Preparation of P-type indium phosphide films by close space vapor transport

InP films have been grown by close space transport employing 0.8 mol% PCl₃ in H₂. For deposition on InP single crystals, 700C source and 650C substrate temperatures produced epitaxial films on (100), polycrystalline films on (111)A, and powdery layers on (111)B. Growth rates are 6 to 10 |Gmm/hr on (100) InP and ~50 μm/hr on (111)A InP. Regardless of InP source doping, deposits exhibit net donor concentrations of 5×10¹⁷ to 1×10¹⁸cm^?3. Zn doping with 0.02 to 0.5 mol% Zn(C₂H₅)₂ in the gas phase resulted in partially compensated p-InP with net acceptor concentrations up to 7×10¹⁸cm^?3. Polycrystalline films have been grown on Mg-coated carbon or molybdenum substrates at 700C source and 590C substrate temperatures. Growth rates lie between 40 and 50 μm/hr. Substantial recrystallization and grain growth are observed after 2 day anneals at 950C under 5 atm of phosphorus.     

Wet Chemical Treatment and Mg Doping of p-InP Surfaces for Ohmic Low-Resistive Metal Contacts

Manufacturing a low-resistive Ohmic metal contact on p-type InP crystals for various applications is a challenge because of the Fermi-level pinning via surface defects and the diffusion of p-type doping atoms in InP. Development of wet-chemistry treatments and nanoscale control of p-doping for InP surfaces is crucial for decreasing the device resistivity losses and durability problems. Herein, a proper combination of HCl-based solution immersion, which directly provides an unusual wet chemical-induced InP(100)c(2 × 2) atomic structure, and low-temperature Mg-surface doping of the cleaned InP before Ni-film deposition is demonstrated to decrease the contact resistivity of Ni/p-InP by the factor of 10 approximately as compared to the lowest reference value without Mg. Deposition of the Mg intermediate layer on p-InP and postheating of Mg/p-InP at 350 °C, both performed in ultrahigh-vacuum (UHV) chamber, lead to intermixing of Mg and InP elements according to X-ray photoelectron spectroscopy. Introducing a small oxygen gas background (O₂ ≈ 10⁻⁶ mbar) in UHV chamber during the postheating of Mg/p-InP enhances the indium outdiffusion and provides the lowest contact resistivity. Quantum mechanical simulations indicate that the presence of InP native oxide or/and metal indium alloy at the interface increases In diffusion.     

Improved Transport J c in MgB2 Tapes by Graphene Doping

Graphene is a special form of carbon which can effectively enhance the critical current density J _c of MgB₂. In this work, a systematic investigation on the impact of sintering conditions and doping level was carried out for graphene-doped MgB₂ tapes. It is found that an appropriate addition level, i.e., 8 at% in this work, is very critical to obtain a high J _c in graphene-doped samples. The critical field and pinning force are improved obviously due to the graphene doping. The magnetic J _c of samples sintered at 800 °C with 8 at% graphene doping reached 1.78 × 10⁴ A/cm², at 5 T, 20 K. At the same time, the transport J _c was up to 2.38 × 10⁴ A/cm² at 10 T, 4.2 K. The lattice distortion caused by C substitution and residual C at the grain boundaries were thought to be the major factors affecting the J _c of graphene-doped MgB₂ samples.     

Mixage ionique sur des structures Au/InP

The reactions induced by Zn⁺ implantations near the interface of Au/InP contacts have been studied by using scanning electron microscopy, X-ray diffraction, He⁺ Rutherford backscattering, secondary ion mass spectrometry and current-voltage measurements. A 5 × 10¹⁴ Zn ions cm^-2 dose does not induce compound formation but accelerates the growth of Au₃In and Au₂P₃ patches during post-annealing treatment. After a 5 × 10¹⁵ Zn ions cm^-2 implantation, many compounds, different from those obtained by a thermal anneal, are detected. These compounds, which depend on the implantation temperature (25 or 200°C), have a layered structure. In this case no Au₂P₃ is observed. However, for the range of doses (from 10¹⁴ to 5 × 10¹⁵ Zn ions cm^-2), the temperatures of implantation (25 and 200°C) and the range of annealing temperatures (from 320 to 450°C) that were studied, no contact with a low resistivity is formed. The electrical properties are in fact limited by an InP layer damaged by the ion implantation in which the zinc atoms are trapped in an electrically inactive form.     

