Photo-Electron Spectroscopy

Atomic Layer Deposition (ALD) is similar to CVD except that the ALD reaction occurs at lower temperature and it is broken into two half-reactions, keeping the precursor materials separated during the reaction. The separation of the precursors is important to achieve a self-limiting surface reaction to enable precise thickness control and ensure uniform coatings and compactness even in complicated 3D structures.

The present implementation of the technique is suitable for mm to cm-size samples as those typically studied by spectroscopy and microscopy.

ALD has a rich history in microelectronics. It is studied as a potential technique to deposit high-k (high permittivity) gate oxides, high-k memory capacitor dielectrics, ferroelectrics, and metals and nitrides for electrodes and interconnects.

The number of materials that can be prepared by ALD has tremendously increased in the past decade. Notably, it can deposit a wider variety of materials, including important functional oxides like Al₂O₃, MoO₃, SiO₂ and HfO₂.

Scanning Tunneling Microscopy (STM) allows imaging conductive surfaces at the atomic scale. It is possible to characterize the distribution of surface terraces and steps, as well as to determine the atomic arrangement of (ordered) surface (over)structures.

In STM, an atomically sharp tip is scanned on a surface at a few-angstrom distance, while a bias voltage is applied between these two electrodes, so that a current flows due to the quantum tunneling effect. The intensity of the tunneling current depends exponentially on the tip-surface distance and can therefore be used to reconstruct a morphologic image.

STM is a local technique: while high-resolution can be achieved on small (nanometer sized) areas, information on large-scale (micron sized or more) is lost, and measurements have to be repeated systematically on several regions of the sample to get statistically relevant information.

Due to stability performances, STM experiments are typically time-consuming. The technique is applicable both in air and in vacuum. Ultra-high-vacuum (UHV) is required for the characterization of delicate, atomically clean systems and for performing measurement at cryogenic temperature.

The STM signal is not purely topographic, but brings also information on the local density of electronic states. Scanning tunneling spectroscopy (STS) is an extension of STM that provides information about the density of electrons in a sample as a function of their energy. Inelastic tunneling spectroscopy (IETS) is a challenging extension for the investigation of vibrational states at liquid helium temperature. The STM tip can also be used to manipulate single atoms and molecules.

By acquiring sequences of consecutive images, STM can also be used to investigate at the atomic scale dynamical processes occurring on the surface of conductive samples, with a typical acquisition time of few tens of seconds per image. To further extend the range of accessible details in this kind of measurements, NFFA-Europe makes for the first time available to external users the access to a FastSTM option for high-speed imaging with a VT-STM microscope at CNR-IOM.  Thanks to this option, it is now possible to image with atomic resolution dynamical processes as chemical reactions, diffusion and growth, with a frame rate up to 100 images per second on regions few-nanometer wide.

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.