Donor Formation Research Articles

Formation of donors in germanium–silicon alloys implanted with hydrogen ions with different energies

The distributions of hydrogen-containing donors in Ge1–xSix (0 ≤ x ≤ 0.06) alloys implanted with hydrogen ions with an energy of 200 and 300 keV and a dose of 1 × 1015 cm–2 are studied. It is established that, at the higher ion energy, the limiting donor concentration after postimplantation heat treatment (275°C) is attained within ~30 min and, at the lower energy, within ~320 min. In contrast to donors formed near the surface, a portion of hydrogen-containing donors formed upon the implantation of ions with the higher energy possess the property of bistability. The limiting donor concentration is independent of the ion energy, but decreases from 1.3 × 1016 to 1.5 × 1015 cm–3, as the Si impurity content in the alloy is increased from x = 0.008 to x = 0.062. It is inferred that the observed differences arise from the participation of the surface in the donor formation process, since the surface significantly influences defect-formation processes involving radiation-induced defects, whose generation accompanies implantation.

Electrically active light-element complexes in silicon crystals grown by cast method

Electrically active light-element complexes called thermal donors and shallow thermal donors in silicon crystals grown by the cast method were studied by low-temperature far-infrared absorption spectroscopy. The relationship between these complexes and either crystal defects or light-element impurities was investigated by comparing different types of silicon crystals, that is, conventional cast-grown multicrystalline Si, seed-cast monolike-Si, and Czochralski-grown Si. The dependence of thermal and the shallow thermal donors on the light-element impurity concentration and their annealing behaviors were examined to compare the crystals. It was found that crystal defects such as dislocations and grain boundaries did not affect the formation of thermal or shallow thermal donors. The formation of these complexes was dominantly affected by the concentration of light-element impurities, O and C, independent of the existence of crystal defects.

Unraveling the Fundamental Mechanisms of Solvent-Additive-Induced Optimization of Power Conversion Efficiencies in Organic Photovoltaic Devices.

The realization of controllable morphologies of bulk heterojunctions (BHJ) in organic photovoltaics (OPVs) is one of the key factors enabling high-efficiency devices. We provide new insights into the fundamental mechanisms essential for the optimization of power conversion efficiencies (PCEs) with additive processing to PBDTTT-CF:PC71BM system. We have studied the underlying mechanisms by monitoring the 3D nanostructural modifications in BHJs and correlated the modifications with the optical analysis and theoretical modeling of charge transport. Our results demonstrate profound effects of diiodooctane (DIO) on morphology and charge transport in the active layers. For small amounts of DIO (<3 vol %), DIO promotes the formation of a well-mixed donor-acceptor compact film and augments charge transfer and PCE. In contrast, for large amounts of DIO (>3 vol %), DIO facilitates a loosely packed mixed morphology with large clusters of PC71BM, leading to deterioration in PCE. Theoretical modeling of charge transport reveals that DIO increases the mobility of electrons and holes (the charge carriers) by affecting the energetic disorder and electric field dependence of the mobility. Our findings show the implications of phase separation and carrier transport pathways to achieve optimal device performances.

KOtBu: A Privileged Reagent for Electron Transfer Reactions?

Many recent studies have used KOtBu in organic reactions that involve single electron transfer; in the literature, the electron transfer is proposed to occur either directly from the metal alkoxide or indirectly, following reaction of the alkoxide with a solvent or additive. These reaction classes include coupling reactions of halobenzenes and arenes, reductive cleavages of dithianes, and SRN1 reactions. Direct electron transfer would imply that alkali metal alkoxides are willing partners in these electron transfer reactions, but the literature reports provide little or no experimental evidence for this. This paper examines each of these classes of reaction in turn, and contests the roles proposed for KOtBu; instead, it provides new mechanistic information that in each case supports the in situ formation of organic electron donors. We go on to show that direct electron transfer from KOtBu can however occur in appropriate cases, where the electron acceptor has a reduction potential near the oxidation potential of KOtBu, and the example that we use is CBr4. In this case, computational results support electrochemical data in backing a direct electron transfer reaction.

