Reduction In Band Bending Research Articles

Enhancing the performance of InGaN-based near-infrared light-emitting diodes under a weakly polarized electric field

This work presents a simulation analysis of nitride multi-quantum-well near-infrared light-emitting diodes (LEDs) with an emission wavelength of 1300 nm using Crosslight-APSYS software. The polarization electric field at the quantum wells was effectively tuned by adopting an Al0.02Ga0.19In0.79N quaternaries compound as the barrier for the In0.78GaN quantum wells, resulting in a near-zero polarization electric field. This approach significantly reduced band bending and increased the overlap of electron-hole wave functions within the quantum wells. Consequently, the internal quantum efficiency (IQE) in the nitride near-infrared LED improved, and the droop effect was significantly reduced. The Auger recombination mechanism that influences the droop effect was further analyzed. This provides an efficient approach for the development of high-performance InGaN-based near-infrared light-emitting diodes with high indium content.

Origin, Nature, and Location of Defects in PM6:Y6 Organic Solar Cells

AbstractTargeted strategies to overcome defects in organic semiconductors require insight into their identity and origin. Here the formation, nature, and location of defects is studied in PM6:Y6 organic solar cells by sensitive EQE measurements. Exposure of the active layer to ambient atmosphere and H2O‐saturated compressed air indicates that a trace constituent in ambient air causes the formation of defects. By exposing the active layer to O3‐enriched air, O3 is identified as the species creating defects in PM6:Y6 blends. Aging of complete inverted (n–i–p) configuration solar cells in H2O‐saturated compressed air also increases the defect response. This is attributed to a reduced band bending at the PM6:Y6 | MoO3 hole‐collecting contact, caused by a change in work function of MoO3 interacting with the H2O, which allows more defect states to be filled and available for photoexcitation. By measuring energy resolved‐electrochemical impedance spectroscopy and by fabricating semitransparent cells, regular architecture cells, and semitransparent cells with an optical spacer−mirror stack it is found that defects originate predominantly from PM6 and are located near the top electrode, independent of device polarity. Because O3 is omnipresent in ambient atmosphere, albeit in small amounts, it likely causes defects in many organic semiconductors exposed to ambient air.

Implementation of Tunneling Junction Passivated Contact Concept in Flexible CIGS Solar Cells

AbstractIn today's state‐of‐the‐art high‐efficiency silicon solar cells need to be inserted a thin insulating layer in order to reduce the recombination losses between the carrier transport layer and Si surface, which can form a tunneling junction (TJ), thus increasing the performance of the TJ solar cells comparable with the pn junction structure. However, the copper indium gallium selenium (CIGS) solar cells inevitably lead to the losses of the carrier transport due to the interface which is widely assumed as the pn‐heterojunction. Herein, the TJ solar cells, aiming to enhance the performance of the solar cells, are fabricated by inserting the TiO2 between CIGS/CdS interface deposited by atomic layer deposition (ALD). By inserting the TiO2 insulating layer, the CIGS/TiO2/CdS structure can be effectively reduced the interface recombination, which leads to a reduced band bending in the p‐CIGS surface and compromises its field‐effect passivation. As a result, the CIGS solar cell with the tunneling junction achieves the 15.57% based on the stainless steel (SS) substrate, by introducing a barrier layer and doping NaF. These results provide an important preliminary foundation for the development of the CIGS solar cells with the tunneling junction structure.

