The ability to control the resistivity of the wide band gap semiconductor B12As2 by doping with silicon was verified. The electrical properties of nominally undoped and Si-doped rhombohedral B12As2 thin films on semi-insulating 6H-SiC (0001) substrates prepared by chemical vapor deposition were subjected to Hall effect measurements. Varying the Si concentration in the B12As2 thin films from 7×1018to7×1021at.∕cm3 (as measured by secondary ion mass spectrometry) decreased the resistivities of the p-type B12As2 films from 2×105to10Ωcm. The resistivities of the B12As2 films were decreased by one to two orders of magnitude after rapid thermal annealing for 30s in argon. The spatial distribution of the hydrogen concentration was measured before and after annealing. No changes were detected, casting doubt on hydrogen as being the cause for the change in the resistivities of the B12As2 films with annealing.

Tantalum-ruthenium diffusion barriers for contacts to SiC were investigated in this work. Stable specific contact resistances of (2 ± 1) x 10-5 Ω cm2 and (4 ± 2) x 10-5 Ω cm2 were measured on p-type 4H SiC for Al/Ni and Ni ohmic contacts, respectively, when they were beneath Ru-rich Ta-Ru barriers aged at 350 °C for 3000 h in air. Annealed Ni ohmic contacts on n-SiC aged at 350 °C in air for 1000 h (the longest time tested) are also very stable. Pull tests revealed greatly improved adhesion between layers in metallization stacks that contained Ta-Ru barriers in place of previously studied Ta-Ru-N barriers. A 5 nm Ta layer inserted between the Ru-rich Ta-Ru barriers and Au was found to further improve the adhesion of the metallization stacks.

We have investigated the electrical and metallurgical behavior of ohmic contacts to p-InAs0.80P0.20. Auger depth profiling reveals that Ru, Ti, and V have better thermal stability against reaction than Pt, Pd, and Ni on p-InAsP. However, contacts with Pd deposited as the first layer exhibit lower specific contact resistances than contacts with Ti, V, Ni, Ru, or Pt as the first layer. For this reason, multilayer contacts were studied, adding Au as the top layer to minimize the metal sheet resistance. Transmission electron microscopy indicates that Pd/Ru/Au contacts aged for 3 days exhibit a uniform and shallow reaction with p-InAsP, with the Pd consuming only 4 nm of the semiconductor and the Ru serving as an effective diffusion barrier. Specific contact resistances of 3.8 x 10(-6) ohm cm(2) as deposited and 1.7 x 10(-6) ohm cm(2) for contacts aged at 250 degrees C for 90 days in an evacuated quartz tube were measured for the Pd/Ru/Au contacts. (c) 2006 The Electrochemical Society.

Palladium, Pt, and Cr∕Pt contacts to the wide band gap icosahedral boride semiconductor B12As2 have been studied. All Pd and Pt contacts exhibited nonlinear I-V characteristics, while Cr∕Pt contacts were Ohmic. The specific contact resistance was reduced from 6Ωcm2 as-deposited to 3×10−4Ωcm2 after the Cr∕Pt contacts were annealed at 750°C for 30s in Ar. Annealing at 600°C or higher drastically reduced the semiconductor sheet resistance, whether annealing was performed before or after metallization. This apparent activation of the semiconductor is a likely cause for the improvement in the Ohmic contacts with annealing.

The thermal stability of various gate metallizations on AlGaAsSb/InAs, with and without an InAs cap, was investigated. A W/Au gate metallization was found to be a good candidate for stable gate metallizations directly on AlGaAsSb. Ti/Pt/Au (30/40/80 nm) and Co/Si/Co/Si/Co gates are thermally stable on InAs. Cross-sectional transmission electron microscopy showed that degradation in the current–voltage characteristics of aged samples is associated with metal/InAs reactions.

We report a shallow, thermally stable Pt∕W∕Au (2∕50∕145nm) Ohmic contact to p-InGaSb prepared using a brief (NH4)2S rinse as part of the premetallization surface treatment. Cross-sectional transmission electron microscopy reveals that the Pt∕W∕Au contacts have better thermal stability than previously reported Pd∕W∕Au contacts, with the Pt∕W∕Au contacts remaining shallow even after they are aged at 250°C for 3days. The specific contact resistances of as-deposited Pt∕W∕Au, Pd∕W∕Au, Co∕W∕Au, Cu∕W∕Au, W∕Au, Cr∕W∕Au, and Ag∕W∕Au contacts are also compared, and the (NH4)2S rinse is found to partially relieve Fermi level pinning at the contact∕p-InGaSb interface.

The relationship between the specific contact resistance of nonalloyed Pd∕W∕Au ohmic contacts to p-In0.25Ga0.75Sb and premetallization surface preparations is examined. The resistance of ohmic contacts is minimized when p-InGaSb is exposed briefly to a dilute (NH4)2S solution. This treatment minimizes the thickness of any residual oxide or sulfide layer on the semiconductor and avoids excessive etching of the semiconductor, which would make the contact less shallow. A specific contact resistance of 5.9×10−7Ωcm2 is achieved for a Pd∕W∕Au (2∕50∕145nm) contact that consumes no more than 5nm of InGaSb.

