Atomic-layer-by-layer Molecular Beam Epitaxy Research Articles

Atomic-Layer Engineering of La2-xSrxCuO4-La2-xSrxZnO4 Heterostructures.

The fabrication of trilayer superconductor-insulator-superconductor (SIS) Josephson junctions with high-temperature superconductor (HTS) electrodes requires atomically perfect interfaces. Therefore, despite great interest and efforts, this remained a challenge for over three decades. Here, we report the discovery of a new family of metastable materials, La2-xSrxZnO4 (LSZO), synthesized by atomic-layer-by-layer molecular beam epitaxy (ALL-MBE). We show that LSZO is insulating and epitaxially compatible with an HTS compound, La2-xSrxCuO4 (LSCO). Since the "parent" compound La2ZnO4 (LZO) is easier to grow, here we focus on this material as our insulating layer. Growing LZO at very low temperatures to reduce cation interdiffusion makes LSCO/LZO interfaces atomically sharp. We show that in LSCO/LZO/LSCO trilayers, the superconducting properties of the LSCO electrodes remain undiminished, unlike in previous attempts with insulator barriers made of other materials. This opens prospects to produce high-quality HTS tunnel junctions.

A Method to Probe the Interfaces in La2−xSrxCuO4-LaSrAlO4-La2−xSrxCuO4 Trilayer Junctions

C-axis trilayer cuprate Josephson junctions are essential for basic science and digital circuit applications of high-temperature superconductors. We present a method for probing the interface perfection in La2−xSrxCuO4 (LSCO)-LaSrAlO4 (LSAO)-La2−xSrxCuO4 trilayer junctions. A series of LSCO-LSAO superlattices with atomically smooth surfaces and sharp interfaces were grown by the atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) technique. We have systematically varied the thickness of LSCO and LSAO layers with monolayer precision. By studying the mutual inductance and electrical transport in these superlattices, we detect the non-superconducting (“dead”) layers at the interfaces and quantify their thicknesses. Our results indicate that two optimally doped LSCO monolayers just above and below the one monolayer LSAO barrier are no longer superconducting, rendering the actual barrier thickness of five monolayers. Next, we have shown that introducing a protective highly-overdoped LSCO layer reduces the thickness of dead layers by one or two monolayers.

Methods to Create Novel La2−xSrxCuO4 Devices with Multiple Atomically Sharp Interfaces

We present methods to create devices that utilize the high-temperature superconductor La2-xSrxCuO4 grown by atomic layer-by-layer molecular beam epitaxy (ALL-MBE). The ALL-MBE synthesis technique provides atomically precise interfaces necessary for the tunnel junctions, Josephson junctions, and dyon detection devices that will be considered. A series of microfabrication processing steps using established techniques are given for each device, and their details are discussed. These procedures are easily extended to generate more complex designs and could be suitable for a wider variety of materials.

Optimization of La2−xSrxCuO4 Single Crystal Film Growth via Molecular Beam Epitaxy

Atomic layer-by-layer molecular beam epitaxy (ALL-MBE) combined with ozone is one of the best methods to fabricate single-crystal thin films of complex oxides. Cuprate such as La2−xSrxCuO4 (LSCO) is a representative complex-oxide high-temperature superconductor (HTS) material. Our group utilizes this method to produce high-quality single-crystal HTS films with atomically smooth surfaces and interfaces. In addition, ALL-MBE enables us to engineer multilayer heterostructures with atomic precision. This allows the fabrication of tunnel junctions, various nanostructures, and other HTS devices of interest for superconducting electronics. We have synthesized over three thousand LSCO thin films in the past two decades. These films’ structural and electronic properties have been studied and characterized by various methods. Here, we distill the extensive experience we accumulated into a step-by-step protocol to fabricate atomically perfect LSCO films. The recipe includes substrate preparation, ozone generation and distillation, source calibration, the in situ monitoring of the film synthesis, post-growth annealing, and ex situ characterization. It discloses a reproducible way to fabricate single-crystal LSCO films for basic research and HTS electronic applications.

