Reduced Surface State Research Articles

Uniform Tendency of Surface Dipoles Across Silicon Doping Levels and Types of H‐Terminated Surfaces

AbstractThe termination of surface‐dangling bonds on silicon through hydrogen atoms, also known as Si–H, can achieve chemical passivation and reduce surface states in the electronic bandgap, thus altering electronic properties. Through a comprehensive study of doping levels (1014–1020 cm−3) and types (n and p), a consistent surface dipole trend induced by Si–H termination is discovered. It is achieved by redistributing surface charges and establishing thermal equilibrium with the chemical bond. To resolve this, the surface work function, surface electron affinity, and the energy difference between the valence band and the Fermi level are measured by employing the Kelvin probe, X‐ray photoelectron spectroscopy, and photoelectron yield spectroscopy methods. These findings are further validated through ab initio simulations. This finding has immense implications not only for eliminating electronic defects at semiconductor interfaces, which is crucial in microelectronics but also for developing and engineering hybrid interfaces and heterojunctions with controlled electronic properties.

Tunable C2 Products via Photothermal Steam Reforming of CO2 over Surface-Modulated Mesoporous Cobalt Oxides.

The conversion of CO2 to high-value products by renewable energy is a promising approach for realizing carbon neutralization, but the selectivity and efficiency of C2+ products are not satisfying. Herein, we report the controllable preparation of highly ordered mesoporous cobalt oxides with modulated surface states to achieve efficient photothermal water-steam reforming of CO2 to C2 products with high activity and tunable selectivity. Pristine mesoporous Co3O4 exhibited an acetic acid selectivity of 96% with a yield rate of 73.44 μmol g-1 h-1. By rationally modifying mesoporous Co3O4 surface states, mesoporous Co3O4@CoO delivered a radically altered ∼100% ethanol selectivity with a yield rate of 14.85 μmol g-1 h-1. Comprehensive experiments revealed that the pH value could strongly influence the selectivity of C2 products over mesoporous cobalt oxides. Density functional theory verified that reduced surface states and rich oxygen vacancies on surface-modified mesoporous cobalt oxides could facilitate further variation of C2 products from acetic acid to ethanol.

Correlating Oxidation State and Surface Ligand Motifs with the Selectivity of CO2 Photoreduction to C2 Products.

The design for non-Cu-based catalysts with the function of producing C2+ products requires systematic knowledge of the intrinsic connection between the surface state as well as the catalytic activity and selectivity. In this work, photochemical in-situ spectral surface characteristic technique combined with the first principle calculations (DFT) were applied to investigate the relationships among the composition of surface states, coordinated motifs, and catalytic selectivity of the titanium oxynitride catalyst. By adopting the process of CO2 photoreduction, we found that C2 product selectivity is positively correlated with the surface Ti2+/Ti3+ ratio while the surface oxidation state is regulated and controlled by coordinated motifs of N-Ti-O/V[O], which can reduce the potential dimerization energy barriers of *CO-CO* and promote the spontaneous formation of the subsequent *CO-CH2* intermediate. On the contrary, relatively high or low N/O coordination ratios will cause imbalance of coordination motifs in reducing surface states and abate the hydrogenation barriers of the methoxy group, thus facilitating methane formation. Such phenomenon provides a new design perspective and by such, researchers make a step close to the accurate production of photo-transformed CO2 products.

Improved device performance of vertical GaN-on-GaN nanorod Schottky barrier diodes with wet-etching process

In this work, a deliberate etching-based top-down approach is proposed to fabricate the GaN nanorod (NR) Schottky barrier diode (SBD). As a key step during the fabrication, the impact of the wet-etching process on device performance is systematically studied. By virtue of the reduced surface states at the sidewall, the performance of NR SBD with the wet-etching process is substantially improved, delivering a forward turn-on voltage of 0.65 V, a current density of ∼10 kA/cm2 at 3 V, an ideality factor of 1.03, an ON/OFF current ratio of ∼1010, and no severe current collapse, along with a reverse breakdown voltage of 772 V.

