Mechanisms of carriers transport in Ni/n-SiC, Ti/n-SiC ohmic contacts

A mechanism of carriers transport through metal-semiconductor interface created by nickel or titanium-based ohmic contacts on Si-face n-type 4H-SiC is presented herein. The mechanism was observed within the temperature range of 20 °C ÷ 300 °C which are typical for devices operating at high current density and at poor cooling conditions. It was found that carriers transport depends strongly on concentration of dopants in the epitaxial layer. The carriers transport has thermionic emission nature for low dopant concentration of 5×10¹⁶ cm^?3. The thermionic emission was identified for moderate dopant concentration of 5×10¹⁷ cm^?3 at temperatures higher than 200 °C. Below 200 °C, the field emission dominates (for the same doping level of 5×10¹⁷ cm^?3). High dopant concentration of 5×10¹⁸ cm^?3 leads to almost pure field emission transport within the whole investigated temperature range.     

Highly transparent and conducting In doped CdO synthesized by sol-gel solution processing

Cadmium oxide (CdO) is a much-studied wide gap semiconductor with an inherent high mobility of?>?100 cm²/Vs, high electron concentration of?>?10²¹ cm^?3 and a wide optical transparency window of?>?1800 nm. These unique properties make CdO a potential transparent conductor for full spectrum photovoltaics. However, in order to achieve optimum material properties for optoelectronic applications, CdO was grown by vacuum-based physical or chemical vapor deposition methods. In this work, we explored the application of a low-cost sol-gel spin coating method to achieve highly conducting and transparent CdO thin films doped with 0–10% In (CdO:In). We find that while as-grown CdO:In films are nanocrystalline/amorphous with a high resistivity of?~?1 Ω-cm, polycrystalline and highly conducting films can be obtained after optimized annealing at?≥?400 °C. However, the electron concentration n saturates at?~?5?×?10²⁰ cm^?3 for In concentration?>?5% (or N_In?~?1.9?×?10²¹ cm^?3). This low activation of In may be attributed to the high density of native defects and/or impurities incorporated in the sol-gel process. With 5% In doping, we obtained a low resistivity of ρ?~?2.5?×?10^–4 Ω-cm and a high mobility μ?~?50 cm²/Vs. These values of σ and µ are better than those reported for other TCOs synthesized by solution processes and comparable to conventional commercial TCOs grown by physical vapor deposition methods. Benefiting from their high mobility, these sol-gel CdO:In films are optically transparent over a wide spectral range up to λ?>?1800 nm, making them promising as transparent conductors for optoelectronic devices utilizing the infrared photons.

     

Editorial

Abstract

Silicon thin films, phosphorus doped in situ, have been deposited on to glass substrates using low pressure chemical vapour deposition at 550°C. The doping level is determined by adjusting the phosphine/silane molar ratio. Using this method a wide range of concentration is controllable. For a gas molar ratio varying between 4×10^?8 and 4 × 10^?4, phosphorus atomic incorporation is in the range 3 × 10¹⁶?3 × 10²⁰ cm^?3. The resistivity of the layers varies from 8·3 × 10⁵ to 1·5 × 10^?3 Ω cm. Lightly doped samples were passivated by hydrogenation, and Hall measurements were carried out, showing a marked improvement of the electrical properties. A lightly in situ doped drain thin film transistor suitable for active matrix applications was fabricated which exhibited good electrical properties.

MST/3335     

Effect of film thickness on properties of aluminum doped zinc oxide thin films deposition on polymer substrate

A series of aluminum doped zinc oxide thin films with different thickness (25–150 nm) were deposited on indium tin oxide coated polyethylene terephthalate substrates by radio frequency magnetron sputtering method at room temperature. The structural, optical and electrical properties of the films were investigated by X-ray Diffractometer, UV–Vis spectrometer and Hall Effect Measurement System. All the obtained films were polycrystalline with a hexagonal structure and a preferred orientation along [002] direction with the c-axis perpendicular to the substrate surface. The optical energy band gap (E_g) values of the films were found to be in the range from 3.36 to 3.26 eV, and their average optical transmissions were about 75 % in the visible region. The films had excellent electrical properties with the resistivities in the range from 2.78 × 10^?5 to 2.03 × 10^?4 Ω cm, carrier densities more than 3.35 × 10²¹ cm^?3 and Hall mobilities between 5.77 and 11.13 cm²/V s.     