Synthesis of La2−xSrxCuO4 films via atomic layer-by-layer molecular beam epitaxy

Atomic layer-by-layer molecular beam epitaxy (ALL-MBE) is a sophisticated technique to synthesize high-temperature superconductor (HTS) materials. ALL-MBE produces single-crystal HTS films with atomically smooth surfaces and interfaces, as well as precise multilayer heterostructures engineered down to a single atomic layer level. This enables the fabrication of tunnel junctions, nanowires, nanorings, and other HTS devices of interest. Our group has focused on ALL-MBE synthesis and materials science of La2−xSrxCuO4 (LSCO), a representative HTS cuprate. In the past two decades, we have synthesized over three thousand LSCO thin films and characterized them by a range of analytical techniques. Here, we present in full detail a systematic process for the synthesis and engineering of atomically perfect LSCO films. The procedure includes the preparation of substrates, calibration of the elemental sources, the recipe for ALL growth of LSCO films without any secondary-phase precipitates, post-growth annealing of the films, and ex situ film characterization. This report should aid replication and dissemination of this technique of synthesizing single-crystal LSCO films for basic research as well as for HTS electronic applications.

Method to create cuprate tunnel junctions with atomically sharp interfaces

We present a method for producing tunnel junctions from cuprate thin films grown by atomic layer-by-layer molecular beam epitaxy (ALL-MBE). Our approach utilizes microfabrication processing and relies on the atomically precise interfaces provided by the ALL-MBE synthesis technique. The method is suitable for a wide range of junction dimensions and materials and allows for more complex designs such as superconducting quantum interference devices and junction arrays. Fully superconducting electrodes ensure that the contact resistances in the devices created by this method remain very low.

The Quest for High(er) Tc in BaBiO3-Based Heterostructures

Almost all known superconductors with high Tc have layered structure and are quasi two-dimensional (2D) electronically. Among those that are not, hole-doped BaBiO3 has the highest Tc (and also the highest among all oxides other than cuprates). Coincidentally, Bi is a valence-skipping element and a natural candidate for a negative-U center, with the potential of enhancing superconductivity, as often pointed out by Ted Geballe. With this motivation, we have been trying to synthesize quasi-2D embodiments of BaBiO3. The technique we use is atomic-layer-by-layer molecular beam epitaxy (ALL-MBE), which allows for “digital” synthesis of atomically precise heterostructures and superlattices. We have succeeded in synthesizing some novel, artificial, metastable, layered compounds such as Ba3Bi2LaO9. So far, neither has shown superconductivity, but we will persist in trying.

Is there a path from cuprates towards room-temperature superconductivity?

A brief account is presented of an extensive experiment performed at Brookhaven National Laboratory, aimed at understanding the nature of high-temperature superconductivity in cuprates. Over the course of the last 12 years, over 2000 films of the prototypical high- $$T_\mathrm{c}$$ superconductor, $$\hbox {La}_{2-{x}}\hbox {Sr}_{{x}}\hbox {CuO}_{4}$$ , have been synthesized using atomic-layer-by-layer molecular beam epitaxy (ALL-MBE), characterized by a range of techniques, and patterned into devices. These were used to measure accurately the key physical parameters in both the superconducting and the normal states, and establish their precise dependence on doping, temperature, and external fields. The results bring in some great surprises, challenge the commonly held beliefs, rule out many theoretical models, and point to a new path for raising $$T_\mathrm{c}$$ even further.

The Demise of Superfluid Density in Overdoped La2−x Sr x CuO4 Films Grown by Molecular Beam Epitaxy

We have synthesized La2−xSrxCuO4 thin films using atomic layer-by-layer molecular beam epitaxy (ALL-MBE). The films are high-quality—singe crystal, atomically smooth, and very homogeneous. The critical temperature (Tc) shows a very little (<1 K) variation within a film of 10×10 mm 2 area. The large statistics (over 2000 films) is crucial to discern intrinsic properties. In these films, we have measured the absolute value of the magnetic penetration depth λ with the accuracy better than 1 % and mapped densely the entire overdoped side of the La2−xSrxCuO4 phase diagram. A new scaling law is established accurately for the dependence of Tc on the superfluid density. The scaling we observe is incompatible with the standard Bardeen-Cooper-Schrieffer picture and points to local pairing.

