Aluminum induced in situ crystallization of amorphous SiC

Transcription

1 Aluminum induced in situ crystallization of amorphous SiC Author Wang, Li, Dimitrijev, Sima, Tanner, Philip, Zou, Jin Published 2009 Journal Title Applied Physics Letters DOI Copyright Statement 2009 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters and may be found at dx.doi.org/ / Downloaded from Link to published version Griffith Research Online

2 Aluminum induced in situ crystallization of amorphous SiC Li Wang, 1,a) Sima Dimitrijev, 1 Philip Tanner, 1 and Jin Zou 2 1 Queensland Microtechnology Facility and Griffith School of Engineering, Griffith University, Nathan, Qld. 4111, Australia 2 Centre for Microscopy and Microanalysis and School of Engineering, The University of Queensland, St Lucia, Qld. 4072, Australia Experimental evidence of aluminum induced in situ crystallization of amorphous SiC is presented. The deposition of SiC films on Si substrates was performed using low pressure chemical vapour deposition method at 600 ºC with concurrent supply of Al(CH 3 ) 3 and H 3 SiCH 3. Transmission electron micrographs confirm the presence of nano-crystals, whereas capacitance-voltage measurements demonstrate that the deposited films are p type doped. A crystallization mechanism is proposed based on the classic theory of nucleation in the growth rate limited regime. The introduction of Al(CH 3 ) 3 enhances the surface reaction and increases the supersaturation, which reduces the activation energy for nucleation. a) Author to whom correspondence should be addressed. Electronic mail: l.wang@griffith.edu.au. 1

3 P type amorphous Si x C 1-x (a-sic) and nanocrystalline silicon-carbide (nc-sic) films have attracted considerable research interest as a material for constructing high efficiency silicon solar cells. 1-4 Compared to a-sic, nc-sic has higher electrical conductivity, which can significantly improve the solar cell efficiency. 4-6 To date, a number of methods have been developed to crystallize deposited amorphous SiC layer, including ion-beam induced crystallization, 7,8 alternating aluminum layer induced crystallization, 9,10 and laser induced crystallization. 11 These methods add complicated steps and thermal budget to the process. Ion-beam induced crystallization combines Al ion implantation and high temperature annealing at 1500ºC to achieve p type Al doped nc-sic, with the risk that the hole mobility might be reduced by radiation damage. 7,8 Al layer induced crystallization requires a separate Al layer deposition and annealing treatment as well as etching of the Al layer after crystallization is finished. 9 In addition, Al atoms tend to diffuse into substrate and change its doping concentration. 9 Although laser crystallization is a popular method to crystallize amorphous films, it is only effective for layers thinner than certain thickness and excessive laser intensity may also create graphite and Si rather than SiC grains. 11 There is no evidence to date of a convenient in situ process for crystallization of amorphous SiC that provides damage-free p type films, although such a technique is urgently needed to achieve nc-sic for the practical applications. In this letter, we provide experimental evidence that crystallization of a-sic occurs when Al, Si, and C-containing precursors are supplied concurrently in a low temperature low pressure chemical vapour deposition (LPCVD) process. A mechanism is also proposed to explain the crystallization process that occurs during the film growth. 2

4 The SiC films were deposited on (100) Si wafers (with a resistivity of 5-15 Ω cm) using H 3 SiCH 3 (methylsilane, hereafter abbreviated as MS) as a single precursor in a hot wall LPCVD reactor (base pressure lower than mbar).prior to deposition, standard RCA cleaning was carried out to clean the surface of the Si wafers. The deposition of SiC film was performed for 30 h at a growth temperature of 600 ºC. The flow rate of MS was maintained at 9.5 sccm and the process pressure was fixed at 0.06 mbar for all deposition runs. Two deposition processes are compared. The first process produces 150 nm intrinsic a-sic on a Si substrate using MS as the single precursor. In the second process, Al(CH 3 ) 3 (trimethylaluminum, hereafter abbreviated as TMA) with a flow rate of 0.15 sccm was added as a means to dope SiC p type, resulting in a film with a thickness of 240 nm. Compared with other known acceptor atoms, Al has a shallower level in SiC with an ionization energy of around 200 mev. 8 TMA is a common p type doping source for single-crystal SiC films. 12,13 Cross-section high resolution transmission electron microscopy (HRTEM, FEI Tecnai F30) and capacitance-voltage (C-V) techniques were employed to characterize the deposited SiC films. To enable C-V measurements, the SiC surface was cleaned and aluminum was thermally evaporated and patterned into cm 2 electrodes surrounded by a much larger area electrode acting as a low impedance second contact. High frequency capacitance-voltage (HFCV) measurements were conducted at room temperature by pulsing the applied voltage from 0 V to the series of values in order to accurately measure the semiconductor depletion width. A computer-controlled HP4284A LCR meter was used and all the measurements were performed under lighttight and electrically-shielded conditions. Figure 1 shows typical selected area electron diffraction (SAED) patterns and HRTEM images on cross-section SiC films deposited by two different processes. 3

