Topological states and phase transitions in Sb2Te3-GeTe multilayers

Topological insulators (TIs) are bulk insulators with exotic ‘topologically protected’ surface conducting modes. It has recently been pointed out that when stacked together, interactions between surface modes can induce diverse phases including the TI, Dirac semimetal, and Weyl semimetal. However, currently a full experimental understanding of the conditions under which topological modes interact is lacking. Here, working with multilayers of the TI Sb2Te3 and the band insulator GeTe, we provide experimental evidence of multiple topological modes in a single Sb2Te3-GeTe-Sb2Te3 structure. Furthermore, we show that reducing the thickness of the GeTe layer induces a phase transition from a Dirac-like phase to a gapped phase. By comparing different multilayer structures we demonstrate that this transition occurs due to the hybridisation of states associated with different TI films. Our results demonstrate that the Sb2Te3-GeTe system offers strong potential towards manipulating topological states as well as towards controlledly inducing various topological phases

Thickness dependence of electron-electron interactions in topological p-n junctions

Electron-electron interactions in topological p-n junctions consisting of vertically stacked topological insulators are investigated. n-type Bi2Te3 and p-type Sb2Te3 of varying relative thicknesses are deposited using molecular beam epitaxy and their electronic properties measured using low-temperature transport. The screening factor is observed to decrease with increasing sample thickness, a finding which is corroborated by semi-classical Boltzmann theory. The number of two-dimensional states determined from electron-electron interactions is larger compared to the number obtained from weak-antilocalization, in line with earlier experiments using single layers

Four-Terminal Ferroelectric Schottky Barrier Field Effect Transistors as Artificial Synapses for Neuromorphic Applications

In this paper, artificial synapses based on four terminal ferroelectric Schottky barrier field effect transistors (FE-SBFETs) are experimentally demonstrated. The ferroelectric polarization switching dynamics gradually modulate the Schottky barriers, thus programming the device conductance by applying negative or postive pulses to imitate the excitation and inhibition behaviors of the biological synapse. The excitatory post-synaptic current can be modulated by the back-gate bias, enabling the reconfiguration of the weight profile with high speed of 20 ns and low energy (< 1 fJ/spike) consumption. Besides, the tunable long term potentiation and depression show high endurance and very small cycle-to-cycle variations. Based on the good linearity, high symmetricity and large dynamic range of the synaptic weight updates, a high recognition accuracy (92.6%) is achieved for handwritten digits by multilayer perceptron artificial neural networks. These findings demonstrate FE-SBFET has high potential as an ideal synaptic component for the future intelligent neuromorphic network

Vertical Ge Gate-All-Around Nanowire pMOSFETs With a Diameter Down to 20 nm

In this work, we demonstrate vertical Ge gate-all-around (GAA) nanowire pMOSFETs fabricated with a CMOS compatible top-down approach. Vertical Ge nanowires with diameters down to 20 nm and an aspect ratio of ~11 were achieved by optimized Cl 2 -based dry etching and self-limiting digital etching. Employing a GAA architecture, post-oxidation passivation and NiGe contacts, high performance Ge nanowire pMOSFETs exhibit low SS of 66 mV/dec, small DIBL of 35 mV/V and a high $\text {I}_{ \mathrm{\scriptscriptstyle ON}}/\text{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ratio of ${2.1}\times {10}^{{6}}$ . The electrical behavior was also studied with temperature-dependent measurements. The deviation between the experimental SS and the ideal kT/q $\cdot$ ln10 values stems from the density of interface traps $(\text {D}_{\text {it}})$ . Our measurements suggest that lowering the top contact resistance is a key to further performance improvement of vertical Ge GAA nanowire transistors

Epitaxy of direct bandgap group IV heterostructure lasers

We demonstrate epitaxial growth of direct bandgap group IV GeSn/SiGeSn double heterostructures and multi quantum wells. While both designs offer high structural quality and strong light emission, multi quantum wells benefit from a smaller number of defects at the active region