Showing papers on "Depletion region published in 2022"

Control of electronic band profiles through depletion layer engineering in core–shell nanocrystals

Abstract: Fermi level pinning in doped metal oxide (MO) nanocrystals (NCs) results in the formation of depletion layers, which affect their optical and electronic properties, and ultimately their application in smart optoelectronics, photocatalysis, or energy storage. For a precise control over functionality, it is important to understand and control their electronic bands at the nanoscale. Here, we show that depletion layer engineering allows designing the energetic band profiles and predicting the optoelectronic properties of MO NCs. This is achieved by shell thickness tuning of core-shell Sn:In2O3-In2O3 NCs, resulting in multiple band bending and multi-modal plasmonic response. We identify the modification of the band profiles after the light-induced accumulation of extra electrons as the main mechanism of photodoping and enhance the charge storage capability up to hundreds of electrons per NC through depletion layer engineering. Our experimental results are supported by theoretical models and are transferable to other core-multishell systems as well.

Interpretation of Mott–Schottky plots of photoanodes for water splitting

Abstract: A large body of literature reports that both bismuth vanadate and haematite photoanodes are semiconductors with an extremely high doping density between 1018 and 1021 cm−3. Such values are obtained from Mott–Schottky plots by assuming that the measured capacitance is dominated by the capacitance of the depletion layer formed by the doping density within the photoanode. In this work, we show that such an assumption is erroneous in many cases because the injection of electrons from the collecting contact creates a ubiquitous capacitance step that is very difficult to distinguish from that of the depletion layer. Based on this reasoning, we derive an analytical resolution limit that is independent of the assumed active area and surface roughness of the photoanode, below which doping densities cannot be measured in a capacitance measurement. We find that the reported doping densities in the literature lie very close to this value and therefore conclude that there is no credible evidence from capacitance measurements that confirms that bismuth vanadate and haematite photoanodes contain high doping densities.

Li+ Conductivity of Space Charge Layers Formed at Electrified Interfaces Between a Model Solid-State Electrolyte and Blocking Au-Electrodes.

Abstract: The formation of space charge layers in solid-state ion conductors has been investigated as early as the 1980s. With the advent of all-solid-state batteries as an alternative to traditional Li-ion batteries, possibly improving performance and safety, the phenomenon of space charge formation caught the attention of researchers as a possible origin for the observed high interfacial resistance. Following classical space charge theory, such high resistances result from the formation of the depletion layers. These layers of up to hundreds of nanometers in thickness are almost free of mobile cations. With the prediction of a Debye-like screening effect, the thickness of the depletion layer is expected to scale with the square root of the absolute temperature. In this work, we studied the temperature dependence of the depletion layer properties in model solid Ohara LICGC Li+ conducting electrolytes using electrochemical impedance spectroscopy. We show that the activation energy inside the depletion layer increases to ca 0.42 eV compared to ca 0.39 eV in the bulk electrolyte. Moreover, the proportionality between temperature and depletion layer thickness, correlating to the Debye length, is tested and validated.

A comparison of Au/n-Si Schottky diodes (SDs) with/without a nanographite (NG) interfacial layer by considering interlayer, surface states (Nss) and series resistance (Rs) effects

Abstract: In this paper, an organic interlayer, Rs, and Nss on the transport- mechanisms (TMs), both the Au/n-Si (MS) and Au/(Nanographite-PVP/n-Si (MPS) (SDs) were performed onto the same Si-wafer in same-conditions. Some electrical parameters of them have been investigated. The interface-traps/states (D it /N ss) were extracted from the I F –V F data as function of energy (Ec–Ess). These results show that the N ss for MPS is much-lower than MS SD and increase from the midgap-energy towards the E c like U-shape. Double-logarithmic I F –V F graphs of them show three linear-regimes for low, intermediate, and high-voltages and in these regimes, TM are governed by ohmic, trap/space charge limited currents (TCLCs/SCLCs), respectively. All these results show that (NG:PVP) interlayer leads to an increase in rectifier-ratio (RR = I F /I R ), BH, R sh, and decrease in N ss, reverse saturation-current (I o), and n. Thus, (NG:PVP) can be successfully utilized as interfacial layer with high performance characteristics.

