Silicon Carbide Formation by Methane Plasma Immersion Ion Implantation Into Silicon Zhenghua An,A) Ricky K

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Silicon carbide formation by methane plasma immersion ion implantation into silicon Zhenghua An,a) Ricky K. Y. Fu, and Peng Chen Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong Weili Liu State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Metallurgy, Chinese Academy of Science, Shanghai 200050, China Paul K. Chub) Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong Chenglu Lin State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Metallurgy, Chinese Academy of Science, Shanghai 200050, China ͑Received 30 January 2003; accepted 19 May 2003; published 30 June 2003͒ Silicon carbide ﬁlms were synthesized by methane plasma immersion ion implantation into silicon and their properties were investigated. The molecular ions dissociate upon entry into the sample surface and our simulation results show that the implanted hydrogen peak is located at about twice as deep as the implanted carbon. The ﬁlms undergo a transformation from hydrogenated amorphous ␤ silicon carbide to -SiC with increasing annealing temperature. The amount of SiuC bonds increases with annealing temperature whereas the CuC bonds change in an opposite manner. At high temperature, a large amount of ␤-SiC forms and graphitization takes place within the remaining carbon clusters. For the sample annealed at 1300 °C, SiuC bonds are detected by Raman spectroscopy, and our data indicate that ␤-SiC grains may contribute to the strong photoluminescence behavior. © 2003 American Vacuum Society. ͓DOI: 10.1116/1.1591741͔

I. INTRODUCTION a-Si1ϪxCx :H films are typically synthesized employing ͑ ͒6 Hydrogenated amorphous silicon carbide (a-Si1ϪxCx :H) plasma enhanced chemical vapor deposition PECVD or 7 is an interesting material used in applications such as solar radio-frequency glow discharge. In this work, a-Si1ϪxCx :H cells, photoreceptors, and light-emitting diodes ͑LEDs͒.1 Its films were produced by implanting carbon and hydrogen into band gap can be modulated by varying the carbon fraction silicon wafers using plasma immersion ion implantation and, as a result, it is a potential material for multicolor vis- ͑PIII͒. Methane was used as the precursors for both carbon ible LEDs. Nevertheless, a-Si1ϪxCx :H is a rather complex and hydrogen. One of the advantages of this technique com- system, particularly with respect to characterization, due to pared to conventional PECVD is that the energy of the im- the nature of the chemical bonds and disorder.2 Besides to- pinging ions is independent of other plasma parameters and a pological and compositional disorder, an additional source of bombarding energy in excess of 10 keV is possible by ap- variability arises from the ability of carbon to form sp3, sp2, plying high-voltage pulses to the substrate. This provides an and even sp1 hybridization.3 Consequently, the properties of extra degree of freedom in controlling the deposition param- a-Si1ϪxCx :H are highly dependent on the choice of the eters such as ion mixing and film properties. The films syn- deposition technique and conditions, and so the bonding thesized by this technique were characterized using several characteristics are still not well understood, especially from analytical techniques and the results show that the film un- ␤ the theoretical perspective. Results from Monte Carlo dergoes a transformation from a-Si1ϪxCx :H to -SiC after calculations4 performed on amorphous stoichiometric SiC annealing at a high temperature. and molecular-dynamics calculations5 in amorphous-SiC (a-SiC) clusters are even more controversial. Further work, from both the theoretical and experimental points of view, is II. EXPERIMENT needed to elucidate the formation mechanism and detailed The details of the PIII system have been described structure of silicon carbide materials formed by different elsewhere.8 Boron-doped p-type Si ͑100͒ wafers with resis- techniques under different conditions. tivity of 1–20 ⍀ cm were loaded into the chamber. Prior to film deposition, the vacuum chamber was pumped to a base ͒ a Also at: State Key Laboratory of Functional Materials for Informatics, pressure of 9ϫ10Ϫ4 Pa and then the samples were cleaned Shanghai Institute of Metallurgy, Chinese Academy of Science, Shang- using argon-ion sputtering for 5 min. During deposition, hai 200050, China. ͒ b Author to whom correspondence should be addressed; electronic mail: methane (CH4) gas was bled into the chamber to a pressure Ϫ [email protected] of about 2ϫ10 1 Pa. The applied voltage, repetition rate,

