Ni Overlayers Research Articles

Pulsed Laser And Ion Beam Surface Modification of Sintered, Alpha-Sic

ABSTRACTPulsed laser annealing and ion beam mixing have been used as surface modification techniques to enhance the physical properties of polycrystalline α-SiC. Thin Ni overlayers (20 nm - 100 nm) were evaporated onto the SiC surface. The specimens were subsequently irradiated with pulses of a ruby or krypton fluoride (KrF) excimer laser or bombarded with high energy Xe+ or Si+ ions. Both processes are non-equilibrium methods and each has been shown to induce unique microstructural changes at the SiC surface which are not attainable by conventional thermal treatments. Under particular (and optimum) processing conditions, these changes considerably increased the mechanical properties of the SiC; following laser irradiation, the fracture strength of the SiC was increased by as much as 50%, but after ion beam mixing, no strength increase was observed.High resolution cross-section transmission electron microscopy (X-TEM), scanning electron microscopy (SEM), and Rutherford backscattering techniques were used to characterize the extent of mixing between the Ni and the SiC as a result of the surface modification.

Ion-beam and laser mixing of nickel overlayers on silicon carbide

We have investigated ion-beam and laser mixing of Ni overlayers on silicon carbide substrates using cross-section and analytical techniques of electron microscopy, and ion scattering and channeling techniques. The thickness of the overlayers was varied from 200 to 500 Å on both crystalline and polycrystalline substrates of silicon carbide. The thickness of the ion-beam mixed region was found to increase with increasing dose of the implanted ions. The laser mixing was obtained only within a critical window of the pulse energy density (1.15±1.25 J cm−2) of a ruby laser (wavelength 0.693 μm, and pulse duration 28×10−9 s). The structure of the mixed region was found to be amorphous, with occasional presence of crystalline nickel silicide (Ni2Si) islands. The laser-mixed SiC specimens showed a considerable improvement (>35%) in their fracture strength.

An exploratory study of the reactive Ni-GaAs (1 1 0) interface

Soft X-ray photoemission spectroscopy measurements have been carried out on cleaved n-type GaAs (1 1 0) surfaces covered with Ni overlayers ranging in thickness from 0.05 to 53 Å. The results of these room temperature measurements show that we have band bending effects occurring in conjunction with strong interfacial chemical reactions. Deconvolution of the Ga 3 d core line into substrate and metallic components shows dissolution of the substrate at the interface with Ga diffusing into the surface of the metal overlayer for the intermediate coverages (1–15 Å). Observation of the As 3 d core level shows out-diffusion of As to the surface over the entire Ni coverage range. Using this deconvolution scheme we are able to follow the band bending of the Schottky barrier formed here up to the 8 Å coverage. The Schottky barrier height is 1.0 ± 0.1 eV for this overlayer thickness.

Thermal stability of amorphous Ni-Nb on semiconductor substrates

The thermal stability of amorphous metal films is of prime importance in high temperature semiconductor device applications. Suitable thin film deposits of amorphous Ni-Nb, 58 at % Ni have been prepared by sputter deposition onto Si, GaAs, and GaP substrates. The amorphous character and crystallization behavior of the films have been monitored by x-ray diffraction (XRD). Polycrystalline Au and Ni overlayers were found to lower the NiNb crystallization temperature by about 100 and 150°C, respectively. In addition, Auger depth profiles were used to monitor the various interdiffusion reactions between the amorphous metal, polycrystalline metal overlayers and semiconductor substrates. For example, the feasibility of NiNb as a diffusion barrier between Au and a semiconductor substrate was found to depend on the substrate. Following a one hour 500°C anneal, very little interdiffusion was observed in the Au/NiNb/Si system. However on a GaAs or GaP substrate the Au and NiNb interdiffused extensively. These studies have demonstrated the viability of amorphous metal films as effective diffusion barriers in semiconductor contact applications, but emphasize the need for careful selection of compatible material combinations.

Reaction of Amorphous Minb Films with Crystalline Metal Overlayers

AbstractIn many cases the stability of amorphous films is influenced by interaction with metallic crystalline overlayers. Such interactions between Au, Ni, Nb and Ta overlayers and a-(Ni-Nb) films are reported. During interdiffusion Au overlayers reacted with a-(Ni-Nb) to form two different adjacent crystalline layers. In order to study the influence of relaxation of the amorphous film on overlayer reaction several a-(Ni-Nb) samples were pre-annealed prior to Au deposition. High depth resolution Rutherford Backscattering Spectrometry (RBS) demonstrates that preannealing lowers the diffusion poefficient of Au in a-(Ni-Nb) at 4500C from 7.5×10−22 m2/s to 8.7×10−23 m22/s. During interdiffusion Ta was discovered to be substantially more inert than Au. For example, negligible interdiffusion between Ta and a-(Ni-Nb) at 505°C after 25 hours implies a diffusivity of less than 5×10−24 m2/s. These observations allow assessment of some of the requirements for increasing the stability of crystalline-amorphous metal film layered structures.

