A novel violet/blue light-emitting device based on Ce2Si2O7

Rare-earth silicates are highly efficient materials for silicon-based light sources. Here we report a novel light-emitting device based on Ce2Si2O7. Intense violet/blue electroluminescence was observed, with a turn-on voltage of about 13 V. The violet/blue emission is attributed to 4f–5d transitions of the Ce3+ ions in Ce2Si2O7, which are formed by interfacial reaction of CeO2 and Si. Electroluminescence and photoluminescence mechanisms of the Ce2Si2O7 light-emitting device are also discussed.

Scientific RepoRts | 5:16659 | DOI: 10.1038/srep16659 plane. The lattice structure is also investigated by transmission electron microscopy (TEM). As shown in Fig. 1(b), the thickness of Ce 2 Si 2 O 7 is about 50 nm. There is an amorphous SiO x layer at the Ce 2 Si 2 O 7 /Si interface, with a thickness of about 5 nm. During the annealing process, CeO 2 was reduced to CeO 2−x , while Si was oxidized. Then, SiO x and CeO 2−x reacted at high temperature, resulting in Ce 2 Si 2 O 7 with various crystallographic orientations.
The lattice structure of Ce 2 Si 2 O 7 observed is different from previous reports 18,26,27 . We attribute this to the strong dependence of the crystallinity and orientation on deposition and annealing conditions 27-29 . To confirm this, the as-deposited CeO 2 films with different thicknesses were annealed in reducing ambient (Ar:H 2 = 97:3) for 1 hour. As shown in Fig. 2(a), there are various crystallographic orientations of Ce 2 Si 2 O 7 when 300 nm CeO 2 film was annealed at 1020 °C. However, the intensity of the (008) diffraction  peak increases with decreasing the thickness of CeO 2 . In Fig. 2(b), various crystallographic orientations of Ce 2 Si 2 O 7 are exhibited when a 70 nm CeO 2 film was annealed at 1000 °C. Unlike the 300 nm film, here the (008) diffraction peak initially increases with the annealing temperature up to 1020 °C, and then decreases while the annealing temperature is further raised to above 1060 °C. Similarly, as shown in Fig. 2(c), this peak first increases with the annealing ambient flow rate is increased, and then decreases once the flow rate up is above 5000 sccm. In summary, these observations show that the deposition and annealing conditions are critical factors for synthesizing Ce 2 Si 2 O 7 . Figure 3 shows the PL spectrum of the CeO 2 film annealed in Ar:H 2 ambient. The intense violet/blue emission consists of two peaks centered at about 390 nm (3.18 eV) and 410 nm (3.02 eV), respectively. Both the shape and the position of the peaks are similar to the results obtained by Choi 18 . However, these emission bands are not observed in the as-deposited films or the samples annealed in O 2 ambient. Combined with crystal structure analysis mentioned above, the violet/blue emission bands are attributed to the formation of Ce 2 Si 2 O 7 , and correspond to the Ce 3+ transitions from the relaxed lowest 5d excited state to 4f ground states. The energy separation of the two peaks is about 2000 cm −1 , which matches well with the theoretical value of spin-orbit splitting between the 4f ground state 2 F 5/2 and 2 F 7/2 30 . The excitation bands located at 295 nm (4.2 eV) and 324 nm (3.83 eV) can be assigned to the 4f-5d transitions 31 . Based on these assignments, a simplified energy level diagram is sketched in Fig. 3. In order to estimate the optical band gap (E g opt ) of Ce 2 Si 2 O 7 , we measured the absorption spectrum of the sample in the visible range, and obtain a value of 3.6 eV by extrapolating the spectrum using Tauc's relation 32 : where α is the absorption coefficient, h is the Planck's constant, ν is the photon frequency, and C is a constant.
