A new structure of In-based ohmic contacts to n-type GaAs

Electrical, structural and reaction characteristics of In-based ohmic contacts to n-GaAs were studied. Attempts were made to form a low-band-gap interfacial phase of InGaAs to reduce the barrier height at the metal/semiconductor junction, thus yielding low-resistance, highly reliable contacts. The contacts were fabricated by e-beam sputtering Ni, NiIn and Ge targets on VPE-grown n(+)-GaAs film (approximate to 1 mu m, 2 x 10(18) cm(-3)) in ultrahigh vacuum as the structure of Ni(200 Angstrom)/NiIn(100 Angstrom)/Ge(40 Angstrom)/n(+)-GaAs/SI-GaAs, followed by rapid thermal annealing at various temperatures (500-900 degrees C). In this structure, a very thin layer of Ge was employed to play the role of heavily doping donors and diffusion limiters between In and the GaAs substrate. Indium was deposited by sputtering NiIn alloy instead of pure In in order to ensure In atoms to be distributed uniformly in the substrate; nickel was chosen to consume the excess indium and form a high-temperature alloy of Ni3In. The lowest specific contact resistivity (rho(c)) of (1.5 +/- 0.5)x 10(-6) cm(2) measured by the Transmission Line Method (TLM) was obtained after annealing at 700 degrees C for 10 s. Auger sputtering depth profile and Transmission Electron Microscopy (TEM) were used to analyze the interfacial microstructure. By correlating the interfacial microstructure to the electronical properties, InxGa1-xAs phases with a large fractional area grown epitaxially on GaAs were found to be essential for reduction of the contact resistance.

Persistent Photoconductivity in n-type GaN

The persistent photoconductivity（PPC） phenomena in n-type GaN Films grown by metalorganic chemical vapor deposition（MOCVD） have been studied. After using some testing and analysis methods, such as the double crystal X-ray diffraction（DCXRD）, the photolumineseence（PL） spectra, etc, it is found that the issue which influences PPC in n-type GaN is not relative to the dislocations and yellow band （YB）, and is caused by the doping level of Si most likely.

Observation of deep electron states in n-type Al-doped ZnS1-xTex grown by molecular beam epitaxy

Deep level transient spectroscopy (DLTS) technique was used to investigate deep electron states in n-type Al-doped ZnS1-xTex epilayers grown by molecular fiction epitaxy (MBE), Deep level transient Fourier spectroscopy (DLTFS) spectra of the Al-doped ZnS1-xTex (x = 0. 0.017, 0.04 and 0.046. respectively) epilayers reveal that At doping leads to the formation of two electron traps at 0.21 and 0.39 eV below the conduction hand. 1)DLTFS results suggest that in addition to the rules of Te as a component of [lie alloy as well as isoelectronic centers, Te is also involved in the formation of all electron trip, whose energy level relative to the conduction hand decreases a, Te composition increases.

Modulation magnesium-doping in AlGaN/GaN superlattices

Low resistivity of p-type Mg-doped AlGaN/GaN superlattices (SLs) is demonstrated. The resistivity of the SLs is less than 0.6 Omega .cm. and the measured hole concentration is higher than 1x10(18)cm(-3). The resistivity of SLs is much lower, and the hole concentration of SLs is much higher, than that of bulk GaN and AlGaN, The electrical properties of the SLs are less sensitive than the conventional bulk lavers.

In situ doping of 3C-SiC grown on (0001) sapphire substrates by LPCVD

The heteroepitaxial growth of n-type and p-type 3C-SiC on (0001) sapphire substrates has been performed with a supply of SiH4+C2H4+H-2 system by introducing ammonia (NH3) and diborane (B2H6) precursors, respectively, into gas mixtures. Intentionally incorporated nitrogen impurity levels were affected by changing the Si/C ratio within the growth reactor. As an acceptor, boron can be added uniformly into the growing 3C-SiC epilayers. Nitrogen-doped 3C-SiC epilayers were n-type conduction, and boron-doped epilayers were p-type and probably heavily compensated.

