Crystallinity and Sub-Band Gap Absorption of Femtosecond-Laser Hyperdoped Silicon Formed in Different N-Containing Gas Mixtures

Femtosecond (fs)-laser hyperdoped silicon has aroused great interest for applications in infrared photodetectors due to its special properties. Crystallinity and optical absorption influenced by co-hyperdoped nitrogen in surface microstructured silicon, prepared by fs-laser irradiation in gas mixture of SF6/NF3 and SF6/N2 were investigated. In both gas mixtures, nitrogen and sulfur were incorporated at average concentrations above 1019 atoms/cm3 in the 20–400 nm surface layer. Different crystallinity and optical absorption properties were observed for samples microstructured in the two gas mixtures. For samples prepared in SF6/N2, crystallinity and light absorption properties were similar to samples formed in SF6. Significant differences were observed amongst samples formed in SF6/NF3, which possess higher crystallinity and strong sub-band gap absorption. The differing crystallinity and light absorption rates between the two types of nitrogen co-hyperdoped silicon were attributed to different nitrogen configurations in the doped layer. This was induced by fs-laser irradiating silicon in the two N-containing gas mixtures.

Besides post-processing methods, high crystallinity is achieved in hyperdoped silicon during the process of preparation by ion implantation [24]. In our previous work, a method of fs-laser co-doping technique in SF 6 /NF 3 used to obtain hyperdoped silicon showed the ability to simultaneously possess high crystallinity and strong sub-band gap absorption by co-hyperdoping with nitrogen and sulfur [25]. The co-hyperdoped nitrogen from SF 6 /NF 3 is deemed to effectively improve the crystallinity by repairing defects in the doped layer [26][27][28][29][30][31][32]. Based on this result, whether other N-containing gas (e.g., N 2 ) can also introduce similar effects on crystallinity raises potential future research questions. If N 2 does offer positive outcomes, this will create additional choices for N-containing gas used in the fs-laser co-hyperdoping method; even if outcomes are not favorable, the different effects from NF 3 will help researchers to understand the mechanisms for obtaining improved crystallinity via a nitrogen co-doping method. This work presents recent progress on this issue. Hyperdoped silicon was fabricated by fs-laser irradiation in SF 6 /N 2 gas-mixtures, and its crystallinity and light absorption properties were investigated. Although super-statured nitrogen and sulfur are also incorporated into the doped layer for gas mixtures of SF 6 /N 2 , different crystallinity and optical absorption rates were observed for samples prepared in gas mixtures of SF 6 /NF 3 and SF 6 /N 2 . The configurations of nitrogen doped in silicon introduced by the fs-laser irradiation in the two N-containing gas mixtures, and their different effects on the crystallinity and light absorption is discussed.

Materials and Methods
Silicon wafers (p (100), ρ = 1-3 Ω/cm 2 , d = 250 ± 10 µm) were cleaned to remove organic and metallic contaminants by the Radio Corporation of America (RCA) standard process. The cleaned Silicon wafers were placed in a stainless-steel vacuum chamber and irradiated by a Yb:KGW fs-laser (515 nm, 190 fs, and 1 kHz) at normal incidence. The chamber was filled with gas mixture of SF 6 /NF 3 or SF 6 /N 2 at 70 kPa. The laser beam was focused to a spot size of 60 µm in diameter on the samples with a 250-mm focal length lens. The silicon wafer was mechanically translated by stepper motors, and the areas of the samples could be achieved at 10 × 10 mm 2 . The stepper motors translated at a speed of 500 µm/s, and the scanning line interval chose the radius of the laser spot (about 30 µm). In this way, any given spot in the irradiated region was exposed to about 350 laser pulses.
The morphology of the textured surface of the fs-laser hyperdoped silicon was observed by a scanning electron microscope (SEM, Hitachi, Tokyo, Japan). The average concentrations of the doped nitrogen and sulfur in the surface layer were detected by secondary ion mass spectrometry (SIMS, Xevo TQD, SCIEX, Framingham, MA, USA) measurements. The crystal properties of samples were examined by a confocal Raman spectroscopy (excited by a He-Ne laser of 633 nm, Horiba Jobin Yvon XploRA, Paris, France). The structures of nitrogen in silicon lattices were explored by a Fourier transform infrared (FTIR) spectrometer (Nicolet Nexus 470, Nicolet Nexus, Madison, WI, USA). Finally, using a spectrophotometer (Varian Cary 5E UV-VIS-NIR, Varian Cary, Palo Alto, CA, USA) equipped with an integrating sphere, we collected the transmission (T) and reflection (R) spectra of samples in the wavelength of 250-2500 nm, respectively, and determined the absorptance (A) by A = 1 − R − T.

