MOCVD Growth of GeTe/Sb 2 Te 3 Core–Shell Nanowires

: We report the self-assembly of core–shell GeTe/Sb 2 Te 3 nanowires (NWs) on Si (100), and SiO 2 /Si substrates by metalorganic chemical vapour deposition, coupled to the vapour–liquid– solid mechanism, catalyzed by Au nanoparticles. Scanning electron microscopy, X-ray diffraction, micro-Raman mapping, high-resolution transmission electron microscopy, and electron energy loss spectroscopy were employed to investigate the morphology, structure, and composition of the obtained core and core–shell NWs. A single crystalline GeTe core and a polycrystalline Sb 2 Te 3 shell formed the NWs, having core and core–shell diameters in the range of 50–130 nm and an average length up to 7 µ m.


Introduction
Interest in chalcogenide nanowires (NWs) continues to grow due to their potential capacity to undergo low energy crystalline-amorphous (order-disorder) phase transition [1,2], for several applications such as memory, logic, and sensor devices [3][4][5][6][7][8][9]. Such phase change materials have attracted significant attention, being a promising media for non-volatile phase-change memories (PCM), in addition to optical memories such as (re)writable compact discs (CD) and digital video discs (DVD) [10][11][12]. The realization of PCM devices is, however, still limited due to the requirements of alloy composition, structure, small cell size, high scalability, and low power consumption. The major setback for attaining high-density PCM devices is the operation currents and power consumption needed to prepare the system in the amorphous physical state associated with high resistance reset state [13]. The reduction of the operation currents is desirable because it can realize faster amorphization of material with less power consumed and enable higher storage density, faster memory switching, and higher reliability.

Experimental Section
The growth of the NWs was performed with an Aixtron AIX200/4 MOCVD reactor (Aixtron SE, Herzogenrath, Germany), employing the VLS mechanism induced by Au NPs with average diameter sizes of 10, 20, 30, and 50 nm, respectively. The Au NPs were dispersed from a colloidal solution by Ted Pella © on Si (100) and Si/SiO 2 substrates by a simple drop-casting method. For the growth on Si (100) substrates, the native oxide was removed prior to dispersion of the Au NPs by immersion in an HF 5% solution. The employed metalorganic precursors were electronic-grade tetrakisdimethylamino germanium (Ge[N(CH 3 ) 2 ] 4 , TDMAGe), diisopropyltelluride ((C 3 H 7 ) 2 Te, DiPTe), antimonytrichloride (SbCl 3 ), and bis(trimethylsilyl) telluride (Te(SiMe 3 ) 2 , DSMTe), transported to the MOCVD reactor by an ultra-purified N 2 carrier/process gas. The GeTe core NWs were obtained with the reactor temperature (T), reactor pressure (P), and growth duration (t) varied in the range of 300-400 • C, 50-300 mbar, and 60-120 min, respectively. The MOCVD growth of the Sb 2 Te 3 shell over the GeTe core NWs was carried out at room temperature.
The morphology of the NWs was studied by a Zeiss Supra 40 field emission scanning electron microscope (SEM), (Cark Zeiss, Oberkochen, Germany) in both plan and cross-section mode. The structural analysis was performed by X-ray diffraction (XRD) experiments, using an ItalStructures HRD3000 diffractometer system (Ital Structures Sas, Riva de Garda, Italy) to evaluate the average crystal structure of the NWs. The experimental XRD curves were analyzed by a best-fit procedure based on the Rietveld method [34]. The vibrational properties of the NWs were also studied by collecting Raman maps with a Thermo Scientific DXR2xi Raman Imaging Microscope (ThermoFischer Scientific, Waltham, MA, USA), equipped with a 50× objective and a 532 nm excitation laser. The maps were acquired with a step size of 0.5 µm, and each point spectrum resulted from 40 accumulations of 50 ms acquisitions. The local microstructure, growth direction, and composition were studied by high-resolution transmission electron microscopy (HR-TEM). To make the NWs suitable for TEM observations, the as-grown NWs were transferred directly from the substrate by mechanical rubbing on the TEM grid. TEM analyses were performed on a Cs-probe-corrected TEM JEOL ARM200CF microscope (JEOL Ltd, Akishima, Japan), operated in scanning mode, Scanning Transmission Electron Microscope (STEM) at a primary beam energy of 200 keV and equipped with a GIF Quantum ER energy filter by GATAN for electron energy loss spectroscopy (EELS), (Gatan Inc., Pleasanton, CA, USA) spectroscopy.

