Induced Band Bending Research Articles

Synergistic photoelectrochemical detection of Cd2+ with carbon-bismuth oxybromide composites: Mechanistic insights from DFT calculations

The existences of hazardous heavy metals like cadmium (Cd) in water bodies necessitates rapid, accurate on-site detection strategies. Herein, a novel photoelectrochemical scheme for sensitive detection of Cd2+ was proposed based on the hybrid carbon-bismuth oxybromide materials (C/Bi4O5Br2 and CNTs/BiOBr). The effects of ultraviolet light (UV-light) illumination during the Cd2+ deposition and desorption stages were analyzed, revealing enhanced photocurrent responses and detection sensitivity compared with those non-illumiantion. The optimal sensitivity of 0.42079 μA/ppb (C/Bi4O5Br2 under both deposition and desorption stages) and 0.15634 μA/ppb (CNTs/BiOBr under desorption stage only) were observed with UV illumination within a Cd2+ range of 10–100 ppb. Density functional theory calculation elucidated that Cd2+ adsorption on the semiconductor surface reduced the badgaps of Bi4O5Br2 and BiOBr, which would benefit electron transfer and redox reactions. Additionally, the interaction between Cd and the semiconductor produced a Schottky barrier, which prolonged the lifetime of the photo-generated electron-hole pairs, where the electrons migrated on the semiconductor surface formed electron capture traps and induced band bending, therefore facilitated the electrons in the conduction band to react with Cd2+ deposited on the electrode surface. The study provides mechanistic insights into the photoelectrochemical processes in enhancng precise and sensitive on-site heavy metal ions detection.

Bi2O3@MoS2 heterojunction for enhanced photoelectrocatalytic hydrogen evolution

A Z-type Bi2O3@MoS2 heterojunction was successfully synthesized using a hydrothermal method. Bi2O3 microflowers served as a growth support for MoS2. The Bi2O3@MoS2-10 heterojunction demonstrated optimal photoelectrocatalytic performance as a result of adjusting the loading of MoS2. It exhibited excellent electrocatalytic hydrogen evolution performance, with an overpotential of only 223 mV vs. RHE. This value is approximately half that of pure MoS2 and one-third of Bi2O3. Under illumination, the overpotential for the hydrogen evolution reaction of the Bi2O3@MoS2-10 heterojunction decreased by ∼ 35 mV when the current density reached −10 mA cm−2, compared to dark conditions. The improved performance can be attributed to the growth of MoS2 on the Bi2O3 support, which prevents the aggregation of MoS2 nanosheets and results in a layered structure with a large surface area. Furthermore, the introduction of Bi2O3 as a co-catalyst lowers the bandgap energy of the heterojunction, facilitating efficient separation and rapid transfer of photogenerated electrons and holes. As a result, this growth induces band bending near the heterojunction interface, leading to the rearrangement of energy levels and the redistribution of charges through the interaction between Bi2O3 and MoS2. Moreover, photogenerated electrons can efficiently transfer to the heterojunction surface, participating in the reduction of H+/H3O+ ions to produce H2. This enhances the photoelectrocatalytic hydrogen evolution performance of the Bi2O3@MoS2-10 heterojunction.

PH-Dependent photocatalytic performance of faceted BiOBr semiconductor particles in degradation of dyes

Bismuth oxyhalides, such as bismuth-oxy-bromide (BiOBr), show high performance in photocatalytic oxidation of water pollutants. Process conditions, and in particular the pH of the solution, largely affect the photocatalytic efficacy, which is fundamentally poorly understood. We prepared {001} faceted bismuth-oxy-bromide (BiOBr), and determined by advanced AFM analysis that the surface charge of the {001} facet of BiOBr is slightly positive at acidic pH (pH 3) and significantly negative and increasing in negative surface charge in the order pH 6 < pH 9. Decomposition of MB or RhB by illumination of BiOBr is strongly favored by basic conditions (pH 9-10), as evident from discoloration experiments, and determination of the TOC as a function of time of illumination. Reference experiments show the pH dependent degradation profiles cannot be explained by differences in quantities of adsorption of the dye, but rather by the lifetime and/or quantity of BiOBr related photoexcited (surface) charge carriers (holes and electrons) – correlating with surface charge. Using MeOH and other scavengers, we show oxidation of MB or RhB is mainly induced by superoxide anions (•O2− radicals) at low pH and by holes or hydroxyl radicals at high pH – in agreement with surface charge induced band bending. This study provides novel understanding of the pH dependence of the rates of photocatalytic degradation of dyes using BiOBr photocatalysts.

