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.

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.

CVD is a thermal process because a certain thermal energy is needed to decompose the gas precursors and reassemble them in the material to be synthetize. Generally,  in a CVD process, the substrate is maintained at high temperature while it is exposed to the volatile precursor/s, usually carried by an inert carrier gas (like Ar). High temperatures are usually required to decompose the precursors and to reassemble the products in the growing material.The use of a catalyst may allow to decrease the process temperature and to select the location and the nanostructure of the growning film. Together with the carrier and precursor gasses, other gasses, like etching gasses (like H₂, NH₃, …), may be used to avoid the formation of undesired species and to enhance the synthesis of the desired material. In high pressure CVD (atmospherics pressure) the use of an inert carrier gas helps to avoid gas-phase reactions (homogenous deposition) and favor the synthesis trough surface processes (heterogeneous deposition). When CVD is done at low pressure, the process is called Low Pressure CVD (LPCVD). In this case the reactions in the gas-phase of the precursors are reduced and the higher diffusivity of the precursor species favor to decorate cavities and led to get homogeneous coatings when a topography is present. The homogeneity is also spurred by the process temperature, which favors the mobility of the atomic/radical species along the surface. Process pressures are usually in the millitorr-torr range and process temperatures range is 400-800C (depending on the energy needed to decompose the precursors). To favor the precursor gas decomposition and to lower the CVD temperature, the CVD process may be assisted by a plasma, and in this case the technique is referred as plasma-enhanced CVD (PECVD). This usually allows lowering the temperature of the process to a few hundred centigrade degrees, which can be run on less temperature resistant substrates or without setting off temperature triggered unwanted processes. 

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