Changes In Trap Densities Research Articles

Strain distribution and electrical characteristics of flexible low-temperature polysilicon thin film transistors under biaxial bending

This paper investigates the strain distribution of flexible p-channel low temperature polycrystalline silicon thin film transistors under biaxial bending. The strain distribution of LTPS TFTs under different bending conditions is analysed by finite element analysis. \U0001d6c6r and \U0001d6c6θ are essentially equal and constant within the load ring. The finite element analysis shows that when the biaxial bending strain condition is 2.5%, the strain distribution is mainly concentrated at the contact angle between the electrode and the oxide layer. This strain is significantly greater than the overall strain of the device, which is the main reason for the bending performance of the device. We found that under biaxial bending, the threshold voltage shifts towards the negative axis as the biaxial strain increases. Within this strain range, these electrical properties are evident under biaxial strain conditions. Comparing theoretical threshold voltage shifts based on energy band changes with experimental results under biaxial strain, it was found that changes in trap density should also contribute to the threshold voltage shift.

Charge‐Carrier Trapping and Radiative Recombination in Metal Halide Perovskite Semiconductors

AbstractTrap‐related charge‐carrier recombination fundamentally limits the performance of perovskite solar cells and other optoelectronic devices. While improved fabrication and passivation techniques have reduced trap densities, the properties of trap states and their impact on the charge‐carrier dynamics in metal‐halide perovskites are still under debate. Here, a unified model is presented of the radiative and nonradiative recombination channels in a mixed formamidinium‐cesium lead iodide perovskite, including charge‐carrier trapping, de‐trapping and accumulation, as well as higher‐order recombination mechanisms. A fast initial photoluminescence (PL) decay component observed after pulsed photogeneration is demonstrated to result from rapid localization of free charge carriers in unoccupied trap states, which may be followed by de‐trapping, or nonradiative recombination with free carriers of opposite charge. Such initial decay components are shown to be highly sensitive to remnant charge carriers that accumulate in traps under pulsed‐laser excitation, with partial trap occupation masking the trap density actually present in the material. Finally, such modelling reveals a change in trap density at the phase transition, and disentangles the radiative and nonradiative charge recombination channels present in FA0.95Cs0.05PbI3, accurately predicting the experimentally recorded PL efficiencies between 50 and 295 K, and demonstrating that bimolecular recombination is a fully radiative process.

Wet oxidation growth of hafnium doped tantalum oxide films with different composition deposited on silicon substrate

As-deposited Ta-HfO2 films produced via co-sputtering using different Ta sputtering powers (120, 150, and 180 W) with a fixed HfO2 power (160 W) were transformed to ternary HfxTayOz films after wet oxidation at 800 °C. An increase in the Ta sputtering power encouraged amorphous to crystalline phase transformation. Crystallization has assisted film growth but densification happened in the film sputtered at 180 W, yielding a reduction in thickness. Results indicated that formation of thick TaSiOx-rich interfacial layer (IL) at the interface has triggered the thickest film at 150 W while a thinner SiOx-rich IL was formed in the film at 180 W. Changes in dielectric constant with respect to the IL thickness were reported. The decrease in IL formation at 180 W was due to accumulation of nitrogen at interface for nitridation. Dissimilar from that at 150 W, a higher amount of oxygen vacancies would trap the nitrogen species, decreasing the nitridation activity. The trapping of nitrogen happened together with H2, affecting passivation capability of interface defects. Corresponding changes in trap density were discussed.

