Deep-level States Research Articles

Impacts of PbI2 on high-efficiency perovskite solar cells: exploring intercalation orientations and defects

The parallel and vertical intercalations of the layered PbI2 in CsPbI3 result in type-I and type-II band alignments, respectively. The deep-level states caused by certain defects in PbI2 are not active recombination centers.

Contaction of atoms for outstanding dielectric characteristics in KX-passivated polymer dielectrics

High-power, high-integration, and high-energy storage density electrical devices and film capacitors require polymer dielectrics with high dielectric characteristics. However, polymer dielectrics accumulate significant charge during operation, causing local electric field distortion, flashover and breakdown. The deep-level defect state (electron/hole accumulation center) fundamentally affects the dielectric strength. Unfortunately, research on passivating deep-level defects in polymer dielectrics is limited. In this study, KX (X = F, Cl, Br, I) effectively passivated deep-level defect states in polymer dielectrics. The cation dipole effect on polymers was found to be the deep-level defect passivation principle. The principle of anion passivation of deep-level defects by suspending anions and cations is the key to producing more stable C covalent bonds. Such bonds transfer the local charge from C with the lone electron pair that originally contains the hanging bond. Therefore, some transfer occurs, and the anion implements effective passivation. We also investigated the passivation mechanism of KX anion-passivated polymer dielectrics. We found that anions can transform bonding-state defects into antibonding-state defects at deep-level defects in the gap. Therefore, the energy of the defect state rises and disappears in the deep band gap. This study addressed the design of high-performance polymer dielectrics, providing a reference for experimental and theoretical analyses.

Surface morphological and optical evolution of RF sputtered AZO films for optoelectronic devices

Herein, aluminium-doped zinc oxide (AZO) films were fabricated onto borosilicate glass and fluorine-doped tin oxide (FTO) substrates by RF sputtering. Surface morphology and scaling studies of these films revealed the formation of large grain in AZO films fabricated onto the glass substrates. Several fractal parameters of all samples have been evaluated to understand the deeper morphological insights of observed surfaces. Optical investigations revealed a higher transmittance of ∼93% in the visible region for FTO/AZO films. Estimated values of Urbach energy are found to be 0.20 eV and 0.17 eV for glass/AZO and FTO/AZO films, respectively. Important optical constants such as refractive indices, extinction coefficients, etc have been investigated to explore the optical mechanism of these films. Furthermore, photoluminescence studies suggest the reduction in the deep-level emission defect states for AZO films fabricated onto FTO substrate. These results propose that AZO films may be suitable for use in optoelectronics.

Thin Film Photodetectors Based on Zinc Oxide Nanoinks

Zinc oxide (ZnO) has many useful properties for electronics and optoelectronics including wide band gap, large exciton binding energy, low-cost, ease of processing and availability [1]. This has led to increased interest and development of thin film electronics, transparent conductors, solar cells, light-emitting diodes, lasers, photodetectors and various sensors based on ZnO materials [2, 3]. The high surface areas and tunable properties of ZnO nanostructures make them particularly suitable for applications such as sensing and photonic devices [4, 5].In this work, we present results on nanostructured ZnO thin film photodetectors fabricated using nanoparticle inks (nanoinks) obtained via planetary ball milling (PBM) of bulk powders. PBM [6] is an emerging solution-based nanofabrication approach that can quickly produce nanoink suspensions at low-cost by nanoscale grinding, without complex processing and suitable for thin film coating of various materials on different substrates [7]. The thin film photodetector devices were fabricated by depositing PBM ZnO nanoink onto flat insulating glass substrates followed by contact formation as shown schematically in Fig. 1a: PBM was performed using ZnO powder in ethylene glycol (EG) or deionized (DI) water solvent (a.k.a. colloidal grinding) with zirconia grinding beads. The grinding speed and time were varied between 200 and 1000 rpm and 10 min. and 60 min., respectively. A few μL of the resulting ZnO nanoink was used to coat the substrate surface and dried at ~ 100 °C. Lastly, two electrical contacts to the resulting films were made using silver paint and copper tape. Analysis of ZnO films after deposition showed they consist of nanostructured particles with sizes reaching below 100 nm, as displayed in the scanning electron microscopy (SEM) and atomic force microscopy (AFM) images in Fig. 1b, depending on grinding conditions (speed, time).The optical properties of the ZnO thin films were evaluated via photoluminescence measurements (Fig. 1c), which, in addition to interband transitions in the UV, displayed longer wavelength emission peaks due to surface and bulk defect states. Such deep level states are dependent on grinding parameters and thus allow the accessible spectrum for the nanostructured ZnO films to be extended into the visible region in a tunable manner. Two-terminal photoconductance data of the ZnO PBM nanoink thin film devices were obtained using a probe station and precision source-measure unit, with and without illumination, under ambient atmosphere and at room temperature. Fig. 1d shows current vs. voltage curves obtained for a typical photodetector device, which display current increasing proportional to incident light intensity. This behavior can be explained by electron-hole pair creation and desorption of surface oxygen species (leading to vacancies that act as donors) upon photon absorption, which leads to an increase in conductance. This is consistent with previous studies where ZnO thin films have been used for photoconductive sensor applications, validating PBM nanoink as a suitable synthesis technique for the active material in photodetectors.Compared to standard ZnO thin films, the PBM nanoink method allows both particle dimensions and surface states to be tailored and optimized for different photodetector applications in a straightforward manner by adjusting grinding conditions. In particular, both UV and visible light detection can be tuned via the solution-based ZnO nanoink approach presented without additional material/chemical processing. Such PBM nanoinks thus offer the potential of realizing low-cost photodetectors and multifunctional thin film coatings for applications in optoelectronics, imaging, environmental monitoring and communications.

