Defect States Research Articles

Mn Doped MoO3 with Self-Assembled Nanoflowers Structure and Enhanced Gas Sensing Properties to Triethylamine

Increasing quality of life requires low power consumption and reliable gas sensing technology for real-time monitoring of the environment. Herein, based on the principle of ion compensation and charge compensation, Mn-doped MoO3 self-assembled nanoflowers were designed and prepared, and their gas-sensing performance were studied. Benefiting from abundant defective sites and surface chemical state changes, the sensor exhibits superior characteristics for triethylamine detection, including ultrahigh response (436.9), short response time (7 s), small detection limit (1 ppm), and remarkable selectivity. The gas-sensitive mechanism of M-MoO3 was explained from the points of view of charge compensation and ion compensation, and it was proved that the incorporation of Mn into MoO3 was an effective way to improve its gas sensitivity. This work provides a potential strategy for widespread triethylamine detection and provides new ideas for the design of high-performance sensors.

Mg-doped SrTaO2N as a visible-light-driven H2-evolution photocatalyst for accelerated Z-scheme overall water splitting

Perovskite SrTaO2N is one of the most promising narrow-bandgap photocatalysts for Z-scheme overall water splitting. However, the formation of defect states during thermal nitridation severely hinders the separation of charges, resulting in poor photocatalytic activity. In the present study, we successfully synthesize SrTaO2N photocatalyst with low density of defect states, uniform morphology and particle size by flux-assisted one-pot nitridation combined with Mg doping. Some important parameters, such as the size of unit cell, the content of nitrogen, and microstructure, prove the successful doping of Mg. The defect-related carrier recombination has been significantly reduced by Mg doping, which effectively promotes the charge separation. Moreover, Mg doping induces a change of the band edge, which makes proton reduction have a stronger driving force. After modifying with the core/shell-structured Pt/Cr2O3 cocatalyst, the H2 evolution activity of the optimized SrTaO2N:Mg is 10 times that of the undoped SrTaO2N, with an impressive apparent quantum yield of 1.51% at 420 nm. By coupling with Au-FeCoOx modified BiVO4 as an O2-evolution photocatalyst and [Fe(CN)6]3−/[Fe(CN)6]4− as the redox couple, a redox-based Z-scheme overall water splitting system is successfully constructed with an apparent quantum yield of 1.36% at 420 nm. This work provides an alternative way to prepare oxynitride semiconductors with reduced defects to promote the conversion of solar energy.

Advanced spectroscopic methods for probing in-gap defect states in amorphous SiNx for charge trap memory applications

Silicon nitride (SiNx) serves as the charge trap layer in current 3D NAND flash memory devices. The precise formation mechanism and electronic structure of localized defect trap states in SiNx remain elusive. Here, we present a refined experimental methodology to elucidate the in-gap defect states and the band gaps in amorphous SiNx thin films. Our approach integrates high-resolution reflection electron energy loss spectroscopy (REELS) and spectroscopic ellipsometry (SE) for comprehensive analysis. By systematical analysis, we aim to provide a robust method for determining in-gap electronic states in SiNx. We investigated two different SiNx films prepared by plasma-enhanced chemical vapor deposition and sputtering. Our analysis revealed several distinct in-gap states and determined band gap energies. This approach not only provide advanced spectroscopic methods to characterize the defect electronic states in SiNx, but also applicable to other large band gap semiconductors or dielectrics to predict device-level characteristics for future devices.

Radiative recombination model for BiSeI microcrystals: unveiling deep defects through photoluminescence

Abstract Pnictogen chalcohalides are semiconductors that have emerged as promising materials for energy conversion due to their exceptional optoelectronic properties. Their electronic configuration (ns2), particularly for Bi- and Sb-based compounds, can be a key factor in efficient carrier transport and defect tolerance, similarly, to Pb-perovskites. In the present study, the Bi-containing chalcohalide, bismuth selenoiodide (BiSeI) was synthesized via isothermal heat treatment of binary precursors in evacuated quartz ampoules. The synthesized BiSeI microcrystals exhibited a characteristic needle-like morphology and a near-stoichiometric composition. Both indirect and direct band gap energies of BiSeI were determined by ultraviolet–visible–near-infrared diffuse reflectance spectroscopy, with room temperature values of 1.17 eV and 1.29 eV, respectively. This study presents the first experimental investigation of the photoluminescence properties of BiSeI microcrystals resulting in a recombination model involving multiple defect states. This work provides valuable insights into the defect structure and recombination mechanisms within BiSeI, paving the way for further exploration of its potential in optoelectronic devices.

