Shallow electronic states

The dark ESR spectra of doped a-Si H in Fig. 5.10 show resonances near g = 2, with different line shapes and g-values from those of the dangling bond (Stuke 1977). These lines are attributed to band tail states because they are observed when the Fermi energy is moved up to the band tails by doping and also in the low temperature LESR spectra of undoped a-Si H, when electrons or holes are optically excited into the band tails. The larger g-shift for the valence band tail states than for the conduction band states is expected from Eq. (4.12). 

More sensitive ESR measurements over a wider magnetic field range find additional resonances in phosphorus- and arsenic-doped material, examples of which are shown in Fig. 5.11. The extra lines (two for phosphorus and four for arsenic) are due to the hyperfine interaction of the electron bound to the donor (Stutzmann and Street 1985). The ESR spectra have exactly the number of lines and relative intensities expected from the nuclear spins of and for phosphorus and arsenic atoms. The splitting of the lines, is proportional to the electron density at the nucleus and is a measure of the localization length, r, of the donor. 

The donor electrons are thermally excited into the larger density of conduction band states at elevated temperatures, reducing the neutral donor concentration. Donor ionization occurs in crystalUne siUcon near 20 K, but the corresponding effect in a-Si H begins at about 200 K. The ESR spin density of the neutral phosphorus donor 

O Phosphorus - band tail Phosphorus - donor Arsenic - band tail Arsenic - donor 

There is no sign of an ESR hyperfine interaction in boron-doped a-Si H, so that there is little information about the acceptor states. It may be that boron acceptors have an unexpectedly small hyperfine interaction. A more likely explanation is that virtually all the acceptors are ionized. The valence band tail is much broader than the conduction band and the Fermi energy remains further from the band edge, so that the probability that a hole occupies an acceptor is much smaller. The 

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A substitutional donor or acceptor in a crystalline semiconductor results in the formation of a shallow electronic state. For the specific case of the donor, the extra electron is bound to the charged... 

Several shallow defects near the band edges have been postulated in these alloy systems, but none of them directly involve the presence of hydrogen. The most accepted interpretatj.0n of these defects is in terms of strained bonds on the group IV atoms. It haj also been suggested that neutral two-fold coordinated Si and Ge atoms may be responsible for these shallow electronic states. 

In an unsensitized grain, shallow trap states provided by crystal imperfections are important in the trapping of both electrons and holes. Hamilton assumes that the fraction of holes trapped is approximately 1, that is, the concentration of mobile holes is near 0. Nucleation to form silver is inefficient, and a high level of free-electron/trapped-hole recombination occurs. There is a certain probability, however, that a trapped electron will unite with a silver ion to form an atom which may either dissociate back into electron and silver ion or trap another electron and, with a second Ag, form a silver atom pair. This pair is relatively stable and can grow by... 

The first step describes the excitation of a quasi-bound vibrational level in the excited electronic state with quantum numbers (m, n ). The second step represents the dissociation of the intermediate compound due to coupling to the continuum induced by energy redistribution inside the shallow well. 

The Coulomb interaction between the electron and the donor core is, of course, present in an amorphous semiconductor and binds an electron in much the same way, so the shallow donor state is preserved. The effective mass theory for dopants cannot be applied directly to amorphous semiconductors, because it is formulated in terms of the momentum-space wavefunctions of the crystal. It is not immediately obvious that the effective mass has any meaning in an amorphous... 

A comparison of Fig. 5.12 and the defect density in Fig. 5.9 shows that the total density of the band tail electrons - neutral donors plus occupied intrinsic band tail states-is about ten times less than the density of deep defects induced by the doping. This is a remarkable result because it implies that almost all the donors are compensated by deep defects. However, before considering the consequences of this observation, it is helpful to discuss an alternate experimental technique for measuring the density of shallow electrons or holes, because of the possibility that ESR is missing some of the carriers due to electron pairing or broadening of the resonance. 

Thus, within 1 ps, the room temperature current is entirely due to carriers occupying states shallower than about 0.3 eV, which are present only in the doped layer. The undoped material, with its Fermi energy deep in the gap, has a minimum carrier release time of order 10" s. The sweep-out current arises from the emission of the shallow electrons occupying the intrinsic band tail states, the donor states, and... 

The triplet of frequencies gives rise to a beating in the time-domain spectrum as seen for a powder sample of ZnO in Figure 1. The separation of the two satellite lines provides a direct measure of the hyperfme coupling constant between the muon and the electron. This is 500 20 kHz, which is 0.011% of the free-muonium value of 4463 MHz, indicating immediately a small electron spin density at the site of the muon and an extended waveflmction associated with a shallow donor state. 

The influence of deep-level states or traps on the statistics of electron-hole recombination was first described by Shockley and Read and Hall. Deep-level states, as their name implies, lie close to the middle of the energy bandgap of the semiconductor. Due to the large energy separation from the valence-band and conduction-band edges, deep-level states are not fully ionized at room temperature. In contrast, shallow-level states are those considered to be fully iordzed at room temperature due to thermal excitation. 

Long-lived CS- ions may also be formed at large intemuclear distances in the shallow potential wells of CS (a6fI). Indeed, the sextet is located well below its parent neutral state [i.e. CS(a 5fl)] at these distances. Here again, the J = 7/2 fine component of this sextet has no counterpart in the lower electronic states of the CS-anion. This potential well is due to polarization effects. More than ten vibrational states were calculated to be bound there. 

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