Direct Imaging of Dopant Distribution in Polycrystalline ZnO Films.

Two fundamental requirements of transparent conductive oxides are high conductivity and low optical absorptance, properties strongly dependent on the free-carrier concentration of the film. The free-carrier concentration is usually tuned by the addition of dopant atoms; which are commonly assumed to be uniformly distributed in the films or partially segregated at grain boundaries. Here, the combination of secondary ion mass spectroscopy at the nanometric scale (NanoSIMS) and Kelvin probe force microscopy (KPFM) allows direct imaging of boron-dopant distribution in polycrystalline zinc oxide (ZnO) films. This work demonstrates that the boron atoms have a bimodal spatial distribution within each grain of the ZnO films. NanoSIMS analysis shows that boron atoms are preferentially incorporated into one of the two sides of each ZnO grain. KPFM measurements confirm that boron atoms are electrically active, locally increasing the free-carrier concentration in the film. The proposed cause of this nonuniform dopant distribution is the different sticking coefficient of Zn adatoms on the two distinct surface terminations of the ZnO grains. The higher sticking coefficient of Zn on the c+ surface restricts the boron incorporation on this side of the grains, resulting in preferential boron incorporation on the c- side and causing the bimodal distribution.

Height-resolved quantification of microstructure and texture in polycrystalline thin films using TEM orientation mapping.

A method is presented for the quantitative investigation of microstructure and texture evolution in polycrystalline thin films based on in-plane automated crystal orientation mapping in transmission electron microscopy, from the substrate up. To demonstrate the method we apply it to the example of low pressure metal-organic chemical vapor deposited ZnO layers. First, orientation mapping is applied to standard cross-section and plan-view transmission electron microscopy samples of films, illustrating how plan-view samples both reduce the occurrence of grain overlap that is detrimental to reliable orientation mapping and also improve sampling statistics compared to cross-sections. Motivated by this, orientation mapping has been combined with a double-wedge method for specimen preparation developed by Spiecker et al. (2007) [1], which creates a large area plan-view sample that traverses the film thickness. By measuring >10,000 grains in the film, the resulting data give access to grain size, orientation and misorientation distributions in function of height above the substrate within the film, which are, in turn, the inputs necessary for quantitative assessment of growth models and simulations. The orientation data are directly related to microstructural images, allowing correlation of orientations with in-plane and out-of-plane grain sizes and shapes. The spatial correlation of the entire data set gives insights into previously unnoticed growth mechanisms such as the presence of renucleation or preferred misorientations. Finally, the data set can be used to guide targeted, local studies by other transmission electron microscopy techniques. This is demonstrated by the site-specific application of nano-beam diffraction to validate the presence of coherent [21̄1̄0]/(011̄3) twin boundaries first suggested by the orientation mapping.