Carrier Depletion Layer Research Articles

Recent Progress on Functionalized Graphene Quantum Dots and Their Nanocomposites for Enhanced Gas Sensing Applications.

Gas-sensing technology has witnessed significant advancements that have been driven by the emergence of graphene quantum dots (GQDs) and their tailored nanocomposites. This comprehensive review surveys the recent progress made in the construction methods and applications of functionalized GQDs and GQD-based nanocomposites for gas sensing. The gas-sensing mechanisms, based on the Fermi-level control and charge carrier depletion layer theory, are briefly explained through the formation of heterojunctions and the adsorption/desorption principle. Furthermore, this review explores the enhancements achieved through the incorporation of GQDs into nanocomposites with diverse matrices, including polymers, metal oxides, and 2D materials. We also provide an overview of the key progress in various hazardous gas sensing applications using functionalized GQDs and GQD-based nanocomposites, focusing on key detection parameters such as sensitivity, selectivity, stability, response and recovery time, repeatability, and limit of detection (LOD). According to the most recent data, the normally reported values for the LOD of various toxic gases using GQD-based sensors are in the range of 1-10 ppm. Remarkably, some GQD-based sensors exhibit extremely low detection limits, such as N-GQDs/SnO2 (0.01 ppb for formaldehyde) and GQD@SnO2 (0.10 ppb for NO2). This review provides an up-to-date perspective on the evolving landscape of functionalized GQDs and their nanocomposites as pivotal components in the development of advanced gas sensors.

Enhanced electrical characteristics of Zr/diamond Schottky barrier diode with cerium hexaboride interfacial layer

To fabricate a performance-enhanced Schottky barrier diode (SBD) on a diamond, a cerium hexaboride (CeB6) interfacial layer was introduced between the diamond and the Zr/Au electrode. The inserted CeB6 layer was characterized using X-ray photoelectron spectroscopy. Introducing the CeB6 layer significantly improved the performance of the diamond SBD compared with a sample without the layer. The ideality factor (n)/SBH (ΦB) values of the SBD with and without the CeB6 interlayer were 2.24/1.92 eV and 2.82/1.65 eV, representing an improvement of 20.5% and 16.4% for the ideality factor and Schottky barrier height, respectively. The reverse breakdown voltage increased from −86 to −110.5 V, indicating an improvement of 28.5% for breakdown voltage. Additionally, the reverse current of the SBD with CeB6 was suppressed, making it more reliable and stable than that without CeB6 before the breakdown threshold. The carrier concentration and depletion layer width were increased from 1.135 × 1014 cm−3 and 206.7 nm to 2.646 × 1014 cm−3 and 227.6 nm, respectively, with the introduction of CeB6. These results demonstrate the advantages of inserting of CeB6 in vertical diamond SBDs and show the potential of SBDs for future power rectifiers.

Effect of defect-rich Co-CeOx OER cocatalyst on the photocarrier dynamics and electronic structure of Sb-doped TiO2 nanorods photoanode

This work demonstrates the in-depth mechanism of enhanced photoelectrochemical (PEC) water oxidation of Sb-doped rutile TiO2 nanorods (NRs) photoanode coupled with oxygen vacancy defect-rich Co-doped CeOx (Co-CeOx) oxygen evolution reaction (OER) cocatalyst. The defect-rich Co-CeOx cocatalyst modification improves the conductivity, light absorption, charge transfer efficiency, and surface photovoltage generation of the Co-CeOx/Sb-TiO2 hybrid NRs photoanode. The Co-CeOx cocatalyst also serves as the surface passivating overlayer for the Sb-TiO2 photoanode, which suppresses the surface states mediated recombination of electron-hole pairs in the NRs. The PEC studies further indicate that Co-CeOx cocatalyst induces remarkably large band bending at the semiconductor/electrolyte interface and shortens the carrier diffusion length and depletion layer width, facilitating the rapid separation and transportation of the photocarriers for the surface PEC reactions. The experimental and theoretical studies confirm that the Co-doping in CeOx cocatalyst enhances the surface oxygen vacancy defects, which provides active catalytic sites for OH− adsorption and charge transportation for enhanced OER kinetics. The density functional theory (DFT) calculations demonstrate a higher conductivity of the Co-CeOx cocatalyst, advantageous for rapid charge transfer capability during PEC reactions. The synergy between all these merits helps the optimized Co-CeOx/Sb-TiO2 photoanode to deliver a maximum photocurrent density of 1.41 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (VRHE) and an ultra-low turn on potential (Von) of 0.1 VRHE under AM 1.5G solar illumination compared to the Sb-TiO2 NRs (0.96 mA cm−2 at 1.23 VRHE and Von = 0.42 VRHE). This work demonstrates the design of an efficient defect-rich cocatalyst modified photoanode for solar energy harvesting.

