Intragap States Research Articles

Non-Invasive On-Chip Light Observation by Contactless Waveguide Conductivity Monitoring

Photonic technologies lack non-invasive monitoring tools to inspect the light inside optical waveguides. This is one of the main barriers to large scale integration, even though photonic platforms are potentially ready to host thousands of elements on a single chip. Here, we demonstrate non-invasive light observation in silicon photonics devices by exploiting photon interaction with intra-gap energy states localized at the waveguide surface. Light intensity is monitored by measuring the electric conductance of the silicon core through a capacitive access to the waveguide. The electric contacts are located at suitable distance from the waveguide core, thus introducing no measurable extra-photon absorption and a phase perturbation as low as 0.2 mrad, comparable to thermal fluctuations below 3 mK. Light monitoring with a sensitivity of -30 dBm and a dynamic range of 40 dB is demonstrated in waveguides and high-Q resonators, and for the tuning of coupled-resonator optical filters. This approach realizes a ContactLess Integrated Photonic Probe (CLIPP), that is simple, inherently CMOS compatible, non-invasive and scalable to hundreds of probing points per chip. The CLIPP concept provides a viable route to real-time conditioning and feedback control of densely-integrated photonic systems.

Realization of the population inversion in a conjugated polymer by a single or double stimulating pulse

By applying a single or double stimulating pulse to a conjugated polymer, we theoretically realize the population inversion between the generated intragap states. The simulations are performed within the framework of an extended version of the one-dimensional Su–Schrieffer–Heeger tight-binding model combined with a nonadiabatic evolution method. In discussions, we mainly compare the stimulating intensity thresholds between the two different pumped modes, at which the population inversion in polymer is reached. It is found that, the stimulating intensity threshold can be remarkably reduced by employing a double pulse rather than a single one. Effect of the lasting time for each pulse on the threshold is also discussed. We then propose a new optical-pumped mode to realize the population inversion in polymers, which is potentially of great importance for polymer lasers.

Confirmation of intrinsic electron gap states at nonpolar GaN(1-100) surfaces combining photoelectron and surface optical spectroscopy

The electronic structure of GaN(1–100) surfaces is investigated in-situ by photoelectron spectroscopy (PES) and reflection anisotropy spectroscopy (RAS). Occupied surface states 3.1 eV below the Fermi energy are observed by PES, accompanied by surface optical transitions found in RAS around 3.3 eV, i.e., below the bulk band gap. These results indicate that the GaN(1–100) surface band gap is smaller than the bulk one due to the existence of intra-gap states, in agreement with density functional theory calculations. Furthermore, the experiments demonstrate that RAS can be applied for optical surface studies of anisotropic crystals.

Colloidal Bi2S3Nanocrystals: Quantum Size Effects and Midgap States

Among solution‐processed nanocrystals containing environmentally benign elements, bismuth sulfide (Bi2S3) is a very promising n‐type semiconductor for solar energy conversion. Despite the prompt success in the fabrication of optoelectronic devices deploying Bi2S3nanocrystals, the limited understanding of electronic properties represents a hurdle for further materials developments. Here, two key materials science issues for light‐energy conversion are addressed: bandgap tunability via the quantum size effect, and photocarrier trapping. Nanocrystals are synthesized with controlled sizes varying from 3 to 30 nm. In this size range, bandgap tunability is found to be very small, a few tens of meV. First principles calculations show that a useful blueshift, in the range of hundreds of meV, is achieved in ultra‐small nanocrystals, below 1.5 nm in size. Similar conclusions are envisaged for the class of pnictide chalcogenides with a ribbon‐like structure [Pn4Ch6]n(Pn = Bi, Sb; Ch = S, Se). Time‐resolved differential transmission spectroscopy demonstrates that only photoexcited holes are quickly captured by intragap states. Photoexcitation dynamics are consistent with the scenario emerging in other metal–chalcogenide nanocrystals: traps are created in metal‐rich nanocrystal surfaces by incomplete passivation by long fatty acid ligands. In large nanocrystals, a lower bound to surface trap density of one trap every sixteen Bi2S3units is found.

Visible and Infra-red Light Emission in Boron-Doped Wurtzite Silicon Nanowires

Silicon, the mainstay semiconductor in microelectronic circuitry, is considered unsuitable for optoelectronic applications owing to its indirect electronic band gap, which limits its efficiency as a light emitter. Here we show the light emission properties of boron-doped wurtzite silicon nanowires measured by cathodoluminescence spectroscopy at room temperature. A visible emission, peaked above 1.5 eV, and a near infra-red emission at 0.8 eV correlate respectively to the direct transition at the Γ point and to the indirect band-gap of wurtzite silicon. We find additional intense emissions due to boron intra-gap states in the short wavelength infra-red range. We present the evolution of the light emission properties as function of the boron doping concentration and the growth temperature.

