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Abstract
Epitaxial growth of erbium-doped cerium oxide (Er:CeO2) is achieved on Si (111) substrates by the cooperative integration of Si and oxide molecular beam epitaxy (MBE) technologies. Lattice matching between CeO2 and Si provides an attractive opportunity to build dilutely doped Er-based optical devices on a Si chip. The CeO2 host crystal is optically transparent for the telecom C-band wavelength and has quite a small magnetic moment, which serves as a disturbance-free environment for the two-level system formed in the doped Er. After the systematic optimization of the growth conditions for stoichiometric Er:CeO2; i.e., (Er + Ce)/O = 1/2, we varied the Er concentration in a range of 1 ~4%. The doped Er showed well-defined optical transitions at the wavelength of 1.533 μm irrespective of the Er concentration. With decreasing Er concentration, enhancement of the luminescence intensity, narrowing of the spectral width, and an increase in the radiative lifetime were observed. The results suggest that CeO2 on Si is promising as a platform for the doped-Er-based optical devices and their quantum optics applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Cerium oxide (CeO2, ceria) has been a key material for a wide range of applications. Examples include its use as a chemical–mechanical polishing agent for microelectronic device wafers and optical materials [1] and in oxygen storage/release devices utilizing the redox reaction in non-stoichiometric CeO2-x [2–4]. In microelectronics, CeO2 as a gate dielectronic material could exceed the performance of traditional SiO2 [5–9]. This is because CeO2 has a high dielectric constant (k > 26) and large bandgap (~6 eV). In addition, CeO2 takes the cubic fluorite structure with lattice matching to the Si (111) surface (Δa0/a0 ~0.36%). In optoelectronic devices, CeO2 is promising as a transparent host crystal for photons emitted from other lanthanoid elements embedded in it. In fact, erbium-doped CeO2 (Er:CeO2) metal-oxide-semiconductor (MOS) devices with low driving voltages have been studied vigorously as photon emitters on the Si platform [10–13]. However, those studies on optoelectronic devices were carried out by using polycrystalline [10,11] or nanoparticle Er:CeO2 [12,13], while epitaxial CeO2 has been exploited in the field of microelectronics [5–7].
For quantum optics applications, CeO2 is expected to be advantageous as a host crystal for the two-level system formed in doped lanthanoids such as Er because it can serve as a magnetically purified host crystal. A major source of population decoherence in the two-level system is a fluctuation of the magnetic moment around it [14–16]. This fluctuation is mainly induced by nuclear/electron spin moments of the constituent elements of the host crystal. Cerium is the only element among the lanthanoids whose stable isotopes all have a zero nuclear spin moment. As for oxygen, only 17O (abundance of 0.04%) has a nuclear spin moment (−1.894μN). Therefore, it is expected that the intrinsic coherence time of the two-level system is preserved in the CeO2 host crystal.
The present study focuses on the potential application of Er:CeO2 to quantum optical devices, and we start this article with the epitaxial growth of Er:CeO2 on Si(111) substrate. After optimization of the growth conditions for stoichiometric Er:CeO2; i.e., (Er + Ce)/O = 1/2, we varied the Er concentration in the range of 1 - 4%. Characterization of the film structure by scanning transmission electron microscopy (STEM) revealed that the entire Er:CeO2 layer is single crystal with the cubic fluorite structure except for the interface region, where a-few-nanometer-thick amorphous layer is formed. The doped Er showed well-defined optical transitions at the wavelength of 1.533 μm, irrespective of the Er concentration. The Er concentration dependences of the luminescence intensity, the spectral width, and the radiative lifetime will be discussed in the context of Er-Er interactions in the CeO2 host crystal.
2. Experiments
After standard wet- and thermal-cleaning of Si substrates, single-crystal Er:CeO2 films (30-nm-thick) were grown on Si (111) surfaces with 7 × 7 reconstruction by molecular beam epitaxy (MBE) at a growth temperature of 640°C. Erbium and Ce fluxes were generated by using e-gun evaporators: purities of the metal sources are 99.997% and 99.993% for Er and Ce, respectively. RF activated oxygen (O*) was simultaneously supplied with an O2 flow rate of 0.1 - 0.3 sccm, and the resultant chamber pressure was ~10−6 Pa during the growth. The deposition rate of 0.01Å/s for Er was controlled by electron impact emission spectroscopy (EIES) [17], and that of 0.03 - 1.00 Å/s for Ce was regulated by the quartz crystal microbalance (QCM). The growth surface was monitored in real time by reflection high-energy electron diffraction (RHEED) with an acceleration voltage of 20 kV.
