Deep-level States Research Articles

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.

Elucidating the Role of Alkali Metal Carbonates in Impact on Oxygen Vacancies for Efficient and Stable Perovskite Solar Cells.

Effectively suppressing nonradiative recombination at the SnO2/perovskite interface is imperative for perovskite solar cells. Although the capabilities of alkali salts at the SnO2/perovskite interface have been acknowledged, the effects and optimal selection of alkali metal cations remain poorly understood. Herein, a novel approach for obtaining the optimal alkali metal cation (A-cation) at the interface is investigated by comparatively analyzing different alkali carbonates (A2CO3; Li2CO3, Na2CO3, K2CO3, Rb2CO3, and Cs2CO3). Theoretical calculations demonstrate that A2CO3 coordinates with undercoordinated Sn and O on the surface, effectively mitigating oxygen vacancy (VO) defects with increasing A-cation size, whereas Cs2CO3 exhibits diminished preferability owing to enhanced steric hindrance. The experimental results highlight the crucial role of Rb2CO3 in actively passivating VO defects, forming a robust bond with SnO2, and facilitating Rb+ diffusion into the perovskite layer, thereby enhancing charge extraction, alleviating deep-level trap states and structural distortion in the perovskite film, and significantly suppressing nonradiative recombination. X-ray absorption spectroscopy analyses further reveal the effect of Rb2CO3 on the local structure of the perovskite film. Consequently, a Rb2CO3-treated device with aperture area of 0.14 cm2 achieves a notable efficiency of 22.10%, showing improved stability compared to the 20.11% achieved for the control device.

Enhancing electrical and optical properties of anatase TiO2 nanotubes through electrochemical lithiation

In presented paper, anatase TiO2 nanotubes (NTs) were obtained by anodic oxidation of Ti-foil followed by subsequent annealing in air at 400 °C. After thermal treatment, TiO2 nanotubes (NTs) were electrochemically lithiated by means of galvanostatic (GS) discharge, as a part of GS cycling, at 25°C and 55 °C. Microstructural properties of the as-prepared material were observed by scanning and transmission electron microscopy (SEM, TEM), while changes in chemical bonding with lithiation were examined by X-ray photoelectron spectroscopy (XPS). Specific electrical conductivity, obtained by 4-point probe method, showed multifold increase after Li-ion insertion in TiO2 NTs, at both temperatures. By using diffuse reflectance spectroscopy (DRS), the decrease in energy gap, from 3.04 eV for the as-prepared TiO2 NTs to 2.81 eV for lithiated TiO2 NTs, was observed. The photoluminescence (PL) measurements suggest that Li-ion intercalation led to suppression of deep-level trap states within the bandgap, of TiO2 NTs, and promoted shallow defects associated to F-centers. Open-circuit photovoltage decay (OCVD) measurements confirm the promotion of shallow defect states. Furthermore, HER measurements indicate that the electrochemical lithiation is a promising strategy for enhancing the catalytic performance of TiO2.

Synergy Between Dynamic Behavior of Hydrogen Defects and Non-Radiative Recombination in Metal-Halide Perovskites.

In organic-inorganic hybrid perovskite solar cells (PSCs), hydrogen defects introduce deep-level trap states, significantly influencing non-radiative recombination processes. Those defects are primarily observed in MA-PSCs rather than FA-PSCs. As a result, MA-PSCs demonstrated a lower efficiency of 23.6% compared to 26.1% of FA-PSCs. In this work, both hydrogen vacancy (VH -) and hydrogen interstitial (Hi -) defects in MAPbI3 bulk and on surfaces, respectively are investigated. i) Bulk VH - defects have dramatic impact on non-radiative recombination, with lifetime varying from 67 to 8ns, depending on whether deprotonated MA0 are ion-bonded or not. ii) Surface H-defects exhibited an inherent self-healing mechanism through a chemical bond between MA0 and Pb2+, indicating a self-passivation effect. iii) Both VH - and Hi - defects can be mitigated by alkali cation passivation; while large cations are preferable for VH - passivation, given strong binding energy of cation/perovskite, as well as, weak band edge non-adiabatic couplings; and small cations are suited for Hi - passivation, considering the steric hindrance effect. The dual passivation strategy addressed diverse experimental outcomes, particularly in enhancing performance associated with cation selections. The dynamic connection between hydrogen defects and non-radiative recombination is elucidated, providing insights into hydrogen defect passivation essential for high-performance PSCs fabrication.

