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

Abstract

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.

Introduction

Silicon-based light-emitting devices have attracted considerable attention, since silicon has many advantages such as low fabrication cost, mature fabrication and processing technology and high electrical and thermal conductivities¹. In the past decades, integrations of ZnO, GaN and other III-V semiconductors with silicon were extensively studied for novel optoelectronic applications because of the direct and wide band gaps of these materrials^2,3,4. However, the lattice and thermal mismatch between these materials and Si still presents a significant challenge^5,6. Alternatively, rare-earth doped SiO₂ has attracted a lot of interest due to their high luminescence efficiency and wide spectral range spanning from ultraviolet (UV) to infrared (IR). More importantly, SiO₂ is naturally compatible with Si technology^7,8. Previously, efficient visible light-emitting devices based on rare-earth doped metal-oxide-semiconductor (MOS) structures have been demonstrated^9,10,11. However, the solubility of rare-earth ions in Si-based materials achieved so far is still relatively low (10¹⁶−10²⁰ cm⁻³)¹². In fact, even if the solubility could be improved, since the rare-earth ions (e.g. Er³⁺, Eu³⁺, Ce³⁺) are optically inactive at high concentrations due to the formation of clusters^13,14,15, such a concentration quenching would still be a significant barrier for further improving performance of rare-earth doped Si-based materials. Recently, it was shown that formation of rare-earth silicates (e.g. Er₂SiO₅, Eu₂SiO₅, Ce₂Si₂O₇) could enhance their photoluminescence efficiency^16,17,18. It was found that in these silicates, the rare-earth ions are 100% activated and the solubility can reach 10²² cm^{−3 19,20,21}.

Ce₂Si₂O₇ is a promising material for UV optoelectronic devices, with an emission band similar to that of ZnO and GaN. In addition, different emission wavelengths could be obtained from the energy transfer between Ce³⁺ and other rare-earth ions (such as Er³⁺, Tb³⁺, Dy³⁺, Sm³⁺)^10,11,22,23. Although photoluminescence (PL) of Ce₂Si₂O₇ has been investigated by several groups^16,18,24,25, there has been no reports on electroluminescence (EL) of Ce₂Si₂O₇ so far. In this paper, Ce₂Si₂O₇ is synthesized by interfacial reaction of CeO₂ thin film and Si wafer in reducing ambient. Violet/blue emissions at about 390 and 410 nm from Ce₂Si₂O₇ were obtained. The internal quantum efficiency of Ce₂Si₂O₇ was found to be about 37% at room temperature. Finally, we fabricated Ce₂Si₂O₇ light-emitting devices (LEDs) and achieved a turn on voltage of 13 V, while the maximum EL intensity was obtained at a driving voltage of 25 V.

Results and Discussion

In order to investigate the effect of annealing ambient on the structural change of CeO₂ films, X-ray diffraction (XRD) patterns of as-deposited and annealed CeO₂ films were measured. As shown in Fig. 1(a), the crystal structure of the samples depends strongly on the annealing ambient. The as-deposited film contains a broad reflection around 28.6°, which corresponds to the (111) plane of CeO₂. When the as-deposited film was annealed in O₂ ambient, the diffraction peak at 28.6° becomes higher and narrower and the (200) plane of CeO₂ can be identified. The Ce₂Si₂O₇ phase is obtained after the as-deposited CeO₂ film was annealed in reducing ambient and the highest reflection peak is identified as the (008) plane. The lattice structure is also investigated by transmission electron microscopy (TEM). As shown in Fig. 1(b), the thickness of Ce₂Si₂O₇ is about 50 nm. There is an amorphous SiO_x layer at the Ce₂Si₂O₇/Si interface, with a thickness of about 5 nm. During the annealing process, CeO₂ was reduced to CeO_2−x, while Si was oxidized. Then, SiO_x and CeO_2−x reacted at high temperature, resulting in Ce₂Si₂O₇ with various crystallographic orientations.

Figure 1

(a) XRD of as-deposited and annealed CeO₂ films at 1060 °C for 1 hour, (b) TEM image of the Ce₂Si₂O₇ film.

