Characterization of defects in thick semiconducting amorphous YMnO₃

In recent years amorphous semiconducting oxides have gained a lot of attention both from science and industry [1]. The rapid development in this field can be illustrated with the example of amorphous oxide semiconductor thin film transistors. Although invented only approximately one decade ago, they are already commercialized for active-matrix liquid crystal display backplane applications. Comparing to single-crystalline and polycrystalline materials, processing of amorphous oxides benefits from the easiness of production on the large, both plastic and glass areas (e.g. for solar cells) at low temperatures. In addition, thanks to amorphous nature, the problem of scattering (e.g. of light or carriers) and trapping at the defects of crystal structure and/or grain boundaries is avoided. “Amorphous”, however, does not mean “defect free” and as in the case of crystals, defects significantly influence the electronic, vibrational, optical and transport properties of the material. In this work, we present an attempt to identify and characterize defects in amorphous YMnO₃, a well-known multiferroic when in crystalline form [2], deposited on sapphire by multiple techniques. We use X-ray diffractometry, Rutherford backscattering spectroscopy, scanning electron and atomic force microscopy to define composition, structural and surface properties of the oxides. Optical properties in the range between 210 and 1000 nm are measured and modelled by variable angle spectroscopic ellipsometry. Open volume defects are probed by positron annihilation spectroscopy (PAS). Comparison of results obtained on as-grown films and films annealed at T= 400°C for 5 and 10 hours in air allows for qualitative description of defects formed in YMnO₃. According to the results of PAS presented in Figure 1, we consider following scenarios: (1) two types of negatively charged, vacancy-type defects are formed during the growth of which one was annealed out and (2) one type of negatively charged, vacancy-type defect is formed during the growth which agglomerates to bigger and stable form during annealing. Our research enables qualitative description of the defects in the amorphous semiconducting films. Obtained results might be used for (1) further optimization of film processing in order to reduce amount of defects, or for (2) defect engineering in order to prepare a film with tuned properties.
[1] T. Kamiya, H. Hosono, NPG Asia Mater. 2 (2010) 15-22
[2] B. B. Van Aken, T. T. M. Palstra, A. Filippetti, N. A. Spaldin, Nat. Mater. 3 (2004) 164-170