Chemical Reaction at the Ni/Inp (110) and Ni/GaAs (110) Interfaces

ABSTRACTThe chemical reaction at the Ni/InP (110) and Ni/GaAs (110) interfaces produced by sequential deposition of thin Ni overlayers onto cleaved semiconductor surfaces has been investigated with valence band (VB) and core level photoemission and Auger spectroscopies using synchrotron radiation as the excitation source. By monitoring changes in the VB, P 2p, In 4d, Ga 3d, As 3d, and Ni 3p photoemission spectra and the lineshape of the P LVV Auger transition during the initial stage of Schottky barrier formation, we found that for both interfaces the first few Å of Ni react strongly with the surface resulting in the formation of a nickel phosphide or nickel arsenide. At the same time, segregation of metallic In or Ga is observed.

Electronic structure and magnetism of Ni overlayers on a Cu(001) substrate

Our recently developed linearized augmented-plane-wave thin-film method is used to determine the electronic structure and magnetism of Ni overlayers on Cu(001). Accurate ab initio self-consistent spin-polarized semirelativistic band calculations are reported for (i) a clean five-layer Cu(001) slab and (ii) the same Cu slab plus one or two $p(1\ifmmode\times\else\texttimes\fi{}1)$ layers of Ni on either side. Results presented include charge and spin densities, work function, band structures, projected density of states, magnetic moments, and direct and transferred hyperfine fields. Both surface and interface effects are found to be important. The Ni overlayers are not magnetically dead: The Ni layer adjacent to the Cu has its moment decreased from the bulk value to 0.39${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for the single Ni overlayer and to 0.47${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for the two-Ni-thick layers; the surface Ni layer in the two-layer Ni on Cu film has its moment increased somewhat to 0.68${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. This reduction in moment for the interface Ni arises primarily from charge transfer onto Ni sites from the Cu substrate. By contrast, the increase in moment of the Ni surface atoms arises in large part due to the dehybridization of the $p$ electrons from the $d$-band electrons; these $p$ electrons become more delocalized and spill out into the vacuum region. A similar effect was also observed for an unsupported Ni monolayer. In the case of the Ni monolayer on Cu, the total number of Ni electrons is almost the same as for bulk Ni. Here, the loss of electrons due to the dehybridization of $p$ electrons is nearly canceled by the increase from its interface with the Cu substrate; the decrease in magnetic moment (to 0.39${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$) agrees with electron-capture experiments.

Abstract: Magnetism of Ni overlayers on a Cu(001) substrate

Abstract: Magnetism of Ni overlayers on a Cu(001) substrate

Core level binding energy shifts in Ni on Au and Au on Ni overlayers

Core level binding energy shifts of the Ni-2 p and Au-4ƒ lines have been measured for Ni on Au and Au on Ni overlayers down to mean coverages of less than 0.1 monolayers. When normalized to the maximum shift at submonolayer coverage the Ni on Au and Au on Ni shifts show the same dependence as function of the monolayer coverage. Using the thermodynamical approach for the calculation of binding energy shifts in metals, recently developed by Mårtensson and Johansson, the submonolayer shifts together with experimental results of core level binding energy shifts in dilute Ni Au and Au Ni alloys are used to calculate surface segregation energies in these alloys. They are compared with semiempirical determinations of these energies by Seah.

Development ot ohmic contacts for GaAs devices using epitaxial Ge films

Ohmic contacts to n-type GaAs have been developed using epitaxial Ge films on GaAs alloyed with Ni overlayers by solid-state diffusion at temperatures of 450/spl deg/C-650/spl deg/C. These contacts have applications to high reliability, high temperature microwave devices. Reflection electron diffraction of the Ge layers prior to deposition of the Ni overlayers reveals the presence of high quality single-crystalline films. Even after sintering, there is very little penetration of Ge into GaAs in the absence of Ni. With the presence of a Ni overlayer, significant interdiffusion between Ge and GaAs is revealed by Auger electron spectroscopic profiles. These results, together with the current-voltage characteristics of similar contacts prepared on p-type GaAs, indicate the presence of a Ge-doped n/SUP +/ layer at the Ni/Ge-GaAs interface. Ohmic contacts using epitaxial Ge films with Ta and Mo overlayers have also been investigated.

Smooth and continuous Ohmic contacts to GaAs using epitaxial Ge films

Ohmic contacts to n-type GaAs have been developed using epitaxial Ge films on GaAs alloyed with Ni overlayers by solid-state diffusion at temperatures of 450–550 °C. The contacts are smooth and continuous, showing no evidence of phase separation or other surface structure. Interdiffusion at the Ge-GaAs and Ni/Ge-GaAs interfaces was examined by Auger electron spectroscopy (AES) sputter profiling techniques. An abrupt profile is observed at both the as-deposited and sintered Ge-GaAs interface. With the presence of a Ni overlayer, significant interdiffusion between Ge and GaAs is revealed by AES profiles. These results, together with the current-voltage (I-V) characteristics of similar contacts prepared on p-type GaAs, indicate the presence of a Ge-doped n+ layer at the Ni/Ge-GaAs interface.