Session Q28: Applications of Semiconductors, Dielectrics, Complex Oxides

Anatomy of a Nanoscale Conduction Channel Reveals the Mechanism of a High-Performance Memristor

Two major challenges for resistance memory devices (memristors) based on conductivity changes in oxide materials are better performance and understanding of the microscopic picture of the switching. After researchers' relentless pursuit for years, tantalum oxide-based memristors have rapidly risen to be the top candidate, showing fast speed, high endurance and excellent scalability. While the microscopic picture of these devices remains obscure, by employing a precise method for locating and directly visualizing the conduction channel, here we observed a nanoscale channel consisting of an amorphous Ta(O) solid solution surrounded by crystalline Ta$_{2}$O$_{5}$. Structural and chemical analyses of the channel combined with temperature dependent transport measurements revealed a unique resistance switching mechanism: the modulation of the channel elemental composition, and thus the conductivity, by the cooperative influence of drift, diffusion and thermophoresis, which seem to enable the high switching performance observed. (Miao*, Strachan*, Yang* \textit{et al.}, Advanced Materials. DOI: 10.1002$/$adma201103379 (2011))   

Low-frequency noise properties in nanodiodes

Terahertz (THz) detection by a novel type of unipolar nanodiodes, known as self-switching diodes (SSDs), has recently been demonstrated at room temperature up to 1.5 THz (App. Phys. Lett. Vol. 98, 223501, 2011). Since noise property is also of great importance for THz detection, here we have fabricated thousands of SSDs connected in parallel in order to increase the signal-to-noise ratio. Because of the planar nature of the SSDs, no specific metal interconnects are needed, and hence the device speed and voltage sensitivity are unaffected. We study the low-frequency noise spectra and noise equivalent power (NEP) at room and elevated temperatures. The exceptional possibility for the SSD to have an intrinsic zero threshold voltage enables very low noise at frequencies below 1 kHz, without being much affected by 1/$f $noise. We find that the NEP is comparable to the commercial Schottky diode detector. The activation energy extracted from the temperature dependence is approximately 0.27 eV, which we will compare with the barrier height in the SSD channel as well as the conduction band offset in the InGaAs/InAlAs structure used in this work. We show evidence that the observed 1/$f$ noise properties at room and elevated temperatures seem to support Hooge's mobility fluctuation theory.   

Understanding the resistive switching in thin-film Ta-O memristors by their d.c. switching characteristics

Tantalum oxide (Ta-O) memristor is a promising candidate for resistive switching memory (RRAM) technology as they have demonstrated outstanding features such as high endurance, high speed, and low power. However, the responsible mechanisms remain vague partly due to difficulties in characterizing the amorphous film structure, nanoscale active regions, coupled ionic and electronic transport, and intertwined electrochemical and thermochemical processes. Rich information about Ta-O memristors has been revealed by microscopic structural and chemical characterizations of Ta-O conduction channels combined with temperature-dependent transport measurements. As an alternative approach, we took perspectives from a statistical study of the switching behavior under d.c. excitation. We identified distinctive behaviors in device switching characteristics depending on the chemical compositions of conductance channel, and found close correlations with previous temperature-dependent transport measurements and X-ray photoemission (XPS) characterizations. We were able to gather further insight into the microscopic switching mechanisms based on these observations, revealing the granularity of the switching phenomena.   

\textit{In-situ} MBE and ALD deposited HfO$_{2}$ on In$_{0.53}$Ga$_{0.47}$As

The semiconductor industry is calling for innovative devices offering high performance with low power consumption. High-$\kappa $ dielectrics/metal gates on high carrier mobility channels are now strong contenders in the post Si CMOS application. Hafnium-based oxide has been employed as the gate dielectric in Si CMOS since 45 nm node and InGaAs is a leading candidate for channel materials. However, reports of HfO$_{2}$ on InGaAs are scant, and surface treatments using H$_{2}$S or trimethylaluminum are claimed to be required for achieving high quality HfO$_{2}$(high-$\kappa )$/InGaAs interface. In this work, HfO$_{2}$ has been \textit{in-situ} deposited on $n$- and $p$-In$_{0.53}$Ga$_{0.47}$As using both molecular-beam-epitaxy (MBE) and atomic-layer- deposition (ALD), without using any interfacial passivation layer or surface treatments. The HfO$_{2}$/In$_{0.53}$Ga$_{0.47}$As metal-oxide-semiconductor capacitors (MOSCAPs) all exhibit outstanding thermal stabilities ($>$ 800$^{\circ}$C), low leakage currents ($\sim $ 10$^{-8}$ A/cm$^{2}$ at 1 MV/cm), and good CV characteristics. Moreover, the MOSCAPs have shown spectra of interfacial trap densities (D$_{it}$'s) with no discernible peaks at mid-gap, confirmed by temperature-dependent conductance method.   

