Boron implanted in silicon

Figure 36 Comparison of normal incidence profiling in vacuum to 60° incident profiling with oxygen flooding. Profiles of boron implanted in silicon. (From Ref. 122.)...

When we have a system with more than one species, as in the case of boron implantation in silicon, it is also important to identify the final location of the implanted atom, critical in this particular example, since the activation of the dopant will depend on this configuration. The different possibilities for a stable configuration of boron in a silicon lattice, together with their occurrence probability are shown in Table 1. It must be emphasized with high probability that a boron atom has to occupy a substitutional site, called B. ... 

In subsequent experiments, Biersack, et al. (29) used the boron (n,alpha) reaction to show the effect of pre- and post-irradiation damage on boron implantation profiles. By post-irradiating a boron implant in silicon with 200 keV H2, a migration of the boron to the Induced damage sites was observed. In the same paper, diffusion and trapping of lithium ions in niobium were reported. Using the lithium (n,alpha) reaction, they mapped irradiation Induced crystal defects through a depth of several micrometers with respect to several sample treatment conditions. 

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 the semiconductor industry, SIMS has been particularly useful for the depth profiling of dopants that are present in silicon in very low concentrations. As an example, a SIMS depth profile for boron implanted into silicon is shown in Figure 27. One of the significant features is that we can detect about 101S boron atoms/cm3 in a silicon matrix of 5 x 1022 atoms/cm3. This illustrates an ability to detect 20 ppb. Also, the method spans 5 orders of magnitude in boron concentration. No other technique can span such a large range accurately. 

Fig. 3. Configurations of implanted boron atoms in silicon Complex + Si. ...

Subsurface structures in silicon were also studied using apertureless s-SNOM in the IR range. Lahrech et al. have shown successfully that implanted boron lines in silicon can be detected with a lateral resolution of -400 mn, even in the absence of any topographical contrast [47]. Knoll and Keilmann have performed near-field... 

Boron implant with laser anneal. Boron atoms are accelerated into the backside of the CCD, replacing about 1 of 10,000 silicon atoms with a boron atom. The boron atoms create a net negative charge that push photoelectrons to the front surface. However, the boron implant creates defects in the lattice structure, so a laser is used to melt a thin layer (100 nm) of the silicon. As the silicon resolidihes, the crystal structure returns with some boron atoms in place of silicon atoms. This works well, except for blue/UV photons whose penetration depth is shorter than the depth of the boron implant. Variations in implant depth cause spatial QE variations, which can be seen in narrow bandpass, blue/UV, flat fields. This process is used by E2V, MIT/LL and Samoff. 

An ion-implanted standard and the MBE sample were depth profiled under the same conditions, and the secondary ions were analysed in a quadrupole mass spectrometer. The data from the ion-implanted standard was used to find the useful ion yield and thus the instrumental sensitivity for boron-in-silicon in the MBE sample. The quantified data appear in Figure 4.9. 

Fig. 16. Infrared absorption spectra for boron-implanted silicon after passivation in either monatomic hydrogen or deuterium. The specimens were passivated at 150°C for 1 h (per surface), and the spectral resolution is 4 cm-. From Johnson (1985). 

Generally speaking most of the shallow impurity levels which we shall encounter are based on substitution by an impurity atom for one of the host atoms. An atom must also occupy an interstitial site to be a shallow impurity. In fact, interstitial lithium in silicon has been reported to act as a shallow donor level. All of the impurities associated with shallow impurity levels are not always located at the substitutional sites, but a part of the impurities are at interstitial sites. Indeed, about 90% of group-VA elements and boron implanted into Si almost certainly take up substitutional sites i.e., they replace atoms of the host lattice, but the remaining atoms of 10% are at interstitial sites. About 30% of the implanted atoms of group-IIIA elements except boron are located at either a substitutional site or an interstitial site, and the other 40% atoms exist at unspecified sites in Si [3]. The location of the impurity atoms in the semiconductors substitutional, interstitial, or other site, is a matter of considerable concern to us, because the electric property depends on whether they are at the substitutional, interstitial, or other sites. The number of possible impurity configurations is doubled when we consider even substitutional impurities in a compound semiconductor such as ZnO and gallium arsenide instead of an elemental semiconductor such as Si [4],... 

Implantation. Ziegler and co-workers (1,14,24,25) introduced NDP by determining the range and shape of boron implantation distributions in intrinsic and doped silicon wafers. With the resultant profiles, they were able to calculate diffusion coefficients for boron in crystalline, amorphous, and arsenic-doped silicon. Since little experimental data existed for the case of boron to judge the validity of the current range theories, the shape of the boron profiles from NDP were of great interest. NDP and other techniques have since been able to show that a Pearson IV model rather than a gaussian profile is required to describe accurately the Implant distribution (21,26-28). 

Most impurity and defect states in SiC can be considered as deep levels. Both capacitance and admittance spectroscopy provide data on these deep levels which can act as donor or acceptor traps. Bulk 6H-SiC contains intrinsic defects which are thermally stable, up to 1700 °C. In epitaxial films of 6H-SiC a deep acceptor level is seen in boron-implanted samples but not when other impurities are implanted. Other centres, acting as electron traps, are also seen in p-n junction and Schottky barrier structures. Irradiation of 6H-SiC produces 6 deep levels, reducing to 2 after annealing. Only limited studies have been carried out on the 3C-SiC polytype, in the form of epitaxial films on silicon substrates. No levels were seen in thick films but electron traps were seen in thin n-type films and a hole trap (structural defect) was found to be a mobility killer. Neutron irradiation produces defects most of which can be removed by annealing. Two levels were found in Al-implanted 4H-SiC. 

The electronic industry benefits from boron trichloride in many applications. It is used in the production of optical fibers, as a p-type dopant for thermal diffusion in silicon, and for ion implantation. 

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