The inset shows high-magnification TEM of the interface region marked by the yellow box.

The lattice structure of Ce₂Si₂O₇ 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^27,28,29. To confirm this, the as-deposited CeO₂ films with different thicknesses were annealed in reducing ambient (Ar:H₂ = 97:3) for 1 hour. As shown in Fig. 2(a), there are various crystallographic orientations of Ce₂Si₂O₇ when 300 nm CeO₂ film was annealed at 1020 °C. However, the intensity of the (008) diffraction peak increases with decreasing the thickness of CeO₂. In Fig. 2(b), various crystallographic orientations of Ce₂Si₂O₇ are exhibited when a 70 nm CeO₂ 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 Ce₂Si₂O₇.

Figure 2

(a) XRD of as-deposited CeO₂ films with different thicknesses annealed at 1020 °C for 1 hour, (b) XRD of 70 nm CeO₂ films annealed at various temperatures for 1 hour, (c) XRD of 70 nm CeO₂ films annealed at 1040 °C by different annealing ambient flow rate.

Figure 3 shows the PL spectrum of the CeO₂ film annealed in Ar:H₂ 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 Choi¹⁸. However, these emission bands are not observed in the as-deposited films or the samples annealed in O₂ ambient. Combined with crystal structure analysis mentioned above, the violet/blue emission bands are attributed to the formation of Ce₂Si₂O₇ and correspond to the Ce³⁺ 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 ²F_5/2 and ²F_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 transitions³¹. 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 Ce₂Si₂O₇, 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 relation³²:

Figure 3

PL and PLE spectra of the CeO₂ film annealed in Ar:H₂ ambient with the excitation wavelength of 325 nm, the PLE was measured with a detection wavelength of 390 nm.

The inset shows the energy levels diagram of Ce₂Si₂O₇. All the spectra were measured at room temperature.

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, Ce³⁺ activator concentration is limited to about 0.5 at% in most host materials^33,34,35. However, our results show that Ce³⁺ ions in Ce₂Si₂O₇ are almost 100% activated. This is similar to self-activated materials such as CeF₃ and CeCl₃ ³⁶. We find that Ce³⁺ emission is enhanced significantly by the formation of Ce₂Si₂O_7. To quantify this enhancement, we measured the internal quantum efficiency of Ce₂Si₂O₇ by using an integrated sphere³⁷. 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 Ce³⁺. 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 processes³⁸.

Figure 4

PL spectra from Ce₂Si₂O₇ in the temperature range from 10 to 300 K with the excitation wavelength of 325 nm.

The inset shows the PL decay of Ce₂Si₂O₇.

Next, we fabricated Ce₂Si₂O₇ 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 μA at a bias voltage of −10 V is observed. These values illustrate the excellent rectification performance achieved in this device.

Figure 5

EL spectra of the LED at various forward biases of 16–25 V.

The insets show the structure diagram of the LED and EL photos of the device at different voltages.

Figure 6

Current-voltage characteristic of the device.

The inset shows the energy band diagram of the devices and the charge transfer process.

Based on the energy level diagram of Ce₂Si₂O₇ 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 Ce₂Si₂O₇ side by tunneling through the SiO_x barrier. Meanwhile, the holes are injected into Ce₂Si₂O₇ from ITO and accumulated at Ce₂Si₂O₇/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 Ce₂Si₂O₇. These emitted photons with an energy (hυ) approximately equals to the energy difference between 4f and 5d states of Ce³⁺. 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 nm³⁹. 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 Ce₂Si₂O₇ was demonstrated. Ce₂Si₂O₇ was formed after the Si-based CeO₂ film was annealed in reducing ambient. Intense violet/blue emission was observed from Ce₂Si₂O₇ and the PL emission bands are located around 390 and 410 nm, which are attributed to 4f–5d transitions of Ce³⁺. More importantly, Ce₂Si₂O₇ LEDs were fabricated and strong violet/blue EL emission was observed. The turn on voltage of Ce₂Si₂O₇ LED is 13 V and the maximum EL intensity was obtained at 25 V.

Methods

CeO₂ 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:H₂ = 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.