ARTICLE pubs.acs.org/JPCB
Identification of Sublattice Damages in Swift Heavy Ion Irradiated N-Doped 6H-SiC Polytype Studied by Solid State NMR E. Viswanathan,† D. Kanjilal,‡ K. Sivaji,*,† and S. Ganapathy*,§ †
Materials Science Centre, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India Inter-University Accelerator Centre, Aruna Asaf Ali Marg, P.O. Box 10502, New Delhi 110067, India § Central NMR Facility, National Chemical Laboratory, Pune 411008 and Centre of Advance Study in Crystallography and Bio-Physics, University of Madras, Guindy Campus, Chennai 600 025, India ‡
ABSTRACT: We have studied N-doped 6H-SiC in its pristine and Swift Heavy Ion (SHI) irradiated (150 MeV Ag12þ ions) forms by solid state Nuclear Magnetic Resonance (NMR) at 7.01 T using 13C and 29Si as probe nuclei under magic angle spinning. We show that increased levels of nitrogen doping, than used before, lead to the observation of Knight shifts emanating from an increase in electron density in the conduction band, which in 13C far exceed those in 29Si MAS spectra. We have rationalized the differential effects in the MAS spectra and site-dependent paramagnetic shifts in terms of the nitrogen doping at the A, B, and C lattice sites. N-doping has a profound effect on 29Si spinlattice relaxation, and the site-dependent relaxation behavior is attributed to a difference in conduction electron properties at the different lattice sites. 29Si T1 measurements serve to identify the sublattice damages in SHI irradiated 6H-SiC. By determining the spinlattice relaxation rates as a function of the SHI irradiation ion fluences, the change in relaxation behavior is correlated to the damage production mechanism. The sublattice damage leads to discernable changes in the interaction between the mobile unpaired electrons in the conduction band and the nuclear site, which profoundly influence the NMR relaxation properties. Our relaxation studies also provide evidence for site-dependent localized effects and a decrease in carrier spin density in the conduction band for the SHI irradiated 6H-SiC.
’ INTRODUCTION Silicon carbide (SiC) is an industrially important semiconducting material which has been extensively studied for more than a century. SiC exists in various polytypic forms, and as many as 200 polytypes are known to exist. The SiC structure is built from silicon and carbon atoms which are covalently linked in tetrahedral geometry, and these polytypes themselves differ structurally by the stacking sequence of a particular polytype. Different polytypes are possible by varying the stacking sequences of tetrahedrally bonded SiC bilayers along the c-axis.13 Among these, the polytypes which are structurally prominent and which have attracted the largest attention are the 3C-, 4H-, and 6H-SiC polytypes. Figure 1 shows the structural model of the 6H-SiC polytype which in the Ramsdell notation has the stacking sequence ABCACB. The structural distinction among these three prominent polytypes is best described by the number of nonequivalent silicons (or carbons) which makes up the repeat unit for the three-dimensional ordering of a given polytype. For the 6H-polytype, there are three distinct sites at which the silicons and carbons reside, and these are called the A, B, and C sites. These crystallographic sites differ only in their second next-nearest neighbors because the first neighbors are always tetrahedral. The structural diversity endows silicon carbide with distinct physical and chemical properties. The high thermal conductivity4 is a very important r 2011 American Chemical Society
property which leads to its application in high frequency and high power devices.3,5 Other potential applications of silicon carbide include its use in high-temperature, radiation-resistant devices and nonvolatile-memory applications.6 Although the 3C-, 4H-, and 6H-SiC polytypes are well studied and have generated great interest for recent technological advances, research carried out during the past two decades suggests that the 6H-SiC-based devices exhibit better performance characteristics because of the high reverse current and breakdown voltage they exhibit when compared to 4H-SiC-based devices. It may also be noted that while 4H-SiC is more widely commercially available current research is also focused more on 6H-SiC. Nitrogen doping in silicon carbide has been shown to modify its physical and electronic properties. For the wideband gap semiconductor, nitrogen doping serves to create shallow energy levels below the conduction band, thus reducing the bulk resistance and enhancing the reverse current and breakdown voltage. Nitrogen doping is often sought in the fabrication of SiCbased devices for application in extreme conditions, namely, aerospace, high-temperature, and reactor applications. In this Received: February 11, 2011 Revised: May 12, 2011 Published: May 13, 2011 7766
dx.doi.org/10.1021/jp201367b | J. Phys. Chem. B 2011, 115, 7766–7772
The Journal of Physical Chemistry B
Figure 1. (a) 6H-polytype silicon carbide (6H-SiC). Silicon and carbon atoms are in yellow and gray colors, respectively. (b) Schematic band structure diagram for the nitrogen-doped silicon carbide.
