Impurities, in silicon

Table 3. Equilibrium Segregation Coefficients for Impurities in Silicon...

Of course, the most reliable and accurate method of quantitative analysis is to calibrate each element with standards prepared in matrices similar to the unknown being analyzed. For a survey technique that is used to examine such a wide variety of materials, however, standards are not available in many cases. When the technique is used mainly in one application (typing steels, specifying the purity of alloys for a selected group of elements, or identifying impurities in silicon boules and... 

NAA is a quantitative method. Quantification can be performed by comparison to standards or by computation from basic principles (parametric analysis). A certified reference material specifically for trace impurities in silicon is not currently available. Since neutron and y rays are penetrating radiations (free from absorption problems, such as those found in X-ray fluorescence), matrix matching between the sample and the comparator standard is not critical. Biological trace impurities standards (e.g., the National Institute of Standards and Technology Standard Rference Material, SRM 1572 Citrus Leaves) can be used as reference materials. For the parametric analysis many instrumental fiictors, such as the neutron flux density and the efficiency of the detector, must be well known. The activation equation can be used to determine concentrations ... 

Unfortunately, no reliable estimate of cr is available for any hydrogen species. Since the hydrogen donor level seems to be somewhere near midgap, it is appropriate to recall the range covered by the cr values measured for various deep impurities in silicon (Milnes, 1973, Chapter 10), namely, cr 10-14 - 10 21 cm2. Such values would give r0 values in (22) of the order of microseconds to seconds at 200°C if eD = em. At room temperature, on the other hand, values as long as hours could occur if eD is well below em or o-+e is very small. The range of possibilities for other conceivable carrier emission processes (H°— H + h, H+— H° + h, etc.) is presumably similar. 

For more details on the roles of metal impurities in silicon solar cells and efforts to engineer materials that are less affected by these impurities, see S. Dubois, O. Palais, M. Pasquinelli, S. Martinuzzi, and C. Jassaud, J. Appl. Phys. 100 (2006), 123502, and T. Buonassisi, A. A. Istratov, M. A. Marcus, B. Lai, Z. H. Cai, S. M. Heald, and E. R. Weber, Nature Materials 4 (2005), 676. 

T. G. Brown and D. G. Hall, Radiative Isoelectronic Impurities in Silicon and Silicon-Germanium Alloys and Superlattices J. Michel, L. V. C. Assail, M. T. Morse, and L. C. Kimerling, Erbium in Silicon K Kanemitsu, Silicon and Germanium Nanoparticles... 

T. G. Brown and D. G. Hall, Radiative Isoelectronic Impurities in Silicon and Silicon-Germanium Alloys and Superlattices... 

As an example of the study of vacancies and self-interstitial impurities by the continued fraction expansion of Eq. (S.2S), we mention the work of Kauffer et al. These authors consider impurities in silicon and set up a model tight-binding Hamiltonian with s p hybridization, which satisfactorily describes the valence and conduction bands of the perfect crystal. A cluster of 2545 atoms is generated, and vacancies (or self-interstitial impurities) are introduced at the center of the cluster. One then takes as a seed state an appropriate orbital or symmetrized combination of orbitals, and the recursion method is started. Though self-consistent potential modifications are neglected in this paper, the model leads to qualitatively satisfactory results within a simple physical picture. 

There are several impurities in mc-Si, including carbon, oxygen, nitrogen, and iron. Carbon is one of the major impurities in silicon feedstock. When the carbon content exceeds its solubility limit in silicon, it will precipitate to form SiC particles in a directional solidification process. It has been experimentally... 

The assessed thermodynamic properties of liquid and solid Si-based solution can be directly applied to evaluate the influence of third element on the solubility of the main impurity in silicon melt. For example, the effect of the impurity element on the solubility of C in pure Si melt can be evaluated by the following equation ... 

Here 2k 0 and 3are, respectively, the distribution coefficient of i in Si-i binary and Si-i-j ternary systems. Calculated A values for different impurities in pure Si are shown in Fig. 13.23. Most of the common impurities in pure Si have a positive contribution to the impurity distribution coefficient, i.e., the appearance of secondary impurities will increase the distribution coefficient of the primary impurity in silicon. 

Fig. 13.27. Calculated concentration gradients of impurities in silicon melt...

Docekal B., Broekaert J. A. C., Graule T., Tschopel P. and Tolg G. (1992) Determination of impurities in silicon carbide powders, Fresenius J Anal Chem 342 113-117. 

This paper describes the configuration of oxygen and carbon impurities in silicon and techniques which can be used to improve the quality and quantitative accuracy of IR absorption spectra obtained on FT-IR instruments. It also discusses instrument-to-instrument... 

---