5 Figure 1(a) and 1(b) are the SAED patterns and HRTEM image taken from the SiC film deposited by MS only; while Fig. 1(c) and 1(d) were taken from the SiC film deposited with the concurrent supply of MS and TMA. The diffused diffraction rings shown in Fig. 1(a) demonstrate that the intrinsic SiC film is pure amorphous, which is further confirmed by the HRTEM image [Fig. 1(b)]. The diffraction spots in Fig. 1(a) and 1(c) are due to the underlying Si substrates. In the case of the Al-doped SiC film, with the introduction of TMA, multi-rings with spots are observed in the SAED patterns as shown in Fig. 1(c) and lattice fringes are found in Fig. 1(d), indicating that the SiC film contains polycrystals. The only difference between the two deposition processes is the presence of TMA. It is clear that the introduction of TMA is responsible for the structural transition and the growth of nc-sic. FIG. 1. (a) SAED patterns of the intrinsic a-sic, (b) HRTEM image of the intrinsic a-sic, (c) SAED patterns of the Al-doped nc-sic, (d) HRTEM image of the Al-doped nc-sic. Capacitance-voltage techniques were used to determine the SiC doping type and level, with the results shown in Fig. 2. At positive applied voltages, the n-si substrate surface is accumulated and thus the observed decrease in capacitance seen in Fig. 2 is due to the increase in SiC depletion width at the SiC/Al contact. This indicates that the SiC is p type with a doping level around 2x10 18 cm -3, determined from the slope of A/C 2 vs V plot, where A is the device area. The voltage axis intercept of this plot also reveals a built-in potential of about 2 V for the SiC/Al contact, which is further evidence that the nc-sic is p type. 4

6 FIG. 2. High frequency capacitance-voltage data of the Al-doped nc-sic deposited on n-type Si substrate. Metal induced layer exchange (MLILE) is a commonly used crystallization method for converting amorphous Si (a-si) into crystalline Si (c-si) An Al layer is generally formed on the substrate prior to a-si layer deposition, so that during the post-annealing treatment Si atoms diffuse into the Al layer and when Si solubility with the aluminium is exceeded and a certain supersaturation is built, Si nuclei are formed. 16 The difference in the Gibbs free energy of a-si and c-si is assumed as the driving force. 17 Similar mechanism has been proposed to explain Al-induced crystallization of a-sic using the layer change method, 9,10 in which Al atoms diffuse into a-sic during the post-annealing and crystallize it, but this mechanism is not applicable to the in situ crystallization caused by the co-supply of Al, Si, and C- containing precursors during the LPCVD process. The most similar process was in the case of amorphous Ge (a-ge), where crystallization of a-ge was achieved by the cosupply of Al and Ge at a fixed deposition temperature by rf sputtering. 18 It was proposed that the Al-induced crystallization originates from fourfold-coordinated Al atoms in the a-ge network, acting as crystallization seeds. 18 In our process, the mechanism of Al induced in situ crystallization of a-sic is proposed based on the classic theory of nucleation in the growth rate-limited regime. From a theoretical point of view, the Gibbs Thomson relation indicates that the lower the supersaturation, the larger the nucleus size. With increasing supersaturation, the critical nucleus size decreases and the activation energy for nucleation is reduced. Based on deposition experiments conducted at various temperatures using MS as the only precursor, it is found that the deposition rate is exponentially dependant on the 5

