Shallow thermal donors

We are faced with two interconnected problems related to the intelligibility of the presentation. The first one concerns the nomenclature of the centres other than isolated atoms and the second the labelling of the optical transitions. These problems are not trivial, [5], but not as severe for H-like centres as for deep centres. The different notations for the shallow thermal donor complexes in silicon, discussed in Sect. 6.4.2, are however, a counter-example of this statement. In this book, on the basis of the present knowledge, names of centres, in direct relation with their atomic structure, have been privileged, but the usual label has however been indicated. When the exact structure is not simple and when there exist an acronym, like TDD for thermal double donor , it has been used. The labelling by their excited states of the transitions of the shallow donor centres and of similar species, whose spectra... 

The possibility of passivation of the TDDs by hydrogen has been investigated, but this point and the results obtained by optical spectroscopy will be discussed in the next section, with the properties of the shallow thermal donors. 

Fig. 5. Capacitance and current transient spectra from -type, CZ grown Si annealed for 18h at 450°C to form the shallow, oxygen thermal donors. (Chantre et al., 1987). Hydrogenation at 200°C passivates the electrical activity of these thermal donors (Chantre et at, 1987).

Shallow donors (or acceptors) add new electrons to the CB (or new holes to the VB), resulting in a net increase in the number of a particular type of charge carrier. The implantation of shallow donors or acceptors is performed for this purpose. But this process can also occur unintentionally. For example, the precipitation aroimd 450°C of interstitial oxygen in Si generates a series of shallow double donors called thermal 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 Hall effect measm ements. 

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.

The impurity atoms used to form the p—n junction form well-defined energy levels within the band gap. These levels are shallow in the sense that the donor levels He close to the conduction band (Fig. lb) and the acceptor levels are close to the valence band (Fig. Ic). The thermal energy at room temperature is large enough for most of the dopant atoms contributing to the impurity levels to become ionized. Thus, in the -type region, some electrons in the valence band have sufficient thermal energy to be excited into the acceptor level and leave mobile holes in the valence band. Similar excitation occurs for electrons from the donor to conduction bands of the n-ty e material. The electrons in the conduction band of the n-ty e semiconductor and the holes in the valence band of the -type semiconductor are called majority carriers. Likewise, holes in the -type, and electrons in the -type semiconductor are called minority carriers. 

The exposure of n-type LPE GaAs layers to a hydrogen plasma for three hours at 300°C induces a neutralization of five deep electron traps at c - 0.13 eV, c - 0.36 eV, c - 0.38 eV, c - 0.54 eVand c - 0.73 eV (Pearton and Tavendale, 1982). The thermal stability of these neutralized centers is lower than for EL2 neutralization and can be compared with the shallow donors one. 

In MBE grown GaAs three dominant electron traps are usually observed Ml at c - 0.17 eV, M3 at c - 0.28 eV and M4 at c - 0.45 eV. Exposure of MBE grown material to a hydrogen plasma for 30 minutes at 250°C completely passivates these three deep levels as shown in Fig. 10 (Dautremont-Smith et al., 1986). After five minute anneals at 400°C or 500°C, the passivation remains complete while the shallow donors are fully reactivated. A five minute annealing at 600°C partially restores the electrical activity of M3. Therefore the thermal stability of the neutralization of deep levels in MBE material is much higher than in other materials and is compatible with most technological treatments. 

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,... 

The literature abounds with reports of thermal activation energies for shallow donors in GaN, obtained from Hall effect measurements over a range of temperatures, above and below room temperature, though their interpretation is rendered problematic by a number of complicating factors. At low temperatures there is clear evidence for impurity band conduction (see, for example, [31]) which severely limits the temperature range over which data may usefully be fitted to the standard equation for free carrier density n in terms of the donor density ND and compensating acceptor density NA ... 

Using ion implantation in GaN, Zolper et al reported n-type doping with oxygen and obtained a relatively shallow donor level at 29 meV [4], SIMS measurements did not show any measurable redistribution with rapid thermal annealing (RTA) at 1125°C. However, the activation efficiency of the implanted dopants was very poor. 

Extrinsic semiconductors ate those in which the carrier concentration, either holes or electrons, are controlled by intentionally added impurities called dopants. The dopants are termed shallow impurities because their energy levels lie within the band gap close to one or other of the bands. Because of thermal excitation, -type dopants (donors) are able to donate electrons to the conduction band and p-type dopants (acceptors) can accept electrons from the valence band, the result of which is equivalent to the introduction of holes in the valence band. Band gap widening/narrowingmay occur if the doping changes the band dispersion. At low temperamres, a special type of electrical transport known as impurity conduction proceeds. This topic is discussed in Section 7.3. 

The lithium dopant in ZnO may occupy interstitial sites (Lk) or may substitute for the Zn site (Lizn) acting as a shallow donor or as an accepter, respectively. The optical depth, measured from the edge of the valence band, amounts to about 1 eV and the thermal depths to 0.2 - 0.4 eV. ... 

Becanse shallow dopant atoms are readily ionized at room temperature, the electron concentration, n, in an n-type semicondnctor is closely approximated by the concentration of donor atoms, N, in the lattice. Rigoronsly, the electron concentration is given by the sum of the electrons thermally generated from the Si atoms and those generated by the thermal ionization of dopants. However, becanse n is so small for most common semicondnctors, n = -F Aj Aj for any reasonable dopant concentration (10 -10 dopant atoms cm ). Similarly, for a p-type semicondnctor, the hole concentration is approximately equal to the acceptor concentration, N. This approximation holds becanse shallow acceptors are essentially all ionized at room temperatme, and the intrinsic hole concentration, p, is generally negligible compared to the number of holes that are generated by the dopants. Clearly, control over the dopant density of a semicondnctor allows the manipulation of the carrier concentrations. 

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