We thank Miss I. Kuna for assistance with the 3CS-resist exposures;
Mr. J. Hofstetter for carrying out the HPLC investigations;
and Siemens Systemtechnik;
Berlin;
for providing the MESFET specimens shown in Fig. 7. This work was supported in part by the Bundesministerium for Forschung und Technologie (BMFT;
FRG);