Band offset

The electronic properties of the interface depend in part on how the conduction and valence bands line up when they come into contact. The energy of the band offsets are obtained from X-ray 

Efii and E are the band gaps of the two materials ( qi being the larger) and AE. and AEy are the band offsets. For the nitride overlayers, the sum on the right hand side agrees with the silicon nitride band gap of 5.3 eV within about 0.2 eV, which is as accurate as can be expected for this type of measurement. The conduction band offset is nearly twice that of the valence band and the interface is illustrated in Fig. 9.18. 

Similar measurements have been made for the interface with a-Ge H and a-Si C H, although there is more disagreement between the various sets of data (Evangelisti 1985). The band offset between a-Si H and crystalline silicon is particularly interesting. Photoemission 

Interest in the electronic properties of interfaces centers around a-Si H/Si3N4, because this combination is used in multilayers (Section 9.4) and field effect transistors (Section 10.1.2). The electronic structure of the interface is illustrated in Fig. 9.18. Apart from the band offset which confines carriers to the a-Si H layer, the distribution of localized interface states and the band bending are the main factors which govern the electronic properties of the interface. The large bulk defect density of the SijN also has an effect on the electronic properties near the interface. Band bending near the interface may result from the different work functions of the two materials or from an extrinsic source of interface charge - for example, interface states. 

Transient photoconductivity measurements of the depletion width, as described in Section 9.1.3, show that there is an electron accumulation layer at the interface with SijN4 (Street et al. 1985b). In contrast, an oxide interface (either a native or deposited oxide) has a depletion layer (Aker, Peng, Cai and Fritzsche 1983). The band bending causes similar changes in the conductance of the films as is described for adsorbed molecules in Section 9.2.2. 

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Some of tliese problems are avoided in heterojunction bipolar transistors (HBTs) [jU, 38], tlie majority of which are based on III-V compounds such as GaAs/AlGaAs. In an HBT, tlie gap of tlie emitter is larger tlian tliat of tlie base. The conduction and valence band offsets tliat result from tlie matching up of tlie two different materials at tlie heterojunction prevent or reduce tlie injection of tlie base majority carriers into tlie emitter. This peniiits tlie use of... 

W.R.L.Lambrecht, B.SegaJl and O.K.Andersen, Self-consistent dipole theory of heterojunction band offsets , Phys. Rev. B41 2813 (1990). 

At the interface of the nitride (Ef, = 5.3 eV) and the a-Si H the conduction and valence band line up. This results in band offsets. These offsets have been determined experimentally the conduction band offset is 2.2 eV, and the valence band offset 1.2 eV [620]. At the interface a small electron accumulation layer is present under zero gate voltage, due to the presence of interface states. As a result, band bending occurs. The voltage at which the bands are flat (the flat-band voltage Vfb) is slightly negative. 

To facilitate good charge transport in an OLED, the organic materials must satisfy three key requirements they must have a high mobility for either electrons or holes, a good injection efficiency from the contact electrode, and suitable band offsets with other organic layers within the device. These processes are discussed in detail by, for example, Kalinowski [73] and Greenham and Friend [74],... 

After discussing semiconductor/metal heterostructures let us now consider the case of semiconductor/semiconductor heterojunchons. The semiconductors will be characterized by the band gaps fg and E, where we arbitrarily assume that E > E, and with valence-HOMO band offset A v, dehned as the difference between VBM and HOMO, and the conduction-LUMO band offset A c, dehned as the difference between CBM and LUMO (see Fig. 4.28). Both AE v and AE c are dehned as posihve. 

On-state insulator reliability due to much smaller conduction band offset of SiC to SiO compared with that of silicon [6-8] ... 

Material Dielectric Constant Critical Field (MV/cm) Operating Field (MV/cm) (MV/cm) Conduction Band Offset to Si (eV) ... 

Calculated conduction band offsets of b b-k oxides to Si reported by Robertson [16]. Source [11]. 

A serious potential problem related to the use of high-k gate dielectrics in SiC power MOSEETs can be encountered due to the much smaller (sometimes-negative) conduction band offsets between SiC and high-k metal oxides compared with silicon dioxide (see Table 5.1). Time-dependent dielectric breakdown (TDDB) and hot... 

