Conducting tip AFM nanoindentation:
an original technology of nanolithogtaphy developped to study original transport properties at the nanometer scale
The magnetic nanostructure realization paves the way for active research fields such as nanospintronics. Novel effects arise when typical coherence length of transport and magnetism meets with sample scaling. A technological bottleneck impeding physic progress is the ability to connect individual nanometric objects.
An original nano-fabrication technology has been developed in the laboratory: A new tool for nanolithography based on an atomic force microscope coupled with a high performance current measurement system. The AFM tip is used to create an aperture by indentation through a thin insulator (indentation mask) deposited on a conducting structure (conducting thin film, nanowires, cluster assembly…). The indentation process is real-time controlled by measuring the current flowing between the AFM tip and the underlying conducting structure. The nano-hole is then filled by a metal to create a nano-contact on the underlying sructure. The full process includes some optical UV lithography steps to allow transport measurement through individual nano-contacts or single nano-objects [1].
Figure 1: Examples of applications of our AFM based nanolithography :
Magnetic tunnel nanojunctions
Heterostructures are tested on small scale (typically 30x30 nm²) for which the tunnel barriers are completely free from heterogeneities. This technology has been used successfully to realize spin filters based on ferromagnetic insulating oxides such as NiFe2O4[2] or BiMnO3[3] and multiferroic
based tunnel junctions with La0.1Bi0.9MnO3 [4].
Figure 2: Scheme of a magnetic tunnel nanojunctions
Connection of a single nanometric embedded cluster for spin dependant transport studies
Clusters assemblies generally available exhibit very large density (3 nm diameter clusters separated of 3 nm). Sub-10 nm nanocontacts were demonstrated on this type of systems booming the emerging field of spin transport through nanometric objects [5].
Figure 3: scheme of a magnetic tunnel nanojunctions :a) Scheme of the nanolithography on our cluster system, b) representation of the nanocontact on the scale of a TEM image of the real clusters system, c) transport measurement (dI/dV as a function of V) at 4K through a single cluster.
Spin torque based high-frequency generation
Nanoscaled structures allow high current densities to be injected (>10^7 A/cm2),
necessary to obtain high-frequency generation associated to GMR effects. These structures are nano-injectors on GMR multilayers, or GMR nanowires connected individually by the nanoindentation process [6,7].
Figure 4: Scheme of single nanowire connection by AFM based nanolithography
Molecular electronic
The links between the molecular structure and the associated transport properties
have triggered many researches worldwide. This task needs the elaboration of devices involving only few molecules. The target resolution for lithographic processes has to be in the nanometric range. Depending on the nature of the molecule (tunnel or metalliclike behaviors), different magneto resistive effects are expected.
Advanced characterization atomic force microscopy:
Beside classical AFM based characterizations (topography, Manetic Force Microscopy…), we have developed more specific techniques related to our research projects.
Spreading resistance microscopy
The conducting tip atomic force microscope (CT-AFM) adds to the toolbox to assess the transport properties of materials and heterostructures at the nanometric scale. The principle is based on contact mode AFM while a tip-sample bias is applied. An external system allows the amplification and the measurement of the current flowing during AFM imaging, giving simultaneous mappings of the sample topography and of the spreading resistance. The experimental set up was especially designed for AFM and has been developed by the research team of F. Houzé at the “Laboratoire de Genie Electrique de Paris”: it allows high performance current measurement (5kHz bandwith) down to the pA over a wide range of 10 decades.
Piezoresponse force microscopy
The Piezoresponse-AFM or so called PFM adds to the toolbox to assess the local ferroelectric properties of ultrathin films such as those taking place in tunnel junctions. The principle is based on the measurement of the piezoelectric response (contraction/extension) of the ferroelectric material while it is vibrating under an ac electric field applied locally between the AFM tip and a bottom electrode. The technique allows imaging of the ferroelectric domains structure at the nanoscale or the local measurement of hysteretic cycles.
Examples of applications of advanced characterization:
Highly spin polarized current sources
The CT-AFM is used to validate the nature of the transport in ultrathin films such as those acting as a tunnel barrier [8,9]. The CT-AFM is also used to map the electrical homogeneity of these films.
Figure 5: Simultaneous topography and conductance mapping of epitaxial La2/3Ca1/3MnO3 thin film.
Multiferroics
An example is the multiferroic material BiFeO3 epitaxially grown on the half metallic manganite La2/3Sr1/3MnO3. The PFM was at work to demonstrate the preserved ferroelectric nature of the BiFeO3 down to the thickness of 2 nm [10] compatible with tunnel transport.
Figure 6: Simultaneous topography, domain orientation mapping and domain wall imaging of a 70 nm epitaxial multiferroic BiFeO3 film
2D electron gas from oxide heterostructure interfaces
Combined with a “cross section” sample preparation, CT-AFM allows the direct visualization of charge carriers confinement at the LaAlO3-(001) SrTiO3 interface discriminating real interfacial phenomenon from unintentional bulk doping by oxygen vacancies [11].
Figure 7: Cross section observation by CT-AFM of the interface of epitaxial LaAlO3 on SrTiO3 a)Topography, b) Conduction mapping for deposition at 10-6 mbar and c) 10^-3 mbar.
References
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