Electrochemical deposition, characterisation of metal films, and the modification of electrodes by near-field photolithography

Cobalt films of various thickness were electrodeposited from a 0.4M sulphamate solution onto single crystal n-GaAs substrates. The growth of the films was found to be from a uniform distribution of instantaneously formed nuclei. AIM and STM measurements suggested that the films were continuous, with a uniform grain size of approximately 30nm. The saturated surface roughness of the films was itself found to saturate at approximately 6nm for films of above 30nm thickness. Electron diffraction indicated a predominantly hexagonal close packed structure. Epitaxy was also found, with the cobalt c-axis aligned with the GaAs [110] and [110] directions, and with the cobalt <1120>directions parallel to the GaAs [001] direction. The average thickness of the film, obtained by calibrated ion milling, was found to be close to the values obtained by integration of the deposition current. The barrier height of the rectifying Co/GaAs interface was measured as 0.88#+-#0.05V. Metal coated optical fiber probes, prepared by melt-pulling, were used as 100nm apertures for near-field electrochemical photolithography. In situ STM feedback was used to position a probe within 100nm of a Co-on-n-GaAs working electrode. Etching of two parallel features was achieved by the anodic biasing of the electrode and using the carriers generated by the near-field illumination. The line width of approximately 2#mu#m was consistent with the diffusion length of holes in GaAs. A shear-force feedback system was used to position a probe within a few nanometers of a p-InP electrode. An anodic pre-treatment was used to sensitise the surface to light, prior to the near-field deposition of cobalt from a sulphate solution. Granular deposits were produced, within 2.5#mu#m of the centre of illumination. A new transmission X-ray absorption method was developed, in which the electrodeposited cobalt-on-n-GaAs films were used as both the sample and detector. The new method produced comparable results to the established, total-electron-yield technique. The discrepancies in the absorption spectra obtained for films of different thickness were explained using models based on the surface roughness measurements. A potentially fundamental problem, caused by a contribution of Auger electrons from the metal to the photocurrent measured by the new system, was considered. Experimental evidence, however, suggested that the contribution was negligible. MXCD calculations, performed using the X-ray spectra obtained using both methods, were used to derive values of the spin and orbital moments of the electrodeposited films. The average spin moment was found to be 1.77 by the new method and 1.43 by the total electron yield method, compared to a theoretical value of 1.52. The value obtained for the orbital moment was 0.4 in both cases, compared to the theoretical value of 0.15. The orbital moment is affected by the bonding in the film, which could explain the discrepancy. (A statistical error of 0.14% was estimated from the fluctuations in a horizontal section of the spectra. This is much lower that systematic errors caused by surface roughness and calibration drift, and was therefore not quoted). (author)Available from British Library Document Supply Centre-DSC:DXN037597 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo