1. Introduction

Rare earth (Re³⁺) doped semiconductor materials have attracted much attention due to their potential applications in solid-state lasers, light-emitting diodes (LEDs) and optical communications [1–4]. Particularly, since the near infrared (NIR) emission (1550 nm) of Er³⁺ ions lies at the minimum loss window of the silica optical waveguide and can be directly coupled to system-on-chip optical communication network, it is interesting to study the NIR emission property of Er³⁺ and develop light emitting devices for realizing Si-based monolithic optoelectronic integrations [5]. So far, Er³⁺ doped silicon dioxide as a Si-compatible material has been extensively investigated [6]. To improve the NIR emission of Er³⁺-doped SiO₂ films devices, Si nanocrystals were co-doped into the matrix as sensitizers, transferring the absorbed energy by Si nanocrystals to the nearby Er³⁺ ions [5,7]. However, the low electroluminescence (EL) efficiency and undesirable high driving voltages caused by the SiO₂ insulator layer still hinder the further development of Er³⁺-related Si-based light sources under electrical pumping [8,9]. Therefore, exploring desirable matrix materials and developing Er³⁺-related LEDs with high EL efficiency, low driving voltages and low cost is of great significance. As a kind of wide band-gap oxide semiconductor, TiO₂ thin film is considered to be a prominent candidate for Re³⁺-related Si-based LEDs due to its Si compatibility, chemical stability, electrical conductivity and low cost [10,11]. Anatase phase TiO₂ has been reported to be a good host for Re³⁺-related EL devices [12]. Recently, Er³⁺doped TiO₂ EL device with low driving voltages was fabricated by magnetron sputtering, and the Er³⁺-related EL mechanisms were analyzed in detail [13,14]. Moreover, it was reported that Er³⁺-related EL can be enhanced by co-doping Y into the TiO₂:Er³⁺ film, due to the less symmetric crystal field and reduced concentration quenching [15].

The sol-gel method has been widely used to prepare thin films. However, to our knowledge, there are few reports on LEDs based on Re³⁺ doped TiO₂ films prepared by sol-gel method [16,17], since it is difficult to obtain high-quality films by sol-gel method because of inevitable gel film shrinkage and gas evolution during drying and annealing process. In this work, TiO₂:Er³⁺ films were prepared by using a facile sol-gel technique combining with the spin-coating and post-annealing treatments. The crystal quality can be improved by controlling the annealing temperature and an intense photoluminescence (PL) can be observed both in visible and NIR range. Furthermore, we developed the EL devices based on the prepared TiO₂:Er³⁺ films and NIR EL can be clearly detected due to the efficient energy transfer (ET) from TiO₂ host to Er³⁺ ions. The EL device can be operated under the voltage as low as 5.6 V with low power consumption.

2. Experimental section

The TiO₂:Er³⁺ films used in this work were prepared by using sol-gel method with spin-coating and post-annealing process [18]. In a typical experiment, 22 ml TBOT, 56 ml ethanol, 8 ml acetylacetone, 3 ml deionized water, several drops of HNO₃ and a certain amount of Er(NO₃)₃·5H₂O (2 mol%) were mixed together. After vigorous stirring at 60 °C for 4 hrs, the precursor was sealed and aged at room temperature for a week to form uniform gel. Then, the as-prepared Er³⁺ doped TiO₂ gel was spin-coated onto clean 1.1×1.1 cm²-sized n⁺-Si (0.001 Ω·cm) substrates with a speed of 5000 rpm for 60 seconds to form TiO₂:Er³⁺ films. Subsequently the TiO₂:Er³⁺ films were dried at 100 °C for 4 hrs, followed by annealing at different temperatures in air atmosphere for 1 hr. For preparing the electrodes of devices, aluminum electrodes at the backside of n⁺-Si substrates were deposited by thermal evaporation, followed by heat-treatment in N₂ atmosphere at 400 °C for 30 minutes to make ohmic contacts. Finally, dot-shaped ITO electrodes with diameters of 1.5 mm were sputtered on the top of the thin films using magnetron sputtering through a circle shadow mask. The EL devices with 700, 800 and 900 °C annealed TiO₂:Er³⁺ films are briefly labeled as 7-TLED, 8-TLED and 9-TLED, respectively.

