High Tunneling Magnetoresistance in Magnetic Tunnel Junctions with Subnanometer Thick Al2O3 Tunnel Barriers Fabricated Using Atomic Layer Deposition.

Pinhole-free and defect-free ultrathin dielectric tunnel barriers (TBs) are a key to obtaining high-tunneling magnetoresistance (TMR) and efficient switching in magnetic tunnel junctions (MTJs). Among others, atomic layer deposition (ALD) provides a unique approach for the fabrication of ultrathin TBs with several advantages including atomic-scale control over the TB thickness, conformal coating, and a low defect density. Motivated by this, this work explores the fabrication and characterization of spin-valve Fe/ALD-Al2O3/Fe MTJs with an ALD-Al2O3 TB thickness of 0.55 nm using in situ ALD. Remarkably, high TMR values of ∼77 and ∼90% have been obtained, respectively, at room temperature and at 100 K, which are comparable to the best reported values on MTJs having thermal AlOx TBs with optimized device structures. In situ scanning tunneling spectroscopy characterization of the ALD-Al2O3 TBs has revealed a higher TB height (Eb) of 1.33 ± 0.06 eV, in contrast to Eb ∼ 0.3-0.6 eV for their AlOx TB counterparts, indicative of significantly lower defect concentrations in the former. This first success of the MTJs with subnanometer thick ALD-Al2O3 TBs demonstrates the feasibility of in situ ALD for the fabrication of pinhole-free and low-defect ultrathin TBs for practical applications, and the performance could be further improved through device optimization.

Ultrahigh Brightening of Infrared PbS Quantum Dots via Collective Energy Transfer Induced by a Metal-Oxide Plasmonic Metastructure.

We demonstrate that a solution-processed heterojunction interface formed via the addition of a thin buffer layer of CdSe/ZnS quantum dots (QDs) to a functional metal oxide plasmonic metastructure (FMOP) can set up a collective interquantum dot energy-transport process, significantly enhancing the emission of infrared PbS quantum dots. The FMOP includes a Schottky junction, formed via deposition of a Si layer on arrays of Au nanoantennas and a Si/Al oxide charge barrier. We show when these two junctions are separated from each other by about 15 nm and the CdSe/ZnS quantum dot buffer layer is placed in touch with the Si/Al oxide junction, the quantum efficiency of an upper layer of PbS quantum dots can increase by about 1 order of magnitude. These results highlight a unique energy circuit formed via collective coupling of the CdSe/ZnS quantum dots with the hybridized states of plasmons and diffraction modes of the arrays (surface lattice resonances) and coupling between such resonances with PbS QDs via lattice-induced photonic modes.

Switching On/Off Negative Capacitance in Ultrathin Ferroelectric/Dielectric Capacitors.

Ferroelectric (FE) and dielectric (DE) insulator bilayer stacks provide a promising gate for low-power microelectronic devices. To fully realize the FE polarization switching, the DE layer must be ultrathin in the FE/DE bilayer stack. Motivated by this, this work presents the first successful fabrication and characterization of Fe/FeOx/Al2O3/Fe FE/DE bilayer capacitors using in vacuo atomic layer deposition (ALD) with a total FE/DE stack thickness <3-4 nm. A key tuning parameter in generating the FE/DE bilayer capacitors is the thickness of an Al wetting layer between the bottom Fe electrode and the ALD-Al2O3 DE layer. At a large thickness in exceeding 1.0 nm, high-quality conventional DE capacitors of 2.2 nm thick ALD-Al2O3 were obtained with dielectric constant (εr) ∼8.0 that is close to εr ∼ 9.2 for the Al2O3 bulk single crystal with an effective oxide thickness of 1.0 nm. By reducing the Al wetting layer thickness to below 1.0 nm, a thin ferroelectric FeOx interfacial layer of a thickness of 1-2 nm forms, enabling the achievement of a FeOx/Al2O3 FE/DE bilayer capacitor with static negative capacitance. Since all ferroelectric materials are piezoelectric, we show that a dynamic switching on/off of the negative capacitance can be achieved under the application of an external force on the ultrathin FE/DE capacitors through manipulation of the electric dipoles. This result not only provides a viable approach for generating ultrathin FE/DE bilayer capacitors but also offers a promising solution to low-power consumption microelectronics.

Plasmonic WS2 Nanodiscs/Graphene van der Waals Heterostructure Photodetectors.

Two-dimensional material van der Waals (vdW) heterostructures provide an excellent platform for design of novel optoelectronics. In this work, transition-metal dichalcogenide WS2 nanodiscs (WS2-NDs) of lateral dimension of 200-400 nm and layer number of 4-7 were synthesized on graphene using a layer-by-layer, transfer-free chemical vapor deposition. On this WS2-NDs/graphene vdW heterostructures, localized surface plasmonic resonance (LSPR) was achieved, resulting in remarkably enhanced light absorption as compared to the counterpart devices with a continuous WS2 layer (WS2-CL/graphene). Remarkably, the photoresponsivity of 6.4 A/W on the WS2-NDs/graphene photodetectors is seven times higher than that (0.91 A/W) of the WS2-CL/graphene vdW heterostructures at an incident 550 nm light intensity of 10 µW/cm2. Furthermore, the WS2-NDs/graphene photodetectors exhibit higher sensitivity to lower lights. Under 550 nm light illumination of 3 µW/cm2, which is beyond the sensitivity limit of the WS2-CL/graphene photodetectors, high photoresponsivity of 8.05 A/W and detectivity of 2.8 × 1010 Jones are achieved at Vsd = 5 V. This result demonstrates that the LSPR WS2-NDs/graphene vdW heterostructure is promising for scalable high-performance optoelectronics applications.

