A tunable red photoluminescence from a-Si:H/a-SiNx multilayers was modulated in the wavelength range of 800–640 nm by controlling the thickness of the a-Si:H sublayer from 4 to 1.5 nm. Subsequent annealing was used to improve red photoluminescence without recrystallization of the amorphous silicon sublayers. The significant enhancement of red emission was found to depend on the decomposition of the Si–H bond in a-Si:H sublayers. Based on the absorption measurement, Raman, and FTIR spectra, the origin of light emission is ascribed to the silicon dangling bonds associated with hydrogen in a-Si:H sublayers, and the mechanism of light emission is suggested from the radiative recombination between the electrons existing at the negatively charged levels of silicon dangling bond and holes at the valence band.
© 2013 OSA
Note that the key technologies based on III-Nitride [1–3] and InGaP [4,5] semiconductors are used for achieving state-of-the-art results in visible light-emitting diodes (LEDs). Although the quantum efficiency as high as 50% has been reported in III-Nitride [1,6], there are still lots of problems for them applied in monolithic optoelectronic integrated circuits. The current work focuses on the visible light generation on silicon platform. In order to realize monolithic optoelectronic integrated circuits by using the mature Si technology, one of the challenging topics is to develop an efficient Si-based light source operating at room temperature [7–9]. Due to the indirect band gap, bulk silicon as a light emitter material usually has low efficiency of light emission and cannot be used as a light source. In recent years, amorphous and nanocrystalline silicon particles embedded in silicon oxide or silicon nitride have attracted a large number of researchers for their high-efficient light emission [10–13]. The 7% photoluminescence quantum efficiency for near-infrared emission has been achieved in Si-rich silicon nitride films . For SiOx films with buried Si nanocrystals on Si nanopillar array, it is reported that the electroluminescence external quantum efficiency is over 0.2%, which is about one order of magnitude higher than the same device made on smooth Si substrate under a power conversion ratio of 1 times 10−4 . And the high-efficiency PL as well as optical gain and light amplification have been observed in the Si nanocrystals embedded in Si-rich oxide matrix . Compared with the silicon particles embedded in dielectric matrix (SiNx or SiO2), a better control of the growth and size of the nanocrystalline silicon grains could be obtained in the Si/SiO2 or Si/SiNx multilayers by adjusting the Si sublayer thickness [15,16]. And it has been reported that the intensity and position of the luminescence peak for Si/SiO2 or Si/SiNx multilayers could be modulated by controlling the thickness of ultrathin Si sublayer [17–19]. However, due to extremely high potential barrier of SiO2, the efficiency of injection of carriers in silicon nanostructures is limited, resulting in low electroluminescence efficiency. On the other hand, because of possessing smaller band gap and higher intensive visible PL, the hydrogenated amorphous silicon and silicon nitride (a-Si:H/SiNx) multilayers have attracted a great interest [20,21]. So far, many theoretical models, such as the quantum confinement effect, interface-states-assisted radiative recombination and defect luminescence have been used to describe the PL mechanisms [22–25]. But it seems that luminescent characteristics are quite different from case to case and the luminescence mechanism is still an open question. The further investigation of the luminescence mechanism is needed to be elucidated.
In this paper, a series of a-Si:H/ a-SiNx multilayers samples with different thickness of a-Si:H sublayer were prepared by plasma-enhanced chemical-vapor deposition (PECVD) method. Post-thermal annealing was carried out and the photoluminescence characters were investigated. We found that the position of photoluminescence can be changed by controlling the thickness of a-Si:H sublayer, and the intensity of photoluminescence can be enhanced as increasing the annealing temperature. Based on the Raman, FTIR, and absorption measurement, the mechanism of the photoluminescence was discussed.
A series of a-Si:H/ a-SiNx multilayers were prepared on Si (100) wafers and quartz plates in a conventional parallel-plate radio-frequency (40.68 MHz) glow discharge system by alternatively repeating the silicon deposition and silicon nitride deposition. The a-Si:H layers were deposited by using the gas mixture of SiH4 and H2 as the precursor with flow rate of 1.5 and 160 standard cubic centimeter per minute (SCCM), respectively. The a-SiNx layers were deposited by using the gas mixture of SiH4 and NH3 as the precursor with the flow rate of 2.5 and 15 SCCM, respectively. The thickness of a-SiNx sublayer is controlled at 4 nm. The thickness of a-Si:H sublayer changes from 4 to 1.5 nm. During the growth process, the r.f. power and substrate temperature was controlled at 30 W and 250°C. The pressure in the chamber was kept at 60 Pa and 17 Pa for a-Si:H and SiNx layers, respectively, by controlling the pumping speed. After deposition the samples were, individually, annealed in the conventional furnace at the temperature of 450°C, 600°C, and 800°C for 1 hr in nitrogen ambient. The microstructures of the samples were revealed by Raman scattering and cross-section transmission electron microscopy (TEM). A Fourier transform infrared (FTIR) spectroscope was used to record the bonding configurations of the samples. Absorption spectroscope was employed to record the absorption properties in the multilayers. PL measurements were carried out at room temperature using He–Cd laser with a wavelength of 325 nm as an excitation source.
