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Preparation of SiO2 films with embedded Si nanocrystals by reactive r.f. magnetron sputtering
Thin Solid Films 330 (1998) 202±205
Preparation of SiO2 ®lms with embedded Si nanocrystals by reactive r.f. magnetron sputtering H. Seifarth*, R. GroÈtzschel, A. Markwitz, W. Matz, P. Nitzsche, L. Rebohle Research Center Rossendorf Inc., Institute for Ion Beam Physics and Materials Research, P.O. Box 510119, D-01314Dresden, Germany Received 17 October 1997; accepted 20 March 1998
Abstract SiOx ®lms with a nominal x-value (1 # x # 2) were deposited on ¯at-surface silicon substrates by reactive r.f. magnetron sputtering at substrate temperatures of 20 and 5008C, respectively. X-ray diffraction and high resolution TEM investigations of SiOx ®lms with x 1:45 and x 1 show that as-deposited ®lms have an amorphous structure. After annealing, a nucleation of Si nanocrystals was found with increasing size at increasing initial Si concentration and annealing temperature. The weak photoluminescence in the visible region of asdeposited SiOx ®lms increases remarkably by annealing with dependence on x. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Nanostructures; Plasma processing; Silicon oxide; Sputtering
1. Introduction Although silicon is the most important material in semiconductor device fabrication, it has been regarded as unsuitable for light emitting devices due to its indirect optical transition. However, Si nanocrystals embedded in an amorphous surrounding, e.g. SiO2, exhibit a quasi-direct band gap as a result of quantum con®nement of electron and hole wave functions. They show a visible photoluminescence (PL) depending on the crystal size [1]. During the last decade, Si-rich SiO2 ®lms have been prepared by r.f. magnetron co-sputtering from a SiO2 glass plate target, on which some Si single-crystal chips were placed for investigating their PL properties [2-7]. In this contribution we show that the reactive r.f. magnetron sputtering from a Si-plate target in an Ar/O2 atmosphere is a useful method for depositing Si-rich SiO2 ®lms with a continuous control of the Si concentration. Results of Si nanocrystal nucleation and PL properties are presented. 2. Experimental details Thin ®lm deposition was performed in a NORDIKO 2000 sputtering device using a 13.6 MHz r.f. planar magnetron with a silicon target (8-inch diameter, 99.99% purity) arranged in a distance of 75 mm from the substrate plate. Flat-surface silicon wafers were used as substrates. Before * Corresponding author.
starting the deposition process, the residual gas pressure in the chamber was about 7 £ 1025 Pa. The following deposition conditions were used: substrate temperature Ts 20 and 5008C, respectively, r.f. sputtering power Pr:f: 1:5 kW, argon pressure pAr 3:5 £ 1021 Pa. For the deposition of the SiOx ®lms (x # 2) oxygen gas was fed into the chamber, additionally. Because the reactive sputtering process exhibits the behaviour of a positive feedback, the oxygen mass ¯ow has to be controlled by an external regulating circuit for adjustment and stabilization of the set point as described in detail in Refs. [8±11]. In our experiments the set point is characterized by the normalized optical emission intensity I (l 251:9 nm) of Si atoms sputtered from the target and excited by the plasma. This intensity is proportional to the sputter rate of Si atoms. Fig. 1 shows I as a function of the oxygen mass ¯ow for a constant sputtering power Pr:f: 1:5 kW using a regulating circuit (inset in Fig. 1) consisting of the optical emission spectrometer VM 3000 (Varity Instruments) and the reactive sputtering controller REACTAFLO (Megatech). The set point I determines the composition x of the deposited SiOx ®lms and was adjusted at the REACTAFLO as a nominal value corresponding to the desired value of x. 3. Results and discussion The composition x of SiOx ®lms deposited by using different set points was measured by Rutherford backscattering spectroscopy (RBS) and, after Si crystal nucleation,
0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved. PII S0040-609 0(98)00609-9
H. Seifarth et al. / Thin Solid Films 330 (1998) 202±205
203
Fig. 1. Set point-curve: normalized optical emission intensity I (l 251:9 nm) of sputtered Si atoms as a function of the oxygen mass ¯ow controlled by a regulating circuit. The values of x characterize the composition of SiOx ®lms, which were deposited by using the set points marked by arrows. The inset shows the regulating circuit.
