Morphological, structural and electrical properties of pentacene thin films grown via thermal evaporation technique

Fatin Nor Ahmad, Yusmar Palapa Wijaya, Khairul Anuar Mohamad, Nafarizal Nayan, Megat Muhammad Ikhsan Megat Hasnan, Afishah Alias, Bablu Kumar Ghosh Faculty of Electrical and Electronic Engineering, Universiti Tun Hussein Onn Malaysia, Malaysia Microelectronics and Nanotechnology-Shamsuddin Research Center (MiNT-SRC), Universiti Tun Hussein Onn Malaysia, Malaysia Faculty of Applied Science and Technology, Universiti Tun Hussein Onn Malaysia, Malaysia Faculty of Engineering, Universiti Malaysia Sabah, Malaysia


INTRODUCTION
In this new era of science and technology, organic electronics is experiencing rapid growth in the tremendously exciting area of research and development in organic electronics. Organic molecular crystals are assembled from the intermolecular interaction with weak van der Waals forces, and multiple crystalline packaging states commonly exist in the active layer of a semiconductor device. Pentacene (C22H14) is one of the p-type organic semiconductor materials that are widely used in the optical and electronics fields. It is a polycyclic aromatic hydrocarbon made up of five fused benzene rings with the appearance of purple or deep blue solid color in powder form. Nevertheless, when pentacene is exposed to light, air, or chemicals, it will change to green color due to degradation. Pentacene's crystalline structure is triclinic, involving thin-film, bulk, and single-crystal phases [1]. When ultraviolet (UV) or visible light is absorbed, the compound will be excited. When the number of acene rings increases, the bandgap of pentacene decreases and the temperature increases. Pentacene has unique properties compared with other linear acenes, such as benzene, naphthalene, anthracene, tetracene, hexacene, and heptacene. Among organic materials, pentacene thin films exhibit the highest mobility of up to 35 cm 2 /Vs [2]- [5] in transistors, lending to their potential application in inexpensive electronic devices due to their low cost [6], flexibility, large area, light weight, and low-temperature processing [7]- [9]. These materials are deposited on various substrates including silicon, transparent plastic, and glass. The fabrication of optoelectronic devices on transparent substrates such as, glass or indium tin oxide (ITO)-coated glass, is attracting interest for applications in photo-detectors and photo-multiplications for broadband and narrowband response-ability using organic-only or organic-inorganic heterojunction diode structures.
There are a number of deposition processes for growing pentacene thin films. From reported studies, pentacene thin films are mostly grown using physical deposition, such as thermal evaporation [7], [10]- [14], pulsed laser deposition (PLD) [15], [16], and organic molecular beam deposition (OMBD) [16], whereas chemical deposition, such as ink-jet printing, dip coating [17], and solution-based spin coating [18], depends on preparation conditions and the material's nature. The physical evaporation techniques operate by changing the phase of the pentacene from a solid phase to the vapor phase and converting again to a solid phase on the substrate. Meanwhile, chemical deposition is strongly dependent on the chemistry of the solution, pH value, and viscosity. Soluble pentacenes, such as 6,13-bis(triisopropylsilylethynyl) pentacene, also known as TIPSpentacene, have gained consideration for the deposition of thin films from solutions. The selection of technique will affect the film's quality and the overall appearance on the substrate [19].
This work reports the processes involved in the deposition of pentacene thin films via thermal evaporation, emphasizing on the inorganic conductive oxides with a transparent substrate. The influence of deposition time and film thickness with constant pentacene's weight and vacuum pressure was discussed on the morphological and structural properties of pentacene films. Finally, we investigated the electrical properties of the pentacene thin films with varying deposition times using a metal-organic metal structure for the development of a heterojunction configuration of pentacene and conductive oxide for photo-diode applications. Figure 1 shows the flowchart of the deposition process of pentacene thin films on ITO-coated glass using the thermal evaporation technique. Pentacene powder (Sigma-Aldrich) was used without any other further purification. The substrate was an ITO-coated glass with sheet resistance of 10 Ω/sq cut into 1.5cm2cm. The ITO-coated glass substrate was rinsed with acetone, methanol, and deionized water using ultrasonic bath for 10 minutes in 50 ml of each solution. Then, the substrates was dried slowly at room temperature using an air dust blower. 1293 powder in a tungsten boat and the substrate holder was about 10 cm. The applied current was kept constant at 38 A for each deposition time of 20 min, 30 min, and 60 min to optimize the condition of the thin films at room temperature. The vacuum system's pressure and the pentacene's powder weight were 6.1810 -2 Pa and 0.04 g, respectively. For electrical characterization, the top-contact was aluminium (Al). An Al wire was placed on the tungsten boat and deposited using thermal evaporation onto the pentacene thin film. Figure 2(b) shows the metal-organic-metal structure with pentacene thin film as an active layer. The thickness of the pentacene thin film was measured using field-emission scanning electron microscopy (FESEM) (JEOL JSM-7600F) with a 15 kV accelerating voltage through the FESEM crosssection. X-ray diffraction (XRD) was used to measure the structure of the pentacene thin film using Cu Kα radiation (λ=0.154060 nm) and analyzed using HighScore software. The 2θ range was 5°-40° with slit=½ omega 0.5°. Atomic force microscopy (AFM) was used using a contact mode to examine the morphology of the grains of the pentacene on ITO-coated glass. The scan size was 1 µm and the scan was analyzed using Spisel32 software. Meanwhile, the current-voltage (I-V) and current density-voltage (J-V) measurement of the metal-pentacene-metal structure was carried out using Oriel Sol1A software.  Figure 3(c) shows that another layer was detected, with a thickness of 131 nm, which was the bulk phase changing from the thin-film phase [1]. Figure 3(d) shows the relationship between deposition time and the thickness of the thin film. Thus, thickness increased gradually from 356 nm to 452 nm and 581 nm when the time for the evaporation process increased [20]. The longer deposition time revealed a thicker pentacene thin film.