Degradation study of carrier selective contact silicon solar cells with ageing: Role of silicon surface morphology

Ageing effect on the performance of Ag/ITO/MoOx/n-Si/LiFx/Al carrier selective contact (CSC) silicon solar cells has been investigated based on silicon surface morphology (planar, different pyramid sizes, and chemical polishing of pyramids). The cells with small silicon pyramids (~2 µm) have shown better stability with time than the medium/large pyramids (~5/~8 µm). The chemical polishing of pyramid peaks and valleys has further reduced the cells ageing effect due to minimization of the microstructural stress (tensile in the valleys and compressive at the peaks) at the MoOx/n-Si interface, which has been verified indirectly with cells fabricated on a planar surface. The cell’s performance degradation based on morphology points out the instability of the MoOx layer and Schottky barrier reduction, which results in hole-transport hindrance at the MoOx/n-Si junction with the S-shape in J-V graphs under illumination. The ageing effect on device performance is further investigated using the Sentaurus TCAD simulations with the MoOx layer work function variation, and the study indicates the band bending reduction (minimal carrier inversion) and hole transport barrier at the junction region.

Photo-Enhanced Ionic Conductivity in Solid-State Oxygen Electrochemical Systems

Oxide based Ionic and mixed Ionic – Electronic conductors play strategic roles in applications ranging from energy conversion and storage to thin-film nanoionic devices such as fuel cells, batteries, permeation membranes and nano-electronic memory and sensing devices. Most cost effective production methods for fabricating both bulk and thin films samples often result in the formation of polycrystalline microstructures. Grain boundaries possess significantly different properties to the bulk and often block ion migration, a ubiquitous problem to overcome for a variety of electrochemical energy storage and conversion devices particularly important for low temperature applications. Space-charge potential barriers, featuring depletion zones and band bending, forming at the interface between adjacent grains have been used to explain their blocking nature in polycrystalline semiconductors. In solar cells and in photoelectrochemistry, above bandgap light is well known to contribute to reduced band bending at interfaces by providing additional charge carriers to screen potential barriers. Here, we show that the same principle can be applied as well to ionic conductors and that we can decrease the grain boundary impedance contribution in such systems. We illustrate photoenhanced ionic conduction in 3% Gd-doped ceria upon illumination of a thin film by UV light, whose photon energy is slightly above ceria’s bandgap. We further demonstrate that this effect is not caused by heat or electronic conductivity. Our considerations are based on electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent spectroscopy (IMPS) measurements, performed on polycrystalline and epitaxial samples, and backed by theoretical considerations of grain boundary potential height and space charge regions. Figure 1

Near-Complete Suppression of Oxygen Evolution for Photoelectrochemical H2O Oxidative H2O2 Synthesis.

Solar energy-assisted water oxidative hydrogen peroxide (H2O2) production on an anode combined with H2 production on a cathode increases the value of solar water splitting, but the challenge of the dominant oxidative product, O2, needs to be overcome. Here, we report a SnO2-x overlayer coated BiVO4 photoanode, which demonstrates the great ability to near-completely suppress O2 evolution for photoelectrochemical (PEC) H2O oxidative H2O2 evolution. Based on the surface hole accumulation measured by surface photovoltage, downward quasi-hole Fermi energy at the photoanode/electrolyte interface and thermodynamic Gibbs free energy between 2-electron and 4-electron competitive reactions, we are able to consider the photoinduced holes of BiVO4 that migrate to the SnO2-x overlayer kinetically favor H2O2 evolution with great selectivity by reduced band bending. The formation of H2O2 may be mediated by the formation of hydroxyl radicals (OH·), from 1-electron water oxidation reactions, as evidenced by spin-trapping electron paramagnetic resonance (EPR) studies conducted herein. In addition to the H2O oxidative H2O2 evolution from PEC water splitting, the SnO2-x/BiVO4 photoanode can also inhibit H2O2 decomposition into O2 under either electrocatalysis or photocatalysis conditions for continuous H2O2 accumulation. Overall, the SnO2-x/BiVO4 photoanode achieves a Faraday efficiency (FE) of over 86% for H2O2 generation in a wide potential region (0.6-2.1 V vs reversible hydrogen electrode (RHE)) and an H2O2 evolution rate averaging 0.825 μmol/min/cm2 at 1.23 V vs RHE under AM 1.5 illumination, corresponding to a solar to H2O2 efficiency of ∼5.6%; this performance surpasses almost all previous solar energy-assisted H2O2 evolution performances. Because of the simultaneous production of H2O2 and H2 by solar water splitting in the PEC cells, our results highlight a potentially greener and more cost-effective approach for "solar-to-fuel" conversion.

