Nitrides, point defects

Diffusion in the Presence of Excess Point Defects. Oxidation-Enhanced Diffusion. Oxidation generally enhances the diffusion of group III and group V elements except for antimony (Figure 13). Oxidation-enhanced diffusion is generally observed by depositing a silicon nitride mask on the silicon surface that will prohibit oxidation in the regions that it covers. 

For most group III nitride crystals, their perfection is still far from ideal. Therefore, the thermal conductivity is determined by point defects in the case of single crystals and by point defects and grain... 

Point defects can drastically lower the thermal conductivity of the important carbide and nitride high thermal conductivity ceramics. In this respect, oxygen, which is a common impurity, has been found to be very important. For example, silica (Si02), an impurity in silicon nitride (Si3N4), formed by oxidation at high temperatures in air, can react to produce substitutional defects and vacancies in the following way ... 

Point Defects Nitrogen and boron are ubiquitous atomic-scale impurities in synthetic diamonds. The free energy of formation of nitrides and borides in the solvent/catalyst melt will determine the extent to which these impurities are... 

There is strong evidence that interstitial solutes such as C and N are attracted to the point defects produced by irradiation. Little and Harries demonstrated that the amount of free nitrogen, indicated by the height of the Snoek internal friction peaks, decreased with increasing irradiation fluence, such that it was zero by fluences of about 2 x 10 n/cm. This was attributed to trapping of free N or precipitation of nitrides at point defects or defect clusters. 

Saarinen K. (2000) Characterization of native point defects in GaN by positron annihilation spectroscopy, in III-V Nitride Semiconductors Electrical, Structural and Defects... 

Up to now, we have discussed mostly model intermetallic compounds with simple crystal structures (generally cubic LI2, B2,. . . ) and containing two metal species. We shall now present briefly some properties of point defects in more exotic systems, of considerable interest the A15 superconductors, transition-metal carbides and nitrides, and III-V semiconductors (e.g. GaAs). 

Refractory Compounds. Refractory compounds resemble oxides, carbides, nitrides, borides, and sulfides in that they have a very high melting point. In some cases, they form extensive defect stmctures, ie, they exist over a wide stoichiometric range. For example, in TiC, the C Ti ratio can vary from 0.5 to I.O, which demonstrates a wide range of vacant carbon lattice sites. 

The specific heat of a semiconductor has contributions from lattice vibrations, free carriers and point and extended defects. For good quality semi-insulating crystals only the lattice contribution is of major significance. Defect-free crystals of group III nitrides are difficult to obtain, and thus the specific heat measurements are affected by the contributions from the free carriers and the defects. While the specific heat of AIN is affected by the contribution of oxygen impurities, the data for GaN and InN are affected by free electrons, especially at very low temperatures. 

Native defects have sometimes been invoked not just as sources of compensation, but as sources of doping. The nitrogen vacancy in GaN is a prime example for a long time the nitrogen vacancy was thought to be the source of n-type conductivity in GaN. As early as 1983 it was pointed out that unintentional incorporation of oxygen was a more likely explanation [1], Still, it is only recently that unintentional impurities have become widely accepted as the source of n-type conductivity, thanks in part to contributions from first-principles theory (see Datareview A8.1). In this Datareview we will describe some of those theoretical results, for vacancies as well as other native defects (self-interstitials and antisites). Experimental information about native defects in the nitrides is very scarce at this time we will include references where available. 

The strength of a brittle material is proportional to the fracture toughness and indirectly proportional to the square root of the defect size. The defect size of the materials can be reduced by optimised processing. [55,82], The highest measured mean three-point-bending strength for silicon nitride was 2000 MPa [85]. This corresponds to a defect size of about 5 pm. Materials with a strength level of 1400 to 1500 MPa usually have a defect size of 10 pm [55,83], i.e. these materials have... 

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