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Laser hyperdoped silicon with high crystallinity in doped layer is obtained by method of femtosecond laser irradiation in NF3/SF6 gas mixture. SIMS measurement indicates nitrogen and sulfur dopants are incorporated into the top layer of about 300 nm at concentrations above 1019 cm−3, SAD and Raman Spectrum results show that there is approximately crystalline silicon in the doped layer. The crystallinity in doped layer of the laser co-hyperdoped silicon is almost same as that of the annealed sample at 800 K prepared in SF6. The improvement of crystallinity is attributed to the hyperdoped nitrogen, which is effectively on repairing the defects in the doped layer. The light absorption is more than 95% over the range of 300-1100 nm, and about 80% in the 1200-2500 nm wavelength range.
© 2016 Optical Society of America
1. Introduction
Ultra-fast laser pulse irradiation as an effective technique for functionalizing silicon materials has aroused great interest recently [1–5 ]. Fs-laser pulse irradiation of silicon under certain gases can lead to special microstructures on the surface [1–3 ], and introduce doping by supersaturated impurities in the surface layer [4,5 ]. These changes in the silicon surface induce some unique optical properties, such as strong sub-band-gap absorption [4,6–8 ] and high infrared radiation [9], which have garnered significant attention for applications in high efficiency solar cells [10,11 ], infrared photodiodes [12–14 ], and plane blackbody with ultrahigh emissivity [9,15 ]. However, during the extreme non-equilibrium process of the intense light-matter interaction induced by fs-laser pulse irradiation, a volume of defects and phase transformations are induced in the doped layer [3,12,16 ], which deteriorated the crystallinity of the samples significantly, leading to a degradation in the performance of the devices [12,14,17 ]. High temperature annealing, one of the most common methods, can reduce defects and recover the crystallinity of these materials [12,14 ]. Unfortunately, it leads to large variations in the atom structures of the dopants in silicon lattices, which weakens the unique optical properties and greatly limits its applications [8,12 ]. Therefore, a proper method of simultaneously obtaining high crystallinity and strong sub-band-gap absorption in hyperdoped silicon is essential for applications based on its unique properties.
Through ion implantation followed by nano-second (ns) pulse laser irradiation at proper fluence, Kim et al., in 2006, obtained the first single-crystal sulfur-hyperdoped layer [18]. Nevertheless, owing to the absence of microstructures on the surface, the unique sub-band-gap absorption of such a flat silicon is lower than that of a spiked surface obtained by fs-laser irradiation [18]. More recently, with fs-laser ablation followed by ns-laser melting, Franta et al. obtained a spiked sample with high crystallinity that exhibits a strong sub-band-gap absorption, but the fabrication processes followed by ns-laser melting are complex [19]. It is still a challenge to prepare such a spiked hyperdoped sample during the fabrication process (without any post-processing). In our previous reports, using fs-laser pulses irradiation in a nitrogen trifluoride (NF3) ambient, we fabricated nitrogen hyperdoped silicon that possesses higher crystallinity in the doped layer [4,20 ]. The improvement in the crystallinity is attributed to the special effects of the hyperdoped nitrogen, which effectively immobilize the dislocations and suppress the formation of large vacancies [4,21,22 ]. The result suggests that fs-laser hyperdoped silicon possesses good crystallinity, and strong sub-band-gap absorption could be achieved by co-doping of supersaturated nitrogen and sulfur. In this work, we report our recent results on improving the crystallinity in the doped layer of fs-laser hyperdoped silicon during the fabrication process instead of during post-processing. Supersaturated sulfur and nitrogen were successfully incorporated into the surface layer of silicon by fs-laser pulse irradiation in a gas mixture of NF3 and sulfur hexafluoride (SF6). As expected, through such a co-hyperdoping method, the samples possess a strong sub-band-gap absorption and good crystallinity in the doped layer without any post-processing treatment.
2. Experiments
Double-polished silicon wafers (p-type, 1-3 Ω/cm2, (100) orientation) were cleaned by Radio Corporation of America (RCA) standard process to remove organic and metallic contaminants. Then, the cleaned wafers were placed in a stainless steel vacuum chamber, which was filled with a gas mixture of SF6/NF3 at 70 kPa. They were irradiated at normal incidence with a Yb:KGW fs-laser (515 nm, 190 fs, and 1 kHz). The laser beam was focused to a spot size of 60 µm in diameter on samples with a 250 mm focal length lens. Translating the silicon wafer by stepper motors in continuous raster scan pattern at a speed of 500 µm/s, we fabricated micro-structured silicon with an area of 10 × 10 mm2.
