216 research outputs found
Optical spin injection and spin lifetime in Ge heterostructures
We demonstrate optical orientation in Ge/SiGe quantum wells and study their
spin properties. The ultrafast electron transfer from the center of the
Brillouin zone to its edge allows us to achieve high spin-polarization
efficiencies and to resolve the spin dynamics of holes and electrons. The
circular polarization degree of the direct-gap photoluminescence exceeds the
theoretical bulk limit, yielding ~37% and ~85% for transitions with heavy and
light holes states, respectively. The spin lifetime of holes at the top of the
valence band is found to be ~0.5 ps and it is governed by transitions between
heavy and light hole states. Electrons at the bottom of the conduction band, on
the other hand, have a spin lifetime that exceeds 5 ns below 150 K. Theoretical
analysis of the electrons spin relaxation indicates that phonon-induced
intervalley scattering dictates the spin lifetime.Comment: 5 pages, 3 figure
Optical properties of highly n-doped germanium obtained by in situ doping and laser annealing
High n-type doping in germanium is essential for many electronic and optoelectronic applications especially for high performance Ohmic contacts, lasing and mid-infrared plasmonics. We report on the combination of in situ doping and excimer laser annealing to improve the activation of phosphorous in germanium. An activated n-doping concentration of 8.8  ×  1019 cm−3 has been achieved starting from an incorporated phosphorous concentration of 1.1  ×  1020 cm−3. Infrared reflectivity data fitted with a multi-layer Drude model indicate good uniformity over a 350 nm thick layer. Photoluminescence demonstrates clear bandgap narrowing and an increased ratio of direct to indirect bandgap emission confirming the high doping densities achieved
Probing the in-plane electron spin polarization in Ge/Si0.15 Ge0.85 multiple quantum wells
We investigate spin transport in a set of Ge/Si0.15Ge0.85 multiple quantum wells (MQWs) as a function of the well thickness. We exploit optical orientation to photogenerate spin-polarized electrons in the discrete energy levels of the well conduction band at the Γ point of the Brillouin zone. After diffusion, we detect the optically oriented spins by means of the inverse spin-Hall effect (ISHE) taking place in a thin Pt layer grown on top of the heterostructure. The employed spin injection/detection scheme is sensitive to in-plane spin-polarized electrons, therefore, by detecting the ISHE signal as a function of the photon energy, we evaluate the spin polarization generated by optical transitions driven by the component of the light wave vector in the plane of the wells. In this way, we also gain insight into the electron spin-diffusion length in the MQWs. The sensitivity of the technique to in-plane spin-related properties is a powerful tool for the investigation of the in-plane component of the spin polarization in MQWs, which is otherwise commonly inaccessible
Ultrafast valley relaxation dynamics in monolayer MoS2 probed by nonequilibrium optical techniques
We study the exciton valley relaxation dynamics in single-layer MoS2 by a combination of two nonequilibrium optical techniques: time-resolved Faraday rotation and time-resolved circular dichroism. The depolarization dynamics, measured at 77 K, exhibits a peculiar biexponential decay, characterized by two distinct time scales of 200 fs and 5 ps. The fast relaxation of the valley polarization is in good agreement with a model including the intervalley electron-hole Coulomb exchange as the dominating mechanism. The valley relaxation dynamics is further investigated as a function of temperature and photoinduced exciton density. We measure a strong exciton density dependence of the transient Faraday rotation signal. This indicates the key role of exciton-exciton interactions in MoS2 valley relaxation dynamics
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