Photoluminescence of atomic layer deposited ZrO2:Dy3 + thin films
Introduction
High-quality thin dielectric films are of great importance for many modern applications, such as active waveguides [1], nanophosphors [2], [3], high-k dielectrics for transistors [4], [5], and memory applications [6], just to name a few. Among the variety of thin film deposition techniques, atomic layer deposition (ALD) has the advantage of producing uniform conformal coatings on large-area surfaces due to a self-terminating nature of the alternating, complementary surface reactions [7]. In a number of applications, such thin films doped with different impurities are needed whereas rare earth (RE) impurities which are frequently used for tailoring the magnetic or luminescent properties of materials or controlling the phase stability are of particular importance. During the last decade, a number of works on ALD of metal oxides with RE doping have been published. Many of those address the stabilization of high-permittivity ZrO2 and HfO2 phases [8]. Some works have described also luminescence properties of RE-doped ALD-grown films, such as Y2O3:Er3 + [9], Al2O3:Er3 + [10] and SiO2:Tb3 + [11]. The basic idea in the ALD of doped oxides is to apply cyclically both host metal and impurity precursors. Sometimes analogous strategy has been applied also in the case of other deposition techniques, such as e-beam evaporation [12]. In order to achieve lower average impurity concentrations, one may need to deposit alternating layers of thinner impurity-rich and thicker pure oxide. Therefore it is necessary that either active impurity centers in a suitable concentration are formed already during deposition or appropriate spatial (re-)distribution of active dopants can be obtained at least during the subsequent high-temperature annealing of the layered structure. Alternative, more elaborate approach, suitable for obtaining small impurity concentrations, is the implantation of RE ions into nominally pure ALD oxide film [13], [14].
In this work, we describe preparation of luminescent ZrO2:Dy thin films by the ALD-method and report their structure-dependent photoluminescence (PL) properties. Due to its spectral properties (two almost equally strong emission bands around 490 and 590 nm), the luminescence of Dy3 + is promising for application in economic white light sources [15], [16], [18] and new solid state lasers operating in the blue or yellow spectral region [19]. On the other hand, zirconia with its low phonon frequencies and endurance against environmental influences is an attractive host for RE emitters, if compared to some traditional wide-gap matrices, such as SiO2 or Al2O3. In addition, RE dopants can contribute to the stabilization of the metastable ZrO2 phases, similarly to yttrium [20]. In these metastable phases the solubility of RE dopants is higher. Moreover, the selection of the host crystalline phase may allow some spectral tuning of the RE3 + emission bands. Although the PL emission of Dy3 + in ZrO2 host (prepared by using sol-gel or co-precipitation techniques) has been reported in several cases [16], [17], the crystal field splitting of Dy3 + transitions caused by ZrO2 surrounding has not been observed, either due to amorphous nature of samples or instrumental limitations.
Section snippets
Experimental details
The ZrO2:Dy films were grown from ZrCl4 and Dy(thd)3 (thd = 2,2,6,6-tetramethyl-3,5-heptanedionato) as metal precursors and H2O as an oxygen source on p-Si (100) substrates in a flow-type low-pressure ALD reactor at 300 °C [21]. ZrCl4, Dy(thd)3 and H2O were volatilized at 170, 160 and 23 °C, respectively, while the films were deposited as stacks of alternating layers of undoped ZrO2 with a thickness of 1.5–2.7 nm and ternary Zr1 -xDyxOy with a thickness of 0.5–0.8 nm. An ALD cycle repeated for
Structural characterization
As shown in Section 3.2, annealing of as-grown samples at temperatures up to 900 °C was necessary to obtain reasonably strong Dy3 + emission. For this reason, the structural characterization was performed on as-grown samples and on samples annealed at 900 °C. Two sets of samples, S1 and S2, were studied in this way. A film thickness of 80 nm and refractive index of 2.06 at 633 nm were obtained from spectroscopic ellipsometry studies for as-grown samples S1. Respective values for samples S2 were 137
Conclusions
The results of this study demonstrate that Dy-doped ZrO2 films for photoluminescence emitters can be prepared using atomic layer deposition route, which is based on ZrCl4, Dy(thd)3 and H2O precursors, and is followed by annealing at 900 °C.
For application perspective, it may be important that the selection of the ZrO2 crystal structure allowed considerable adjustment of the Dy3 + emission spectrum. Moreover, Dy3 + luminescent probe demonstrated a better performance, compared to Raman-scattering,
Acknowledgments
This work was partially supported by the Estonian Research Council (projects PUT170 and ETF9283), Estonian Ministry of Education and Research (projects IUT2-24 and IUT34-27) and European Regional Development Fund (Centre of Excellence “Mesosystems: Theory and Applications”, TK114). The authors are grateful to Dr. Peeter Ritslaid for XRF measurements and Dr. Kaupo Kukli for fruitful discussions.
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