Shallow impurity

Shallow impurities have energy levels in the gap but very close to a band. If an impurity has an empty level close to the VB maximum, an electron can be thennally promoted from the VB into this level, leaving a hole in the VB. Such an impurity is a shallow acceptor. On the other hand, if an impurity has an occupied level very close to the CB minimum, the electron in that level can be thennally promoted into the CB where it participates in the conductivity. Such an impurity is a shallow donor. 

The Bolir radius is very large, 3-5 nm, and tlie shallow impurity wavefunction extends over a large portion of the crystal. Doping up to tlie Tnetallic limit consists in implanting a sufficiently high concentration of donors so tliat tlie shallow-donor wavefunctions overlap, creating a half-filled impurity band in which tlie electrons move freely. 

The responsivity and g-r noise may be analyzed to obtain background photon flux and temperature dependence of responsivity, noise, and detectivity. Typically, n > p, and both ate determined by shallow impurity levels. The minority carrier density is the sum of thermal and optical contributions. 

Band gaps of semiconductors carrier lifetimes shallow impurity or defect detection sample quality and structure... 

InP, in the range 10 —10 cm . Boron, phosphorus, and other shallow impurities can be detected in silicon in concentrations approaching 10 cm . Copper contamination at Si surfaces has been detected down to 10 cm levels. ... 

A celebrated derivation of the temperature dependence of the mobility within the hopping model was made by Miller and Abrahams 22. They first evaluated the hopping rate y,y, that is the probability that an electron at site i jumps to site j. Their evaluation was made in the case of a lightly doped semiconductor at a very low temperature. The localized states are shallow impurity levels their energy stands in a narrow range, so that even at low temperatures, an electron at one site can easily find a phonon to jump to the nearest site. The hopping rate is given by... 

There are several reports of the use of directed ion sources to hydrogen passivate both shallow impurities (Horn et al., 1987 Martinuzzi et al., 1985) and deep defect levels in silicon (Dube and Hanoka, 1984 Hanoka... 

The passivation of deep level defects and shallow impurities in semiconductors by hydrogen has been studied extensively in recent years (Pearton et al., 1987, 1989 Haller, 1989). For Si in most cases, complexing with hydrogen eliminates the electrical activity of a defect.Once passivated, the... 

Much of the microscopic information that has been obtained about defect complexes that include hydrogen has come from IR absorption and Raman techniques. For example, simply assigning a vibrational feature for a hydrogen-shallow impurity complex shows directly that the passivation of the impurity is due to complex formation and not compensation alone, either by a level associated with a possibly isolated H atom or by lattice damage introduced by the hydrogenation process. The vibrational band provides a fingerprint for an H-related complex, which allows its chemical reactions or thermal stability to be studied. Further, the vibrational characteristics provide a benchmark for theory many groups now routinely calculate vibrational frequencies for the structures they have determined. 

The implantation of hydrogen into silicon or crystal growth in a hydrogen atmosphere introduces vibrational bands that have been ascribed to lattice defects decorated with hydrogen. While IR experiments were begun —10 years before similar studies of passivated shallow impurities, the structures of the complexes that result from H+ implantation are not well understood. This subject has been reviewed previously by Pearton et al. (1987, 1989). Here, the central experimental results will be summarized. A recent uniaxial stress study (Bech Nielsen etal., 1989) of several of the vibrational features will be discussed in Section IV.3. 

Haller, E.E. (1989). Proc. Third Inti. Conference on Shallow Impurities in Semiconductors, ed. Monemar B., Inst, of Phys. Conf. Series 95, 425. 

Contrary to the case of shallow impurities, the interaction between deep level defects and hydrogen has been the subject of a few detailed studies. The reasons for that can be found in the lack of detailed understanding of the defect itself involved in the interaction. In some cases, these defects are still the subject of studies with sometimes controversial interpretations. Moreover, the concentration of deep level defects involved in the hydrogen complexes is relatively low, which makes experimental investigations for local structure analysis very difficult. In this section, we present the set of data establishing the neutralization of defects or deep impurities by hydrogen. 

There is still a lack of data on the possibility of neutralizing shallow impurities in materials other than GaAs, Inp, GaP and AlGaAs. In III-V materials, some double acceptors such as nickel or the 78/203 meV acceptors in GaAs are known. One should determine whether they behave as... 

Briddon, P., and Jones, R. (1989). Proceedings of the Third International Conference on Shallow Impurities in Semiconductors, Linkoping, 1988, edited by B. Monemar. IOP Conf. Ser. (IOP London, 1989), p. 459. 

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 resistivity of the SI wafers has been measured to be in excess of 10" Q-cm at room temperature [35]. As mentioned earlier, a deep level defect was found to have an activation energy of 1.4 eV. MESFETs manufactured on these wafers show an increased performance in the sense of reduced trapping, which the authors explain as being primarily due to a reduction of the shallow impurities in the material. 

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