On Local Pairs vs. BCS: Quo Vadis High- T c Superconductivity

Since the discovery of high-temperature superconductivity in cuprates, proposals have been made that pairing may be local, in particular in underdoped samples. Here, we briefly review evidence for local pairs from our experiments on thin films of La 2−x Sr x CuO 4, synthesized by atomic layer-by-layer molecular beam epitaxy (ALL-MBE).

High temperature interface superconductivity

High-Tc superconductivity at interfaces has a history of more than a couple of decades. In this review we focus our attention on copper-oxide based heterostructures and multi-layers. We first discuss the technique, atomic layer-by-layer molecular beam epitaxy (ALL-MBE) engineering, that enabled High-Tc Interface Superconductivity (HT-IS), and the challenges associated with the realization of high quality interfaces. Then we turn our attention to the experiments which shed light on the structure and properties of interfacial layers, allowing comparison to those of single-phase films and bulk crystals. Both ‘passive’ hetero-structures as well as surface-induced effects by external gating are discussed. We conclude by comparing HT-IS in cuprates and in other classes of materials, especially Fe-based superconductors, and by examining the grand challenges currently laying ahead for the field.

High Temperature Interface Superconductivity

We use atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) to deposit atomically smooth films of cuprate superconductors and other complex oxides. Bilayers, multilayers, and superlattices are grown with atomic precision and virtually perfect interfaces. This has allowed a discovery and in-depth study of high-temperature interface superconductivity, which is briefly reviewed here.

Insights from the study of high-temperature interface superconductivity

A brief overview is given of the studies of high-temperature interface superconductivity based on atomic-layer-by-layer molecular beam epitaxy (ALL-MBE). A number of difficult materials science and physics questions have been tackled, frequently at the expense of some technical tour de force, and sometimes even by introducing new techniques. ALL-MBE is especially suitable to address questions related to surface and interface physics. Using this technique, it has been demonstrated that high-temperature superconductivity can occur in a single copper oxide layer-the thinnest superconductor known. It has been shown that interface superconductivity in cuprates is a genuine electronic effect-it arises from charge transfer (electron depletion and accumulation) across the interface driven by the difference in chemical potentials rather than from cation diffusion and mixing. We have also understood the nature of the superconductor-insulator phase transition as a function of doping. However, a few important questions, such as the mechanism of interfacial enhancement of the critical temperature, are still outstanding.

Artificial superlattices grown by MBE: Could we design novel superconductors?

We review briefly the technique of atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) that allows us to deposit atomically smooth single-crystal thin films of cuprate superconductors and various other complex oxides. We give a couple of examples showing how this technique has been used to synthesize ‘artificial’ high- T c superconductors – metastable high- n members of known cuprate families or superlattices of various known (natural, stable) compounds. We also report briefly on the present status of our state-of-the-art ALL-MBE system, and suggest that this technique could be useful in search for new natural or artificial superconducting materials.

Rheed Studies of a-Axis Oriented DyBa2Cu3O7 Films Grown by All-MBE

ABSTRACTReal-time, in-situ surface monitoring by reflection high-energy electron diffraction (RHEED) has been the key enabling component of atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) of complex oxides. RHEED patterns contain information on crystallographic arrangements and long range order on the surface; this can be made quantitative with help of numerical simulations. The dynamics of RHEED patterns and intensities reveal a variety of phenomena such as nucleation and dissolution of secondary-phase precipitates, switching between growth modes (layer-by-layer, step-flow), surface phase transitions (surface reconstruction, roughening, and even phase transitions induced by the electron beam itself), etc. Some of these phenomena are illustrated here, using as a case study our recent growth of atomically smooth a-axis oriented DyBa2Cu3O7 films.

