The effect of oxygen in WN x films on thermal stability of WN x /GaAs interfaces

The investigation of thermal stability of WN _x /GaAs Schottky contacts prepared by selective ion implantation of nitrogen into tungsten film has been demonstrated. Auger electron spectroscopy was used for characterization of thermal stability of the interfaces after rapid thermal annealing at 950 C for 10s. Significant oxygen influence on migration of Ga atoms from the substrate into metallization systems was observed.     

Rapid thermal annealing of Cu/WN x /Si structures

The thermal stability of Cu/WN _x /Si multilayer structures fabricated by using reactive d.c. magnetron sputtering was studied using rapid thermal annealing (RTA) and several analytic methods. The analytic methods included four-point resistance measurements, X-ray diffraction analysis, Rutherford backscattering spectrometry, and optical and scanning electron microscopy. The barrier performance of WN _x -layers is compared to the performance of sputtered W layers. It is shown that a 15-nm thick tungsten nitride (WN _x ) layer prevents the destructive reactions up to 700 °C for a 12-s RTA process in a nitrogen atmosphere.     

Molybdenum silicide schottky contacts to GaAs

The thermal stability of MoSi _x Schottky contacts to GaAs has been studied. The contacts were analysed by Rutherford backscattering spectroscopy (RBS), X-ray diffraction (XRD), sheet resistance, and current-voltage measurements. We have studied MoSi _x with the compositionsx=0, 0.3, 0.6, 1, and 2. The films were annealed at various temperatures up to 850°C. The thermal stability of the films is found to bex-dependent. The best composition for thermally stable Schottky contacts is Mo₅Si₃. Good Schottky characteristics were retained with annealing temperatures up to 850°C for 30 min.     

Development of refractory ohmic contact materials for gallium arsenide compound semiconductors

Recent strong demands for optoelectronic communication and portable telephones have encouraged engineers to develop optoelectronic devices, microwave devices, and high-speed devices using hetero-structural GaAs-based compound semiconductors. Although the GaAs crystal growth techniques had reached a level to control the compositional stoichiometry and crystal defects on a nearly atomic scale by the advanced techniques such as molecular beam epitaxy and metal organic chemical vapor deposition techniques, development of ohmic contact materials (which play a key role to inject external electric current from the metals to the semiconductors) was still on a trial-and-error basis.Our research efforts have been focused to develop low resistance, refractory ohmic contact materials to n-type GaAs using the deposition and annealing techniques, and it was found the growth of homo-or hetero-epitaxial intermediate semiconductor layers (ISL) on the GaAs surface was essential for the low resistance ohmic contact formation. In this paper, two typical examples of ohmic contact materials developed by forming ISL were given. The one was refractory NiGe-based ohmic contact material, which was developed by forming the homo-epitaxial ISL doped heavily with donors. This heavily doped ISL was discovered to be formed through the regrowth mechanism of GaAs layers at the NiGe/GaAs interfaces during annealing at elevated temperatures. To reduce the contact resistance further down to a value required by the device designers, an addition of small amounts of third elements to NiGe, which have strong binding energy with Ga, was found to be essential. These third elements contributed to increase the carrier concentration in ISL. The low resistance ohmic contact materials developed by forming homo-epitaxial ISL were Ni/M/Ge where a slash ‘/’ denotes the deposition sequence and M is an extremely thin (∼5 nm) layer of Au, Ag, Pd, Pt or In. The other was refractory In_xGa_1−xAs-based ohmic contact materials which were developed by forming the hetero-epitaxial ISL with low Schottky barrier to the contacting metals by growing the In_xGa_1−xAs layers on the GaAs substrate by sputter-depositing In_xGa_1−xAs targets and subsequently annealing at elevated temperatures. To reduce the contact resistance, it was found that this In_xGa_1−xAs (ISL) layer had to have In compositional gradient normal to the GaAs surface: the In concentration being rich at the metal/In_xGa_1−xAs interface and poor close to the In_xGa_1−xAs/GaAs interface. This concentration graded ISL reduced both the barrier heights at the metal/ISL and ISL/GaAs interfaces and reduced the contact resistance. The ohmic contact materials developed by forming hetero-epitaxial ISL was In_0.7Ga_0.3As/Ni/WN₂/W. These contact materials formed refractory compounds at the interfaces, which was also found to be essential to improve thermal stability of ohmic contacts used in the GaAs devices.     

Solid-State Reactions during Thermal Oxidation of Vanadium-Modified GaAs Surfaces

It is shown that vanadium layers deposited by magnetron sputtering onto GaAs accelerate surface oxide growth during thermal oxidation. The processes occurring during thermal oxidation on the surface and at the semiconductor–metal interface are interpreted under the assumption that vanadium deposition leads to the formation of an interfacial V _x Ga _y As _z layer, which plays a key role in determining the mechanism of V/GaAs oxidation.     

