Phase Change Ge-Rich Ge–Sb–Te/Sb2Te3 Core-Shell Nanowires by Metal Organic Chemical Vapor Deposition

Abstract

Ge-rich Ge–Sb–Te compounds are attractive materials for future phase change memories due to their greater crystallization temperature as it provides a wide range of applications. Herein, we report the self-assembled Ge-rich Ge–Sb–Te/Sb2Te3 core-shell nanowires grown by metal-organic chemical vapor deposition. The core Ge-rich Ge–Sb–Te nanowires were self-assembled through the vapor–liquid–solid mechanism, catalyzed by Au nanoparticles on Si (100) and SiO2/Si substrates; conformal overgrowth of the Sb2Te3 shell was subsequently performed at room temperature to realize the core-shell heterostructures. Both Ge-rich Ge–Sb–Te core and Ge-rich Ge–Sb–Te/Sb2Te3 core-shell nanowires were extensively characterized by means of scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, Raman microspectroscopy, and electron energy loss spectroscopy to analyze the surface morphology, crystalline structure, vibrational properties, and elemental composition.

Interface Analysis of MOCVD Grown GeTe/Sb2Te3 and Ge-Rich Ge-Sb-Te/Sb2Te3 Core-Shell Nanowires

“…Figure 1 a,b shows the top-view SEM images of the GT and GGST core NWs grown on Si and SiO 2 substrates, respectively. It is useful to recall here the results of the morphological analysis from our previous works [ 30 , 31 ], Au catalyst NPs were observed at the NWs’ tips, confirming that the growth occurs via the VLS mechanism; the as-grown GT NWs were of an average length and diameter of about 5 µm and 50 nm, while the GGST NWs were about 1.40 µm long and 60 nm in diameter, respectively; the diameter of the NWs turned out to be directly dependent on the size of the catalyzed Au NPs [ 30 , 31 ]. Figure 1 c,d depicts the GT/ST and GGST/ST core-shell NWs having shells of about 10 nm thickness, and the insets show the magnified view of a single NW.…”

“…Figure 1 c,d depicts the GT/ST and GGST/ST core-shell NWs having shells of about 10 nm thickness, and the insets show the magnified view of a single NW. The 10 nm shell revealed a lower granularity in comparison to the 30 nm shell [ 30 , 31 ]. The shell was continuously deposited all over the core NWs.…”

“…The growth of the GGST and GT core NWs was carried out with an Aixtron AIX200/4 MOCVD reactor (Aixtron SE, Herzogenrath, Germany), employing the Vapor–Liquid–Solid (VLS) mechanism catalyzed by Au nanoparticles (NPs), with average sizes of 20, 30, and 50 nm. Details about the growth methodology have been previously reported [ 30 , 31 ]. The GT core NWs were obtained with optimized reactor temperature (T), reactor pressure (P), and a growth duration (t) of 400 °C, 50 mbar, and 60 min, respectively.…”

“…The required precursor pressures were 4.42 × 10 −3 mbar for tetraisdimethylamino germanium (Ge[N(CH 3 ) 2 ] 4 , TDMAGe), 5.12 × 10 −5 mbar for antimony trichloride (SbCl 3 ) and 6.98 × 10 −3 mbar for diisopropyl telluride ((C 3 H 7 ) 2 Te, DiPTe. The chemical composition of the GGST NWs has been previously estimated by electron energy loss spectroscopy (EELS) to be 35% Ge, 10% Sb, 55% Te, corresponding to the atomic ratio of Ge:Sb:Te = 3:1:5, with the Ge concentration higher than regular Ge 2 Sb 2 Te 5 [ 30 ].…”

“…Core-shell NWs formed by two chalcogenide materials with different phase change characteristics, namely Ge 2 Sb 2 Te 5 /GeTe, have also been proposed as multi-level PCM memory cells [ 29 ]. Indeed, the growth and the properties of Ge-rich Ge-Sb-Te/Sb 2 Te 3 (GGST/ST) and GeTe/Sb 2 Te 3 (GT/ST) PCM core-shell NWs have been recently reported by our group [ 30 , 31 ].…”