Results and Discussion
The first step of our investigation addressed the growth of the GeTe core NWs. Different precursors and different growth parameters were investigated. In particular, using the DSMTe and TDMAGe precursors, we investigated the growth process conditions ranges: T = 380-400 • C, P = 50-300 mbar, with t = 120 min, TDMAGe partial pressure = 4.48 × 10 −3 , and DSMTe partial pressure = 8.06 × 10 −3 . Under non-optimized conditions, only a relatively low fraction of Au NPs gave rise to the VLS process (see Supplementary Material, Figure S1). The best results in terms of morphological characteristics (length and density of NWs) were obtained for T = 400 • C and P = 50 mbar. However, a large density of unreacted Au NPs (see Supplementary Material, Figure S2) was observed. Also, XRD measurements revealed that the NWs were basically formed by Germanium (see Supplementary Material, Figure S3). Replacing the DSMTe precursor with the DiPTe precursor and optimized growth conditions, i.e., T = 400 • C, P = 50 mbar, t = 60 min, DiPTe partial pressure = 8.58 × 10 −3 mbar, TDMAGe partial pressure = 3.35 × 10 −3 mbar, the growth of GeTe NWs was achieved (see Figure 1a,b). Au catalyst NPs were observed at the NWs tips, confirming that the growth occurs via the VLS mechanism. Also, SEM observations indicated that the NWs grew with high density and unevenly distributed both on Si (100) and SiO 2 substrates, with a certain dispersion in length and diameter (see Supplementary Material, Figures S4 and S5). The density of the NWs increased with increasing the Au NPs sizes. Also, larger NPs produced NWs with larger diameters, consistently with the VLS growth mechanism, and no material growth was found to occur without Au NPs. The effect of the Au NPs size on the diameter of the GeTe NWs is illustrated in Figure S6. The results of the large-area XRD analysis on as-grown GeTe core NWs are presented in Figure 2a. The data were analyzed by Rietveld refinement, taking into account not only the peak position, but also the existing background and peak broadening, using the opensource software Maud [36][37][38]. The patterns were simulated using the rhombohedral GeTe structure [39] and cubic Au [40]. The presence of cubic GeTe [41] cannot be excluded. The simulation of the diffracting maxima at a low angle allowed us to extract the lattice parameters of the NWs, which resulted to be a = 8.28 Å and c = 10.55 Å. Moreover, the different intensity of the low-angle diffracting maxima, compared to the powder pattern, suggested that the NWs exhibit a preferential orientation. The data of the GeTe/Sb2Te3 core-shell structures are presented in Figure 2b. Scattering from crystallized Sb2Te3 was evident, as shown by the numerous diffracting maxima assigned to rhombohedral Sb2Te3 [42], with the following lattice parameters, as extracted from Rietveld refinement: a = 4.22Å and c = 30.46Å. The Sb2Te3 crystals also showed a preferential orientation, which was analyzed in more detail by HR-TEM analysis (see in the corresponding section). A In the second step of the core-shell fabrication, the GeTe NWs were coated with a Sb 2 Te 3 layer using the combination of SbCl 3 and DSMTe precursors, under the MOCVD growth conditions: T = room temperature, P = 15 mbar, t = 90 min, SbCl 3 partial pressure = 2.23 × 10 −4 mbar, DSMTe partial pressure = 3.25 × 10 −4 mbar, leading to the conformal growth of a uniform and continuous layer on the NWs core (see Figure 1c,d and Supplementary Material Figure S7) [35]. For the shell, it was necessary to lower the deposition temperature down to room temperature to obtain the conformal NWs coating, although with a granular morphology. It was also possible to obtain the Sb 2 Te 3 coating at higher deposition temperatures, but this was detrimental for the conformality, since different voids appeared and the tendency to form crystalline clusters was clearly enhanced. The obtained core-shell NWs have overall diameters in the range of (80-130) nm (see Supplementary Materials Figure S8) and lengths in the range of several microns. The presented results show that, by an appropriate selection of growth conditions, a relatively high density of NWs (i.e., the efficiency of growth catalyzed by Au NPs) and a relatively good control in terms of reproducibility and dispersion of their morphology were achieved.
