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Abstract
The application of silicon nanocrystals (Si-NC) is somewhat limited due to their low luminescence intensity. Therefore, it is of interest to investigate methods for enhancing the luminescence intensity of Si-NC. In this study, phosphorus (P)-doped Si-NC with two different doping methods were prepared by electron beam thermal evaporation: in-situ doping (during synthesis) and ex-situ doping (after synthesis). The photoluminescence (PL) intensity and crystallinity of Si-NC can be enhanced through phosphorus doping. Moreover, a comparison between two different methods of Si-NC doping reveals that the luminescence intensity of in-situ P-doped Si-NC is superior to that of ex-situ P-doped Si-NC, which is increased by an order of magnitude compared to the PL intensity of undoped Si-NC.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, silicon nanocrystals (Si-NC) have been widely utilized in microelectronics and optoelectronics due to their tunable band gap, unique optical and electrical properties, which has set off a boom in the research of Si-NC [1–5]. The potential application of Si-NC depends mainly on the improvement of its performance, which can be achieved through doping with positively (phosphorus) or negatively (boron) charged impurities to enhance its optical and electrical propertie [6–15]. In 2000, Atsushi et al. [16] prepared p-doped Si-NC by co-sputtering using phosphosilicate glass substrate as a target, showing that low concentration of phosphorus doping could reduce the number of Si-NC interface defects and improve the PL intensity. In 2009, Hao et al. [17] deposited Si-rich oxide films containing P by magnetron co-sputtering Si, SiO2, and P2O5 targets and diffused P into the interior of Si-NC by high-temperature annealing. Their findings illustrate that the presence of P enhances the phase separation of Si-rich oxides and Si crystallization, ultimately leading to an increase in the PL intensity of Si-NC. In 2015, Xiang et al. [18] doped phosphorus atoms into Si/SiO2 multilayers using phosphine as a dopant by plasma chemical vapor deposition, suggesting that appropriate P doping can effectively fill the carrier capture centers in the interface of Si nanocrystals, thereby improving the photoluminescence intensity of Si-NC.
In this paper, P-doped Si-NC multilayer films were prepared by a thermal evaporation method using SiO and Si as raw materials and black phosphorus quantum dots as a dopant. Three kinds of P-doped Si-NC were prepared by varying the doping method: doped during synthesis (in-situ doping), annealed with the film face up, and annealed with the film face down after synthesis (ex-situ doping).
2. Experiment
The undoped Si-NC and P-doped Si-NC multilayer films in this work were made onto a p-type Si <100> (0.5-1.0 Ω. cm) substrate. A certain amount of black phosphorus quantum dots with a concentration of 0.1 mg/ml was spin-coated on the p-type Si substrate surface as a dopant. The p-type Si substrate with and without black phosphorus quantum dots were placed in an electron beam thermal evaporation coater to alternately deposit SiO and Si (the thickness is 3 nm for the SiO layer and 2 nm for the Si layer with a period of 20 times) to obtain 100 nm multilayer films. In this study, we have prepared four samples. Figure 1 illustrates the structural schematic of these samples. The multilayers film without black phosphorus quantum dots were annealed at 1100°C for 1 h under an N2 atmosphere, the undoped Si-NC was obtained (S1). A certain amount of black phosphorus quantum dots were spin-coated onto the undoped Si-NC surface, followed by annealing with the surface facing up and down for 1 hour at 300°C. Subsequently, P-doped Si-NC were obtained through annealing with doping film face up after synthesis (S2) and doping film face down after synthesis (S3). A certain amount of black phosphorus quantum dots were spin-coated on the p-type Si substrate, followed by the preparation and annealing of multilayer films in an N2 atmosphere at 1100 °C for 1 hour. This resulted in the synthesis of P-doped Si-NC (S4).
Fig. 1. Schematic diagram of structures prepared by different doping methods. (a) Undoped Si-NC. (b) P-doped Si-NC prepared by annealing with doping film face up at 300°C after synthesis. (c) P-doped Si-NC prepared by annealing with doping film face down at 300°C after synthesis. (d) P-doped Si-NC doped during synthesis.
