Electronic Structure of ZnO Surfaces

Motivated by the application of ZnO in gas sensors and catalysis and by the more general desire to understand surface properties of ionically bonded solids, electronic properties of ZnO surfaces have been investigated for many years [20,76-80]. An overview of the early work on ZnO surface properties is included in the book of Henrich and Cox [81]. 

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 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 

The variation of the ionization potential with surface orientation evident from the literature data corresponds well with a systematic study by Ranke using a cylindrical GaAs single crystal [91]. This revealed that the electron affinity of the cation terminated (111) surface of GaAs (corresponding to wurtzite (0001)) is 0.4-0.5eV lower than those of other surface terminations. The same variation is observed at single crystal surfaces of CdTe and 

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