Grazing-incidence X-ray diffraction from multilayers, taking into account diffuse scattering from rough interfaces

Received: 21 May 1997: Accepted 30 July 1997     

Structural stability of heat-treated CoN/CN soft X-ray multilayers fabricated by dual-facing-target sputtering

286 , 176 (1996)]. (1) The interdiffusion critical wavelengths were calculated as 2.00–2.04 nm at temperatures ranging from 473 to 523 K, which is equal to those of Co/C multilayers within the experimental error, indicating that the interdiffusion behaviours in the CoN/CN multilayers are still decided by the thermodynamic properties of the Co-C system. (2) The effective interdiffusivities and macroscopic diffusion coefficients are smaller. (3) The activation energy for diffusion is larger. The features imply that it is possible to improve the thermal stability of Co/C multilayers by doping with N atoms. The high-temperature annealing results imply that the destructive threshold of the CoN/CN multilayers is 100–200 °C higher than that of Co/C multilayers. The small-angle X-ray diffraction of CoN/CN soft X-ray multilayers indicates that the period expansion of the multilayers is only 4% at 400 °C, and the interface pattern still exists even if they were annealed at 700 °C. The large-angle X-ray diffraction and transmission electron microscopy analysis reveal that the crystalline process is significantly retarded if doped with N atoms, leading to a smaller grain size at higher annealing temperatures. The significant improvement of the thermal stability can be interpreted with Raman spectroscopy and X-ray photoelectron spectroscopy analysis. The Raman spectra give the evidence that the formation of the sp³ bonding in the CN sublayers can be suppressed effectively by doping with N atoms, and thus the period expansion resulting from the changes in the density of CN layers can be decreased considerably. The X-ray photoelectron spectra give information about existence of the strong covalent bonding between N atoms and the ionic bonding between Co and N atoms, which can slow down the tendency of the structural relaxation. The interstitial N atoms decrease the mobility of Co atoms, and thus the fcc Co and hcp Co coexist even though the annealing temperature is much higher than the phase transformation temperature of 420 °C, leading to the suppression of the grain growth. Received: 29 May 1997/Accepted: 8 September 1997     

Self-organized structuring of W/C multilayers on Si substrate

After aging at room temperature for several months W/C multilayers (20 periods, single layer thicknesses in the nanometer range) grown on Si-(111) substrates by pulsed laser deposition (PLD) developed homogeneously wrinkled surfaces. Their structures were studied by optical microscopy, atomic force microscopy and X-ray diffractometry. Typical dimensions of debonded areas are some 100 μm in length, about 40 μm in width and 2–3 μm in height. The formation of wrinkles is accompanied by an increase in the free surface by 1–2%. Stress relaxation is considered the driving force of this phenomenon. Received: 26 July 1999 / Accepted: 29 July 1999 / Published online: 16 September 1999     

Sub-ps laser microstructuring of soft X-ray Mo/Si multilayer gratings

The sub-picosecond laser microstructuring of multilayer gratings is presented in this paper. A micromachining system operating with a 0.5 ps KrF laser at 248 nm was used to etch grating structures with a groove width of 1–2 μm in Mo/Si and Si/Mo multilayers. Atomic force microscopy, scanning electron microscopy and X-ray reflectivity were used to characterize the microetched patterns. The ω-scans around the 1st Bragg maximum show symmetric satellites up to 3rd order, with positions corresponding to the grating period. The use of sub-picosecond laser pulses minimizes the thermally affected zone and enhances the quality of the etched features. Short pulse laser processing is advantageous for the fabrication of high spatial resolution microstructures required in X-ray optics. Received: 21 May 2002 / Accepted: 19 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Email: dpapa@iesl.forth.gr     

Effect of interfacial-roughness replication on the diffuse X-ray reflection from periodical multilayers

The correlation of roughness profiles of different interfaces in a periodical multilayer affects the reciprocalspace distribution of the diffusely reflected X-ray intensity. This distribution has been calculated by means of the Distorted-Wave Born Approximation (DWBA) method and compared with the results of X-ray reflectometry on periodical metallic multilayers. It has been shown that the measurement of the reciprocal-space distribution of diffusely scattered intensity is a suitable non-destructive method to investigate the roughness replication in multilayers.     

Surface modification of exchange-coupled Co/NiOx magnetic bilayer by bias sputtering

We have investigated the effect of bias voltage on sheet resistance, surface roughness and surface coverage of Co/NiO_x magnetic bilayer. In addition, interface topography and corrosion resistance of the Ta/Co/Cu/Co/NiO_x/Si(1 0 0) system have been studied for Co layers deposited at an optimum bias voltage. Atomic force microscopy (AFM) and four point probe sheet resistance (R_s) measurement have been used to determine surface and electrical properties of the sputtered Co layer at different bias voltages ranging from 0 to −80 V. The Co/NiO_x bilayer exhibits a minimum surface roughness and low sheet resistance value with a maximum surface coverage at V_b=−60 V resulted in a slight increase of magnetic resistance and its sensitivity for the Co/Cu/Co/NiO_x/Si(1 0 0) magnetic multilayers, as compared with the same magnetic multilayers containing unbiased Co layers. The presence of Ta protection layer improves the corrosion resistance of the multilayers by three orders of magnitude in a humid environment.     