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.     

Current carrying capacities of NbAlGe bulk samples in magnetic fields up to 10 T

Results of critical current density measurements of bulk NbAlGe both as-cast and heat-treated samples are given in this paper. In steady magnetic fields up to 10 T, the current densities 10³?3 × 10⁴Acm^?2 have been reached. The optimal heat treatment of samples led to the enhancement of T_c in all cases, but its influence on j_c was not significant and unambiguous. In some cases a small enhancement of j_c, and in other cases lower j_c was observed after sample annealing.     

Performance of Pd–Ge based ohmic contacts to n-type GaAs

The performance of Pd–Ge based ohmic contacts, with and without Ti–Pt or Ti–Pt–Au capping layers, has been investigated. The contacts were deposited by electron beam evaporation, then characterized electrically using a modified transmission line method (TLM) and structurally using both cross-section and plan-view transmission electron microscopy (TEM). Although both capped and non-capped contact structures underwent the same phase transformations during annealing, capped contacts had significantly better contact resistances (a minimum value of 4×10-7 Ω cm² was achieved) – almost three orders of magnitude better. The superior performance is attributed to the capping layers providing protection for the Pd–Ge layers during contact processing, where the metallization was exposed to a CF₄–O₂ plasma, oxyen descumming, organic solvents and deionized water. Non-capped contacts exhibited PdGe decomposition and oxidation of exposed Ge. Long-term reliability testing of capped contacts showed virtually no change in contact resistance at 235°C (1350 h) and a sevenfold increase after ageing at 290°C for 370 h. There were no phase changes during ageing; the increase in contact resistance was attributed to interdiffusion between Ge and GaAs. This revised version was published online in July 2006 with corrections to the Cover Date.     

Grain boundary self-diffusion in evaporated Au films at low temperatures

Direct self-diffusion measurements in vapor-deposited polycrystalline Au films have been made using ¹⁹⁵Au radioactive tracer and an r.f. back-sputtering technique for serial sectioning. A temperature range of 117°–177°C was investigated. It has been demonstrated that self-diffusion in thin Au films at these low temperatures takes place by rapid transport of the tracer atoms along the grain boundaries. The grain boundary self-diffusion parameters are Q_b=1.0±0.1 eV and δD_b⁰ = 9 × 10^?10 cm³/sec, which compare well with those in bulk polycrystalline Au.     

Effects of In-situ and Ex-situ Heat-Treatment Procedures on?the?Transport Properties of the MgB2 Superconducting Thin Films Fabricated by Ultrasonic Spray Pyrolysis (USP) System

Superconducting MgB₂ thin films have been prepared using Ultrasonic Spray Pyrolysis (USP) system. 2.4 MHz USP system and various solutions which contain different Mg, B and de-ionized water and LAPSA concentrations and gas atmospheres were used to obtain 500 nm to 1.0 ??m thick MgB₂ films. Some of the films produced were heat treated in-situ in the spraying chamber during deposition and some of them were annealed ex-situ in the tube furnace under Mg vapor. T _c and T _zero of the samples were obtained to be 39.5 K and 37.4, 39.5 and 37.0 for ex-situ and in-situ prepared films, respectively. The highest critical current density was obtained to be 4.12×10⁶ A?cm^?2 for ex-situ annealed films and 4.01×10⁶ A?cm^?2 for in-situ produced films. The result obtained indicated that the ex-situ preparation method provides improvement in the transport and magnetic properties.     