Effects of thermal and electrical stress on defect generation in InAs metal–oxide–semiconductor capacitor

Defects in HfO2 dielectric film caused by indium and arsenide diffusion from InAs were investigated. To investigate the dissociation of InAs during post-deposition annealing (PDA) at 600 °C, we analyzed the ratio of the elements on the surface of the oxide layer and the chemical states by using time-of-flight secondary-ion mass spectroscopy and X-ray photoelectron spectroscopy, respectively. In–As bonding was dissociated and In and As atoms were diffused through the HfO2 layer from InAs. Fortunately, the diffusion and trap density could be controlled by using a 1-nm-thick Al2O3 passivation layer.In addition, we used the nitridation process to control the trap density. We evaluated the thermal and electrical stability of three samples—HfO2/InAs, HfO2/Al2O3/InAs, and nitrided HfO2/Al2O3/InAs—by analyzing the change in trap density before and after PDA at 600 °C and the stress-induced leakage current. In conclusion, the passivation layer effectively improved the thermal and electrical stability, whereas the nitridation process using NH3 gas did not. Moreover, although nitridation could reduce the interfacial defect states, due to structure distortion, it induced the degradation of the device.

Simulation of metastable changes in time resolved photoluminescence of Cu(In,Ga)Se2 thin film solar cells upon light soaking treatment

Cu(In,Ga)Se2 thin film solar cells are known to show a metastable behavior of their open circuit voltage when exposed to light. Time resolved measurements of the photoluminescence decay after pulsed excitation (TRPL) reveal that also the photoluminescence decay (PL) changes with conditioning, yielding shorter effective lifetimes after annealing under illumination (light soaking) than after annealing in the dark. By using time resolved device simulations we applied a model that explains the TRPL metastable behavior. For this we used a model of capture and reemission of minority charge carriers via two characteristic trap states near the conduction band, while also accounting for changes in trap density and doping concentration caused by light soaking. To further understand the mechanisms behind PL decay we analyse the influence that the charge carrier dynamics play in TRPL decay by a variation of select simulation parameters.

(Invited) AlGaN/GaN HEMT Reliability and Trap Detection Using Optical Pumping

Changes in the drain current that correspond to charge trap energies in the band-gap of AlGaN/GaN High Electron Mobility Transistors (HEMTs) were measured when illuminated by below band-gap light. The energy of the applied wavelength of light affects the trapping and de-trapping of carriers within the band-gap, providing an indicator of trap densities. These changes were compared on HEMTs with gate lengths of 0.14 & 0.17 µm, before and after electrically stressing under on-state (VG = 0 V), off-state (VG =-5 V), and typical operating conditions (VG = -2V) indicating a change in trap density as a result of electrical stressing. Changes in trap densities were minimal after both off-state and on-state stressing but significant trap creation in the range EC¬-0.4-0.6 eV were observed in HEMTs exhibiting gradual degradation during stressing. Energy levels corresponding to these values in the literature have been suggested to correlate GaN and NGa substitutional defects, as well as GaI interstitials.

Trap detection in electrically stressed AlGaN/GaN HEMTs using optical pumping

AlGaN/GaN High Electron Mobility Transistors (HEMTs) with gate lengths of 0.14–0.17μm were electrically stressed under on-state (VG=0V), off-state (VG=−5V) and typical operating conditions (VG=−2V) and subsequently illuminated with below band-gap light to monitor changes in drain current corresponding to the changes in trapping and de-trapping of carriers within the band-gap. The changes in drain current are indicators of a change in trap density as a result of electrical stressing, since the energy from a specific wavelength of light fills traps whose activation energies are less than or equal to that of the light source. Changes in trap densities were minimal after both off-state and on-state stressing but significant trap creation in the range EC-0.4–0.6eV were observed in HEMTs exhibiting gradual degradation during stressing. Energy levels corresponding to these values in the literature have been suggested to correlate with GaN and NGa substitutional defects, as well as GaI interstitials.

Dye adsorption, desorption, and distribution in mesoporous TiO2 films, and its effects on recombination losses in dye sensitized solar cells

