Phase transition of copper(II) phthalocyanine thin films characterized by a near-field microwave microprobe

Abstract

Metal phthalocyanine is one of the promising organic compounds due to the possibility of applications in electrooptical devices, photoconducting agents, photovoltaic cell elements, nonlinear optics, electrocatalysis, and other photoelectronic devices. Several metal substituted phthalocyanines have been widely investigated. Among the metal substituted phthalocyanines, copper (II) phthalocyanine (Cu-Pc) has been found to have superior properties. Cu-Pc is a p-type semiconductor and mobility changes could be achieved by employing controlled substrate heating temperature during deposition. The crystal phase and the grain orientation of Cu-Pc depend on the heat-treatment temperatures and substrate heating conditions [1]. In order to better characterize the electrical properties of theαand the β-phase of Cu-Pc thin films, we take advantage of the nondestructive evaluation capabilities of a near-field microwave microprobe (NFMM) [2]. NFMM techniques with high sensitivity have been developed for the microwaveand millimeter-wave ranges. An important ability of the NFMM is nondestructive and contactless characterization of thin films, in particular, the characterization of electrical properties of organic thin films. Organic multilayer thin films have attracted considerable interest in regard to various display applications. Nondestructive and contactless characterization techniques are very useful for these applications. NSMM technique, which directly measures the physical properties such as surface resistance of organic thin films shows practical promise. To study the phase transition of Cu-Pc thin films, wemeasured the surface resistance using a NFMM by measuring the microwave reflection coefficient S11. The crystal structure of Cu-Pc thin films transformed from the α-phase of the orthorhombic crystal to the thermally stable β-phase of themonoclinic crystal as the substrate heating temperatures increased. The surface resistance depended on the crystal structures of the Cu-Pc thin films. As the phase changed from the α-phase to the β-phase, the surface resistance of the Cu-Pc thin films decreased.