Doping donor-acceptor compensation

Another disadvantage of NTD technique is the simultaneous creation of both donors and acceptors giving place to a compensation in the doping. The compensation is the donor/acceptor ratio (in p-type) which depends mainly on the isotopic composition and on cross-sections for neutron absorption. 

When both donors and acceptors are present, compensation results, whereby the electrons supplied by the donor are given to the acceptor. Thus, the free carrier concentration can be considerably reduced below that expected from introducing a known donor or acceptor if the opposite type of dopant is unintentional. For example, semi-insulating (SI) InP (used as a substrate for epitaxial growth) can be made by incorporating low levels of Fe3+ as a deep acceptor (reduced to Fe2+) to compensate for unintentional n-type doping in the sample [19]. 

Host cations are easily oxidized and reduced [transition metals (Fe203, Mn203, etc.)]. In both donor (higher valence) and acceptor (lower valence) doping electronic compensation (donors by electrons and acceptors by holes) will be preferred to structural (vacancies and interstitials) compensation. 

In comparison to the research in n-type oxide semiconductors, little work has been done on the development of p-type TCOs. The effective p-type doping in TCOs is often compensated due to their intrinsic oxide structural tolerance to oxygen vacancies and metal interstitials. Recently, significant developments have been reported about ZnO, CuA102, and Cu2Sr02 as true p-type oxide semiconductors. The ZnO exhibits unipolarity or asymmetry in its ability to be doped n-type or p-type. ZnO is naturally an n-type oxide semiconductor because of a deviation from stoichiometry due to the presence of intrinsic defects such as Zn interstitials and oxygen vacancies. A p-type ZnO, doped with As or N as a shallow acceptor and codoped with Ga or Zn as a donor, has been recently reported. However, the origin of the p-type conductivity and the effect of structural defects on n-type to p-type conversion in ZnO films are not completely understood. 

The effects of deliberately added donors, such as titanium, and acceptors, such as iron and magnesium, on electrical conductivity have been studied. Doping with aliovalent ions affects the concentration of intrinsic defects and, in consequence, the diffusivity of A1 and O. In the case of variable-valency dopants, changes in p0l change the fraction of dopants in the aliovalent state and the nature and concentration of the defects. For example, the dopant Ti substitutes for A1 and, in the fully oxidized state, produces the defect TiA1, compensated by Va", so that... 

The doping can be affected by native defects such as vacancies (Vo and VN), self-interstitials (Gai and Nj) and antisites (GaN and Ng,). Such defects may cause self-compensation, e.g. when one tries to dope the material p-type, certain native defects which act as donors may spontaneously form and compensate the deliberately introduced acceptors. In GaN, a specific native defect was long believed to play an even more important role the nitrogen vacancy, which acts as a donor, was thought to occur in large concentrations, thereby causing unintentional n-type conductivity. 

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