Matthias Grube

High Area Capacity Lithium-Sulfur Full-cell Battery with Prelitiathed Silicon Nanowire-Carbon Anodes for Long Cycling Stability

We show full Li/S cells with the use of balanced and high capacity electrodes to address high power electro-mobile applications. The anode is made of an assembly comprising of silicon nanowires as active material densely and conformally grown on a 3D carbon mesh as a light-weight current collector, offering extremely high areal capacity for reversible Li storage of up to 9 mAh/cm2. The dense growth is guaranteed by a versatile Au precursor developed for homogenous Au layer deposition on 3D substrates. In contrast to metallic Li, the presented system exhibits superior characteristics as an anode in Li/S batteries such as safe operation, long cycle life and easy handling. These anodes are combined with high area density S/C composite cathodes into a Li/S full-cell with an ether- and lithium triflate-based electrolyte for high ionic conductivity. The result is a highly cyclable full-cell with an areal capacity of 2.3 mAh/cm2, a cyclability surpassing 450 cycles and capacity retention of 80% after 150 cycles (capacity loss <0.4% per cycle). A detailed physical and electrochemical investigation of the SiNW Li/S full-cell including in-operando synchrotron X-ray diffraction measurements reveals that the lower degradation is due to a lower self-reduction of polysulfides after continuous charging/discharging.

Mesoscopic analysis of leakage current suppression in ZrO2/Al2O3/ZrO2 nano-laminates

Metal-Insulator-Metal capacitors, with ZrO2/Al2O3/ZrO2 (ZAZ)-nanolaminate thin-films as a dielectric layer, exhibit reduced leakage currents compared to corresponding capacitors based on pure ZrO2 while maintaining a sufficiently high dielectric constant for the DRAM application. This work is a comparative study demonstrating how the incorporation of a small amount of Al is responsible for the suppression of crystallization during deposition. Extensive electrical characterization leads to the identification of a defect band which conductive atomic force microscopy shows to be formed along crystallite grain boundaries, extending through the entire ZrO2-film. The incorporation of a sub-layer of Al2O3 prevents these grain boundaries resulting in an effective reduction of leakage currents, despite the film being in the nanocrystalline phase, necessary for it to exhibit the required high dielectric constant. A transport model based on phonon assisted trap to trap tunneling is proposed.

Reconfigurable Nanowire Electronics-Enabling a Single CMOS Circuit Technology

Reconfigurable nanowire transistors are multifunctional switches that fuse the electrical characteristics of unipolar n- and p-type field effect transistors (FETs) into a single universal type of four-terminal device. In addition to the three known FET electrodes the fourth acts as an electric select signal that dynamically programs the desired polarity. The transistor consists of two independent charge carrier injection valves as realized by two gated Schottky junctions integrated within an intrinsic silicon nanowire. The transport properties that provide unipolar n- and p-type behavior will be elucidated. Further, solutions to the major device challenges toward the implementation of these novel transistors at the circuit level are proposed, by exploiting specific nanowire geometries and dimensions. These include methods that deliver equal on-currents and symmetric transfer characteristics for n- and p-type, and that eliminate supra-linear output characteristics at low source-drain biases. We will further show that circuits built of these symmetric transistors successfully exhibit complementary operation. Finally, the prospects in building reconfigurable circuits and systems will be briefly summarized.

Local charge transport in nanoscale amorphous and crystalline regions of high-k (Zr02)0.8(Al2O3)0.2 thin films

The charge transport in (Zr02)0.8(Al2O3)0.2 thin films consisting of nanoscale crystallites in an amorphous matrix were investigated by conductive atomic force microscopy. Local current-voltage curves were obtained either from multiple current images for different biases or by ramping the tip bias at a specific location. Comparison of both approaches for amorphous matrix sites implies that they are equivalent. Current-voltage curves for crystallite and amorphous sites are different, but repeated voltage ramps at amorphous sites yield curves as obtained at pristine crystallites. This suggests charge transport through leakage paths along defects at grain boundaries and along stress-induced defects at amorphous sites.