regard, current research is focused on studying the effects of different dopants and radiation damages in doped silicon carbide materials. Even though a number of characteristic investigations on irradiated 6H-SiC have already been reported,714 detailed insight into the damage accumulation mechanism, due to fast moving ions at high-energy regions, has not been gathered. Furthermore, the mechanism of energy transfer and subsequent defect productions in a semiconductor are not clearly understood. In this direction, new studies, which provide atomic level delineation about the sublattice damages created by irradiation, are warranted. Solid state NMR spectroscopy has been used in the past to study different polytypes of silicon carbide. By and large, the solid state NMR studies reported so far deal with the characterization of neat and doped 3C-, 4H-, and 6H-SiC using 13C and 29Si as probe nuclei.1521 A recent study by Hartman et al.22 concerns the spectral and spinlattice relaxation behavior of the 4H- and 6H-SiC at high N-dopant levels and resonance assignments in magic angle spinning (MAS) spectra.22 Solid state NMR studies of radiation damages due to neutron,2326 γ-ray,27,28 R-particle,29 electron,27,28,30 and heavy ion irradiation27,28,3133 have been reported only in the case of other materials. As of yet, to the best of our knowledge, there is no solid state NMR report about the effects of radiation damage in any of the silicon carbide polytypes. Considering the importance of heavy ion irradiation in semiconducting materials, such as the N-doped silicon carbide, and the insights that a powerful spectroscopic technique like solid state NMR can provide, we have undertaken the present study. Our work specifically focuses on the heavily nitrogen-doped 6H-SiC additionally subjected to 150 MeV Ag12þ swift heavy ion irradiation (SHI). For the first time we have studied the nature of radiation damage at the sublattice level in 6H-SiC using 13C and 29 Si MAS NMR and site-resolved 29Si spinlattice relaxation. As we show, our studies provide new atomic level insight on radiation-induced damage in 6H-SiC.
ARTICLE
’ MATERIALS AND METHODS Undoped and nitrogen-doped silicon carbide materials grown by the physical vapor transport (PVT) method34 of thickness 0.25 and 0.48 mm were used. Nitrogen doping was accomplished during the crystal growth under an argon atmosphere by mixing nitrogen gas in appropriate composition to ensure that the pristine N-doped materials were homogeneous in nature. A square 5 5 mm2 area of the nitrogen-doped sample was subjected to swift heavy ion irradiation, and this was subsequently used for the solid state NMR studies. For the irradiation, a high energy of 150 MeV heavy ions Ag12þ was used with ion fluences 8 1012 and 2 1013 ions/cm2. This was achieved using the 15 UD Pelletron accelerator at Inter University Accelerator Centre, New Delhi, and the beam current density was approximately 0.5 pnA/cm2. Four different irradiation conditions were realized with these two different fluences and irradiating the samples at two different temperatures, namely, 300 and 80 K. The depth penetration of irradiation in the sample could not be determined experimentally. However, by independent SRIM (Stopping and Range of Ions in Matter)35 calculations for the same 150 MeV silver (Ag) ion irradiation in a silicon carbide matrix, it is ascertained that the irradiation would cause a penetration depth of 13 μm all across the surface. For the surface-irradiated material, both the bulk and the surface constitute one integrated material for which the NMR observations were made. The pristine undoped and nitrogen-doped 6H-SiC samples were characterized using Raman spectroscopy. Raman spectra were obtained using a Renishaw system equipped with an argon ion laser as a source (excitation wavelength of 514.5 nm). The energy losses of the 150 MeV silver (Ag) ions in the silicon carbide matrix were calculated through the Monte Carlo based simulation code Stopping and Range of Ions in Matter (SRIM).35 Solid State NMR. 29Si and 13C Magic Angle Spinning (MAS) experiments were performed at the Larmor frequency of 59.621 and 75.476 MHz, respectively, on a Bruker-300 (7.01 T) NMR spectrometer equipped with an AVANCE-III console. The MAS spectra were acquired at ambient probe temperature (298 K) in the Bloch decay mode using a 27° Ernst angle pulse of duration 1.2 μs and a pulse repetition delay of 5 s. 29Si and 13C MAS spectra could be acquired in this way to achieve maximum signalto-noise ratio within our available experimental measurement time. This was mainly warranted by our inability to acquire 29Si and 13C spectra under fully relaxed conditions, owing to the prohibitively long 29Si and 13C relaxation times of 6H-SiC. 29Si and 13C were acquired with 8 K data points and a 32.05 kHz spectral window. Signal averaging was done over 1200 and 2400 scans for the 29Si and 13C experiments, respectively, and the free induction decays were apodized with an exponential line broadening of 1025 Hz prior to Fourier transformation. For 29Si spinlattice relaxation time (T1) measurements, a comb of 100 π/2 (2.5 μs) pulses with a fixed delay between the pulses was used to saturate the 29Si longitudinal magnetization. The magnetization Mz(τ) after a recovery time τ was monitored using a π/2 pulse. Typically 40 mg of samples was filled in 4 mm zirconia rotors which were spun at 6 kHz in all the experimental measurements. 29Si and 13C chemical shifts were externally referenced to TMS. Spinlattice relaxation times were determined by online processing of the time domain saturation recovery data and offline fitting of the signal intensities using ORIGIN 7.5. 7767
dx.doi.org/10.1021/jp201367b |J. Phys. Chem. B 2011, 115, 7766–7772
The Journal of Physical Chemistry B
ARTICLE
Figure 3. 13C (left) and 29Si (right) of MAS spectra of pristine-undoped (bottom) and nitrogen-doped (top) 6H-SiC. The peak assignments for the silicon and carbon resonances in the undoped sample are indicated (vide text). Here A, B, and C denote the three crystallographic nonequivalent sites (Si, C) in the repeat structure of 6H-SiC.
