Polar Discontinuity Research Articles

Termination control of the interface dipole inLa0.7Sr0.3MnO3/Nb:SrTiO3(001) Schottky junctions

In order to investigate the interface termination dependence of perovskite band alignments, we have studied the Schottky barrier height at ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}/\text{Nb}:{\text{SrTiO}}_{3}$ (001) heterointerfaces. As the $\text{Nb}:{\text{SrTiO}}_{3}$ semiconductor was varied from ${\text{TiO}}_{2}$ termination to SrO termination by variable insertion of a ${\text{SrMnO}}_{3}$ layer, a large systematic increase in the Schottky barrier height was observed. This can be ascribed to the evolution of the interface dipole induced to screen the polar discontinuity at the interface, which gives a large internal degree of freedom for tuning band diagrams in oxides.

Charge ordering and magnetism in quarter-filled Hubbard-Holstein model

We study a two-dimensional Hubbard-Holstein model with phonons treated in the adiabatic limit. A Hartree-Fock decomposition is employed for the Hubbard term. A range of electronic densities are discussed with special emphasis on the quarter filling $(n=0.5)$. We argue that the quarter-filled system is relevant for the electronic properties observed at the interface between ${\text{LaAlO}}_{3}$ and ${\text{SrTiO}}_{3}$, where half-electron per unit cell is transferred to the ${\text{TiO}}_{2}$ layer as a consequence of the polar discontinuity at the interface. In addition to presenting the overall phase diagrams, we identify an interesting charge-ordered antiferromagnetic phase for $n=0.5$, which was also reported recently in the ab initio study of the ${\text{LaAlO}}_{3}{\text{-SrTiO}}_{3}$ interface.

High-Tc superconducting thin films with composition control on a sub-unit cell level; the effect ofthe polar nature of the cuprates

Inspired by the work of Ohtomo and Hwang in 2004, we shed new light on thin films of layered cupratehigh-Tc superconductors (HTS). In principle all HTS materials consist of charged perovskite-like layerswhich in thin films can lead to polar discontinuities at the interfaces of different materials. Theresulting charge redistribution has to occur but we expect it to be far more complex than in theLaAlO3/SrTiO3 system since copper can be multivalent. This makes it hard to predict what will happen interms of transport or even magnetic properties compared to the ‘simple’ insulatorLaAlO3. Nevertheless, we point out that the picture of systems of charged layers is importantand necessary to fully understand heterostructures of these complex materials.

Phase diagram of a two-component Fermi gas with resonant interactions

The pairing of fermions lies at the heart of superconductivity and superfluidity. The stability of these pairs determines the robustness of the superfluid state, and the quest for superconductors with high critical temperature equates to a search for systems with strong pairing mechanisms. Ultracold atomic Fermi gases present a highly controllable model system for studying strongly interacting fermions. Tunable interactions (through Feshbach collisional resonances) and the control of population or mass imbalance among the spin components provide unique opportunities to investigate the stability of pairing-and possibly to search for exotic forms of superfluidity. A major controversy has surrounded the stability of superfluidity against an imbalance between the two spin components when the fermions interact resonantly (that is, at unitarity). Here we present the phase diagram of a spin-polarized Fermi gas of (6)Li atoms at unitarity, experimentally mapping out the superfluid phases versus temperature and density imbalance. Using tomographic techniques, we reveal spatial discontinuities in the spin polarization; this is the signature of a first-order superfluid-to-normal phase transition, and disappears at a tricritical point where the nature of the phase transition changes from first-order to second-order. At zero temperature, there is a quantum phase transition from a fully paired superfluid to a partially polarized normal gas. These observations and the implementation of an in situ ideal gas thermometer provide quantitative tests of theoretical calculations on the stability of resonant superfluidity.

Polar Discontinuity Doping of theLaVO3/SrTiO3Interface

We have investigated the transport properties of LaVO_{3}/SrTiO_{3} Mott-insulator-band-insulator heterointerfaces for various configurations. The (001)-oriented n-type VO_{2}/LaO/TiO_{2} polar discontinuity is conducting, exhibiting a LaVO3 thickness-dependent metal-insulator transition and low temperature anomalous Hall effect. The (001) p-type VO_{2}/SrO/TiO_{2} interface, formed by inserting a single layer of bulk metallic SrVO3 or SrO, drives the interface insulating. The (110) heterointerface is also insulating, indicating interface conduction arising from electronic reconstructions.

