Density Of Trapping Centers Research Articles

Co-doped ZnSe: Mn, Cu quantum dots (QDs)—Eco-friendly synthesis, optical and structural properties besides lattice strain/dislocation density

Cu-Mn co-doped ZnSe quantum dots were eco-friendly synthesized at different synthesis conditions using water-based microwave-activated (MWIR) method. Energy band gaps $$E_{\text{gap}}^{\text{DASF}}$$ were determined using high-precision derivation of absorption spectrum fitting (DASF) method; the obtained results show that $$E_{\text{gap}}^{\text{DASF}}$$ values are within the range of 3.49–3.77 eV. Depending on the synthesis conditions, a decreasing trend of energy gap was observed with the increasing of MWIR time and ambient temperature. Morphologies and structural characterizations were done by XRD, EDX, FESEM, map and TEM, which revealed formation of ZnSe nanoparticles with mean crystallite size of ~ 2–5 nm. Also, Urbach energies were estimated, which their very small values in compare with $$E_{{{\text{gap}}}}$$ imply to the sharp band edges and so their good crystallinity nature. Moreover, refractive index, dielectric constant and nonlinear optical susceptibility of each sample were determined; results imply that these nanoparticles have a high potential in optoelectronic applications. Also, structural characteristics such as dislocation density, lattice strain and size of NCs were evaluated upon the Scherrer, Williamson–Hall and Williamson–Smallman methods. The results of structural features obtained from these approaches are highly inter-correlated and show same trends with the variation of synthesis conditions. Results show that the size of QDs is varied tunable by changing the MWIR time, ambient temperature and Mn dopant percentage and the as-synthesized ZnSe QDs and doped QDs have similar cubic zinc blend structure. Also, TEM result reveals that ZnSe nanoparticles are spherical in shape with an average grain size of about 5 nm. Upon the different performed opto-structural criteria, sample with Mn = 4% has the best crystallinity as good candidate in optical and mechanical applications. There were anomalies in different properties at Mn = 1.5 and 2% attributed to their higher density of lattice imperfections and surface trap centers related to how Mn ions incorporated to the ZnSe host lattice.

Charge transport features of CdTe-based X- and [formula omitted]-ray detectors with Ti and TiO[formula omitted] Schottky contacts

The X- and γ-ray detectors, formed as Ti/p-CdTe/Au/Cu and TiOx/p-CdTe/MoOx diode structures with Ti and TiOx Schottky contacts, created by electron-beam evaporation and DC reactive magnetron sputtering of Ti, respectively, have been investigated. The structural, electrical and spectroscopic properties of the Schottky diodes were studied by X-ray diffraction, I–V characteristic and isotope spectrum measurements, respectively. The current transport processes in the Ti/p-CdTe/Au/Cu and TiOx/p-CdTe/MoOx heterostructures at relatively low voltages were well described by the models of the carrier generation–recombination in the space-charge region and space-charge limited current. Comparing the experimental data and calculation results, the energies of generation–recombination centers and hole traps, density of discrete trapping centers were determined, and charge carrier lifetimes. A rapid increase in the reverse current due to the Poole–Frenkel emission was observed and this was attributed to the presence of a thin intermediate insulator layer in the p-CdTe/MoOx interface (near-contact contact). The I–V characteristics of the Schottky diodes have been analyzed using the model of space-charge limited current taking into account the Poole–Frenkel effect. The TiOx/p-CdTe/MoOx detectors were tested using 241Am isotope emission.

The Effect of Density and Concentration of Trapping Centers on Charge Carrier Mobility in Polymer Dielectrics

The spectra of the currents of thermostimulated depolarization and the time dependences of the electret potential difference were measured in polymers irradiated with soft X-rays of electric gas discharge. The results of measurements are used to determining the mobility of electrons and holes in polymer cable and capacitor dielectrics having different values of density and concentration of electrically active trapping centers.

Capabilities of CdTe-Based Detectors With ${\mathrm {MoO}}_{x}$ Contacts for Detection of X- and $\gamma$ -Radiation

The charge transport mechanism and spectrometric properties of the X-ray and $\gamma $ -ray detectors, fabricated by the deposition of molybdenum oxide thin films onto semi-insulating p-CdTe crystals were studied. The current transport processes in the ${\mathrm {Mo}}{-}{\mathrm {MoO}}_{x}/{\mathrm {p}}{-}{\mathrm {CdTe/MoO}}_{x}{-}{\mathrm {Mo}}$ structure are well described in the scope of the carrier’s generation in the space-charge region and the space-charge-limited current models. The lifetime of charge carriers, the energy of hole traps, and the density of discrete trapping centers were determined from the comparison of the experimental data and calculations. Spectrometric properties of ${\mathrm {Mo}}{-}{\mathrm {MoO}}_{x}/{\mathrm {p}}{-}{\mathrm {CdTe/MoO}}_{x}{-}{\mathrm {Mo}}$ structures were also investigated. It is shown that the investigated heterojunctions have demonstrated promising characteristics for practical application in X-ray and $\gamma $ -ray detector fabrication.

