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A single layer of dense Si quantum dots with average size of 4 nm sandwiched in amorphous SiN layers was prepared by laser crystallization of ultrathin amorphous Si film followed by subsequently thermal annealing. The electroluminescent diodes were fabricated by evaporating Al electrodes on back sides of p-Si substrates and the top surface of samples. Room temperature electroluminescence can be detected with applying the negative voltage around 10V on the top gate electrode and the luminescent intensity is increased with increasing the applied voltage. It was found that the integrated luminescent intensity is linearly proportional to the injection current which suggested the intensity depends on the concentrations of injected carriers after Fowler-Nordheim tunneling through amorphous SiN barriers. The influence of the amorphous SiN with different band gap on the device performance was also discussed briefly.
©2009 Optical Society of America
1. Introduction
Light emission from Si-based materials is currently one of the challengeable research subjects in order to realize the next generation of monolithic optoelectronic integrations by using the mature Si technology [1–3]. Due to the indirect band gap, bulk Si has very low luminescence efficiency and can not be used as a light source. In order to circumvent the inability of bulk Si material, many approaches have been proposed to get the strong light emission from Si-based materials, such as porous Si [4], Si quantum dots [5–6], Si-based alloys [7–9], rare-earth doped Silica films [10] in which, the Si quantum dots embedded in surrounding matrix is believed to be one of the most potential candidates.
So far, photoluminescence (PL) and electroluminescence (EL) characteristics have been investigated extensively, especially in Si quantum dots (QDs)/SiO2 system [11–15]. Compared with the large band gap of SiO2 (8.9eV), amorphous SiN film has low injection barriers both for electrons and holes and the bipolar injection is possible to get efficient emission in Si QDs/SiN structures[16–17]. Actually, a strong EL was reported in Si QDs embedded in a-SiN matrix [18]. In our previous work, a single layer of Si QDs on amorphous SiN (a-SiN) layer was fabricated by the method combining the pulsed laser irradiation on the ultrathin amorphous Si (a-Si) films and subsequently thermal annealing [19]. It was found that a dense Si QDs array can be formed and the size of formed Si QDs can be well controlled by the a-Si film thickness. An intense photoluminescence was observed at room temperature and the emission peak can be tunable with the film thickness. Pulsed laser crystallization of a-Si thin films is a low temperature process due to the short pulse length and optical absorption depth which can avoid the high temperature damages in device fabrication. In this work, the electroluminescent devices were fabricated based on a-SiN/Si QDs/a-SiN sandwiched structures. The sandwiched or multilayered structures have the advantages such as the size controllability, high areal density of Si QDs and the single barrier thickness instead of the wide distribution of barrier thicknesses in homogeneous layer which can be helpful for improving the electroluminescence efficiency [13, 20]. By controlling the treatment conditions, Si QDs with an average size of 3–4nm can be achieved with the areal density as high as 1012/cm2. Electroluminescence can be observed from the sandwiched structures containing a single layer Si QDs. It is found that the post thermal annealing at a moderate temperature (700°C) can obviously enhances the EL intensity and the influence of amorphous SiN barrier layer with different Si/N ratio was investigated.
2. Experimental
Amorphous SiN(20nm)/amorphous Si(4nm)/ SiN(30nm) sandwiched structures were prepared in plasma enhanced chemical vapor deposition system with radio frequency (rf) of 13.56.MHz. The substrate temperature and the rf power was kept at 250°C and 30W, respectively. The chamber pressure was about 20Pa during the deposition process. Amorphous SiN insulating layer was deposited by using silane and ammonia gas mixtures while a-Si:H layer was deposited by using silane diluted by Ar gas. Crystalline Si wafers and fused quartz glass were used as substrates for various measurements. In order to get the a-SiN barrier layers with different optical band gap, two kinds of samples were prepared. The flow rate of ammonia gas is 40sccm and 20sccm for sample A and sample B, respectively while the gas flow rate of silane is kept at 8sccm during the whole deposition process. Their optical band gaps were deduced from the thick a-SiN films deposited under the same conditions according to the Tauc plots based on measurement results using the UV-VIS-NIR spectrophotometer [7].