Tailoring optical and electrical properties of PVP-I2 complexes through gamma irradiation-induced structural defect Engineering for optoelectronic applications

This study investigates the complexation of iodine with polyvinylpyrrolidone (PVP) subjected to gamma irradiation at doses of 0, 10, 20, and 30 kGy. The complex formation involves I2 acting as a Lewis acid with electron-rich PVP as a Lewis base through a charge transfer mechanism. UV–visible spectroscopy revealed a red shift of the absorption peak at 337 nm for irradiated PVP-I2 samples, indicating stronger interactions between the triiodide ion (I3¯) and irradiated PVP. FTIR analysis confirmed the formation of new C≡N moieties arising from PVP-I2 complexation. The FT-Raman analysis revealed a new C≡N peak at 2360 cm−1 in irradiated PVP-I2 samples, with increasing intensity correlating to higher irradiation doses, indicating enhanced electron delocalization due to complex formation. XRD analysis showed that PVP-I2 samples irradiated at 0 and 10 kGy exhibited higher crystallinity than those irradiated at 20 and 30 kGy, suggesting that high-dose irradiation causes PVP radiolysis and ring opening. Dielectric measurements demonstrated enhanced polarizability and ionic conductivity of irradiated PVP-I2 films, related to structural disorder promoting charge dissociation and ion mobility. The direct bandgap decreased from 2.75 eV for unirradiated samples to 2.55 eV for samples irradiated at 30 kGy, indicating the creation of new optically active defect states. Urbach energy increased from 0.85 eV to 1.55 eV with increasing irradiation dose, confirming increased disorder in the PVP structure. These findings reveal that gamma irradiation significantly alters the optical and electrical properties of PVP-I2 films by creating defects and disrupting structure, offering new insights for tailoring these materials for optoelectronic applications.

Suppressed nonradiative electron–hole recombination in bismuth halide perovskite Cs3Bi2Cl9: Time domain ab initio analysis

Time-domain density functional theory, coupled with non-adiabatic molecular dynamics simulations, was employed to explore the defect characteristics and the associated nonradiative recombination processes in the bismuth halide perovskite Cs3Bi2Cl9. Our findings indicate that Cs3Bi2Cl9 inherently exhibits p-type semiconductor behavior, with vacancies at the Cs and Bi sites acting as predominant shallow acceptor defects. Although Cl vacancy and interstitial Cl defects introduce trap states within the bandgap of Cs3Bi2Cl9, the by-defect electron–hole (e-h) recombination is substantially impeded, which is attributed to the remarkable local structural deformations associated with the BiCl63− octahedral compression around the defects, which further results in decoupling between the defect state and the band edge state. As a result, the enhanced delocalization of defect states leads to a notably small wave function overlap between defect states and band edge states, as well as weak nonadiabatic couplings dominated by low-frequency phonons. Our study offers crucial insights into the mechanism of defect-mediated e-h recombination in bismuth-based perovskites and provides guidelines for designing efficient optoelectronic devices based on these materials.

Solution-processed Tb: Zr co-doped In2O3 thin film transistor and its dual effect on improving photostability

In this paper, Tb: Zr co-doped In2O3 TFTs were fabricated using an aqueous route. The dual effect of Tb: Zr co-doping on improving photostability was addressed. At a 5 mol% co-doping concentration, the ΔVth under NBIS first decreased and then increased as Tb: Zr ratio increased. The optimized co-doped sample (1: 2) showed a ΔVth of −3.22 V, compared to the undoped sample of −9.5 V and the single-doped samples (1: 0 and 0: 1) of −4.4 V and −4.15 V respectively. The defect state was evaluated using μ-PCD. The peak value and τ2 first decreased and then increased with an increase in Zr ratio, and reached their minimum values when the Tb: Zr co-doping ratio was 2: 1 and 1: 2, respectively. This suggested that co-doping can introduce more deep level states for the rapid recombination of photogenerated carriers and reduce shallow level states, leading to superior photostability of the co-doped devices. Further characterization by UV-Vis and XPS indicated that the increase in band gap and the decrease in oxygen vacancy were also contributing factors to improved device stability. These results highlight the great potential of solution-processed Tb: Zr co-doped In2O3 TFT for applications in low-cost and high-stability electronics.