Resistive switching phenomena of tungsten nitride thin films with excellent CMOS compatibility

We report the resistive switching (RS) characteristics of tungsten nitride (WN_x) thin films with excellent complementary metal-oxide-semiconductor (CMOS) compatibility. A Ti/WN_x/Pt memory cell clearly shows bipolar RS behaviors at a low voltage of approximately ±2.2 V. The dominant conduction mechanisms at low and high resistance states were verified by Ohmic behavior and trap-controlled space-charge-limited conduction, respectively. A conducting filament model by a redox reaction explains the RS behavior in WN_x films. We also demonstrate the memory characteristics during pulse operation, including a high endurance over >10⁵ cycles and a long retention time of >10⁵ s.     

The synthesis and properties of epitaxial layers of (Si<Subscript>2</Subscript>)<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>(GaAs)<Subscript>x</Subscript> solid solutions on silicon substrates

Epitaxial layers of (Si₂)_1−x (GaAs)_x (0≤x≤0.85) solid solutions were grown by liquid phase epitaxy in a temperature interval from 850 to 700°C from a tin-based solution melt confined between two horizontal polycrystalline silicon substrates. The dependence of the quality and parameters of the epilayers on the process conditions have been studied. The obtained Si-(Si₂)_1−x (GaAs)_x structures can be used in various microelectronic and photoconversion devices, replacing analogous but more expensive GaAs-based structures. Experimental data on the electrical properties and photoelectric characteristics of the Si-(Si₂)_1−x (GaAs)_x structures are presented.     

Lattice mismatch and surface morphology studies of In x Ga1−x As epilayers grown on GaAs substrates

In _x Ga_1−x As (0·06≤x≤0·35) epilayers were grown on GaAs substrates by atmospheric pressure metal organic chemical vapour deposition technique. Surface morphology and lattice mismatch in the InGaAs/GaAs films of different compositions were studied. Cross-hatched patterns were observed on the surface of the epilayers for bulk alloy composition up tox≈0·25. Forx>0·3, a rough textured surface morphology was observed.     

The Role of the Thermal Oxide in GaAs-Based Photonic Bandgap Waveguide Microcavities

Recent work on optical microcavities is concisely reviewed in this article. Optical microcavities can be fabricated using III–V compound semiconductors by introducing a large periodic refractive-index variation along the axis of GaAs waveguides. Strong optical confinement in the microcavities is achieved by oxidizing the underlying III–V material and further optimized by preferentially etching away the oxide to form an air-bridge structure. The III–V thermal oxide is obtained from the wet thermal oxidation of high Al-content Al_xGa_1–xAs material. The critical role of the thermal oxide in the design and fabrication of the waveguide microcavities is outlined and the characterization of the properties of the thermal oxide salient to the fabrication of the devices is described. Finally, preliminary transmission data demonstrating the successful realization of these microcavities are presented.     

Effect of composition on the physicochemical properties of nickel aluminium hydrotalcites

Ni-Al hydrotalcite compounds with the structural formula [Ni_1–x Al _x (OH)₂] ^x+ [(CO₃) _x/2 ·mH₂O] ^x– were synthesized in the composition range Al/Ni+Al (x)=0.20–0.50 by sequential precipitation. The samples were characterized by X-ray diffraction, infrared absorption, thermogravimetry-differential scanning calorimetry-evolved gas analysis, transmission electron microscopy and BET surface-area measurements. The crystallinity and the lattice parameters of the samples are significantly affected with respect to composition. Hydrothermal treatments performed on the aged samples enhanced the crystallinity. The nitrate ion is also present along with carbonate ion as the charge-balancing anion in the interlayer space. The thermal stability of these materials increased with decrease in Ni/Al atomic ratio. Thermal calcination of these compounds yielded non-stoichiometric NiO whose crystallinity and thermal stability depends on the composition as well as on calcination temperature.     

Peculiarities of the formation and thermal stability of barrier contacts in high-sensitivity silicon carbide detector diodes

The effect of rapid thermal treatment at T=1000°C on the formation of \textTiB_x - n-\textSiC6H(00\text1^-- ){\text{TiB}}_x - n--{\text{SiC6H(00}}\mathop {\text{1}}\limits^-- ) barrier contacts and Ni-n-SiC6H(0001) ohmic contacts was studied. In the former case, thermal treatment neither disturbs the layer structure nor reduces the thermal stability of the barrier contacts. The rapid annealing of an Ni-n-SiC6H(0001) structure results in the formation of a stable ohmic contact. At the same time, this treatment does not change the parameters of the static current-voltage characteristics of the \textAu - \textTiB_x - n-\textSiC6H(00\text1^-- ){\text{Au}} - {\text{TiB}}_x - n--{\text{SiC6H(00}}\mathop {\text{1}}\limits^-- ) Schottky diodes. These thermally stable diodes are characterized by a sensitivity of ∼3300–3500 mV/mW at an incident radiation power of 10–7 W and are capable of operating at a microwave power of up to 1 W. The dynamic range of a linear portion of the conversion characteristic reaches up to 50 dB.     

