Differential Resistivity Measurements Research Articles

High temperature arsenic doping of CdHgTe epitaxial layers

AbstractExperimental results on solid‐state arsenic doping of the n‐type bulk and ISOVPE epitaxial CdXHg1‐XTe (X = 0.19 ÷ 0.3) alloys are presented. The arsenic doped thin epitaxial CdxHg1‐xTe films (nAs ≈ 5 · 1016 ÷ 1 · 1020 cm‐3; d = 2 ÷ 5 μm) obtained by RF sputtering in a mercury glow discharge were used as As diffusion sources. The arsenic diffusion and activation were carried out at temperatures T = 500 ÷ 600 °C under Hg vapour pressure. Immediately after the high temperature treatment all samples were annealed to annihilate point defects. The SIMS analysis was used for determination of the quantitative admixture distribution of As in the diffusion area. The arsenic electrical activity has been evaluated by means of differential Hall, resistivity and thermoemf measurements. The analysis of experimental data obtained as well as their comparison with previously obtained results has been performed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Detailed interpretation of electron transport in n-GaN

We report on temperature-dependent differential Hall-effect and resistivity measurements, between 10 and 300 K, on two silicon doped GaN epitaxial layers grown by two different metalorganic chemical vapor deposition sources on sapphire substrates. Reactive ion etching has been used to enable Hall measurements to be taken as a function of film thickness, for 1.62 and 3.92 μm thick films. Temperature-dependent Hall experiments indicate classical donor freeze-out in the doped region, while the depth profile measurements show that in the undoped layer, the Hall electron density passes through a minimum before increasing again at lower temperatures. Such behavior is indicative of impurity conduction in this region. Using a model based upon a doped layer with one type of shallow donor, plus compensating acceptors, in parallel with an interface layer which shows impurity-band conduction, a simultaneous fitting of mobility and carrier concentration has been undertaken to quantify the contribution of different scattering mechanisms, and the densities of the donors and acceptors in the doped region were also found. An important result is that for the carrier concentration fitting, when the temperature dependence of the activation energy is taken into account, the fitted energies are closer to those predicted by free and bound carrier screening. Finally the Fermi level was found to be in the range ∼56–84 meV below the conduction band minimum at room temperature and moved closer to the donor levels at low temperatures.

Location of the minimum of the differential tunneling resistance $ \mathsf {R(V)}$ in a superconductor-degenerate semiconductor Schottky contact

Measurements of differential resistance in a superconductor-degenerate semiconductor junction Nb - n + + GaAs at T = 1.6 K show close similarity to those for a conventional superconductor-insulator- normal metal junction, except for the position of the minimum which is located at 3.6 meV. Using a simple model for the charge screening at the Schottky barrier, we give an argument why this minimum is by far displaced with respect to the superconducting gap energy ( Δ g = 1.5 meV for bulk Nb). We argue that a rebuilding of the density of states takes place at the barrier, due to the imperfect metal screening in the degenerate semiconductor. Energy states close to the degenerate semiconductor Fermi energy are depleted at the barrier and are not available for tunneling, up to an energy Eg which adds to the superconducting gap Δ g .

Evidence of an impurity band at an n-GaN/sapphire interface

We report on temperature-dependent differential Hall-effect and resistivity measurements, between 10 and 300 K, on a silicon-doped GaN epitaxial layer grown by metalorganic chemical vapor deposition on a sapphire substrate. Reactive ion etching has been used to enable Hall measurements to be taken as a function of film thickness. Temperature-dependent Hall experiments indicate classical donor freeze-out in the doped region, while the depth profile measurements show that close to the interface, the Hall electron density passes through a minimum before increasing again at lower temperatures. Such behavior is indicative of impurity conduction in this region. Over the whole temperature range, the mobility does not change appreciably for the first 1.35 μm of removed doped material, but then falls rapidly over the remaining undoped region. Using a model based upon one donor, a deep acceptor and one conducting layer, a simultaneous fitting of mobility and carrier concentration has been undertaken to quantify the contribution of different scattering mechanisms, and the densities of the donors and acceptors were also found.