Low-Mass WIMP Sensitivity and Statistical Discrimination of Electron and Nuclear Recoils by Varying Luke-Neganov Phonon Gain in Semiconductor Detectors

Amplifying the phonon signal in a semiconductor dark matter detector can be accomplished by operating at high voltage bias and converting the electrostatic potential energy into Luke-Neganov phonons. This amplification method has been validated at up to |E|=40 V/cm without producing leakage in CDMS?II Ge detectors, allowing sensitivity to a benchmark WIMP with mass M _χ =8 GeV/c² and σ=1.8×10^?42 cm² (with significant sensitivity for M _χ >2 GeV/c²) assuming flat electronic recoil backgrounds near threshold. Furthermore, for the first time we show that differences in Luke-Neganov gain for nuclear and electronic recoils can be used to discriminate statistically between low-energy background and a hypothetical WIMP signal by operating at two distinct voltage biases. Specifically, 99% of events have p-value <10^?8 for a simulated 20?kg-day experiment with a benchmark WIMP signal with M _χ =8 GeV/c² and σ=3.3×10^?41 cm².     

Diffusion kinetics of Au through Pt films about 2000 and 6000 Å thick studied with Auger spectroscopy

The diffusion kinetics of Au through Pt films were studied because of the frequent use of PtAu metallizations in semiconductor technology and because Au is often undesirable in active semiconductor regions etc. that are in contact with the Pt. PtAu couples with approximately 2000 Å and 6000 Å Pt films were heat treated between 250° and 350°C in 1 atm N₂ ambient. Surface composition and depth profiles were measured using Auger spectroscopy and ion milling. Au was found to diffuse initially through thin Pt films (< 6000 Å) by grain boundary migration and more than 10¹⁵ atoms cm^?2 of Au crossed the Pt film when the bulk of the Pt contained very little Au (?1 at.%). For 2250 Å Pt films on Au, the time t(X = 0.5) for half-saturation of the Pt surface with Au was given by t(0.5) = 1.2 X 10^?7X exp (0.96 eV/kT) min, where X is the fractional Au concentration in the first 7 Å of the surface. This diffusion rate is relatively fast; e.g.t(0.5)≈7 min for a 2250 Å Pt film at 350°C. At t(0.5) the bulk of the Pt contained less than 1 at.% Au and t(0.5) was proportional to Pt film thickness near 2000 to 6000 Å.     

Structural and electrical properties of V-doped ZnO prepared by the solid state reaction

This paper reports the synthesis, crystal structure and electrical conductivity properties of vanadium (V)-doped zinc oxide (ZnO) powders (i.e. Zn_1?2X V _X O binary system, x = 0, 0.0025, 0.005, 0.0075 and in the range 0.01 ≤ x ≤ 0.15). I-phase samples, which were indexed as single phase with a hexagonal (wurtzite) structure in the V-doped ZnO binary system, were determined by X-ray diffraction (XRD). The limit solubility of V in the ZnO lattice at this temperature is 3 mol % at 950 °C. The impurity phase at 950 °C was determined as ZnV₂O₆ when compared with standart XRD data. The research focused on single I-phase ZnO samples which were synthesized at 950 °C because of the limit of the solubility range is widest at this temperature. It was observed that the lattice parameters a and c decreased with V doping concentration. The electrical conductivity of the pure ZnO and single I-phase samples were studied using the four-point probe dc method at temperatures between 100 and 950 °C in an air atmosphere. The electrical conductivity values of pure ZnO and 3 mol % V-doped ZnO samples at 100 °C were 2.75 × 10^?6 and 7.94 × 10^?5 Ω^?1 cm^?1, and at 950 °C they were 3.4 and 54.95 Ω^?1 cm^?1, respectively. In other words, the electrical conductivity increased with V doping concentration.     