Evaluation of bond covalency and bond valence in Sr-doped perovskite-type compounds

Formulas for decomposing of complex crystals to a sum of binary crystals are described and applied to the study of bond covalency in La1-xSrxFeO3 (0.0 less than or equal to x less than or equal to 0.9) and Ca1-xSrxMnO3 (0.0 less than or equal to x less than or equal to 0.5). The bond valence is treated by bond-valence sums scheme. The results indicate that, for both compounds, with the increasing doping level, the bond covalency and bond valence show the same trend, namely, larger bond covalency corresponds to higher bond valence. For La1-xSrxFeO3, with the increase of doping level, the bond covalency of La-O, Ca-O decreases in the orthorhombic (0.0 less than or equal to x less than or equal to 0.2) and rhombohedral (0.4 less than or equal to x less than or equal to 0.7) systems, then increases slightly for the cubic (0.8 less than or equal to x less than or equal to 0.9) system, but that of Fe-O increases for all crystal systems. A sharp decrease in bond covalency was observed where the crystal changes from orthorhombic to rhombohedral, while a smooth trend was seen for the rhombohedral-to-cubic transition. On the other hand, for orthorhombic Ca1-xSrxMnO3, the bond covalency of Ca-O, Sr-O, and Mn-O (4-coordinate site) decreases with the increasing doping level, that of Mn-O (2-coordinate site) increases.

In situ, continuous monitoring of the photoinduced superhydrophilic effect: Influence of UV-type and ambient atmospheric and droplet composition

A controlled-atmosphere chamber, combined with a CCTV system, is used to monitor continuously the change in shape of water droplets on the self-cleaning commercial glass, Activ, and a sol-gel TiO2 substrate during their irradiation with either UVA or UVC light. This system allows the photoinduced superhydrophilic effect (PSH) exhibited by these materials to be studied in real time under a variety of different conditions. UVA was less effective than UVC in terms of PSH for both titania-coated glasses, and plain glass was unaffected by either form of UV irradiation and so showed no PSH activity. With UVA, ozone increased significantly the rate of PSH for both substrates, but had no effect on the wettability of plain glass. For both titania substrates and plain glass, no PSH activity was observed under an O-2-free atmosphere. A more detailed study of the PSH effect exhibited by Activ revealed that doping the water droplet with either an electron acceptor (Na2S2O8), electron donor (Na2S2O4), or simple electrolyte (KCl) in the absence of oxygen did not promote PSH. However, when Activ was UV irradiated, while immersed in a deoxygenated KCl solution, prior to testing for PSH activity, only a small change in contact angle was observed, whereas under the same conditions, but using a deoxygenated persulfate-containing immersion solution, it was rendered superhydrophilic. The correlation between organic contaminant removal and surface wetting was also investigated by using thick sol-gel films coated with stearic acid; the destruction of SA was monitored by FTIR and sudden wetting of the surface was seen to coincide with the substantial removal of the organic layer. The results of this work are discussed in the context of the current debate on the underlying cause of PSH.

Evaluation of the piezoresistance properties of p-type silicon

This paper investigates the characteristics of silicon piezoresistors with various doping concentrations and Length/Width dimensions at micro level. The silicon piezoresistors have been produced by conventional fabrication methods. The measurements are conducted on silicon test chips where p type resistors are fabricated on n type (100) silicon substrates along the <110> direction. A four point bending setup has been designed and fabricated for characterizing the piezoresistor sets. The four point bending setup is used to apply uniform uniaxial stress along the <110> direction. This experimental result demonstrates a good linear relationship between resistance change and stress applied. The effect of doping concentration on temperature sensitivity is also investigated.