For chemical and structural characterization after the growth, we performed X-ray photoelectron spectroscopy (XPS), Rutherford back scattering (RBS), X-ray diffraction (XRD), and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) measurements. The XPS measurements were carried out ex-situ by using monochromatic Al Kα (1486.6 eV) line as X-ray source (operated at 200 W) and detection angle of 45 degree from a sample normal. We use Cu Kα1 (λ = 1.5406 Å) for XRD 2θ−ω scan. The RBS measurements were performed with the incident 4He++ ions accelerated at 2.275 MeV with normal (160°) and grazing (100°) detection angles. The STEM measurements were carried out using 200-kV-accelerated electrons with an aberration-corrected JEOL JEM-ARM200F microscope.
Moreover, optical properties of the doped Er ions in the CeO2 host crystal were investigated by photoluminescence (PL), photoluminescence excitation (PLE), and time-resolved PL (TR-PL) spectroscopies at 4 K. The excitation source was a continuous-wave tunable laser (1500 – 1520 nm with spectral width of 400 kHz). The excitation laser was focused to a spot with a ~3-μm diameter through an objective lens with a numerical aperture of 0.40. The PL signals were collected by the same lens and detected with a spectrometer equipped with a liquid-nitrogen-cooled InGaAs linear array (resolution of 80 μeV) for spectral-domain measurements and a superconducting single photon detector (timing jitter of 25 ps) for time-domain measurements.
3. Results and discussion
First, to achieve epitaxial and stoichiometric Er:CeO2 layers, i.e., (Er + Ce)/O = 1/2, on Si (111) substrates, the growth was carried out with various Ce/O2 flux ratios. Here, the growth temperature, layer thickness, Er deposition rate, and O2 flow rate were fixed at 640 °C, 30 nm, 0.01 Å/s, and 0.1 sccm, respectively. Figure 1 shows the RHEED patterns for the 30-nm-thick Er:CeO2 films grown with various Ce deposition rates (0.03 to 0.82 Å/s). The incident electron beam was parallel to the [11] axis of the Si substrate ([11]Si). As the Ce deposition rate increases, the pattern changes from halo-like [Fig. 1(a)] to streaky [Fig. 1(c)]. Further increases in the Ce rate leads to a spotty ring pattern [Fig. 1(e)]; structures of the films are amorphous, single-crystalline, and polycrystalline for excessive, optimal and insufficientoxidations, respectively. The stoichiometric composition of the film grown with the Ce rate of 0.33 Å/s is further confirmed by RBS results as described later.
Fig. 1 RHEED patterns after Er:CeO2 growth: Ce rate dependence. Er fluxes are supplied at a constant rate (0.01 Å/s). (a-e) O*+O2 are supplied with a fixed O2 flow rate of 0.1 sccm. (f) Ce is supplied at a rate of 1.00 Å/s, which is three times higher than that in (c). Oxygen flow rate is also increased so that the Ce/O2 ratio becomes equal to that in (c).
As long as the appropriate Ce/O2 ratio was used, films with identical quality were achieved when we grew them at a three times higher rate [Fig. 1(f)]. Specifically, the Ce and O2 flow rates were increased to 1.00 Å/s and 0.3 sccm, respectively, while the Er rate (0.01 Å/s) was kept constant. This means that a higher growth rate is advantageous for achieving a more dilute doping of Er, and equivalently, a less pronounced Er-Er interaction.
Although epitaxial films were prepared, there remains a possibility that some competing phases (e.g., hexagonal or bixbyite Ce2O3) were formed other than the fluorite Er:CeO2 [18]. To confirm the formation of Er:CeO2, we investigated the film structure by XRD. Figure 2(a) shows a 2θ−ω scan XRD pattern for an Er:CeO2 specimen. The d value calculated from the peak position is 3.11 Å, which agrees well with the spacing between the fluorite CeO2 (111) planes [19]. The thickness of the Er:CeO2 layer estimated from the intervals of the satellite peaks in the XRD pattern is 26.4 nm. In addition, to determine the valence of Ce (Ce4+ for CeO2 or Ce3+ for Ce2O3), we measured the XPS spectrum for the Ce 3d core level as shown in Fig. 2(b). All the peaks in the Ce 3d spectrum can be assigned to tetravalent Ce [20]. These XRD and XPS results indicate that the epitaxial layer is composed of Er:CeO2 and amount of the Ce2O3 (and/or Er:Ce2O3) phases, if any, is negligibly small.
