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We deposited electric charge in periodic manner in form of square lattices of points in Ge–As–Se chalcogenide semiconductor films by electron beam. Various types of surface reliefs are observed as a result of interaction of electron beam with the semiconductor thin films. For the films where charge removal is facilitated by conducting gold underlayer shape of surface relief is similar for all deposited doses per point. For the films with no gold underlayer, where charge removal is hindered, the shape of local relief depends on the dose per point and at higher doses also on the lattice period. When the lattice period is larger than interaction volume of electron in the material the identical surface reliefs consisting of spire and crater are formed during each charge deposition. When the lattice period is smaller than the interaction volume of the electron identical surface reliefs are formed only in every second charge deposition event. As a result highly organized checkerboard-like lattices are formed. We suggest that a reason for the appearance of such complex periodic structures is charge accumulation in these films. When the ratio between the size of space charge region and lattice period is small enough there would be influence of already deposited charge on the charge deposition in the next step. This opens possibility to fabricate structures on the surface of these materials by combining charge already accumulated in the material with charge deposited in consecutive step. We also show that it is possible to experimentally define the size of space charge region in chalcogenide thin films. [ABSTRACT FROM AUTHOR]

The processes of surface relief formation on Ge x Se 100-x films under their point irradiation by an electron beam have been studied. Electron-induced effects in Ge x Se 100-x films are interpreted in framework of a two-layer charge model. The distribution of the surface potential in the locally irradiated regions of the Ge x Se 100-x film have been established using the Kelvin probe force microscopy (KFPM). The KPFM measurements reveal that the driving force behind mass transfer is not localized charge in the film itself, but rather the gradient of the electric field created by the charge. The potential distribution derived from the measured KPFM illustrates the distribution of gradient of the electric field. We find that the direction of the field gradient sets the direction of mass transfer during point exposure with electron beam. [ABSTRACT FROM AUTHOR]

On the basis of experimental data of dependence of the output photocurrent on the exciting light flow intensity at wide operating voltage range there has been made an estimation of current amplification factor of two-barrier photodiode Ag-NGaAs-nGaInAs-Ag-structure. The increase in amplification factor with the voltage increase is explained by photogeneration processes in dilated space charge layer. Such structures, having amplifying property, are of interest for creating of optical signals reception devices.

Lateral bulk heterojunctions (LBHJ) provide a tool to directly probe the active area of photovoltaic devices using microscopy techniques. Here, we use scanning photocurrent microscopy (SPCM) to probe an organic photovoltaic (OPV) device with poly[{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl} (PTB7):[6,6]phenyl-C61-butyric acid methyl ester (PC 61 BM). The effects of the additive 1,8-diiodooctane (DIO) on the recombination dynamics and morphology are probed in real space in a LBHJ structure. By using SPCM, we can see a larger increase in the space charge region for samples with DIO when compared to those without DIO. This indicates that the additive improves film morphology leading to increased charge extraction efficiency and decreased recombination. [ABSTRACT FROM AUTHOR]

Organic semiconductors are widely investigated for their application in photovoltaics and photodetectors. We show that the efficiency of these devices is strongly influenced by the position of the space charge region, due to unintentional doping, and wavelength-dependent absorption properties in bulk heterojunctions. Spray-coated P3HT:PCBM bulk heterojunction photodiodes with thicknesses up to 4.2 μm and semitransparent top contact enable the characterization of exciton generation and separation in both irradiation directions. A large difference in external quantum efficiency (EQE) is observed for top and bottom illuminated configurations and is explained by a bias dependent arrangement of the space charge region at the two contact electrodes. Numerical drift–diffusion simulations allow to get insight into first order mechanisms behind the spectral features of EQE data in highly-doped organic photodiodes. [ABSTRACT FROM AUTHOR]

Cu(In,Ga)Se 2 (CIGS) absorbers with several Ga/III, Ga/(In+Ga) ratio, profiles were deposited by the so-called “multi layer precursor method” consisting of Ga-Se/In-Se/Cu-Se stacks. The effects of average Ga/III ratios and back slopes of the Ga/III profiles on the photovoltaic performance were investigated. The optimum average Ga/III ratio of approximately 0.36 leads to the appropriate average band-gap energy as well as sufficiently long carrier lifetime in CIGS absorber, thus enhancing open-circuit voltage. In addition, the back slope of 0.50 μm −1 , defined as the ratio of the change in Ga/III ratio to the change in the depth range from 1 to 2 μm from CIGS surface, induces back surface field of 0.34 V/μm, thereby increasing short-circuit current density. Ultimately, the CIGS absorber on flexible stainless steel substrate with the double graded Ga/III profile (the average Ga/III of 0.36 and the back slope of 0.50 μm −1 ) was fabricated by the multi layer precursor method. This results in the 14.36%-efficient CIGS solar cell on the flexible substrate without an anti-reflective layer. [ABSTRACT FROM AUTHOR]

