This is the accepted version of the article: Scarafagio M., Tallaire A., Tielrooij K.-J., Cano D., Grishin A., Chavanne M.-H., Koppens F.H.L., Ringuedé A., Cassir M., Serrano D., Goldner P., Ferrier A.. Ultrathin Eu- and Er-Doped Y2O3 Films with Optimized Optical Properties for Quantum Technologies. Journal of Physical Chemistry C, (2019). 123. : 13354 - . 10.1021/acs.jpcc.9b02597. Available at: https://dx.doi.org/10.1021/acs.jpcc.9b02597 Marion Scarafagio*a,e, Alexandre Tallairea, Klaas-Jan Tielrooijb,c, Daniel Canob, Andrey Grishina, Marie-Hélène Chavannea, Frank H. L. Koppensb,d, Armelle Ringuedéa, Michel Cassira, Diana Ser- ranoa, Philippe Goldnera and Alban Ferrier*a,e a Institut de Recherche de Chimie Paris (IRCP), Université PSL, Chimie ParisTech, CNRS, 75005 Paris, France b ICFO – Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, Castelldefels, Barcelona, Spain c Current affiliation: Catalan Institute of Nanoscience and Nanotechnology (ICN2), Barcelona Institute of Science and Technolo- gy, Campus UAB, Bellaterra, 08193 Barcelona, Spain d ICREA Institucio Catalana de Recerca I Estudis Avancats, 08010 Barcelona, Spain e Sorbonne Universités, Faculté des Sciences et Ingénierie, UFR 933, 75005 Paris, France ABSTRACT: Atomic Layer Deposited (ALD) Y2O3 thin films have been thoroughly investigated for optical or electronic applications. The coherent spectroscopy of lanthanide ions doped into this material has also recently attracted an increas- ing interest in the field of quantum technologies for which they are considered promising candidates in quantum memo- ries or as spin-photon interface. However, these most demanding applications require a deep control over the local posi- tioning of the ions and their close environment in the crystalline matrix. This study focuses on the structural as well as optical properties of Eu3+ and Er3+ dopants in Y2O3 using photoluminescence (PL), luminescence decay times and inhomo- geneous linewidth (Γinh) measurements within this particular context. While as-grown ALD films do not provide an ideal host for the emitters we demonstrate that by optimizing the deposition conditions and using appropriate annealing post treatments, narrow inhomogeneous lines can be obtained for the 7F0 ↔ 5D0 transition of Eu3+ even for nanoscale films. Furthermore, about 1.5 ms lifetime has been measured for the infrared telecom transition of Er in ultrathin films (< 10 nm) which is an order of magnitude higher than in nanoparticles of the same size. These results validate optimized rare-earth doped ALD Y2O3 films as a suitable platform for photonics applications where few nm thick films with well localized emit- ters are mandatory. This approach provides the first building blocks towards the development of more complex devices for quantum sensing or hybrid structures coupled to other systems such as 2D materials. ing. Indeed, their optical emission consists of narrow and well-resolved lines that stem from the shielded 4f-4f transi- Rare earth (RE) oxides represent a technologically useful tions that are only weakly perturbed by the crystalline envi- class of materials that can address a variety of applications 1,2,3 4,5 ronment. Thanks to this, RE doped oxide crystals are the such as photonics , protective coatings , laser media, 6 1,7 state-of-the-art materials for long storage time quantum catalysts , scintillators and phosphors. They have also been 13 14 memories , entanglement storage and quantum state tele- highly studied for microelectronics in which high permittivi- 15 portation between a telecom photon and a crystal. ty dielectric films could advantageously replace SiO2 as gate 8,9 While bulk crystals are preferably used for QT, a strong in- insulators. RE elements (Sc, Y and La-Lu) have similar 16,17 terest has also emerged for RE ions inside nanostructures. chemical properties and close atomic radius allowing easily 18 The availability of nanoscale particles or thin films, can substituting one to another in a crystalline matrix and giving open up many opportunities in this field. The coupling of access to a wide range of compositions as well as high doping these systems to resonators or hybrid structures as well as levels. the fabrication of waveguides and photonic structures is Recently, RE doped oxide crystals have attracted an in- 16,19 greatly facilitated in this form. The ability to develop creasing attention for quantum technologies (QT) that integrated quantum devices compatible with the silicon promise to overcome classical limits in a large range of appli- industry is also an important goal in the development of cations including communication, sensing, time stamping 20 10 QT. In addition, the thin film approach allows optimizing and computing. RE doped oxide crystals feature, at cryo- precursor consumption and gives access to compositions and genic temperatures, a combination of exceptionally long 11 12 phases that can hardly be reached by bulk growth from the optical and spin coherence times, e.g. 2.6 ms and 6 hours melt. However high crystallinity and purity films are required respectively together with a high PL