Electrical Activation Processes in Ion Implanted SiC Device Structures K.A. Jones1, M.H. Ervin1, P.B. Shah1, M.A. Derenge1, R.D. Vispute2, T. Venkatesan2, and J.A. Freitas3 1 3 Army Research Lab - SEDD, 2800 Powder Mill Rd, Adelphi, MD 20873 2 University of Maryland, Physics Dept., College Park, MD 20742 Naval Research Lab, Code 6877, 4555 Overlook Ave., Washington, DC 20375 Abstract: Electrically activating implanted dopants requires annealing temperatures above that where the Si evaporates preferentially. Methods for preventing this are discussed, and it is shown that AlN can be used as an annealing cap for temperatures as high as 1600°C. For the higher temperatures required to activate acceptors, a dual BN/AlN cap can be used. Defects such as the DI defect, however, cannot be annealed out, and it is shown that this defect is probably a deep donor that can act as a hole trap, and it is most likely an extended defect such as a dislocation loop. is impossible to create an atmosphere that is in equilbrium with both the exposed implanted and unimplanted regions. We have demonstrated that AlN can behave as an effective annealing cap; it can survive annealing temperatures as high as 1600°C for times as long as 30 min, it does not react with the SiC, and it can be removed selectively with a warm KOH etch [2]. Although 1600°C seems to be sufficient for activating the n-type dopant, N, much evidence exists suggesting that higher temperatures are required to activate most of the p-type implants. We attempted to use an amorphous graphite cap, which can be removed easily by oxidizing it, but it crystallizes and can only be removed by ion milling it off. When we deposited it or Al2O3 on AlN to prevent the AlN from decomposing thermally, they were not strong enough to prevent the evaporation of N and/or Al, and blow-holes were created in them [3]. We have shown that BN films are strong enough to stop the AlN from decomposing, and they can be ion milled off, and then the AlN film can again be preferentially removed in warm KOH [4]. In this paper we will show that much can be learned about the activation processes in N, Al, and Al/C implanted samples as they can be annealed for long times at high temperatures without the preferential of Si and the subsequent degradation it causes. Introduction Because the rate of the diffusion of dopants is too low to be technologically useful even at temperatures as high as 1800°C [1], SiC can only be doped by ion implantation. Thus, to create an isolated p-n junction in an n-type film and at the same time retain the planar technology, the pregions must be fabricated by localized ion implantation. This is also true for the fabrication of junction barrier Schottky (JBS) diodes and double diffused MOSFET's. Even when diodes are made from p-n layered structures by etching away the surrounding p-region, the areas around them are often implanted to decrease the electric field in the vicinity of the junction to prevent premature breakdown. This is known as junction termination extension or JTE. The dopants must not only be implanted, they have to be activated by thermally annealing the sample. However, the temperatures required to activate the dopants are above temperatures where an appreciable amount of Si evaporates preferentially. Si3N4 capping layers cannot be used as they are to prevent the preferential evaporation of As from GaAs that is being annealed to activate implanted ions because it, too, evaporates at the high temperatures used to activate the implanted dopants in SiC, typically ~ 1600°C. Some have tried to prevent the Si from evaporating by covering the device wafer with another SiC wafer, but a hermetic seal is not formed, and SiC is expensive. Others place SiC powder in the annealing chamber or introduce SiH4 to create a Si overpressure, but this only increases the Si deposition rate and does nothing to the rate of Si evaporation. Moreover, the implanted region is in a higher energy state so it Experimental Procedure After lightly doped p-type films were