Asymmetric interface profiles in LaVO3∕SrTiO3 heterostructures grown by pulsed laser deposition

Surface segregation effects play an important role in the growth of traditional III-V semiconductor heterointerfaces. Here we show that segregation processes can also set an upper limit to the obtainable interface sharpness in perovskite oxide heterostructures. In particular, the structure of LaVO3∕SrTiO3 superlattices was studied on the atomic scale by electron microscopy and spectroscopy. The vanadate layers exhibit a growth asymmetry, with diffuse lower and atomically abrupt upper interfaces, caused by preferential Sr surface segregation. Switching the SrTiO3 termination layer does not change the interface abruptness, which excludes the interfacial polar discontinuity as the driving force for the observed growth asymmetry.

Electronic interface reconstruction at polar-nonpolar Mott-insulator heterojunctions

We report on a theoretical study of the electronic interface reconstruction (EIR) induced by polarity discontinuity at a heterojunction between a polar and a nonpolar Mott insulator, and of the two-dimensional strongly correlated electron systems (2DSCESs) which accompany the reconstruction. We derive an expression for the minimum number of polar layers required to drive the EIR, and discuss key parameters of the heterojunction system which control 2DSCES properties. The role of strong correlations in enhancing confinement at the interface is emphasized.

Photoemission Spectroscopy of Oxide Interfaces

This is a copy of the slides presented at the meeting but not formally written up for the volume. MBE growth of transition-metal oxide thin films using the pulsed laser deposition (PLD) technique has opened up new opportunities in the electronic structure studies of correlated systems using photoemission spectroscopy [1]. In this talk, I will present the results of photoemission studies on interfaces between a band insulator and a Mott insulator. In SrTiO3/LaTiO3 superlattices [2], metallic states were observed and systematic spectral weight transfer between the coherent part and incoherent part of the Ti 3d band was found to take place [3] as inferred from the electron-energy-loss spectroscopy study [4] and predicted theoretically [5]. For TiO2/VO2 interfaces, on the contrary, no metallic state was observed when VO2 was in the low-temperature insulating phase. The different behaviours are attributed to the absence of polarity discontinuity at the interface:electronic reconstruction;occurs at the polarity-discontinuous SrTiO3/LaTiO3 interface and does not at the polarity-continuous TiO2/VO2 interface. In entirely polar heterostructures LaAlO3/LaTiO3 [6] and LaAlO3/LaVO3 [7], soft x-ray and hard x-ray core-level photoemission measurements revealed that the Ti and V valences change at the topmost layers of the interfaces, suggesting electronic reconstruction competing with chemical reconstruction. This work was done in collaboration with M. Takizawa, H. Wadati, K. Maekawa, A. Chikamatsu, S. Minohara, A. Maniwa, H. Kumigashira, M. Oshima, M. Kawasaki, H. Koinuma, Y. Takata, K. Horiba, M. Matsunami, S. Shin, M. Yabashi, K. Tamasaku, Y. Nishino, D. Miwa, T. Isikawa, K. Shibuya, M. Lippmaa, Y. Nagao, Y. Muraoka, Z. Hiroi, Y. Hotta, T. Susaki, H.Y. Hwang, G.W.J. Hassink, and G. Rijnders.

Polarization-induced valence-band alignments at cation- and anion-polar InN∕GaN heterojunctions

The authors show that the existence of polarization discontinuities at polar III-nitride heterointerfaces can lead to large core-level shifts of photoelectrons and modification of apparent valence-band offsets. In this letter, large Ga core-level shifts, resulting from the interface dipole fields, have been directly measured by photoelectron spectroscopy on In∕Ga-polar (0001)- and N-polar (0001¯)-oriented InN∕GaN heterojunctions with monolayer abrupt, nearly fully relaxed lattices. Combined with the photoelectron spectroscopic measurements of InN and GaN bulk epilayers, the determined valence-band offsets are 1.04 and 0.54eV for In∕Ga- and N-polar heterojunctions, respectively.

Investigation on Photoluminescence Blue Shift of InGaN/GaN Quantum Wells Induced by Surface Band Bending

The effects of GaN cap layer thickness and doping type on photoluminescence (PL) are investigated for InGaN/GaN quantum wells (QWs), which are deposited on a C-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD). As for the results, the QWs capped with an unintentional doped (un-doped) GaN layer show a linear PL blue shift behavior with decreased cap layer thickness, while for the QWs capped with a heavy n- or p-doped GaN layer none of the similar behaviors are observed until the cap layer is less than 25 nm. These phenomena are well explained by introducing the effects of upward surface band bending. Surface polarization discontinuity and surface states are considered as the origins of the surface band bending. Furthermore, the actual surface band bending value of the un-doped GaN is also extracted as Eb = 1.54 ±0.08 eV from the linear PL blue shift behavior.