Hierarchically constrained carrier dynamics in red-LED illuminated type-II InAs/GaSb superlattices

We report on positive and negative persistent photo-effects observed in some red light emitting diode (LED)-illuminated type-II InAs/GaSb superlattices (SLs) grown on a p-type GaSb. By analyzing the time dependence of the transverse resistance during and after the illumination, we show that the rise and decay curves are logarithmic, a behavior which points to hierarchically constrained carrier dynamics. Accordingly, negative persistent effects are explained by diffusion and trapping of photo-excited carriers in the p-type buffer layer and their subsequent tunneling back to the SL. On the other hand, positive persistent effects are explained by a low density of majority-carrier trapping centers in the buffer layer. Hence, persistent photo-effects upon red-LED irradiation provide a diagnostic of the quality of the superlattice-buffer interface.

Persistent luminescence property of rare earth doped BaMg2Al6Si9O30 phosphor

Several rare earth single-doped and co-doped BaMg2Al6Si9O30 phosphors were synthesized by high temperature solid-state reaction. Their persistent luminescence property was studied by means of afterglow time and thermoluminescence curves. Single-doped phosphor with 0.1mol% ratio Eu2+ replacing Ba2+ obtained 143s’ afterglow time. When doping concentration was increased from 0.1% to 0.4%, thermoluminescence peek around 75°C rose nearly linearly, but afterglow time was shorted to 97s at 0.2% and 74s at 0.4%. This indicated that trap center density increased slower than luminescence center density in BaMg2Al6Si9O30:Eu phosphor. Relative low afterglow performance could be ascribed to a relative low ratio of replaceable cation (Ba2+ in our case), and this deduction could also suit to other host matrices.

Comparison of the role of holes and electrons in hysteresis and threshold voltage stability of organic field effect transistors

AbstractElectrical measurements on copper phthalocyanine (CuPc)‐based organic field effect transistors using SiO2 dielectric revealed hysteresis and threshold voltage instability to be due to the combined effect of both hole‐ and electron‐trapping mechanisms, with the electron‐trap density being higher than hole‐trap density. Hysteresis was found to increase further after modifying the SiO2 surface with octadecyltrichlorosilane (OTS) while it reduced drastically after polystyrene (PS) modification. Individual contribution of holes and electrons to hysteresis and the cause of increased/reduced trapping of the same for interface modified devices were identified. The increase in hysteresis with OTS‐modified devices was attributed to increase in hole‐trap density arising from a larger grain‐boundary density of CuPc films. Minimum grain‐boundary density and efficient removal of trap centers at dielectric/semiconductor interface using PS modification resulted in minimum hysteresis. Comparison of hole‐ and electron‐trap densities estimated for measurements under different ambient conditions, like high humidity conditions and oxygen flow, showed that hole‐trap density increased with increase in humidity. Under oxygen flow, hysteresis, and hole‐trap density were found to reduce considerably, which could be attributed to creation of excess holes that readily fill hole traps, thereby resulting in increased mobile carrier concentrations.

Effect of deposition conditions and thermal annealing on the charge trapping properties of SiNx films

The density of charge trapping centers in SiNx:H films deposited by plasma enhanced chemical vapor deposition is investigated as a function of film stoichiometry and postdeposition annealing treatments. In the as-deposited films, the defect density is observed to increase with an increasing N/Si ratio x in the range of 0.89–1.45, and to correlate with the N–H bond density. Following the annealing in the temperature range of 500–800 °C, the defect density increases for all N/Si ratios, with the largest increase observed in the most Si rich samples. However, the defect density always remains highest in the most N rich films. The better charge storage ability suggests the N rich films are more suitable for the creation of negatively charged nitride films on solar cells.