Dynamical study on the stimulated absorption and emission in a coujugated polymer

By applying a femtosecond electric pump pulse to a poly(p-phenylene vinylene) (PPV) molecule, we theoretically investigate the dynamical processes for its stimulated absorption and emission. The simulations are performed within the framework of an extended version of one-dimensional Su-Schrieffer-Heeger tight-binding model combined with a nonadiabatic evolution method. Firstly, we set the molecule initially lying in the ground state, by which we give the relation between different stimulated transition modes and the photoexciting pulse. Analysis of the final states shows that we can only obtain some electron-hole binding states by an external photoexcitation for the molecule, which includes exciton, biexciton, and high-energy exciton. We have calculated their yields and find that they are determined by the photoexciting energy. In addition, based on the experimental observations, we separately investigate the effect of the photoexciting intensity on the yields of biexciton and high-energy exciton. The calculated results are consistent with the corresponding experimental speculations. Finally, by setting the molecule lying in an exciton or a biexciton, we focus on the stimulated emission process between their generated intragap states. Effects of the photoexciting energy and intensity on them are separately analyzed. These results might be of great importance for further improving the optical applications of polymers, especially for optimizing the polymer photovoltaic and laser properties.

Electronic Theory of Ultrafast Spin Dynamics in NiO

Electronic Theory of Ultrafast Spin Dynamics in NiO NiO is a good candidate for ultrafast magnetic switching because of its large spin density, antiferromagnetic order, and clearly separated intragap states. In order to detect and monitor the switching dynamics, we develop a systematic approach to study optical second harmonic generation (SHG) in NiO, both at the (001) surface and in the bulk. In our calculations NiO is modeled as a doubly embedded cluster. All intragap d-states of the bulk and the (001) surface are obtained with highly-correlational quantum chemistry and propagated in time under the influence of a static magnetic field and a laser pulse. We find that demagnetization and switching can be best achieved in a subpicosecond regime with linearly rather than circularly polarized light. We also show the importance of including an external magnetic field in order to distinguish spin-up and spin-down states and the necessity of including magnetic-dipole transitions in order to realize the process in the centrosymmetric bulk. Having already shown the effects of phonons in the SHG for the bulk NiO within the frozen-phonon approximation, and following the same trail of thoughts, we discuss the role of phonons in a fully quantized picture as a symmetrylowering mechanism in the switching scenario and investigate the electronic and lattice temperature effects.

Dual-Color Electroluminescence from Dot-in-Bulk Nanocrystals

The emission color from colloidal semiconductor nanocrystals (NCs) is usually tuned through control of particle size, while multicolor emission is obtained by mixing NCs of different sizes within an emissive layer. Here, we demonstrate that recently introduced "dot-in-bulk" (DiB) nanocrystals can emit two-color light under both optical excitation and electrical injection. We show that the effective emission color can be controlled by adjusting the relative amplitudes of the core and shell emission bands via the intensity of optical excitation or applied bias in the cases of photoluminescence (PL) and electroluminescence (EL), respectively. To investigate the role of nonradiative carrier losses due to trapping at intragap states, we incorporate DiB NCs into functional light-emitting diodes and study their PL as a function of applied bias below the EL excitation threshold. We show that voltage-dependent changes in core and shell emissions are not due to the applied electric field but rather arise from the transfer of charges between the anode and the NC intragap trap sites. The changes in the occupancy of trap states can be described in terms of the raising (lowering) of the Fermi level for reverse (direct) bias. We find that the applied voltage affects the overall PL intensity primarily via the electron-trapping channel while bias-induced changes in hole-trapping play a less significant role, limited to a weak effect on core emission.

First Principle Study of Capping Energies and Electronic States in Stoichiometric and Nonstoichiometric PbSe Nanoclusters

A large variety of PbSe nanoclusters have been modeled at the DFT level, to study their structure, their affinity for different ligands and their electronic properties, also depending on surface passivation. The clusters are extracted from the bulk rock salt structure with cubic, prism, truncated cubic, cuboctahedral and octahedral shape and they are fully relaxed, before computing the addition energies of methylamine and formate anions in different positions, to model the process of surface passivation. Then the density of states of all the clusters is computed, to study in particular the band gap and the behavior of the so-called intragap states, which affect the photophysical properties of the nanoparticles, also acting as trap states for charge carriers. We confirm the strong relationship between nanocluster off-stoichiometry and intragap states: such states can be localized on the surface, in the bulk or delocalized over the nanoparticle, according to the source of off-stoichiometry. The ability of different ligands to eliminate the intragap states are tested and discussed, also proposing nonstandard capping molecules.