Characteristics of the pentacene structure
Interestingly, Figure 4 reveals the presence of X-ray diffraction (XRD) patterns for pentacene thin films grown on ITO-coated glass with different deposition times. The films exhibited one phase with a peak of diffraction of the first order at 5.78°, corresponding to 15.5 Å of lattice spacing, which confirmed the thinfilm phase. Furthermore, XRD indicated a second spacing of a second phase at a diffraction angle of 6.18°, corresponding to the lattice spacing of 14.5 Å. This pattern showed the presence of pentacene in the bulk phase. It showed five polymorphs, labeled as the triclinic crystalline phase, for the pentacene deposits on the ITO-coated glass substrate. (00k) represents the thin-film phase, while (00k') represents the bulk phase. The pentacene thin film only possessed crystallinity on the (001) plane at deposition time of 20 min and 30 min.  nucleation [13]. Nevertheless, both thin-film and bulk phases are crystalline and firmly structured. The crystalline size, D, for each peak of pentacene with different deposition times included in Table 1 by using Debye-Scherrer formula [21], where λ is the wavelength of the X-ray radiation used (0.15406 nm), β is the full width at half maximum (FWHM), and θ is the Bragg's diffraction angle of the main peak in the XRD spectra. However, the XRD peak can be widened through the defects and internal stress, so these obtained crystalline size values are comparable to the other reported values in the literature [22]- [26]. The AFM result shows the growth condition of the pentacene thin films using a scan size of 11µm 2 . It has small changes in film morphology with various deposition times. The pentacene films were well deposited for all deposition times but not homogenous due to the bulk phase of the pentacene grains. As verified in the XRD measurement, the bulk phase only emerged in the deposition time of 60 min due to the nucleation of the pentacene layer. From the overall AFM images, pentacene films deposited on ITO-coated glass exhibited a similar island formation with modular grains without visible dendrite crystallinity. The relationship between grain size and surface roughness is formed as Figure 5. The result indicates that as the pentacene film's roughness gradually increased, the grain size slowly increased; thus, the large grain size gave a fine crystalline structure for the pentacene film [10]. By increasing the deposition time, the roughness and grain size increased, with a better crystalline structure. The different roughness and grain size of pentacene on the ITO substrate indicated that the surface energy affected the film deposited at different deposition times.

Electrical characteristics of the pentacene thin film
The electrical characteristics of an Al/pentacene/ITO structure with three thicknesses of 356 nm, 452 nm, and 712 nm are provided in Figure 6. The Al/pentacene/ITO with 356 nm thickness revealed a rectification characteristic. The I-V curve changed to ohmic behavior for structures with 452 nm and 712 nm thicknesses, as shown in Figure 6(a) [28]. As the tip was forced harder into the sample, conductance increased. From the resistivity measurement using the gradient of the I-V curve, the trend of the pentacene thin film's resistivity showed an increase with the increase of the pentacene thin film's thickness. The resistivity values were 1.7210 10 , 2.6110 10 , and 3.4510 11 .cm for pentacene thickness of 356, 452, and 712 nm, respectively. The relationship between current density as a function of the pentacene's thickness is presented in Figure 6(b). Current density increased when the thickness was thinner with increasing forward-bias voltage.
where the largest current density of about 5.5210-5 A/cm 2 was obtained for voltage bias of +20 V with 356nm of thickness.

CONCLUSION
In summary, the pentacene's morphological and structural studies were done with deposition time of 20 min, 30 min, and 60 min using thermal evaporation. A longer deposition time of 60 min resulted in the formation of similar islands with modular grains on the pentacene thin film compared with 20 min and 30 min of deposition time. A comparison of the third-, fourth-, and fifth-order diffraction peaks revealed a higher diffraction intensity for pentacene thin film on ITO-coated glass and the presence of thin-film and bulk phases. Grain size and surface roughness increased as deposition time increased. The electrical characteristics of the metal/pentacene/metal structure revealed changes from the rectification to the ohmic behavior as the pentacene's thickness increased. In addition, current density was influenced by the nature of the pentacene thin film. Thus, deposition time is an essential parameter in determining the molecular orientation and the electrical characteristics of pentacene for two-terminal device performance.