In situ atomic layer deposition and electron tunneling characterization of monolayer Al 2 O 3 on Fe for magnetic tunnel junctions

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 Al 2 O 3 TBs both directly on Fe films and with an ultrathin Al wetting layer. In situ characterization of the ALD Al 2 O 3 TB was carried out using scanning tunneling spectroscopy (STS). Despite a moderate decrease in TB height E b with reducing Al wetting layer thicknesses, a remarkable E b of ∼1.25 eV was obtained on 1 Å thick ALD Al 2 O 3 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

Wrapping cytochrome c around single-wall carbon nanotube: engineered nanohybrid building blocks for infrared detection at high quantum efficiency

Biomolecule cytochrome c (Cty c), a small molecule of a chain of amino acids with extraordinary electron transport, was helically wrapped around a semiconductive single-wall carbon nanotube (s-SWCNT) to form a molecular building block for uncooled infrared detection with two uniquely designed functionalities: exciton dissociation to free charge carriers at the heterojunction formed on the s-SWCNT/Cty c interface and charge transport along the electron conducting chain of Cty c (acceptor) and hole conducting channel through s-SWCNT (donor). Such a design aims at addressing the long-standing challenges in exciton dissociation and charge transport in an SWCNT network, which have bottlenecked development of photonic SWCNT-based infrared detectors. Using these building blocks, uncooled s-SWCNT/Cyt c thin film infrared detectors were synthesized and shown to have extraordinary photoresponsivity up to 0.77 A W−1 due to a high external quantum efficiency (EQE) in exceeding 90%, which represents a more than two orders of magnitude enhancement than the best previously reported on CNT-based infrared detectors with EQE of only 1.72%. From a broad perspective, this work on novel s-SWCNT/Cyt c nanohybrid infrared detectors has developed a successful platform of engineered carbon nanotube/biomolecule building blocks with superior properties for optoelectronic applications.This work was supported by ARO contract No. ARO-W911NF-12-1-0412, and NSF contracts Nos. NSF-DMR-1105986 and NSF EPSCoR-0903806, and was matching supported by the State of Kansas through Kansas Technology Enterprise Corporation. S.R. thanks the financial support from the Army Research Office-Young Investigator Award (W911NF-14-1–0443) for nanocarbon study. We thank Melisa Xin and Dr. Tanya Simms for their assistance in fabrication of electrodes on devices and AFM characterization, respectively

Atomically Thin Al2O3 Films for Tunnel Junctions

Metal-insulator-metal tunnel junctions are common throughout the microelectronics industry. The industry standard AlOx tunnel barrier, formed through oxygen diffusion into an Al wetting layer, is plagued by internal defects and pinholes which prevent the realization of atomically thin barriers demanded for enhanced quantum coherence. In this work, we employ in situ scanning tunneling spectroscopy along with molecular-dynamics simulations to understand and control the growth of atomically thin Al2O3 tunnel barriers using atomic-layer deposition. We find that a carefully tuned initial H2O pulse hydroxylated the Al surface and enabled the creation of an atomically thin Al2O3 tunnel barrier with a high-quality M−I interface and a significantly enhanced barrier height compared to thermal AlOx. These properties, corroborated by fabricated Josephson junctions, show that atomic-layer deposition Al2O3 is a dense, leak-free tunnel barrier with a low defect density which can be a key component for the next generation of metal-insulator-metal tunnel junctions

Atomically Thin Al2 O3 Films for Tunnel Junctions

Metal-insulator-metal tunnel junctions are common throughout the microelectronics industry. The industry standard AlOx tunnel barrier, formed through oxygen diffusion into an Al wetting layer, is plagued by internal defects and pinholes which prevent the realization of atomically thin barriers demanded for enhanced quantum coherence. In this work, we employ in situ scanning tunneling spectroscopy along with molecular-dynamics simulations to understand and control the growth of atomically thin Al2O3 tunnel barriers using atomic-layer deposition. We find that a carefully tuned initial H2O pulse hydroxylated the Al surface and enabled the creation of an atomically thin Al2O3 tunnel barrier with a high-quality M-I interface and a significantly enhanced barrier height compared to thermal AlOx. These properties, corroborated by fabricated Josephson junctions, show that atomic-layer deposition Al2O3 is a dense, leak-free tunnel barrier with a low defect density which can be a key component for the next generation of metal-insulator-metal tunnel junctions