Õ Õ Õ Õ Õ Õ 1375 J. Vac. Sci. Technol. B 21„4…, Jul Aug 2003 1071-1023 2003 21„4… 1375 5 $19.00 ©2003 American Vacuum Society 1375 1376 An et al.: Silicon carbide formation by methane plasma 1376 and pulse width were Ϫ15 kV, 50 Hz, and 550 ␮s, respectively. The plasma was generated near the substrate by self- ignited glow discharge when the Ϫ15 kV voltage pulse was applied to the sample. Since an external plasma source was not used in our experiments, the plasma was quenched when the sample voltage was turned off and deposition ceased almost instantaneously. In this way, plasma generation and substrate biasing were synchronized and pure implantation could be accomplished. The work reported here is different from previous attempts9 involving the use of externally pow- ered rf glow discharge in which the plasma was sustained continuously allowing both deposition ͑sample voltage turned off͒ and implantation ͑sample voltage turned on͒ during the process. The carbon implant dose is estimated to be about 1017 cmϪ2 based on similar experiments conducted in the same instrument. After implantation, the samples were an- FIG. 1. Simulated carbon and hydrogen distributions in Si for the ϩ ͑ ͒ nealed at 400 °C– 1300 °C in a nitrogen ambient with about CH4 -implanted silicon 15 keV based on TRIM. 1% oxygen ͑contamination due to minor leakage in our fur- nace determined from previous experiments͒. To determine predicted by TRIM. Increased broadness also results from the the properties of the implanted samples, they were character- ﬁnite rise and fall times of the voltage pulses, during which ized using techniques such as Fourier transform infrared the ions do not acquire the full voltage. ͑FTIR͒ spectroscopy, photoluminescence ͑PL͒, and Raman Figure 2 exhibits the FTIR spectra acquired from samples spectroscopy. The PL spectra were measured at room tem- implanted for 90 min and, subsequently, heat treated for 30 perature and the light source was a He–Cd laser operated at min. A bare silicon wafer was used as a reference to subtract a wavelength of 325 nm. The PL signal was analyzed using a the background signals. A weak and broad absorption peak at single monochromator and detected by a liquid-nitrogen- Ϫ about 750 cm 1, detected from the as-implanted sample, can cooled InSb photodetector hooked up to a lock-in ampliﬁer. be assigned to amorphous SiC (a-SiC). However, the Raman spectra were acquired on a RM3000 Raman spec- amount of a-SiC is quite small as inferred from the weak trometer with the wavelength of the excitation source set at intensity. For shorter PIII times ͑30 and 60 min͒, this absorp- 488 nm. tion peak is even weaker ͑not shown here͒ indicating that little SiC exists in these as-implanted samples. The peak at

III. RESULTS AND DISCUSSION

Based on our previous studies on methane plasma in our instrument, the dominant ions in the plasma are assumed to ϩ be primarily CH4 that will dissociate upon entry into the target since the bonding energy of carbon and hydrogen is on the order of ϳeV whereas the kinetic energy of the ions is about 1000 times larger. The initial energies of carbon and ϩ hydrogen projectiles are given by ͓mC /(mC 4mH)͔E0 and ϩ 10 ͓mH /(mC 4mH)͔E0 , respectively, where E0 is the im- ϩ plantation energy of the CH4 ions and mC and mH are the atomic masses of carbon and hydrogen, respectively. Figure 1 depicts the calculated depth profiles of carbon and hydrogen implanted into silicon independently based on TRIM. The projected range of hydrogen ͑81 nm͒ is about twice that of carbon ͑41 nm͒. It should, however, be noted that plasma implantation is different from conventional beamline ion implantation ͑that is, the profiles derived by TRIM͒ due to the lack of energy filtering, and Fig. 1 only renders an approxi- mation of the real distributions in the plasma-implanted samples. In addition, taking into account the existence of ϩ ϩ ϩ other coimplanted ions such as CH3 ,CH2 ,CH, and smaller amounts of Cϩ and Hϩ, the carbon and hydrogen FIG. 2. FTIR spectra acquired from CH4 plasma-implanted and annealed distributions will be a little deeper and broader than those samples (600 °C, 800 °C, 1000 °C, and 1300 °C for 30 min͒.