Self-cleaning perovskite type catalysts for the dry reforming of methane

Gas-to-liquid processes are generally used to convert natural gas or other gaseous hydrocarbons into liquid fuels via an intermediate syngas stream. This includes the production of liquid fuels from biomass-derived sources such as biogas. For example, the dry reforming of methane is done by reacting CH4 and CO2, the two main components of natural biogas, into more valuable products, i.e., CO and H2. Nickel containing perovskite type catalysts can promote this reaction, yielding good conversions and selectivities; however, they are prone to coke laydown under certain operating conditions. We investigated the addition of high oxygen mobility dopants such as CeO2, ZrO2, or YSZ to reduce carbon laydown, particularly using reaction conditions that normally result in rapid coking. While doping with YSZ, YDC, GDC, and SDC did not result in any improvement, we show that a Ni perovskite catalyst (Na0.5La0.5Ni0.3Al0.7O2.5) doped with 80.9 ZrO2 15.2 CeO2 gave the lowest amount of carbon formation at 800 °C and activity was maintained over the operating time.

A Computational Approach for Simulating P-type Silicon Piezoresistor Using Four Point Bending Setup

The piezoresistance effect is defined as change in resistance due to applied stress. Silicon has a relatively large piezoresistance effect which has been known since 1954. A four point bending setup is proposed and designed to analyze the piezoresistance effect in p-type silicon. This setup is used to apply uniform and uniaxial stress along the <110> crystal direction. The main aim of this work is to investigate the piezoresistive characteristic of p-type resistors as a function of doping concentrations using COMSOL Multiphysics. Simulation results are compared with experimental data.

A Pragmatic Approach to Ethical Inquiry on Transhuman Athletes and Gene Doping in Sport

Gene doping is the most recent addition to the list of banned practices formulated by the World Anti-doping Agency. It is a subset of doping that utilizes the technology involved in gene therapy. The latter is still in the experimental phase but has the potential to be used as a type of medical treatment involving alterations of a patient‘s genes. I apply a pragmatic form of ethical inquiry to evaluate the application of this medical innovation in the context of sport for performance-enhancement purposes and how it will affect sport, the individual, society and humanity at large. I analyze the probable ethical implications that will emerge from such procedures in terms of values that lie at the heart of the major arguments offered by scholars on both affirmative and opposing sides of the debate on gene doping, namely fairness, autonomy and the conception of what it means to be human.

Growth and characterisation of p-type delafossite transparent conducting thin films for heterojunction applications

In the present studies, various copper delafossite materials viz; CuAlO2, CuGaO2, CuFeO2 , CuGa1-xFexO2, CuYO2 and CuCaxY1-xO2 were synthesised by solid state reaction technique. These copper delafossite materials were grown in thin film form by rf magnetron sputtering technique. In general copper delafossites exhibit good optical transparency. The conductivity of the CuYO2 could be improved by Ca doping or by oxygen intercalation by annealing the film in oxygen atmosphere. It has so far been impossible to improve the p-type conductivity of CuGaO2 significantly by doping Mg or Ca on the Ga site. The ptype conductivity is presumed to be due to oxygen doping or Cu Vacancies [6]. Reports in literature show, oxygen intercalation or divalent ion doping on Ga site is not possible for CuGaO2 thin films to improve the p-type conductivity. Sintered powder and crystals of CuFeO2 have been reported as the materials having the highest p-type conductivity [14, 15] among the copper and silver delafossites. However the CuFeO2 films are found to be less transparent in the visible region compared to CuGaO2. Hence in the present work, the solid solution between the CuGaO2 and CuFeO2 was effected by solid state reaction, varying the Fe content. The CuGa1-xFexO2 with Fe content, x=0.5 shows an increase in conductivity by two orders, compared to CuGaO2 but the transparency is only about 50% in the visible region which is less than that of CuGaO2 The synthesis of α−AgGaO2 was carried out by two step process which involves the synthesis of β-AgGaO2 by ion exchange reaction followed by the hydrothermal conversion of the β-AgGaO2 into α-AgGaO2. The trace amount of Ag has been reduced substantially in the two step synthesis compared to the direct hydrothermal synthesis. Thin films of α-AgGaO2 were prepared on silicon and Al2O3 substrates by pulsed laser deposition. These studies indicate the possibility of using this material as p-type material in thin film form for transparent electronics. The room temperature conductivity of α-AgGaO2 was measured as 3.17 x 10-4 Scm-1and the optical band gap was estimated as 4.12 eV. A transparent p-n junction thin film diode on glass substrate was fabricated using p-type α-AgGaO2 and n-ZnO.AgCoO2 thin films with 50% transparency in the visible region were deposited on single crystalline Al2O3 and amorphous silica substrates by RF magnetron sputtering and p type conductivity of AgCoO2 was demonstrated by fabricating transparent p-n junction diode with AgCoO2 as p-side and ZnO: Al as n-side using sputtering. The junction thus obtained was found to be rectifying with a forward to reverse current of about 10 at an applied voltage of 3 V.The present study shows that silver delafossite thin films with p-type conductivity can be used for the fabrication of active devices for transparent electronics applications.

