Band Tailing Effect Research Articles

Design and Simulation of a High-Performance Tunneling Field Effect Transistor-Based Biosensor Using a Heterojunction Electron-Hole Bilayer

Abstract This study introduces a novel dielectrically-modulated heterojunction electron-hole bilayer tunnel field-effect transistor (DM-HEHBTFET) for bio-sensing applications. The device features a Ga0.85Sb0.15As/Ga0.8In0.2As heterojunction and a p-type pocket in the channel, achieving a remarkably low threshold voltage (VT) of 20 mV, an average subthreshold slope (SS) of 5.7 mV/dec, and a leakage current (IOFF) as low as 5 × 10⁻¹¹ A/μm. The staggered bandgap in the heterostructures enhances electric field control, enabling lower gate voltage operation. Furthermore, the strategically positioned nanogap cavities in non-overlapping regions of the top and bottom gates effectively mitigate gate control issues over the channel, ensuring improved device performance. A modified design, the modified DM-HEHBTFET, is also proposed, featuring source and drain regions engineered with Ga0.85Sb0.15As/Ga0.8In0.2As heterojunctions. This design mitigates leakage current and improves the average subthreshold slope (SS). For biomolecules with a dielectric constant of 12, the modified biosensor exhibits a drain current sensitivity (Scurrent) of 2.6e4, average SS=2.7 mV/dec, and IOFF=1e-12 A/μm. The device's performance is assessed by examining steric hindrance and band tailing effects. The modified biosensor outperforms recent DM-TFET biosensors, making it a promising candidate for low-power, high-switching speed bio-sensing.

Quantification of the strong, phonon-induced Urbach tails in β-Ga2O3 and their implications on electrical breakdown

In ultrawide bandgap (UWBG) nitride and oxide semiconductors, increased bandgap (Eg) correlates with greater ionicity and strong electron–phonon coupling. This limits mobility through phonon scattering, localizes carriers via polarons and self-trapping, broadens optical transitions via dynamic disorder, and modifies the breakdown field. Herein, we use polarized optical transmission spectroscopy from 77 to 633 K to investigate the Urbach energy (Eu) for many orientations of Fe- and Sn-doped β-Ga2O3 bulk crystals. We find Eu values ranging from 60 to 140 meV at 293 K and that static (structural defects plus zero-point phonons) disorder contributes more to Eu than dynamic (finite temperature phonon-induced) disorder. This is evidenced by lack of systematic Eu anisotropy, and Eu correlating more with x-ray diffraction rocking-curve broadening than with Sn-doping. The lowest measured Eu are ∼10× larger than for traditional semiconductors, pointing out that band tail effects need to be carefully considered in these materials for high field electronics. We demonstrate that, because optical transmission through thick samples is sensitive to sub-gap absorption, the commonly used Tauc extraction of a bandgap from transmission through Ga2O3 >1–3 μm thick is subject to errors. Combining our Eu(T) from Fe-doped samples with Eg(T) from ellipsometry, we extract a measure of an effective electron–phonon coupling that increases in weighted second order deformation potential with temperature and a larger value for E||b than E||c. The large electron–phonon coupling in β-Ga2O3 suggests that theories of electrical breakdown for traditional semiconductors need expansion to account not just for lower scattering time but also for impact ionization thresholds fluctuating in both time and space.

Design of MoO3 Porous Back Contact for High Efficiency CZTSSe Thin-film Solar Cells

The introduction of in-situ anodized MoO3 porous arrays with tailored structural parameters as the rear interface contact has a positive impact on enhancing the solar cell performance. The optimized device efficiency increased from 6.31% to 9.00% (in reference to molybdenum-based cells), resulting in a 32% increase in JSC and a 64% increase in FF. The results indicate that at a 10V oxidation voltage, the MoO3 pore size is relatively larger, facilitating the formation of a well-interpenetrating structure and contact interface with CZTSSe. In turn, assists in carrier interface separation and transfer, effectively suppressing the recombination of separated carriers. It extends carrier lifetime, reduces band tailing effects, and lowers urbach energy, thus improving the overall performance of CZTSSe devices.