\textit{In-situ} photoemission analyses of ALD-oxide/In$_{x}$Ga$_{1-x}$As (001) interfaces

High-$\kappa $ dielectrics on high carrier mobility channels, such as In$_{x}$Ga$_{1-x}$As, are now being considered for CMOS technology beyond 15 nm node. The initial bonding of high-$\kappa $/InGaAs determines the value and the distribution of interfacial density of states (D$_{it})$ within the In$_{x}$Ga$_{1-x}$As band gap, key to the device performance. In this work, atomic layer deposited (ALD) HfO$_{2}$ and Al$_{2}$O$_{3}$ on MBE-grown In$_{x}$Ga$_{1-x}$As (001) have been \textit{in-situ} and \textit{ex-situ} carried out to investigate the initial stage of interfacial reactions by high resolution photoemission spectroscopy using synchrotron radiation and monochromatic Al Ka x-ray sources. Comparing the results with the corresponding electrical measurements (C-V and G-V at various temperatures), Fermi level unpinning in the oxide/In$_{x}$Ga$_{1-x}$As hetero-structure may be attributed to the exclusion of the As-As and the As-O bonding during the initial interfacial formation.   

High Temperature Seebeck Coefficient and Electrical Resistivity of Ge$_{2}$Sb$_{2}$Te$_{5}$ Thin Films

Phase-change memory (PCM) is a promising memory technology in which a small volume of a chalcogenide material can be reversibly and rapidly switched between amorphous and crystalline phases by an electrical pulse that brings it above crystallization ($\sim $ 150-200 C) or melting ($\sim $ 700 C) temperature. The large temperature levels involved and small dimensions of PCM devices give rise to very large temperature gradients ($\sim $ 10 K/nm and higher) which result in strong thermoelectric effects. High-temperature characterization of the temperature-dependent thermoelectric properties of these materials is therefore critical to understand for the operation of these devices but to date there is only limited experimental data available. We have performed simultaneous measurements of Seebeck coefficient and electrical resistance of thin films of GST with different thicknesses, deposited on silicon dioxide, from room temperature to $\sim $ 600 C, under small temperature gradients. Two-point current-voltage (I-V) measurements were performed using a semiconductor parameter analyzer. The resistance of the material and the Seebeck voltage (open-circuit voltage) are calculated from the slope and intercept of the I-V characteristics. The details of the measurements and S(T) and R(T) results for the GST thin film samples will be presented and discussed.   

Thermoelectric Effects in Simulations of Phase Change Memory Mushroom Cells

Phase change memory is a potential candidate for the future of high-speed non-volatile memory, however significant improvements in cell design is crucial for its success in the mainstream market. Due to the asymmetric geometry of phase change mushroom cells and the high temperature gradients generated, thermoelectric effects play a key role in determining energy consumption, cell performance, and reliability. In this study, rotationally symmetric 2D finite element simulations using COMSOL Multiphysics are implemented for GeSbTe (GST). Temperature dependent material parameters (electrical conductivity, thermal conductivity, heat capacity, and Seebeck coefficient) are included in the model for accuracy. Switching the direction of current shows a large change in peak molten volume within the cell, as well as current and power consumption.   

Light-controlled plasmon switching using hybrid metal-semiconductor nanostructures

We show a method for the dynamic control over the plasmon resonance frequencies in a hybrid metal-semiconductor nanoshell structure with silver core and TiO$_{2}$ coating. We temporarily change the dielectric function of TiO$_{2}$ using pump laser pulse operating at bandgap or above. This generates free electron-hole pairs in TiO$_{2 }$that alter the dielectric environment for the silver core. The probed surface plasmon frequency lying below bandgap appears to be blue-shifted due to the altered dielectric environment. We calculate the magnitude of the plasmon resonance wavelength shift as a function of electron-hole pair density and obtain shifts up to 126 nm at wavelengths of around 460 nm. Using these results, we propose a model of a light-controlled surface plasmon polariton (SPP) switch.   

Direct-bandgap infrared light emission from tensilely strained germanium nanomembranes

Silicon, germanium, and related alloys, which provide the leading materials platform of electronics, are extremely inefficient light emitters because of their indirect fundamental energy bandgap. This basic materials property has so far hindered the development of group-IV photonic active devices, including diode lasers, thereby significantly limiting our ability to integrate electronic and photonic functionalities at the chip level. We show that Ge nanomembranes can be used to overcome this materials limitation. Theoretical studies have predicted that tensile strain in Ge lowers the direct energy bandgap relative to the indirect one. We demonstrate [1] that mechanically stressed nanomembranes allow for the introduction of sufficient biaxial tensile strain to transform Ge into a direct-bandgap, efficient light-emitting material that can support population inversion and therefore provide optical gain. \\[4pt] [1] J. R. S\'{a}nchez-P\'{e}rez, C. Boztug, F. Chen, F. Sudradjat, D. M. Paskiewicz, RB. Jacobson, M. G. Lagally, and R. Paiella, PNAS web published Nov, 14, 2011   

A Tunable Terahertz Detector Based On Self Assembled Plasmonic Structure on a GaAs 2DEG

The use of tunable gated gratings on 2DEG structure has been well known methods for compact frequency sensitive THz detection based on the resonant absorption of the 2D Plasmon. The resonant frequency is dependent on system dimension and the tunability of that dimension by depletion gating. Here we attempt to improve detector sensitivity, tunability and remove polarization dependence through the development of a gated grid design. To satisfy the requirement for imaging applications of device dimensions on the order of $<$ 1 micron over a detector area of 4 mm$^{2}$, we have fabricated gated grid plasmonic structure on 2DEG material by using nanosphere self-assembly lithography. This fabrication method has not been widely developed for III-V processing but allows us to achieve large area sensitive detectors with tunability in the 1-4 THz range. In this paper we will discuss the characterization of the devices as a function of gate bias, magnetic field, and temperature using FTIR and THz time domain measurements.   

