Visible electroluminescence (EL) with two composite bands, i.e., a violet band and a green-yellow band has been observed from Si-implanted silicon nitride thin films. By varying the intensity ratio of the two composite EL bands in terms of the injection current, strong white-color EL can be achieved at certain injection currents (e.g., ~265 mA/cm2). The observed transition in EL color from violet to white under different injection conditions is studied based on the understanding that the violet band is originated from silicon nitride matrix while the green-yellow band is related to the implanted Si. The Si-implanted silicon nitride thin film offers the possibility of electrically tunable white-light Si-based light emitters.
© 2010 OSA
Recently, Si-rich nitride (SRN) has attracted great interest in fabrication of high-performance low-cost light emitting devices compatible with the mainstream silicon technology, which are essential to optical interconnects in monolithic optoelectronic integrated circuits. In addition to light emissions from Si nanocrystals formed by the excess Si in SRN , various defect states in the silicon nitride host or at the surface of Si nanocrystals can also be responsible for the light emission process as luminescence centers [2–4]. As compared with oxide-based counterparts, the SRN materials have smaller barriers for electron/hole injections, which offer potentials to fabricate low-voltage Si-based light emitters. Because of its merits, SRN is the preferred active material in Si-based light emitter applications. Visible electroluminescence (EL) covering the whole white light spectrum has been observed from various SRN-based structures [1,2,5–7]. However, the mechanisms of white-color EL from SRN are still under debate. It is believed that the white light emissions are combined contributions from various luminescence centers related to the formation of Si nanocrystals, and it may be that the silicon nitride host also takes part in the light emission.
Si ion implantation into Si3N4 films is one of the direct approaches to introduce excess Si into silicon nitride, which can control the distribution and density of the excess Si atoms or the subsequent Si nanocrystals by modifying the implantation dose and energy. Optically or electrically excited light emissions from Si-implanted Si3N4 thin films have been studied in our previous works [5,8,9]. In the present work, a strong white-color EL has been demonstrated from Si3N4 thin films with a single implantation of Si ions by electrically tuning the violet and green-yellow bands under a high injection current of ~265 mA/cm2. An EL color transition from violet to white with increasing injection current was observed, which is attributed to the relative evolution of the violet and green-yellow bands.
2. Experimental details
A 30-nm thick Si3N4 film was deposited on a p-type (100) Si wafer with the resistivity of 9-12 ohm∙cm by low-pressure chemical vapor deposition (LPCVD). Subsequently, Si ions of 4 × 1016 atoms/cm2 were implanted into the Si3N4 thin film at the energy of 12 keV at room temperature. The depth profile of the implanted Si ions in the Si3N4 thin film can be obtained by a Monte Carlo simulating program of the stopping and range of ions in matter (SRIM) . As shown in Fig. 1(a) , the implanted Si ions distribute throughout the entire 30 nm Si3N4 film with the concentration peak at ~14 nm below the film surface. The peak excess Si concentration is ~2.4 × 1022 cm−3. The Si-implanted thin film was annealed at 1100 °C for 1 hour in flowing N2 gas. To fabricate the metal-insulator-semiconductor (MIS)-like light emitter structure as shown in Fig. 1(b), a 120-nm indium tin oxide (ITO) film with the diameter of 1.2 mm as the transparent electrode was deposited on the Si-implanted film by rf sputtering and a 200-nm Al layer as the back ohmic contact was coated on the Si wafer backside by E-beam evaporation. Both electrical and EL measurements were performed on the device at room temperature.
