Fluctuations Of Band Edges Research Articles

Design and analysis of hetero-dielectric Junctionless-TFET(JL-TFET) with N+ pocket as label free biosensors

This paper includes sensitivity assessment of label-free biosensors using hetero dielectric Junctionless-TFET (HD-JL-TFET) thorough TCAD simulator. The fundamental structure, operation and design of a Junctionless-TFET (HD-JL-TFET) as biosensor are investigated in this paper. For the purpose of detecting the biomolecule, a nano-gap is added close to the source end between the gate and channel. To test the sensing potential, we adjusted the charge density and material dielectric constant (K) by comprehensive calibrated device simulation. For several biomolecules, the device’s sensitivity was examined as surface potential, electron tunnelling rate, and conduction-valence band edge fluctuation. Additionally, the Id versus VGS features, the sensitivity to the drain current, and the ION/IOFF fluctuation are also examined. By contrasting neutral or charged biomolecules using various dielectric constants, the sensitivity characteristics of positive, negative, and neutral biomolecules are examined. The development of biosensors, which enable the rapid and precise detection of multiple biomolecules, has revolutionized the field of bioanalysis.

Electrical transport in epitaxially grown undoped and Si-doped degenerate GaN films

This study investigates the electrical transport properties of undoped and Si-doped, degenerate GaN heteroepitaxial films grown on sapphire by reactive rf sputtering of GaAs (and Si) in Ar-N2 mixture. The room temperature electrical measurements showed that the resistivity of undoped GaN film grown at 100% N2 was ∼2 × 105 Ω cm, which reduced to ∼1 Ω cm in Si-doped film, revealing the effect of Si doping. With decrease of N2 from 100% to 75%, the carrier concentration of Si-doped films increased from ∼7 × 1018 cm−3 to ∼2 × 1019 cm−3, but remained practically unchanged as N2 was decreased to 20%, which is explained by effects due to saturation of Si doping and increase of Ga interstitials as well as compensation by N interstitials and Ga vacancies. Undoped and Si-doped films grown below 20% N2 displayed similar carrier concentrations (∼1020 cm−3), due to dominance of Ga interstitials. Both undoped and Si-doped films were degenerate and displayed increase of mobility with carrier concentration and temperature, which was analyzed by the combined effect of ionized impurity and dislocation scattering, using compensation ratio as fitting parameter. At carrier concentrations ≲1019 cm−3, the mobility was governed by both ionized impurity and dislocation scattering, while at higher carrier concentrations, ionized impurity scattering was found to dominate, limited by compensation due to acceptors. In spite of the degenerate character, the films displayed a small decrease of carrier concentration with temperature, along with a nearly linear decrease of mobility, which are explained by a marginal increase of compensation ratio with decrease of temperature, taking into account the band edge fluctuation effects. These features of electrical transport have not been much explored for heteroepitaxial, n-type degenerate GaN films, possessing high density of dislocations and point defects.

Reversible Photomodulation of Two-Dimensional Electron Gas in LaAlO3/SrTiO3 Heterostructures.

Long-lived photoinduced conductance changes in LaAlO3/SrTiO3 (LAO/STO) heterostructures enable their use in optoelectronic memory applications. However, it remains challenging to quench the persistent photoconductivity (PPC) instantly and reproducibly, which limits the reversible optoelectronic switching. Herein, we demonstrate a reversible photomodulation of two-dimensional electron gas (2DEG) in LAO/STO heterostructures with high reproducibility. By irradiating UV pulses, the 2DEG at the LAO/STO interface is gradually transformed to the PPC state. Notably, the PPC can be completely removed by water treatment when two key requirements are met: (1) the moderate oxygen deficiency in STO and (2) the minimal band edge fluctuation at the interface. Through our X-ray photoelectron spectroscopy and electrical noise analysis, we reveal that the reproducible change in the conductivity of 2DEG is directly attributed to the surface-driven electron relaxation in the STO. Our results provide a stepping-stone toward developing optically tunable memristive devices based on oxide 2DEG systems.

Thermal fluctuations and carrier localization induced by dynamic disorder in MAPbI3 described by first-principles based tight-binding model

Halide perovskites are strongly influenced by large amplitude anharmonic lattice fluctuations at room temperature. We develop a tight-binding model for dynamically disordered ${\mathrm{MAPbI}}_{3}$ based on density functional theory calculations to calculate electronic structure for finite temperature crystal structures at the length scale of thermal disorder and carrier localization. The model predicts individual Hamiltonian matrix elements and band structures with high accuracy, owing to the inclusion of additional matrix elements and descriptors for non-Coulombic interactions. We apply this model to electronic structure at length and timescales inaccessible to first-principles methods, finding an increase in band gap, carrier mass, and the subpicosecond fluctuations in these quantities with increasing temperature as well as the onset of carrier localization in large supercells induced by thermal disorder at 300 K. We identify the length scale ${L}^{*}=5$ nm as the onset of localization in the electronic structure, associated with decreasing band edge fluctuations, increasing carrier mass, and Rashba splitting approaching zero.