Time-resolved second harmonic generation study of buried semiconductor heterointerfaces using soliton-induced transparency

The transient second harmonic generation (SHG) and linear optical reflectivity (LOR) signals measured simultaneously in reflection from GaAs/GaSb/InAs and GaAs/GaSb heterostructures revealed a new mechanism for creating self-induced transparency in narrow bandgap semiconductors at low temperatures, which is based on the dual-frequency electro-optic soliton propagation. The mechanism takes account of the photo-Dember field solitary wave, which traps both the fundamental and SHG pulses, slowing their velocity down to that of the solitary wave. The trapped light pulses maintain the amplitude of the solitary wave and hence create a condition, at which the self-reinforcing nonlinear optical polarization (dual-frequency electro-optic soliton) can propagate through the semiconductor. This allows the ultrafast carrier dynamics at buried semiconductor heterointerfaces to be studied.   

Room temperature ballistic transport in InSb quantum well structures

We report significant advancements in InSb/AlInSb quantum well (QW) heterostructures for room temperature nanoelectronic applications. InSb/AlInSb heterostructures have phenomenally high room temperature mobility but display intrinsic parallel conduction in the buffer layer limiting exploitation for nanostructured devices where deep isolation etch processing is impractical. We demonstrate a strategy to reduce the parasitic conduction by the insertion of a pseudomorphic barrier layer of wide-band-gap alloy below the QW.\footnote{A.M. Gilbertson, P.D. Buckle, T. Ashley, L.F. Cohen, Phys Rev B \textbf{84}, 075474 (2011).} Mesoscopic geometric nanocrosses fabricated from such material clearly demonstrate ballistic transport at room temperature, as evidenced by very significant negative bend resistance (NBR). We have studied the interplay between sidewall and bulk scattering at 300K in relation to quantum calculations. DC measurements in the non-equilibrium (hot carrier) regime reveal that electrons remain ballistic at current densities in excess of 10$^{6}$ A/cm$^{2}$.   

Spectral and Spatial Response of Sulfur-Hyperdoped n+/p Silicon Photodiodes

Pulsed laser melting of implanted silicon can enable doping well above equilibrium concentrations. Sulfur doping leads to a deep donor state that may form an impurity band at high enough concentrations. Photodiodes formed from sulfur-hyperdoped n+ layers on a p-type wafer have shown external quantum efficiency of much greater than 100\%, as well as enhanced infrared response. In this paper we report on optoelectronic characterization of diodes prepared by implantation of 10$^{15}-10^{16}$ sulfur/cm2 into a p-type wafer, followed by nanosecond pulsed laser melting and recrystallization. Experimental results from wavelength-dependent diode response, spatial quantum efficiency mapping, intensity dependent efficiency, and current-voltage techniques will be reported. We will also discuss potential models for the observed behavior.   

The mesoscopic chaotic cavity as a rectifying heat engine

We present an exactly solvable model of a mesoscopic heat engine that works using the principle of rectifying thermal fluctuations applied to a nonlinear system. The system is a chaotic mesoscopic cavity where the contact transmission to leads is energy-dependent. This energy-dependence is generic in mesoscopic conductors, and leads to an intrinsic nonlinearity. The cavity is coupled capacitively to another conductor, held at a different temperature. The nonlinear cavity rectifies the thermal fluctuations, leading to a hot spot rectified electrical current that is proportional to the asymmetry in the energy-dependence of the contacts, and to the temperature difference. We will discuss the maximum power produced by the system, as well as the efficiency of the engine by comparing it to the heat current that passes between the coupled systems. Possible practical energy-harvesting applications will be proposed.   

Thermal properties of CuGaS$_2$ from first principles

We have investigated in the past the specific heats of monatomic and binary semiconductors and their dependence on temperature and isotopic mass, both experimentally and theoretically. The theoretical calculations were performed \textit{ab initio} with LDA exchange correlations. We are at present carrying over these investigations to ternary materials, in particular to those with chalcopyrite structure. We present here results involving the dependence of the specific heat and other physical properties (lattice parameters, volume thermal expansion, phonon dispersion) of the chalcopyrite CuGaS$_2$ on temperature and on the isotopic masses of the three constituent atoms. Particular emphasis is paid to the maxima of $C_p/T^3$ found at low temperatures which correspond to the deviation of Debye's law related to transverse acoustic phonons near the zone boundary. The calculations were performed with the ABINIT and VASP codes within the local density approximation for exchange and correlation. The results are shown to be in excellent agreement with the experimental data. Correlation with similar compounds such as CuAlS$_2$ and CuInS$_2$ is discussed.