F-centre luminescence in nanocrystalline CeO2

Nanocrystalline CeO2 powders were synthesized by two cost-effective methods: the self-propagating room temperature (SPRT) method and the precipitation method. Differently prepared samples exhibited different temperature-dependent photoluminescence (PL) in the ultraviolet and visible regions. The PL signals originated from different kinds of oxygen-deficient defect centres with or without trapped electrons (F0, F+ or F++ centres). The temperature-dependent PL spectra were measured using different excitation lines, below (457, 488 and 514 nm) or comparable (325 nm) to the ceria optical band gap energy, in order to investigate the positions of intragap localized defect states. Evidence for the presence of F+ centres was supported by the signals observed in electron paramagnetic resonance (EPR) measurements. Based on PL and EPR measurements it was shown that F+ centres dominate in the CeO2 sample synthesized by the SPRT method, whereas F0 centres are the major defects in the CeO2 sample synthesized by the precipitation method. The luminescence from F++ states, as shallow trap states, was registered in both samples. Energy level positions of these defect states in the ceria band gap were proposed.

Possible Bose-condensate behavior in a quantum phase originating in a collective excitation in the chemically and optically doped Mott-Hubbard system UO2+x

X-ray pair distribution function (pdf) and U L${}_{3}$ extended x-ray absorption fine structure (EXAFS) and neutron pdf measurements that give identical results for UO${}_{2}$ show U(VI)-oxo moieties with x rays for mixed valence U${}_{4}$O${}_{9}$ and U${}_{3}$O${}_{7}$, in contrast to the neutron data that indicate only U(V) sites with no short U-O bonds as well as other differences. In addition, although the EXAFS spectra of UO${}_{2}$ are essentially identical at 30, 100, and 200 K, those of the UO${}_{2+x}$ compounds exhibit different nearest-neighbor U-O distributions at each temperature. Further tunneling polaron-type behavior is found in the broadening of the features of the O K-edge x-ray absorption spectra (XAS) of the UO${}_{2+x}$ compounds. Raman spectra of powders also show a large increase in scattering cross section with increasing O content that would originate in a change in the electronic structure that increases the overall polarizability. The XAS and Raman also show that U${}_{4}$O${}_{9}$ does not behave as a linear combination of the UO${}_{2}$ and U${}_{3}$O${}_{7}$ fluorite endpoints. The properties induced by mobile rather than static charged quasiparticles were explored by optical pumping of the metal-to-metal charge-transfer transition. The temperature dependence of 4.71 eV pump--1.57 eV probe reflectivity on UO${}_{2}$ that initially populates the U 6$d$-dominated portion of the upper Hubbard band (UHB) shows a sharp 28-\ensuremath{\mu}sec lifetime peak at 25 K that may be associated with the fluctuations of its antiferromagnetic transition. Pumping at 3.14 eV into the 5$f$-dominated portion of the UHB shows an analogous 2.8-\ensuremath{\mu}sec peak but also a plateau bracketing this peak that ends in a cusp at 50--60 K and an abrupt change in the hardening rate of a novel 12--15 GHz phonon that is the signature for the quasiparticle quantum phase. The different results for the different excitation channels indicate a highly specific nonthermal relaxation mechanism. These results constitute the first observation of a distinct phase of photoinduced quasiparticles that is sufficiently coupled to the lattice to undergo a gap-opening transition. When the intragap state is probed with a terahertz time domain spectroscopy (TTDS) measurement 33 psec after a 3.14 excitation pulse, it shows increased absorption in the 0.5--1.1 THz range with a decrease in temperature from \ensuremath{\sim}30 to 10 K instead of the expected decrease, a result consistent with the presence of a condensate. These results are too extreme to originate in the dynamical, nonadiabatic, coupled charge-transfer--phonon/tunneling polaron scenario previously used for doped Mott-Hubbard insulators with intermediate electron-phonon coupling and therefore indicate novel physics. One possibility that could cause all of these behaviors is that a collective, dynamical, charge transfer-coupled Peierls distortion involving the 2 U(V) \ensuremath{\leftrightarrow} U(IV)+U(VI)-oxo excitation occurs coherently over an entire domain to cause the atoms in this domain to condense into a system with Bose-Einstein or Bose-Einstein-Hubbard properties.