Variational Calculations of Donor Binding Energy in Rectangular Wurtzite Aluminium Gallium Nitride / Gallium Nitride Quantum Wires

AbstractWe present variational calculations of donor binding energy in rectangular wurtzite aluminium gallium nitride / gallium nitride quantum wires. We explicitly take into account the effect of spontaneous and piezoelectric polarization on the energy levels of donors in quantum wires. Wurtzite structure nitride semiconductors have spontaneous polarization even in the absence of externally applied electric fields. They also have large piezoelectric polarization when grown as pseudomorphic layers. The magnitude of both polarization components is of the order of 1013 electrons per cm2, and has a non-trivial effect on the potential profile seen by electrons. Due to the large built-in electric fields resulting from the polarization discontinuities at heterojunctions, the binding energies of donors is a strong function of the position inside the quantum wire. The potential profile in the 0001 direction can vary by as much as 1.5eV due to polarization effects for vertical dimensions of the quantum wire up to 20 angstroms. The probability density of electrons tends to concentrate near the minimum of the conduction band profile in the 0001 direction. Donors located close to this minimum tend to have a larger concentration of electron density compared to those located closer to the maximum. As a consequence, the binding energy of the former are higher compared to the latter. We use Lorentzian variational wavefunctions to calculate the binding energy as a function of donor position. The confinement potential enhances the binding by a factor of about 3 compared to donors in bulk nitride semiconductors, from about 30 meV to about 90 meV. The variation of binding energy with position is calculated to be more than 50% for typical compositions of the quantum wire regions. Our calculations will be useful for understanding device applications involving n-type doped nitride semiconductor quantum wires.

Charge compensation and mixed valency inLaAlO3∕SrTiO3heterointerfaces studied by the FLAPW method

The electronic structures and properties of heterointerfaces in the perovskite oxide superlattices ${\mathrm{LaAlO}}_{3}∕{\mathrm{SrTiO}}_{3}\phantom{\rule{0.3em}{0ex}}[001]$ are presented using the first-principles all-electron full-potential linearized augmented plane-wave (FLAPW) method. Superlattices with three types of interfaces, (i) electron-doped, (ii) hole-doped, and (iii) both electron- and hole-doped, are studied and compared. For the electron-doped interface, the mixed valency of Ti along with the Jahn-Teller effect are found to explain the metallicity, in agreement with experiment, whereas for the hole-doped interface metallicity is found, in contrast to experiment. Oxygen vacancies introduce an additional $n$-type carrier to compensate the holes present at the interface, which now becomes insulating and agrees well with experiment. For a system with both types of interfaces, the metallicity found for the unrelaxed structure is changed to insulating after relaxation is included. For all cases, the relaxation results in a complicated buckled geometry, which is a good indication of the adjustment due to polarity discontinuity. Our findings support recent experimental results, namely, (i) the mixed-valence character of Ti at the electron-doped interface, and (ii) the existence and importance of oxygen vacancies at the hole-doped interface.

Determination of polarization charge density on interface of AlGaN∕GaN heterostructure by electroreflectance

Electroreflectance spectra of AlGaN∕GaN heterostructure were obtained at various bias voltages (Vdc). Franz-Keldysh oscillations (FKOs) were observed above band gap of AlGaN. The strength of electric field in AlGaN (FAlGaN) was evaluated from the periods of the FKOs, yielding the strength of FAlGaN as a function of Vdc. A positive fixed polarization charge σp arises as a result of the polarization discontinuity across the heterointerface. The neighboring electrons are then attracted by the established σp to form a two-dimensional electron gas (2DEG) at the interface. The experimental value of σp, as obtained by completely depleting the 2DEG using a negative Vdc, is found to be reasonably close to the theoretical value.

Numerical simulation of transconductance of AlGaN/GaN heterojunction field effect transistors at high temperatures

Based on the investigation of the influence of temperatures on parameters, including polarization, electron mobility, thermal conductivity and conduction band discontinuity at the interface between AlGaN and GaN, the temperature dependence of transconductance for AlGaN/GaN heterojunction field effect transistors (HFETs) has been obtained by using a quasi-two-dimensional approach and the calculated results are in good agreement with the experimental data. The reduction in transconductance at high temperatures is primarily due to the decrease in electron mobility in the channel. Calculations also demonstrate that the self-heating effect becomes serious as environment temperature increases.

