Charge Width Research Articles

Fabrication and characterization of thermal evaporated n-Si/ p-ZnTe thin film heterojunction diodes

The paper reports the fabrication and detailed electrical characterization of thermal evaporated n-Si/p-ZnTe thin film heterojunction diodes. The heterojunction diodes were prepared by depositing ZnTe films on n-Si substrates. The conduction mechanism, barrier height, space charge density and width of the depletion region were determined by I–V and C–V characteristics of the heterojunction diodes. The bandgap and activation energies of n-Si and p-ZnTe were also determined and a theoretical band diagram of n-Si/p-ZnTe heterojunction was drawn based on Anderson's model.

Comparison of charge distributions in CIGS thin-film solar cells with ZnS/(Zn,Mg)O and CdS/i-ZnO buffers

The commonly used CdS/i-ZnO buffer system in Cu(In,Ga)Se 2 (CIGS) thin-film solar cells was substituted by ZnS/(Zn,Mg)O. ZnS has a higher transmission in the short wavelength range due to the higher bandgap energy E g = 3.7 eV compared to CdS with E g = 2.4 eV. Unfortunately, in our experiments the resulting gain in short-circuit current density j SC as the result of reduced absorption losses in the blue wavelength region is mostly accompanied by a decrease in open-circuit voltage V OC of the devices with ZnS buffer. This contribution discusses possible explanations for the systematically lower open-circuit voltages of the devices with a ZnS buffer layer. The carrier collection properties of the devices with a ZnS buffer were investigated by electron beam induced current measurements in the junction configuration. The maximum of the collection probability for ZnS cells is located in the CIGS bulk and not near the buffer/CIGS interface like for solar cells with CdS buffer. Additionally, we observed a larger space charge width compared to devices with a CdS buffer. This finding concurs with the considerably lower capacitance values and also lower charge densities in ZnS-buffered devices, as determined by capacitance voltage measurements. Based on these findings, the main reason for the lower open-circuit voltages of our ZnS devices is that the charge densities are lower than for the CdS/i-ZnO cells.

High resolution resonant recombination measurements using evaporative cooling technique

We report on a method significantly improving the energy resolution of dielectronic recombination (DR) measurements in electron beam ion traps (EBITs). The line width of DR resonances can be reduced to values distinctly smaller than the corresponding space charge width of the uncompensated electron beam. The experimental technique based on forced evaporative cooling is presented together with test measurements demonstrating its high efficiency. The principle for resolution improvement is elucidated and the limiting factors are discussed. This method opens access to high resolution DR measurements at high ion-electron collision energies required for innermost shell DR in highly charged ions (HCI).

Morphological and Electrochemical Properties of Crystalline Praseodymium Oxide Nanorods

Highly crystalline Pr6O11 nanorods were prepared by a simple precipitation method of triethylamine complex at 500°C. Synthesized Pr6O11 nanorods were uniformly grown with the diameter of 12–15 nm and the length of 100–150 nm without any impurities of unstable PrO2 phase. The Pr6O11 nanorod electrodes attained a high electrical conductivity of 0.954 Scm−1 with low activation energy of 0.594 eV at 850°C. The electrochemical impedance study showed that the resistance of electrode was significantly decreased at high temperature, which resulted from its high conductivity and low activation energy. The reduced impedance and high electrical conductivity of Pr6O11 nanorod electrodes are attributed to the reduction of grain boundaries and high space charge width.

Emittance growth due to static and radiative space charge forces in an electron bunch compressor