Temperature-Dependent Characteristics of HgCdTe Mid-Wave Infrared E-Avalanche Photodiode

Abstract: This paper presents the characteristics of HgCdTe mid-wavelength infrared (MWIR) electron-initiated avalanche photodiodes (e-APDs) as a function of temperature under different biases. The devices show a low dark current density of the order of ${10^{ - 7}} \text{A/cm}^2$ at 80 K when the reverse bias voltage is below 4 V, with an exponential gain above 100 at − 8 V. Low excess noise factor of around 1.2 is also demonstrated for the devices at 80 K. The dark current and gain characteristics at different temperatures are analyzed, along with the device simulation results. A varied-temperature impact ionization model for MWIR e-APD devices is adopted based on the experimental results. In addition, a significant increase of the dark current in the APD device is observed at high temperature under large reverse bias, which can be associated with the local electric field redistribution due to the accumulation of holes in the depletion region at high temperatures. The analysis presented in this work paves the way to achieve a high operating temperature (HOT) of the HgCdTe APD with further optimization.

Realization of Double‐Gate Junctionless Field Effect Transistor Depletion Region for 6 nm Regime with an Efficient Layer

Abstract: Herein, the authors suggest a junctionless field effect transistor with an embedded p‐type layer (EPL‐JLT) near the drain channel side, employing calibrated structure simulations to obtain a complete depletion region in a 6 nm channel length. The incorporation of a p‐type layer improves leakage current (I OFF) and subthreshold swing (SS) for a 6 nm regime structure at 5.9 eV work function (WF) while the ON current (I ON) diminishes a little. This considerable achievement in the leakage current enables obtaining multiple threshold voltages (V TH) by adjusting the gate WF. The goal aids in creating optimized structures, which is impossible in silicon JLFETs (Si‐JLT) due to their requirement for large WFs to obtain a depletion region even at higher channel lengths. The proposed device has a leakage current of 1 nA μm−1 even at a 5.1 eV WF. The scaling of the EPL‐JLT for different channel lengths is investigated.

Identifying and Removing the Interfacial States in Metal-Oxide-Semiconductor Schottky Si Photoanodes for the Highest Fill Factor.

Abstract: A critical bottleneck for realizing an efficient Schottky type Si photoelectrode is minimizing the charge extraction losses across the heterointerface via reducing the unfavorite defects. This requires a clear microscopic insight into the correlation between interfacial features and photoconversion. Herein, by taking the n-Si/oxide (MOx)/Ni as the prototype, the heterointerface with the different characteristics and its effects on charge transportation and the corresponding photoelectric/photoelectrochemical (PEC) behaviors were clarified. An ultra-thin AlOx layer can effectively diminish the interfacial pinning of n-Si/Ni and significantly facilitate the photoconversion; meanwhile, it results in some unexpected donor-like deep defects at around 0.59 eV below the conduction band of n-Si, which could be ionized under a reverse bias and cause about 10% photogenerated charge recombination. Fortunately, these deep defects can be further eliminated by cooperating AlOx with a thin Au layer. The AlOx/Au dual-interlayer can remove almost all unexpected defects and maximize the efficiency of the electric field for charge extraction from semiconductor Si for the surface catalytic reaction. Eventually, the n-Si/SiOx/AlOx/Au/Ni/NiFeOx photoanode exhibited a record fill factor of 0.75 for the corresponding photoelectric device and an applied bias photon-to-current efficiency of 3.71% for PEC water oxidation. This study provides definite insights into interfacial electronic states and elaborates their crucial role in solar photoelectric conversion.

Carrier‐Transport Mechanism in Organic Antiambipolar Transistors Unveiled by Operando Photoemission Electron Microscopy

Abstract: Organic antiambipolar transistors (AATs) have partially overlapped p–n junctions. At room temperature, this p–n junction induces a negative differential transconductance in an AAT. However, the detailed carrier‐transport mechanism remains unclear. Herein, an operando photoemission electron microscopy is used to tackle this issue owing to the technique's ability to visualize conductive electrons in real time during transistor operation. Notably, it is observed that when the AAT is on, a depletion layer forms at the lateral p–n junction. The visualized depletion layer shows that both p‐ and n‐type channels have pinch‐off states in the gate voltage range when the AAT is in on state. The steep potential gradient at the lateral p–n interface enhances the electron conduction from n‐type to p‐type semiconductor. Another significant finding is that most electrons are considered to recombine with the accumulated holes in the p‐type semiconductor, affording the reduction of photoemission intensity by ≈80%. This technique provides a thorough understanding of carrier transport in AATs, further improving the device performance.