Microbiosensor for the detection of acetate in electrode-respiring biofilms

The goal of this work was to develop a microbiosensor to measure acetate concentration profiles inside biofilms in situ. The working principle of the microbiosensor was based on the correlation between the acetate concentration and the current generated during acetate oxidation by Geobacter sulfurreducens. The microbiosensor consisted of a 30-µm carbon microelectrode with an open tip as a working electrode, with G. sulfurreducens biofilm on the tip and a pseudo Ag/AgCl reference electrode, all enclosed in a glass outer case with a 30-µm tip diameter. The microbiosensor showed a linear response in the 0–1.6mM acetate concentration range with a 79±8µM limit of detection (S/N=2). We quantified the stirring effect and found it negligible. However, the interfering effect of alternative electron donors (lactate, formate, pyruvate, or hydrogen) was found to be significant. The usefulness of the acetate microbiosensor was demonstrated by measuring acetate concentration depth profiles within a G. sulfurreducens biofilm. The acetate concentration remained at bulk values throughout the biofilm when no current was passed, but it decreased from the bulk values to below the detection limit within 200µm when current was allowed to pass. The zero acetate concentration at the bottom of the biofilm showed that the biofilm was acetate-limited.

MAT2B promotes adipogenesis by modulating SAMe levels and activating AKT/ERK pathway during porcine intramuscular preadipocyte differentiation

Intramuscular fat (IMF) has been demonstrated as one of the crucial factors of livestock meat quality. The MAT2B protein with MAT2α catalyzes the formation of methyl donor S- adenosylmethionine (SAMe) to mediate cell metabolism including proliferation and apoptosis. However, the regulatory effect of MAT2B on IMF deposition is still unclear. In this study, the effect of MAT2B on adipogenesis and its potential mechanism during porcine intramuscular preadipocyte differentiation was studied. The results showed that overexpression of MAT2B promoted adipogenesis and significantly up-regulated the mRNA and protein levels of adipogenic marker genes including FASN, PPARγ and aP2, consistently, knockdown of MAT2B inhibited lipid accumulation and down-regulated the mRNA and protein levels of the above genes. Furthermore, flow cytometry and EdU-labeling assay indicated that MAT2B regulate adipogenesis was partly due to influence intracellular SAMe levels and further affect cell clonal expansion. Also, increased expression of MAT2B activated the phosphorylations of AKT and ERK1/2, whereas knockdown of MAT2B blocked AKT signaling and repressed the phosphorylation of ERK1/2. Moreover, the inhibitory effect of LY294002 (a specific PI3K inhibitor) on the activities of AKT and ERK1/2 was partially recovered by overexpression of MAT2B in porcine intramuscular adipocytes. Finally, Co-IP experiments showed that MAT2B can directly interact with AKT. Taken together, our findings suggested that MAT2B acted as a positive regulator through modifying SAMe levels as well as activating AKT/ERK signaling pathway to promote porcine intramuscular adipocyte differentiation.

Recombination activity associated with thermal donor generation in monocrystalline silicon and effect on the conversion efficiency of heterojunction solar cells

The recombination properties of the carrier lifetime-limiting center formed during the generation of oxygen-related thermal donors (so called “old” thermal donors) in n-type Czochralski silicon were determined over a wide range of thermal donors' concentrations. The procedure involved (1) determining the various energy levels associated with dopants with the help of temperature Hall effect measurements, (2) clarifying which energy level limits the carrier lifetime by temperature lifetime spectroscopy, and (3) determining the recombination parameters of the involved defect from room-temperature carrier lifetime curves. Our results support the fact that a deep energy level in the range of 0.2–0.3 eV below the conduction band limits the carrier lifetime. The second family of thermal donors, featuring bistable properties, was tentatively identified as the corresponding defect. From the obtained experimental data, the influence of the defect on the amorphous/crystalline silicon heterojunction solar cell conversion efficiency was simulated. It is observed that for extended donor generation, the carrier lifetime is reduced by orders-of-magnitude, leading to unacceptable losses in photovoltaic conversion efficiency. A key result is that even for samples with thermal donor concentrations of 1015 cm−3—often met in seed portions of commercial ingots—simulations reveal efficiency losses greater than 1% absolute for state-of-the-art cells, in agreement with recent experimental studies from our group. This result indicates to crystal growers the importance to mitigate the formation of thermal donors or to develop cost-effective processes to suppress them at the ingot/wafer scale. This is even more critical as ingot cool-down is likely to be slower for future larger ingots, thus promoting the formation of thermal donors.

