Charge Trapping Sites Research Articles

Correlation between grain size and device parameters in pentacene thin film transistors

We develop a general approach to precisely extract the device parameters in top-contact pentacene thin film transistors. The charge trap sites are clarified by analyzing the grain size dependence of the device parameters. The channel mobility and threshold voltage are limited by the charge traps in the channel region, most of which are located not at the grain boundaries but at the organic/insulating-layer interface. The contact resistance decreases by increasing the grain size and is controlled by the charge traps in the contact region, which are suggested to be concentrated at the grain boundaries and at the metal/organic interface.

P-associated defects in the high-κinsulatorsHfO2andZrO2revealed by electron spin resonance

We report on the observation by electron spin resonance of P-related point defects in nanometer-thick $\mathrm{Hf}{\mathrm{O}}_{2}$ films on (100)Si after annealing in the range $500--900\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and in $\mathrm{Zr}{\mathrm{O}}_{2}$ powder---two oxides prominent in current high-$\ensuremath{\kappa}$ insulator research. Based on the principal $g$ matrices and hyperfine tensors inferred from consistent $X$-, $K$-, and $Q$-band spectra simulations and comparison with established P-associated defects in silica, both centers appear similar in nature and are assigned to a ${\mathrm{P}}_{2}$-type defect---a P substituting a Hf or Zr atom. Both centers were observed in the monoclinic phase of the high-$\ensuremath{\kappa}$ oxides, with the unpaired electron strongly localized on the P atom. Within the concern about dopant penetration out of Si into the high-$\ensuremath{\kappa}$ layers on top, identification of the dopant-associated defects in the latter appears crucial to which the present basic results provide fundamental access. The centers may operate as detrimental charge trapping sites.

Observation of Charge Trapping Site within High-k Thin Films by Scanning Capacitance Microscopy

Observation of Charge Trapping Site within High-<i>k</i> Thin Films by Scanning Capacitance Microscopy

Origin of photomultiplication in C60 based devices

In this manuscript, the origin of the photomultiplication effect was studied in C60 based devices by evaluating the wavelength dependent external and internal quantum efficiencies under various biases. The effect of materials with disordered structures on the photomultiplication effect was determined by intentionally integrating both ordered and disordered material structures into one organic solar cell device with a configuration of indium tin oxide/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)∕pentacene∕C60 fullerene/bathocuproine (BCP)/Al. Our results show that both the disordered structure of C60 and the charge trapping effect at the C60∕PEDOT:PSS interface contribute to the photomultiplication effect. By studying the C60-only single layer device, the charge trapping sites are identified to be at the C60 and PEDOT:PSS interface. The interfacial traps behave as an electronic valve that enables a significant increase in electron injection, which causes the photomultiplication phenomena. Quantitative comparisons indicate that photomultiplication induced by C60 disordered structure is much less significant than that by charge trapping at the interface.

Effect of electric field on the photoluminescence intensity of single CdSe nanocrystals

An investigation of the effect of an applied electric field on the photoluminescence (PL) intensity of single CdSe nanocrystals has revealed a measurable field induced PL modulation for a large fraction of the nanocrystals studied. The field induced intensity modulation characteristics (i.e. modulation sign and depth) were observed to vary from particle to particle, and even for different time periods for the same particle in many cases. Simultaneous intensity and frequency resolved PL measurement show that the PL intensity modulation is in fact due to an electric field effect on the PL quantum yield. The results are consistent with a model in which the energies of surface charge trapping sites are modulated by the applied electric field, causing in turn a modulation of the rates of exciton quenching by these sites. The complex observed field effects can be explained by the superposition of the applied and internal electric fields due to deeply trapped charges on the surface of the nanoparticle.

Oxygen vacancy induced charge trapping and positive bias temperature instability in HfO 2nMOSFET

The charge trapping and positive bias temperature instability (PBTI) are investigated at different post deposition annealing conditions (PDA) in HfO 2 nMOSFET. Pulse based measurements (Pulsed Id-Vg and “Pulse on the fly”) are performed to characterize charge trapping effect. Compared with NH 3 PDA, the NH 3 + O 2 PDA shows significant reduction of charge trap sites in HfO 2, which causes the improvement of device performance and reliability. The significant improvement after additional annealing can be explained by the passivation of oxygen vacancies in HfO 2.