by multiple angle ellipsometry with a SE400 ellipsometer (Sentech Instruments GmbH). The Bruggemann mixed medium model [12] was used for the determination of the volume fraction of Si crystals. Some x-values are attached to the set point curve in Fig. 1. As-deposited SiOx ®lms are considered to be amorphous with a random mixture of Si±SinO42n tetrahedra, which distribution depends on x and n is varied from 0 to 4. Xray diffraction (XRD) patterns were taken with Cu-Ka radiation (Siemens D5000) using the grazing incidence technique (angle of incidence 18). Typical XRD results are shown in Fig. 2. The samples deposited at Ts 20 and 5008C show typical patterns of amorphous structures. The increased signal at about 558 in Fig. 2b is a result of the scattering from the single crystalline substrate and not an effect of the amorphous layer. During annealing at T 10008C for 1 h in an N2 atmosphere, the exceeding Si forms Si nanocrystals in a surrounding SiO2 network. The sample with the high Si concentration corresponding to x 1 (Ts 208C) in Fig. 2a shows distinct peaks of crystalline silicon. The ®rst three re¯ections are clearly visible, indications of further re¯ections are seen, additionally. However, the sample with the remarkable lower Si concentration corresponding to x 1:45 (Ts 5008C) in Fig. 2b shows only ®rst indications of re¯ections from crystalline silicon after annealing. The results indicate that the number of Si nanocrystals is the
Fig. 2. XRD pattern of as-deposited and annealed samples for deposition temperatures of Ts 208C (a) and Ts 5008C (b). The inset in (b) shows the modelling of the measured section at Si(111). Considering a mean crystallite size between 4 and 5 nm the spectrum is well described as a sum of the pure amorphous and nanocrystalline scattering.
Fig. 3. HRTEM micrograph of Si nanocrystals embedded in the SiO2 matrix. Some nanocrystals that are showing (111) lattice planes mainly are marked by circles. The corresponding diffraction pattern is shown in the inset.
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Fig. 4. PL spectra of as-deposited and annealed SiOx ®lms with different values of x using 250 nm excitation. (a) SiOx ®lm with slight silicon excess corresponding to 1:9 , x , 2, dSiOx 190 nm, Ts 208C. (b) x 1:85, dSiOx 190 nm, Ts 208C (Ð, W, X) and x 1:45, dSiOx 650 nm, Ts 5008C (O).
