Avery Green

Surface oxidation of the topological insulator Bi2Se3

An understanding of the aging and oxidation of the (0001) surface of Bi2Se3 is critical to a comprehensive physical picture of its topologically protected surface states. Here, the authors contribute new experimental observations about the aging and oxidation process. The authors find that surface aging in ambient conditions occurs in two major steps. Within 2 h of exfoliation, a series of ∼3.2 A high islands are observed by atomic force microscopy over approximately 10% of the surface. Subsequently, patch growth stops, and oxidation begins after the 2 h and continues until one quintuple layer has been oxidized. X-ray photoelectron spectroscopy shows no sign of oxidation before ∼120 min of exposure to air, and the oxygen 1 s peak, as well as oxidized Se 3d and Bi 4d peaks, are clearly present after ∼190 min of ambient exposure. Variable angle spectroscopic ellipsometry indicates that the oxidation of a full quintuple layer occurs on the time scale of days. These results are in good agreement with the time...

Optical critical dimension metrology for directed self-assembly assisted contact hole shrink

Abstract. Directed self-assembly (DSA) is a potential patterning solution for future generations of integrated circuits. Its main advantages are high pattern resolution (∼10  nm), high throughput, no requirement of high-resolution mask, and compatibility with standard fab-equipment and processes. The application of Mueller matrix (MM) spectroscopic ellipsometry-based scatterometry to optically characterize DSA patterned contact hole structures fabricated with phase-separated polystyrene-b-polymethylmethacrylate (PS-b-PMMA) is described. A regression-based approach is used to calculate the guide critical dimension (CD), DSA CD, height of the PS column, thicknesses of underlying layers, and contact edge roughness of the post PMMA etch DSA contact hole sample. Scanning electron microscopy and imaging analysis is conducted as a comparative metric for scatterometry. In addition, optical model-based simulations are used to investigate MM elements’ sensitivity to various DSA-based contact hole structures, predict sensitivity to dimensional changes, and its limits to characterize DSA-induced defects, such as hole placement inaccuracy, missing vias, and profile inaccuracy of the PMMA cylinder.

Fermi Level Manipulation through Native Doping in the Topological Insulator Bi2Se3

The topologically protected surface states of three-dimensional (3D) topological insulators have the potential to be transformative for high-performance logic and memory devices by exploiting their specific properties such as spin-polarized current transport and defect tolerance due to suppressed backscattering. However, topological insulator based devices have been underwhelming to date primarily due to the presence of parasitic issues. An important example is the challenge of suppressing bulk conduction in Bi2Se3 and achieving Fermi levels ( EF) that reside in between the bulk valence and conduction bands so that the topologically protected surface states dominate the transport. The overwhelming majority of the Bi2Se3 studies in the literature report strongly n-type materials with EF in the bulk conduction band due to the presence of a high concentration of selenium vacancies. In contrast, here we report the growth of near-intrinsic Bi2Se3 with a minimal Se vacancy concentration providing a Fermi level near midgap with no extrinsic counter-doping required. We also demonstrate the crucial ability to tune EF from below midgap into the upper half of the gap near the conduction band edge by controlling the Se vacancy concentration using post-growth anneals. Additionally, we demonstrate the ability to maintain this Fermi level control following the careful, low-temperature removal of a protective Se cap, which allows samples to be transported in air for device fabrication. Thus, we provide detailed guidance for EF control that will finally enable researchers to fabricate high-performance devices that take advantage of transport through the topologically protected surface states of Bi2Se3.

Rapid optical determination of topological insulator nanoplate thickness and oxidation

The stability of 2D antimony telluride (Sb2Te3) nanoplates in ambient conditions is elucidated. These materials exhibit an anisotropic oxidation mode, and CVD synthesized samples oxidize at a much faster rate than exfoliated samples investigated in previous studies. Optical measurement techniques are introduced to rapidly measure the oxidation modes and thickness of 2D materials. Auger characterization were conducted to confirm that oxygen replaces tellurium as opposed to antimony under ambient conditions. No surface morphology evolution was detected in AFM before and after exposure to air. These techniques were employed to determine the origin of the thickness dependent color change effect in Sb2Te3. It is concluded that this effect is a combination of refractive index change due to oxidation and Fresnel effects.

Thickness and Rotational Effects in Simulated HRTEM Images of Graphene on Hexagonal Boron Nitride

Recent studies have shown that when graphene is placed on a thin hexagonal boron nitride (h-BN) substrate, unlike when it is placed on a typical SiO2 surface, it can closely approach the ideal carrier mobility observed in suspended graphene samples. This study further examines the epitaxial relationship between graphene and h-BN substrate with high-resolution transmission electron microscopy simulation. Virtual monolayer and multilayer stacks of h-BN were produced with a monolayer of graphene on top, on bottom, and in between h-BN layers, in order to study this interface. Once the simulations were performed, the phase contrast image and Moiré pattern created by this heterostack were analyzed for local and global intensity minima and maxima. In addition, h-BN substrate thickness and rotations between h-BN and graphene were probed and analyzed. The simulated images produced in this work will be used to help understand subsequent transmission electron microscopy images and electron energy-loss studies.

Pulsed-N2 assisted growth of 5-20 nm thick β-W films

A technique to deposit 5-20 nm thick β-phase W using a 2-second periodic pulse of 1 sccm-N2 gas on Si(001) and SiN(5 nm)/Si(001) substrates is reported. Resistivity, X-ray photoelectron spectroscopy and X-ray reflectivity were utilized to determine phase, bonding and thickness, respectively. X-ray diffraction patterns were utilized to determine the crystal structure, lattice constant and crystal size using the LeBail method. The flow rate of Nitrogen gas (continuous vs. pulsing) had significant impact upon the crystallinity and formation of β-phase W.