Ruomeng Huang

Nonpolar resistive switching in Cu/SiC/Au non-volatile resistive memory devices

Amorphous silicon carbide (a-SiC) based resistive memory (RM) Cu/a-SiC/Au devices were fabricated and their resistive switching characteristics investigated. All four possible modes of nonpolar resistive switching were achieved with ON/OFF ratio in the range 106–108. Detailed current-voltage I-V characteristics analysis suggests that the conduction mechanism in low resistance state is due to the formation of metallic filaments. Schottky emission is proven to be the dominant conduction mechanism in high resistance state which results from the Schottky contacts between the metal electrodes and SiC. ON/OFF ratios exceeding 107 over 10 years were also predicted from state retention characterizations. These results suggest promising application potentials for Cu/a-SiC/Au RMs.

Area Selective Growth of Titanium Diselenide Thin Films into Micropatterned Substrates by Low-Pressure Chemical Vapor Deposition.

The neutral, distorted octahedral complex [TiCl4(SenBu2)2] (1), prepared from the reaction of TiCl4 with the neutral SenBu2 in a 1:2 ratio and characterized by IR and multinuclear (1H, 13C{1H}, 77Se{1H}) NMR spectroscopy and microanalysis, serves as an efficient single-source precursor for low-pressure chemical vapor deposition (LPCVD) of titanium diselenide, TiSe2, films onto SiO2 and TiN substrates. X-ray diffraction patterns on the deposited films are consistent with single-phase, hexagonal 1T-TiSe2 (P3̅m1), with evidence of some preferred orientation of the crystallites in thicker films. The composition and structural morphology was confirmed by scanning electron microscopy (SEM), energy dispersive X-ray, and Raman spectroscopy. SEM imaging shows hexagonal plate crystallites growing perpendicular to the substrate, but these tend to align parallel to the surface when the quantity of reagent is reduced. The resistivity of the crystalline TiSe2 films is 3.36 ± 0.05 × 10–3 Ω·cm with a carrier density of 1 × 1022 cm–3. Very highly selective film growth from the reagent was observed onto photolithographically patterned substrates, with film growth strongly preferred onto the conducting TiN surfaces of SiO2/TiN patterned substrates. TiSe2 is selectively deposited within the smallest 2 μm diameter TiN holes of the patterned TiN/SiO2 substrates. The variation in crystallite size with different diameter holes is determined by microfocus X-ray diffraction and SEM, revealing that the dimensions increase with the hole size, but that the thickness of the crystals stops increasing above ∼20 μm hole size, whereas their lengths/widths continue to increase.

Non-aqueous electrodeposition of functional semiconducting metal chalcogenides: Ge2Sb2Te5 phase change memory

We report a new method for electrodeposition of device-quality metal chalcogenide semiconductor thin films and nanostructures from a single, highly tuneable, non-aqueous electrolyte. This method opens up the prospect of electrochemical preparation of a wide range of functional semiconducting metal chalcogenide alloys that have applications in various nano-technology areas, ranging from the electronics industry to thermoelectric devices and photovoltaic materials. The functional operation of the new method is demonstrated by means of its application to deposit the technologically important ternary Ge/Sb/Te alloy, GST-225, for fabrication of nanostructured phase change memory (PCM) devices and the quality of the material is confirmed by phase cycling via electrical pulsed switching of both the nano-cells and thin films.

Controlling the nanostructure of bismuth telluride by selective chemical vapour deposition from a single source precursor

High quality, nanostructured Bi2Te3, with an unprecedented degree of positional and orientational control of the material form on the nanoscale, is readily obtained by low pressure chemical vapour deposition using a new molecular precursor. This system offers a convenient method that delivers key structural requirements necessary to improve the thermoelectric efficiency of Bi2Te3 and to develop the nascent field of topological insulators.

Non-aqueous electrodeposition of p-block metals and metalloids from halometallate salts

A versatile electrochemical system for the non-aqueous electrodeposition of crystalline, oxide free p-block metals and metalloids is described, and it is demonstrated that by combining mixtures of these reagents, this system is suitable for electrodeposition of binary semiconductor alloys. The tetrabutylammonium halometallates, [NnBu4][InCl4], [NnBu4][SbCl4], [NnBu4][BiCl4], [NnBu4]2[SeCl6] and [NnBu4]2[TeCl6], are readily dissolved in CH2Cl2 and form reproducible electrochemical systems with good stability in the presence of a [NnBu4]Cl supporting electrolyte. The prepared electrolytes show a wide potential window and the electrodeposition of indium, antimony, bismuth, tellurium and selenium on glassy carbon and titanium nitride electrodes has been demonstrated. The deposited elements were characterised by scanning electron microscopy, energy dispersive X-ray analysis and powder X-ray diffraction. The compatibility of the reagents permits the preparation of a single electrolyte containing several halometallate species which allows the electrodeposition of binary materials, as is demonstrated for InSb. This room temperature, ‘bottom-up’ electrochemical approach should thus be suitable for the one-pot deposition of a wide range of compound semiconductor materials.