larger the higher the initial Si concentration of the SiOx ®lm, and that the low temperature deposition is favourable for the formation of Si nanocrystals, because of quenched in defects and/or stress, which relax upon annealing. This ®nding is consistent with the results of several other investigators who have shown that the size of the crystals can be controlled by varying the initial Si concentration in the SiOx ®lm at a low substrate temperature Ts, [4]. The diffraction lines of silicon are strongly broadened, which is characteristic for small crystallite sizes. The mean size of the Si nanocrystals was estimated from the line width by the Scherrer formula [13]. A value of 4.5±5 nm was obtained for the sample in Fig. 2a. For the sample in Fig. 2b, a direct estimation of the line width was impossible. A model calculation for the Si(111) re¯ection (inset in Fig. 2b) reveals that a crystal size between 4 and 5 nm describes the measured spectrum satisfactory. Silicon nanocrystals, which are embedded in amorphous SiO2 have been directly observed by high resolution transmission electron microscopy (HRTEM). Fig. 3 shows the cross-sectional bright ®eld HRTEM micrograph (Philips CM300, U 300 kV, line resolution 0:14 nm, magnification 500K) of the same sample, of which the XRD pattern is presented in Fig. 2a. The diffraction pattern (inset in Fig. 3) of a region where the HRTEM micrograph
originates from, shows three rings corresponding to the (111), (200) and (311) lattice planes of polycrystalline silicon. This observation con®rms the existence of Si nanocrystals and corresponds well with the XRD results including the decreasing ring intensity with increasing diffraction angle. In the bright ®eld HRTEM picture such Si nanocrystals are visible, which are randomly well oriented to show the (111) lattice planes. In this way, only a fraction of the present nanocrystals is observed. The number and crystallinity of the nanocrystals are con®rmed by HRTEM dark ®eld analysis. The Si nanocrystals have nearly spherical shape, some of them are marked in Fig. 3 by circles. Their size ranges from 3 to 8 nm resulting in a mean crystal size of 5 nm, which is in good agreement with the value obtained with XRD (Fig. 2). Furthermore, polycrystalline nanoclusters are not observed. This part of the HRTEM micrograph is representative for the nanocrystal size of the sample, however, other regions of the specimen show higher or lower nanocrystal densities. It has to be noted that the detection limit of nanocrystals is ,2 nm by the used HRTEM imaging set-up. Fig. 4a shows the PL spectra of SiOx ®lms with a slight Si excess corresponding to 1:9 , x , 2 deposited at Ts 208C. The as-deposited SiOx ®lms show only very weak PL in the visible region at 460 nm and around 680 nm, as well as, an ultraviolet peak at 285 nm. After annealing at T 6008C for 3 h in an N2 atmosphere the intensity of the blue and the ultraviolet peak is highly increased. A second annealing procedure at T 5008C for 1 h in an Ar/ 7%H2 atmosphere results in a signi®cant decrease of the PL intensity of the ultraviolet peak, whereas the intensity of the blue peak remains nearly unchanged. However, the peak at 460 nm is broadened and shifted to 500 nm. It should be noted that stoichiometric as-deposited SiO2 ®lms show only minor PL around 460 nm, which disappears completely after annealing at T 5008C for 1 h in an N2 atmosphere. The PL spectra of SiOx ®lms change signi®cantly with increasing Si concentration. As shown in Fig. 4b, the PL spectrum of SiOx ®lms with x 1:85 deposited at Ts 208C and annealed at T 6008C for 3 h in an N2 atmosphere is dominated by a broad emission between 450 and 700 nm. The ultraviolet PL peak at 285 nm is still present, but with a very low intensity. After a second heat treatment at T 10008C for 1 h in an N2 atmosphere the PL in the blue-green region is diminished and a peak around 720 nm becomes dominant. No PL in the visible region was detected for SiOx ®lms with x 1:45 deposited at Ts 5008C and annealed at T 10008C for 1 h in an N2 atmosphere. Instead of it, a very intense infrared peak at 830 nm can be observed. SiOx ®lms with a very large Si concentration corresponding to x 1 do not show any PL in the investigated wavelength region between 250 and 900 nm. It is well known from the literature, that oxygen de®ciency induced defect centres (ODC) in SiO2 like the neutral oxygen vacancy (B2-center) [14] or the 2-fold co-ordinated Si (Si20-center) [15] exhibit an excitation band at 250 nm and