Suppressing interface recombination via element diffusion regulation towards high-efficiency Cd-free Cu(In,Ga)Se2 solar cells

Magnetron-sputtered Zn1-xMgxO (ZMO) films suffer from severe elemental diffusion to Cu(In,Ga)Se2 (CIGS), which limits the further improvement of device performance. Herein, we introduced Zn(O,S) (ZOS) as a barrier layer, and we controlled the diffusion of Zn within an appropriate range by adjusting the annealing temperature of ZOS. The results indicate that Zn can occupy copper vacancies (VCu) in CIGS and easily diffuse through VCu. A moderate amount of Zn at the interface acts as a charge compensator, reducing the band tailing effect and promoting the carrier collection efficiency of photovoltaic (PV) devices. However, excessive Zn doping induces additional defects dominated by ZnCu, ZnGa and ZnIn, leading to severe nonradiative recombination. Consequently, the Cd-free CIGS solar cell produced based on these insights exhibited a 49.4-mV reduction in the Voc deficit (Voc,def) as well as a notable increase in the power conversion efficiency (PCE) to 18.0 %. This study is the first to reveal the mechanism of defects caused by Zn diffusion, and these findings provide theoretical support for the study of Cd-free thin-film solar cells.

Physics-based compact current model for schottky barrier transistors at deep cryogenic temperatures including band tail effects and quantum oscillations

This paper presents a compact model for the DC current of Schottky barrier field-effect transistors at deep cryogenic temperatures, close to absolute zero kelvin. The proposed model is physics based and calculates the injection current over a device’s Schottky barriers, by considering physical effects at these temperatures (e.g. quantum oscillations, band tail effect, phonon-assisted tunneling, etc.). For model verification, it is compared to measurements performed on nanowire Schottky barrier transistors at a temperature of 5.5K.

Influence of implantation assisted Ni doping on structural and optical properties of WO2.72 films

In this paper, structural and optical properties of WO2.72 thin films implanted by 150 keV Ni ions having ion fluences in the range of 1015 to 1016 ions/cm2 have been investigated. Structural investigations (Raman and XRD) suggest the formation of the crystalline monoclinic phase of pristine films. However, after ion implantation, the crystalline quality degrades as a function of ion fluences, and complete amorphization is observed at highest fluence. Further, the bandgap decreases upon ion implantation as a result of band tailing effect. Optical investigation under UV excitation delineates the existence of NUV, band-to-band transition and blue green emission bands in WO2.72 thin films. Implantation-assisted Ni doping blueshifted the new NUV emission peaks due to the enhancement in free electron concentration in the conduction band (CB). The overall hue of the emitted light investigated using Commission International de l’Eclairage (CIE) diagram demonstrates the transition from light to deep blue region upon Ni implantation.

Preparation and Numerical Optimization of TiO2:CdS Thin Films in Double Perovskite Solar Cell

This work focuses on preparing TiO2, CdS, and composite TiO2:CdS thin films for photovoltaic applications by thermal evaporation. The suggested materials exhibit very good optical and electrical properties and can play a significant role in enhancing the efficiency of the device. Various microscopy and spectroscopy techniques were considered to investigate the optical, morphological, photoluminescence, and electrical properties. FTIR confirms the material identification by displaying some peaks in the fingerprint region. UV Vis spectroscopy yields high transmission (80–90%) and low absorbance (5–10%) within the spectral region from 500 nm to 800 nm for the composite thin films. The optical band gap values for CdS, TiO2, and TiO2:CdS thin films are 2.42 eV, 3.72 eV, and 3.6 eV. XRD was utilized to analyze the amorphous nature of the thin films, while optical and SEM microscopy were employed to examine the morphological changes caused by the addition of CdS to TiO2. The decrease in the bandgap of the composite thin films was determined by the Tauc plot, which is endorsed due to the band tailing effects. Photoluminescence spectroscopy depicts several emission peaks in the visible region when they are excited at different wavelengths, and the electrical measurement enhances the material conductivity. Furthermore, the proposed electron transport materials (TiO2, CdS, TiO2:CdS) were simulated with different perovskite materials to validate their design by employing the SCAPS-1D program and assess their performance in commercial implementation. The observed results suggest that TiO2:CdS is a promising candidate to be used as an ETM in PSC with enhanced productivity.

