Donor States Research Articles

Direct determination of band-gap renormalization in degenerately doped ultrawide band gap β- Ga2O3 semiconductor

${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ is emerging as a promising wide band-gap semiconductor for high-power electronics and deep ultraviolet optoelectronics. It is highly desirable to dope it with controllable carrier concentrations for different device applications. This work reports a combined photoemission spectroscopy and theoretical calculation study on the electronic structure of Si doped ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ films with carrier concentration varying from $4.6\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ to $2.6\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. Hard x-ray photoelectron spectroscopy was used to directly measure the widening of the band gap as a result of occupation of conduction band and band-gap renormalization associated with many-body interactions. A large band-gap renormalization of 0.3 eV was directly observed in heavily doped ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. Supplemented with hybrid density functional theory calculations, we demonstrated that the band-gap renormalization results from the decrease in energy of the conduction band edge driven by the mutual electrostatic interaction between added electrons. Moreover, our work reveals that Si is a superior dopant over Ge and Sn, because $\mathrm{Si}\phantom{\rule{0.28em}{0ex}}3s$ forms a resonant donor state above the conduction band minimum, leaving the host conduction band mostly unperturbed and a high mobility is maintained though the doping level is high. Insights of the present work have significant implications in doping optimization of ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ and realization of optoelectronic devices.

Peculiarities of erbium incorporation into ZnO microrods at high doping level leading to upconversion and the morphology change. Influence on excitonic as well as shallow donor states

Heavily Er-doped zinc oxide (ZnO) microrods with nominal compositions, ZnO:Er(2, 10, 30%), were prepared by the hydrothermal growth method. The crystallographic phases present in the as grown and annealed materialswere determined. The presence of the hexagonal Wurtzite ZnO phase in the form of the hexagonal shape nanoplatelets and microrods was confirmed. Moreover, the Er2O3phase in the form of nanosheets has been revealed as well. Erbium is partly incorporated into the ZnO hosts. It was thinning upon erbium doping level contributing to erbium oxide. This effect was even more pronounced upon annealing in air. Moreover, the influence of erbium doping and annealing on paramagnetic shallow donor centers has been studied. There were two types of these centers, both affected by the erbium presence. The increase of the neutral zinc vacancies amount upon erbium doping has been proven. Annealing in air moderated the charge distribution between them. Moreover, upconversion processes between the two erbium ions were observed and explained. The surface distribution of excitonic emission centers was studied. The larger the erbium content the weaker the exciton luminescence. The smaller the ZnO microrods the stronger the exciton emission.

CMOS-compatible direct laser writing of sulfur-ultrahyperdoped silicon: Breakthrough pre-requisite for UV-THz optoelectronic nano/microintegration

Direct laser writing/ultra-hyperdoping technology was harnessed to fabricate a universal CMOS-compatible Si-based broadband (UV-THz) n-p junction detector, with its spectral response controlled by external liquid helium-nitrogen cooling. Near-far IR (2–40 μm) temperature-dependent photoconductivity (PC) was revealed in a biased n-p junction, made of n-type sulfur-ultrahyperdoped Si (uh-Si, sulfur content ∼1 at. %) sub-micron-thick layer on a surface of a p-doped Si substrate. The observed broadband photoconductivity is provided by a dense quasi-continuum series of sulfur-impurity donor states near the conduction band bottom (so-called “intermediate band”, the bandwidth ≈0.6 eV), corresponding to IR-absorbing neutral and singly-ionized substitutional atomic and cluster centers of sulfur. The “intermediate” donor band supports the gradual “blue” spectral migration of PC maximum due to thermal ionization of deeper states at the temperature increasing in the range of 5–250 K. As a result, in the temperature range of 77–300 K complementary THz (wavenumbers <100 cm−1) spectroscopy indicates the predominating Drude-like conduction-band response of electrons and their gradually raising density ∼1019–1020 cm−3. Illustrating the thermal-ionization depletion of the donor sulfur states from the THz-probed plasma side, this unveils the unprecedently high concentration of electrically-active donor sulfur centers ∼1020 cm−3 and related donor-state density ∼1020 states/eV·cm3 in the uh-Si sample. Overall, these advances enable in situ laser writing of universal liquid helium/nitrogen cooled Si nano/micro/macrodetectors with broad – near-far IR and even THz – spectral response, crucial for photovoltaics, thermal and bio-imaging.

