Combining surface-sensitive techniques and density functional theory to unravel the structural properties and the chemical composition of Sn-Sb-Te thin films. The role of oxidation

V. BILOVOL; H. H. MEDINA; A. V. GIL REBAZA; A. M. MUDARRA; L. A: ERRICO

Trujillo (virtual)

Simposio; III Simposio Internacional de Nanociencia y Nanotecnología (Salud-Medio Ambiente-Energía); 2021

Universidad de Trujillo

Conferencista invitado para charla plenaria.Chalcogenide-based materials (those that contain one of the elements from VI group of the periodic table, such as S, Se, Te) are versatile systems for many technological applications but, when presented as thin films, such materials are particularly attractive. One example of such applications where chalcogenide-based materials have leading positions in terms of fulfillment are phase change- based memories (PCM). The big data era requires real-time processing of huge amounts of information and hence generating a need for new computing technologies. In this concern, PCM´s are the most promising candidates for application in the emerging nonvolatile memory technologies. These memories are fast, scalable, and present low power consumption. Particularly, PCM alloys obtained as thin films exhibit a fast and reversible phase transformations (switching) between crystalline and amorphous states. The amorphous phase exhibits a high electrical resistivity, in contrast to the crystalline phase, characterized by a lower resistivity (the electrical conductivity of this type of materials can change in a factor 107 when it transforms from the amorphous to the crystalline phase). One material that received considerable attention due to its potential application as a base material in phase change memories is Ge2Sb2Te5 (GST). In the amorphous-to-stable crystalline transition in thin films, GST presents at first a metastable cubic phase. This cubic version of the GST thin films underlies the fast amorphous-crystalline transition exploited in the PCM applications. In this process, the crystallization temperature is a critical parameter. In this concern, it was reported that the crystallization temperature of GST can be changed by impurities such as O, N, or C. and then devices based on such materials can be affected significantly by presence of oxygen improving or worsening their performance. For instance, it was found that in uncapped GST films Sb and Ge get oxidized and, in the case of GST films exposed to air, the crystallization occurred at a lower temperature than the non-exposed ones (ΔT=20 C). On the other hand, the controlled introduction of metallic impurity atoms such as Al, Cu, Ag or Sn in the chalcogenide hosts is expected to improve the performance of the PCM-based memories. Therefore, the study of such films at the basic level becomes an important task to have precise control over the properties of the material.In the last years, attention is being devoted to another compound of the chalcogenide family, SnSb2Te4 (SST). SST also exhibits a metastable cubic phase when transitioning from amorphous to stable trigonal lattice with an attractive transition temperature. But, as far as we know, no similar studies have been reported as in the case of GST. To elucidate the properties of SST films, the detailed understanding of their chemical composition and its structure are of fundamental importance. In this work three surface-sensitive techniques, grazing incidence x-ray diffractometry (GIXRD), Mössbauer Spectroscopy (MS) and x-ray photoelectron spectroscopy (XPS), are employed to study structural and chemical properties of uncapped Sn-Sb-Te thin films with nominal composition SnSb2Te4 grown by pulsed laser deposition technique and air exposed. GIXRD showed that the grown film was polycrystalline and adopts a NaCl-type crystalline structure. 119Sn MS demonstrated the presence of two tin environments, the expected one for Sn(II) located at the sites of the NaCl type of the Sn-Sb-Te film, and also a fraction of Sn(IV) that was attributed to the presence of amorphous or embryos of tin dioxide. XPS shows that uncapped Sn-Sb-Te films are prone to oxidation and that the topmost layers (at least about 10 nm tickness) of the studied film exposed to oxygen become depleted of Te and transformed into Sn-O, Sb-O and Te-O. Underneath approximately 10 nm and down to, at least, 40-50 nm, the sample is mostly Sn-Sb-Te crystalline phase keeping the inalterable distribution of the elements albeit with deviation from the stoichiometric SnSb2Te4 with a notorious lacking of Te. Density functional theory-based calculations support the hypothesis that Te-vacancies sites are occupied by oxygen atoms during the natural oxidation process of Sn-Sb-Te film. Additionally, our calculations demonstrated that only the substitution of Te atoms by oxygen ones induces a semiconducting behavior of the otherwise metallic Sn-Sb-Te host.