Distribution Of Defect States Research Articles

Effect of interlayer thickness on electrochemical properties and corrosion resistance of sputtered TiNxnm/Tixnm multilayer thin films

Three multilayers [TiNxnm/Tixnm/p-type Si (100)] (x = 2.5, 5, 10) were fabricated using a dc-magnetron sputtering to evaluate surface and interfacial effect on electrochemical properties for possible application in supercapacitor electrode materials. The (111) and (101) planes of TiN and Ti were observed at 36.50° and 40.19° respectively. TA, LA, and TO phonon modes were found at 254.12, 306.15, and 637.94 cm−1, respectively. The defect state distribution of Ti, N, and O was investigated from PL. XPS confirmed the Ti at. % of TiNxnm/Tixnm multilayers decreased from 43.07 to 12.04 and N at. % increased from 42.79 to 72.53 % as the x value decreased from 10 nm to 2.5 nm. The areal specific capacitance (Ca) and energy density (E) of the TiNxnm/Tixnm multilayer electrode increased with decreasing multilayer thickness. E varied from 0.27 to 1.37 mWhcm−2 and the power density (P) from 5.04 to 25.4 mWcm−2 for the multilayers. The diffusion coefficient (D) and total accumulated charge (Qin) decreased from 6.48 × 10−17 to 1.56 × 10−18 cm2/s and 1.82 to 1.64 C/cm2, respectively, for TiNxnm/Tixnm multilayers. The capacitive contribution was the highest in x = 10 (2.181 Av−1s). However, the diffusion-limited contribution was maximum in x = 5 (3.541 Av−1/2s1/2). The cyclic stability maintained at 85.4 % capacitive retention over 1000 cycles and the expansive potential window (up to 0.8 V in 1 M H2SO4) makes the multilayer thin films ideal for supercapacitor electrodes. Among all the samples, the TiN2.5nm/Ti2.5nm exhibited the highest Ca, E, and diffusion coefficient, originating from the smaller layer thickness, leading to more ohmic conductivity and facilitating the supercapacitive performance.

Tuning of interface quality of Al/CeO2/Si device by post-annealing of sol-gel grown high-k CeO2 layers

A comprehensive study has been done on the influence of post-deposition annealing temperature on high-k cerium oxide (CeO2) layer grown on n-type silicon (Si) substrate and its resultant interface states have been studied for Al/CeO2/Si metal-oxide-semiconductor (MOS) devices. The high-k CeO2 thin films were deposited by spin-coating and sintered at different annealing temperatures (Ta) in the range of 400–900 °C. The parameters such as fixed charge density (Qeff), dielectric constant (k) of the layers, flat-band voltage (VFB), interface defect density (Dit), etc., of the MOS device were evaluated from CV and I-V measurements. A minimum value of flat band shift (∼0.05 V) with lower Qeff (−4.81 × 1011 C/cm2) have been achieved for the Ta of 600 °C. The k and Dit were evaluated to be 22 and 1.29 × 1012 cm−2, respectively at the Ta of 600 °C. In addition, the CV measurements showed a very small hysteresis and very low frequency dispersion for the Ta of 600 °C sample. Energy distribution of defect states was evaluated and it was maximum towards the bottom of the conduction band. This shows that the 600 °C is the optimum annealing temperature, which results in high quality interface, and the electron affinity of the corresponding CeO2 layers was found to be 3.29 eV as evaluated from ultraviolet photoelectron spectroscopy (UPS). Further, a maximum value of minority carrier lifetime (147 μs) has been achieved for the samples annealed at Ta of 400 °C, indicating that the post-annealing temperature plays a significant role on the properties of CeO2 films deposited by sol-gel process. Thus, the present study demonstrates the possibility of sol-gel grown high k-CeO2 layers suitable for MOS like devices.

Probing Electronic Band Structures of Dielectric Polymers via Pre-Breakdown Conduction.

