Shallow Gap States Research Articles

Deep and Shallow Gap States in Reduced and n-Type Doped m-ZrO2

Deep and Shallow Gap States in Reduced and n-Type Doped <i>m</i>-ZrO₂

Carrier trapping centers in a two-dimensional silica bilayer: Strongly localized shallow gap states and resonances induced by oxygen vacancies

Structural and electronic properties of neutral native defects in a two-dimensional ${\mathrm{SiO}}_{2}$ bilayer (2D-${\mathrm{SiO}}_{2}$) are examined using the SIESTA ab initio approach. We identify scissor and rotation modes of oxygen atoms, in the middle of Si-O-Si chains, as low-energy structural excitations responsible for the response of the 2D-${\mathrm{SiO}}_{2}$ lattice to the formation of all native defects in the present study. Furthermore, we find that oxygen native defects (single vacancies and interstitials) should be the most abundant defect species in thermal-equilibrated samples. Regarding native-defect electronic states, single oxygen vacancies and interstitials are found to be amphoteric trapping centers in 2D-${\mathrm{SiO}}_{2}$. Silicon vacancies and interstitials introduce several strongly localized states spanning a large fraction of the gap. Generally, we identify a marked tendency for the appearance of strongly spatially localized defect states, be their energies shallow or deep within the band gap, as well as the emergence of strongly localized resonances in the valence and/or conduction bands. Strong spatial localization of defect states, a hallmark of systems displaying trapping and polaronic effects, is a result of quantum confinement and enhanced Coulombic effects in this 2D system. Carrier trapping and polaron formation are thus expected to be a common feature of defect states and added carriers in 2D-${\mathrm{SiO}}_{2}$.

Unprecedentedly Large Photocurrents in Colloidal PbS Quantum-Dot Solar Cells Enabled by Atomic Layer Deposition of Zinc Oxide Electron Buffer Layer

Due to the excitonic nature, colloidal PbS quantum-dot solar cells have suffered from lower photocurrent densities than expected from the absorber band gap. The heterojunction between solution-processed ZnO and PbS quantum-dots has been predominantly explored for photovoltaic applications. However, the deeper conduction band minimum of typical PbS quantum-dots than that of solution-processed ZnO imposes a high electron barrier, limiting the short-circuit current densities of the resulting solar cells mostly below 30 mA/cm2. Here, we report that atomic layer deposition (ALD) of ZnO buffer at a low temperature can favor the interfacial band alignment and boost the photocurrent density over 35 mA/cm2 at PbS quantum-dot band gap of 1.18 eV. From our band structure analysis, the electron barrier with ALD-ZnO can be 0.55 eV lower compared to that with sol–gel ZnO. Furthermore, photoactivation of shallow gap states formed by hydroxyl species in ALD-ZnO induces band bending and efficient electron tunneling from PbS to ZnO. Due to the improved band alignment, the device with ALD-ZnO exhibits a significantly enhanced lifetime compared to that with sol–gel ZnO upon constant illumination at 1-sun.

TiO2 Nanotubes Synthesis with Dominant {001} Exposed Facets for Efficient Photocatalytic and Photoelectrochemical Water Oxidation Applications

The photoreactivity of TiO2 nanotube arrays (TiNT) is mainly related to the light absorption and photogenerated charge carriers separation efficiencies. Recently, engineering of the crystal facets has gained increasing attention because of its exceptional ability to extend the photoresponse toward visible light and to separate photogenerated charges. Here, TiNT with high crystallinity and percentage of exposed {001} facets was synthesized using a two-step anodization process in ethylene glycol based electrolyte containing 2 wt% of water and 0.27 M NH4F. Decreasing the potential of the second anodization (from 50 to 20 V) resulted in an increase in crystallinity, crystallite size, and percentage of exposed {001} facets. Because trap states limit the photoreactivity performance of TiNT by facilitating the recombination of photogenerated electron/hole pairs, correlation between crystallinity, preferential crystalline orientation, gap states characteristics (density and location), and photoreactivity performance was investigated. TiNT with a high percentage of {001} exposed facets, consisting mainly of shallow gap states with low density, significantly improve photoelectrochemical water oxidation and photocatalytic efficiencies.

