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Carbon nanotubes (CNTs) have great potential for future high-performance and energy-efficient transistor technology. To realize this potential, methods to dope the CNTs need to be developed to achieve low parasitic resistance of the transistor. Two key issues present themselves: (a) understanding the doping mechanism of the various methods and (b) stability of the doping method. For instance, although studies on molybdenum oxide (MoOx) demonstrate its ability to heavily dope nanomaterials, the interaction between MoOx and the CNT is unclear. Here, we observe an unstable effect of MoOx on the CNT and demonstrate dielectric passivation as a means to preserve the doping strength. The semiconducting CNTs exhibit greater than 103× reduction in resistance after stably doped with MoOx. By exploiting the instability of MoOx, we delve deeper into clarifying the doping mechanism. The relationship between the time-dependent material property of MoOx and the change in the electrical measurements of CNT devices is investigated to study the role of work function in doping the CNTs. We conclude that the doping mechanism of MoOx on the CNT is due to bandgap modulation by charge transfer, which occurs due to the difference in work function between MoOx and the CNT. [ABSTRACT FROM AUTHOR]

Phase Change Memory (PCM) is a leading candidate for next generation data storage, but it typically suffers from high switching (RESET) current density (20–30 MA/cm2). Interfacial Phase Change Memory (IPCM) is a type of PCM using multilayers of Sb2Te3/GeTe, with up to 100× lower reported RESET current compared to the standard Ge2Sb2Te5-based PCM. Several hypotheses involving fundamentally new switching mechanisms have been proposed to explain the low switching current densities, but consensus is lacking. Here, we investigate IPCM switching by analyzing its thermal, electrical, and fabrication dependencies. First, we measure the effective thermal conductivity (∼0.4 W m−1 K−1) and thermal boundary resistance (∼3.4 m2 K GW−1) of Sb2Te3/GeTe multilayers. Simulations show that IPCM thermal properties account only for an ∼13% reduction of current vs standard PCM and cannot explain previously reported results. Interestingly, electrical measurements reveal that our IPCM RESET indeed occurs by a melt-quench process, similar to PCM. Finally, we find that high deposition temperature causes defects including surface roughness and voids within the multilayer films. Thus, the substantial RESET current reduction of IPCM appears to be caused by voids within the multilayers, which migrate to the bottom electrode interface by thermophoresis, reducing the effective contact area. These results shed light on the IPCM switching mechanism, suggesting that an improved control of layer deposition is necessary to obtain reliable switching. [ABSTRACT FROM AUTHOR]

In this work, we introduce a novel nonalloyed contact structure for n-GaAs and n-In0.53Ga0.47As by using single metals in combination with a thin dielectric to tune the effective metal/III-V work function toward the conduction band edge. We reduced the effective Schottky barrier height ([uppercase_phi_synonym]B,eff) of Al/GaAs from 0.75 to 0.17 eV through the use of a thin atomic layer deposition Al2O3. Barrier height reduction was verified for a variety of metals (Y, Er, Al, Ti, and W) through direct measurements and deduced from increased diode current and reduced contact resistance. Similar results were observed on n-In0.53Ga0.47As. Two possible underlying mechanisms are discussed: one based on the formation of a dielectric dipole and the other based on the blocking of metal induced gap states. This structure has applications as a nonalloyed low resistance ohmic contact for III-V metal-oxide-semiconductor field-effect transistors (MOSFETs) or high electron mobility transistors (HEMTs), and as a near zero barrier height contact for III-V Schottky barrier field-effect transistors or diodes. [ABSTRACT FROM AUTHOR]