Figure 2. Raman spectra of 6H-SiC. (a) Pristine undoped and (b) pristine N-doped.
’ RESULTS AND DISCUSSION Figure 2 shows the Raman spectra of the pristine undoped and N-doped 6H-SiC samples. The spectra depict the transverse optical mode E1(TO), E2(TO), and A1(LO) longitudinal mode at 788, 797, and 966 cm1, respectively, and these match well with the literature values of the hexagonal 6H-SiC. The effects of nitrogen doping can be clearly seen in the Raman spectrum. The LO mode for the nitrogen-doped sample is observed to be broadened. The doping of nitrogen influences the LO mode, and the A1(LO) mode line shape, in particular, depends strongly on the nitrogen concentration in 6H-SiC.36 Raman results thus confirm that our SiC materials are highly crystalline and conform to the hexagonal structure of the 6H polytype. 13
C and 29Si MAS NMR of Silicon Carbide. Nitrogen-Doped 6H-Polytype SiC. In our efforts to assess the effects of swift heavy
ion irradiation on the NMR spectra of silicon carbide, we first analyze the 13C and 29Si MAS spectra of our nitrogen-doped materials. First, the results obtained at our N-dopant concentration of 1.2 1019 atoms/cm2 provide a basis for comparison with earlier reports on the N-doped 6H-SiC at levels of 0.914.5 1017 atoms/cm2. Second, although nitrogen doping is known to affect the spectral and relaxation response of 6H-SiC,16,22,37 the effects of nitrogen doping at the level of 1.2 1019 atoms/cm2 that we have used must be first ascertained before effects of irradiation are discerned from our solid state NMR measurements. 13 C and 29Si MAS spectra of pristine samples without and with nitrogen doping are shown in Figure 3. A three-line spectrum is noticed in each case for the undoped sample and is characteristic of the 6H polytype for which both the carbon and silicon reside at three crystallographic nonequivalent locations, referred to as the A, B, and C sites in the structure.16,18,20,22,38 A near mirror reflection of the three-line pattern in the 29Si and 13C spectra can be seen, and their isotropic chemical shifts (δiso) (Table 1) are found to be identical to those reported earlier16,18,20,22,38 for the 6H-SiC polytype. The expected 1:1:1 intensity, due to the equal occupancy for the three silicon and the three carbon sites in the 6H polytype, is also readily apparent from the observed signal intensities. Both 13C and 29Si spectra depict sharp resonances (Δν = 1025 Hz) due to the high crystallinity of the material. 13 C and 29Si signal assignments are also shown in Figure 3 on the 13 C and 29Si MAS spectra. These peak assignments are based on
Table 1. 29Si and 13C MAS NMR Spectral Data of 6HPolytype SiCa 29
sample undoped
N-doped (1.2 1019 atoms/cm2)
13
Si
C
δiso (ppm)
Δν (Hz)
site
δiso (ppm)
14.7
10
A
15.8
C
26
20.9
11
B
20.7
A
23
25.3
10
C
23.4
B
15
13.9
70
A
18.6
C
175
21.3
79
B
23.9
A, B
225
24.5
104
C
site
Δν (Hz)
a
Chemical shifts are referenced to TMS. The site assignments for the undoped sample are as per ref 22.
the work of Hartman et al.22 who employed a combination of electronic structure calculations, 13C29Si cross-polarization experiments, and chemical shift anisotropy determinations to correct difference in the literature reports.17,18,20,21,39 13 C and 29Si MAS spectra for the nitrogen-doped 6H-SiC, recorded under fully relaxed conditions, are compared with the spectra of the undoped sample in Figure 3. As seen, both 29Si and 13 C resonances show marked changes in chemical shift and line width, being quite severe in the carbon spectra. For the N-doped 6H-SiC the three-line pattern with a near 1:1:1 integrated intensity is retained in the 29Si MAS spectrum (Figure 3b), but the silicon resonances shift to different extents. Noticeably, N-doping causes a high-frequency shift for the two outer resonances corresponding to the A and C silicon sites (A, þ0.7 ppm; C, þ0.8 ppm) and a low-frequency shift for the middle resonance (B, 1.1 ppm). On the other hand, the 13C spectrum is more affected by N doping than the 29Si spectrum and exhibits a twoline pattern with a large high-frequecy shift for the A and C carbon sites (A, þ3.2; C, þ2.8 ppm) and a small high-frequency shift for the B site (B, þ0.6 ppm). The signal overlap of B and C sites causes a two-line spectrum with a 2:1 integrated intensity. It may be noted that 6H-SiC has been studied previously at low levels of nitrogen doping (0.914.5 1017 atoms/cm2),16 and no significant changes in chemical shift were detected in the 29Si and 13C MAS spectra. The high-frequency shift that we observe in the 29Si and 13C MAS spectra for the N-doped 6H-SiC can be traced to a typical paramagnetic shift (Knight Shift). Such a paramagnetic shift has been noticed in N-doped 4H-SiC22 with differential effects in 13C and 29Si MAS spectra. It is important to note that whereas low 7768
dx.doi.org/10.1021/jp201367b |J. Phys. Chem. B 2011, 115, 7766–7772
The Journal of Physical Chemistry B
ARTICLE
Table 2. 13C MAS NMR Spectral Data of SHI-Irradiated N-Doped 6H-SiC
Figure 4. 13C (left) and 29Si (right) MAS NMR spectra of N-doped 6HSiC, subjected to swift heavy ion irradiation. (a) Pristine, (b) 8 1012 ions/cm2 at 300 K, (c) 8 1012 ions/cm2 at 80 K, (d) 2 1013 ions/cm2 at 300 K, and (e) 2 1013 ions/cm2 at 80 K.
