Ion Implantation

Activation of Nitrogen, Phosphorus, Aluminum, and Boron Implants in SiC

M. A. Capano, R. Santhakumar, J. A. Cooper, Jr., and M. R. Melloch

Supported by the Office of Naval Research

Because of the extreme stability of silicon carbide, it is not feasible to dope the crystal by thermal diffusion as is commonly done in silicon processing. Instead, dopants are introduced by ion implantation. Once implanted into the crystal, the dopant atoms occupy interstitial positions in the lattice and must be transferred to substitutional sites to become electrically active. This "activation" is accomplished by high temperature annealing in an inert ambient such as argon.

Nitrogen and phosphorus are typical n-type dopants in SiC, while aluminum and boron are standard p-type dopants. Implantation is usually conducted with the sample at an elevated temperature (600 - 800 C) to provide some in-situ annealing of lattice damage caused by the implant. The implanted sample is subsequently annealed at temperatures between 1000 - 1700 C for times between 5 and 90 minutes to activate the dopants. The dynamics of activation depends both on the dopant species (i.e. aluminum vs boron, nitrogen vs phosphorus) and upon the SiC polytype (i.e. 4H vs 6H).

In Figs. 1 - 3 below, we plot sheet resistivity of n-type implanted layers in 4H and 6H-SiC as a function of anneal time and temperature. Within each plot, all samples are fabricated on the same wafer and are implanted together at the same time, with the sample maintained at 650 C. Activation is performed in an argon ambient. Sheet resistivity is obtained using the transmission line method (TLM) with ohmic contacts established using unannealed nickel.

Figure 1 shows sheet resistivity of nitrogen implants in 6H-SiC at three anneal temperatures as a function of anneal time [1]. At each temperature there exists an optimum anneal time, and sheet resistivity increases if the activation anneal is continued beyond the optimum time. At temperatures of 1200 C and above, the optimum anneal time is less than five minutes, much shorter than has been recognized to date. At 1050 C the optimum anneal time is about 30 minutes, and at 900 C the optimum anneal time is between 36 and 48 hours. The minimum sheet resistivity increases as anneal temperature is reduced, but acceptable sheet resistivities can be attained at temperatures as low as 900 C.

Figure 1. Sheet resistivity of nitrogen in 6H-SiC as a function of anneal time and temperature. At any given anneal temperature, there exists an optimum anneal time. At temperatures of 1200 C and above, the optimum anneal time is less than five minutes. Significant activation can be obtained for anneal temperatures as low as 900 C. Doses and energies are 3.8x10¹⁵ cm^-2 at 30 keV and 7.1x10¹⁵ cm^-2 at 70 keV.

Activation of nitrogen implants in 4H-SiC requires higher anneal temperatures than in 6H-SiC. Figure 2 shows sheet resistivity of nitrogen in 4H-SiC as a function of anneal time and temperature [2]. In contrast to 6H-SiC where sheet resistivities below 1000 Ohms can be obtained at anneal temperatures of 1050 C, achieving the same resistivity in 4H-SiC requires anneal temperatures in excess of 1400 C.

Figure 2. Sheet resistivity of nitrogen-implanted layers in 4H-SiC as a function of anneal time and temperature. To obtain resistivities below 1000 Ohms it is necessary to anneal at temperatures of 1400 C or higher.

Phosphorus is an excellent n-type dopant in 4H-SiC when implanted at high doses, such as for source and drain regions of MOSFETs. Figure 3 shows sheet resistivity of phosphorus in 4H-SiC as a function of anneal time and temperature [2]. Notice that sheet resistivities below 1000 Ohms can be obtained at annealing temperatures as low as 1200 C, and sheet resistivities below 100 Ohms can be obtained by annealing at 1700 C.

Figure 3. Sheet resistivity of phosphorus-implanted layers in 4H-SiC as a function of anneal time and temperature. Resistivities below 1000 Ohms can be obtained at anneal temperatures as low as 1200 C, and resistivities below 100 Ohms can be obtained by annealing at1700 C.

P-type dopants, aluminum and boron, require much higher temperatures for efficient activation. Figure 4 shows activation percentage for boron in 4H-SiC as a function of anneal temperature [3], as determined from C-V measurements. Notice that temperatures in excess of 1650 C are required for efficient activation. Figure 5 shows how activation percentage depends on anneal time [3]. Anneal times in excess of 30 minutes are needed for the activation of boron to approach a steady-state. Although not shown here, aluminum implants typically achieve the same degree of activation at anneal temperatures about 100 C lower than boron.

Figure 4. Activation of boron in 4H-SiC as a function of anneal temperature. All anneals were conducted for 40 minutes in argon. Efficient activation requires annealing at temperatures above 1650 C.

Figure 5. Activation of boron in 4H-SiC as a function of anneal time for two temperatures. Efficient activation requires annealing for at least 30 minutes.

[1] J. N. Pan, J. A. Cooper, Jr., and M. R. Melloch, "Activation of Nitrogen Implants in 6H-SiC," J. Electronic Mat'ls., Vol. 26, p. 208, March 1997.

[2] M. A. Capano, R. Santhakumar, M. K. Das, J. A. Cooper, and M. R. Melloch, "Phosphorus and Nitrogen Impantation into 4H-Silicon Carbide," Electronic Materials Conference, Santa Barbara, CA, June 30 - July 2, 1999.

[3] M. A. Capano, S-H. Ryu, M. R. Melloch, and J. A. Cooper, Jr., "Dopant Activation and Surface Morphology of Ion Implanted 4H- and 6H-Silicon Carbide," J. Electronic Mat'ls., Vol. 27, p. 370, April 1998.

Return to the Basic Studies page?
Return to the Wide Band Gap Research homepage?