Shallow acceptor level

Shallow acceptor levels lie close to the valence band and take up electrons from it to create holes in the valence band and produce p-type semiconductors. Interstitial nonmetal atoms often generate shallow acceptor levels because anion formation involves taking up extra electrons. Acceptor levels are said to be ionized when they take electrons from the valence band, creating holes in the process. The energy of a neutral acceptor atom is different to that of an ionized acceptor. The electrons on the ionized anions are often trapped and do not contribute to the conductivity. 

Due to the lack of a shallow acceptor level in 4H-SiC and a relatively low hole mobility, the intrinsic base resistance of a 4H-SiC BJT is very high for example, a base epilayer sheet resistance of 46 kQ per square was reported for a 4H-SiC BJT in [6]. The high intrinsic base resistance in 4H-SiC BJTs results in a very severe emitter current crowding effect [7], which limits most of the current flow to within 1 jum from the edge of the emitter regions [8]. This indicates that tight cell pitches are necessary to achieve a low specific on-resistance, as shown in Figure 6.2(a) [6]. 

The presence of a shallow acceptor level in GaN has been attributed to C substituting on an N site by Fischer et al [7], In luminescence experiments on GaN from high temperature vapour phase epitaxy in a C-rich environment donor-acceptor and conduction-band-to-acceptor transitions have been distinguished in temperature dependent experiments. From the separation of both contributions an optical binding energy of 230 meV close to the value of effective mass type acceptors was obtained. Hole concentrations up to 3 x 1017 cm 3 were achieved by C doping with CCU by Abernathy et al [10], In addition Ogino and Aoki [17] proposed that the frequently observed yellow luminescence band around 550 nm should be related to a deep level of a C-Ga vacancy complex. The identification of this band, however, is still very controversial. 

The bivalent substitutional impurities of group-IVA elements such as C, Si, Ge, Sn, or Pb also produced double shallow acceptor levels with the ionization energy of 0.721 eV for C, 0.919 eV for Si, 0.792 eV for Ge, 1.034 eV for Sn, and 1.283 eV for Pb, respectively. Some bivalent substitutional impurities of another type of group-VIIIA elements such as Ne, Ar, Kr, or Xe did not produce any energy levels in the band gap by the substituting host O atom. As expected, the acceptor levels produced by the impurities of group-VA and -IVA elements at the O site were single or double acceptors, respectively. Quantitave analysis of these shallow acceptors produced by the monovalent and bivalent substitutional impurities will be made in Section 4.2. 

Hamiltonian (3.26) will be used to explain the principles of the calculation of the shallow acceptor levels in these crystals. For a magnetic field B, derived... 

The electron effective masses mn at the CB minimum at k = 0 are generally smaller than the ones for k /Z 0 CB minima, as can be judged from Table 3.6. The Luttinger VB parameters have been determined by many authors, though biased in some cases by the values used in the most recent calculations of the shallow-acceptor levels. The situation is complicated by the fact that for semiconductors like InSb, where there is an interaction between the valence and the conduction bands, effective Luttinger VB parameters 7j have been defined by [82] as ... 

The wave function character of a shallow acceptor level is similar to that of the valence band maximum state, consisting of anion p, cation p, and cation d orbitals. Among the group V elements substituting the anion site in ZnO, N has the lowest p orbital energy as shown by first-principles calculations within the local density approximation, and therefore produces the lowest acceptor level [108]. However, the No acceptor level is deep ( 0.3 eV above the valence band) as the valence band maximum of ZnO is low, compared to, for example, GaN. 

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