nitrogen dopant levels affect the relaxation behavior of 13C and 29Si spins16,18,20,22,38 a high level of N-doping (1.2 1019 atoms/cm2) is warranted to observe the paramagnetic shifts in the MAS spectra. As shown schematically in Figure 1b, and as alluded to in the case of N-doped 4H-SiC,22 the paramagnetic shift can be traced to the effect of the unpaired electrons in the conduction band. In the band structure of N-doped silicon carbide (Figure 1b), the shallow donor level lies below the conduction band, and as the nitrogen doping at the silicon carbide lattice site occurs an extra electron is promoted to the conduction band from the donor level through the ionization mechanism. Considering that each nitrogen in the donor state contributes one unpaired electron spin to the conduction band, the net unpaired electron spin density in the conduction band is strongly related to the level of nitrogen doping in the material. In fact a linear relationship between the paramagnetic shift and N-dopant concentration has been noticed earlier in the case of 4H-SiC.22 At high nitrogen dopant levels, such as the one we have used, the conduction electron spin density is quite large as to cause the observed Knight shifts in both 13C and 29Si MAS spectra. The paramagnetic interaction affects both the spectral and relaxation behavior of the spins and causes the observed high-frequency shift for the resonance and an enhancement in the spinlattice relaxation rate. In the case of N-doped 6H-SiC, clearly Knight Shift effects manifest both in 13C and 29Si MAS spectra, and nitrogen doping is clearly seen to have a much greater effect in 13C spectra than in 29 Si spectra. It has been shown from electronic structure calculations22 that nitrogen doping can occur at any of the lattice sites occupied by silicon or carbon or both. As noted in the case of 4H-SiC,22 the replacement of carbon sites by nitrogen leads to delocalized defect bands, and this will not make any new states in the forbidden band gap, in contrast to silicon-site replacement which leads to a strong local effect that is less effective at long distances. That the effects are localized in the case of silicon site doping and delocalized in the case of carbon site doping implies that doping should affect carbon sites throughout the material and cause a larger effect due to conduction band electrons to be seen in 13C spectra. The density of delocalized electrons at the conduction band has a direct bearing on how small or large the observed Knight shift is and in the case of the N-doped 6H-SiC that we have studied the 13C NMR observation point to a high delocalized electron spin density. The Knight shifts seen in 6H-SiC can be compared with those observed earlier for 4H-SiC at a comparable nitrogen dopant level (1.4 109 atoms/cm2).22 While the effects were similar in the case of 4H-SiC, being greater in 13C than in 29Si spectra, siteresolved Knight shifts were hard to discern in the 29Si spectra due
fluencea
δiso (ppm)
Δν (Hz)
A
23.9
183
B
18.4 23.9
160 188
18.4
199
23.6
105
18.5
224
23.9
122
18.3
44
C D
A: 8 1012 ions/cm2 at 300 K. B: 8 1012 ions/cm2 at 80 K. C: 2 1013 ions/cm2 at 300 K. D: 2 1013 ions/cm2 at 80 K. a
to the lack of resolution for the two sites of the 4H-polytype. On the other hand, in the case of 6H-SiC there are three sites, and these are also sufficiently resolved at the comparable N-dopant level (1.2 1019 atoms/cm2) we have used. The large chemical shift dispersion across the three sites (10.7 ppm), as against the smaller dispersion for the two sites in 4H-polytype (3 ppm),22 entails site-resolved Knight shifts to be observed in our case. Finally, we make a remark about the line broadening especially in the 13C spectra. Previous “hole burning” experiments22 in N-doped 4H-SiC established the line broadening of the 13C MAS spectrum to be inhomogeneous. We believe that the 13C MAS spectrum of our N-doped 6H-SiC is likewise inhomogeneously broadened. Such an inhomogeneously broadened line shape has been shown in a similar instance to be governed by the probability density function for electron carrier concentration in the bulk sample.40 SHI Irradiated N-Doped 6H-SiC. We show in Figure 4 the 13C and 29Si MAS spectra of swift heavy ion (150 MeV Ag12þ) irradiated 6H-SiC. The ion fluences employed for the irradiation are indicated in the figure. As seen, both 13C and 29Si MAS spectra depict the same signal multiplicity as in the unirradiated sample irrespective of the fluence employed, and furthermore, the chemical shifts remain unchanged. This clearly indicates that any change in carrier concentration must be small so that it does not lead to an observable change in Knight shifts. However, as we show later, profound effects can be seen in 29Si spinlattice relaxation. 13C MAS spectra of SHI-irradiated 6H-SiC are too broad to assess the influence of irradiation (Table 2). However, as we have mentioned earlier, 29Si MAS spectra suffer less broadening upon nitrogen doping, and this has allowed us to qualitatively assess the effects of irradiation by an analysis of the MAS line shapes. 29 Si MAS spectra exhibit noticeable variations in the relative intensity of the peaks due to irradiation. 29Si relaxation measurements and T1 ratio estimates for the silicon sites show that the relative intensities are not altered due to relaxation effects in these spectra acquired under rapidly pulsed conditions with the Ernst angle optimized for the N-doped sample. As the damage can occur at one or more of the three different lattice silicon sites and the extent of damage due to irradiation can vary depending upon the fluence employed, we ought to expect that the site populations would be affected. This is reflected in the 29Si MAS spectra. Over and above the changes in relative intensity of the peaks, we also notice that the MAS line shapes additionally contain a broad component which can be ascribed to the amorphous content present along the ion track due to heavy 7769