A KrF excimer laser operating at 248nm wavelength with 30ns pulse duration was used to crystallize the sandwiched structures prepared on p-Si substrates (1–3 Ohm-cm) in air ambient. Only a single pulse with laser fluence around 0.5 J/cm2 was employed and the corresponding laser spot area is 5×3mm2. After the laser irradiation, all samples were subjected to a thermal annealing at 700°C for 1hr under N2 ambient in order to relax the stressed film network. The film microstructures were characterized by Raman scattering spectroscopy and transmission electron microscopy (TEM). The planar TEM samples were prepared by lifting of the laser crystallized films from the quartz substrates using HF solution and then moving to the copper grid for TEM observations. Room temperature photoluminescence was detected by using Ar+ laser with wavelength of 488nm as an excitation source. The sandwiched structures after laser and thermal annealing were used to construct electro-luminescent diodes by evaporating Al electrodes on the top surface of films and the bottom of the p-Si substrates. The device structure is similar with the case with Si/SiO2 multilayers structures reported previously [21]. For EL measurements, the DC voltage was applied on the samples and the forward biased condition is defined as the gate electrode (Au) is negatively biased (Vg < 0). All the luminescence results were corrected for the spectral response of the system.
3. Results and discussion
Figure 1(a) gives the planar TEM images for laser crystallized SiN/Si/SiN sandwiched structures with a-SiN layers deposited with ammonia gas flow rate of 40sccm (Sample A). The formation of Si QDs can be clearly identified which is consistent with the Raman measurements that the laser induced crystallization of a-Si films occurs when the laser fluence is larger than 0.2J/cm2. The average size of Si QDs is around 4nm and the area density can be estimated about 1.2×1012/cm2. The cross-sectional TEM image of the same sample was given in Fig. 1(b). It is found that the sandwiched structures are still kept after laser and thermal treatments and a single layer containing dense Si QDs is formed with the initial a-Si layer and the size can be well confined by the adjacent a-SiN layers. Inset of Fig. 1(b) is the high resolution TEM image which shows a crystallized Si dot with size around 4nm. The spherical Si dot with (111) orientation faces can be well identified. It is noticed that the a-SiN layers show the featureless structures in the cross-sectional TEM images which indicates that the amorphous SiN layers used in our case is stable against the laser crystallization and the subsequently thermal annealing.
Room temperature electroluminescence (EL) signals under the gate voltage of -10V can be detected from laser crystallized sandwiched structures as shown in Fig. 2. The luminescence band is located around 600nm and it is quite broad with the full width at half maximum (FWHM) larger than 120nm. It is found that the EL signals can only be obtained under the forward bias conditions (negative voltage applied to the top gate electrode). As a reference, The EL spectrum from as-deposited sandwiched structures measured under the same gate voltage (-10V) is also given in the same figure. The EL intensity from as-deposited sample is stronger than that from laser crystallized one and the EL peak is centered at 680nm. The EL band shape is also quite different from that obtained from laser crystallized sample which implies that the luminescence may originated from the different mechanism due to the formation of Si QDs after laser crystallization.
It is found that the EL intensity is very weak for samples only treated by laser crystallization. However, the luminescence intensity can be significantly enhanced after the subsequently thermal annealing at 700°C for 1hr. Figure 3(a) gives the EL spectra for sample A after the subsequently thermal annealing. The EL intensity is increased by more than one order of magnitude after thermal annealing while the EL peak energy and shape is almost unchanged under the low gate voltage compared with that sample treated only by laser crystallization. The similar phenomena have also been observed in photoluminescence measurements as reported in our previous work [18]. It looks like that subsequently thermal annealing can relax the film structures and reduce the non-radiative defect states generated during the laser treatment process which can enhances the luminescence efficiency obviously. Thermal annealing may also helpful for finishing the crystallization process to get well-defined crystallized Si QDs which can improve the carrier transportation characteristics of the Si QDs based light emitting devices and consequently enhance the luminescence intensity.