Robertson, J., Electronic Structure and Band Offsets in High-K Oxides, IWGI 2001, Tokio, pp. 76-77. 

Fig. 1 Band offsets, i.e., relative HOMO/LUMO energies, for two representative type II polymer junctions, i.e., the TFB F8BT and PFB F8BT heterojunctions. Both are fluorene-based polymer materials [26,33]. In this chapter, we focus on the TFB F8BT junction.

Fig. 8. Calculated and measured emission spectra of YP04 Pr3+ from Peijzel et al. (2005a). The bars in the upper spectrum give information on the positions and intensities calculated for the zero-phonon lines, while the spectrum is obtained by superimposing a Gaussian band (offset 600 cm-1, width 1000 cm-1) on the zero-phonon lines.

Scheme 5.1 Electronic energy levels of selected IH-V and II-VI semiconductors using the valence band offsets from Reference 32 (VB valence band, CB conduction band).

Fig. 1.11. Energy-level alignment between ZnO and (Zn,Mg)0 as determined by optical spectroscopy [100]. The energy-level alignment agrees with a recent indirect determination using photoelectron spectroscopy [105]. The difference of the band gaps is almost fully accomplished by a conduction band offset...

Fig. 4.1. Example energy band diagrams for a semiconductor/metal contact and and a semiconductor p/n-heterocontact. The Schottky barrier height for electrons B,n is given by the energy difference of the conduction band minimum Ecb and the Fermi energy Ey. The valence and conduction band offsets A/ An and AEcb are given by the discontinuities in the valence band maximum Eyb and the conduction band minimum, respectively...

The CdS/ZnO interface is of particular importance in Cu(In,Ga)Se2 thin film solar cells because it is used in the standard cell configuration (Fig. 4.2). A first experimental determination of the band alignment at the ZnO/CdS interface has been performed by Ruckh et al. [102]. The authors have used ex-situ sputter-deposited ZnO films as substrates. The interface formation was investigated by stepwise evaporation of the CdS compound from an effusion cell. Photoelectron spectroscopy revealed a valence band offset of A Vb = 1.2eV. An identical value of 1.18eV has been derived using first-principles calculations [103]. With the bulk band gaps of CdS and ZnO of 2.4 and... 

Table 4.1. Valence band offsets at interfaces of II-VI compounds determined by photoelectron spectroscopy...

To study the influence of the preparation conditions on the interface properties, a number of different interfaces have been prepared. Details of the preparation and the determined valence band offsets are listed in Table 4.2. The experiments include not only both deposition sequences, but also interfaces of Al-doped ZnO films, which have been conducted to elucidate the role of the undoped ZnO film as part of the Cu(In,Ga)Se2 solar cell. Details of the experimental procedures and a full set of spectra for all experiments are given in [70]. Table 4.2 includes a number of interfaces between substrates of undoped ZnO films and evaporated CdS layers (ZOCS A-D). In a recent publication [90] different values were given for the valence band offsets, as the dependence of BEvb(CL) on the deposition conditions was not taken into account in this publication. 

The experimentally determined valence band offsets span quite a large range from A Vb = 0.84 — 1.63 eV. The variation of 0.8 eV is considerably larger than the experimental uncertainty, which is 0.1 eV for most experiments with only a few exceptions. Experiments with a larger uncertainty have been omitted. 

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. 4.23. Valence band offsets for CdS/ZnO, CdS/(Zn,Mg)0 and CdS/ZnO Al interfaces as determined by photoemission experiments. Solid symbols are for sputter deposition of the oxides onto CdS, open symbols are for deposition of CdS onto the oxides. The value from Ruckh et al. [102] is included (diamonds). The circled numbers serve to classify the different values as described in the text...

The interfaces prepared by sputter deposition of ZnO (filled square) or (Zn,Mg)0 (filled triangles) exhibit a valence band offset of AEyb = 1.2 eV. The ZnO and (Zn,Mg)0 films were prepared at room temperature in pure Ar and therefore exhibit a large disorder and a large BEve(Zn 2p3/2)- Compared with the interface with reverse deposition sequence, the offset is 0.35 eV larger. This indicates a rather strong influence of the deposition sequence on the band alignment at the CdS/ZnO interface, which is most likely related to the amorphous nucleation layer when ZnO is deposited onto CdS. 

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