Passivation Of Surface States Research Articles

Effects of Oxygen Annealing of β-Ga2O3 Epilayers on the Properties of Vertical Schottky Barrier Diodes

Electrical characteristics of vertical Schottky barrier diodes (SBDs) fabricated on as-grown and oxygen annealed β-Ga2O3 (001) epilayers were investigated. SBDs on as-grown epilayer showed anomalous reverse leakage characteristics. Annealing of β-Ga2O3 epilayers in an oxygen-containing environment up to 40 min immensely reduced the reverse leakage current. The specific on-resistance (Ron) of the SBD remained very close to that of the as-grown sample for annealing up to 20 min and increased almost by 25 times for annealing up to 40 min. Simulations of reverse tunneling characteristics, assuming oxygen vacancy type surface states, explained both the magnitude and the shape of anomalous reverse leakage. The diffusion of oxygen during annealing passivated the oxygen vacancy type surface states at first (for 20 min annealing)-resulting in two orders of reduction in leakage with minimal change in Ron. Further annealing (up to 40 min) subsequently reduced the epilayer net carrier concentration from 3.3 × 1016 to 2.9 × 1015 cm−3 -resulting in immense change in both reverse leakage and Ron. Thus, oxygen annealing proved to be a vital technique for the passivation of surface states and to reduce the net carrier concentration, which allows the modulation of reverse leakage of β-Ga2O3 SBDs.

Passivating Surface States on Water Splitting Cuprous Oxide Photocatalyst with Bismuth Decoration.

To enhance the visible light photocatalystic activity of Cu2O(100) surface, we performed first-principles calculations on the structural, electronic and optical properties of a bismuth (Bi)-decorated Cu2O(100) surface (Bi@Cu2O(100)). It is shown that the Bi prefer to be loaded to the hollow sites among four surface oxygen atoms and tend to individual dispersion instead of aggregating on the surface due to the lowest formation energy and larger distance between two Bi atoms at the surface than the Bi clusters; the coverage of around 0.25 monolayer Bi atoms can effectively eliminate the surface states and modify the band edges to satisfy the angular momentum selection rules for light excited transition of electrons, and the loaded Bi atoms contribute to the separation of photogenerated electron-holes. The relative positions between the band edges and the redox potentials are suitable for photocatalytic hydrogen production from the redox water, and moreover, the optical absorption spectrum indicates a positive response of the Bi0.25@Cu2O(100) to visible light, implying that the Bi0.25@Cu2O(100) is a promising visible light photocatalyst.

Investigation on the impact of hydrogen on the passivation of silicon surface states in clean and copper contaminated conditions

Hydrogen is identified as a useful technique to passivate defects within crystalline silicon. However, the effect of hydrogen passivation for a silicon surface is normally characterized as a reduction in surface recombination velocity (SRV), which is not enough to reflect the detailed changes of electronic properties, such as defect density, defect energy levels, and capture cross section, of silicon surface states. In this paper, we utilized the transient capacitance measurement to characterize the detailed electronic properties of silicon surface states before and after hydrogenation. The differences, in terms of the effects of hydrogenation on silicon surface states, either in copper contaminated conditions or clean conditions, are presented and discussed.

Integrating photocatalysis and thermocatalysis to enable efficient CO2 reforming of methane on Pt supported CeO2 with Zn doping and atomic layer deposited MgO overcoating

CO2 reforming or dry reforming of methane (DRM) produces syngas with a low carbon footprint, but the efficiency and stability of DRM remains a challenge. Herein, we report an efficient photo-thermo-chemical DRM (PTC-DRM) process on a Pt supported CeO2 catalyst with Zn doping and surface atomic layer deposition (ALD)-enabled MgO overcoating using concentrated sunlight as the energy input. Under 30 suns irradiation at 600 °C, high syngas production rates of 356 and 516 mmol g−1 h−1 for H2 and CO are achieved, which are more than 9 and 3 times larger than those obtained in the thermally driven DRM. Moreover, the light illumination stabilizes the dry reforming process without deactivation, which results from the in situ generation of oxygen vacancy on CeO2 by photo-induced electrons that enables stable CO2 thermo-activation. The ALD coating also reduces surface charge recombination through passivating surface states, thereby enhancing photocatalytic activity.

