• 097511 - SEMICONDUCTOR NANOSTRUCTURES

The course provides a conceptual framework for understanding the essential Physics of low-dimensional semiconductors where quantum confinement and strain effects are exploited for tailoring electronic and optical properties.

The physical properties of a variety of semiconducting materials, including compound semiconductors and heterostructures, will be analysed on the base of their electronic bandstructure outlining their potentiality in opto-electronics, photovoltaics and spintronics applications.

The effect of quantum confinement in 2-dimensional (quantum wells), 1-dimensional (quantum wires) 0-dimensional (quantum dots) heterostructures will be analyzed also in view of their application in intersubband photodetectors and optical modulators.

Strain effects on the bandstructure will be addressed with a focus on strained-Si technology and strain effects on lasing both in III-V and group IV semiconductors.

During the course students will:

• learn the basic principle of tight-binding and k-dot-p bandstructure calculations also by means of numerical models implemented in MATLAB;
• learn how to extract the main electronic and optical properties of Group IV, III-V and II-VI semiconductors from their bandstructure;
• understand the relevant effects of quantum confinement on the optical and electronic properties of semiconductors;
• understand the key elements of strain-engineering for bandstructure modification.

Semiconductor bandstructures

Bandstructure of group IV and compound (III-V, II-VI) semiconductors. Tight binding model of the bandstructure: implementation in MATLAB. The k-dot-p model: bandgap dependence of the effective mass. Symmetry of conduction (valence) band minima (maxima) and their effect on the selection rules for optical absorption: optical spin orientation . The effective mass approximation and the effective mass tensor. Density of states effective mass and conductivity effective mass. Cyclotron resonance measurements.

Semiconductors alloys

Semiconductor alloys: the virtual crystal approximation. Case studies using the tight binding model implemented in MATLAB. Use of semiconductors alloys in multi-junction solar cells.

Quantum confinement in semiconductor heterostructures

Band‑offset in heterostructures, type I , II and III band‑alignment. Experimental determination of band‑offsets by X-ray photoelectron spectroscopy. Theoretical calculation of the band offset using the Jaros model.

Quantum confinement effects in semiconductor heterostructures. The Schrödinger equation for the envelope function: energy levels and density of states in 2D (quantum wells), 1D (quantum wires) and 0D (quantum dots).

Strain engineering

Lattice mismatch in heterostructures. Elastic strain in cubic semiconductors. Deformation potentials for hydrostatic and uniaxial strain. Strained silicon technology and strain effects on lasing in III-V and group IV semiconductors.

The student will benefit from having a background in Solid State Physics (bandstructure of crystalline solids), Semiconductor Physics (Fermi level, electrons and holes, effective mass approximation) and Quantum Mechanics ( Schrödinger equation and selection rules for optical transitions).

Students will be evaluated by oral examination.

The student will be expected to discuss the physical phenomena, such as strain and quantum confinement effects, which give rise to certain optical/electronic properties. Emphasis is placed on the comprehension of fundamental physical phenomena, also in simplified systems, rather than the reproduction of text-book derivations.