The electronic band structure, especially the defect states at the conduction band tail, dominates electron transport and electrical degradation of a dielectric material under an extremely high electric field. However, the electronic band structure in a dielectric is barely well studied due to experimental challenges in detecting the electrical conduction to an extremely high electric field, i.e., prebreakdown. In this work, the electronic band structure of polymer dielectric films is probed through an in situ prebreakdown conduction measurement method in conjunction with a space-charge-limited-current spectroscopic analysis. An exponential distribution of defect states at the conduction band tail with varying trap levels is observed in accordance with the specific morphological disorder in the polymer dielectric, and the experimental defect states show also a favorable agreement with the calculated density of states from the density functional theory. The methodology demonstrated in this work bridges the molecule-structure-determined electronic band structure and the macro electrical conduction behavior with a highly improved understanding of material properties that control the electrical breakdown, and paves a way for guiding the modification of existing material and the exploration of novel materials for high electric field applications.

Quantitative analysis of defect states in InGaZnO within 2 eV below the conduction band via photo-induced current transient spectroscopy

This work investigates the function of the oxygen partial pressure in photo-induced current measurement of extended defect properties related to the distribution and quantity of defect states in electronic structures. The Fermi level was adjusted by applying a negative gate bias in the TFT structure, and the measurable range of activation energy was extended to < 2.0 eV. Calculations based on density functional theory are used to investigate the changes in defect characteristics and the role of defects at shallow and deep levels as a function of oxygen partial pressure. Device characteristics, such as mobility and threshold voltage shift under a negative gate bias, showed a linear correlation with the ratio of shallow level to deep level defect density. Shallow level and deep level defects are organically related, and both defects must be considered when understanding device characteristics.

Model and data of optically controlled tunable capacitor in silicon single-photon avalanche diode

This paper reports the photocapacitance effect of silicon-based single-photon avalanche diodes (SPADs), and the frequency scattering phenomenon of capacitance. The test results of the small-signal capacitance–voltage method show that light can cause the capacitance of a SPAD device to increase under low-frequency conditions, and the photocapacitance exhibits frequency-dependent characteristics. Since the devices are fabricated based on the standard bipolar-CMOS-DMOS process, this study attributes the above results to the interfacial traps formed by Si–SiO2, and the illumination can effectively reduce the interfacial trap lifetime, leading to changes in the junction capacitance inside the SPAD. Accordingly, an equivalent circuit model considering the photocapacitance effect is also proposed in this paper. Accurate analysis of the capacitance characteristics of SPAD has important scientific significance and application value for studying the energy level distribution of device interface defect states and improving the interface quality.

Deep level defects in low-pressure chemical vapor deposition grown (010) β-Ga2O3

This study provides the full-bandgap evaluation of defect state distributions in beta phase gallium oxide (β-Ga2O3) grown by low-pressure chemical vapor deposition (LPCVD) on (010) β-Ga2O3 substrates at high growth of up to 20 µm/h. Deep-level optical spectroscopy and deep-level transient spectroscopy measurements applied to Ni/β-Ga2O3 Schottky diodes revealed the presence of a previously unreported defect state at EC-3.6 eV, which dominated the overall trap distribution in LPCVD grown material. However, states at EC-0.8 eV, EC-2.0 eV, and EC-4.4. eV were also detected, similar to prior studies on β-Ga2O3 grown by other methods, with similar or lower concentrations for the LPCVD samples. The EC-0.8 eV and EC-2.0 eV states were previously connected to residual Fe impurities and gallium vacancies, respectively. The total concentration of traps in the LPCVD material was on par with or lower than the state-of-the-art metal–organic chemical vapor deposition-grown materials despite the much higher growth rate, and the distribution of states showed negligible dependence on SiCl4 flow rate and doping concentration. These results demonstrate that the high growth rate of LPCVD-grown β-Ga2O3 is very promising for achieving thick, low defect density, and high-quality layers needed for multi-kV device applications.

