S. Rawal

Structure and magnetism of cobalt-doped ZnO thin films

The structure and magnetic properties of Co-doped ZnO films are discussed in relation to cobalt doping levels and growth conditions. Films were deposited by pulsed-laser deposition (PLD) from ZnO targets containing cobalt concentrations from 0 to 30?at.%. The structure of the films is examined by x-ray diffraction (XRD) and transmission electron microscopy (TEM), and optical absorption is used to infer the substitution of cobalt inside the ZnO lattice. Magnetic properties are characterized by superconducting quantum interference device (SQUID) magnetometry. Films doped with cobalt concentrations of a few per cent appear to be composed of two magnetic components: a paramagnetic component and a low-field ferromagnetic component. Films doped with 30% cobalt show a larger FM signature at room temperature with clear hysteretic shape, but films grown at low pressure are plagued by the precipitation of metallic cobalt nanoparticles within the lattice which can be easily detected by XRD. These particles are well oriented with the ZnO crystal structure. By increasing the base pressure of the vacuum chamber to pressures above 1?10?5?Torr, metallic cobalt precipitates are undetectable in XRD scans, whereas the films still show an FM signature of ~0.08??B/Co. Depositions in the presence of oxygen background gas at 0.02?mTorr decreases the magnetization. The decreased magnetization with oxygen suggests that the activation of ferromagnetism depends on defects, such as oxygen vacancies, created during growth. Optical absorption measurements show a sequential increase in the Co+2 absorption peaks in these films, along with an almost linearly increasing bandgap with cobalt concentration suggesting a large solubility of cobalt in ZnO. Bright-field TEM imaging and electron diffraction do not show signs of precipitation; however, dark-field imaging shows circular areas of varying contrast which could be associated with cobalt precipitation. Therefore, the possibility that ferromagnetism results from secondary phases cannot be ruled out.

Band gap properties of Zn1- xCdxO alloys grown by molecular-beam epitaxy

Optical absorption and reflectance measurements are performed to evaluate compositional and temperature dependences of band gap energies of Zn1−xCdxO alloys grown by molecular-beam epitaxy. The compositional dependence of the band gap energy, determined by taking into account excitonic contributions, is shown to follow the trend Eg(x) = 3.37−2.82x+0.95x2. Incorporation of Cd was also shown to somewhat slow down thermal variation of the band gap energies, beneficial for future device applications.

Properties of W-Ge-N as a diffusion barrier material for Cu

The properties of W–Ge–N thin films are reported, focusing on issues relevant to their use as diffusion barriers for Cu metallization on silicon. The amorphous W–Ge–N thin films were deposited on thermally grown SiO 2 / Si using reactive sputter deposition. This was followed by in situ deposition of Cu films. Annealing studies for W–Ge–N were then carried out in a vacuum to investigate Cu diffusion and barrier film crystallization. X-ray diffraction was used to assess the crystallinity of the films upon annealing. The results show that W–Ge–N has a recrystallization temperature that is higher that that for WNx. Auger electron spectroscopy was used to measure the depth profile of Cu diffusion through the barrier layer. Little or no Cu diffusion was detected for a relatively high annealing temperature. The W–Ge–N films were conductive, although the resistivity is somewhat higher than that for WNx. The results suggest that W–Ge–N may be an attractive diffusion barrier material for Si or SiGe devices.

Properties of Ta–Ge–(O)N as a diffusion barrier for Cu on Si

The properties of Ta–Ge–(O)N as a diffusion barrier for Cu on silicon have been investigated. Ta–Ge–(O)N was deposited on single crystal p‐Si(001) by reactive sputtering. This was followed by in situ deposition of Cu. Diffusion barrier tests were conducted by subsequent annealing of individual samples in Ar atmosphere at higher temperature. The films were characterized by x-ray diffraction, Auger electron spectroscopy, and four-point probe. The results indicate that Ta–Ge–(O)N fails after annealing at 500°C for 1h compared to Ta(O)N which fails after annealing at 400°C for 1h indicating better diffusion barrier properties.

Mechanism for radiative recombination in ZnCdO alloys

Temperature dependent cw- and time-resolved photoluminescence combined with absorption measurements are employed to evaluate the origin of radiative recombination in ZnCdO alloys grown by molecular-beam epitaxy. The near-band-edge emission is attributed to recombination of excitons localized within band tail states likely caused by nonuniformity in Cd distribution. Energy transfer between the tail states is argued to occur via tunneling of localized excitons. The transfer is shown to be facilitated by increasing Cd content due to a reduction of the exciton binding energy and, therefore, an increase of the exciton Bohr radius in the alloys with a high Cd content.

Ge∕HfNx diffusion barrier for Cu metallization on Si

The properties of Ge∕HfNx have been investigated relative to its use as a diffusion barrier for Cu metallization. The Ge∕HfNx bilayer was grown on p-Si (001) substrates by reactive sputtering, followed by in situ deposition of Cu. Individually annealed films at different temperatures (400–700°C, 1h) were characterized for evidence of Cu transport through the barrier bilayer to the Si substrate. The annealed structures were characterized by x-ray diffraction, energy-dispersive spectroscopy, and high-resolution transmission electron microscopy. The results indicate superior diffusion barrier properties of Ge∕HfNx for Cu metallization on Si compared to that for HfNx (7nm).

Comparative study of HfNx and Hf–Ge–N copper diffusion barriers on Ge

The diffusion barrier properties of HfNx and Hf–Ge–N thin films for Cu metallization on Ge are examined. The diffusion barrier films were deposited by reactive sputtering on p-Ge (001) single crystal substrates with varying thicknesses. Cu thin films were then deposited in situ on the diffusion barrier. The multilayer film structure was subsequently annealed in an Ar atmosphere. X-ray diffraction was used to determine the film crystallinity and identify intermetallic phases due to reactions involving the film and substrate. The HfNx and Hf–Ge–N diffusion barrier films remained amorphous for annealing temperatures up to 700°C. At thickness of 50nm, the HfNx films showed superior diffusion barrier properties as compared to Hf–Ge–N based on the appearance of secondary phases due to reactions and changes in the Cu morphology. These results suggest that HfNx is an effective barrier material for Cu integration on Ge.

A collective dynamics description of dipolar interactions and the coercive field of magnetic nanoparticles

The effect of dipolar interactions on the coercive field of Ni nanoparticles embedded as layers in a Al2O3 host matrix is discussed. The results for two sets of 5 layer samples with different interlayer spacings and a set of single layer samples are compared for samples with particle size varying from 3 nm (single domain) to 60 nm (multiple domain). The dipolar interactions are strongest in the samples with shorter interlayer distances and weakest for the single layer samples. Our observation that dipolar interactions increase the coercive field and decrease the critical diameter separating single domain from multiple domain behavior reinforces a description based on collective dynamics.