W.A.M. Aarnink

Controlled preparation of all high-Tc SNS-type edge junctions and DC SQUIDs

High-Tc SNS-type Josephson junctions and DC SQUIDs were successfully fabricated using hetero-epitaxially grown multilayers of YBa2Cu3Ox and PrBa2Cu3Ox. These layers are c-axis oriented and hence edges of the multilayers give rise to a current flow in the ab-plane between the electrodes of a Josephson junction. The necessary structuring was done by Ar ion beam etching. The individual junctions exhibit a supercurrent up to 80 K. The IcRn-product of these junctions usually has a lower limit of 8 mV at 4.2 K. Voltage modulation of the first DC SQUIDs can be observed up to 66 K. Details on the fabrication and measurements are presented.

Determination of complex dielectric functions of ion implanted and implanted‐annealed amorphous silicon by spectroscopic ellipsometry

Measuring with a spectroscopic ellipsometer (SE) in the 1.8–4.5 eV photon energy region we determined the complex dielectric function (ϵ = ϵ1 + iϵ2) of different kinds of amorphous silicon prepared by self‐implantation and thermal relaxation (500 °C, 3 h). These measurements show that the complex dielectric function (and thus the complex refractive index) of implanted a‐Si (i‐a‐Si) differs from that of relaxed (annealed) a‐Si (r‐a‐Si). Moreover, its ϵ differs from the ϵ of evaporated a‐Si (e‐a‐Si) found in the handbooks as ϵ for a‐Si. If we use this ϵ to evaluate SE measurements of ion implanted silicon then the fit is very poor. We deduced the optical band gap of these materials using the Davis–Mott plot based on the relation: (ϵ2E2)1/3 ∼ (E− Eg). The results are: 0.85 eV (i‐a‐Si), 1.12 eV (e‐a‐Si), 1.30 eV (r‐a‐Si). We attribute the optical change to annihilation of point defects.

Nondestructive determination of damage depth profiles in ion‐implanted semiconductors by spectroscopic ellipsometry using different optical models

A several‐parameter fitting of spectroscopic ellipsometry data is developed to characterize near‐surface layers in semiconductors damaged by implantation. The damage depth profiles are described by either rectangular, trapezoid‐type, or coupled half‐Gaussian (realistic) optical models. The rectangular model has three parameters: the average damage level, the effective thickness of the implanted layer, and the thickness of the native oxide. The trapezoid‐type model is enhanced with a fourth parameter, the width of the amorphous/crystalline interface. The realistic optical model consists of a stack of layers with fixed and equal thicknesses. The damage levels are determined by a depth profile function (presently coupled half‐Gaussians). Five parameters are used: the position of the maximum, the height, and two standard deviations of the profile, plus the thickness of the native oxide. The complex refractive index of each layer is calculated from the actual damage level by the Bruggeman effective medium appr...

Angle-resolved X-ray photoelectron spectroscopy (ARXPS) and a modified Levenberg-Marquardt fit procedure: a new combination for modeling thin layers

The combination of angle-resolved X-ray photoelectron spectroscopy (ARXPS) and a modified Levenberg-Marquardt (LM) fit procedure has been used to study a native oxide layer on a clean Si(100) substrate. Numerical calculations show that with an aperture of 3° or 9° of the electron analyser, the photoelectron take-off angle should not exceed 80° or 70°, respectively, as compared to normal take-off angles. At larger photoelectron take-off angles, the effect of the aperture on the photoelectron energy distribution may not be neglected. We show how absolute ARXPS measurements in which the same XPS feature is considered at several electron take-off angles are an alternative for relative ARXPS film thickness measurements, avoiding large errors in the quantitative results. Models for the composition and thickness of the oxide layer have been developed. Also, the errors in the parameters of these models have been calculated. It can be concluded that the native oxide layer on silicon is 27 ± 1 (±-5%) A thick and that the ratio of the silicon atom concentration in the substrate to that in the native oxide layer is 3.7 ±- 0.3 (±-8%), values that agree well with the literature. This report shows that the combination of ARXPS and a LM fit procedure is well suited to study ultra-thin layers and gives reliable results.

Epitaxial YBa2Cu3Ox thin films on sapphire with a PrBa2Cu3Ox buffer layer

Highly c‐axis‐oriented YBa2Cu3Ox thin films have been grown on (1102) sapphire substrates by a modified off‐axis magnetron rf‐sputtering technique. To prevent the interaction and to improve the lattice match between film and substrate a thin layer of 10–60 nm of PrBa2Cu3Ox was used as a buffer layer. The diffusion process was effectively blocked and the epitaxy was enhanced. The Jc (77 K) has been measured to be 4×105 A/cm2 which is significantly higher than the Jc of films directly grown on Al2O3 (∼8×104 A/cm2). The surface resistance of such films at 11 GHz and 4.2 K was measured to be 80 μΩ for 50‐nm thin films. These results are comparable to the best results reported so far on sapphire with other buffer layers.