The crystal structures and phases of the TiO₂:Er³⁺ films were characterized by X-ray diffraction (XRD). The cross-sectional morphology of the device was investigated by using a field emission scanning electron microcopy (FE-SEM) and a transmission electron microscopy (TEM). Photoluminescence excitation (PLE), PL and EL spectra were recorded by a fluorescence spectrophotometer equipped with a Xenon lamp and a 325 nm He-Cd laser as the excitation sources. A photomultiplier tube and a liquid-nitrogen-cooled InGaAs photodiode were used as detectors to detect visible and NIR regions, respectively. It should be pointed out that visible and NIR intensities in figures cannot be directly compared. All measurements were performed at room temperature.

3. Results and discussion

The XRD patterns of TiO₂:Er³⁺ films annealed at different temperatures are shown in Fig. 1(a). The result shows that samples annealed at 600-800 °C crystallized in anatase phase TiO₂, which is consistent with the standard X-ray diffraction JCPDS 01-0562. The intensity of the XRD peaks of the samples is gradually increased, which indicates that the crystallinity of anatase phase TiO₂:Er³⁺ thin film is improved with increasing the annealing temperature. When the annealing temperature reaches 900 °C, the anatase phase disappears and dominant diffraction peaks (27.1°, 35.9°, 40.9° and 54.1°) can be assigned to the rutile phase TiO₂ (JCPDS 21-1276). The similar phase transformation from anatase to rutile has also been reported in previous literatures [19,20]. According to the XRD results, the full width at half maximum (FWHM) of the peaks decreases with the annealing temperature, which indicates that the crystallite size becomes larger gradually. Based on FWHM of the strongest diffraction peaks, the crystallite size has been roughly estimated by using Scherrer’s equation [20]. It is found that the crystallite size is estimated to be 10.5, 13.1, 18.1 and 59.5 nm for sample annealed at 600, 700, 800 and 900 °C, respectively. Figure 1(b) displays the typical high-resolution cross-sectional TEM (HRTEM) image of the TiO₂:Er³⁺ thin film annealed at 800 °C. The interplannar spacing is ∼3.53 Å, which corresponds to the distance of the (101) plane of the anatase phase TiO₂. In addition, it can be found that there is an amorphous SiO₂ layer (about 3 nm) between the TiO₂:Er³⁺ film and Si substrate. Actually, the formation of the amorphous SiO₂ layer is nearly inevitable during the heat treatment in air ambient [14].

 

Fig. 1. (a) XRD patterns of TiO₂:Er³⁺ thin films annealed at different temperatures. (b) HRTEM image of the cross section of the TiO₂:Er³⁺ thin film annealed at 800 °C.

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Figure 2(a) shows the PL spectra of TiO₂:Er³⁺ thin films under over-band-gap excitation using a 325 nm He-Cd laser. PL emissions of Er³⁺ ions in the visible range as well as in the NIR region were detected, including green band (510-580 nm), red band (640-690 nm) and NIR band (1400-1700nm) which are identified as the ²H_11/2/⁴S_3/2→⁴I_15/2, ⁴F_9/2→⁴I_15/2, and ⁴I_13/2→⁴I_15/2 transitions of Er³⁺ ions. As the annealing temperature rises to 800 °C, the PL intensity is gradually enhanced, which can be ascribed to the crystal quality of anatase phase TiO₂:Er³⁺ improving. However, the PL intensity decreases sharply at 900 °C. According to the XRD results, the phase transformation from anatase to rutile phase occurs at 900 °C. It is reported that rutile phase TiO₂:Er³⁺ with high symmetry of crystal field around Er³⁺ ions has low ET efficiency [19]. Therefore, we attribute the decreased fluorescence to rutile phase transformation at high temperature.