H. Seifarth et al. / Thin Solid Films 330 (1998) 202±205
emission bands around 285 and 460 nm. We think that the observed PL can be explained as follows: during deposition, ODC and especially precursor defects like the E 0 -center are produced, in which only the ODC contribute to the observed PL around 285 and 460 nm. In the case of stoichiometric SiO2 ®lms the SiO2 network recovers during annealing and the weak PL observed after deposition vanishes. In contrast to this the annealing of SiOx ®lms with a slight Si excess corresponding to 1:9 , x , 2 leads to a transformation of precursor defects into ODC increasing the PL in the blue and ultraviolet spectral region. Another competing process is the segregation of the excess Si into amorphous Si clusters and Si nanocrystals due to annealing. This process should be favoured by high annealing temperatures and dominates for a high Si excess. Therefore, the strong blue and ultraviolet PL is observed only for SiOx ®lms with 1:9 , x , 2. The PL of SiOx ®lms with higher Si concentration could be attributed to band to band recombination in Si clusters or Si nanocrystals. In this case a redshift of the observed PL with increasing cluster size is expected as shown in Fig. 4b. However, the presence of Si nanocrystals could be detected only for x 1, and so the existence of other luminescence centres, especially for 1 , x , 1:9, cannot be excluded. 4. Summary SiOx ®lms with 1 # x # 2 were deposited on silicon substrates by reactive r.f. magnetron sputtering at the substrate temperatures Ts 20 and 5008C, respectively, using an external regulating circuit for adjustment and stabilization of the setpoint. The specimens were investigated with RBS, XRD and HRTEM. The as-deposited ®lms show typical XRD pattern for amorphous structures, whereas the exceeding Si forms nanocrystals in a surrounding SiO2 network during annealing at Ts 10008C for 1 h in an N2 atmosphere. The annealed samples with a high Si concentration corresponding to x 1 show distinct XRD
205
peaks of crystalline silicon. The mean size of the Si nanocrystals, which have nearly spherical shape, was estimated to be about 5 nm by XRD and HRTEM. The analysis results indicate that the number of Si nanocrystals is a function of the Si concentration of the SiOx ®lm. The as-deposited SiOx ®lms show only weak PL in the visible region, which vanishes in stoichiometric SiO2 ®lms during annealing due to the recovering of the SiO2 network. For SiOx ®lms with x , 2, the PL changes remarkably by the annealing procedure due to the transformation of precursor defects into ODC or nucleation of Si nanocrystals depending on x. References [1] Y. Kanemitsu, T. Ogawa, K. Shiraishi, K. Takeda, Phys. Rev. B 48 (1993) 4883. [2] M. Yamamoto, R. Hayashi, K. Tsunetomo, K. Kohno, Y. Osaka, Jpn. J. Appl. Phys. 30 (1991) 136. [3] Y. Osaka, K. Tsunetomo, F. Toyomura, H. Myoren, K. Kohno, Jpn. J. Appl. Phys. 31 (1992) L365. [4] S. Hayashi, T. Nagareda, Y. Kanzawa, K. Yamamoto, Jpn. J. Appl. Phys. 32 (1993) 3840. [5] K. Kohno, Y. Osaka, F. Toyomura, H. Katayama, Jpn. J. Appl. Phys. 33 (1994) 6616. [6] Q. Zhang, S.C. Bayliss, D.A. Hutt, Appl. Phys. Lett. 66 (1995) 1977. [7] S. Yoshida, T. Hanada, S. Tanabe, N. Soga, Jpn. J. Appl. Phys. 35 (1996) 2694. [8] S. Berg, T. Larsson, C. Nender, H.-O. Blom, J. Appl. Phys. 63 (1988) 887. [9] T. Larsson, H.-O. Blom, C. Nender, S. Berg, J. Vac. Sci. Technol. A 6 (1988) 1832. [10] S. Berg, H.-O. Blom, M. Moradi, C. Nender, T. Larsson, J. Vac. Sci. Technol. A 7 (1989) 1225. [11] H. Seifarth, Thin Solid Films 172 (1989) 61. [12] H.G. Tompkins, A User's Guide to Ellipsometry, Academic Press, New York, 1993, pp. 246±248. [13] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures, Wiley, New York. 1974. Pp. 656±687. [14] R. Tohmon, Y. Shimogaichi, H. Mizuno, Y. Ohki, Phys. Rev. Lett. 62 (1989) 1388. [15] L. Skuja, J. Non. Cryst. Solids 149 (1992) 77.
