Chin‐An Chang

Molecular‐beam epitaxy (MBE) of In1−xGaxAs and GaSb1−yAsy

Films of In1−xGaxAs and GaSb1−yAsy over the entire composition ranges have been grown on (100) GaAs, InAs, and GaSb substrates by MBE. In situ observations by high‐energy electron diffraction have revealed a variety of surface reconstructions and correlated the growth process with the lattice mismatch. The compositions are governed by the relative rates of In and Ga in In1−xGaxAs, but primarily by that of Sb in GaSb1−yAsy because of its dominant incorporation over As. In these alloys, Sn is found to be a donor throughout In1−xGaxAs but an amphoteric impurity in GaSb1−yAsy.

In1−xGaxAs‐GaSb1−yAsy heterojunctions by molecular beam epitaxy

Smooth films of n‐In1−xGaxAs and p‐GaSb1−yAsy were grown by molecular beam epitaxy. As a function of the compositions, x and y, the lattice constants vary linearly while the energy gaps show a downward bowing. Abrupt heterojunctions made of these alloys with close lattice matching exhibit a series of current‐voltage characteristics which change from rectifying to Ohmic as x and y are reduced. The relative location of the band‐edge energies of the two semiconductors at the interface is shown to account for the unusual characteristics observed experimentally.

Enhanced Cu‐Teflon adhesion by presputtering prior to the Cu deposition

Adhesion of Cu to Teflon has been studied by depositing Cu to Teflon with and without a presputtering prior to the Cu deposition. Without presputtering, a weak adhesion is observed, with a value of 1 g/mm, which fails the scotch tape test. With a presputtering using 500 eV Ar+ ions, the adhesion rapidly increases, becoming evident after a sputtering of 10 s, and reaches maximal increases of 50 times at longer sputtering times. All the Cu films deposited after presputtering show strong adhesion, and can only be removed by forceful scratching with sharp tools. The presputtering was shown to change both the surface morphology of Teflon, with the deposited Cu following the morphologies created, and the interface chemical bonding between Cu and Teflon as revealed by x‐ray photoelectron spectroscopy. Exposure of the presputtered Teflon to air prior to the Cu deposition shows no effect on the strong adhesion obtained. An interface bonding model is suggested for the enhanced adhesion observed.

Characteristics of AuGeNi ohmic contacts to GaAs

Abstract We have studied AuGeNi ohmic contacts to n-type MBE grown GaAs epitaxial-layer with doping in the (1016−1019) cm−3 range, and found several new effects: (a) Contact resistivity exhibit a weak dependence on carrier concentration (much weaker than 1/ND depencence); (b) We find evidence for a high resistivity layer under the contact at least several thousands angstroms deep, which dominate the contact resistance in most cases; (c) We find a peripheral zone around the contact, about 1 μm wide which differs chemically from the GaAs epi-layer; (d) SIMS analysis reveals a deep diffusion into the GaAs of Ni and Ge; (e) Correlation between density of GeNi clusters in the contact and the contact resistivity are found; (f) Temperature measurements justify that tunneling is responsible for the ohmic contact. We discuss also the validity of the transmission line method and the commonly accepted model of the contact.

Adhesion studies of metals on fluorocarbon polymer films

Adhesion of metal films to several fluorocarbon polymer films is studied for Cu, Cr, Ti, Al, and Au. The polymers include polytetrafluoroethylene (PTFE), fluoroethylenepropylene (FEP), and a copolymer containing a perfluoroalkoxy group (PFA), all deposited on Cr/SiO2 substrates by spin coating. Peel strengths of the metal strips on these polymers are compared and are taken as measures of the metal–polymer adhesion. Among the polymers, FEP gives the highest peel strengths to metals, with PTFE the lowest. Among the metals, Ti gives the highest peel strength for each polymer, followed by Cr, with Cu being the lowest. The peel strengths for Ti on FEP, PFA, and PTFE are 85, 75, and 20 g/mm, respectively. Those for Cu to the polymers are around 5 g/mm or less. The strong adhesion for the metals on both FEP and PFA is attributed to the high concentration of carbon sites with three fluorine neighbors, by considering the electronegativities among the various carbon sites in the polymer chains. Among the metals, so...

