Defect annealing kinetics in irradiated 6H–SiC

Nuclear Instruments and Methods in Physics Research B 166±167 (2000) 410±414

www.elsevier.nl/locate/nimb

Defect annealing kinetics in irradiated 6H±SiC W.J. Weber *, W. Jiang, S. Thevuthasan Paci®c Northwest National Laboratory, MSIN K2-44, P.O. Box 999, Richland, WA 99352, USA

Abstract Isochronal and isothermal annealing of ion-irradiation damage on the Si sublattice in 6H±SiC has been investigated experimentally by in situ Rutherford backscattering spectrometry in channeling geometry (RBS/C). At low ion ¯uences corresponding to dilute concentrations of irradiation-induced defects, complete recovery of disorder on the Si sublattice can occur below room temperature. The implantation of helium impedes the defect recovery processes at low temperatures. Below room temperature, the thermal recovery of defects on the Si sublattice has an activation energy on the order of 0:25  0:1 eV. Recovery of disorder on the Si sublattice above 570 K has an activation energy on the order of 1:5  0:3 eV. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.72.Cc; 61.72.Ji; 61.80.Jh; 61.82.Fk Keywords: Silicon carbide; Defects; Annealing; Ion irradiation; RBS/Channeling

1. Introduction Silicon carbide (SiC) is a promising candidate material for electronic device fabrication and structural components of advanced nuclear energy systems. Thus, investigations of the accumulation and recovery of irradiation damage in SiC are of scienti®c and technological interest. There have been numerous investigations of the temperature dependence of ion-beam-induced damage and recovery in 6H±SiC in recent years, and the results of these studies have been recently reviewed and summarized [1,2]. Additionally, molecular dy-

*

Corresponding author. Tel.: +1-509-375-2299; fax: +1-509375-2186. E-mail address: [email protected] (W.J. Weber).

namic (MD) simulations of damage production and accumulation in SiC [3,4] are providing fundamental parameters and new insights into the nature of the stable defects produced. It is now well established that below a critical temperature, which is dependent on the irradiation conditions (i.e., ion mass, energy, ion ¯ux) and ranges from about 300 to 600 K [1,2], there is an irradiationinduced crystalline-to-amorphous transformation. Amorphization in SiC appears to occur primarily by a homogeneous process that is dominated by a defect-stimulated amorphization process [1,2], which occurs from the accumulation of ion-beaminduced defects. Several models [2,5] have been used to describe the temperature dependence of amorphization in SiC in terms of a single-activated process associated with simultaneous recovery. The activation energy associated with simulta-

0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 8 6 8 - X

W.J. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 410±414

neous recovery during amorphization of 6H±SiC has been estimated to be 0.1 [2], 0.3 [2] or 0.6 eV [5], depending on which model is applied. Obviously, the activation energies for defect recombination and migration will be critical parameters in the kinetics of damage accumulation and amorphization. Isochronal annealing has been used in several recent studies [6±9] to characterize the recovery behavior in 6H±SiC irradiated with di€erent ions. This paper presents the most recent results of both isochronal and isothermal annealing studies of irradiation-induced structural disorder on the Si sublattice in 6H±SiC irradiated at low to intermediate damage levels.

411

radiated with Si‡ , C‡ and He‡ ions over a range of ¯uences. Isothermal anneals were performed on di€erent specimens, each irradiated with 550 keV C‡ to 8 ions/nm2 at 180 K to produce a constant damage state (i.e., defect concentration), over a sequence of time intervals (5 s to several hours) at 180, 220, 263, 300, 570 and 870 K, respectively. Channeling measurements after each isochronal or isothermal annealing step were made at temperatures well below the anneal temperature to minimize the defect recombination during data acquisition.

3. Results and discussion 2. Experimental procedures The ion-irradiation and damage recovery experiments, along with subsequent in-situ ion beam analyses, were performed using a 3.4 MV tandem accelerator facility within the Environmental Molecular Sciences Laboratory (EMSL) at the Paci®c Northwest National Laboratory (PNNL). The 6H±SiC single crystal wafers used in this study were obtained from Cree Research. The irradiation experiments were carried out with either 550 keV Si‡ ions (incident angle 30° o€ the surface normal), 550 keV C‡ ions (60° incident angle), or 390 keV He‡ ions (60° incident angle) over a range of ion ¯uences at low temperatures (between 160 and 190 K). The large incident angles were used to produce near-surface damage that could be analyzed by ion-beam techniques. Pro®les of atomic disorder on the Si sublattice were measured in situ by 2.0 MeV He‡ Rutherford backscattering spectrometry in a á0 0 0 1ñ axial channeling geometry (RBS/C), as described previously [7,8]. The integrated disorder and the disorder at the damage peak were obtained from the RBS/C spectra under the assumption of a linear dechanneling approximation from the defects induced in the irradiated crystals. Isochronal and isothermal annealing were used to study the damage recovery in situ using RBS/C. The isochronal anneals were performed for 20 min in a sequence of annealing steps, from the irradiation temperature up to 1170 K, for samples ir-