Insights Into to the KX (X = Cl, Br, I) Adsorption-Assisted Stabilization of CsPbI2 Br Surface.

Despite the excellent optoelectronic properties, organic-inorganic hybrid perovskite solar cells (PSCs) still present significant challenges in terms of ambient stability. CsPbI2 Br, a member of all-inorganic perovskites, may respond to this challenge because of its inherent high stability against light, moisture, and heat, and therefore has gained tremendous attraction recently. However, the practical application of CsPbI2 Br is still impeded by the notorious phenomenon of photoinduced halide segregation. Herein, by applying first-principles calculations, the stability, electronic structure, defect properties, and ion-diffusion properties of the stoichiometric CsPbI2 Br (110) surface and that with the adsorption of KX (X = Cl, Br, I) are systematically investigated. It is found that the adsorbed KX can serve as an external substitute of the halogen vacancies on the surface, therefore inhibiting halogen segregation and improving the stability of the CsPbI2 Br surface. The KX can also eliminate deep-level defect states caused by antisites, thereby contributing to the promoted optoelectronic properties of CsPbI2 Br. The mechanistic understanding of surface passivation in this work can lay the foundation for the future design of CsPbI2 Br PSCs with optimized optoelectronic performance.

Combination of InZnO/InGaZnO Bi-layered channels prepared by atomic layer deposition and ozone-based gate-stack formation for guaranteeing high field-effect mobility and long-term stability of thin film transistors

Bi-layered channel structures comprising In–Zn–O (IZO) prompt and In–Ga–Zn-O (IGZO) prime layers were introduced to enhance the carrier mobility of oxide thin film transistors (TFTs) by atomic layer deposition (ALD) technique. Considering that hydrogen-related species in the gate stack may convert the semiconducting channel with In-rich composition into a more conducting layer during the ALD process, ozone oxidant was chosen for the formation of Al2O3 protection and gate insulator layers. The TFT using a IZO/IGZO bi-layered channel exhibited the highest carrier mobility of 50.8 cm2/Vs when the film thickness and ALD sub-cyclic ratio between the In and Zn precursors for the IZO prompt layer were designed to be 3 nm and 2:1, respectively. The optimum cationic compositions of the IZO and IGZO layers were determined to be 1.3:1 (In:Zn) and 1.4:1:2 (In:Ga:Zn), respectively. The turn-on position of the fabricated device was located at near 0 V without additional heat treatment. Furthermore, the device using the optimum IZO/IGZO bi-layered channel exhibited threshold voltage shifts (ΔVTH) of +1.8 V under a positive bias stress (PBS) condition that corresponded to an improvement over a single IGZO channel device (+4.5 V). From the numerical analyses on the sub-gap states below the conduction band edge and trap densities in the mid-gap regions, it was found that the electron trapping sites especially located in deep-level states could be reduced by implementing a bilayer channel configuration. Alternatively, the ΔVTH further decreased to +1.3 V during the PBS at an elevated temperature of 80 °C because of the migration of hydrogen from the gate stack toward the active channel during the measurement, demonstrating the synergic impact of bi-layered channel structures and gate-stack formation conditions.

