Ultrafast-Spectroscopy

Transmission Electron Microscopes (TEM) and Scanning Transmission Electron Microscopes (STEM)  are designed for high-resolution imaging and analysis.
In both cases, high energy electrons, incident on ultra-thin samples called TEM lamella (typically <100nm), travel through the specimen.
Depending on the density, crystallinity, orientation, etc. of the material present, electrons are scattered differently, giving rise to an image of the sample with different contrast features according to specimen properties and to the microscope setup.
Usually, the magnified image of the sample is focused on a CCD or CMOS camera. On the same system is also possible to acquire the electron diffraction produced by a specific area of the sample.

Modern TEM are equipped with a Cold Field Emission Gun (FEG) or with a Schottky FEG, making them suitable for materials science, nanotechnology, and various fields requiring detailed structural and compositional analysis in a local nanoscale. Indeed, point resolution below 0.3 nm at 200kV in TEM mode can be reached, according to the
instrument configuration.

In the STEM mode, electrons pass through the specimen, but, the electron optics focus the beam into a narrow spot which is scanned over the sample in a raster. The rastering of the beam across the sample makes these microscopes suitable for analysis techniques.

According to the specific configuration (see details and the specific availability below, referred to each instrument and facility), the microscope can be equipped with a selection of multiple detectors enabling different techniques, such as Energy-Dispersive x-ray Spectroscopy analysis (EDS), electron energy loss spectroscopy (EELS), STEM detectors in bright field (BF), annular bright field (ABF), annular dark field (ADF) for diffraction contrast, high-angle annular dark field (HAADF) for Z-contrast.

EELS/EDS system control software enables point-by-point signal acquisitions, for the analysis of spectra and maps.
Sample preparation (see details and the specific availability below, referred to each instrument and facility) is a crucial part in TEM experiments.

High quality TEM specimens have a thickness that is comparable to the mean free path of the electrons that travel through the samples, which may be only a few tens of nanometres. Preparation of TEM specimens is specific to the material under analysis and the desired information to obtain from the specimen. Sample preparation laboratories are equipped with the basic tools (diamond saw, polisher, dimpler, ultrasonic cutter, precision ion polishing system, plasma cleaner) commonly used in conventional thinning procedures.

Luminescence optical spectroscopy is a classical spectroscopy to characterize excited states in semiconductors. It is based on an optical excitation with an excess energy with respect to the band gap. The various pathways of de-excitation give rise to a re-emitted photon distribution with energy related to the excited states.

The lineshape as a function of the re-emitted photon energy is generally modeled by the product of the absorption times the photoexcited electron and hole distribution functions at energies coupled by the photon. In this approximation a common temperature of electrons and holes is postulated and by fitting the high energy tail of the spectrum the carriers temperature can be retrieved.

The PhotoLumiscence also provides a scenario for the exciton dynamics; in particular, varying the flux of the photon beam, the PhotoLuminescence intensity is super-linear in the case of exciton-exciton interaction.

Moreover, PhotoLuminescence at low temperature is a powerful tool to characterize point defects in semiconductors, and it is a fundamental characterization to improve the quality of the semiconductors in electronic devices.

The Spreading Resistance Profiling (SRP) technique is an established methodology that is in use in both industry and research for measuring resistance profiles and, following calibration, the density profiles of active carriers in state-of-the-art semiconductor geometries. Two metallic probes, between which a controlled potential (small V_SRP=5 mV, for Si or Ge) is applied, are used to quantify the charge carrier distributions as a function of depth. Depth information is obtained by stepping down the probes along a bevelled surface of the sample. During the measurement a resistance profile is acquired, which is computed and converted into resistivity and carrier concentration by means of calibration curves (mobility parameters of monocrystalline materials). A particular algorithm is also applied which corrects for sampling volume effects in case of ultra-shallow profiles.

Spectroscopic Ellipsometry (SE) is a contact-free, nondestructive method for the characterization of the dielectric and optical properties (refractive index, absorption coefficient, anisotropy) and structural properties (roughness, thickness and intermix of layers) in the thickness range of < 1 nm to few µm.

Ellipsometry is an accurate technique for calculating materials’ parameters, including complex refractive index or dielectric function. The samples are measured in reflection and the information is given based on the change of the polarization of the reflected light. Standard SE methods measure the Fresnel reflection coefficients as a function of wavelength only. While variable angle SE (VASE) measures the sample’s coefficients in s- and p-polarized light as a function of wavelength and angle of incidence. In addition, thanks to the vertical configuration it can be measured also the sample transmittance to be coupled with the ellipsometric data during fitting. All measurements can be done with a motor stage on a 4x4cm area.

Depending on the spectral coverage of a VASE instrument, dielectrics, semiconductors, and metals can be characterized. Typical applications are the characterization of manufactured thin films used in devices. Sample size can vary from 1x1cm up to a 6” wafer.

Finally, the measurements, thanks to a closed cell, can be performed in an N₂ environment in the case of air-sensitive samples and or by varying the temperature in the range -90°C up to 600°C to identify phase changes.