A comparison of mixed phase titania photocatalysts prepared by physical and chemical methods: The importance of the solid–solid interface

Mixed phase titania photocatalysts, such as Degussa P25, typically show higher photocatalytic activity than pure phase titania, as reported by many researchers. Our previous experimental results indicate that the solid–solid interface is a key factor in enhancing the photocatalytic reactivity of mixed phase titania and may be the location of catalytic “hot spots”. In this study, titania photocatalysts consisting of varying amount of anatase and rutile phases are prepared by reactive dc magnetron sputtering and by a modified sol–gel method. These titania materials are characterized with a variety of techniques and are tested in the photocatalytic degradation of acetaldehyde. Mixed phase titania thin films prepared by magnetron sputtering have a high density of anatase–rutile interfaces and display the highest photocatalytic activity among the catalysts tested. Studies by electron paramagnetic resonance spectroscopy show a unique distribution of charge trapping sites which are characteristic of the sputtered films. The role of anatase–rutile interface to separate charge and improve the photoactivity of mixed phase materials is discussed.

Observation of a P-associated defect in HfO2 nanolayers on (100)Si by electron spin resonance

Electron spin resonance analysis has detected a P-donor related point defect in nanometer-thick HfO2 films on (100)Si after annealing in the range of 500–900°C. Based on the principal g matrix (axial; g‖=1.9965; g⊥=1.9975) and hyperfine tensor values (A1=1425±10G, A2=1245±10G, and A3=1160±10G) inferred from consistent K- and Q-band spectrum simulations, the center is assigned to a P2-type defect—a P substituting a Hf atom—similar to P2 in silica, where the unpaired spin is strongly localized on the P atom. The annealing impact is linked to the onset of crystallization enabling substitutional positioning of the P impurities. The centers may act as detrimental charge trapping sites.

Behavior of Local Charge-Trapping Sites in La2O3–Al2O3 Composite Films under Constant Voltage Stress

We have investigated the behavior of local charge-trapping sites in La2O3–Al2O3 composite films under electrical stress by observing local leakage current by conductive atomic force microscopy (C-AFM). The total area of leakage spots was observed to increase in current images obtained when repeating C-AFM observation at a constant substrate voltage. This phenomenon can be explained by the local trapping of holes in the film in which the Fowler–Nordheim tunneling current becomes predominant. On the other hand, in the area without leakage spots, average leakage current gradually decreases, due to the electron trapping in the film. The present C-AFM observation clearly reveals different behaviors of trapping sites for electrons and holes; the trapped holes are more easily detrapped than the electrons, and the number of electron-trapping sites is more effectively reduced by additional thermal oxidation than the number of holes.

Fabrication of SONOS-Type Flash Memory with the Binary High-k Dielectrics by the Sol-Gel Spin Coating Method

We fabricated the binary high- nanocrystal memory using a very simple sol-gel spin coating method and 900°C rapid thermal annealing (RTA). From the transmission electron microscopy identification, the nanocrystals were formed as the monolayered charge trapping site after 900°C RTA and the size was ca. . We verified the electrical properties in terms of program-erase speed, charge retention, and endurance. The sol-gel device exhibited the long charge retention time of with only 2.5% charge loss, and good endurance performance for program/erase cycles up to .

有機薄膜トランジスタにおける表面・界面の制御

Insulator-semiconductor and electrode-semiconductor interfaces are crucial to the performance of organic thin-film transistors (TFTs). This paper reviews how to control the interfaces for improving the performance of p-type and n-type organic TFTs and p-type polymer TFTs. In particular, modifying the gate insulator surface by using self-assembled monolayers (SAMs) is our main focus. Changes in the water contact angle and surface energy by forming SAMs on the gate insulator surface are shown, and the TFT performance in the organic semiconductor layers grown on the modified surfaces is presented. Crystal growth of the organic semiconductors and charge trap sites on the gate insulator surface are discussed. They explain the improved TFT performance such as field effect mobility and current on/off ratio.