In this work, we investigate a low temperature boron (B) and phosphorus (P) activation in amorphous (α)-Ge using metal-induced crystallization technique. Eight candidates of metals (Pd, Cu, Ni, Au, Co, Al, Pt, and Ti) are used to crystallize the α-Ge at a low temperature. Resistivity measurement, transmission electron microscopy, and x-ray diffraction analyses reveal behaviors of the metal-induced dopant activation process using the metals reacting with α-Ge. It is revealed that Co achieves the highest B and P activation ratio in Ge below 360 °C with a slow diffusion rate. This method can be utilized to activate gate, source, and drain of transistors on upper layers in three-dimensional integrated circuits, where low temperature processing is critical. [ABSTRACT FROM AUTHOR]

Schottky barrier height modulation in metal/Ge Schottky junction was demonstrated by inserting an ultrathin interfacial silicon nitride layer. The SiN interfacial layer suppressed strong Fermi level pinning in metal/Ge Schottky junction, which resulted in effective control of Schottky barrier height. Metal/SiN/Ge Schottky diode was systematically investigated in terms of SiN thickness dependence and metal work function dependence. At an optimal SiN thickness, Ohmic contact between metal and Ge was obtained as a result of Fermi level depinning, and almost ideal Schottky barrier height determined by the work function difference between the metal and Ge was achieved. This technology was finally applied to metal source/drain Ge metal-oxide-semiconductor field-effect-transistors with low source/drain resistance. [ABSTRACT FROM AUTHOR]

We developed a fully analytical ballistic theory of carbon nanotube field effect transistors enabled by the development of an analytical surface potential capturing the temperature dependence and gate and quantum capacitance electrostatics. The analytical ballistic theory is compared to the experimental results of a ballistic transistor with good agreement. The validated analytical theory enables intuitive circuit design, provides techniques for parameter extraction of the bandgap and surface potential, and elucidates on the device physics of drain optical phonon scattering and its role in reducing the linear conductance and intrinsic gain of the transistor. Furthermore, a threshold voltage definition is proposed reflecting the bandgap-diameter dependence. Projections for key analog and digital performances are discussed. [ABSTRACT FROM AUTHOR]

Crystallization behavior of scalable phase change materials can be studied on nanoscale structures. In this paper, high density ordered phase change nanodot arrays were fabricated using the lift-off technique on a self-assembled diblock copolymer template, polystyrene-poly(methyl-methacrylate). The size of the nanodots was less than 15 nm in diameter with 40 nm spacing. This method is quite flexible regarding the patterned materials and can be used on different substrates. The crystallization behavior of small scale phase change nanodot arrays was studied using time-resolved x-ray diffraction, which showed the phase transition for different materials such as Ge15Sb85, Ge2Sb2Te5, and Ag and In doped Sb2Te. The transition temperatures of these nanodot samples were also compared with their corresponding blanket thin films, and it was found that the nanodots had higher crystallization temperatures and crystallized over a broader temperature range. [ABSTRACT FROM AUTHOR]

A full-band analytical model of the equilibrium carrier density for single-wall semiconducting carbon nanotubes (sCNTs) is presented. The carrier density, which is a fundamental property of all semiconductors, is obtained using a semiempirical method for degenerate positions of the Fermi level and shows good agreement with numerical tight-binding results. The quantum capacitance is subsequently derived from the carrier density and used to develop a C-V model with good agreement with experimental quantum capacitance measurements. An analytical model of the gate coupling function of sCNTs is also reported which relates the internal surface potential with the external applied gate voltage. The diameter temperature and Fermi level dependency, and the essential properties of carbon nanotubes device physics are captured in these analytical equations. [ABSTRACT FROM AUTHOR]

Threshold switches, which typically exhibit an abrupt increase in current at an onset voltage, have been used as selector devices to suppress leakage current in crosspoint arrays of two-terminal resistive switching memory devices. One of the most important metrics for selector devices is the leakage or low-voltage current, which limits the maximum achievable size of the crosspoint memory array. Here, we show that for self-heating-triggered threshold switches, there is an intrinsic lower limit to the leakage current resulting from the need to avoid an electric field-induced breakdown of the active material. We provide a quantitative theoretical estimate of this limit for NbOx threshold switches, one of the most widely studied selectors, and provide a plausible explanation for the experimentally observed leakage currents in NbOx. Our results provide some guidelines for achieving minimum leakage currents in threshold switches. [ABSTRACT FROM AUTHOR]