Two‐Step Cooling Strategy for Synergistic Control of CuZn and SnZn Defects Enabling 12.87% Efficiency (Ag,Cu)2ZnSn(S,Se)4 Solar Cells

AbstractAbundant intrinsic defects and defect clusters in Cu2ZnSn(S,Se)4 (CZTSSe) solar cells lead to severe nonradiative recombination and limited photoelectric performance. Therefore, developing effective method to suppress the detrimental defects is the key to achieve high‐efficiency solar cell. Herein, a convenient two‐step cooling strategy in selenization process is reported to suppress the CuZn and SnZn defects and defect clusters synergistically. The results show that rapid cooling during section from selenization temperature to turning temperature can inhibit the volatilization of Sn and restrain the corresponding Sn‐related defects, while slow cooling during the subsequent temperature section can reduce the degree of Cu‐Zn disorder. Benefitting from the synergistic effect of two‐step cooling, a significantly lowered concentration of SnZn and CuZn defect and their defect clusters [2CuZn+SnZn] in absorber is observed, meanwhile, a reduced band tailing effect and promoted carrier collection efficiency of the photovoltaic device is obtained. Finally, a device with improved open‐circuit voltage (Voc) of 505.5 mV and efficiency of 12.87% is achieved. This study demonstrates the impact of cooling process on defects controlling for the first time and provides a simple and effective new strategy for intrinsic defect control, which may be universal in other inorganic thin film solar cells.

Modulation Effect of the Zn/Sn Ratio on Performance of Chloride-Fabricated CZTSSe Cells for Boosting 10%-Beyond Efficiencies

The cation ratio in Cu-poor and Zn-rich CZTSSe absorbers is key to decide the photovoltaic performance of CZTSSe solar cells, while its toleration and the influence mechanism is still not clear. In this work, for single-layer-structured CZTSSe absorbers spin-coated from metal chloride precursors with constant Cu and Zn values, the Zn/Sn ratio is regulated in the range of 0.8∼1.6. It is found that the Zn/Sn ratio has great influence on the morphology and electrical properties of the CZTSSe absorbers and thus the performance parameters of the CZTSSe cells. Comparatively, the results show that a 1.2 Zn/Sn ratio can boost the photovoltaic efficiency to 10.65% with a relatively high open-circuit voltage (VOC) of 481 mV and a reduced VOC-deficit of 585 mV. Detailed studies indicate that the 1.2 Zn/Sn ratio could favor the large-grain growth, increase the carrier concentration, suppress the carrier recombination, and reduce the band-tail effects and Urbach energy, thus leading to the highest efficiency with superior photovoltaic performance. The optimization of the Zn/Sn ratio in chloride precursor-coated CZTSSe absorbers would be beneficial for in-depth understanding of the effect mechanism of the element ratio on device performance.

Steep Switching Si Nanowire p-FETs With Dopant Segregated Silicide Source/Drain at Cryogenic Temperature

Fully silicided source/drain Si gate-all-around (GAA) nanowire (NW) p-FETs with NW diameter of 5 nm are fabricated and characterized from room temperature (RT) down to 5.5 K. Thanks to the improved electrostatics by the scaled NW and 3D GAA structure, close to ideal transfer characteristics are obtained at both RT and 5.5 K with a sharp switching. Benefiting from less defects in Si created by the implantation into silicide (IIS) process, the band tail effects and neutral defects scattering are suppressed. Therefore, the fabricated Si GAA NW p-FETs provide very low subthreshold swing SS of 3.4 mV/dec in the weak inversion region and an average SS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> of 14 mV/dec measured from the off-state to the threshold voltage, as well as an improved transconductance G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> at 5.5 K.

