Shallow donors ionization energy

Shallow levels play an important part in electronic conductivity. Shallow donor levels lie close to the conduction band in energy and liberate electrons to it to produce n-type semiconductors. Interstitial metal atoms added to an insulating ionic oxide often act in this way because metal atoms tend to ionize by losing electrons. When a donor level looses one or more electrons to the conduction band, it is said to be ionized. The energy level representing an ionized donor will be lower than that of the un-ionized (neutral) donor by the same amount as required to move the electron into the conduction band. The presence of shallow donor levels causes the material to become an w-type semiconductor. 

The same sort of considerations will apply to vacancies. For instance, an anion vacancy may give rise to a set of shallow donor levels just below the lower edge of the conduction band. If the vacancy is created by removing a neutral nonmetal atom from the crystal, the electrons that were on the anion are transferred to the conduction band to produce an n-type semiconductor. The energies of neutral and ionized vacancies are slightly different. 

Xiu et al. [23] reported in Bi-doped ZnO films by molecular-beam epitaxy that Bi-induced acceptor ionization energy was estimated to be 0.185-0.245 eV by photoluminescence measurements based on the donor-acceptor pair peak position in the Bi-doped ZnO films Bi in ZnO films had positive charge state determined by X-ray photoelectron spectroscopy measurements, indicating that BiZn at Zn sites, rather than Bio at O sites, was formed in the films. BiZn itself, however, is a donor. The origin of the shallow acceptor states was, therefore, identified as a donor-acceptor pair such as BiZn-VZn-0, or BiZn-2VZn complexes. 

An exciton bound to a shallow neutral donor of interstitial zinc (Fig 1 a) and of interstitial lithium (Fig. lb) is presented, for example, in our spectra. In some instances the radiative recombination of an exciton bound to a neutral defect may not lead to the ground state of the respective defect but to an excited state of the carrier at this occupied center (2 - electron transition). In a hydrogenic model we can calculate an ionization energy of the neutral donor state of interstitial zinc to 0.05 eV and of interstitial lithium to 0.033 eV. 

In zinc oxide, various impurities of both substitution and interstitial type act as shallow donor centers with an ionization energy Ed = 0.03 - 0.05 eV (for example, interstitial hydrogen, the substitutional impurities of the 3-rd group of the periodic table of elements and so on). 

Figure 6 displays the energy levels of common donors and acceptors with reference to the band edges of two semiconductors. If the energy levels of these donors and acceptors are close to the conduction or valence bands, respectively, they are called shallow donors or acceptors. Dopants with energy levels that are further away from either band are called deep dopants. While ionization of shallow dopants is usually complete at room temperature, ionization of deep dopants generally does not occur at room temperature. Shallow dopant materials are considered primarily in the remainder of this section. 

The FEs can bind to neutral shallow impurities and become bound excitons (BEs), with a value of Eex slightly larger than the one of the FE. The difference is called the localization energy E oc of the BE. For the P donor, it is 4 meV in silicon, but 75 meV in diamond. E oc is given approximately by Haynes empirical rule [20] as 0.1 A, where A is the ionization energy of the impurity. BEs are created by laser illumination of a semiconductor sample at an energy larger than Eg and the study of their radiative recombination by PL... 

The corresponding ionization energies are depicted in the energy diagram in the left-hand panel of Fig. 2.8. The common convention is to label the donor states by their charge before ionization, whereas acceptor states are labeled by their charge after ionization. Hence, represents a shallow donor, and a deep acceptor. 

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