Evolution of short intense electron bunches passing through bunch-compressing beam lines is studied using the UAL (Unified Accelerator Libraries) string space charge formulation [R. Talman, Phys. Rev. ST Accel. Beams 7, 100701 (2004); N. Malitsky and R. Talman, in Proceedings of the 9th European Particle Accelerator Conference, Lucerne, 2004 (EPS-AG, Lucerne, 2004); R. Talman, Accelerator X-Ray Sources (Wiley-VCH, Weinheim, 2006), Chap. 13]. Three major configurations are studied, with the first most important and studied in greatest detail (because actual experimental results are available and the same results have been simulated with other codes): (i) Experimental bunch compression results were obtained at CTF-II, the CERN test facility for the ``Compact Linear Collider'' using electrons of about 40 MeV. Previous simulations of these results have been performed (using TraFiC4* [A. Kabel et al., Nucl. Instrum. Methods Phys. Res., Sect. A 455, 185 (2000)] and ELEGANT [M. Borland, Argonne National Laboratory Report No. LS-287, 2000]). All three simulations are in fair agreement with the data except that the UAL simulation predicts a substantial dependence of horizontal emittance ${ϵ}_{x}$ on beam width (as controlled by the lattice ${\ensuremath{\beta}}_{x}$ function) at the compressor location. This is consistent with the experimental observations, but inconsistent with other simulations. Excellent agreement concerning dependence of bunch energy loss on bunch length and magnetic field strength [L. Groening et al., in Proceedings of the Particle Accelerator Conference, Chicago, IL, 2001 (IEEE, New York, 2001), http://groening.home.cern/groening/csr_00.htm] confirms our understanding of the role played by coherent synchrotron radiation (CSR). (ii) A controlled comparison is made between the predictions of the UAL code and those of CSRTrack [M. Dohlus and T. Limberg, in Proceedings of the 2004 FEL Conference, pp. 18\char21{}21, MOCOS05, available at http://www.JACoW.org], a code with similar capabilities. For this comparison an appropriately new, 50 MeV, ``standard chicane'' is introduced. Unlike CSRTrack (which neglects vertical forces) the present simulation shows substantial growth of vertical emittance. But ``turning off'' vertical forces in the UAL code (to match the CSRTrack treatment) brings the two codes into excellent agreement. (iii) Results are also obtained for 5 GeV electrons passing through a previously introduced ``standard chicane'' [Coherent Synchrotron Radiation, CSR Workshop, Berlin 2002, http://www.desy.de/csr] [of the sort needed for linear colliders and free electron lasers (FEL's) currently under design or construction]. Relatively little emittance growth is predicted for typical bunch parameters at such high electron energy. Results are obtained for both round beams and ribbon beams (like those actually needed in practice). Little or no excess emittance growth is found for ribbon bunches compared to round bunches of the same charge and bunch width. The UAL string space charge formulation (like TraFic4 and CSRTrack) avoids the regularization step (subtracting the free-space space charge force) which is required (to remove divergence) in some methods. Also, by avoiding the need to calculate a retarded-time, four-dimensional field history, the computation time needed for realistic bunch evolution calculations is modest. Some theories of bunch dilution, because they ascribe emittance growth entirely to CSR, break down at low energy. In the present treatment, as well as CSR, all free-space Coulomb and magnetic space charge forces (but not image forces), and also the centrifugal space charge force (CSCF) are included. Charge-dependent beam steering due to CSCF, as observed recently by Beutner et al. [B. Beutner et al., in Proceedings of FEL Conference, BESSY, Berlin, Germany, 2006, MOPPH009], is also investigated.

Evaluation of electron beam induced current profiles of Cu(In,Ga)Se 2 solar cells with different Ga-contents

The measurement of electron beam induced current profiles in junction configuration (JEBIC) is a settled method for several semiconductor devices. We discuss the JEBIC method in the light of the special conditions present in the case of thin film Cu(In,Ga)Se 2 solar cells. Our previously published results indicate that the charge state of defects close to or at the Cu(In,Ga)Se 2/CdS interface depends on the minority carrier distribution, which changes strongly during a scan of the cross section with an electron beam. The charge distribution influences the electrostatic potential and therewith the collection of minority carriers. Here, we present an evaluation method of JEBIC profiles that accounts for this effect. Monte Carlo simulations of the carrier generation help us to consider in detail the influence of surface recombination. We determine the diffusion length, space charge width, surface- and back contact recombination velocity of Cu(In (1-r),Ga r)Se 2 devices with different Ga-contents r from JEBIC line scans.

Ammonia sensitivity of amorphous carbon film/silicon heterojunctions

The amorphous carbon film/n-Si (a-C∕Si) junctions have been fabricated by magnetron sputtering. The results show that these junctions have good rectifying properties and high ammonia (NH3) gas sensitivity. For a given reverse bias voltage, the resistance of the junction can increase by 100 times rapidly when exposed to NH3 gas. This phenomenon may be attributed to the change of the space charge width of the junction, which is caused by the adsorption of NH3 gas molecules. This study shows that these a-C∕Si junctions have potential application as NH3 gas detect sensors.

Effect of gas pressure on current-voltage characteristics of amorphous carbon film/silicon heterojunction

Amorphous carbon film/n-Si (a-C∕Si) junctions have been fabricated by direct current magnetron sputtering and their current-voltage (I-V) characteristics have been investigated. The results show that the gas pressure has a large effect on the reverse bias I-V characteristics of the junctions. For example, the reverse current can increase by 3300% when the gas pressure decreases from 100000to100Pa. The effect of gas pressure may be attributed to the physisorption process of gas molecules which increases the space charge width and changes the surface states of the junction.