Heavy Alkali Treatment of Cu(In,Ga)Se2Solar Cells: Surface versus Bulk Effects

AbstractChalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post‐deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open‐circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single‐phase Cu‐alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries.

Critical analysis of step‐graded polarisation engineered electron‐blocking layer optimisation for InGaN MQW laser diode

Here, the authors have optimised electron-blocking layer (EBL) with step graded design for a laser diode (LD) with an emission wavelength of 450 nm. Step graded EBL contain layers of varying compositions from In0.04Ga0.96N to Al0.2Ga0.8N with the total thickness of 30 nm. The electron leakage has reduced from ∼2272 to ∼6.6 A cm−2, while hole transportation has improved by 2.185at 10 kA cm−2 injected current density. The light output power per facet of reference LD is 146.8 mW, which the authors have enhanced to 247.7 mW in authors’ step-graded EBL structure. The slope efficiency has increased by ∼61.9%. Also, the EBL energybarrier hasincreased by 324.2 meV, while the hole transportation barrierhas reduced by 77.1 meV. The reduction in hole transportation barrier and increase in EBL barrier plays the vital role in the improved performance of LDs. The built-in potential affects the EBL band bending. Electric fields due to polarisation, ionised impurity charge distribution, and externally applied bias are investigated. It is observed that the step graded EBL structurereduces the built-in potential at the EBL interface, thus, resulting in reduced band bending as compared to reference laser structure.

(Invited) Material Considerations for the Development of III-Nitride Power Devices

AlGaN is of high interest for high-power electronics based on vertical structures due to its high breakdown field and electron mobility. Recent studies in GaN based p-n junctions indicate a dielectric strength in the range of 3.3–3.75 MVcm-1, resulting in a Baliga’s figure of merit several times larger than that of SiC. However, high breakdown voltages (>4kV) are currently confined to GaN p-n junctions indicating GaN and AlGaN surface instabilities. Consequently, defect-free metal-nitride interface and passivation of Al/GaN are necessary for breakthroughs in high power vertical Al/GaN Schottky diodes. Accordingly, we report defect-free behavior in Ni/GaN Schottky diodes with unity ideality factor and fabricated by photolithography. They also exhibit high temperature stability (>600 oC) and low leakage with forward and reverse I-V-T characteristics successfully modeled by a single homogeneous barrier (0.7 eV) by ATLAS simulations. It is significant that the forward and reverse characteristics could be modeled by a common set of parameters without a defect based second diode to model the reverse leakage. XPS chemical and electronic analysis was employed to determine the surface treatment necessary to obtain such a Schottky interface. Consequently, we demonstrate the suitability of GaN surface and interface with metals for high power electronics. Although a near-ideal diode may be obtained on GaN, the relatively low barrier height limits the GaN Schottky diode to operating voltages <1000V. Increasing the barrier by controlling the Fermi level pinning and producing a camel junction by surface Mg doping is possible. Passivation is the next challenge to implementing Al/GaN based high power electronics. Silicon nitride has emerged as the standard passivation material for GaN and low Al composition AlxGa1-xN (x<0.3). However, the bandgap of silicon nitride is lower than AlGaN for x>0.75 and the feasibility of silicon nitride functioning as an insulating dielectric (by providing appropriate electron/hole barriers) in Al rich AlGaN and AlN is uncertain. Similarly, reduction of surface states by silicon nitride passivation has not been reported for Al rich AlGaN (x>0.3). In this work, we have employed X-ray photoelectron spectroscopy to determine the band offsets and interface Fermi level at the hetero-junction formed by stoichiometric silicon nitride deposited on metal polar AlxGa1-xN (of varying Al composition ‘x’) via high temperature low pressure chemical vapor deposition. Silicon nitride is found to form a type II staggered band alignment with AlGaN for all Al compositions (0<x<1) and presents an electron barrier into AlGaN even at higher Al compositions where Eg(AlGaN)>Eg(Si3N4). No band bending is observed in AlGaN for x<0.6 and indicating excellent passivation beyond x=0.3. Further a reduced band bending (by 1 eV relative to free surface) is observed for x>0.6. The Fermi level in silicon nitride is found to be at 3 eV with respect to its valence band and is likely due to silicon (≡Si0/-1) dangling bonds. The presence of band bending for x>0.6 is seen as a likely consequence of Fermi level alignment at Si3N4/AlGaN hetero-structure and not due to interface states indicating likely passivation on AlGaN beyond x=0.6. Photoelectron spectroscopy is corroborated by capacitance-voltage measurements. A shift in the interface Fermi level from the conduction band in Si3N4/n-GaN to the valence band in Si3N4/p-GaN is observed which strongly indicates no or greatly reduced mid-gap interface states. Further, required compensation of spontaneous polarization charge is typically performed by oppositely charged surface states/defects on free surface is hypothesized to be replaced by oppositely charged silicon dangling bonds (≡Si0/-1) after passivation. This hypothesis is supported by characterization of silicon nitride on N-polar GaN. In conclusion, HT LPCVD silicon nitride is found to be a suitable passivation for Al rich AlGaN.

