N. Nakagawa

Why some interfaces cannot be sharp

A central goal of modern materials physics and nanoscience is the control of materials and their interfaces to atomic dimensions. For interfaces between polar and nonpolar layers, this goal is thwarted by a polar catastrophe that forces an interfacial reconstruction. In traditional semiconductors, this reconstruction is achieved by an atomic disordering and stoichiometry change at the interface, but a new option is available in multivalent oxides: if the electrons can move, the atoms do not have to. Using atomic-scale electron energy loss spectroscopy, we have examined the microscopic distribution of charge and ions across the (001) LaAlO3/SrTiO3 interface. We find that there is a fundamental asymmetry between the ionically compensated AlO2/SrO/TiO2 interface, and the electronically compensated AlO2/LaO/TiO2 interface, both in interfacial sharpness and charge density. This suggests a general strategy to design sharp interfaces, remove interfacial screening charges, control the band offset and, hence, markedly improve the performance of oxide devices.

Atomic-scale imaging of nanoengineered oxygen vacancy profiles in SrTiO3

At the heart of modern oxide chemistry lies the recognition that beneficial (as well as deleterious) materials properties can be obtained by deliberate deviations of oxygen atom occupancy from the ideal stoichiometry. Conversely, the capability to control and confine oxygen vacancies will be important to realize the full potential of perovskite ferroelectric materials, varistors and field-effect devices. In transition metal oxides, oxygen vacancies are generally electron donors, and in strontium titanate (SrTiO3) thin films, oxygen vacancies (unlike impurity dopants) are particularly important because they tend to retain high carrier mobilities, even at high carrier densities. Here we report the successful fabrication, using a pulsed laser deposition technique, of SrTiO3 superlattice films with oxygen doping profiles that exhibit subnanometre abruptness. We profile the vacancy concentrations on an atomic scale using annular-dark-field electron microscopy and core-level spectroscopy, and demonstrate absolute detection sensitivities of one to four oxygen vacancies. Our findings open a pathway to the microscopic study of individual vacancies and their clustering, not only in oxides, but in crystalline materials more generally.

COMPACT LASER MOLECULAR BEAM EPITAXY SYSTEM USING LASER HEATING OF SUBSTRATE FOR OXIDE FILM GROWTH

A high-temperature, oxygen compatible, and compact laser molecular beam epitaxy (laser MBE) system has been developed. The 1.06 μm infrared light from a continuous wave neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was used to achieve a wide range and rapid control of substrate temperature in ultrahigh vacuum and at up to 1 atm oxygen pressure. The maximum usable temperature was limited to 1453 °C by the melting point of the nickel sample holder. To our knowledge, this is the highest temperature reported for pulsed laser deposition of oxide films. The efficient laser heating combined with temperature monitoring by a pyrometer and feedback control of the Nd:YAG laser power by a personal computer made it possible to regulate the substrate temperature accurately and to achieve high sample heating and cooling rates. The oxygen pressure and ablation laser triggering were also controlled by the computer. The accurate growth parameter control was combined with real-time in situ surface structure monitor...

Growth mode mapping of SrTiO3 epitaxy

We have mapped the growth mode of homoepitaxial SrTiO3 thin films as a function of deposition rate and substrate temperature during pulsed laser deposition. The transition from layer by layer growth to step flow growth was mapped by making 260 depositions, 3 monolayers each, on a single substrate. The growth mode was determined by time-resolved reflection high-energy electron diffraction. An atomically smooth surface was regenerated after each deposition by annealing the sample at temperatures above 1200 °C. The depositions were performed at an oxygen pressure of 10−6 Torr and covered a temperature range from 900 to 1380 °C. The effective activation energies of surface migration on Ti- and Sr-terminated surfaces were determined from the mapping results.

Magnetocapacitance and exponential magnetoresistance in manganite–titanate heterojunctions

We present a rectifying manganite–titanate heterojunction exhibiting a magnetic field tunable depletion layer. This creates a large positive magnetocapacitance, a direct measure of the field-induced reduction of the effective depletion width across the junction. Furthermore, the reduction of the junction barrier shifts the forward bias characteristics, giving exponentially enhanced differential magnetoresistance, occurring despite the absence of a spin filter. These results provide a unique probe of a Mott insulator∕band insulator interface, and further suggest electronic devices incorporating the magnetic field sensitivity of these strongly correlated electron materials.

Step-flow growth of SrTiO3 thin films with a dielectric constant exceeding 104

The use of SrTiO3 films in cryogenic high-frequency applications has been limited by the low dielectric constant er of thin films (≈103) when compared to the bulk value of over 104. We show that the extension of the pulsed laser deposition technique to temperatures well above 1000 °C, coupled with in situ reflection high energy electron diffraction monitoring, makes it possible to grow SrTiO3 films in the step-flow mode. Films grown in this mode showed at 4.2 K a maximum er of 12 700, which could be tuned by 80% by applying a bias voltage of ±1 V.

In vacuo photoemission study of atomically controlled La 1 − x Sr x Mn O 3 thin films: Composition dependence of the electronic structure

We have investigated change in the electronic structures of atomically controlled

Parallel integration and characterization of nanoscaled epitaxial lattices by concurrent molecular layer epitaxy and diffractometry

{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}

In situ photoemission characterization of terminating-layer-controlled La0.6Sr0.4MnO3 thin films

(LSMO) thin films as a function of hole-doping levels

Atomically flat (110) SrTiO3 and heteroepitaxy

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