The morphology of the textured surface was observed using a scanning electron microscope (SEM). The nitrogen and sulfur doping concentration and distribution in the surface layer were determined by secondary ion mass spectrometry (SIMS), which was equipped with a CAMECA 4F device using a 14.5 keV Cs+ primary beam. The crystal structure features of the samples were examined by confocal Raman spectroscopy (excited by a He-Ne laser) and transmission electron microscopy (TEM) combined with selected area diffraction (SAD). The total hemispherical (specular and diffuse) reflectance (R) and transmittance (T) of the samples were examined using a spectrophotometer over the wavelength range of 250–2500 nm (Varian Cary 5 E UV–VIS–NIR). The light absorptance (A) was obtained by A = 1-R-T.
3. Results and Discussion
With fs-laser irradiation in a gas mixture of 35 kPa NF3 and 35 kPa SF6, a rough surface morphology consisting of sharp spikes is formed, as shown in Fig. 1(a)–1(c) . Similar to the morphology of samples prepared in NF3 and SF6 ambient [4,6 ], the height of the spikes increased as the laser fluence increased. In contrast to the spikes formed in SF6 that have nanoparticles on the surface of spikes (Fig. 1(d)), those formed in the NF3/SF6 gas mixture exhibit a relative smoother surface without any large particles (Fig. 1(c)), which may benefit the subsequent deposition of electrodes on the surface layer. The nanoparticles might originate as particulate debris deposited onto the surface during the laser irradiation process [23], and the relatively smoother surface formed in the gas mixture is attributed to the special cleansing effects of NF3 [4,24 ].
Fig. 1 SEM images of fs-laser hyperdoped silicon formed at a fluence of (a) 5.6, (b) 8.6, (c) 12.1 kJ/m2 in a gas mixture of NF3/SF6, and (d) 12.1 kJ/m2 in 70 kPa SF6. All the samples are viewed at 45° to the normal.
As we mentioned above, the supersaturated impurities can be doped into silicon during fs-laser irradiation, so sulfur and nitrogen are expected to co-hyperdope into our samples. Figure 2 is the SIMS measurement results of the micro-structured silicon formed in the gas mixture (NF3/SF6, 35/35 kPa), which indicates the doping concentration and distribution of impurities (nitrogen and sulfur) in the surface layer. In order to verify the SIMS data, two different locations were measured for both the impurities, each detected over an area of about 250 × 250 μm2 and containing dozens of conical spikes. Thus, the SIMS data reflects the average distribution of impurities in the doped layer. From Fig. 2, it can be seen clearly that both the sulfur and nitrogen are hyperdoped into the surface layer of the micro-structured silicon. The concentrations of sulfur and nitrogen dopants in the uppermost 300 nm layer are all more than 1019 atoms/cm3, only a little lower than that prepared in NF3 and SF6 [4,6 ], which are all several orders of magnitude above their solid solubility in silicon crystals [22,25 ]. Additionally, with an increase in the depth, the doping concentrations of the impurities decreases, which is also similar to that of the samples prepared in NF3 and SF6.
Fig. 2 SIMS measurements of sulfur and nitrogen on the surface layer of the sample prepared in a gas mixture of NF3 and SF6 with a laser fluence of 8.6 kJ/m2.
Figure 3 is the TEM image of the nitrogen and sulfur co-hyperdoped silicon, formed in a gas mixture of NF3/SF6 at a fluence of 8.6 kJ/m2. The spikes consist of a core of undisturbed silicon covered with a surface layer of about 1 μm thickness on the top and less than 0.5 μm on the side. The SAD patterns obtained from different positions (marked as 1, 2, 3) in the microstructures all consist of sharp diffraction spots, which indicates that both the surface layer and the core area are mostly single crystalline silicon (c-Si) [4,6,16 ]. The small diffraction spots in the SAD patterns (marked as 1, 2) are attributed to the small defects and dislocations that may be formed during the rapid re-solidification process [6,16 ]. For the hyperdoped silicon prepared in SF6, the doped layer is composed of silicon nano-crystals, polycrystalline silicon and pores, and the disordered corresponding SAD also indicates poor crystallinity (more crystallinity information about silicon prepared using SF6 silicon can be found in [6]). Therefore, we can conclude that the crystallinity in the doped layer of our samples formed in the gas mixture of NF3/SF6 is much better than that of samples formed in SF6. Upon comparing with the SIMS profiles results (in Fig. 2), the hyperdoped depth (about 1 μm) is almost the same as the thickness of the surface layer on the top of spikes, which also indicates that both the impurities exist in the surface layer of the structures.
Fig. 3 (Left) cross-section TEM image of the nitrogen and sulfur co-hyperdoped silicon, and (right) the corresponding SAD patterns obtained from different positions of the structure.
