Search

Single-crystalline Zn3N2 thin films have been grown on MgO (100) and YSZ (100) substrates by plasma-assisted molecular beam epitaxy. Depending on growth conditions, the film orientation can be tuned from (100) to (111). For each orientation, x-ray diffraction and reflection high-energy electron diffraction are used to determine the epitaxial relationships and to quantify the structural quality. Using high-temperature x-ray diffraction, the Zn3N2 linear thermal expansion coefficient is measured with an average of (1.5 ± 0.1) × 10−5 K−1 in the range of 300–700 K. The Zn3N2 films are found to be systematically n-type and degenerate, with carrier concentrations of 1019–1021 cm−3 and electron mobilities ranging from 4 to 388 cm2 V−1 s−1. Low-temperature Hall effect measurements show that ionized impurity scattering is the main mechanism limiting the mobility. The large carrier densities lead to measured optical bandgaps in the range of 1.05–1.37 eV due to Moss–Burstein band filling, with an extrapolated value of 0.99 eV for actual bandgap energy. [ABSTRACT FROM AUTHOR]

This paper presents the realisation of anemometers based on periodic structures elaborated by flash evaporation technique on polyimide substrate using Bi2Te3–Sb2Te3(P) and Bi2Te3–Bi2Se3(N) materials which have a high figure of merit. These anemometers are based on both Seebeck and Joule effects. We elaborate a particular geometry in order to improve the sensitivity of periodic structure sensors. The measurement method involves first the use of the Joule effect to generate the temperature difference between numerous thermoelectric junctions along the sensor, and second the use of the Seebeck effect to detect these thermal gradients. On these periodic structures we studied the sensitivity of various geometries and we obtained maximum sensitivity on the periodic structure SP1 which have an arm length of 1.2 mm. We show the possibility of measuring gas flow velocity with a sensor of low power consumption and slightly sensitive to room temperature. These results are very promising to develop original microstructures based on Seebeck and Joule effect. [Copyright &y& Elsevier]

A unique bulk-origin perpendicular magnetic anisotropy (PMA) is reported in thin films of the cubic ferromagnetic semiconductor GdxSm1−xN. PMA behavior is clearly signaled by in-vs-out-of-plane magnetic hysteresis showing out-of-plane remanence of ∼ 80 % of the saturation magnetization in films with up to 30% Gd. Anomalous Hall effect data show further that the conduction-band spin imbalance shows complete 100% remanence to retain half-metallic conduction even as the out-of-plane applied field is reduced to zero, in stark contrast to the 80% magnetization remanence. The unusual occurrence of PMA in this cubic material is discussed as stabilized by a mix of two in-plane crystal orientations in the epitaxial (111)-oriented films. [ABSTRACT FROM AUTHOR]

We report the homoepitaxial growth by molecular beam epitaxy of high quality nonpolar m-plane ZnO and ZnO:N films over a large temperature range. The nonintentionally doped ZnO layers exhibit a residual doping as low as ∼1014 cm-3. Despite an effective incorporation of nitrogen, p-type doping was not achieved, ZnO:N films becoming insulating. The high purity of the layers and their low residual n-type doping evidence compensation mechanisms in ZnO:N films. [ABSTRACT FROM AUTHOR]

This paper discusses the results of high temperature resistivity and Hall effect studies of Mg-doped GaN and AlxGa1−xN epilayers (0.05 < x < 0.23). The studied samples were grown by molecular beam epitaxy on low temperature buffers of GaN and AlN deposited on a sapphire substrate. The experiments were carried out at temperatures ranging from 300 up to 1000 K. Up to a certain critical temperature TC (around 800 K), a typical increase of the conduction processes due to the excitation of impurity states has been observed with an activation energy of about EA = 200 meV. However, at this critical temperature TC, an annealing effect was observed in all the investigated samples. At this critical temperature, the increase in the free carrier concentration as a function of time leads to an irreversible decrease in the samples' resistivity of more than 60%. The observed temperature dependences of the electrical transport properties are analyzed in the frame of an impurity model including shallow donors and Mg-related acceptors (with EA ∼ 200 meV). In some cases, an additional conduction channel not related to free carriers in the valence band must be taken into account. This can lead to an incorrect determination of hole concentration in the valence band, an important parameter in the process of radiative recombination. [ABSTRACT FROM AUTHOR]

AlyGa1−yN quantum dots (QDs) have been grown by molecular beam epitaxy on AlxGa1−xN (0001) using a 2-dimensional–3-dimensional growth mode transition that leads to the formation of QDs. QDs have been grown for Al compositions y varying between 10% and 40%. The influence of the active region design [composition y, QD height, and bandgap difference (ΔEg) between the AlxGa1−xN cladding layer and the AlyGa1−yN QDs] is discussed based on microscopy, continuous wave photoluminescence (PL), and time-resolved PL (TRPL) measurements. In particular, increasing y leads to a shift of the QD emission toward shorter wavelengths, allowing covering a spectral range in the UV from 332 nm (UVA) to 276 nm (UVC) at room temperature (RT). The low-temperature (LT) internal quantum efficiency of the QD ensembles was estimated from TRPL experiments at 8 K and values between 11% and 66% were deduced. The highest internal quantum efficiency (IQE)-LT is found for the QDs with higher Al content y. Then, the PL spectrally integrated intensity ratios between RT and LT were measured to estimate the IQE of the samples at RT. The PL ratio is higher for larger ΔEg, for QDs with y of 0.1 or 0.2, and high PL intensity ratios up to 30% were also measured for QDs with larger y of 0.3 and 0.4. RT IQE values between 5% and 20% are deduced for AlyGa1−yN QDs emitting in the 276–308 nm range. [ABSTRACT FROM AUTHOR]