stability without blink- 1 whereas surface effects play an important role in decoher- trated in Figure 1(b). The self-limited regime was obtained ence. for moderate temperatures of around 200-400 °C. Indeed, in RE:Y2O3 is currently the only nanosystem to exhibit long this temperature range the film thickness increases linearly 18,21,22 optical and spin coherent lifetimes. Furthermore, thin at about 0.21 Å per cycle. Furthermore, the linear behaviour films of Er:Y2O3 have for example been integrated into a new and the zero offset visible in Figure 1(b) indicate a low nucle- demonstrate control of the energy transfer from erbium ions ation activation energy. The films were deposited on (100) Si 23 into graphene. The ability to grow few nanometer thick wafers with an excellent thickness uniformity (better than films combining high crystalline quality, optimal optical 1 %) (Figure 1(c)). In order to improve the crystallinity of the properties and precise spatial localisation is a key point to films and the optical properties of the emitters, we investi- address to capitalize on these new hybrid devices. gated the process conditions and the use of a post-growth Several techniques have been used to grow Y2O3 as thin annealing treatment under O2 atmosphere (600 °C-1100 °C). 9 films on a variety of substrates including Pulsed Laser Depo- The crystallinity of the films was evaluated using X-ray 24 25 sition (PLD) , Physical Vapour Deposition (PVD) , Atomic Diffraction (XRD) in a Panalytical XPert Pro diffractometer 8,26,27,28 Layer Deposition (ALD) , Metalorganic Chemical Va- with an incident beam Ge monochromator, and a stainless 29 pour Deposition (MOCVD) or Molecular Beam Epitaxy steel sample holder. Film thickness was measured by white 30,31 (MBE). Among those, ALD possesses many assets for light interferometry in the range 250-1000 nm with an Ocean 34 depositing thin films with an accurate thickness control, the Optics NanoCalc system using Y2O3 optical constants. FEG possibility to vary the doping level as well as a good uni- Scanning Electron Microscope (FEG-SEM) images of the formity on standard silicon wafers or on any technologically surface of the films were recorded using a ZEISS Leo 15030 relevant material platform. While the ALD synthesis of Y2O3 microscope. Photoluminescence (PL) measurements were thin films is relatively well-known, the optical properties of performed in a Renishaw InVia micro-PL apparatus with a RE elements (such as Eu or Er) doped into this matrix have 50 × objective and a 532 nm laser as excitation source. Ca- yet not been thoroughly investigated within the context of thodoluminescence (CL) analysis was carried out at room 32,33 temperature in a Horiba HClue system attached to a conven- QT. In particular no lifetime or inhomogeneous linewidth of sub 50 nm RE:Y2O3 ALD films have been reported to our tional SEM (Zeiss EVO MA15) using a 10 kV electron accelera- knowledge. tion voltage and a 20 nA beam current. For the analysis we In study, we aim at demonstrating the potential of this ma- used a 600-groove grating with a 150 µm slit size for im- terial system as a platform for photonics in particular for very proved resolution. Decay measurements and time resolved low thicknesses (< 20 nm). We determine the optimized PL were also recorded using a tunable optical parametric deposition parameters but also the effect of post treatment oscillator pumped by a Nd:YAG Q-switched laser (Ekspla on the film crystallinity. Furthermore, the structural analysis NT342B-SH) with 6 ns pulse length, a Jobin-Yvon HR250 is correlated to optical properties in order to clarify how the monochromator and a photomultiplier tube. The detection deposition temperature affects the local environment of RE time constant was 20 µs and data was recorded on a digital ions. oscilloscope (Lecroy 332A). 7 5 In this paper we report for the first time narrow inhomo- Inhomogeneous linewidths of the F0 ↔ D0 transition were 7 5 measured in reflection mode in a helium bath cryostat (Janis geneous linewidth of the F0 ↔ D0 of Eu in sub 100 nm films. Furthermore we demonstrate that our strategy is versatile to CTI-Cryogenics model CCS-150, LakeShore model 330 tem- other lanthanide ions since we observe a significant en- perature controller) between 10 and 12 K. A low laser intensi- -2 hancement of the lifetime of Erbium telecom transition even ty (1 mWcm ) was used to avoid saturation of the transition for 10 nm thick films. and spectral hole burning. We performed fluorescence exci- tation (PLE) experiments by monitoring the red lumines- 5 7 cence of the D0 → F2 transition (around 612 nm) for various excitation wavelengths around 580.883 nm (516 098 GHz) provided by a Sirah Matisse DS laser, with a linewidth of 200 Eu and Er doped Y2O3 thin films were grown in a Picosun kHz. The red luminescence was filtered using at least 3 band Sunale R200 using conventional β-diketonate precursors: pass filters and sent to a high sensitivity photomultiplier Y(tmhd)3 and Eu(tmhd)3 / Er(tmhd)3. They are thermally (PMT R5108 Thorlabs). In order to improve the signal to stable and volatile and can