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CHAPTER 1

A HYBRID INORGANIC-ORGANIC HETEROJUNCTION DEVICE: STRUCTURAL MORPHOLOGY AND ELECTRICAL

RESPONSE

Abdul Ismail Abdul Rani1, Khairul Anuar Mohamad2, Pungut Ibrahim1, Ismail

Saad1, Ghosh Bablu Kumar1, Chee Fuei Pien3, Afishah Alias4

1Electrical and Electronic Engineering, Faculty of Engineering, Universiti

Malaysia Sabah, Jalan UMS, 88400 Kota Kinabalu, Sabah, Malaysia.

2Faculty of Electrical and Electronic Engineering, Universiti Tun Hussein Onn Malaysia 86400 Parit Raja, Batu Pahat, Johor, Malaysia.

3Faculty of Science and Natural Resources, Universiti Malaysia Sabah,

Jalan UMS, 88400 Kota Kinabalu, Sabah, Malaysia.

4Faculty of Applied Science and Technology, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Batu Pahat, Johor, Malaysia.

ABSTRACT

Organic-inorganic heterojunctions have a promising future due to their efficient optical and electrical properties, which make them technologically useful for optoelectronics applications. The aim of developing hybrid devices was to combine the significant advantages of each component that forms a heterojunction. It helps to decrease or overcome the drawbacks hindering to achieve the ideal behaviour of synergic effect that results in the development of new systems with new properties. Inorganic semiconductor materials offer a broader absorption of spectrum compared to the organic semiconductors. Their characteristic resemblance to green technology, optical and biocompatible properties presents improvements of response and sensitivity of electronic devices. Wide range resistance, high optical transparency in the visible light region and wide band-gap energy, enhances

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the application of oxide material for optoelectronics application. Combining such organics semiconductors with inorganics semiconductors of wide optical band-gap is a promising for optoelectronics applications and radio frequency identification (RFID) tags. Investigation of structural morphology and analysis of electrical response are presented to state the device performance based on a hetero-structure device configuration.

Keywords: Hybrid inorganic, heterojunction, optical device, inorganic-

organic semiconductor

1.1 INTRODUCTION Hybrid structure devices are receiving tremendous worldwide attention due to their potential solutions for semiconductor electronics. The combination of inorganic-organic semiconductor materials has unlocked new achievements in the performance of semiconductor devices such as photovoltaic cells, diodes, photodetectors and transistors [1–6]. Few decades ago, inorganic semiconductors such as silicon (Si), germanium (Ge), gallium arsenide (GaAs), and metals such as aluminum (Al) and copper (Cu) were the cores of the semiconductor industry. Most of the conventional semiconductors are made of inorganic materials due to their good electrical conductivity, high charge carrier mobility and wide band-gap energy [6–9]. However, inorganic semiconductors such as transparent conductive oxides (TCOs) possess drawbacks including poor mechanical flexibility, complex process and low reproducibility. This makes it difficult for the design engineers to accommodate inorganic materials with desired conditions for diverse applications. Thus, organic semiconductors were introduced in the electronic industry, since they are capable to replace the inorganics materials as an active layer with a better performance and low-cost procedures. The first highly conductive polymer was found in 1977, where the bromine was doped with trans-polyacetylene system [10]. During the early stages, the stability and performance of the organic semiconductors were very poor and were not significant enough to surpass the contribution of inorganic semiconductor such as amorphous silicon (a-Si). However, due to mass research towards organic materials during the past few decades, drastic improvements in the fabrication process has increased the performance and stability of the organic materials. The modern synthesis and extraction techniques applied to organic semiconductor materials have diversified their properties and applications.