Perovskite-type compounds as electrode catalysts for cathodic reduction of oxygen

The perovskite-type compounds (Ln, M)M′O₃ (Ln: lanthanoid, M: alkaline earth, M′: transition metal) are synthesized, and the related perovskite formation region is determined. The resistivities of these compounds at 25°C decrease by several orders of magnitude when they are doped with Ca, Sr or Ba. The resistivity minima range from 10^?4 to 10^?1 Ωcm, depending on the composition. The oxygen ion diffusion constants are evaluated from the electrode potentials in a KOH solution. The diffusion constant for Nd_0.8Sr_0.2 CoO₃ is 1.4 × 10^?11 cm²/s at 25°C. These compounds are examined as potential oxygen electrode materials for alkaline solution metal fuel cells, and (Nd, Sr)CoO₃ provides good performance for several hundred hours.     

Improvement of Ge/Pd/GaAs ohmic contact by In layer

The presented work describes behaviour of contact structures of the Ge/Pd type with the In layer deposited on the surface of the GaAs substrate plate prior to the metallization. The most suitable structure by contact resistivity and thermal stability is Ge(40 nm)/Pd(20 nm)/In(22 nm). This structure shows minimal contact resistivity 2 × 10⁻⁶ Ωcm². Raman spectroscopy and XPS spectroscopy was used for the contact structure analysis. After thermal annealing, the metallization contains GePd phase and a thin germanium layer remains at the surface. Very slight reaction of indium with the substrate (creation of a ternary phase InGaAs) has been proved. Germanium and palladium diffuse into the GaAs substrate, the surface layer of GaAs is doped by Ge and Pd is built in the GaAs crystal structure instead of arsenic.     

Radiation-induced optical emission quenching in light-probed silica core fibers

The radiation-induced visible emission (400–750 nm) intensity in an optical fiber with a KU-1 silica glass core (OH group content, 1000 ppm) was measured in the fiber irradiated in a pulse mode [BARS-6 reactor; pulse duration, 80 μs; dose per pulse, <5.5×10¹² cm^?2 (9 Gy); dose rate, <7×10¹⁶ cm^?2 s^?1 (1.1×10⁵ Gy/s)]. The fiber probed by laser pulses (at 532 and 632 nm) with increasing intensity showed a decrease in the radiation-induced emission intensity in the regions of wavelength both greater and lower than and equal to the probing light wavelength.     

Electrical,optical and magnetoresistive behavior of nanostructured ZnO:Cu thin films deposited by sputtering

Copper doped ZnO (ZnO:Cu) nanostructured films with magnetoresistive behavior were produced by growing ZnO/Cu/ZnO arrays at room temperature (RT) by the sputtering technique on corning glass substrates. The arrays were made with two electrical insulating ZnO films of 50 and 105 nm, and a Cu film of 5 nm, both materials were deposited at RT by the RF- and DC-sputtering technique, respectively. The processing method involves two stages that proceed in the course of the growth process, the main one is originated by the non-equilibrium regime of the sputtering technique, and the second is the diffusion-redistribution of the intermediate Cu film towards the neighborhood ZnO layers aided by the nanocrystalline films character. The influence of applying an additional annealing stage to the arrays in N₂ atmosphere at 250 and 350 °C by periods of 30 min were studied. The resistivity of the ZnO:Cu films can be varied from 0.0034 to 2.83 Ω-cm, corresponding to electron concentrations of 1.12?×?10²¹ and 7.85?×?10¹⁷ cm^?3 with carrier mobility of 1.6 and 2.8 cm²/V s. Measured changes on the magnetoresistance behavior of the films at RT were of ?R?~?3% for annealed samples with electron concentration of 1.12?×?10²¹ cm^?3. The X-ray diffraction measurements show that the films are comprised of nanocrystallites with dimensions between 13 and 20 nm in size with preferred (002) orientation. The transmittance of the films in the visible region was of 83% with an optical band gap of ~?3.3 eV for the low-resistivity samples.     