Preparation of SiO2 ®lms with embedded Si nanocrystals by reactive r.f. magnetron sputtering H. Seifarth*, R. GroÈtzschel, A. Markwitz, W. Matz, P. Nitzsche, L. Rebohle Research Center Rossendorf Inc., Institute for Ion Beam Physics and Materials Research, P.O. Box 510119, D-01314Dresden, Germany Received 17 October 1997; accepted 20 March 1998
Abstract SiOx ®lms with a nominal x-value (1 # x # 2) were deposited on ¯at-surface silicon substrates by reactive r.f. magnetron sputtering at substrate temperatures of 20 and 5008C, respectively. X-ray diffraction and high resolution TEM investigations of SiOx ®lms with x 1:45 and x 1 show that as-deposited ®lms have an amorphous structure. After annealing, a nucleation of Si nanocrystals was found with increasing size at increasing initial Si concentration and annealing temperature. The weak photoluminescence in the visible region of asdeposited SiOx ®lms increases remarkably by annealing with dependence on x. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Nanostructures; Plasma processing; Silicon oxide; Sputtering
1. Introduction Although silicon is the most important material in semiconductor device fabrication, it has been regarded as unsuitable for light emitting devices due to its indirect optical transition. However, Si nanocrystals embedded in an amorphous surrounding, e.g. SiO2, exhibit a quasi-direct band gap as a result of quantum con®nement of electron and hole wave functions. They show a visible photoluminescence (PL) depending on the crystal size [1]. During the last decade, Si-rich SiO2 ®lms have been prepared by r.f. magnetron co-sputtering from a SiO2 glass plate target, on which some Si single-crystal chips were placed for investigating their PL properties [2-7]. In this contribution we show that the reactive r.f. magnetron sputtering from a Si-plate target in an Ar/O2 atmosphere is a useful method for depositing Si-rich SiO2 ®lms with a continuous control of the Si concentration. Results of Si nanocrystal nucleation and PL properties are presented. 2. Experimental details Thin ®lm deposition was performed in a NORDIKO 2000 sputtering device using a 13.6 MHz r.f. planar magnetron with a silicon target (8-inch diameter, 99.99% purity) arranged in a distance of 75 mm from the substrate plate. Flat-surface silicon wafers were used as substrates. Before * Corresponding author.
starting the deposition process, the residual gas pressure in the chamber was about 7 £ 1025 Pa. The following deposition conditions were used: substrate temperature Ts 20 and 5008C, respectively, r.f. sputtering power Pr:f: 1:5 kW, argon pressure pAr 3:5 £ 1021 Pa. For the deposition of the SiOx ®lms (x # 2) oxygen gas was fed into the chamber, additionally. Because the reactive sputtering process exhibits the behaviour of a positive feedback, the oxygen mass ¯ow has to be controlled by an external regulating circuit for adjustment and stabilization of the set point as described in detail in Refs. [8±11]. In our experiments the set point is characterized by the normalized optical emission intensity I (l 251:9 nm) of Si atoms sputtered from the target and excited by the plasma. This intensity is proportional to the sputter rate of Si atoms. Fig. 1 shows I as a function of the oxygen mass ¯ow for a constant sputtering power Pr:f: 1:5 kW using a regulating circuit (inset in Fig. 1) consisting of the optical emission spectrometer VM 3000 (Varity Instruments) and the reactive sputtering controller REACTAFLO (Megatech). The set point I determines the composition x of the deposited SiOx ®lms and was adjusted at the REACTAFLO as a nominal value corresponding to the desired value of x. 3. Results and discussion The composition x of SiOx ®lms deposited by using different set points was measured by Rutherford backscattering spectroscopy (RBS) and, after Si crystal nucleation,
0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved. PII S0040-609 0(98)00609-9
H. Seifarth et al. / Thin Solid Films 330 (1998) 202±205
203
Fig. 1. Set point-curve: normalized optical emission intensity I (l 251:9 nm) of sputtered Si atoms as a function of the oxygen mass ¯ow controlled by a regulating circuit. The values of x characterize the composition of SiOx ®lms, which were deposited by using the set points marked by arrows. The inset shows the regulating circuit.