In order to study the crystal properties of these samples further, we performed a Raman analysis, and the results are shown in Fig. 4 . It shows the normalized Raman spectra of the samples formed in NF3, SF6, and the gas mixture (NF3/SF6, 35/35 kPa), and those of the crystalline silicon and the annealed sample formed in SF6 for comparison. From the inset, we can see that the silicon polymorph peaks are indicative of Si-III (387 cm−1 and 443 cm−1) and Si-XII (401 cm−1) [16], which are observed clearly in the Raman spectrum of the sample formed in SF6. They almost vanish in the nitrogen hyperdoped silicon formed in NF3 and the gas mixture. The intensities of the amorphous silicon (a-Si) Raman modes (470 cm−1, broad peaks at 100–200, 300, and 460–495 cm−1) [16] also decline greatly. We know that compared to crystalline silicon, silicon polymorphs show different properties. For example, a-Si a has a large number of defects, Si-XII has an indirect band gap of 230 meV, and Si-III is a semimetal, all of which are not beneficial for the photo-electric performance of devices, and are usually removed by various methods before the samples are used [6,14,16 ]. Using a flash lamp, we annealed the sample prepared in SF6 at 800 K for 30 min in forming gas (95% N2, 5% H2, 300 sccm), which is reported to effectively remove the silicon polymorphs [6]. After annealing, the Raman spectra show that most of the a-Si and almost all the Si-III and Si-XII in the doped layer are removed, which is consistent with a previous report [6]. More importantly, the Raman spectrum of the nitrogen co-hyperdoped silicon is almost the same as that of the annealed silicon prepared in SF6, which may suggest that the crystallinity recovered or maintained by the nitrogen co-hyperdoping is comparable to that by the high temperature annealing process. The presence of a little residual a-Si in the Raman spectra of both the samples prepared in the gas mixture and the annealed silicon prepared in SF6 is attributed to the small amount of defects and dislocations, as indicated by the SAD patterns of the doped layer in Fig. 3.
Fig. 4 Raman spectra of hyperdoped silicon formed in SF6, NF3 and the gas mixture; the annealed hyperdoped silicon formed in SF6 and crystalline silicon are shown as a comparison. The inset highlights the Raman modes and all the hyperdoped samples are fabricated with a laser fluence of 8.6 kJ/m2.
With the intent of improving the crystallinity of silicon, researchers introduced nitrogen into the silicon lattices [21,22 ]. The reason, as Sawada and Kawakami reported, is that the nitrogen in the silicon crystals could combine with the vacancies to form nitrogen-vacancies complexes, which would largely reduce the concentration of the vacancies [27]. Goss et al. also reported that the complexes effectively locked dislocations, and suppressed void formation because of the formation of nitrogen-vacancy centers, all of which may form as the nitrogen concentration exceeds that of the vacancy [27]. However, through traditional methods such as growing the silicon in nitrogen ambient, the concentration of nitrogen doping is only about 1015 cm3 [4,22 ]. In our case, it can be more than 1019 cm3, which is several orders of magnitude higher than that obtained by traditional methods. Because of this high concentration, we can reasonably infer that these repairing effects from the doped nitrogen will be largely enhanced in the nitrogen hyperdoped silicon. This enhanced repairing effect has been confirmed in the hyperdoped silicon that was formed in the NF3 ambient, which exhibits a higher quality of crystallinity in the doped layer due to the doping of super-statured nitrogen at a concentration of 3.5 × 1019 cm3. More structural information through Raman analysis and TEM measurements on the hyperdoped silicon prepared in NF3 can be found in [4] and [16]. Therefore, for our samples formed in the gas mixture, the higher crystallinity in the doped layer is also attributed to the co-hyperdoped nitrogen.
It is noted that through our method of nitrogen co-hyperdoping, the crystallinity of the doped layer is recovered or maintained during the process of fs-laser fabrication, rather than by a post-processing method, e.g., high temperature annealing and ns-laser melting. This is crucial and important for the applications of the fs-laser hyperdoped silicon. In spite of recovery of crystallinity of the samples by thermal treatment, various studies have shown that it significantly deactivates the sub-band-gap absorption, and the photoelectric response obtained in the sulfur-hyperdoped silicon also decreases at higher temperature (above 850 K) [3,5,6,12 ]. From first-principles calculations, it is determined that the atomic structures of the dopants in the silicon lattices are changed during the annealing process, which weakens its unique optical properties and limits its applications [8]. Here, the good crystallinity of our samples makes the process of annealing unnecessary, so the active structures of the dopants in the silicon crystals, leading to unique properties such as strong sub-band absorption, can be retained.