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Now, organic semiconductors are widely commercialized as high end product such as organic light-emitting diodes (OLEDs), organic field effect transistors (OFETs) and solar cells [11–13]. In the recent years, technology of hybrid material system consisting of inorganic-organic semiconductors has sufficiently evolved and is becoming popular among researchers worldwide. A hybrid material system combines advantages of organic and inorganic materials along with improved physical and electrical performances. The inorganic semiconductors form huge crystalline networks with strong covalent or ionic bond when deposited onto the substrate layer. The organic semiconductors with smaller molecules, layers itself onto the inorganic film and produces a strong grip at the interface. This property leads to a higher electrons and holes carrier mobility in the device [14]. Various studies have been carried out to prove the significant contribution of hybrid semiconductor towards global electronics evolution. Improvements in terms of configuration, chip size, device area, material thickness and other parameters are being tested from years to years to obtain stability and best performance of the hybrid devices. This research will focus on a hybrid inorganic-organic diode composed of two materials with different structural and optical properties. The diode is combination of zinc oxide (ZnO) and polytriarylamine (PTAA) conjugated polymer acting as n-type and p-type semiconductor materials, respectively. The semiconductor configuration, allows more charge transfer between the interfaces of thin film, increases the recombination rate of holes and electrons. Thus, enhancing its ability to perform as a diode in optoelectronic applications. Figure 1 shows the diode configuration of organic-inorganic heterojunction.

Figure 1: Hybrid diode configuration of ITO/ZnO-PTAA/Al

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1.2 THIN FILM MATERIALS

1.2.1 Inorganic Semiconductor Zinc Oxide (ZnO) Zinc oxide (ZnO) is a well-recognized semiconductor material with remarkable characteristics and wide applications in semiconductor industry. It presents a wide bad-gap of 3.3 eV with high binding energy of 60 meV [15–17]. Moreover, high thermal and mechanical stability at room temperature increase its potential to be used in electronics and optoelectronic technology [15]. Additionally, ZnO has an excellent optical transmittance with approximate value of ~80% [16–19]. The crystalline structure of ZnO makes it useful in the semiconductor technology. The crystalline orientation of zinc and oxygen atoms held by ionic and covalent bonds, form a very smooth layered framework [19]. This layer of crystalline network enhances performance of the device by increasing the charge transfer in the layer. In addition, zinc oxide also exists as hexagonal structures, as shown in Figure 2. It has a lattice parameters number of a = 0.3296 and c = 0.52065 nm [20]. The tetrahedral coordination in ZnO crystal, results in a non-symmetric structure and consequently give it the piezoelectric and pyroelectric properties [20].

Figure 2: Hexagonal structure of zinc oxide

Moreover ZnO is associated with a large band gap, that provides electrical properties such as higher breakdown voltages, ability to sustain large electric fields, lower noise generation and high-power operation [17]. In case of

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sufficiently low electric fields, energy gained by the electrons is also low as compared to the thermal energy of electrons and thus, the energy distribution of electrons is unaffected by such low electric field. The scattering rates determine the electron mobility, therefore it can also be determined using the electron distribution function. In this function, the electron mobility is independent on applied electric field and it obeys Ohm’s law. The electron distribution function significantly deviates from its equilibrium value, when energy gained by electrons through applied electric field is no longer negligible as compared to the thermal energy of electrons [21,22]. Furthermore, transient transport occurs when the device dimensions are decreased to submicron levels. At short and critical periods of time, there is a minimal or no energy loss during transport at the gate terminal of field effect transistor (FET) or through the base terminal of bipolar junction transistor (BJT). The transient transport is characterized by onset of ballistic or velocity overshoot phenomenon [23,24]. A device operation become crucial at higher frequencies, when it exceeds the linear scaling dimensions, since electron drift velocity becomes higher than the steady state value.