Temperature Dependence of Intrinsic Channel Mobility in Indium-Gallium-Zinc-Oxide TFTs

This work reports on the development of a new TCAD material and device model for Indium-Gallium-Zinc-Oxide TFTs that accounts for operation from 150 K to room temperature, over applied gate and drain bias conditions up to 10 V. Specified defect state distributions include oxygen vacancy donors, conduction band-tail acceptor states, and acceptor-like interface traps. The presented model allows the TCAD simulation to account for the behavior of defect states according to defined energy distributions, thus properly regulating the amount of free channel charge. An intrinsic channel mobility defined as only a function of temperature was found to accurately represent extended state transport. The intrinsic channel mobility was 19 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V•s at room temperature, with a reduction to 9 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V•s at T = 150 K. The functional dependence reflects a thermally-activated diffusive mobility with a distinct activation energy E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> = 40 meV, that is shown to be independent of the Fermi level.

Defect Passivation of Efficient Sb2(S,Se)3 Solar Cells with Ultrathin, Insulating Polymers

Interfacial passivation is an effective strategy to suppress recombination and improve the stability of efficient solar cells. Here, Sb2(S,Se)3, a promising candidate for next‐generation, solution‐processed photovoltaics, is carefully studied focusing on the defect state distribution and recombination mechanism. Ultrathin polymer layers were introduced to the n‐i‐p Sb2(S,Se)3 thin‐film solar cells. The insulating polymers can effectively fill the grain boundaries and pinholes, which physically restrain the leakage channels in the film and reduce the surface roughness. By adding this passivation layer, the device exhibits fewer interface states while spatially separating electron holes localized at the interface. The average fill factor of these optimized devices presents an 8% improvement, compared with the control devices. The maximum efficiency reaches a state‐of‐the‐art efficiency of 8.9%. These results again prove the concept of metal‐insulator‐semiconductor, and revealed the mechanism of polymer passivation enhanced photovoltaic performance.

Defect density of states in natural and synthetic MoS2 multilayer flakes

It has become important to identify and study dominant defects in different forms of transition metal dichalcogenide materials, which are being explored for use in electronic devices. We have investigated the density and distribution of deep defect states in natural and synthetic multilayer MoS2 (m–MoS2) flakes using temperature-dependent admittance spectroscopy. The flakes sandwiched between suitable electrodes with an Au|m-MoS2|ZnO structure act as good quality diodes suitable for capacitance-based studies. The defect density of states (DOS) show Gaussian distribution, and density was found to be approximately 1014 and 1013 cm−3 eV−1 in the natural and synthetic MoS2 flake devices, respectively. Both types of flakes showed a deep level around 0.8 eV below the conduction band edge with a Gaussian disorder parameter of around 33 and 30 meV, respectively, at room temperature, indicating a common origin corresponding to these defect states. The synthetic MoS2 flake device shows the appearance of an additional defect state at around 0.7 eV, which is probably related to a stoichiometric defect. Our results point to the possible occurrence of a large lattice relaxation of donors with associated trap levels deep within the gap. Our results demonstrate an excellent non-destructive method of deriving defect DOS in multilayer flakes.

Nonlinear optical performance and optical limiting of germanate glasses modified with PbF2 and B2O3 induced by nanosecond pulsed laser

In this work, GeO2 modified with PbF2 and B2O3 glasses were synthesized by conventional melting and quenching technique to investigate the effect of modifier network on its linear and nonlinear optical properties. Linear optical analysis was indicated decreased band gap energy (∼0.14 eV) and Urbach energy (∼0.12 eV) with increasing of Pb/B molar ratio in the glass structure. Beside, decreasing Urbach energy with increase of Pb/B molar ratio indicated that the increasing large atomic radius in the glass matrix led to accumulation of the defect states localized inside the band gap. Open aperture Z-scan results exhibited that the germanate glasses had decreasing normalized transmittance and increasing nonlinear absorption coefficient with increasing of the Pb/B molar ratio. The higher optical limiting effect was in line with the nonlinear absorption behaviors of the germanate glasses. These observations are associated with increased two-photon and free-carrier absorptions with increasing of the PbF2 modifier content which led to the reduction of the band gap energy and changing distribution of the localized defect states which facilitate the nonlinear absorption. Our results indicate that 70GeO2 + 20PbF2 + 10B2O3 glasses are very good candidate as optical limiter applications in optoelectronic applications.

