PREPARATION AND CHARACTERIZATION OF NANOSTRUCTURES FOR PHOTONICS
-Development of: 1) design and preparation of QD structures for photonics and 2) structural, electrical and optical characterization of the structures for their optimization;
-In the framework of common projects, 3) transfer of expertise to interested institutions and 4) provision of QD nanostructures with controlled properties to the scientific and industrial community.
Quantum dot (QD) nanostructures based on III-V compounds have great relevance in the fabrication of telecom devices of interest for the Information Society. They play an important role also in the fields of Quantum Information and Quantum Computing. QDs structures may have operation wavelengths in the 0.98 um and 1.3-1.6 um windows of photonic interest and may be grown on GaAs substrates; this opens the way to the fabrication of high-yield and high-performance vertical cavity surface-emitting lasers. Said structures are also of huge interest for the study of physical properties of zero-dimensional systems.
The research deals with: 1) the MBE preparation of III-V QD nanostructures, 2) the engineering of band structures of said structures in order to tune the RT emission in the windows of interest and to increase the emission efficiency, 3) the study of structural, electrical and optical properties of the structures.
The expertise of the group covers a wide range of topics, that are: 1) MBE of advanced III-V epitaxial structures and, in particular, of QD nanostructures; 2) structural characterization by AFM, TEM and advanced X-ray techniques; 3) electrical characterization by means of space-charge techniques, such as C-V, DLTS, admittance and scanning capacitance microscopy as well as by I-V and Hall effect measurements; 4) optical characterization by photoluminescence. Besides the experimental techniques mentioned above, photolithographic and metallization ones are available to prepare structures for characterization.
In order to study QD nanostructures a number of cooperation have been established with CNR Institutes (IFN, INFM-NNL) and academic groups (Universities of Milano Bicocca, Pavia, Firenze and Roma La Sapienza). In the framework of the SANDiE Network of Excellence (6th FP) cooperations with Universities of Sheffield (optical characterization of laser structures) Lancaster, Leuven, Valencia (optical properties of QD structures), Cádiz and Edinburgh (structural characterization) have been set up.
Among other results, it has been recently proposed and demonstrated that by engineering the band structure of InAs/InGaAs QDs, by means of the strain induced by metamorphic buffers (QD strain-engineering) and by enhancing the barriers that confine the carriers into QDs, both emission wavelength and efficiency can be optimized and RT emission is obtained up to 1.59 um from nanostructures grown on GaAs substrates, a result of wide interest for photonic devices and rarely reported so far.
DESIGN, PREPARATION, AND STUDY OF FUNCTIONAL MAGNETIC MATERIALS
o Preparation and properties optimisation of:
-magnetic thin films, exchange-spring magnets, and nanostructures for applications in recording media, and micro/nano-electromechanical systems
-intermetallic ferromagnetic-shape-memory materials, magnetocaloric materials, and hard magnetic materials for applications as sensors/actuators, eco-compatible refrigerants and permanent magnets
-magnetic oxides for application in spintronic devices.
o Theoretical studies on magnetism and superconductivity; modelling of magnetism in ultra-thin films and of magnetization processes in multi-sublattice systems.
o Realization of measuring techniques and innovative instruments.
The phenomena displayed by novel nanostructured and bulk magnetic materials can be widely exploited in a variety of emerging applications ranging from data storage to nano-medicine, or from sensoristic to room temperature magnetic refrigeration. The research activity of the group that has a pluriennial expertise on hard magnetic materials, measurements and modeling of magnetic anisotropy and magnetization processes, are today mainly devoted to:
-the preparation of hard magnetic epitaxial thin films (e.g. FePt L10-based) and the study of the correlation between functional, morphological/interfacial and microstructural properties for applications in memories and MEMS
-the realization of new nanostructured architectures: patterned and exchange-spring systems aimed at realising ultra-high density magnetic recording media with improved storage capacities: smaller bits with enhanced thermal stability and limited coercivity
-the preparation of thin films and bulk materials for spintronic and the study of the correlation between nanostructuration, composition, and functional properties
-the preparation of ferromagnetic shape memory materials, the study of structure and magnetism and their pressure dependence in order to improve the giant magnetoelastic (giant magnetic field induced strain) and magnetocaloric properties
-the identification and study of new magnetocaloric materials exploitable in room-temperature magnetic refrigeration (eco-compatible alternative to conventional gas-compression refrigeration technique)
- the identification and magnetic study and of new intermetallic compounds that can be expoited as high performance permanent magnets at high temperature.