Preface

The reliability of GaAs microwave devices is directly related to the integrity of Schottky and ohmic contacts for Schottky barrier devices and metal/semiconductor field effect transistor devices. The interface analysis of these device structures using surface analysis techniques has become extremely important in the study of the degradation of these devices. The research reported here focuses on three different metallic systems, namely Au/In, Au-12 wt.% Ge and Ni/AuGe, for both Schottky and ohmic contacts. The three metallic systems were evaporated onto 〈100;〉- oriented GaAs substrates (N_D=3×10¹⁷cm^-3) in an ultrahigh vacuum system. These samples were thermally aged by keeping them at 150°C for 500h. Current-voltage and capacitance-voltage measurements were made on as-deposited and thermally aged samples. The ideality factor decreased in all the samples. There was an apparent large increase in barrier height in AuGe/GaAs and Ni/AuGe/GaAs Schottky diodes. There was an insignificant change in the contact resistivity of ohmic contacts after thermal aging.The changes in the electrical characteristics of these device structures are explained on the basis of the formation of an oxide layer after thermal aging. A comparison of the Auger depth profiles of the as-deposited and the thermally aged samples substantiates the electrical observations. However, Au/In/GaAs Schottky diodes do not show the existence of an oxide layer at the interface. The out-diffusion of indium to the surface might have removed the oxygen from the interface to result in an Au-GaAs interface in the thermally aged sample. A slight increase in the barrier height of this sample is due to the Au-GaAs interface rather than the In-GaAs interface.     

Beryllium diffusion and influence on the luminescent and electrical properties of indium phosphide

The diffusion of beryllium in indium phosphide (InP) has been studied. The temperature dependence of the diffusion coefficient can be described by the formula D = 6.3 × 10^?5exp(?1.4/kT) [cm²/s], which yields D = 1.07 × 10^?11 cm²/s at T = 768°C. The results of measurements of the luminescence and electrical properties show that beryllium is a shallow acceptor with an activation energy of 0.03 ± 0.005 meV.     

Electrical and microstructural investigation of Au/Pd/ri ohmic contacts for AIGaAs/GaAs heterojunction bipolar transistors

Abstract

The microstructure and electrical properties of as deposited and annealed Au (400 nm)/Pd (75 nm)/Ti (10 nm) contact structures to p type GaAs, C doped with a concentration of 5 × 10¹⁸ and 5 × 10¹⁹ cm^?3, have been investigated using transmission electron microscopy, and current-voltage measurements as afunction of temperature in the range 198–348 K. The specific contact resistivities have also been measured using the transmission line method. It was found that increasing the epilayer doping level by an order of magnitude, from 5 × 10¹⁸ to 5 × 10¹⁹ cm^?3, caused the dominant current transport mechanism to change from thermionic field emission to field emission. For the lower level doped epilayers generationrecombination within the depletion region was found to be the dominant current transport mechanism for temperatures below 289 K. The contacts to the more highly doped epilayers (C doped, 5 × 10¹⁹ cm^?3) had specific contact resistivities of 0·08 ± 0·03 Ωmm and 0·05 ± 0·06 Ωmm, respectively. These values, together with a minimal metal penetration in the semiconductor of <15 nm, indicate that these contacts are suitable for heterojunction bipolar device applications.

MST/3325     

Superconducting and metallurgical properties of bronze processed Nb3Sn wires with Ni and Zn additions and comparison with Ta and Ti additions

Critical current density measurements up to 23 T of nineteen core Nb₃Sn wires with simultaneous addition of Ni to the core and Zn to the Cu-Sn bronze matrix have revealed a considerable increase of J_c at fields above 11 T. For a wire with the composition Nb-0.6 wt% Ni/Cu-10 wt% Sn-3 wt% Zn, reacted at 750°C for 64 h, J_c in the layer was determined to 1.3 × 10⁵A cm^?2 at 14 T and to 4 × 10⁴A cm^?2 at 19 T. Comparison with Ta and Ti core-alloyed Nb₃Sn wires, also performed in the present study, shows very similar results in J_c and J_c vs. ε up to the highest fields.Composition profiles for Sn and for Ni, Ta and Ti additives in the A15 layers were studied by Auger spectroscopy. For Sn a concentration gradient across the layer (from ≈ 25 at% to ≈ 22 at% Sn) was observed, with the highest Sn content occurring at the interface with the bronze. The presence of the additives in the layer was detected as well by Auger analysis as by X-ray diffractometry.     