Compact model for MFIS-NCFETs considering deep-level interface trap states

Compact model for MFIS-NCFETs considering deep-level interface trap states

Analysis of interface trap induced ledge in β-Ga2O3 based MOS structures using UV-assisted capacitance–voltage measurements

A ledge feature in the capacitance–voltage (CV) profiles of Ga2O3 MOS (metal–oxide–semiconductor) capacitors is investigated using UV-assisted CV measurements. A model is presented whereby the capacitance ledge is associated with carrier trapping in deep-level states at the Al2O3/Ga2O3 interface. Following UV assisted emptying of interface traps at a constant bias, a voltage ramp toward flatband results in a CV ledge when the trap recombination current becomes equal to the quasi-static sweep charging current. The ledge continues until all the traps below the corresponding pinned surface potential have been filled. Varying the UV energy varies the ledge voltage range and allows a density of states to be determined as a function of energy. A broad interface state peak with maximum density ∼8 × 1012 cm−2 eV−1 for deep trap energies lying between 2.4 and 4.1 eV below the conduction band (CB) edge is extracted. Using the conductance method, the interface trap density is also found to rise toward the CB edge in the range 0.25–0.45 eV below the CB edge, reaching a maximum density of ∼1 × 1012 cm−2 eV−1. Combining these two techniques, an interface trap distribution is estimated for almost the entirety of the bandgap of Ga2O3. This novel technique probes deep interface states where standard methods fail to quantify interface states reliably.

Enhancing energy storage performance of polyethylene via passivation with oxygen atoms through C–H vacancy carbonylation

Low energy density of polymer film capacitors is regarded as one of the most serious drawbacks facing growing demands for equipment integration and miniaturization. Herein, ultraviolet light and ozone (UVO) surface modification is utilized to simultaneously improve dielectric constant and breakdown strength of polyethylene (PE) films. As a result, after 3 min of UVO treatment, an enhanced recoverable energy density of 4.79 J/cm3 with a charge-discharge efficiency of >95% is obtained under 650 MV/m at room temperature (RT). Significantly, stable energy storage performance under 200 MV/m is maintained throughout a broad temperature range from −90 °C to 90 °C and during 20,000 cycles of charge-discharge procedures. According to first-principles calculations and thermally stimulated depolarization current measurements, formation of carbonyl groups (C=O) after UVO treatment could effectively passivate initial deep-level defect states caused by H vacancies, which explains the improvement in capacitive energy storage. Moreover, the metalized UVO-modified PE exhibits valuable breakdown self-clearing ability, and the self-cleared specimen maintains stable energy storage performance over 20,000 cycles at 200 MV/m and RT. This work offers an effective and user-friendly method for enhancing comprehensive dielectric characteristics of PE-based materials and potential for applications in modern power systems and electronic devices.

Coupling effects of interface charge trapping and polarization switching in HfO2-based ferroelectric field effect transistors

HfO2-based ferroelectric field-effect transistors (FeFETs) are regarded as one of the most promising non-volatile memory technologies in the future. However, the charge trapping phenomenon during the program/erase operation is still a challenge. In this work, we comprehensively investigate the behaviors of semiconductor/insulator interface charge trapping in HfO2-based FeFETs. Through analyzing the effects of the spatial distribution of interface traps and the polarization switching speed, the coupling effects of semiconductor/insulator interface charge trapping and polarization switching are recognized. We also find that the band tail state traps have much less influence on the electrical characteristics of the FeFETs than the deep level state traps. Through engineering the devices with band tail state traps with concentrations as small as possible, the influences of charge trapping could be effectively suppressed. Moreover, the gate voltage (VG) scanning rate has a significant influence on the interface charge trapping process due to the time dependent change of ferroelectric polarization. The largest memory window could be obtained by carefully choosing the VG scanning rate of the FeFETs based on the polarization switching speed. This work represents a key step for realizing highly reliable HfO2-based FeFETs.

Capacitance–voltage extraction method for the deep-level defect distribution in organic photodiode

This paper proposes a method to extract deep-level trap states of the organic photodiode by capacitance–voltage (CV) measurement. The relationship between the trapped charge density and the surface potential can be determined by solving Poisson's equation, while employing Gauss's theorem to establish a correlation between the charge density and the CV characteristics. Consequently, deep-level trap states can be analytically obtained by the conventional CV measurement. Experimental results on P3HT:PCBM devices demonstrate that the deep trap distribution obtained by this method can be well connected with the capacitance–frequency method. Furthermore, our CV method yields a total trap concentration, which closely aligns with that obtained through Mott–Schottky relation. In conclusion, this method provides an effective approach for quantifying deep trap state density of organic photodiode.