by multiple angle ellipsometry with a SE400 ellipsometer (Sentech Instruments GmbH). The Bruggemann mixed medium model [12] was used for the determination of the volume fraction of Si crystals. Some x-values are attached to the set point curve in Fig. 1. As-deposited SiOx ®lms are considered to be amorphous with a random mixture of Si±SinO42n tetrahedra, which distribution depends on x and n is varied from 0 to 4. Xray diffraction (XRD) patterns were taken with Cu-Ka radiation (Siemens D5000) using the grazing incidence technique (angle of incidence 18). Typical XRD results are shown in Fig. 2. The samples deposited at Ts 20 and 5008C show typical patterns of amorphous structures. The increased signal at about 558 in Fig. 2b is a result of the scattering from the single crystalline substrate and not an effect of the amorphous layer. During annealing at T 10008C for 1 h in an N2 atmosphere, the exceeding Si forms Si nanocrystals in a surrounding SiO2 network. The sample with the high Si concentration corresponding to x 1 (Ts 208C) in Fig. 2a shows distinct peaks of crystalline silicon. The ®rst three re¯ections are clearly visible, indications of further re¯ections are seen, additionally. However, the sample with the remarkable lower Si concentration corresponding to x 1:45 (Ts 5008C) in Fig. 2b shows only ®rst indications of re¯ections from crystalline silicon after annealing. The results indicate that the number of Si nanocrystals is the
Fig. 2. XRD pattern of as-deposited and annealed samples for deposition temperatures of Ts 208C (a) and Ts 5008C (b). The inset in (b) shows the modelling of the measured section at Si(111). Considering a mean crystallite size between 4 and 5 nm the spectrum is well described as a sum of the pure amorphous and nanocrystalline scattering.
Fig. 3. HRTEM micrograph of Si nanocrystals embedded in the SiO2 matrix. Some nanocrystals that are showing (111) lattice planes mainly are marked by circles. The corresponding diffraction pattern is shown in the inset.
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Fig. 4. PL spectra of as-deposited and annealed SiOx ®lms with different values of x using 250 nm excitation. (a) SiOx ®lm with slight silicon excess corresponding to 1:9 , x , 2, dSiOx 190 nm, Ts 208C. (b) x 1:85, dSiOx 190 nm, Ts 208C (Ð, W, X) and x 1:45, dSiOx 650 nm, Ts 5008C (O).