Electrostatic characteristics of a high-k stacked gate-all-around heterojunction tunnel field-effect transistor using the superposition principle

Abstract: We use the superposition method to model the electrostatic characteristics of a high-k stacked gate-all-around heterojunction tunneling field-effect transistor (TFET). The heterojunction is formed from Ge/Si material in the source/channel, respectively. The modeling is accomplished by considering the space-charge regions at the source–channel and drain–channel junctions and in the channel region. The surface potential in the channel region is obtained by applying the superposition principle derived in the source/drain region by solving the two-dimensional (2D) or one-dimensional (1D) Poisson’s equation, respectively. Furthermore, the electric field and the drain current are modeled by using the surface potential and the Kane model, respectively. The results are confirmed using ATLAS technology computer-aided design (TCAD) simulations.

Stacked Single Gate SOI 4H‐SiC Junctionless FET with a Buried P‐type 4H‐SiC layer

Abstract: This study describes a single-gate silicon on insulator junctionless metal–oxide–semiconductor field-effect transistor (SJL-MOSFET) with inserted buried 4H-SiC p-type layer (B-SJL-MOSFET). The embedded p-type layer is placed at the bottom of the active regions, achieving the full depletion of the channel in the off-state mode. The p-type layer affects the depletion region of the channel, which helps us have a full depletion area with a lower gate electrode work function. The main reason for using SiC material instead of silicon is the higher electrostatic integrity and a shorter natural length than silicon, resulting in a better short channel effect (SCE). In addition, the high-k oxide is stacked for achieving lower natural length and better subthreshold swing (SS). The p-type layer improves the leakage current (Ioff) by ≈105 in the event that it slightly diminishes the on-state current (Ion) and improves the SS. Improvement of off-current is due to higher barriers between source-channel side and reduces the parasitic BJT in the off-mode.

Bidirectional Bias Response Ultraviolet Phototransistors With 4H-SiC NPN Multi-Layer Structure

Abstract: Silicon Carbide (SiC) Ultraviolet (UV) phototransistors with bidirectional bias response were fabricated and analyzed in this letter. The results showed that the fabricated UV phototransistor exhibited stable photoelectric response characteristics and transistor characteristics under both forward and reverse voltages, owing to the structure design of the epi-layers, while the mechanism of the bidirectional response was also studied. The difference in photocurrent under different bias voltages and wavelengths of UV illumination were mainly due to the variation of the absorption coefficient with wavelength and the change in the width of the space charge regions, which ultimately led to a difference in the number of photo-generated carriers. The time response characteristics of the detector showed that there were time delays when the detector was turned on and turned off, which was caused by the photo minority carriers in the depletion layer of the base-collector junction.

Stacked Single‐Gate Silicon on Insulator 4H‐SiC Junctionless Field‐Effect Transistor with a Buried P‐Type 4H‐SiC Layer

Abstract: This study describes a single‐gate silicon on insulator junctionless metal–oxide–semiconductor field‐effect transistor (SJL‐MOSFET) with inserted buried 4H‐SiC p‐type layer (B‐SJL‐MOSFET). The embedded p‐type layer is placed at the bottom of the active regions, achieving the full depletion of the channel in the off‐state mode. The p‐type layer affects the depletion region of the channel, which helps us have a full depletion area with a lower gate electrode work function. The main reason for using SiC material instead of silicon is the higher electrostatic integrity and a shorter natural length than silicon, resulting in a better short channel effect (SCE). In addition, the high‐k oxide is stacked for achieving lower natural length and better subthreshold swing (SS). The p‐type layer improves the leakage current (Ioff) by ≈105 in the event that it slightly diminishes the on‐state current (Ion) and improves the SS. Improvement of off‐current is due to higher barriers between source‐channel side and reduces the parasitic BJT in the off‐mode.

Structural design optimization of 279 nm wavelength AlGaN homojunction tunnel junction deep-UV light-emitting diode

Abstract: We demonstrated the structural optimization of AlGaN tunnel junction (TJ) deep-ultraviolet light-emitting diodes by changing the thickness and impurity concentrations of p+-type and n+-type AlGaN constituting the TJ. By decreasing the total thickness of the TJ to 23 nm, the operating voltage reached a minimum of 8.8 V at 63 A cm−2. Further decrease in TJ thickness markedly increases the operating voltage. This finding implies that the depletion layer width becomes greater than the TJ thickness if it is smaller than 12 nm. Therefore, we conclude that the TJ thickness must be greater than the depletion layer width.

