Valence band offset

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... 

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. 

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).

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. 

The valence band offsets for deposition of ZnO Al on CdS are 1.6eV when the ZnO Al films are prepared using pure Ar, which leads to degenerately doped material (CSZA-A and CSZA-D). Deposition of ZnO Al films with a sputter gas containing 10 % 02 results in a low doped material (see Sect. 4.2.3.1). For such deposition conditions (CSZA-B) a valence offset of 1.2eV is obtained. 

The valence band offsets determined for the ZnO Al/CdS interfaces (1.4 0.1 eV) are 0.2-0.4eV larger than the values obtained for interfaces where undoped ZnO or (Zn,Mg)0 films have been used as substrate. This points toward an influence of the A1 content in the ZnO film on the band alignment. An explanation for this cannot be given yet. 

The results presented in this section further illustrate that there is a considerable dependence of the band alignment at the CdS/ZnO interface on the details of its preparation. An important factor is the local structure of the ZnO film. There is considerable local disorder when the films are deposited at room temperature in pure Ar, deposition conditions that are often used in thin film solar cells. It is recalled that the disorder is only on a local scale and does not affect the long range order of the films, as obvious from clear X-ray diffraction patterns recorded from such films (see discussion in Sect. 4.2.3.3). Growth of sputter deposited ZnO on CdS always results in an amorphous nucleation layer at the interface. The amorphous nucleation layer affects the valence band offset. 

To give an individual value for the band alignment is not possible. Structurally well-ordered interfaces, which are obtained e.g., by deposition of CdS onto ZnO layers deposited at higher temperatures and/or with the addition of oxygen to the sputter gas, show a valence band offset of A TV is = 1.2 eV in good agreement with theoretical calculations [103]. Sputter deposition of undoped ZnO at room temperature in pure Ar onto CdS also leads to a valence band offset of 1.2 eV. In view of the observed dependencies of the band offsets this agreement is fortuitous, as the influence of the local disorder and of the amorphous nucleation layer most likely cancel each other. 

To determine the valence band offset at the interface, the binding energies of the core levels are plotted in dependence on deposition time in Fig. 4.30. Core... 

According to our experience, it is more difficult to determine a reliable valence band offset for the Cu(In,Ga)Se2/ZnO interface than for the CdS/ZnO interface. This is related to the lower substrate core-level intensities because of the presence of multiple cations. The substrate intensity might, therefore, be already completely suppressed when the Zn 2p and the O Is derived valence band maxima (see filled circles and squares in Fig. 4.30) reach the same value, and, therefore, reflect a proper ZnO valence band maximum (end of the amorphous nucleation layer). This difficulty is not present in the data set in Fig. 4.30 and for a deposition time of 64 s a valence band offset of A/ y vis = 2.15 0.1 eV can be determined. In another experiment, we have derived a slightly smaller valence band offset of AEyb = 1.98 0.2 eV [70]. The larger uncertainty is due to the above-mentioned difficulties. 

The valence band offsets determined in our group are very close to values reported in literature. Platzer-Bjorkman et al. have determined A/ A is = 2.2 0.2 eV for ALD10-ZnO deposited onto CuInSe2 or Cu(In,Ga)Se2 [143, 144]. Weinhardt et al. give a valence band offset for ILGAR11-ZnO on CuIn(S,Se)2 substrates of A Vb = 1.8 0.2eV [60]. The comparable values for the different interface preparation and substrate compositions suggest a rather small variation of the band alignment with these parameters. 

Fig. 4.33. Energetic positions of valence band maxima in the Cu(In,Ga)Se2/ CdS/ZnO sequence showing the transitivity of band alignment. The valence band offsets for CdS/ZnO and Cu(In,Ga)Se2/ZnO are discussed in this chapter. The valence band offset for the Cu(In,Ga)Se2/CdS interface is taken from literature [36,106,123,125]...

For all investigated interfaces the valence band offset can be estimated by the alignment of the Fermi levels of thick In2S3 and ZnO Al prepared under the same conditions used in the interface experiment. This is related to the very small band bending observed at the interfaces (<0.2eV) and concurs... 

It has been found for various III/V heterostructures that bandoffsets can be obtained via an internal reference rule for transition metals [18,19], This method was applied to predict the valence-band offset of the GaN/AIN heterostructure [20], However, as demonstrated by Heitz et al [10], the internal reference rule fails for the GaN/GaAs heterostructure. 

Cl. 3 Band offsets at interfaces between AIN, GaNand InN TABLE 1 Valence band offsets for GaN and AIN interfaces. 

TABLE 3 Valence band offsets for interfaces between AN and InN. 

TABLE 5 Valence band offsets between III-V nitrides and various substrates. 

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