According to several previous studies, Ce 3+ activator concentration is limited to about 0.5 at% in most host materials [33][34][35] . However, our results show that Ce 3+ ions in Ce 2 Si 2 O 7 are almost 100% activated. This is similar to self-activated materials such as CeF 3 and CeCl 3 36 . We find that Ce 3+ emission is enhanced significantly by the formation of Ce 2 Si 2 O 7. To quantify this enhancement, we measured the internal quantum efficiency of Ce 2 Si 2 O 7 by using an integrated sphere 37 . We obtained a value of about 37% at room temperature. To reveal the excitation and emission mechanisms, we measured the PL spectra as a function of temperature. As shown in Fig. 4, the PL intensity increases as the sample temperature is decreased. We also measured PL decay time at temperatures of 77 and 300 K, respectively, as shown in the inset of Fig. 4. The PL decays exponentially, with the time constant slightly changes from 17 to 21 ns as the temperature is decreased from 300 to 77 K. This very short lifetime is in agreement with the electric-dipole allowed 4f-5d transition rate of Ce 3+ . Since non-radiative centers are frozen at low temperatures, we attribute the decrease of the PL intensity and the decay time to the energy transfer to defect centers by non-radiative recombination processes 38 .
Next, we fabricated Ce 2 Si 2 O 7 LEDs with the structure diagram shown in Fig. 5. Violet/blue EL emission is clearly observed when a positive voltage is applied on the indium tin oxide (ITO) layer. The turn-on voltage of the device is found to be as low as 13 V. The EL spectra (Fig. 5) are in good agreement with the PL spectra (Fig. 3). The EL intensity increases with the forward voltage, and the maximum EL intensity is obtained at 25 V. Figure 6 presents a typical current-voltage (I-V) characteristics. The forward current of the device reaches up to 0.26 mA when the forward bias is 25 V, while a reverse leakage current of 6.4 μ A at a bias voltage of − 10 V is observed. These values illustrate the excellent rectification performance achieved in this device.
Based on the energy level diagram of Ce 2 Si 2 O 7 developed in this work, we propose the following mechanism of the LED operation: As seen from the inset of Fig. 6, an asymmetrical energy barrier is formed at the junction interface. When a positive voltage is applied between the two electrodes (forward bias), the electrons accumulated in the Si/SiO x interface are swept to Ce 2 Si 2 O 7 side by tunneling through the SiO x barrier. Meanwhile, the holes are injected into Ce 2 Si 2 O 7 from ITO and accumulated at Ce 2 Si 2 O 7 / SiO x interface due to the SiO x barrier. As a result, the EL originates from the recombination between the injected electrons and holes in Ce 2 Si 2 O 7 . These emitted photons with an energy (hυ ) approximately equals to the energy difference between 4f and 5d states of Ce 3+ . Hence, in this design, the SiO x layer functions as a carrier blocking layer. For high-performance and reliable device operation, the ideal thickness of SiO x layer should be less than 2 nm 39 . Therefore, the turn-on voltage can be further decreases, considering the 5 nm SiO x layer used in this device.
In summary, a new method of synthesizing Ce 2 Si 2 O 7 was demonstrated. Ce 2 Si 2 O 7 was formed after the Si-based CeO 2 film was annealed in reducing ambient. Intense violet/blue emission was observed from Ce 2 Si 2 O 7 , and the PL emission bands are located around 390 and 410 nm, which are attributed to 4f-5d transitions of Ce 3+ . More importantly, Ce 2 Si 2 O 7 LEDs were fabricated and strong violet/blue EL emission was observed. The turn on voltage of Ce 2 Si 2 O 7 LED is 13 V and the maximum EL intensity was obtained at 25 V.  Methods CeO 2 thin films were deposited on P-type Si (100) substrates by electron beam evaporation (EVA 450). The as-deposited films were annealed in reducing ambient (Ar:H 2 = 97:3) at various temperatures for 1 hour. We fabricated the LEDs as schematically illustrated in the inset of Fig. 5. ITO and Ag electrodes were deposited on the surface of the film and the back side of Si substrates, respectively, both by magnetron sputtering. The PL spectra were measured by a He-Cd laser with a 325 nm excitation wavelength, and the PL excitation (PLE) spectrum was measured with a fluorescence spectrometer (FLS920) using a 450 W xenon lamp as the excitation source. The crystal structures were characterized by XRD using Cu Kα radiation (Bruker D8 ADVANCE), and morphology of the samples was determined by TEM (Hitachi, H8100 200 kV). The internal quantum efficiency of samples is measured using an F-3018 integrating sphere with a 335 nm laser excitation. The PL decay was measured by using a nanosecond xenon flash lamp at 325 nm excitation wavelength and detected by a time-correlated single photon-counting system. The EL spectra of the devices and I-V characteristics were measured by a system of an ACTON 150 CCD spectrometer and a Keithley 2410, respectively.