Double Donor Research Articles

Electrical and optical properties of iron in GaN, AlN, and InN

Iron is a common trace impurity in the group-III nitrides. Iron is also intentionally introduced in III-nitride electronic devices to create semi-insulating substrates and in the context of spintronics and quantum information applications. Despite the wide-ranging consequences of iron's presence in III-nitrides, the properties of iron impurities in the nitrides are not fully established. We investigate the impact of iron impurities on the electrical and optical properties of GaN, AlN, and InN using first-principles calculations based on a hybrid functional. We report formation energies of substitutional and interstitial iron impurities as a function of the Fermi-level position. We also investigate complexes of Fe with substitutional oxygen on the nitrogen site, with nitrogen vacancies and with hydrogen interstitials. In GaN and AlN, iron on the cation site is amphoteric. We discuss the role of the Fe-induced acceptor level and its impact on nonradiative recombination in the context of loss mechanisms in light emitters, and current collapse in high-electron-mobility transistors. In InN, we find the iron interstitial to be the most favorable configuration, where it acts as a shallow double donor.

Sequential Single Lung Transplant (SLT) with or without Ex-Vivo Lung Perfusion Maximizes on Donor to Transplant Conversion Rates

Purpose A double lung donor may be split, with a single lung transplanted sequentially into two different recipients by one team. This minimizes utilization of hospital staff resources, day time theater access and supports donor to transplant conversion rates. This is a retrospective institutional review of the first 12 cases assessing the impact of extended ischemic time (for the delayed second lung transplant recipient) on length of stay (LOS), intermediate term graft function and survival. Methods Twelve sequential single lung transplants were performed from 6 donors between August 2016 and September 2017. Basic data for donors and recipients including outcomes (ischemic time, ICU stay, total hospital stay, 30 day mortality, 12-month survival including pulmonary function tests (PFTs)) were recorded. Borderline lungs were implanted to the EVLP circuit and reperfused using acellular supplemented Steen Solution (Vitrolife, Goteborg, Sweden). Recipients were categorized into 2 groups: G1 for patients who received the 1st lung, hence having a shorter ischemic time (whether they had EVLP or not), G2 for patients who received the 2nd lung, hence a longer ischemic time. 3 patients received EVLP reconditioned lungs. Results Donor data (n = 6) were: age, 44.2 ± 8.33 years; female/male, 5:1; cause of death: intracranial haemorrhage 4(66.6%), hypoxic brain injury 1(16.6%) and traumatic brain injury 1(16.6%). Mean LAS Score was similar for both groups (44.9 G1 and 41.7 G2, p value=0.64). Mean ischemic time from donor cross clamp to reperfusion was 389 minutes for G1 and 708 minutes for G2. There was no significant difference in ICU stay: 14.7 days vs 6.0 days with a quicker discharge from ITU in the G2(p value=0.20). The mean Hospital LOS was 32.3 vs 20.3 days, (p value= 0.35) Mean FEV1 at 3 months in G1 was 2.4 L (79.7%) and in G2 was 2.0 L (68.4%) (p value=0.38). No all cause 30 day mortality in both groups. At 1-year 2 patients had died from an unexpected bradycardic cardiac arrest. Conclusion SLT if limited resources are available maximize on donor to transplant conversion rates. Donor ischemic lung time is not a deterrent in the era of EVLP. SLT into two recipients by a single theater service is feasible, cost-effective and safe.

One-dimensionally extended oxygen vacancy states in perovskite oxides

We elucidate as-yet-unidentified spatially extended oxygen vacancy states in $\mathit{AB}{\mathrm{O}}_{3}$ perovskite oxides using first-principles calculations. When $B$ sites are occupied with ${d}^{0}$ transition-metal ions, charged oxygen vacancies are found to be long ranged in one direction because of charge transfer accompanied with atomic displacement along a chainlike structure. This finding offers two important results. First, it explains the reason why accurate calculations of oxygen vacancy formation energies necessitate very large supercells in ${d}^{0}$ perovskite and related oxides. It is remarkable especially for ${\mathrm{KNbO}}_{3}$ and ${\mathrm{WO}}_{3}$ on account of their anomalous Born effective charges. Second, such elongated vacancy states tend to show lower formation energies and thereby shallower donor levels. Our discussion is extendable to systems other than perovskites. Since the chainlike structure also appears in anatase ${\mathrm{TiO}}_{2}$ but not in its rutile counterpart, it could be a reason why the oxygen vacancy is a double shallow donor in the former while constructing a deep state in the latter.