The insight into the critical role of photoexcitation in manipulating charge carrier migration in piezo-photocatalytic S-scheme heterojunction

Piezo-photocatalytic S-scheme heterojunctions, consisting of a piezoelectric N-type semiconductor and a conventional N-type semiconductor, have garnered substantial interest due to their capacity for facilitating charge carrier separation and transport within piezo-photocatalytic processes. However, previous studies on the piezo-photocatalytic of S-scheme heterojunctions primarily focus on the coupling mechanism between piezoelectricity and semiconductor properties, aimed at manipulating charge carrier migration. This leaves scant attention to the modulation mechanism induced by photoexcitation. In this work, we aim to investigate the photoexcitation-induced modulation mechanisms in piezo-photocatalytic S-scheme heterojunctions of ZnO/Cs2AgBiBr6 nanorod array (ZC NRA) for the first time, using a light-tailoring irradiation strategy. ZnO is a rational choice for constructing the heterojunctions due to its wide band gap (3.23 eV), which allows the photoexcitation state to be modulated by UV irradiation. The degradation kinetic constant of optimized ZC NRA under simulated sunlight irradiation with ZnO excitation is 2.52 times higher than that under visible light (λ > 420 nm) without ZnO excitation. The piezo-polarization induced band bending structures that underlie the enhanced mechanisms of piezo-photocatalytic S-scheme heterojunctions are switched with the presence or absence of ZnO photoexcitation. This study reveals the photoexcitation-induced modulation mechanisms in piezo-photocatalytic S-scheme heterojunctions, complementing the neglected piece in the puzzle of piezo-phototronics effect. It also paves the way for the use of lead-free halide double perovskite in piezo-photocatalytic conversion of solar and mechanical energy.

Sub-5 nm Contacts and Induced p-n Junction Formation in Individual Atomically Precise Graphene Nanoribbons.

This paper demonstrates the fabrication of nanometer-scale metal contacts on individual graphene nanoribbons (GNRs) and the use of these contacts to control the electronic character of the GNRs. We demonstrate the use of a low-voltage direct-write STM-based process to pattern sub-5 nm metallic hafnium diboride (HfB2) contacts directly on top of single GNRs in an ultrahigh-vacuum scanning tunneling microscope (UHV-STM), with all the fabrication performed on a technologically relevant semiconductor silicon substrate. Scanning tunneling spectroscopy (STS) data not only verify the expected metallic and semiconducting character of the contacts and GNR, respectively, but also show induced band bending and p-n junction formation in the GNR due to the metal-GNR work function difference. Contact engineering with different work function metals obviates the need to create GNRs with different characteristics by complex chemical doping. This is a demonstration of the successful fabrication of precise metal contacts and local p-n junction formation on single GNRs.

Direct Evidence for Sulfur-Induced Deep Electron and Hole Traps in Titania and Implications for Photochemistry

Numerous studies show that sulfating titania narrows its band gap, facilitating longer-wavelength photochemistry, but there are contradictory reports on whether this improves or degrades UV photocatalysis and whether reactions proceed via electron- or hole-mediated pathways. The widely proposed role of sulfur is that it induces deep electron traps, which increase hole lifetimes. There is, however, no direct evidence for unoccupied states deep in the band gap. By contrast, transient absorption spectroscopy indicates that sulfur induces hole traps. We present experiments on sulfur-free and sulfated titania in which dissociation of hydrogen generates electrons that fill the lowest unoccupied states. The energy of these electrons relative to the conduction band minimum (CBM) was measured with diffuse reflectance spectroscopy in the infrared and UV–vis ranges. For all commercial sulfur-containing anatase materials, conversion of tridentate sulfate species into sulfur substituted on lattice sites occurred under highly oxidizing conditions above 400 °C and led to partially unoccupied states ∼2.8 eV below the CBM. We assign this deep trap state to sulfur atoms substituted on a titanium lattice site with a formal charge of S5+ in non-stoichiometric TiO2+x, based on agreement between the experiment and the predicted UV–vis spectrum of Harb, Sautet, and Raybaud, using HSE06 density functional perturbation theory. Our band structure calculations demonstrate that titanium vacancies (or excess oxygen) are necessary to create partially unoccupied states, and X-ray diffraction Rietveld analysis confirms the existence of these vacancies. The partial occupancy of these states, along with sulfur’s ability to switch oxidation states, explains their role as both electron (S5+ + e– → S4+) and hole (S5+ + h+ → S6+) traps, reconciling previous work. We discuss how relative rates of electron vs hole trapping can enhance or degrade activity depending on the pathway and the TiO2+x non-stoichiometry. We consider how increasing the dopant concentration can induce band bending or pin the Fermi level and shift the redox reactions that are thermodynamically accessible.

