Doping unintentional

Autodopiag occurs whea dopants are unintentionally released from a substrate through diffusion and evaporation, and subsequently reiacorporated during the deposition layer. Epitaxial layers are typically doped at concentrations of lO " -10 atoms/cm. The higher levels of doping are used in bipolar technology where the epilayer forms the transistor base. The epitaxial layer can be up to several hundred micrometers, and as thin as 0.05—0.5 p.m. Uniformities of 5% are common. 

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]. 

The 71Ga and 14N spectra of several of these films also showed partially-resolved shoulders shifted to higher frequency and having shorter T relaxation times that were attributed to Knight shifts in more heavily unintentionally doped regions of the film. These Knight shifts were observed in other GaN film samples [53] and will be discussed in more detail in Sects. 3.4.3 and 3.4.4, where MAS-NMR was used to improve the resolution in polycrystalline powders of h-GaN. Section 3.3.2 also shows 71Ga and 14N MAS-NMR spectra of GaN. 

As mentioned in Sect. 3.4.3, very broad Knight shifted peaks were also observed by 71Ga MAS-NMR in samples of h-GaN either with unintentionally-doped shallow donors (primarily Si and O) or intentionally doped with 0.13% Ge,... 

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. 

Specific results for donor and acceptor doping have been reviewed. The main conclusions for n-fype GaN are that (i) nitrogen vacancies are not responsible for unintentional n-type conductivity ... 

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. 

Baur et al [3] first observed a defect-related PL exhibiting an NP line at 0.931 eV in their unintentionally doped samples. They attributed this PL to the 3T2 — 3A2 transition of V3+. Kaufmann et al [15] performed intentional doping of GaN with V by ion implantation. They found a PL at 0.82 eV after post-growth annealing, which they attribute to a V-related radiation defect. 

By this standard, PMBE-grown GaN with best room temperature mobilities of 300 to 410 cm2/V s [43,44] has yet to reach the quality of GaN grown by MOVPE or HVPE [45,46] where mobilities up to 900 cm2/V s have been reached. Despite these deficiencies, GaN with very low carrier concentration and exceedingly low levels of yellow luminescence have been routinely achieved by MBE. In RMBE, unintentionally doped GaN layers can be highly resistive, the highest mobilities (300 K) reported for undoped layers being 230 cm2/V s for free electron concentrations of 2 x 1017 cm 3 [10], n-Type doping with Si yields mobilities (300 K) or 255 cm2/V s and 150 cm2/V s for free electron concentrations of 5 x 1017 cm 3 [41] and 5 x 10l cm 3 [10], respectively. 

Oxygen incorporation on an N-lattice site also acts as a donor in GaN. Furthermore, oxygen is thought to play a role in the background n-type conductivity in unintentionally doped material [6], The redistribution of O has also been studied with annealing up to 1125°C. Here again, no measurable redistribution is seen and an upper limit of 2.7 x 1013 cm2/s can be set on the diffusivity of O in GaN at 1125°C. 

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