Chemical vapour deposition of antimony chalcogenides with positional and orientational control: precursor design and substrate selectivity

A series of alkylchalcogenostibines, Me2SbSenBu, MeSb(SenBu)2, Sb(SenBu)3 and MeSb(TenBu)2, have been designed and synthesised as potential precursors for chemical vapour deposition (CVD) by reaction of nBuELi (E = Se, Te) with the appropriate halostibine, Me3−nSbCln (n = 1, 2, 3), and characterised by 1H, 13C{1H} and 77Se{1H} or 125Te{1H} NMR spectroscopy as appropriate. MeSb(SenBu)2 and MeSb(TenBu)2 are very effective single source precursors for the low pressure CVD of high quality crystalline thin films of Sb2Se3 and Sb2Te3, respectively, confirmed by scanning electron microscopy, energy dispersive X-ray spectroscopy, Raman spectroscopy and thin film X-ray diffraction. Hall conductivity, carrier mobility, carrier density and, in the case of Sb2Te3, Seebeck coefficient measurements reveal electronic characteristics comparable with Sb2E3 deposited by atomic layer deposition or molecular beam epitaxy, suggesting materials quality and performance suitable for incorporation into electronic device structures. Choice of substrate and deposition conditions were found to significantly affect the morphology and preferred orientation of Sb2Te3 crystallites, enabling deposition of films with either 〈1 1 0〉 or 〈0 0 1〉 alignment. Use of micro-patterned substrates allowed selective deposition of crystalline 2D micro-arrays of Sb2Te3 onto exposed TiN surfaces only.

Switching kinetics of SiC resistive memory for harsh environments

Cu/a-SiC/Au resistive memory cells are measured using voltage pulses and exhibit the highest ROFF/RON ratio recorded for any resistive memory. The switching kinetics are investigated and fitted to a numerical model, using thermal conductivity and resistivity properties of the dielectric. The SET mechanism of the Cu/a-SiC/Au memory cells is found to be due to ionic motion without joule heating contributions, whereas the RESET mechanism is found to be due to thermally assisted ionic motion. The conductive filament diameter is extracted to be around 4nm. The high thermal conductivity and resistivity for the Cu/a-SiC/Au memory cells result in slow switching but with high thermal reliability and stability, showing potential for use in harsh environments. Radiation properties of SiC memory cells are investigated. No change was seen in DC sweep or pulsed switching nor in conductive mechanisms, up to 2Mrad(Si) using 60Co gamma irradiation.

Nanoscale arrays of antimony telluride single crystals by selective chemical vapor deposition

Arrays of individual single nanocrystals of Sb2Te3 have been formed using selective chemical vapor deposition (CVD) from a single source precursor. Crystals are self-assembled reproducibly in confined spaces of 100 nm diameter with pitch down to 500 nm. The distribution of crystallite sizes across the arrays is very narrow (standard deviation of 15%) and is affected by both the hole diameter and the array pitch. The preferred growth of the crystals in the <1 1 0> orientation along the diagonal of the square holes strongly indicates that the diffusion of adatoms results in a near thermodynamic equilibrium growth mechanism of the nuclei. A clear relationship between electrical resistivity and selectivity is established across a range of metal selenides and tellurides, showing that conductive materials result in more selective growth and suggesting that electron donation is of critical importance for selective deposition.

Contact resistance measurement of Ge2Sb2Te5 phase change material to TiN electrode by spacer etched nanowire

Ge2Sb2Te5 (GST) phase change nanowires have been fabricated using a top-down spacer etch process. This approach enables controls over the dimension and location of the nanowires without affecting the electrical properties. Phase change devices based on these nanowires have been used to systematically investigate the contact resistance between GST phase change material and TiN metal electrodes. The specific contact resistance was found to be 7.96 × 10−5 Ω cm2 for crystalline GST and 6.39 × 10−2 Ω cm2 for amorphous GST. The results suggest that contact resistance plays a dominant role in the total resistance of GST memory device in both crystalline and amorphous states.

Compliance-free ZrO2/ZrO2 - x /ZrO2 resistive memory with controllable interfacial multistate switching behaviour

A controllable transformation from interfacial to filamentary switching mode is presented on a ZrO2/ZrO2 − x/ZrO2 tri-layer resistive memory. The two switching modes are investigated with possible switching and transformation mechanisms proposed. Resistivity modulation of the ZrO2 − x layer is proposed to be responsible for the switching in the interfacial switching mode through injecting/retracting of oxygen ions. The switching is compliance-free due to the intrinsic series resistor by the filaments formed in the ZrO2 layers. By tuning the RESET voltages, controllable and stable multistate memory can be achieved which clearly points towards the capability of developing the next-generation multistate high-performance memory.