Revealing and Eliminating the Light‐Soaking Issue in Metal Oxide‐Based Inverted Organic Solar Cells

AbstractInverted organic solar cells (i‐OSCs) provide an exciting opportunity for commercialization owing to their excellent device air stability. However, light soaking (LS) issue generally occurs in metal oxide based i‐OSCs, causing drastically decreased performance. The underlying root of LS effect is not clearly clarified until now. Herein, it is demonstrated that the surface oxygen defects on metal oxide nanoparticles, such as chemisorbed superoxide (O2−) and hydroxide (OH) dangling bonds, are the main reasons for LS issue in i‐OSCs. The O2− layer induces band bending at the cathode interface and increases the work function (WF) of metal oxide, thus leading to inefficient charge transport. The dangling bonds serve as interfacial trap states and cause non‐radiative recombination, thus leading to the reduced open circuit voltage (Voc). With ultraviolet (UV) illumination, the surface oxygen defects are interacted with photogenerated carriers, thereby improving the photovoltaic performance. Additionally, UV pretreatment of metal oxide films is employed to eliminate the LS issue and the resulting device yields significantly improved fill factors from 50.20% to 73.50% in the pristine SnO2 based i‐OSCs. This study reveals the origin of LS effect in i‐OSCs and proposes a suggested model for LS mechanism.

Effect of ground state charge transfer and photoinduced charge separation on the energy level alignment at metal halide perovskite/organic charge transport layer interfaces

One of the most promising routes for the future fabrication of solution-processable high-performance solar cells is to employ metal halide perovskites as photoactive material combined with organic semiconductors as charge extraction layers. An essential requirement to obtain high device performance is a proper energy level alignment across the device interfaces. Here, we investigate the interface between a triple cation perovskite and a prototypical electron acceptor molecule. Photoemission spectroscopy reveals a ground state charge transfer induced band bending on either side of the junction, which significantly alters the charge extraction barriers as compared to assumed vacuum level alignment and flat-band conditions. In addition, we demonstrate that upon white light illumination, the energy levels of the organic layer exhibit rigid shifts by up to 0.26 eV with respect to those of the perovskite, revealing a non-constant energy offset between the frontier energy levels of the two materials. Such level shifts of the organic transport layer are fully reversible upon switching on/off the light, indicating an electrostatic origin of this phenomenon caused by unbalanced distribution of photogenerated charge carriers. We therefore stress the importance of determining the energy level alignment at perovskite-based interfaces not only in the electronic ground state (dark) but also under device operating conditions (operando) to enable for a reliable correlation with the device performance.

Exploiting multiferroicity of TbFeO3 nanoparticles for hydrogen generation through photo/electro/photoelectro-catalytic water splitting

Hydrogen is a potential future energy source that could replace conventional fuel and provide the necessary energy. Multiferroic materials are the most likely choices for water splitting due to their ferroelectric characteristics and ability to function as magnetically recoverable catalysts. Multiferroic terbium orthoferrite nanoparticles were synthesized at low temperature by the polymeric citrate precursor route to study the photocatalytic, electrocatalytic and photoelectrochemical activity towards hydrogen production. Powder X-ray diffraction revealed successful formation of orthorhombic TbFeO3 nanoparticles. A larger aspect ratio of 3.9 and a bandgap of 2.13 eV were seen in the elongated TbFeO3 nanoparticles, which contributes to the photo/electro-catalytic activity. Magnetic and ferroelectric studies revealed weak ferromagnetism and ferroelectric polarization of 0.037 μC cm−2 in TbFeO3 nanoparticles, confirming multiferroicity. Visibly active and multiferroic TbFeO3 nanoparticles showed notable hydrogen evolution of 1.44 mmol h−1 g−1. In electrocatalytic and photoelectrochemical water splitting investigations, TbFeO3 nanoparticles demonstrated current densities of 30 and 60 mA cm−2, respectively. EIS, TRPL, transient photocurrent and Mott-schottky measurements were used to examine charge transfer kinetics. The high H2 evolution and good OER/HER tests were attributed to increased charger separation efficiency due to ferroelectricity induced band bending.

