Acceptor doping

Acceptor dopants are impurity ions of a lower valence than that of the parent ions, as when small amounts of A1203 are incorporated into Ti02, so that the Al3+ ion substitutes for Ti4+. The acceptor species has an effective negative charge, Al i in this example, and the introduction of acceptor species tends to introduce counterbalancing 

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An incidental conclusion discussed in Section 5 is that exposure of freshly etched silicon surfaces to a given ambient of plasma products seems always to produce a chemical potential of hydrogen just beneath the surface that varies only modestly (no more than a factor ten) over a wide range of the bulk donor or acceptor doping. 

For the reasons we have just been discussing, we shall focus attention on the uptake of hydrogen by samples hydrogenated by exposure to plasma products for times of the order of an hour at 300°C and shall analyze the data on the assumption that the surface chemical potential / for given external and surface conditions is roughly independent of donor or acceptor doping. However, our conclusions will be tentative, since presently available data are limited and both the assumption of local equilibration and that of constant surface p need further checking. 

Acceptor doping, as in lithium oxide doping of nickel oxide, produces p-type thermistors. The situation in nickel-oxide-doped Mn304 is similar but slightly more complex. This oxide has a distorted spinel structure (Supplementary Material SI), with Mn2+ occupying tetrahedral sites and Mn3+ occupying octahedral sites in the crystal, to give a formula Mn2+[Mn3+]204, where the square parentheses enclose the ions in octahedral sites. The dopant Ni2+ ions preferentially occupy... 

In the case of acceptor doping, a similar change in the electroneutrality equation is required. Consider acceptor doping by a monovalent ion A+ due to reaction with A2X to introduce Am defects, once again assuming that Frenkel defects are not important. The original electroneutrality Eq. (7.12) ... 

Insertion of the equilibrium constants will result in equations that can be solved for [h ] as a function of px2, which, in turn, can be used to determine the values of the other defects as a function of pXl (Section 7.10.2). The diagrams for donor and acceptor doping of MX in which electronic defects dominate over Schottky equilibrium (Fig. 8.2a and 8.2c) can be compared to that for undoped material (Fig. 7.11), redrawn here (Fig. 8.2b). 

In acceptor-doped material, the diagram is virtually unchanged for dopant levels below that of the intrinsic defects, that is, less than 1020 defects m-3 in the examples given above. When the concentration of acceptors passes this quantity, the [e ] = [h ] plateau lengthens and divides with [h ] greater than [e ]. The separation of these... 

Acceptor doping of La2Cu04 is simply carried out by replacing the La3+ ion with a similar sized divalent ion, typically an alkaline earth ion such as Ca2+, Sr2+, or... 

The average valance of the Ti component can be pushed toward 4.0 by valence induction, simply by increasing the amount of Li present relative to that of titanium, to give a formula Li1+xTi2 x04. This is equivalent to acceptor doping of the phase. The additional Li+ ions enter the octahedral sites, as the tetrahedral sites are already occupied, to a maximum of Li [Li (nTi nJCL and the acceptor doping is balanced by holes. In order to write formal defect equations for this situation, it is convenient... 

At greater degrees of reduction, all of the Pr ions are in the trivalent state, and the oxide is in essence an acceptor-doped oxide with oxygen vacancy compensation. Any further reduction must then be accomplished by the transformation of Ce4+ to Ce3+, repeating the previous cycle ... 

B-Site Substitution Acceptor doping of the normally insulating per-ovskite structure SrTi03 has been widely explored for the purpose of fabricating mixed conductors. Replacement of part of the Ti4+ by a lower valence acceptor cation such as Fe3+ leads to enhanced total conductivity with greatly enhanced O2- migration. 

Acceptor doping in perovskite oxides gives materials with a vacancy population that can act as proton conductors in moist atmospheres (Section 6.9). In addition, the doped materials are generally p-type semiconductors. This means that in moist atmospheres there is the possibility of mixed conductivity involving three charge carriers (H+, O2-, and h ) or four if electrons, e, are included. 

The situation can be illustrated with respect to the acceptor-doped perovskite structure SrZrC>3, with Y3+ substituted for Zr4+ to give compositions SrZrj YVC>3-0.5.V The doping reaction can be written ... 

Acceptor doping, as in lithium oxide doping of nickel oxide, leads to the production of holes and produces p-typc thermistors ... 

The magnetic structure becomes more complex when doped materials are considered. Acceptor doping of LaCoCL by incorporation of an alkaline earth cation, Ca, Sr, Ba, in place of La, as in La SpCoCL now introduces holes into the system. These are generally located on the Co3+ ions to form Co4+. The Co3+ ions are thought to be mainly in the IS state and Co4+ in the HS state, with an electron configuration for this 3d5 ion of t2g e2. Further studies are needed to completely resolve the spin and charge distribution. 

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