dx.doi.org/10.1021/jp201367b |J. Phys. Chem. B 2011, 115, 7766–7772
The Journal of Physical Chemistry B
ARTICLE
Figure 5. Deconvolution of the 29Si MAS NMR spectrum of swift heavy ion irradiated N-doped 6H-SiC at the highest fluence (2 1013 ions/cm2 at 80 K).
Figure 7. 29Si MAS signal evolution as a function of the saturation recovery delay (τ) for the C-site silicon of undoped (Δ) and nitrogendoped (O) 6H-SiC and (0) N-doped irradiated with 2 1013 ions/cm2 at 80 K. The straight lines represent the “best fit” relaxation curves using eq 1.
Figure 6. Electronic and nuclear energy losses versus the penetration depth of 150 MeV Ag12þ ions in silicon carbide as calculated using SRIM code.
ion irradiation. This is seen more clearly in the 29Si MAS spectrum of the highly damaged sample (2 1013 ions/cm2 at 80 K), shown in Figure 5, for which the deconvolution41,42 of the spectrum acquired under rapidly pulsed conditions clearly reveals the presence of a strong broad component. We have recently presented evidence for partial amorphous content in SHI-irradiated 6H-SiC from Raman studies.43 Line broadening in the LO mode of Raman scattering was observed, and this showed the presence of amorphous content in the SHI-irradiated samples. Similarly, it has been shown that evolution of damages at every sublattice site influence the NMR line width.25 The observed changes in NMR line shape can be correlated to the creation of sublattice damages by the irradiation and the ion fluence used. The high energy heavy ions moving at very high velocity cause irradiation damage through losses in its energy in two ways, namely, the electronic energy losses (Se) and nuclear energy losses (Sn) emanating from elastic/inelastic collision process, respectively. These two processes operate in different energy regimes, and this can be seen in Figure 6 where we have presented the results from an independent Monte Carlo simulation of the energy losses due to 150 MeV Ag ion irradiation in a silicon carbide matrix. It can be seen that the electronic energy loss dominates at the surface, whereas at the stopping regime of the Ag12þ ions the nuclear energy loss dominates. When taken over the entire range of heavy Ag12þ ion penetration, the ratio between Se and Sn is found to be still very high (297.4 eV/nm), and hence we see an overall dominance of the electronic loss. This implies that 150 MeV Ag12þ heavy ion irradiation in silicon carbide causes only a partial amorphous conversion. This is in good accord with 29Si MAS results which provide evidence for sublattice damages and their accumulation to cause partial
amorphous content in the SHI-irradiated N-doped 6H-SiC without any change of its structure. 29 Si SpinLattice Relaxation in N-Doped and SHI-Irradiated 6H-SiC. We have measured the spinlattice relaxation times (T1) in pristine undoped, N-doped, and SHI-irradiated 6H-SiC samples under MAS conditions using the saturation recovery method. 29Si was chosen mainly because relaxation times at all the three sites (A, B, C) could be determined, and this was not feasible with 13C in the N-doped and SHI-irradiated 6H-SiC samples. Moreover the peak signal-to-noise ratio is inherently superior in 29Si MAS spectra, and this has allowed reliable estimates of spinlattice relaxation times to be made. The recovery of 29Si magnetization as a function of the saturation recovery delay time (τ) for the undoped and N-doped 6H-SiC samples with N-doped irradiated sample with 2 1013 ions/cm2 at 80 K is shown in Figure 7 for one of the silicon sites. The relaxation data obtained at each silicon site were analyzed, and the spinlattice relaxation times were determined using the following expression. " # t n ð1Þ Mz ðtÞR exp T1 Equation 1 denotes the stretched exponential (n) form of the conventional relaxation function, and we have used this in accordance with previous determinations of spinlattice relaxation times in N-doped silicon carbide.16,38 We have not used “n” as an empirical parameter. The stretched exponential form takes into account the conduction electrons and localized paramagnetic centers and their contributions thereof to the relaxation rate. The factor n is a good indicator of how strongly or weakly the conduction electron mechanism contributes to T1. Using eq 1 and by nonlinear least-squares fit of the relaxation data T1 values were determined for all three silicon sites (Table 3). Spinlattice relaxation times for the undoped 6H-SiC are very long (>2700 s), and this can be ascribed to the lack of any relaxation mechanism in the rigid material. By comparison, the spinlattice relaxation is drastically affected in the nitrogendoped sample. There is nearly a 5-fold decrease in T1 at our N-dopant concentration of 1.2 1019 atoms/cm2. The large depression in T1 that we observe is attributable to the very high 7770
dx.doi.org/10.1021/jp201367b |J. Phys. Chem. B 2011, 115, 7766–7772
The Journal of Physical Chemistry B Table 3.
