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Dipartimento di Fisica - Politecnico di Milano

4 - Epitaxial growth and nanostructure fabrication

Magnetic nanostructures

The activity is focused on the growth of nanostructured artificial materials (typically layers of nano- or subnanometric thickness) and on their characterization by means of several spectroscopy and microscopy techniques.

In particular, magnetic materials spatially confined down to nanometric scales are investigated. Typical examples are given by ultrathin films with antiferromagnetic coupling, oxides thin films, ferromagnet/semiconductor interfaces, rare earths multilayers and transition metals multilayers.

These innovative structures are synthesized by vacuum deposition on single crystal substrates, using either Pulsed Laser Deposition (PLD) or Molecular Beam Epitaxy (MBE).

The samples are characterized in-situ by means of diagnostic tools such as diffraction of low/high energy electrons (LEED and RHEED, respectively), Auger spectroscopy (AES), x-ray and ultraviolet photoemission spectroscopy (XPS and UPS), photoelectron diffraction (XPD). The electronic and magnetic properties are investigated in-situ by means of electron spectroscopies - also with spin resolution - and techniques based on the magneto-optical Kerr effect (MOKE). The latter technique is also performed with spatial resolution (ex-situ) and variable temperature (down to 10 K), for the investigation of statistical fluctuations in the process of magnetic hysteresis.

A further activity devoted to the lateral definition of nanostructures by means of lithographic techniques (both with light and electron beams), together with an ion beam etching system (IBE), has been recently started. Deposition by sputtering is also available for the realization of electrical contacts and for the growth of different materials, insulators too. Transport and magneto-transport properties are investigated by performing measurements taking advantage of a helium cryostat with a superconducting magnet.

Dealing with micro- and nano-structures, it is certainly important to have access to microscopy techniques. In such a field, the activities deal both with well-estabilished optical microscopies (confocal microscopy) and scanning probe techniques, such as Scanning Near-field Optical Microscopy (SNOM), Scanning Tunneling, Atomic and Magnetic Force Microscopy (STM, AFM and MFM), and finally with chemical sensitive electron microscopy (SAM, Scanning Auger Microscopy).

All the research lines are integrated within the activities of the Center of Excellence for NanoEngineered Materials and Surfaces (NEMAS), instituted by the Italian Ministry for University and Research (MIUR) at Politecnico di Milano, and of the Interuniversity Center (Politecnico di Milano and Università di Milano Bicocca) LNESS (Laboratory of Epitaxial Nanostructures on Silicon and for Spintronics).

 

Semiconductor nanostructures

The laboratory for heterostructures and nanostructures on silicon, located at the centre LNESS in Como, focuses on the fabrication and characterization of semiconductor thin films for microelectronic and optoelectronic applications. The laboratory has a substantial infrastructure, consisting of complementary state to the art deposition and analysis techniques. It is also equipped with clean-room facilities for photolithography, wet chemical processing, electron beam lithography and reactive ion etching.

One of the main strengths of the laboratory is undoubtedly the variety of epitaxial deposition techniques, ranging from molecular beam epitaxy (MBE) for compound semiconductors and oxides, ultra-high vacuum magnetron sputter epitaxy of group IV semiconductors to plasma assisted chemical vapour deposition.
Of particular relevance is low-energy plasma-enhanced chemical vapour deposition (LEPECVD). This process is ideally suited for the production of virtual substrates, consisting of relaxed SiGe alloy buffer layers on Si. These artificial substrates are used as platforms for subsequent epitaxy steps, with numerous applications in micro- and optoelectronics and photonics. Thus the laboratory is active in the fabrication of high-mobility SiGe/Si heterostructures, specializing on strained-Ge quantum wells with record low-temperature mobilities, photodetectors and modulators based on SiGe, and SiGe waveguides.Another area of major interest to the laboratory is monolithic integration of III-V semiconductors on Si substrates for applications in optoelectronics and especially high-efficiency solar cells.
Here, the laboratory’s expertise on oxide epitaxy by MBE provides a means for an alternative virtual substrate concept distinct from the aforementioned SiGe buffers. With the epitaxial oxide activity we also explore alternative concepts for gate oxides and novel devices based on thin oxide/Si combinations.

The laboratory also has a strong activity in nanodevices, where device fabrication is accomplished by combining electron beam lithography with optical lithography and reactive ion etching. The same techniques are used for substrate patterning for precise positioning of semiconductor nanostructures during self-assembled epitaxial growth.

The work on epitaxial hetero- and nanostructures is complemented by research on  nanocrystalline silicon and its applications in low-cost photovoltaic cells and sensors.