Orange/Red Photoluminescence Enhancement Upon SF6 Plasma Treatment of Vertically Aligned ZnO Nanorods.

Although the origin and possible mechanisms for green and yellow emission from different zinc oxide (ZnO) forms have been extensively investigated, the same for red/orange PL emission from ZnO nanorods (nR) remains largely unaddressed. In this work, vertically aligned zinc oxide nanorods arrays (ZnO nR) were produced using hydrothermal process followed by plasma treatment in argon/sulfur hexafluoride (Ar/SF6) gas mixture for different time. The annealed samples were highly crystalline with ~45 nm crystallite size, (002) preferred orientation, and a relatively low strain value of 1.45 × 10−3, as determined from X-ray diffraction pattern. As compared to as-deposited ZnO nR, the plasma treatment under certain conditions demonstrated enhancement in the room temperature photoluminescence (PL) emission intensity, in the visible orange/red spectral regime, by a factor of 2. The PL intensity enhancement induced by SF6 plasma treatment may be attributed to surface chemistry modification as confirmed by X-ray photoelectron spectroscopy (XPS) studies. Several factors including presence of hydroxyl group on the ZnO surface, increased oxygen level in the ZnO lattice (OL), generation of F–OH and F–Zn bonds and passivation of surface states and bulk defects are considered to be active towards red/orange emission in the PL spectrum. The PL spectra were deconvoluted into component Gaussian sub-peaks representing transitions from conduction-band minimum (CBM) to oxygen interstitials (Oi) and CBM to oxygen vacancies (VO) with corresponding photon energies of 2.21 and 1.90 eV, respectively. The optimum plasma treatment route for ZnO nanostructures with resulting enhancement in the PL emission offers strong potential for photonic applications such as visible wavelength phosphors.

Hydrogen Peroxide Assisted Synthesis of Highly Luminescent Sulfur Quantum Dots

AbstractAn H2O2‐assisted top‐down approach is used to synthesize brightly luminescent, color‐tunable sulfur quantum dots (SQDs), with a photoluminescence quantum yield of up to 23 %. The formation of SQDs involves dissolution of bulk sulfur powder into small particles in an alkaline environment in the presence of polyethylene glycol, followed by H2O2‐assisted etching of polysulfide species, which has the advantage of the passivation of surface states. This synthetic strategy allows us to simultaneously control the final size of SQDs, to tune their emission color, and to improve their emission quantum yield by eliminating surface traps. Down‐conversion white light emitting diodes were also fabricated using blue emissive SQDs and orange emissive copper nanoclusters, with CIE color coordinates of (0.33, 0.32) and a high color rendering index of 91. The water‐soluble, highly luminescent SQDs are promising luminescent materials that can be produced from abundant precursor materials.

Hydrogen Peroxide Assisted Synthesis of Highly Luminescent Sulfur Quantum Dots.

An H2 O2 -assisted top-down approach is used to synthesize brightly luminescent, color-tunable sulfur quantum dots (SQDs), with a photoluminescence quantum yield of up to 23 %. The formation of SQDs involves dissolution of bulk sulfur powder into small particles in an alkaline environment in the presence of polyethylene glycol, followed by H2 O2 -assisted etching of polysulfide species, which has the advantage of the passivation of surface states. This synthetic strategy allows us to simultaneously control the final size of SQDs, to tune their emission color, and to improve their emission quantum yield by eliminating surface traps. Down-conversion white light emitting diodes were also fabricated using blue emissive SQDs and orange emissive copper nanoclusters, with CIE color coordinates of (0.33, 0.32) and a high color rendering index of 91. The water-soluble, highly luminescent SQDs are promising luminescent materials that can be produced from abundant precursor materials.

