Deep donors and acceptors

If the dopant level is within 2kT of Ec or Ey, it will be (almost) fully ionized at room temperature - this is referred to as a shallow dopant. For deep donors and acceptors, the degree of ionization can be calculated with the following equations ... 

Fig. 2.8 Left Energy diagram for a hypothetical a metal oxide, indicating several shallow and deep donor and acceptor levels. Right n-Type oxide with optically active deep acceptw levels. After excitation, the holes have to hop to the interface via neighboring acceptor levels...

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]. 

Charge-transfer spectra represent one of the most important classes of spectra for analytical chemistry since the molar absorptivities tend to be very large. Charge-transfer can occur in substances, usually complexes that have one moiety that can be an electron donor and another that can be an electron acceptor. Both the donor and acceptor must have a small difference in their energy levels so that the electron can be readily transferred from the donor to the acceptor orbitals and back again. One example is the well-known, deep-red color of the iron (III) thiocyanate ion. The process appears to be... 

In a study that addressed the effect of doping on quantum dots, the donor and acceptor levels were found to be practically independent of particle size [De3]. In other words, shallow impurities become deep ones if the dot size is reduced. Experimental observations show that the luminescence is not affected by doping if a thermal diffusion process, for example using a POCl3 source, is used [Ell]. Implantation, in contrast, is observed to effectively quench the PL [Tal4]. If the pores are filled with a medium of a large low-frequency dielectric constant, such as water or any other polar solvent, it is found that deep impurity states still exist,... 

Donor and acceptor levels are assumed to remain practically unaffected by particle size and thus become deep levels in confined crystallites. 

The donor electron level, cd, which may be derived in the same way that the orbital electron level in atoms is derived, is usually located close to the conduction band edge level, ec, in the band gap (ec - Ed = 0.041 eV for P in Si). Similarly, the acceptor level, Ea, is located close to the valence band edge level, ev, in the band gap (ea - Ev = 0.057 eV for B in Si). Figure 2-15 shows the energy diagram for donor and acceptor levels in semiconductors. The localized electron levels dose to the band edge may be called shallow levels, while the localized electron levels away from the band edges, assodated for instance with lattice defects, are called deep levels. Since the donor and acceptor levels are localized at impurity atoms and lattice defects, electrons and holes captured in these levels are not allowed to move in the crystal unless they are freed from these initial levels into the conduction and valence bands. 

Fig. 2. Schematic energy band diagrams showing the use of a p -n junction to study capture and emission processes at a deep level. The junction is shown at steady state with a reverse bias of V volts (a) and with 0 volts (b). The width of the space-charge region under these conditions is ) and WTO). Immediately after the reverse bias is switched from 0 to V volts a nonequilibrium condition exists in which electrons occupying traps within the space-charge region are emitted to the conduction band and swept out (c). The shallow donor and acceptor that must be present have been omitted for clarity.

The expectation that substitutional impurities behave as effective mass donors and acceptors in a Si H is borne out by experiment. However, doping also induces deep states in a-Si H and these are discussed first. 

This type of transition has been extensively investigated in ZnS doped with a monovalent d" cation (Cu+, Ag+, Au+) (usually called the activator) and a trivalent ion such as AP+ (coactivator) substituted for divalent zinc. The coactivator can also be Cl substituted for. The monovalent cations create deep acceptor states, Al + or Cl form shallow donor levels (0.1 eV for Al) (Figure 13). When electrons are transferred from the valence to the conduction band, for instance, under electron beam irradiation, they are trapped by the coactivator while holes formed in the valence band are captured by Cu+ or Ag+, which are oxidized to the divalent state. The energy of the photons emitted depends on the energy difference between the donor and acceptor levels and on the acceptor-donor distance ... 

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. 

F. 2.6 Energy levels of shallow and deep donors (SD, DD) and acceptors (SA, DA) in a semiconductor. Deep donor ot acceptor states can also occur below or above midgap, respectively. Midgap states (RC) are often very efficient recombination centers and can be either donor- or acceptor-like in nature... 

Analysis of the dephasing of the out-of-phase echo for radical pairs produced by photo-excitation can give the interspin distance. This technique has been applied predominantly in studies related to photosynthesis. Following creation of the radical pair, a two-pulse spin echo is created. Crucial to the interpretation is that there is phase coherence between eigenstates present in the photo-induced spin-correlated radical pair (84). The spin-spin interaction causes deep modulation in the out-of-phase echo, whereas the normal, in-phase, echo vanishes (84). Distances in the range of 25 - 34 A have been measured to define distances between donor and acceptors in the photosystem (84-96). 

As in the case of GaAs, semi-insulating GaP can be obtained by doping with shallow donors and acceptors and with deep centers. Resistivities at room temperature are of the order of 10 -10 S2 cm. T)fpical data at high temperatures are shown in Fig. 4.1-91. 

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