Real-time synchrotoron radiation X-ray diffraction and abnormal temperature dependence of photoluminescence from erbium silicates on SiO 2 / Si substrates

The erbium silicate formation processes during annealing in Ar gas were monitored by synchrotron radiation grazing incidence X-ray diffraction (GIXD) in real time and the optical properties of the silicates were investigated by photoluminescence measurements in spectral and time-resolved domains. The GIXD measurements show that erbium silicates and erbium oxide are formed by interface reactions between silicon oxide and erbium oxides deposited on silicon oxide by reactive sputtering in Ar gas and O2/Ar mixture gas ambiences. The erbium silicates are formed above 1060 °C in Ar gas ambience and above 1010 °C in O2/Ar gas ambience, and erbium silicides are dominantly formed above 1250 °C. The I15/2-I13/2 Er3+ photoluminescence from the erbium oxide and erbium silicate exhibits abnormal temperature dependence, which can be explained by the phonon-assisted resonant absorption of the 532-nm excitation photons into the 2H11/2 levels of Er3+ ions of the erbium compounds.


Introduction
Erbium silicates have received much attention as light emitting materials in silicon photonics [1][2][3].They have been fabricated on Si and SiO 2 /Si substrates by various methods, including wet chemical methods and radio frequency (rf) reactive magnetron sputtering [1].Photoluminescence (PL) experiments have shown that intense PL is obtained from erbium silicates Er 2 SiO 5 and Er 2 Si 2 O 7 formed on SiO 2 /Si substrates by rf-reactive magnetron sputtering at room temperature with subsequent rapid thermal annealing at high temperatures [2,3].To obtain more intense PL from these erbium silicates on the substrate at room temperature, it is important to precisely characterize their structural and optical properties and their formation processes in detail.
In this work, we characterized the formation of erbium silicates on silicon oxide films in real time using synchrotron radiation grazing incidence X-ray diffraction (GIXD) in beamline BL24XU of SPring-8.We also characterized light emission from the erbium silicates by photoluminescence measurements as a function of temperature.

Experiments
The GIXD experiments were performed using the z-axis goniometer of the beamline.The 0.124 nm x-ray wavelength was used at incident angle α from 0.1°to 0.4°.The 100-nm-thick erbium oxides were deposited on SiO 2 /Si(100) substrates by rf-magnetron sputtering at room temperature and were annealed at the temperatures of up to 1350℃ in Ar ambience using a furnace that enables us to monitor GIXD during the annealing.Standard Si powders were used for calibration of instrumental resolution.Photoluminescence measurements were mainly performed on the annealed samples by pumping at 532 nm of a semiconductor laser at excitation power of 4.5 mW between 4 and 573 K.

Results and discussion
Figure 1 shows a typical x-ray powder pattern obtained from the samples annealed in Ar ambience at 1250℃.As seen in the pattern, the peaks are well assigned to the monosilicate of Er Figure 2 shows the evolution of x-ray diffraction obtained during the annealing of the as-deposited Er 2 O 3 .The x-ray diffraction experiments revealed that the as-deposited erbium oxide, which is amorphous, spontaneously transformed into a mixture of crystalline erbium silicates (Er 2 SiO 5 and Er 2 Si 2 O 7 phases) at about 600℃.The Er 2 Si 2 O 7 phase disappeared at 1060℃ and the Er 2 SiO 5 phase was maintained.Therefore, the Er 2 O 3 reacts with SiO 2 and forms erbium mono and disilicate at 1010℃ and the disilicate disappear above the 1060 ℃, indicating that the monosilicate is thermally stable, as confirmed by the data in Fig. 1, but that the disilicate is not thermally stable above 1100℃.
Figure 3 shows Williamson-Hall plots derived from the x-ray diffraction pattern obtained from the sample annealed at 1250℃.The Williamson-Hall plots of the X-ray powder patterns indicate that the Er 2 SiO 5 crystals formed at 1250℃ are about 55 nm in size with strain of 0.25%.
The PL measurements show that the erbium silicate nanocrystals formed by annealing at 1350℃ in Ar gas ambience on the SiO 2 /Si substrate have an abnormal temperature dependence of PL from Er +3 ions at 1530 nm as a function of the excitation wavelength of the pump light with increasing temperature: the PL peak intensity at 1530 nm decreases from 4 to 50 K, but increases up to 573 K at a factor of 5 with respect to the minimum intensity at 50 K for the 532-nm pump, while the peak intensity monotonically decreases from 4 to 300 K for the 790-nm -1072-Extended Abstracts of the 2010 International Conference on Solid State Devices and Materials, Tokyo, 2010, pp1072-1073 D-9-1 pump, as is normally observed in erbium-doped materials.The reversed temperature dependence for the 532-nm pump clearly indicates a mechanism of phonon-assisted sensitization to Er +3 4f levels in the erbium silicates.The intense PL from Er +3 ions above room temperature suggests the erbium silicate nanostructures are promising as light emitters for Si-based optoelectronic devices.

Conclusion
We explored the structural evolution of erbium oxide deposited on Si O 2 /Si substrate using GIXD and found that erbium mono and disilicate nanostructures, which are nanoscle and have strain 0.25%, are formed through the solid-solid reaction at the interface between erbium oxide and SiO 2 by annealing below 1350℃.The nanostructures of erbium silicates exhibit abnormal temperature dependence of 1.5-μm photoluminescence; i.e., the PL intensity becomes larger with increasing temperature up to 300℃.

Figure 1 :
Figure 1: X-ray powder diffraction patterns obtained at α = 0.2°from the sample annealed at 1250℃ in Ar ambience.

Figure 2 :
Figure 2: Evolution of powder X-ay diffraction patterns obtained at α = 0.2 °during the annealing of the as-deposited Er 2 O 3 on SiO 2 /Si substrate.

Figure 3 :
Figure 3: Williamson-Hall plots for the samples annealed at 1250 ℃.The plots are linearly fitted (dotted line).The broadening of diffraction peaks is defined by the integral breadth (= area/amplitude) of the fitted pseudo-Voigt function.