Effect of tetravalent ions dopants and CoO surface modification on hematite nanorod array for photoelectrochemical degradation of Orange-II dye

Abstract The surface modified tetravalent ions doped hematite nanorod photoanodes has been synthesized by successive hydrothermal and spin coating approaches. The effect of tetravalent ion (Sn, Ti, and Zr) doping and high temperature annealing on to the morphological, structural and photoelectrochemical properties has been investigated. Photoelectrochemical analyses indicate tetravalent doping (Sn, Ti, and Zr) and high-temperature annealing (800 °C) showed higher photocurrent and improved activity towards Orange-II dye degradation, compared to the pristine hematite film. Amongst tetravalent dopants, Zr4+ doped hematite can significantly enhance the photocurrent density (1.37 mA cm−2 at 1.23 V vs. RHE) as well as the Orange-II dye degradation activity (93% under one sun illumination in 270 min). Kinetic parameters are also investigated by first-order rate equation. Further, Zr–Fe2O3 photoanode modified with the appropriate composition of CoOx and specific structural features demonstrated improvement in photocurrent from 1.6 to 1.78 mA cm−2 (at 1.4 V vs. RHE). The efficient charge carrier separation and generated hydroxyl radicals led to enhancements of the Orange-II dye degradation efficiency up to 97% within 270 min. The significant decrease in chemical oxygen demand values suggests the removal of Orange-II dye. This work demonstrates that enhancement in photoelectrochemical activity is due to the combined effect of passivation of surface states and the formation of CoOx/Zr–Fe2O3 heterojunction.

Surface passivation of InGaN nanorods using H3PO4 treatment for enhanced photoelectrochemical performance

Due to the high density of surface states introduced in the nanorods growth process and subsequent exposure to air, electrons will be trapped in these states and recombine with photo-generated holes, which leads to a high loss of photo-generated carriers and creates a large onset potential for carrier transport. Here, we report a simple and low-cost surface treatment method, which utilizes H3PO4 to passivate surface states in the as-grown InGaN nanorods. Using optimized surface passivation condition, surface trapping states in as-grown InGaN nanorods can be removed by eliminating surface Indium aggregation, reducing the surface oxides, and saturating the dangling bonds. Hence, carrier recombination mediated by these surface states is weakened, as well as applied bias potential required for carrier transport is lowered. The optimized H3PO4-treated nanorods photoanode shows a larger photovoltage and a smaller charge transfer resistance to the electrolyte. Resultantly, a significant negative shift of onset potential from 0.7 to 0.3 V vs. RHE for the treated photoanode is achieved. Moreover, the optimized H3PO4-treated photoanode exhibits a high photocurrent density of 18 mA/cm2 at 1.23 V vs. RHE and a maximum applied bias photon-to-current efficiency value of 1.09%, whereas as-grown InGaN photoanode reaches only 0.64%.

Wet-Chemical Synthesis of Surface-Passivated Halide Perovskite Microwires for Improved Optoelectronic Performance and Stability.

One-dimensional (1D) halide perovskite materials with intrinsic high carrier mobility and long diffusion length hold great promises for high-performance optoelectronic devices, in which the passivation of the surface defects is of significance for further boosting its optoelectronic performance as well as its moisture stability. Herein, we demonstrate a simple room-temperature wet-chemical synthetic protocol for perovskite microwires with controlled morphologies and passivated surface states. This strategy allows for facile assembly of hydrophobic 1 H,1 H-perfluorooctylamine (PFA) molecules on the surface of the perovskite microwires owing to the coordination binding between the amino groups of PFA and Pb2+. Both steady and time-resolved photoluminescence measurements revealed that the passivation of PFA greatly benefit for the inhibition of the photogenerated carriers recombination. The constructed perovskite microwire-based photodetectors have shown increased detectivity of 4.99 × 1011 jones and responsivity of 1.27 A/W (light power density of 1 mW/cm2). Moreover, the hydrophobic fluorocarbon alkyl chains endow the perovskite microwires with higher resistance toward moisture. Such coating of a water-resisting layer resulted in greatly enhanced stability of perovskite microwires under the humidity of 55 ± 5% over 30 days. We thus believe that our work is of importance for the development of 1D halide perovskite photodetectors with highly improved performance and stability.