An effective air heat-treatment strategy of precursor films in search of high open-circuit voltage for efficient CZTSSe cells

Rationally controlling the electronic property of the absorbers is effective but challenging in overcoming the drawbacks of large open-circuit voltage (VOC) deficit for high efficient Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Herein, selecting chlorides as metal precursor, for the ambient-air processed devices, an approach of heat-treating the Cu2ZnSnS4 (CZTS) precursor films to regulate the electronic properties of CZTSSe absorbers is proposed to effectively increase the VOC towards high efficient cells. It is found that though the temperature of heat-treatment can lead to similar CZTSSe absorber films in composition and crystallinity, the final photovoltaic performance of the cells are dramatic difference. An efficiency of 9.20% with a large VOC of 464 mV was achieved from the 400 oC treated precursor film, whereas the non-treated cells obtained 6.38% efficiency and 399 mV VOC, realizing the VOC deficit decreased from 711 to 594 mV. Detailed investigations indicate a proper heat-treating temperature can significantly improve the electric property of the absorber layers, making it possess reduced band-tailing effects, lower Urbach energy, larger recombination resistance and longer carrier lifetime for enhancing the device efficiency. Such effective approach of directly heat-treating the precursor films to rationally regulate electric properties of the absorbers might provide a new way to identify the origin of the VOC deficit for high efficient CZTSSe solar cells.

Defects passivation and crystal growth promotion by solution-processed K doping strategy toward 16.02% efficiency Cu(In,Ga)(S,Se)2 solar cells

Unsatisfactory crystal quality and deep-level defects are detrimental for Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells. In this work, an in-suit K doping strategy is implemented by introducing KF into the CIGSSe precursor solution to promote crystallization and passivate the deep defects. The effects of K on the solution-processed CIGSSe devices are systematic investigated. It has been discovered that K incorporation results in better crystallization, modified CIGSSe surface and fewer detrimental defects. Benefit from the reduction of the band-tailing effects, suppressed defects density and passivated interface recombination, the solution-processed CIGSSe solar cell achieve systematic improvement in open-circuit voltage and fill factor, enabling a champion efficiency of 16.02%, which is the highest value for alkali metal doped CIGSSe solar cells fabricated by solution-processed method. The effective in-suit K doping tactic opens an avenue for developing highly efficient solution-processed CIGSSe solar cells.

Photoluminescence spectroscopy of excitonic emission in CsPbCl3 perovskite single crystals

We analyzed low-temperature photoluminescence (PL) and the band edge excitonic emission in bulk halide perovskite CsPbCl3 single crystals. Strong near band edge excitonic emission is observed at 420 nm and a series of excitonic states was observed. Temperature-dependent PL measurements on cesium lead chloride over a wide temperature range from 13 K to 295 K revealed significant electron phonon coupling with a Huang-Rhys parameter ranging from 1.34 to 3.38. A primary experimental finding is that the near band edge PL emission shows existence of multiple sharp features at low temperatures that are attributable to free excitons, bound excitons, and their phonon replicas. The PL also shows evidence of band tailing effects due to disorder. We investigated the recombination mechanism of photogenerated charge carriers by means of time-resolved PL (TR-PL) spectroscopy at low temperatures and various excitation powers and observed competing mono- and bimolecular recombination processes. A dynamic model is proposed to describe the TR-PL. We observed both long-lived and short-lived recombination where the short-lived time decay is attributed to free exciton recombination, whereas the long-lived decay to bound exciton recombination involving traps due to intrinsic disorder.