The isochronal recovery of the relative integrated Si disorder on the Si sublattice is shown in Fig. 1 as a function of annealing temperature for three di€erent Si‡ ion ¯uences. For low ion ¯uences (0.1 and 0.5 ions/nm2 ), complete recovery of the irradiation-induced disorder (freely migrating defects) occurs at 300 K. At an ion ¯uence of 1.0 ions/nm2 , there is still signi®cant recovery above room temperature. The results suggest that there may be another recovery stage between 470 and 670 K, which is followed by gradual recovery until full recovery is obtained at 1170 K. The amount of recovery at 300 K is comparable for irradiation

Fig. 1. Isochronal recovery of disorder on the Si sublattice for 6H±SiC irradiated with 550 keV Si‡ ions to several ¯uences at 160 K.

412

W.J. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 410±414

¯uences of 0.5 and 1.0 ions/nm2 , which may suggest that the number of freely migrating defects at the damage peak saturates at a ¯uence of 0.5 ions/ nm2 under these Si‡ irradiation conditions. Above this ¯uence, more stable defect clusters must form or amorphization occurs, similar to the results reported for B‡ -irradiated SiC [1]. The isochronal recovery of the relative integrated Si disorder on the Si sublattice is shown in Fig. 2 as a function of annealing temperature for three di€erent He‡ ion ¯uences. Although some damage recovery occurs between the irradiation temperature and 300 K, complete recovery of the He‡ damage does not occur at 300 K, as in the case for the Si‡ (Fig. 1), even though the level of He‡ -induced disorder is comparable to the level of Si‡ -induced disorder. This suggests that the residual damage after annealing at 300 K (Fig. 2) may be associated with trapped helium or other complex defects, rather than freely migrating defects. The formation of trapped or complex defects, such as He-defect clusters, inhibits full recovery of the He‡ -induced damage at low temperatures. Similar to the Si‡ results (Fig. 1), the results in Fig. 2 suggest additional recovery stages above 470 K. The activation energies for defect recovery processes on the Si sublattice have been estimated using samples irradiated under identical conditions

Fig. 2. Isochronal recovery of disorder on the Si sublattice for 6H±SiC irradiated with 390 keV He‡ ions to several ¯uences at 190 K.

(550 keV C‡ at 180 K) to a ¯uence of 8.0 C‡ /nm2 . These irradiated samples provided a constant damage state (defect concentration) for isochronal and a series of isothermal anneals. The isochronal and isothermal annealing results are shown in Fig. 3(a) and (b), respectively. The isochronal annealing results cover the full disorder range of all the isothermal annealing steps. Isothermal annealing at 180 K for over 2 h after irradiation shows no evidence for recovery. The solid lines in Fig. 3(b) are exponential ®ts to the isothermal annealing data, which assumes a ®rst-order annealing process. A combination of exponential ®ts was used for the data at 570 and 870 K. The isochronal and isothermal recovery data can be used to separate di€erent kinetic processes and isolate a single-activated process with constant activation energy. Since the initial defect concentration and pro®le are the same for each annealing experiment, the activation energies can be determined under rather unrestrictive assumptions using the following expression, which is applicable for a combination of isochronal and isothermal anneal data [10,11], ln…Dsi † ˆ ln…Dtj † ‡ E=kTa ÿ E=kTj ;

…1†

where Dsi is the time interval during isothermal annealing at temperature, Ta , needed to anneal the same amount of defects in the jth isochronal pulse of time Dtj at a temperature, Tj . A plot of ln …Dsi † versus 1/kTj for a single-activated process has a slope of )E. Arrhenius plots obtained from the

Fig. 3. Recovery of the relative disorder on the Si sublattice, at the damage peak, in 6H±SiC irradiated with 550 keV C‡ ions at 180 K to 8.0 ions/nm2 : (a) isochronal and (b) isothermal annealing.