3. Results and discussion
Figure 1 shows the typical cross-section TEM micrograph of the multilayer structure. The white and dark regions in the micrograph correspond to a-Si:H with a thickness of 4 nm, and to a-SiNx with a thickness of 4 nm, respectively. As seen in the micrograph, the interfaces of layered structure are flat and abrupt.
Figure 2(a) shows the room-temperature PL spectra of the as-deposited a-Si:H/a-SiNx multilayers with different thickness of the a-Si:H sublayer. It can be seen that the peak position of PL varies from 800 nm to 640 nm as decreasing the thickness of a-Si:H sublayer from 4 nm to 1.5 nm, which indicates that the light emission should be caused by the a-Si:H sublayer and the blue shift of PL peak energy arise from the decreasing of a-Si:H thickness. In order to further investigate the origin of the PL blue shift with the decrease of a-Si:H sublayer thickness, we measured the absorption spectra of the samples and evaluated the optical band gap Eg by Tauc plot . From Fig. 2(b) one can see that the experimental band gap increases with the decrease of a-Si:H sublayer. In fact, for one-dimensional confined a-Si:H film, the energy band gap can be also calculated by the theoretical model E(eV) = 1.8 + 2.2d−2, where d is the thickness of the a-Si:H sublayer , as is shown in Fig. 2(b). It is shown that the experimental band gap has a similar change tendency with the theoretical results as decreasing the thickness of a-Si:H sublayer, which implies that the blue shift of the band gap may be related to the quantum confinement effect. But it can be seen that the experimental band gap is higher than the corresponding theoretical value. According to Ref , the deviation of band gap between the experimental and theoretical results should be caused by the existence of hydrogen in the a-Si:H sublay which widens the band gap. From Fig. 2(b), we noted that the PL energy for all samples is smaller than the corresponding band gap, which is different from that reported by Lo and Ma [18,29] where the optical band gap is coincide with the experimental light emission energy and the light emission is attributed to the recombination of electrons and holes in the band tail of a-Si:H sublayer.
In order to gain more insight on the light emission mechanism of the a-Si:H/a-SiNx multilayers samples, the PL spectra of samples annealed at different temperatures were examined. Figure 3(a) shows the PL spectra of a-Si:H/a-SiNx multilayers with 2 nm a-Si:H sublayer annealed at different temperatures. We can see that the intensity of PL can be enhanced by increasing annealing temperature from 450°C to 600°C, and it is reduced as further increasing the annealing temperature to 800°C. Besides the change of PL intensity, it can also be found that peak position of PL varies from 700 nm to 760 nm as increasing the annealing temperature to 600°C, and the peak position of PL come back to 720 nm as further increasing the annealing temperature to 800°C. Figure 3(b) shows the experimental band gap evaluated by Tauc plot and the PL peak energy as the function of annealing temperature. It can be also found that the value of PL peak energy is lower than that of the corresponding band gap, which is similar with the as-deposited a-Si:H/a-SiNx multilayers as discussed in Fig. 2, implying that the origin of light emission for samples annealed at different temperatures should be related with a-Si:H sublayer.
Figure 4(a) shows the Raman spectra for a-Si:H/a-SiNx multilayers samples with 2 nm a-Si:H sublayer post-treated at different temperatures. The signals at 160 cm−1 and 480 cm−1 represent the transverse-acoustic (TA) vibration mode and transverse-optical (TO) vibration mode of amorphous silicon, which indicates that no Si nanocrystals exist in samples and the light emission of a-Si:H/a-SiNx multilayers before and after annealing should be caused by the a-Si:H sublayer. Compared with the as-deposited sample the signal has a slight change for the samples annealing at 450°C and 600°C. While for the sample annealed at 800°C, the TO mode of amorphous silicon slightly up-shifted, and a weak shoulder around 500 cm−1 can be obtained, which indicates that the network of a-Si:H becomes more ordered as a large of silicon dangling bond becomes of the Si–Si bonds . Meanwhile, it is well known that the TA mode reflects the mediate-distance order of the a-Si:H films, an increase in the ratio of the intensity of the TA band to that of the TO band, ITA/ITO, manifests the increase in the intermediate-range disorder [31,32]. From Fig. 4(b), it can be found that the value of ITA/ITO increases from 0.13 to 0.2 as increasing annealing temperature from 450°C to 600°C, and it is reduced to 0.13 as further increasing the annealing temperature to 800°C, which indicates that the rearrangement and relaxation of the film structures take place during the annealing treatment process.
