Formation of planar n+pockets in GaAs for mixer diode fabrication

A novel technique has been developed to produce n
⁺
"pockets" in semi-insulating GaAs bulk material. This technique has produced thick pockets of highly conducting epitaxial material on the substrate surface. The pockets were formed by the growth of a liquid-phase epitaxial (LPE) layer into holes which had been etched into the substrate. Surface uniformity was obtained by chemo-mechanically polishing the substrate surface under tightly controlled conditions. Polishing rates as low as 0.2 µm/min have been obtained. Photographs taken of the pocket cross-sectional area have revealed that growth occurred throughout the entire pocket region. Growth was even found to have occurred along the irregularly shape walls of the pockets. The continuous growth throughout the pockets coupled with the subsequent polishing of the substrate have produced exceptionally smooth and planar surfaces. Thicknesses as great as 10 µm have been obtained for the n
⁺
pockets using this technique. Mixer diodes have been fabricated onto these layers and tested. Preliminary dc measurements taken on these devices have yielded a zero-biased cutoff frequency (F
co
) of 800 GHz with a series resistance (R
s
) of 6 Ω and a zero-biased capacitance (C
0
) of 30 fF.