J. Vac. Sci. Technol. B, Vol. 21, No. 4, JulÕAug 2003 1377 An et al.: Silicon carbide formation by methane plasma 1377 about 1100 cmϪ1 is believed to arise from the native oxide (SiO2) and oxide incorporated into the sample. After annealing, the peak at ϳ750 cmϪ1 increases and becomes narrower. At temperature higher than 800 °C, the signal becomes much stronger and narrower. Furthermore, it shifts to a higher frequency of about 800 cmϪ1 and is indicative of the formation of ␤-SiC. Based on this trend, the film has apparently undergone a transformation from a-SiC to ␤-SiC at a high annealing temperature. On the other hand, when the temperature exceeds 600 °C, an absorption peak appears on the left-hand side (1080 cmϪ1) of the oxide peak suggesting Ͻ Ͻ the formation of silicon suboxide (SiOx ,0 x 2), and its intensity increases with annealing temperature. Our results are consistent with those described in another report11 in which it was demonstrated that oxygen greatly promoted the formation of SiC. For the sample annealed at 1300 °C, the absorption peak at ϳ1080 cmϪ1 increases significantly and the peak at 1100 cmϪ1 disappears. This is because more oxygen has been incorporated into the film to form the suboxide. Meanwhile, the crystalline grain of ␤-SiC is reduced after annealing at such a high temperature resulting in a small shift of the absorption peak (ϳ800 cmϪ1) toward high frequencies. Figure 3 shows the room-temperature PL spectra acquired from samples implanted for 90 min and subsequently annealed. The as-implanted sample exhibits negligible PL signals in Fig. 3͑a͒. After annealing at 400 °C or 600 °C, no significant change can be observed. However, after annealing at 800 °C or 1000 °C, the samples begin to exhibit clear PL bands that are consistent with the aforementioned FTIR data. After annealing at 800 °C and 1000 °C, SiOx precipitates have formed. These two samples have different SiC struc- ͑ ͒ tures (a-SiC for 800 °C, but ␤-SiC for 1000 °C) but the FIG.3. a PL spectra acquired from CH4 plasma-implanted and annealed ͒ ͑ ͒ same PL band and intensity, suggesting that SiO , rather samples (600 °C, 800 °C, 1000 °C, and 1300 °C for 30 min . b Gaussian x fittings for the PL band from the sample annealed at 1300 °C for 30 min. than SiC, is the primary origin of the PL band. After annealing at 1300 °C, much stronger PL is observed as shown in Fig. 3͑a͒͑note that its intensity has been divided by 10͒ and content. In order to obtain information of the CuC bonds in it is visible to the naked eyes. It appears that the higher the our samples, Raman measurements were performed. Figure 4 amount of incorporated oxygen in the film, the larger the PL shows the Raman spectra from samples implanted and an- intensity. However, the oxide alone cannot cause such a large nealed at different temperatures. The conditions adopted in increase in the PL intensity and also the PL peak location has the Raman measurements were the same for all of the shifted. There should be another mechanism. In Fig. 3͑b͒, the samples and so the peak intensities measured from the dif- broad PL band is deconvoluted into two components at ferent samples can be directly compared. The presence of a ϳ430 nm and ϳ500 nm, respectively. The component at ϳ Ϫ1 Raman band at 1500 cm indicates that CuC bonds ex- ϳ430 nm agrees well with the PL band for samples annealed ist in the films, thus suggesting carbon segregation and amor- at lower temperatures, and so it can be ascribed to silicon phous carbon film formation at SiC grain boundaries. An- oxide. The other component at ϳ500 nm is similar to the other Raman peak appears at ϳ2800 cmϪ1 indicating the results from porous SiC reported by Bao et al.12 and Matsu- presence of SiuH bonds. The increase of the background moto et al.13 Thus, it is believed that it originates from crys- toward high frequencies originates from the PL effects under talline SiC in the film and the size of crystalline SiC is on the laser irradiation. Both Raman bands (ϳ1500 cmϪ1 and nanoscale resulting in quantum confinement. This is consis- ϳ2800 cmϪ1) and PL effects observed in the Raman spectra tent with the shift of the absorption peak in the FTIR spectra. diminish with annealing temperature. When the annealing The reason for not seeing a clear SiC component in the PL temperature reaches 1000 °C, the CuC band decreases pre- band for samples annealed at a lower temperature may be cipitously and the Si–H Raman peak disappears completely. that SiC in those samples are amorphous or have defects. This can be explained as follows. In the as-implanted 9 Volz et al. observed the presence of CuC bonds in sto- sample, the excessive carbon has no choice but to segregate ͑ ichiometric Si1ϪxCx or even Si1ϪxCx with a smaller carbon due to its extremely low solubility in silicon below