Transmission electron study of heteroepitaxial growth in the BiSrCaCuO system

Films of Bi2Sr2CaCu2O8 and Bi2Sr2CuO6 have been grown using Atomic-Layer-by-Layer Molecular Beam Epitaxy (ALL-MBE) on lattice-matched substrates. These materials have been combined with layers of closely related metastable compounds like Bi2Sr2Ca7Cu8O20 (2278) and rare-earth-doped compounds like Bi2Sr2DyxCa1–xCu2O8 (Dy: 2212) to form heterostructures with unique superconducting properties, including superconductor/insulator multilayers and tunnel junctions. Transmission electron microscopy (TEM) has been used to study the morphology and microstructure of these heterostructures. These TEM studies shed light on the physical properties of the films, and give insight into the growth mode of highly anisotropic solids like Bi2Sr2CaCu2O8.

Angle-resolved photoemission spectroscopy study of Bi2Sr2CaCu2O8+ delta thin films.

We report angle-resolved photoemission spectroscopy (ARPES) results from Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} (2212) single-crystal thin films grown with atomic layer-by-layer molecular beam epitaxy (ALL-MBE). This was made possible by adapting the top-post cleaving technique to thin films of 2212. We have observed clear photoemission features from these films that disperse through the Fermi level. We have observed the superconducting energy gap with ARPES in thin films. Both the magnitude of the gap as well as its anisotropy are consistent with the results from bulk crystals. Furthermore, we observe the pile-up and dip in spectral weight associated with the gap. These results open possibilities for future research using thin-film samples in this materials system that take advantage of the ability to grow metastable materials with ALL-MBE.

High-Temperature Superconducting Multilayers and Heterostructures Grown by Atomic Layer-By-Layer Molecular Beam Epitaxy

Atomic layer-by-layer molecular beam epitaxy (ALL-MBE) ofhigh-tem­ perature superconductors (HTSC) and other complex oxides has been developed. Thin films with atomically flat surfaces and abrupt interfaces can be produced an atomic layer at a time. Samples are engineered by stacking molecular layers of diff erent compounds, by adding or omitting atomic monolayers, and by doping within specified monolayers. Novel artificial HTSC compounds such as Bi2Sr2 Ca7CugOx (2278), as well as various heterostructures , have been synthesized in this way. This unique synthetic capability has allowed several fundamental physics issues, such as the dimensionality of the HTSC state, long-range proximity effects, resonant tunneling with a specified number of hops, etc, to be addressed. Trilayer Josephson junctions with uniform and reproducible properties have been fabricated; phase-locked operation of two junctions has also been demonstrated. Finally, titanate slabs only 4-A thick have been grown without pin-holes and shown to provide tunneling barriers for c-axis transport.

Molecular Beam Epitaxy of Single Crystal Colossal Magneto-Resistive Material

AbstractWe have grown films of (LaSr)MnO3 (LSMO) and (LaCa)MnO3 (LCMO) using atomic layer-by-layer molecular beam epitaxy (ALL-MBE). Depending on growth conditions, substrate lattice constant and the exact cation stoichiometry, the films are either pseudomorphic or strain relaxed. The pseudomorphic films show atomically flat surfaces, with a unit cell terrace structure that is a replica of that observed on the slightly vicinal substrates, while the strain relaxed films show bumpy surfaces correlated with a dislocation network. All films show tetragonal structure and exhibit anisotropic magnetoresistance, with a low field response, (1/R)(dR/dH) as large as 5 T−1.

Engineering of ultrathin barriers in high T C, trilayer Josephson junctions

Josephson junctions with ultrathin (25–40 Å) barriers were fabricated using high TC, trilayer films grown by atomic layer-by-layer molecular beam epitaxy (ALL-MBE). The films consisted of top and bottom electrodes of Bi2Sr2CaCu2O8, separated by a single molecular layer of a metastable compound, Bi2Sr2(Ca, Sr, Bi, Dy)n−1CunO2n+4, with n=5 to 11. Systematic variation of Bi or Dy doping on Ca sites in the barrier layer provided four orders of magnitude of tuning of both the junction critical current and the normal state resistance, while keeping their product approximately constant, near 0.5 mV. Barrier and junction engineering, on an atomic length scale, has been demonstrated for the first time.