Core properties and mobility of the basal screw dislocation in wurtzite GaN: a density functional theory study

We have performed first principles simulations, based on density functional theory (DFT), to investigate the core properties of the basal a-type screw dislocation in wurtzite gallium nitride. Our calculations demonstrate that the fully coordinated shuffle core configuration is the most energetically favourable. The calculated electronic structure of the a-type screw dislocation was found to exhibit exclusively shallow gap states which are not associated with any extended metallization. This may explain why a-type screw dislocations are less detrimental to the performance of GaN based electronic devices than c-type screw dislocations.

Electronic structure of threading dislocations in wurtzite GaN

AbstractWe have carried out atomistic simulations, based on density functional theory, to investigate the atomic and electronic structures of the three types of prismatic threading dislocations in hexagonal gallium nitride. Screw and mixed threading dislocations were demonstrated to introduce both deep and shallow gap states, while most of core configurations of the edge dislocation introduce solely shallow states. The higher electrical activity of both screw and mixed dislocations, compared to the edge one, is correlated with the high dispersion of their energy levels, within the GaN bandgap. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Deep and Shallow TiO2 Gap States on Cleaved Anatase Single Crystal (101) Surfaces, Nanocrystalline Anatase Films, and ALD Titania Ante and Post Annealing

Using photoelectron spectroscopy (PES), deep (around 1.2 eV below Fermi level) and shallow (around 0.2 eV below Fermi level) gap states are investigated in differently prepared TiO2 samples: In situ cleaved single anatase crystal TiO2 (101) surface, sintered slurry of nanocrystalline anatase, amorphous atomic layer deposited (ALD) titania, and nanocrystalline anatase transformed by annealing from ALD titania. Deep gap states are generally attributed to under-coordinated Ti atoms due to oxygen defects. The origin of shallow gap states is unclear. PES on in situ cleaved anatase TiO2 (101) surfaces show in part no or weak emission from deep, but always weak emission from shallow gap states. Amorphous ALD titania initially is free of gap states, but deep gap states are easily induced by exposure to synchrotron radiation, while shallow gap states do not form. Exposure to synchrotron radiation also induced deep gap states in in situ cleaved single crystal (101) surfaces and in the nanoporous anatase films, wher...

Engineering of the energetic structure of the anode of organic photovoltaic devices utilizing hot-wire deposited transition metal oxide layers

In this work we use hydrogen deposited molybdenum and tungsten oxides (chemically described as H:MOxx≤3 where M=Mo or W) to control the energetics at the anode of bulk heterojunction (BHJ) organic photovoltaics (OPVs) based on poly(3-hexylthiophene):[6,6]-phenyl butyric acid methyl ester (P3HT:PC71BM) blends. Significantly improved current densities and open circuit voltages were achieved as a result of improved hole transport from the P3HT highest occupied molecular orbital (HOMO) toward indium tin oxide (ITO) anode. This was attributed to the formation of shallow gap states in these oxides which are located just below the Fermi level and above the polymer HOMO and thus may act as a barrier-free path for the extraction of holes. Consequently, these states can be used for controlling the energetic structure of the anode of OPVs. By using ultraviolet photoelectron spectroscopy it was found that dependent on the deposition conditions these gap states and work function of the metal oxides may be tailored to contribute to the precise alignment of the HOMO of the organic semiconductor (OSC) with the Fermi level of the anode electrode resulting in further enhancement of the device performance.

Electronic structure and metallization effects at threading dislocation cores in GaN

We have carried out a complete and consistent study on core configurations of the three types of threading dislocations in wurtzite GaN. Their atomic and local electronic structures were analyzed and systematically compared, at the level of Density Functional Theory. Screw and mixed threading dislocations were found to introduce both deep and shallow gap states, while most of core configurations of the edge dislocation introduce only shallow states. We demonstrated that the existence of an extended one-dimensional metallization, associated with unoccupied gap states, is a specific feature of threading screw dislocations. The extended metallization along with the high dispersion of the energy gap states are at the origin of the experimentally observed high electric conduction along threading screw dislocations in GaN. The presence of nitrogen vacancies in the core of screw dislocations is predicted to enhance their electric conduction.

