Probing inhomogeneities in nanoscale organic semiconductor films: Depth profiling using slow positron beam and X-ray reflectivity techniques Priya Maheshwari a , D. Bhattacharya b , S.K. Sharma a , S. Mukherjee a , S. Samanta c , S. Basu b , D.K. Aswal c , P.K. Pujari a,n a Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India b Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India c Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India article info Article history: Received 8 May 2014 Received in revised form 28 August 2014 Accepted 1 September 2014 by A. Pinczuk Available online 16 September 2014 Keywords: A. Organic semiconductors C. Defects and nanostructure E. Positron annihilation spectroscopy abstract Depth profiling studies in 200 nm organic semiconductor (OSC) films on quartz substrate have been carried out using slow positron beam and X-ray reflectivity (XRR) techniques with the objective of examining structural inhomogeneities in as-deposited film and those annealed at high temperature. Grazing incidence X-ray diffraction and atomic force microscopy measurements are carried out to examine the crystallinity and surface morphology, respectively. In general, annealing is seen to modify the morphology and nanostructure. However, a significant inhomogeneity in nanostructure, marked by a disordered layer with low density region is observed in the film annealed at 200 1C from positron as well as XRR measurements. This study highlights the sensitivity of these techniques to defects and inhomogeneities in nanoscale that may have profound influence on device performance. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Organic thin films are primary building blocks for the next generation organic electronics due to significant advantages of flex- ibility, low cost, large area applications and ease of tailoring material properties [1–4]. In recent years, there has been an increasing interest in organic semiconductor (OSC) thin film technology due to many intriguing applications such as chemical and gas sensors, organic transistors, organic photovoltaic cells and organic solar cells [5–9]. It has been seen that electronic, optical and even magnetic properties of organic thin films strongly depend on their structure and morphology [10,11]. Therefore, ordered, smooth and defect free surfaces are prere- quisites for achieving high performance electronic devices. Fabrication of the organic thin films is one of the most crucial aspects in OSC industry. The growth behavior of OSC molecules depends on the nature of substrate, its temperature, deposition technique, rate of deposition and post deposition annealing treatment [12–16]. There are number of studies using variety of techniques addressing the growth behavior and its effect on the performance of organic devices [3,17]. In this regard, structural disorder/defects are seen to strongly influence the charge mobility in OSC devices. These disorder may act as trapping sites for charge carriers and result in reduced charge mobility leading to deteriorating device performance. This necessitates defect charac- terization in thin OSC films for optimum device performance. Metal phthalocyanines (MPcs) are a class of OSC consisting of a central metal atom bound to a π-conjugated ligand. They are well known for their high thermal and chemical stability [18,19]. Extensive studies on the growth of nanoparticles, nanoflowers, nanorods, nanoribbons, molecular orientation under different growth conditions and the effect of annealing temperature on the morphology have been carried out [20–22]. Gentry et al. have shown the dependence of grain size and its distribution on the deposition temperature [23]. There are studies related to the effect of thickness and deposition temperature on the structural and opto-electronic properties of ZnPc thin films [24]. Other reported work on MPcs include the effect of molecular orientation on charge transport and gas sensing characteristics of thin films [25]. In addition to structural properties, photoconductivity behavior, opto-electronic and magnetic properties [10,26] have been studied for different MPc thin films. It is therefore, essential to have extensive knowledge of the structural properties and control over film structure for tailoring properties like charge transport, opto-electronic response and stability of the electronic devices [27]. Techniques such as atomic force microscopy (AFM) and scanning electron microscopy (SEM) are often used for structural characterization, although mainly Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ssc Solid State Communications http://dx.doi.org/10.1016/j.ssc.2014.09.001 0038-1098/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ91 22 25595326; fax: þ91 22 25505151. E-mail address: pujari@barc.gov.in (P.K. Pujari). Solid State Communications 200 (2014) 22–28