Scanning Tunneling Microscopy

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.

Ultrafast spectroscopy (or Pump-probe spectroscopy) is a technique used to study the dynamics of excited states in various materials, such as molecules, semiconductors, and solids.

It involves the use of two laser pulses. The first laser pulse, the "pump," excites the sample, elevating electrons from the ground state to an excited state. This initiation creates a non-equilibrium condition in the sample. After a controlled delay, a second laser pulse, the "probe," is sent through the sample. The probe pulse interrogates the sample to gather information about the transient states and the evolution of the excited states over time.

By varying the delay time between the pump and probe pulses, researchers can create a time-resolved picture of the dynamic processes occurring in the material. This time resolution can be on the order of femtoseconds (10^-15 seconds), allowing scientists to observe ultrafast phenomena.

These techniques provide insights into ultrafast processes, such as electronic transitions, high-order harmonics generation, molecular vibrations, and energy transfer mechanisms, that occur on very short time scales. It helps in understanding the fundamental properties of materials, such as the relaxation times of excited states, carrier dynamics in semiconductors, topological effects and the mechanisms of photochemical reactions.

Other possible studies can cover the world of solar cells, light-emitting devices, and photocatalysts, where understanding the dynamics of excited states is essential for improving efficiency and performance. Moreover, in biophysics and biochemistry, pump-probe spectroscopy can be used to investigate the dynamics of biomolecules, such as the folding and unfolding of proteins, photosynthesis mechanisms, and the behavior of molecular motors.

PES (Photo-electron Spectroscopy) is a general term that includes X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA),  and Ultraviolet Photoelectron Spectroscopy (UPS), that are the most used techniques for the investigation of the chemical properties of the material. In PES, a monochromatic photon beam impinges on the sample and extracts photoelectrons. These photoelectrons are then collected by an electrostatic lens system; their energy dispersion is then measured by an electron energy analyzer. The collected data form the so-called electron dispersion curve (EDC) or photoemission spectrum. From the analysis of the photoemission spectrum, it is possible to derive the chemical composition (stoichiometry) of the material under study and several information about the chemical state of each element, such as oxidation state, chemical bonds, etc. PES can be exploited as a surface chemical analysis technique: the valence band DOS (assessable by UPS) reveals the orbital contributions through cross-section analysis (photon energy dependence) and core level photoemission (assessable by XPS) is sensitive to the local environment of a given atomic species in the first atomic layers of the sample.

With increasing photon energy in the hard x-ray regime (HAXPES) it is possible to increase the probing depth, allowing access to information beyond the surface, such as buried interfaces. In this regime, characterizations of realistic electronic devices is possible, combined with in-situ measurements of electrical transport in the device. For electrons with kinetic energy around 5 keV, typical probing depth can range from 12 nm (e.g. Pt) to 30 nm (e.g. SiO2).

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.

SPELEEM microscope uses low-energy electron as information carriers. Unlike scanning microscopy, electrons are collected simultaneously from an illuminated area of several tens of micrometers. The image formed by the magnification lenses can be acquired even in video-rate. SPELEEM is capable of combining microscopy, spectroscopy and diffraction to obtain a comprehensive characterization of conductive and semiconductive samples from the meso- to the nanoscale. Thanks to its versatility, SPELEEM can be employed to study model systems in the fields of nanotechnology, material science, catalysis, energy storage, organic and inorganic thin films, and low-dimensional materials.

SPELEEM is equipped with an hemispherical energy analyzer and two complemetary illumination sources - an electron gun and a He I/II UV lamp. Using the UV lamp, the following operational modes are available.

• Photo-Emission Electron Microscopy (PEEM): the photoemitted electrons are collected by the optical elements to form a magnified image on a 2D electron detector. Electrons can be filtered in energy and acceptance angle while traveling through the optical path. The contrast mechanism is given by the physics behind the photoemission process and the chemical state of the probed matter.
• Micron-sized Angle-Resolved PhotoEmission Spectroscopy (µARPES): by displaying the backfocal plane of the objective lens on the 2D electron detector, the microscope give access to a high-quality reciprocal space map of the photoemitted electrons. The probed kinetic energy of the photoemitted electrons can be filtered and varied to collect a complete characterization of the occupied band structure of the specimen. The photoemitted electron beam can be segmented to a few-micron sized area, enabling the collection of ARPES maps from a controlled portion of the sample.

Using an electron gun, LEEM mode can be implemented in the same instrument.

SPELEEM experimental facility can accommodate conductive and semiconductive samples hosted by Elmitec and Flag-style sample holders. Heating stages are provided in the microscope chamber and in the preparation chamber. Other equipment available includes a gas line with precision leak valves, e-beam evaporator, preparation chamber with noble-gas sputtering, fast entry lock for sample loading.