be purchased with high purity noise ratio the detection was performed with a lock-in ampli- 3+ (99.9 % STREM Chemicals). They however require a strong fier. The lifetime of the 1.5 µm Er transition in ultra-thin 26 oxidizing agent that is provided by ozone (O3). To ensure films was characterized in a confocal microscopy setup, an efficient vaporization, the temperature of the tanks con- where we excited with 532 nm laser light and measured the taining the precursors was maintained at 160 °C for Y(tmhd) 3 emission at 1.5 µm using a single photon counter (ID Quan- and at 150 °C for the dopants throughout the deposition run. tique 230 NIR). A square wave modulation to the 532 nm The doping level of the oxide films was finely tuned by se- excitation laser is applied and time-correlated single photon quentially introducing pulses of the dopant elements in the counting using a PicoHarp 300 is performed. Each laser trig- standard yttrium oxide cycle. A typical ALD sequence con- ger event and each detected 1.5 µm photon event have been sisted of a 3 s Y(tmhd)3 pulse followed by a 3 s purging pulse recorded and used to construct a histogram during a time of N2; a 3 s ozone pulse also followed by a 3 s N2 purge (Fig- period of 10 minutes. The decay of the histogram after time ure 1(a)). For a theoretical doping level of 5 %, when consid- zero (when the laser switches off) represents the lifetime of ering an equivalent sticking coefficient for all precursors, 5 the erbium transition. ALD cycles of Eu(tmhd)3 or Er(tmhd)3 were introduced for every 95 ALD cycles of Y(tmhd)3. The final thickness was precisely controlled in the range 10-375 nm by the number of cycles based on the calculated growth rate per cycle as illus- 2 Figure 1 (a) Schematics of a typical ALD cycle used to grow RE-doped Y2O3 thin films, (b) Film thicknesses measured by white light interferometry as a function of cycle number. The red line is a linear fit. (c) Image of a 2-inch Si wafer covered by a uniform Y2O3 ALD coating and showing a mirror-like surface. to this limited annealing temperature but requires further study. Additionally, on Figure 2(c), we observed that the diffraction RE sesquioxides are polymorphic materials with at least 5 peaks are shifted towards higher angles with respect to the 35 different crystallographic structures. In the case of Y2O3, reference which suggests a smaller lattice parameter than for a bulk material and thus compressive strain. Despite the lack monoclinic and cubic phases are usually observed. The cubic one (space group Ia-3) presents two cation sites with the of epitaxy, strain could be induced by the difference of lattice parameter between Si and Y2O3 which is about 2.4 % be- local symmetry C2 and C3i whereas the monoclinic phase (space group C2/m) have three crystallographically distinct tween a double Si lattice cell and a single oxide cell. The 35,36 difference in thermal expansion coefficients (α) also induces cation sites, each having point group symmetry Cs. For QTs, the cubic phase is preferred because the longest optical strain upon heating and cooling since α is almost 3 times 5 7 coherence lifetime (T2) of the D0 → F0 optical transition of higher for Y2O3 than for Si. While annealing at a moderate Eu has been measured for ions inserted in the C2 sites of the temperature of 600 °C increased the shift towards higher cubic phase. angles (lower graph of Figure 2(c)), it is noticeable that at higher temperatures, the trend became opposite with the XRD analysis of the 5 % Eu doped Y 2O3 films annealed at different temperatures is presented in Figure 2 and compared diffraction angles still remaining higher than the reference (upper graph of Figure 2(c)). Both contributions (lattice with Y2O3 nanoparticles of the cubic phase obtained by ho- mogeneous precipitation and annealed at 1200 °C. The 37 parameter and thermal mismatch) should contribute to tensile strain which is not in agreement with the XRD result. agreement between the reference powder and the films dif- fraction peaks (Figure 2(a)) indicates that polycrystalline It is thus believed that additional competitive mechanisms related to the presence of impurities in the films are at work Y2O3 has been grown mostly in the cubic phase with unde- tectable contribution from the monoclinic phase. The rela- that could reduce the effective cell parameter. A moderate tive intensity of the different peaks with respect to the ran- temperature of 600 °C is enough to allow for the release of domly oriented nanopowders shows that diffraction from the contaminants such as carbon, hydrogen or other impurities (400) crystallographic planes is enhanced as compared to that can reach up to several % in ALD films grown with 38 RE(tmhd)3 precursors, thus possibly leading to a reduction that from (222), (440) and (622) planes indicating a strong (100) texturing. This is consistent with previous reports on of cell parameter. Further annealing at higher temperatures 38 has a beneficial effect on the crystallinity of the films and this Y2O3 films grown by ALD on silicon. The effect of annealing is particularly obvious when looking