W.J. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 410±414

combination of isochronal and isothermal annealing data (Fig. 3) using Eq. (1) are shown in Fig. 4. At low temperatures (below room temperature), the estimated activation energy is on the order of 0:25  0:1 eV. At higher temperatures (between about 570 and 870 K), the estimated activation energy is about 1:5  0:3 eV, which is in agreement with the activation energy (1.6 eV) previously determined by Primak et al. [12] for recovery of neutron damage in 6H±SiC in this same temperature range. Since the initial defect concentration and pro®le are constant, the isothermal annealing data obtained at low temperatures (Fig. 3(b)) can also be analyzed by the method of cross-cut [11], where the di€erent times and temperatures needed to reach a given level of recovery (i.e., reduced disorder) are related by the expression ln …ti † ˆ lnC ‡ E=kTi ;

…2†

where C is a constant and ti is the isothermal annealing time at temperature Ti to reach a given level of disorder. Arrhenius plots for two crosscuts, based on Eq. (2), of the isothermal annealing data in Fig. 3(b) are shown in Fig. 5. Although there is limited data, the estimated activation energy is on the order of 0:2  0:1 eV, which is in agreement with the results in Fig. 4.

413

Fig. 5. Arrhenius plot, based on Eq. (2), for two cross-cuts of isothermal annealing results.

The data in Fig. 3 suggest another recovery stage may be present between 350 and 570 K, but the current data could not be used to estimate an activation energy for this process. The results indicate that there are at least two distinct activation energies and associated recovery processes responsible for the observed thermal annealing on the Si sublattice. Additional studies are planned to determine the activation energies for recovery on both the Si and C sublattices in greater detail.

4. Conclusions

Fig. 4. Arrhenius plot, based on Eq. (1), of logarithmic annealing time in isothermal steps versus 1/kT in isochronal annealing.

In situ RBC/C has been employed in isochronal and isothermal annealing studies of disorder recovery processes in irradiated 6H±SiC. For low (dilute) defect concentrations introduced by 550 keV Si‡ irradiation at 160 K, complete recovery is observed at 300 K. However, defects introduced by He‡ irradiation on the Si sublattice are more dicult to anneal at room temperature, which suggests trapping of the implanted helium may inhibit defect recombination or recovery processes. The activation energies have been estimated for two recovery stages on the Si sublattice in 6H±SiC. At low temperatures … 6 300 K), the activation energy for thermal recovery is estimated to be 0:25  0:1 eV; while at higher temperatures (570±

414

W.J. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 410±414

870 K), the activation energy for thermal recovery is estimated to be 1:5  0:3 eV. Acknowledgements This work was supported by the Division of Materials Science, Oce of Basic Energy Sciences, US Department of Energy under Contract DEAC06-76RLO 1830. Operational support for the EMSL accelerator laboratory was provided by the Oce of Biological and Environmental Research, US Department of Energy under Contract DEAC06-76RLO 1830. References [1] E. Wendler, A. Heft, W. Wesch, Nucl. Instr. and Meth. B 141 (1998) 105. [2] W.J. Weber, N. Yu, L.M. Wang, N.J. Hess, Mater. Sci. Eng. A 253 (1998) 62.

[3] R. Devanathan, T. Diaz de la Rubia, W.J. Weber, J. Nucl. Mater. 253 (1998) 47. [4] R. Devanathan, W.J. Weber, T. Diaz de la Rubia, Nucl. Instr. and Meth. B 141 (1998) 118. [5] L.L. Snead, S.J. Zinkle, in: I.M. Robertson, G.S. Was, L.W. Hobbs, T. Diaz de la Rubia (Eds.), Microstructure Evolution During Irradiation, Mater. Res. Soc. Proc. 439, Materials Research Society, Pittsburgh, PA, 1997, p. 595. [6] W.J. Weber, N. Yu, L.M. Wang, N.J. Hess, J. Nucl. Mater. 244 (1997) 258. [7] W. Jiang, W.J. Weber, S. Thevuthansan, D.E. McCready, Nucl. Instr. and Meth. B 143 (1998) 333. [8] W. Jiang, W.J. Weber, S. Thevuthasan, D.E. McCready, J. Nucl. Mater. 257 (1998) 295. [9] W.J. Weber, W. Jiang, S. Thevuthasan, D.E. McCready, in: S.J. Zinkle, G.E. Lucas, R.C. Ewing, J.S. Williams (Eds.), Microstructure Processes in Irradiated Materials, Mater. Res. Soc. Proc. 540, Materials Research Society, Warrendale, PA, 1999, p. 159. [10] C.J. Meechan, J.A. Brinkman, Phys. Rev. 103 (1956) 1193. [11] A.C. Damask, G.J. Dienes, Point Defects in Metals, Gordon and Breach, New York, 1963. [12] W. Primak, L.H. Fuchs, P.P. Day, Phys. Rev. 103 (1956) 1184.