Analysis Of Carrier Transport Research Articles

Analyzing Carrier Density and Hall Mobility in Impurity‐Free Silicon Virtually Doped by External Defect Placement

AbstractImpurity doping at the nanoscale for silicon is becoming less efficient with conventional techniques. Here, an alternative virtual doping method is presented for silicon that can achieve an equivalent carrier density while addressing the primary limitations of traditional doping methods. The doping for silicon is carried out by placing aluminum‐induced acceptor states externally in a silicon dioxide dielectric shell. This technique can be referred to as direct modulation doping. The resistivity, carrier density, and mobility are investigated by Hall effect measurements to characterize the carrier transport using the new doping method. The results thereof are compared with carrier transport analysis of conventionally doped silicon at room‐temperature, demonstrating a 100% increase in carrier mobility at equal carrier density. The sheet density of hole carriers in silicon due to modulation doping remains nearly constant, ≈4.7 × 1012 cm−2 over a wide temperature range from 300 down to 2 K, proving that modulation‐doped devices do not undergo carrier freeze‐out at cryogenic temperatures. In addition, a mobility enhancement is demonstrated with an increase from 89 cm2 Vs−1 at 300 K to 227 cm2 Vs−1 at 10 K, highlighting the benefits of the new method for creating emerging nanoscale electronic devices or peripheral cryo‐electronics to quantum computing.

Analysis of Carrier Transport at Zn1-xSnxOy/Absorber Interface in Sb2(S,Se)3 Solar Cells.

This work explores the effect of a Zn1-xSnxOy (ZTO) layer as a potential replacement for CdS in Sb2(S,Se)3 devices. Through the use of Afors-het software v2.5, it was determined that the ZTO/Sb2(S,Se)3 interface exhibits a lower conduction band offset (CBO) value of 0.34 eV compared to the CdS/Sb2(S,Se)3 interface. Lower photo-generated carrier recombination can be obtained at the interface of the ZTO/Sb2(S,Se)3 heterojunction. In addition, the valence band offset (VBO) value at the ZTO/Sb2(S,Se)3 interface increases to 1.55 eV. The ZTO layer increases the efficiency of the device from 7.56% to 11.45%. To further investigate the beneficial effect of the ZTO layer on the efficiency of the device, this goal has been achieved by five methods: changing the S content of the absorber, changing the thickness of the absorber, changing the carrier concentration of ZTO, using various Sn/(Zn+Sn) ratios in ZTO, and altering the thickness of the ZTO layer. When the S content in Sb2(S,Se)3 is around 60% and the carrier concentration is about 1018 cm-3, the efficiency is optimal. The optimal thickness of the Sb2(S,Se)3 absorber layer is 260 nm. A ZTO/Sb2(S,Se)3 interface with a Sn/(Zn+Sn) ratio of 0.18 exhibits a better CBO value. It is also found that a ZTO thickness of 20 nm is needed for the best efficiency.

Programmable Optoelectronic Synaptic Transistors Based on MoS2/Ta2NiS5 Heterojunction for Energy-Efficient Neuromorphic Optical Operation.

The optoelectronic synaptic transistors with various functions, broad spectral perception, and low power consumption are an urgent need for the development of advanced optical neural network systems. However, it remains a great challenge to realize the functional diversification of the systems on a single device. 2D van der Waals (vdW) materials can combine unique properties by stacking with each other to form heterojunctions, which may provide a strategy for solving this problem. Herein, an all-2D vdW heterojunction-based programmable optoelectronic synaptic transistor based on MoS2/Ta2NiS5 heterojunctions is demonstrated. The device implements reconfigurable, multilevel non-volatile memory (NVM) states through sequential modulation of multiple optical and electrical stimuli to achieve broadband (532-808nm), energy-efficient (17.2 fJ), hetero-synaptic functionality in a bionic manner. The intrinsic working mechanisms of the photogating effect caused by band alignment and the interfacial trapping defect modulation induced by gate voltage are revealed by Kelvin-probe force microscopy (KPFM) measurements and carrier transport analysis. Overall, the (opto)electronic synaptic weight controllability for combined in-sensor and in-memory logic processors is realized by the heterojunction properties. The proposed findings facilitate the technical realization of generic all 2D hetero-synapses for future artificial vision systems, opto-logical systems, and Internet of Things (IoT) entities.