Suppressing the formation of double‐layer in Cu 2 ZnSnSe 4 ( CZTSe ) absorber layer by facile heating process through nontoxic selenium atmosphere

The Cu2ZnSnSe4 (CZTSe) absorber layer is typically prepared by post-selenization with a metal precursor. In the process of selenization, the loss of SnSex is a common phenomenon, resulting in both an atomic- ratio change and double-layer distribution of the absorber layer. This change affects the film properties. Additionally, excessive deviation from stoichiometry causes the formation of secondary-phase compounds. Moreover, the double-layer distribution reduces the carrier transport between the Mo back electrode and the CZTSe absorber film, inhibiting the effectiveness of the CZTSe solar cell. To address these problems, this study used CuxSe and ZnxSn1−x targets as the sputtering target materials to reduce the loss of SnSex during the selenization of precursor films. In addition, the effect of heating rate on the atomic ratio of the absorber layer was explored by adjusting the heating rate, which is one of the selenization parameters. The results showed that faster heating rates reduced the loss of SnSex, adjusted the Zn/Sn ratio in the absorber layer, and decreased ZnSe-related defects. In this way, the double-layer distribution was improved, air holes were reduced, and crystal structure characteristics of the films were enhanced. Photoluminescence (PL) spectroscopy showed that the signal of the ZnSe-related defect decreased, and the band tail effect became insignificant. The CZTSe absorber layer fabricated under different heating rates is used to prepare the CZTSe solar cell with a photoelectric conversion efficiency ranging from 0.51% to 5.6%.

Optimizing the Ratio of Sn4+ and Sn2+ in Cu2ZnSn(S,Se)4 Precursor Solution via Air Environment for Highly Efficient Solar Cells

The use of different Sn valence states (such as Sn4+ and Sn2+) in the Cu2ZnSn(S,Se)4 (CZTSSe) precursor solution is especially important for the quality of the subsequent growth of the CZTSSe films. The latest study has found that replacing SnCl2·2H2O with anhydrous SnCl4 can remarkably improve the performance of CZTSSe solar cells, but it needs to be operated in the glovebox. Herein, for the precursor solution, SnCl4·5H2O powder is used instead of anhydrous SnCl4 in air environment, and the proportion of Sn4+ and Sn2+ precursor solutions is further systematically studied. When the ratio of Sn4+ to Sn2+ is 1:1, a uniform, compact, and noncracking CZTSSe thin film is obtained, effectively alleviating the interface recombination and reducing the concentration of deep‐level defects. In particular, the concentration of CuZn antisite defects is decreased by an order of magnitude, and the carrier recombination and band tail effect are alleviated. When JSC is maintained, VOC and FF are considerably improved. Finally, CZTSSe thin‐film solar cells are fabricated with an efficiency of over 11%. Herein, the feasibility of controlling the ratio of Sn4+ to Sn2+ in the CZTSSe precursor solution for higher efficiency of CZTSSe thin‐film solar cells is demonstrated.

Improved charge transfer dynamics of Antimony doped TiO2/rGO nanocomposites

The present work focuses on the charge transfer process in the Titanium dioxide (TiO2) lattice due to Antimony (Sb) doping and compositing it with reduced graphene oxide (rGO). Herein, we have prepared Sb doped TiO2 (STO) samples with different concentrations of Sb by solvothermal method and investigated its charge dynamics. From the TEM and HRTEM analysis spherical morphology of TiO2 nanoparticles and d-spacing variation in lattice due to Sb doping was observed. Optical measurements showed improved absorption in visible range as well as a low band tailing effect in the STO samples. XPS peak shift and PL intensity variations confirmed the formation of oxygen defects in the TiO2 lattice. EIS and Hall measurement investigation demonstrated slightly decreased electrical properties due to the heavy doping of Sb ions. Integration of STO NPs with rGO improved the electrical conductivity (1.2 times), accelerated the charge transfer process (1.8 times) as well as enhanced the mobility (2.3 times) in the TiO2 lattice.