Electronic Structure of ZnO Quantum Dots Studied by High-Frequency EPR, ESE, ENDOR and ODMR Spectroscopy

High-frequency electron paramagnetic resonance (EPR), electron spin echo (ESE), electron-nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) were applied for the investigation of the electronic properties of ZnO colloidal quantum dots (QDs) which consist of a ZnO nanocrystal core and Zn(OH)2 shell. Shallow donors (SDs) in the form of interstitial atoms of lithium and sodium, as well as substituent of the aluminum have been identified and the spatial distribution of their electronic wave functions has been determined in the regime of quantum confinement. Hyperfine interactions as monitored by ENDOR quantitatively reveal the transition from semiconductor to molecular properties upon reduction of the size of the nanoparticles. We studied the process of charge separation in the quantum dot when excited by UV light resulting in the formation of shallow donors and deep acceptors in the paramagnetic state, which recombine when heated, accompanied by thermoluminescence go back again in the non-paramagnetic state. It was shown that in all studied quantum dots deep acceptors are present positioned near the interface and including one sodium atom. ODMR techniques, which is based on EPR detection via photoluminescence or via tunneling afterglow that can be observed after preliminary X-ray or UV irradiation proved to be very useful to study colloidal ZnO NCs. The higher sensitivity of ODMR via afterglow allowed characterization of ZnO QDs dispersed in transparent media, which is an important advantage, since these systems are more relevant for a number of practical applications. A direct evidence of Co (Mn) interaction with SD in the core and hyperfine coupling with 1H in the shell of QDs has been demonstrated in Mn (Co)-doped ZnO QDs, which are promising classes of diluted magnetic semiconductors.

Deep-level transient spectroscopy at platinum/titanium-dioxide hydrogen sensors

Abstractauthoren Deep traps in platinum/mesoporous-titanium-dioxide hydrogen sensors are studied by deep-level transient spectroscopy (DLTS) before, during, and after the gas detection. This is possible for atmospheric hydrogen volume fractions in the low ppm regime where the Schottky barrier at the Pt/TiO2 interface is retained. In the absence of hydrogen, two deep levels with binding energies of 470 and 720 meV are found. Their occurrence increases as hydrogen is offered, but to a smaller extent than the hydrogen-induced doping density. An additional level develops during hydrogen exposure at a binding energy of ≈870meV. After the sensing sequence, the sample recovers only partially, but the deep trap densities can be reset by annealing in dry air. The data suggest that besides donor formation, exposure of mesoporous TiO2 to hydrogen also generates deep traps. The results furthermore show that despite the large interfacial disorder, which in the present case is essential for the functionality, DLTS can be used to obtain information about deep traps generated during utilization of the device as gas sensor.

Mechanism of formation of ultrashallow thermal donors in carbon-doped oxygen-rich monocrystalline silicon preannealed to introduce hydrogen

We previously reported on ultrashallow thermal donors (USTDs) in carbon-doped oxygen-containing monocrystalline silicon (Czochralski-grown, CZ-Si) crystals that were preannealed to introduce hydrogen at 1300 °C, and then annealed at 480 °C. In this study, the formation mechanism of the USTDs was evaluated. It was observed that an increase in the intensity of UTSDs leads to a reduction in that of hydrogen-related shallow thermal donors [STD(H)s], and the sum of the area intensities of the lines in the transmission spectra of USTDs and STD(H)s is nearly constant when the silicon crystals are annealed for longer than 10 h at 480 °C. We also found some thermally activated processes linked to the formation of USTDs. We thus conclude that the mechanism is composed of the high-speed formation of STD(H)s in the first stage and carbon modulation of the electronic structure of STD(H)s in the second stage.

Peculiarities of the hydrogenated In(AsN) alloy

The electronic properties of In(AsN) before and after post-growth sample irradiation with increasing doses of atomic hydrogen have been investigated by photoluminescence. The electron density increases in In(AsN) but not in N-free InAs, until a Fermi stabilization energy is established. A hydrogen ϵ+/− transition level just below the conduction band minimum accounts for the dependence of donor formation on N, in agreement with a recent theoretical report highlighting the peculiarity of InAs among III–V compounds. Raman scattering measurements indicate the formation of N–H complexes that are stable under thermal annealing up to ∼500 K. Finally, hydrogen does not passivate the electronic activity of N, thus leaving the band gap energy of In(AsN) unchanged, once more in stark contrast to what has been reported in other dilute nitride alloys.