Realizing high performance solar water oxidation for Ti-doped hematite nanoarrays by synergistic decoration with ultrathin cobalt-iron phosphate nanolayers

Hematite (α-Fe2O3) is one of the most promising photoanode materials for solar-to-hydrogen (STH) energy conversion by photoelectrochemical (PEC) water splitting. However, the practical application of hematite in PEC water splitting is severely hindered by the intrinsically low charge separation efficiency and sluggish water oxidation kinetics. Here, we reported Ti-doped hematite photoanode nanoarrays decorated with cobalt-iron phosphate (CoFePi) ultrathin nanolayers greatly enhanced bulk and surface charge separation efficiency, as well as passivate surface states for promoting the PEC water oxidation activity. The synergistic interactions of CoFePi nanolayers with the Ti-Fe2O3 photoanode yield current densities of 1.75 mA/cm2 and 2.15 mA/cm2 in 0.1 M KOH and 1 M KOH respectively at 1.23 VRHE under AM 1.5 G illumination, which is 2.3 and 3.1 fold higher than bare hematite photoanode, and also even higher than Ti-Fe2O3 and Ti-Fe2O3 coated with cobalt phosphate (CoPi), iron phosphate (FePi), phosphate (Pi), respectively. A low turn-on voltage of 0.68 VRHE and high incident photon-to-electron conversion efficiency of 46.6% (IPCE, 1.23 VRHE, λ = 340 nm) are achieved with the Ti-Fe2O3/CoFePi photoanode. Collectively, the results demonstrated that the synergistic Ti-Fe2O3/CoFePi photoanode is promising for PEC water oxidation for hydrogen production.

Simultaneously improving solar water oxidation kinetics and passivating surface states of hematite by loading an amorphous Ni doped cobalt phosphate layer

High surface states density of hematite photoanodes results in their low water oxidation kinetics and high surface electrons-holes recombination. To overcome these inherent drawbacks, various methods have been adopted, especially loading oxygen evolution catalysts and depositing oxide passivation layers. We report here an efficient way to promote the photocurrent of Fe2O3 photoanodes via depositing a thin layer of Co0.84Ni0.16-Pi. With the Co0.84Ni0.16-Pi deposit on its surface, the photocurrent density of Fe2O3 increases by ca. 42% at 0.23 V vs. Ag/AgCl, and the onset potential shifts 200 mV cathodically. In contrast, Co-Pi@Fe2O3 photoanode shows only 20% enhancement in photocurrent density under otherwise identical condition. The dark current densities of the photoanodes give an evidence that both Co0.84Ni0.16-Pi and Co-Pi are good oxygen evolution catalysts. Moreover, different from sparsely distributed Co-Pi nanoparticles, a 2–3 nm amorphous Ni doped cobalt phosphate layer can be also an effective passivation layer for surface states of hematite, which has been demonstrated by the analyses of Mott-Schottky plots and electrochemical impedance spectroscopy. This work demonstrates the dual roles of an amorphous oxygen evolution co-catalyst on hematite photoanodes and provides a simple method for designing highly efficient photoanodes.

Surface State Passivation and Optical Properties Investigation of GaSb via Nitrogen Plasma Treatment.

GaSb is one of the most suitable semiconductors for optoelectronic devices operating in the mid-infrared range. However, the existence of GaSb surface states has dramatically limited the performance of these devices. Herein, a controllable nitrogen passivation approach is proposed for GaSb. The surface states and optical properties of GaSb were found to depend on the N passivation conditions. Varying the plasma power during passivation modified the chemical bonds of the GaSb surface, which influenced the emission efficiency. X-ray photoelectron spectroscopy was used to quantitatively demonstrate that the GaSb oxide layer was removed via treatment at a plasma power of 100 W. After nitrogen passivation, the samples exhibited enhanced emission. Free exciton emission was the main factor leading to this enhanced luminescence. An energy band model for the surface states is used to explain the carrier radiative recombination processes. This nitrogen passivation approach can suppress surface states and improve the surface quality of GaSb-based materials and devices. The enhancement in exciton-related emission by this simple approach is important for improving the performance of GaSb-based optoelectronic devices.

