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.     

The effect of Cr barrier on interfacial reaction of Au/Zn/Au/Cr/Au contacts to p-type InGaAs/InP

Alloy-type metal is widely used to reduce contact resistance in optoelectronic devices. Among the alloy-types, Au/Zn is one of the most common metallization systems. In this paper, we studied the alloy morphology of p-InP/p-InGaAs/Au/Zn/Au/Cr/Au systems. We found that the amount of Au-Zn alloy depended upon the thickness of the Cr layer. When Cr thickness was reduced to 135 Å, both Au-rich and GaAs-rich excessive compound formation started to occur. The Au diffusion punched through the InGaAs layer and penetrated into the InP. Comparison of Au/Zn/Au and Au/Zn/Au/Cr/Au suggested that the top Au layer maybe very influential during the alloy reaction. The Au-Zn alloy was significantly less in the Au/Zn/Au than that in the Au/Zn/Au/Cr/Au.     

Unit cell parameters of wurtzite InP nanowires determined by x-ray diffraction

High resolution x-ray diffraction is used to study the structural properties of the wurtzite polytype of InP nanowires. Wurtzite InP nanowires are grown by metal-organic vapor phase epitaxy using S-doping. From the evaluation of the Bragg peak position we determine the lattice parameters of the wurtzite InP nanowires. The unit cell dimensions are found to differ from the ones expected from geometric conversion of the cubic bulk InP lattice constant. The atomic distances along the c direction are increased whereas the atomic spacing in the a direction is reduced in comparison to the corresponding distances in the zinc-blende phase. Using core/shell nanowires with a thin core and thick nominally intrinsic shells we are able to determine the lattice parameters of wurtzite InP with a negligible influence of the S-doping due to the much larger volume in the shell. The determined material properties will enable the ab initio calculation of electronic and optical properties of wurtzite InP nanowires.     

Wettability of SiC and alumina particles by liquid Bi under liquid Al

In this paper a new experimental method to measure contact angle of liquid metals on solid particles under an immiscible second liquid metal is described. This includes efficient mixing of the three-phase system (two immiscible liquid metals with solid dispersed particles) with subsequent solidification. The micrographs of the cross sections of samples are analyzed for the distribution of the particles between the bulk of the two metals and the interface between them. As an alternative, exact positions of the particles can be measured at the interface between the two solidified metals. These two measurements are used to estimate the contact angle of one liquid metal on the solid particles under another, immiscible liquid metal. As an example, contact angle of a liquid Bi-rich alloy on SiC and alumina particles is measured under liquid Al-rich alloy. The liquid Bi-rich alloy was found to wet perfectly (with zero contact angle) alumina particles under liquid Al-rich alloy, while the contact angle on SiC is found to be around 88°. These contact angle data are shown to be relevant to control the stability of liquid metallic emulsions formed between the two immiscible liquid metals stabilized by the solid particles.     

Study by AES and EELS spectroscopies of antimony and phosphorus evaporated on massive indium and on cleaned InP

In this paper, we give some results related to interaction mechanism between the elements V such as antimony or phosphorus with the metal indium. We used both powerful spectroscopy methods the Auger electron spectroscopy (AES) and the electron energy loss spectroscopy (EELS) for which the spectra were recorded in direct mode N(E). The antimony was evaporated on pure In metal or on cleaned InP surface involving the In metal because of its cleaning by the argon ion bombardment at low energy 300 eV. The antimony flow composed of Sb₄ species arrived with a thermal energy on the In metal surface. Such an energy was sufficient to their diffusion into the In matrix because of the low melting point of In metal (123 °C). A nucleation phenomenon occurred between Sb₄ and the In metal to form small islands of antimony metal in bulk. Further antimony evaporation enabled to increase the size of these islands towards the surface. However, the antimony evaporated on cleaned InP reacted chemically with the In metal distributed on the InP surface to form a thin layer of InSb. The inner stoichiometric layers of InP and the size of Sb₄ species and also the stability of InP versus the temperature impeded the interdiffusion phenomenon of antimony to occur deeply into the InP matrix. The InSb layer played the role to stabilise the surface of the InP compound versus the heating at 450 °C and the electron irradiation of 4 KeV energy. But, the phosphorus evaporation on In metal or on cleaned InP led to form chemical bonds InP. The phosphorus flow included chemical species P and P₂ with a thermal energy able to stimulate the chemical reactivity process between indium and phosphorus to form the InP compound.     