Band Alignment at n-GaN/Electrolyte Interface Explored by Photo-Induced Offset of Open-Circuit Potential for Efficient Water Splitting

For semiconductor photoelectrochemistry to split water upon sunlight irradiation, alignment of semiconductor band edges with respect to redox potentials for the evolution of hydrogen and oxygen from water is of crucial importance. Two essential factors are involved in the alignment: 1) the position of semiconductor band edges with respect to the standard electrode potential and 2) the band bending inside the depletion region of the semiconductor adjacent to an electrolyte. Both of these are related to the atomistic nature at the semiconductor/electrolyte interface: the former is affected by the electrical double layer at the interface and the latter is determined by the energy level of mid-gap states originating from the chemical bonds of the semiconductor exposed to the electrolyte. These may be affected by a lot of factors such as pH of the electrolyte, an oxidized layer and atomistic disorder on the semiconductor surface, and crystal defects inside the semiconductor. Due to such complexity, it is not straightforward to characterize the band alignment at the interface. We here propose a method for clarifying such band alignment at the semiconductor/electrolyte interface. For a n-type GaN electrode, open-circuit potential (OCP) moves to the negative direction with increased light intensity incident on its surface. For an ideal situation like a Schottky junction between a metal and a semiconductor, the relationship between OCP and the logarithm of light intensity will follow a linear relationship owing to the accumulation of light-induced carriers inside the semiconductor and the reduction of band bending, approaching a situation with nearly flat band under a sufficiently-large light intensity. For a n-type GaN electrode in a pH=14 electrolyte with NaOH, a similar behavior was observed: OCP with respect to a Ag/AgCl reference electrode moved to the negative direction with a logarithmic increase in the intensity (from 10-5 to 102 mW/cm2) of a Xe lamp irradiated on the GaN surface. For a n-type GaN electrode with its surface reduced in an electrolyte by cathodic current, OCP stayed at approximately 1.0 eV below the flat-band potential (FBP) under a light intensity range from 10-5 to 10-3 mW/cm2. Above this intensity range, OCP increased drastically and exhibited an almost linear behavior with the logarithm of light intensity from 0.1 mW/cm2 until it reached FBP with 102 mW/cm2. Here, the FBP value of -1.4 V was measured by Mott-Schottky plot under dark in a different experiment. Such a behavior would indicate that the native defect states in n-GaN existed at 1.0 eV below the conduction band edge and these states pinned the quasi-Fermi level (qFL) under the light intensity of 10-3 mW/cm2.We have extended the analysis for n-GaN electrodes with a variety of surface modification: anodic oxidation of the surface in an electrolyte, irradiation with Ar atomic beam and high energy electron beam. Such treatments changed the value of OCP under low light intensity, indicating that such surface modification created surface states depending on the treatment and those states pinned qFL until the states are filled with a sufficient amount of light-induced carriers, i.e., unpinning of qFL. Above a threshold light intensity which was dependent on the surface modification, OCP started to approach to FBP with an almost linear manner with the logarithm of light intensity. The tendency suggests that OCP of n-GaN was strongly affected by the surface states which was dependent on the method of surface modification, and the intense light irradiation anyhow make the band alignment close to flat-band. When the surface of n-GaN was covered by a continuous layer such as transparent conducting oxide (TCO), the saturation value of OCP under a large light intensity was quite apart from the FBP for a naked n-GaN electrode, indicating that the TCO modified the band alignment at the n-GaN/electrolyte interface completely by both the electrical double layer on the TCO surface and the band offset between TCO and n-GaN. Such behavior is not only of interest from a scientific point viewpoint but also a suggestion that we can modify the alignment between the semiconductor band edges and redox potentials in order to facilitate an photoelectrochemical reaction with an appropriate surface modification on the semiconductor surface.