Highly Conductive Nitrogen‐Doped sp2/sp3 Hybrid Carbon as a Conductor‐Free Charge Storage Host

AbstractIt is commonly accepted that the increased degree of graphitization leads to a higher electrical conductivity of carbon materials. However, more and more evidence reveals that heteroatom doping on carbon host can also improve the conductivity, owing to the dopant atoms contributing to higher charge delocalization and density of donor states near Fermi level. The reality is, such conductivity improvement from doping is often overwhelmed by graphitized carbon. Although heteroatom‐doped carbon is widely used as active materials in the fields of energy storage and electrocatalysis, which still requires extra carbon‐based conductive additives to enhance the overall conductivity. In this stu, it is demonstrated that the electrical conductivity of finely designed nitrogen‐doped carbon is even beyond the commercialized carbon conductors over 3.5 times, endowing such conductive agent‐free electrode material an excellent performance in an all‐solid‐state flexible supercapacitor. The theoretical simulation further demonstrates that N‐doped sp2/sp3 hybrid carbon can migrate the Fermi level to the conduction band, leading to an n‐type conductivity due to the additional electrons caused by the N dopant.

Semiconducting Behaviour and Corrosion Resistance of Passive Film on Corrosion-Resistant Steel Rebars.

Chloride-induced corrosion of steel rebars is one of the major causes of the premature failures of reinforced concrete structures served in different environments. This paper investigates the semiconducting behaviour and corrosion resistance of the passive film formed on the corrosion-resistant rebars exposed to simulated concrete pore solutions with different pH values and chloride concentrations. The electronic properties of the passive film were studied using potentiodynamic measurements and capacitance measurements (Mott-Schottky analysis). The results indicate that, firstly, the passive film of corrosion-resistant steel rebar shows n-type semiconducting behaviour with shallow and deep donor states in the band gap during passivation, and the deep donor energy level of corrosion-resistant steel rebar passive film is not sensitive to the decreasing pH value; secondly, under the same conditions, the passive film of corrosion-resistant rebars has a larger negative flat-band potential and thicker space charge layer than hot-ribbed rebars; thirdly, n-type semi-conductivity at a higher potential disappears once the chloride concentration at the rebar surface attains the chloride threshold value; and finally, a reverse charge layer forms on the surface of CR rebar at 0.50 V potential.

Multifunctional bandgap-reduced ZnO nanocrystals for photocatalysis, self-cleaning, and antibacterial glass surfaces

A set of ZnO nanocrystals assembly with tuneable reduced bandgaps was prepared via thermal decomposition of ZnO2 precursor. A detailed Raman and EPR analysis revealed ZnO rich in oxygen vacancies with concentration varying upon annealing temperature. The structural defect features corroborated with the bandgap variations indicate photocatalytic response in the visible-light, which was evaluated by using monochromatic LEDs (377, 401, and 429 nm wavelengths) for a correct assessment of the photocatalytic activities of samples in the close vicinities of their bandgaps. It was revealed, that bandgap reduced ZnO exhibits only little yet negligible photocatalytic activity towards Methylene Blue discoloration under 429 nm diode. The Photocatalytic experiments using scavengers support the model that oxygen vacancies easily trap photo-excited electrons (whether VO+ and VO++ state) and, if the energy level of this donor state is close enough, the trapped electron could easily thermalize to the conduction band as well as recombine with photo-excited holes. Furthermore, oxygen vacancies rich ZnO nanocrystals treated glasses were prepared and their photocatalytic-induced self-cleaning property, wettability, and antibacterial activities were evaluated under both UV and visible-light. Besides excellent antibacterial activities against both S. aureus and E. coli, which was achieved even in dark, both photocatalytically-induced self-cleaning ability and wettability provided yet another indirect evidence of the inability of bandgap-reduced ZnO to become activated under visible-light irradiation.

A Multilateral Donation that Boomerangs Home: Analysing the Donor State Advantage in UN Procurement

Bilateral aid procurement is politicised and strongly favours suppliers from donor countries. Does multilateral development assistance eliminate the procurement bias favouring donor countries because international bureaucrats make procurement decisions? Existing evidence from the World Bank, which delegates procurement responsibilities to aid recipient countries, cannot answer our theoretical question. Using official data from 20 UN organisations during the 2013–2018 period and applying regression and mediation analysis, we find that the procurement of international organisations still favours donor countries when international bureaucrats make procurement decisions. We identify donor state representation within the UN staff as a key stepping stone linking donation to procurement bias. In contrast, member states whose nationals are heads of a UN bureaucracy do not enjoy procurement advantage, suggesting that UN procurement bias operates through an informal bottom-up channel. Our paper contributes to the debates on the independence of international organisations in the context of multilateral development assistance and procurement.