Morphology, defects and polymer concentration related nonlinear absorption and optical limiting properties of electrospun polyamide 6 nanofibers

AbstractThe effects of morphology, defects and polymer concentration on nonlinear absorption characteristics of electrospun polyamide 6 (PA 6) nanofibers were studied systematically. Scanning electron microscopy measurements showed that a deviation from uniform nanofibers to irregular cob‐web nanofibers morphology occurred with increasing PA 6 concentration and fiber diameter. Open aperture Z‐scan results indicated that the thinner uniform PA 6 nanofibers present higher nonlinear absorption. This observation is mainly attributed to a greatest light scattering for thicker uniform nanofibers, considering the linear optical analysis of the PA 6 nanofibers. As a secondary factor, this could be associated with defect state distribution within the band gap, which facilitates the contribution of two photon absorption (TPA) and free carrier absorption (FCA) to nonlinear absorption. Moreover, the thinner uniform nanofibers exhibit higher optical limiting behavior. Polyamide 6 nanofibers attract more attention for biomedical applications due to their good biocompatibility. However, to our knowledge, no study exists on PA 6 nanofibers nonlinear optical properties and their dependency on nanofiber morphology. Therefore, in line with the reported results, this report reveals that PA 6 nanofibers are a promising material for optoelectronic applications as well as biomedical applications.

Physical properties of nanosized (x)NiO/(1−x)CdFe2O4 composites

This study investigates the optical and magnetic properties of nanosized (x)NiO/(1−x)CdFe2O4 (x = 0.1, 0.2, 0.3, 0.4, 0.5, and 1) composites. The samples were synthesized using the wet coprecipitation technique. The X-ray Diffraction patterns demonstrated the formation of Nickel Oxide (NiO) and Cadmium Ferrite (CdFe2O4) phases without any chemical reaction between them. Fourier Transform Infrared results confirm the formation of pure NiO, pure CdFe2O4, and nanosized NiO/CdFe2O4 composites. Transmission Electron Microscopy micrographs show agglomeration between the nanoparticles, demonstrating the magnetic interaction between the oxide and ferrite phases. High-Resolution Transmission Electron Microscopy results verify the presence of NiO and CdFe2O4 phases in each nanosized composite without impurities, which is consistent with the XRD data. The ultraviolet–visible spectroscopy results confirmed that HCl is a better solvent than ethanol in investigating the optical properties of nanosized oxide/ferrite composites. Photoluminescence spectra exhibited asymmetricity owing to the presence of various emission peaks, which reveals the distribution of defect states within the bandgap of the nanosized composites. The Mössbauer spectra of the nanosized composites comprised an asymmetrical doublet at 300 K and an asymmetrical centered doublet within minor sextets at 77 K. Vibrating Sample Magnetometer study showed a shift at 77 K, which can be attributed to the spontaneous exchange bias effect. The magnetic parameters extracted from the M − H loops of all the synthesized nanosized composites at 77 K are higher than those at 300 K.

Performance improvement of flexible ultraviolet photodetectors based on ZnO nanorod arrays by hydrothermal method with assistance of polyethyleneimine

Investigations on the high-performance flexible ultraviolet photodetectors have been becoming one of the interesting research focuses in recent years. ZnO is considered as suitable material for ultraviolet photodetection due to its wide bandgap (~3.37 eV). In this work, ZnO nanorod arrays were prepared using the hydrothermal method at the temperature of 70 ℃ on flexible substrates. Polyethyleneimine (PEI) was added into hydrothermal solution as the growth additive to achieve the performance improvement of the ZnO nanorod arrays ultraviolet photodetector. SEM, XRD, absorption spectroscopy, Raman spectroscopy and XPS were employed to characterize the morphology, crystallinity, optical properties and chemical composition. The introduction of PEI adjusts the growth process of ZnO nanorod arrays and affects the distribution of defect states, which brings out the improvement of the optoelectronic performance of ZnO nanorod arrays ultraviolet photodetectors. Compared with pure ZnO, the higher photo to dark current ratio and responsivity, and quicker time response speed are demonstrated in the PEI-assisted ZnO nanorod arrays photodetectors. And more photoresponse parameters are also discussed and compared. Furthermore, the photodetectors also display outstanding mechanical flexibility and robustness under multiple bending cycle tests. Our work presents a potential approach for improving the optoelectronic performance of simple-operated flexible ultraviolet photodetectors.