The PL spectra of Si-NC were measured by a fluorescence spectrophotometer (HORIBA + FluoroMax-4LC) at room temperature. The absorption spectra were measured by a UV-visible spectrophotometer (UV-1900PC type). The structure and composition of Si-NC were characterized by XRD (Evolution 220) and XPS (Shimadzu Axis supra+).
3. Results and discussion
Figure 2 displays the XPS spectra of Si 2p in P-doped Si-NC prepared via different doping methods. Specifically, Fig. 2(a) illustrates the Si 2p spectra of P-doped Si-NC that was annealed with the doping film facing upwards at a temperature of 300°C after synthesis (the inset depicts a local enlargement of Si°). In contrast, Fig. 2(b) shows the Si 2p spectra of P-doped Si-NC that was annealed with the doping film facing downwards at a temperature of 300°C after synthesis, while Fig. 2(c) presents the Si 2p spectrum for P-doped Si-NC doped during synthesis. The results showed the appearance of Si° (99.3 eV) and Si4+ (103.2 eV) after annealing, which indicates the SiO/Si layer has phase separation during heat treatment to form Si-NC and SiO2[19]. Si° (99.3 eV) and Si4+ (103.2 eV) exhibit more distinct features in Fig. 2(b) and Fig. 2(c) compared to Fig. 2(a). This indicates that the conversion rate of Si-NC prepared by annealing with doping film face down at 300°C by ex-situ doping and in-situ doping is higher due to the introduction of P which promotes the phase separation of Si-rich oxide films during annealing, thereby promoting the crystallization of Si. In S2, the intensity of the Si° signal is significantly lower than that in S3 and S4 due to the sublimation of P during hot annealing with doping film face up after synthesis. The sublimated P either deposits on the wall of the annealing furnace or discharges into the atmosphere with protective gas after cooling, failing to enter Si nanocrystals and thus not promoting their growth. Therefore, there is less Si° component in the Si nanocrystals doping film face up by ex-situ doping (S2).
Fig. 2. XPS spectra of the Si 2p for P-doped Si-NC prepared by different doping methods. (a) P-doped Si-NC prepared by annealing with doping film face up at 300°C after synthesis. (b) P-doped Si-NC prepared by annealing with doping film face down at 300°C after synthesis. (c) P-doped Si-NC doped during synthesis.
The XPS spectra of P 2p of P-doped Si-NC prepared by different doping methods are shown in Fig. 3. In the P-doped Si-NC, the P 2p signal at 134.3 eV is considered to be the diffusion of P atoms into the oxide and subsequent formation of a P-O bond. However, the P 2p signal at 129.6 eV is attributed to the diffusion of phosphorus atoms into Si-NC, forming P-Si bonds. The intensity of the P 2p peak at 129.6 eV increases more significantly in Fig. 3(b) and Fig. 3(c) than in Fig. 3(a), indicating the successful doping of phosphorus atoms into Si-NC after high-temperature annealing.
Fig. 3. XPS spectra of the P 2p for P-doped Si-NC prepared by different doping methods. (a) P-doped Si-NC prepared by annealing with doping film face up at 300°C after synthesis. (b) P-doped Si-NC prepared by annealing with doping film face down at 300°C after synthesis. (c) P-doped Si-NC doped during synthesis.
Figure 4 shows the XRD of undoped Si-NC and P-doped Si-NC prepared by different doping methods, where the black curve is the S1 sample, the red curve reparents the S2 sample, and the blue curve stands for the S3 sample, the green curve points out the S4 sample. All four samples exhibit strong diffraction peaks at 28.3° and 47.4°, which correspond to the standard Bragg peaks of Si (111) and Si (220), respectively [20], indicating that Si-NC was generated in the annealed samples. After P doping, the crystallinity of Si (111) in Si-NC is significantly enhanced compared to undoped Si-NC (S1). Among the annealing P-doped Si-NC samples, film face down annealing resulted in higher crystallinity than film face up. Moreover, P-doped Si-NC doped during synthesis (S4) exhibited the strongest crystallinity in Si (111). The reason is that the P atoms enter the multilayer film structure during in-situ doping, which promotes the phase separation of the Si-rich oxide film and facilitates the crystallization of Si. These results are consistent with those observed in Fig. 2 for the Si 2p spectrum of the doped samples. However, the stronger diffraction peaks appearing at 51.6° and 55.5° which are caused by the Si substrate.