In Situ Ligand Bonding Management of CsPbI3 Perovskite Quantum Dots Enables High‐Performance Photovoltaics and Red Light‐Emitting Diodes

AbstractTo fine‐tune surface ligands towards high‐performance devices, we developed an in situ passivation process for all‐inorganic cesium lead iodide (CsPbI3) perovskite quantum dots (QDs) by using a bifunctional ligand, L‐phenylalanine (L‐PHE). Through the addition of this ligand into the precursor solution during synthesis, the in situ treated CsPbI3 QDs display significantly reduced surface states, increased vacancy formation energy, higher photoluminescence quantum yields, and much improved stability. Consequently, the L‐PHE passivated CsPbI3 QDs enabled the realization of QD solar cells with an optimal efficiency of 14.62 % and red light‐emitting diodes (LEDs) with a highest external quantum efficiency (EQE) of 10.21 %, respectively, demonstrating the great potential of ligand bonding management in improving the optoelectronic properties of solution‐processed perovskite QDs.

In Situ Ligand Bonding Management of CsPbI3 Perovskite Quantum Dots Enables High-Performance Photovoltaics and Red Light-Emitting Diodes.

To fine-tune surface ligands towards high-performance devices, we developed an in situ passivation process for all-inorganic cesium lead iodide (CsPbI3 ) perovskite quantum dots (QDs) by using a bifunctional ligand, L-phenylalanine (L-PHE). Through the addition of this ligand into the precursor solution during synthesis, the in situ treated CsPbI3 QDs display significantly reduced surface states, increased vacancy formation energy, higher photoluminescence quantum yields, and much improved stability. Consequently, the L-PHE passivated CsPbI3 QDs enabled the realization of QD solar cells with an optimal efficiency of 14.62 % and red light-emitting diodes (LEDs) with a highest external quantum efficiency (EQE) of 10.21 %, respectively, demonstrating the great potential of ligand bonding management in improving the optoelectronic properties of solution-processed perovskite QDs.

Highly Sensitive UV Photodiode Composed of β-Polyfluorene/YZnO Nanorod Organic-Inorganic Hybrid Heterostructure.

The highly sensitive ultra-violet (UV) photodiode was demonstrated on the organic-inorganic hybrid heterostructure of β-phase p-type polyfluorene (PFO)/n-type yttrium-doped zinc oxide nanorods (YZO-NRs). The device was fabricated through a simple fabrication technique of β-phase PFO coating onto YZO-NRs that had been directly grown on graphene by the hydrothermal synthesis method. Under UV illumination (λ = 365 nm), the device clearly showed excellent photoresponse characteristics (e.g., high quantum efficiency ~690%, high photodetectivity ~3.34 × 1012 cm·Hz1/2·W−1, and fast response time ~0.17 s). Furthermore, the ratio of the photo current-to-dark current exceeds 103 even under UV illumination with a small optical power density of 0.6 mW/cm2. We attribute such superb photoresponse characteristics to both Y incorporation into YZO-NRs and conformation of β-phase PFO. Namely, Y dopants could effectively reduce surface states at YZO-NRs, and β-phase PFO might increase the photocarrier conductivity in PFO. The results suggest that the β-phase p-PFO/n-YZO-NR hybrid heterostructure holds promise for high-performance UV photodetectors.

Layer-dependent dielectric permittivity of topological insulator Bi2Se3 thin films

Dielectric properties and their evolutions are fundamental to reveal the optical responses of topological insulators and to guide the design of related devices. Here, we systematically investigate the layer-dependent dielectric permittivity of Bi2Se3 over an ultra-broad spectral range (0.73–6.43 eV) by combining the spectroscopic ellipsometry, energy-loss function (ELF), critical point (CP) analysis, and density function theory. Results demonstrate that the epsilon-near-zero point of the real permittivity and the ELF peak exhibit blue shifts, indicating the resonance frequencies of surface plasmon modes move towards the higher energy, due to the enhanced hybridization between the opposite surfaces. The joint density of states dominates the layer-dependent increase in imaginary permittivity as the thickness increases. We also find that the CP transition positions in band structures are layer-independent, and the CP center energies exhibit red shifts resulted by the reduced surface state gap with the thickness increasing.