Macroscopic and microscopic electrical characterizations of high-k ZrO2 and ZrO2/Al2O3/ZrO2 metal-insulator-metal structures

In order for sub-10 nm thin films of ZrO2 to have a dielectric constant larger than 30 they need to be crystalline. This is done by either depositing the layer at higher temperatures or by a postdeposition annealing step. Both methods induce high leakage currents in ZrO2 based dielectrics. In order to understand the leakage a thickness series of ultrathin ZrO2 and nanolaminate ZrO2/Al2O3/ZrO2 (ZAZ) films, deposited by atomic layer deposition, was investigated. After deposition these films were subjected to different rapid thermal annealing (RTA) processes. Grazing incidence x-ray diffraction and transmission electron microscopy yield that the crystallization of ZrO2 during deposition is dependent on film thickness and on the presence of an Al2O3 sublayer. Moreover, the incorporation of Al2O3 prevents crystallites from spanning across the entire film during RTA. C-V and I-V spectroscopies show that after a 650 °C RTA in N2 the capacitance equivalent oxide thickness of 10 nm ZAZ films is reduced to 1.0 nm w...

Influence of composition and bottom electrode properties on the local conductivity of TiN/HfTiO2 and TiN/Ru/HfTiO2 stacks

HfTiO2 layers of various stoichiometries where deposited by physical vapor depostion on TiN and TiN/Ru bottom electrodes (BE) in order to determine the influence of composition, conduction band offset, and BE morphology on the overall leakage current characteristics. Current-voltage spectroscopy, transmission electron microscopy, electron energy loss spectroscopy, and conductive atomic force microscopy studies show increased leakage current and charge trapping with increased Ti content. The interplay of conduction band offset and trap density were studied. The influence of Ru bottom electrode roughness on the leakage current is higher than the influence of Ti content and low conduction band offset.

Applicability of molecular beam deposition for the growth of high-k oxides

Following the demand of replacing conventional dielectrics in the semiconductor industry, a material screening for new high-k dielectrics is necessary. In this article, the molecular beam deposition is presented as a versatile and valuable tool for growing dielectric films. ZrO2 was chosen as an example to demonstrate the capability of molecular beam deposition to grow thin high-k dielectrics in a metal-insulator-metal stack. A k-value from 21 to 26 could be achieved for as-grown films. This could be improved even further up to 30 by performing postdepositions anneals that result in a capacitance equivalent thickness of 1.5 nm at a leakage current density of 1.5×10−7 A/cm2. In addition, the crystallization behavior of ZrO2 was investigated.

Molecular beam deposited zirconium dioxide as a high-κ dielectric for future GaN based power devices

Molecular beam deposited zirconium dioxide (ZrO2) was assessed as high-κ gate dielectric for future GaN based devices. To compare and study electrical and structural properties, thin ZrO2 films were deposited on three different substrates, n++-c-plane GaN, p-(100) Si, and TiN. The films were fabricated by electron beam evaporation from a single stoichiometric ZrO2 target. A substrate-independent phase transition from amorphous ZrO2 to the tetragonal/cubic phase was identified by gracing incidence x-ray diffractometry. Finally, monoclinic ZrO2 emerged with increasing film thickness. As found by x-ray photoelectron spectroscopy, ZrO2 formed an abrupt interface to both GaN and TiN without intermixture. Dielectric constants in the range of 14–25 were extracted from capacitance versus voltage measurements for as-deposited ZrO2 films. The leakage currents of ZrO2 on GaN resembled their counterparts on Si as well as on TiN.

Structural and dielectric properties of sputtered SrxZr(1−x)Oy

Over the past years, high-k dielectrics have been incorporated into modern semiconductor devices. One example is ZrO2, which has been introduced in memory applications. This paper elucidates some difficulties with pure ZrO2 like unintended crystallization during the growth of the dielectric and the evolution of the monoclinic phase, which reduces the k-value. The admixture of Sr is shown as a solution to circumvent those issues. A detailed structural analysis for a varying stoichiometry ranging from pure ZrO2 to the perovskite SrZrO3 is given. The detected crystal structures are correlated to our observations of the dielectric properties obtained by an electrical characterization.

Investigation of zirconium oxide based high-k dielectrics for future memory applications

Zirconium oxide based high-k dielectrics are studied for their possible future integration in Dynamic Random Access Memory (DRAM) Metal-Insulator-Metal (MIM) capacitors. To better understand their electrical properties a combined study is presented considering both their macroscopic and mesoscopic electrical behavior. Molecular beam deposition is used to deposit thin films of ZrO2 as well as (ZrO2) x(Al2O3)1−x. Three different ZrO2 deposition methods are compared and evaluated by capacitance voltage (CV) and current voltage (IV) measurements. These deliver valuable information for benchmarking. To further understand the leakage current mechanism in nanometer thin zirconate based films conductive atomic force microscopy is performed. IV curves are taken with high spatial resolution enabling a detailed comparison between the leakage paths at crystallites and in the amorphous matrix. Through this method the evolution of the leakage path formation can be traced.