Mixed Ionic–Electronic Conducting Li4Ti5O12 as Anode Material for Lithium Ion Batteries with Enhanced Rate Capability – Impact of Oxygen Non-Stoichiometry and Aliovalent Mg2+-Doping Studied by Electron Paramagnetic Resonance

Abstract Spinel-type Li4Ti5O12 is an attractive anode material for lithium-ion battery applications owing to its high specific energy and almost vanishing mechanical strain upon Li-(de)intercalation. The impact of oxygen non-stoichiometry and aliovalent Mg-doping on the defect chemistry of Li4Ti5O12 has been characterized by quantitative electron paramagnetic resonance (EPR) spectroscopy. The results obtained demonstrate that the Mg2+-ions are amphoterically incorporated at the here studied concentrations on both lithium (MgLi •) and titanium (Mg′′Ti) sites. Mg2+-doping thus simultaneously results in the formation of donor (MgLi •) and acceptor (Mg′′Ti) centers. Furthermore, upon both oxygen deficiency and Mg2+-doping, the concentration of `polaronic'-type Ti3+-states is increased, accounting for the enhanced electronic conductivity.

Sulfur doping of M/In2O3 (M=Al,W) nanowires with room temperature near infra red emission

We have investigated the growth of Al doped In2O3 nanowires via the vapor-liquid-solid mechanism at 800°C using Au as a catalyst. We find that the Al is not incorporated into the cubic bixbyite crystal structure of In2O3 but nevertheless was detected in the form of Al2O3. The nanowires had metallic like conductivities and exhibited photoluminescence at 2.3 eV which shifted to 1.5 eV after exposure to H2S above 500°C due to the formation of β-In2S3 and deep donor to acceptor transitions with a lifetime of ≈1 μs. The near infra red emission was also observed in W/In2O3 but not in W/SnO2 core-shell nanowires after processing under H2S at 600°C, confirming it is related to β-In2S3. The nanowires remain one dimensional up to 900°C due to the shell which is interesting for the fabrication of high temperature nanowire sensors.

On a chaotic potential at the surface of a compensated semiconductor under conditions of the self-assembly of electrically active defects

Natural irregularities of the electric potential on the surface of a semiconductor under conditions of the partial self-assembly of electrically active defects, i.e., on the formation of donor–acceptor pairs in depletion layers, are studied. The amplitude and character of the spatial distribution of the chaotic potential on the surface of a semiconductor in the cases of localized and delocalized states are determined. The dependence of the amplitude of the chaotic potential on the degree of compensation of the semiconductor is obtained.

Application of the Drago-Wayland equation for prediction of enthalpies of formation of donor–acceptor complexes in the systems of group IIIA and VA element methyl compounds

The possibility to use the Drago–Wayland equation for prediction of enthalpies of formation of donor–acceptor complexes in the systems of Group IIIA and VA element methyl compounds has been considered and substantiated.

Phenol-substituted tetrapyrazinoporphyrazines: pH-dependent fluorescence in basic media.

Tetrapyrazinoporphyrazines (TPyzPzs) bearing one, two, four or eight 3,5-di(tert-butyl)-4-hydroxyphenol moieties were synthesized as zinc(II) complexes and metal-free derivatives. The deprotonation of the phenol using tetrabutylammonium hydroxide induced the formation of a strong donor for intramolecular charge transfer that switched OFF the red fluorescence (λF ∼660 nm) of the parent zinc TPyzPzs. The changes were fully reversible for TPyzPzs with one to four phenolic moieties, and an irreversible modification was observed for TPyzPzs substituted with eight phenols. The sensors were anchored to lipophilic particles in water, and a pKa approximately 12.5-12.7 was determined for the phenolic hydroxyl based on fluorescence changes in different buffers. In addition, a novel concept for fluorescence OFF-ON-OFF switching in metal-free TPyzPzs bearing phenolic moieties upon addition of specific amounts of base was demonstrated.