High temperature and low pressure chemical vapor deposition of silicon nitride on AlGaN: Band offsets and passivation studies

In this work, we employed X-ray photoelectron spectroscopy to determine the band offsets and interface Fermi level at the heterojunction formed by stoichiometric silicon nitride deposited on AlxGa1-xN (of varying Al composition “x”) via low pressure chemical vapor deposition. Silicon nitride is found to form a type II staggered band alignment with AlGaN for all Al compositions (0 ≤ x ≤ 1) and present an electron barrier into AlGaN even at higher Al compositions, where Eg(AlGaN) &amp;gt; Eg(Si3N4). Further, no band bending is observed in AlGaN for x ≤ 0.6 and a reduced band bending (by ∼1 eV in comparison to that at free surface) is observed for x &amp;gt; 0.6. The Fermi level in silicon nitride is found to be at 3 eV with respect to its valence band, which is likely due to silicon (≡Si0/−1) dangling bonds. The presence of band bending for x &amp;gt; 0.6 is seen as a likely consequence of Fermi level alignment at Si3N4/AlGaN hetero-interface and not due to interface states. Photoelectron spectroscopy results are corroborated by current-voltage-temperature and capacitance-voltage measurements. A shift in the interface Fermi level (before band bending at equilibrium) from the conduction band in Si3N4/n-GaN to the valence band in Si3N4/p-GaN is observed, which strongly indicates a reduction in mid-gap interface states. Hence, stoichiometric silicon nitride is found to be a feasible passivation and dielectric insulation material for AlGaN at any composition.

Sub-band gap photo-enhanced secondary electron emission from high-purity single-crystal chemical-vapor-deposited diamond

Secondary-electron-emission (SEE) current measured from high-purity, single-crystal (100) chemical-vapor-deposited diamond is found to increase when sub-band gap (3.06 eV) photons are incident on the hydrogenated surface. Although the light does not produce photoemission directly, the SEE current increases by more than a factor of 2 before saturating with increasing laser power. In energy distribution curves (EDCs), the emission peak shows a corresponding increase in intensity with increasing laser power. However, the emission-onset energy in the EDCs remains constant, indicating that the bands are pinned at the surface. On the other hand, changes are observed on the high-energy side of the distribution as the laser power increases, with a well-defined shoulder becoming more pronounced. From an analysis of this feature in the EDCs, it is deduced that upward band bending is present in the near-surface region during the SEE measurements and this band bending suppresses the SEE yield. However, sub-band gap photon illumination reduces the band bending and thereby increases the SEE current. Because the bands are pinned at the surface, we conclude that the changes in the band levels occur below the surface in the electron transport region. Sample heating produces similar effects as observed with sub-band gap photon illumination, namely, an increase in SEE current and a reduction in band bending. However, the upward band bending is not fully removed by either increasing laser power or temperature, and a minimum band bending of ∼0.8 eV is established in both cases. The sub-band gap photo-excitation mechanism is under further investigation, although it appears likely at present that defect or gap states play a role in the photo-enhanced SEE process. In the meantime, the study demonstrates the ability of visible light to modify the electronic properties of diamond and enhance the emission capabilities, which may have potential impact for diamond-based vacuum electron sources, particle detectors, and other electronic devices.

