MS-14-P-3377 TEM characterization of In-free transparent conductive oxides: the case of Al:ZnSnO

Novel optoelectronic technologies, such as organic light emitting devices or flexible solar cells, require electrodes that are flexible, transparent and conductive. Materials such as In-Sn-O (ITO) and In-Ga-Zn-O (IGZO) both gather those three required properties and have been extensively studied in the last years. However the downside is that they are made out of scarce and expensive indium. Therefore there is a need from the market to find alternative solutions. Amorphous transparent oxide semiconductors are a relatively new class of materials, which could fulfil the requirements for replacing In in transparent conductive oxides (TCO), and in particular Zn-Sn-O (ZTO) is an excellent candidates as it is inexpensive, abundant and non-toxic.

This work presents the development and characterization of amorphous Al-doped ZTO grown by co-sputtering ZnO:Al and SnO₂ with varying Sn/Zn composition ratio. 150 nm layers were simultaneously deposited on glass substrates and TEM Cu grids with thin C film for top-view characterization by STEM (on FEI Technai Osiris, Fig. 1). Same conditions were used for deposition of 300 nm thick layers for optical and electrical characterization. Hall mobility and free carrier concentration were determined by Hall-effect measurements using the Van der Pauw configuration. Optical transmission and absorptance spectra in the range from 320 to 2000 nm were determined using a UV-Vis-NIR spectrophotometer equipped with an integrating sphere. TEM lamellae were extracted by FIB for x-section.

A clear variation of the structural, electrical and optical properties was observed, indicating an ideal Sn/Zn ratio of 4.3 (measured by EDX), which was then used to deposit uniform layers by tuning the sputtering power of each target and rotating the substrate. Furthermore, although amorphous, the layer presented a columnar structure, which could explain a lower conductivity compared to crystalline layers (Fig.2). Hydrogen is known to act as a shallow electron donor in several TCOs and thus to improves their electrical properties. H₂ plasma treatments were applied to the AZTO films under temperature and time conditions ranging from 50 to 200 °C and from 1 to 5 minutes, respectively. Electrical measurements show an improved behaviour after treatment, whereas optical transmittance degrades. SEM observations reveal that reduction of tin oxide is the responsible for the degraded optical properties. SIMS was also performed to assess a possible H penetration into the layer, but only revealed the oxygen depletion in the top 100 nm of the layer due to oxide reduction. This shows that EM studies play a key role in linking film morphology and physical processes occurring during H₂ plasma treatment as well as electrical and optical properties.