Electronic properties of anatase-TiO 2 codoped by cation-pairs from hybrid density functional theory calculations

We have attempted to modulate the band gap of anatase-titania using hybrid density functional theory (DFT) calculations to examine cation codoping at two Ti sites by Al with either Nb or Ta, and by Cu with either Mo or W to achieve significant gap narrowing via creation of shallow gap states, without creating recombination centres. Cu- and Mo/W-doped systems exhibit larger gap-narrowing than those of Al- and Nb/Ta-codoped TiO 2 owing to strong d–d repulsion in the former two cases. At the same time, codoping serves to decrease transparent conducting behavior relative to monodoping. Upon increasing the doping concentration, the gap-narrowing mechanisms and transparent conducting characteristics do not change.

N-Type Transition Metal Oxide as a Hole Extraction Layer in PbS Quantum Dot Solar Cells

The n-type transition metal oxides (TMO) consisting of molybdenum oxide (MoO(x)) and vanadium oxide (V(2)O(x)) are used as an efficient hole extraction layer (HEL) in heterojunction ZnO/PbS quantum dot solar cells (QDSC). A 4.4% NREL-certified device based on the MoO(x) HEL is reported with Al as the back contact material, representing a more than 65% efficiency improvement compared with the case of Au contacting the PbS quantum dot (QD) layer directly. We find the acting mechanism of the hole extraction layer to be a dipole formed at the MoO(x) and PbS interface enhancing band bending to allow efficient hole extraction from the valence band of the PbS layer by MoO(x). The carrier transport to the metal anode is likely enhanced through shallow gap states in the MoO(x) layer.

Dependence of mobility on shallow localized gap states in single-crystal organic field-effect-transistors

Abstract In order to optimize the performance of molecular organic electronic devices it is important to study the intermolecular density of states and charge transport mechanisms in the environment of crystalline organic material. Using this approach in Field Effect Transistors (FETs) we show that material purification improves carrier mobility and decreases density of the deep localized electronic state. We also report a general exponential energy dependence of the density of localized states in a vicinity of the mobility edge (Fermi energies up to ∼7 times higher than the thermal energy ( kT )) in a variety of the extensively purified molecular organic crystal FETs. This observation and the low activation energy of the order of ∼ kT suggest that molecular structural misplacements of the sizes that are comparable with thermal molecular modes rather than impurity deep traps play a role in formation of these shallow states. We find that the charge carrier mobility in the FET nanochannels, μ eff , is parameterized by two factors, the free-carrier mobility, μ 0 , and the ratio of the free carrier density to the total carrier density induced by gate bias. Crystalline FETs fabricated from rubrene, pentacene, and tetracene have a high free-carrier mobility, μ 0 ∼50 cm 2 /Vs, at 300 K with lower device μ eff dominated by localized shallow gap states. This relationship suggests that further improvements in electronic performance could be possible with enhanced device quality.

Investigation of the atomic core structure of the ($ \vec a + \vec c $)‐mixed dislocation in wurtzite GaN

AbstractBy using a tight‐binding based ab‐initio method, we have investigated the atomic and electronic core structures of the threading ($ \vec a + \vec c $)‐mixed dislocation in wurtzite gallium nitride. For this dislocation, two core configurations were found to occur: a 5/7‐atoms ring and a complex double 5/6‐atoms ring configurations. Both core configurations were found to induce in the bandgap as well deep as shallow gap states. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Interaction of Oxygen with Threading Dislocations in GaN

A review is given of the results of first principles calculations used to investigate the structures and electronic properties of screw and edge dislocations in GaN. The atoms at the core of the full core screw dislocation possess heavily strained bonds leading to deep gap states. Removing the first shell of Ga and N atoms gives a screw dislocation with a small open core consisting of {101-0} type surfaces. Therefore open-core screw dislocations induce only shallow gap states. In the same way we found the core of the threading edge dislocation to be reconstructed without any deep states. The interaction of oxygen with the cores of open-core screw and edge dislocations is considered and it is found that the impurity has a strong tendency to be bound by Ga vacancies leading to three types of defect trapped in the strain field. We suggest that the most stable defect leads to a poisoning of growth centres on the walls of nanopipes.