1.2.2 Organic Semiconductor Polytriarylamine (PTAA) The development of polymeric organic semiconductors has attracted great interest in semiconductor industry, as these materials can be used to fabricate electronic components such as OLEDs [25]. Polytriarylamine (PTAA) is one of the potential contenders to become an active layer of a semiconductor device. PTAA is a solution-based polymer which presents several advantages such as stability at room temperature, soluble in chloroform or toluene and it only requires low temperatures for annealing process [26,27]. PTAA are highly soluble, amorphous semiconducting polymers and can achieve stable charge carrier mobility in the order of 5.0 × 10-3 cm2/Vs in both top and bottom gate transistors architectures [28,29]. Figure 3 shows the chemical structure of PTAA. The presence of amine nitrogen in polymer backbone prevents efficient delocalization of π electrons between adjacent phenyl units. This, limits effective conjugate length and causes low lying highest occupied molecular orbital (HOMO) energy levels and excellent oxidative stability [28,30]. The non-planar, rotationally-free and large linkage angle backbone prevents the optimal

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intermolecular π-electron aromatic stacking. This leads to an amorphous microstructure that limits the charge carrier mobility to lower values as compared to highly ordered crystalline materials [28].

Figure 3: Chemical structure of Polytriarylamine (PTAA)

1.3 PHYSICAL STRUCTURE AND MORPHOLOGY

1.3.1 Physical Structure The X-ray diffraction (XRD) method utilizes a rapid analytical technique. It is primarily used for the phase identification of a crystalline material and can provide information on unit cell dimensions. The advantages of using X-rays diffraction is its ability to penetrate thick samples to obtain diffraction pattern. Moreover, it provides higher diffraction angles to determine lattice parameters with greater precision. The data from X-ray diffraction can also be used to study the size of thin film grains by using Scherrer equation [26,27],

𝑑 = 0.9𝜆

𝛽𝑐𝑜𝑠𝜃 (1)

where d is an average grain size, λ is the wavelength of X-rays, β is full width half maximum (FWHM) and θ is Bragg angle.

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Figure 4: XRD diffraction patterns for (a) ZnO and (b) Polytriarylamine,

PTAA at room temperature

The X-ray diffraction patterns for ZnO and PTAA are clearly shown in Figure 4. X-ray is diffracted and scattered in many directions depending on the structure and orientation of the crystal, thus leading to distribution of bumps in a wide range of 2θ. In amorphous state, the broad peak is formed from low intensity X-rays that scatter individually while passing through a smaller lattice plane of a crystal. A broad diffraction peak of ZnO is observed at 34.195° with FWHM value of 0.308. Meanwhile, the PTAA film depicts a broad peak between the angles of 21° to 26° which agrees well with the research literature [26,27,31]. The FWHM values obtain for PTAA film are within the range of 0.181 to 0.243, as the orientation of XRD pattern show amorphous phase of PTAA.

1.3.2 Surface Morphology The surface morphology of thin films can be studied using an advanced material microscope (Hirox, USA). The images of ITO, ZnO and PTAA film are shown in Figure 5 and Figure 6, respectively. Different type of lens

(a)

(b)

34.195°

24.425°

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is used to observe the morphological structure of the thin films with low, mid and high ranges. Low range lenses have a magnification power of 35-250x and a horizontal view ranging from 8.71-1.22 mm. The mid-range presents a 140-1000x magnification power in the range 82.18-0.31 mm ranging view. The high range lens has a magnification power of 350-2500x, along with a horizontal view of 0.87-0.12 mm. The orientation of zinc nano-rod in a ZnO thin film are stacked closely with each other, where it is held strongly by the ionic bond. ZnO is deposited using an RF sputtering method at a high vacuum state of 9.0 mTorr, which produces homogeneous deposition [16,32]. The PTAA is deposited by spin coating method with different rpm speeds, ranging from 1000 to 5000 rpm [33,34]. It forms a polymeric lump during the pre-deposition process, as it binds with chloroform [35]. The polymeric lump is held by a strong covalent bond of π- π chain relation. The smoothness of the surface varies differently with the deposition speed of the PTAA on the ZnO film. In this case, spin coating method is the best method to cast the PTAA as it will obtain a

uniform thickness of PTAA [36]. Therefore, a greater spin speed will result in a thinner film deposition. The centripetal force combines with the

surface tension of the liquid PTAA, pulls the coating into an even covering. Meanwhile, the evaporation of coating leaves the desirable thin film onto the ZnO layer for a homogeneous surface.