Effects of NH3 flow on structural and optical properties of ZnO films grown by atmospheric-pressure chemical vapor deposition

Structural and optical properties of zinc oxide (ZnO) films doped with nitrogen (N) grown on r-plane sapphire substrates by the atmospheric-pressure chemical vapor deposition using zinc, water, and ammonia (NH3) as source materials have been studied. The small amount of NH3 flow was found to be effective for improving the film orientation and surface morphology. Photoluminescence (PL) spectra of the as-grown undoped and N-doped ZnO films were composed of a near-band-edge emission with two peaks at ~376 and ~386 nm and a green band (GB) emission with a peak at ~540 nm. PL and PL excitation (PLE) measurements for the as-grown N-doped films grown at the different NH3 flow rates indicated the possibility of the contribution of the N and/or hydrogen (H) related defect to both the appearance of the peak at ~386 nm and the excitation process of the GB emission. The comparison between the PL and photoacoustic (PA) spectra showed that the N atom occupying oxygen site (NO) or its complex defect is the most conceivable candidate for the non-radiative center responsible for the intra-band-gap PA absorption. After post annealing under the different conditions, a red band (RB) emission appeared at ~720 nm besides the GB emission. Both the RB and GB emissions observed for the annealed films were found to be effectively excited by the band-to-band (inter-band-gap) and free exciton transitions. Regardless of the difference in annealing condition, all the PLE spectra for the RB emissions showed a weak excitation band extending from 460 nm to 680 nm, suggesting the formation of the deep impurity level probably due to the NO acceptor. It was also confirmed by X-ray photoelectron spectroscopy that the NO deep acceptors were formed in the films, but passivated by the H atoms and/or compensated by the H related donors and/or by the nitrogen molecule substituting oxygen site [(N2)O] double donors.

Even‐Parity Excited States in Infrared Emission, Absorption, and Raman Scattering Spectra of Shallow Donor Centers in Silicon

The Lyman absorption spectra of different shallow dopants in elemental semiconductors at liquid‐helium temperature show typically transitions from the even‐parity ground state to odd‐parity excited states. Such transitions are dipole allowed and observed by absorption spectroscopy. Knowledge on the excited states with other type of symmetry can be derived from absorption and Raman spectra of a heavily doped semiconductor, or at experimental conditions enabling optical dipole‐allowed transitions from or into even‐parity states. Here, we report on the experimental observation of impurity transitions involving different excited even‐parity (ns‐ and nd‐) states in silicon crystals: 1) doped with group‐V hydrogen‐like donors in samples with large infrared absorbance, and 2) doped with the magnesium double donor evoking a thermal population of the lowest even‐parity excited states.

Electron emission and capture by oxygen-related bistable thermal double donors in silicon studied with junction capacitance techniques

It has been recently suggested that oxygen-related bistable thermal double donors (BTDDs) are responsible for the reduction of minority carrier lifetime and conversion efficiency of novel amorphous-crystalline Si heterojunction solar cells with their base formed from n-type Czochralski-grown (Cz) silicon [M. Tomassini et al., J. Appl. Phys. 119, 084508 (2016)]. To test this hypothesis, we have studied processes associated with carrier emission and capture by BTDDs in p+-n and Schottky barrier diodes on n-type Cz-Si materials with the use of junction capacitance techniques. By means of deep level transient spectroscopy, we have detected electron emission signals from the deep donor state of the BTDD-0 and BTDD-1 centers. The values of activation energy for electron emission (Eem) have been determined as 1.01 ± 0.01 and 0.91 ± 0.01 eV for the BTDD-0 and BTDD-1 centers, respectively. Such high Eem values are very unusual for defects in Si. We have carried out measurements of electron capture kinetics and associated shallow donor–deep donor transformations for the BTDD-0 and BTDD-1 defects at different temperatures in the diodes with different doping levels. Energy barriers for the capture-transformation processes have been determined. It is argued that BTDDs are responsible for carrier trapping in n-type Cz-Si crystals but are not effective recombination centers.