(Keynote) Photoinduced Charge Transfer from Quantum Dots Measured By Cyclic Voltammetry

Measuring and modulating charge transfer processes at quantum dot interfaces are crucial steps in the development of quantum dot photocatalysis. In this work, cyclic voltammetry under illumination is demonstrated to measure the rate of photoinduced charge transfer from CdS quantum dots by directly probing the changing oxidation states of a library of model charge acceptors. The voltammetry data demonstrates the presence of long-lived electron donor states generated by native photodoping of the QDs as well as the relationship between driving force and rate of charge transfer. Changes to the voltammograms under illumination follow mechanistic predictions from classic zone diagrams and electrochemical modeling allows for measurement of the rate of productive electron transfer, giving rate constants (104 M-1s-1) that are distinct from the ps dynamics measured by conventional optical spectroscopy methods and that are more closely connected to the quantum yield of photoinduced chemical transformations.

Dynamics of the Energy Transfer Process in Eu(III) Complexes Containing Polydentate Ligands Based on Pyridine, Quinoline, and Isoquinoline as Chromophoric Antennae.

In this work, we investigated from a theoretical point of view the dynamics of the energy transfer process from the ligand to Eu(III) ion for 12 isomeric species originating from six different complexes differing by nature of the ligand and the total charge. The cationic complexes present the general formula [Eu(L)(H2O)2]+ (where L = bpcd2– = N,N′-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate; bQcd2– = N,N′-bis(2-quinolinmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate; and bisoQcd2– = N,N′-bis(2-isoquinolinmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate), while the neutral complexes present the Eu(L)(H2O)2 formula (where L = PyC3A3– = N-picolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate; QC3A3– = N-quinolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate; and isoQC3A3– = N-isoquinolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate). Time-dependent density functional theory (TD-DFT) calculations provided the energy of the ligand excited donor states, distances between donor and acceptor orbitals involved in the energy transfer mechanism (RL), spin-orbit coupling matrix elements, and excited-state reorganization energies. The intramolecular energy transfer (IET) rates for both singlet-triplet intersystem crossing and ligand-to-metal (and vice versa) involving a multitude of ligand and Eu(III) levels and the theoretical overall quantum yields (ϕovl) were calculated (the latter for the first time without the introduction of experimental parameters). This was achieved using a blend of DFT, Judd–Ofelt theory, IET theory, and rate equation modeling. Thanks to this study, for each isomeric species, the most efficient IET process feeding the Eu(III) excited state, its related physical mechanism (exchange interaction), and the reasons for a better or worse overall energy transfer efficiency (ηsens) in the different complexes were determined. The spectroscopically measured ϕovl values are in good agreement with the ones obtained theoretically in this work.

Microstructural effects on the thermoelectric performance of Ge0.962Bi0.038Te1.057

Thermoelectrics is a renewable energy method, capable of a direct conversion of waste heat into useful electricity, which can be applied for tackling global warming, high pollution levels and dilution of fossil energy sources. For this purpose novel and highly efficient thermoelectric compositions should be developed and analyzed under practical operation conditions. In the current research the effects of grain size and thermal treatment under operation conditions on the thermoelectric performance of Ge 0.962 Bi 0.038 Te 1.057 were investigated. Following hot-pressing both positive electronic and lattice effects were identified for sub-micron grained samples, enabling very high figure of merit, ZT , values of up to 1.8. These values were slightly degraded down to ≈ 1.4 following thermal treatment, mainly due to grain coarsening, highlighting the requirement of grain growth inhibition for maximizing the potential of this composition. • Nano-grained Ge 0.962 Bi 0.038 Te 1.057 was found to exhibit a very high ZT of 1.8 following hot-pressing. • This impressing ZT was attributed to both enhanced phonon scattering and electronic donor states at grain boundaries. • Reduction of ZT to 1.4 was noticed after thermal treatment, highlighting the requirement of grain growth inhibition

The Forthright Political Stance of Sport for Development in Korean Official Development Assistance

The rapid growth of Sport for Development (SFD) policies, practices, and organizations over the past two decades has been widely studied. However, by far the preponderance of attention has been focused on SFD initiatives and analyses concentrated in traditional Western donor states, where they have been characterized by a focus on social development objectives and civil society-led modalities. By contrast, SFD efforts emanating from non-Western countries have been relatively neglected. This paper contributes to correcting this imbalance by focusing on the important and distinctive Korean approach to SFD within its growing Official Development Assistance (ODA) programmes. In contrast to mainstream Western SFD, Korean SFD has been forthrightly oriented towards support for top-down, state-based objectives. Moreover, it has reflected a distinct hybrid character, blending a superficial prioritization of global social development norms exemplified by the United Nations Millennium Development Goals and Sustainable Development Goals with strong and persistent characteristics of the Korean developmental state in its approach to sport.

