Phase formation and growth behavior of Co‐Ge thin films

Abstract

The potential use of germanide thin films as self‐aligned metallization in Ge‐based Complementary Metal‐Oxide Semiconductor (CMOS) technology has drawn interest in the solid‐state reactions of Ge and metal films [1‐3]. In order to integrate these germanide thin films as a contact material, a complete analysis of the solid‐state reactions of amorphous Ge (a‐Ge) and metals is required. The solid‐state reactions in the Co‐Ge thin films systems previously examined by in situ x‐ray diffracting annealing experiments only probed the crystal structures present in the film and did not identify the morphological variations of the process [1]. Furthermore, to the authors knowledge previous works Co‐Ge thin film systems have only analyzed systems with Ge in excess of Co. In this study, the phase formation and crystallization behavior of a‐Ge and Co thin film layers are investigated by ex situ transmission electron microscopy (TEM) coupled with in situ x‐ray diffraction (XRD) annealing experiments. Four different specimens were prepared, alternating the stacking order and film thickness (200 nm and 30 nm), in order to explore the influence of the free surface on the crystallization and the phase reactions with Co in excess of a‐Ge. Thin film layers of a‐Ge and Co were deposited via electron beam evaporation onto (100) silicon wafers with the native oxide film present on the surface (~1–2 nm). The thin film specimens were characterized by TEM before and after annealing during the phase formation process. Bright field TEM images of the 200 nm Co on top of 30 nm a‐Ge stack are shown in Figure 1: (a) as‐deposited, (b) annealed at 280°C and (c) 400°C. Upon heating to 280°C, the a‐Ge layer reacts with the Co layer to form CoGe, which then undergoes an additional phase transformation to CoGe 2 with continued heating to 400°C. The influence of the free surface ( fs ) on the crystallization process was examined on the 200 nm a‐Ge and 30 nm Co film stack. The scanning electron microscopy images of the specimens after annealing to 400°C are shown in Figure 2. (a) The specimen with a‐Ge at the free surface (a‐Ge fs ) shows two distinct contrast indicating the presence of two different crystalline phases near the surface, while (b) the specimen with Co at the free surface (Co fs ) shows a more homogeneous contrast throughout the surface. Both specimens have voids visible at the surface, which formed during annealing. By viewing the specimens in cross‐section, the presence of large voids is visible in (c) the specimen a‐Ge fs , but the voids are not present in (d) the specimen with Co fs . The resulting formation of voids in only one of two specimens indicates the free surface has a direct influence of the phase formation process.