Mechanism of Defect Passivation in Sb2Se3 Solar Cells via Buried Selenium Seed Layer

AbstractQuasi‐1D antimony selenide (Sb2Se3) is known for its stable phase structure and excellent light absorption coefficient, making it a promising material for high‐efficiency light harvesting. However, the (Sb4Se6)n ribbons align horizontally, increasing defect interference and limiting vertical carrier transport. Herein, a novel strategy of burying selenium (Se) seed layers to reduce lattice mismatch at the heterojunction interface, promote crystal orientation, mitigate deep donor defects, increase P‐type carrier concentration, and purify the PN junction, is proposed. Admittance spectroscopy reveals that Sb2Se3 solar cells with Se seed layers have higher activation energies for defect states and significantly lower defect densities (1.2 × 1014, 2.7 × 1014, and 1.3 × 1015 cm−3 for D1, D2, and D3) compared to an order of magnitude higher densities in Sb2Se3 solar cells without a Se seed layer. First‐principles calculations support these findings, showing that Se seed layers create a Se‐rich environment, reducing selenium vacancies (VSe), antimony on selenium sites (SbSe), and interface defects. This dual passivation mechanism suppresses defect formation and activation, increasing carrier concentration and open‐circuit voltage (VOC). Ultimately, employing this novel method, a VOC of 498.3 mV and an efficiency of 8.42%, the highest performance reported for Sb2Se3 solar cells prepared via vapor transport deposition (VTD), are achieved.

Amorphous InZnSnO thin-film-transistors performance enhancement with low-energy Xenon Pulsed-light annealing

The electrical characteristics and stability of amorphous indium-zinc-tin oxide (IZTO) thin-film transistors (TFTs) were significantly improved in this study using low-energy Xenon pulsed-light annealing (PLA). In comparison to conventional furnace annealing (FA), PLA treatment significantly reduced the processing temperature and annealing time. Besides, the defect states are obviously reduced from 1.73×1012 cm-2 to 6.53×1011 cm-2 by PLA treatment, resulting in improved device electrical characteristics and enhanced reliability, including improved carrier mobility from 20.08 to 39.61 cm2/V∙s, threshold voltage from 0.82 to 0.24 V, subthreshold swing (S.S.) from 0.54 to 0.24 V/decade, and the ON/OFF current ratio from 2.32×107 to 7.31×108. In addition, the enhanced stability is observed under both positive gate-bias stress (PGBS) of VGS = 30 V and negative gate-bias stress (NGBS) of VGS = –30 V for a stress duration of 3000 s. The threshold voltage shift (ΔVTH) is reduced from 8.31 V to 1.65 V and from –3.93 V to –1.51 V, respectively. This is the first time low-energy PLA was conducted for IZTO TFT performance improvement.

Multiple defect states engineering towards high thermoelectric performance in GeTe-based materials

GeTe is a promising material for thermoelectric (TE) applications. However, its low Seebeck coefficient and high thermal conductivity in the mid-temperature range of 298–550 K have hindered its widespread use in TE devices. In this work, we show that a combination of band engineering, Fermi level tuning, and miscibility gap exploration can significantly improve the energy conversion performance of GeTe. Bi provides the optimized carrier concentration and induces band convergence, while Ga possibly forms a resonant state. The synergistic effects of Bi and Ga significantly improve the Seebeck coefficient from 38 µVK−1 for undoped GeTe to over 120 µVK−1 for Bi and Ga-containing materials at 298 K and provide a high power factor over a wide temperature range. Phonon scattering is strengthened by optimizing the microstructure through Pb-induced spinodal decomposition and enhanced point defect scattering due to increased dopant solubility, resulting in a very low lattice thermal conductivity of about 0.5 Wm-1K−1 at 600 K for the triple-doped samples. The maximum thermoelectric figure of merit ZT was significantly increased to 2.1 at 600 K for the Ge0.87Pb0.05Bi0.06Ga0.02Te sample due to the improved power factor and reduced lattice thermal conductivity. Additionally, the average figure of merit ZTave for this sample achieved a very high value of 1.4 for Ge0.87Pb0.05Bi0.06Ga0.02Te at a temperature gradient of 475 K (Tc = 298 K). The developed material shows great potential for use in energy conversion devices, with an estimated energy conversion efficiency exceeding 17 %.