Fig. 5 shows the light absorption properties of the fs-laser hyperdoped silicon that are formed in ambient of SF6, NF3, and the gas mixture, and that of the annealed silicon prepared in SF6 and crystalline silicon are shown for comparison. All the hyperdoped silicon are prepared at a fluence of 8.6 kJ/m2. Over the wavelength range of 300–1100 nm, the absorption of all the hyperdoped silicon are largely increased compared to that of crystalline silicon, which is attributed to the multiple reflection of the special microstructures [4,6 ]. In the near-infrared region (1100–2500 nm), the nitrogen co-hyperdoped silicon exhibits a strong sub-band-gap absorption of about 80%. It is much higher than that of the sample prepared in NF3, which is about 30%, and only a little lower than that of the sample prepared in SF6 (about 90%). As we mentioned above, because of the poor crystallinity of the silicon prepared in SF6, high temperature annealing is an essential step before it is used for photoelectric devices, which causes the strong sub-band-gap absorption to decrease significantly, e.g. it is only about 20% after annealing at 800 K (as shown in Fig. 5). Through nitrogen co-hyperdoping, the sample shows higher crystallinity, almost the same as the annealed sample prepared in SF6 (see Fig. 4). Meanwhile, the sub-band-gap absorption is close to that of the unannealed silicon prepared in SF6. Such a unique optical absorption property is obviously crucial for the applications of the hyperdoped silicon in the field of infrared devices.
Fig. 5 Light absorption of the hyperdoped silicon prepared in different atmospheres and that of the untreated silicon.
The strong sub-band-gap absorption of the fs-laser hyperdoped silicon, as previously reported, is mainly attributed to the hyperdoped impurities (such as S, Se, Te), which lead to the formation of impurity levels in the band-gap of silicon [7,8,29 ]. In this work, for the sample formed in a gas mixture of NF3/SF6, the supersaturated nitrogen and sulfur doping in silicon are confirmed by the SIMS results. According to our recent first-principles calculations [8,30 ], similar to the super-saturated sulfur doped in silicon, some of the nitrogen atomic structures can also introduce nitrogen-related electronic levels in the silicon band gap. Hence, the strong sub-band-gap absorption of our nitrogen co-hyperdoped silicon is attributed to both the hyperdoped sulfur and nitrogen. However, our calculations also show that the sub-band-gap absorption introduced by nitrogen is lower than that of sulfur, which can also be seen clearly through a comparison of the absorption of samples prepared in NF3 and SF6 in Fig. 5. This difference in the doping by the two impurities decreases the entire sub-band-gap absorption with an increase in the nitrogen concentration. However, the decrease in the absorption is lesser than that induced by the annealing treatment at high temperature, and this requires further studies. On the other hand, a lower concentration of nitrogen doping is insufficient for obtaining higher crystallinity. Therefore, a proper concentration of nitrogen and sulfur, which depend on the partial pressure ratio of SF6 and NF3 in the gas mixture, is necessary and a key point for the high crystallinity and strong sub-band-gap absorption in fs-laser hyperdoped silicon. We tried several pressure ratios of SF6 and NF3, and found 35:35 to be optimum. In a future study, the infrared photoelectric properties of the samples prepared at these pressure ratios (35:35) will be studied.
4. Conclusion
In summary, we improved the crystallinity of fs-laser hyperdoped silicon with a novel method of nitrogen co-hyperdoping. By fs-laser irradiation of silicon in a gas mixture of NF3 and SF6, both sulfur and nitrogen are incorporated into the surface layer at concentrations that are several orders of magnitude higher than their solid solubility in silicon crystals. Although the fs-laser hyperdoped silicon is prepared in a non-thermal equilibrium process, SAD and Raman analysis show that the nitrogen co-hyperdoped samples exhibit higher crystallinity in the doped layer. The reason is attributed to the special repairing effects of the supersaturated nitrogen doping. More importantly, the fs-laser co-hyperdoped silicon shows strong sub-band-gap optical absorption, up to 80%. The good crystallinity in the hyperdoped layer and strong sub-band-gap absorption is beneficial for using fs-laser hyperdoped silicon in infrared device applications. Our method offers an avenue for obtaining spiked functional hyperdoped silicon possessing higher crystallinity and unique optical properties without any post-processing.
Acknowledgments
This work are supported by the National Basic Research Program of China (973 Program) under Grant No. 2012CB934200, National Natural Science Foundation of China (NSFC) under Grant No. 51071048, and the Open Research Fund of Key Laboratory of Neutron Physics, CAEP (No. 2014BB07).
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