Persistent Ion Accumulation at Interfaces Improves the Performance of Perovskite Solar Cells.

The mixed ionic–electronic nature of lead halide perovskites makes their performance in solar cells complex in nature. Ion migration is often associated with negative impacts—such as hysteresis or device degradation—leading to significant efforts to suppress ionic movement in perovskite solar cells. In this work, we demonstrate that ion trapping at the perovskite/electron transport layer interface induces band bending, thus increasing the built-in potential and open-circuit voltage of the device. Quantum chemical calculations reveal that iodine interstitials are stabilized at that interface, effectively trapping them at a remarkably high density of ∼1021 cm–3 which causes the band bending. Despite the presence of this high density of ionic defects, the electronic structure calculations show no sub-band-gap states (electronic traps) are formed due to a pronounced perovskite lattice reorganization. Our work demonstrates that ionic traps can have a positive impact on device performance of perovskite solar cells.

When Flexoelectricity Drives Triboelectricity.

Triboelectricity has been known since antiquity, but the fundamental science underlying this phenomenon lacks consensus. We present a flexoelectric model for triboelectricity where contact deformation induced band bending at the nanoscale is the driving force for charge transfer. This framework is combined with first-principles and finite element calculations to explore charge transfer implications for different contact geometry and materials combinations. We demonstrate that our ab initio based formulation is compatible with existing empirical models and experimental observations including charge transfer between similar materials and size/pressure dependencies associated with triboelectricity.

Defects, band bending and ionization rings in MoS2

Chalcogen vacancies in transition metal dichalcogenides are widely acknowledged as both donor dopants and as a source of disorder. The electronic structure of sulphur vacancies in MoS2 however is still controversial, with discrepancies in the literature pertaining to the origin of the in-gap features observed via scanning tunneling spectroscopy (STS) on single sulphur vacancies. Here we use a combination of scanning tunnelling microscopy and STS to study embedded sulphur vacancies in bulk MoS2 crystals. We observe spectroscopic features dispersing in real space and in energy, which we interpret as tip position- and bias-dependent ionization of the sulphur vacancy donor due to tip induced band bending. The observations indicate that care must be taken in interpreting defect spectra as reflecting in-gap density of states, and may explain discrepancies in the literature.

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.

Synergetic surface charge transfer doping and passivation toward high efficient and stable perovskite solar cells

SummaryOrganic-inorganic lead halide perovskite solar cells (PSCs) have received much attention in the last few years due to the high power conversion efficiency (PCE). Generally, perovskite/charge transport layer interface and the defects at the surface and grain boundaries of perovskite film are important factors for the efficiency and stability of PSCs. Herein, we employ an extended benzopentafulvalenes compound (FDC-2-5Cl) with electron-withdrawing pentachlorophenyl group and favorable energy level as charge transfer molecule to treat the perovskite surface. The FDC-2-5Cl with pentachlorophenyl group could accept the electrons from perovskite as a p-type dopant, and passivate the surface defects. The p-type doping effect of FDC-2-5Cl on perovskite surface induced band bending at perovskite surface, which improves the hole extraction from perovskite. As a result, the PSC with FDC-2-5Cl treatment achieves a PCE of 21.16% with an enhanced open-circuit voltage (Voc) of 1.14 V and outstanding long-term stability.