Dielectric Engineering for Manipulating Exciton Transport in Semiconductor Monolayers.

The dielectric screening from the disordered media surrounding atomically thin transition metal dichalcogenides (TMDs) monolayers modifies the effective defect energy levels and thereby the transport and energy dynamics of excitons. In this work, we study this effect in WSe2 monolayers for different combinations of surrounding dielectric media. Specifically, we study the source of the anomalous diffusion of excitons in the WSe2 monolayer and attribute the anomaly to the modification of the energy distribution of defect states in different disordered dielectric environments. We use this insight to manipulate exciton transport by engineering the dielectric environment using a graphene/hexagonal boron nitride (h-BN) moiré superlattice. Finally, we observe that the effect of dielectric disorder is even more significant at high excitation fluences, contributing to the nonequilibrium phonon drag effect. These results provide an important step toward achieving control over the exciton energy transport for next-generation opto-excitonic devices.

Defect characterization in floating body transistors using a single pulse charge pumping method

Field-effect transistors on a floating body channel have been used for a range of applications, such as three-dimensional nand flash and high-performance devices on silicon-on-insulator wafers. Conventional techniques to characterize the defect states cannot be implemented because those transistors do not have body contacts. A single-pulse charge pumping (SPCP) method was introduced to characterize the defect state distribution of the floating body transistors. The results extracted from the SPCP method on the three-terminal transistors (without body contact) agreed well with those from a conventional charge pumping method or SPCP on four-terminal transistors (with body contact). The validity of the SPCP measurement of the floating body devices was demonstrated by monitoring the increase in the defect state of the three-terminal poly-Si transistors density during the bias stress test. The SPCP method could detect the defect states of floating body transistors with high resolution.

Imaging Vacancy Defects in Single-Layer Chromium Triiodide.

As a van der Waals magnetic semiconductor, chromium triiodide (CrI3) is widely considered for its high research value and potential applications. Defects in CrI3 are inevitably present and significantly alter the material properties. However, experimental identification of defects of CrI3 at the atomic level is still lacking. Here for the first time, we carried out a scanning tunneling microscopy (STM) study and density functional theory calculations to explore the intrinsic defects in monolayer CrI3 grown by molecular beam epitaxy. The three most common types of intrinsic point defects, i.e., I vacancy (VI), Cr vacancy (VCr), and multiatom CrI3 vacancy (VCrI3) with distinct spatial distributions of the localized defect states, are identified and characterized by high-resolution STM. Moreover, defect concentrations are estimated based on our experiments, which agree with the calculated formation energies. Our findings provide vital knowledge on the types, concentrations, electronic structures, and migration mechanism of the intrinsic point defects in monolayer CrI3 for future defect engineering of this novel 2D magnet.

(Invited) Photoluminescent ZnO Thin Films By Photochemical Gel Conversion Under VUV Irradiation