Analysis of electromechanical couplings and nonlinear carrier transport in flexoelectric semiconductors

In recent years, a linearization method has been extensively employed to investigate the electromechanical fields and carrier distribution in flexoelectric semiconductors, where the assumption of a small perturbation of carrier concentration is adopted. However, this method fails to accurately describe the realistic physical process in which a considerable variation of carrier concentration takes place. Based on fully coupled nonlinear equations, this paper presents a finite element approach to study the electromechanical couplings and nonlinear carrier transport in flexoelectric semiconductors. This method is applied to calculate the electrostatic potential in a bent piezoelectric semiconductive nanowire (NW) going beyond simple considerations and to simulate the nonlinear current–voltage (I–V) characteristics of a mechanically loaded flexoelectric p–n junction. The results indicate that the inherently nonlinear drift of carriers gives rise to the asymmetric distribution of the electric potential relative to the NW axis in the upper body. Flexoelectricity brings about a remarkable enhancement in output voltage and is responsible for the linear variation of electric potential along the length direction of the NW unless close to two ends. Furthermore, the barrier height and I–V relations of a flexoelectric p–n junction can be effectively tuned by mechanical forces due to the flexoelectric effect, the effect of which relies on the size of the p–n junction configuration. This work is a good starting point to comprehend the coupling of flexoelectricity and nonlinear carrier transport in static and dynamic cases, and offers an effective approach to numerically deal with the issues involved in flexoelectronics and piezoelectronics at the nanoscale.

Analysis of Carrier Transport in Quantum Dot/Metal‐Oxide Phototransistors via Light‐Mediated Interfacial Modeling

AbstractQuantum dot (QD) hybrid phototransistors based on high‐mobility channel semiconductors have attracted great interest due to their outstanding photonic characteristics with diverse and broadband optoelectronic functionalities. However, difficulties for precise control of electronic coupling between QD particles and lack of QD/semiconductor interface analysis have limited the use of these platforms into practical optoelectronic applications. Here, the authors report a new strategy to high‐performance phototransistors based on CdSe QD/amorphous oxide semiconductor heterostructures using trap‐reduced chelating chalcometallate surface ligands by optimizing the QD layer thickness and interface engineering to enhance photogenerated charge transfer properties. Furthermore, based on performance parameters extracted from experimental structure and electrical characteristics, Technology Computer‐Aided Design (TCAD) modeling reveals carrier flow between the QDs in the semiconductor layer in agreement with various spectroscopic data. These modeling results indicate that interface engineering of QD with chalcometallate ligand results in very efficient photo‐induced charge transfer characteristics. Thus, high‐performance chalcometallate ligand QD‐based phototransistors are successfully realized with enhanced responsivity (2.12 × 104 A W−1) and photodetectivity (2.74 × 1016 Jones) compared with conventional monodentate ligand functionalized (SCN−) QD‐based devices (8.79 × 103 A W−1 and 8.8 × 1010 Jones, respectively). This is the demonstration of systematic analysis of QD‐based hetero‐structured phototransistors with light‐mediated interfacial physical modeling and electrochemical measurement.

Simulation analysis of high-field carrier transport in wide-bandgap semiconductors considering tunable band structures and scattering processes

Though the high breakdown electric field of wide-bandgap semiconductors is usually attributed to their large bandgap, the impacts of other band structure parameters and scattering processes on impact ionization phenomena have not been clarified yet. This study computationally analyzes the effects of band structures and scattering rates on the high-field carrier transport properties such as impact ionization coefficients and drift velocity in wide-bandgap semiconductors. For that purpose, this study adopts Monte Carlo simulations in which the E–k dispersion and scattering rates are directly tuned. Simulations with varied band structures indicate that an average of the group velocity in the whole Brillouin zone is a dominant factor determining the impact ionization coefficients rather than the effective mass at the band edge. In addition, the Brillouin zone width has critical impacts when Bloch oscillations occur, which significantly suppress impact ionization. As for scattering mechanisms, the roles of inelastic scattering processes including impact ionization in energy relaxation are discussed. It is also revealed that elastic scattering contributes to energy relaxation processes through transitions of electrons to higher bands. This mechanism leads to the unintuitive positive temperature dependence of impact ionization coefficients when Bloch oscillations occur. These results obtained by the theoretical analyses in this study can serve as basic physical insight to understand the behaviors of impact ionization coefficients in wide-bandgap semiconductors.