Growth of InAs/InP core-shell nanowires with various pure crystal structures

We have studied the epitaxial growth of an InP shell on various pure InAs core nanowire crystal structures by metal-organic vapor phase epitaxy. The InP shell is grown on wurtzite (WZ), zinc-blende (ZB), and {111}- and {110}-type faceted ZB twin-plane superlattice (TSL) structures by tuning the InP shell growth parameters and controlling the shell thickness. The growth results, particularly on the WZ nanowires, show that homogeneous InP shell growth is promoted at relatively high temperatures (～500?°C), but that the InAs nanowires decompose under the applied conditions. In order to protect the InAs core nanowires from decomposition, a short protective InP segment is first grown axially at lower temperatures (420-460?°C), before commencing the radial growth at a higher temperature. Further studies revealed that the InP radial growth rate is significantly higher on the ZB and TSL nanowires compared to WZ counterparts, and shows a strong anisotropy in polar directions. As a result, thin shells were obtained during low temperature InP growth on ZB structures, while a higher temperature was used to obtain uniform thick shells. In addition, a schematic growth model is suggested to explain the basic processes occurring during the shell growth on the TSL crystal structures.     

Interfacial reactions between palladium thin films and InP

Palladium appears to be an important component in ohmic contact metallizations to III–V semiconductors. Very little is known about its interaction with InP. Consequently, the reaction between a thin layer of Pd (100 nm) and an InP substrate has been studied at annealing temperatures ranging from 250–450 °C for up to 30 sec, i.e., typical annealing conditions encountered during contact fabrication. Palladium reacts readily with InP, initially forming an amorphous ternary phase, which transforms to crystalline Pd₂InP on annealing. Pd₂InP has an ordered cubic structure, with a lattice parameter of 0.830 nm, and grows epitaxially on InP. Microtwins, 2–3 atomic layers thick, have been identified in the ternary phase and these form along the (110) and ( 10) planes.     

First-principles study of the electronic properties of wurtzite, zinc-blende, and twinned InP nanowires

The electronic properties of zinc-blende, wurtzite, and rotationally twinned InP nanowires were studied using first-principles calculations. The results show that all the simulated nanowires exhibit a semiconducting character, and the band gap decreases with increasing the nanowire size. The band gap difference between the zinc-blende, wurtzite, and twinned InP nanowires and bulk InP can be described by ΔE(g)(wire) = 0.88/D(1.23), ΔE(g)(wire) = 0.79/D(1.22) and ΔE(g)(twin) = 1.3/D(1.19), respectively, where D is the diameter of the nanowires. The valence band maximum (VBM) and conduction band minimum (CBM) originate mainly from the p-orbitals of the P atoms and s-orbitals of the In atoms at the core regions of the nanowires, respectively. The hexagonal (2H) stacking inside the cubic (3C) stacking has no effect on the electronic properties of thin InP nanowires.     

Threshold Fluence Measurement for Laser Liftoff of InP Thin Films by Selective Absorption

We explore conditions for achieving laser liftoff in epitaxially grown heterojunctions, in which single crystal thin films can be separated from their growth substrates using a selectively absorbing buried intermediate layer. Because this highly non‐linear process is subject to a variety of process instabilities, it is essential to accurately characterize the parameters resulting in liftoff. Here, we present an InP/InGaAs/InP heterojunction as a model system for such characterization. We show separation of InP thin films from single crystal InP growth substrates, wherein a ≈10 ns, Nd:YAG laser pulse selectively heats a coherently strained, buried InGaAs layer. We develop a technique to measure liftoff threshold fluences within an inhomogeneous laser spatial profile, and apply this technique to determine threshold fluences of the order 0.5 J cm^?2 for our specimens. We find that the fluence at the InGaAs layer is limited by non‐linear absorption and InP surface damage at high powers, and measure the energy transmission in an InP substrate from 0 to 8 J cm^?2. Characterization of the ejected thin films shows crack‐free, single crystal InP. Finally, we present evidence that the hot InGaAs initiates a liquid phase front that travels into the InP substrate during liftoff.

     

The effect of electron beam sterilization on the physical properties of the bioresorbable polymer coatings on the titanium 6-aluminum 4-vanadium substrate

The aim of the work is to determine the physical properties of titanium 6-aluminum 4-vanadium alloy with poly (glycolide-ϵ-caprolactone) coating after electron beam sterilization. First, the metal substrate is machined with grade 120 and 320 grinding papers. Some of the samples are subjected to anodic oxidation. Then, the samples are coated with a biodegradable polymer layer of poly (glycolide-ϵ-caprolactone). Samples with polymer coatings are subjected to electron beam sterilization. To evaluate the effect of sterilization on physical properties of modified titanium alloy the scanning electron microscopy and atomic force microscopy, adhesion studies of the polymer coating to the metal substrate and wettability tests are applied. On the basis of the obtained results, an increase of the contact angle value is found both after applying the polymer coating to the surface of the tested titanium 6-aluminium 4-vanadium alloy as well as after electron beam sterilization. In addition, a slight increase of the adhesion in sterilized samples comparted to non-sterilized is observed. In scanning electron microscopic observations, traces of machining on the surface of the metal substrate and the continuity of the polymer coatings before and after sterilization are found. In the atomic force microscopic studies in relation to the initial state, a very good mapping of the surface topography of the samples with a homogeneous coating is found.     