Analysis of Nitrogen-Doping Effect on Sub-Gap Density of States in a-IGZO TFTs by TCAD Simulation.

In this work, the impact of nitrogen doping (N-doping) on the distribution of sub-gap states in amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) is qualitatively analyzed by technology computer-aided design (TCAD) simulation. According to the experimental characteristics, the numerical simulation results reveal that the interface trap states, bulk tail states, and deep-level sub-gap defect states originating from oxygen-vacancy- (Vo) related defects can be suppressed by an appropriate amount of N dopant. Correspondingly, the electrical properties and reliability of the a-IGZO TFTs are dramatically enhanced. In contrast, it is observed that the interfacial and deep-level sub-gap defects are increased when the a-IGZO TFT is doped with excess nitrogen, which results in the degeneration of the device’s performance and reliability. Moreover, it is found that tail-distributed acceptor-like N-related defects have been induced by excess N-doping, which is supported by the additional subthreshold slope degradation in the a-IGZO TFT.

Peak-tracking scanning capacitance force microscopy with multibias modulation technique

Scanning capacitance force microscopy (SCFM) is a good method for capacitance measurements using electrostatic force detection. However, to obtain an entire capacitance–voltage (C–V) curve by SCFM, a sweep of a direct current (DC) bias voltage is required at a certain fixed point on a sample surface during scan suspension, and thus the measurements become very time-consuming when we want to observe some types of image related with C–V characteristics. In this paper, we propose peak-tracking scanning capacitance force microscopy (PTSCFM) for the purpose of extracting the main feature of the C–V curve without DC voltage sweep. In PT-SCFM, alternating current voltages at three different angular frequencies, ω 1, ω 2, and , are applied together with DC voltage, , to generate an electrostatic force, and high-order components at the angular frequencies of and , which represent a voltage derivative of a capacitance () and a second-order derivative of the capacitance (), respectively, are extracted from the electrostatic force. Then, a DC voltage, , giving the peak of is determined from to be adjusted to nullify the component using a feedback controller. From the obtained values of and at , the C–V curve can be outlined. In PT-SCFM, the distributions of those values are simultaneously imaged together with a topography without sweep, and when we operate PT-SCFM under various modulation frequency conditions, analyses similar to those based on the frequency dependence of the C–V property are realized. We have applied the PT-SCFM to a microcrystalline material to discuss the effects of surface depletion and deep-level states, from which the validity of PT-SCFM has been examined.

Band-engineered Zn2TiO4 nanowires for hydrogen generation from water using visible light: A first-principles study

The electronic structure and optical properties of inverse-spinel Zn2TiO4 nanowires and bulk material were investigated for hydrogen generation from water by visible-light photocatalysis using first-principles density calculations. In our theoretical studies, the bandgap of the Zn2TiO4 nanowires was much smaller than that of the bulk material. New intermediate states appeared in the mid-bandgap as a result of Zn and Ti atoms on the surface of ZnO-terminated and ZnTiO-terminated nanowires. These deep-level states could become recombination centers for photogenerated electron–hole pairs, indicating that these two types of nanowires would no longer meet the requirements for photocatalytic water splitting. In contrast, the electronic states arising from oxygen elements on the surface of the TiO-terminated nanowires resulted in an upward movement of the edge of the valence band, whereas the surface electronic states made no difference to the edge of the conduction band. Moreover, the optical property calculations showed that the optical absorption edge of the nanowire would be red-shifted. These calculations revealed that inverse-spinel Zn2TiO4 nanowires were appropriate for visible-light photocatalytic water splitting reactions.

