Far-infrared optical properties of Ge/SiGe quantum well systems

Abstract

The microelectronic industry is nowadays fully dominated by silicon, mostly thanks to the outstanding thermal and mechanical properties exhibited by this material and by its native oxide, needed for the realization of the CMOS platform. On the other hand, the electronic structure of silicon makes it unexploitable in any application involving optics or opto-electronics. The main problem is the indirect nature of the silicon band gap which prevents radiative recombination between conduction and valence bands, resulting in the almost total absence of good silicon-based emitters and detectors. These devices, including lasers, are today realized mainly in compounds of III-V semiconductors and therefore a monolithic photonic integration with the mainstream electronic platform is still far from being achieved, since the employed materials are not compatible with silicon. Several techniques have been investigated in order to obtain III-V materials on silicon, like heteroepitaxial growth or wafer bonding, but the most desirable approach remains the realization of opto-electronic devices in silicon-compatible materials directly grown on silicon substrates. In this context, the recent breakthroughs in the epitaxial deposition techniques opened the possibility to realize a large number of artificial structures with attracting properties from many points of view. The state of the art in semiconductor deposition now allows the growth of periodic systems of few atomic layers of di erent composition in which the charges are con ned in low-dimensional structures called quantum wells (QWs). The quantum effects arising from their low-dimensional nature can be exploited in several manners, for example with the aim of obtaining radiative emission from silicon-compatible materials. In fact, in these periodic heterostructures, dipole-active transitions between the confined levels have clearly demonstrated their potential for infrared light emission. Each level in a QW is constituted by a two-dimensional electron energy level called subband, and therefore the transitions involving them are named inter-subband (ISB) transitions. One important property of ISB light emitters is that they are unipolar devices, i.e. they do not require the use of semiconductors with a direct bandgap because either electrons or holes are involved in the emission process, and the radiative emission does not depend on electron-hole recombination between conduction and valence bands. This property makes ISB transition design a viable route toward the realization of silicon-based monolithic light emitters, employing silicon-compatible group-IV semiconductor materials such as germanium (Ge) and tin (Sn), even if with indirect band gap. With the employment of these materials, light emitters realized on silicon (Si) wafers could be integrated in the CMOS technology platform and thus, although working at low temperatures and in the far-infrared range, they have the potential to open the field of silicon photonics beyond the telecom technology to several applications. After the first demonstration in 1994 of the quantum cascade laser (QCL), coherent ISB emitters based on electron injection-tunneling between coupled QWs are now an established technology, although QCL are still realized only in III-V semiconductor heterostructures. The polar nature of these compounds limits the emission efficiency specially at room temperature because of the strong interaction with optical phonons, which generates high-rate non-radiative electron relaxation. Moreover, the strong coupling with optical phonons in III-V emitters leads to a forbidden emission range in the far-infrared energy region named THz gap, in which any radiative emission is prevented. This means that nowadays compact and efficient emitters in the range of 0.3-10 THz (around 1-40 meV) do not exist yet, excluding the possibility to exploit THz radiation for a number of important applications. In fact, the THz region of the electromagnetic spectrum is of great interest in many fields both from a theoretical point of view and for technological applications. In this context, the present work of thesis focused on the realization and characteriza- tion of Ge/SiGe QW heterostructures featuring the needed properties for the implementation in THz emitter devices. After the optimization of the deposition process is conducted and the suitable strategy to obtain high quality structures is adopted, several Ge/SiGe QW samples with different characteristics have been grown via chemical-vapor deposition (CVD) and a thorough study is conducted on both structural and optical properties. Two different kinds of QW structures have been investigated: the first one is a multi quantum well (MQW) modulation-doped structure which consists in the periodic repetition of symmetric elements of single QWs separated by thick barriers to prevent any coupling between them. This structure has been used to the seminal study of the inter- subband (ISB) transition properties in Ge QWs and to highlight the main features useful in THz technology. Basing on the results obtained from the MQW samples, a second type of structure has been considered, and direct-doped asymmetric coupled quantum well (ACQW) samples have been realized. Here, exploiting the peculiarities of the ACQW structure, the possibility to realize a quantum fountain (QF) device has been investigated, mainly studying the ISB transition characteristics in terms of relaxation times and population inversion under optical pumping. The structural characterization conducted on the realized samples testi es the high quality that can be achieved with the state-of-the-art techniques employed in the deposition process. The result is the excellent crystalline quality of the samples with low lattice defects. Different investigation techniques also highlight the high precision the periodic structures are realized, with abrupt interfaces of roughness below the nanometer scale. The optical characterization discussed in the present work started with a steady-state investigation, which highlights the very good agreement between the experimental data and the theoretical predictions. The spectra analyses allowed to identify the main optical characteristics of the investigated structures, testifying the high quality of the grown samples. After the optical characterization in the steady-state conditions, investigation on the carrier dynamics has been performed with pump-probe spectroscopy measurements, in order to study the system dynamics under optical pumping. The obtained relaxation times of the excited states are found to be suitable for the realization of THz emitter devices based on population inversion, e.g. the QF. Although it was not possible to obtain population inversion in our samples due to thermal limitations, we developed a model to calculate the material gain G(!) and the model parameters have been fitted to reproduce the experimental data at different pumping powers. Their values have been extrapolated to higher powers in order to identify the possibility to achieve population inversion in our structures. The result of this procedure showed how a net positive material gain can be achieved in our systems for pumping powers above 500 kW/cm2. This value is only slightly higher than what reported for III-V systems, and can be easily lowered down with a proper cavity realization for high modal gain. Starting from the results obtained in the ACQW structures characterization, in fact, the realization of optical resonators for positive modal gain can be possibly obtained as a long term objective and it represents the natural continuation of the present work.