The techniques of micromachining are used for the manufacturing of a thermal inclinometer, which requires no solid proof mass and has a low-cost production. The sensor principle is as follows: a heating resistor creates a symmetrical temperature profile and two thermocouples, placed symmetrically on both sides of the heater, measure a differential temperature. When an acceleration is applied on the sensitive axis x of the sensor, the convection heat transfer and the temperature profile become asymmetric and the differential temperature was shown to be proportional to the acceleration and to the Grashof number. Platinum resistor deposited by electron beam evaporation and flash evaporated Bi0.5Sb1.5Te3(p)/Bi2Se0.3Te2.7(n) thermocouples are used, respectively, as heater and temperature sensors on a polyimide substrate. These ternary materials present higher figures of merit than 10−3 K−1 and the thermocouple sensitivity is 440 μV K−1. Two types of sensors have been manufactured: the first one is deposited on a continuous polyimide film and the second one on a micromachined film, which permits to limit the energy consumption by a factor of 2 and to obtain a heater temperature rise of 3.44 K mW−1 and a sensitivity of 1410 μV g−1 (1g=9.81 m s−2) for a heating power of 100 mW. [Copyright &y& Elsevier]

We have studied Be-acceptors in a p-type GaInNAs epilayer using magneto-luminescence spectroscopy. The band edge photoluminescence (PL) spectra at T=7K contain two features: the first is associated with the free exciton while the second with the conduction band to acceptor (CB → A) transition. The intensity of the latter decreases with increasing temperature while the excitonic feature survives up to T=250 K. From the energies of the two PL features, as well as the exciton binding energy in GaInNAs, we determined the Be-acceptor binding energy to be equal to 42 meV. The energy of the CB → A feature varies linearly with magnetic field B and has a slope of 5:5 × 10-4 eV/T. [ABSTRACT FROM AUTHOR]

The electroluminescence (EL) characteristics of a set of InAs/GaInNAs quantum dot (QD) light-emitting diodes with varying In and N contents are analyzed. Room-temperature EL around 1.5 μm is obtained with 15% In and 2% N in the QD capping layer. It is shown that the addition of N results in a degradation of the external efficiency, ηext, probably due to an increase in the nonradiative recombination in the QD heterostructure and an increase in the carrier escape from the QD to the capping layer, which yield a degradation of the current injection efficiency into the QD. Nevertheless, ηext can be partially recovered if a postgrowth rapid thermal annealing is performed, although this also results in a blueshift in the EL peak wavelength. The different contributions to the EL spectra are also analyzed and identified by looking at their dependence on injected current and temperature. It is found that N-containing devices show two radiative transitions. The lowest energy transition has been ascribed to the QD ground state recombination, whereas the higher energy transition has been attributed to recombination of carriers confined in the capping layer. Moreover, the carrier loss mechanisms responsible for the quenching of the EL in the dilute nitride-based devices are studied. It is found that the EL thermal quenching has an activation energy which can be ascribed to carrier escape from the QD to the capping layer. [ABSTRACT FROM AUTHOR]

The effect of UV-activated hydrogenation on the performance of GaInNAs solar cells is presented. A proof-of-principle investigation was performed on non-optimum GaInNAs cells, which allowed a clearer investigation of the role of passivation on the intrinsic nitrogen-related defects in these materials. Upon optimized hydrogenation of GaInNAs, a significant reduction in the presence of defect and impurity based luminescence is observed as compared to that of unpassivated reference material. This improvement in the optical properties is directly transferred to an improved performance in solar cell operation, with a more than two-fold improvement in the external quantum efficiency and short circuit current density upon hydrogenation. Temperature dependent photovoltaic measurements indicate a strong contribution of carrier localization and detrapping processes, with non-radiative processes dominating in the reference materials, and evidence for additional strong radiative losses in the hydrogenated solar cells. [ABSTRACT FROM AUTHOR]

We report the growth by molecular beam epitaxy of highly resistive GdN, using intentional doping with magnesium. Mg-doped GdN layers with resistivities of 10³ Ω cm and carrier concentrations of 1016cm-3 are obtained for films with Mg concentrations up to 5 x 1019 atoms/cm³. X-ray diffraction rocking curves indicate that Mg-doped GdN films have crystalline quality very similar to undoped GdN films, showing that the Mg doping did not affect the structural properties of the films. A decrease of the Curie temperature with decreasing the electron density is observed, supporting a recently suggested magnetic polaron scenario [F. Natali, B. J. Ruck, H. J. Trodahl, D. L. Binh, S. Vezian, B. Damilano, Y. Cordier, F. Semond, and C. Meyer, Phys. Rev. B 87, 035202 (2013)]. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL), optical pumping, and reflectance studies of nominally undoped and p-type GaInNAs epilayers are presented. The PL peak energy of the nominally undoped sample exhibits an S-shaped dependence on temperature for T < 50 K. This is attributed to recombination of bound excitons localized on traps. The energy of the PL circular-polarization maximum coincides with the energy of the free-exciton related reflectance feature at all temperatures. In heavily p-type samples the S-shaped temperature-dependence of the PL energy disappears, and the PL peak and circular polarization maximum coincide with the reflectance feature at all temperatures, indicating that the PL is free-exciton-like. [ABSTRACT FROM AUTHOR]