Moreover part of the activity is devoted to simulations and theory: e.g. Monte Carlo simulation of bidimensional spin models for the description of ultrathin films, theoretical studies on the superconductivity in MgB2 and on the coexistence of superconductivity and antiferromagnetism in heavy fermions systems.
Coherently with the tradition of the group, part of the activity is also devoted to the realisation of measuring techniques and new instruments: e.g. generation of high pulsed magnetic fields (up to 50 T) and development of advanced coils (>50 T).
GROWTH, CHARACTERIZATION AND PROCESSING OF ADVANCED NANOSTRUCTURED MATERIALS
(i) Preparation and characterization of materials for electronics and (bio-)sensor applications; (ii) novel protocols for the structural and optical characterization at the nanometer scale; (iii) ab initio simulations of nanostructured surfaces.
The development of advanced nano-structured materials and processing optimization for the design of nanoelectronic devices and advanced sensors is a strategic field. In this frame the present research activity deals with the growth of electronic materials with tailored properties for specific applications and the design of experimental set-up for the nanoscale. The experimental approach is complemented by the first principles study of electronic and structural properties of materials and structures.
The most relevant topics can be outlined as follows:
i. MOVPE preparation and structural, optical and electric characterization of SiC thin films and nanowires grown on (001) and (111) oriented Si substrates; High-k ferroelectric thin film oxides for high frequency applications, to be used in MIM structures; Structural study of C-MOS implantation profiles in As:Si and In:SiGe layers by XSW and TEM; Analysis of residual impurity type and density by studying the p+ gettering in epi-Si for EPROM memories
ii. Correlation between point defects and optical properties in single III-V and IV-IV semiconductor nanowires by SEM-CL and electron beam irradiation; controlled band-gap modulation in diluted hydrogenated nitrides by electron beam writing; failure analysis of electron devices by CL, EBIC and EL; TEM, SEM-CL and HRXRD study of optical and structural properties in multi-layered SiGe/Si islands for the design of single electron transistors
iii. First principles and semiempirical methods for electronic and structural properties of materials, with particular emphasis on surfaces and interfaces of wide-gap semiconductors, and related nanostructures; theoretical study of functionalization feasibility and stability, also in humid environment; characterization of the functionalized system electronic and structural properties, with the final goal of (bio-)sensing and solar device design strategies.
The results obtained up to now raised interest among different national (CNR and UNI) and international groups, and also at the industrial level: we are defining collaboration research projects to be submitted to CNR, MUR and FP7. As an acknowledgment to the activity and efforts for the research community in Material Science, the group of Electron Microscopy has received 1.200.000 EUR from a private foundation to buy a novel transmission electron microscope capable of electronic spectroscopy and imaging, compositional analysis and chemical contrast, at the nano- and sub-nanoscale.
HYBRID ORGANIC-INORGANIC NANOMATERIALS FOR MANUFACTURING
Expertise development in preparing, qualifying and functionalizing organic-inorganic hybrid materials.
In recent years hybrid organic-inorganic (HOI) materials have been widely recognized as a rapidly emerging research area in the field of advanced functional nanomaterials. They offer the potential to combine desirable physical properties of both organic and inorganic components within a single composite, thus allowing not only the design of new materials and compounds for research, but also the development of innovative technologies due to their improved or unusual features.
The research activity of the Institute in this field is focused on developing expertises in the preparation and functionalization of HOI, with main attention to:
A) Self-assembling organic-inorganic perovskites
Such perovskites are crystalline hybrids that provide a pathway to an alternating framework of semiconducting inorganic sheets and organic layers. At present HOI perovskites [R-NH3)MX3 and (R-NH3)2MX4] based on alkylammonium cations [R=CnH(2n+1)] and divalent-metal halides (M=Cu, Sn and X=Cl,Br) are under investigation. The research is aimed at i) optimizing the preparation of thin films by single source thermal ablation and spin coating, ii) studying their structural, optical and electrical properties, iii) evaluating the functional properties as a function of the chemical composition in view of potential application in optoelectronics and large area flexible electronics, iv) developing simple test devices and demonstrators to pursue the synergy between materials and device technologies. Scientists from the Information Engineering Dept. of the Parma Universtity contribute through their microelectronic design and modelling expertise.