Microstructural analysis of Pd/Pt/Au/Pd ohmic contacts to InGaP/GaAs

Microstructural evolution during the annealing of Pd/Pt/Au/Pd p-ohmic contacts (with and without a thin layer of Zn) to InGaP/GaAs HBTs has been studied using transmission electron microscopy (TEM). Metal layers were deposited by electron beam evaporation directly onto the InGaP emitter layer with the intention of consuming the InGaP during annealing to contact the heavily C-doped p-type GaAs (3×10¹⁹ cm⁻³) base layer below. Initial reaction between Pd and Pt and InGaP formed a five-component amorphous layer (Pd, Pt, In, Ga and P), which crystallized to (Pt_xPd_1−x)5(In_yGa_1−y)P (0≤x, y≤1) at the interface between Pt and the amorphous layer. Annealing at temperatures ≥415°C caused complete decomposition of the InGaP and partial decomposition of the GaAs base layer, producing a contact consisting of (Pt_xPd_1−x)5(In_yGa_1−y)P, PtAs₂ and PdGa. The attainment of low contact resistances did not depend on the presence of Zn. Minimum values of 0.10−0.12 Ω mm were achieved after annealing at 415–440°C for contacts both with and without Zn. This revised version was published online in July 2006 with corrections to the Cover Date.     

Novel direct MOCVD growth of In x Ga1−x As and InP metamorphic layers on GaAs substrates

A review of the technique of direct growing high-quality In_xGa_1−xAs or InP buffer layers on GaAs substrates by metal-organic chemical vapor deposition (MOCVD) is given. This low-temperature growth method benefits the improvement of metamorphic device performance. In this work, a simple and novel method of directly deposited thin In_xGa_1−xAs or InP buffer layers (< 1 μm) on GaAs substrates is presented, instead of strained-layer superlattice, two-step, graded or CS (compliant substrates) methods, while maintaining low dislocations, high crystal quality, and uniform and mirror like surfaces. For the direct growth technique of In_xGa_1−xAs on GaAs, we found an excellent quality In_0.54Ga_0.46As buffer of rms surface roughness of only 0.686 nm by AFM and FWHM of 925 arcsec by XRD can be obtained at a low growth temperature of 440^∘C with a constant Ga/In_gas partial pressure of 5. The superior results are mainly due to the use of low temperature growth technology. In addition, we also found this growth technology is available to directly grow InP buffer layers on GaAs. Experimental results conclude that the growth temperature of 480^∘C in harmony with the V/III ratios range of 130–210 is a suitable window to directly grow InP on GaAs substrates.     

Schottky contacts of refractory metal nitrides on gallium nitride using reactive sputtering

Schottky contacts of refractory metal nitrides formed by reactive sputtering on n-type gallium nitride (GaN) were electrically evaluated, including film resistivity, Schottky characteristics and thermal stability. For the metal nitrides of TiN_x, MoN_x and ZrN_x, resistivities of 108.3, 159.0 and 270.0 μΩcm were obtained, respectively. Current-voltage (I-V) characteristics showed that the ideality factor varied from 1.03 to 1.16, while the Schottky barrier height (SBH) varied from 0.66 to 0.79 eV for the three kinds of Schottky contacts. Especially for the ZrN_x contact, the ideality factor and SBH were improved after annealing at 800 °C for 30 s. Schottky contact utilizing a refractory metal nitride on GaN shows its potential to develop thermally stable GaN devices.     

Multivalent substitution in a quasi-binary Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>(II-Mn-IV)<Subscript>x</Subscript>As solid solution

A process of the multivalent substitution of Mn atoms for cations in Ga_1−x Mn_xAs (a base material) is proposed. According to this approach, the two-cation ternary compound with a chalcopyrite structure is used as a component forming a solid solution with GaAs. Based on the successful prediction and growth of high-temperature chalcopyrite ferromagnets, it is expected that the content of Mn in Ga_1−x Mn_xAs can be increased. Quasi-binary (GaAs)_1−x (ZnGeAs₂)_x:Mn and (GaAs_1−x (CdSiAs₂)_x:Mn solid solutions are considered as candidate materials possessing higher Curie temperatures as compared to that of Ga_1−x MnxAs.     

Misfit dislocations and stress in In<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As/GaAs heterostructures

We have estimated the elastic properties of In_{1 − x} Ga _x As/GaAs heterostructures and the characteristics of misfit dislocations in such heterostructures: misfit dislocation spacing, Burgers vector length in various interfaces, surface density of dangling bonds, film/substrate interface energy, critical film thickness below which pseudomorphic growth is possible without misfit dislocations, elastic strain energy of the film-substrate system, average elastic strain of a thin-film island as a function of its radius, thermal stresses induced by the thermal-expansion and lattice mismatches between the layers in contact, and crack length in the film.     

