1. Introduction

Although an intense research effort towards efficient light emission from silicon-based materials has been ongoing, the persistent absence of an efficient silicon-based light source has still prevented the realization of silicon photonic chips. From low-dimensional silicon, such as nanocrystal silicon, to silicon-rich silicon oxide, silicon-rich silicon nitride (SiN_x), and rare earth doped silicon materials, photoluminescence (PL) or electroluminescence (EL) has been demonstrated with a low efficiency [1–6]. Within these materials, SiN_x film has received extensive attention as a candidate material for silicon-based light source because of its good luminescence properties and compatibility with CMOS techniques [3,6]. However, the power efficiency of SiN_x-based light emitting devices (LEDs) is too low to fulfill the demands of optical interconnections [3,7–9]. To achieve highly efficient LEDs, several methods, such as introducing surface plasmons (SPs) [10,11], surface roughing [12], nano-patterned substrate [13], and NH₃ plasma treatment [14] have been applied to circumvent the problems. Among those methods, increasing the radiative recombination rate by coupling excitons to SPs in metal nanostructures has attracted a great deal of attention [10,11,15,16]. Enormous efforts on SPs enhanced emission efficiency from InGaN-based luminescent materials, which were the widely-used state-of-the-art high efficiency blue-green-yellow emitters, have been made in the last decades [16–21]. This addition of metallic layers not only enhanced the photon density near the SPs frequency, but also tuned the emission wavelength, both of which can improve the emission of InGaN-based LEDs with different wavelength via the modulation of the dipole resonance peaks of metal [17,19]. By modifying the double-metallic Au/Ag thickness, Zhao et al. achieved a maximum enhancement of ~8.3 times for Purcell factor in the green spectral regime [19], from which the radiative recombination rate as well as the emission efficiency of InGaN-based LEDs can be improved significantly. Despite of the abundant works on the emission efficiency improvement of InGaN-based light emitters by SPs, there have been few reports on SPs enhanced EL of SiN_x-based LEDs, which also emitted in the region of blue-green-yellow [10,11]. Here, we report the enhancement of EL intensity of SiN_x-based LEDs by introducing a silver (Ag) islands film in a device structure. The introduced Ag islands film obviously enhanced the external quantum efficiency of electroluminescent device of SiN_x films, which could get the appropriate surface morphology for effective light extraction and current-injection efficiency.

2. Experimental section

SiN_x LEDs with an ITO/SiO₂/SiN_x/Ag/p/p⁺-Si/Al multilayer structure, as shown in Fig. 1(a) , were prepared for EL investigation. A boron doped p-type epitaxial silicon layer on a heavily doped p-type (100) silicon wafer was employed as a substrate, where the thickness and resistivity of the epitaxial layer were 11.4 μm and 2.5 Ω•cm, respectively. Ag was deposited on the substrate by an electron beam evaporation (EBE) system (the original thickness of Ag layer was ~10 nm read from a film thickness monitor), and then annealed in argon atmosphere at 400 °C for 120 seconds to form islands film, as shown in Fig. 1(b). According to incomplete statistics, the average dimension of Ag nanoparticles was ~70 nm in diameter, as shown in Fig. 1(c), where most of them were in the range of 50-80 nm. The average surface distance (d) between Ag nanoparticles and the surface coverage (η_SC) of them were about 26 nm and ~55.0%, respectively. The SiN_x film of about 60 nm was deposited on the Ag islands film by plasma-enhanced chemical vapor deposition (PECVD). The reactant gas sources were nitrogen-diluted 10% silane and ammonia gas. And the flow rate of the nitrogen-diluted 10% silane and ammonia were maintained at 100 sccm and 30 sccm, respectively. The plasma power, growth temperature, and deposition pressure were fixed at 20 W, 300 °C, and 16 Pa, respectively. After the SiN_x deposition, a SiO₂ layer of 25 nm was evaporated on the SiN_x film by EBE system. At last, an indium tin oxide (ITO) electrode of about 100 nm was sputtered on the SiO₂ through a mask to define the device area. An Al layer of 150 nm was evaporated on the back of silicon substrate. We also prepared the reference devices without Ag islands film using the same process. Due to the roughness of underneath Ag islands layer, the sequent deposited SiN_x and ITO layers also became rough, as shown in Figs. 1(d) and 1(e). The area root-mean-square roughness (R_rms) of ITO electrode for the devices with Ag islands film was estimated in the range of 5-10 nm from the measurement of atomic force microscope (AFM, not shown here). However, the SiN_x and ITO layers (as shown in Fig. 1(f)) for SiN_x LEDs without Ag islands layer were flat (R_rms<2 nm) comparing with the one with Ag islands layer. To measure the absorbance of a SiN_x/Ag multilayer, we also prepared a SiN_x/Ag/quartz structure film under the same deposited condition with the LEDs.

