Lateral conductivity in two-layer molecular heterostructures based on phthalocyanine

In this work the effect of thermal annealing on the structure of thin vacuum sublimed films of boron subphthalocyanine chloride, as well as on their electro-physical properties, was demonstrated.

The surface topology of the SubPcBCl films was studied depending on the type of substrate, on which the deposition has been carried out, by means of interferometry, atomic force microscopy and X-ray diffraction. It was shown that the films consist of grains of nearly spherical shape. The surface roughness is approximately order of magnitude less that the maximal drop of heights across the scanned area. Comparing the roughness of the films deposited on different substrates one can conclude that smoother films grow on the ceramic surfaces, while more rough films are obtained on metallic underlayer.

It was found out that initial SubPcBCl layers are amorphous regardless of their thickness and materials of deposition surface. Successive annealing of films in air up to the temperatures close to beginning of sublimation initiates gradual increase of the roughness, growth of surface clusters, at Т>180 оС it causes possible breakage of continuousness of the layer due to formation of objects, whose height is comparable with nominal thickness of film. Formation of such objects over the basic layer, which becomes thinner, is accompanied by the appearance of diffraction maxima in X-ray spectra. This allows to identify the crystalline structure of the films.

The observed structural transformations of the films affect their specific conductivity. Dark and light-assisted conductivity in non-annealed films differ by two orders of magnitude, whilst dark current corresponds to few picoampheres. Successive annealing leads to significant decrease in the absolute photocurrent; dark conductivity increases slightly. Since, based on obtained data, the annealing of thin SubPcBCl films causes deterioration of surface quality and continuousness of the films, whereas its influence on specific conductivity is ambiguous, the use of amorphous films for fabrication of sandwich-type devices is preferred.