Analysing space charge-limited conduction in Au/n-InP Schottky diodes

The theory of space charge limited currents (SCLC) was investigated at the range of 160–300 K temperatures. Current–voltage measurements show that the current flow is space charge limited and influenced by traps. At low voltages, the current density behavior suggests ohmic conductivity, while at higher voltages, a region of space-charge-limited current conduction was observed. Some essential parameters such as trap concentration, characteristic temperature and the trap center density was obtained from the temperature dependent current density in accordance with the theory for the exponential trap distributions. It was concluded that SCLC measurements may also be used to investigate the properties of deep levels in high electron mobility-InP semiconductors with insulators in the presence of interfacial trap states.

Optical and electrical quality improvements of undoped InAs∕GaSb superlattices

The performance and operating temperature of infrared (IR) detectors are largely limited by thermal generation and noise processes in the active region of the device. Particularly, excess background charge carriers enhance dark currents and depress the detector figures of merit. Therefore, reducing the overall defects and background carriers in the undoped region of p-i-n diodes is an important issue for developing high-operating temperature IR detectors. In this article, the authors discuss how several postgrowth annealing conditions and interface shutter sequences are optimized to reduce the density of nonradiative defect trap centers and background carriers and studied their relevance to the photoluminescence (PL) emission qualities of typical mid-IR InAs∕GaSb superlattices (SLs). Among the several in situ postgrowth annealing temperatures investigated, the SLs annealed at 450°C had the highest carrier density and the lowest PL intensity, while the SLs annealed at 475°C had the lowest carrier density and the highest PL intensity. In situ annealing did not change the carrier type, but slightly reduced the carrier densities as compared to the as-grown SL sample of 1.8×1011cm−2. While in situ annealing improves the surface morphological quality and the PL intensity, ex situ annealing creates many surface pits and significantly reduces the PL intensity. The carrier density was sensitive to the variation in interface composition control. With a minor variation in the interface shutter sequence, the carrier density dramatically increased from ∼2×1011to5×1012cm−2, and the corresponding mobility decreased from 6600to26cm−2∕Vs, indicating a degradation of the interfacial quality. All of the SL samples investigated for this study were p-type.

The Effects of Rare-Earth Co-Dopants on Green Long-Lasting Phosphorescence and Thermoluminescence Properties of Ca8Mg ( SiO4 ) 4Cl2 : Eu2 + , RE3 +

A series of green long-lasting phosphorescence materials was systematically investigated by long-lasting phosphorescence and thermoluminescence spectra. The green phosphorescence predominates at 502 nm due to the transition of the ion. It strongly depends on the co-doped rare-earth ion, and its performance decreases in the order ; for the Yb ion, the phosphorescence is totally quenched. The thermoluminescence properties show that there are generally four peaks around 330, 370, 435, and 470 K in the temperature range of 293–577 K, associated with the foreign defects due to the aliovalent substitution of rare-earth ions. The relationship of the long-lasting phosphorescence and thermoluminescence properties show that the difference in trap depth and the density of charge trapping centers and trapped charges, associated with thermoluminescence characteristics in the temperature range of 300–400 K, is responsible for the regular variation in the green long-lasting phosphorescence. The thermoluminescence spectrum of is fitted by the general kinetic model. The most appropriate trap depth in the present host is estimated to be around 0.8 eV, and the kinetic-order parameter is 1.14.

Effect of Sn addition on the photoconductivity of narrow-gap Sb2Se3 films

Steady state and transient photocurrent measurements have been performed on thermally evaporated Sb2Se3:Sn films. The DC conductivity increases accompanied by a decrease in the DC activation energy and the pre-exponential factor with increasing Sn content. A red shift in the reflectivity maxima was observed with an increase in Sn concentration. Sn addition inhibits photodegradation with a decrease in the photosensitivity. The transient decay process fits well to a stretched exponential function. Values of fit parameters, decay time constant, and the dispersion parameter are reported for the Sb2Se3:Sn system. Bimolecular recombination is predominant for the Sb2Se3 films, while it changes towards a monomolecular process with increase in the Sn content. An increase in the density of hole trapping centers with the inclusion of Sn as a charged entity is proposed to explain the results.

Surface electroluminescence phenomena correlated with trapping parameters of insulating polymers

Electroluminescence (EL) phenomena are closely linked to the space charge and degradation in insulating polymers, and dominated by the luminescence and trap centers. EL emission has been promising in defining the onset of electrical aging and in the investigation of dissipation mechanisms. Generally, polymeric degradation reveals the increment of the density of luminescence and trap centers, so a fundamental study is proposed to correlate the EL emission of insulating polymers and their trapping parameters. A sensitive photon counting system is constructed to detect the weak EL. The time- and phase-resolved EL characteristics from different polymers (LDPE, PP and PTFE) are investigated with a planar electrode configuration under stepped ac voltage in vacuum. In succession, each sample is charged with exposing to multi-needle corona discharge, and then its surface potential decay is continuously recorded at a constant temperature. Based on the isothermal relaxation current theory, the energy level and density of both electron and hole trap distribution in the surface layer of each polymer is obtained. It is preliminarily concluded that EL phenomena are strongly affected by the trap properties, and for different polymers, its EL intensity is in direct contrast to its surface trap density, and this can be qualitatively explained by the trapping and detrapping sequence of charge carriers in trap centers with different energy level.

