Dielectric properties of ultrathin AlO (1.1-4.4 nm) in metal-insulator-metal (M-I-M) Al/AlO/Al trilayers fabricated in situ using an integrated sputtering and atomic layer deposition (ALD) system were investigated. An M-I interfacial layer (IL) formed during the pre-ALD sample transfer even under high vacuum has a profound effect on the dielectric properties of the AlO with a significantly reduced dielectric constant (ε) of 0.5-3.3 as compared to the bulk ε ∼ 9.2. Moreover, the observed soft-type electric breakdown suggests defects in both the M-I interface and the AlO film. By controlling the pre-ALD exposure to reduce the IL to a negligible level, a high ε up to 8.9 was obtained on the ALD AlO films with thicknesses from 3.3 to 4.4 nm, corresponding to an effective oxide thickness (EOT) of ∼1.4-1.9 nm, respectively, which are comparable to the EOTs found in high-K dielectrics like HfO at 3-4 nm in thickness and further suggest that the ultrathin ALD AlO produced in optimal conditions may provide a low-cost alternative gate dielectric for CMOS. While ε decreases at a smaller AlO thickness, the hard-type dielectric breakdown at 32 MV/cm and in situ scanning tunneling spectroscopy revealed band gap ∼2.63 eV comparable to that of an epitaxial AlO film. This suggests that the IL is unlikely a dominant reason for the reduced ε at the AlO thickness of 1.1-2.2 nm but rather a consequence of the electron tunneling as confirmed in the transport measurement. This result demonstrates the critical importance in controlling the IL to achieving high-performance ultrathin dielectric in MIM structures.

We report synthesis and time-resolved transient absorption measurements of TiS3 nanoribbons. TiS3 nanoribbons were fabricated by direct reaction of titanium and sulfur. Dynamics of the photocarriers in these samples were studied by transient absorption measurements. It was found that following ultrafast injection of nonequilibrium and hot photocarriers, the thermalization, energy relaxation, and exciton formation all occur on a subpicosecond time scale. Several key parameters describing the dynamical properties of photocarriers, including their recombination lifetime, diffusion coefficient, mobility, and diffusion length, were deduced.

A B S T R A C TAB-stacked bilayer graphene has attracted considerable attention due to its feasibility of band gap tuning. Although synthesis of bilayer graphene on Cu has been reported using chemical vapor deposition (CVD) through a layer-by-layer growth mechanism, the process is long and complicated due to lack of catalytic assistance of Cu to the second graphene layer growth. Here we show that theoretical modeling demonstrates an alternative synchronous growth of bilayer graphene on Cu is possible by passivating the top graphene nuclei edges with hydrogen to allow carbon diffusion underneath the top graphene nuclei for bottom graphene layer formation. Moreover, such a growth mechanism has been achieved experimentally in a facile CVD method by simply controlling the H 2 pressure.Bilayer graphene with high coverage of over $95% and a high AB stacking ratio of up to $90% has been obtained within a short growth time of 30 min. Also, graphene with single, double and multiple layers can be obtained by simply controlling the hydrogen pressure.This result represents the demonstration of the fast synchronous AB-stacked bilayer graphene growth, which is important to scalable manufacture of graphene with controllable layer number and stacking required for practical applications.

Electron tunneling through high-quality, atomically thin dielectric films can provide a critical enabling technology for future microelectronics, bringing enhanced quantum coherent transport, fast speed, small size, and high energy efficiency. A fundamental challenge is in controlling the interface between the dielectric and device electrodes. An interfacial layer (IL) will contain defects and introduce defects in the dielectric film grown atop, preventing electron tunneling through the formation of shorts. In this work, we present the first systematic investigation of the IL in AlO dielectric films of 1-6 Å's in thickness on an Al electrode. We integrated several advanced approaches: molecular dynamics to simulate IL formation, in situ high vacuum sputtering atomic layer deposition (ALD) to synthesize AlO on Al films, and in situ ultrahigh vacuum scanning tunneling spectroscopy to probe the electron tunneling through the AlO. The IL had a profound effect on electron tunneling. We observed a reduced tunnel barrier height and soft-type dielectric breakdown which indicate that defects are present in both the IL and in the AlO. The IL forms primarily due to exposure of the Al to trace O and/or HO during the pre-ALD heating step of fabrication. As the IL was systematically reduced, by controlling the pre-ALD sample heating, we observed an increase of the ALD AlO barrier height from 0.9 to 1.5 eV along with a transition from soft to hard dielectric breakdown. This work represents a key step toward the realization of high-quality, atomically thin dielectrics with electron tunneling for the next generation of microelectronics.

Magnetic tunnel junctions (MTJs), formed through sandwiching an ultrathin insulating film (so-called tunnel barrier or TB), with ferromagnetic metal electrodes, are fundamental building blocks in magnetoresistive random access memory (MRAM), spintronics, etc. The current MTJ technology employs physical vapor deposition (PVD) to fabricate either amorphous AlOx or epitaxial MgO TBs of thickness around 1 nm or larger to avoid leakage caused by defects in TBs. Motivated by the fundamental limitation in PVD in, and the need for atomically thin and defect-free TBs in MTJs, this work explores atomic layer deposition (ALD) of 1-6 Å thick Al2O3 TBs both directly on Fe films and with an ultrathin Al wetting layer. In situ characterization of the ALD Al2O3 TB was carried out using scanning tunneling spectroscopy (STS). Despite a moderate decrease in TB height Eb with reducing Al wetting layer thicknesses, a remarkable Eb of ∼1.25 eV was obtained on 1 Å thick ALD Al2O3 TB grown directly on an Fe electrode, which is more than twice of that of thermal AlOx TB (∼0.6 eV). Achieving such an atomically thin low-defect TB represents a major step towards improving spin current tunneling in MTJs.