Surface Morphology
As in SF 6 /NF 3 , fs-laser irradiation in a gas mixture of SF 6 /N 2 also induces arrays of sharp spikes in the surface of silicon, as shown in Figure 1. While the spikes formed in a gas mixture of SF 6 /NF 3 have a relatively smoother surface (Figure 1a), those prepared in SF 6 /N 2 exhibit a rough surface with overlying numbers of nm-granular structures (Figure 1b). The difference in the degree of spike surface roughness is attributed to different effects of NF 3 and N 2 during the fs-laser fabrication processes [26].

Nitrogen Co-Doping Characters
Similar to that in SF 6 /NF 3 [25], the hyperdoped silicon formed in SF 6 /N 2 also co-doped with supersaturated sulfur and nitrogen in the surface layer. The average doping concentrations of sulfur and nitrogen over a depth range of 20-400 nm in the surface layer are shown in Table 1.
Compared with the average doping concentration of nitrogen in the silicon materials prepared in SF 6 /NF 3 , those introduced by SF 6 /N 2 are slightly lower. The lower doping concentration exceeded 10 19 atoms/cm 3 of the surface layer, which is several orders of magnitude above its solid solubility in silicon crystals [26,30]. Additionally, the co-doping concentrations of sulfur induced by the two gas mixtures are very similar. The difference of the doped nitrogen induced by the two N-containing gas mixtures was determined by the doping method. This will also affect the crystallinity of the doped layer. with the average doping concentration of nitrogen in the silicon materials prepared in SF6/NF3, those introduced by SF6/N2 are slightly lower. The lower doping concentration exceeded 10 19 atoms/cm 3 of the surface layer, which is several orders of magnitude above its solid solubility in silicon crystals [26,30]. Additionally, the co-doping concentrations of sulfur induced by the two gas mixtures are very similar. The difference of the doped nitrogen induced by the two N-containing gas mixtures was determined by the doping method. This will also affect the crystallinity of the doped layer.

Crystal Properties
The crystallinity of the laser hyperdoped silicon influenced by supersaturated nitrogen from SF6/NF3 and SF6/N2 were detected by Raman diagnosis. The results are shown in Figure 2. Results show normalized Raman spectra of hyperdoped silicon prepared in gas mixture of SF6/NF3 and SF6/N2 at different pressure ratios. Compared with sample prepared in SF6, an improved crystallinity with a small amount of amorphous silicon and little polymorph silicon was obtained for samples formed in SF6/NF3. Hyperdoped silicon prepared in SF6/N2 showed that the crystallinity in the doped layer was little improved by the co-hyperdoped nitrogen. As the pressure ratio of N2 in SF6/N2 increased, that of SF6 decreased. The Raman peaks assigned to polymorphic BC8 structure (Si-III, 387 cm −1 and 443 cm −1 ) [20,21,25,26] remained nearly unchanged, and the amorphous silicon (a-Si, broad peaks at 300 cm −1 and 460-495 cm −1 ) [20,21,25,26] increased. Furthermore, as the pressure ratio of N2 increased, the width of Raman peaks at 520 cm −1 also increased, which implies that the lattice stress increased and/or the grain size decreased [23]. Therefore, we can draw the conclusion that the hyperdoped nitrogen introduced by gas mixtures of SF6/NF3 and SF6/N2 show different effects on the crystallinity in the doped layer.
As reported, nitrogen can be doped in silicon lattices in different configurations, such as single nitrogen atoms, paired nitrogen atoms, and nitrogen molecules [26,[31][32][33]. With the irradiation of fs-laser pulses, nitrogen atoms were dissociated from NF3 and then doped in silicon lattices at a substitutional or interstitial site (single atoms and paired atoms). The super-statured nitrogen atoms in the doped layer combine with vacancies to form various nitrogen vacancies complexes [26,32,33].