Ab initio investigation of possible lower-energy candidate structure for cationic water cluster (H2O) 12+ via particle swarm optimization method

Detecting the underlying performance of hydrated electrons and hydroxyl radicals in the cationic water cluster can greatly help to understand the inter reaction mechanism in the liquid water and aqueous solutions. Based on our previous (H2O)10+ research, we have paid attention to more problems of larger cationic clusters in this work, including the existence of hemibonded type, long-range correction functions, and hydrogen-bonded site analyses. The lower-energy structures of the cationic water cluster (H2O)12+ have been comprehensively explored, and more experienced functions are introduced to check the ground state and vibration spectrum. Unlike the configuration regularity of neutral (H2O)12 clusters and small cationic water clusters, those new-found structures for (H2O)12+ are inclined to adopt three dimension (3D) cage-like structures and the H2O-OH2 structure appears in the higher energy isomer. The calculation reveals that the lowest stable isomer is the 3D cage structure W14 predicted at MP2 level, which has not been reported yet. In the thermal simulation, structure transition from the cage-like to the ring-like occurs at T > ≈256 K, and the two dimension (2D) ring-like structure occupies a dominant position at high temperature range. The infrared spectra explain that the difference of the spectra between the 2D net structures and 3D cage-structures is mainly caused by the weight fluctuation of single acceptor-single donor (AD), double acceptor-single donor (AAD), and single acceptor-double donor (ADD) sites in these isomers. This further gives a similarity relation between (H2O)12+ and H+(H2O)12 clusters in the shape of the network and spectral characteristics. By molecular orbitals and topological analysis, we find that the lone pair orbital on hydroxyl radical dominates the reactivity and stability of cationic system. The present research may be helpful for exploring the evolution law of the larger cationic water clusters in the future.

Induced defect levels of P and Al vacancy-complexes in [formula omitted]-SiC: A hybrid functional study

The electronic behaviour of high-dose phosphorus implanted in 4H-SiC is mainly desirable to obtained lower sheet resistance of 4H-SiC. Al doping on the other hand acts as an acceptor, improves the dielectric properties of 4H-SiC and has very low diffusivity in SiC. Using a hybrid density functional theory, we investigated the properties of Al and P defect-complexes in 4H-SiC a wide band-gap semiconductor that is promising for applications in high-frequency and high-temperature electronic device. We show that vacancy-complexes formed by PSi and AlSi are more energetically stable than those formed by PC and AlC. The defects with silicon vacancy are predicted to experience more lattice distortion compared to those formed with carbon vacancy. While vacancy-complexes formed with PSi or PC and VC induced double donor levels, vacancy-complex formed with substitution of P and VSi induced negative-U charge state ordering. The Al with VC related vacancy-complexes induced deep single donor and acceptor levels, and Al with VSi induced only acceptor and negative-U ordering.

Radii of Rydberg states of isolated silicon donors

We have performed high field magnetoabsorption spectroscopy on silicon doped with a variety of single and double donor species. The magnetic field provides access to an experimental magnetic length, and the quadratic Zeeman effect, in particular, may be used to extract the wave-function radius without reliance on previously determined effective mass parameters. We were, therefore, able to determine the limits of validity for the standard one-band anisotropic effective mass model. We also provide improved parameters and use them for an independent check on the accuracy of effective mass theory. Finally, we show that the optically accessible excited-state wave functions have the attractive property that interactions with neighbors are far more forgiving of position errors than (say) the ground state.

Further investigations of the deep double donor magnesium in silicon

The deep double donor levels of substitutional chalcogen impurities in silicon have unique optical properties which may enable a spin/photonic quantum technology. The interstitial magnesium impurity (Mg$_i$) in silicon is also a deep double donor but has not yet been studied in the same detail as have the chalcogens. In this study we look at the neutral and singly ionized Mg$_i$ absorption spectra in natural silicon and isotopically enriched 28-silicon in more detail. The 1s(A$_1$) to 1s(T$_2$) transitions, which are very strong for the chalcogens and are central to the proposed spin/photonic quantum technology, could not be detected. We observe the presence of another double donor (Mg$_{i*}$) that may result from Mg$_i$ in a reduced symmetry configuration, most likely due to complexing with another impurity. The neutral species of Mg$_{i*}$ reveal unusual low lying ground state levels detected through temperature dependence studies. We also observe a shallow donor which we identify as a magnesium-boron pair.