 

Fig. 2. (a) PL spectra of TiO₂:Er³⁺ thin films annealed at different temperatures. (b) PLE spectra of TiO₂:Er³⁺ thin films by monitoring the emission at 1530 nm. (c) The schematic diagram of the energy levels in the TiO₂:Er³⁺ thin film under 325 nm excitation.

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In order to understand the PL mechanism of Er³⁺-related emissions, the PLE measurement has been carried out by monitoring the NIR emission at 1530 nm. PLE spectra of TiO₂:Er³⁺ thin films annealed at different temperatures are shown in Fig. 2(b). All PLE spectra have an intense UV band, resulting from the band-to-band transition of TiO₂. Weak peaks at 489, 521 and 652 nm are assigned to Er³⁺ transitions from the ground state ⁴I_15/2 to ⁴F_7/2, ²H_11/2 and ⁴F_9/2 excited states, respectively [see the inset of Fig. 2(b)]. The wide UV excitation band implies that the Er³⁺-related emissions under over-band-gap excitation is due to the indirect sensitization process through the ET from band-to-band transition of TiO₂ to surrounding Er³⁺ ions. The proposed mechanisms for the emissions of TiO₂:Er³⁺ thin film can be described in Fig. 2(c). Under a 325 nm laser excitation, the electrons in TiO₂ matrix are excited from valance band to conduction band, followed by the ET to metastable states of Er³⁺ ions such as ²H_2/9 level, etc. The electrons on the ²H_2/9 level of Er³⁺ ions are not stable that they will non-radiatively relax to ²H/_11/2, ⁴S/_3/2, ⁴F_9/2 and ⁴I_13/2, respectively. Finally, the transitions from the excited states to ground state (⁴I_15/2) in Er³⁺ ions occur, giving green, red and NIR emissions, respectively.

In order to realize the LEDs based on sol-gel prepared TiO₂:Er³⁺ films, aluminum as a bottom electrode and ITO as a top electrode were deposited on the back and front of the TiO₂:Er³⁺/n⁺-Si hetero-structure to form EL devices. The structure of TiO₂:Er³⁺ EL device is shown in Fig. 3(a). Figure 3(b) displays a typical SEM image of the cross section of EL device with TiO₂:Er³⁺ film annealed at 800 °C. The thicknesses of the TiO₂:Er³⁺ film and ITO layer are about 101 and 224 nm, respectively. The EL measurement was performed at room temperature with ITO positively biased (forward bias).

 

Fig. 3. (a) The structure of TiO₂:Er³⁺ EL device. (b) The SEM image of the cross section of TiO₂:Er³⁺ EL device.

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Figure 4(a) shows the EL spectra of the TiO₂:Er³⁺ film EL devices, including visible emission bands at 383, 410, 526, 550 and 660 nm, respectively, and NIR one at 1530 nm from transitions of Er³⁺ ions. For both 7-TLED and 9-TLED, very weak visible light emission band of Er³⁺ ions was observed and no NIR emission (1530 nm) band was detected at the marked voltage/current. When a voltage of about 20 V is applied, extremely weak NIR light can be detected for 7-TLED and 9-TLED (not shown in here). Compared with 7-TLED and 9-TLED devices, the 8-TLED shows an intense visible and NIR EL emissions at a relatively low driving voltage/current of 6.4 V/1.35 mA. The inset in Fig. 4(a) is the corresponding photograph of EL from 8-TLED. Figure 4(b) displays the NIR and visible EL spectra of 8-TLED acquired at different forward voltages. According to the EL spectra, the integrated intensity of each EL peak was calculated. As shown in Fig. 4(c), the normalized integrated EL intensity of each peak increases rapidly with injection current. It indicates that more and more carriers are injected into the TiO₂:Er³⁺ films with increasing the injection current, and they transfer their energy to the single-typed Er³⁺ luminescence center via ET or impact excitation process more efficiently as discussed later. Consequently, the EL intensity is enhanced obviously. It is worth mentioning that the onset voltage/current of EL of 8-TLED is about 5.6 V/0.66 mA. Briefly compared to other reported devices, e.g. TiO₂:Sm³⁺ LED with 12 V/2 mA onset voltage/current of EL [17], TiO₂:Er³⁺ LED (9 V/4 mA) [15], our device (5.6 V/0.66 mA) shows lower input electrical power (I×V). Our results indicate that the LED under low power consumption can be achieved based on sol-gel prepared films, which has not been reported before. It is worth noting that the external quantum efficiency of our device is still very low and it can be further improved by optimizing the device structures and parameters.