Passivation of Surface States of AlGaN Nanowires Using H3PO4 Treatment To Enhance the Performance of UV-LEDs and Photoanodes

Surface states serve as additional charge-carrier-trapping centers and create an energy barrier at the semiconductor–electrolyte interface. This in turn may severely reduce the internal quantum efficiency of AlxGa1–xN nanowire ultraviolet light-emitting diodes (UV-LEDs) and solar-to-hydrogen energy conversion efficiency of photoelectrodes used in photoelectrochemical water splitting applications. These states also cause Fermi-level pinning and band bending, leading to Shockley–Read–Hall nonradiative recombination. Hence, surface states need to be passivated. In the present study, we used phosphoric acid to passivate the surface states in AlGaN nanowires. The internal quantum efficiency of the near-band-edge emission peak of the chemically treated nanowires was 7%, whereas that of the as-grown nanowires was 3%. Suppression of the oxide layers was achieved, as indicated by the reduced intensity of the O 1s peak. The higher carrier lifetime of 3.2 ns of the treated nanowires compared to the lifetime of 2.6 n...

Optical absorption and oxygen passivation of surface states in III-nitride photonic devices

III-nitride surface states are expected to impact high surface-to-volume ratio devices, such as nano- and micro-wire light-emitting diodes, transistors, and photonic integrated circuits. In this work, reversible photoinduced oxygen desorption from III-nitride microdisk resonator surfaces is shown to increase optical attenuation of whispering gallery modes by 100 cm−1 at λ = 450 nm. Comparison of photoinduced oxygen desorption in unintentionally and n+-doped microdisks suggests that the spectral changes originate from the unpinning of the surface Fermi level, likely taking place at etched nonpolar III-nitride sidewalls. An oxygen-rich surface prepared by thermal annealing results in a broadband Q improvement to state-of-the-art values exceeding 1 × 104 at 2.6 eV. Such findings emphasize the importance of optically active surface states and their passivation for future nanoscale III-nitride optoelectronic and photonic devices.

Lifetime degradation of n-type Czochralski silicon after hydrogenation

Hydrogen plays an important role in the passivation of interface states in silicon-based metal-oxide semiconductor technologies and passivation of surface and interface states in solar silicon. We have shown recently [Vaqueiro-Contreras et al., Phys. Status Solidi RRL 11, 1700133 (2017)] that hydrogenation of n-type silicon slices containing relatively large concentrations of carbon and oxygen impurity atoms {[Cs] ≥ 1 × 1016 cm−3 and [Oi] ≥ 1017 cm−3} can produce a family of C-O-H defects, which act as powerful recombination centres reducing the minority carrier lifetime. In this work, evidence of the silicon's lifetime deterioration after hydrogen injection from SiNx coating, which is widely used in solar cell manufacturing, has been obtained from microwave photoconductance decay measurements. We have characterised the hydrogenation induced deep level defects in n-type Czochralski-grown Si samples through a series of deep level transient spectroscopy (DLTS), minority carrier transient spectroscopy (MCTS), and high-resolution Laplace DLTS/MCTS measurements. It has been found that along with the hydrogen-related hole traps, H1 and H2, in the lower half of the gap reported by us previously, hydrogenation gives rise to two electron traps, E1 and E2, in the upper half of the gap. The activation energies for electron emission from the E1 and E2 trap levels have been determined as 0.12, and 0.14 eV, respectively. We argue that the E1/H1 and E2/H2 pairs of electron/hole traps are related to two energy levels of two complexes, each incorporating carbon, oxygen, and hydrogen atoms. Our results show that the detrimental effect of the C-O-H defects on the minority carrier lifetime in n-type Si:O + C materials can be very significant, and the carbon concentration in Czochralski-grown silicon is a key parameter in the formation of the recombination centers.