Characterization of the Si:H network during transformation from amorphous to micro- and nanocrystalline structures

Optical, structural, and electrical characterizations of the Si:H network have been performed during its changes occurred by the increasing H2 dilution to the SiH4 ensemble in hot-wire chemical vapor deposition (HWCVD). A rapid structural transformation from a mostly amorphous phase to comprehensive micro/nanocrystallinity was attained at a relatively low H2 dilution, even at a low substrate temperature, because of the associated abundance of atomic H in HWCVD. However, elevated H2 dilution induces enormous polyhydrogenation and formation of lesser dense network full of voids, mostly around the tiny micro/nanocrystallites. This highly defective grain boundary zone provides high density of electronic trapping centers and contributes significantly to the transport of carriers. The overall electrical transport in the amorphous-micro-nano-crystalline heterogeneous Si:H network has been accounted in the framework of a three-phase model comprised of amorphous and crystalline (micro- and nano-) components while the grain boundary being the third phase, separately contributing to a potential energy barrier in between. The three-phase model to the electrical phenomena of nanocrystalline structures convincingly explains the dual activation energies arising in the electrical conduction with distinguishably different temperature dependent behaviors from that occurs in the conventional amorphous matrix.

UV Schottky photodiode on boron-doped CVD diamond films

We report on experimental study of photosensitivity and Q-DLTS spectra of polycrystalline CVD diamond UV photodetectors. The measured characteristics of Schottky photodiode on boron-doped diamond films are compared with those obtained for planar photoconductive structures (photoresistor type) based on undoped CVD diamond. The Schottky photodiode exhibited a sharp cut-off in photoresponse with spectral discrimination ratio (between wavelengths of 190 nm and 700 nm) as high as 5 · 10 5 at zero bias voltage (at zero dark current). The photodiode showed the maximum of photoresponse at wavelength < 190 nm, and a low density of trapping and recombination centers as evaluated with the Q-DLTS technique. The devices demonstrated the photoresponsivity at 190 nm from 0.03 to 0.1 A/W with quantum yield of 0.20 to 0.67 in closed circuit, while the photovoltage ≥ 1.6 V was measured in open circuit regime. Another type of UV detector, the planar photoconductive structures with interdigitizing ohmic electrodes fabricated on undoped diamond film and operated under a bias voltage, revealed a higher density of (surface) defect centers and the maximum photoresponse at ∼ 210 nm wavelength. A strong influence of UV light illumination on the Q-DLTS spectra of the planar photoconductive structures was observed. This effect can be used for development of new UV detectors and dosimeters based on the Q-DLTS signal measurements.

Transparent conductive Al-doped ZnO films for liquid crystal displays

Al-doped zinc oxide (ZnO:Al) films were applied to liquid crystal displays (LCDs) as transparent electrodes substituting indium tin oxide (ITO). While the ZnO:Al-based twisted nematic LCD cell showed similar operational behavior to ITO-based counterpart, its electro-optical (EO) and residual dc (r-dc) characteristics were somewhat improved. Capacitance-voltage relations suggested that these improved EO and r-dc characteristics of the ZnO:Al-based LCD cell are due to the substantially lower density of charge carrier trapping centers in the polyimide layer∕electrode interface region, demonstrating high application potential of ZnO:Al films as transparent electrodes of LCDs.