Preparation and properties of all high T/sub c/ SNS-type edge DC SQUIDs

High-T/sub c/ SNS-type Josephson junctions and DC SQUIDs were successfully fabricated using hetero-epitaxially grown multilayers of YBa/sub 2/Cu/sub 3/O/sub x/ and PrBa/sub 2/Cu/sub 3/O. These layers are c-axis oriented, and hence edges of the multilayers give rise to a current flow in the ab-plane between the electrodes of a Josephson junction. The necessary structuring was done by Ar ion beam etching. The individual junctions exhibit a supercurrent up to 80 K. The I/sub c/R/sub n/ product of these junctions usually has a lower limit of 8 mV at 4.2 K. Voltage modulation of the first DC SQUIDs can be observed up to 66 K. The voltage modulation for various bias currents investigated at 4.2 K noise measurements were performed. Details on the fabrication and measurements are presented.

305)SrTiO3 as substrate for coherently tilted epitaxial YBa2Cu3Ox thin films

High Tc superconducting YBa2Cu3Ox (YBCO) thin films have been prepared on (305) SrTiO3 (STO) substrates. X‐ray diffraction analysis and Rutherford backscattering experiments reveal that the c‐axis of the layers is directed along the [001] STO axis. Bragg reflection measurements from YBCO lattice planes with high h, k, and l indices confirm that the film growth is epitaxial and almost single domain. For the critical current density jc(77 K) values of 2×106 and 1×103 A/cm2 have been found in the [010] and [501] YBCO directions, respectively.

Noise properties of direct current SQUIDs with quasiplanar YBa2Cu3O7 Josephson junctions

We describe the noise performance of dc SQUIDs fabricated with quasiplanar ramp‐type Josephson junctions on the basis of c‐axis‐oriented YBa2Cu3O7/PrBa2Cu3O7 thin‐film heterostructures. The noise spectrum of the dc SQUIDs was measured with dc‐ and ac‐bias schemes at different temperatures and showed values below 10−5 Φ0/Hz1/2 down to frequencies of about 1 Hz at 70 K. Up to now for the magnetic fluxnoise and the energy resolution obtained at 1 kHz and 77 K the best values were 2.5×10−6, Φ0/Hz1/2 and 3×10−31 J/Hz, respectively. A study of the white and 1/fnoises of the SQUIDs was performed. The influence of magnetic flux, bias current, high static magnetic fields, and aging on the SQUID noise were investigated. The junctions and devices do not degrade due to aging in air or thermal cycling.

Quantitative analysis of X-ray photo emission spectra, acquired on c-axis oriented high-Tc superconducting YBa2Cu3O7-delta thin films

Angle-resolved X-ray photoemission spectroscopy (ARXPS) was performed on c-axis oriented high-Tc superconducting YBa2Cu3O7−δ thin films. The layered structure of the YBa2Cu3O7−δ films was used to develop a model for the quantitative analysis of the ARXPS experiments. Our XPS results may be compared to spectra taken on YBa2Cu3O7−δ single-crystal surfaces. On the spectra features are supposed that are assigned to a thin non-superconducting surface layer. For the first time, relative ARXPS measurements show that the interface between the superconducting YBa2Cu3O7−δ film and the surface layer is formed by the atomic Y layer. The surface layer consists mainly of BaCO3 and C, A small volume fraction (≈20%) contains Ba- and Cu-oxides, probably in the form of BaCuO2. From absolute ARXPS measurements, the thickness of the surface layer was calculated to be 0.93 ± 0.06 nm. It is shown that the surface roughness of our films is on the order of 0.6 nm. A good agreement between theory and experiment has been found in this report.

Epitaxial multilayers of YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox

Heteroepitaxial multilayers of YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox have been made by sputtering. No degradation of the transition temperature and the critical current density due to the presence of the PrBa2Cu3Ox layer could be observed. By using high-resolution transmission electron microscopy the atomic details of the interfaces and the defect structures have been studied. These films showed a perfectly stacked lattice just above the interface between film and substrate. The orientation of the c-axis perpendicular to the substrate was fairly perfect. The structural faults are mainly distributed in the middle and overlying layers. The dominant defects in our films seems to be stacking faults which give rise to nano-sized coherent anti-phase domains with the 1-2-3 structure. Rutherford backscattering spectroscopy, secondary ion mass spectroscopy, and scanning Auger microscopy were used to examine the interdiffusion between layers. Within the experimental resolution of 7 nm no interdiffusion is visible between YBa2Cu3Ox and PrBa2Cu3Ox layers.