Entropy Reduced Charge Transport and Energy Loss in Interfacial Zones of Polymer Nanocomposites

Polymer nanocomposites have higher energy storage density, much lower conductivity, and energy loss than the polymer matrices. The excellent performance may be originated from the deep-level localized states introduced by the interfacial zones between nanofillers and polymer matrices. However, the essential relation between the deep-level localized states and the composition, architecture, and configuration of polymer chains is still not clear. We investigate the temperature dependences of electrical conductivities of three types of energy storage polymer nanocomposites and find that they all obey the Meyer-Neldel (MN) compensation rule and the prefactor has an exponential relationship with the MN energy. It is found that the kinetic energy of electrons in the extended states of nanocomposites is the MN energy, which is related to the excitation of electrons by optical phonons. We propose to redefine the energy of localized states by the energy of optical phonons and the entropy of molecular conformations. It is found that the shape parameter of localized state distribution is related to the optical phonons and controlled by the composition, structure, and configuration of polymer chains, while the density of localized states is determined by the conformation and aggregate structure. We propose an optical phonon and entropy activated charge transport model, and the obtained conductivity equation is consistent with the experimental results. The entropy of molecular conformations can be used as a key factor to tailor the conductivity and energy loss of polymer nanocomposites. It provides a new perspective to study how to improve the energy storage performance of polymer nanocomposites.

The nonlinear optical absorption and optical limiting properties of noble metal-doped ZnO films

Herein, pure ZnO, Ag-doped ZnO, and Cu-doped ZnO films with excellent ultrafast (femtosecond) nonlinear absorption/limiting behavior were deposited on quartz substrates by magnetron sputtering. From the open-aperture Z-scan result, under femtosecond pulses, the strong two-photon absorption (TPA) behavior of metal-doped ZnO was found, and the absorption coefficient was found to be several times that of pure ZnO, which will expand the design and application strategies of metal-doped wide-gap semiconductor materials in the field of nonlinear devices. Moreover, the samples exhibited superior optical limiting behavior with respect to the optical modulation characteristics of the input–output laser. The mechanism of optical limiting effects is induced by the TPA-induced reverse saturable absorption of the deep level and low excited state processes.

Accelerated Learning in Wide-Band-Gap AlN Artificial Photonic Synaptic Devices: Impact on Suppressed Shallow Trap Level.

Artificial synaptic platforms are promising for next-generation semiconductor computing devices; however, state-of-the-art optoelectronic approaches remain challenging, owing to their unstable charge trap states and limited integration. We demonstrate wide-band-gap (WBG) III-V materials for photoelectronic neural networks. Our experimental analysis shows that the enhanced crystallinity of WBG synapses promotes better synaptic characteristics, such as effective multilevel states, a wider dynamic range, and linearity, allowing the better power consumption, training, and recognition accuracy of artificial neural networks. Furthermore, light-frequency-dependent memory characteristics suggest that artificial optoelectronic synapses with improved crystallinity support the transition from short-term potentiation to long-term potentiation, implying a clear emulation of the psychological multistorage model. This is attributed to the charge trapping in deep-level states and suppresses fast decay and nonradiative recombination in shallow traps. We believe that the fingerprints of these WBG synaptic characteristics provide an effective strategy for establishing an artificial optoelectronic synaptic architecture for innovative neuromorphic computing.

Effect of Cation Occupancy Ordering in Double Perovskites To Overcome Hurdles in Carrier Transport: Cs2AgBiBr6 as a Case Study

In spite of environmental stability and toxic-free merits, the performance of double perovskite solar cells (Cs2AgBiBr6 to be specific) has not excelled due to their large indirect band gap, low absorption of solar spectrum, and more importantly hurdles in carrier transport processes. A large number of intrinsic defects and self-trapped excitons (STE) appear in the double perovskite due to a disordered arrangement of Ag/Bi octahedra in the perovskite lattice. In this work, we investigate the role of cation ordering in Cs2AgBiBr6 through the use of phenethylammonium bromide (PEABr) in the precursor toward suppression of STEs and deep-level defect states, such as antisite defects of Ag+/Bi3+, for an improved carrier conduction process. While cation ordering has been verified from X-ray diffraction studies and Raman spectroscopy, a reduction in the defect density and inhibition of STE formation have been witnessed from optical spectroscopy results. The transport gap, obtained from scanning tunneling spectroscopy and density of states spectra thereof, has been found to increase, suggesting the defect states to become suppressed. The effect of cation ordering toward an efficient carrier transport in the ordered lattice of Cs2AgBiBr6 has been demonstrated by Kelvin probe force microscopy and conductive atomic force microscopy. Finally, a fair improvement in photovoltaic parameters was observed in devices based on the ordered double perovskite.