Anthracene-dibenzofuran based electron transport type hosts for long lifetime multiple resonance pure blue OLEDs

In this work, we report two simple and efficient electron transport (ET) type host materials, namely 2-(3-(10-phenylanthracen-9-yl)-phenyl)dibenzo[b,d]furan (m-PPDF) and 2-(4-(10-phenylanthracen-9-yl)phenyl)dibenzo[b,d]furan (p-PPDF) based on 9,10-diphenylanthracene and dibenzofuran moieties. The designed host materials show excellent thermal stability with thermal decomposition temperature (Td) above 351 °C. The computational studies and carrier transport analysis proved good electron-transporting features of these hosts. The synthesized materials were utilized as hosts for multiple resonance emitters (DABNA-NP-TB) based blue OLEDs. The fabricated OLED device based on p-PPDF host and DABNA-NP-TB dopant shows a high external quantum efficiency (EQE) of 7.03% with a narrow spectrum in deep blue color and Commission International de l’Eclairage (CIE) coordinate of (0.136, 0.076). Further, the operational lifetime at 95% initial luminescence decay (LT 95 ) of the p-PPDF based host device shows 85 h at 600 cd/m 2 , which is increased by 1.7 times compared with the well-known bipolar host (PhPC). Further, we analyzed that such a long lifetime is attributed to the improved stability of electron transfer type-designed hosts. • We report long lifetime fluorescent host materials. • Anthracene-dibenzofuran core host for multi-resonance blue emitter. • EQE of 7.03%, CIE of (0.136, 0.076) and 85h(LT 95 ) at 600nit are reported.

High-efficiency organic photovoltaic cells processed using a non-halogen solvent

In this paper we describe high-performance PM6:BTP-eC9–based organic photovoltaic (OPV) cells prepared using non-halogen solvents, with the goal of minimizing any potential environmental pollution. We investigated three green solvents (toluene, o-xylene, and 1,2,4-trimethylbenzene) as replacements for the commonly used chloroform. Using UV–Vis spectroscopy, atomic force microscopy, two-dimensional grazing incidence wide-angle X-ray spectroscopy, and carrier transport analysis, we optimized the blend films and investigated their optoelectronic properties and those of their respective devices. Among our tested devices, the OPV cell prepared with toluene as the solvent and 1,8-diiodooctane (DIO) as the additive displayed the best performance, with a power conversion efficiency (15.8 ± 0.20%) comparable with that of the chloroform-derived OPV (16.2 ± 0.1%). Thus, it is feasible to replace halogen solvents with non-halogen solvents when preparing OPV devices.

Analysis of charge carrier transport in p-n junctions with optimized impurity distributions

Analysis of charge carrier transport in p-n junctions with optimized impurity distributions

Enabling high efficiency and long lifetime of pure blue phosphorescent organic light emitting diodes by simple cyano modified carbazole-based host

Herein, we report two simple and efficient high triplet energy bipolar host materials named 2'-(9H-carbazol-9-yl)-[1,1′-biphenyl]-2-carbonitrile (CzBPCN) and 9-(2′-cyano-[1,1′-biphenyl]-2-yl)-9H-carbazole-3-carbonitrile (CNCzBPCN) by connecting carbazole type donor and cyano acceptor on 2 and 2′ positions of a biphenyl linker, respectively. The effect of cyano substitution on carbazole building block on photophysical and electroluminescence properties was unveiled. Both the compounds revealed high triplet energy (ET) of above 3.0 eV attributed to the interrupted interchromophoric electronic interactions between the donor and acceptor units. The computational studies and carrier transport analysis proved that the compounds secured good bipolar charge transporting feature. The applicability of these materials as hosts for blue phosphorescent organic light emitting diodes (PhOLEDs) was tested. Interestingly, the CNCzBPCN hosted device exhibited superior performance with three fold improved external quantum efficiency (EQEmax) of 20.2% over the CzBPCN hosted device which showed EQEmax of 7.1%. Importantly, the CNCzBPCN hosted device demonstrated 11 fold extended operational lifetime of 121 h up to 50% of its intimal luminance (L0 = 200 cd/m2) compared to CzBPCN. We believe that this work demonstrates the cost-effective and efficient high triplet energy bipolar host materials for simultaneous achievement of high EQE over 20% and long operational lifetime of blue PhOLEDs.

Self-consistent gain calculations and carrier transport analysis for split-well direct-phonon terahertz quantum cascade lasers

A split-well direct phonon THz quantum cascade laser scheme supporting a clean three-level system was analyzed using nonequilibrium Green’s functions. In this system, most of the electrons occupy the lower three levels, and thermally activated leakage channels are suppressed almost entirely. The simulations showed that the gain values were deeply affected by the changes in the devices’ interface roughness and impurity scattering strengths. We concluded that in addition to the low oscillator strength and relatively low doping level, these are the mechanisms responsible for the temperature performance limitations of this scheme.

