MOVPE and characterization of rhombohedral Ge1Sb2Te4/Si(111)

Chalcogenide alloys in the Ge-Sb-Te system exhibit a large contrast in their optical and electrical properties when their phases switch from the amorphous to the meta-stable cubic crystalline state. Up to now, they have been applied to rewritable optical data storage media[1] such as compact discs (CDs), digital versatile discs (DVD) and blu-ray discs (BRD). Currently, the exploitation of their large change in resistivity upon switching is in the focus of attention for next generation non-volatile memories. However, a high energy imput is required for switching and the resistance especially of the amorphous state drifts with time[2]. Therefore, there is a growing interest in memory material systems switching between two distinct crystalline states avoiding the amorphous state and melting and recrystallization processes.The so-called interfacial phase change memory (iPCM) based on GeTe-Sb2Te3 superlattices[3] was reported to be a suitable approach to reduce energy consumption in data storage applications. Here, the resistance change is reported to be evoked by a displacement of Ge atoms at the GeTe-Sb2Te3 interfaces and the structural changes are field induced. Only a fraction of the energy for switching is consumed. The hexagonal superlattices are deposited by sputter deposition or molecular beam epitaxy and are highly textured. In this work, epitaxial Ge-Sb-Te films crystallizing in the thermodynamically stable rhombohedral phase have only lately been accessible. Ge1Sb2Te4 layers were deposited by the industrially relevant method MOVPE on Si (111) substrates[4]. The alloy is expected to have properties similar to those of the superlattices mentioned above. Therefore, the characteristics of this novel material and its potential for non-volatile memory applications is of great interest. In this contribution, we will report shortly on the growth and then mainly center on the structural and electrical characteristics of this new epitaxial material.Growth was carried out in a single wafer horizontal reactor (AIX 200, AIXTRON) on Si (111) substrates using nitrogen as the carrier gas[5]. Further information is given in[4]. The samples were characterized structurally by X-ray diffractometry, aberration corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy dispersive X-ray (EDX) spectroscopy and electron back scatter diffraction (EBSD). The intrinsic electrical characteristics of the material were characterized by removing Ge1Sb2Te4 platelets from their growth substrate and transferring them to coplanar striplines. It was found, that Ge1Sb2Te4 grows epitaxially and its unit cell consists of 3 seven layered blocks each of which is separated by van der Waals gaps. Alternating anion and cation planes as well as the gaps are found in parallel to the Si (111) substrate surface. Resistive switching measurements were performed without any detectable structural deterioration. Current densities exhibit values from 10-6 down to 10-11A/µm3 at 1V bias switching. Complete structural and electrical characterization will be presented