Magnetization of (100) Cu‐Ni, (100) Cu‐Co, and (100) Ni‐Co superlattices deposited on silicon using a Cu seed layer

The magnetization of (100)Cu‐Ni, (100)Cu‐Co, and (100)Ni‐Co superlattices has been studied. The structures were grown on the (100)Cu/Si substrates, with the (100)Cu epitaxially grown on the (100)Si as the seed. The (100)oriented superlattices show different magnetization curves from the (111) structures and from bulk (100)Ni and Co films. A much enhanced magnetization is observed for the (100)Cu‐Ni superlattice with the field perpendicular to the film plane. The remanence is larger than that for the field parallel to the film plane. Negative remanence and coercivity are observed for the (100)Cu‐Co and Ni‐Co structures. The magnetization curve for the (100)Ni‐Co superlattice is further shown to resemble a superposition of the Cu‐Ni and Cu‐Co ones. Upon heating the Ni‐Co structure to 400 °C to consume more Ni than Co, a magnetization curve similar to that of the Cu‐Co one is observed.

Novel method of patterning YBaCuO superconducting thin films

A unique method of patterning YBaCuO thin films based on the inhibition of superconductivity by Si‐YBaCuO intermixing has been developed. In the experiment, a thin Si film was first evaporated on a MgO substrate and subsequently patterned using laser direct‐write etching. Multilayered YBaCuO thin films were then deposited by e‐beam evaporation and annealed in a rapid thermal annealing system for 30–90 s at 980 °C. The YBaCuO film deposited on the silicon regions became insulating. Auger depth profiling measurements indicate that Si‐YBaCuO intermixing had occurred in these areas. Between the insulating regions, narrow YBaCuO superconducting lines were formed. For both 10‐μm‐wide, 1‐mm‐long and 2.5‐μm‐wide, 80‐μm‐long lines, the Tc was observed above 76 K. The critical current density of the lines was measured to be 300 A/cm2 at 75 K. This patterning technique may be useful for fabrication of high Tc superconducting interconnects and devices.

Epitaxy of (100) Cu on (100) Si by evaporation near room temperatures: In‐plane epitaxial relation and channeling analysis

The epitaxial growth of (100) Cu on (100) Si reported recently using evaporation is analyzed to determine the epitaxial relation between Cu and Si, and also the crystalline quality of the Cu films. A 45° rotation between the (100) plane of Cu and that of Si around their (001) axis is shown to be needed for the lattice match. Such an epitaxial relation is confirmed by the grazing angle x‐ray diffraction, with the [010] of Cu parallel to the [011] of Si. The channeling analysis of a 2‐μm‐thick Cu film shows a 10% minimum near the surface.

Reaction between Cu and TiSi2 across different barrier layers

The reaction between Cu and TiSi2 is studied with and without barriers, Cu being used for interconnect and TiSi2 as the gate silicide for the metal‐oxide‐semiconductor devices. The barriers include Ta, TiN, and W. Without a barrier, Cu reacts with TiSi2 below 300 °C, forming Cu silicides. An improvement in thermal stability by 50–100 °C is obtained using the barriers, with TiN/Ti being the most effective. A combined use of these barriers, with a final structure of Ta/Cu/Ta/W/TiN/Ti/TiSi2 /Si, suppresses the Cu‐TiSi2 reaction until above 600 °C. The reaction mechanisms involved, and their relation with the reactions between Cu and other silicides, are discussed.

Reaction between Cu and PtSi with Cr, Ti, W, and C barrier layers

The Cu/PtSi metallurgy is studied for reaction and thermal stability using several barrier layers, Cr, Ti, W, and amorphous C. Using preformed PtSi and Cr, Ti, and W barrier layers, Cu is found to react with PtSi around 350 °C. The results are compared with those using similar barriers for the Al/PtSi structure, where an improvement in thermal stability by 50–150 °C is observed. The low thermal stability of the Cu/PtSi structures is attributed to the high affinity of Cu to Si, with the Cu silicide formation starting around 200 °C for a Cu/Si structure. Using an amorphous carbon barrier for the Cu/PtSi structure, a small amount of Cu silicide is observed at 400 °C, but not at 600 °C. Migration of Cu into the structure, however, makes uncertain the effectiveness of the carbon barrier. The results are compared with those of Al/C/PtSi, Al/C/Pd/Si, and C/Cu/SiO2 to understand the mechanism involved.