As-implanted profiles

In this section we will review the current knowledge of ion damage accumulation and chemical profiles in as-implanted single-crystal SiC. We will investigate the effects of the postimplant annealing process in Section 4.3. Results for the 4H- and 6H-SiC polytypes will also be presented. 

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Figure 7. Experimental profile of boron implanted in silicon showing the as-implanted profile and the profile after laser annealing. (Reproduced with permission from Reference 29, copyright 1978, American Institute of Physics).

In this chapter, we review the current status of doping of SiC by ion implantation. Section 4.2 examines as-implanted depth profiles with respect to the influence of channeling, ion mass, ion energy, implantation temperature, fluence, flux, and SiC-polytype. Experiments and simulations are compared and the validity of different simulation codes is discussed. Section 4.3 deals with postimplant annealing and reviews different annealing concepts. The influence of diffusion (equilibrium and nonequilibrium) on dopant profiles is discussed, as well as a comprehensive review of defect evolution and electrical activation. Section 4.4 offers conclusions and discusses technology barriers and suggestions for future work. 

Random and channeled implanted profiles in 6H-SiC are shown in Figure 4.11(a). These profiles correspond to implants performed at RT with 1.5 MeV AF to a low fluence ( 10 cm" ). The flux is not accurately defined because of the experimental constraints, as explained in [74]. In fact, most of the studies concerning channeled implants make use of ion beams in single- spot configurations and have to face the problems related to nonhomogeneous flux distribution within the beam spot. The... 

Figure 4.10 Damage and doping profiles for the same set of random as-implanted 6H-SIC samples, (a) Comparison between RBS-C measured and MC-BCA simulated damage profiles. (From [73]. 1999 Elsevier B.V. Reprinted with permission.) (b) Comparison between SIMS measured and MC-BCA simulated chemical profiles. (From [72]. 2001 Material Science Forum. Reprinted with permission.)...

Figure 23 Depth profile of F in Ti02 rutile single crystals implanted with 200-keV F ions, (a) As-implanted, (b) 600°C-annealed, and (c) 1200°C-annealed samples.

Inserting values for D+ and r into the expression for L+, a value of approximately 1000 A is obtained, and this is typical for metals. One can then find an estimate of the efficiency e of the moderator by multiplying the implantation profile P(x), equation (1.9), by the probability that a positron reaches the surface from a depth x, exp(—x/L+), and integrating over all values of x. Then, since pimpL+ -C 1, the efficiency may be written as... 

An excellent way to create standards is ion implantation of the elements of interest into the matrix. This works exceptionally well for semiconductors since one can usually start with high-purity single-crystal materials that represent the matrix of interest. Also the use of Eq. (4.8) is well suited for this purpose since ion implanters usually quote doses in atoms per square centimeter. However, Eq. (4.5) serves just as well by converting the matrix concentration to atoms per cubic centimeter. In this procedure, the implant profile is sputtered through, the implant element secondary ions and the matrix element secondary ions are each summed, and the depth of the sputter profile is determined, usually by using a stylus profilome-ter. The sensitivity factor is then calculated from... 

This Datareview discusses the redistribution of typical dopant atoms in GaN during the implant activation anneal. Secondary ion mass spectroscopy (SIMS) spectra of the impurity profiles (impurity concentration versus depth into the sample) before (as-implanted) and after annealing are presented. SIMS analysis is the primary method of characterising impurity distributions in semiconductors [2], This information can be used to roughly estimate a diffusivity, D , of the dopant at the temperatures studied by invoking the relationship... 

Si is the dopant most commonly used to create n-type GaN. FIGURE 1 shows the SIMS profile for 2 Si in GaN as-implanted and annealed (1050°C). Despite the interference in the mass 28 SIMS signal from 2 N2, the annealed Si profile demonstrates no measurable redistribution. Using a conservative estimate... 

FIGURE 1 SIMS profile of28 Si-implanted (100 keV, 5 x 1014 cm 2) GaN as-implanted and annealed at 1050°C for 15 s. No measurable redistribution is seen at the peak of the annealed sample. 

The difference in the annihilation ratio for positronium at the surface and in the sample is used to measure the effective range of positronium. Implantation profiles for a range of incident energies (density 1 g/cm3) were calculated. In this simulation the fraction that stops within a diffusion length of the surface can reach it and annihilates into 3 photons the remainder annihilate 10% into three photons and 90% into two photons, as shown in Figure 7.4. The fractions are chosen as an example. A short effective range appears as a sharp transition from surface measurement to bulk measurement value. 

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