Development of p-type transparent semiconducting oxides for thin film transistor applications

Semiconductor physics has developed significantly in the field of re- search and industry in the past few decades due to it’s numerous practical applications. One of the relevant fields of current interest in material science is the fundamental aspects and applications of semi- conducting transparent thin films. Transparent conductors show the properties of transparency and conductivity simultaneously. As far as the band structure is concerned, the combination of the these two properties in the same material is contradictory. Generally a trans- parent material is an insulator having completely filled valence and empty conduction bands. Metallic conductivity come out when the Fermi level lies within a band with a large density of states to provide high carrier concentration. Effective transparent conductors must nec- essarily represent a compromise between a better transmission within the visible spectral range and a controlled but useful electrical con- ductivity [1–6]. Generally oxides like In2O3, SnO2, ZnO, CdO etc, show such a combination. These materials without any doping are insulators with optical band gap of about 3 eV. To become a trans- parent conductor, these materials must be degenerately doped to lift the Fermi level up into the conduction band. Degenerate doping pro- vides high mobility of extra carriers and low optical absorption. The increase in conductivity involves an increase in either carrier concen- tration or mobility. Increase in carrier concentration will enhance the absorption in the visible region while increase in mobility has no re- verse effect on optical properties. Therefore the focus of research for new transparent conducting oxide (TCO) materials is on developing materials with higher carrier mobilities.

Fabrication and characterization of chemical sensors made from nanostructured films of poly(o-ethoxyaniline) prepared with different doping acids

Chemical sensors made from nanostructured films of poly(o-ethoxyaniline) POEA and poly(sodium 4-styrene sulfonate) PSS are produced and used to detect and distinguish 4 chemicals in solution at 20 mM, including sucrose, NaCl, HCl, and caffeine. These substances are used in order to mimic the 4 basic tastes recognized by humans, namely sweet, salty, sour, and bitter, respectively. The sensors are produced by the deposition of POEA/PSS films at the top of interdigitated microelectrodes via the layer-by-layer technique, using POEA solutions containing different dopant acids. Besides the different characteristics of the POEA/PSS films investigated by UV-Vis and Raman spectroscopies, and by atomic force microscopy.. it is observed that their electrical response to the different chemicals in liquid media is very fast, in the order of seconds, systematical, reproducible, and extremely dependent on the type of acid used for film fabrication. The responses of the as-prepared sensors are reproducible and repetitive after many cycles of operation. Furthermore, the use of an "electronic tongue" composed by an array of these sensors and principal component analysis as pattern recognition tool allows one to reasonably distinguish test solutions according to their chemical composition. (c) 2007 Published by Elsevier B.V.

Effect of Ta2O5 doping on the electrical properties of 0.99SnO(2)center dot 0.01CoO ceramic

The effect of Ta2O5 doping in 0.99SnO(2). 0.01CoO on the microstructure and electrical properties of this ceramic were analyzed in this study. The grain size was found to decrease from 6.87 mu m to 5.68 mu m when the Ta2O5 concentration increased from 0.050 to 0.075 mol%. DC electrical characterization showed a dramatic increase in the current loss and decrease in the non-linear coefficient with the increase of the Ta2O5 concentration. The conduction mechanism is by thermionic emission and the potential barriers are of Schottky type, separated by a thin film. (C) 2000 Kluwer Academic Publishers.