 

Fig. 1 (a) The schematic of the devices we fabricated. (b) Scanning electron microscopy (SEM) image of Ag islands film. (c) Dimension distribution of Ag nanoparticles. SEM images of (d) SiN_x film deposited on Ag islands film, (e) ITO electrode of the device with Ag islands layer, and (f) ITO electrode of the reference device.

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The EL and PL spectra of SiN_x -based LEDs were recorded by an Acton SpectraPro-2500i monochrometer coupled to a photomultiplier tube driven by a dc power source and a 325 nm He-Cd laser, respectively. The absorption of films was measured by a HITACHI U-4100 spectrophotometer. The current-voltage (I-V) characteristics of devices were measured by a Keithley 4200 SCS semiconductor parameter analyzer.

3. Results and discussion

Both SiN_x LEDs with and without Ag islands film emit a broad band EL from 400 to 750 nm. EL spectra of both SiN_x LEDs were measured as a function of the injection current, as shown in Fig. 2(a) . It is obviously that the intensity of EL from both SiN_x LEDs increases gradually with the injection current. However, the SiN_x LEDs with Ag islands layer can inject current higher than 40 mA, while the reference SiN_x LEDs are broken down if the injection current is higher than 25 mA. Under the same injection current, the EL intensity of SiN_x LEDs with Ag islands film is always higher than that of the reference devices. Moreover, the EL spectrum shape and main peak position of the two SiN_x LEDs are different. The reference LEDs contain one band located at about 600 nm, while the EL spectra of SiN_x LEDs with Ag islands layer emit at main band peak located at ~550 nm. In order to show clearly, the EL intensity of SiN_x LEDs without Ag islands film was multiplied twice. The blueshift of EL main peak by the addition of Ag islands film may result from its improved carrier injection, as carriers, especially for holes, can be injected into a higher energy level along the band tails of SiN_x due to the enhanced local electrical fields surrounding Ag nanoparticles [22,23]. This blueshift of EL peak was also observed by increasing the injected current in our early works [3]. The EL enhancement factor (I_Ag/I_R) is shown as a function of the injection current in Fig. 2(b). I_Ag and I_R are the EL intensities from SiN_x LEDs with and without Ag islands layer, respectively. Due to the weak signal/noise ratio of EL from the reference devices, the spectrum shape of EL enhancement factor is not smooth but quite rough. The EL enhancement factor of ~14 is observed at ~550 nm at a current of 25 mA, as shown in Fig. 2(b). As higher intensity current can be injected, the output intensity of SiN_x LEDs with Ag islands layer is expected to be improved much more. Figure 2(c) gives the integrated EL intensity of both SiN_x LEDs as a function of the applied voltage. No EL signals can be detected under a low voltage for the reference devices. The turn-on voltage decreases significantly by the addition of Ag islands film. Besides, the output intensity of SiN_x LEDs with Ag islands layer increases much faster than that of the reference devices. At the applied voltage of 6 V, the integrated EL intensity of SiN_x LEDs with Ag islands layer is about 480 times stronger than that of the reference devices. Considering the difference of the injection currents, the ratio of the integrated EL intensity to the injection current as a function of the applied voltage for both LEDs is shown in Fig. 2(d). The external quantum efficiency (η_EQE) is defined by the ratio of the light output power (P_out) to the injected current (I) [10,11], and the integrated EL intensity is in direct proportion to the P_out. Consequently, the EQE can be compared quantitatively by the comparation of the ratio of the integrated EL intensity to the injection current under a same voltage or current. Clearly, the external quantum efficiency of SiN_x LEDs with Ag islands layer is enhanced much more compared with the reference devices. And the external quantum efficiency of SiN_x LEDs with Ag layer increases with the applied voltage, while that of the reference ones decreases. Comparing the EQE of both devices at a same applied voltage, an improvement of EL efficiency of at least one order of magnitude can be obtained by the addition of Ag islands films, as shown in Fig. 2(d).

 

Fig. 2 (a) EL spectra and (b) EL enhancement factor (I_Ag/I_R) of both SiN_x LEDs with and without Ag islands layer, where I_Ag and I_R stand for the EL intensities of devices with and without Ag, respectively. (c) Integrated EL and (d) efficiency curves as a function of the applied voltage for both SiN_x LEDs.