The Study on Charge-trapping Mechanism in Nitride Storage Flash Memory Device

AbstractIn this work, the charge-trapping distributions of polysilicon-oxide-nitride-oxide-silicon (SONOS) structure are studied. The trapping energy level of SiNx films with different composition ratio deposited by low-pressure chemical vapor deposition (LPCVD) were first characterized by photoluminescence (PL) measurement. Moreover, using F-N/CHE program and charge pumping techniques, the vertical location and the lateral distribution of programmed charges are investigated in the nitride films with different composition ratio. The study offers strong evidence that the density of charge-trapping levels in the Si-rich nitride is higher than the standard nitride. A simple qualitative model and calculation explains that the trapping level distributions in the SiNx films are shallower by increasing relative Si-content. Furthermore, we have observed the nitride trap vertical location was changed by adjusted Si/N composition ratio. And the lateral distribution of hot electron programmed charges in the modified nitride is broader than that in the standard nitride because it offered more charge-trapping sites and shallower charge-trapping levels. In summary, the study can help researchers to understand the nitride charge-trapping mechanism and the analysis of optical/electrical characteristics.

Electronic Excitation of Polyfluorenes: A Theoretical Study

We present systematic, theoretical investigations on structure-property correlations in polyfluorenes (PFs) derived mainly from the chain morphology, oligomer length, and chemical substitutent. Both the vertical absorptions and the vibrational contributions to electronic absorption and fluorescence spectra have been calculated. The effect of temperature on the nature of photoexcitations of PFs has been demonstrated. It is found that the vibronic (electronic and vibrational) structures of PFs are morphology-dependent. beta-phase oligofluorenes (beta-(FL)(n)) and ladder-type poly(p-phenylene) (LPPP) oligomers show a red shift compared to the spectra of alpha-(FL)(n). The asymmetry of the absorption and fluorescence spectra in alpha-(FL)(n) and the fluorenone (FLO) defect oligofluorenes alpha-(FL)(n)(-)(m)(FLO)(m) is significantly more pronounced than that in planarized beta-(FL)(n) and LPPP oligomers. By properly taking into account the anharmonic torsion potentials resulting from the strong electronic and nuclear coupling in the oligofluorenes, we have reasonably reproduced the experimentally observed spectroscopic features. The low-energy on-chain chemical defect sites such as FLO units act as charge-trapping sites for singlet excitations, are the predominantly lighting-emitting species, and thus alter the blue light-emitting properties of PFs whereas the blue-light-emitting properties of PFs are hardly influenced by the hole-transporting molecules. The optical properties of PFs have been predicted by the finite-size calculations. Energy gaps of PFs are estimated by extrapolations from excitation energies of oligofluorenes up to 21 FL units.

Interfacial Phenomena Affecting Charge Transport In Small Molecule Organic Thin-Film Transistors

ABSTRACTOrganic semiconductors exhibiting p-, n-type, or ambipolar majority charge transport are grown on six different bilayer dielectric structures consisting of various spin-coated polymers / HMDS on 300 nm SiO2/p+-Si, and are characterized by AFM, SEM, and WAXRD, followed by field-effect transistor (FET) electrical characterization. It is observed that in case of air-sensitive n-type semiconductors, dielectric surface modifications induce large variations in the corresponding OTFT performance parameters although the film morphologies and microstructures remain similar. In marked contrast, the device performance of air-stable n-type and p-type semiconductors is not significantly affected by the same dielectric surface modifications. This study provides key information on the chemical origin of the charge trapping sites at the FET dielectric-semiconductor interface. In parallel, bottom-contact FETs of n-type oligothiophenes were investigated by a combination of microscopy/electrical measurements and new insights for the poor electron injection efficiency from the Au contacts are presented

Persistent Photoinduced Changes in Charge States of Donors and Acceptors in Hydrothermally Grown ZnO

ABSTRACTBulk ZnO grown by the hydrothermal technique was investigated using electron paramagnetic resonance (EPR), photoluminescence (PL), and infrared absorption (FTIR) techniques. Isolated subsitutional lithium is the dominant acceptor and could be detected using EPR or PL. A large concentration of neutral Li+-OH− centers were observed using FTIR data. EPR spectra assigned to Mn, Co, Ni, Fe, and Group III (Al, Ga) donors were also observed. Photoinduced changes in the charge states of the different deep and shallow centers were produced using 325 nm light, and the stability of these changes were monitored with EPR during pulsed thermal anneals. The charge-state changes for some defects were persistent and remained up to 300 K. These impurities, when present in device structures, may act as stable charge trapping sites.