The measurement of inhomogeneous conductivity in optically crystallized, amorphous Ge2Sb2Te5 (GST) films is demonstrated via scanning microwave impedance microscopy (MIM). Qualitative consistency with expectations is demonstrated in spots crystallized by focused coherent light at various intensities, exposure times, and film thicknesses. The characterization of process imperfections is demonstrated when a mask is used to optically pattern the nanoscale features of crystalline GST in the amorphous film. These measurements show the ability of MIM to resolve partial crystallization, patterning faults, and other details in optically patterned GST. [ABSTRACT FROM AUTHOR]

Phase change memory (PCM) is an emerging data storage technology; however, its programming is thermal in nature and typically not energy-efficient. Here, we reduce the switching power of PCM through the combined approaches of filamentary contacts and thermal confinement. The filamentary contact is formed through an oxidized TiN layer on the bottom electrode, and thermal confinement is achieved using a monolayer semiconductor interface, three-atom thick MoS2. The former reduces the switching volume of the phase change material and yields a 70% reduction in reset current versus typical 150 nm diameter mushroom cells. The enhanced thermal confinement achieved with the ultra-thin (∼6 Å) MoS2 yields an additional 30% reduction in switching current and power. We also use detailed simulations to show that further tailoring the electrical and thermal interfaces of such PCM cells toward their fundamental limits could lead up to a sixfold benefit in power efficiency. [ABSTRACT FROM AUTHOR]

Niobium oxide (NbOx) two-terminal threshold switches are potential candidates as selector devices in crossbar memory arrays and as building blocks for neuromorphic systems. However, the physical mechanism of NbOx threshold switches is still under debate. In this paper, we show that a thermal feedback mechanism based on Poole-Frenkel conduction can explain both the quasi-static and the transient electrical characteristics that are experimentally observed for NbOx threshold switches, providing strong support for the validity of this mechanism. Furthermore, a clear picture of the transient dynamics during the thermal-feedback-induced threshold switching is presented, providing useful insights required to model nonlinear devices where thermal feedback is important. [ABSTRACT FROM AUTHOR]

An AC stress test was performed to investigate the accompanying electronic effects in a HfO2 resistive random access memory during the SET transition, which featured a sudden decrease in resistance. Comparing the DC and AC measurement results indicated the pronounced influence of interrupted stress on both the mean values and variations of time to SET. First-principles calculations suggested that the charge states (+2, +1, or neutral) of oxygen vacancies affect the migration barrier for forming oxygen vacancy clusters. Therefore, a charge-state-dependent SET model is proposed to include the additional electronic effects induced by the dynamics of electron trapping and detrapping in oxygen vacancies during AC stress. A trimodal Weibull fitting based on the proposed model reproduced the experimental time to SET distributions obtained in a wide range of AC stress conditions. [ABSTRACT FROM AUTHOR]

Gold catalyzed vapor-liquid-solid method is widely applied to III-V nanowire (NW) growth on Si substrate. However, the easy oxidation of Si, possible Si contamination in the NWs, high defect density in the NWs, and high sensitivity of the NW morphology to growth conditions largely limit its controllability. In this work, we developed a buffer layer technique by introducing a GaAs thin film with predefined polarity as a template. It is found that samples grown on these buffer layers all have high vertical NW yields in general, due to the single-orientation of the buffer layers. Low temperature buffer with smoother surface leads to highest yield of vertical NWs, while high temperature (HT) buffer with better crystallinity results in perfect NW quality. The defect-free property we observed here is very promising for optoelectronic device applications based on GaAs NW. Moreover, the buffer layers can eliminate Si contamination by preventing Si-Au alloy formation and by increasing the thickness of the Si diffusion barrier, thus providing more flexibility to vertical NW growth. The buffer layer technique we demonstrated here could be easily extended to other III-V on Si system for electronic and photonic applications [ABSTRACT FROM AUTHOR]

