Epitaxial growth of SrTiO₃ on silicon by molecular beam epitaxy has opened up the route to the integration of functional complex oxides on a silicon platform. Chief among them is ferroelectric functionality using perovskite oxides such as BaTiO₃. However, it has remained a challenge to achieve ferroelectricity in epitaxial BaTiO₃ films with a polarization pointing perpendicular to the silicon substrate without a conducting bottom electrode. Here, we demonstrate ferroelectricity in such stacks. Synchrotron X-ray diffraction and high-resolution scanning transmission electron microscopy reveal the presence of crystalline domains with the long axis of the tetragonal structure oriented perpendicular to the substrate. Using piezoforce microscopy, polar domains can be written and read and are reversibly switched with a phase change of 180°. Open, saturated hysteresis loops are recorded. Thus, ferroelectric switching of 8- to 40-nm-thick BaTiO₃ films in metal-ferroelectric-semiconductor structures is realized, and field-effect devices using this epitaxial oxide stack can be envisaged.

The crystallization behavior of ultrathin phase change films was studied using time-resolved x-ray diffraction (XRD). Thin films of variable thickness between 1 and 50nm of the phase change materials Ge2Sb2Te5 (GST), N-doped GST, Ge15Sb85, Sb2Te, and Ag- and In-doped Sb2Te were heated in a He atmosphere, and the intensity of the diffracted x-ray peaks was recorded. It was found for all materials that the crystallization temperature increases as the film thickness is reduced below 10nm. The increase depends on the material and can be as high as 200°C for the thinnest films. The thinnest films that show XRD peaks are 2nm for GST and N-GST, 1.5nm for Sb2Te and AgIn-Sb2Te, and 1.3nm for GeSb. This scaling behavior is very promising for the application of phase change materials to solid-state memory technology.

Chalcogenide films with reversible amorphous-crystalline phase transitions have been commercialized as optically rewritable data-storage media, and intensive effort is now focused on integrating them into electrically addressed non-volatile memory devices (phase-change random-access memory or PCRAM). Although optical data storage is accomplished by laser-induced heating of continuous films, electronic memory requires integration of discrete nanoscale phase-change material features with read/write electronics. Currently, phase-change films are most commonly deposited by sputter deposition, and patterned by conventional lithography. Metal chalcogenide films for transistor applications have recently been deposited by a low-temperature, solution-phase route. Here, we extend this methodology to prepare thin films and nanostructures of GeSbSe phase-change materials. We report the ready tuneability of phase-change properties in GeSbSe films through composition variation achieved by combining novel precursors in solution. Rapid, submicrosecond phase switching is observed by laser-pulse annealing. We also demonstrate that prepatterned holes can be filled to fabricate phase-change nanostructures from hundreds down to tens of nanometres in size, offering enhanced flexibility in fabricating PCRAM devices with reduced current requirements.

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The solid-state reaction and agglomeration of thin nickel-silicide films was investigated from sputter deposited nickel films ͑1-10 nm͒ on silicon-on-insulator ͑100͒ substrates. For typical anneals at a ramp rate of 3°C / s, 5-10 nm Ni films react with silicon and form NiSi, which agglomerates at 550-650°C, whereas films with a thickness of 3.7 nm of less were found to form an epitaxylike nickel-silicide layer. The resulting films show an increased thermal stability with a low electrical resistivity up to 800°C. © 2010 American Institute of Physics. ͓doi:10.1063/1.3384997͔Nickel-silicides are currently used as contacting materials in state-of-the-art microelectronic devices.1,2 Feature size has shrunk to a few tens of nanometers, and for nickel monosilicide ͑NiSi͒ layers, this results in a particularly severe tendency to agglomerate, 3 leading to a large increase in the electrical resistance of the contact, and an increased mobility of the nickel as it starts to move on the defects, both resulting in a low yield. Since the agglomeration of thin films is driven by a minimization of interface energy, it is expected that thinner films will agglomerate faster ͑i.e., have a lower agglomeration temperature͒. 4 In this letter, we show that this holds true only for films with a thickness of at least 5 nm ͑as-deposited thickness of the nickel layer͒, while for thinner layers the resulting nickel-silicide layer is much more resistant to agglomeration.Nickel films with a thickness between 1 and 10 nm were sputter deposited onto lightly p-doped ͑ =14-22 ⍀ cm͒, Radio Corporation of America ͑RCA͒ cleaned, and HF dipped silicon-on-insulator substrates, with a top layer of 117 nm of Si ͑100͒. The deposition chamber was first evacuated to 10 −4 Pa during deposition, the samples were mounted on a rotating carousel to ensure a uniform deposition thickness. An argon pressure of 0.5 Pa and a sputtering power of 100 W were used, resulting in a deposition rate of 0.04 nm/s. After deposition, the samples were annealed in a high-purity He atmosphere, from 100 to 850°C at a rate of 3°C / s, and the surface roughness ͑using laser light scattering, recording the intensity of nonspecular reflection of the laser light͒, and the sheet resistance ͑using a four point probe͒ were recorded in situ. The thickness of both the as-deposited and annealed films was determined using x-ray reflectivity and cross section transmission electron microscopy, resulting in the reported thicknesses with a precision of Ϯ0.2 nm.An overview of the in situ sheet-resistance is shown in Fig. 1. All of the samples with more than 3.7 nm of nickel ͑6, 8, and 10 nm are shown͒, exhibit a sheet resistance qualitatively similar to what was previously reported 5 for 10 nm layers of nickel on Si ͑100͒; a complex phase sequence of high-resistive metal rich nickel-silicides at low temperatures, and the formation of the low-resistive NiSi phase at 400-450°C. This layer then agglomerates at 550-650°C, leading to the observed increase in sheet resistance. In contrast, for the thinn...