3. Results and discussion
Figure 2 shows the EL spectra from the Si-implanted thin film obtained with different injection currents under negative voltages. The photographs of the corresponding light emissions are shown in the figure also. Under the low injection current of about −9 mA/cm2, the EL spectrum exhibits an EL peak at ~2.9 eV (425 nm) and an EL shoulder at around 2.2 eV (560 nm); as the EL peak at ~2.9 eV is predominant, a violet light emission can be observed by the naked eye in the dark. As the injection current is increased, the EL becomes stronger with simultaneous increases in intensities of the two EL peaks. However, the intensity of the EL peak at ~2.2 eV increases faster than that of the EL peak at ~2.9 eV, causing a change in the EL spectrum shape. Under an injection current of −265 mA/cm2, the intensity of the peak at ~2.2 eV is even greater than that of the peak at ~2.9 eV, and a bright white light emission with the estimated external quantum efficiency in the order of 10−4-10-3% can be observed by the naked eye. To the best of our knowledge, such an interesting current-dependent EL device tunable from violet to white has not been reported for SRN materials previously.
As shown in Fig. 2, the above-mentioned two EL peaks can be represented simply by two Gaussian peaks, i.e., the EL spectra can be deconvoluted into two major composite EL bands including the violet band at ~2.9 eV (425 nm) and the green-yellow band at ~2.2 eV (560 nm). Note that the other EL bands including the ultraviolet and near infrared bands are not fitted here due to their low intensities (the intensities are less than 5% of that of the major EL bands) . The peak positions (i.e., the peak wavelength λpeak or the peak energy Epeak) and the peak full width at half maximum (FWHM) of the two composite bands, the ratio (Imax-GY / Imax-V) of the peak intensity of the green-yellow band to that of the violet band, and the integrated-EL-intensity ratio (SGY / SV) of the green-yellow band to the violet band as a function of the injection current are shown in Fig. 3 (a) – (d), respectively. As shown in Fig. 3(a), the peak positions of the two EL bands are insensitive to the injection current. On the other hand, as can be seen in Fig. 3(b), the FWHM of the green-yellow band is larger than that of the violet band. When the injection current is increased from −26 mA/cm2 to −265 mA/cm2, the FWHM of the green-yellow band decreases from 0.70 eV to 0.56 eV, while the FWHM of the violet band increases from 0.44 eV to 0.55 eV. The peak intensities of both bands increase with injection current simultaneously. However, the peak intensity of the green-yellow band increases faster than that of the violet band, leading to an increase in the peak-intensity ratio (Imax-GY / Imax-V) with injection current as shown in Fig. 3(c). For low injection currents (e.g., < −61 mA/cm2), Imax-GY / Imax-V < 0.55, the violet band is dominant. However, at high injection current of −265 mA/cm2, Imax-GY / Imax-V = 1.1, showing that the peak intensity of the green-yellow band is even slightly greater than that of the violet band. The integrated-EL-intensity ratio (SGY / SV) of the green-yellow band to the violet band, which includes the effects of both the peak intensity and the FWHM, also increases as the injection current is increased, as shown in Fig. 3(d). Obviously, the changes in Imax-GY / Imax-V and FWHM with injection current lead to a change in the spectrum shape or the color. In fact, the color transition from violet at the low injection currents to white at the high injection currents is mainly due to the changes in Imax-GY / Imax-V. Similar tunable two-band EL at other wavelengths was observed from an organic double-heterostructure  and a two-substance structure of silicon nitride and silicon carbide , where the EL tuning was dependent on the active layer thickness  and was related to the radiative recombination location in the structures .
The radiative recombination of the injected electrons and holes should be affected by the carrier transport in the Si-implanted thin film. Figure 4(a) shows the current-voltage (I-V) characteristic of the Si-implanted thin film under negative voltages. The current conduction in the Si-implanted thin film at large voltages at which the EL can be excited follows a power law, I = α(V-Vth)ζ, where I and V are the magnitudes of the current and voltage, respectively, Vth is the global threshold voltage, α is a coefficient and ζ is a scaling exponent . The current conduction can be explained by the conductive percolation paths formed by the closely distributed excess Si atoms and defects introduced during the Si ion implantation, which serve as the tunneling nodes for the injected carriers . With the percolation networks, the ITO electrode and the Si substrate can be electrically connected. And thus, the collective charge transport in the Si-implanted films can be improved as compared with the silicon nitride thin film without Si implantation, which is essential to achieve EL from Si-implanted silicon nitride thin film. The integrated EL intensity in the measured wavelength range is summarized in Fig. 4(b) as a function of the injection current. As can be seen in the figure, the integrated EL intensity shows a linear relationship with injection current, which suggests that the radiative recombination of the transported electrons and holes occurs at the location of the corresponding luminescence centers along the conductive percolation paths .
