Photoluminescence of atomic layer deposited ZrO₂:Dy^3 + thin films

Highlights

• Atomic layer deposition of luminescent Dy-doped ZrO₂ thin films was demonstrated.

• Dy^3 + luminescence was significantly activated only after high-temperature annealing.

• Correlation between luminescent and structural properties was obtained.

• Dy^3 + luminescent probe showed superior performance compared to Raman-scattering.

• Presence of several quenching processes was deduced from luminescence behavior.

Abstract

Atomic layer deposition based on alternate cycling of ZrCl₄, Dy(thd)₃ and H₂O as precursors was applied for preparation of nanocrystalline ZrO₂:Dy thin films. Photoluminescence (PL) properties of Dy^3 + in the ZrO₂ films were studied at several laser excitations. Substantial activation of Dy^3 + PL required thermal treatment at 900 °C. As a result of annealing, thinner (~ 80 nm) films with higher Dy content retained relatively high amount of tetragonal phase and remained crack-free. In thicker (~ 140 nm) films, considerable amount of monoclinic phase was formed and a peculiar microscale cracking pattern was developed along with phase segregation. It is demonstrated that the crystal structure of ZrO₂ significantly influences the Dy^3 + emission spectrum and, at least for ZrO₂-type matrices, Dy^3 + is an excellent luminescent microprobe in comparison with micro-Raman scattering. A Förster-like PL decay profile allowed a conclusion that the self-quenching due to cross-relaxation between Dy^3 + ions had a marked impact on emission intensity.

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:Er^3 + [9], Al₂O₃:Er^3 + [10] and SiO₂:Tb^3 + [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 Dy^3 + 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 RE^3 + emission bands. Although the PL emission of Dy^3 + 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 Dy^3 + transitions caused by ZrO₂ surrounding has not been observed, either due to amorphous nature of samples or instrumental limitations.