Quantum point contact microscopy

We introduce quantum point contact microscopy (QPCM) as a novel method for surface characterization, where the conductance through a quantum point contact formed by a metal atom between the tip of a scanning tunneling microscope and the surface is mapped across the surface. Application of QPCM to copper and gold (111) shows reproducibly atomic resolution, on gold (111) the alternating atomic stacking of the surface reconstruction is observed in real space. The perspectives for chemical sensitivity in QPCM images are demonstrated for an iron-platinum surface alloy where we observe local variations of the transport current due to changes in the chemical environment of the point contact.     

Flux-assisted infiltration of liquid Al-6063 into TiC beds in air

This study details trials to produce aluminium metal matrix composites reinforced with TiC particles by means of a flux-assisted infiltration technique. Whilst no infiltration of TiC beds occurred, by using a K-Al-F flux infiltration was successful at temperatures as low as 680°C. Some reaction of TiC with the Al matrix, forming TiAl_2.3Si_0.1 and Al₄C₃, was observed in the microstructure along with flux trapped within the Al-6063 matrix. DSC showed exothermic oxidation of TiC to occur, until the flux melts at 545°C arresting and preventing further oxidation by spreading over, coating and cleaning the particle surfaces. As soon as the flux melts, it also starts dissolving the oxide layer on the Al alloy and prevents any re-oxidation by isolating the surface from the surrounding atmosphere. Sessile drop experiments suggest that when the alloy melts and the oxide layer has been dissolved by the flux, intimate contact occurs between the liquid and the particles. The low tensions for the solid/flux and liquid metal/flux interfaces facilitates spreading and wetting of liquid Al on the TiC particles, followed by infiltration of the bed and the displacement of the flux to the outer surfaces of the sample.     

镀锌钢的液态金属脆现象及其在电阻点焊过程中的表现

液态金属脆是指通常具有韧性的固体金属或者合金与液态金属直接接触且受到拉伸应力时,其塑性降低并发生脆性断裂的现象。钢在液态锌中会发生液态金属脆现象,这在镀锌钢的热拉伸实验中得到了证实。此外,研究人员发现在镀锌高强钢的电阻点焊过程中也会出现液态金属脆现象,表现为在焊点表面出现大量裂纹,这些裂纹对焊点性能存在潜在危害。本文回顾了镀锌钢液态金属脆现象的热拉伸实验研究,阐明了影响脆化现象的实验因素;综述了镀锌钢在电阻点焊过程中发生液态金属脆现象的研究进展,分析了产生裂纹的位置及其影响因素,并总结了可能的解决方案。     

Model of intensive inter-phase interaction between high temperature melt and cold volatile fluid

The fragmentation of the molten metal drop in the cold volatile liquid was studied. Two cases of water surface temperature were investigated: a—lower than its critical meaning during direct contact with melt, and b—higher than the critical one. The criterion determining existence of the fragmentation was presented for the first case. In the second case, that is being more complicated, possibility of the direct contact between the surrounding liquid and the melted metal surface and development of the instability on both surfaces were studied. It was shown that the fragmentation mechanism for the second case is strongly dependent on the characteristic time of the direct water-melt contact. The characteristic time must be sufficient enough for liquid water to interact with the melt before generation of the vapor layer between two surfaces. The process that follows the contact of water with the melted metal and a high pressure region formation was considered. The fragmentation mechanism, based on the analogy with the known problem, when a body with a flat circular nose impacts upon a flat liquid surface, was presented. Mean velocity and mean width of the generated jets from the melt surface were calculated.     

An investigation of Au/Mn contacts to p-type InP

The performance of a Au/Mn contact metallization to p-type InP has been reported. Electrical resistance measurements done on annealed contacts have been correlated to the accompanying microstructural changes, by means of electron microscopy and X-ray diffraction techniques. Manganese was found to react readily with the underlying InP, leading to the formation of Mn₂P followed by MnP. Subsequent outward diffusion of indium towards the gold layer led to the formation of Au₃In, which replaced the original gold layer. Inward diffusion of gold resulted in the formation of an Au-In-Mn ternary phase at the MnP-InP interface. This phase may have supplied the necessary manganese for InP doping required to lower the contact resistance. A minimum resistance of 6 x 10^-4Ωcm² was obtained.     