The results of the large-area XRD analysis on as-grown GeTe core NWs are presented in Figure 2a. The data were analyzed by Rietveld refinement, taking into account not only the peak position, but also the existing background and peak broadening, using the open-source software Maud [36][37][38]. The patterns were simulated using the rhombohedral GeTe structure [39] and cubic Au [40]. The presence of cubic GeTe [41] cannot be excluded. The simulation of the diffracting maxima at a low angle allowed us to extract the lattice parameters of the NWs, which resulted to be a = 8.28 Å and c = 10.55 Å. Moreover, the different intensity of the low-angle diffracting maxima, compared to the powder pattern, suggested that the NWs exhibit a preferential orientation. The data of the GeTe/Sb 2 Te 3 coreshell structures are presented in Figure 2b. Scattering from crystallized Sb 2 Te 3 was evident, as shown by the numerous diffracting maxima assigned to rhombohedral Sb 2 Te 3 [42], with the following lattice parameters, as extracted from Rietveld refinement: a = 4.22Å and c = 30.46Å. The Sb 2 Te 3 crystals also showed a preferential orientation, which was analyzed in more detail by HR-TEM analysis (see in the corresponding section). A shoulder at the right side of the (015) reflection of Sb 2 Te 3 , a peak at around 2θ = 29.6 • , could be attributed to the (202) main reflection of the GeTe structure, confirming that the core GeTe NWs preserve their crystallinity after the Sb 2 Te 3 deposition.  [40], GeTe [39], and Sb2Te3 [42] are also added for comparison.
Micro Raman mapping was performed to analyze the vibrational modes of the deposited GeTe core and GeTe/Sb2Te3 core-shell NWs. Figure 3a shows a large area brightfield optical image of GeTe NWs on the SiO2 substrate with 20 nm Au NPs. The high magnification micrograph revealed that the NWs have a length up to about 5 µm (Figure 3b). The two-dimensional (2D) map in Figure 3c was obtained over the entire region of Figure  3b by measuring a series of Raman spectra with a step-size of 0.5 µm (see the red points indicating the measuring array in Figure 3b). The map's colour representation refers to a point-by-point correlation with the Raman spectrum, averaged over the full length of the NW, identified on the upper part of the map and reported in Figure 3d. The mean spectrum reveals features similar to those of crystalline GeTe, confirming the well-defined crystalline structure of the NWs. In particular, the peak at about 98 cm −1 assigned to the E mode is specific to the distorted octahedral Ge sites, and it is not present in the spectra of the amorphous material, while the two high-intensity bands at about 126 and 142 cm −1 can be assigned to the A1 mode, attributed to vibrations of Ge atoms in distorted and defective octahedral sites [43,44]. Two more features at 276 and 300 cm −1 could be identified in the spectrum and ascribed to Ge-Ge stretching vibrations, typically observed for Ge nanocrystals [45].  [40], GeTe [39], and Sb 2 Te 3 [42] are also added for comparison.