Recombination rates through bare, partially dyed, and fully dyed mesoporous TiO2 films have been measured, as well as the penetration kinetics of dye molecules into the films. We give evidence that homogeneous partial dye coverage cannot be achieved by short dyeing times or dilute dye solutions, due to the high sticking coefficient of the dyes on TiO2. A new method has been developed which gives homogenous partial dye coverage, based on full dyeing followed by partial desorption. Using this method we show that dyeing with N719, C101, or TG6 dyes provides only minor blocking of recombination, relative to bare TiO2. We also show that recombination rates across the bare TiO2 surface can vary strongly after treatment with mild base, acid, or a Lewis acid/base buffer. We find these treatments can change the density of trap states as well as the conduction band edge potential. We present a method, based on charge extraction, that corrects for changes in trap density when estimating shifts in the conduction band potential. The combined results suggest that the industrially favored fast dyeing procedures can lead to higher recombination rates, especially in the absence of the typical TiCl4 treatment, and under low (e.g. indoor) light levels. The results also explain some anomalous features seen in studies of exciton and hole conduction along partial coverage dye layers which were previously assumed to be homogenous.

Investigation of Resistive Switching at SrRuO3/Cr-doped SrZrO3/Metal Junctions

AbstractWe have investigated charge transport properties with current perpendicular to the plane across the interface between a Cr-doped SrZrO3 perovskite oxide film and different top electrode metals. The measured resistance showed a wide variation depending on the top electrode metal, and suggested the interfacial properties with respect to contacting electrode metal kinds are responsible for the change in I-V characteristics (IVCs). The observed I-V curves were modeled with the equation I(V) = aV+bV2, where the variation of a was related to the work function of the metal whereas the variation of b was related to the oxygen affinity of the metal, consistent with space charge limited conduction through a defected interface. Additionally, the resistive switching was attributed to a change in both the a and the b coefficients. We speculate that voltage (or current) induced vacancy motion can account for a change in trap density in the oxide near the metallic electrical contact, and thus trapping or detrapping of charge carriers, showing the conductivity modulation across the junction.

Effect of Sintering Time of Sintered EaTiO<sub>3</sub> on Electrical Conductivity

The relationship between the electrical conductivity of BaTiO3 ceramics and sintering time was studied. SiO2 (2wt%) and La2O3 (0.02wt%) were added to a mixture of BaCO3 and TiO2, and sintered at 1300°C for 1 to 10h in an oxygen atmosphere. With an increase in sintering time the electrical conductivity decreased, and the PTC effect became less evident. AC conductivities also decreased with sintering time, and showed a maximum value at the temperature at which PTC effect was observed in DC conductivity measurement. These experimental results can be explained by the change in trap density at the grain boundaries above and below the crystal transition point.

Dependence of deep-level parameters in ion-implanted GaAs MESFET's on material preparation

A detailed study of the characteristics and origin of deep level traps in device-quality Si-implanted GaAs and in MESFET's has been made using an ultrasensitive optical DLTS technique. Efforts were made to elucidate the conditions which reduce or enhance trap concentrations. The effects of etching and long-term baking of substrates prior to implantation and prolonged annealing after implantation were investigated. These measurements were complemented by low-temperature photoluminescence measurements to establish the corresponding variation in the quality of the material. Several electron and hole traps were commonly observed, ranging in activation energy from 0.21 to 1.1 eV. It is believed that some of these centers are distributed in energy. Among others, two hole traps with activation energies of 0.43 and 0.72 eV and a 0.57-eV electron trap are dominantly present. The first is believed to be due to Cu, the second is probably related to a native defect and the electron trap seems to originate from lattice damage. The other centers are possibly complexes related to lattice damage and vacancies. Under certain conditions, changes in trap densities could be correlated with corresponding changes in some device parameters.

Pulse Excitation Studies of Gain and Trapping in Photoconductors

Excitation of photoconductors by single pulses of monoenergetic photons shows that the integrated gain over one pulse event is approximately equal to the steady-state gain, except in materials for which the steady-state gain itself is a function of photon flux. Measurement of the peak pulse current as a function of pulse length permits the determination of trap density in a simple manner, which agrees well with values measured independently by thermally stimulated conductivity. A model based on a quasiuniform trap distribution is shown to be capable of predicting the observed dependence of peak pulse current on pulse length without the use of any adjustable parameters. Changes in trap density by several orders of magnitude with variations in ambient atmosphere are demonstrated for certain cadmium sulfide crystals.