Figure 3(a) shows the RBS depth profile for the Er:CeO2 layer for composition analysis. A uniform composition was accomplished in the entire layer except for the interface region. Remarkably, uniform doping of 1% Er was achieved for the film grown when the rates of Er and Ce were 0.01 and 1.00 Å/s, respectively, and the O2 flow rate was 0.3 sccm. Cross-sectional TEM and STEM observations also confirmed the high crystallinity of the Er:CeO2 layer with a very flat surface [Fig. 3(b), (c)]. They also indicate that the Er:CeO2 layer is free from any unidentified phases, orientations, or stacking sequences. However, in the HAADF-STEM image [Fig. 3(b), (c)], an amorphous layer can be seen at the interface between the Er:CeO2 layer and the Si substrate. The formation of this interface layer, which is peculiar to the growth of CeO2 on Si substrates, is widely known, and so far, two models have been proposed and discussed [6,21–24]. One is that solid state reaction occurs already at the very early stages of the growth [22–24]. The other is that a redox reaction takes place at the CeO2/Si interface, leading to formation of the amorphous layer consisted of CeO2-x and SiO2, after the growth [6,21]. Interface engineering for controlling such solid state or redox reactions is crucial for electronic devices. In contrast, for waveguide-type optical devices without current injection, such an amorphous layer formed at the interface may not deteriorate the device performance. Instead, it may positively serve as a blocking layer for energy transfer from the Er ions in CeO2 to the Si substrates [25].
Fig. 3 (a) Depth profile of each constituent element determined by RBS. Cross-sectional images obtained by (b) TEM and (c) HAADF-STEM.
Subsequently, optical characteristics of the grown Er:CeO2 layer with various Er concentration were investigated. Again, the Er concentration was varied by changing the Ce and O2 supply rates while the Ce/O2 flux ratio and Er deposition rate (0.01 Å/s) were kept constant. Figures 4(a-c) show the PLE color plots for Er concentration dependence. The Er concentrations (1, 2, and 4%) were estimated by RBS measurements. An optical emission in the Er ions, which originates from the transition between the Stark-level manifolds of 4I13/2 and 4I15/2 formed by the crystal field splitting, is observed at a wavelength of around 1.53 μm for all the Er concentrations. Moreover, the PL peak and absorption positions completely agree with each other irrespective of the Er concentration. The excitation spectrum detected at 1.533 μm for the 1% sample, which is equivalent to an absorption spectrum, is shown in Fig. 4(d). In the measured wavelength range, the most intense emission and absorption appear at 1.533 and 1.512 μm, respectively, for all the samples. This indicates that the Er ions are certainly located at the Ce sites in the CeO2 lattice.
Fig. 4 PLE color plots for Er:CeO2/Si(111) with Er concentrations of (a) 4%, (b) 2%, and (c) 1%. The measurements were performed at 4K. (d) Excitation spectrum detected at 1533 nm.
Figure 5(a) shows the PL spectra, which correspond to slices of Figs. 4(a-c) at the resonant excitation condition (λexc = 1.5122 μm). The emission intensity of the PL peak at around 1.533 μm clearly increases with decreasing Er concentration. This is owing to an enhancement of the quantum efficiency of the PL emission for the lower Er concentration. Specifically, the reduction of the Er concentration makes each Er ion more isolated and suppresses the Er-Er interactions, leading to less energy transfer. Moreover, the degree of inhomogeneous broadening of the spectra is reduced with decreasing Er concentration, and the spectra become narrowed. The emission lifetime with decreasing Er concentration [Fig. 5(b)] is prolonged by the same mechanism. The lifetime of the 1% Er:CeO2 sample was 1.5 ms. This value is comparable to the lifetime of the 1% Er in the epitaxial Sc2O3 host crystal in our previous work [26,27]. As we have also reported, Er ions diluted by three orders of magnitude (~0.001 at%) and doped in a Y2SiO5 (YSO) single crystal shows the intrinsic emission lifetime of about 11 ms [16], longer by a factor of five or one order magnitude. Nevertheless, the lifetime is monotonically prolonged with decreasing Er concentration in the % range [Fig. 5(b)], and hence, the present Er:CeO2/Si system provides a promising route to achieve the intrinsic lifetime on Si chips by further reducing the Er concentration.
Fig. 5 (a) PL spectra under the resonant excitation for Er:CeO2/Si(111) with various Er concentrations. (b) Radiative lifetime as a function of Er concentration. Solid circles and squares indicate Er:CeO2 and Er:Sc2O3 [26,27], respectively. The black dotted line shows the intrinsic lifetime of Er [16] and the red dotted curve is the guide to the eye.
4. Conclusion
In conclusion, we achieved an epitaxial Er:CeO2 layer on Si (111) substrates by MBE. The Er:CeO2 layer was single-crystalline and had a cubic fluorite structure, although an amorphous layer was formed at the interface region. The doped Er showed well-defined optical transitions at the wavelength of 1.533 μm. With decreasing Er concentration, enhancement of the luminescence intensity, narrowing of the spectral width, and an increase of the radiative lifetime were observed. In the most dilutely doped case (1% Er), a fairly long emission lifetime of 1.5 ms was achieved. Our present results indicate that the CeO2 is one of the promising platform material for Er-based optical devices on Si chips and their quantum optics applications, although characterization of the coherence time of the two-level system in the Er:CeO2 is still ongoing.
Funding
JSPS KAKENHI (JP15H04130, JP16H01057 and JP16H03821).
Acknowledgments
The authors thank Satoru Adachi and Yoshiharu Krockenberger for fruitful discussions.
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