Midinfrared intersubband absorption in lattice-matched AlInN∕GaN multiple quantum wells

We report the observation of midinfrared intersubband (ISB) absorption in nearly lattice-matched AlInN∕GaN multiple-quantum-wells. A clear absorption peak is observed around 3μm involving transitions from the conduction band ground state to the first excited state. In addition to ISB absorption, photoluminescence experiments were carried out on lattice- matched AlInN∕GaN single quantum wells in order to determine the spontaneous polarization discontinuity between GaN and Al0.82In0.18N compounds. The experimental value is in good agreement with theoretical predictions. Our results demonstrate that the AlInN∕GaN system is very promising to achieve crack-free and low dislocation density structures dedicated to intersubband devices in the 2–4μm wavelength range.

Determination of the polarization discontinuity at the AlGaN∕GaN interface by electroreflectance spectroscopy

The total polarization discontinuity ΔP at an Al0.31Ga0.69N∕GaN heterojunction has been determined by electroreflectance spectroscopy. This technique is based on the analysis of the Franz–Keldysh oscillations observed above the AlGaN band gap, yielding the barrier electric field strength as a function of the applied bias voltage. The threshold field strength, where the two-dimensional electron gas (2DEG) is depleted, corresponds to a ΔP of 1.1×1013e∕cm2 which is only 85% of the theoretical prediction. Applying the same optical method, the 2DEG density at the heterointerface can be accurately determined, as proven by comparison to Shubnikov–de Haas measurements.

An Efficient Adaptive Polarimetric Processor with an Embedded CFAR

To improve the detection performance of surveillance radars with polarization diversity, we developed an adaptive polarimetric processor and compared it with other polarimetric processors. We derived our adaptive polarimetric processor, called the polarization discontinuity detector (PDD), from the generalized likelihood ratio (GLR) test principle for the unspecified target component. We derived closed-form expressions of its probabilities of detection and false alarm, and compared its performance to that of the adaptive polarization canceller (APC) and Kelly's GLR processor. The PDD had a performance similar to Kelly's GLR in Gaussian clutter, and both the PDD and Kelly's GLR, which have embedded constant false alarm rates (CFARs), outperformed the APC, especially when the target polarization state was close to the clutter's polarization state. The important difference is that the PDD is much simpler than Kelly's GLR for hardware/software implementation, because the PDD does not require a costly two-parameter filter bank to cover the unknown target polarization state as Kelly's GLR does.

Boundary effects on the optical properties of InGaN multiple quantum wells

We examine the issues of spontaneous and piezoelectric polarization discontinuity on the optical properties of 3.0-nm-thick indium gallium nitride (InGaN) multiple quantum wells (MQWs). A quench of band-edge emission from the cap GaN layer is observed when the photoexcitation source is changed from a 355- to a 248-nm laser. The interband transitions from the InGaN wells exhibit a linear dependence on the 1) spectral blue shift of ∼8.5×10/sup -18/ meV /spl middot/ cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and 2) change of the internal field of ∼3×10/sup -14/ meV /spl middot/ cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with the injected carrier density up to N/sub inj/∼10/sup 19/ cm/sup -3/ at 77 K. These observations are attributed to the redistribution of photogenerated carriers in the InGaN wells due to the polarization discontinuity at the QW interface and the surface band bending effect. By incorporating an additional boundary condition of surface Fermi-level pinning into the Poisson equation and the band-structure analysis, it is shown the emission from the InGaN-GaN MQWs is dominant by the recombination between the high-lying subbands and the screening of internal field effects.

Free electron distribution in AlGaN/GaN heterojunction field-effect transistors

A detailed calculation of the free electron concentration and conduction and valence band edges of AlGaN/GaN heterojunction field-effect transistors is presented. The model is based on a self-consistent solution of the Schrödinger, Poisson, and charge balance equations and includes the effect of exchange correlation on the Coulomb interaction. It also includes surface acceptor and donor states populated according to Fermi–Dirac statistics. The piezoelectric and spontaneous polarization discontinuities across the material interfaces are rigorously taken into account. The influence of the polarization discontinuity on the magnitude of the charge in the two-dimensional electron gas is investigated. From charge conservation, it is shown that the polarization discontinuity does not behave as dopants in the same manner as substitutional impurities. Any free electrons within the structure must originate from some other source, either from the surface through surface donors, or from the bulk through unintentional n-type dopants. The present model allows for δ doping and multiple AlGaN/GaN interfaces.

Evidence for nonlinear macroscopic polarization in III–V nitride alloy heterostructures

We provide explicit rules to calculate the nonlinear polarization for nitride alloys of arbitrary composition, and hence, the bound sheet charge induced by polarization discontinuity at the interfaces between different alloy and binary (epi)layers. We then present experimental results and simulations of polarization-related quantities in selected nitride-alloy-based heterostructure systems. The agreement of experiment and simulation, also in comparison to previous approaches, strongly suggests that the macroscopic polarization of nitride alloys is indeed nonlinear as a function of composition.