B) Organic functionalized metal oxides nanostructures
Nanostructured metal oxides (NMO) represent a very interesting class of materials for the manufacturing since they have different promising functionalities as, for instance, solar cells, photocatalysts and gas-sensors. Nowadays the development of novel preparation methods allows NMOs to be obtained with various morphologies (e.g. nanowires, nanobelts, nanosheets, comb-like structures, tetrapods, highly ordered opals and inverted opals), whose unique properties can be exploited for different applications. Grafting organic functional groups on the surface of NMOs results in hybrid composites where the inorganic functional properties may be improved or extended by the inorganic component. At present the activity at IMEM is focused on nanocrystalline ZnO and SnO2, and well assessed materials, such as TiO2. Aims of the research are i) the development of novel preparation processes suitable for large area (tens of square centimeters) NMO deposition, ii) the NMO functionalization through the coupling with organic molecules (e.g. phthalocyanines or perylene-based radicals), and iii) the qualification of the obtained hybrids with respect to their functionalities in photocatalysis, photovoltaics and gas sensing applications.
ADVANCED POWER SYSTEMS
- Realization of low bandgap solar cells to be used in high efficiency tandem solar cells or in thermophotovoltaic generators
- Realization of selective emitters and-or infrared selective filters for thermophotovoltaic conversion
o The research activity concerns the preparation by Metal Organic Chemical Vapour Deposition (MOCVD) technique of low bandgap photovoltaic cells and the characterization with electrical and structural characterization techniques. Low bandgap photovoltaic cells are employed in thermo-photovoltaic generators or in "tandem" solar cells absorbing the low energy range of the solar spectrum.
Due to the low bandgap (Eg=0.67 eV) the germanium based photovoltaic cells are suitable for such applications. At the present state the p-n junction in low bandgap material as Ge and GaSb is obtained only with diffusion process, which leads to cells with low values of open circuit voltage Eoc and high surface carrier recombination. Epitaxial techniques provide a more precise definition of doping and doping profiles and higher cell efficiencies.
Homoepitaxial Ge/Ge, Ge/GaAs, GaAlAs/GaAs e InGaP/GaAs junctions to be used in high efficiency tandem solar cells based on multiple integrated cells are prepared and studied.
o In order to improve the efficiency in the thermophotovoltaic conversion selective emitter materials are prepared and studied in cooperation with university groups. Selective emitters absorb the energy emitted from a radiant source at high (1200-1550 °C) temperature in a broad spectral range and emit in a much narrower range. Their emission range as a function for the temperature is studied by spectroscopic techniques.
In addition to the selective emitters, selective filters that transmit the radiation in a suitable range at wavelengths near 1.5 micron and reflects the unwanted radiation can be employed. Selective filters working in the infrared range can be realised by multilayers coatings of materials with different refraction index.
Infrared selective filters made of multilayers of NiO2 and SiO2 are realised in cooperation with other research centers and studied by optical techniques.
MATERIAL DEVELOPMENT FOR SIGNAL ACQUISITION DEVICES
The objective of the working group concerns the development of materials for the realization of sensors for security and safety (food and environment control). The research deals with both the material preparation and the realization of prototypes.
The activity is organized in three sections.
A) Gas sensors. Market requires sensors of high sensitivity, selectivity and stability: these requirements can be fulfilled by the realization of sensors based on metal-oxide nanowires because of the high surface to volume ratio and the single crystalline structure. Planned activities: i) growth of SnO2 and In2O3 nanowires, ZnO nanostructures (nanowires, tetrapods, nanocombs,...); ii) structural, electrical and optical characterization of the nanowires; iii) preparation and functional characterization of gas sensors based on metal-oxide nanostructures; iv) nanowires doping for selectivity enhancement; v) ZnO deposition by sol-gel technology.