Magnetic Intragap States and Mixed Parity Pairing at the Edge of Spin-Triplet Superconductors

We show that a spontaneous magnetic moment may appear at the edge of a spin-triplet superconductor if the system allows for pairing in a subdominant channel. To unveil the microscopic mechanism behind such an effect, we combine numerical solution of the Bogoliubov-de Gennes equations for a tight-binding model with nearest-neighbor attraction, and the symmetry based Ginzburg-Landau approach. We find that a potential barrier modulating the electronic density near the edge of the system leads to a nonunitary superconducting state close to the boundary where spin-singlet pairing coexists with the dominant triplet superconducting order. We demonstrate that the spin polarization at the edge appears due to the inhomogeneity of the nonunitary state and originates in the lifting of the spin degeneracy of the Andreev bound states.

Contribution of Quasiparticles in the Subgap States to the Surface Impedance of Superconductors

The surface impedance of a superconductor has been calculated using the extended Mattis-Bardeen theory, in which the complex gap energy are taken into account. It is found that the surface resistance of the superconductor increases with increasing magnitude of the imaginary part of the gap energy. It is also found that the surface resistance of an NbN film calculated by the extended Mattis-Bardeen equation quantitatively agrees well with the measured one. It is shown that temperature dependence of not only values but also residual numbers of quasiparticles of superconducting resonators agree well with those calculated by the extended Mattis-Bardeen equation. It is also shown that phonon-assisted gradual relaxation of excess quasiparticles passing through the intragap states is dominant rather than the direct recombination into Cooper pairs.

Preventing Interfacial Recombination in Colloidal Quantum Dot Solar Cells by Doping the Metal Oxide

Recent research has pushed the efficiency of colloidal quantum dot solar cells toward a level that spurs commercial interest. Quantum dot/metal oxide bilayers form the most efficient colloidal quantum dot solar cells, and most studies have advanced the understanding of the quantum dot component. We study the interfacial recombination process in depleted heterojunction colloidal quantum dot (QD) solar cells formed with ZnO as the oxide by varying (i) the carrier concentration of the ZnO layer and (ii) the density of intragap recombination sites in the QD layer. We find that the open-circuit voltage and efficiency of PbS QD/ZnO devices can be improved by 50% upon doping of the ZnO with nitrogen to reduce its carrier concentration. In contrast, doping the ZnO did not change the performance of PbSe QD/ZnO solar cells. We use X-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, transient photovoltage decay measurements, transient absorption spectroscopy, and intensity-dependent photocurrent measurements to investigate the origin of this effect. We find a significant density of intragap states within the band gap of the PbS quantum dots. These states facilitate recombination at the PbS/ZnO interface, which can be suppressed by reducing the density of occupied states in the ZnO. For the PbSe QD/ZnO solar cells, where fewer intragap states are observed in the quantum dots, the interfacial recombination channel does not limit device performance. Our study sheds light on the mechanisms of interfacial recombination in colloidal quantum dot solar cells and emphasizes the influence of quantum dot intragap states and metal oxide properties on this loss pathway.

Low-bias electron transport properties of germanium telluride ultrathin films

The nanometer-scale size-dependent electronic transport properties of crystalline (c-) and amorphous (a-) germanium telluride (GeTe) ultrathin films sandwiched by titanium nitride (TiN) electrodes are investigated using ab initio molecular dynamics, density functional theory, and Green's function calculations. We find that a-GeTe ultrathin films scaled down to about 38 Å (12 atomic layers) still show a band gap and the electrical conductance is mainly due to electron transport via intra-gap states. If the ultrathin films are further scaled, the a-GeTe band gap disappears due to overlap of the two metal induced gap states (MIGS) regions near the TiN electrodes, leading to sharp increase of a-GeTe conductance and significant decrease of c-GeTe/a-GeTe conductance ratio. The c-GeTe/a-GeTe conductance ratio drops below one order of magnitude if the ultrathin films are scaled below about 33 Å, making it difficult to reliably perform read operations in thin film based phase change memory devices. This overlap of the MIGS regions sets up the ultimate scaling limit of phase change memory technology. Our results suggest that the ultimate scaling limit can be pushed to even smaller size, by using phase change material with larger amorphous phase band gap than a-GeTe.

Depth-resolved cathodoluminescence spectroscopy of silicon supersaturated with sulfur

We investigate the luminescence of Si supersaturated with S (Si:S) using depth-resolved cathodoluminescence spectroscopy and secondary ion mass spectroscopy as the S concentration is varied over 2 orders of magnitude (1018–1020 cm−3). In single-crystalline supersaturated Si:S, we identify strong luminescence from intra-gap states related to Si self-interstitials and a S-related luminescence at 0.85 eV, both of which show a strong dependence on S concentration in the supersaturated regime. Sufficiently high S concentrations in Si (>1020 cm−3) result in complete luminescence quenching, which we propose is a consequence of the overlapping of the defect band and conduction band.