Frequency response optimization of P-I-N photodiode based on InGaAsN lattice matched to GaAs for High-Speed photodetection applications

Abstract: This paper reports on pin photodiode frequency response optimization based on InxGa1-xAs1-yNy quaternary lattice matched to GaAs. Two transparent layers are placed on p-side and n-side in order to manage the photodiode frequency response limitations. The lattice matching condition is calculated in order to obtain stable structure. The physical and optical parameters calculations are performed at room temperature showing the impact of nitrogen on the absorption coefficient. A stable structure, having 2 % of nitrogen and 6% of indium, allows to achieve a cutoff frequency of about 116 GHz and a capacitance of 5.21fF while the quantum efficiency is 41.59% for a depletion region thickness of about 0.55 µm. However, in case of depletion region thickness of about 0.625 µm, the cutoff frequency degrades to 98 GHz while the capacitance diminishes to 4.58fF and the quantum efficiency increases to 51.56%. In addition, a comparative study with literature results has been carried out in order to show the advantages of the proposed photodiode. This comparison affirms that the proposed photodiode based on InGaAsN lattice matched to GaAs exhibits high-speed photo-detection. This work allowed us to obtain a p-i-n photodiode with stable structure suitable for photo-detection at 1.15 µm.

Comment on “Enhanced Charge Selectivity via Anodic-C60 Layer Reduces Nonradiative Losses in Organic Solar Cells”

Abstract: Understanding interface-related phenomena is important for improving the performance of thin-film solar cells. In ACS Appl. Mater. Interfaces2021, 13, 12603–12609, Pranav et al. report that incorporating a thin C60 interlayer at the MoO3 anode results in reduced surface recombination of electrons, which is ascribed to a decreased electron accumulation near the anode on account of an increased built-in voltage. Here, we offer an alternative explanation: the introduction of a C60 interlayer renders the MoO3 contact Ohmic. The reduced anode barrier simultaneously increases the built-in voltage, minimizes nonradiative voltage losses upon the extraction of majority carriers (holes), and suppresses minority-carrier (electron) surface recombination, the latter being the result of hole accumulation and associated band bending near the Ohmic hole contact. We therefore argue that Ohmic contact formation suppresses both majority- and minority-carrier surface recombination losses, whereas the built-in voltage per se does not play a major role in this respect.

Visualization of depletion layer in AlGaN homojunction p–n junction

Abstract: We analyzed the p–n junction of an aluminum gallium nitride (AlGaN) homojunction tunnel junction (TJ) deep-ultraviolet light-emitting diode by phase-shifting electron holography. We clearly obtained a phase image reflecting the band alinement of the p–n homojunction and derived a depletion layer width of approximately 10 nm. In addition, the observed depletion layer width for the AlGaN TJ was in good agreement with the simulated one reflecting the diffusion profile of Mg and Si, thus enabling a discussion on the electrical conduction mechanism for an AlGaN p–n junction.

X-ray performance of a small pixel size sCMOS sensor and the effect of depletion depth

Abstract: Abstract In recent years, scientific Complementary Metal Oxide Semiconductor (sCMOS) devices have been increasingly applied in X-ray detection, thanks to their attributes such as high frame rate, low dark current, high radiation tolerance and low readout noise. We tested the basic performance of a backside-illuminated (BSI) sCMOS sensor, which has a small pixel size of 6.5 μm × 6.5 μm. At a temperature of -20°C, The readout noise is 1.6 e - , the dark current is 0.5 e - /pixel/s, and the energy resolution reaches 204.6 eV for single-pixel events. The effect of depletion depth on the sensor's performance was also examined, using three versions of the sensors with different deletion depths. We found that the sensor with a deeper depletion region can achieve a better energy resolution for events of all types of pixel splitting patterns, and has a higher efficiency in collecting photoelectrons produced by X-ray photons. We further study the effect of depletion depth on charge diffusion with a center-of-gravity (CG) model. Based on this work, a highly depleted sCMOS is recommended for applications of soft X-ray spectroscopy.

Carrier capture and emission by substitutional carbon impurities in GaN vertical diodes

Abstract: A model was developed for the operation of a GaN pn junction vertical diode which includes rate equations for carrier capture and thermally activated emission by substitutional carbon impurities and carrier generation by ionizing radiation. The model was used to simulate the effect of ionizing radiation on the charge state of carbon. These simulations predict that with no applied bias, carbon is negatively charged in the n-doped layer, thereby compensating n-doping as experimentally observed in diodes grown by metal-organic chemical vapor deposition. With reverse bias, carbon remains negative in the depletion region, i.e., compensation persists in the absence of ionization but is neutralized by exposure to ionizing radiation. This increases charge density in the depletion region, decreases the depletion width, and increases the capacitance. The predicted increase in capacitance was experimentally observed using a pulsed 70 keV electron beam as the source of ionization. In additional confirming experiments, the carbon charge-state conversion was accomplished by photoionization using sub-bandgap light or by the capture of holes under forward bias.

