Acceptor impurities solubility

The acceptor impurities have a much weaker dependence of solubility on growth rate than nitrogen [39]. This is consistent with the fact that the diffusion mobility is higher for the acceptors in silicon carbide (see Chapter 7). In contrast to the donors, acceptor impurities tend to increase their solubility at elevated temperatures [10,40,41]. The increase of growth... 

A high solubility limit is appropriate to beryllium, the group II acceptor. It reaches 1020 cm 3 [40]. However, beryllium is usually not employed as a dopant in the sublimation growth. Firstly, it has an exceptionally high diffusivity in SiC (see Chapter 7). For the typical growth time of about 10-60 min the diffusion front will penetrate as far as 10- 100 pm. In addition, the activation energy of Be acceptors is considerable and this dopant has a tendency to self-compensation [51]. The results of a comprehensive study of impurity solubility in SiC performed by Vodakov, Mokhov and co-authors [40] are presented in TABLE 3. 

The behaviour of isolated interstitial hydrogen, as discussed in Section B, provides crucial information about interaction with impurities. Since both the solubility and the diffusivity of hydrogen in n-type GaN are low, hydrogen-donor complexes will rarely form, and we focus on complexes with acceptors. 

The dielectric constant and the loss tangent of alumina are affected by impurities, e.g.. Si, Ti, Mg, and Ca. Substitution of either Si or Ti for Al in alumina creates donor levels at the impurity sites and acceptor levels at the compensating cation vacancies. Conversely, the substitution of either Mg or Ca for Al creates acceptor levels at the impurity sites and donor levels at the compensating interstitials. These donors and acceptors contribute charge carriers which affect the dielectric and loss characteristics of alumina. Since the solubility of Si in AljOj is limited, excess SiOj leads to the formation of a glassy phase at the grain boundaries, and to ionic conduction. 

Exactly the opposite occurs, namely the conversion of an ex situ parameter to an in situ one, if foreign components become sufficiently mobile. The corresponding incorporation reaction then becomes reversible. Under such conditions it is natiurally better to speak of solubility equihbria. Important examples are segregation equilibria of impurities at very high temperatures, another refers to the incorporation of protonic defects in oxides by the dissolution of H2O. Materials interesting in this respect are CaO-doped Z1O2 [196] or acceptor-doped perovskites [197], such as the Fe-doped SrTiOs discussed above. (As before we regard the acceptor dopant... 

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