Synthesis and Optical Properties of Europium‐Doped ZnS: Long‐Lasting Phosphorescence from Aligned Nanowires

AbstractQuasi‐aligned Eu2+‐doped wurtzite ZnS nanowires on Au‐coated Si wafers have been successfully synthesized by a vapor deposition method under a weakly reducing atmosphere. Compared with the undoped counterpart, incorporation of the dopant gives a modulated composition and crystal structure, which leads to a preferred growth of the nanowires along the [01$\bar 1$0] direction and a high density of defects in the nanowire hosts. The ion doping causes intense fluorescence and persistent phosphorescence in ZnS nanowires. The dopant Eu2+ ions form an isoelectronic acceptor level and yield a high density of bound excitons, which contribute to the appearance of the radiative recombination emission of the bound excitons and resonant Raman scattering at higher pumping intensity. Co‐dopant Cl– ions can serve not only as donors, producing a donor–acceptor pair transition with the Eu2+ acceptor level, but can also form trap levels together with other defects, capture the photoionization electrons of Eu2+, and yield long‐lasting (about 4 min), green phosphorescence. With decreasing synthesis time, the existence of more surface states in the nanowires forms a higher density of trap centers and changes the crystal‐field strength around Eu2+. As a result, not only have an enhanced Eu2+ 4f65d1–4f7 intra‐ion transition and a prolonged afterglow time been more effectively observed (by decreasing the nanowires' diameters), but also the Eu2+ related emissions are shifted to shorter wavelengths.

Characterisation of single crystal CVD diamond particle detectors for hadron physics experiments

AbstractCharacterisation results from a single‐crystal CVD diamond detector are reported, which is under development for heavy‐ion particle identification and minimum‐ionizing particles timing in nuclear physics experiments. The charge collection efficiency is about 100%, never obtained from polycrystalline CVD‐diamond detectors. An energy resolution of 20 keV (ΔE/E ≈ 0.004) is achieved using a mixed nuclide α‐source, which is comparable to the energy resolution of silicon pin diode detectors. Using low impedance broadband electronics and a ToF technique, where holes or electrons drift separately inside the diamond bulk, the saturation velocity, mobility and lifetime of both charge carriers are estimated. In addition the thermoluminescent properties of the new material are discussed in comparison to polycrystalline diamond. A much lower density of trapping centres has been found for single‐crystal CVD diamond. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Charge trapping at high doses in an active matrix flat panel dosimeter

Active matrix flat panel dosimeters (AMFPDs) have a high density of charge trapping centers (/spl sim/3/spl times/10/sup 15//cm/sup 3/) which lead to both lag and ghosting in reconstructed dose measurements. Lag is due to charge release within the detector and is a function of the lifetime of the trapping centers within the detector. Ghosting, defined as excess charge not produced by charge release, is a phenomenon believed to be caused by reduced charge trapping probability due to the trapped charge population density being greater after exposure to a radiation field than previous to the exposure. This paper examines the characteristics of a large AMFPD (26 cm /spl times/ 26 cm) detector for long exposure times (up to 1 minute) and high doses (up to 3 Gy). This detector was operated in a direct detection mode. Nonlinear response functions were mapped as a function of exposure time and dose for an X-ray beam where the accelerating potential of the electrons was 6 megavolts (MV). Additionally, the impulse response functions (IRFs) were measured at 6 MV for the same set of exposure times and doses used for the measurement of the nonlinear mapping. These IRFs are shown to be functions of both exposure time and dose delivered to the detector. The reproducibility of dose measurements using procedures derived from this paper is discussed. The temporal transfer function (TTF), defined as the absolute value of the Fourier transform of the IRF, is introduced and several examples of the TTFs are shown and discussed.

Physical and electrical characterization of polysilicon vs. TiN gate electrodes for HfO 2 transistors

The effects of pre-deposition substrate treatments and gate electrode materials on the properties and performance of high- k gate dielectric transistors were investigated. The performance of O 3 vs. HF-last/NH 3 pre-deposition treatments followed by either polysilicon (poly-Si) or TiN gate electrodes was systematically studied in devices consisting of HfO 2 gate dielectric produced by atomic layer deposition (ALD). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) using X-ray spectra and Electron Energy Loss Spectra (EELS) were used to produce elemental profiles of nitrogen, oxygen, silicon, titanium, and hafnium to provide interfacial chemical information and to convey their changes in concentration across these high- k transistor gate-stacks of 1.0–1.8 nm equivalent oxide thickness (EOT). For the TiN electrode case, EELS spectra illustrate interfacial elemental overlap on a scale comparable to the HfO 2 microroughness. For the poly-Si electrode, an amorphous reaction region exists at the HfO 2/poly-Si interface. Using fast transient single pulse (SP) electrical measurements, electron trapping was found to be greater with poly-Si electrode devices, as compared to TiN. This may be rationalized as a result of a higher density of trap centers induced by the high- k/poly-Si material interactions and may be related to increased physical thickness of the dielectric film, as illustrated by HAADF-STEM images, and may also derive from the approximately 0.5 nm larger EOT associated with polysilicon electrodes on otherwise identical gate stacks.