Surface passivation of GaAs (0 0 1) by Hg2Cl2 nanoplates combined with hexadecanethiol

Surface states in III–V semiconductor materials have detrimental effects on their optical and electronic properties, and the passivation of GaAs surface has become of longstanding interest. Here, we demonstrated a two-step process to greatly reduce surface states of GaAs (0 0 1) by a combination of Hg2Cl2 and alkanethiol. Firstly, uniformly distributed Hg2Cl2 nanoplates with a size around 200 nm were deposited on GaAs surface by the incubation of the etched wafer into the mercury (II) chloride solution, without the need of a reducing agent. Secondly, hexadecanethiol (HDT) molecules were assembled on the Hg2Cl2-GaAs hybrid surface, which decreases the density of surface states through electron transfer processes. Noticeably, after such two steps, a significant enhancement of photoluminescence (PL) signal was noted. It may be due to the fact that components of As2O3 and As0 which are identified as major sources of the surface states are reduced considerably, or even disappear. Chemical state changes of mercury species are expected to play a key role in achieving the surface passivation. Obtained hybrid GaAs materials with considerably improved photonic signals open a new avenue for the fabrication of electronic and optoelectronic devices.

Wet etching of dilute nitride GaInNAs, GaInNAsSb, and GaNAsSb alloys lattice-matched to GaAs

We have studied the etching of GaInNAs, GaInNAsSb, and GaNAsSb alloys by NH4OH, H2SO4, and H3PO4 based solutions. NH4OH based solutions resulted in smooth surface, while other solutions created rougher and granular surfaces. The etch rates were found to increase with the Sb content. For GaInNAs, x-ray photoelectron spectroscopy revealed the enrichment of In on the etched surfaces, indicating In or In oxides having a smaller removal rate compared to Ga or Ga oxides. The enrichment of In was associated with smoother surfaces after etching and an enhanced photoluminescence caused by lower surface recombination due to reduced surface state density.

Defect-Induced Epitaxial Growth for Efficient Solar Hydrogen Production.

Epitaxial growth suffers from the mismatches in lattice and dangling bonds arising from different crystal structures or unit cell parameters. Here, we demonstrate the epitaxial growth of 2D MoS2 ribbon on 1D CdS nanowires (NWs) via surface and subsurface defects. The interstitial Cd0 in the (12̅10) crystal plane of the [0001]-oriented CdS NWs are found to serve as nucleation sites for interatomically bonded [001]-oriented MoS2, where the perfect lattice match (∼99.7%) between the (101̅1) plane of CdS and the (002)-faceted in-plane MoS2 result in coaxial MoS2 ribbon/CdS NWs heterojunction. The coaxial but heterotropic epitaxial MoS2 ribbon on the surface of CdS NWs induces delocalized interface states that facilitate charge transport and the reduced surface state. A less than 5-fold ribbon width of MoS2 as hydrogen evolution cocatalyst exhibits a ∼10-fold H2 evolution enhancement than state of the art Pt in an acidic electrolyte, and apparent quantum yields of 79.7% at 420 nm, 53.1% at 450 nm, and 9.67% at 520 nm, respectively.

High-Performance Photoconductivity and Electrical Transport of ZnO/ZnS Core/Shell Nanowires for Multifunctional Nanodevice Applications

We report the electrical and optical properties of ZnO/ZnS core/shell nanowire (NW) devices. The spatial separation of charge carriers due to their type II band structure together with passivation effect on ZnO/ZnS core/shell NWs not only enhanced their charge carrier transport characteristics by confining the electrons and reducing surface states in the ZnO channel but also increased the photocurrent under ultraviolet (UV) illumination by reducing the recombination probability of the photogenerated charge carriers. Here the efficacy of the type-II band structure and the passivation effect are demonstrated by showing the enhanced subthreshold swing (150 mV/decade) and mobility (17.2 cm2/(Vs)) of the electrical properties, as well as the high responsivity (4.4×10(6) A/W) in the optical properties of the ZnO/ZnS core/shell NWs, compared with the subthreshold swing (464 mV/decade), mobility (8.9 cm2/(Vs)) and responsivity (2.5×10(6) A/W) of ZnO NWs.