Highly thermally stable TiN nanocrystals as charge trapping sites for nonvolatile memory device applications

TiN nanocrystals formed by a co-sputtering method have been investigated as discrete charge traps for metal–oxide–nitride–oxide–silicon-type nonvolatile memory devices. The formation of isolated TiN nanocrystals embedded in Al2O3 was confirmed by transmission electron microscopy, x-ray photoelectron spectroscopy, and x-ray diffraction analyses. In addition, superior thermal stability of TiN nanocystals embedded in Al2O3 was confirmed. Compared to the control samples without TiN nanocrystals, Al2O3 layers with TiN nanocrystals exhibited wider capacitance–voltage hysteresis and this in turn showed better charge trapping characteristics due to the incorporation of TiN nanocrystals into Al2O3.

A 2-bit highly scalable nonvolatile memory cell with two electrically isolated charge trapping sites

A highly scalable 2-bit nonvolatile memory (NVM) cell using two electrically isolated charge trapping sites is proposed and demonstrated by numerical device simulation. The operational mechanisms including read, program, erase and inhibit in an array structure are studied in detail. This double storage capability per single cell and highly scalable structure is very suitable for high density nanometric NVM applications.

Surface Potential Investigation on Nano Domain Formation in Lithium Tantalate Single Crystal

AbstractThe surface potential distribution following polarization reversal by scanning probe microscopy is examined in order to determine the mechanism of anti-parallel polarization reversal and ring-shaped domain formation in single-crystal lithium tantalate. Lithium vacancy lattice defects are identified as charge-trapping sites, with the resultant surface potential distribution governing the formation of ring-shaped nano-domains. The behavior of this charge injection is shown to be dependent on atmospheric humidity.

Photochemical charge transfer and trapping at the interface between an organic adlayer and an oxide semiconductor.

Identification of charge transfer and trapping sites on semiconducting oxide surfaces is of fundamental importance in furthering the field of heterogeneous photocatalysts. Using scanning tunneling microscopy, electron energy loss spectroscopy, and photodesorption, we observed both electron trapping and hole transfer events on the (110) surface of TiO2 rutile. UV irradiation of a saturated monolayer of trimethyl acetate (TMA) on TiO2(110) at room temperature resulted in hole transfer to the carboxylate group, followed by (CH3)3C-COO bond cleavage and desorption of CO2 and isobutene/isobutane. Hole transfer to TMA proceeded in the absence of a gas-phase electron scavenger (which is typically O2) because the accompanying photogenerated electrons could be trapped at the surface as Ti3+ cations bound to bridging OH groups. The extent of electron trapping, gauged by electron spectroscopy, correlated directly with the yields of photodesorption fragments resulting from the hole transfer channel. Charge at the Ti3+ sites was titrated in the dark via a reaction between O2 and the Ti3+-OH groups.

Efficient Green-Light-Emitting Diodes from Silole-Containing Copolymers

Two random copolymers based on poly(di-n-hexylfluorene-co-4,4-diphenyldithienosilole) (PF-DTS) were synthesized by Suzuki coupling reaction. The presence of a small portion (10 mol% for PF9-DTS, 5 mol% for PF19-DTS) of electron-deficient dithienosilole moiety in the copolymers increases their electron affinity and facilitates charge recombination through its function as a charge-trapping site for both holes and electrons. Photoluminescence (PL) studies show that color tuning through efficient Förster energy transfer from the higher-energy fluorene segments to the lower-energy dithienosilole-containing segments can be achieved as well. A double-layer device using PF9-DTS as the emitting layer and an in-situ polymerized bis-tetraphenylenebiphenyldiamine-perfluorocyclobutane (BTPD-PFCB) as the hole-transporting layer demonstrates a low turn-on voltage at 4.6 V, a brightness greater than 25 900 cd/m2, and a maximum external quantum efficiency of 1.64%, which could be attributed to both the improved charge injection and charge recombination in this polymer.