Charge state evolution of 2 MeV/u sulfur ion passing through thin carbon foil

Charge state distribution and its evolution for 2.0MeV/u sulfur ions after passing through 0.9, 1.1, 1.5, 2.0, 3.0, 4.7, 6.9 and 10μg/cm2 carbon foils have been extensively studied following our previous paper [M. Imai, M. Sataka, K. Kawatsura, K. Takahiro, K. Komaki, H. Shibata, H. Sugai, K. Nishio, Nucl. Instr. and Meth. B 230 (2005) 63] to derive data for all the initial charge states between 6+ and 14+. Measured charge state distributions, their mean charge states and distribution widths do not flat off to establish equilibrium within the measured target thickness, and an overshooting feature of the distribution width for S6+ projectile is observed around 1μg/cm2 in the target thickness. Two kinds of calculations, one based on rate equations accounting only for single charge transfer and the other applying ETACHA code, show good agreements with the experimental evolutions of mean charge and distribution width.

Semiconducting properties of the anodic films grown over PbIn alloy electrodes

Abstract Semiconducting properties of oxide film grown over PbIn binary alloy electrodes in alkaline medium is elucidated. It has been observed that anodic film formed over Pb0.3 wt%In alloy in the potential range of −0.75 to 1.25 V (vs. Hg/HgO) shows better short circuit photocurrent density (3.2 mA/cm2) than the oxide film over pure Pb electrode (2.2 mA/cm2). The observed photoactivity is correlated with the interfacial parameters of the nonstoichiometric oxide–electrolyte interface using Gartner Butler model. Effect of varying concentration of In dopant in the PbO matrix on the interfacial properties is reported. It is shown that combined effect of space charge width and diffusion length of the minority carriers can describe the observed photoactivity efficiently.

Temperature Effect on Read Current in a Two-Bit Nitride-Based Trapping Storage Flash EEPROM Cell

The temperature effect on the read current of a two-bit nitride-storage Flash memory cell is investigated. In contrast to a conventional silicon-oxide-nitride-oxide (SONOS) cell with uniform Fowler-Nordheim (FN) programming, a significant high-V/sub T/ state read current increase, which results in the read window narrowing at high temperature, is observed in a channel hot electron (CHE) programmed cell. The increment of high-V/sub T/ state leakage current shows a positive correlation with program/erase threshold voltage window. Since the temperature effect is very sensitive to a locally trapped charge profile, a two-dimensional simulation with a step charge profile is employed to characterize the relationship between current increment and both charge width and charge density.

Investigation of Channel Hot Electron Injection by Localized Charge-Trapping Nonvolatile Memory Devices

A novel measurement method to extract the spatial distribution of channel hot electron injection is described. The method is based on characterization of localized trapped-charge in the nitride read-only memory (NROM) device. The charge distribution is determined by iteratively fitting simulated subthreshold and gate induced drain leakage (GIDL) currents to measurements. It is shown that the subthreshold and the GIDL measurements are sensitive to charge trapped over the n+ junction edge. Their characteristics are determined by the trapped charge width, density and location and the associated fringing field. Extremely high sensitivity of the GIDL measurement to localized charge over the n+ junction is demonstrated. The extracted charge distribution width is shown to be /spl sim/40 nm, located over the junction edge.

Dye sensitization of antimony-doped CdS photoelectrochemical solar cell

Sb-doped CdS single crystal was used as a photoanode to fabricate a photoelectrochemical solar (PECS) cell. The three organic dyes; eosin, thymol blue and rhodamin 6G were used as sensitizers in (PECS) cell. In the absence of the dye, the results showed that with Sb-doped CdS single crystal electrode, a higher power conversion efficiency ∼9.27% has been achieved compared to ∼5.7–7.4% for pure crystal. Application of the dye in PECS cell increases the efficiency to about 13%. The efficiency reaches its maximum value when the dye concentration is (2.5×10 −5)M, sufficient to cover the surface of the semiconductor electrode with a continuous monolayer of the dye. Exceeding this value resulted in a gradual decrease of the efficiency from its maximum value. Mott–Schottky plots gave a doping density of 3.14×10 17 cm −3 and a space charge width of ∼4.95×10 −6 cm for the sample used. A flat-band potential equal to −0.84 V, independent of both frequency and pH, was also predicted. Cyclic voltammetry (c.v.) measurements showed an anodic current peak at 0.4 V vs. SCE. The disappearance of this peak after excess addition of the reducing agent Na 2S, indicates that this peak is due to the PEC corrosion of the semiconductor electrode.

Transient response of organic light-emitting devices: A Monte Carlo simulation study

A Monte Carlo simulation method has been conducted to study transient response of electroluminescence in single-layer light-emitting devices. The results from the numerical method have been compared with our recent (transient) experimental results. The simulation results supported the role and contribution of accumulated charges at the metal–semiconductor interfaces in device operation. Several parameters like barrier heights and mobilities of charge carriers, width and distribution of the accumulated space charge regions in the device, have been used in the simulation work. Additionally, barriers for charge injection have been considered to be modified by the presence of space charges near the injecting interfaces. A recipe has been established to estimate different parameters of device operation from the comparison between simulation and experimental results.