Ohmic contacts to p-type InGaSb have been investigated. The factors that influence the contact resistance, thermal stability, and shallowness of the contacts are examined. The most desirable contact studied in this work employs three layers. A very thin layer of palladium is deposited on the p-InGaSb first and is found to lower the resistance at the metal/semiconductor interface. The next layer is W, which is predicted to be in thermodynamic equilibrium with InGaSb and which serves as a diffusion barrier to protect the semiconductor from the reaction with the final capping layer. The final capping layer is a 100 or 150 nm Au layer. The Au lowers the metal sheet resistance, which we have found both experimentally and through modeling to influence the contact resistance measurements, and the Au layer provides a contact surface that does not oxidize. The contact resistance of the as-deposited Pd/W/Au (5/50/145 nm) contact is 0.08 Ω mm (corresponding to a specific contact resistance of <3×10−7 Ω cm2), while the more thermally stable Pd/W/Au (5/145/100 nm) contact exhibits a contact resistance of 0.08 Ω mm only after annealing at 250 °C for 3 h, in both cases on a p-In0.25Ga0.75Sb layer with a semiconductor sheet resistance of approximately 300 Ω/□. The thermal stability of the Pd/W/Au contacts was also examined. The Pd/W/Au (5/145/100 nm) contacts remained shallow and exhibited no measurable electrical degradation when aged at 250 °C in N2 for 100 h, while they survived at 250 °C for 14 days in sealed, evacuated, quartz tubes.

In recent years, Au/Ni/p-GaN ohmic contacts annealed in air have been studied extensively because they provide low specific contact resistances and high transparency. In this article we focus on the environmental and thermal degradation that we have observed in these contacts. When the contacts were not protected from the environment, degradation of the contacts always occurred over a period of days, and after sitting 30 days in the laboratory at room temperature, the current–voltage characteristics of the originally ohmic contacts were nonlinear and the contacts were significantly more resistive. To pinpoint the degradation mechanism, samples were stored at room temperature in dry nitrogen, dry oxygen, dry air, air saturated with water vapor, nitrogen gas saturated with water vapor, or vacuum. These experiments revealed that water vapor was the cause of the room temperature degradation. Since no change in the sheet resistance of the p-type GaN was observed upon aging, four point probe measurements and x-ray photoelectron spectroscopy depth profiles were carried out to determine the interaction between water vapor and the p-type NiO present in the annealed contact metallization. The measurements indicated that hydroxyl groups were incorporated in the NiO, leading to a reduction in its conductivity and presumably a decrease in its hole concentration. Aging studies at 200 °C further revealed poor thermal stability of the contacts not only in water vapor but also in nitrogen gas and vacuum, and this degradation was again linked to a degradation in the conductivity of the NiO component of the contact metallization.

Ultrathin films (<30 nm) of Pr0.7Sr0.3MnO3 on LaAlO3 have been studied using transmission electron microscopy (TEM). It was shown that the films are highly uniform and defect-free, and that they are coherently strained to the smaller lattice parameter of the substrate, resulting in a tetragonal expansion perpendicular to the film plane and a change of crystal structure from the ordered orthorhombic of bulk materials to a simple tetragonal perovskite. The variation of the tetragonality with distance from the interface was also determined from high-resolution TEM images.

Resistance and magnetoresistance in compressively strained epitaxial Pr_2/3Sr_1/3MnO_3 ultrathin films have been studied. The samples were first demagnetized in different ways so that different magnetic structures were created, such as random domain and single domain states. Very large difference in resistance in zero applied magnetic field was observed between different states. The large change of resistance between states is attributed to spin-dependent scattering at the domain walls. We have shown for the first time that large domain wall resistance can be obtained in strained ultrathin manganite films and the result cannot be explained by the double-exchange model.

Resistance related to magnetic domain walls in compressive-strained epitaxial manganite ultrathin films has been studied. The samples were demagnetized in different ways to induce either multidomain or single domain states. Very large difference in resistance was observed between the two states, which was attributed to the domain wall resistance. The magnitude of the domain wall resistance was found to be different in different manganite compounds. We have shown that large domain wall resistance can be obtained in strained ultrathin manganite films and the result cannot be simply explained by the existing models.

Anisotropic magnetoresistance (AMR) in strained Pr0.67Sr0.33MnO3 thin films has been studied by measuring the resistance as a function of the angle between the applied magnetic field direction and the film normal with the current always perpendicular to the magnetic field. The results show that both compressive- and tensile-strained ultrathin films (50–150 Å) exhibit unusually large AMR, but with opposite signs. In contrast, the almost strain free films show much smaller AMR over all the temperature and field ranges studied. The AMR decreases rapidly as the film thickness increases due to the gradual release of strain.

