Fermi level position

Ga( Zn)silicon The two-electron acceptor impurity of Zn is present in silicon only in the form of neutral (ZnO) or doubly ionized Zn centers depending on the Fermi-level position. Broadening of the spectra corresponding to the above centers indicates that the local symmetry of these centers is not cubic... 

Most impurities can occur in different charge states we will see that H in Si can occur as H+, H°, or H. Which charge state is preferred depends on the position of the Fermi level, with which the defect can exchange electrons. Relative formation energies as a function of Fermi level position can be calculated and tell us which charge state will be preferred in material of a certain doping type. Section V will discuss charge states in detail. 

Fig. 19 Schematic level diagram around the Fermi level, for the infinite DNA wire studied by Gervasio et al. [94]. The Fermi level positioned in the middle of the gap was chosen as the zero of energy. The highest occupied "band" is constituted of a manifold of 12 states localized on the 12 Gua bases contained in the periodic unit and originated from the Gua-HOMO. Gyt-localized states (n Cyt) appear as another manifold at 3.16 eV above the HOMO-band. Flowever, empty electron states due to metal counterions and phosphates are revealed at 1.28 eV above the FIOMO-band (see also [92]), and the ground-to-excited-state transitions are therefore related to charge transfer between the inner and outer helix (adapted from [94] with permission Copyright 2002 by the American Physical Society)...

Apart from the Fermi level position, palladium oxide shows behaviour similar to the GaO unit. Adsorption of the methane molecule is apparently weak, the overall shape of the spectrum does not change significantly while an additional feature due to carbon appears with similar shape and in a... 

This corresponds to a Frenkel type defect behavior. Negative defect formation enthalpies corresponds to unstable crystals (spontaneous defect formation) and indicate the limit for physically reasonable Fermi level positions. 

Erhart and Albe also calculated zinc diffusion in ZnO [130]. The results are displayed in Fig. 1.18 together with a comparison to experimental data. Depending on chemical potential and Fermi level position either zinc vacancy or zinc interstitial diffusion can dominate. In the case of n-type material, where the Fermi level is close to the conduction band, zinc diffusion is mostly accomplished via the vacancy mechanism. 

Electronic surface properties including Fermi level positions, work functions, and ionization potentials of sputter-deposited ZnO and Al-doped ZnO films in dependence on deposition parameters. The results provide insight into aspects of doping, surface chemistry, and terminations. 

The core and valence levels in Fig. 4.6 show comparable binding energy shifts in dependence on deposition conditions. The shifts are mainly due to shifts of the Fermi level position at the surface. The Fermi level position with respect to the valence band maximum is directly measured as the binding energy of the valence band maximum. Values for magnetron-sputtered ZnO and ZnO Al thin films are shown in Fig. 4.12 in dependence on oxygen content in the sputter gas and substrate temperature. 

The work functions and ionization potentials of sputter-deposited ZnO and ZnO Al films are shown in Fig. 4.13. The different Fermi level positions of ZnO and ZnO Al for deposition at room temperature in pure Ar are also observed in the work function. The undoped films prepared under these conditions have a work function of 4.1eV, while the Al-doped films show values of 3.2eV. The difference is almost of the same magnitude as for the Fermi level position and, therefore, explained by the different doping level. Also the ionization potentials are almost the same under these preparation conditions. The work function of the undoped material is close to the value reported by Moormann et al. for the vacuum-cleaved Zn-terminated (0001) surface [20], The same authors report a work function of 4.95 eV for the oxygen terminated ZnO(OOOl) surface, which is in good agreement with the values obtained for films deposited with >5% oxygen in the sputter gas. Since the Fermi level position of the undoped ZnO films does not depend on the oxygen content in the sputter gas (Fig. 4.12), the different work functions correspond to different ionization potentials. 

The experimental procedure for the determination of the valence band offsets directly relies on the core level to valence band maximum binding energy differences BEvb(CL) as described in Sect. 4.1.3 and Fig. 4.3. The corresponding values for the Zn2p3/2 and the Cd3ds/2 core level are therefore included in Table 4.2. These values are determined directly from the respective interface experiments. With two exceptions (CSZA-E and ZACS-C), the values for the Zn2p3/2 core level show the same dependence on deposition conditions as given in Fig. 4.15. For these two exceptions, also the Fermi level position... 

Fig. 4.39. Top Estimation of band alignment (middle section) from the Fermi level positions measured at the surfaces of thick In2S3 and ZnO Al films in dependence on deposition conditions.

On the CP side, the situation at the interface and the spatial variation of the energy-level positions away from the contact depend on two parameters the Fermi level position <[>p and the energy of either the top of the valence band I (the ionization potential) for the holes, or the bottom of... 

For a doped semiconductor, the Fermi level position will be shifted from mid-gap, because the doping process will vary the tendency of the solid to either gain or lose electrons. For example, if donors are added to an intrinsic semicondnctor, the material will be more likely to lose electrons. The Fermi level of an n-type semiconductor will thus move closer to the vacuum level (i.e. will become more negative on the electrochemical potential scale) (Figure 9(b)). Similarly, if acceptors are added to an intrinsic material, the Fermi level will become more positive, because this phase will now have an increased tendency to accept electrons from another phase (Figure 9(c)). 

Figure 9 A schematic representation of the Fermi level position in a semiconductor, (a) The Fermi level in an intrinsic semiconductor at absolute zero is approximately at mid-band gap. (b) In an n-type semiconductor, the electron concentration in the conduction band is greater than in an intrinsic sample. The Fermi level is thus closer to the conduction band for an n-type sample, (c) In a p-type semiconductor, the Fermi level position is moved closer to the valence band. Thus, a p-type semiconductor is more likely to accept electrons from another phase than is the corresponding n-type semiconductor...

In essence, this is analogous to shifting the reference level of an aqueous solution to pH = 0. Changes in pH could then be calculated relative to the amount of acid in a 1.0 M H+ solution, as opposed to calculating pH changes relative to a 10 M H+ solution for neutral water. The result for the Fermi level position versus electron concentration is, of course, identical no matter which formula is used. It is only a matter of convenience as to which reference level is used in the calculation of E. Similarly, we could choose to refer the Fermi level position to the energy of the top of the valence band, fvb In this case, the expression for E g is ... 

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