Metastable defect curing by alkaline earth metal in chalcogenide thin-film solar cells

This study investigates the use of an alkaline earth metal precursor (MgF2) to enhance the performance of chalcogenide-based Cu(In,Ga)Se2 (CIGS) solar cells with a chemically bath deposited-Zn(O,S) (CBD-Zn(O,S)) buffer layer via post-deposited treatment (PDT). The optimal substrate temperature and layer thickness are 570 °C and 5 nm, and the light soaking (LS) treatment does not be required in this condition. The morphological properties and chemical reaction at the p-n junction of CIGS/CBD-Zn(O,S) are examined as a function of MgF2 PDT layer thickness. As the MgF2 PDT layer thickness increases, the CIGS surface becomes rough with vigorously agglomerated Cu clusters owing to the substantially high substrate temperature, which increases the incorporation of In-Se bonds and the oxygenation rate of MgF2. Density functional theory (DFT) clarifies the improved cell efficiency without the need for LS treatment (MgF2 PDT, 5 nm) by calculating the defect-related electronic behavior. The MgF2 phase effectively passivates metastable defect Cu-Se vacancy defects (VCu-Se), related to the LS effect without the additional formation of deep-level defect states into the CIGS bandgap. Moreover, VCu-Se states exert the most influence on the LS effect, and the control of defect states in the CIGS layer (not the buffer layer) is crucial for cell efficiency.

Pulsed Low-Frequency Noise Characterization on Al/ZnO/Al ReRAM to Investigate the Role of Deep-Level Oxygen Vacancy State in HRS Leakage

Pulsed Low-Frequency Noise Characterization on Al/ZnO/Al ReRAM to Investigate the Role of Deep-Level Oxygen Vacancy State in HRS Leakage

Electrical properties and defect analysis of MAPbI3 thin films grown on TiO2 layer through a two-step drying process

We studied the electrical properties and defect states of methylammonium lead iodide perovskite (MAPbI3) thin film grown onto the TiO2 layer at a temperature of 120 °C via a two-step drying process. The TiO2 layer with thickness of 20 nm were formed on fluorine doped tin oxide/glass at 120 °C via atomic layer deposition. The MAPbI3 thin film showed α-phase structure with band gap energy of about 1.61 eV. For the MAPbI3/TiO2 with Ag top and bottom electrodes, the resistance switching phenomena were enhanced compared to the MAPbI3 film without the TiO2 layer. In the MAPbI3/TiO2, two deep level states of E1(Ec-0.38 eV) and H2 (Ev+0.82 eV) appeared from deep level transient spectroscopy measurements. The defect states of E1 and H2 are related to iodine interstitial (Ii) and iodine antisite (IPb), respectively. In the MAPbI3/TiO2, the defect of Ii appeared as the ionized states of Ii+ or Ii−, so that it was found that the defect state of Ii caused the MAPbI3/TiO2 to have the resistance random access memory property by the migration of the ionized states.

Physical passivation of deep-level defect states in polymer dielectrics for outstanding dielectric properties

The limited dielectric strength of polymer dielectrics hampers their reliable use in highly integrated electrical systems and high-energy storage film capacitors. This constraint is closely associated with the presence of deep-level defect states in polymer dielectrics. Here, we present an efficient and stable approach for the physical passivation of deep-level defect states aimed at enhancing the dielectric properties of polymer dielectrics. Employing inductively coupled plasma technology with argon as the carrier gas, the polymer undergoes substantial decreased deep-level defect states, as evidenced by nano-micro-region defect measurements. Consequently, the localized charge accumulation in the treated polymer dielectric is significantly reduced, resulting in a remarkable 62% increase in flashover voltage. The breakdown strength is also enhanced, and the treated material exhibits long-term stability. Moreover, materials featuring passivated deep-level defect states can boost discharge energy density without compromising efficiency. For instance, PTFE with passivated deep-level defect states maintains a discharge energy density of 1.18 J/cm3 and a charge-discharge efficiency of 95.06% at 200 °C and 300 MV/m. By focusing on deep-level defect states passivation, we introduce a strategy for receiving polymers with exceptional performance and stability.