Zero-Bias Anomaly in a Nanowire Quantum Dot Coupled to Superconductors

We studied the low-energy states of spin-1/2 quantum dots defined in InAs/InP nanowires and coupled to aluminum superconducting leads. By varying the superconducting gap Δ with a magnetic field B we investigated the transition from strong coupling Δ << T(K) to weak-coupling Δ >> T(K), where T(K) is the Kondo temperature. Below the critical field, we observe a persisting zero-bias Kondo resonance that vanishes only for low B or higher temperatures, leaving the room to more robust subgap structures at bias voltages between Δ and 2Δ. For strong and approximately symmetric tunnel couplings, a Josephson supercurrent is observed in addition to the Kondo peak. We ascribe the coexistence of a Kondo resonance and a superconducting gap to a significant density of intragap quasiparticle states, and the finite-bias subgap structures to tunneling through Shiba states. Our results, supported by numerical calculations, own relevance also in relation to tunnel-spectroscopy experiments aiming at the observation of Majorana fermions in hybrid nanostructures.

Fermi level shift and doping efficiency inp-doped small molecule organic semiconductors: A photoelectron spectroscopy and theoretical study

We study the mechanism of molecular doping of the organic small molecule N,N,N${}^{\ensuremath{'}}$,N${}^{\ensuremath{'}}$-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) doped with the fluorinated fullerene ${\mathrm{C}}_{60}{\mathrm{F}}_{36}$ or the acceptor molecule 2,2${}^{\ensuremath{'}}$-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ). Varying the doping concentration, photoemission spectroscopy measurements show a comparable Fermi level shift for both dopants. The doping efficiency, defined as the ratio of free charge carriers (holes) to acceptors, is estimated from the depletion layer thickness in metal/intrinsic/$p$-doped structures. For low concentrations, we observe rather high doping efficiencies of up to 36$%$ for ${\mathrm{C}}_{60}{\mathrm{F}}_{36}$, whereas for both dopants the doping efficiency strongly decreases with increasing doping concentration down to less than 10$%$. By numerically solving the charge neutrality equation using a classical semiconductor physics approach and comparing the results to the ultraviolet photoelectron spectroscopy data, we show that for very low concentrations doping is hindered by deep intragap states. In particular, the calculations can statistically explain the strong decrease of the doping efficiency for high doping concentrations.

Tuning Radiative Recombination in Cu-Doped Nanocrystals via Electrochemical Control of Surface Trapping

The incorporation of copper dopants into II-VI colloidal nanocrystals (NCs) leads to the introduction of intragap electronic states and the development of a new emission feature due to an optical transition which couples the NC conduction band to the Cu-ion state. The mechanism underlying Cu-related emission and specifically the factors that control the branching between the intrinsic and impurity-related emission channels remain unclear. Here, we address this problem by conducting spectro-electrochemical measurements on Cu-doped core/shell ZnSe/CdSe NCs. These measurements indicate that the distribution of photoluminescence (PL) intensity between the intrinsic and the impurity bands as well as the overall PL efficiency can be controlled by varying the occupancy of surface defect sites. Specifically, by activating hole traps under negative electrochemical potential (the Fermi level is raised), we can enhance the Cu band at the expense of band-edge emission, which is consistent with the predominant Cu(2+) character of the dopant ions. Furthermore, we observe an overall PL "brightening" under negative potential and "dimming" under positive potential, which we attribute to changes in the occupancy of the electron trap sites (that is, the degree of their electronic passivation) that control nonradiative losses due to electron surface trapping.

Contribution of Co3+ ions to the high-temperature magnetic and electrical properties of GdCoO3

The temperature dependence of the static magnetization of polycrystalline rare-earth cobaltite GdCoO3 is measured in the temperature range 2–800 K. The magnetic behaviors of GdCoO3 and Gd3+ are found to be different at temperatures above room temperature, which is caused by the appearance of a contribution from Co3+ ions at high temperatures. The temperature dependence of the magnetic susceptibility of GdCoO3 is determined by the magnetization of rare-earth gadolinium ions and the additional paramagnetic contribution induced by the thermally excited magnetic terms of Co3+ ions. The LDA + GTB method is used to calculate the electronic structure of GdCoO3 in the temperature range 0–300 K with allowance for strong electron correlations. The energy spectrum of GdCoO3 is found to have intragap states that decrease the dielectric gap width with increasing temperature.