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The enhanced EL intensity of SiN_x LEDs with Ag islands layer was expected as increasing the radiative recombination rate by coupling excitons to SPs. In order to understand the detail mechanism of enhancement from SPs, the extinction spectra and PL spectra of SiN_x films with and without Ag islands layer were characterized, as shown in Fig. 3 . The normalized extinction spectra shown in Fig. 3(a) are attributed to the localized surface plasmon resonance (LSPR) of Ag islands film. The resonance absorption peak of Ag islands is located at about 460 nm while after depositing SiN_x on Ag layer it shifts to about 555 nm due to the large dielectric constant of SiN_x film. Of course, the resonance peak of Ag islands in our metal-insulator-semiconductor (MIS) devices should redshift more considering the larger dielectric constant of silicon substrate than that of quartz. Moreover, we note that the resonance absorption peak from Ag islands is just near the EL peak of SiN_x LEDs. Thus, the enhancement of EL intensity can be attributed to the coupling between the dipole moment of excitons in SiN_x film and localized electric field of LSPs in Ag islands film [10,11]. The PL spectra confirm this result further, as shown in Fig. 3(b). From the inset of Fig. 3(b), the maximum PL enhancement factor is about 2 at about 600 nm. It is the coupling of the dipole moment of excitons in SiN_x to the local electric field of LSPs in Ag islands film that leads to an increase in the radiative recombination rate of excitons, consequently increasing the PL/EL intensity. However, the EL enhancement factor at ~600 nm and the maximum EL enhancement factor at ~500 nm are about 5 and 16, respectively, as can be seen in Fig. 2(b), while the maximum PL enhancement is only ~2. Since the EL enhancement factor is larger than the PL enhancement factor, the coupling of excitons and LSPs in Ag islands film is considered as a partial factor for the enhancement of EL intensity. Furthermore, the redshift of PL peak by the addition of Ag islands film may result from the scattering enhancement of long wavelength by LSPs, where the dipole resonance peak of LSPs has a similar wavelength with that of PL peak for the device with Ag, as shown in Fig. 3.

 

Fig. 3 (a) Normalized extinction spectra of Ag islands film and SiN_x film on the Ag layer deposited on quartz. (b) PL spectra of single SiN_x film and Ag/SiN_x structure, inset is the PL enhancement factor.

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On the other hand, considering that the existence of Ag islands in devices can enhance the inhomogeneous local electric fields at the interface between Ag layer and SiN_x layer [22,23], the carrier injection efficiency would be enhanced significantly. Due to the existence of an additional path of recombination mentioned above, carriers can transfer their energies into LSPs rapidly keeping the carrier density in the luminescence layer low even though the injection level was improved significantly. Consequently, the increasing tunneling probability of carrier into SiN_x film can also result in an enhanced EL intensity. Figure 4 shows the current-voltage (I-V) characteristics of SiN_x LEDs with and without Ag islands layer. The EL signals from both devices can be detected when the injected current is ~5 mA, but the threshold voltage is ~3.3 V for the device with Ag layer and ~6.1 V for the reference device. The lower threshold voltage of the device with Ag layer indicates that Ag islands film can significantly enhance the carrier injection into SiN_x film. However, considering the transferring of carriers between Ag nanoparticles, this enhancement of carrier injection efficiency might not be so much distinctly. Furthermore, we have noticed the roughening of SiN_x and ITO films deposited on Ag islands film, which can also enhance the EL intensity of SiN_x LEDs. As shown in Figs. 1(c) and 1(d), the surface roughness and crystalline grain of ITO film in the device with Ag islands layer are larger than those of ITO film in the reference device. The rougher surface of ITO can increase the transparency of ITO electrode and light extraction efficiency of the device [12,24]. In general, the roughening induced by the addition of Ag islands film plays an important role in the improvement of EL performance of SiN_x-based LEDs, which not only increases the light extraction efficiency, but also enhances the carrier injection efficiency significantly.

 

Fig. 4 Current-voltage (I-V) characteristics of SiN_x LEDs with Ag layer and the reference devices.

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4. Conclusion

The enhancement of EL intensity and EQE are observed for SiN_x-based LEDs with Ag islands film. One order magnitude of EQE enhancement is achieved in the devices. We attributed this significant increase mainly to the improvement in radiative recombination rate of excitons in SiN_x, the increased carrier injection and light extraction efficiency due to the addition of Ag islands film. Our experimental results provide an alternative approach towards the fabrication of SiN_x-based or Si-based LEDs with high EL extraction efficiency. And further improvement of the EL performance of SiN_x-based LEDs can be achieved by the optimization of the surface coverage and/or the morphology of Ag particles.