Potential of La-Doped SrTiO3 Thin Films Grown by Metal-Organic Vapor Phase Epitaxy for Thermoelectric Applications.

La-doped SrTiO3 thin films with high structural quality were homoepitaxially grown by the metal-organic vapor phase epitaxy (MOVPE) technique. Thermogravimetric characterization of the metal-organic precursors determines suitable flash evaporator temperatures for transferring the liquid source materials in the gas phase of the reactor chamber. An adjustment of the charge carrier concentration in the films, which is necessary for optimizing the thermoelectric power factor, was performed by introducing a defined amount of the metal-organic compound La(tmhd)3 and tetraglyme to the liquid precursor solution. X-ray diffraction and atomic force microscopy verified the occurrence of the pure perovskite phase exhibiting a high structural quality for all La concentrations. The electrical conductivity of the films obtained from Hall-effect measurements increases linearly with the La concentration in the gas phase, which is attributed to the incorporation of La3+ ions on the Sr2+ perovskite sites by substitution inferred from photoemission spectroscopy. The resulting structural defects were discussed concerning the formation of occasional Ruddlesden-Popper-like defects. The thermoelectric properties determined by Seebeck measurements demonstrate the high potential of SrTiO3 thin films grown by MOVPE for thermoelectric applications.

Widely tunable, efficient 2 µm laser in monocrystalline Tm3+:SrF2.

We report on the growth, spectroscopy, and laser operation of monocrystalline Tm3+:SrF2. Spectroscopic investigations confirmed the presence of broad absorption and emission bands caused by inequivalent doped sites, introduced by charge compensation effects which also caused the clusterization of doping ions in the lattice. We obtained continuous-wave laser emission at about 2 µm, with efficiencies comparable with other Tm-doped crystals. We also achieved an uninterrupted tuning range of 180 nm between 1.8 and 2 µm. This characterization indicates that SrF2 enhances the cooperative mechanisms between Tm ions, helping to obtain remarkable laser performances at low doping concentrations.

Electronic materials with a wide band gap: recent developments.

The development of semiconductor electronics is reviewed briefly, beginning with the development of germanium devices (band gap E g = 0.66 eV) after World War II. A tendency towards alternative materials with wider band gaps quickly became apparent, starting with silicon (E g = 1.12 eV). This improved the signal-to-noise ratio for classical electronic applications. Both semiconductors have a tetrahedral coordination, and by isoelectronic alternative replacement of Ge or Si with carbon or various anions and cations, other semiconductors with wider E g were obtained. These are transparent to visible light and belong to the group of wide band gap semiconductors. Nowadays, some nitrides, especially GaN and AlN, are the most important materials for optical emission in the ultraviolet and blue regions. Oxide crystals, such as ZnO and ß-Ga2O3, offer similarly good electronic properties but still suffer from significant difficulties in obtaining stable and technologically adequate p-type conductivity.

Low-cost, broadly tunable (375-433 nm & 746-887 nm) Cr:LiCAF laser pumped by one single-spatial-mode diode.

We describe a low-cost, compact, and efficient Cr(3+):LiCaAlF(6) (Cr:LiCAF) laser that is broadly tunable in the near infrared (746-887 nm) and blue/ultraviolet (375-433 nm) regions of the optical spectrum. The pump source is a 660 nm single-mode-diode with 145 mW of output power. A melt-grown Cr:LiCAF crystal with an extremely high figure of merit above 2000 was used as the gain medium. This enabled the construction of a high-Q-cavity with continuous wave (cw) lasing thresholds as low as 3 mW, slope efficiencies as high as 54%, and output powers up to 63 mW. The stored intracavity power levels were above 30 W. By placing a beta-barium-borate crystal at a second intracavity focus, we could obtain tunable cw blue/ultraviolet radiation with output powers up to 3.5 mW around 400 nm. When mode-locked using a saturable Bragg reflector around 800 nm, the Cr:LiCAF laser produced 95 fs pulses with average powers of 33 mW and peak powers of 3.58 kW at a repetition rate of 85.5 MHz. Extracavity second harmonic generation enables generation of femtosecond pulses around 400 nm. These results demonstrate the possibility to grow Cr:LiCAF crystals with passive losses below 0.15% per cm, which enables construction of low-cost and complexity systems that are pumped only with one low-power single-mode-diode.