at the XRD pattern reorganization partially releases the accumulated stress. Further analysis of the chemical composition by XPS for without normalization (Figure 2(b)). The intensity of the (400) diffraction peak is increased from 600 to 800 °C sug- example would be needed to confirm this explanation. gesting an improvement of the crystalline quality. However, A direct consequence of the induced stress is the apparition above 950 °C, additional peaks (visualized by red arrows in of cracks as illustrated in the SEM images of Figure 3. Alt- Figure 2(a)) appeared and were attributed to the formation hough the as-deposited films always showed a smooth sur- face whatever their thickness, annealing at high tempera- of a silicate phase (such as Y2SiO5 or Y2Si2O7) due to the 39 reaction of the oxide with the silicon wafer. This sets an tures (> 800 °C) led to the appearance of a network of cracks extending along specific crystalline directions particularly for upper limit for the annealing temperature to allow for an improvement of the crystalline quality without formation of thicker layers (> 100 nm). This indicates again the presence of thermally induced stress and is another limitation to the parasitic phases. Deposition on other substrate materials or with a diffusion barrier could potentially provide a solution use of post-growth annealing treatments. However thinner of thermally induced stress and is another limitation to the use 3 3+ Figure 2 (a) XRD pattern normalized to the (400) diffraction peak, of Eu :Y2O3 films annealed at different temperatures. The 3+ films thickness is 300 nm. Eu :Y2O3 cubic phase nanoparticles produced by homogeneous precipitation are used as a reference. Silicate related peaks are highlighted by red arrows. (b) XRD pattern shown without normalization. (c) and (d) evolution of the (400) diffraction peak position with annealing temperature. * indicates XRD reflection from sample holder. of post-growth annealing treatments. However thinner oxide The films were analysed by standard luminescence char- 3+ films annealed up to 950 °C do not exhibit cracks or parasitic acterization in PL using a 532 nm laser to excite Eu ions to 5 phase formation which is encouraging for exploiting the the D1 manifold. As expected, the resulting spectra are dom- 5 7 optical properties of doped RE emitters as long as film thick- inated by emission from the D0 → F2 transition at 612 nm ness remains low enough. (Figure 5(a)) corresponding to ions in the C2 site of the cubic 37 structure of yttria. For as-grown films or films annealed at a low temperature, an additional contribution from the same transition but in a monoclinic lattice cell environment is Optical properties of RE oxides thin films or nanocrystals 42 visible at around 622 nm. When increasing the annealing can differ from their bulk counterparts since due to a high temperature, this emission tends to disappear while the lines surface to volume ratio, an important part of RE ions may be get narrower as judged by the linewidths (FWHM) reported submitted to a varying crystalline environment. In this study, 3+ in Figure 5(b). Although the emission remains broader by a Eu ions were used for structural investigation. Indeed, the 3+ 7 5 factor of about 2 than that of a reference Eu :Y2O3 ceramic narrow F0 → D0 absorption line enables to probe the disor- 40,41 with the same composition, a strong improvement of the der and thus the crystalline quality of the host material. 5 7 film crystallinity with annealing is confirmed. Moreover, we Moreover the main emission transitions D0 → F2, are forced 5 7 observe a clear improvement of the luminescence intensity electric dipole transitions (ED) whereas the D0 → F1 pre- 42,43 with the annealing temperature. Indeed an increase up to 10 sents a magnetic dipole (MD) character. Slight modifica- has been observed between films of the same batch as grown tions of the symmetry are thus revealed by looking at the and annealed at 950°C. At temperatures higher than 950 °C, ratio between those 2 transition types since ED are much PL spectrum of the Eu doped Y2O3 film shows new sets of more sensitive to a distortion of the local surroundings than 5 emission lines (Figure 5(c)). This is consistent with the para- MD. For a low symmetry site like C2 in cubic Y2O3, the D0 → 44 7 sitic silicate phase due to reaction with the Si substrate, F2 transition usually dominates the emission spectrum (Fig- previously observed in XRD. The drastic change in crystalline ure 4). 