133 research outputs found
Breakdown of step-flow growth in unstable homoepitaxy
Two mechanisms for the breakdown of step flow growth, in the sense of the
appearance of steps of opposite sign to the original vicinality, are studied by
kinetic Monte Carlo simulations and scaling arguments. The first mechanism is
the nucleation of islands on the terraces, which leads to mound formation if
interlayer transport is sufficiently inhibited. The second mechanism is the
formation of vacancy islands due to the self-crossing of strongly meandering
steps. The competing roles of the growth of the meander amplitude and the
synchronization of the meander phase are emphasized. The distance between
vacancy islands along the step direction appears to be proportional to the
square of the meander wavelengthComment: 7 pages, 9 figure
Step velocity tuning of SrRuO3 step flow growth on SrTiO3
Taking advantage of vicinal (001) SrTiO3 substrates with different mean
terrace widths, the heteroepitaxial growth of SrRuO3 in the step flow mode has
been mapped as a function of mean step velocity. Transition between stable and
unstable step flow is shown to occur at a well-defined critical step velocity,
with a step bunching instability observed below this threshold. The ability to
pass from unstable to stable step flow during growth upon increasing the mean
step velocity is demonstrated. This result is discussed in terms of a
stress-based driving force for step bunching in competition with an effective
step-up adatom current.Comment: 4 pages, 3 figure
Effect of kink-rounding barriers on step edge fluctuations
The effect that an additional energy barrier E_{kr} for step adatoms moving
around kinks has on equilibrium step edge fluctuations is explored using
scaling arguments and kinetic Monte Carlo simulations. When mass transport is
through step edge diffusion, the time correlation function of the step
fluctuations behaves as C(t) = A(T) t^{1/4}. At low temperatures the prefactor
A(T) shows Arrhenius behavior with an activation energy (E_{det} + 3 epsilon)/4
if E_{kr} epsilon,
where epsilon is the kink energy and E_{det} is the barrier for detachment of a
step adatom from a kink. We point out that the assumption of an Einstein
relation for step edge diffusion has lead to an incorrect interpretation of
step fluctuation experiments, and explain why such a relation does not hold.
The theory is applied to experimental results on Pt(111) and Cu(100).Comment: 11 pages, 4 eps figure
Interface Electronic Structure in a Metal/Ferroelectric Heterostructure under Applied Bias
The effective barrier height between an electrode and a ferroelectric (FE)
depends on both macroscopic electrical properties and microscopic chemical and
electronic structure. The behavior of a prototypical electrode/FE/electrode
structure, Pt/BaTiO3/Nb-doped SrTiO3, under in-situ bias voltage is
investigated using X-Ray Photoelectron Spectroscopy. The full band alignment is
measured and is supported by transport measurements. Barrier heights depend on
interface chemistry and on the FE polarization. A differential response of the
core levels to applied bias as a function of the polarization state is
observed, consistent with Callen charge variations near the interface.Comment: 9 pages, 8 figures. Submitted to Phys. Rev.
Time-resolved PhotoEmission Spectroscopy on a Metal/Ferroelectric Heterostructure
In thin film ferroelectric capacitor the chemical and electronic structure of
the electrode/FE interface can play a crucial role in determining the kinetics
of polarization switching. We investigate the electronic structure of a
Pt/BaTiO3/SrTiO3:Nb capacitor using time-resolved photoemission spectroscopy.
The chemical, electronic and depth sensitivity of core level photoemission is
used to probe the transient response of different parts of the upper
electrode/ferroelectric interface to voltage pulse induced polarization
reversal. The linear response of the electronic structure agrees quantitatively
with a simple RC circuit model. The non-linear response due to the polarization
switch is demonstrated by the time-resolved response of the characteristic core
levels of the electrode and the ferroelectric. Adjustment of the RC circuit
model allows a first estimation of the Pt/BTO interface capacitance. The
experiment shows the interface capacitance is at least 100 times higher than
the bulk capacitance of the BTO film, in qualitative agreement with theoretical
predictions from the literature.Comment: 7 pages, 10 figures. Submitted to Phys. Rev.
Magneto-ionic control of spin polarization in multiferroic tunnel junctions
Magnetic tunnel junctions (MTJs) with Hf0.5Zr0.5O2 barriers are reported to
show both tunneling magnetoresistance effect (TMR) and tunneling
electroresistance effect (TER), displaying four resistance states by magnetic
and electric field switching. Here we show that, under electric field cycling
of large enough magnitude, the TER can reach values as large as 10^6%.
Moreover, concomitant with this TER enhancement, the devices develop electrical
control of spin polarization, with sign reversal of the TMR effect. Currently,
this intermediate state exists for a limited number of cycles and understanding
the origin of these phenomena is key to improve its stability. The experiments
presented here point to the magneto-ionic effect as the origin of the large TER
and strong magneto-electric coupling, showing that ferroelectric polarization
switching of the tunnel barrier is not the main contribution
Active Based-Metasurfaces for Mid-Infrared Optoelectronics Devices
We develop low-temperature (450°C) deposition conditions for vanadium di-oxide phase change material. It permits implementation of tunable mid-infrared meta-surfaces on quantum cascade lasers based heterostructures
Spin electronic magnetic sensor based on functional oxides for medical imaging
8th Spintronics Symposium , AUG 09-13, 2015 , San Diego, CAInternational audienceTo detect magnetic signals coming from the body, in particular those produced by the electrical activity of the heart or of the brain, the development of ultrasensitive sensors is required. In this regard, magnetoresistive sensors, stemming from spin electronics, are very promising devices. For example, tunnel magnetoresistance (TMR) junctions based on MgO tunnel barrier have a high sensitivity. Nevertheless, TMR also often have high level of noise. Full spin polarized materials like manganite La0.67Sr0.33MnO3 (LSMO) are attractive alternative candidates to develop such sensors because LSMO exhibits a very low 1/f noise when grown on single crystals, and a TMR response has been observed with values up to 2000%. This kind of tunnel junctions, when combined with a high Tc superconductor loop, opens up possibilities to develop full oxide structures working at liquid nitrogen temperature and suitable for medical imaging. In this work, we investigated on LSMO-based tunnel junctions the parameters controlling the overall system performances, including not only the TMR ratio, but also the pinning of the reference layer and the noise floor. We especially focused on studying the effects of the quality of the barrier, the interface and the electrode, by playing with materials and growth condition
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