Impact Thickness on Structural and Electrical Characterization of Nickel Phthalocyanine Thin Films

Thin films of Nickel Phthalocyanine have been prepared by evaporation technique for (50 350 nm) of thickness. XRD studies show that the thin films have single crystalline structure for low thicknesses with (100) orientation and the crystallite size increased with increased thickness. Also from the AFM technique for NiPc films, the roughness was determined and the grain size increases with increasing of thickness from except at thickness 350 nm. The studies of electrical properties, morphology and orientations of the crystallites are important to understand and predict the nature of the films and essential for their successful applications in solar cell and sensors. The electrical properties of these films were studied with different thickness, NiPc has three activation energy. Carrier’s concentration and mobility was calculated. Hall measurements showed that all the films are p-type.


Introduction
Nowadays, semiconducting organic materials are very important since they have successful application in optical and electronic devices. One of these materials is Phthalocyanines (Pcs); it has significant properties which made it a good alternative for development electronic devices. These materials have been used in the gas sensor and in electronic devices because it possesses many advantageous properties such as thermal, chemical and photochemical stability, excellent film growth and good optical and electronic properties [1]. Their thermally stable nature makes them suitable for thin film deposition by thermal sublimation.
These materials have also shown promise for photoconductive and photovoltaic response [2]. Phthalocyanines are a class of planar aromatic organic compounds that have attracted a great deal of attention for quite some time because of their unique properties such assemiconductivity, photoconductivity, photochemical reactivity, chemical stability, electrochromism, bio-organic and catalytic activity and their application in the field of colour display technology and gas sensors.
The electrical, optical and structural properties of phthalocyanine thin films depend on different parameters such as evaporation rate, substrate temperature and post-deposition annealing [3] [4]. In addition to their excellent photoconductive properties, Pc-s have the advantages of being very stable against thermal and chemical decomposition and present very intenseoptical absorption in the visible region. These properties similarity to chlorophyll have many applications in solar cell. Also, they have the potential to serve as active material for molecular electronic devices such as electrochromic displays, chemical sensors and optical data storage. Furthermore, interests in Pc compounds have recently been renewed due to the discovery that they form molecular metals after partial oxidation [5]. The physical properties of thin films materials depend very much on the structure properties; therefore, they have always high priority. The knowledge of the composition of the film is important to understand and predict the nature of the films [6]. Thin films of Phthalocyanines are chemically and thermally durable and are prepared by vacuum thermal evaporation [7] [8] [9]. NiPc is thermally stable and its thin film can be deposited by thermal evaporation without dissociation [10] [11]. A number of analytic techniques are available for the composition characterization of thin films. These include X-ray diffraction studies, AFM and electrical studies. The objective of this paper is to study the effect of thickness on the structural, compositional and electrical studies of Nickel phthalocyanine (NiPc) thin films prepared by vacuum evaporation technique.

Experimental
Nickel phthalocyanine thin films, with thickness (50, 110, 265 and 350) nm, were deposited on cleaned glass substrate (type corning, China) with dimensions (7.5 × 2.5 × 0.1) cm by thermal evaporation technique (Edwaed coating unit model 306 A) under high vacuum with pressure of (6 × 10 −5 ) mbar with deposition rate of about 20 nm/min. A molybdenum boat was used as a source for the evaporation of the material. Set a distance of 15cm to separate the substrate and the boats. The films were characterized by X-ray diffraction technique using (Philips X-ray diffractometer) with CuK α radiation at wavelength (1.5406) A˚. Atomic

X-Ray Diffraction XRD
X-ray diffraction pattern of NiPc thin film with different thickness (50, 110, 265 and 350) nm are shown in Figure 1 respectively. XRD pattern indicates that all the samples are single crystal and that it has β-crystalline phase [12]. The difference between αand β-phases is attributed to the tilt angle of b-axis of the unit cell. The structure of the NiPc thin film is determined as tetragonal with preferential orientation along the (100) direction. Well-defined diffraction peaks by (100) give the direction of the preferential orientation as deposited film (JCPDS, file No. 11-0744). The diffraction peaks are agreement with the previous observations [1]- [10]. The spacing between the planes (d) has been calculated using the Bragg's formula: as shown in where k = 0.94 is a constant, λ -the wavelength of x-ray, β-the full width half maximum and θ-the diffraction angle, the grain size increases from (20.85 -33.5 nm) for thickness (50 -265 nm) and then decreased to 23.2 at 350 nm as in The dislocation density δ defined as the length of dislocation lines per unit volume of the crystal and can be evaluated from the particle size D by the relation [17].      where n is a factor, when equal unity giving minimum dislocation density. The microstrain is related to the lattice misfit, which in order depends on the deposition conditions. The microstrain ε is calculated using the relation [17]: cos 4 ε β θ = (4) Figure 2 and Figure 3 represent the grain size and dislocation density δ and the microstrain ε of prepared thin films evaporated with different thicknesses on glass substrates. It was observed that the dislocation density and microstrain exhibit a decreasing trend with the deposition thin film thickness (except at 350 nm) which represents better lattice quality.

Atomic Force Microscope
This section of studies includes the effect of thickness on the morphology of   Table 2. It is observed that the roughness increase with thickness increase, and this is agreement with other researchers [18] [19] [20].
where ΔE is the thermal activation energy and k B is the Boltazmann's constant. A plot of lnσ against (1000/T) yields a straight line whose slope can be used to determine the thermal activation energy of the film. Figure 5 shows  figure, there are three semiconductor distinct linear parts, which correspond to three activation energies ΔE 1 , ΔE 2 and ΔE 3 . ΔE 1 corresponds to extrinsic region and represents transition process for carriers within localized states in the energy gap and this suggests the existence of high density of localized states in the energy gap, and ΔE 2 and ΔE 3 corresponds to intrinsic region and represents the carriers transport across the grain boundaries by thermal excitation. The change in the slope and hence the activation energy is interpreted as a change from extrinsic to intrinsic conduction [21]. The value of the thermal activation energy ΔE 1 , ΔE 2 and ΔE 3 is shown in Table 3, which is in good agreement with those obtained by other workers [22] [23]. The room temperature conductivity increases as the thickness increases and this is attributed to the change in the degree of crystallinity. Also, as given in Figure 6, the activation energy varies as the thickness increases and this is because of the improved crystallinity with the increase of the grain size.

Conclusions
Thin films of Ni-Pc were prepared by the thermal evaporation method on glass substrates with different thickness successfully. From the x-ray diffraction studies, it is observed that the structure is single crystalline β-phase film-oriented preferentially (100) plane for all thickness. The structure properties are found to be very sensitive to the thickness of thin film. The grain size increases with in-