Diffusion amorphization and interface properties of FeB multilayers

Received: 3 September 1996/Accepted: 6 January 1997     

Thermal annealing of a-Si:H/a-SiNx:H multilayers

The crystallinity of Si/SiN_x multilayers annealed by a rapid thermal process and furnace annealing is investigated by a Raman-scattering technique and transmission electron microscopy. It is found that the crystallization temperature varies from 900 °C to 1000 °C when the thickness of a-Si:H decreases from 4.0 nm to 2.0 nm. Raman measurements imply that the high crystallization temperature for the a-Si:H sublayers originates from the confinement modulated by the interfaces between a-Si:H and a-SiN_x:H. In addition to the annealing temperature, the thermal process also plays an important role in crystallization of a-Si sublayers. The a-Si:H sublayers thinner than 4.0 nm can not be crystallized by furnace annealing for 30 min, even when the annealing temperature is as high as 1000 °C. In contrast, rapid thermal annealing is advantageous for nucleation and crystallization. The origin of process-dependent crystallization in constrained a-Si:H is briefly discussed. Received: 11 April 2001 / Accepted: 20 June 2001 / Published online: 30 August 2001     

Structure and mechanical properties of titanium nitride/carbon nitride multilayers

The structure and mechanical properties of the multilayers consisting of 5-73 nm thick titanium nitride (TiN) and 4.6 nm thick carbon nitride (CN) have been investigated. It has been found that the CN layers are amorphous and the TiN layers thinner than 17 nm are amorphous. The TiN layers become crystallized as the thickness is increased to 30 nm or thicker. The hardness from the composite response of the multilayered films and their substrates determined using continuous stiff measurement is smaller than the film-only hardness (without substrate effects) calculated using Bhattacharya-Nix empirical equation. The hardness increases with raising the thickness of TiN layers. With the crystallization of the TiN layer, the multilayers become even harder than that calculated based on the rule of mixtures. However, no enhancement in hardness has been observed when the TiN layers are amorphous.     

Photoluminescence properties of size-selected Si nanocluster films prepared by laser ablation

Size-dispersed Si nanocluster films have been synthesized by a method of crossing an Ar gas beam perpendicularly to a silicon cluster beam that is produced by a laser ablation technique. Flight directions of the nanoclusters are changed due to Ar gas collisions, and smaller nanoclusters are deflected further from the axis of the primary cluster beam. The size-dispersed nanocluster films exhibit strong red photoluminescence (PL) after exposure to the air. The PL peak energy changes between 1.42 and 1.72 eV depending on the sample position. The average diameter of the oxidized nanoclusters characterized by transmission electron microscopy is 10 nm at the position of the primary cluster beam axis and becomes smaller as deviated from the axis. The relation between the PL peak energy and the size of the oxidized Si nanoclusters is discussed. Received: 4 May 2000 / Accepted: 9 May 2000 / Published online: 13 July 2000     

Dome-to-pyramid transition induced by alloying of Ge islands on Si(001)

Received: 19 October 1998/Accepted: 19 October 1998     

Fabrication, morphology, and photoluminescence properties of GaN nanowires and nanorods by ammoniating Ga2O3/V films on Si(1 1 1)

GaN nanowires and nanorods have been successfully synthesized on Si(1 1 1) substrates by magnetron sputtering through ammoniating Ga₂O₃/V films at 900 °C in a quartz tube. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectrum were carried out to characterize the structure, morphology, and photoluminescence properties of GaN sample. The results show that the GaN nanowires and nanorods with pure hexagonal wurtzite structure have good emission properties. The growth direction of nanostructures is perpendicular to the fringes of (1 0 1) plane. The growth mechanism is also briefly discussed.     

Field electron emission from HfNxOy thin films deposited by direct current sputtering

HfN_xO_y thin films were deposited on Si substrates by direct current sputtering at room temperature. The samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). SEM indicates that the film is composed of nanoparticles. AFM indicates that there are no sharp protrusions on the surface of the film. XRD pattern shows that the films are amorphous. The field electron emission properties of the film were also characterized. The turn-on electric field is about 14 V/μm at the current density of 10 μA/cm², and at the electric field of 24 V/μm, the current density is up to 1 mA/cm². The field electron emission mechanism of the HfN_xO_y thin film is also discussed.     

