Deep and shallow electronic states associated to doping, contamination and intrinsic defects in ε-Ga2O3 epilayers

Deep and shallow electronic states in undoped and Si-doped ε-Ga2O3 epilayers grown by MOVPE on c-oriented Al2O3 were investigated by cathodoluminescence, optical absorption, photocurrent spectroscopy, transport measurements, and electron-paramagnetic-resonance. Nominally undoped films were highly resistive, with a room temperature resistivity varying in the range 107- 1013 Ωcm depending on the carrier gas used during growth. Films grown with He carrier were generally more resistive than those grown with H2 carrier and exhibited a Fermi level located at about 0.8 eV below the conduction band edge, which tends to shift deeper with temperature. This can tentatively be attributed to the combined action of deep donors (probably carbon impurities and oxygen vacancies) and deep acceptors (Ga vacancies and related complexes), which compensate residual shallow donors. There are strong experimental hints that nitrogen also behaves as deep acceptor. Room temperature resistivity as low as 0.42 Ωcm and electron concentrations around 1018 cm−3 were obtained by silicon doping. Si was confirmed to act as shallow donor with sufficiently high solubility. A variable range hopping conduction was observed in a wide temperature interval in the n-type layers, and compensation by native acceptors also plays a major role on conduction mechanisms. Previous evaluations of curvature and anisotropy of the conduction band are confirmed, which allows for the estimation of the electron effective mass. The present experimental data are discussed considering the theoretical predictions for point defect formation in the ε-polymorph as well as literature data on extrinsic and intrinsic defects in β-Ga2O3

Si and Sn doping of ε-Ga2O3 layers

Low resistivity n-type e-Ga2O3 epilayers were obtained for the first time either by adding silane to the gas phase during the metal organic vapour phase epitaxy deposition or by diffusing Sn in nominally undoped layers after the growth. The highest doping concentrations were few 1018 cm−3 and about 1017 cm−3 for Si and Sn doping, with corresponding resistivity below 1 and 10 Ω cm, respectively. Temperature dependent transport investigation in the range of 10-600 K shows a resistivity behavior consistent with the Mott law, suggesting that conduction through localized states dominates the electrical properties of Si- and Sn-doped samples. For both types of dopants, two different mechanisms of conduction through impurity band states seem to be present, each of them determining the transport behavior at the lower and higher temperatures of the measurement range.Low resistivity n-type e-Ga2O3 epilayers were obtained for the first time either by adding silane to the gas phase during the metal organic vapour phase epitaxy deposition or by diffusing Sn in nominally undoped layers after the growth. The highest doping concentrations were few 1018 cm−3 and about 1017 cm−3 for Si and Sn doping, with corresponding resistivity below 1 and 10 Ω cm, respectively. Temperature dependent transport investigation in the range of 10-600 K shows a resistivity behavior consistent with the Mott law, suggesting that conduction through localized states dominates the electrical properties of Si- and Sn-doped samples. For both types of dopants, two different mechanisms of conduction through impurity band states seem to be present, each of them determining the transport behavior at the lower and higher temperatures of the measurement range

Emotional modulation of episodic memory in school-age childern: emotional items and their associated contextual details.

International audienc

Growth and characterization of buried GaSb p-n junctions for photovoltaic applications

Structures composed of a p(++)(Zn) GaAs layer deposited by MOVPE on a n(Te)-doped GaSb substrate were fabricated, with the purpose of obtaining GaSb p-n homo-junctions, through the diffusion of Zn into the substrate, for photovoltaic applications. Different Zn doping levels and post-growth annealing parameters were investigated. The junctions were characterized from the electrical point of view by I-V measurements, while Zn diffusion profiles were studied by SIMS analysis. The effective achievement of a GaSb buried junction, whose profile is characterized by a limited Zn diffusion into the substrate, was evidenced. Efficiency measurements by a solar simulator on 5x5 mm(2) samples were also performed in order to investigate the photovoltaic properties of the structur

Task 3 Report - PCBs in the Environment Near the Oak Ridge Reservation - A Reconstruction of Historical Doses and Health Risks

This report presents the results of an in-depth assessment of historical releases of polychlorinated biphenyls (PCBs) from the Oak Ridge Reservation (ORR) and risks of adverse health effects in local populations. The study was conducted by ChemRisk, a service of McLaren/Hart, Inc., for the Tennessee Department of Health. The project team (1) investigated releases of PCBs from the government sites, (2) evaluated PCB levels in environmental media in the area, (3) described releases of PCBs from other sources in the area, and (4) evaluated potential human exposures and health impacts associated with the historical presence of these contaminants in the environment. Beginning in the 1940s, PCBs were used extensively on the ORR and throughout the U.S. as a fire retardant in electrical components. PCBs were also used as cutting fluids for lubrication and cooling during metal working operations. Using information specific to the ORR, the project team estimated health risks for five off-site populations: (1) farm families that raised beef, dairy cattle, and vegetables on the flood plain of East Fork Poplar Creek (EFPC); (2) individuals who may have purchased beef and milk from cattle raised in the EFPC flood plain; (3) commercial and recreational fish consumers; (4) individuals that may have consumed turtles; and (5) users of surface water for recreation. Noteworthy features of the study include a two-dimensional analysis of uncertainty and variability in the non-cancer risk estimates and an assessment of the uncertainty in PCB toxicology thresholds. Conservative estimates of cancer risks from the ORR releases of PCBs to consumers of fish from Watts Bar Reservoir and the Clinch River range from less than 1 in a 1,000,000 to 2 in 10,000. Three or less excess cases of cancer would be expected to occur among individuals who consumed fish from these local waters since the 1940's. Persons who consumed large amounts of fish from the Clinch R. and Watts Bar were also at risk from non -cancer effects of PCBs. However, for Watts Bar, these risks were mainly due to sources of PCBs other than the ORR; the releases from the ORR appear to have placed an additional one to two percent of the total number of fish consumers potentially at risk. This percentage corresponds to approximately 1,000-2,000 fish consumers over the last 50 years. This report is one in a set of eight technical reports on the Oak Ridge Offsite Dose Reconstruction

Combined impact of B2H6 flow and growth temperature on morphological, structural, optical, and electrical properties of MOCVD-grown B(In)GaAs heterostructures designed for optoelectronics

BGaAs/GaAs epilayers and BInGaAs/GaAs quantum well (QW) have been prepared using metal–organic chemical vapor deposition under different growth conditions, and their physical and structural properties have been examined. SEM-EDS investigation showed a dependence of surface properties of the ternary compound on the growth conditions. High-resolution X-ray diffraction evidenced a tensile strain for the ternary alloys whatever the growth condition, while the quaternary QW always shows a compressive strain state. Room temperature optical absorption allowed to follow the variation of the bandgap with boron incorporation. Photoluminescence measurements confirmed the carrier-localization phenomenon and its dependence with the growth conditions. Deposition temperature and diborane (B2H6) flow rate are with particularly significant effects on the optical properties: lower diborane flow rate and high growth temperature enhance the radiative emission. Computer simulation using localized state ensemble model quantitatively relates the lattice inhomogeneity to the optical properties and suggests a way to engineer the localization phenomenon and avoid clustering effects. Electrical investigations by current–voltage, capacitance and conductance methods have been performed for the first time on selected BGaAs samples. The ideality factor of Schottky barriers has been determined, while their height and film doping level could only be approximately estimated. Such physical properties make boron-based alloys very promising for applications in multijunction solar cells