From Mott insulator to superconductivity in (TMTTF) 2BF 4: high pressure transport measurements

Within the generic (pressure, temperature) phase diagram of (TM) 2X compounds, we have investigated the crossover between the spin density wave and superconducting ground states by transport measurements under pressure. Superconductivity is reported for the first time in (TMTTF) 2BF 4 below 1.38 K, in the pressure range 33.5–37.5 kbar, and coexists with the spin density wave phase, present alone at lower pressure. Differential resistance measurements support the idea of a phase segregation with the proportion of the superconducting phase (related to the measured critical current) increasing with pressure.

Transport properties of reactive-ion-etching-induced p-to-n type converted layers in HgCdTe

In this work, the effect of the reactive ion etching (RIE)-induced p-to-n type conversion process on the transport properties of HgCdTe is investigated. Magnetic-field-dependent differential Hall and resistivity measurements have been performed to determine the n-type doping profile and temperature-dependent transport properties of carriers resulting from the RIE-induced type conversion. The study examined Hg1-xCdxTe with x = 0.23 and x = 0.31 for both vacancy-doped and gold-doped p-type epilayers grown by liquid phase epitaxy (LPE) on lattice-matched CdZnTe, which were partly converted to n-type by a RIE process. Analysis using quantitative mobility spectrum analysis (QMSA) reveals that RIE type conversion results in a damaged surface layer characterized by moderate mobility electrons and a bulk n-type region exhibiting higher electron mobility. The surface and bulk electrons show a distinct temperature dependence. It can be observed that, generally, the concentration and mobility of the surface electrons are similar for all samples studied and are independent of temperature in the measured temperature range. In contrast, the bulk electrons exhibit classical behavior with characteristics that are strongly dependent on temperature and consistent with high-quality HgCdTe material. Differential Hall results indicate that the n-type dopants resulting from the RIE process are distributed into the sample via a diffusion process. The results suggest a p-to-n type conversion mechanism involving neutralization of the p-type dopants and diffusion of extrinsic n-type dopants from the surface.

Magnetic properties ofNi2.18Mn0.82Ga Heusler alloys with a coupled magnetostructural transition

Polycrystalline Ni2.18Mn0.82Ga Heusler alloys with a coupled magnetostructural transition are studied by differential scanning calorimetry, magnetic and resistivity measurements. Coupling of the magnetic and structural subsystems results in unusual magnetic features of the alloy. These uncommon magnetic properties of Ni2.18Mn0.82Ga are attributed to the first-order structural transition from a tetragonal ferromagnetic to a cubic paramagnetic phase.

Simultaneous Impurity-Band and Interface Conduction in Depth-Profiled n-GaN Epilayers

We report on temperature-dependent differential Hall-effect and resistivity measurements, between 10 and 300 K, on two Si-doped GaN epitaxial layers grown by MOCVD on sapphire substrates. These experiments indicate parallel conduction paths in our layers, while depth profiling using plasma-etching shows that two paths are simultaneously present: an impurity band in the Si-doped region and a conducting layer at the GaN/sapphire interface.

Electrical reliability of electrically conductive adhesive joints: dependence on curing condition and current density

The use of electrically conductive adhesives as interconnection materials in electronic assembly process is increasingly becoming a vital part of the electronics industry. Flip-chip joining technique using conductive adhesives has been identified as a key technology for future electronics assembly and manufacturing. The purpose of the present work is to investigate optimum conditions to achieve the best electrical performance in conductive adhesive joints. This study shows a comparison of electrical performance in conductive adhesive joints at various current densities with different curing conditions. Differential scanning calorimetry and resistance measurement were used to monitor curing condition in conductive adhesives. Accelerated life testing of conductive adhesive joints made of the selected conductive adhesive using different curing conditions was performed with various current densities. The current-induced degradation of conductive adhesive joints was investigated using optical microscopy and electrical resistance measurements. Results show a strong dependence of curing condition and current density on electrical performance of adhesive joints. Additionally, sample cured for less time exhibited better quality than sample cured for more time at high current densities. It is also found that conducting particles move with the current-induced aging, which shows that the migration of conducting particles can induce the failure of conductive adhesive joint during the current-induced resistance increase.

A novel resistance measurement technique in the field of directional solidification

A brief review of existing diagnostic methods in directional solidification is first presented. We then discuss the potential of global and differential resistance measurements to determine the interface position and velocity in liquid metals and semiconductors. An electronic design allowing the measurements to be carried out with maximum accuracy is proposed. This design is characterized both at low and high temperatures and it is shown that the peak to peak noise on the resistance signal can be kept of the order of 0.1 μ Ω. In the tin based system used in our investigations, this amounts to an accuracy in terms of solid–liquid interface position in the micrometer range, which represents a significant improvement with respect to existing techniques. We found that such an accuracy allows us to characterize in detail the heat transfer phenomena within our directional solidification furnace, and to identify the thermal lag of the device in the translation and stabilization stages.