Unintentional doping effect in Si-doped MOCVD β-Ga2O3 films: Shallow donor states

High-quality β-Ga2O3 films were epitaxially grown by using metalorganic chemical vapor deposition (MOCVD) with different donor concentrations, and their shallow donor states were investigated by the temperature-dependent Hall measurement and secondary ion mass spectroscopy (SIMS) analysis. Two donor levels with ionization energies of ∼36 and ∼140 meV were extracted. It is found that the unintentional doping (UID) effects in MOCVD contribute substantially to both these two levels: the first donor level may come not only from silicon doping but also from unintentional substitution of carbon for Ga sites, and the second donor level probably comes from doubly charged defects related to unintentional H doping. By analyzing the relationship between the growth conditions and donor states, combined with density functional theory calculations, it is found that reducing the oxygen partial pressure during the growth might be a feasible way to reduce the UID effect. This work paves the way to the precise control of carrier density in Si-doped MOCVD β-Ga2O3 films.

The role of point defects related with carbon impurity on the kink of log J–V in GaN-on-Si epitaxial layers

Carbon impurity as point defects makes key impact on the leakage in GaN-on-Si structures. GaN-based epitaxial layers with different point defects by changing carbon-doped concentration were used to investigate the point defects behavior. It was found that leakage mechanisms correspond with space-charge-limited current models at low voltages, and after 1st kink, electron injection from silicon to GaN and PF conduction play a key role in the leakage of both point defects case with low carbon and high carbon doped. In addition, high carbon in GaN-on-Si epitaxial layers obtained lower leakage and larger breakdown voltage. The slope of log J–V has two kinks and effective energy barrier E a has two peaks, 0.4247 eV at about 300 V and 0.3485 eV at about 900 V, respectively, which is related to accepted states and donor states related with carbon impurity. While the slope of log J–V has one kink and effective energy barrier E a has one peak, 0.4794 eV at about 400 V of low carbon in GaN-on-Si epitaxial layers, indicating only field-induced accepted ionized makes impact on leakage. The comparative results of more donor trap density in high carbon indicate point defects related with carbon impurity play a key role in the kinks of log J–V slope.

Quantum rate efficiency of the charge transfer mediated by quantum capacitive states

It has been demonstrated that the transfer of electrons between donor and acceptor states is a room-temperature quantum mechanical event wherein quantum capacitive states per se determine the rate of the charge transfer. This analysis establishes quantum capacitance as a key concept governing the rate efficiency with which electrons are transferred between states. This rate efficiency has been particularly demonstrated using redox-active switches assembled over metallic electrodes, where the transfer of charge between the electrode and redox states occurs in a diffusionless regime. This analysis formed the basis for the quantum rate theory of electron transmittance, which predicts (and experimentally confirms) the existence of a limiting value for this charge transfer resistance that complies with the conductance quantum (a conductance constant value of ∼ 77.5 μS or resistance of ∼ 12.9 kΩ). In this study, we evaluated how the quantum rate concept applies to electrochemical reactions in which the transfer of electrons occurs between electrode and redox-free states in solution (electrolyte) mediated by quantum capacitive states within the interface. The quantum capacitive mediation of the electron transfer reaction demonstrates an improvement in electronic communication, with capacitive states effectively acting as a non-adiabatic bridge with a quantum efficiency enabling electrons to hop following a tunnelling mechanism. The quantum efficiency of electron transport surpasses the traditional diffusion-controlled transfer of electrons within a charge transfer resistance limit that complies with ∼ 12.9 kΩ, leading to a maximum electrode-mediated quantum rate efficiency. Applications of the concept allow us to design molecular interfaces with quantum mechanical efficiency for harvesting electrons from the solution phase to solid-state electrodes.

Electrical and optical properties of hydrogen plasma treated β-Ga2O3 thin films

The behavior of H in β-Ga2O3 is of substantial interest because it is a common residual impurity that is present in β-Ga2O3, regardless of the synthesis methods. Herein, we report the influences of H-plasma exposure on the electric and optical properties of the heteroepitaxial β-Ga2O3 thin films grown on sapphire substrates by chemical vapor deposition. The results indicate that the H incorporation leads to a significantly increased electrical conductivity, a greatly reduced defect-related photoluminescence emission, and a slightly enhanced transmittance, while it has little effect on the crystalline quality of the β-Ga2O3 films. The significant changes in the electrical and optical properties of β-Ga2O3 may originate from the formation of shallow donor states and the passivation of the defects by the incorporated H. Temperature dependent electrical properties of the H-incorporated β-Ga2O3 films are also investigated, and the dominant scattering mechanisms at various temperatures are discussed.

