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During the last four decades, a remarkable research effort has been made to understand the physical properties of Si:Er material, as it is considered to be a promising approach towards improving the optical properties of crystalline Si. In this paper, we present a summary of the most important results of that research. In the second part, we give a more detailed description of the properties of Si/Si:Er multinanolayer structures, which in many aspects represent the most advanced form of Er-doped crystalline Si with prospects for applications in Si photonics.
A possibility to realize optical transitions within the I-4(15/2) ground state of Er3+ ion in Si, between levels split by crystal-field, has been investigated by pump-probe technique. The study has been conducted in the THz range on a sublimation MBE-grown Si/Si:Er multinanolayer structure, which allows to take advantage of the preferential formation of a single type of Er-related centers. We present preliminary results, which show absorption band around the wavelength of 43 mu m. A resonant transition at this wavelength is predicted for this material from high-resolution photoluminescence measurements. The experimentally observed absorption decay time is most likely due to non-radiative recombination by phonon emission.
We report on picosecond, time-resolved measurements of the vibrational relaxation and decay pathways of the Si–H and Ge–H stretching modes in hydrogenated amorphous silicon-germanium thin films (a-SiGe:H). It is demonstrated that the decay of both modes has a nonexponential shape, attributable to the local environment of the Si–H and Ge–H bonds. Temperature dependent measurements of the ensemble averaged population decay time T1 are used to demonstrate that the stretch modes relax to Si(Ge)-H bending modes and that the excess energy is dissipated into a combination of bulk vibrations. The influence of the mixed character Si-Ge bulk vibrations upon the relaxation dynamics is discussed.
We present experimental evidence on the donor level related to optical properties of the Er3+ ion in crystalline silicon. Using two-color spectroscopy with a free-electron laser we provide a direct link between the identified level in the bandgap and the optical properties of EP,. The investigation is performed in sublimation MBE-grown Si/Si:Er multinanolayer structure, which allows us to take advantage of the preferential formation of a single Er-related center. Quenching of the Er-related 1.5 mu m photoluminescence, due to ionization of the donor state with energy E-D approximate to 225 meV, is demonstrated. A microscopic model of the PL quenching mechanism as Auger type energy transfer between excited Er3+ ions and free carriers optically ionized from the Er-related donor states is put forward.
Using the Dutch free electron laser FELIX, we have investigated vibrational relaxation in free standing porous silicon (p-Si) films. Pump–probe measurements resonant with the SiH, SiH2 and O3SiH stretching modes yield temperature dependent measurements of the decay rates which demonstrate that all the modes decay via at least one internal defect mode with the excess vibrational energy distributed among the Si–Si bath phonons in a fourth order decay process.
Influence of oxygen on optical activity and thermal stability of the 1.5 µm emission of Er3+ ions in Si is well documented. Here we support this predominantly phenomenological notion with microscopic evidence. Using two-color and pump-probe spectroscopy with a free-electron laser we establish a direct link between the 1.5 µm emission from Er3+ ions and the 9 µm vibrational band of interstitial oxygen in Si. We show that: i) intensity of the 1.5 µm emission quenches when the 9 µm oxygen vibration is resonantly excited, and ii) the lifetime of the 9 µm vibrational band is affected by Er presence. Both findings evidence microscopic relation of interstitial oxygen and Er and their close proximity in Si matrix.
The gain recovery time of a bound-to-continuum terahertz quantum cascade laser is measured using a stimulated emission photocurrent technique. At low temperatures a value similar to 50 ps is found. This tends to decrease with rising temperature, and suggests that class B laser dynamics should be expected from these devices. Both the value and the trend in the gain recovery time are found to be consistent with hopping transport through the miniband. Power-dependent measurements of the photocurrent clearly show the effects of inhomogeneous broadening of the gain transition
We report the direct determination of nonradiative lifetimes in Si∕SiGe asymmetric quantum well structures designed to access spatially indirect (diagonal) interwell transitions between heavy-hole ground states, at photon energies below the optical phonon energy. We show both experimentally and theoretically, using a six-band k∙p model and a time-domain rate equation scheme, that, for the interface quality currently achievable experimentally (with an average step height ⩾1 Å), interface roughness will dominate all other scattering processes up to about 200 K. By comparing our results obtained for two different structures we deduce that in this regime both barrier and well widths play an important role in the determination of the carrier lifetime. Comparison with recently published experimental and theoretical data obtained for mid-infr...
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