High-performing heterojunction devices were achieved by bringing an aluminum-doped ZnO (AZO) into contact with a p-Si substrate. A thin transparent AZO film was directly coated on pillar-array patterned Si and spontaneously formed a rectifying junction without any intentional doping process. Si pillar-arrays were designed to have 5 μm width with variation in periods (7 μm and 10 μm) used to modulate the surficial lengths. The light response is directly proportional to the surficial length enhancement. AZO/Si heterojunction devices showed strong dependence on the incident wavelengths. At a wavelength of 600 nm, the highest response ratio of 70,900% was achieved. We found that the locational superposition of the space charge region and the photo-generated region is crucial for light-reactive responses. We suggest an efficient geometric design scheme for highly efficient light-absorbers. [ABSTRACT FROM AUTHOR]

Admittance of MIS structures based on graded-gap n-HgCdTe ( x = 0.31-0.32) grown by molecular beam epitaxy (MBE) is investigated in a wide temperature range (8-300 K). It is shown that the temperature and frequency dependences of the differential resistance of space charge region for structures with a graded-gap layer are qualitatively similar to those for structures without a graded-gap layer. It is found that for MIS structures based on MBE n-HgCdTe ( x = 0.31-0.32), regardless of the presence of a graded-gap layer, the differential resistance of space charge region is limited by the processes of Shockley-Read generation in the temperature range of 25-100 K. [ABSTRACT FROM AUTHOR]

Radiation detectors based on radiation-hardened semiconductor such as silicon carbide (SiC), have received considerable attention in many applications such as in outer space, high energy physics experiments, gas and oil prospection, and nuclear reactors. In the frame work of the European project I_SMART (Innovative Sensor for Material Ageing and Radiation Testing), we demonstrated for the first time the reliability of thermal neutron detectors realized by standard ion implantation of boron atoms to form a neutron converter layer (NCL). Two types of detectors were realized; the first was implanted by aluminum to create the p^ + - layer, and then implanted by boron (^10B) to realize the NCL. The second type was based on p^ + - \epitaxial layer, and was implanted by ^10B into the aluminum metallic contact in order to avoid implantation-related defect within the sensitive area. Both kinds of detectors reveal to respond to thermal neutrons and gamma rays, showing consistent counting rates as a function of bias voltages, radiation intensity and type of shielding. [ABSTRACT FROM PUBLISHER]

Abstract: Cu(In,Ga)Se2 (CIGS) is one of the most promising materials to fabricate low-cost and high-efficiency thin film solar cells. In this work, CIGS films were deposited by the so-called “multi-layer precursor method” using multi-layer co-evaporation of material sources. Based on the simulated and experimental results, the optimum averaged Ga/III, Ga/(In+Ga), in space charge region (SCR) controlling the carrier recombination near the junction and back surface Ga/III grading forming back surface field have drastic influence on cell performance. The CIGS absorber layer with double Ga/III grading profile (averaged Ga/III in SCR; 0.38 and the height of the back surface Ga/III grading; 0.33) is readily achieved by multi-layer precursor method. This leads to the improvement of efficiency of the CIGS solar cell up to 15.30% without anti-reflective layer. [Copyright &y& Elsevier]

In this study, the surface characteristic of oxide films on Ti-6Al-4V alloy formed by an anodic oxidation treatment in HSO/HPO electrolyte at potentials higher than the breakdown voltage was evaluated. Morphology of the surface layers was studied by scanning electron microscope. The results indicated that the diameter of pores and porosity of oxide layer increase by increasing the anodizing voltage. The thickness measurement of the oxide layers showed a linear increase of thickness with increasing the anodizing voltage. The EDS analysis of oxide films formed in HSO/HPO at potentials higher than breakdown voltage demonstrated precipitation of sulfur and phosphor elements from electrolyte into the oxide layer. X-ray diffraction was employed to exhibit the effect of anodizing voltage on the oxide layer structure. Roughness measurements of oxide layer showed that in spark anodizing, the Ra and Rz parameters would increase by increasing the anodizing voltage. The structure and Corrosion properties of oxide layers were studied using electrochemical impedance spectroscopy (EIS) techniques, in 0.9 wt.% NaCl solution. The obtained EIS spectra and their interpretation in terms of an equivalent circuit with the circuit elements indicated that the detailed impedance behavior is affected by three regions of the interface: the space charge region, the inner compact layer, and outer porous layer. [ABSTRACT FROM AUTHOR]

A key issue in the development of high-performance semiconductor devices is the ability to properly measure active dopants at the nanometer scale. In a p-n junction, the abruptness of the dopant profile around the metallurgical junction directly influences the electric field. Here, a contacted nominally symmetric and highly doped ( N A = N D = 9 × 10 18 cm -3 ) silicon p-n specimen is studied through in situ biased four-dimensional scanning transmission electron microscopy (4D-STEM). Measurements of electric field, built-in voltage, depletion region width, and charge density are combined with analytical equations and finite-element simulations in order to evaluate the quality of the junction interface. It is shown that all the junction parameters measured are compatible with a linearly graded junction. This hypothesis is also consistent with the evolution of the electric field with bias as well as off-axis electron holography data. These results demonstrate that in situ biased 4D-STEM can allow a better understanding of the electrostatics of semiconductor p-n junctions with nm-scale resolution.