Micro Raman mapping was performed to analyze the vibrational modes of the deposited GeTe core and GeTe/Sb 2 Te 3 core-shell NWs. Figure 3a shows a large area brightfield optical image of GeTe NWs on the SiO 2 substrate with 20 nm Au NPs. The high magnification micrograph revealed that the NWs have a length up to about 5 µm (Figure 3b). The two-dimensional (2D) map in Figure 3c was obtained over the entire region of Figure 3b by measuring a series of Raman spectra with a step-size of 0.5 µm (see the red points indicating the measuring array in Figure 3b). The map's colour representation refers to a point-by-point correlation with the Raman spectrum, averaged over the full length of the NW, identified on the upper part of the map and reported in Figure 3d. The mean spectrum reveals features similar to those of crystalline GeTe, confirming the well-defined crystalline structure of the NWs. In particular, the peak at about 98 cm −1 assigned to the E mode is specific to the distorted octahedral Ge sites, and it is not present in the spectra of the amorphous material, while the two high-intensity bands at about 126 and 142 cm −1 can be assigned to the A1 mode, attributed to vibrations of Ge atoms in distorted and defective octahedral sites [43,44]. Two more features at 276 and 300 cm −1 could be identified in the spectrum and ascribed to Ge-Ge stretching vibrations, typically observed for Ge nanocrystals [45]. mode is specific to the distorted octahedral Ge sites, and it is not present in the spectra of the amorphous material, while the two high-intensity bands at about 126 and 142 cm −1 can be assigned to the A1 mode, attributed to vibrations of Ge atoms in distorted and defective octahedral sites [43,44]. Two more features at 276 and 300 cm −1 could be identified in the spectrum and ascribed to Ge-Ge stretching vibrations, typically observed for Ge nanocrystals [45].    Figure 4 shows the results of Raman mapping on the GeTe/Sb 2 Te 3 core-shell NWs. A single NW was localized and selected by means of optical microscopy in dark-field modality ( Figure 4a) and then mapped with a 0.5 µm step-size array of Raman spectra (Figure 4b). Figure 4c reports the spectra corresponding to the Sb 2 Te 3 background (black line, blue area of the map) and the NW (red line, green to red contrast in the map). The background spectrum is characterized by 3 main features at about 69, 112, and 165 cm −1 , associated with the A 1 1g (LO), E 2 g (TO), and A 2 1g (LO) modes of Sb 2 Te 3 , respectively [46][47][48]. These peaks are also present in the NW spectrum, together with those already identified for GeTe at 123, 141, and 273 cm −1 (slightly displaced with respect to those of Figure 3d).
Coatings 2021, 11, x FOR PEER REVIEW 6 of 10 4b). Figure 4c reports the spectra corresponding to the Sb2Te3 background (black line, blue area of the map) and the NW (red line, green to red contrast in the map). The background spectrum is characterized by 3 main features at about 69, 112, and 165 cm −1 , associated with the A 1 1g (LO), E 2 g(TO), and A 2 1g(LO) modes of Sb2Te3, respectively [46][47][48]. These peaks are also present in the NW spectrum, together with those already identified for GeTe at 123, 141, and 273 cm −1 (slightly displaced with respect to those of Figure 3d). The HR-TEM analysis confirmed that the NWs synthesized with Au NPs have a welldefined core-shell structure, where the core is composed of a single crystalline GeTe, surrounded by a polycrystalline Sb2Te3 shell. Figure 5 shows a STEM micrograph of a core GeTe NW with the Au nanoparticle still on the tip. From the high-resolution image of the NW portion, it is revealed that the core GeTe NWs are composed of a single crystalline core, as depicted in Figure 5b. In the inset of Figure 5b, the relative orientations of the Fast-Fourier-Transform (FFT) patterns demonstrate the interplanar distances. By indexing this diffraction pattern, a rhombohedral GeTe phase was recognized, consistent with recent studies of GeTe NWs and the XRD results. EELS confirmed the presence of only Ge and Te along the whole NW, with a ratio compatible with a 1:1 proportion of Ge and Te (see Supplementary Material Figure S9a). The HR-TEM analysis confirmed that the NWs synthesized with Au NPs have a well-defined core-shell structure, where the core is composed of a single crystalline GeTe, surrounded by a polycrystalline