Fabrication of high-breakdown GaN Schottky barrier diodes over deeply-etched crystal surfaces

We report in this paper an effective method to recover the defects on GaN surfaces following inductively coupled-plasma (ICP) deep-etching using KOH treatment with optimized concentration. GaN Schottky barrier diodes are fabricated over the unintentionally doped GaN buffer region of LED epi-wafers with this method, which achieves a high breakdown over 200 V as revealed in our measurement. This was accompanied with a dramatically reduced leakage current by four orders of magnitude. Temperature-variable current–voltage characterization of the fabricated Schottky junctions and the subsequent carrier transport analysis indicate that the surface concentration of the n-type dopants reduces by more than 20-folds, confirming the effectiveness of the KOH treatment in removing the ICP-induced surface defects.

Optical testing method analysis of carrier transport in GaAs PCSS

Measurement of the carrier transport in the GaAs photoconductive semiconductor switch (PCSS) is important for the study of the GaAs PCSS current filament and the design of improved GaAs PCSS devices. In this study, an optical testing method was proposed for the measurement of the time-dependent carrier density distribution in the GaAs PCSS. The measured electron density curves show that the carrier density distribution between the two electrodes is nonuniform for Δt = 0, and as Δt increase from 0.6 ns to 6 ns, a large number of carriers concentrate near the anode of the GaAs PCSS. These phenomena are analyzed in detail and the measured curves are found to be in good agreement with the simulation results.

V-Pits-Induced Photoresponse Enhancement in AlGaN UV-B Photodetectors on Si (111)

We demonstrate the influence of surface terminated V-pits in tuning dark current and spectral responsivity of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</sub> N-based UV-B photodetectors with metal-semiconductor-metal geometry on Si (111) substrate. We show that the V-pit morphological defects contribute to a large internal gain in these photodetectors, thereby leading to a substantial enhancement in external quantum efficiency (EQE) at relatively low applied biases. For photodetectors fabricated on metal organic chemical vapor deposition grown Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</sub> N epilayers with a surface pit density of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , an EQE of 100% was measured at a meager bias of 1.7 V, which increased significantly with bias. The EQE, photo-to-dark current ratio, and UV-to-visible rejection ratio measured 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> %, 1.2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> , and 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , respectively, at 5 V. The evidence of localized enhancement of photoresponse at the surface terminations of V-pits is exemplified by UV-assisted conductive atomic force microscopy. Temperature-dependent carrier transport analysis under dark and UV illumination revealed cumulative contributions of pit-induced thermionic field emission and hole-trapping-induced gain to the observed large EQE. This work presents the highest value of responsivity for III-nitride UV-B detectors at a given bias.

Analysis of Carrier Transport in Tunnel-Junction Vertical-Cavity Surface-Emitting Lasers by a Coupled Nonequilibrium Green’s Function–Drift-Diffusion Approach

This work investigates carrier transport in tunnel junctions for vertical-cavity surface-emitting lasers (VCSELs). The study is performed with a quantum-corrected semiclassical approach, where tunneling is described rigorously with a nonequilibrium Green's function formalism based on a multiband description of the electronic structure. Validated with experimental results, the proposed approach provides a quantum -kinetic perspective of the tunneling process and paves the way toward a comprehensive theory of VCSELs, bridging the gap between semiclassical and quantum simulations.

Conductive filament evolution dynamics revealed by cryogenic (1.5 K) multilevel switching of CMOS-compatible Al2O3/TiO2 resistive memories

Non-volatile resistive switching devices are considered as prime candidates for next-generation memory applications operating at room temperature and above, such as resistive random-access memories or brain-inspired in-memory computing. However, their operability in cryogenic conditions remains to be mastered to adopt these devices as building blocks enabling large-scale quantum technologies via quantum–classical electronics co-integration. This study demonstrates multilevel switching at 1.5 K of Al2O3/TiO2-x resistive memory devices fabricated with complementary metal-oxide-semiconducto-compatible processes and materials. The I–V characteristics exhibit a negative differential resistance (NDR) effect due to a Joule-heating-induced metal-insulator transition of the Ti4O7 conductive filament. Carrier transport analysis of all multilevel switching I–V curves show that while the insulating regime follows the space charge limited current (SCLC) model for all resistance states, the conduction in the metallic regime is dominated by SCLC and trap-assisted tunneling for low- and high-resistance states respectively. A non-monotonic conductance evolution is observed in the insulating regime, as opposed to the continuous and gradual conductance increase and decrease obtained in the metallic regime during the multilevel SET and RESET operations. Cryogenic transport analysis coupled to an analytical model accounting for the metal-insulator-transition-induced NDR effects and the resistance states of the device provide new insights on the conductive filament evolution dynamics and resistive switching mechanisms. Our findings suggest that the non-monotonic conductance evolution in the insulating regime is due to the combined effects of longitudinal and radial variations of the Ti4O7 conductive filament during the switching. This behavior results from the interplay between temperature- and field-dependent geometrical and physical characteristics of the filament.