Enhanced Photocarrier Collection in Bismuth Vanadate Photoanode through Modulating the Inner Potential Distribution

Abstract: Photoelectrochemical (PEC) synthesis provides a promising approach to produce green fuels, while the low energy conversion efficiency limits its application due to photocarrier recombination. Photocarrier recombination can be inhibited by building potential gradients in the photoelectrode with heterojunctions. However, despite the achieved progress, only a small amount of photocarriers can be collected and take PEC reactions. One of the main reasons is that the potential gradient provided by heterojunctions is limited in the depletion region, while photocarrier recombination mainly occurrs in the charge diffusion region. To break through the limitation of band engineering technique, herein, polarized BaTiO3 nanoparticles are embedded in BiVO4 composite photoanode to modulate the inner potential distribution so that more photocarriers can be successfully collected. The results show that the energy conversion efficiency has been significantly improved, as evidenced by an improved PEC H2O2 production concentration of 836 µmol L–1 under simulated sunlight illumination, which is 2.6 times of pure BiVO4 in the same situation. This work demonstrates a feasible strategy to enhance photocarrier collection efficiency by modulating the inner potential distribution in photoelectrode with an embedded polarizer, which can significantly enhance the energy conversion efficiency without extra energy consumption.

Characterization of Low Gain Avalanche Detector Prototypes’ Response to Gamma Radiation

Abstract: Motivated by the need for fast timing detectors to withstand up to 2 MGy of ionizing dose at the High Luminosity Large Hadron Collider, prototype low gain avalanche detectors (LGADs) have been fabricated in a single pad configuration, 2 × 2 arrays, and related p-i-n diodes, and exposed to Co-60 sources for study. Devices were fabricated with a range of dopant layer concentrations, and for the arrays, a variety of inter-pad distances and distances from the active area to the edge. Measurements of capacitance versus voltage and leakage current versus voltage have been made to compare pre- and post-irradiation characteristics in gain layer depletion voltage, full bulk depletion voltage, and breakdown voltage. Conclusions are drawn regarding the effects of the gammas on both surface and interface states and on their contribution to acceptor removal through non-ionizing energy loss from Compton electrons or photoelectrons. Comparison of the performances of members of the set of devices can be used to optimize gain layer parameters.

Impact of 100 MeV high-energy proton irradiation on β-Ga2O3 solar-blind photodetector: Oxygen vacancies formation and resistance switching effect

Abstract: β-Ga2O3 based solar-blind photodetectors have strong radiation hardness and great potential applications in Earth's space environment due to the large bandgap and high bond energy. In this work, we investigated the photoelectric properties influence of β-Ga2O3 photodetector irradiated by 100 MeV high-energy protons which are the primary components in the inner belt of the Van Allen radiation belts where solar-blind photodetectors mainly worked. After proton irradiation, due to the formation of more oxygen vacancies and their migration driven by bias at the metal/semiconductor interface, transportation of carriers transforms with electron tunneling conduction for low-resistance state and thermionic emission for high resistance state. As a result, the current–voltage curves of β-Ga2O3 solar-blind photodetectors exhibit apparent hysteresis loops. The photoresponsivity of β-Ga2O3 photodetectors slightly increases from 1.2 × 103 to 1.4 × 103 A/W after irradiation, and the photoresponse speed becomes faster at a negative voltage while slower at positive voltage. The results reveal the effects of high-energy proton irradiation on β-Ga2O3 solar-blind photodetectors and provide a basis for the study of their use in a radiation harsh environment.

Terahertz radiation from propagating acoustic phonons based on deformation potential coupling.

Abstract: We report on new THz electromagnetic emission mechanism from deformational coupling of acoustic (AC) phonons with electrons in the propagation medium of non-polar Si. The epicenters of the AC phonon pulses are the surface and interface of a GaP transducer layer whose thickness (d) is varied in nanoscale from 16 to 45 nm. The propagating AC pulses locally modulate the bandgap, which in turn generates a train of electric field pulses, inducing an abrupt drift motion at the depletion edge of Si. The fairly time-delayed THz bursts, centered at different times (t1T H z, t2T H z, and t3T H z), are concurrently emitted only when a series of AC pulses reach the point of the depletion edge of Si, even without any piezoelectricity. The analysis on the observed peak emission amplitudes is consistent with calculations based on the combined effects of mobile charge carrier density and AC-phonon-induced local deformation, which recapitulates the role of deformational potential coupling in THz wave emission in a formulatively distinct manner from piezoelectric counterpart.