29
ARTICLE
Si SpinLattice Relaxation Data of 6H-Polytype SiCa,b site A
B
T1 (s)
n
T1 (s)
2728 ( 10%
0.7 ( 0.06
499 ( 5%
0.6 ( 0.03
(A) 8 1012 ions/cm2 at 300 K
482 ( 3%
(B) 8 1012 ions/cm2 at 80 K
534 ( 6%
(C) 2 1013 ions/cm2 at 300 K (D) 2 1013 ions/cm2 at 80 K
sample undoped nitrogen doped (1.2 1019 atoms/cm2)
C n
T1 (s)
n
6168 ( 70%
0.7 ( 0.16
2974 ( 3%
0.9 ( 0.03
937 ( 4%
1.0 ( 0.05
324 ( 8%
0.7 ( 0.05
0.9 ( 0.04
1197 ( 15%
0.9 ( 0.07
377 ( 9%
0.6 ( 0.04
0.6 ( 0.04
1389 ( 2%
0.8 ( 0.03
419 ( 3%
0.8 ( 0.04
587 ( 12%
0.8 ( 0.07
1916 ( 5%
0.7 ( 0.04
469 ( 13%
0.8 ( 0.07
849 ( 7%
1.1 ( 0.09
2525 ( 6%
0.7 ( 0.05
544 ( 9%
0.3 ( 0.02
swift heavy ion irradiated
a
T1 determined using eq 1. b “n” denotes the exponential stretching factor. A, B, and C denote the three nonequivalent silicon sites in the structure.
Figure 8. Variation of the 29Si spinlattice relaxation rate (T11) of A, B, and C site silicons with respect to the swift heavy ion irradiation. A: Pristine N-doped 6H-SiC. B: 8 1012 ions/cm2 at 300 K. C: 8 1012 ions/cm2 at 80 K. D: 2 1013 ions/cm2 at 300 K. E: 2 1013 ions/cm2 at 80 K.
level of nitrogen doping in our case. T1 values of N-doped 6HSiC also show that the relaxation is most efficient for the C-site and least efficient for the B-site, whereas it is intermediate for the A-site. Such site-dependent relaxation behavior was also noticed in 6H-SiC at lower N-dopant levels.16 The mechanism of 29Si nuclei relaxation in N-doped 6H-SiC is mainly due to the interaction between the conduction electron and the observed 29 Si spins and the modulation due to electron mobility. The recovery of 29 Si magnetization as a function of the saturation recovery delay time (τ) for the SHI-irradiated N-doped 6H-SiC samples for each silicon site was analyzed, and the spinlattice relaxation times were determined using eq 1. The T1 values determined from best fits to the relaxation data are given in Table 3 along with error estimates for n as well as T1. The T1 results show that site-dependent relaxation behavior is observed, and the relative relaxation efficiency among the three silicon sites remains unchanged irrespective of the heavy ion fluence employed. Since the irradiation has not altered the 6HSiC structure, the observed site-dependent relaxation behavior can be ascribed in the same way to the conduction electron mechanism. This is also borne out from the stretched exponential factor (n) determined from T1 analysis, which in many cases exceeds 0.5 and indicates that the observed 29Si spinlattice relaxation is by and large conduction electron driven. The low
symmetry hexagonal B site in 6H-SiC has a weak interaction with the conduction band electrons, whereas the high symmetry quasi-cubic sites interact strongly with the conduction band electrons.16,38 The observed 35-fold increase in T1 for the B site over the A and C sites is a direct consequence of the above site-dependent conduction band electron interaction. A striking feature of our relaxation studies is that over and above the order of magnitude decrease in T1 due to N-doping SHI irradiation causes a further decrease in the spinlattice relaxation rate for all three silicon sites, the degree of which increases for all the sites in fluence order 8 1012 (300 K), 8 1012 (80 K), 2 1013 (300 K), and 2 1013 (80 K) ions/cm2 (Table 3). In Figure 8 we have plotted the 29Si spinlattice relaxation rate of the A, B, and C sites against the heavy ion fluence taken in the above order. A near linear dependence with respect to ion fluence can be noticed. The observed decrease in relaxation rates due to SHI irradiation can be rationalized based on the defect trapping mechanism.44 As defect states are created in the irradiated material, these defect states can trap an electron in the conduction band and cause a reduction in the conduction band electron density. This reflects a weaker electronnuclear interaction seen by the observed 29Si spins, thereby causing a decrease in the relaxation efficiency. As the relaxation rates are determined by the conduction electron mechanism, which has a direct bearing on the carrier concentration, our results clearly indicate a progressive decrease in carrier concentration with increasing severity of the swift heavy ion irradiation. That this occurs at all three lattice sites is also revealed from our relaxation studies. The decreased relaxation rate and the decreased relaxation efficiency we observe for the B site additionally indicate that the sublattice damage is more severe at this lattice site. Our 29Si spinlattice relaxation studies thus establish that SHI irradiation causes sublattice damage with a larger decrease in the carrier concentration at the low symmetry hexagonal B sites over the high symmetry quasi-cubic A and C silicon sites.