Rationalizing device performance of perylenediimide derivatives as acceptors for bulk-heterojunction organic solar cells

Abstract This study examines the correlation between the structure of four perylenediimide (PDI) derivatives and their optoelectronic behaviors. Three of them with one, two or four PDI moieties conjugated peripherally to porphyrin core by phenylene linker, denoted as nPDI-ZnP (n = 1,2,4), are investigated as acceptors on the charge transport property and device performance in non-fullerene (NF) bulk-heterojunction (BHJ) organic photovoltaic cells with a commercial polymer PTB7-Th as the donor material. Besides, the parent PDI molecule is studied as a control. Detailed charge carrier transport analysis is carried out on these PTB7-Th:Acceptor BHJs. Among all acceptor molecules, PTB7-Th:4PDI-ZnP film possesses the most desirable transport properties among the NF BHJs, and the best device performance with a power conversion efficiency (PCE) of 8.35%. To quantify the transport behaviors, the charge imbalance factor Δ is introduced to describe the global electron and hole balance in different electric fields. We discover that PTB7-Th:4PDI-ZnP has the least Δ and thereby gives rise to a BHJ cells with the highest fill factor and PCE.

Analysis of Carrier Transport in n-Type Hg1−xCdxTe with Ultra-Low Doping Concentration

Mercury cadmium telluride (HgCdTe, or MCT) with low n-type indium doping concentration offers a means for obtaining high performance infrared detectors. Characterizing carrier transport in materials with ultra low doping (ND = 1014 cm−3 and lower), and multi-layer material structures designed for infrared detector devices, is particularly challenging using traditional methods. In this work, Hall effect measurements with a swept B-field were used in conjunction with a multi-carrier fitting procedure and Fourier-domain mobility spectrum analysis to analyze multi-layered MCT samples. Low temperature measurements (77 K) were able to identify multiple carrier species, including an epitaxial layer (x = 0.2195) with n-type carrier concentration of n = 1 × 1014 cm−3 and electron mobility of μ = 280000 cm2/Vs. The extracted electron mobility matches or exceeds prior empirical models for MCT, illustrating the outstanding material quality achievable using current epitaxial growth methods, and motivating further study to revisit previously published material parameters for MCT carrier transport. The high material quality is further demonstrated via observation of the quantum Hall effect at low temperature (5 K and below).

Analysis of carrier transport and band tail states in p-type tin monoxide thin-film transistors by temperature dependent characteristics

Tin monoxide (SnO) has drawn much attention in recent years due to its high hole mobility, transparency, and potential for mass production. However, due to its metastable nature, the deposited film often contains multi-phases such as metallic tin and tin dioxide, which may degrade its electrical properties. Here, we presented the temperature dependent characteristics of p-type SnO thin-film transistors. The hole transport mechanism is dominated by band conduction at high temperatures and variable-range hopping at low temperatures. The maximum activation energy was found to be 308 meV, which denotes a bandgap of around 0.6 eV. The density of states was found to be 1.12 × 1021 cm−3 eV−1 at VG = −80 V, and 6.75 × 1020 cm−3 eV−1 at VG = 0 V, respectively.

Performance and carrier transport analysis of In0.7Ga0.3As quantum-well MOSFETs with Al2O3/HfO2 gate stack

In this paper, we have fabricated and characterized In0.7Ga0.3As quantum-well (QW) metal-oxide-semiconductor field-effect-transistors (MOSFETs). We have employed the gate dielectric of the Al2O3/HfO2 (0.6/2nm) bi-layer stack by ALD. The fabricated device with Lg=4μm exhibits a record maximum transconductance (gm_max) in excess of 520μS/μm at >1μm region, and reasonably good electrostatic integrity, such as SS=110mV/decade and DIBL=43mV/V. Also, we have investigated the gate length scaling behavior in terms of output, transconductance, and transfer characteristics. In particular, our devices feature very uniform values of the electrostatic integrity, such as SS=100–110mV/decade, VT=−0.25V to −0.2V and DIBL=40–50mV/V, as Lg decreases from 10μm to 4μm. Furthermore, we have explored the impact of source resistance (RS) onto the device characteristics of the InGaAs QW MOSFETs. In doing so, we have modeled both measured extrinsic transconductance (gm_ext) and intrinsic transconductance (gm_int) as a function of Lg.