Carrier Depletion near the Grain Boundary of a SiC Bicrystal

Silicon carbide (SiC) bicrystals were prepared by diffusion bonding, and their grain boundary was observed using scanning transmission electron microscopy. The n-type electrical conductivity of a SiC single crystal was confirmed by scanning nonlinear dielectric microscopy (SNDM). Dopant profiling of the sample by SNDM showed that the interface acted as an electrical insulator with a ~2-μm-thick carrier depletion layer. The carrier depletion layer contained a higher number of oxygen impurities than the bulk crystals due to the incorporation of oxygen from the native oxide film during diffusion bonding. Density functional theory calculations of the density of states as a function of the bandgap also supported these findings. The existence of a carrier depletion layer was also confirmed in a p-type polycrystalline SiC ceramic. These results suggest that the electrical conductivity of SiC ceramics was mostly affected by carrier depletion near the grain boundary rather than the grain boundary itself.

Spectral response, carrier lifetime, and photocurrents of SiC photocathodes

Silicon carbide (SiC) photocathode is one of the candidates for energy conversion from the solar light to hydrogen gas. The conversion efficiencies using SiC photocathodes are still low for practical use. In this study, to find origins of low conversion efficiency, we evaluated carrier lifetimes and depletion layer widths in SiC photocathodes and examined relationship of them with photocurrents. In addition, we observed spectral response of the photocurrents from the photocathodes. From these results, we found that enhancement of the carrier lifetime and the depletion layer width is effective for increase of the conversion efficiency for 4H- and 6H-SiC. 3C-SiC would have defects reducing the effective carrier lifetime, and thus decrease of such defects is essential for increase of the conversion efficiency using 3C-SiC.

The electronic, chemical and electrocatalytic processes and intermediates on iron oxide surfaces during photoelectrochemical water splitting

We re-assess experimental soft X-ray absorption spectra of the oxygen K-shell which we recorded operando from iron oxide during photoelectrochemical water splitting in KOH electrolyte. In particular, we refer to recently reported transitional electron hole states which originate within the charge carrier depletion layer of the iron oxide and on the iron oxide surface. For the latter we find that an intermediate oxy-peroxo species is formed on the iron oxide with increasing bias potential, which disappears upon further polarization of the electrode, concomitantly with the evolution and disappearance of the aforementioned surface state. The oxygen spectra contain also the spectroscopic signatures of the electrolyte water, the position of which changes with increasing bias potential towards lower X-ray energies, revealing the breaking and formation of hydrogen bonds in the water during the experiment. Combined with potential dependent impedance spectroscopy data we are able to sketch the molecular structure of chemical intermediates and their charge carrier dynamics.

Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution

Phase-locked ultrashort pulses in the rich terahertz (THz) spectral range have provided key insights into phenomena as diverse as quantum confinement, first-order phase transitions, high-temperature superconductivity, and carrier transport in nanomaterials. Ultrabroadband electro-optic sampling of few-cycle field transients can even reveal novel dynamics that occur faster than a single oscillation cycle of light. However, conventional THz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielectric functions averaged over the size, structure, orientation and density of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved THz spectroscopy (20 - 50 THz) to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM). We trace the time-dependent dielectric function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast ($<$50 fs) formation of a local carrier depletion layer.

Analysis of the forward and reverse bias I-V and C-V characteristics on Al/PVA:n-PbSe polymer nanocomposites Schottky diode

This paper presents the fabrication and characterization of the Al/PVA:n-PbSe Schottky diode. I-V characteristics have been measured at different temperatures in the forward bias. The behavior study of the series resistance (RS), the ideality factor (n), the effective barrier height (Φb), the Richardson constant (A*), and the leakage current with the temperature have emphasized an inhomogeneity of the barrier height and a tunneling mechanism assisted by traps in the SBD. In C-V measurements, in the reverse bias, the Al/PVA:n-PbSe has been performed as a function of temperature and frequency. The values of barrier height (ΦC-V), the built-in–voltage (Vbi) and carrier concentration (ND) and depletion layer width (W) have been calculated at different temperatures in reverse bias. The barrier inhomogeneities of the Al/PVA:n-PbSe contact has been explained on an assumption of a Gaussian distribution of barrier heights by using the potential fluctuation model.

