Chiara defended a thesis entitled
Silicon-Germanium heterostructures for Terahertz emission
The Terahertz (THz) region of the electromagnetic spectrum (electromagnetic frequencies 0.3 – 10 THz, wavelengths 1 mm – 30 mm) is still poorly exploited with respect to its potential, due to the lack of compact sources, powerful enough to favor their commercial use. The photon energies involved are indeed too low for semiconductor lasers and too high for electronic devices. The Quantum Cascade Lasers (QCLs), exploiting optical transitions among the discrete levels arising in semiconductor quantum wells (QWs), have been invented exactly with the purpose of bridging the “THz gap”. However, they are presently made of III-V compound semiconductors, whose polar behavior hinders emission in the range 5-10 THz (Reststrahlen band) and prevents room-temperature operation.
Silicon (Si) and Germanium (Ge) may be employed to solve both the issues, given the less effective electron-phonon interaction of group IV materials, which produces almost temperature insensitive non-radiative lifetimes (competitors of radiative emission), thus allowing room-temperature operation. Moreover, emission is expected in the whole 1-10 THz range, since it does not exist any Reststrahlen band of forbidden light propagation for these materials. As a further advantage, a QCL made of Si and Ge would be compatible with the mainstream microelectronics based on Si, therefore allowing for mass-scale distribution. For these important motivations, the final goal of the European project “FLASH”, where this thesis work can be inserted, is the realization of a Ge/SiGe QCL working in the THz range.
Among the different designs, electron-doped Ge QWs sandwiched by Si1-xGex barriers with high Ge content (x>0.8) have been predicted to be the most promising heterostructures, although several growth challenges, mostly related to the high lattice mismatch between Si and Ge, must be addressed.
Thanks to the progresses made during the last decades in the field of epitaxial growth, high-quality thick stacks of alternating layers can now be realized. However, given the novelty of the Ge/SiGe system, it becomes crucial a full characterization of the material, starting from basic structures.
For this purpose, the thesis work focuses on the optical characterization of different Ge/SiGe QW structures for the investigation of fundamental material properties needed for the QCL realization.
Different designs of n-doped Ge/Si1-xGex (0.81 < x < 0.87) heterostructures have been grown by Ultra-High Vacuum CVD, which provides precision at the atomic scale, as confirmed by a first structural characterization performed on most of the investigated samples.
The optical characterization has been carried out by Fourier Transform Infrared Spectroscopy (FTIR), providing the absorption spectra and, hence, information on the energy of the quantized levels and the oscillator strengths. By comparing rectangular QWs and asymmetric structures, e.g. asymmetric-coupled QWs (ACQWs) (where two wells having different width are separated by a thin barrier), we could verify the breaking of the parity selection rule. Moreover, from the investigation of the ACQWs we could evaluate the condition for wavefunction tunnelling through thin barriers, crucial information for a cascade architecture. Theoretical predictions based on a self-consistent Schrodinger-Poisson’s solver accurately reproduced the experimental data, thus enabling high model reliability.
Rectangular QWs have been also investigated in Pump-Probe Spectroscopy experiments, using a Free Electron Laser as optical pump, to estimate the non-radiative lifetimes and to evaluate the major scattering channels affecting the radiative emission in this material system. We found that the electron-optical phonon interaction is much less effective in our Ge/SiGe structures than in bulk Ge and, therefore, long non-radiative lifetimes (10-30 ps) have been measured in a wide range of powers and up to a temperature 100 K, thus confirming the feasibility of the material for THz emission at high temperatures.
To further validate the potential of SiGe heterostructures as THz emitters, asymmetric-coupled QWs designed as three level systems, have been studied for photoluminesce (PL) emission. Although low PL efficiencies have been measured, this experiment represented an important milestone, being the first evidence of photon emission from n-Ge/SiGe structures.
As a parallel route to the QCL realization, parabolic potentials have been realized with the purpose of THz emission up to room-T. Such structures may prove advantageous in terms of high efficiencies, since a single and intense peak is expected at the bare harmonic oscillator potential, regardless electron distribution and electron density (Kohn’s theorem). Different graded parabolic QWs have, thus, been optically characterized by FTIR to verify the Kohn’s prediction for future applications.