4 environment of the Eu emitters indeed leads to the appear- ance of strongly shifted emissions. Figure 3 SEM images of the surface of 100 nm thick films (a) and (b) and 375 nm thick films (c) and (d) before (left) and Figure 4 The main optical levels of europium ions, and the 5 7 7 5 after (right) high temperature annealing of 900°C. two main transitions D0 → F2 and F0 ↔ D0. CL analysis of the films allows assessing emissions in a wider spectral range due to the broad excitation energies of the incoming electron beam. As illustrated in Figure 5(d) we confirmed the improvement of crystalline quality at moder- Modifying the synthesis conditions was attempted in order ate temperature and the clear change in emission spectrum to push further the performance of our material. At first, we at temperatures above 950 °C. Additionally, a broad charge investigated a variation of O3 pulse duration between 3 and 9 transfer band which proceeds from transfer of an electron s (Figure 6(a)). The intensity of the XRD peak was highest for from the valence band to the 4f states of RE ions shows up in a 6 s long pulse but it shifted to even higher angles. For long- the UV range (250-350 nm) particularly at high annealing er pulse durations the (400) XRD peak was barely visible, temperatures. This is consistent with a change in the crystal- consistent with a decrease in crystallinity. Moreover, prefer- line environment due to the formation of a new phase and ential growth along the [111] direction occurred since the the appearance of defect levels in the band gap. Besides, on main XRD peak is now (222) (not presented here). A small the CL spectrum we observe an intense peak at 980 nm not and broad contribution also appeared in the bottom of the Si 3+ attributed to Eu ions transitions but possibly coming from peak at around 32.7° (Figure 6(a)). We attribute it to the 3+ Yb impurities. Although this element has not been previ- monoclinic phase as in the literature, the oxygen content has ously introduced in the reactor chamber, the ALD precursors been reported to strongly influence the monoclinic to cubic 45 have 3N purity (99.9 %) only due to the difficulty in separat- phase ratio. ing different RE elements. It is thus probably the origin of PL analysis confirmed these results. Indeed PL shows that this contamination. Inductively Coupled Plasma Mass Spec- shorter O3 pulse durations led to fluorescence emission from 3+ trometry (ICP-MS) of Y(tmhd)3 precursor diluted in ethanol Eu ions in a cubic environment (main peak at 612 nm) (not shown here) confirmed that significant trace impurities whereas longer O3 pulses confirm the appearance of the 3+ of Yb were present. This is a potential limitation for the use monoclinic phase with emission at around 622 nm associated of our thin films in QT since the presence of RE impurities with a broadening of the main peak. Therefore, short O3 with an electron or nuclear spin could induce unwanted pulses length of 3 s or 6 s seemed to be the most appropriate 3+ decoherence of close-by Eu ions. Synthesis with higher to obtain a good crystalline quality and to limit the deposi- purity precursors (4N or more) that are available but at a tion time. much higher cost could be needed. Secondly, lengthening the N2 purge time between each From this first structural and spectral analysis, it is found precursor pulse from 3 to 5 s was also attempted. The results that Eu:Y2O3 thin films with a controlled thickness and dop- shown in Figures 6(c) and 6(d) indicate that it did not im- ing can be obtained on Si by ALD and that their crystalline prove the film quality but instead led to a decrease of the and optical properties can be significantly improved by an- intensity of the (400) diffraction peak and a broadening of nealing at temperatures up to 950 °C. The formation of para- the main Eu emission. In fact, too long a purge pulse can lead sitic phases due to reaction with the silicon substrate and reaction with the oxidant which can explain that moderate stress leading to cracking for thicker films are limitations to purging times are required to maintain a high crystalline the use of higher temperatures. In addition, the purity of quality. commercial standard grade precursors is probably not suffi- Finally, the deposition temperature was also varied be- cient to achieve the highest purity required for quantum tween 250 and 400 °C, which are the limits of the ALD win- 38 applications. dow for this precursor. The results are presented in Figure 5 3+ Figure 5 (a) Room-temperature PL spectra of 100 nm-thick Y2O3 films doped with 5 % Eu showing the main Eu emission lines 5 7 in a cubic or monoclinic environment. A ceramic with the same composition is used as a reference. (b) FWHM of the D0  F2 transition at 612 nm plotted as a function of annealing temperature. (c) Comparison of PL spectra for films annealed at 900 and 1000 °C showing a drastic change in the ion environment. (d) Room-temperature CL over a wide spectral range for films an- 3+ nealed at different temperatures and in which Yb impurities are detected. 