Characterisation of n-type γ-CuCl on Si for UV optoelectronic applications

The use of co-evaporation of ZnCl₂ with CuCl in order to achieve n-type conductivity in CuCl is reported herein. Linear current–voltage (IV) characteristics in the range of ±4 V have been measured using Cu–Au electrical contacts. Room temperature Hall effect measurements show some evidence of a mixed conduction mechanism. On average the samples exhibit n-type conductivity with a bulk electron carrier concentration n ～1 × 10¹⁶ cm^?3 and Hall mobility μ ～ 29 cm²v^?1s^?1 for a CuCl sample doped with a nominal 3 mole % ZnCl₂. By use of an in situ CaF₂ capping layer, transmission >90% is achieved. At room temperature a strong Z₃ free excitonic emission occurs at ～385 nm using both photoluminescence and x-ray excited optical luminescence, indicating the high optical quality of the doped material.     

Direct Measurements of the Strong Refractive Nonlinearity of Impurity Origin in Heavily-doped GaAs

Abstract

A strong dispersive nonlinearity below the band gap in heavily doped bulk n-GaAs is observed for differnet donor impurities. Negative refractive index changes of up to ?5 × 10^?3 are obtained in the spectral range 880–900 nm, induced by light of the same wavelength at an incident intensity of about 5 × 10⁵ W cm^?2. Since the lifetime of the nonlinearity is ～ 10^?10 s, it is suggested that a bistable device exploiting this effect could be constructed with a switching energy of (1?5) × 10^?14 J μm^?2.     

Structural,optical and electrical properties of SnO<Subscript>2</Subscript>:F thin films deposited by spray pyrolysis for application in thin film solar cells

Fluorine doped tin oxide (FTO) thin films with adequate properties to be used as transparent electrical contact for PV solar cells were synthesised using the spray pyrolysis technique, which provides a low cost operation. The deposition temperature and the fluorine doping have been optimized for achieving a minimum resistivity and maximum optical transmittance. No post-deposition annealing treatments were carried out. The X-ray diffraction study showed that all the FTO films were polycrystalline with a tetragonal crystal structure and preferentially oriented along the (200) direction. The grain size ameliorates with the increase in substrate temperature. The samples deposited with the substrate temperature at 440 °C and fluorine content of 20 wt % exhibited the lowest electrical resistivity (1.8 × 10^?4 Ω cm), as measured by four-point probe. Room-temperature Hall measurements revealed that the 20 wt% films are degenerate and exhibit n-type electrical conductivity with carrier concentration of ~4.6 × 10²⁰ cm^?3, sheet resistance of 6.6 Ω/□ and a mobility of ~25 cm² V^?1 s^?1. In addition, the optimized growth conditions resulted in thin films (~500 nm thickness) with average visible transmittance of 89 % and optical band-gap of 3.90 eV. The electrical and optical characteristics of the deposited films revealed their excellent quality as a TCO material.     

Two-photon pumped upconversion and nonlinear optical properties of a new lasing dye: HMASPS

Abstract

Two-photon pumped frequency upconversion optical properties and two-photon absorption (TPA) induced nonlinear absorption of a new dye, trans-4-[p-(N-hydroxyethyl-N-methylamino)styryl]-N-methylpyridinium ptoluene sulfonate (abbreviated to HMASPS) have been experimentally investigated. This new dye exhibits strong superradiation properties when the pumping power is above its threshold. The superradiation upconversion efficiency from 900 to 1150 nm and the nonlinear absorption from 720 to 1100 nm have been measured. The largest effective molecular TPA cross-section was measured to be 44.3 × 10^?48 cm⁴·s·photon^?1at 920 nm. At 1064 nm, it was 2.77 × 10^?48 cm⁴·s·photon^?1, much smaller than that at 920 nm. The highest conversion efficiency is 7% at 990 nm, whereas 4.2% at 1064 nm.     