Figure 5: Optical images of surface view at the interface of ITO and ZnO

with (a) low range, 35×; (b) mid-range, 350×; (c) high range, 2000×

(a)

(c)

(b)

ITO ZnO

ZnO nanorods

2000 µm 200 µm

2 µm

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Figure 6: Optical images of surface view of PTAA layered on ZnO thin film; a) low range; b) mid-range; c) high range

1.4 ELECTRICAL PROPERTY

1.4.1 Current-Voltage Characteristics A current-voltage (I-V) relationship is used to define the electrical characteristics of a thin film semiconductor. Figure 7 shows the characteristic curves of relationship between the applied voltage and current flow through the device. The hybrid diode with a configuration of ITO/ZnO-PTAA/Al shows a typical characteristic diode behavior at both forward and reverse bias conditions. On the contrary, the current of the hybrid device decreases to negative ranges under reverse bias condition. This indicates that the diodes possess the Schottky behavior on both interfaces.

2000 µm

(a)

(c)

(b) ZnO

PTAA

2 µm

200 µm

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Figure 7: I-V characteristic of ZnO-PTAA based diode from -4 V to 4 V

range The turn-on voltage is associated with the layer thickness produced using different spin coating speeds. The threshold or turn-on voltages obtained at spinning speed ranges of 1000 to 5000 rpm are 2.47 V, 2.42 V, 1.66 V, 3.87 V and 3.92 V, respectively as shown in Figure 7. The highest turn on voltage was obtain at the spinning speed of 3000 rpm. However, the unstable and fluctuating results for I-V curves arise, when the voltage is above than 2 V. This happens due to the morphological structure of the active layer surface and due to electron diffusion at the interface layer [3,37]. The high turn on voltage of 1.66 V can be associated with the crystalline structure resulted from the bonding of ZnO and PTAA layers. This also enhances the charge carrier mobility in the diode. In case of higher speed of 1000 and 2000 rpm, the I-V plots indicate an exponential increasing current at higher applied voltages and a stable increment at room temperature.

Cu

rren

t, A

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1.4.2 Electrical Parameters of Inorganic-Organic Diode The combination of organic-inorganic material had significantly presented the characteristics of Schottky behavior. The phenomenon can be better explained by using the thermionic emission (TE) theory. It can be done by extracting results from the semi-log of the I-V characteristics. Based on the theory , the diode current can be expressed as following [38],

𝐼 = 𝐼𝑠𝑒 [(𝑞(𝑉−𝐼𝑅𝑠)

𝑛𝑘𝑇) − 1] (2)

where q is the charge, V is the applied voltage across the space-charge region, IS is the saturation current at reverse bias, RS is series resistance, n is the diode ideality factor, k is the Boltzmann constant and T is temperature in Kelvin. By using Is, barrier height, Φb and ideality factor, n of the diodes is determined by [38, 39].

Φ𝑏 =𝑘𝑇

𝑞ln(

𝐴𝐴∗𝑇2

𝐼𝑠) (3)

𝑛𝑑 =𝑞

𝑘𝑇

𝑑𝑉

𝑑 ln (𝐼) (4)

where A is the contact area of the diode and A* is the Richardson constant which equal to 32 A cm-2 K-2 [38,40]. Ideality factor, n is the relationship on the closeness of fabricated diode performance with the ideal diode equations. Figure 8 shows the semi-log plot of the ZnO/PTAA diode.

