Undoped ZnO

In Fig. 3.8, typical IRSE spectra of a ZnO bulk sample and a ZnO film on sapphire are plotted. In the /- -spectrum of the ZnO bulk sample a plateau with S 45° can be seen, which corresponds to the bands of total reflection (reststrahlen bands), which occurs between the Ei(TO)- and E (LO)-mode frequencies [123]. The small dip within the plateau is caused by the loss in p-reflectivity, and localizes the Ti(LO)- and E (LO)-mode frequencies. The derivative-like structure in the / -spectrum of the bulk ZnO sample at u 650cm-1 is caused by the anisotropy Re Re j (Sect. 3.3) [38]. 

In the / -spectrum of the ZnO thin film, a similar plateau as in the 3 -spectrum of the ZnO bulk sample is present. However, the phonon modes of the sapphire substrate introduce additional features, for example atw 510, 630, and -900 cm 1 [38,123]. The spectral feature at w 610 cm-1 is called the Berreman resonance, which is related to the excitation of surface polari-tons of transverse magnetic character at the boundary of two media [73]. In the spectral region of the Berreman resonance, IRSE provides high sensitivity to the A (LO)-mode parameters. For (OOOl)-oriented surfaces of crystals with wurtzite structure, linear-polarization-dependent spectroscopic 

Temperature-dependent Raman data were reported for the E -mode of flux-grown ZnO platelets in the temperature range from T 15 K to T 1050K [127], and for the T -mode, the E -mode, and the MP-mode at w 332 cm-1 of a ZnO bulk sample in the temperature range from T 300 K to T 700 K [43] In Fig. 3.11 the unpolarized Raman spectra and the temperature-dependence of the phonon-mode frequencies from [43] are 

A and B are model parameters representing the high-temperature linear slope (du /8T t oo) and effective phonon-mode temperature (l hui(Q) jfc ), respectively. w(0) is the phonon-mode frequency at T = OK. Table 3.5 summarizes the best-model parameters reported in [43]. In [127], a linear temperature-dependence with dw[E2 ]/ T = —1.85 x 10 2cm 1 K 1 was reported for temperatures above RT. 

The pressure-dependence of ZnO phonon-mode frequencies measured by Raman scattering was reported in [37, 130]. From that the Griineisen parameters 7j 

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G. Xiong, J. Wilkinson, B. Mischuck, S. Tiizemen, K.B. Ucer, and R.T. Williams, Control of p- and n-type conductivity in sputter deposition of undoped ZnO, Appl. Phys. Lett., 80 1195-1197, 2002. 

The mobility and resistivity data of single crystalline zinc oxide samples (measured at room temperature) from different authors, which were reported from 1957 to 2005, are displayed in Fig. 2.6 as a function of the carrier concentration (part of these data were taken from [67]). Undoped ZnO crystals exhibit carrier concentrations as low as 1015 cm-3, while indium-doped crystals reach carrier concentrations up to 7 x 1019cm-3. The mobility data show a large scattering between carrier concentrations of 1017 to 5 x 1018cm-3. This is caused by the fact that zinc oxide is a compound semiconductor that is not as well developed as other semiconducting compounds. For instance, only... 

Figure 2.14 shows a compilation of mobility and resistivity data of doped and undoped ZnO films prepared in the last 25 years as a function of the carrier... 

Fig. 2.14. (a) Mobilities and (b) resistivities of doped and undoped ZnO films vs. the carrier concentration (part of the data were taken from [67,157]). The films were deposited by magnetron sputtering (filled square, open square, open triangle down), MOCVD (open diamond) or pulsed-laser deposition (open circle). The dashed lines (B) are a theoretical estimation of the mobility and the resistivity of Bellingham et al. [158]. The full lines (F) are the calculated data from the semiempirical fit according to (2.24), while the dotted lines (1, 2) represent theoretical mobility calculations according to (2.15-2.22)... 

Fig. 4.2. Structure and energy band diagram of a Cu(In,Ga)Se2 (CIGS) thin-film solar cell. The ZnO window layer typically consists of a combination of a nominally undoped ZnO and a highly doped ZnO layer...