Crystal Properties
The crystallinity of the laser hyperdoped silicon influenced by supersaturated nitrogen from SF 6 /NF 3 and SF 6 /N 2 were detected by Raman diagnosis. The results are shown in Figure 2. Results show normalized Raman spectra of hyperdoped silicon prepared in gas mixture of SF 6 /NF 3 and SF 6 /N 2 at different pressure ratios. Compared with sample prepared in SF 6 , an improved crystallinity with a small amount of amorphous silicon and little polymorph silicon was obtained for samples formed in SF 6 /NF 3 . Hyperdoped silicon prepared in SF 6 /N 2 showed that the crystallinity in the doped layer was little improved by the co-hyperdoped nitrogen. As the pressure ratio of N 2 in SF 6 /N 2 increased, that of SF 6 decreased. The Raman peaks assigned to polymorphic BC8 structure (Si-III, 387 cm −1 and 443 cm −1 ) [20,21,25,26] remained nearly unchanged, and the amorphous silicon (a-Si, broad peaks at 300 cm −1 and 460-495 cm −1 ) [20,21,25,26] increased. Furthermore, as the pressure ratio of N 2 increased, the width of Raman peaks at 520 cm −1 also increased, which implies that the lattice stress increased and/or the grain size decreased [23]. Therefore, we can draw the conclusion that the hyperdoped nitrogen introduced by gas mixtures of SF 6 /NF 3 and SF 6 /N 2 show different effects on the crystallinity in the doped layer.
As reported, nitrogen can be doped in silicon lattices in different configurations, such as single nitrogen atoms, paired nitrogen atoms, and nitrogen molecules [26,[31][32][33]. With the irradiation of fs-laser pulses, nitrogen atoms were dissociated from NF 3 and then doped in silicon lattices at a substitutional or interstitial site (single atoms and paired atoms). The super-statured nitrogen atoms in the doped layer combine with vacancies to form various nitrogen vacancies complexes [26,32,33]. These complexes effectively improved the crystallinity of the hyperdoped silicon by locking dislocations and suppressing the formation of large defects [25,26,29,30]. Although the gas mixture of SF 6 /N 2 also introduced supersaturated nitrogen in the doped layer, this hyperdoped nitrogen exists in silicon lattices as other nitrogen configurations. As Takao reported, during the process of silicon growth in N 2 , nitrogen molecules doped in silicon showed minimal interaction with silicon lattices [34]. Even with fs-laser irradiation, the N 2 could not be dissociated into single nitrogen atoms and doped in silicon. This is attributed to the laser fluence of 12.1 kJ/m 2 (approximately 10 7 W/m 2 ), far from the ionization threshold of N 2 (≥10 14−15 W/m 2 ) [35,36]. These complexes effectively improved the crystallinity of the hyperdoped silicon by locking dislocations and suppressing the formation of large defects [25,26,29,30]. Although the gas mixture of SF6/N2 also introduced supersaturated nitrogen in the doped layer, this hyperdoped nitrogen exists in silicon lattices as other nitrogen configurations. As Takao reported, during the process of silicon growth in N2, nitrogen molecules doped in silicon showed minimal interaction with silicon lattices [34]. Even with fs-laser irradiation, the N2 could not be dissociated into single nitrogen atoms and doped in silicon. This is attributed to the laser fluence of 12.1 kJ/m 2 (approximately 10 7 W/m 2 ), far from the ionization threshold of N2 (≥10 14−15 W/m 2 ) [35,36]. To further understand the structure of the co-hyperdoped nitrogen in silicon introduced by fs-laser irradiation in SF6/NF3 and SF6/N2, IR detection was performed on the samples using a Fourier transform infrared (FTIR) spectrometer. In the results shown in Figure 3, the spectra of samples prepared in SF6/NF3, showed several IR bands at 462, 511, 568, 613, 646, 670, 737, 817, 894, and 960 cm −1 . For the spectra of samples prepared in SF6/N2, there were only two obvious IR bands at 613 and 737 cm −1 . According to calculations, the IR band at 960 cm −1 is assigned to a nitrogen di-interstitial pair (Ni-Ni); the IR bands at 568, 646, and 670 cm −1 are assigned to local vibrational modes of (Ni-Ni) self-interstitial complexes sub-situational nitrogen pairs (Ns-Ns), (Ni-Ni) vacancy complexes, and (Ns-Ns) vacancy complexes, respectively [23,26,32]. The IR bands at 613 and 737 cm −1 are connected with crystalline silicon [23,32]. All of the IR bands assigned to nitrogen-containing complexes (568, 646, and 670 cm −1 ) were unobserved in the spectra of samples prepared in SF6/N2. The FTIR spectra measurement