Zinc oxide (ZnO), as a wide band gap (3.37 eV) semiconductor, are useful materials for wide range of applications in thin-film transistor (TFT), photovoltaics, photocatalysts, sensors, and so on. Large exciton binding energy (60 meV) at room temperature (RT) makes ZnO a promising luminescent material especially in optoelectronic devices such as UV lasers and light emitting diodes (LED). Among them, ZnO nanocrystals (quantum dots) has attracted increasing attention because their optical properties can be tuned by varying the crystal size. However, its nanoscale crystallites tend to aggregate to undergo Ostwald ripening due to their high surface energy. To stabilize them, surface modification are usually necessary. At the same time, the whole processes for nanocrystal synthesis and film fabrication become complicated.Here, we report a simple novel photochemical gel conversion approach for one-step preparation of ZnO thin films with an average nanocrystals in about 2-3 nm size (inset in Fig. 1a), by employing vacuum ultraviolet (VUV) light irradiation in dry N2. Systematic characterization is performed to understand the surface chemistry and the optical properties of ZnO nanocrystals. Thus obtained ZnO films show unique absorption peak at 294 nm (4.2 eV) (Fig. 1a) and strong tunable photoluminescence in the range of 386-496 nm when expose to UV light, from violet, blue to bluish-green upon increasing time of VUV irradiation (Fig. 1b). Clearly blue-shifted absorption peaks of the VUV treated samples are indicative of quantum-confinement effect, as contrasted to the broad character of the annealed film, which originates from band structure of bulky ZnO. The extension of VUV time only results in increase of absorbance, with no change of the peak wavelength, indicating population increase but no crystal growth by Ostwald ripening during the VUV irradiation. Therefore, the color change of the emitted light should be associated with the change of defect state distribution, rather than the size effect. However, such photoluminescence from defects was fairly strong to be clearly recognized by naked eyes, so that the university logo could be patterned by photomasking VUV to be seen under black light was demonstrated (inset in Fig. 1b). Figure 1

Influence of growth temperature on defect states throughout the bandgap of MOCVD-grown β -Ga2O3

The influence of growth temperature on the distribution (concentrations and energy levels) of individual defect states in metal organic chemical vapor deposition-grown, Si-doped β-Ga2O3 is investigated. A combination of deep level thermal transient/optical spectroscopies and admittance spectroscopy (AS) was used to quantitatively monitor the evolution of trap states throughout the ∼4.8 eV bandgap. States are observed at EC-0.12 eV by AS; at EC-0.4 eV by deep level transient spectroscopy; and at EC-1.2 eV, EC-2.0 eV, and EC-4.4 eV by deep level optical spectroscopy, and showed different dependencies on growth temperatures ranging from 800 °C to 920 °C. The EC-0.4 eV and EC-4.4 eV states both displayed a strong reduction in its concentration with increasing growth temperature, whereas no consistent trends were seen for the states at EC-1.2 eV and 2.0 eV over the temperature range studied. In contrast, the concentration of the EC-0.12 eV trap monotonically increased over the same range of increasing growth temperature, which tracked a slight, monotonic increase in overall Si concentration measured by secondary ion mass spectroscopy with growth temperature. The opposing trends in concentrations for some of these states shifted the dominant deep level in the bandgap from the EC-4.4 eV state at the lowest growth temperature explored here to the EC-0.12 eV state at the highest growth temperature. The shifting dominance of various bandgap states can have important ramifications on β-Ga2O3 device behavior, and the different trends for these deep levels cannot only guide further growth optimization but also advance the identification of their physical sources.

Evidence for narrow mid-gap defect band in amorphous selenium from low-temperature anomalous electrical properties

Electrical properties of amorphous selenium films (~2.1 μm thick) were investigated at different temperatures between 225 K and room temperature. The samples were prepared using thermal evaporation method. The capacitance and conductance were measured and analyzed to study their changes with temperature and voltage. An anomalous peak is found in the conductance- and capacitance-temperature curves at ~245 K. The phenomenon can be linked to the distribution of defect states near the mid-gap and the change in ‘effective transport energy’ at which the transport takes place.

Electrical Defect State Distribution in Single Crystal ZnO Schottky Barrier Diodes

The characterization of defect states in a hydrothermally grown single crystal of ZnO was performed using deep-level transient spectroscopy in the temperature range of 77–340 K. The native intrinsic defect energy level within the ZnO band gap occurred in the depletion region of ZnO Schottky barrier diodes. A major defect level was observed, with a thermal activation energy of 0.27 eV (E3) within the defect state distribution from 0.1 to 0.57 eV below the conduction band minimum. We confirmed the maximum defect concentration to be 3.66 × 1016 cm−3 at 0.27 eV (E3). As a result, we clearly confirmed the distribution of density of defect states in the ZnO band gap.