implanted approximately uniformly to 3x1019 cm-3 to a depth of ~ 0.3 µm, AlN films were deposited on them to a thickness of ~200 nm using pulsed CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan 2003 American Institute of Physics 0-7354-0149-7/03/$20.00 694 laser deposition (PLD). The samples were annealed at various temperatures, TA, for 30 min up to a maximum temperature of 1600°C, and then the sheet resistance, RSH, was determined as a function of the measurement temperature, TM. The same procedure was followed for the implantation of Al or Al and C into a lightly doped n-type film. They were implanted with 1020 cm-3 Al or 1020 cm-3 Al and 1020 cm-3 C to a depth of ~ 0.5 µm. In addition, the catholdoluminescence spectra were recorded. Sheet resistivity (Ohm/square) 1000000 Resistivity [Ω-cm] 0.02 Room Temperature 0 1500 1550 1600 EA = 250 meV 10000 Calculated EA = 191 meV 50 100 150 200 250 300 at the same temperature. RSH decreases with increasing TM because the Al acceptor is so deep, but it is not clear just how deep. When a room temperature mobility of 50 cm2/V⋅s is assumed [5], it appears that the best fit for the RSH vs TM curves in Fig. 2 for the samples that have been completely activated after a 1650°C anneal is 250 meV. This is substantially larger than the generally accepted value of 192 meV [6], and the best fit depth for the model would be even deeper if, as expected, the mobility is lower in implanted layers than it is in films that are grown epitaxially. It has been suggested that co-implanting C assists in the activation because it improves the chances that the Al will occupy a Si site rather than a C site [7]. Another explanation is that the Al is more easily incorporated into the lattice because it can more easily surround itself with four C atoms when the C is implanted and not already bonded to four Si atoms [8]. This is consistent with the fact that the C implants do not noticeably reduce RSH for the 1650°C anneal as at this high TA kinetic factors no longer affect the incorporation of the Al. We have also noted in our studies of SiC coimplanted with Si that RSH is larger than for those implanted only with Al and annealed at the same temperature. The difference is larger at the lower TA 's, and there is little difference at the higher TA's. This can be explained by the increased difficulty the Al has in surrounding itself with four C atoms at lower temperatures, and that the kinetic effects are not much of a factor at the higher TA's. The catholdoluminescence spectrum for a coimplanted sample annealed at 1300°C is shown in Fig. 3. This spectrum and all the other spectra are qualitatively the same indicating that a 125 °C 1450 Al / C 1650 ºC Figure 2. The sheet resistance of Al and Al and C implanted layers plotted as a function of the measurement temperature for layers annealed for 30 min at 1500, 1600, or 1650°C. Room Temperature 50 °C 75 °C 1400 Al 1650 ºC Measurement temperature (Celsius) 0.01 1350 Al / C 1600 ºC 0 0.05 0.03 Al / C 1500 ºC 100000 1000 Results and Discussion That N implants are almost completely activated by a 1600°C anneal is demonstrated in Fig. 1 where RSH is plotted as a function of the measurement temperature for samples annealed different annealing temperatures. It is seen that RSH changes very little between TA = 1500°C and TA = 1600°C, and the RSH vs TM curve is very close to what one would predict for complete activation. In addition RSH decreases 0.04 Al 1500 ºC 1650 Anneal Temperature [°C] Figure 1. The sheet resistance of N implanted layers plotted as a function of the annealing temperature for layers annealed for 30 min and measured at room temperature, 50, 75, 125°C. with increasing TM for these two TA's reflecting the decrease in the mobility with increasing TM. For the lower TA's of 1300 and 1400°C, RSH increases with increasing TM implying that some of the carriers are excited from levels below that of the N donor level. These levels could be defect levels that have not yet been annealed out. The increase in carrier density with