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Figure 8: Semi-log plots of ZnO/PTAA diode

The ideality factor obtained was varies at different spin speeds of PTAA thin film layer. It was observed that, among the spin speeds, the ideality factor of PTAA with spin speed of 1000 rpm is 3.93, which is the nearest value to 1. The diode, deviates from its ideality due to the interface layer of organic PTAA. A higher ideality factor can only be achieved by the presence of oxide at the interface and other factors such as the state density and series resistance [38]. The state density and series resistance in the device may affect the value of ideality factor, depending on the linearity of semilog plot at the forward bias interface [38]. The barrier height obtain was the barrier height at the interface of organic layer with the electrode, PTAA/Al. The electrical parameters are

Sem

ilog

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summarized in Table 1. The highest barrier height obtained was at a spin speed of 4000 rpm with the value of 0.82 eV. A greater energy is needed for the electron to pass through the interface of PTAA/Al thus lowering the leakage current at the reverse bias region. When the barrier height of active layer-metal electrode was lowered, it would induce ohmic behavior instead of rectifying Schottky behavior. Therefore, it decreases the efficiency of diodes in terms of current-voltage relationship.

Table 1: Saturation current (Is), ideality factor (n) and B\barrier height (Φb) of ITO/ZnO-PTAA/Al based heterojunction diode

Spin Speed (rpm)

Saturation Current, Is (μA)

Ideality Factor, n

Barrier Height, Φb (eV)

1000 1.160 3.93 0.78

2000 5.070 7.33 0.74

3000 8.130 5.02 0.73

4000 0.235 4.63 0.82

5000 0.873 9.65 0.78

1.5 SUMMARY ZnO-PTAA based Schottky diode has been successfully fabricated by combining RF sputtering and spin coating techniques to fabricate ZnO and PTAA thin films. It has been demonstrated that the threshold voltage of fabricated diodes is ranges from 1.66 to 3.92 V with a peak value is at spin speed of 3000 rpm. For ideality factor, spin speed of 1000 rpm shows an ideal diode behavior where the value is the nearest value approaching to 1. Meanwhile, at a spin speed of 4000 rpm, a highest value of barrier height of 0.82 eV is obtained. This result in more energy requirements for the charge carriers to excite to another energy level. The combination of organic-inorganic semiconductors has significantly impacted semiconductor technology. It is believed that, with a further research, organic-inorganic hybrid semiconductor devices will embark a new revolution of world. It will not only revolutionize the electronic industry but will also help to generate clean energy as a tool for harvesting energy and energy converter. Thus, it will decrease the manufacturing cost of conventional diode and will replacing existing electronic devices.

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REFERENCES [1] Park, C. H., Lee, H. S., Lee, K. H., Kim, D. H., Kim, H. R., Lee, G. H., Kim, J. H., & Im, S., (2011). Organic/oxide hybrid complementary thin-film transistor inverter in vertical stack for logic, photo-gating, and ferroelectric memory operation, Organic Electronics, 12(9), 1533-1538. [2] Hammer, M. S., Deibel, C., Pflaum, J., & Dyakonov, V., (2010). Effect of doping of zinc oxide on the hole mobility of poly(3- hexylthiophene) in hybrid transistors, Organic Electronics, 11(9), 1569-1577. [3] Kimura, M., Kojima, K., Mukuda, T., Kito, K., Hayashi, H., Matsuda, T., Hiroshima, Y., & Miyasaka, M., (2014). Hybrid-type temperature sensor using thin-film transistors, IEEE Journal of the Electron Devices Society, 2(6), 182-186. [4] Chen, Q., Marco, N. D., Yang, Y. M., Song, T. B., Chen, C. C. Zhao, H., Hong, Z., Zhou, H., & Yang, Y., (2015). Under the spotlight: The organic-inorganic hybrid halide perovskite for optoelectronic applications, Nano Today, 10(3), 355–396. [5] Faltakh, H., Mahdouani, M., Hemdana, I., Dkhil, S. B., Bourguiga, R., & J. Davenas, (2015). Extraction of different parameters of hybrid solar cell based on PVK/silicon nanowires, Superlattices and Microstructures, 79, 166-179. [6] Banerjee, D. & Chattopadhyay, K. K., (2018). Perovskite photovoltaics. In S. Thomas & A. Thankappan (Eds.), Hybrid inorganic organic perovskites (pp. 123-162). Retrieved from Academic Press. [7] Shao, Y., Yuan, Y., & Huang, J., (2016). Correlation of energy disorder and open-circuit voltage in hybrid perovskite solar cells, Nature Energy, 1, 15001–15006. [8] Hima, A., Khechekhouche, A., Kemerchou, I., Lakhdar, N., Benhaoua, B., Rogti, F., Telli, I., & Saadoun, A., (2018). GPVDM simulation of layer thickness effect on power conversion efficiency of CH3NH3PbI3 based planar heterojunction solar cell, International Journal of Energetica, 3(1), 37–41. [9] Deng, W., Fang, H., Jin, X., Zhang, X., Zhnag, X., & Jie, J., (2018).