Observations regarding deep-level states causing p-type doping in AlTiO gate and positive threshold voltage shift in AlGaN/GaN high electron mobility transistors

Application of ternary oxides has emerged as a potential enabler to achieve enhancement mode (normally-OFF) operation in AlGaN/GaN high electron mobility transistors (HEMTs). However, it is not well understood what leads to the 2-Dimensional Electron Gas depletion or positive threshold voltage shift by the integration of these oxides in the gate stack. In this work, an electro-optical experiment-based method is used to probe the underlying mechanism. For experiments, AlGaN/GaN metal–oxide–semiconductor high electron mobility transistors with gate stack consisting of AlTiO (ternary p-type oxide), Al2O3, TiO2 (undoped binary oxides), and NiO (conventional p-type oxide) are used. Optical exposure with wavelengths falling in the UV regime (365 nm) was found to induce a negative threshold voltage shift in AlTiO based devices. Experimentation on different GaN buffer stacks, different gate oxides, and selective UV exposure only to the gate region established the phenomenon to be governed by properties of the ternary oxide. This was further confirmed by the fact that the negative threshold voltage shift with UV exposure was proportional to the positive shift achieved as a function of Al% in AlTiO. The negative shift was found to be due to de-ionization of deep-level negative states in AlTiO, which resulted from the presence of Al at Ti sites ([Al]Ti′). These negatively ionized deep-level states at room temperature result in p-type doping of the oxide, leading to the positive threshold voltage shift in AlTiO gate based HEMTs when compared with HEMTs with Al2O3 or TiO2 as gate oxide.

Recent Progress on Perovskite Surfaces and Interfaces in Optoelectronic Devices.

Surfaces and heterojunction interfaces, where defects and energy levels dictate charge-carrier dynamics in optoelectronic devices, are critical for unlocking the full potential of perovskite semiconductors. In this progress report, chemical structures of perovskite surfaces are discussed and basic physical rules for the band alignment are summarized at various perovskite interfaces. Common perovskite surfaces are typically decorated by various compositional and structural defects such as residual surface reactants, discrete nanoclusters, reactions by products, vacancies, interstitials, antisites, etc. Some of these surface species induce deep-level defect states in the forbidden band forming very harmful charge-carrier traps and affect negatively the interface band alignments for achieving optimal device performance. Herein, an overview of research progresses on surface and interface engineering is provided to minimize deep-level defect states. The reviewed subjects include selection of interface and substrate buffer layers for growing better crystals, materials and processing methods for surface passivation, the surface catalyst for microstructure transformations, organic semiconductors for charge extraction or injection, heterojunctions with wide bandgap perovskites or nanocrystals for mitigating defects, and electrode interlayer for preventing interdiffusion and reactions. These surface and interface engineering strategies are shown to be critical in boosting device performance for both solar cells and light-emitting diodes.

Improving the Catalytic CO2 Reduction on Cs2AgBiBr6 by Halide Defect Engineering: A DFT Study.

Pb-free double halide perovskites have drawn immense attention in the potential photocatalytic application, due to the regulatable bandgap energy and nontoxicity. Herein, we first present a study for CO2 conversion on Pb-free halide perovskite Cs2AgBiBr6 under state-of-the-art first-principles calculation with dispersion correction. Compared with the previous CsPbBr3, the cell parameter of Cs2AgBiBr6 underwent only a small decrease of 3.69%. By investigating the adsorption of CO, CO2, NO, NO2, and catalytic reduction of CO2, we found Cs2AgBiBr6 exhibits modest adsorption ability and unsatisfied potential determining step energy of 2.68 eV in catalysis. We adopted defect engineering (Cl doping, I doping and Br-vacancy) to regulate the adsorption and CO2 reduction behavior. It is found that CO2 molecule can be chemically and preferably adsorbed on Br-vacancy doped Cs2AgBiBr6 with a negative adsorption energy of −1.16 eV. Studying the CO2 reduction paths on pure and defect modified Cs2AgBiBr6, Br-vacancy is proved to play a critical role in decreasing the potential determining step energy to 1.25 eV. Finally, we probe into the electronic properties and demonstrate Br-vacancy will not obviously promote the process of catalysis deactivation, as there is no formation of deep-level electronic states acting as carrier recombination center. Our findings reveal the process of gas adsorption and CO2 reduction on novel Pb-free Cs2AgBiBr6, and propose a potential strategy to improve the efficiency of catalytic CO2 conversion towards practical implementation.