Fig. 1. (a)Planar TEM images for laser crystallized SiN/Si QDs/SiN sandwiched structures of Sample A. Inset of figure (a) is the high resolution TEM image and the formation of Si QDs can be clearly identified. (b) The cross-sectional TEM image of the same sample which shows the sandwiched structures and the size of Si QDs can be well confined by the film thickness.
Fig. 2. Electroluminescence (EL) spectra of as-deposited sample and laser crystallized sample under laser fluence of 0.5 J/cm2 without the subsequently thermal annealing. The EL signals were measured by applied the gate voltage of -10V.
As shown in Fig. 3(a), the EL intensity is gradually increased with increasing the gate voltage. The turn-on voltage for the EL device, defined as the gate voltage at which the EL signals can be detected in our measurement setup, is less than 10V. It is found that the EL peak is located around 600nm under the low gate voltage but it is slightly blue shifted to 550nm with increasing the gate voltage. It has been reported in Si QDs/SiO2 system that the EL peak position is blue-shifted first and then tends to saturate with increasing the applied voltage [15]. They attributed it to the size distribution of the Si QDs and the possible quantum-confined Stark effect. In our case, the EL band around 580nm can be ascribed to the radiative recombination of injected electron-hole pairs via Si QDs or the luminescent centers within Si QDs/SiN interfaces. Since the Si QDs with small size can only be excited under the high gate voltage due to the large splitting energy level according to the quantum size effect which emit the light with short wavelength (high photon energy), the EL intensity in the short wavelength side will be enhanced which causes the blue-shift of the EL spectrum as seen in Fig. 3(a). Considering the broad EL band, it may consist of a few sub-bands with different origins. The relative intensity of each bands may changed under the different gate voltages which can also result in the shift of the EL peak by overlapping into a one broad band. The further investigation is needed to address this point more clearly.
Fig. 3. (a)EL spectra of sample A and (b) sample B after laser crystallization and the subsequently thermal annealing at 700°C for 1hr. The EL signals were measured under the various gate voltages. Dot line in (a) indicates the slight blue shift of the band peak energy with increasing the gate voltage.
Since the luminescence intensity is dominated by the radiative recombination probability in Si QDs as well as the injection efficiency of electron and holes, the amorphous SiN barrier has a strong influence on the device performance. In order to further understand the EL characteristics from SiN/Si QDs/SiN sandwiched structures, we prepared sample B with amorphous SiN layers deposited by using the small ammonia gas flow rate (20 sccm). The optical band gap is 2.8eV and 2.3eV for amorphous SiN film deposited with 40 sccm and 20sccm ammonia gas flow rate together with 8 sccm silane gas, respectively, which means the SiN barrier height of sample B is lower than that of sample A. Figure 3(b) gives the EL spectra for sample B after laser and thermal annealing under the same parameters. It is found that the turn-on voltage is reduced to 7V for sample B which can be attributed to the low injection barrier height by using amorphous SiN layers with small band gap. The fact that the EL peak position for sample B is still around 600nm suggests that the EL from our sandwiched structures is originated from Si QDs other than from amorphous SiN layers. Recently, the direct current white thin film electroluminescent devices based on hydrogenated amorphous SiN films with 30nm thickness were studied and the emission wavelength is usually in blue-green light region due to the large band gap of amorphous SiN materials and the devices can emit light wither the forward or reverse bias [22]. In our case, amorphous SiN was deposited under high gas ratio of ammonia to silane which was used as barrier material as in our previous work [23]. We found that the luminescence can be changed with changing the Si/N ratio which results in the emission wavelength in a range of 620–510nm even in Si-rich SiN films. The almost same EL band between sample A and B and the different in EL spectra for samples with and without Si QDs (as shown in Fig. 2) indicate that the Si QDs layer plays an crucial role in the electroluminescence from crystallized sandwiched samples though we can not completely rule out the influences from the amorphous SiN layers.