Performance improvement mechanisms of i-ZnO/(NH4)2Sx-treated AlGaN MOS diodes

The intrinsic ZnO (i-ZnO) film was deposited by a vapor cooling condensation system and used as the dielectric layer of the AlGaN metal–oxide–semiconductor (MOS) diodes. Before the deposition of the i-ZnO dielectric layer, the AlGaN surface was treated using (NH4)2Sx solution. In view of the lattice match of the i-ZnO film and the reduced surface state density of the (NH4)2Sx-treated surface, the quality of the i-ZnO/AlGaN interface was improved. According to the experimental results, the i-ZnO/(NH4)2Sx-treated MOS diodes revealed the lower leakage current, the lower interface state density, and the high electrical performances compared with the i-ZnO/untreated ones. Furthermore, the X-ray photoelectron spectroscopy and the charge neutrality level model were used to analyze the conduction band offset and the valence band offset of the i-ZnO/AlGaN interface. The valence band offset of the i-ZnO film contacted with the untreated and the (NH4)2Sx-treated AlGaN layer was 1.53eV and 1.96eV, respectively. The conduction band offset of the i-ZnO film contacted with untreated and (NH4)2Sx-treated AlGaN layers was 0.77eV and 1.20eV, respectively. The mechanisms of the enhanced conduction band offset were attributed to the effective reduce of interface states by using the (NH4)2Sx surface treatment.

Temperature and composition dependence of photoluminescence dynamics in CdSxSe1−x (0 ≤ x ≤ 1) nanobelts

CdSxSe1−x nanobelts with uniform and controllable composition are successfully fabricated by a one-step vapor-liquid-solid process. Temperature-dependent steady and time-resolved transient photoluminescence (PL) are studied for the obtained CdSxSe1−x nanobelts as well as pure CdS and CdSe nanobelts prepared in same method. For all samples, both band edge and surface state emission are found to contribute to the observed PL spectra. However, the emission originated from surface state is quite weak for CdSxSe1−x nanobelts as compared to CdS and CdSe. The reduced surface state emission in CdSxSe1−x nanobelts is ascribed to exciton localization due to random potential fluctuations. This attribution is further confirmed by the time-decay PL profiles where a very fast process of exciton transfer from interior to surface of nanobelts, observed in CdS and CdSe nanobelts, is absent for CdSxSe1−x nanobelts. These observations reveal that the surface effect of CdSxSe1−x nanobelts is less significant than that in CdS and CdSe nanobelts, which consequently leads to enhanced band-edge emission in CdSxSe1−x nanobelts.

Band Alignment and Performance Improvement Mechanisms of Chlorine-Treated ZnO-Gate AlGaN/GaN Metal–Oxide–Semiconductor High-Electron Mobility Transistors

The intrinsic ZnO (i-ZnO) film deposited by a vapor cooling condensation system was used as the gate dielectric layer of the AlGaN/GaN MOS-HEMTs. The chlorine surface treatment was utilized to obtain a high-quality i-ZnO/AlGaN interface due to the reduced surface state density. The chlorine-treated MOS-HEMTs showed the better direct current and pulsed output performances than those of the untreated MOS-HEMTs. The resulting unit gain cutoff frequency and the maximum frequency of oscillation were 9.5 and 19.4 GHz, respectively. The Hooge's coefficient was 7.23 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> , when the chlorine-treated ZnO-gate MOS-HEMTs operated at 100 Hz and the gate-source voltage of -4 V. Compared with the untreated MOS-HEMTs, the chlorine-treated MOS-HEMTs revealed better performances. The valence-band offset of i-ZnO/AlGaN was measured by X-ray photoelectron spectroscopy. The valence-band offset of the i-ZnO film on the untreated and the chlorine-treated AlGaN was 1.53 and 2.05 eV, respectively. The conduction-band offset of the i-ZnO film on the untreated AlGaN and the chlorine-treated AlGaN was deduced to be 0.77 and 1.29 eV, respectively. The improved performances of the chlorine-treated MOS-HEMTs and the enhanced conduction-band offset of the i-ZnO/AlGaN interface were attributed to the decrease of Ga dangling bonds and the passivation of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> vacancies on the AlGaN surface by using the chlorine surface treatment.

Synchronously Reduced Surface States, Charge Recombination, and Light Absorption Length for High-Performance Organic Dye-Sensitized Solar Cells

We employ the 4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) segment as a conjugated spacer to construct an extremely high-molar-absorption-coefficient organic chromophore for dye-sensitized solar cells, exhibiting a high power conversion efficiency of 8.95% measured under irradiation of 100 mW cm(-2) AM1.5G sunlight. Our comparative experiments have proved the prominent merit of employing CPDT instead of the prevailing 2,2'-dithiophene (DT) as the building block for the further dye design. We also have demonstrated that a controllable coassembling of dye molecules and electrolyte components on semiconducting nanocrystals can reduce surface states and inhibit charge recombination synchronously.