Preparation of rare earth CeO<Subscript>2</Subscript> thin films using metal organic decomposition method for metal-oxide–semiconductor capacitors

Investigation of metal organic decomposed rare earth cerium oxide thin films deposited on Si substrate by sol–gel spin coating technique was carried out. The structural properties have been examined by using XRD, Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The XRD confirms the cubic phase of CeO₂ thin films with (111) plane observed at 28.54°. The FTIR and EDAX spectra confirm the formation of CeO₂ films with atomic percentage of 19.39 and 54.82% of Ce and O₂, respectively. Thickness of 60.11 nm of CeO₂ film measured by cross sectional FESEM image, the average roughness of ~0.6 nm of 400?°C annealed CeO₂ films were observed from AFM micrograph. The MOS capacitors were fabricated by using Ti/Au bilayer metal contact depositing by E-beam evaporator on CeO₂/Si thin film for electrical measurements. Capacitance and conductance voltage measurement was carried out to determine the effective oxide charges (Q_eff), interface trap density (D_it) and dielectric constant (k) and are 2.48?×?10¹² cm^?2, 1.26?×?10¹² eV^?1cm^?2 and ~39, respectively. The effective metal work function of 5.68 for Ti/Au bilayer is observed to be higher than the work function of Ti or Au metals in vacuum.     

CuTa2O6 - crystal growth and characterization

The compound CuTa₂O₆ has been prepared as crystals from a Cu/O melt and found to be tetragonal ($\text{a}\text{= 7.510}\text{A}\text{?}$, $\text{c}\text{= 7.526}\text{A}\text{?}$) rather than cubic as reported in the literature. The coefficient of thermal expansion between room temperature and 1000°C was found to be 8.0 × 10^?6°C^?1. Electrical resistivity measurements on a crystal showed semiconductor behavior between room temperature ($\text{? = 2 × 10}{}^3\text{Ω}\text{cm}$) and 140°K ($\text{? = 7 × 10}{}^6\text{Ω}\text{cm}$) with an activation energy of E_A = 0.2 eV. Magnetic measurements between 4.2°K and room temperature showed Curie-Weiss behavior with a change in μeff at 120°K. For T>120°K, μeff = 1.76μ_B and θ_p = 0°K while for T<120°K μeff = 1.91 μB and θ_p = ?15°K.     

Effect of doping with transition and rare-earth metals on the electrical and optical properties of AgGaGe<Subscript>3</Subscript>Se<Subscript>8</Subscript> single crystals

We have studied the electrical, optical, and thermoelectric properties of AgGaGe₃Se₈ single crystals grown by the Bridgman-Stockbarger method and doped with transition and rare-earth metals during growth. AgGaGe₃Se₈ is an anisotropic p-type semiconductor with a band gap of ～2.25 eV (T ～ 290 K), which decreases slightly, to ～2.20 eV, on doping with Nd, Gd, and Er (N ～ 5 × 10¹⁸ cm^?3). The high concentration of stoichiometric silver vacancies (N ～ 1.8 × 10²⁰ cm^?3) and random distribution of the Ga and Ge atoms over the cation sites give rise to static disorder, so that the structure of AgGaGe₃Se₈ approaches that of disordered systems. This leads to the formation of electronic states in the band gap, intrinsic edge broadening, and, as a consequence, a drop in transmission. Depending on the concentration and nature of the dopant, doping of AgGaGe₃Se₈ may lead to structural ordering and bleaching or may increase the destructive factor at high doping levels (N > 10¹⁸ cm^?3). ⁶⁰Co gamma irradiation to a dose of 1.2–1.3 kGy reduces the absorption coefficient of AgGaGe₃Se₈ crystals doped with high concentrations of Mn and Cu (N > 5 × 10¹⁸ cm^?3). A model is proposed which explains the observed effects.