Effects of nanoscale size dependent parameters on lattice thermal conductivity in Si nanowire

The effects of nanoscale size dependent parameters on lattice thermal conductivity are calculated using the Debye-Callaway model including transverse and longitudinal modes explicitly for Si nanowire with diameters of 115, 56, 37 and 22 nm. A direct method is used to calculate the group velocity for different size nanowire from their related calculated melting point. For all diameters considered, the effects of surface roughness, defects and transverse and longitudinal Gruneisen parameters are successfully used to correlate the calculated values of lattice thermal conductivity to that of the reported experimental curve. The obtained fitting value for mean Gruneisen parameter has a systematic dependence on all Si nanowire diameters changing from 0·791 for 115 nm diameter to 1·515 for the 22 nm nanowire diameter. The dependence also gave a suggested surface thickness of about 5–6 nm. The other two parameters were found to have partially systematic dependence for diameters 115, 56, and 37 nm for defects and 56, 37 and 22 nm for the roughness. When the diameters go down from 115 to 22 nm, the concentration of dislocation increased from 1·16 × 10¹⁹cm⁻³ to 5·20 × 10¹⁹cm⁻³ while the surface roughness P found to increase from 0·475 to 0·130 and the rms height deviation from the surface changes by about 1·66 in this range of diameter. The diameter dependence also indicates a strong control of surface effect in surface to bulk ratio for the 22 nm wire diameter.     

Low indium content In–Zn–O system towards transparent conductive films: structure,properties and comparison with AZO and GZO

In this work, low content indium doped zinc oxide (IZO) thin films were deposited on glass substrates by RF magnetron sputtering using IZO ceramic targets with the In₂O₃ doping content of 2, 6, and 10 wt%, respectively. The influences of In₂O₃ doping content and substrate temperature on the structure and morphology, electrical and optical properties, and environmental stability of IZO thin films were investigated. It was found that the 6 wt% doped IZO thin film deposited at 150?°C exhibited the best crystal quality and the lowest resistivity of 9.87?×?10^?4 Ω cm. The corresponding Hall mobility and carrier densities were 9.20 cm² V^?1 s^?1 and 6.90?×?10²⁰ cm^?3, respectively. Compared with 2 wt% Al₂O₃ doped ZnO and 5 wt% Ga₂O₃ doped ZnO thin films, IZO thin film with the In₂O₃ doping content of 6 wt% featured the lowest surface roughness of 1.3 nm. It also showed the smallest degradation with the sheet resistance increased only about 4.4% at a temperature of 121?°C, a relative humidity of 97% for 30 h. IZO thin film with 6 wt% In₂O₃ doping also showed the smallest deterioration with the sheet resistance increased only about 2.8 times after heating at 500?°C for 30 min in air. The results suggested that low indium content doped ZnO thin films might meet practical requirement in environmental stability needed optoelectronic devices.     

Characterization of ohmic contacts to InP

The results of a study of the electrical and metallurgical properties of thin metallic layers deposited on InP for use as ohmic contacts are presented. The layers were heat treated at temperatures up to 550°C and were examined with Auger electron spectroscopy. For contact to n-type InP three thin film systems were investigated: gold, nickel and a composite Ni/Au/Ge layer. Nickel was found to produce ohmic behavior in the Ni/Au/Ge/InP system with a minimum specific contact resistance r_c of 3×10^?5 Ω cm² for a net doping of 3×10¹⁶ cm^?3. For contact to p-type InP a film consisting of Au/Mg was investigated. For heat treatment of the Au/Mg/InP system above 350°C, r_c decreased as the temperature of the heat treatment increased and the surface morphology exhibited increasing signs of alloying at higher temperatures. The smoothest surface was obtained at 446°C for 50 min with r_c≈1×10^?4Ω cm² for a net doping of 6×10¹⁷ cm^?3.     

The variation of the features of SnO2 and SnO2:F thin films as a function of V dopant