Majority Carrier Properties of Single Crystal Cu2−xZnSnSe4 with Varying Copper Composition

The dependence of carrier concentration on copper content in Cu2−xZnSnSe4 single crystals, grown without fluxing agents, is presented over the composition range Cu/(Zn + Sn) of 0.77–0.93 and Zn/Sn from 1.3 (low Cu content) to 1.3 (high Cu content). Single phase mono‐crystals with facets up to 5 mm are synthesized from high‐purity elements and validated using Laue diffraction. A direct optical band gap of 0.98 eV is observed for a crystal with Cu/(Zn + Sn) = 0.85. Van der Pauw–Hall resistance analysis shows decreasing hole concentration, from 1019 cm−3 to 2 × 1015 cm−3, with decreasing copper content over the composition range. Hole mobility varies from 50 cm2 V−1 s−1 to 160 cm2 V−1 s−1, with no dependence on copper concentration. A linear temperature dependence of the Seebeck coefficient is found for all crystals. The Seebeck coefficient–carrier concentration dependence is used to determine the valence band density of states, NV, and corresponding effective mass, μh*. Crystals with Cu/(Zn + Sn) < 0.9 have NV = 4 × 1018 cm−3 and μh* = 0.4 m0 while crystals with elevated copper concentration, Cu/(Zn + Sn) > 0.9, exhibit higher density of states, NV = 3 × 1019 cm−3 and hole effective mass μh* = 0.9 m0. The increased valence band density of states at elevated copper composition is consistent with more Cu–Zn exchange sites contributing to bulk defects and band edge fluctuations. Temperature‐dependent conductivity measurements indicate a transition from metallic‐type to semiconductor‐type conduction at Cu/(Zn + Sn) < 0.9. At lower copper content, for a crystal with Cu/(Zn + Sn) = 0.83 and p = 1017 cm−3 the activation energy for intrinsic conduction is 60 meV. This behavior is attributed to the copper vacancy defect, VCu, and is comparable to reported values obtained in thin films and by first‐principles predictions.

Carrier transport properties in a thin-film Cu2ZnSnSe4 solar cell

We report the measurement of majority carrier concentration, depletion width, mobility, and resistivity in a thin-film based Cu2ZnSnSe4 (CZTSe) photovoltaic device. The carrier transport properties were measured using coordinated admittance spectroscopy and capacitance-voltage technique. The bias dependence of the modified dielectric relaxation in the absorber of the CZTSe solar cell was investigated to extract the mobility and resistivity. Hall measurement was also performed at room temperature for the verification of carrier concentration, resistivity, and mobility. The temperature dependent resistivity and mobility exhibit thermally activated behaviors characterized by a thermal activation energy ≈ 60 meV. The positive temperature dependence of the mobility indicates a carrier-transport impeding effect caused by the band-edge fluctuation in poly-crystalline CZTSe, whose magnitude is measurable by the aforementioned activation energy.

Photon‐Induced Reversible Phase Transition in CsPbBr3 Perovskite

AbstractStructure reorganization within perovskite materials has attracted much attention due to its assisted appealing features in optoelectronic devices, such as achieving continue‐wave laser and performance enhancement in photovoltaic devices. Unfortunately, the difficulty of controlling reorganization processing and unclear underlying mechanisms impose an impediment for taking advantage of the structural reorganization in pursuit of distinctive functions in perovskite‐based devices. In this work, using above‐bandgap illumination with a small energy threshold (1.6 mW cm−2) triggering phase transition from orthorhombic to tetragonal in CsPbBr3 is first reported. This photon‐induced structure reorganization is reversible and presents a fast and controllable response (<0.5 s) to light on/off. Raman spectroscopy and density functional theory calculations reveal that such a dynamic structure reorganization is caused by the transition of torsion direction in Pb–Br octahedral, while the diffusion potential difference induced local Coulombic field is proved to drive this process. The findings provide a deep understanding for universal structure reorganization under irradiation in perovskite materials and encourage further study of the novel functions associated with structure reorganization inducing temporal behaviors in optoelectronic devices, for example variations in dielectric constant and band edge fluctuation induced Rashba effects, which show a significant influence on perovskite‐based optoelectronic devices.