Study of Electrical Characteristics for Dual-Gate TFTs With Asymmetric Defect Distributions and Gate Work Functions

Combined effects of asymmetric defect distributions and asymmetric gate work functions (WFs) on the performances of self-aligned dual-gate poly-Si TFTs are investigated. Normally, small grains with plentiful grain boundaries (GBs) or other structural defects appear at different positions of the poly-Si film, which is dependent on the growth process of the film. Subgap states of acceptor-like tail, acceptor-like deep-level, donor-like tail, and donor-like deep-level states are used to emulate the defects. Two sets of density of states (DOS) are employed. We find that defects at different positions of the source-side and drain-side channels exhibit different influences on TFT performance and the influences are dependent on the WFs of the gates. TFTs with a higher gate WF can have a higher tolerance to the depth of the defect region. Besides the electrical characteristics, the combined effects of defects and gate WFs on current density distributions and electric field distributions in the channel regions are explored. The performance variations caused by the asymmetric defects along with asymmetric gate WFs can be explained.

Investigations of defects in TiO2/HgTe/MoO3 heterostructures and their influence on transport properties

The need for simple-to-manufacture, solution-processable, tunable infrared active optoelectronic materials led to the development of infrared colloidal quantum dots, whose band gaps may be easily regulated by dimensional constraints resulting from the quantum confinement. Due to electronic deep-level trap states that limit effective mobility and function as effective recombination centers, nanocrystal (NC)-based optoelectronic devices still perform less well than what is predicted theoretically. The performance of NC-based optoelectronic devices can be considerably improved by identifying and passivating these defect levels. In this study, we use low-temperature I–V, C–V, C–F and the microcontroller-based deep level transient spectroscopy (DLTS) system to investigate the defect levels in the FTO/TiO2/HgTe/MoO3/Au device. The I–V measurements at low temperatures demonstrated the existence of exponential trap states, with an activation energy of 0.24 eV and a trap density of . Low-temperature C–V and C–F measurements established the existence of deep trap states. With DLTS, we have located two deep trap levels (traps 1 and 2), with energies of 0.25 and 0.46 eV, capture cross-sections and and a concentration of , respectively. The surface states at the HgTe NCs and the oxygen vacancies at the TiO2 are the leading causes of the trap levels, which are primarily present at the interface of the TiO2/HgTe heterojunction. The transport mechanism in these heterojunction devices was happening mainly through these interface trap states. Passivating these defect levels is vital to increase the device effectiveness.

Optically Reconfigurable Complementary Logic Gates Enabled by Bipolar Photoresponse in Gallium Selenide Memtransistor.

To avoid the complexity of the circuit for in-memory computing, simultaneous execution of multiple logic gates (OR, AND, NOR, and NAND) and memory behavior are demonstrated in a single device of oxygen plasma-treated gallium selenide (GaSe) memtransistor. Resistive switching behavior with RON /ROFF ratio in the range of 104 to 106 is obtained depending on the channel length (150 to 1600nm). Oxygen plasma treatment on GaSe film created shallow and deep-level defect states, which exhibit carriers trapping/de-trapping, that lead to negative and positive photoconductanceat positive and negative gate voltages, respectively. This distinguishing feature of gate-dependent transition of negative to positive photoconductance encourages the execution of four logic gates in the single memory device, which is elusive in conventional memtransistor. Additionally, it is feasible to reversibly switch between two logic gates by just adjusting the gate voltages, e.g., NAND/NOR and AND/NAND. All logic gates presented high stability. Additionally, memtransistor array (1×8) is fabricated and programmed into binary bits representing ASCII (American Standard Code for Information Interchange) code for the uppercase letter "N". This facile device configuration can provide the functionality of both logic and memory devices for emerging neuromorphic computing.

Open-Shell Diradical-Sensitized Electron Transport Layer for High-Performance Colloidal Quantum Dot Solar Cells.

The zinc oxide (ZnO) nanoparticles (NPs) are well-documented as an excellent electron transport layer (ETL) in optoelectronic devices. However, the intrinsic surface flaw of the ZnO NPs can easily result in serious surface recombination of carriers. Exploring effective passivation methods of ZnO NPs is essential to maximize the device's performance. Herein, a hybrid strategy is explored for the first time to improve the quality of ZnO ETL by incorporating stable organic open-shell donor-acceptor type diradicaloids. The high electron-donating feature of the diradical molecules can efficiently passivate the deep-level trap states and improve the conductivity of ZnO NP film. The unique advantage of the radical strategy is that its passivation effectiveness is highly correlated with the electron-donating ability of radical molecules, which can be precisely controlled by the rational design of molecular chemical structures. The well-passivated ZnO ETL is applied in lead sulfide (PbS) colloidal quantum dot solar cells, delivering a power conversion efficiency of 13.54%. More importantly, as a proof-of-concept study, this work will inspire the exploration of general strategies using radical molecules to construct high-efficiency solution-processed optoelectronic devices.