Defect related anomalous mobility of small polarons in dielectric oxides at the example of congruent lithium niobate

Polarons play a major role in the description of optical, electrical and dielectrical properties of several ferroelectric oxides. The motion of those particles occurs by elementary hops among the material lattice sites. In order to compute macroscopic transport parameters such as charge mobility, normal (i.e. Fickian) diffusion laws are generally assumed. In this paper we show that when defect states able to trap the polarons for long times are considered, significant deviations from the normal diffusion behaviour arise. As an example of this behavior, we consider here the case of lithium niobate (LN). This can be considered as a prototypical system, having a rich landscape of interacting polaron types and for which a significant wealth of information is available in literature. Our analysis considers the case of a stoichiometric, defect-free lithium niobate containing a certain concentration of small electron polarons hopping on regular Nb sites, and compares it to the material in congruent composition, which is generally found in real-life applications and which is characterized by a large concentration of antisite NbLi defects. While in the first case the charge carriers are free polarons hopping on a regular Nb sublattice, in the second case a fraction of polarons is trapped on antisite defects. Thus, in the congruent material, a range of different hopping possibilities arises, depending on the type of starting and destination sites. We develop a formalism encompassing all these microscopic processes in the framework of a switching diffusion model which can be well approximated by a mobile–immobile transport model providing explicit expressions for the polaron mobility. Finally, starting from the Marcus–Holstein’s model for the polaron hopping frequency we verify by means of a Monte Carlo approach the diffusion/mobility of the different polarons species showing that, while free polarons obey the laws for normal diffusion as expected, bound polarons follow an anomalous diffusion behaviour and that in the case of the congruent crystal where mixed free and bound polaron transport is involved, our expressions indeed provide a satisfactory description.

Non-Traditional Response to Intensive NBS and NBIS Treatments on IGZO TFTs

This work provides an interpretation of defect and trap state behavior following the application of intensive negative-bias stress (NBS) and negative-bias illumination stress (NBIS) on IGZO TFTs. Staggered bottom-gate TFTs were fabricated with a 50 nm IGZO film sandwiched between a 50 nm gate oxide and a 50 nm passivation oxide layer. The working metal was molybdenum for the gate electrode and source/drain contacts. A passivation anneal was done and an alumina capping layer was deposited via ALD to promote electrical stability; full process details are provided in a previous report. Devices were tested with a Keysight B1500 parameter analyzer to establish initial device characteristics, with +20 V PBS and -10 V NBS stress applications for 20 ks having a negligible response. Intensive stress testing was then done for 10 ks at either high bias (-20 V NBS) or with the addition of illumination (-10 V NBIS at 410 nm). The responses to intensive NBS and NBIS applications were both non-traditional and distinctive.

The influence of intrinsic point defects on the electronic band structures and swelling behaviors of 4H-SiC

In this work, both the first principles calculations and molecular dynamics simulations are adopted to investigate the formation ability of various intrinsic point defects in 4H-SiC as well as the electronic band structure changes and the swelling effects caused by them. Our results demonstrate that the classical MEAM potential has an unsatisfactory performance on the defect formation energies as compared with the ab initio calculations based on the advanced SCAN functional, but provides a reasonable description on the volumetric swelling caused by single point defect. Both the SCAN and MEAM calculations indicate that the VC monovacancy and the CSi antisite defect will cause the lattice shrinkage, while the VSi monovacancy, the SiC antisite, the IC and the ISi interstitials will induce the volumetric swelling. Besides that, first principles calculations reveal that monovacancies and interstitials would induce defective electronic states near the Fermi level, while antisites have negligible influence on the electronic band structures as compared with the pristine case. Furthermore, the molecular statics relaxations are employed to explore the influence of point defect densities on the swelling behaviors of 4H-SiC, which reveals that the volume swelling ratios present good linear relationships with the increased density of the VC monovacancy, CSi antisite, SiC antisite and IC interstitial defects, but express complex non-linear tendencies with the VSi monovacancy and ISi interstitial defects. Therefore, our research deepens the understanding of the point defects’ effects on the electronic properties and swelling behaviors of 4H-SiC.