Theoretical explanation of scanning tunneling spectrum of cleaved (110) surface of InGaAs

The cross-sectional (110) surface of In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As/InP hetero-structure grown by molecular beam epitaxy on an InP (001) substrate is characterized by the cross-sectional scanning tunneling microscopy (XSTM). The cleaved (110) surface across the interface between the In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As layer and InP layer is atomically flat but displays slight different image contrast between the two neighbor regions. The scanning tunneling spectroscopy (STS) is used to measure the current/voltage (&lt;i&gt;I-V&lt;/i&gt;) spectra. The &lt;i&gt;I-V&lt;/i&gt; data of the InGaAs surface and InP (110) surface show the different characteristics. The voltage range of zero-current plateau (apparent band gap) in the &lt;i&gt;I-V&lt;/i&gt; spectrum of InP displays the values close to its energy band gaps whereas the plateau ranges in the spectra of In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As are by contrast generally 50% larger than the energy band gap of In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As. The above phenomenon implies the different physical pictures on the tunneling of two surfaces. In the case of InP, the flat band model is feasible since the band edge states existing in the InP (110) surface can prevent the surface from being affected by the tip –induced band bending (TIBB) effect. In contrast, the TIBB effect must be taken into account to explain the &lt;i&gt;I-V&lt;/i&gt; spectra of the In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As (110) surface. A statistical analysis of the &lt;i&gt;I-V&lt;/i&gt; data of In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As reveals that the width of current plateau in the &lt;i&gt;I-V&lt;/i&gt; spectrum is generally between 1.05 eV and 1.20 eV and the current onset points (turn-points) with the plateau for the different spectra are slightly different from each other. We are able to explain quantitatively the above features based on the three-dimensional TIBB model given by Feenstra (&lt;ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1116/1.1606466"&gt;2003 &lt;i&gt;J.&lt;/i&gt; &lt;i&gt;Vac. Sci. Technol. B&lt;/i&gt; &lt;b&gt;21&lt;/b&gt; 2080&lt;/ext-link&gt;). Our calculation reveals that the parameter of density of surface states (DOSS) is a sensitive parameter responsible for the &lt;i&gt;I-V&lt;/i&gt; features mentioned above. According to an appropriate assignment of the value of DOSS, which is generally taken in the scope of (0.8–3.0) × 10&lt;sup&gt;12&lt;/sup&gt; (cm&lt;sup&gt;2&lt;/sup&gt;·eV)&lt;sup&gt;–1&lt;/sup&gt;, we well predict both the width and the onset points of the current-plateau. Moreover, the model also reproduces the line-shapes of the &lt;i&gt;I-V&lt;/i&gt; spectra measured on In&lt;sub&gt;0.53&lt;/sub&gt;Ga&lt;sub&gt;0.47&lt;/sub&gt;As.

Origin of the tunable carrier selectivity of atomic-layer-deposited TiOx nanolayers in crystalline silicon solar cells

Titanium oxide (TiOx) nanolayers grown by atomic layer deposition are investigated with respect to their application as carrier selective contacts for crystalline silicon (c-Si) solar cells. Although TiOx is known to act as an electron contact, in this work the selectivity of TiOx layers is found to be widely tunable from electron to hole selective depending on deposition conditions, post-deposition treatments, and work function of the metal electrode used. Using TiOx and an intrinsic hydrogenated amorphous silicon buffer layer, solar cell test structure exhibiting open-circuit voltages (Voc) as high as 720 and 650 mV are shown for electron and hole selective contacts, respectively. Surface photovoltage and capacitance-voltage measurements reveal that carrier selectivity is correlated with the amount of c-Si band bending induced by TiOx, which are governed not only by the effective work function difference at the Si/TiOx interface, but also by the negative fixed charge present in the TiOx layer. This new finding is in contrast to the previous model for carrier transport where selectivity is determined only by the asymmetric band offsets at the Si/contact interface. It highlights the influence of induced band bending to produce carrier depletion/inversion conditions, and the importance of its selectivity effect in a c-Si absorber.

Edge induced band bending in van der Waals heterojunctions: A first principle study

The dangling bond free nature of two-dimensional (2D) material surface/interface makes van der Waals (vdW) heterostructure attractive for novel electronic and optoelectronic applications. But in practice, edge is unavoidable and could cause band bending at 2D material edge analog to surface/interface band bending in conventional three-dimensional (3D) materials. Here, we report a first principle simulation on edge band bending of free standing MoS2/WS2 vdW heterojunction. Due to the imbalance charges at edge, S terminated edge causes upward band bending while Mo/W terminated induces downward bending in undoped case. The edge band bending is comparable to band gap and could obviously harm electronic and optoelectronic properties. We also investigate the edge band bending of electrostatic doped heterojunction. N doping raises the edge band whereas p doping causes a decline of edge band. Heavy n doping even reverses the downward edge band bending at Mo/W terminated edge. In contrast, heavy p doping doesn’t invert the upward bending to downward. Comparing with former experiments, the expected band gap narrowing introduced by interlayer potential gradient at edge is not observed in our free-standing structures and suggests substrate’s important role in this imbalance charge induced phenomenon.