JVSTB-MicroelectronicsandNanometer Structures 1378 An et al.: Silicon carbide formation by methane plasma 1378

clusters. Usually, the graphitization effect takes place in amorphous diamondlike carbon film due to the relative sta- bility of graphite. Figure 4͑b͒ depicts the enlarged Raman band of the sample annealed at 1000 °C. Although we cannot deconvolute the peak completely and assign each component accurately because of its high complexity, it is quite evident that graphitization has taken place for the carbon clusters in the films, as indicated by the splitting of the Raman peak in the graphitic G and disordered D line9 shown in Fig. 4͑b͒. Nevertheless, on account of the reaction with silicon atoms to form SiC, the band intensity decreases and thus graphitization is greatly limited for the remaining carbon clusters. In addition, during annealing, hydrogen diffuses toward the surface from the defects produced by implantation. At annealing temperatures exceeding 800 °C, hydrogen has almost outdif- fused completely. Generally, Raman spectroscopy is not very sensitive to SiuC bonds, but nonetheless, we observe a band at ϳ800 cmϪ1 from the 1300 °C sample that can be assigned to ͓ ͑ ͔͒ the SiuC vibrational mode Fig. 4 c . This confirms that a large amount of SiuC is present in this sample, consistent with the FTIR results. At ϳ620 cmϪ1, another peak can be seen in Fig. 4, however, owing to the complex bonding en- vironment in the film, we do not have sufficient knowledge at this moment for the correct assignment. This peak was also reported in Ref. 2. More work is being conducted to further unveil the details.

IV. CONCLUSION In recapitulation, we have performed methane PIII into silicon to form a-Si1ϪxCx :H. FTIR data reveal that the ﬁlms ␤ undergo a transformation from a-Si1ϪxCx :H to -SiC with increasing annealing temperature, and SiuC bonds increase with the annealing temperature while CuC bonds change in the opposite direction. At a high annealing temperature, a large amount of ␤-SiC forms and graphitization takes place for the remaining carbon clusters. SiuC is detectable by Raman spectroscopy from the sample annealed at 1300 °C and ␤-SiC grains are believed to contribute to the strong PL behavior.

ACKNOWLEDGMENT The work was ﬁnancially supported by Hong Kong Re- search Grants Council CERG CityU 1052/02E ͑CityU desig- nation No. 9040689͒.

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