V doped SnO₂ and SnO₂:F thin films were successfully deposited on glass substrates at 500 °C with spray pyrolysis. It was observed that all films had SnO₂ tetragonal rutile structure and the preferential orientation depended on spray solution chemistry (doping element and solvent type) by X-ray diffraction measurements. The lowest sheet resistance and the highest optical band gap, figure of merit, infrared (IR) reflectivity values of V doped SnO₂ for ethanol and propane-2-ol solvents and V doped SnO₂:F films were found to be 88.62 Ω–3.947 eV–1.02 × 10^?4 Ω^?1–65.49 %, 65.35 Ω–3.955 eV–8.54 × 10^?4 Ω^?1–72.58 %, 5.15 Ω–4.076 eV–6.15 × 10^?2 Ω^?1–97.32 %, respectively, with the electrical and optical measurements. Morphological properties of the films were investigated by atomic force microscope and scanning electron microscope measurements. From these analysis, the films consisted of nanoparticles and the film morphology depended on doping ratio/type and solvent type. It was observed pyramidal, polyhedron, needle-shaped and spherical grains on the films’ surfaces. The films obtained in present study with these properties can be used as front contact for solar cells and it can be also one of appealing materials for other optoelectronic and IR coating applications.     

Low-resistance and highly transparent Ag/IZO ohmic contact to p-type GaN

The electrical, structural, and optical characteristics of Ag/ZnO-doped In₂O₃ (IZO) ohmic contacts to p-type GaN:Mg (2.5 × 10¹⁷ cm^− 3) were investigated. The Ag and IZO (10 nm/50 nm) layers were prepared by thermal evaporation and linear facing target sputtering, respectively. Although the as-deposited and 400 °C annealed samples showed rectifying behavior, the 500 and 600 °C annealed samples showed linear I-V characteristics indicative of the formation of an ohmic contact. The annealing of the contact at 600 °C for 3 min in a vacuum (~ 10^− 3 Torr) resulted in the lowest specific contact resistivity of 1.8 × 10^− 4 Ω·cm² and high transparency of 78% at a wavelength of 470 nm. Using Auger electron spectroscopy, depth profiling and synchrotron X-ray scattering analysis, we suggested a possible mechanism to explain the annealing dependence of the electrical properties of the Ag/IZO contacts.     

Semi-insulating GaN:C epilayers grown by metalorganic vapor phase epitaxy using propane as a carbon source

The influence of propane present in a reactor at various stages of GaN growth by metalorganic vapor phase epitaxy (MOVPE) on sapphire substrates on the character of epitaxial process and the properties of epilayers has been studied. Doped GaN epilayers with carbon concentration 5 × 10¹⁸ cm^–3 characterized by high crystalline perfection, an atomically smooth surface, and electric breakdown voltage above 500 V at a doped layer thickness of 4 μm have been obtained.

     

Synthesis and properties of rare earth doped lamp phosphors

Red, blue and green emitting lamp phosphors such as Eu³⁺ doped Y₂O₃ (red phosphor), Eu²⁺ doped Ba_0·64Al₁₂O_18·64, BaMgAl₁₀O₁₇ and BaMg₂Al₁₆O₂₇ (blue phosphors) and Ce_0·67Tb_0·33MgAl₁₁O₁₉ and Eu²⁺, Mn²⁺ doped BaMgAl₁₀O₁₇ (green phosphors) have been prepared by the combustion of the corresponding metal nitrates (oxidizer) and oxalyl dihydrazide/urea/carbohydrazide (fuel) mixtures at 400°–500°C within 5 min. The formation of these phosphors has been confirmed by their characteristic powder X-ray diffraction patterns and fluorescence spectra. The phosphors showed characteristic emission bands at 611 nm (red emission), 430–450 nm (blue emission) and 515–540 nm (green emission). The fine-particle nature of the combustion derived phosphors has been investigated using powder density, particle size and BET surface area measurements. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

A study of the electrical characteristics,before electroforming,of thin SiOx films with laterally-spaced electrodes

The sample structure comprises two laterally-spaced electrodes joined by a thin dielectric film. The electrodes make a blocking contact to the dielectric with a barrier height ～ 0.75 eV. The voltage-current characteristic is studied between 213 to 413 K. Below room temperature the effect of hopping conduction on such contacts is explained. At room temperature and at high fields where hopping conduction is expected to be less effective, the conduction is controlled by the contact. Above room temperature and at low fields localized state conduction (hopping) becomes effective again with activation energy ～ 0.07 eV and an estimated charge carrier mobility of 1.3×10^?2cm²V^?1sec^?1. The density of ionizable impurities is estimated to be in the range 1 to 6×10¹⁸cm^?3 and the density of surface states is of the order of 1×10¹³cm^?2.