(Invited) Characterization of Phases and Defects in Compound Semiconductors

Defects in semiconductors can be difficult to characterize, especially in complex semiconductors such as the chalcogenides. This talk presents several different and relatively unusual methods for their study along with examples of results. The talk will focus on semiconductors used for photovoltaic applications but the talk should illustrate methods that would be of use with any semiconductor material. Specifically the talk will address the use of admittance spectroscopy, nuclear magnetic resonance (NMR), scanning microwave impedance microscopy (SMIM), scanning tunneling spectroscopy (STS) and others to study defects in CdTe, CuInSe2 (CIGS), and Cu2ZnSnS4 (CZTS), and related compounds. Examples of results include the identification of two defects in the energy gap of CIGS by admittance spectroscopy, band edge fluctuations in CIGS by STS and their impact on the ability to obtain atomic resolution images, organized point defect structures in CZTS by NMR, and grain and grain boundary behaviors in CdTe by SMIM. Other examples will be included as time permits.

Simultaneous blocking of minority carrier and high energy phonon in p-type skutterudites

Skutterudites are among the most promising thermoelectric materials. However, the strong bipolar effects and a lack of high energy phonon scattering make the thermoelectric performance of p-type skutterudites far below that of n-type skutterudites. Here, we reveal that the distribution of cations spatially fluctuate in p-type skutterudites, leading to a band edge fluctuation with small valence-band offsets, multi-scale strain field fluctuation, and formation of nanostructures. It further results in a significant suppressed bipolar effect, and wide-frequency spectrum phonon scattering. A 38% enhanced power factor and reduced lattice thermal conductivity approaching to the theoretical minimum at 723 K lead to a remarkable theoretical conversion efficiency of 13.3%, reaching the highest records in p-type lead-free thermoelectric materials. Our results provide a new thought to realize high performance in polynary thermoelectric materials.

Scanning tunneling spectroscopy of epitaxial silver indium diselenide

Abstract We report on the measurement of the electronic properties of (112) AgInSe 2 (AIS) by scanning tunneling microscopy (STM) and spectroscopy (STS). Current–voltage STS measurements show an average band gap of about 1.3 eV and n-type behavior. The IV data also shows band edge fluctuations and a region of states near the valence band edge that decay well into the gap. We compare STS determined band fluctuations in AIS to those previously measured in CuInSe 2 (CIS) and find that the fluctuations are smaller in scale in AIS than CIS.

Photoreflectance Study of Strained GaAsN/GaAs T-junction Quantum Wires Grown by Metal-Organic Vapor Phase Epitaxy

Strained GaAsN T-junction quantum wires (T-QWRs) with different N contents grown on GaAs by two steps metal-organic vapor phase epitaxy in [001] and [110] directions, namely QW1 and QW2 respectively, have been investigated by photoreflectance (PR) spectroscopy. Two GaAsN T-QWRs with different N contents were formed by T-intersection of (i) a 6.4-nm-thick GaAs0.89N0.011 QW1 and a 5.2-nm-thick GaAs0.968N0.032 QW2 and (ii) a 5.0-nm-thick GaAs0.985N0.015 QW1 and a 5.2-nm-thick GaAs0.968N0.032 QW2. An evidence of a one-dimensional structure at T-intersection of the two QWs on the (001) and (110) surfaces was established by PR resonances associated with extended states in all the QW and T-QWR samples. It is found that larger lateral confinement energy than 100 meV in both of [001] and [110] directions were achieved for GaAsN T-QWRs. With increasing temperature, the transition energy of GaAsN T-QWRs decreases with a faster shrinking rate compared to that of bulk GaAs. Optical quality of GaAsN T-QWRs is found to be affected by the N-induced band edge fluctuation, which is the unique characteristic of dilute III-V-nitrides.

Modelling of the effects of conduction band fluctuations caused by nitrogen clustering in GaInNAs materials

AbstractIt has been observed experimentally that the band edge photoluminescence of GaInNAs Quantum well (QW) materials is broadened resulting from band‐tailing, localised states or conduction band edge fluctuations. In this paper we develop a model for N compositional fluctuations causing conduction band edge fluctuations which localise the electrons into the resulting quantum dots (QDs). The electron dynamics in the QDs and QW states are examined using a rate equation approach and the carrier populations presented as a function of barrier height and temperature. This mechanism could lead to broad gain in GaInAsN QW structures which could be useful for broad band SOAs for optical communications (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence properties of In(Zn)P alloy core/ZnS shell quantum dots

Chemically synthesized InP/ZnS core/shell quantum dots (QDs) are studied using time-resolved photoluminescence spectroscopy and x-ray diffraction. Zinc stearate, which is added during the synthesis of the InP core, significantly improves the optical characteristics of the QDs. The luminescence quantum yield (QY) reaches 60%–70% and the emission is tunable from 485 to 586 nm by varying the Zn2+:In3+ molar ratio and growth temperature. The observed increased Stokes shift, luminescence decay time, and QY in the presence of Zn are rationalized by the formation of an In(Zn)P alloy structure that causes band-edge fluctuation to enhance the confinement of the excited carriers.