We have developed a highly sensitive integrated capacitance bridge for quantum capacitance measurements. Our bridge, based on a GaAs HEMT amplifier, delivers attofarad (aF) resolution using a small AC excitation at or below kBT over a broad temperature range (4-300 K). We have achieved a resolution at room temperature of [formula] for a 10 mV ac excitation at 17.5 kHz, with an improved resolution at cryogenic temperatures, for the same excitation amplitude. We demonstrate the utility of our bridge for measuring the quantum capacitance of nanostructures by measuring the capacitance of top-gated graphene devices and cleanly resolving the density of states. [ABSTRACT FROM AUTHOR]

The conduction mechanism of metal oxide resistive switching memory is debated in the literature. We measured the I-V characteristics below the switching voltages through TiN/HfOx/Pt memory stack and found the conduction cannot be described by the commonly used Poole-Frenkel model, because the fitted dielectric constant and the trap energy are unreasonable as compared to their known values. Therefore, we provide an alternate viewpoint based on a trap-assisted-tunneling model. Agreement of the bias polarity/temperature/resistance state-dependent conduction behavior was achieved between this model and experimental data. And insights for the multilevel capability due to the control of tunneling distance were obtained. [ABSTRACT FROM AUTHOR]

HfOx/AlOx bilayer resistive switching devices were fabricated for the study of the switching dynamics of the metal oxide memory. An exponential voltage-time relationship was experimentally observed as follows: the programming pulse widths need for switching exponentially decreased with the increase in the programming pulse amplitudes. Two following programming schemes were proposed to modulate the high resistance state values: (1) exponentially increase the programming pulse width; (2) linearly increase the programming pulse amplitude. Although both of these schemes were effective to achieve the target resistance, the transient current response measurements suggest the second scheme consumes considerably less energy in the programming. A field-driven oxygen ions migration model was utilized to elucidate the above experimentally observed phenomenon. [ABSTRACT FROM AUTHOR]

We demonstrate the integration of a nonalloyed contact structure developed for III-V semiconductors to the source/drain of an In0.53Ga0.47As metal-oxide-semiconductor field effect transistor (MOSFET) with atomic layer deposition (ALD) Al2O3 high-κ dielectric. This nonalloyed contact utilizes single metals in combination with dielectric dipoles to tune the metal/semiconductor Femi level pinning toward the semiconductor's band edge for minimal contact resistance and barrier height. The metal/III-V semiconductor effective barrier heights have been reduced by as much as 0.40 eV using SiN and 0.55 eV using Al2O3 dielectrics. With this InGaAs nMOSFET as a starting point, this contact can be applied to other III-V n-type MOSFETs, p-type MOSFETs, and Schottky Barrier field effect transistors. [ABSTRACT FROM AUTHOR]

In this paper, high-performance back-gated carbon nanotube field-effect transistors based on transferred aligned carbon nanotubes were fabricated and studies found that the on/off ratio can reach 107 and the current density can reach 1.6 μA/μm after electrical breakdown. In addition, chemical doping with hydrazine was used to convert the p-type aligned nanotube devices into n-type. These devices were further utilized to demonstrate various logic circuits, including p-type metal-oxide-semiconductor inverters, diode-loaded inverters, complementary metal-oxide-semiconductor inverters, NAND, and NOR gates. This approach could work as the platform for future nanotube-based nanoelectronics. [ABSTRACT FROM AUTHOR]