Enhanced luminescence efficiency of wet chemical route synthesized InP-based quantum dots by a novel method: Probing the humidity sensing properties

In indium phosphide quantum dots (InP QDs), the presence of surface states due to unbonded indium or phosphorus atoms quenches its photoluminescence (PL) efficiencies. Hence, it is imperative to passivate these surface states to achieve high photoluminescence efficiencies of InP nanocrystals. In this work, a novel post-synthesis nascent H chemical treatment to enhance the quantum yield of single-pot, wet chemical route synthesized InP QD's is reported for the first time. The main advantages of this post-synthesis treatment are: (i) it is easy, inexpensive and reproducible and (ii) it does not involve harsh chemical treatment viz. dipping of InP QDs in HF-based solutions, nor it requires annealing at high temperatures which may be detrimental to the fragile structure of InP. The significant increment in PL intensity upon aforementioned hydrogen treatment is due to the passivation of surface states and structural recrystallization mechanism that promotes radiative recombination of electrons and holes and hence higher lifetime values. An enhancement in PL intensity of as-synthesized InP QDs upon nascent hydrogen treatment is quite remarkable and is even better than that accomplished by InP-ZnS core-shell QDs with similar size-distribution. The hydrophobicity of H-treated InP QD's is found to be more than untreated InP thus implying higher compactness and structural rigidity similar to as achieved by InP-ZnS core-shell QDs. Mechanisms related to nascent hydrogen treatment, photo-oxidation and PL enhancement and its effect on the surface stoichiometry of InP QDs are discussed via structural, morphological, compositional and optical studies corroborated by various complementary techniques viz. SEM, HRTEM, EDAX,NMR, DLS, Zeta Potential, Contact Angle, PL and Transient Absorption respectively. The application of InP films as a humidity sensor has been demonstrated and untreated InP film in particular shows higher sensitivity values upon exposure to humid treatment as compared with H-treated InP and InP-ZnS core-shell QD's respectively.

Ultrasmall NiFe-Phosphate Nanoparticles Incorporated α-Fe2O3 Nanoarrays Photoanode Realizing High Efficient Solar Water Splitting

The practical application of hematite (α-Fe2O3) in solar water splitting is severely limited by the highly charge recombination rate though its abundant reserves and suitable bandgap of ∼2.1 eV. This work describes the synthesis of ultrasmall NiFe-phosphate (NFP) nanoparticles incorporated α-Fe2O3 nanoarrays photoanode via a facile dip-coating and annealing process to demonstrate combined effects on enhanced photoelectrochemical (PEC) water oxidation. The NFP uniformly decorating on the surface of hematite nanorods not only could improve water oxidation kinetics and charge separation efficiency, but also could suppress the charge recombination in company with the surface states passivation. Furthermore, the phosphate (P) in the NFP nanoparticles could also play a synergistic effect on promoting the multiproton-coupled electron transfer (PCET) process for the PEC water oxidation. All of these lead to ∼140 mV cathodic shift of onset potential, ∼2.3-fold enhancement of the photocurrent and excellent long-term stability at 1.23 VRHE in 0.1 M KOH solution for α-Fe2O3/NFP photoanode. Along with these advantages, the NFP nanoparticles may possess new opportunities for modulating PEC water oxidation performances in hematite and other metal oxide photoanodes.

In3+-doped BiVO4 photoanodes with passivated surface states for photoelectrochemical water oxidation

In3+-doped BiVO4 film photoanodes with passivated surface states for efficient photoelectrochemical water oxidation.

Stable Ta2O5 Overlayers on Hematite for Enhanced Photoelectrochemical Water Splitting Efficiencies

AbstractHematite (α‐Fe2O3) is one of the most promising photoanodes for water oxidation, however the efficiencies of current hematite materials remain low. Surface trap states are often reported as one of the factors which limit the activity of hematite photoelectrodes, often leading to undesirable surface pinning and trap‐mediated recombination. The deposition of ultra‐thin Al2O3 overlayers is known to enhance hematite activity through passivation of surface states, however Al2O3 is rapidly degraded at normal hematite operating pH values (pH≈13). This study reports atomic layer deposition (ALD) of Ta2O5 thin films as stable, passivating overlayers on a range of hematite photoelectrodes and demonstrates that enhanced activity correlates with observed changes in trap‐state dynamics.