7. At 250 °C, the (400) XRD diffraction peak is barely visible, Figures 8(a) and (b) present the PL spectra recorded for 7 5 while its intensity increases when the temperature is in- different excitation wavelengths around the F0 → D2 transi- creased. In addition, PL emission is broad. This indicates that tion. As for Eu doped bulk cubic-phase yttria ceramics, the 5 7 a low deposition temperature leads to poor crystallinity. most intense PL signal at 612 nm ( D0 → F2) is observed for a 37 Although higher temperatures appear as more appropriate, resonant excitation wavelength of 465.2 nm. For other we also observe on the XRD spectrum that the films’ texture excitation wavelengths a significant variation of the PL line changed from (100) to (111) in agreement with a previous shape is also observed as illustrated on the normalized spec- 38 study. tra of Figure 8(b), which is not expected for ions in a single As a consequence the optimal deposition conditions to cubic phase. Indeed at shorter excitation wavelengths, emis- obtain higher quality oxide films were 3 s to 6 s O3 pulse, a 3 sion at 625 nm is enhanced as compared to that at 612 nm. 3+ s N2 purge and a deposition temperature between 300 °C and We attribute this change to the selective excitation of Eu 350 °C. ions in the monoclinic phase of yttria, which supports again the fact that a residual amount of unwanted phase is present in our films. This interpretation is also confirmed by the 5 7 noticeable blue shift of the D0 → F0 transition from 580 nm to about 578 nm for shorter excitation wavelengths, which is In order to analyse further the optical properties of RE ions consistent with values reported in the literature for the mon- 46,47 into our optimized Y2O3 ALD-grown thin films, we prepared oclinic phase. These emissions promoted by the 462.2 nm layers containing 5% Eu into a 100-nm thick film and an- excitation wavelength are however broad which indicates nealed them at different temperatures. Within the context of that the monoclinic phase is highly disordered and probably 46 QT it is essential that the lifetime of the emission of these present only as nanodomains. Figure 8(c) presents the ions is as long as possible and, additionally, the lines should variation of the normalized integrated PL in the range 570 to be narrow and weakly inhomogeneously broadened. 640 nm as a function of the excitation wavelength for differ- ent annealing temperatures (i.e. PLE). Although not required 6 Figure 6 Effect of the ozone pulse duration from 3 s to 9 s (a,b) and the N2 purge pulse duration (c, d) on the XRD pattern (left) 3+ and room-temperature PL emission spectra (right) of 100 nm-thick Y2O3 films doped with 5% Eu and annealed 2 h at 900 °C. High resolution optical linewidth measurements are a 3+ to optically activate Eu ions, annealing greatly improved powerfull tool for material characterization. According to the 3+ the PL performance of the films since the contribution due to literature on bulk Eu :Y2O3 ceramics, a single lorentzian with disorder observed under short excitation wavelength de- a line center around 516 098 GHz (580.883 nm in vacuum) is creased significantly. Indeed, at 950°C a decrease of the expected corresponding to the maximum resonant absorp- 5 7 48,49,50 monoclinic emission is observed. The annealing post treat- tion of the D0 → F0 transition. However, for films ment has thus a dual beneficial effect by both improving the annealed at temperatures below 800 °C, we only observed a crystalline quality of the cubic phase while also decreasing broad line (Peak B) centered at 518 200 GHz (578.52 nm the monoclinic phase proportion. vacuum) on Figure 9. This large frequency shift indicates 3+ 3+ To get a better idea of the properties of Eu ions doped that Eu ions are located in a significantly different envi- into our optimized Y2O3 thin film, we also performed tempo- ronment. The central frequency seems to be compatible with 3+ rally resolved measurements. The emission’s lifetime of the previous studies on Eu doped ultra-small monoclinic Y2O3 5 46 D0 level was estimated from the fluorescence decay present- nanocrystals and confirms our hypothesis. Indeed, no sig- ed in Figure 8(d) and reported in Table 1 for films annealed at nature of the 3 different sites is detected indicating a large different temperatures. For the as-grown films, we clearly disorder as observed for nanocrystals with crystallites-size 46 observed a non-exponential behaviour at the beginning of below 5 nm. Another argument in favor of a disordered the decay curve indicating a non-radiative de-excitation structure below 800°C is the asymmetric shape of the curve pathway. This fast component is probably due to the pres- with a longer tail in the high-energy side that prevents any fit ence of organic residues trapped in the film and due to the with a pure lorentzian or a gaussian model. Indeed, this poor crystallinity. After the annealing post-treatment, the peculiar lineshape has already been observed for Eu doped decay curves present an exponential behaviour and a signifi- into amorphous oxide glass and has been attributed to the 51 cant increase in the lifetime constant. In fact, fluorescence large variation of the crystal field parameter. lifetime reaches 0.8 ms, a value close to that measured for a For films annealed at temperatures above 800 °C, an addi- bulk transparent ceramic (0.9 ms) which is encouraging for tional narrow peak (Peak A) appeared at 516 150 GHz the foreseen application in QTs. (580.824 nm vacuum), i.e. much closer to the expected value 3+ of Eu ions in the C2 sites of cubic Y2O3. The relative peak 7 Figure 7 Effect of the ALD deposition temperature on the XRD pattern (a) and room-temperature PL emission spectra (b) of 100 3+ nm-thick Y2O3 films doped with 