Ground and space processing of single-crystalline organic thin films

Physical vapor transport (PVT) crystal growth experiments are being conducted with nonlinear optical organic materials in semi-closed ampoules which allow a vacuum to penetrate into the growth chamber, removing impurities and decomposition products from the growth interface. One material being grown is N,N-dimethyl-p-(2,2-dicyanovinyl) aniline (DCVA). Two samples of DCVA were flown in separate, seeded PVT chambers on Space Shuttle mission STS-59 in April 1994. The growth parameters were set to reproduce laboratory experiments that yielded small, bulk crystals. Surprisingly, however, the results from the STS experiment were, for both samples, the production of single-crystalline thin films. A second set of 8 PVT cells with DCVA was flown on STS-69 in September 1995. Several parameters were changed to study their influences on the film growth process and, in summary, the film growth process was found to be very robust. Films were obtained in all cases, albeit with varying quality. An evaluation of the flight- and ground-test data revealed a slower heat-up rate than was typical of the laboratory experiments. No background pressure measurements were made, but the temperature profile is an indication of a much higher background nitrogen pressure than the millitorr range used in the laboratory. A 4-year research grant has been obtained to study this film-growth phenomenon. Ground-based experiments are beginning to be carried out involving the varying of source and substrate temperatures, nitrogen background pressure, and the substrate material.