increasing TM more than offsets the decrease in the mobility. However, the activation of the p-type implant, Al, is quite different as shown in Fig. 2 where RSH is measured as a function of TM for TA = 1500, 1600, and 1650°C. It is seen that RSH in all instances decreases with increasing TM, and the samples co-implanted with C have a smaller RSH than those implanted only with Al and annealed 695 Egilisson et al [9] have convincingly shown that the DI defect is a deep donor 0.35 eV above the valence band. The holes that are trapped out by it are now deeper. This can also explain why our RSH curves appear to be associated with an acceptor that is deeper than that attributed to Al in epitaxially grown material. As noted above, the modeled curve would require an acceptor depth greater than 0.25 eV if the carrier mobility is lower in the implanted than in an epitaxially grown layer, as is expected. The DI defect has been associated with a divacancy [10], but we believe it is a more extensive defect such as a dislocation loop, and the dislocation loop increases in size as TA increases [8]. This is consistent with our EPR results showing that the implanted Al acceptor does not behave in the same way one in an epitaxial film does, and a new peak emerges that is associated with an extended defect as it has little anisotropy and is broad. It is also consistent with our Rutherford backscattering spectroscopy (RBS) results that show the χmin initially decreases as TA increases, but it begins to increase at the larger TA's. We attribute this increase to the nucleation and growth of dislocation loops. Figure 3. Cathodoluminescence spectrum of a coimplanted sample annealed at 1300°C for 30 min. 1300°C anneal is sufficient to optically activate the implants. It is not well understood why significantly higher temperatures are required to electrically activate them. There are, however, subtle differences particularly in the relative peak heights of the peaks associated with a free electron recombining with a hole bound to an Al acceptor near 3.0 eV, and the two peaks associated with the DI defect near 2.9 eV [7]. As shown in Fig. 4, the peak near 3.0 eV decreases in intensity as TA increases. One explanation is that the hole is trapped out by the DI defect and the ability for the DI defect to act as a hole trap increases with TA. Also, its ability to trap the holes is impeded slightly by the presence of implanted C as the relative CL peak height of the co-implanted samples is a little higher for the same annealing temperature. 1650 1. R.F. Davis, G. Kelner, M. Shur, J.W. Palmour, and J.A. Edmond, Proc. IEEE 79, 677 (1991). 2. K.A. Jones, P.B. Shah, K.W. Kirchner, R.T. Lareau, M.C. Wood, M.H. Ervin, R.D. Vispute, R.P. Sharma, T. Venkatesan, and O.W. Holland, Mater. Sci. & Eng. B61/62, 281 (1999). 3. K.A. Jones, M..A. Derenge, P.V. Shah, T.S. Zheleva, M.H. Ervin, K.W. Kirchner, M.C. Wood, C. Thomas, M.G. Spencer, O.W. Holland, and R.D. Vispute. 4. L.B. Ruppalt, S. Stafford, D. Yuan, K.A. Jones, M.H. Ervin, K.W. Kirchner, T.S. Zheleva, M.C. Wood, B.R. Geil, E. Forsythe, R.D. Vispute, and T. Venkatesan, Solid State Electron. 47, 253 (2002). 5. W. J. Schaffer, G. H. Negley, K. G. Irvine, and J. W. Palmour, Mater. Res. Soc. Proc., 339, 595, (1994). 6. T. Troffer, M. Schadt, T. Frank, H. Itoh, G. Pensly, J. Heindl, H.P.Strunk and M. Maier, Phys. Stat. Sol. A162, 277 (1997). 7. J.H. Zhao, K. Tone, S.R. Weiner, M.A. Caleca, H. Du and S.P. Withrow, IEEE Electron Dev. Lett., 18, 375 (1997). 8. K.A. Jones, M.A. Derenge, M.H. Ervin, P.B. Shah, J.A. Freitas, R.D. Vispute, R.P. Sharma, and G.I. Gerardi, to be published. 9. T. Egilisson, J.P. Bergman, I.G. Ivanov, A. Henry, and E. Janzen, Phys. Rev. B59, 1956 (1999). 10. L. Patrick and W.J. Choyke, Phys. Rev. B5, 3253 (1972). 1650 Al Implanted 1500 1400 1600 1300 1500 RT 1400 1300 RT Al & C Implanted Figure 4. Cathodoluminescence spectra in the vicinity of 2.9 and 3.0 eV for Al implanted and co-implanted samples annealed at various temperatures. 696
© Copyright 2024 Paperzz