Improvement of the Stability of Quantum-Dot Light Emitting Diodes Using Inorganic HfOx Hole Transport Layer.

One of the major challenges in QLED research is improving the stability of the devices. In this study, we fabricated all inorganic quantum-dot light emitting diodes (QLEDs) using hafnium oxide (HfOx) as the hole transport layer (HTL), a material commonly used for insulator. Oxygen vacancies in HfOx create defect states below the Fermi level, providing a pathway for hole injection. The concentration of these oxygen vacancies can be controlled by the annealing temperature. We optimized the all-inorganic QLEDs with HfOx as the HTL by changing the annealing temperature. The optimized QLEDs with HfOx as the HTL showed a maximum luminance and current efficiency of 66,258 cd/m2 and 9.7 cd/A, respectively. The fabricated all-inorganic QLEDs exhibited remarkable stability, particularly when compared to devices using organic materials for the HTL. Under extended storage in ambient conditions, the all-inorganic device demonstrated a significantly enhanced operating lifetime (T50) of 5.5 h, which is 11 times longer than that of QLEDs using an organic HTL. These results indicate that the all-inorganic QLEDs structure, with ITO/MoO3/HfOx/QDs/ZnMgO/Al, exhibits superior stability compared to organic-inorganic hybrid QLEDs.

Solar driven photocatalytic colored co–TixOyNz amorphous film prepared through air plasma treatment of sputtered titanium film

Colored co–titanium oxides and oxynitride amorphous thin film were fabricated through air plasma treatment of sputtered titanium films. Air plasma treatment led to the removal of the passive film that developed on the titanium film surface, and changes in the structural and optical properties of the films. The amorphous nature, grain size, and thickness of the films remained invariant after the treatment process. The surface roughness of the films increased upon plasma treatment. UV-vis spectroscopy study showed the reduction of the bandgap energy of the films after the air plasma treatment and the maximum reduction was found for the thinner film. The 60min air plasma treatment of the thinner film (AD5: film with 5 min deposition time) leads to the changes in bandgap from 4.11 eV to 3.26 eV, and significant Urbach energy values pointed to the formation of defect states. XPS analysis highlighted that the plasma treatment process led to the formation of titanium oxides (TiO2, Ti2O3) and oxynitrides (TiON), accompanied with Ti2+ defect states in the bandgap. The degradation of methylene blue dye solution with plasma-treated film (PT5) as the photocatalyst resulted in 88% degradation efficiency under 240min irradiation of solar simulator. The degradation has also been checked with natural solar illumination and is found to have a 90% degradation efficiency in 480min.

Iron single atoms and nitrogen vacancies on graphitic carbon nitride synergically shallowing deep electron traps towards efficient photocatalytic ozonation

The coupling of photocatalysis and ozonation has been approved a very efficient strategy for hydroxyl radicals (•OH) production and wastewater decontamination, yet it remains challenging to enrich surface-reaching charges around reaction centers to accelerate this process. This paper explored the strategy of single-atoms (SAs) modification and defects control, aiming to enhance the activity of graphite carbon nitride (CN) in the visible light photocatalytic ozonation. It was found that the presence of iron SAs on CN greatly altered the impact of nitrogen vacancies (NVs), which behaved as recombination centers of photogenerated charge carriers and deteriorated activity of CN, while positively promoting the separation and migration of photogenerated carriers in the case for Fe-CNV (consist of iron SAs and NVs) catalysts. Based on photoluminescence (PL) emission, time-resolved PL spectra, ultrafast transient absorption spectroscopy and first-principles calculations, we found that adjacent iron SAs revived electrons trapped at NVs by elevating their energy level and mediating their interfacial transfer, leading to the decreased loss during electron transfer. This finding demonstrates the feasible tailor of energy level alignment for defect states to shallow deep electron traps.