Encapsulated Perovskites: Self‐Structural Healing of Encapsulated Perovskite Microcrystals for Improved Optical and Thermal Stability (Adv. Mater. 21/2021)

A self-sufficient micrometer-level vacuum growth chamber based on encapsulated MAPbBr3 microcrystals is designed by Rui Chen, Zhipeng Wei, and co-workers, as described in article number 2100466. Perovskite materials with enhanced environmental, thermal, and optical stability through the reduction of deep-level trap states due to self-structural healing are observed. This greatly improves the lasing performance and service cycle.

Effect of Sacrificial AlN (Aluminum Nitride) Layer on the Deep Surface States of 4H-SiC.

We investigated the effect of a sacrificial AlN layer on the deep energy level states of 4H-SiC surface. The samples with and without AlN layer have been annealed at 1300 °C for 30 minutes duration using a tube furnace. After annealing the samples, the changes of the carbon vacancy (VC) related Z1/2 defect characteristics were analyzed by deep level transient spectroscopy. The trap energy associated with double negative acceptor (VC(2-/0)) appears at ˜0.7 eV and was reduced from ˜0.687 to ˜0.582 eV in the sacrificial AlN layer samples. In addition, the capture cross section was significantly improved from ˜2.1×10-14 to ˜3.8×10-16 cm-2 and the trap concentration was reduced by approximately 40 times.

Correlation between Optical Localization-State and Electrical Deep-Level State in In0.52Al0.48As/In0.53Ga0.47As Quantum Well Structure.

The peculiar correlationship between the optical localization-state and the electrical deep-level defect-state was observed in the In0.52Al0.48As/In0.53Ga0.47As quantum well structure that comprises two quantum-confined electron-states and two hole-subbands. The sample clearly exhibited the Fermi edge singularity (FES) peak in its photoluminescence spectrum at 10–300 K; and the FES peak was analyzed in terms of the phenomenological line shape model with key physical parameters such as the Fermi energy, the hole localization energy, and the band-to-band transition amplitude. Through the comprehensive studies on both the theoretical calculation and the experimental evaluation of the energy band profile, we found out that the localized state, which is separated above by ~0.07 eV from the first excited hole-subband, corresponds to the deep-level state, residing at the position of ~0.75 eV far below the conduction band (i.e., near the valence band edge).

Semiconducting ZnO＆WS2 heterojunction composite films: Fabrication, characterization and ultrafast nonlinear properties

In this work, a series of WS2, ZnO base films and ZnO&WS2 (ZnO@WS2 and WS2@ZnO) heterojunction composite films were synthesized by the single/two-step Radio Frequency Magnetron Sputtering. Interestingly, it is found that structure, morphology and growth mechanism of the samples depends on the regulation different experimental parameters. The result of morphology characterization demonstrated that the standing WS2 with different morphology depend on growth time and the multi-directional growth of the ZnO based film. And using the WS2 base films promoted the growth of ZnO sheet-like morphology. The result of X-ray diffraction characterization showed that the location and the shape of the characteristic peaks existed in the ZnO&WS2 heterojunction composite films are analogous to ZnO (002) and WS2 (002)/(101) alone. Furthermore, the ultrafast nonlinear optical behavior and dynamics of the samples were studied. The “W” and “M” type image of open-aperture Z-scan curve were analyzed. From the results, the samples showed multiphoton absorption induced the ground state or excited state saturated absorption or reverse saturated absorption behavior. And the two-photon absorption and three-photon absorption coefficients of ZnO@WS2 heterojunction composite films reached 1.93 × 10−9 m/W, −2.22 × 10−23 m3/W2, respectively. The bi-exponential decay of the positive signal assigned to the pump-induced deep-level or excited state absorption. Moreover, these findings indicate that the excitation wavelength increases and the detection wave band widen. The study shows that the carrier lifetime of ZnO@WS2 heterojunction composite films is significantly higher than that of the WS2 base films. ZnO&WS2 heterojunction composite films will be promising candidates in nonlinear optical material.