Temperature characteristics of AlGaN/GaN MOS-HEMT with Al2O3 gate dielectric

A metal-oxide-semiconductor high-electron-mobility transistor (MOS-HEMT) with atomic layer deposition (ALD) of 10nm Al2O3 gate dielectric is manufactured. The superiorities in saturation current and leakage current of the novel MOS-HEMT device are verified by comparison of the MOS-HEMT with traditional MES-HEMT devices at room temperature. The statistical analysis of the features of temperature variation of the device characteristics in the 30—180℃ range, which shows the distinction in degration degree by temperature on the two kinds of devices, is also illustrated. The conclusion that the device saturation current and the transconductance degradations are mainly caused by transport characteristic degradation is reached at. Thus, the conclusion can be drawn that all statistical and analytical results witness the significant role played by the reduced surface states in the surface of the AlGaN in optimizing the characteristics of the device.

Effects of interfacial sulfidization and thermal annealing on the electrical properties of an atomic-layer-deposited Al2O3 gate dielectric on GaAs substrate

In this study we investigated the interfacial chemistry occurring between an atomic-layer-deposited Al2O3 high-k film and a GaAs substrate and the impact of sulfidization and thermal annealing on the properties of the resultant capacitor. We observed that sulfide passivation of the Al2O3∕GaAs structure improved the effect of Fermi level pinning on the electrical characteristics, thereby providing a higher oxide capacitance, smaller frequency dispersion, and reduced surface states, as well as decreased interfacial charge trapping and gate leakage currents. Photoemission analysis indicated that the (NH4)2S-treated GaAs improved the quality of the as-deposited Al2O3 thin film and preserved the stoichiometry of the dielectric during subsequent high-temperature annealing. This behavior was closely correlated to the diminution of GaAs native oxides and elemental arsenic defects and their unwanted diffusion. In addition, thermal processing under an O2 atmosphere, relative to that under N2, decreased the thickness of the Al2O3 gate dielectric and relieved the gate leakage degradation induced by metallic arsenic; as a result, superior dielectric reliability was attained. We discuss the underlying thermochemical reactions that account for these experimental observations.

Enhanced Ultraviolet Emission from ZnS-Coated ZnO Nanowires Fabricated by Self-Assembling Method

A simple chemical route for ZnS-coated ZnO nanowires with preferential (002) orientation is reported. Sodium sulfide and zinc nitrate were employed to supply S and Zn atoms at 60 degrees C to form ZnS-coated ZnO nanowires structures. Electron diffraction measurement shows that the ZnO/ZnS core-shell nanostructure is single crystalline. Interesting features are found in the photoluminescence (PL) spectra of ZnS-coated ZnO nanostructures. After coating, the UV emission of nanorods is dramatically enhanced at the expense of the green emission. The core/shell structure with higher band gap shell material and reduced surface states should be responsible for this PL enhancement.

Device interference in GaAs quantum wire transistors and its suppression by surface passivation using Si interface control layer

In order to establish feasibility of high density integration of gate-controlled GaAs nanodevices, this article investigates device interference in GaAs-based quantum wire transistors (QWRTrs) by using a side-gating test structure and attempts to suppress the observed anomalously large side-gating with surface passivation using a silicon interface control layer (Si ICL). QWRTrs were formed on AlGaAs∕GaAs etched quantum wires (QWRs) and were controlled by nanometer sized Schottky wrap gates. A Schottky side gate was formed at a distance dsg from the QWR. When dsg was large, the QWRTr showed weak side gating which can be explained by the electrostatic side gating. However, when the side gates was placed close to the nanowire with dsg&amp;lt;500nm, anomalously large side gating started to take place which cannot be explained by the electrostatic side gating. On the basis of detailed measurements of side-gating behavior and side-gate leakage currents at various temperatures, the anomalous side gating was explained by a model in which occupation of deep traps at the back AlGaAs∕GaAs interface of the QWR is modulated by tunneling injection of electrons from the side-gate edge resulting from strong Fermi level pinning by surface states. Based on this model, attempts to reduce surface states by surface passivation were made. Formation of the Si ICL structure on a regrown thin GaAs cap layer completely removed the anomalous side-gating effect.