Effects of substrate temperature, pressure, and high evaporation rates on nucleation, epitaxy, and structure of palladium thin films

Abstract

Using transmission and selected-area diffraction electron microscopy, investigations of vapor deposited Pd thin films on (001) NaCl substrates revealed that the degree of epitaxy (based on 100% for a continuous, single-crystal film) increases with substrate temperature, vacuum, and evaporation rate. At 10 -5 torr and an evaporation rate of approximately 1000 Å/sec, Pd film volume epitaxially oriented ranged from 0–60% in the substrate temperature range 50°–400°C. At 10 -8 torr for this same evaporation rate, the epitaxial volume ranged from 20–100% (single-crystalline) in the temperature range 100°–350°C. At an evaporation rate of roughly 3000 Å/sec, the epitaxial volume ranged from 25–100% in the temperature range 80°–200°C at 10 -8 torr. The grain size was observed to increase regularly with substrate temperature and vacuum at 1000 Å/sec evaporation rate, but to vary only slightly between 100°C and 200°C substrate temperature at 3000 Å/sec, with the films becoming single crystalline above 200°C at 10 -8 torr. The nucleation appeared to be homogeneous at the 1000 Å/sec rate while that for the higher rate (3000 Å/sec) appeared generally more heterogenous, suggesting a possible vapor-phase contribution to the nucleation process. Films could be prepared at the higher evaporation rate which were essentially continuous and single crystalline at approximately 200 Å in thickness. The distributions of dislocations and microtwins were observed to correspond to the size and distribution of converging nuclei in discontinuous thin films rather than having a misfit association. The misfit appears to have been accommodated by the close-packing of atoms composing the initial nucleation sites.