Revised identification of the G-levels in gold doped Si by Laplace deep level transient spectroscopy

In this study, we re-examine the electronic levels G1-G4 of gold-hydrogen complexes in Si by Laplace deep level transient spectroscopy. In Au doped n- and p-type Si, we analyse the depth profiles of the levels after wet-chemical etching, study their annealing behaviour and detect changes of their emission rates in the electrical field. We give evidence that G1, G3, and G2 are the double acceptor, acceptor, and donor level of the Au complex with one hydrogen atom, whereas G4 belongs to the Au complex with two hydrogen atoms and is probably an acceptor level.

Red fruit, orange fruit, orange fruit, red fruit: genome editing in tomato.

Red fruit, orange fruit, orange fruit, red fruit: genome editing in tomato.

Singlet-triplet minus mixing and relaxation lifetimes in a double donor dot

We measure singlet-triplet mixing in a precision fabricated double donor dot comprising 2 and 1 phosphorus atoms separated by 16 ± 1 nm. We identify singlet and triplet-minus states by performing a sequential independent spin readout of the two electron system and probe its dependence on magnetic field strength. The relaxation of singlet and triplet states is measured to be 12.4 ± 1.0 s and 22.1 ± 1.0 s, respectively, at Bz = 2.5 T.

Relaxation times measured for singlet and triplet electron spin states between two donor qubits

Two single-electron spin qubits based on single and double donor quantum dots controllably produce singlet and triplet states transitions in the combined system of two electron spins. The lifetimes were measured using single spin readout.

Electrically active induced energy levels and metastability of B and N vacancy-complexes in 4H–SiC

Electrically active induced energy levels in semiconductor devices could be beneficial to the discovery of an enhanced p or n-type semiconductor. Nitrogen (N) implanted into 4H–SiC is a high energy process that produced high defect concentrations which could be removed during dopant activation annealing. On the other hand, boron (B) substituted for silicon in SiC causes a reduction in the number of defects. This scenario leads to a decrease in the dielectric properties and induced deep donor and shallow acceptor levels. Complexes formed by the N, such as the nitrogen-vacancy centre, have been reported to play a significant role in the application of quantum bits. In this paper, results of charge states thermodynamic transition level of the N and B vacancy-complexes in 4H–SiC are presented. We explore complexes where substitutional N/N or B/B sits near a Si (V) or C (V) vacancy to form vacancy-complexes (NV, NV, NV, NV, BV, BV, BV and BV). The energies of formation of the N related vacancy-complexes showed the NV to be energetically stable close to the valence band maximum in its double positive charge state. The NV is more energetically stable in the double negative charge state close to the conduction band minimum. The NV on the other hand, induced double donor level and the NV induced a double acceptor level. For B related complexes, the BV and BV were energetically stable in their single positive charge state close to the valence band maximum. As the Fermi energy is varied across the band gap, the neutral and single negative charge states of the BV become more stable at different energy levels. B and N related complexes exhibited charge state controlled metastability behaviour.

Design of butterfly type organic dye sensitizers with double electron donors: The first principle study

In this work, we designed a series of butterfly type organic dyes, named ME07-ME13 by introducing such as triphenylamine, phenothiazine, coumarin groups etc. as electron donors and further investigated their absorption spectra using density functional theory (DFT) and time-dependent DFT (TDDFT). All designed dyes cover the entire visible absorption spectrum from 300 to 800nm. It's fascinating that ME13 molecule has two absorption peak and the molar coefficient of two absorption peaks are above 4.645×104M−1·cm−1. The light absorption area of ME13 exhibits an increment of 16.5–19.1% compared to ME07-ME12. Furthermore, we performed a detailed analysis on their geometrical and electronic properties, including molecular structures, energy levels, light harvesting efficiency (LHE), driving force (ΔGinject), regeneration (ΔGregen),electron dipole moments (μnormal), intermolecular electron transfer and dye/(TiO2)38 system electron transitions. The results of calculation reveal that double coumarin donors in ME13 are promising functional groups for butterfly type organic dye sensitizers. It is expected that the design of double donors can provide a new strategy and guidance for the investigation in high efficiency dye-sensitized devices.