’ CONCLUSIONS In the present work, we have studied the radiation damages at the sublattice level in nitrogen-doped 6H-SiC using 29Si and 13C MAS NMR. Our MAS studies on pristine N-doped 6H-SiC show that pronounced Knight shifts are observed at the high nitrogen dopant concentration of 1.2 1019 atoms/cm2 that we have used. The sublattice damage in swift heavy ion (150 MeV Ag12þ) irradiated 6H-SiC has been studied by spectral and relaxation 7771
dx.doi.org/10.1021/jp201367b |J. Phys. Chem. B 2011, 115, 7766–7772
The Journal of Physical Chemistry B measurements. 29Si MAS spectral analysis reveals partial amorphous content in the SHI-irradiated 6H-SiC, and this has been validated by SRIM. 29Si MAS based spinlattice relaxation measurements have enabled site-resolved T1 measurements to be made and their relaxation behavior addressed and understood in both the N-doped and SHI-irradiated 6H-SiC. For the N-doped material, 29Si spinlattice relaxation efficiency is found to be quite different at the A, B, and C sites, and this is attributed to the variation in the conduction band electron spin density at these sites. In the case of SHI-irradiated 6H-SiC, our relaxation studies show that sublattice damage occurs at the three silicon sites, but the damage is more severe at the low symmetry hexagonal B lattice silicon site. Our 29Si spinlattice relaxation results also indicate a progressive decrease in carrier concentration with increasing severity of the swift heavy ion irradiation.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (S.K);
[email protected] (S.G).
’ ACKNOWLEDGMENT The authors gratefully acknowledge the support of Inter University Accelerator Centre (IUAC), New Delhi, for providing the Accelerator facility and financial support in the form of a Junior Research Fellowship to E.V. (UPUF-39306). S.G. thanks the Council of Scientific and Industrial Research, New Delhi, for support under Emeritus Scientist Scheme (HRDG:21(0701)/ 07/EMR-II). ’ REFERENCES (1) Ramsdell, L. S. Am. Mineral. 1944, 29, 431. (2) Ramsdell, L. S.; Kohn, J. A. Acta Crystallogr. 1952, 5, 215. (3) Saddow, S. E.; Agarwal, A. Advances in Silicon Carbide Processing and applications; Artech House, INC.: 685 Canton Street, Norwood, MA 02062, 2004. (4) Casady, J. B.; Johnson, R. W. Solid-State Electron. 1996, 39, 1409. (5) Raynaud, C. J. Non-Cryst. Solids 2001, 280, 1. (6) Li, C.; Duster, J. S.; Kornegay, K. T. IEEE Electron Device Lett. 2003, 24, 72. (7) Intarasiri, S.; Yu, L. D.; Singkarat, S.; Hallen, A.; Lu, J.; Ottosson, M.; Jensen, J.; Possnert, G. J. Appl. Phys. 2007, 101, 084311. (8) Jiang, W.; Weber, W. J.; Thevuthasan, S. Damage Response To Irradiation Temperature and Ion Fluence in Cþ-Irradiation 6H-SiC. Mater. Res. Soc. Symp. Proc. 1999. (9) Weber, W. J.; Jiang, W.; Thevuthasan, S.; MeCready, D. E. Accumulation and Recovery of Irradiation Effects in Silicon Carbide. Mater. Res. Soc. Symp. Proc. 1999. (10) Jiang, W.; Zhang, Y.; Shutthanandan, V.; Thevuthasan, S.; Weber, W. J. Appl. Phys. Lett. 2006, 89, 261902. (11) Jiang, W.; Nachimuthu, P.; Weber, W. J.; Ginzbursky, L. Appl. Phys. Lett. 2007, 91, 091918. (12) Ling, C. C.; Chen, X. D.; Gong, M.; Yang, C. L.; Ge, W. K.; Wang, J. N. Phys. B 2006, 376377, 374. (13) Benyagoub, A. Nucl. Instrum. Methods Phys. Res., Sect. B 2008, 266, 2766. (14) Benyagoub, A.; Audren, A. Nucl. Instrum. Methods Phys. Res., Sect. B 2009, 267, 1255. (15) Hartman, J. S.; Richardson, M. F.; Sherriff, B. L.; Winsborrow, B. G. J. Am. Chem. Soc. 1987, 109, 6059. (16) Hartman, J. S.; Narayanan, A.; Wang, Y. J. Am. Chem. Soc. 1994, 116, 4019.