larger the higher the initial Si concentration of the SiOx ®lm, and that the low temperature deposition is favourable for the formation of Si nanocrystals, because of quenched in defects and/or stress, which relax upon annealing. This ®nding is consistent with the results of several other investigators who have shown that the size of the crystals can be controlled by varying the initial Si concentration in the SiOx ®lm at a low substrate temperature Ts, [4]. The diffraction lines of silicon are strongly broadened, which is characteristic for small crystallite sizes. The mean size of the Si nanocrystals was estimated from the line width by the Scherrer formula [13]. A value of 4.5±5 nm was obtained for the sample in Fig. 2a. For the sample in Fig. 2b, a direct estimation of the line width was impossible. A model calculation for the Si(111) re¯ection (inset in Fig. 2b) reveals that a crystal size between 4 and 5 nm describes the measured spectrum satisfactory. Silicon nanocrystals, which are embedded in amorphous SiO2 have been directly observed by high resolution transmission electron microscopy (HRTEM). Fig. 3 shows the cross-sectional bright ®eld HRTEM micrograph (Philips CM300, U 300 kV, line resolution 0:14 nm, magnification 500K) of the same sample, of which the XRD pattern is presented in Fig. 2a. The diffraction pattern (inset in Fig. 3) of a region where the HRTEM micrograph
originates from, shows three rings corresponding to the (111), (200) and (311) lattice planes of polycrystalline silicon. This observation con®rms the existence of Si nanocrystals and corresponds well with the XRD results including the decreasing ring intensity with increasing diffraction angle. In the bright ®eld HRTEM picture such Si nanocrystals are visible, which are randomly well oriented to show the (111) lattice planes. In this way, only a fraction of the present nanocrystals is observed. The number and crystallinity of the nanocrystals are con®rmed by HRTEM dark ®eld analysis. The Si nanocrystals have nearly spherical shape, some of them are marked in Fig. 3 by circles. Their size ranges from 3 to 8 nm resulting in a mean crystal size of 5 nm, which is in good agreement with the value obtained with XRD (Fig. 2). Furthermore, polycrystalline nanoclusters are not observed. This part of the HRTEM micrograph is representative for the nanocrystal size of the sample, however, other regions of the specimen show higher or lower nanocrystal densities. It has to be noted that the detection limit of nanocrystals is ,2 nm by the used HRTEM imaging set-up. Fig. 4a shows the PL spectra of SiOx ®lms with a slight Si excess corresponding to 1:9 , x , 2 deposited at Ts 208C. The as-deposited SiOx ®lms show only very weak PL in the visible region at 460 nm and around 680 nm, as well as, an ultraviolet peak at 285 nm. After annealing at T 6008C for 3 h in an N2 atmosphere the intensity of the blue and the ultraviolet peak is highly increased. A second annealing procedure at T 5008C for 1 h in an Ar/ 7%H2 atmosphere results in a signi®cant decrease of the PL intensity of the ultraviolet peak, whereas the intensity of the blue peak remains nearly unchanged. However, the peak at 460 nm is broadened and shifted to 500 nm. It should be noted that stoichiometric as-deposited SiO2 ®lms show only minor PL around 460 nm, which disappears completely after annealing at T 5008C for 1 h in an N2 atmosphere. The PL spectra of SiOx ®lms change signi®cantly with increasing Si concentration. As shown in Fig. 4b, the PL spectrum of SiOx ®lms with x 1:85 deposited at Ts 208C and annealed at T 6008C for 3 h in an N2 atmosphere is dominated by a broad emission between 450 and 700 nm. The ultraviolet PL peak at 285 nm is still present, but with a very low intensity. After a second heat treatment at T 10008C for 1 h in an N2 atmosphere the PL in the blue-green region is diminished and a peak around 720 nm becomes dominant. No PL in the visible region was detected for SiOx ®lms with x 1:45 deposited at Ts 5008C and annealed at T 10008C for 1 h in an N2 atmosphere. Instead of it, a very intense infrared peak at 830 nm can be observed. SiOx ®lms with a very large Si concentration corresponding to x 1 do not show any PL in the investigated wavelength region between 250 and 900 nm. It is well known from the literature, that oxygen de®ciency induced defect centres (ODC) in SiO2 like the neutral oxygen vacancy (B2-center) [14] or the 2-fold co-ordinated Si (Si20-center) [15] exhibit an excitation band at 250 nm and