Interaction of Oxygen with Threading Dislocations in GaN

A review is given of the results of first principles calculations used to investigate the structures and electronic properties of screw and edge dislocations in GaN. The atoms at the core of the full core screw dislocation possess heavily strained bonds leading to deep gap states. Removing the first shell of Ga and N atoms gives a screw dislocation with a small open core consisting of {1010} type surfaces. Therefore open-core screw dislocations induce only shallow gap states. In the same way we found the core of the threading edge dislocation to be reconstructed without any deep states. The interaction of oxygen with the cores of open-core screw and edge dislocations is considered and it is found that the impurity has a strong tendency to be bound by Ga vacancies leading to three types of defect trapped in the strain field. We suggest that the most stable defect leads to a poisoning of growth centres on the walls of nanopipes.

Gap states of polaron and associated optical absorption in the quasi-one-dimensional solid (Pt(en)2)(PtCl2(en)2)(ClO4)4

The energy spectrum and the optical absorption coefficient alpha ( omega ) of a hole polaron in a PtCl chain are numerically calculated on the basis of the Baeriswyl-Bishop model. In intermediate coupling, there appear some shallow gap states as well as deep gap states, both of which give contributions to the absorption. The three-band structure of the absorption in the near- and mid-infrared region is consistent with the experimental data on (Pt(en)2)(PtCl2(en)2)(ClO4)4. The theory also shows that in the far-infrared region there is another absorption near 500 cm-1, which is associated with the shallow gap states.

Signal formation in a-Si:H particle detectors

The response of a hydrogenated amorphous silicon thin film p-i-n diode to protons is presented as a function of the applied bias. A typical pulse shape is also presented. These data are explained on the basis of a model whose main features are fast (<5 ns) electron collection and slow (a few microseconds) hole collection due to multiple trapping transport through the shallow gap states. The overall charge normalization yields a mean pair creation energy in /sub p/ of 3.4-4.4 eV, comparable to 3.63 eV in crystalline silicon despite the larger 1.7-eV gap. To explain this result, a microscopic Monte Carlo calculation, taking into account the actual density of states in a-Si:H, is used to study the energy sharing between ionization and phonon production during hot carrier thermalization. This simulation yields a value of 4.3 eV for in /sub p/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Role of hydrogen in doping mechanism for a-Si:H alloys

Calculations of electronic energies for structural clusters simulating phosphorus and boron substitutional doped and undoped pure and hydrogenated silicon have been performed using CNDO/2 quantum chemical molecular orbital method, respectively. The results show that more energies for implanting impurities are required in hydrogenated Si cluster than that in pure Si cluster, and a band of shallow gap states is induced by the substitutional doping in the hydrogenated system. From the results it is concluded that the hydrogenation is a key factor of efficient substitutional doping in the a-Si:H alloys.

The effect of Te alloying on the electronic gap of a-Se

The effect on the electronic density of gap states in a-Se produced by the isoelectronic additive Te has been studied by a combination of drift mobility and xerographic measurements. Drift mobility experiments probe the shallow gap states which control photoinjected carrier transport. Addition of Te to Se in the nominal range 0–18 wt.% progressively decreases both electron and hole drift mobility while simultaneously increasing dispersion. The number and distribution of deep gap states which control injected carrier range have been determined in the same films from analysis of xerographic residual potentials. Taken together these measurements show that progressive addition of Te to a-Se increases the integrated number of deep traps and broadens the distribution of both deep and shallow transport interactive electronic gap states.