Negative photoinduced current and negative differential characteristics of new optoelectronic sensors with InAs/GaAs nanostructure for visual recognition

Negative photo-induced currents and negative differential characteristics of photoinduced current have been obtained successfully under weak photoexcitations at 300 K for a new optoelectronic sensor including a carrier-storage layer of InAs/GaAs short period superlattice. The phenomena are useful to imitate the selectivity functions in visual recognition, which is based on the difference-of-Gaussian functionlike distribution of light sensitivity in the receptive fields in visual cortex. The phenomena are strongly dependent on the material of the carrier-storage layer, where the photogenerated carriers are separated spatially due to the Schottky forward voltage. The carrier separation is more enhanced as the ratio of electron mobility to hole mobility becomes higher for the material of the carrier-storage layer. This means the In-rich InGaAs material is superior to the Ga-rich one. In addition, compared with In _x Ga_1-x As alloy, the improved surface morphology, the high electron mobility at room temperature and the narrow energy bandgap are observed in the case of InAs/GaAs short period superlattice. That is the InAs/GaAs short period superlattice is superior to In _x Ga_1-x As alloy as the material for the carrier-storage layer.     

Deposition of WNxCy thin films for diffusion barrier application using the dimethylhydrazido (2) tungsten complex (CH3CN)Cl4W(NNMe2)

Tungsten nitride carbide (WN_xC_y) thin films were deposited by chemical vapor deposition using the dimethylhydrazido (2⁻) tungsten complex (CH₃CN)Cl₄W(NNMe₂) (1) in benzonitrile with H₂ as a co-reactant in the temperature range 300 to 700 °C. Films were characterized using X-ray diffraction (XRD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy and four-point probe to determine film crystallinity, composition, atomic bonding, and electrical resistivity, respectively. The lowest temperature at which growth was observed from 1 was 300 °C. For deposition between 300 and 650 °C, AES measurements indicated the presence of W, C, N, and O in the deposited film. The films deposited below 550 °C were amorphous, while those deposited at and above 550 °C were nano-crystalline (average grain size < 70 Å). The films exhibited their lowest resistivity of 840 µΩ-cm for deposition at 300 °C. WN_xC_y films were tested for diffusion barrier quality by sputter coating the film with Cu, annealing the Cu/WN_xC_y/Si stack in vacuum, and performing AES depth profile and XRD measurement to detect evidence of copper diffusion. Films deposited at 350 and 400 °C (50 and 60 nm thickness, respectively) were able to prevent bulk Cu transport after vacuum annealing at 500 °C for 30 min.     

Synthesis and characterization of chromium spinels as potential electrode support materials for intermediate temperature solid oxide fuel cells

Several spinel compositions, i.e. Mn_1+x Cr_2−x O₄ (x = 0.7, 0.5, 0), MnFe _x Cr_2−x O₄ (x = 0.1, 1), MgMnCrO₄ and Mg_1+x Cr_2−x O₄ (x = 0, 0.1) were synthesised and studied in terms of phase analysis, density, stability in reducing atmosphere, electrical conductivity and thermal expansion behaviour. The spinel samples were single phase, with cell parameter values in a good correlation with cation sizes. Most of the studied spinels were found to be unstable under reducing conditions of thermal treatment, except MnCr₂O₄, MgCr₂O₄ and Mg_1.1Cr_1.9O₄. Electrical properties have been investigated by impedance spectroscopy and DC conductivity measurements at temperatures between 200 and 900 °C.     

Crystallization and Glass Transition Kinetics of Na2s-P2s5-Based Super-Ionic Glasses

In the present study, sulfide glass xNa₂S-(100–x)P₂S₅, 35 ≤ x ≤ 55 has been synthesized by the melt quenched technique. All the glass samples are characterized by x-ray diffraction (XRD), differential thermal analysis (DTA), dilatometery, and impedance spectroscopy. The XRD pattern of a sample showing a broad halo indicates its amorphous nature. Dilatometric study demonstrates that for x = 45 and x = 55 mol% Na₂S samples exhibited higher thermal stability than x = 35, x = 40, and x = 50 samples. To understand the crystallization kinetics, DTA was carried out for x = 45 and 55 samples at different heating rates, viz. 10, 20, 30, and 40°C/min. Crystallization kinetics of synthesized glasses for x = 45 and 55 are calculated using Kissinger's model. The thermal expansion coefficient (TEC) of glasses for x = 45 and x = 55 is found to be almost comparable to β-alumina solid electrolyte, which is being used in rechargeable batteries.