Second-harmonic spectroscopy of Ge/Si(001) and Si1-xGex(001)/Si(001)

0.9 Ge_0.1(001)/Si(001) films with SH photon energies 3.1<2hν<3.5 eV near the bulk E₁ critical point of Si(001) or Si_0.9Ge_0.1(001). Ge was deposited on Si(001) by using atomic layer epitaxy cycles with GeH₄ or Ge₂H₆ deposition at 410 K followed by hydrogen desorption. As Ge coverage increased from 0 to 2 monolayers the SH signal increased uniformly by a factor of seven with no detectable shift in the silicon E₁ resonant peak position. SH signals from Si_0.9Ge_0.1(001)/Si(001) were also stronger than those from intrinsic Si(001). Hydrogen termination of the Si_0.9Ge_0.1(001) and Ge/Si(001) surfaces strongly quenched the SH signals, which is similar to the reported trend on H/Si(001). We attribute the stronger signals from Ge-containingsurfaces to the stronger SH polarizability of asymmetric Ge-Si and Ge-Ge dimers compared to Si-Si dimers. Hydrogen termination symmetrizes all dimers, thus quenching the SH polarizability of all of the surfaces investigated. Received: 13 October 1998 / Revised version: 18 January 1999     

Structural and photoluminescence characters of SnO2:Sb films deposited by RF magnetron sputtering

The Antimony-doped tin oxide (SnO₂:Sb) films have been prepared on glass substrates by RF magnetron sputtering method. The prepared samples are polycrystalline films with rutile structure of pure SnO₂ and have preferred orientation of (1 1 0) direction. XRD measurement did not detect the existence of Sb₂O₃ phase and Sb₂O₅ phase; Sb ions occupy the site of Sn ions and form the substitution doping. An intensive UV-violet luminescence peak near 392 nm is observed at room temperature. Photoluminescence (PL) properties influenced by sputtering power and annealing for the SnO₂:Sb films are investigated in detail and corresponding PL mechanism is discussed.     

Microscopic growth mechanisms for carbon and boron-nitride nanotubes

Received: 27 November 1998 / Accepted: 18 December 1998     

The effects of dc bias voltage on the crystal size and crystal quality. of cBN films

For the deposition of cubic boron nitride thin films in Ar–N₂–BF₃–H₂ system by dc jet plasma chemical vapor deposition, the role of dc substrate bias ranging from -70 V to -150 V was investigated. A critical bias voltage was observed for the formation of cBN phase. The cBN content in the film increased with bias voltage and reached a maximum at the bias voltage of -85 V. Increasing the bias voltage further caused a decrease in cBN content and peeling of the films from the substrate. By combining the results of infrared spectroscopy, Raman spectroscopy and X-ray diffraction, the bias voltage was also found to strongly affect the crystal size, crystal quality and residual stress of the deposited films. A bias voltage a little higher than the critical value was demonstrated to be favorable for the deposition of a high-quality cBN film with large crystal size and low residual stress. Received: 13 June 2000 / Accepted: 21 June 2000 / Published online: 23 August 2000     

Ion beam sputter deposited W/Si multilayers: Influence of re-sputtering on the interface structure and structure modification at ultra short periods

X-ray multilayer mirrors of period ranging from 9.6 to 1.7 nm, deposited using ion beam sputtering, have been examined using grazing incidence X-ray reflectivity (GIXRR) and grazing incidence X-ray diffraction. Detailed analysis of GIXRR data revealed that significant amount of re-sputtering of Si layer takes place while W deposition is underway. Re-sputtering is mainly due to bombardment of high-energy neutrals getting reflected from the W target. Due to re-sputtering interface of the multilayer becomes asymmetric. This puts a major hindrance in avoiding the intermixing and achieving sharp interfaces at shorter periods. Maximum thickness of Si which gets lost due to re-sputtering during deposition is ∼0.8 nm. The shortest period multilayer estimated, that could be deposited without intermixing, was 2.7 nm. These results are of significance for developing low period W/Si multilayers.     

Microcharacterization of the surface oxidation of Py/Cu multilayers by scanning X-ray absorption spectromicroscopy

The local surface oxidation of the permalloy surface layer in Py/Cu GMR multilayers on a micron lateral scale has been analyzed by means of a microspot-X-ray absorption spectromicroscope utilizing synchrotron radiation from the Advanced Light Source bending magnet beamline 6.3.2. Additionally, the GMR multilayer samples prepared by dc magnetron sputtering have been analyzed by cross-sectional transmission electron microscopy, hard X-ray reflection and magnetoresistance measurements. The formation of a passivating iron-oxide layer on the sample surface was identified by X-ray absorption near edge structure spectroscopy (XANES) near the Fe-2p edge while no indication for nickel-oxide formation could be found. Small micron-size pits of reduced iron-oxide concentration could be identified by XANES microscopy while the corresponding nickel distribution appeared to be homogeneous. The results are explained in terms of a local breakdown of the passivating oxide layer. Received:16 October 2000 / Accepted: 4 December 2000 / Published online: 21 March 2001     

Effect of elastic anisotropy on the strain fields and band edges in stacked InAs/GaAs quantum dot nanostructures

The effect of elastic anisotropy on the strain fields and confinement potentials in InAs/GaAs quantum dot (QD) nanostructures was investigated for an isolated dot and a stacked multi-layer dots using finite element analysis and model solid theory. The assumption of isotropy tends to underestimate especially hydrostatic strain that is known to modify confinement potentials in conduction band. Consideration of anisotropy results in a wider band gap and shallower potential well as compared with the isotropic model. Since the band gap and potential well depth would be related to opto-electronic properties of quantum dot systems via quantum mechanical effects, it is suggested that consideration of elastic anisotropy in the calculation of strains and band structures is necessary for the design of QD-based opto-electronic devices.