Figure 5 shows the FTIR spectra of a-Si:H/a-SiNx multilayers with a-Si:H sublayer thickness of 2 nm before and after thermal annealing. It can be found that the changes of Si-N bond intensity at ~840 cm−1 are very slight, while the intensity of hydrogen-related absorption modes becomes weaker and weaker as the annealing temperature increases. In order to further study the changes of chemical bond in a-Si sublayer, the density of Si–H bond is determined by integrating the stretching absorption band at ~2180 cm−1 according to Ref . In the inset of Fig. 5, it can be clearly found that the intensity of Si–H bond decreases as the annealing temperature increases, which indicates that the hydrogen effuses from samples and a lot of silicon dangling bond will be formed. Combining the results of Raman and FTIR spectra, it can be concluded that the silicon dangling bond will be formed in a-Si sublayer due to the effusion of hydrogen. As increasing the annealing temperature, the content of silicon dangling bond will be increased , and when the temperature is increased to 800°C, the content will be reduced as a large of silicon dangling bond becomes of the Si–Si bonds. Compared with the change of silicon dangling bond, we noted that the change of PL intensity has a similar tendency that increases with the annealing temperatures from 450°C to 600°C and decreases when annealing temperatures is 800°C. Therefore, it is reasonable to consider that the origin of PL is related to the silicon dangling bond associated with the hydrogen in a-Si:H sublayer.
According to Vaillant et al. , the silicon dangling bond levels diagram for a-Si:H film can be described as shown in Fig. 6. In the diagram, EC and EV represent the edge of the conduction and valance band, respectively. EF is the Fermi level. T30, T3+, and T3- are the neutral, positively and negatively charged silicon dangling bond, respectively. The position of EF are consistent with the position of T30 level at about the mid of the band gap. The effective correlation energy EU between T30 and T3- is about 0.4 eV. It can be seen that the silicon dangling bond levels appear in the band gap, which is much less than the optical band gap of a-Si:H film, and the energy between T3- level and valance band can be expressed as ET = Eg/2 + 0.4 approximately, where Eg is the experimental band gap of a-Si:H film. According to the values of optical band gap, the emission band energy related to electronic transitions between the T3- levels and the valence band is given in Figs. 2(b) and 3(b) for samples with and without thermal annealing. It can be found that the emission band energy related to electronic transitions between the T3- levels and the valence band is compatible with the peak energy of PL, which indicates that the origin of PL in the a-Si:H/a-SiNx multilayers samples should be related to the T3- levels of silicon dangling bond associated with the hydrogen. Consequently, we can suggest that the electrons are mostly photo-excited to the high excited-state energy level above the conduct band corresponding to the laser energy (325 nm) and relaxed to the T3- levels of silicon dangling bond by emitting phonons, and then radiatively recombine with holes at the valence band, as shown in Fig. 6. According to the result of FTIR spectra, as increasing the annealing temperature, the light-emission centers of silicon dangling bonds in a-Si:H sublayer associated with the hydrogen will be increased due to the effusion of hydrogen, which contributes to the increase of PL intensity for the samples annealing at 450°C and 600°C. While, when the annealing temperature is 800°C, the content of silicon dangling bond is reduced as a large of silicon dangling bond become of the Si–Si bonds. As a result, the light-emission centers of silicon dangling bonds are decreased and the PL intensity become weak, which well explains the experimental results that the intensity of PL decreases after annealing at 800°C compared with that annealed at 600°C.
In summary, we have investigated and discussed the properties of photoluminescence from a series of a-Si:H/a-SiNx multilayers samples fabricated by using PECVD method. A tunable red photoluminescence in the range of 800–640 nm is achieved by controlling thickness of a-Si:H sublayer from 4 nm to 1.5 nm. Combining the absorption measurement, Raman and FTIR spectra, the origin of light emission is ascribed to the silicon dangling bond associated with hydrogen in a-Si:H sublayer. Different from the band-to-band radiative recombination, the mechanism of light emission was suggested that the electrons photo-excited to the high excited-state energy level relaxes to the T3- levels of silicon dangling bond, and then radiatively recombine with holes at the valence band. Our results and discussions provide a new insight of luminescence mechanism and useful guidance for the optoelectronic device application.
This work is supported by NSF of China (No. 61274140), the NSF of Guangdong Province (S2011010001853), the Project of DEGP (No. 2012KJCX0075) and the Foundation for Distinguished Young Talents in Higher Education of Guangdong (LYM11090, LYM09101, and LYM10099).
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