Fundamental Understanding of Interface Chemistry and Electrical Contact Properties of Bi and MoS2.

The interface properties and thermal stability of bismuth (Bi) contacts on molybdenum disulfide (MoS2) shed light on their behavior under various deposition conditions and temperatures. The examination involves extensive techniques including X-ray photoelectron spectroscopy, scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS). Bi contacts formed a van der Waals interface on MoS2 regardless of deposition conditions, such as ultrahigh vacuum (UHV, 3 × 10-11 mbar) and high vacuum (HV, 4 × 10-6 mbar), while the oxidation on MoS2 has been observed. However, the semimetallic properties of Bi suppress the impact of defect states, including oxidized-MoS2 and vacancies. Notably, the n-type characteristic of Bi/MoS2 remains unaffected, and no significant changes in the local density of states near the conduction band minimum are observed despite the presence of defects detected by STM and STS. As a result, the Fermi level (EF) resides below the conduction band of MoS2. The study also examines the impact of annealing on the contact interface, revealing no interface reaction between Bi and MoS2 up to 300 °C. These findings enhance our understanding of semimetal (Bi) contacts on MoS2, with implications for improving the performance and reliability of electronic devices.

Plasma-enhanced atomic layer deposition of indium-free ZnSnOx thin films for thin-film transistors

An appropriate synthesis technique for growing high-quality indium-free ZnSnOx (ZTO) films should be developed to achieve high-performance thin-film transistors (TFTs) utilizing ZTO films. This study investigated the growth characteristics and electrical properties of ZTO thin films grown via plasma-enhanced atomic layer deposition (PEALD) using bis(1-dimethylamino-2-methyl-2-propoxide)Sn, diethylzinc, and O2 plasma to optimize the composition and enhance the device performance. Deviations were observed in the growth per cycle when using PEALD for ZTO, compared with binary oxides. In the PEALD of the ZTO films, the introduction of the SnO2 sub-cycle enhanced the mass gain in the ZnO sub-cycle, whereas the mass gain in the SnO2 sub-cycle decreased with the addition of the ZnO sub-cycle. This was attributed to changes in the density of the functional groups on the reaction surface. Precise control over the composition was achieved, enabling the identification of the optimal Zn58Sn42Ox composition. Post-deposition annealing significantly improved the TFT performance, with devices showing enhanced mobility, positive shifts in the threshold voltage, and reduced subthreshold swing values. These improvements stemmed from reductions in oxygen vacancies and sub-gap defect states. These findings highlight the potential of PEALD-grown ZTO films for use in high-performance, cost-effective TFTs, facilitating their integration into modern semiconductor electronics.

Probing defect states in 2D perovskites by two-photon absorption

Probing defect states in 2D perovskites by two-photon absorption

Transport and electronic structure properties of MBE grown Sn doped Ga2O3 homo-epitaxial films

In this work, we report the transport, defect state and electronic structure properties of unintentionally doped (UID) and Sn doped β-Ga2O3 homo-epitaxial thin films grown by molecular beam epitaxy (MBE) with electron density ranging from 2.1 × 1016 to 2.8 × 1019 cm−3. The UID film with an electron density of 2.1 × 1016 cm−3 exhibits a notable RT mobility of 129 cm2/Vs and a peak mobility of 900 cm2/Vs at 80 K, achieving the state-of-the-art level for MBE-grown Ga2O3 films. Temperature dependent Hall measurement reveal that Sn dopants have an activation energy of 56.7 meV. Synchrotron-based photoemission spectroscopy were further used to study insights into the evolution of electronic properties induced by Sn doping. An in-gap defect state was observed at the 1.5 eV above the valence band maximum for the Sn-doped Ga2O3 film. The in-gap state acts as self-compensating centers affecting the overall doping efficiency and mobility. Furthermore, photoemission spectroscopic study also reveals an upward surface band bending existing at the surface region of Sn doped Ga2O3 films. The identification of the in-gap state and surface upward band bending have significant implications for understanding the doping mechanisms in Ga2O3 and its electronic device applications.