Scanning tunneling microscopic analysis of Cu(In,Ga)Se2 epitaxial layers

Scanning tunneling microscopy (STM) measurements have been made on single-crystal epitaxial layers of CuInSe2 grown on GaAs substrates. Results were obtained for as-grown, air-exposed, and cleaned surfaces; in situ cleaved surfaces; surfaces sputtered and annealed in the STM system; and samples prepared by a light chemical etch. Conventional constant-current topographs, current-voltage curves, and current imaging tunneling spectroscopy (CITS) scans were obtained. Topographic images show that the surfaces appear rough on the atomic scale and often exhibit regular features consistent with a previously proposed surface ad-dimer reconstruction. CITS scans show a spatially varying energy gap consistent with band-edge fluctuations on a scale of a few atomic spacings. Energy variations were observed in both band edges. Although quantitative description of the magnitude of these fluctuations is difficult, the fluctuations on the atomic scale appear much larger than observed by methods such as photoluminescence, which average over larger volumes.

The Photocurrent of Dye-Sensitized Solar Cells Enhanced by the Surface Plasmon Resonance

To evaluate the effect of the surface plasmon resonance of Ag nanoparticles on dye-sensitized solar cells, we measured the photocurrent of electrochemical cells using dense dye-adsorbed TiO2 thin films with and without Ag nanoparticles at different adsorbed-dye concentrations. At lower dye concentrations, the photocurrent of the cells with Ag was found to be enhanced by the effect of the surface plasmon resonance compared to that of the cells without Ag. On the other hand, the photocurrent of the cells with Ag decreased at higher dye concentrations, which was caused by increased trap levels and band-edge fluctuations with Ag incorporation. It is revealed that the control of the structure and quantity of Ag particles is important to apply the surface plasmon resonance effect to dye-sensitized solar cells.

An alloy semiconductor system with a tailorableband-tail and its application to high-performance laseroperation: I. A band-states model for an alloy-fluctuatedInGaN-material system designed for quantum well laser operation

This paper presents a model for a new class ofsemiconductor-alloy systems for which the band-tail degree can be tailored. The general model is based on experimentally derived data for the InGaN-alloy system. The temperaturedependence of the photoluminescence decay time for an InGaN QWstructure is fully analysed, verifying the validity of themodel in terms of a band-edge state modified by In compositionalvariation. We found that the band-edge fluctuation can begrown-in between 3 and 170 meV with a singlewell defined quasi-Fermi level. This model is then used to predict the laser performance for an alloy-fluctuated materialssystem. We found that the differential gain and the criticalcarrier density of the inversion distribution are strongly correlated with the fluctuation but in opposite ways. Theutilization of alloy fluctuation is very beneficial for aspecific laser operation condition if there is sufficient fluctuation optimization. The experimental verification ofthe model with regards to the laser operation is described in the companionpaper (part II).

Effects of the incorporation of silver and gold nanoparticles on the photoanodic properties of rose bengal sensitized TiO 2 film electrodes prepared by sol-gel method

Rose bengal-deposited TiO 2 film electrodes bearing dispersed Ag or Au nanoparticles were prepared by the sol-gel method. The dye-induced visible region photoresponse of the electrodes decreased with increasing Ag content up to a mole ratio of Ag/TiO 2 = 0.0207, while the UV photoresponse increased. On the other hand, the dye-induced visible region photoresponse decreased to a less extent by incorporation of a larger amount of Au particles of Au/TiO 2 = 0.06, along with decreased UV photoresponse. The effects of the metal particles on the dye sensitization of the electrodes were discussed in terms of band edge fluctuation induced by the surface metal particles, Schottky barriers at TiO 2/metal interfaces, and surface plasma resonance.

Mobility field dependence in a-Ge:H, a-SiGe:H, compensated a-Si: H, amorphous multilayer Si/SiGe/Si and the long-range potential fluctuation model

Abstract The electric field dependence of the mobility is surveyed by time-of-flight (TOF) experiments on a-Ge: H, a-SiGe: H, compensated a-Si: H, and amorphous multilayer Si/SiGe/Si specimens. For all samples, the mobility was found to have very weak field enhancement by the electric field, less than 60% at 2 × 105 V cm−1. This result is inconsistent with predictions of a long-range potential fluctuation (LRPF) model and questions the validity of the LRPF model in explaining the low mobility in these materials. The rnultilayer specimens are constructed with an a-SiGe: H layer (layers) embedded in an a-Si:H matrix in order to create band edge fluctuations. The multilayer structure is confirmed by a Rutherford backscattering (RBS) experiment. TOF measurements on these samples reveal no change in electron and hole mobilities and the mobility activation energy from that of undoped a-Si: H. This result indicates that the carrier transport behaviour in the presence of long range potential fluctuations may not ...