Infrared Study of C 60 (AlCl 3 ) 2

C 60 (AlCl 3 ) 2 , prepared by interacting C 60 and AlCl 3 in the presence of Cl 2 gas, was examined by differential thermal analysis, infrared spectroscopy, resistivity measurements, and X-ray diffraction. The compound was an electrical insulator and non-crystalline. It exhibited C-Cl coupling and AlCl 3 in the infrared absorption spectrum. The properties of samples exposed to air and heated to 400 °C were studied. Both studies indicated the removal of AlCl 3 and the reduction of C-Cl bonding.

Segregation and diffusion of phosphorus from doped Si1−xGex films into silicon

Segregation and diffusion of P from in situ doped Si1−xGex (0.25⩽x⩽0.8) epitaxial films into Si at 750–850 °C were investigated using secondary ion mass spectroscopy and differential resistance measurements. It was found that the surface P concentration in the diffused layer in Si was higher than the P concentration in the Si1−xGex film, which signifies the segregation of P from the Si1−xGex film into Si. The segregation coefficient, defined as the ratio of the active P concentration in the Si to that in the Si1−xGex film, was about 2.5 at 800 °C in the case of the Si0.75Ge0.25 film as a diffusion source and increased with increasing Ge fraction. It was found that the P diffusion profiles in Si were normalized by x/t, even though the segregation of P occurred, the diffusion coefficient of P depended on the surface P concentration, and the high concentration diffusion characteristics of P in Si were similar to those reported by using a conventional diffusion source.

Anisotropy in microdevices produced by micromachining of cold-rolled NiTi sheets

Abstract The process of fabrication and characterisation of three types of NiTi microdevice are described: a double-beam cantilever of constant beam width and two designs of cantilever of stress-optimised width profile, with thicknesses ranging from 17 to 95 μm produced by cold rolling. The technology of electrolytic photoetching was used as a versatile tool for micromachining. The transformation behavior of the specimens was investigated by differential scanning calorimetry and electrical resistance measurements. Anisotropies caused by rolling-induced textures have been investigated by beam-bending experiments. Strong anisotropies were found in results on thickness-dependent Young’s modulus in the parent phase as well as in the beam-bending characteristics in the rhombohedral and martensitic phase. For beam directions transverse to the rolling direction, beam-bending experiments revealed a thickness dependence of hysteresis associated with the martensitic transformation, which was not observed in the rolling direction. Cantilevers with stress-optimised width profile display favorable beam-bending characteristics compared with cantilevers of constant beam width due to an optimum use of the shape memory effect.

Multiple Andreev reflections in ballistic Nb-InGaAs/InP-Nb junctions

The current–voltage characteristics of ballistic Nb-InGaAs/InP-Nb Josephson junctions have been investigated. At temperatures below 1 K a negative differential conductance, which usually leads to a hysteresis in the current–voltage characteristics, was resolved by connecting an additional external shunt resistor to the junction. The negative differential conductance is explained by heating and conductance enhancement due to multiple Andreev reflections. The structures observed in the differential resistance measurements as a function of the bias voltage are explained by self-detection of Josephson radiation at low bias voltages and subharmonic gap structures at higher bias voltages.

Phosphorus Diffusion From Doped Si1−xGex, Films into Silicon

ABSTRACTPhosphorus diffusion from in-situ doped Si1−xGex epitaxial films into Si at 800°C was investigated using secondary ion mass spectroscopy and differential resistance measurements. The surface P concentration in the diffused layer in Si was higher than the P concentration in the Si1−xGex, film in the present conditions, which signifies the segregation of P from the Si1−xGex, film into Si. The segregation coefficient, defined as the ratio of the active P concentration in the Si to that in the Si1−xGex, film, was about 2.5 in the case of the Si0.75Ge0.25 film as a diffusion source and increased with increasing Ge fraction. The P diffusion profiles in Si were normalized by x/√, even though the segregation of P occurred. The high concentration diffusion characteristics of P in Si were similar to those reported by using conventional diffusion sources.