The barrier capacitance of n-SnS2/n-CdIn2Te4 heterojunction

The results of the study of the electrical characteristics of the n-SnS2/n-CdIn2Te4, obtained by a method of deposition over optical contact, have been presented. It is shown that the current-voltage characteristics and the capacitance-voltage (C-V) characteristics are typical for an abrupt heterojunction. The frequency dependence of the C-V characteristic is established, namely, with increasing frequency of alternating voltage, the barrier capacitance decreases, which is due to the presence of intrinsic structural defects and related localized donor states in the band gap of CdIn2Te4. The obtained results are explained by the frequency-dependent recharging processes of deep donor centers.

Computational study of hyperfine interaction for Zn substitute Ga in β-Ga2O3

By using a gap-optimized, Koopmans-compliant hybrid functional, we investigate Zn at Ga sites in β-Ga2O3. In this study, we found that besides acting as a deep acceptor, ZnGa can also trap another hole to form a donor state. Our calculated results show the Zn substitution at the octahedral Ga site (ZnGa2) is slightly more favorable than at tetrahedral site (ZnGa1). This result is in agreement with the previous experimental study [Journal of Applied Physics 129, 155701 (2021)]. At the neutral charge state of ZnGa, a hole is trapped at an adjacent tetrahedral O site in a small polaron state for both ZnGa1 and ZnGa2. The calculated hyperfine interaction between these trapped-holes and neighbors Ga atoms are in good agreement with the experimental measurements.

Photoinduced Charge Transfer from Quantum Dots Measured by Cyclic Voltammetry.

Measuring and modulating charge-transfer processes at quantum dot interfaces are crucial steps in developing quantum dots as photocatalysts. In this work, cyclic voltammetry under illumination is demonstrated to measure the rate of photoinduced charge transfer from CdS quantum dots by directly probing the changing oxidation states of a library of molecular charge acceptors, including both hole and electron acceptors. The voltammetry data demonstrate the presence of long-lived charge donor states generated by native photodoping of the quantum dots as well as a positive correlation between driving force and rate of charge transfer. Changes to the voltammograms under illumination follow mechanistic predictions from the ErCi' zone diagram, and electrochemical modeling allows for measurement of the rate of productive electron transfer. Observed rates for photoinduced charge transfer are on the order of 0.1 s-1, which are distinct from the picosecond dynamics measured by conventional transient optical spectroscopy methods and are more closely connected to the quantum yield of light-mediated chemical transformations.

A Theoretical Modelling of International Migration on the Explanation of the Nature and Motives of Labour Migrants From Ukraine to Poland

This article considers the concepts and theories of international migration which answer questions about the current trends of migration flows. Particular emphasis has been placed on the transformation of established perceptions of women’s migration as, under the influence of globalisation in the modern system of international coordinates, there is a trend that is interpreted as the “feminisation” of international migration. Designing theories and concepts of international migration on an explanation of the motives and specifi cs of the adaptation of Ukrainian labour migrants in Poland offers a basis upon which to argue that each of them is appropriate for application. In addition, a theoretical modelling of migration policy has been presented. In practice, national governments use such models of migration policy as: the complete exclusion of immigration into a country or profess a “closed-door policy”, differentiated exclusion/segregation, integration as a gradual and weaker form of assimilation, the so-called “selective policy”, and the assimilation, multicultural or “open-door policy”. Future scenarios for the modellingof migration flows represented by leading international organisations have been designed. The multidimensional MIPEX Index, according to which Poland’s policy on migrants is assessed as “equality on paper”, is lower than the EU average, and, with a score of an estimated 40 points out of a possible 100, has been used to assess the trends of modern migration policy of the Republic of Poland. The new migration model of Poland leans towards a model of a differentiated integration of migrants rather than a multicultural policy or assimilation. It has been concluded that migration policy should be based on the principle of “tripartism” scenarios (the ‘win-win-win’ concept) and benefit the donor state, the recipient state, and the migrants themselves.

Simple accurate model of silicon donor arrays

Donors in silicon can now be positioned with an accuracy of about one lattice constant, making it possible to form donor arrays for quantum computation or quantum simulation applications. However, the multivalley character of the silicon conduction band combines with central cell corrections to the donor states to convert small atomic scale imperfections in donor placement into strong interdonor hybridization disorder. We present a simple model that is able to account for central cell corrections accurately, and use it to assess the impact of donor positional disorder on donor array properties in both itinerant and localized limits. Our results show that donor arrays in silicon simulate strongly disordered one-dimensional electrons.