The violet band and green-yellow band observed here are the same as or similar to the two major bands observed from multiply-Si-implanted Si3N4 films [5,9]. The violet band is attributed to the radiative recombination between the defect state related to the Si dangling bond (≡Si0) located in the middle of silicon nitride band gap and the bonding state (σ) of the Si-Si≡ unit that is close to the valence band edge [2,6,9,16]. And the green-yellow band is due to the transition from the ≡Si0 state to the state related to the nitrogen dangling bonds ( = N–) in the valence band tail, which is located at 0.8 eV above the valence band maximum [9,16,17].
EL tunable to the higher energy side with increasing excitation condition has been reported [7,18,19]. The phenomenon could be explained by the luminescence of higher energies excited by more energetic carriers at higher electric fields [7,18]. Nonradiative Auger recombination, Coulomb charging effect and the quantum-confined stark effect that lead to EL quenching could be other possible reasons for the EL blue-shift caused by a larger voltage . However, the evolution of the two EL bands with increasing injection current observed in this work could be a different situation. The evolution of the dominant EL band as well as the EL color with injection current of the Si-implanted film with a single implantation in the present work cannot be observed in the multiply-Si-implanted films with a uniform distribution of the implanted Si ions . The difference in the EL evolution between the single implantation and the multiple implantations suggests that the evolution of the two composite EL bands in the present work might be related to the distribution of excess Si in the silicon nitride thin film. Considering the distribution of the implanted Si ions shown in Fig. 1(a), it can be assumed that the major radiative recombination occurs in the region near the Si substrate for low negative voltages (small injection current) and in the Si-implanted film middle for high negative voltages (large injection current). If this assumption holds, the luminescence centers from Si3N4 host determine the light emission under small injection currents due to the low excess Si concentration near the Si substrate, while the implanted Si of a large concentration in the film middle should play a more important role in EL for large injection currents. Coincidently, it was observed that the Si implantation can produce a photoluminescence (PL) band at 1.82 eV (680 nm), and another PL band at 2.85 eV (435 nm) from the Si3N4 host can also be detected from the Si-implanted Si3N4 film . Although there are some differences between the corresponding EL and PL band positions, which might be due to the different carrier excitation mechanisms, the PL results could imply that the violet EL band is originated from the silicon nitride matrix and the green-yellow EL band is related to the implanted Si. The white-color EL from the Si-implanted film is actually a combined effect of the contributions from both the silicon nitride matrix and the implanted Si. Therefore, if the major radiative recombination region shifts from the few-excess-Si areas to the more-excess-Si areas, a transition in dominant EL band from the violet one to the green-yellow one could occur. On the other hand, the EL quenching of Si-implanted Si3N4 films is more severe for the violet band than the green-yellow band , which can also affect the evolution of the intensity ratio of the violet band to the green-yellow band with increasing injection current. The EL quenching effect could be another possible reason for the transition of the dominant EL band and the EL color tuning from violet to white.
In summary, we have demonstrated a strong white-color EL from a single-Si-implanted silicon nitride thin film under proper injection currents. The EL color transition from violet to white under different injection currents is observed and attributed to the relative intensity evolution of the violet and green-yellow EL bands. The mechanisms for the relative intensity evolution of the EL bands are discussed. It is concluded that the violet EL band is originated from the silicon nitride matrix while the green-yellow band is related to the implanted Si. The observed white-color EL from the Si-implanted Si3N4thin film is a result of the combined effect of the contributions from both the silicon nitride matrix and the implanted Si.
This work has been financially supported by the National Research Foundation of Singapore under project No. NRF-G-CRP 2007-01. Y. Liu would like to acknowledge the Grant under project No. 2008ZC80.
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