Effect of metalloid substitution on the reversible structural relaxation of Fe–Ni–B(-P, Si) metallic glasses

The reversible structural relaxation of Fe 37.5Ni 37.5B 25− x P x ( x=0, 3, 6, 16, 25) and Fe 37.5Ni 37.5B 25− y Si y ( y=0, 5, 10, 15) metallic glasses was investigated using the activation energy spectra (AES) model by differential scanning calorimetry (DSC) and resistivity measurements. From the deduced activation enthalpy spectra for these amorphous alloys, it is concluded that possibilities for atomic movements with characteristic activation energies are created by the substitution of B with P or Si. It is shown that at least one Gaussian contribution to the AES appears on increasing P or Si content, superimposed on an exponential part previously found for Fe–Ni–B glasses. The amplitude of the Gaussian function increases with increasing P or Si content while its position decreases with increasing P and remains constant with increasing Si content. According to isothermal resistivity measurements, for P substitution the reversible atomic rearrangements connected to the Gaussian contribution increase the resistivity while those associated with the exponential part decrease it. For Si substitution both processes decrease the resistivity.

Fabrication and transport properties of long–length doped Bi1.7Pb0.3Sr2Ca2-xMxCu3Oy conductors (M=B, Nb or Ag)

Long lengths of Bi-2223 conductors doped with Ag, Nb and B were fabricated. The recrystallization of the glasses during various heat treatments was studied by differential thermal analysis (DTA), differential scanning calorimetry, X-ray diffraction and resistivity measurements. Activation energies and frequency factors for crystallization were determined by non-isothermal DTA, employing different models. It was found that both the peritectic transition and the reaction rate were dependent on the ambient atmosphere. Kinetic studies under different atmospheres revealed that the thermal stability of Bi-2223 was greatly enhanced under an oxygen atmosphere. Jc measurements at 77 K showed an increase on silver addition. Furthermore it was found that the silver addition does not destroy the superconductivity. © 1998 Chapman & Hall

Fabrication and characterization of mesoscopic superconductor–semiconductor hybrid structures

We have deposited superconducting niobium films on high mobility InAs:GaSb quantum wells. The films are patterned with gaps containing up to four niobium lines. The size of the lines and the spacing between them is less than the electron mean free path in the quantum well and ballistic effects are therefore expected to play a role in the current transport. Differential resistance measurements show evidence of supercurrents of up to 10 μA in each of the hybrid structures as well as enhanced conductivity due to Andreev reflection. The supercurrents are suppressed by magnetic fields of order 50 mT. At higher fields, the resistance minimum associated with Andreev reflection evolves into a resistance maximum for fields such that the cyclotron diameter is smaller than the gaps between the niobium. A detailed explanation of the fabrication process is also presented.

Threshold nonlinear conduction of thin samples of o-TaS3above the Peierls transition temperature

We report precise measurements of differential resistance vs voltage across thin samples of the quasi-one-dimensional conductor ${\mathrm{TaS}}_{3}$ in the vicinity and above the Peierls transition temperature ${\mathrm{T}}_{\mathrm{P}}$. A well-defined threshold voltage for the onset of nonlinear conduction is observed up to 45 K above ${\mathrm{T}}_{\mathrm{P}}$. The effect is associated with depinning of large coherent regions of charge-density waves (CDW) arising due to the fluctuations and extending across the whole sample thickness. Our results show an unexpectedly high contribution of large domains of phase coherence to the ``fluctuative'' conductivity. The thin CDW samples give a unique opportunity to see the effects of extremely long-range fluctuations.

Shape Memory Thin Films of the System Ti- (Ni-Pd-Cu)

ABSTRACTFree-standing shape memory thin films of the system Ti-Ni-Pd-Cu exhibiting the two-way-effect have been fabricated by d.c. magnetron sputtering onto unheated substrates followed by annealing and training processes. Their transformation temperatures were investigated by differential scanning calorimetry and electrical resistivity measurements. By the Ni-Pd substitution the transformation temperatures (austenite/martinsite finish temperature; Af/Mf) could be varied between 32°C/-38°C for the binary TiNi films to 570°C/498°C for the binary TiPd films. By the Ni-Cu substitution the transformation hysteresis (Af/Mf) could be reduced from 70°C for the binary film to 20°C for films with 10 at % Cu. A similar behavior was observed for TiNiPdCu films.