Performance improvement of n-TiO₂/p-Si heterojunction by forming of n-TiO₂/polyphenylene/p-Si anisotype sandwich heterojunction

Abstract

The n-TiO₂/polyphenylene (PPh)/p-Si heterojunction devices were fabricated in which PPh film and TiO₂ top layer were grown on p-Si substrates by diazonium modification method and cathodic electrodeposition, respectively. The XPS, UV–vis diffuse reflectance and STM analyses of the films were performed. After the characterization of deposited films and the fabrications of n-TiO₂/PPh/p-Si sandwich devices, the electrical measurements of nine devices were carried out from the current–voltage (I–V) characteristics, at room temperature. The I–V characteristics of n-TiO₂/PPh/p-Si heterojunctions were compared with TiO₂/p-Si heterojunctions, one of them was analysed in more detailed and it was observed that the n-TiO₂/PPh/p-Si gave better performance than TiO₂/p-Si heterojunctions such that lower ideality factor, higher rectification ratio and more stable reverse current characteristics. Then, the main device parameters of n-TiO₂/PPh/p-Si were compared with many devices reported in literature based on TiO₂/p-Si device and TiO₂ preparation techniques. Furthermore, the rectification ratio of n-TiO₂/PPh/p-Si heterojunction was 9.42 × 10⁵, while it was 5.80 × 10² for TiO₂/p-Si heterojunction. Later, the capacitance-voltage (C–V) and conductance-voltage (G-V) measurements of the n-TiO₂/PPh/p-Si heterojunction was performed depending on applied frequency and bias and it was observed that the values of capacitance and conductance were found a strongly function of bias voltage.

Introduction

In the last two decades, due to its excellent properties titanium oxide (TiO₂) which is a member of metal transition oxide family has been widely studied in various areas such as transparent electrodes, gas sensors, photocatalysts, FETs, MOSFETs, heterojunction photovoltaics, photocatalytic process, etc [[1], [2], [3], [4], [5], [6], [7]]. TiO₂ possess unique physical and chemical properties, including high thermal and chemical stability, high dielectric constant, high refractive index (over 2.3), strong optical absorption, biologically inertness, low toxicity, redox ability, natural geological abundance and low cost [[8], [9], [10], [11], [12]].

TiO₂ has a wide band gap of ~3.2 eV at room temperature, which provides significant performance in the ultraviolet region and it has three known structures under ambient conditions: rutile (it is high refractive index), anatase (it has higher electrical mobility than rutil structure) and brookite. TiO₂ exhibits a typical n-type conductivity behaviour due to the oxygen vacancies, which introduce excess of electrons, which are the predominant defects in TiO₂. These vacancies act as an electron donor. Furthermore, titanium interstitial atoms may cause to n-type conductivity of the semiconductor. The effect of Ti or O defects will result in the reduction of band gap such that the Fermi level will appear near towards the conduction band of TiO₂ [13].

Performance of devices based on TiO₂ or some others oxide materials depends on the structural, electrical or optical properties of the materials. Hence, to improve the performance of device, TiO₂ can be doped with suitable dopants such as metal and non-metal elements. For metal atom dopants, such as Al, Nb, V, Sn, Ge, Fe, Cr, Mn, Cu, and Cr [[14], [15], [16], [17]], both localized and delocalized impurity states will be formed within the band gap of TiO₂. Furthermore, the reliability and performance of heterojunction electronic devices such as metal/semiconductor or metal-oxide-semiconductor (MOS) strongly depend on the interface quality of the materials and the techniques used. The interface between junction materials is the key to heterojunctions.

Although SiO₂ has been used in many studies, as a dielectric material is compatible with silicon, the biggest disadvantage of SiO₂ is its high power consumption and large leakage currents that limit its technological applications. Therefore, there are many alternative oxide materials such as TiO₂ [18,19], SnO₂ [20], CeO₂/TiO₂ [21], Ta₂O₅ [22], HfO₂ [23] used as dielectric material instead of SiO₂ in many electronic applications.

Various methods have been used to preparation of TiO₂ such as chemical vapor deposition (CVD) [24], hydrothermal method [25], hydrolysis [26], atomic layer deposition (ALD) [27], solvent evaporation [28], combustion [29], chemical precipitation [30], pulsed laser deposition technique [31], electrodeposition [32], sol-gel [33,34] and radio frequency sputtering [35]. Since film preparation method may affect the electrical, structural and optical properties of the TiO₂ film or electronic devices, therefore heterojunction device parameters such as ideality factor, barrier height, rectification ratio (RR) may be affected from related method too.

In this paper, we used the electrochemical reduction method for coating n-TiO₂ on p-Si modified with phenyl rings to obtain the n-TiO₂/PPh/p-Si heterojunctions (Fig. 1 (a)) and investigated the I–V and the C/G–V characteristics of n-TiO₂/PPh/p-Si heterojunction at room temperature. It is known that a thin organic, polymer or inorganic interlayer between the junction materials of Schottky type heterojunction devices may improve devices' performance. Namely, this layer can change the device's barrier height, series resistance, ideality factor and distribution of interface states [[36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47]].

Such interface layers can passivate the surface of the wafer or cause the interface to be reconstructed. For this purpose, in this study, a PPh film, which is used as an interface material, was coated on p-Si by covalent modification through aryl diazonium salt chemistry. Then, the positive effect of PPh interlayer formed between n-TiO2 and p-Si on device performance was examined and the results were compared with the literature and the n-TiO₂/p-Si device.