Study and improvement of interfacial properties in a MIS structure based on p-type InP

Indium phosphide is one of the most promising candidates among the available III-V semiconducting compounds for the development of MIS technology. This is based on the availability of InP substrates and the relatively large band gap. Before the deposition of the insulator, the InP surface must be treated and well passivated (Surf Interface Anal 20 (1993) 803; J Appl Phys 67 (1990) 4173). We have shown that a InSb buffer layer can reduce the phosphorus atom migration and the concentration of defects at the interface. We have studied and characterized electrically two series of substrates using p-type InP, the first one with thin and the second with thick insulator films. The results obtained show clearly the reduction of the defects in the thicker structures protected by the InSb buffer layer.     

Joining of Aluminium Metal Matrix Composites

Aluminium metal matrix composites (AlMMCs) offer several advantages relative to monolithic aluminium alloys such as high stiffness, strength, wear resistance, low thermal expansion coefficient, etc. However, despite considerable improvements in developing AlMMCs, the lack of reliable joining methods restrict their greater application. Fusion welding of AlMMCs has not proved successful because high temperature nature of the process normally causes unfavorable reactions between the reinforcement and the matrix, leading to the formation of a variety of defects. On the other hand, solid-state welding and diffusion bonding may not be suitable due to the presence of chemically stable surface layer of aluminium oxide, which, being insoluble in aluminium, inhibits metal-to-metal contact during diffusion bonding. Furthermore, diffusion bonding requires a very smooth and clean contact surface, which is difficult to obtain in industrial applications. As an alternative, transient liquid phase (TLP) diffusion bonding, which operates at a lower temperature, can be used to circumvent the problems associated with the oxide layer. The formation of liquid phase (eutectic) can assist the disruption of the oxide layer and promote metallic contact. The composite material used in the present study consisted of 6061 alloy containing 15 volume % of SiC particulates of 23 μm diameter. TLP bonding was carried out at 560 ˚C in argon atmosphere using copper as an interlayer with different pressures and holding times. TLP-bonded AlMMCs were characterized by optical and scanning electron microscopy, microhardness survey, and shear tests. The results indicated that adequate bond strength could be achieved with suitable bonding parameters such as holding time and initial pressure.     

Structure and composition of aluminum oxide films in contact with the liquid In-Ga eutectic

The oxide layers on the surface of aluminum metal and the aluminum alloys AD-1 and A-5 have been studied by attenuated total reflection infrared spectroscopy. The Al-O and Al-OH surface vibrational modes and A-O modes of the AlO₄ and AlO₆ groups have been identified. The structural inhomogeneity of the surface oxide layers is shown to contribute to their disruption when the sample is brought into contact with the liquid In-Ga eutectic. In situ scanning electron microscopy and X-ray microanalysis have been used to follow the dynamics of oxide layer disruption and the morphological and compositional changes in the eutectic alloy and the surface layer of aluminum.     

1.54-microm TM-mode waveguide optical isolator based on the nonreciprocal-loss phenomenon: device design to reduce insertion loss

We developed a 1.5-microm band TM-mode waveguide optical isolator that makes use of the nonreciprocal-loss phenomenon. The device was designed to operate in a single mode and consists of an InGaAlAs/InP ridge-waveguide optical amplifier covered with a ferromagnetic MnAs layer. The combination of the optical waveguide and the magnetized ferromagnetic metal layer produces a magneto-optic effect called the nonreciprocal-loss phenomenon--a phenomenon in which the propagation loss of light is larger in backward propagation than it is in forward propagation. We propose the guiding design principle for the structure of the device and determine the optimized structure with the aid of electromagnetic simulation using the finite-difference method. On the basis of the results, we fabricated a prototype device and evaluated its operation. The device showed an isolation ratio of 7.2 dB/mm at a wavelength from 1.53 to 1.55 microm. Our waveguide isolator can be monolithically integrated with other waveguide-based optical devices on an InP substrate.     

Effects of heat treatment at 350°C on the adhesion of Cu-Cr alloy films to polyimide

CuCr alloys with varying Cr content were sputter deposited on polyimide films, and the metal/polyimide films were maintained under 350°C/N₂ environment up to 10 hours for the reliability measurements. The Cr contents of the alloy layer prepared were 0, 8.5, 17, 25, 34, and 100 atomic %, respectively. Before exposures to 350°C, the peel strength increased proportionally with the Cr content in the alloy layer up to 17 atomic % and saturated. The failure occurred inside polyimide near the metal/polyimide interface by the cohesive failure mode except for the specimen with no Cr. After exposures to 350°C, the peel strength dropped for all the specimens, but most drastically for the specimens with 8.5% Cr which failed along the Cr-oxide/polyimide interface by the interfacial failure mode. Through AES and XPS analyses, it was shown that the decrease of the peel strength during the heat treatment was primarily caused by the formation of brittle Cr₂O₃ at the metal/polyimide interface, which was accompanied by the reduction of carbidic bonds responsible for the good adhesion.