5% Eu and annealed 2 h at 900 °C. T1 (ms) coefficient mismatch between silicon and yttria. In our case, Samples a tensile strain is expected since Y2O3 film contracts more 5% Eu:Y2O3 transparent than the substrate when the temperature is raised. The influ- ceramic 50 0.9 ence of the thermal stress σT can be estimated by applying 54 the following relation : As-grown ALD film 0.4 ( ) (1) ( ) ALD film annealed at 600 °C 0.7 Where Ef, νf, αf represent the Young modulus, the Poisson's ratio and thermal expansion coefficients for the yttria film ALD film annealed at 700 °C 0.7 whereas αs is the thermal expansion coefficients for the sili- con (Ef = 150 GPa; νf = 0.3; αf = 7.5 ppm/°C and αs = 3.0 55,56 ppm/°C). ALD film annealed at 800 °C 0.7 From equation (1) a tensile stress at room temperature of about 0.9 GPa is expected for films annealed at 950°C. Li et ALD film annealed at 900 °C 0.8 al. have probed the variation of the luminescence spectra 3+ 41 from Eu doped Y2O3 nanotubes under high pressure. Ac- Table 1 Measured fluorescence lifetime of the main emission cording to their results a red shift of about 120 GHz per GPa at 612 nm for different post annealing treatment. is observed under compressive stress and a blue shift under tensile stress. The 100 GHz shift of the peak maximum ob- served for the film annealed at 950 °C (Table 2) thus fits well intensity increased significantly with the annealing tempera- with the calculated tensile stress induced by thermal mis- ture. In order to filter the excitation laser we used three match. spectral filters with different wavelength cut off between 610 and 630 nm. Those filters reduced significantly the 612 nm PL emission of the cubic phase in comparison to the main PL emission of the monoclinic phase (625 nm).The experimental curves can be fitted by two Lorentzian line shapes, which, in agreement with the Stoneham theory, indicate a broadening 52 by diluted point defects. The peak position and the Γinh extracted from the full width at half maximum (FWHM) of the curves are summarized in table 2 for both peaks A and B. The measured Γinh for peak A remains larger by about a factor of 2 in comparison to that extrapolated from transparent 53 ceramics of the same composition (100 GHz). This suggests that the linewidth is only partly due to the mismatch between Eu and Y ionic radii and that additional disorder, defect or impurities also contribute to it. A blue shift of the peak maximum of up to 100 GHz is also observed with an increase of the annealing temperature, possibly re- lated to the generation of thermal stress due to the thermal 8 5 7 Figure 8 Variation of PL intensity around the D0 → F2 emission as a function of excitation wavelength for a 100 nm thick 5 % Eu doped Y2O3 ALD film annealed at 950 °C. (a) Raw data, (b) normalized to the main peak at 612 nm. (c) Variation of the inte- grated PL signal in the range 570-640 nm as a function of the excitation wavelength and for different annealing temperatures. For better comparison the integrated PL has been normalized to that obtained for a 465 nm excitation. (d) Decay curves of the 5 5 7 luminescence from the D0 level for film annealed at different temperatures obtained by monitoring the D0 → F2 emission. nm. The variation of luminescence lifetime of the 1.5 μm 3+ Finally, we also studied the optical properties of RE ions in transition of Er ions doped in films of different thicknesses ultra-thin oxide films. The ability provided by the ALD tech- and annealed at 950 °C is presented in Figure 10. Time-decay nique to finely tune layers’ thickness down to the nanoscale measurements confirmed that the ms long lifetime of the as well as their spatial localization within this layer is indeed transition is preserved for low thicknesses down to 7 nm a particular asset for QTs in which ensembles of RE ions can reflecting the small impact of the surface. Indeed quasi ex- be addressed. ponential decays are observed with decay time constant of Y2O3 films were thus grown with 2 % Er to very low about 1.5 ms. These lifetime constants are shorter than the thicknesses of 7-11 nm by reducing the number of cycles. In 7.7 ms measured for a 1 % Er doped Y2O3 transparent ceramic 3+ this case Er ions were selected since they emit in the useful but remain remarkably long in comparison to the values 57 4 4 telecom band (1.5 μm, I13/2 → I15/2) and have been recently reported for 10 nm large nanocrystals (around 100 μs). successfully exploited in a hybrid device in association with 23 graphene. Besides their strong technological potential, they also exhibit a high sensitivity to hydroxides or other organic contaminants proving a sensitive probe to the local environ- ment in the film. Indeed, the small energy separation be- 4 4 tween the I13/2 and I15/2 levels is sensitive to non-radiative quenching by energy transfer to high frequency vibration of 57,58,59,60 acceptor levels (defects, molecular species etc.). ased on the F rster Dexter theory, the amount of ener- 3+ gy transferred from Er ions to hydroxyl molecules is limited by the distance with the surface and is negligible for distanc- 57 