A physics-based compact model for MoS2 field-effect transistors considering the band-tail effect and contact resistance

In this paper, we present a compact surface-potential-based drain current model in molybdenum disulfide (MoS2) field-effect transistors (FETs). Considering variable range hopping (VRH) transport via band-tail states in MoS2 transistors, an explicit solution for surface potential has been derived and it provides a good description over different regions of operation by comparisons with numerical data. Based on the charge-sheet model (CSM), which applies to drift-diffusion transport, the current expression including contact resistance and velocity saturation effect is developed. Furthermore, the presented model is validated and shows a good agreement with the experimental data for MoS2 FETs.

Cu2ZnSn(S, Se)4 solar cell with slight band tailing states achieves 11.83% efficiency by selenizing sputtered Cu–Zn–Sn–S precursor

The large band tailing states are believed to be a key challenge resulting in large deficit of open-circuit voltage and limit the performances of promising Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. In the study, an effective process is proposed to alleviate the harmful effects of band tailing states and fabricate high performance CZTSSe devices. Cu2ZnSnS4 (CZTS) precursors are sputtered by a CZTS quaternary target and then annealed under a H2Se-containing atmosphere at different temperatures. The phase transformation and grain growth of CZTSSe absorbers in selenization process is illustrated. At the annealing temperature of 500 °C, the best performance device with the efficiency of 11.83% is achieved. The excellent back contact interface and the slight band tailing effect of the champion device are demonstrated by the temperature-dependent J-V and external quantum efficiency (EQE) measurement. The photoluminescence (PL) spectrum illustrates that the donor–acceptor pair formed by charged defects is the dominant reason of band tailing effect.

Simple theoretical analyses of the Burstein–Moss shift (BMS) revisited for n-GaAs semiconductor with and without band-tailing conditions

The Burstein–Moss shift (BMS) has been revisited for n-type GaAs semiconductor with and without band-tailing conditions. Unlike the earlier reported results, the present theoretical analyses of BMS depicted an oscillatory behavior due to the inclusions of the following relevant factors, viz. (a) $$ \overline{k} $$ dependency of the optical matrix elements, (b) consideration of the Fermi integral (FI) methods for the calculation of Fermi energy in non-degenerately doped cases, to a higher-order terms, and (c) band-tailing effects due to heavy doping in degenerately doped condition. In the present communication, attempt has been made to recapitulate the FI of F1/2(η), for non-degenerately doped semiconductor that showed oscillations due to the presence of confluent hyper-geometric function, obtained by the exact integrations method involved with F1/2(η). However, for the heavy doping with band-tailing cases, the FI is found to be complex function of oscillatory amplitude and a phase angle (α). It must be noted that these oscillatory behaviors, demonstrated in the present paper, seemed to be depicted for the first time in both non-degenerately and degenerately doped semiconductors on the literature in the study of BMS.

Influence of Sodium Iodide doped polypyrrole on green synthesized aluminum doped ZnO for the enhanced charge separation at the interface

Different Concentration of Aluminum (1, 3, 5%) was doped in ZnO by green synthesis method using environmental friendly cucum sativa fruit pulp extract as capping agent. Increase in crystallinity was observed for higher concentration of Aluminum in ZnO by X-ray diffraction (XRD). With the increase in Al percentage, band gap gets reduced from 3.14eV to 3.01eV, due to band tailing effect. On the other hand, Polypyrrole(PPy) and Sodium iodide(NaI) doped PPy powder synthesized by chemical polymerization method produced a band gap of 2.83eV and 2.57eV respectively. A simple and economical Successive ionic layer adsorption and reaction (SILAR) method was employed to fabricate 5% Al doped ZnO/NaI-PPy nanocomposite thin film. Characterization of the ZnO reveal enhanced crystallinity, spherical morphology and low band gap which has an influence in the Photoluminescence quenching of the nanocomposite thin film. To reveal the electronic property of ZnO with Al using density functional theory (DFT) was performed. Reduction in band gap well agreed with experimental finding. Our findings suggest that nanocomposite thin film can be fabricated with improved charge separation at the interface which can find application in Photo catalysis, hybrid solar cell and optoelectronic devices.