C–H complex defects and their influence in ZnO single crystal**Project supported by the National Natural Science Foundation of China (Grant No. 61474104).

Infrared absorption local vibration mode (LVM) spectroscopy is used to study hydrogen related defects in n-type ZnO single crystal grown by a closed chemical vapor transport (CVT) method under Zn-rich growth conditions, in which carbon is used as a transport agent. Two C–H complex related absorption peaks at 2850 cm−1 and 2919 cm−1 are detected in the sample. The formation of the C–H complex implies an effect of carbon donor passivation and formation suppression of H donor in ZnO. The influence of the complex defects on the electrical property of the CVT-ZnO is discussed based on Hall measurement results and residual impurity analysis.

Mixed-stack architecture and solvatomorphism of trimeric perfluoro-ortho-phenylene mercury complexes with dithieno[3,2-b:2′,3′-d]thiophene

The formation of the mixed-stack donor–acceptor complex of dithieno [3,2-b:2′,3′-d]thiophene (1) and trimeric perfluoro-ortho-phenylene mercury (I) has been investigated under different conditions. Two solvatomorphs – mixed-stack complexes with a 1:1 donor–acceptor ratio and different solvent molecules in the solid state (dichloromethane (2) and dichloroethane (3)) have been obtained and characterized by experimental methods (FT-IR spectroscopy, differential thermogravimetric analysis, and X-ray crystallography) and quantum-chemical calculations at the density functional theory level. The differences in the solid state packing, thermal stability and potential charge-transfer properties of 2 and 3 are discussed.

Effects of Intracoronary Infusion of Escalating Doses of Cardiac Stem Cells in Rats With Acute Myocardial Infarction

Although c-kit(pos) cardiac stem cells (CSCs) preserve left ventricular (LV) function and structure after myocardial infarction, CSC doses have been chosen arbitrarily, and the dose-effect relationship is unknown. Rats underwent a 90-minute coronary occlusion followed by 35 days of reperfusion. Vehicle or CSCs at 5 escalating doses (0.3×10(6), 0.75×10(6), 1.5×10(6), 3.0×10(6), and 6.0×10(6) cells/heart) were given intracoronarily 4 h after reperfusion. The lowest dose (0.3×10(6)) had no effect on LV function and morphology, whereas 0.75, 1.5, and 3.0×10(6) significantly improved regional and global LV function (echocardiography and hemodynamic studies). These 3 doses had similar effects on echocardiographic parameters (infarct wall thickening fraction, LV end-systolic and end-diastolic volumes, LV ejection fraction) and hemodynamic variables (LV end-diastolic pressure, LV dP/dtmax, preload adjusted maximal power, end-systolic elastance, preload recruitable stroke work) and produced similar reductions in apoptosis, scar size, infarct wall thinning, and LV expansion index and similar increases in viable myocardium in the risk region (morphometry). Infusion of 6.0×10(6) CSCs markedly increased postprocedural mortality. Green fluorescent protein and 5-bromo-2'-deoxyuridine staining indicated that persistence of donor cells and formation of new myocytes were negligible with all doses. Surprisingly, in this rat model of acute myocardial infarction, the dose-response relationship for intracoronary CSCs is flat. A minimal dose between 0.3 and 0.75×10(6) is necessary for efficacy; above this threshold, a 4-fold increase in cell number does not produce greater improvement in LV function or structure. Further increases in cell dose are harmful.

Diketopyrrolopyrrole-based narrow band gap donors for efficient solution-processed organic solar cells

This study involves the development of two new small molecules comprising a diketopyrrolopyrrole (DPP) core flanked with donor units of triphenylamine (TPA-DPP-TPA) and fluorine (FL-DPP-FL) for application in bulk heterojunction (BHJ) organic solar cells (OSCs). The OSCs based on FL-DPP-FL and PC71BM exhibited a PCE of 1.73%, compared to 1.45% for that obtained from devices of TPA-DPP-TPA. The morphological studies reveal that the enhancement in OSCs of FL-DPP-FL is mainly attributed to the improved nanoscale film morphology of the FL-DPP-FL:PC71BM blend, which promoted the formation of smaller domains and greater donor–acceptor interpenetrated networks within the active layer.