Shallow level

Shallow level defects can be understood, at least approximately, based on the hydrogenic model. This approximates the defect as a H atom in the dielectric medium of the semiconductor. An impurity that is chemically similar to the matrix atom it replaces but has one more or one fewer electron and proton (a monovalent substitution) often results in this type of state. Examples of such impurities are P (considered in detail below) or A1 in Si. The hydrogenic model can be applied to any monovalent substitutional point defect in either an elemental or a compound semiconductor and to both n and p type dopants. 

A P atom can be viewed as a Si atom plus a H atom - one extra electron and proton. If one ignores the proton for the moment, an electron added to a Si solid would necessarily wind up in the conduction band, as the valence band is full. Of course, one cannot ignore the extra proton, to which the electron will be attracted. Thus, the electron is not completely free to move about the sohd, as it would normally be in 

A complete analysis of the H atom in a dielectric medium with a relative dielectric constant 8 and effective mass m /m, shows that the atom would have a larger atomic radius, r, and smaller ionization energy E than for H as  

Strain is also well known to govern the incorporation of dopant atoms into solids. An oversized atom tends to be rejected to the surface of a crystal as it grows, (it 

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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. 

Since shallow-level impurities have energy eigenvalues very near Arose of tire perfect crystal, tliey can be described using a perturbative approach first developed in tire 1950s and known as effective mass theoiy (EMT). The idea is to approximate tire band nearest to tire shallow level by a parabola, tire curvature of which is characterized by an effective mass parameter m. ... 

The X-ray emission process followii the excitation is the same in all three cases, as it is also for the electron-induced X-ray emission methods (EDS and EMPA) described in Chapter 3. The electron core hole produced by the excitation is filled by an electron falling from a shallower level, the excess energy produced being released as an emitted X ray with a wavelength characteristic of the atomic energy levels involved. Thus elemental identification is provided and quantification can be obtained from intensities. The practical differences between the techniques come from the consequences of using the different excitation sources. 

In principle all the X-ray emission methods can give chemical state information from small shifts and line shape changes (cf, XPS and AES in Chapter 5). Though done for molecular studies to derive electronic structure information, this type of work is rarely done for materials analysis. The reasons are the instrumental resolution of commercial systems is not adequate and the emission lines routinely used for elemental analysis are often not those most useftil for chemical shift meas-ure-ments. The latter generally involve shallower levels (narrower natural line widths), meaning longer wavelength (softer) X-ray emission. 

Kuroko deposits in Northeast Japan originated from these materials. Antimony, mercury and sulfur in the Hg-Sb deposits in Southwest Honshu may have been derived from the shallow level of the crust under the Shimanto Group. 

Bohrson WA, Reid MR (1998) Genesis of evolved ocean island magmas by deep- and shallow-level basement recychng, Socorro Island, Mexico Constraints from Th and other isotope signatures. J Petrol 39 995-1008... 

Hilton DR, Barling J, Wheiler GE (1990) The effect of shallow-level contamination on the helium isotope systematics of ocean island lavas. Nature 348 59-62... 

Hawkesworth CJ, Kempton PD, Rogers NW, Ellam RM, van Calsteren PW (1990) Continental mantle lithosphere, and shallow level enrichment processes in the Earth s mantle. Earth Planet Sci Lett 96 256-... 

Fig. 9. Arrhenius plots of the free hole concentration p (log p versus 1000/T) in two samples cut from a partially dislocated slice of ultra-pure germanium. The dislocation-free sample contains an acceptor with Ev + 80 meV. The shallow level net-concentration is the same in both samples.

Unfortunately, there has been little computational work on these interesting complexes hence, we will mostly consider complexes formed with shallow-level defects in the next section. There has, however, been a treatment of hydrogen-sulfur complexes in silicon by Yapsir et al. (1988) and a recent treatment of hydrogen-oxygen complexes by Gutsev et al. (1989), which we now describe. 

We proceed now to discuss the analogous hydrogen-related complexes with shallow-level impurities. We will find that the way hydrogen incorporates itself and the character of the complex after it is formed is independent of whether the passivated impurity introduced a deep level or a shallow level. Therefore, many of the basic arguments regarding the complexes will prevail as we consider the shallow-level defects. 

V. Hydrogen—Shallow-Level-Defect Complexes in Silicon... 

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. 

Shallow levels are often found in transition-metal compounds in which the cations can exist in several valence states. The most familiar of these are the transition... 

The lifetime of the core-ionized atom is measured from the moment it emits a photoelectron until it decays by Auger processes or X-ray fluorescence. As the number of decay possibilities for an ion with a core hole in a deep level (e.g. the 3s level) is greater than that for an ion with a core hole in a shallow level (e.g. the 3d level), a 3s peak is broader than a 3d peak. 

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. 

Marty B, Zimmermann L (1999) Volatiles (He, C, N, Ar) in mid-ocean ridge basalts assesment of shallow-level fractionation and characterization of source composition. Geochim Cosmochim Acta 63 3619-3633... 

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