 

Fig. 4. (a) EL spectra of devices. The inset is the EL photograph of 8-TLED at 6.4 V/1.35 mA. (b) EL spectra of 8-TLED at different voltages. (c) The normalized EL intensity of 8-TLED versus the current. (d) I-V characteristic of 8-TLED under reverse and forward bias. The inset is the plot of ln(J) as a function of E. (e) Energy-band diagram of ITO/TiO₂:Er³⁺/SiO₂/n⁺-Si/Al device under forward bias voltage.

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The current density-voltage (J-V) curve of 8-TLED is shown in Fig. 4(d). The device exhibits good rectification characteristics. As the forward bias increases (>4 V), the forward current density increases rapidly, while under the reverse biases, the current is in the order of 10⁻² mA. To understand the carrier transport mechanism, trap-assisted tunneling (TAT) plot based on the ln(J)∼E⁻¹ relation was studied. However, at the electric field of 5-5.8 MV/cm (5.6-6.4 V), the estimated trap energy below the conduction band is about 0.7 eV, which is lower than the value in the previous report about 2.0 eV [12,21]. It is reported that when the thickness of SiO₂ is less than 4 nm, the carrier transport is dominated by direct tunneling of electrons across the SiO₂ barrier [22]. In our case, the thickness of the intermediate SiO₂ layer as thin as 3 nm, therefore the direct tunneling (DT) should be taken into consideration [23]. The inset in Fig. 4(d) is the plot of ln(J) versus E for the SiO₂ layer, which shows a linear relation between ln(J) and E at moderate electric field (5-5.8 MV/cm) [24]. The result indicates DT mechanism works in the EL-enabling voltages. On the basis of the EL and I-V results, the possible EL processes can be proposed. Figure 4(e) illustrates the energy-band diagram of n⁺-Si/SiO₂/TiO₂:Er³⁺ device under forward bias voltage. Under the EL-enabling voltage, a large number of electrons accumulated near the n⁺-Si/SiO₂ interface can be accelerated, tunnel directly through the ultra-thin SiO₂ layer and drift into the TiO₂:Er³⁺ layer, subsequently the conduction band electrons in TiO₂ transfer their energy to the nearby Er³⁺ ions through an efficient ET process. Finally, the radiative transitions from the excited states of Er³⁺ ions to the ground state (⁴I_15/2) emit visible and NIR light. The Re³⁺-related EL mechanism in these devices is also likely to be due to impact excitation by hot electron [7,12]. The energetic electrons tunnel through the SiO₂ layer, drop into the conduction band of TiO₂ and become hot electrons to impact and excite the Er³⁺ ions to metastable levels. The differences in the PL and EL spectra in our case may explained as the different excitation mechanisms. The PL is mainly originated from the ET between TiO₂ and the metastable states of Er³⁺ ions and then causes the characteristic emission peaks in Er³⁺ ions. However, the Er³⁺ ions can be excited both by hot electron impact and through ET from TiO₂ which make the EL spectra become broad since several sharp peaks are merged together especially due to the hot carrier impact process.

4. Conclusion

In conclusion, Er³⁺ doped TiO₂ thin films are prepared by employing a low-cost sol-gel method plus spin-coating and post-annealing treatment. Under a 325 nm laser excitation, anatase phase TiO₂:Er³⁺ annealed at 800 °C has the best PL performance due to its good crystallinity and low crystal symmetry. The corresponding EL device emits Er³⁺-related NIR and visible EL under forward bias. Low operation voltage (less than 7 V) and working current (about 1 mA) mean that our device can be driven under low power consumption. The 8-TLED with low driving voltage/current has comparable performance to devices fabricated by magnetron sputtering. This work could contribute to the development of Er³⁺-related NIR LED sources for optical interconnection.