H. Seifarth et al. / Thin Solid Films 330 (1998) 202±205
emission bands around 285 and 460 nm. We think that the observed PL can be explained as follows: during deposition, ODC and especially precursor defects like the E 0 -center are produced, in which only the ODC contribute to the observed PL around 285 and 460 nm. In the case of stoichiometric SiO2 ®lms the SiO2 network recovers during annealing and the weak PL observed after deposition vanishes. In contrast to this the annealing of SiOx ®lms with a slight Si excess corresponding to 1:9 , x , 2 leads to a transformation of precursor defects into ODC increasing the PL in the blue and ultraviolet spectral region. Another competing process is the segregation of the excess Si into amorphous Si clusters and Si nanocrystals due to annealing. This process should be favoured by high annealing temperatures and dominates for a high Si excess. Therefore, the strong blue and ultraviolet PL is observed only for SiOx ®lms with 1:9 , x , 2. The PL of SiOx ®lms with higher Si concentration could be attributed to band to band recombination in Si clusters or Si nanocrystals. In this case a redshift of the observed PL with increasing cluster size is expected as shown in Fig. 4b. However, the presence of Si nanocrystals could be detected only for x 1, and so the existence of other luminescence centres, especially for 1 , x , 1:9, cannot be excluded. 4. Summary SiOx ®lms with 1 # x # 2 were deposited on silicon substrates by reactive r.f. magnetron sputtering at the substrate temperatures Ts 20 and 5008C, respectively, using an external regulating circuit for adjustment and stabilization of the setpoint. The specimens were investigated with RBS, XRD and HRTEM. The as-deposited ®lms show typical XRD pattern for amorphous structures, whereas the exceeding Si forms nanocrystals in a surrounding SiO2 network during annealing at Ts 10008C for 1 h in an N2 atmosphere. The annealed samples with a high Si concentration corresponding to x 1 show distinct XRD
205
peaks of crystalline silicon. The mean size of the Si nanocrystals, which have nearly spherical shape, was estimated to be about 5 nm by XRD and HRTEM. The analysis results indicate that the number of Si nanocrystals is a function of the Si concentration of the SiOx ®lm. The as-deposited SiOx ®lms show only weak PL in the visible region, which vanishes in stoichiometric SiO2 ®lms during annealing due to the recovering of the SiO2 network. For SiOx ®lms with x , 2, the PL changes remarkably by the annealing procedure due to the transformation of precursor defects into ODC or nucleation of Si nanocrystals depending on x. References [1] Y. Kanemitsu, T. Ogawa, K. Shiraishi, K. Takeda, Phys. Rev. B 48 (1993) 4883. [2] M. Yamamoto, R. Hayashi, K. Tsunetomo, K. Kohno, Y. Osaka, Jpn. J. Appl. Phys. 30 (1991) 136. [3] Y. Osaka, K. Tsunetomo, F. Toyomura, H. Myoren, K. Kohno, Jpn. J. Appl. Phys. 31 (1992) L365. [4] S. Hayashi, T. Nagareda, Y. Kanzawa, K. Yamamoto, Jpn. J. Appl. Phys. 32 (1993) 3840. [5] K. Kohno, Y. Osaka, F. Toyomura, H. Katayama, Jpn. J. Appl. Phys. 33 (1994) 6616. [6] Q. Zhang, S.C. Bayliss, D.A. Hutt, Appl. Phys. Lett. 66 (1995) 1977. [7] S. Yoshida, T. Hanada, S. Tanabe, N. Soga, Jpn. J. Appl. Phys. 35 (1996) 2694. [8] S. Berg, T. Larsson, C. Nender, H.-O. Blom, J. Appl. Phys. 63 (1988) 887. [9] T. Larsson, H.-O. Blom, C. Nender, S. Berg, J. Vac. Sci. Technol. A 6 (1988) 1832. [10] S. Berg, H.-O. Blom, M. Moradi, C. Nender, T. Larsson, J. Vac. Sci. Technol. A 7 (1989) 1225. [11] H. Seifarth, Thin Solid Films 172 (1989) 61. [12] H.G. Tompkins, A User's Guide to Ellipsometry, Academic Press, New York, 1993, pp. 246±248. [13] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures, Wiley, New York. 1974. Pp. 656±687. [14] R. Tohmon, Y. Shimogaichi, H. Mizuno, Y. Ohki, Phys. Rev. Lett. 62 (1989) 1388. [15] L. Skuja, J. Non. Cryst. Solids 149 (1992) 77.