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| Hauptseite > Publikationsdatenbank > MOVPE of crystalline Ge1Sb2Te4 (GST) |
| Hauptseite > Publikationsdatenbank > MOVPE of crystalline Ge1Sb2Te4 (GST) |
| Conference Presentation (Other) | FZJ-2014-04107 |
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2014
Abstract: Chalcogenide phase change materials show significant promise on the road to an ideal memory. Ge-Sb-Te alloys (GST) are considered to be one of the most suitable compounds for this application and are already widely used as non-volatile phase change memory in optical data storage and in first PRAM (Phase-change Random Access Memory) devices today [1, 2]. The reason is the rapid and reversible change between its amorphous and crystalline phase with orders of magnitude differences in reflectivity and electrical resistivity. To this end the compounds along the Sb2Te3 - GeTe pseudobinary line are the most widely used. Nearly all GST layers are deposited in the amorphous phase by sputtering. However, if nanostructures are to be achieved without (reactive ion) etching and its detrimental side effects, a bottom-up approach of single crystalline material is more appropriate [3]. Also a CMOS compatible approach should be adopted, which should be free of catalysts such as Au. Starting point for the investigations are growth conditions in MOVPE at which flat homogeneous coalesced crystalline layers are deposited with only one composition. A special challenge is the incorporation of Germanium. In future the conditions are to be applied to selective area growth of nanostructures such as nanowires, with which a high scalability of structures can be ensured.Deposition was performed in a horizontal low-pressure MOCVD reactor on 2” Si (111) wafers, which were deoxidized by hydrofluoric acid prior to the growth. Pure N2 carrier gas transported the source compounds triethylantimony (TESb) and diethyltelluride (DETe) as metal-organic precursors into the reactor. The hydride digermane (Ge2H6) was chosen as the as the Ge source. The partial pressures of the sources (Ge2H6: 1.46 x 10-2 mbar, TESb: 4.77 x 10-3 mbar, DETe: 1.32 x 10-1 mbar), the total gas flow of 2500 ml/min and the growth time of 60 min were held constant. The reactor pressure was varied between 50 mbar and 350 mbar and growth temperatures in the range of 425°C up to 475°C. The influence of an in-situ treatment of the substrate at growth-temperature prior to deposition was investigated. To this end the substrates were pre-treated with various sources and source combinations for 1 min to 15 min. The influence of a stabilizing pause between the in-situ pre-treatment and the successive GST growth was also studied. The deposited material was characterized by means of scanning electron microscopy, Raman spectroscopy, atomic force microscopy, energy dispersive X-Ray spectroscopy and X-Ray Diffraction to determine topography, crystal structure and composition of the grown layers.It was found that a lower reactor pressure led to a more pronounced growth of flat layers. At lower pressures, the growth temperature needed to be increased slightly on account of the poorer decomposition of the sources. While at a reactor pressure of 250 mbar and T = 420°C the deposited material consists of triangularly shaped crystals with a height of about 1 µm, the samples grown at 50 mbar consisted of nearly coalesced layers of GST with a height around 100 nm and flat surfaces (Figure 1). The samples deposited at 50 mbar exhibit a high crystallinity. Their structure was determined to be Ge1Sb2Te4, crystallized in the hexagonal phase (hcp) (Figure 2).An in-situ deposition of germanium in an antimony atmosphere prior to the growth of GST promotes the growth of flat layers by improving the coverage of the substrate without disturbing the crystallinity or composition on the Si (111) substrate. This is in accordance to the results found for the growth of GST on SiN and SiO2 surfaces [4].
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