Evolution of electron states at an n-type InSb surface in a depletion-layer formation process: Effect of nonparabolicity of the conduction-band dispersion

Advancing adsorption on an n-type InSb(110) surface induces gradual formation of a carrier-depletion layer at the surface. Incorporating a highly nonparabolic (NP) dispersion of the conduction band into the local-density-functional formalism in a complete manner, we investigate evolution of electron states at the n-type InSb surface in the depletion-layer formation process. By comparing the calculated results for the NP dispersion with those for the parabolic (P) dispersion corresponding to the band-edge effective mass, we evaluate the effects of the nonparabolicity on surface electron states. The nonparabolicity is found to make significant differences in the effective one-electron potential and the carrier-density distribution at the surface. The band bending occurs in the closer vicinity of the surface and with a smaller bending magnitude in the NP dispersion than in the P dispersion. This can be explained by the fact that carrier electrons operate to screen positive charges of ionized donors in the depletion layer more effectively in the NP dispersion than in the P dispersion. Accurate treatment of the nonparabolicity is indispensable in evaluating the surface electron states at each stage of the formation process.

4H-SiC(0001) Epilayer 성장 및 쇼트키 다이오드의 전기적 특성

A sublimation epitaxial method, referred to as the Closed Space Technique (CST) was adopted to produce thick SiC epitaxial layers for power device applications. We aimed to systematically investigate the dependence of SiC epilayer quality and growth rate during the sublimation growth using the CST method on various process parameters such as the growth temperature and working pressure. The etched surface of a SiC epitaxial layer grown with low growth rate exhibited low etch pit density (EPD) of and a low micropipe density (MPD) of . The etched surface of a SiC epitaxial layer grown with high growth rate (above ) contained a high EPD of and a high MPD of , which indicates that high growth rate aids the formation of dislocations and micropipes in the epitaxial layer. We also investigated the Schottky barrier diode (SBD) characteristics including a carrier density and depletion layer for Ni/SiC structure and finally proposed a MESFET device fabricated by using selective epilayer process.

Differential capacitance measurements of relaxation-induced defects in InGaAs/GaAs Schottky diodes

The use of a differential capacitance technique for characterizing the relaxation-induced defect states in Schottky diodes has been studied. Based on a proposed equivalent circuit including the effect of potential drop across the carrier-depletion layer, a simple equation of capacitance at different voltages and frequencies is derived and compared with experimental data obtained from relaxed In/sub 0.2/Ga/sub 0.8/As/GaAs samples. It is shown that the carrier-depletion layer will introduce capacitance dispersion over frequency like traps; from it the device's geometric parameters, the resistance of the carrier-depletion layer and the ionization energy of the deep level that gives rise to this resistance can be obtained. The relation between the low-frequency capacitance and reverse voltage can be explained well by the depletion of the free carriers between the Schottky depletion and the carrier-depletion layer. The relaxation-induced traps are believed to be at 0.535 and 0.36 eV, respectively, in the GaAs and In/sub 0.2/Ga/sub 0.8/As regions.

Elementary excitations at doped polar semiconductor surfaces with carrier-depletion layers

We make a comparative analysis of coupled carrier plasmon–polar phonon modes at doped polar semiconductor surfaces in the absence and the presence of a carrier-depletion layer. Our analysis shows how the effect of the depletion layer appears in the spatial structure of each excitation mode visualized in a contour map of the induced charge-density, the coupling character elucidated by the phase relation and the amplitude ratio of the carrier component and the polar-phonon component in the induced charge-density distribution, and the energy-loss intensity due to the surface excitation.

Evolution of electron states at doped semiconductor surfaces in a depletion-layer formation process

Adsorption on a doped semiconductor surface often gives rise to gradual formation of a carrier-depletion layer at the surface. By calculating the carrier density distribution and the effective one-electron potential at each stage with the local-density-functional formalism, we investigate the evolution of electron states at doped semiconductor surfaces through the whole formation process of a carrier-depletion layer. At an early stage of this formation process, the carrier density rises quickly from zero and forms a prominent peak near the surface as a function of distance from the surface. At a late stage, the carrier density rises slowly and monotonically up to its bulk value in its profile curve, and its rising rate significantly depends upon temperature, namely the degree of degeneracy of carriers. After reaching the final stage, the profile curve of the carrier density makes a rigid shift into the inside with its form unchanged with further increase in thickness of the depletion layer.

