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.
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 [
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 force microscopy was used to determine the roughness snd surface morphology of films. Digital electrometer type Keithly 616 and electrical oven have been used for electrical measurements. The Hall coefficient (RH), carrier type and Hall mobility (µH) of films were measured by the circuit contain D.C power supply (0 - 40) V and two digital electrometers (Keithley type 616 to measure the current and voltage
X-ray diffraction pattern of NiPc thin film with different thickness (50, 110, 265 and 350) nm are shown in
2 d sin θ = n λ (1)
as shown in
D = k λ / β cos θ (2)
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 [
Thickness nm | 2θ (degree) | d (A˚) | FWHM | Grain size (nm) | ε * 10−3 | δ * 10−5 Line/nm2 |
---|---|---|---|---|---|---|
50 | 6.9654 | 12.6804 | 0.3986 | 20.85 | 1.735 | 2.298 |
110 | 7.0218 | 12.5787 | 0.3735 | 22.2 | 1.625 | 2.018 |
265 | 7.0068 | 12.6056 | 0.2479 | 33.5 | 1.079 | 0.889 |
350 | 6.9751 | 12.66282 | 0.3583 | 23.2 | 1.559 | 1.857 |
δ = n / D 2 (3)
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 [
ε = β cos θ / 4 (4)
Atomic Force Microscope
This section of studies includes the effect of thickness on the morphology of prepared films were determined by Atomic Force Microscope with different thickness (50, 110, 265 and 350) nm taken in an area of 10 × 10 μm2 as shown in
Thickness (nm) | Roughness Average (nm) | Grain Size (nm) |
---|---|---|
50 | 5.29 | 48.33 |
110 | 6.2 | 87.17 |
265 | 6.31 | 203.28 |
350 | 8.97 | 91.25 |
Electrical properties
The electrical conductivity of NiPc thin films was performed to determine the thermal activation energy. Measurements were carried out in the temperature range 303 to 473 K for films with different thickness ranging from 50 to 350 nm. The temperature dependence of the conductivity can be expressed by Arrhenius equation. [
σ = σ exp ( Δ E / k B T ) (5)
where ΔE is the thermal activation energy and kB is the Boltazmann’s constant. A plot of ln σ against (1000/T) yields a straight line whose slope can be used to determine the thermal activation energy of the film.
figure, there are three semiconductor distinct linear parts, which correspond to three activation energies ΔE1, ΔE2 and ΔE3. ΔE1 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 ΔE2 and ΔE3 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 [
Carrier concentration and Hall mobility have been determined from Hall measurements for NiPc thin films, having different thicknesses which were deposited on glass substrates at 300 K. The Carrier concentration (n) is determined for all film thicknesses and it is of p-type. Hall coefficient (RH) was calculated, then carrier concentration has been calculated for each thickness and it was of p-type. Carrier mobility was determined for each thickness. All these parameters are shown in
Thickness nm | sR.T ´ 10−7 (W∙cm)−1 | ΔE1 (eV) | Temp∙Range (K) | ΔE2 (eV) | Temp∙Range (K) | ΔE3 (eV) | Temp∙Range (K) |
---|---|---|---|---|---|---|---|
50 | 18.524 | 0.051 | 307 - 333 | 0.0691 | 338 - 389 | 0.156 | 400 - 454 |
110 | 6.476 | 0.076 | 333 - 369 | 0.147 | 373 - 413 | 0.477 | 413 - 446 |
265 | 8.654 | 0.017 | 305 - 348 | 0.099 | 348 - 403 | 0.518 | 403 - 458 |
350 | 4.948 | 0.051 | 307 - 353 | 0.241 | 363 - 403 | 0.603 | 418 - 473 |
Thickness (nm) | Carrier Concentration*1012 (cm−3) | Carrier Mobility cm2/v.s. |
---|---|---|
50 | 11.2 | 13.21 |
110 | 2.00 | 54.02 |
265 | 8.68 | 13.32 |
350 | 1.64 | 15.28 |
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 increasing thickness. The change in grain size shows that the properties of NiPc are controlled by its structure and morphology depending on increases the thickness. The surface morphology of prepared films and the crystal morphology vary with thickness. From the electrical properties measurements, there are three semiconductor distinct linear parts, which correspond to three activation energies ΔE1, ΔE2 and ΔE3. ΔE1 corresponds to extrinsic region and ΔE2 and ΔE3 correspond to intrinsic region. Hall effect measurements show that all the films are p-type. The carrier mobility increases with the decrease of the carrier concentration and varies with the increase of film thickness.
The authors declare no conflicts of interest regarding the publication of this paper.
Nasir, E.M., Hussein, M.T. and Al-Aarajiy, A.H. (2019) Impact Thickness on Structural and Electrical Characterization of Nickel Phthalocyanine Thin Films. Advances in Materials Physics and Chemistry, 9, 123-132. https://doi.org/10.4236/ampc.2019.97010