11 research outputs found
Low temperature epitaxial growth of GaP on Si by atomic-layer deposition with plasma activation
International audienc
Etude des propriétés photoélectriques de couches de GaP deposées à basse température par des mesures de conductivité et de photocourant modulé
National audienc
Ytterbium and Europium Complexes of Redox-Active Ligands: Searching for Redox Isomerism
The reaction of (dpp-Bian)Eu<sup>II</sup>(dme)<sub>2</sub> (<b>3</b>) (dpp-Bian is dianion of
1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene; dme is 1,2-dimethoxyethane)
with 2,2′-bipyridine (bipy) in toluene proceeds with replacement
of the coordinated solvent molecules with neutral bipy ligands and
affords europium(II) complex (dpp-Bian)Eu<sup>II</sup>(bipy)<sub>2</sub> (<b>9</b>). In contrast the reaction of related ytterbium
complex (dpp-Bian)Yb<sup>II</sup>(dme)<sub>2</sub> (<b>4</b>) with bipy in dme proceeds with the electron transfer from the metal
to bipy and results in (dpp-Bian)Yb<sup>III</sup>(bipy)(bipy<sup>–</sup>̇) (<b>10</b>) – ytterbium(III) derivative containing
both neutral and radical-anionic bipy ligands. Noteworthy, in both
cases dianionic dpp-Bian ligands retain its reduction state. The ligand-centered
redox-process occurs when complex <b>3</b> reacts with <i>N</i>,<i>N</i>′-bis[2,4,6-trimethylphenyl]-1,4-diaza-1,3-butadiene
(mes-dad). The reaction product (dpp-Bian)Eu<sup>II</sup>(mes-dad)(dme)
(<b>11</b>) consists of two different redox-active ligands both
in the radical-anionic state. The reduction of 3,6-di-<i>tert</i>-butyl-4-(3,6-di-<i>tert</i>-butyl-2-ethoxyphenoxy)-2-ethoxycyclohexa-2,5-dienone
(the dimer of 2-ethoxy-3,6-di-<i>tert</i>-butylphenoxy radical)
with (dpp-Bian)Eu<sup>II</sup>(dme)<sub>2</sub> (<b>3</b>) caused oxidation of the dpp-Bian ligand to radical-anion to afford
(dpp-Bian)(ArO)Eu<sup>II</sup>(dme) (ArO = OC<sub>6</sub>H<sub>2</sub>-3,6-<i>t</i>Bu<sub>2</sub>-2-OEt) (<b>12</b>). The molecular structures of complexes <b>9</b>–<b>12</b> have been established by the single crystal X-ray analysis.
The magnetic behavior of newly prepared compounds has been investigated
by the SQUID technique in the range 2–310 K. The isotropic
exchange model has been adopted to describe quantitatively the magnetic
properties of the exchange-coupled europium(II) complexes (<b>11</b> and <b>12</b>). The best-fit isotropic exchange parameters
are in good agreement with their density functional theory-computed
counterparts
Defect properties of InGaAsN layers grown as sub-monolayer digital alloys by molecular beam epitaxy
International audienceThe defect properties of InGaAsN dilute nitrides grown as sub-monolayer digital alloys (SDAs) by molecular beam epitaxy for photovoltaic application were studied by space charge capacitance spectroscopy. Alloys of i-InGaAsN (Eg = 1.03 eV) were lattice-matched grown on GaAs wafers as a superlattice of InAs/GaAsN with one monolayer of InAs (<0.5 nm) between wide GaAsN (7–12 nm) layers as active layers in single-junction solar cells. Low p-type background doping was demonstrated at room temperature in samples with InGaAsN layers 900 nm and 1200 nm thick (less 1 × 1015 cm−3). According to admittance spectroscopy and deep-level transient spectroscopy measurements, the SDA approach leads to defect-free growth up to a thickness of 900 nm. An increase in thickness to 1200 nm leads to the formation of non-radiative recombination centers with an activation energy of 0.5 eV (NT = 8.4 × 1014 cm−3) and a shallow defect level at 0.20 eV. The last one leads to the appearance of additional doping, but its concentration is low (NT = 5 × 1014 cm−3) so it does not affect the photoelectric properties. However, further increase in thickness to 1600 nm, leads to significant growth of its concentration to (3–5) × 1015 cm−3, while the concentration of deep levels becomes 1.3 × 1015 cm−3. Therefore, additional free charge carriers appearing due to ionization of the shallow level change the band diagram from p-i-n to p-n junction at room temperature. It leads to a drop of the external quantum efficiency due to the effect of pulling electric field decrease in the p-n junction and an increased number of non-radiative recombination centers that negatively impact lifetimes in InGaAsN