A comparative study on the electrical conduction mechanisms of (Ba0.5Sr0.5)TiO3 thin films on Pt and IrO2 electrodes

Electrical conduction mechanisms for Pt/(Ba0.5Sr0.5)TiO3 (BST)/Pt, IrO2/BST/IrO2, and Pt/BST/IrO2 capacitors were studied. The Pt/BST/Pt capacitor shows a Schottky emission behavior with interface potential barrier heights of about 1.5–1.6 eV. The barrier height is largely determined by the surface electron trap states of the BST. The IrO2/BST interface shows an ohmic contact nature due to the elimination of the surface trap states as the result of the formation of strong chemical bonds between the IrO2 and BST which results in the Poole–Frenkel emission conduction mechanism. Pt/BST/IrO2 capacitor shows Schottky emission behavior and a positive temperature coefficient of resistivity (PTCR) effect depending on the bias polarity. The electron trap states at the Pt/BST interface and the positive space charges within the carrier depletion layer result in the PTCR effect.

Structure of tin oxide layers and operating temperature as factors determining the sensitivity performances to NO x

Thin SnO 2 layers have been prepared and characterized by ellipsometry, Auger spectroscopy, XPS and X-ray diffraction analysis for their thickness and structure in order to be tested as NO 2 sensors. The experiments were carried out both in pure nitrogen and in a nitrogen:oxygen (4:1 vol.) mixture. The process of adsorption of NO 2 on tin oxide results in the build up of a carrier depletion layer and therefore a net decrease of the electrical conductivity. In nitrogen:oxygen mixtures this process competes with those pertaining to the oxygen molecule. The maximum sensitivity for the detection of NO 2 in an N 2:O 2 mixture was found at ∼210 °C. This sensitivity position results from the minimum density of acceptors induced at this temperature by the multistep oxygen ionosorption processes.

Effects of etching with a mixture of HCl gas and H2 on the GaAs surface cleaning in molecular-beam epitaxy

I n situ cleaning of GaAs substrates with a HCl gas and hydrogen mixture prior to molecular-beam epitaxy has been investigated. The chemical reaction during etching was monitored using a quadrupole mass spectrometer. After etching, reflection high-energy electron diffraction patterns revealed (2×4) arsenic-stabilized surfaces and (4×2) gallium-stabilized surfaces as reconstructed structures in the gas-etched substrate surface. These structures suggest that the gas-etched substrate surface is atomically flat, resembling an epitaxial layer surface. To study the effect of gas etching, the carrier depletion layer and the residual carbon impurity around the substrate epitaxial interface were measured by capacitance-voltage carrier profiling and secondary-ion mass spectroscopy. After gas etching, the carrier depletion was greatly reduced, from 1.2×1012 to 1×1010 cm−2. The carbon impurity around the interface also decreased by one order of magnitude. We discussed the surface-cleaning mechanism using the atomic hydrogen terminating model of the GaAs surface during etching. We then applied this etching technique to in situ cleaning of semi-insulating GaAs substrates prior to the growth of selectively doped GaAs/N-AlGaAs heterostructures with very thin GaAs buffer layers.

In situ cleaning of GaAs substrates with HCl gas and hydrogen mixture prior to MBE growth

The effect of hydrogen chloride (HCl) gas and hydrogen mixture etching on GaAs surface cleaning prior to MBE growth was studied. The carrier depletion layer formed around the substrate epitaxial layer interface was reduced significantly by the gas etching. The carbon impurity concentration around the substrate epitaxial interface, measured by secondary ion mass spectroscopy (SIMS), also decreased one order of magnitude. Reduced carrier depletion and carbon impurity concentration were almost independent of the etching depth. We thus believe that the surface cleaning effect originates in the chemical reaction between the residual carbon contamination on the substrate and nascent active hydrogen produced by HCl gas decomposition. An atomically flat surface of the gas-etched GaAs substrate was observed using in situ reflection high energy electron diffraction (RHEED).

The criteria of diffusionless solution validity in the theory of exclusion in “super-pure” semiconductors

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Photoelectrochemical characterization of sintered n-CdS pellet

From an analysis of the photoeffects at the n-CdS pellet - S2− S2− electrolyte interface, solid state parameters of the CdS pellet, e.g., flat band potential, donor density, minority carrier diffusion length and depletion layer width have been evaluated. It is stressed that PEC measurements may also be adopted for the determination of bandgap and transition mode of the semiconductor.