The current-voltage (I–V) curves for both samples (A and B) are given in Fig. 4(a). The clear rectification characteristics can be observed and the injection current for sample B is much higher than that for sample A under the forward bias conditions. The large current means the more electron-hole pairs inject into the sandwiched structures to generate the light via recombination. Figure 4(b) gives the plots of ln(I/V2) ~ V-1 for both sample A and Sample B. The straight lines in the voltage above 0.5V suggest the Fowler-Nordheim quantum tunneling mechanism is dominant in the carrier transport process. It should be mentioned that the carrier transport behaviors may affected by the various process simutaneously. It is noted in Fig. 4(b) that an extreme point is shown for sample B which means the characteristics of current versus voltage is changed at high electric field. It is possible that other mechanism such as Frenkel-Poole emission process may affect the transport characteristics as suggested before especially in high electric field region for sample B due to the existence of trap states in amorphous SiN layer with high Si/N ratio [22]. The smaller slope of sample B compared with that of sample A indicates the lower barrier height for carriers due to the amorphous SiN layer with narrow band gap [23].
Fig. 4. (a) Current-voltage plots of sample A and B, respectively. (b) Fowler-Nordheim plots of two samples in the forward bias region (negative voltage).
The plots of the integrated EL intensity as a function of injection current for both samples are shown in Fig. 6. It is found that the integrated intensity for both samples is linearly proportional to the injection current which indicates that the luminescent intensity is controlled by the injected electron-hole pairs. The integrated EL intensity of sample A is much higher (about 5 times) than that of sample B under the same injection current which means the luminescence efficiency in sample A is higher than that in sample B. The higher radiative recombination probability of sample A than that of sample B can be attributed to two possibilities. One is the better confinement effect of carriers in sample A compared with sample B. Since the amorphous SiN barrier layer used in sample A has a large band gap, the injected electrons and holes can be well confined in the Si QDs layer which acts as the potential well in the sandwiched structures. The confined electrons and holes can be easily recombined in Si QDs to emit visible light. For sample B, the electrons and holes can be injected more efficiently under the applied voltage but they can easily leaked from the Si QDs layer due to the low barrier height, which causes the low recombination efficiency. Another reason is due to the strong non-radiative recombination probability in Sample B. Actually, it is mentioned before that at high electric field, the current-voltage characteristics for sample B is different from Fowler-Nordheim tunneling process, the trap-assisted transport process influence the carrier transport behavior due to existence of the defect states in amorphous SiN layers with narrow band gap and this phenomenon can not be observed in sample A. It is also found that the photoluminescence (PL) spectrum for sample A and B is quite different. The PL intensity is very weak in sample B as shown in Fig. 6 suggesting the most of the photo-excited electron-hole pairs non-radiatively recombine via the defect states or interface states. Therefore, the radiative recombination efficiency in sample B is significantly reduced both in photoluminescence and electroluminescence process.
Fig. 5. The plots of the integrated EL intensity as a function of injection current for sample A and sample B, respectively.
Fig. 6. Room temperature photoluminescence spectrum for sample A and sample B after laser crystallization and subsequently thermal annealing.
4. Conclusion
In summary, we have fabricated the electrically driven luminescence diodes based on amorphous SiN/Si QDs/SiN sandwiched structures. The Si QDs with the controllable size and high area density (>1012/cm2) were obtained by laser crystallization and subsequently thermal annealing techniques at a moderate temperature. The EL diodes can operate at room temperature and the turn-on voltage is less than 10V. The luminescence intensity is obviously enhanced by the thermal annealing process and is controlled by the injected carrier concentrations after F-N tunneling through the amorphous SiN barriers. Lowering the barrier height can reduce the trun-on voltage to 7V. It is found that the integrated EL intensity by using amorphous SiN barrier layers with large band gap is obviously higher than that using SiN layers with small band gap which may due to the different band offset and the non-radiative recombination probability.
Acknowlegments
This work was partly supported NSF of China (10874070 and 60425414),“973”project (2007CB613401) and SRFDP (20070284020).
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