ABSTRACT
Phase-change materials (PCMs), which can transition reversibly between crystalline and amorphous phases, have shown great promise for next-generation memory devices due to their nonvolatility, rapid switching periods, and random-access capability. Several groups have investigated phase-change nanowires for memory applications in recent years. The ability to regulate the scale of nanostructures remains one of the most significant obstacles in nanoscience. Herein, we describe the growth and characterization of germanium telluride (GeTe) nanowires, which are essential for phase-change memory devices. GeTe nanowires were produced by combining thermal evaporation and vapor-liquid-solid (VLS) techniques, using 8 nm Au nanoparticles as the metal catalyst. The influence of various growth parameters, including inert gas flow rate, working pressure, growth temperature, growth duration, and growth substrate, was examined. Ar gas flow rate of 30 sccm and working pressure of 75 Torr produced the narrowest GeTe nanowires horizontally grown on a Si substrate. Using scanning electron microscopy, the dimensions, and morphology of GeTe nanowires were analyzed. Transmission electron microscopy and energy-dispersive x-ray spectroscopy were utilized to conduct structural and chemical analyses. Using a SiO2/Si substrate produced GeTe nanowires that were thicker and lengthier. The current-voltage characteristics of GeTe nanowires were investigated, confirming the amorphous nature of GeTe nanowires using conductive atomic force microscopy. In addition, the effects of the VLS mechanism and the Gibbs-Thomson effect were analyzed, which enables the optimization of nanowires for numerous applications, such as memory and reservoir computing.
ABSTRACT
This study demonstrated the deposition of size-controlled gold (Au) nanoclusters via direct-current magnetron sputtering and inert gas condensation techniques. The impact of different source parameters, namely, sputtering discharge power, inert gas flow rate, and aggregation length on Au nanoclusters' size and yield was investigated. Au nanoclusters' size and size uniformity were confirmed via transmission electron microscopy. In general, Au nanoclusters' average diameter increased by increasing all source parameters, producing monodispersed nanoclusters of an average size range of 1.7 ± 0.1 nm to 9.1 ± 0.1 nm. Among all source parameters, inert gas flow rate exhibited a strong impact on nanoclusters' average size, while sputtering discharge power showed great influence on Au nanoclusters' yield. Results suggest that Au nanoclusters nucleate via a three-body collision mechanism and grow through a two-body collision mechanism, wherein the nanocluster embryos grow in size due to atomic condensation. Ultimately, the usefulness of the produced Au nanoclusters as catalysts for a vapor-liquid-solid technique was put to test to synthesize the phase change material germanium telluride nanowires.
ABSTRACT
This work provides useful insights into the development of HfO2-based memristive systems with a p-type silicon bottom electrode that are compatible with the complementary metal-oxide-semiconductor technology. The results obtained reveal the importance of the top electrode selection to achieve unique device characteristics. The Ag/HfO2/Si devices have exhibited a larger memory window and self-compliance characteristics. On the other hand, the Au/HfO2/Si devices have displayed substantial cycle-to-cycle variation in the ON-state conductance. These device characteristics can be used as an indicator for the design of resistive-switching devices in various scenes such as, memory, security, and sensing. The current-voltage (I-V) characteristics of Ag/HfO2/Si and Au/HfO2/Si devices under positive and negative bias conditions have provided valuable information on the ON and OFF states of the devices and the underlying resistive switching mechanisms. Repeatable, low-power, and forming-free bipolar resistive switching is obtained with both device structures, with the Au/HfO2/Si devices displaying a poorer device-to-device reproducibility. Furthermore, the Au/HfO2/Si devices have exhibited N-type negative differential resistance (NDR), suggesting Joule-heating activated migration of oxygen vacancies to be responsible for the SET process in the unstable unipolar mode.
ABSTRACT
Surfaces of polycrystalline α-GeTe films were studied by X-ray photoelectron spectroscopy (XPS) after different treatments in an effort to understand the effect of premetallization surface treatments on the resistance of Ni-based contacts to GeTe. UV-O3 is often used to remove organic contaminants after lithography and prior to metallization; therefore, UV-O3 treatment was used first for 10 min prior to ex situ treatments, which led to oxidation of both Ge and Te to GeOx (x < 2) and TeO2, respectively. Then the oxides were removed by deionized (DI) H2O, (NH4)2S, and HCl treatments. Additionally, in situ Ar+ ion etching was used to clean the GeTe surface without prior UV-O3 treatment. Ar+ ion etching, H2O, and (NH4)2S treatments create a surface richer in Ge compared to the HCl treatment, after which the surface is Te-rich. However, (NH4)2S also oxidizes Ge and gradually etches the GeTe film. All treated surfaces showed poor stability upon prolonged exposure to air, revealing that even (NH4)2S does not passivate the GeTe surface. The refined transfer length method (RTLM) was used to measure the contact resistance (Rc) of as-deposited Ni-based contacts to GeTe as a function of premetallization surface preparation. HCl-treated samples had the highest Rc (0.036 ± 0.002 Ω·mm), which was more than twice that of the other surface treatments. This increase in Rc is attributed to formation of the Ni1.29Te phase at the Ni/GeTe interface due to an abundance of Te at the surface after HCl treatment. In general, treatments that resulted in Ge-rich surfaces offered lower Rc.