Direct Observation of Conductive Polymer Induced Inversion Layer in n‐Si and Correlation to Solar Cell Performance

AbstractHeterojunctions formed by ultrathin conductive polymer [poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate)—PEDOT:PSS] films and n‐type crystalline silicon are investigated by photoelectron spectroscopy. Large shifts of Si 2p core levels upon PEDOT:PSS deposition provide evidence that a dopant‐free p–n junction, i.e., an inversion layer, is formed within Si. Among the investigated PEDOT:PSS formulations, the largest induced band bending within Si (0.71 eV) is found for PH1000 (high PEDOT content) combined with a wetting agent and the solvent additive dimethyl sulfoxide (DMSO). Without DMSO, the induced band bending is reduced, as is also the case with a PEDOT:PSS formulation with higher PSS content. The interfacial energy level alignment correlates well with the characteristics of PEDOT:PSS/n‐Si solar cells, where high polymer conductivity and sufficient Si‐passivation are also required to achieve high power conversion efficiency.

Space charge control of point defect spin states in AlN

One barrier to developing quantum information systems based on impurity point defects is that the desirable spin states of the defects are often unstable for Fermi levels obtained at increased impurity concentrations. The space charge induced band bending near the interface of Si/Mg aluminum nitride (AlN) homojunction is investigated computationally as a method to control the concentration, spin state, and position of such point defects. This is done by solving Poisson's equation with the charge density described by a grand canonical defect chemistry model informed by hybrid-functional density functional theory (DFT) calculations. Previous experimental works have found unintentional carbon and oxygen impurities pervade AlN homojunctions. First principles calculations have predicted the neutral complex between an aluminum vacancy and oxygen impurity on a neighboring nitrogen site (vAl-1ON)0 has a spin triplet configuration, which is stable in a region when the Fermi level is below midgap. From defect equilibrium simulations considering 602 possible defects, vAl-1ON was found to be unstable on the Mg-doped side of the homojunction and isolated oxygen impurities are preferred. On the Si-doped side, vAl-1ON forms but as (vAl-1ON)–2, not (vAl-1ON)0. This makes vAl-1ON a prototypical test case for the proposed strategy. Simulations of the Si/Mg:AlN homojunction showed (vAl-1ON)0 is stabilized within 6 nm of the interface in the Si-doped portion. This result indicates space charge induced band bending enables control over the concentration, spin state, and position of point defects, which is critical to realizing point defect based quantum information systems.

Polar axis oriented crystal growth induced carrier transport characteristics in non-degenerate zinc oxide thin films grown on glass substrates

Abstract The predominance of the methanol, ethanol, and 2-methoxyethanol solvents, with different boiling points and dielectric constants, are observed on the crystal growth orientation, optical behavior, carrier recombination, and carrier transport mechanism in sol-gel spin coated ZnO thin films. Multi-axis oriented crystal growth orientation was observed for methanol processed ZnO film as compared to that along c-axis only while using other solvents. The intensity of c-axis orientation, crystallite size as well as radiative recombination luminescence effects follows a direct (or inverse) relationship with the boiling point (or dielectric constant) of the involved solvent. Diverse nature of stress was seen for the films. Ideal band gap value (3.36 eV) was identified for 2-methoxyethanol processed oxygen defects induced stress free ZnO thin films. Time resolved photoluminescence (TRPL) spectra reveal a significant contribution of ultrafast and fast decay components in ethanol and methanol processed films as compared to the dominance of slow decay components in 2-methoxyethanol processed thin film. The temperature dependent electrical resistivity measurements unfold the coexistence of metallic and semiconducting behavior in all films. Dominant optical phonon scattering and piezoelectric scattering mechanisms provide metallic behavior to the films as confirmed by an increasing resistivity with increased temperature. The dominant grain boundary scattering mechanism controls the mobility of the carriers and results in the semiconducting behavior in all films at different values of higher temperatures. We have quantified the crystallite size induced band bending effect to understand the applicability of ZnO films in optoelectronic devices as well.