that piezo-optic effect is enhanced with increasing film thickness. This may be a consequence of the disturbance of the long range ordering in the nanocrystallites for the film with larger film thickness. The observed effect is explained due to Er ions creating additional local polarizability.

Luminescent spectroscopy and imaging of textured sprayed Er-doped ZnO films in the near ultraviolet and visible regions

The textural dependence of the thermally sprayed Er-doped ZnO films on Er concentration is reported here. The [002] preferred growth orientation of hexagonal phase is obtained at the lower concentrations, while the [100] and [101] directions additionally raised from nearly 5at.% Er content onwards. The cathodoluminescent characteristics of the samples in the near ultraviolet (UV) and visible region depict a complete extinction of the visible emitted bands (λ=445, 526, and 665nm) at 1at.% Er content. Their deactivation below this concentration is explained by a compensation of oxygen defects in the material due to the oxygen-rich medium of the deposition bath. Their reactivation beyond this particular concentration is ascribed to the increase of the Er+3 ion shells whose internal radiative transitions lead to a recovering of these visible emitted bands. The radiative mechanism of the transitions from the (F9∕24) excited states to the (I15∕24) ground state, responsible for the λ=665nm emission, is predominant in that case. The respective normalized intensity of the violet (λ=445nm) and green (λ=526nm) emitted bands exhibits a maximal value for 3at.% Er content, reaching a stabilized regime from about 5at.% Er.

Structure Analyses and Electrical Properties of Er-Doped ZnO Thin Films

Er-doped ZnO thin films which emitted intense infrared light in the vicinity of 1.5 μm were investigated from points of view of the microstructure and electrical properties. The result of X-ray diffraction (XRD) pattern revealed that the crystal lattice of ZnO was apparently expanded by doping of Er ions. Electrical resistance in the direction of thickness of Er-doped ZnO film showed linear behavior, which was resemble to that of undoped ZnO film. Infrared light emission phenomenon of the film was related to the chemical / physical state of Er ions in ZnO matrix.

Luminescent Characteristics of Undoped and Er-Doped ZnO Thin Films

In order to clarify the fundamental luminescent mechanism of undoped and Er-doped ZnO thin films synthesized by sputtering method, cathodoluminescence (CL) from the samples formed on several kinds of substrate were measured. There was no explicit peak identified with luminescence from ZnO crystal defects in undoped sample, on the contrary, three sharp luminescent peaks were observed in the case of Er-doped ZnO film due to the internal transition of the additive Er ions in the CL spectrum. The mechanism was investigated in comparison with photoluminescence (PL).

Influence of Annealing on the 1.5 μm Light Emission of Er-doped ZnO Thin Films and its Crystal Quality

Intensity variation of 1.5 μm light emission at room temperature from Er-doped epitaxial and polycrystal ZnO films depending on annealing temperature (773–1373 K) was studied. As-grown Er-doped epitaxial ZnO film emitted 1.5 μm photoluminescence(PL) higher than as-grown Er-doped polycrystal ZnO. It was found that the annealing in air increases PL intensity and the maximum PL intensity was obtained by annealing at optimal temperature (1073 K). Spectrum shape and intensity of 1.5 μm PL of Er-doped epitaxial ZnO after annealing at 1073 K resembled those of Er-doped polycrystal ZnO after annealing at 1073 K. X-ray diffraction measurement demonstrated that annealing improves crystal quality of Er-doped ZnO film. We assumed that the process of 1.5 μm light emission is dependent on local area placement of Zn and O atoms around Er as well as crystal quality of ZnO.

Sensing characteristics of ZnO-based NO x sensor

Sensing characteristics of ZnO-based sputtered films are studied. The film about 140 nm thick showed high sensitivity to NO x and selectivity against H 2 and CO. The sputtering pressure greatly affected the sensitivity, and the sensitivity maximum was observed at 0.67 Pa. The effect of element doping into ZnO on sensitivity and selectivity to NO x was also studied using co-sputtering technique. Rare earth element doping improved sensitivity and selectivity to NO 2 against NO. Er-doped ZnO film had good selectivity and fast response time to NO 2 above 200°C, and almost linear sensitivity dependence in the sub-ppm concentration range of NO 2.