We report two types of photoconductivity effects observed by illumination of oxygen deficient manganites thin films with UV or visible light. One is the persistent photoconductivity effect observed at low temperature (T < 30 K) when the thin film is in the metallic state. This effect is analog to the persistent photoconductivity observed in the high T c superconductors particularly in oxygen deficient YBaCuO thin films. The other type of effect is a photo-induced transition to metallic state at temperatures T < 100 K observed in Pr2/3Sr1/3MnO3 thin film which stays always in the semiconducting state in the darkness. This photo induced metallic transition from a semiconducting state leads to a non persistent colossal photoconductivity (several orders of magnitude of decrease of the resistivity).

We have studied the strain effects on the structural and magnetotransport properties of Pr0.67Sr0.33MnO3 (PSMO) thin films. The PSMO films were epitaxially grown on LaAlO3 (001), SrTiO3 (001), and NdGaO3 (110) substrates that induce biaxial compressive, tensile, and almost no strain in the films, respectively. The film thickness t, varied between 4–400 nm, was used as another controlling parameter of strain for each type of film. There exist two distinct thickness ranges with different thickness dependence of the magnetotransport properties. For t<20 nm, the zero-field resistance peak temperature (Tp) and the high-field magnetoresistance (HFMR) properties are critically dependent on the thickness and the substrate. For t>20 nm, the Tp and the HFMR ratio show weak t dependence. The results show evidence for the effects of the Jahn–Teller type distortion as well as disorders on the resistive transition temperature and the HFMR.

Strain-induced large low-field magnetoresistance has been observed in very thin Pr 0.67 Sr 0.33 MnO 3 films 1 . To better understand the role of strain in the low-field magnetotransport properties of manganite thin films, we have studied and compared very thin (3–20 nm) Pr 0.67 Sr 0.33 MnO 3 (PSMO), La 0.67 Ba 0.33 MnO 3 (LBMO), La 0.67 Sr 0.33 MnO 3 (LSMO) and La 0.67 Ca 0.33 MnO 3 (LCMO) films grown on different substrates, such as LaAlO 3 (001) (LAO), NdGaO 3 (110) (NGO), and SrTiO 3 (001) (STO). Due to the lattice mismatch between the films and the substrates ranging from −2.6% to +1%, different strains can be imposed to the films. We have found that: (1) large low-field magnetoresistance(LFMR) behaviors are observed in PSMO, LCMO and LSMO thin films on LAO substrates when a magnetic field is applied perpendicular to the film plane, but the maximum LFMR is the largest in PSMO and LCMO samples; (2) most of the films grown on STO substrates show positive MR when a magnetic field is applied perpendicular to the film plane, and when the field is parallel to the film plane all films show negative MR regardless of the substrates; (3) the large low-field MR is strongly dependent on the film thickness and the composition of the manganites. The anomalous low-field MR effect will be discussed based on strain-induced magnetic anisotropy and domain rotation and movement.

Strain effect on the low-field magnetoresistance (LFMR) in epitaxially grown Pr0.67Sr0.33MnO3 thin films has been studied. Very large LFMR and MR hysteresis have been found in compressive-strain ultrathin films grown on LaAlO3 (001) substrates when a magnetic field is applied perpendicular to the film plane. The LFMR ratio as high as 360% at H=1600 Oe and T=30 K was obtained from the MR hysteresis curve. The large LFMR depends strongly on the applied magnetic field direction as well as the film thickness. It is reduced to less than 10% when the film thickness is about 20 nm. In comparison, tensile-strain films on SrTiO3(001) show positive LFMR, and almost strain free films on NdGaO3 (110) show very small LFMR (<2%), at comparable magnetic fields and temperatures. These effects were found to be closely related to the strain-induced magnetic anisotropy.

Review of superconducting (Tc,Hc2(T),Jc), electron transport Rs(T), surface (XPS, UPS) and structural (XRD, RBS) properties of thin films of a novel superconductor BaNbO3−x on different substrates is presented. Superconducting films have been obtained when grown on the Al2O3 with Tc=14K,Jc≈1×104Acm−2 and large Hc2(0)=28T. At the same time, films on NdGaO3 exhibit behavior typical to granular superconductors.

We have studied the anisotropic magnetoresistance (AMR) of strained Pr0.67Sr0.33MnO3 thin films by measuring the MR as a function of the angle between the magnetic field direction and the substrate normal (out-of-plane). The results show that the compressive- and tensile-strained ultrathin films (5-15 nm) grown on LaAlO3 (001) (LAO) and SrTiO3 (001) (STO) substrates show unusually large out-of-plane AMR, but with opposite signs. In contrast, the almost strain-free films on the NdGaO3 (110) substrates show much smaller AMR over all the temperature and field ranges studied. Thick films on LAO and STO substrates also show much smaller AMR.