Erbium doped materials are of great interest in thin film integrated optoelectronic technology, due to their Er3+ intra-4f emission at 1.54 μm, a standard telecommunication wavelength. Er-doped dielectric thin films can be used to fabricate planar optical amplifiers or lasers that can be integrated with other devices on the same chip. Semiconductors, such as silicon, can also be doped with erbium. In this case the Er may be excited through optically or electrically generated charge carriers. Er-doped Si light-emitting diodes may find applications in Si-based optoelectronic circuits. In this article, the synthesis, characterization, and application of several different Er-doped thin film photonic materials is described. It focuses on oxide glasses (pure SiO2, phosphosilicate, borosilicate, and soda-lime glasses), ceramic thin films (Al2O3, Y2O3, LiNbO3), and amorphous and crystalline silicon, all doped with Er by ion implantation. MeV ion implantation is a technique that is ideally suited to dope these materials with Er as the ion range corresponds to the typical micron dimensions of these optical materials. The role of implantation defects, the effect of annealing, concentration dependent effects, and optical activation are discussed and compared for the various materials.

Erbium implanted thin film photonic materials

Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.

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Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

Instabilities in semiconductor heterostructure growth can be exploited for the self-organized formation of nanostructures, allowing for carrier confinement in all three spatial dimensions. Beside the description of various growth modes, the experimental characterization of structural properties, such as size and shape, chemical composition, and strain distribution is presented. The authors discuss the calculation of strain fields, which play an important role in the formation of such nanostructures and also influence their structural and optoelectronic properties. Several specific materials systems are surveyed together with important applications.

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Structural properties of self-organized semiconductor nanostructures

Hot electrons in semiconductors lose their energy very quickly (within picoseconds) to lattice vibrations. Slowing this energy loss could prove useful for more efficient photovoltaic or infrared devices. With their well-separated electronic states, quantum dots should display slow relaxation, but other mechanisms have made it difficult to observe. We report slow intraband relaxation (>1 nanosecond) in colloidal quantum dots. The small cadmium selenide (CdSe) dots, with an intraband energy separation of approximately 0.25 electron volts, are capped by an epitaxial zinc selenide (ZnSe) shell. The shell is terminated by a CdSe passivating layer to remove electron traps and is covered by ligands of low infrared absorbance (alkane thiols) at the intraband energy. We found that relaxation is markedly slowed with increasing ZnSe shell thickness.

https://science.sciencemag.org/content/sci/322/5903/929.full.pdf

Slow electron cooling in colloidal quantum dots.

Good thermoelectric materials possess low thermal conductivity while maximizing electric carrier transport. This article looks at various classes of materials to understand their behavior and determine methods to modify or “tune” them to optimize their thermoelectric properties. Whether it is the use of “rattlers” in cage structures such as skutterudites, or mixed-lattice atoms such as the complex half-Heusler alloys, the ability to manipulate the thermal conductivity of a material is essential in optimizing its properties for thermoelectric applications.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769400009921

Recent developments in bulk thermoelectric materials

We have observed third-harmonic generation associated with intraband transitions in semiconductor quantum dots. The frequency tripling $(12\mathrm{}\mathrm{\ensuremath{\mu}}\stackrel{\ensuremath{\rightarrow}}{m}4\mathrm{}\mathrm{\ensuremath{\mu}}\mathrm{m})$ occurs in the valence band of InAs/GaAs self-assembled quantum dots. We show that the third-harmonic generation is enhanced due to the achievement of the double resonance condition between intraband transitions. A third-order nonlinear susceptibility $|{\ensuremath{\chi}}_{3\ensuremath{\omega}}^{(3)}|$ as large as $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}(\mathrm{m}/\mathrm{V}{)}^{2}$ is measured for one dot plane.

Third-harmonic generation in inas/gaas self-assembled quantum dots

We have investigated the intraband absorption within the conduction band of InAs/GaAs quantum dots. The islands obtained by self-organized epitaxy are modulation doped with a silicon planar doping 2 nm below the dot layer plane. The dots exhibit infrared absorption polarized along the growth axis in the midinfrared spectral range. The absorption is maximum around 150 meV with a large broadening around 130 meV. This broadening is attributed to size fluctuations within the one dot layer plane and the consequent variation of the electron confinement energy with the dot size. The magnitude of the absorption along the growth axis for the one dot layer plane is ≈2.5×10−2% which corresponds to an equivalent absorption cross section σz≈3.1×10−15 cm2. We show that the intraband absorption can also be clearly observed using a photoinduced infrared absorption technique with the doped quantum dots.

Intraband absorption in n-doped InAs/GaAs quantum dots

In situ reflection high-energy electron diffraction along with transmission electron miscoscopy, atomic force microscopy, and photoluminescence spectroscopy have been used to investigate the Ge growth behavior in a standard Ge/Si multilayer structure. It is shown that the decrease of the Ge wetting layer thicknesses in the upper layers of a multilayer structure is the main parameter which leads to the increase of the island size and height. Such an evolution of the Ge wetting layer thickness can be explained by an accumulation of elastic strain in the Si spacer layers induced by the lower Ge wetting layers. This finding opens the route to the realization of a multilayer structure in which the islands have equal size in all layers.

Vertically self-organized Ge/Si(001) quantum dots in multilayer structures

A unipolar semiconductor laser emitting in the mid-infrared spectral region is demonstrated. The laser scheme relies on a simple three-level system in GaAs/AlGaAs asymmetric coupled quantum wells. Population inversion between excited states is achieved by optical pumping of electrons from the ground state with a CO2 laser. Long-wavelength (≈15.5 μm) laser emission is demonstrated. The laser is operated in the pulsed regime up to a temperature of 110 K and with an output peak power ≈0.4 W at 77 K. Unipolar quantum well semiconductor lasers based on this principle are capable of covering the long wavelength mid-infrared spectral region above 12 μm.

Long-wavelength (≈15.5 μm) unipolar semiconductor laser in GaAs quantum wells

We have investigated the polaron dynamics in n-doped InAs/GaAs self-assembled quantum dots by pump-probe midinfrared spectroscopy. A long T1 polaron decay time is measured at both low temperature and room temperature, with values around 70 and 37 ps, respectively. The decay time decreases for energies closer to the optical phonon energy. The relaxation is explained by the strong coupling for the electron-phonon interaction and by the finite lifetime of the optical phonons. We show that, even for a large detuning of 19 meV from the LO photon energy in GaAs, the carrier relaxation remains phonon assisted.

P. Boucaud

Papers

Third-harmonic generation in inas/gaas self-assembled quantum dots

Intraband absorption in n-doped InAs/GaAs quantum dots

Vertically self-organized Ge/Si(001) quantum dots in multilayer structures

Long-wavelength (≈15.5 μm) unipolar semiconductor laser in GaAs quantum wells

Long Polaron Lifetime in InAs/GaAs Self-Assembled Quantum Dots