Band Offset Characterization of the Atomic Layer Deposited Aluminum Oxide on m-Plane Indium Nitride

In this letter, we report the band offset characterization of the atomic layer deposited aluminum oxide on non-polar m-plane indium nitride grown by plasma-assisted molecular beam epitaxy by using x-ray photoelectron spectroscopy. The valence band offset between aluminum oxide and m-plane indium nitride was determined to be 2.83 eV. The Fermi level of indium nitride was 0.63 eV above valence band maximum, indicated a reduced band bending in comparison to polar indium nitride. The band gap of aluminum oxide was found to be to 6.7 eV, which gave a conduction band offset of 3.17 eV.

Free Energy and Electronic Properties of Water Adsorption on the SnO2(110) Surface

A molecular understanding of the adsorption of water on SnO2 surfaces is crucial for several applications of this metal oxide, including catalysis and gas sensing. We have investigated water adsorption on the SnO2(110) surface using a combination of dynamic and static calculations to gain fundamental insight into the reaction mechanism at room temperature. The reaction dynamics are studied by following water adsorption and dissociation on the SnO2 surface with metadynamics calculations at low and high coverage. The electronic structure in the relevant isolated minima is investigated through Mulliken charge analysis and projected density of states analysis. Surface bridging oxygen (Obr) is found to play a decisive role in water adsorption forming rooted hydroxyl groups with the water H atoms. Bond formation with H significantly changes the electronic configuration of Obr and presumably leads to reduced band bending at the SnO2 surface. The free-energy estimation indicates that on a clean SnO2(110) surface at room temperature both associative and dissociative adsorption occur, with the latter being thermodynamically favored. Oxygen coverage strongly affects the ratio between associatively and dissociatively adsorbed H2O, favoring associative adsorption at high oxygen coverage (oxidized surface) and dissociative adsorption at low oxygen coverage (reduced surface). Electronic analyses of isolated surface minima show the existence of two different electron-transfer phenomena occurring at the surface, depending on the water adsorption mechanism. The relevance of these findings in explaining the changes in electric conductivity occurring in SnO2-based gas sensors upon water adsorption is discussed. Whereas associative adsorption leads to electron enrichment of the metal oxide surface, dissociative adsorption induces surface electron depletion. Both mechanisms are consistent with the electrical conductivity changes occurring upon interaction of SnO2 with water, causing cross sensitivity to the latter. The theoretical results form the basis for correlating the existing atomistic models with the experimental data and offer a coherent description of the reaction events on the surface at room temperature.

Surface morphology and electronic structure of halogen etched InAs (1 1 1)

The reaction of halogen-based etchants with n-InAs (111)A and the resulting surface morphology and surface electronic structure are investigated using field emission scanning electron microscopy and Raman spectroscopy. Using the intensity ratio of the unscreened longitudinal optical (LO) phonon to the transverse optical (TO) phonon in the Raman spectrum, a significant reduction in band bending is deduced after exposure of the InAs surface to HCl:H2O, Br–methanol and I–ethanol for moderate times and concentrations. These procedures also lead to smooth and defect-free InAs surfaces. The improvements in surface properties are reversed, however, if the concentrations of the etchants are increased or the etch time is too long. In the worst cases, pit formation and inverted pyramids with {111} side facets are observed. The influence of the etchant concentration and etch time on the morphological and electronic properties of the etched surfaces is reported.