7 substrate temperature as shown in Fig. 3. At a given temperature, the deposition rate increased with increasing MS pressure, for example, at 600 ºC it increased from 5 nm/h with a MS pressure of 0.06 mbar to 130 nm/h with a MS pressure of 0.54 mbar. These trends are evidence of surface reaction limited growth, and the same growth regime was also found for MS by Johnson et al. 19 Under this growth regime, we believe the introduction of TMA enhances the surface reaction and increases the supersaturation, resulting in the reduction of activation energy for nucleation of SiC. FIG. 3. Dependences of the deposition rate of intrinsic SiC films on the substrate temperature and MS pressure. There are experimental results showing that single-crystalline SiC epitaxial growth temperature is decreased by doping acceptor atoms, such as boron and aluminum. 19 It is also experimentally proven that the introduction of TMA decreases the activation energy from 1.50 ev for the intrinsic SiC deposition to 0.38 ev for the Al-doped SiC deposition, 20 which is accompanied by the observation of increasing deposition rate. 20,21 An increase of the deposition rate from around 0.8 Å/min for the intrinsic a- SiC film to around 1.3 Å/min for the Al-doped nc-sic is observed in our experiments, which supports the reduction of activation energy. In conclusion, we have demonstrated that in a 600 ºC LPCVD process, which would otherwise produce an amorphous SiC film, the addition of an Al precursor (TMA) has caused in situ crystallization of the SiC. The presence of nano-crystals is confirmed by cross-section HRTEM image and SAED studies. Capacitance-voltage measurements demonstrate that the crystallized nc-sic film is of p type conductivity with a doping concentration of around cm -3. A crystallization mechanism is 6

8 proposed based on the classic theory of nucleation in the growth rate limited regime. The introduction of Al(CH 3 ) 3 enhances the surface reaction and increases the supersaturation, which reduces the activation energy for nucleation and accelerates the deposition rate. This work was supported by the Australian Research Council. 7

9 References 1 S.Y. Myong and K.S. Lim, Appl. Phys. Lett. 86, (2005). 2 S.Y. Myong, K.S. Lim and J.M. Pearce, Appl. Phys. Lett. 87, (2005). 3 M.W.M. van Cleef, R.E.E. Schropp, and F.A. Rubinelli, Appl. Phys. Lett. 73, 2609 (1998). 4 S. Y. Myong, H. K. Lee, E. Yoon, and K. S. Lim, J. Non-Cryst. Solids 298, 131 (2002). 5 Y. Huang, A. Dasgupta, A. Gordijn, F. Finger, and R. Carius, Appl. Phys. Lett. 90, (2007). 6 S. Miyajima, M. Sawamura, A. Yamada, M. Konagai, J. Non-Cryst. Solids 354, 2350 (2008). 7 V. Heera, K.N. Madhusoodanan, A. Mücklich, W. Skorupa, Diamond Relat. Mater. 12, 1190 (2003). 8 V. Heera, K. N. Madhusoodanan, A. Mücklich, D. Panknin, and W. Skorupa, Appl. Phys. Lett 81, 70 (2002). 9 M. Hossain, M. Yun, V. Korampally, and S. Gangopadhyay, J Mater. Sci.: Mater. Electron 19, 801 (2008). 10 M. Hossain, J.R.S. Perez, J.M.R. Rivera, K. Gangopadhyay, and S. Gangopadhyay, J Mater. Sci.: Mater. Electron 20, S412 (2009). 11 S. Urban, F. Falk, Appl. Surface Sci. 184, 356 (2001). 12 D.L. Larkin, P.G. Neudeck, J.A. Powell, and L.G. Matus, Appl. Phys. Lett. 65, 1659 (1994). 13 A. Schöner and M. Gustafsson, Mat. Res. Soc. Symp. Proc. Vol. 815, J1.3.1 (2004). 14 J. Schneider, J. Klein, M. Muske, S. Gall, and W. Fuhs, Appl. Phys. Lett. 87, (2005). 8

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11 Figure captions FIG. 1. (a) SAED patterns of the intrinsic a-sic, (b) HRTEM image of the intrinsic a- SiC, (c) SAED patterns of the Al-doped nc-sic, (d) HRTEM image of the Al-doped nc-sic. FIG. 2. High frequency capacitance-voltage data of the Al-doped nc-sic deposited on n-type Si substrate. FIG. 3. Dependences of the deposition rate of intrinsic SiC films on the substrate temperature and MS pressure. 10

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