Fig. 4.6. X-ray photoelectron spectra of undoped ZnO (a—c) and of Al-doped ZnO (d—f) prepared by magnetron sputtering. The spectra are excited with monochromatic A1 Ka radiation (hv = 1486.6 eV). ZnO Al films are prepared from a target containing 2 wt % Al. The films are prepared with 100 % Argon as sputter gas either at room temperature (a and d) or at a substrate temperature of 400° C (b and e). Spectra (c) and (f) are recorded from films deposited onto samples held at room temperature in a sputter gas mixture of 50 % argon and 50 % oxygen...

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 ionization potentials of the undoped ZnO films prepared at room temperature are 6.9eV for films deposited in pure Argon and raise to 7.7eV... 

Fig. 4.14. Gracing incidence (

The variation in BEvb(CL) and in the binding energy difference between the two core levels is explained by a superposition of two different effects. To begin with, undoped ZnO films show the highest BEvb(CL) when the films are deposited at room temperature with pure Argon as sputter gas. The... 

Fig. 4.20. Evolution of the CdS and ZnO valence band maxima as derived from the binding energies of the core levels by subtracting BEvb(CL) values determined from the CdS substrate and the thick ZnO film, respectively [71], The different evolution of the Zn 3d and O Is binding energies is attributed to an amorphous structure of the ZnO layer during the initial growth. The thickness of the amorphous layer is 2 nm. The ZnO films were deposited by magnetron sputtering from an undoped ZnO target at room temperature using 5 W dc power...

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 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. 

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. 

A set of spectra, recorded during stepwise deposition of ZnO onto a decapped Cu(In,Ga)Se2 surface is shown in Fig. 4.26. The ZnO film has been sputtered from an undoped ZnO target using 15 W dc power but otherwise the same standard deposition conditions, which have been used for investigation of the CdS/ZnO interface. On a first inspection no changes in the shape of the peaks is observed during deposition. A chemical reaction between Cu(In,Ga)Se2 and ZnO is, therefore, not evident. 

Fig. 4.26. Core levels, Zn LMM Auger level and valence bands recorded using monochromatic A1 Ka radiation during sputter deposition of undoped ZnO onto a decapped Cu(In,Ga)Se2 sample. The deposition times are indicated in seconds...

Fig. 4.30. Evolution of valence band maxima in dependence on ZnO deposition time as derived from core-level binding energies of the spectra shown in Fig. 4.29. The ZnO films were deposited by magnetron sputtering from an undoped ZnO target at room temperature using 15 W dc power. Core level to valence band maxima binding energy differences are comparable to those presented in Fig. 4.15 for ZnO and to those given in [36] for Cu(In,Ga)Se2. The different evolution of the Zn2p and O Is derived valence band positions for ZnO deposition times indicates the presence of an amorphous nucleation layer, as already discussed in Sect. 4.3.2...

Fay et al. [3] used a pressure lower than 1 Torr, but also BoHf, as dopant gas. They, however, did not observe any substantial modification of crystal orientation by doping. Indeed, as illustrated in Fig. 6.35b, the (1120)/(1010) peak ratio observed here is quite high for undoped ZnO films deposited at 155°C, and remains at a high value even when the dopant concentration is increased. Moreover, in this case, the deposition rate does not vary significantly with the introduction of I >21 h , as is shown in Fig. 6.36. 

Fig. 6.43. Variation of band gap Es measured on LP-CVD ZnO B films deposited at 155° C and 0.5mbar and with various doping levels, in function of the carrier density N. The dashed line and the full line are the predicted variations of Eg taking into account the Burstein-Moss effect alone, and both the Burstein-Moss and the band gap narrowing effects, respectively. Eo is the band gap of undoped ZnO and is set for the evaluation here to 3.3 eV. Reprinted with permission from [33]...

Fig. 6.45. Carrier mobility // of various LP-CVD ZnO B samples deposited at IMT Neuchatel as a function of the grain size parameter S. The doping series is the one for which the gas doping ratio B2H6/DEZ has been varied. The doped and undoped series are those for which doped and undoped ZnO films were deposited under a variation of deposition parameters, like substrate temperature or gas ratio HaO/DEZ...

Fig. 7.4. XRD 20 — io scans of PLD grown, nominally undoped ZnO thin films on c-plane (top), a-plane (center), and r-plane (bottom,) sapphire substrates, measured with Ni-filtered Cu-Ko, radiation. The ZnO films were grown at 0.01 mbar O2 and about 650° C...

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