The absorption spectra of the S1, S2, S3, and S4 samples are shown in Fig. 5. The absorption edges of the four samples are all 410 nm. The inset in Fig. 5(a) shows the energy band gap of P-doped Si-NC calculated by the Tauc formula [21].
Where α is the absorbance index, hv = 3.22 × 10−19 J is the incident photon energy, A is a constant, and Eg is the energy band gap of the Si-NC. The band gap of the Si-NC is calculated to be about 3.0 eV, The identical absorption edge of the four samples indicates that their band gaps are equivalent. The comparable widths of derived band gaps for Si nanocrystals in S1-S4 can be attributed to the fact that P doping solely restrains the Si-NC interface, without altering the size of Si nanocrystals. In accordance with the quantum size effect, the band gap of Si nanocrystals also remains constant before and after doping. Therefore, the absorption edge of both undoped and doped synthesized silicon nanocrystals is maintained at 3.0 eV.Figure 6 shows the PL spectra of undoped Si-NC and P-doped Si-NC prepared by different doping methods. The peak position of all four samples (S1, S2, S3, S4) is 465 nm. The Si nanocrystals is direct band-gap semiconductors with an adjustable band gap, and their luminescence peak and intensity are related to the band gap and interface state of silicon nanocrystals [22]. The luminescence peak of silicon nanocrystals is affected by the band gap and interface states. In the P-doped silicon nanocrystals, As shown in Fig. 3(a), the luminescence peak is 465 nm which confirms the XPS results that the P-Si interface state is introduced into the Si nanocrystals. Compared with the PL intensity of undoped Si-NC, the PL intensity of P-doped Si-NC is significantly improved. In the P-doped Si-NC prepared by ex-situ doping, the PL intensity S3 sample increases three times, which is higher than that of the S2 sample. This is due to the fact that most of the black phosphorus sublimates, resulting in only a small amount of P entering Si-NC when annealed with the doping film face up. However, when annealed with the film face down, a portion of sublimated black phosphorus re-enters Si-NC. Nevertheless, there is not much difference in PL intensity between S2 and S3. Moreover, the photoluminescence intensity of P-doped Si-NC during synthesis increases by nearly one order of magnitude compared to undoped Si-NC. This is due to the interaction between P atoms and interface states of Si-NC [23]. During the high-temperature annealing process, phosphorus infiltrates into the Si-NC or Si-NC: SiO2 interface, thereby mitigating interface defects that lead to non-radiative recombination centers. This improves the radiative recombination of Si-NC and ultimately enhances the photoluminescence intensity of silicon nanocrystals.
4. Conclusion
The P-doped Si-NC with various doping methods were fabricated via electron beam thermal evaporation, and the impact of P doping on the luminescence intensity of Si-NC was investigated. We have observed that the introduction of P atoms can facilitate the phase separation of Si nanocrystals, interact with the interface states of Si nanocrystals, mitigate non-radiative recombination defects, and consequently enhance the luminescence intensity of Si nanocrystals. Compared with undoped Si-NC, the PL intensity of P-doped Si-NC was significantly enhanced. The PL intensity of the P-doped Si-NC prepared by ex-situ doping annealing with doping film face down increases three times, while the PL intensity of P-doped Si-NC prepared by in-situ doping can be increased by one order of magnitude.
Funding
National Natural Science Foundation of China (No. 62064001); Science and Technology Foundation of GuiZhou Province (No. ZK [2021]238); Science and Technology Planning Foundation of GuiYang City (No. [2021]43-2).
Acknowledgements
This work was supported from National Natural Science Foundation of China under Grant No. 62064001, the Science and Technology Foundation of GuiZhou Province under Grant No. ZK [2021]238, the Science and Technology Planning Foundation of GuiYang City under Grant No. [2021]43-2.
Disclosures
The authors declare no conflicts of interest
Data availability
Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.
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