In Situ Investigation of Crystallization Pathway of Solution-Processed Magnesium-Alloyed Zinc Tin Oxide Thin Films

Multi-cation metal oxides are a key material class for applications in solar cells, electronics and sensing technologies, as well as batteries and fuel cells. For successful use in these applications, it is crucial to control the material crystallinity. Crystalline metal oxides are valuable for gas sensing applications, where improvement in sensing qualities is linked to the material’s crystal structure and quality. Amorphous-phase metal oxides are commercially used for dielectrics in CMOS and as semiconductors in the display backplane industry due to their excellent electrical properties. In particular, zinc oxide alloys exhibit a wide, direct bandgap in the UV. Here, we choose as a base material amorphous zinc tin oxide, an n-type semiconductor with mobility of about 22 cm²V^-1s^-1 and a bandgap of about 3.0 eV [1,2]. By alloying this material with magnesium, we aim to further increase its bandgap and improve the insulating property of this quaternary alloy. To obtain these improvements, we need to understand and control its crystallization.<br/><br/>Therefore, in this work, we use <i>in situ</i> heating grazing incident x-ray diffraction (GIXRD) to examine the effects of magnesium alloying on crystallization of zinc tin oxide (ZTO) thin films. Four ZTO films - one unalloyed and three alloyed with Mg - are deposited on Si/SiO₂ substrates using solution processing. The Mg metal ratio, [Mg]/([Mg]+[Zn]+[Sn]), was 2.5%, 7.5%, 20% for the three alloyed samples. All films were x-ray amorphous after the deposition process. In order to investigate the crystallization pathways and the effect of Mg incorporation, films were slowly annealed up to and beyond their crystallization temperatures. The evolution of crystalline phases was observed by <i>in situ</i> heating during x-ray diffraction using Rigaku SmartLab. XRD data was collected from 500°C to 750°C using 20°C increments to extract crystallization temperatures. The data for higher temperatures was collected from 550°C to 1050°C using 100°C increments by using new samples. Heating time of 1 hour was set between each increment.<br/><br/>All of the films have a Zn-to-Sn metal ratio of 7:3 which is close to the stoichiometric ratio in inverse spinel Zn₂SnO₄. For this stoichiometry, the inverse spinel is expected to be the most stable phase. However, our <i>in situ </i>XRD shows that the unalloyed film crystallizes predominantly in an orthorhombic structure around 650°C. Similar results have been seen in low-dimensional ZTO structures obtained by solution processing or hydrothermal synthesis [3,4]. At temperatures above 750°C, the unalloyed film begins to show spinel peaks, and this matches the theoretical thermodynamic stability calculations of ZTO systems [5]. When Mg is introduced into the system, a mixed phase film containing inverse spinel and orthorhombic phases forms at temperatures below 750°C. As the Mg percentage increases to 20%, the orthorhombic x-ray peaks completely vanish and the thin film crystallizes into a single spinel phase. This is expected, as spinel is the stable phase for the Mg_2-xZn_xSnO₄ system. After a slow cooling process down to 50°C, the spinel peaks disappear from the unalloyed ZTO thin film, whereas they remain for the 20% Mg-incorporated sample. To summarize, ZTO without Mg first crystallizes into an orthorhombic structure before converting toward the spinel phase at higher temperature, whereas Mg incorporation into ZTO strongly promotes the formation of spinel-phase crystals, over orthorhombic. By tailoring the Mg content, the crystal structure and crystallization dynamics can be controlled to obtain the desired materials properties and function.<br/><br/>[1] Y. Son <i>et al.</i>, <i>J. Mater. Chem. C</i> <b>5</b>, (2017).<br/>[2] C. Allemang <i>et al.</i>, <i>Adv. Electron. Mater.</i> <b>6</b>, 2000195 (2020).<br/>[3] A. Rovisco <i>et al.,</i> <i>ACS Appl. Nano Mater.</i> <b>1</b>, 3986–3997 (2018).<br/>[4] Y. Chen <i>et al.,</i> <i>Nanotechnology</i> <b>23</b>, 415501 (2012).<br/>[5] J. Lee <i>et al.</i>, <i>Journal of Materials Chemistry C</i> <b>1</b>, 6364 (2013).