Transient photocapacitance spectroscopy of deep-levels in (001) β-Ga2O3

Defect levels in (001) β-Ga2O3 are investigated using transient photocapacitance (TPC) spectroscopy. For sub-band photon energies in the range of 1.13–3.10 eV, the TPC signal shows broad optical absorption at room temperature. Using the theoretical Pässler model, deep-level states at E T = 1.15 ± 0.07 eV (Trap 1) and E T = 1.69 ± 0.41 eV (Trap 2) below the conduction bands are demonstrated. The Franck–Condon energies ( D F C) of Trap 1 and Trap 2 are 0.26 ± 0.11 and 0.66 ± 0.55 eV, respectively. TPC measurements have been performed at temperatures ranging from 30 to 360 K. From 150 to 360 K, the TPC signal of Trap 1 decreases as the temperature increases. The decrease in the TPC signal of Trap 1 agrees with the thermal quenching model, and a thermal activation energy of 156 meV is estimated. Moreover, the effective phonon energy of β-Ga2O3 has been extracted. From 30 to 360 K, the effective phonon energy is in the range of 85–126 meV.

Al Coordination and Ga Interstitial Stability in a β-(Al0.2Ga0.8)2O3 Thin Film.

Alloying Al2O3 with Ga2O3 to form β-(AlxGa1-x)2O3 opens the door to a large number of new possibilities for the fabrication of devices with tunable properties in many high-performance applications such as optoelectronics, sensing systems, and high-power electronics. Often, the properties of these devices are impacted by defects induced during the growth process. In this work, we uncover the crystal structure of a β-(Al0.2Ga0.8)2O3/β-Ga2O3 interface grown by molecular beam epitaxy. In particular, we determine Al coordination and the stability of Al and Ga interstitials and their effect on the electronic structure of the material by means of scanning transmission electron microscopy combined with density functional theory. Al atoms can substitutionally occupy both octahedral and tetrahedral sites. The atomic structure of the β-(Al0.2Ga0.8)2O3/β-Ga2O3 interface additionally shows Al and Ga interstitials located between neighboring tetrahedrally coordinated cation sites, whose stability will depend on the number of surrounding Al atoms. The presence of Al atoms near interstitials leads to structural distortions in the lattice and creates interstitial-divacancy complexes that will eventually form deep-level states below the conduction band (Ec) at Ec -1.25 eV, Ec -1.68 eV, Ec -1.78 eV, Ec -1.83 eV, and Ec -1.86 eV for a Ga interstitial surrounded by zero, one, two, three, and four Al atoms, respectively. These findings bring new insight toward the fabrication of tunable β-(AlxGa1-x)2O3 heterostructure-based devices with controlled electronic properties.

Accurate Electrostatic Force Measurements by Atomic Force Microscopy Using Proper Distance Control

Electrostatic force microscopy (EFM) enables us to examine the local electrostatic force between an AFM tip and sample surface, from which an electrical capacitance between them and surface potential can be evaluated, by applying direct current (DC) and alternating current (AC) voltages. From the dependences of the electrostatic force on the DC voltage and on the frequency of AC voltage in EFM, carrier density, carrier type, and deep level states in a semiconductor can also be investigated. Since EFM is basically operated as an atomic force microscopy (AFM), special care should be taken in an effect of the electrostatic force on a distance control between the tip and sample, and robust distance control is necessary even under the strong electrostatic force in order to realize proper measurements of both topography and electrostatic force. In this paper, we have examined the effectiveness of the usage of the oscillation amplitude of a cantilever as a feedback target for the distance control, which is referred to as Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> -mode operation, while the cantilever oscillation frequency is always kept at its primary resonance, like the conventional frequency modulation AFM (FM-AFM) which uses a resonant frequency shift as the feedback target. First, we have verified that a distance change due to the strong electrostatic force was significantly reduced compared with FM-AFM. Secondly, we have confirmed that the Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> -mode operation in EFM realized proper measurements of the dependance of a tip-sample capacitance including a surface depletion capacitance on the DC voltage and on the frequency of AC voltage, indicating good robustness of the method against the strong electrostatic force.