Rhodium-catalyzed asymmetric hydrogenation of β-cyanocinnamic esters with the assistance of a single hydrogen bond in a precise position

With the assistance of hydrogen bonds, the first asymmetric hydrogenation of β-cyanocinnamic esters is developed, affording chiral β-cyano esters with excellent enantioselectivities (up to 99% ee). This novel methodology provides an efficient and concise synthetic route to chiral GABA-derivatives such as (S)-Pregabalin, (R)-Phenibut, (R)-Baclofen. Interestingly, in this system, the catalyst with a single H-bond donor performs better than that with double H-bond donors, which is a novel discovery in the metalorganocatalysis area.

Synthesis and properties of organic sensitizers bearing asymmetric double donor -π- acceptor chains for dye-sensitized solar cells

A novel efficient metal-free sensitizer CVTC-H-CVTP containing asymmetric double D-π-A chains was designed and synthesized for application in dye-sensitized solar cells (DSSCs). The photophysical and electrochemical properties of the target molecule were investigated systematically through comparison of dyes with different single D-π-A structures. In contrast, the sensitizer CVTC-H-CVTP provides improved Jsc and Voc because of wider range of spectral absorption and higher absorption intensity which leads to better photoelectricity conversion efficiency. Therefore, dye with asymmetric double D-π-A shows the most efficient photoelectricity conversion efficiency of 3.20% (Unoptimized, VOC = 0.66 V, JSC = 6.51 mA/cm2, and FF = 74.72%) under simulated AM 1.5 irradiation (100 mW/cm2).

Molecular Engineering of Near Infrared Absorbing Thienopyrazine Double Donor Double Acceptor Organic Dyes for Dye-Sensitized Solar Cells.

The thienopyrazine (TPz) building block allows for NIR photon absorption in dye-sensitized solar cells (DSCs) when used as a π-bridge. We synthesized and characterized 7 organic sensitizers employing thienopyrazine (TPz) as a π-bridge in a double donor, double acceptor organic dye design. Donor groups are varied based on electron donating strength and sterics at the donor-π bridge bond with the acceptor groups varied as either carboxylic acids or benzoic acids on the π-bridge. This dye design was found to be remarkably tunable with solution absorption onsets ranging from 750 to near 1000 nm. Interestingly, the solution absorption measurements do not accurately approximate the dye absorption on TiO2 films with up to a 250 nm blue-shift of the dye absorption onset on TiO2. This shift in absorption and the effect on electron transfer properties is investigated via computational analysis, time-correlated single photon counting studies, and transient absorption spectroscopy. Structure-performance relationships were analyzed for the dyes in DSC devices with the highest performance observed at 17.6 mA/cm2 of photocurrent and 7.5% PCE for a cosensitized device with a panchromatic IPCE onset of 800 nm.

Native interstitial defects in ZnGeN2

A density functional study is presented of the interstitial ${\mathrm{Zn}}_{i}, {\mathrm{Ge}}_{i}$, and ${\mathrm{N}}_{i}$ in ${\mathrm{ZnGeN}}_{2}$. Corrections to the band gap are included by means of the LDA+U method. The Zn and Ge interstitials are both found to strongly prefer the larger octahedral site compared to the two types of tetrahedral sites. The Zn interstitial is found to be a shallow double donor, but it has higher energy than previously studied antisite defects. It has a resonance in the conduction band that is Zn-$s$ like. The Ge interstitial is an even higher energy of formation defect and also behaves as a shallow double donor, but it also has a deep level in the gap corresponding to a Ge-$s$ orbital character while the Ge-$p$ forms a resonance in the conduction band. The nitrogen interstitial forms a split-interstitial configuration, as also occurs in GaN. Its electronic levels can be related to that of a ${\mathrm{N}}_{2}$ molecule. The defect levels in the gap correspond to the ${\ensuremath{\pi}}_{g}$-like lowest unoccupied molecular orbital of the molecule, which here becomes filled with three electrons in the defect's neutral charge state. They are found to prefer a high-spin configuration in the $q=+1$ state. The corresponding transition levels are obtained and show that this is an amphoteric trap level occurring in $+2, +1$, 0, and $\ensuremath{-}1$ charge states. The two possible sites for this split interstitial, on top of Zn or on top of Ge, differ slightly in ${\mathrm{N}}_{2}$ bond length. While the ${\mathrm{N}}_{i}$ defects have the lowest formation energy among the interstitials, it is still higher than that of the antisites. Hence they are not expected to occur in sufficient concentration to affect the intrinsic Fermi level position. In particular, they do not contribute to the unintentional $n$-type background doping.