Atomically-Thin Al2O3 Dielectric Films for Metal-Insulator-Metal Tunnel Junctions

Metal-Insulator-Metal tunnel junctions (MIMTJ) are a core building block for a variety of microelectronics including Magnetic Tunnel Junctions (MTJs) for magnetic memory and Josephson Junctions (JJs) for quantum computers. The performance of MIMTJ devices critically depends on the insulator which should have few defects and an atomic-scale thickness. However, the current state of the art insulators are both high-defect and atomic-scale (thermal or plasma assisted AlOx), or low defect and ultrathin (epitaxial MgO or Al 2O3). In this work, we develop a novel Atomic Layer Deposition (ALD) process which enables the growth of atomically-thin and low-defect density Al2O3 for MIMTJ devices. Exceptional control of the metal-insulator interface is required to achieve this end as any interfacial layer (IL) which develops is catastrophic, introducing defects and impairing the insulator growth. Specifically, two critical issues of pre-ALD IL formation and ALD nucleation on the metal surface were resolved by integrating ALD with sputtering in situ under High Vacuum (HV) along with a pre-ALD H2O pulse to hydroyxlate the Al surface. Ab-initio molecular dynamics simulations were run to shed light on the mechanisms of IL formation in the HV environment and the hydroxylation of the metal surface using this pre-ALD H2O pulse. In tandem, in situ Scanning Tunneling Spectroscopy (STS) quantified the quality of the Al 2O3 as the IL was systematical reduced by optimizing the pre-ALD H2O pulse, sample temperature, and pre-ALD heating time. After optimizations, STS revealed a remarkably high ALD Al2O 3 tunnel barrier height which was constant down to the single monolayer scale of 1 ALD cycle with a band gap comparable to ultrathin epitaxial Al 2O3. In addition, the highest known ALD Al2O 3 dielectric constant, in the ultrathin thickness range, was measured in fabricated capacitors. Amazingly, capacitance fittings along with STS imaging discovered that the IL thickness is sub-monolayer after our optimizations. Thus this work has achieved the first atomically-thin and low defect insulator for MIMTJ devices. Fabricated JJs show promise and preliminary tests reveal that this in situ ALD Al2O3 process can be grown on other metals such as Fe, which is essential for MTJ devices

In situ atomic layer deposition and electron tunneling characterization of monolayer Al2O3 on Fe for magnetic tunnel junctions

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

Synchronous growth of AB-stacked bilayer graphene on Cu by simply controlling hydrogen pressure in CVD process

AB-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 H2 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

Probing the Dielectric Properties of Ultrathin Al/Al₂O₃/Al Trilayers Fabricated Using <i>in Situ</i> Sputtering and Atomic Layer Deposition

Dielectric properties of ultrathin Al₂O₃ (1.1–4.4 nm) in metal–insulator–metal (M–I–M) Al/Al₂O₃/Al trilayers fabricated <i>in situ</i> 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 Al₂O₃ with a significantly reduced dielectric constant (ε_r) of 0.5–3.3 as compared to the bulk ε_r ∼ 9.2. Moreover, the observed soft-type electric breakdown suggests defects in both the M–I interface and the Al₂O₃ film. By controlling the pre-ALD exposure to reduce the IL to a negligible level, a high ε_r up to 8.9 was obtained on the ALD Al₂O₃ 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-<i>K</i> dielectrics like HfO₂ at 3–4 nm in thickness and further suggest that the ultrathin ALD Al₂O₃ produced in optimal conditions may provide a low-cost alternative gate dielectric for CMOS. While ε_r decreases at a smaller Al₂O₃ thickness, the hard-type dielectric breakdown at 32 MV/cm and <i>in situ</i> scanning tunneling spectroscopy revealed band gap ∼2.63 eV comparable to that of an epitaxial Al₂O₃ film. This suggests that the IL is unlikely a dominant reason for the reduced ε_r at the Al₂O₃ 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

Effect of an Interfacial Layer on Electron Tunneling through Atomically Thin Al₂O₃ Tunnel Barriers

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 Al₂O₃ 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 Al₂O₃ on Al films, and in situ ultrahigh vacuum scanning tunneling spectroscopy to probe the electron tunneling through the Al₂O₃. 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 Al₂O₃. The IL forms primarily due to exposure of the Al to trace O₂ and/or H₂O 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 Al₂O₃ 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