es around 2-3 nm, indicating that good luminescence prop- erties should be preserved even for thin layers of about 10 9 Peak A Peak B Samples Central frequency inh Central frequency inh (GHz) (GHz) (GHz) (GHz) 1% Eu:Y2O3 ceramic 516 098 24.2 - - 5% Eu:Y2O3 ceramic - 97 - - 53 (extrapolated) 5% Eu:Y2O3 ALD thin film 516 132  11 202  40 518 378  43 3608  157 annealed at 800°C 5% Eu:Y2O3 ALD thin film 516 182  3 240  10 518 211  27 2459  130 annealed at 900°C 5% Eu:Y2O3 ALD thin film 516 198  2 214  8 518 198  10 1982  110 annealed at 950°C Table 2 Central frequencies and inhomogenenous linewidth Γinh of peaks A and B reported in figure 8 as a function of the an- nealing temperature. The extrapolated value for a 5% Eu doped ceramic annealed at 1200°C is added for comparison. In summary, we have optimized the growth of Eu and Er doped Y2O3 thin films by ALD and evaluated their optical properties within the particular context of quantum technol- ogies. The possibility to produce this material system as thin films with a large range of compositions and doping indeed opens up new opportunities such as the coupling of emitters to resonators and hybrid structures as well as their spatial localization. The use of adapted growth conditions and post- growth thermal annealing confirmed that high quality ho- mogeneous films with narrow and well-resolved emission lines can be produced. Annealing temperatures on silicon were limited to about 950 °C in order to obtain the highest 7 5 Figure 9 Inhomogeneous absorption line of the F0 → D0 properties without the formation of parasitic silicate phases 3+ transition of Eu ions in Y2O3 ALD thin films for different or cracks on the surface. Contamination by other RE ele- annealing temperatures. Films are 168 nm thick and doped ments coming from the β-diketonate precursors were found with 5 % Eu. Measurement temperature is 10 K. Emission of and would probably require that higher purity or even spe- 5 7 the D0 → F2 transition is monitored at wavelengths above cially produced chemicals are used. As-grown ALD films 610 nm as a function of the laser excitation frequency to were mostly of the cubic phase with a small contribution 7 5 probe the F0 → D0 transition. The solid line is a fit of the from a highly disordered monoclinic component leading to a 3+ experimental points with two lorentzian line shapes. Curves different environment around the Eu ions. However, high have been vertically shifted for clarity. temperature annealing led to an almost complete disappear- ance of the monoclinic residual phase and an improvement of the crystalline quality. A detailed spectroscopic study of optimized 100 nm thick 5 7 films showed that optical lifetime (T1) of the main D0 → F2 3+ transition of Eu ions at 612 nm reached values of 0.8 ms close to that measured in transparent bulk ceramics. Besides the inhomogeneous linewidth Γinh of about 200 GHz was only a factor of 2 higher than that expected from the bulk. It is also believed that lineshift of the emission is essentially re- lated to the tensile stress of 1 GPa induced by the post- treatment. Finally we demonstrated that the 1.5 μm telecom 3+ emission of Er ions can be observed in sub 10 nm-thick film grown under optimized conditions and post treatment with a lifetime as high as 1.7 ms. The ability to synthesize films with such a low thickness is particularly relevant in the context of QTs where well- localized emitters are highly desirable. These optical proper- 3+ ties further establish RE :Y2O3 as a promising solid-state 3+ nanoscale platform. It corroborates recent results obtained Figure 10 Decay curves of the 1.5 μm fluorescence of Er ions (2%) doped into ultra-thin Y2O3 films annealed at 950 °C. with nanoparticles that included a long optical coherence 21,22,61 Open scatter: experimental points, solid line: single exponen- time (T2 > 10 μs) even for particles as small as 150 nm and tial fit. For clarity, curves have been vertically shifted. the possibility of all-optical coherent control of the spin 10 3+ 18 levels of Eu ions with ms-long coherence times. Future tween Photons and Spins in a Crystal. Phys. Rev. Lett. work will be dedicated to further harness the possibility of 2017, 118, 210501. integrating ALD thin films into cavities to facilitate the ma- (14) Clausen, C.; Usmani, I.; Bussières, F.; Sangouard, N.; nipulation of emitters at the single ion level. Afzelius, M.; de Riedmatten, H.; Gisin, N. Quantum stor- age of photonic entanglement in a crystal. Nature 2011, 469, 508-512. (15) Bussières, F.; Clausen, C.; Tiranov, A.; Korzh, B.; Verma, V. B.; Nam, S. W.; Marsili, F.; Ferrier, A.; Goldner, P.; Herrmann, H. et al. Quantum teleportation from a * E-mail: marion.scarafagio@chimieparistech.psl.eu; telacom-wavelength photon to a solid-state quantum alban.ferrier@chimieparistech.psl.eu memory Nature Photonics 2014, 8, 775. (16) Zhong, T.; Kindem, J. M.; Bartholomew, J. G.; Rochman, J.; Craiciu, I.; Miyazono, E.; Bettinelli, M.; Cavalli, E.; All authors have given approval to the final version of the Verma, V.; Nam, S. W. et al. 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