more clearly show that super-saturated nitrogen from SF6/NF3 and SF6/N2 doped as different configurations in silicon lattices, and show different effects on defects in the doped layer. Similar to effects on defects, nitrogen configurations also exhibit different effects on the light absorption of the fs-laser hyperdoped silicon. To further understand the structure of the co-hyperdoped nitrogen in silicon introduced by fs-laser irradiation in SF 6 /NF 3 and SF 6 /N 2 , IR detection was performed on the samples using a Fourier transform infrared (FTIR) spectrometer. In the results shown in Figure 3, the spectra of samples prepared in SF 6 /NF 3 , showed several IR bands at 462, 511, 568, 613, 646, 670, 737, 817, 894, and 960 cm −1 . For the spectra of samples prepared in SF 6 /N 2 , there were only two obvious IR bands at 613 and 737 cm −1 . According to calculations, the IR band at 960 cm −1 is assigned to a nitrogen di-interstitial pair (N i -N i ); the IR bands at 568, 646, and 670 cm −1 are assigned to local vibrational modes of (N i -N i ) self-interstitial complexes sub-situational nitrogen pairs (Ns-Ns), (N i -N i ) vacancy complexes, and (Ns-Ns) vacancy complexes, respectively [23,26,32]. The IR bands at 613 and 737 cm −1 are connected with crystalline silicon [23,32]. All of the IR bands assigned to nitrogen-containing complexes (568, 646, and 670 cm −1 ) were unobserved in the spectra of samples prepared in SF 6 /N 2 . The FTIR spectra measurement more clearly show that super-saturated nitrogen from SF 6 /NF 3 and SF 6 /N 2 doped as different configurations in silicon lattices, and show different effects on defects in the doped layer. Similar to effects on defects, nitrogen configurations also exhibit different effects on the light absorption of the fs-laser hyperdoped silicon.  Figure 4 shows the light absorption properties of nitrogen co-hyperdoped silicon prepared in SF6/NF3 (35:35 kPa) and SF6/N2 (35:35 kPa). Compared with the sub-band gap absorption of the silicon materials prepared in SF6, that of the sample formed in SF6/NF3 slightly decreased, but it was nearly unchanged (keeps at 90%) for the silicon materials prepared in SF6/N2. As we noted above, the hyperdoped silicon formed in SF6/NF3 showed higher crystallinity, and the doped nitrogen from SF6/N2 failed to improve crystallinity in the doped layer (see Figure 2b). Samples prepared in SF6/N2 should be annealed in order to recover crystallinity before use in photoelectric devices. Unfortunately, after annealing the samples under a flash lamp at 800 K for 30 min in forming gas (95% N2, 5% H2, 300 sccm), the sub-band gap absorption of the samples prepared in SF6/N2 declined to approximately 25%. This light absorption property is nearly identical to that of samples prepared in SF6, which shows a strong sub-band gap absorption (more than 90%) before annealing, and declines sharply (to 20%-30%) after annealing. Sulfur co-hyperdoped silicon remains a method that can induce stronger sub-band gap absorption. The light absorption properties were determined by the doping method. According to previous research [25,33], similar to sulfur atoms, the hyperdoped single nitrogen atoms in silicon could import defect states into the silicon gap and induce sub-band gap absorption. The hyperdoped paired nitrogen atoms induced no defect states and demonstrate few contributions to sub-band gap absorption. Additionally, the sub-band gap absorption induced by hyperdoped  Figure 4 shows the light absorption properties of nitrogen co-hyperdoped silicon prepared in SF 6 /NF 3 (35:35 kPa) and SF 6 /N 2 (35:35 kPa). Compared with the sub-band gap absorption of the silicon materials prepared in SF 6 , that of the sample formed in SF 6 /NF 3 slightly decreased, but it was nearly unchanged (keeps at 90%) for the silicon materials prepared in SF 6 /N 2 . As we noted above, the hyperdoped silicon formed in SF 6 /NF 3 showed higher crystallinity, and the doped nitrogen from SF 6 /N 2 failed to improve crystallinity in the doped layer (see Figure 2b). Samples prepared in SF 6 /N 2 should be annealed in order to recover crystallinity before use in photoelectric devices. Unfortunately, after annealing the samples under a flash lamp at 800 K for 30 min in forming gas (95% N 2 , 5% H 2 , 300 sccm), the sub-band gap absorption of the samples prepared in SF 6 /N 2 declined to approximately 25%. This light absorption property is nearly identical to that of samples prepared in SF 6 , which shows a strong sub-band gap absorption (more than 90%) before annealing, and declines sharply (to 20%-30%) after annealing. Sulfur co-hyperdoped silicon remains a method that can induce stronger sub-band gap absorption. The light absorption properties were determined by the doping method.  