ARTICLE
(17) Guth, J. R.; Petuskey, W. T. J. Phys. Chem. 1987, 91, 5361. (18) Richardson, M. F.; Hartman, J. S.; Guo, D.; Winsborrow, B. G. Chem. Mater. 1992, 4, 318. (19) Wagner, G. W.; Na, B. K.; Vannice, M. A. J. Phys. Chem. 1989, 93, 5061. (20) David, C. A.; Robin, K. H.; Gregory, L. M.; Derek, P. T. J. Am. Ceram. Soc. 1991, 74, 777. (21) Finlay, G. R.; Hartman, J. S.; Richardson, M. F.; Williams, B. L. J. Chem. Soc., Chem. Commun. 1985, 159. (22) Hartman, J. S.; Berno, B.; Hazendonk, P.; Kirby, C. W.; Ye, E.; Zwanziger, J.; Bain, A. D. J. Phys. Chem. C 2009, 113, 15024. (23) Ring, P. J.; O’Keefe, J. G.; Bray, P. J. Phys. Rev. Lett. 1958, 1, 453. (24) Knutson, C. D.; Hooper, H. O.; Bray, P. J. J. Phys. Chem. Solids 1966, 27, 147. (25) Walter, J. A.; Troup, G. J. J. Nucl. Mater. 1971, 38, 51. (26) Souers, P. C.; Jolly, T. A.; Cline, C. F. J. Phys. Chem. Solids 1967, 28, 1717. (27) Klempt, T.; Schweizer, S.; Schwartz, K.; Kanert, O.; Suter, D.; Rogulis, U.; Spaeth, J. M. Radiat. Eff. Defects Solids 2001, 155, 159. (28) Klempt, T.; Kanert, O.; Suter, D. Phys. Status Solidi B 2003, 236, 151. (29) Farnan, I.; Cho, H.; Weber, W. J. Identifying and Quantifying Actinide Radiation Damage in Ceramics with Radiological Magic-Angle Spinning Nuclear Magnetic Resonance. Mater. Res. Soc. Symp. Proc. 2007. (30) Zogal, O. J.; Beuneu, F.; Vajda, P.; Florian, P.; Massiot, D. Phys. Rev. B 2002, 66, 064101. (31) Dauce, R.; Le Floch, M.; Sangleboeuf, J. C.; Verdier, P. J. NonCryst. Solids 2007, 353, 390. (32) Pellerin, N.; Dodane-Thiriet, C.; Montouillout, V.; Beauvy, M.; Massiot, D. J. Phys. Chem. B 2007, 111, 12707. (33) Stork, H.; Hamburger, A.; Gadke, A.; Fujara, F.; Schwartz, K. J. Phys.: Condens. Matter 2008, 20, 275236. (34) Tupitsyn, E. Y.; Arulchakkaravarthi, A.; Drachev, R. V.; Sudarshan, T. S. J. Cryst. Growth 2007, 299, 70. (35) Ziegler, J. F.; Biersack, J. P.; Littmark, U. The Stopping and Range of Ions in Solids; Pergamon: New York, 1985. (36) Sorieul, S.; Costantini, J.-M.; Gosmain, L.; Thome, L.; Grob, J.-J. J. Phys.: Condens. Matter 2006, 18, 5235. (37) Alexander, M. N. Phys. Rev. 1968, 172, 331. (38) Hartman, J. S.; Narayanan, A.; Wang, Y. NMR Spin Lattice Relaxation In Nitrogen-Doped 6H Silicon Carbide. Mater. Res. Soc. Symp. Proc. 1994. (39) Dybowski, C.; Gaffney, E. J.; Sayir, A.; Rabinowitz, M. J. Colloids Surf., A 1996, 118, 171. (40) Yesinowski, J. P.; Purdy, A. P.; Wu, H.; Spencer, M. G.; Hunting, J.; DiSalvo, F. J. J. Am. Chem. Soc. 2006, 128, 4952. (41) Kassiba, A.; Makowska-Janusik, M.; Boucle, J.; Bardeau, J. F.; Bulou, A.; Herlin-Boime, N. Phys. Rev. B 2002, 66, 155317. (42) Charpentier, S.; Kassiba, A.; Emery, J.; Cauchetier, M. J. Phys.: Condens. Matter 1999, 11, 4887. (43) Viswanathan, E.; Katharria, Y. S.; Selvakumar, S.; Arulchakkaravarthi, A.; Kanjilal, D.; Sivaji, K. Nucl. Instrum. Methods Phys. Res., Sect. B 2011, 269, 1103. (44) Kumar, S.; Katharria, Y. S.; Kanjilal, D. J. Appl. Phys. 2008, 103, 044504.
7772
dx.doi.org/10.1021/jp201367b |J. Phys. Chem. B 2011, 115, 7766–7772