Effect of Al2O3 Seed-Layer on the Dielectric and Electrical Properties of Ultrathin MgO Films Fabricated Using In Situ Atomic Layer Deposition.

Metal/insulator/metal (M/I/M) trilayers of Al/MgO/Al with ultrathin MgO in the thickness range of 2.20-4.40 nm were fabricated using in vacuo sputtering and atomic layer deposition (ALD). In order to achieve a high-quality metal/insulator (M/I) interface and hence high-quality dielectric ALD-MgO films, a 5 cycles (∼0.55 nm) thick ALD-Al2O3 seed layer (SL) was employed to demonstrate the dielectric constant (εr) is ∼8.82-9.38 in 3.30-4.95 nm thick ALD-MgO/SL films, which is close to that of single-crystal MgO εr ∼ 9.80. In contrast, a low εr of 3.55-4.66 for the ALD-MgO films of a similar thickness without a SL was observed. The effective oxide thickness (EOT) of ∼1.40 nm has therefore been achieved in the ultrathin ALD-MgO films, which are comparable to the EOTs of high-K dielectrics such as HfO2. In addition, the leakage current through the M/I/M structure is reduced by more than 1 order of magnitude with implementation of the SL. The high leakage current in the samples without a SL can be attributed to the nonuniform nucleation of the ALD-MgO on the Al surface with a significant portion of the Al surface remaining conductive as confirmed using in vacuo scanning tunneling spectroscopy (STS). With the SL, the STS study has confirmed a tunnel barrier height of 1.50 eV on 0.55 nm MgO with 0.55 nm Al2O3 SL with almost 100% coverage. In addition, molecular dynamics simulations point out the importance of deposition of ultrathin SL that has a significant effect on the initial nucleation of the Mg precursor. This result not only illustrates the critical importance of controlling the M/I interface to obtain high-quality dielectric properties of ultrathin ALD films but also provides an approach to engineering incompatible M/I interfaces using a SL for a high-quality dielectric required for applications in M/I/M tunnel junctions and complementary metal oxide semiconductors.

High-Performance All-Inorganic CsPbCl3 Perovskite Nanocrystal Photodetectors with Superior Stability.

All-inorganic perovskites nanostructures, such as CsPbCl3 nanocrystals (NCs), are promising in many applications including light-emitting diodes, photovoltaics, and photodetectors. Despite the impressive performance that was demonstrated, a critical issue remains due to the instability of the perovskites in ambient. Herein, we report a method of passivating crystalline CsPbCl3 NC surfaces with 3-mercaptopropionic acid (MPA), and superior ambient stability is achieved. The printing of these colloidal NCs on the channel of graphene field-effect transistors (GFETs) on solid Si/SiO2 and flexible polyethylene terephthalate substrates was carried out to obtain CsPbCl3 NCs/GFET heterojunction photodetectors for flexible and visible-blind ultraviolet detection at wavelength below 400 nm. Besides ambient stability, the additional benefits of passivating surface charge trapping by the defects on CsPbCl3 NCs and facilitating high-efficiency charge transfer between the CsPbCl3 NCs and graphene were provided by MPA. Extraordinary optoelectronic performance was obtained on the CsPbCl3 NCs/graphene devices including a high ultraviolet responsivity exceeding 106 A/W, a high detectivity of 2 × 1013 Jones, a fast photoresponse time of 0.3 s, and ambient stability with less than 10% degradation of photoresponse after 2400 h. This result demonstrates the crucial importance of the perovskite NC surface passivation not only to the performance but also to the stability of the perovskite optoelectronic devices.

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.

Printable Nanocomposite FeS2-PbS Nanocrystals/Graphene Heterojunction Photodetectors for Broadband Photodetection.

Colloidal nanocrystals are attractive materials for optoelectronics applications because they offer a compelling combination of low-cost solution processing, printability, and spectral tunability through the quantum dot size effect. Here we explore a novel nanocomposite photosensitizer consisting of colloidal nanocrystals of FeS2 and PbS with complementary optical and microstructural properties for broadband photodetection. Using a newly developed ligand exchange to achieve high-efficiency charge transfer across the nanocomposite FeS2-PbS sensitizer and graphene on the FeS2-PbS/graphene photoconductors, an extraordinary photoresponsivity in exceeding ∼106 A/W was obtained in an ultrabroad spectrum of ultraviolet (UV)-visible-near-infrared (NIR). This is in contrast to the nearly 3 orders of magnitude reduction of the photoresponsivity from ∼106 A/W at UV to 103 A/W at NIR on their counterpart of FeS2/graphene detectors. This illustrates the combined advantages of the nanocomposite sensitizers and the high charge mobility in FeS2-PbS/graphene van der Waals heterostructures for nanohybrid optoelectronics with high performance, low cost, and scalability for commercialization.