Abstract: It has been observed that pixellated CdZnTe detectors fabricated from crystals experience an extending of space charge region in the polarized state when exposed to a high flux of x-rays and a low flux of x-rays. In this case, in some areas of the CZT detector polarization occurs without high incident flux. Results from these studies reveal that at very high photon flux rates, a space charge region with high density develops and extends, consistent with the accumulation of positive space charge due to the trapping of free-carrier holes created by the x-ray irradiation. The extending of space charge region observed without high x-ray has a direct influence on the irradiation hardness and charge collection efficiency of the devices. [Copyright &y& Elsevier]

Within the framework of the mathematical model of Nernst-Planck-Poisson, an attempt is undertaken to theoretically describe the electrodiffusion of ions in the system diffusion layer/monopolar ionexchange membrane, which is accompanied by dissociation of water molecules. The formulas for estimating the current density transferred through a monopolar membrane by hydrogen or hydroxyl ions formed in dissociation of water in the space-charge region are derived. The rate constants and other parameters of dissociation of water molecules in the space-charge region of monopolar membranes under conditions of stabilization of the diffusion layer thickness are calculated. Their comparative analysis with the similar characteristics of bipolar membranes is carried out. For the phosphoric-acid heterogeneous membrane MK-41 in which the polarization conditions in the current density range under study are not so severe and the reaction layer is not being depleted as in the bipolar membrane MB-3 (contains the same phosphoric-acid groups), it is shown that only single-charged phosphoric-acid groups are involved in the water dissociation reaction. For MK-41, the calculated constants of the heterolytic reaction of water molecule dissociation are lower than for the heterogeneous membrane MA-40 containing ternary and quaternary amino groups. It is confirmed that the nature of ionogenic groups in membranes is a factor that determines the rate of water dissociation in systems with ion-exchange membranes. [ABSTRACT FROM AUTHOR]

Abstract: In electrodialysis, a theoretical understanding of overlimiting current (greater ionic transport beyond the electroneutrality limit) has not been clearly demonstrated, despite two hypothetical explanations: the water splitting reaction and electro-convection. To suggest another possible mechanism for this phenomenon, the Nernst-Planck-Poisson equations were solved via the Painlevé equation of the second kind. Numerical simulations found that the initiation of overlimiting current involves a substantial redistribution of the ionic charge near the membrane surface. To maintain a constant ionic charge in the aqueous phase near the membrane while the non-electroneutral space charge region grows, the shrinking electric double layer in the aqueous-phase releases ionic charges toward the membrane. Frequent repetition of this charge redistribution can contribute additional current above the limiting current without a large investment of electrical potential. A sensitivity study found that the relationship between the growth of the space charge region and applied potential is independent of the membrane''s ion exchange capacity and the ionic diffusivity; however, the amount of the ionic charge created in the space charge region was greater with a greater bulk concentration or with an electrolyte solvent of a greater permittivity. These findings are consistent with previous experimental studies in the overlimiting current regime. [Copyright &y& Elsevier]

Abstract: We analyze the channel and surface charge dynamics of a high voltage AlGaN/GaN Heterostructure Field Effect Transistor (HFET) operating as a power switch. We demonstrate that operation in the voltage range exceeding 100V without field plates involves alternating of the surface compensating charge in the gate–drain spacing. Developed model predicts the switching speed limitation of power HFET determined by the surface recharge rate and provides new, voltage-scalable design approach of AlGaN/GaN power devices. [ABSTRACT FROM AUTHOR]

We investigate the temperature dependence of germanium on silicon p-i-n photodetectors in terms of both dark current density and near-infrared responsivity. The dark current increases by nearly a factor 1.6 every 10degC, consistently with carrier generation in the space charge region. The responsivity has a complex trend, its temperature variation depending on wavelength and on the germanium quality. Detectors with a large defect density in the active layer exhibit a reduced responsivity as the temperature increases. [ABSTRACT FROM PUBLISHER]

Abstract: X-ray detectors are fabricated with non-intentionally doped thick epitaxial GaAs layers grown by a chemical technique. They are p+/i/n+ structures in which the highly doped p- and n-type layers are made by ion implantation on both faces of the i epilayers. In order to understand the behaviour of charge collection, we modelize the capacitance–voltage characteristics of such structures, which are then compared with experimental data. [Copyright &y& Elsevier]

State-of-the-art semiconductor devices require accurate control of the full two-dimensional dopant distribution. In this work, we report results obtained on 2D electrical characterization of ultra shallow junctions in Si using off axis electron holography to study two-dimensional effects on diffusion. In particular, the effect of a nitride diffusion mask on lateral diffusion of phosphorous is discussed. Retardation of lateral diffusion of P under the nitride diffusion mask is observed and compared to the lateral diffusion of P under an oxide diffusion mask. The ultra shallow junctions for the study were fabricated by a rapid thermal diffusion process from heavily P doped spin-on-dopants into a heavily B doped Si substrate. These shallow junctions are needed for fabricating source/drain extensions in nanoscale MOSFETs. One-dimensional electrical characterization of the junction was carried out to determine the electrical junction depth and compared to the metallurgical junction depth from SIMS analysis. [Copyright &y& Elsevier]