Photoelectrochemical laser imaging on anodically prepared ?-PbO thin films

The influence of various potential windows on the synthesis of α-PbO and the uniformity of its formation over the entire surface of the lead electrode has been studied by scanning a laser beam over the entire surface and measuring the corresponding photocurrent using the photoelectrochemical laser imaging technique. This technique revealed that a very uniformly distributed highly (110) plane oriented thin film of α-PbO is obtained by potentiodynamic anodization of the lead electrode in an alkaline medium at 80 ∘C in the potential range of −0.32 to +0.08 V vs SCE, giving the highest (73%) quantum yield at 480 nm, an open-circuit photopotential of 800 mV and a short-circuit photocurrent of 5.5 mA cm−2. Gartner analysis suggests that the highest photoactivity achieved with this material is due to the formation of a large space charge width (0.63 m) and diffusion length (0.36 m) of the minority carriers and absorption of almost 100% of the light (480 nm) within the space charge and diffusion regions.

Electrical characteristics of an aluminum/amorphous selenium rectifying contact

Rectifying contacts can be obtained between aluminum and amorphous selenium thin films. It is shown that the interfacial oxide layer makes a decisive contribution to the diode behavior. It is shown that, under forward bias, three regions are visible on the ln I vs V plots. A phenomenological model is proposed to explain these three different domains. It is suggested that under low bias the space charge width increases because of the trapping of carriers at the interface of Al/a-Se. At the saturation of this effect, a recombination process dominates. At high forward bias a series resistance effect becomes dominant. The reverse bias leakage region is due to tunneling of majority carriers via gap states.

Photoelectrochemical properties of dye sensitized Zr-doped SrTiO 3 electrodes

Zr-doped SrTiO 3 was used as a photoanode in a three-electrode electrochemical cell. The three organic dyes Safranin, Fluorescein and Eriochrome black T were used as sensitizers in the cell. The results showed that applying the dyes in photoelectrochemical solar (PECS) cells increased the solar efficiency to 2–3 times its value without dyes. The efficiency reaches its maximum value when the dye concentration is sufficient to cover the surface of the semiconductor electrode with a continuous monolayer. Dye concentration higher than this value resulted in a decrease of the efficiency. Mott-Schottky plots gave a doping density of 1.34 × 10 16cm −3 for the sample and a space charge width of 1.5–2.0 × 10 −6cm (pH dependent). H 2 was produced at zero bias in the presence of Safranin dye at a light intensity of 25mW/cm 2 with an efficiency of about 39%. Both H 2 and O 2 were produced in the stiochiometric ratio. Cyclic voltammetry (c.v.) measurements showed two cathodic current peaks revealing two reduction processes at the semiconductor electrode.

Investigation of semiconducting parameters of PbSn alloy oxide-electrolyte interface by Butler Gartner model

The effect of Sn doping on the photoactivity of anodized PbSn alloy electrodes is discussed. The Butler-Gartner model has been applied to understand the reasons for getting the highest photocurrent with pure lead oxide and the next highest photocurrent with a PbSn alloy oxide containing 0.1 wt.% Sn. The low photoresponse of Sn doped lead oxide is attributed to creation of a small space charge width (5600Å) compared to the large penetration depth (97,000Å) of the incident light.

Influence of substrates on the electrical properties of Cu(In,Ga)Se 2 thin films

Thin films based on Cu(In,Ga)Se 2 prepared on alkali free substrates are compared to films prepared on soda lime glass. On the latter, the presence of sodium species as detected with X-ray photoelectron spectroscopy is correlated with an enhanced formation of Se O, In O and Ga O bonds at the surface after several days air exposure. The electrical conductivity is also one order of magnitude higher for films on soda lime glass. Solar cells prepared on these substrates exhibit increased open circuit voltages. Capacitance-voltage characteristics on junctions prepared on alkali free substrates show an increased space charge width. The observations can be explained in terms of an increased net acceptor density in polycrystalline Cu(In,Ga)Se 2 when prepared on soda lime glass substrates. An alkali-metal-promoted oxidation of the surface is discussed.

Near-infrared electroabsorption in p+/n−/n+ GaSb diodes

Liquid phase epitaxial growth of Te-compensated GaSb results in a very low doped n− GaSb epilayer (n≊1015 cm−3). Be+-implanted photodiodes produced from this material exhibit voltage breakdown values reaching 70 V. Generation-recombination lifetimes in the space charge region around 10−8 s have been deduced from these diode photoelectrical properties. The high value of the space charge width leads to an efficient redshift due to the electroabsorption. The variation of the absorption coefficient value is 1350 cm−1 at 1.72 μm for a variation of the electric field maximum of 1.2×105 V/cm.