Outlook Higher Electron Mobilities in Zinc Oxide

3 Outlook Higher Electron Mobilities in Zinc Oxide 

As mentioned in the preceding section the mobility of degenerately-doped zinc oxide (as well as of other TCO materials and semiconductors) is limited by ionized impurity scattering in homogeneously-doped materials. Since about 30 years it is well known that the mobility can be increased by the so-called modulation doping method, introduced by Dingle et al. [179] for GaAs/C.ai, Af As superlattice structures (for a review see [180]). 

The variation of the work function in the vertical direction of multilayers was investigated by Kelvin probe force microscopy (KPFM) on a fracture cross-section of a film deposited on a silicon substrate, shown in Fig. 2.18. This method is described in detail in [184]. A variation of the work function of about 60-100 meV is clearly seen from the bright-dark contrast as well 

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