Sb 2 Te 3 shell. Figure 5 shows a STEM micrograph of a core GeTe NW with the Au nanoparticle still on the tip. From the high-resolution image of the NW portion, it is revealed that the core GeTe NWs are composed of a single crystalline core, as depicted in Figure 5b. In the inset of Figure 5b, the relative orientations of the Fast-Fourier-Transform (FFT) patterns demonstrate the interplanar distances. By indexing this diffraction pattern, a rhombohedral GeTe phase was recognized, consistent with recent studies of GeTe NWs and the XRD results. EELS confirmed the presence of only Ge and Te along the whole NW, with a ratio compatible with a 1:1 proportion of Ge and Te (see Supplementary Material Figure S9a). NW portion, it is revealed that the core GeTe NWs are composed of a single crystalline core, as depicted in Figure 5b. In the inset of Figure 5b, the relative orientations of the Fast-Fourier-Transform (FFT) patterns demonstrate the interplanar distances. By indexing this diffraction pattern, a rhombohedral GeTe phase was recognized, consistent with recent studies of GeTe NWs and the XRD results. EELS confirmed the presence of only Ge and Te along the whole NW, with a ratio compatible with a 1:1 proportion of Ge and Te (see Supplementary Material Figure S9a).  Further, the STEM images of GeTe/Sb 2 Te 3 core-shell NWs showed a uniform and conformal shell around the NWs. Indeed, a clear interface between the core and the shell region is visible on the core-shell NWs (see Figure 6a). This observation indicated the epitaxial growth of the Sb 2 Te 3 layer on top of the GeTe core NW. In Figure 6b, the highresolution STEM image indicated that the shell is polycrystalline. EELS measurement on the polycrystalline shell confirmed the presence of only Sb and Te, with a ratio compatible with 2:3 of Sb and Te (see Supplementary Material Figure S9b). The GeTe/Sb 2 Te 3 core-shell NWs were found to contain no detectable defects, such as dislocations and stacking faults; the crystal structures of each region of the GeTe/Sb 2 Te 3 core-shell NW heterostructures were in agreement with the XRD results. Further, the STEM images of GeTe/Sb2Te3 core-shell NWs showed a uniform and conformal shell around the NWs. Indeed, a clear interface between the core and the shell region is visible on the core-shell NWs (see Figure 6a). This observation indicated the epitaxial growth of the Sb2Te3 layer on top of the GeTe core NW. In Figure 6b, the highresolution STEM image indicated that the shell is polycrystalline. EELS measurement on the polycrystalline shell confirmed the presence of only Sb and Te, with a ratio compatible with 2:3 of Sb and Te (see Supplementary Material Figure S9b). The GeTe/Sb2Te3 coreshell NWs were found to contain no detectable defects, such as dislocations and stacking faults; the crystal structures of each region of the GeTe/Sb2Te3 core-shell NW heterostructures were in agreement with the XRD results. The above analyses demonstrated the effectiveness of the present approach in achieving a uniform and conformal deposition of Sb2Te3 at room temperature onto the core GeTe NWs, while maintaining a physically and chemically distinct interface, with minimum interdiffusion of the elements.

Conclusions
We presented a bottom-up approach to synthesize core-shell chalcogenide GeTe/Sb2Te3 nanowires, with a diameter down to 80 nm and an average length up to 7 µm. The synthesis of this type of nanoscaled heterostructures could facilitate the understanding of the switching mechanisms in heterostructure-based PCMs and open the way to the realization of advanced microelectronic devices, including multilevel phase-change The above analyses demonstrated the effectiveness of the present approach in achieving a uniform and conformal deposition of Sb 2 Te 3 at room temperature onto the core GeTe NWs, while maintaining a physically and chemically distinct interface, with minimum interdiffusion of the elements.

Conclusions
We presented a bottom-up approach to synthesize core-shell chalcogenide GeTe/Sb 2 Te 3 nanowires, with a diameter down to 80 nm and an average length up to 7 µm. The synthesis of this type of nanoscaled heterostructures could facilitate the understanding of the switching mechanisms in heterostructure-based PCMs and open the way to the realization of advanced microelectronic devices, including multilevel phase-change memories.