The research is supported by the Emilia Romagna networks LARIA and MISTER and by the company Venezia Tecnologie (Ve).
B) X-ray detectors. Security applications as well as medical diagnostics require the availability of X-ray detectors working at room temperature with high efficiency and energy resolution. Similar detector characteristics are also required by the new generation of gamma- ray telescopes. Zinc Cadmium Telluride is the most promising material to realize such devices, due to the high mean Z value, large band-gap energy and good transport properties. Planned activities: i) growth of CZT bulk crystals; ii) material characterization; iii) preparation and functional characterization of CZT-based X-ray detectors; iv) development of front end and read out electronics.
The research is supported by the European Space Agency and the Italian Space Agency, by the companies Softec (Bo), Thales Alenia Space (Mi), Venezia Tecnologie (Ve) and 5Nplus (Canada).
C) Development of systems based on piezoelectric sensors for non-destructive testing of food, in particular Parmigiano Reggiano. The instrumental test is expected to help the test based on human efforts also in the view of quality certification. Activities: i) analysis of the acoustic response of food subjected to mechanical excitation; ii) analysis of the dielectric properties of food in order to study its humidity, solidity and homogeneity.
The research is supported by the Emilia Romagna network TECAL and by the Consorzio Parmigiano Reggiano.
SUPERCONDUCTING MATERIALS FOR POWER APPLICATIONS
- Fabrication of high-quality, low cost HTS (YBCO-based) Coated Conductors (HTS-CC) for power applications.
- Synthesis of new HTS and improvement of the intrinsic properties of the existing HTS (Tc, Jc, Hc).
- Synthesis of new materials with novel electronic properties.
The HTS-CC represent a promising solution to the serious problem of the energy saving.
Although the technology for the fabrication of the HTS-CC is mature, their high production costs prevent a wide market penetration in various sectors like cables, magnets, generators, transformers, motors, etc.
A reel-to-reel apparatus for continuous deposition of YBCO films on metallic tapes (2nd generation Coated Conductors) equipped with a new deposition system (PED, Pulsed Electron Deposition) has been installed at IMEM, in collaboration with Edison.
The research activities to lower the deposition costs, can be summarized in:
- reduction of the complexity of the HTS-CC architecture. In particular, we studied, tested and developed a process for the fabrication of a crack-free, single buffer layer based on doped ceria (CeO2), in substitution of the complex and costly multi-layer structures commonly adopted.
- development of a HTS-CC deposition process entirely based on PED; this technique is characterized by a low investment and maintenance costs, high deposition rate onto large areas, and high rate of material transfer from the target to the substrate, with excellent respect of the stoichiometry.
- optimization of a new system (SNEO, Supersonic Nozzle Enhanced Oxygenations, patent application WO/2005/024088) to improve the oxygenation of YBCO layer. It is based on the focusing effect of the supersonic gas expansion from a conic nozzle, enhancing the oxygen sticking factor on YBCO film and improving its transport properties. The technique can be extended to various applications involving processes of gas absorption.
Thanks to the acquired experience on vacuum deposition techniques and to the flexibility of the PED system, we are extending our interest towards other energy applications (i.e. photovoltaic cells).
A more "basic" research area deals with the synthesis and the characterization of new materials with novel electronic properties (superconductors, cuprates, manganites, multiferroics, etc.)
The main research strategies are:
- To find new HTS systems, also non-cuprates (i.e. sulfides, intercalated graphite, etc), which are promising candidates to host a metallic state and/or superconductivity. The experimental activity is supported by chemical consideration, structure simulation and theoretical studies.
- To prepare new metastable compounds under high pressure and hydrothermal synthesis and to investigate and correlate the microstructure and the electronic properties. The goal is the comprehension of controversial phenomena (i.e., ordering mechanisms and magneto-resistance in manganites, coexistence of ferroelectric and ferromagnetic properties in multiferroics materials, etc.), that implies a detailed study of the material P/T phase diagram and stability region, to produce pure samples (bulk and single crystals), suitable for accurate characterization.
- To improve the intrinsic characteristics and to analyze the interplay between the structure and the electronic properties of existing HTS systems, by chemical substitutions. The comparison between internal (chemical) pressure and external (mechanical) one is also investigated.