Quantum confined Stark effect and corresponding lifetime reduction in a single InxGa1−xN quantum disk

Time-integrated and time-resolved microphotoluminescence studies were carried out on InxGa1−xN quantum disks embedded in GaN nanocolumns grown by molecular beam epitaxy. Emission at ∼3.33 eV from confined states was detected and observed to blueshift with excitation power; a result of charge screening and the quantum confined Stark effect. Due to the reduced band bending and resulting increased overlap of the confined electron and hole wave functions, the lifetime of the emission was measured to decrease with increasing excitation power. The saturation of the blueshift and lifetime reduction follows that of the peak intensity, indicating a Stark screening process.

Thermionic electron emission from nitrogen-doped homoepitaxial diamond

Nitrogen-doped homoepitaxial diamond films were synthesized for application as low-temperature thermionic electron emitters. Thermionic electron emission measurements were conducted where the emission current was recorded as a function of emitter temperature. At a temperature < 600 °C an emission current was detected which increased with temperature, and the emission current density was about 1.2 mA/cm 2 at 740 °C. The electron emission was imaged with photoelectron emission microscopy (PEEM) and thermionic field-emission electron microscopy (T-FEEM). The image displayed uniform electron emission over the whole surface area. Thermionic emission and ultraviolet photoemission spectroscopy were employed to determine the temperature dependent electron emission energy distribution from the nitrogen-doped homoepitaxial diamond films. The photoemission spectra indicated an effective work function of 2.4 eV at 550 °C. These values indicate reduced band bending and establish the potential for efficient electron emission devices based on nitrogen-doped homoepitaxial diamond.

S passivation of GaAs and band bending reduction upon atomic layer deposition of HfO2/Al2O3 nanolaminates

A systematic study of the interface engineering and dielectric properties of nanolaminated hafnium aluminate on GaAs is presented. The dielectrics were deposited using atomic layer deposition of alternating cycles of HfO2 and Al2O3 on GaAs substrates. High resolution x-ray photoelectron spectroscopy (XPS) showed differences in space charge amounts at the interface for the two surface treatments [NH4OH or (NH4)2S]. In-situ XPS analysis shows that chemical bonding to oxygen across the nanolaminate film is independent of the interface formation conditions. In addition, the GaAs surface treated with (NH4)2S shows a decreased band bending and slightly thinner films with respect to NH4OH.

Micro-Raman spectroscopy of disordered and ordered sulfur phases on a passivated GaAs surface

The depletion layer width and band bending of passivated n-type Sn doped GaAs(1 0 0) between subsequent steps of chemical treatment as well as after a single run treatment were investigated by micro-Raman light scattering by longitudinal optical phonons and coupled phonon–plasmon modes. Experiments were carried out ex situ at room temperature. We conclude that all observed lineshape changes are due to band bending and to an amorphous surface phase represented by a broad spectral component. We applied two passivation methods. One was based on (NH 4) 2S x solution and lasted 30 min. The second was based on the S 2Cl 2 solution and lasted 10 s. These enabled identification of surface regions of different amorphousness and for faster passivation places of enlarged and completely reduced band bending.

Effect of Surface Electronic States of p-Type GaN on the Blue-Light-Emitting Diodes

The surface electronic states of p-type GaN and its effect on the light-emitting diodes (LEDs) containing such p-GaN layers are studied. The surface band bending of p-GaN layers prepared from different growth conditions are determined by ultraviolet photoemission spectroscopy and X-ray photoemission spectroscopy. By either way, it is found that the surface band bending depends on the growth conditions and is reduced by increasing Mg concentration, growth temperature, and -rich ambient. A further reduction in band bending is found with a postannealing at in , resulting in a similar reduction of the operation forward voltage of the LED containing such a p-GaN layer. The mechanism involved is discussed.