Thermal conduction in GeSbTe films strongly influences the writing energy and time for phase change memory (PCM) technology. This study measures the thermal conductivity of Ge2Sb2Te5 between 25 and 340 °C for layers with thicknesses near 60, 120, and 350 nm. A strong thickness dependence of the thermal conductivity is attributed to a combination of thermal boundary resistance (TBR) and microstructural imperfections. Stoichiometric variations significantly alter the phase transition temperatures but do not strongly impact the thermal conductivity at a given temperature. This work makes progress on extracting the TBR for Ge2Sb2Te5 films, which is a critical unknown parameter for PCM simulations. [ABSTRACT FROM AUTHOR]

Self-assembling diblock copolymer, polystyrene-b-poly-4-vinylpyridine (PS-b-P4VP), was used as the template for fabricating phase change nanostructures. The high density GeSb nanodots were formed by etching into an amorphous GeSb thin film using silica hard mask which was patterned on top of polymer. The nanodot arrays are 15 nm in diameter with 30 nm spacing. This is smaller than most structures obtained by e-beam lithography. Time-resolved x-ray diffraction studies showed that the phase transition occurred at 235 °C, which is 5 °C lower than blanket GeSb film but higher than that of Ge2Sb2Te5 (150 °C). GeSb showed good temperature stability for fabrication of small memory devices. [ABSTRACT FROM AUTHOR]

SiGe-on-insulator material was fabricated by wafer bonding and hydrogen-induced layer transfer techniques. The transferred SiGe layer is strain relaxed and has a Ge content ranging from 15% to 25%. High-quality strained Si layers were grown on the SiGe-on-insulator substrates by the UHV/chemical vapor deposition process at 550 °C. An electron mobility of 40 000 cm[sup 2]/V s in a modulation-doped Si/SiGe heterostructure was achieved at 30 K on a SiGe-on-insulator substrate. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Threshold switches based on niobium oxide (NbOx) are promising candidates as bidirectional selector devices in crossbar memory arrays and building blocks for neuromorphic computing. Here, it is experimentally demonstrated that the electrical characteristics of NbOx threshold switches can be tuned by engineering the thermal insulation. Increasing the thermal insulation by ∼10× is shown to produce ∼7× reduction in threshold current and ∼45% reduction in threshold voltage. The reduced threshold voltage leads to ∼5× reduction in half-selection leakage, which highlights the effectiveness of reducing half-selection leakage of NbOx selectors by engineering the thermal insulation. A thermal feedback model based on Poole-Frenkel conduction in NbOx can explain the experimental results very well, which also serves as a piece of strong evidence supporting the validity of the Poole-Frenkel based mechanism in NbOx threshold switches. [ABSTRACT FROM AUTHOR]

The measurement of inhomogeneous conductivity in optically crystallized, amorphous Ge2Sb2Te5 (GST) films is demonstrated via scanning microwave impedance microscopy (MIM). Qualitative consistency with expectations is demonstrated in spots crystallized by focused coherent light at various intensities, exposure times, and film thicknesses. The characterization of process imperfections is demonstrated when a mask is used to optically pattern the nanoscale features of crystalline GST in the amorphous film. These measurements show the ability of MIM to resolve partial crystallization, patterning faults, and other details in optically patterned GST. [ABSTRACT FROM AUTHOR]

Phase change memory (PCM) is an emerging data storage technology; however, its programming is thermal in nature and typically not energy-efficient. Here, we reduce the switching power of PCM through the combined approaches of filamentary contacts and thermal confinement. The filamentary contact is formed through an oxidized TiN layer on the bottom electrode, and thermal confinement is achieved using a monolayer semiconductor interface, three-atom thick MoS2. The former reduces the switching volume of the phase change material and yields a 70% reduction in reset current versus typical 150 nm diameter mushroom cells. The enhanced thermal confinement achieved with the ultra-thin (∼6 Å) MoS2 yields an additional 30% reduction in switching current and power. We also use detailed simulations to show that further tailoring the electrical and thermal interfaces of such PCM cells toward their fundamental limits could lead up to a sixfold benefit in power efficiency. [ABSTRACT FROM AUTHOR]