Figure 4 shows the light absorption properties of nitrogen co-hyperdoped silicon prepared in SF6/NF3 (35:35 kPa) and SF6/N2 (35:35 kPa). Compared with the sub-band gap absorption of the silicon materials prepared in SF6, that of the sample formed in SF6/NF3 slightly decreased, but it was nearly unchanged (keeps at 90%) for the silicon materials prepared in SF6/N2. As we noted above, the hyperdoped silicon formed in SF6/NF3 showed higher crystallinity, and the doped nitrogen from SF6/N2 failed to improve crystallinity in the doped layer (see Figure 2b). Samples prepared in SF6/N2 should be annealed in order to recover crystallinity before use in photoelectric devices. Unfortunately, after annealing the samples under a flash lamp at 800 K for 30 min in forming gas (95% N2, 5% H2, 300 sccm), the sub-band gap absorption of the samples prepared in SF6/N2 declined to approximately 25%. This light absorption property is nearly identical to that of samples prepared in SF6, which shows a strong sub-band gap absorption (more than 90%) before annealing, and declines sharply (to 20%-30%) after annealing. Sulfur co-hyperdoped silicon remains a method that can induce stronger sub-band gap absorption. The light absorption properties were determined by the doping method. According to previous research [25,33], similar to sulfur atoms, the hyperdoped single nitrogen atoms in silicon could import defect states into the silicon gap and induce sub-band gap absorption. The hyperdoped paired nitrogen atoms induced no defect states and demonstrate few contributions to sub-band gap absorption. Additionally, the sub-band gap absorption induced by hyperdoped According to previous research [25,33], similar to sulfur atoms, the hyperdoped single nitrogen atoms in silicon could import defect states into the silicon gap and induce sub-band gap absorption. The hyperdoped paired nitrogen atoms induced no defect states and demonstrate few contributions to sub-band gap absorption. Additionally, the sub-band gap absorption induced by hyperdoped single nitrogen atoms is lower than that induced by hyperdoped sulfur, which made the sub-band gap absorption of samples prepared in SF 6 /NF 3 decline slightly [25,33]. For samples prepared in SF 6 /N 2 , supersaturated nitrogen molecules were induced to co-dope in the surface layer, which exhibit few interactions with silicon lattices [34]. The sub-band gap absorption of samples prepared in SF 6 /N 2 was nearly unchanged by the hyperdoped nitrogen molecules, dependent on the co-hyperdoped sulfur atoms in the doped layer. Samples were prepared at several pressure ratios of SF 6 /N 2 , and found that all show a sub-band gap absorption similar to those prepared in SF 6 . The differences of the mechanism on the sub-band gap absorption induced by sulfur atoms and that of those incorporated with nitrogen molecules doping in silicon requires further study.

Conclusions
This study examined different configurations of nitrogen doped in silicon, introduced by N-containing gas of SF 6 /NF 3 and SF 6 /N 2 , and the associated effects on crystallinity and sub-band gap absorption of fs-laser hyperdoped silicon. Although supersaturated nitrogen and sulfur are incorporated into the doped layer for both gas mixtures, the co-hyperdoped nitrogen from SF 6 /NF 3 improved the crystallinity, while samples processing in SF 6 /N 2 showed almost no effects on both crystallinity and sub-band gap absorption. This is because the supersaturated nitrogen molecules induced by SF 6 /N 2 in the doped layer showing little interaction with the silicon lattice. This is largely different from that of induction by SF 6 /NF 3 , which induced supersaturated nitrogen atoms doped in silicon and formed various nitrogen vacancy complexes to improve the crystallinity of the doped layer. The doped nitrogen molecules from SF 6 /N 2 failed to improve crystallinity in hyperdoped silicon; as a comparison, it makes the mechanism and demands for obtaining the improved crystallinity by such nitrogen co-doping technique increasingly clear. The investigations of the effects of nitrogen configuration on the crystallinity and light absorption of hyperdoped silicon will greatly benefit non-equilibrium material manufacturing.