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[28] McCulloch, I., & Heeney, M., (2009). Polytriarylamine semiconductors, Material Matters, 4(3), 70–71. [29] Madec, M. B., Morrison, J. J., Sanchez-Romaguera, V., Turner, M. L., & Yeates, S. G., (2009). Organic field effect transistors from ambient solution processed poly(triarylamine)-insulator blends, Journal of Materials Chemistry, 19(37), 6750–6755. [30] Thelakkat, M., Hagen, J., Haarer, D., & Schmidt, H. W., (1999). Poly(triarylamine)s- synthesis and application in electroluminescent devices and photovoltaics, Synthetic Metals, 102(1-3), 1125–1128. [31] Zhang, F., & Srinivasan, M. P., (2008). Structure-related lower surface resistivity and faster doping of poly(thiophene-3-acetic acid-co-3-hexylthiophene) compared with poly(thiophene-3-acetic acid), Materials Chemistry and Physics, 112(1), 223–225. [32] Ryu, Y. R., Kim, W. J., & White, H. W., (2000). Fabrication of homostructural ZnO p-n junctions, Journal of Crystal Growth, 219(4), 419–422. [33] Wasa, K., Kitabatake, M., & Adachi, H., (2004). Thin film materials technology - Sputtering of compound materials. New York, Springer: William Andrew Inc Publishing. [34] Vossen, J. L., & Kern, W., (2012). Thin film processes II. San Diego, California: Academic Press. [35] Intaniwet, A., Keddie, J. L., Shkunov, M., & Sellin, P. J., (2011). High charge-carrier mobilities in blends of poly(triarylamine) and tips-pentacene leading to better performing X-ray sensors, Organic Electronics, 12(11), 1903-1908. [36] Jayamurugan, P., Ponnuswamy, V., Subba Rao, Y. V., Ashokan, S., & Meenakshisundar, S., (2015). Influence of spin coating rate on the thickness, surface modification and optical properties of water dispersed PPy composite thin films, Materials Science in Semiconductor Processing, 39, 205–210. [37] Yang, S., Lee, J. I., Park, S. H. K., Cheong, W. S., Cho, D. H., Yoon, S. M., Byun, C. W., Hwang, C. S., Chu, H. Y., Cho, K. I., Ahm, T., Choi,

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Y., Yi, M. H., & Jang, J., (2010). Environmentally stable transparent organic/oxide hybrid transistor based on an oxide semiconductor and a polyimide gate insulator, IEEE Electron Device Letters, 31(5), 446-448, [38] Imer, A. G., Tombak, A., & Korkut, A., (2016). Electrical and photoelectrical characteristic investigation of a new generation photodiode based on bromothymol blue dye, Journal of Physics Conference Series, 707(1), 012012. [39] Schroder, D. K. (2006). Semiconductor material and device characterization. New Jersey: John Wiley & Sons. [40] Cheung, S. K., & Cheung, N. W., (1986). Extraction of schottky diode parameters from forward current-voltage characteristics, Applied Physics Letters, 49(20), 85–87.