Shallow donors

In an extrinsic semiconductor, tlie conductivity is dominated by tlie e (or h ) in tlie CB (or VB) provided by shallow donors (or acceptors). If tlie dominant charge carriers are negative (electrons), tlie material is called n type. If tlie conduction is dominated by holes (positive charge carriers), tlie material is called p type. 

Shallow impurities have energy levels in the gap but very close to a band. If an impurity has an empty level close to the VB maximum, an electron can be thennally promoted from the VB into this level, leaving a hole in the VB. Such an impurity is a shallow acceptor. On the other hand, if an impurity has an occupied level very close to the CB minimum, the electron in that level can be thennally promoted into the CB where it participates in the conductivity. Such an impurity is a shallow donor. 

Shallow donors (or acceptors) add new electrons to tire CB (or new holes to tire VB), resulting in a net increase in tire number of a particular type of charge carrier. The implantation of shallow donors or acceptors is perfonned for tliis purjDose. But tliis process can also occur unintentionally. For example, tire precipitation around 450°C of interstitial oxygen in Si generates a series of shallow double donors called tliennal donors. As-grown GaN crystal are always heavily n type, because of some intrinsic shallow-level defect. The presence and type of new charge carriers can be detected by Flail effect measurements. 

The Bolir radius is very large, 3-5 nm, and tlie shallow impurity wavefunction extends over a large portion of the crystal. Doping up to tlie Tnetallic limit consists in implanting a sufficiently high concentration of donors so tliat tlie shallow-donor wavefunctions overlap, creating a half-filled impurity band in which tlie electrons move freely. 

Fig. 1. Band-edge energy diagram where the energy of electrons is higher in the conduction band than in the valence band (a) an undoped semiconductor having a thermally excited carrier (b) n-ty e doped semiconductor having shallow donors and (c) a -type doped semiconductor having shallow acceptors.

Fig. 2. Representation of the band edges in a semiconductor p—n junction where shallow donor, acceptor energies, and the Fermi level are labeled Ejy E, and E respectively, (a) Without external bias is the built-in potential of the p—n junction (b) under an appHed forward voltage F. ...

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,... 

Finally, the use of DNP of shallow donors to enhance both 67Zn and surface ft nuclear polarizations has been demonstrated in ZnO nanoparticles by observation of EPR features rather than direct NMR observation [85, 87]. The electronic wavefunctions of these donors in ZnO have been probed by ENDOR experiments [36, 97], There is much potential for directly observing NMR with the sensitivity greatly enhanced by DNP not only in ZnO but in other nanoparticles as well. 

HYDROGEN NEUTRALIZATION OF SHALLOW-DONOR IMPURITIES IN ARSENIC-DOPED EPILAYERS ON SILICON... 

This chapter is devoted to the energetics and kinetics of the incorporation of hydrogen into the simplest and most studied of its possible hosts, crystalline silicon of high perfection containing known concentrations of shallow donor or acceptor impurities. It undertakes to review what has been learned from experiments about the phenomenological parameters... 

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