The calculation of electronic properties of an Ag/chitosan/n-Si Schottky barrier diode
Introduction
Polymer electronics has attracted considerable interest in recent years with the discovery of conducting polymers, which could be used as active components in electronic devices [1]. A number of devices such as biosensors, electrochemical sensors, polymeric voltaic cell, electroluminescent devices and Schottky diodes have been fabricated and tested by using conducting plastics [2]. The Schottky diode is a semiconductor diode with a low forward voltage drop and a very fast switching action [3]. The electrical characteristics of a Schottky diode are generally controlled by its interface quality. The interface properties have a prevailing influence on the device performance, reliability and stability. The study of interface states is thus important for the understanding of the electrical properties of such devices. Unless specially fabricated, a Schottky barrier diode (SBD) possesses a thin interfacial native oxide layer between the metal and the semiconductor. The existence of such an insulating layer converts the device to a metal/insulator/semiconductor (MIS) diode, and may have a strong influence on the diode characteristics. Moreover, there is a chance that the interface is charged with bias, which will give rise to an additional field in the interfacial layer [4], [5].
Usually, the forward bias current–voltage (I–V) characteristics are linear in the semi-logarithmic scale at low voltages, but deviate considerably from linearity due to the effects of parameters such as the series resistance Rs, the interfacial layer and interface states at sufficiently large applied voltage. The parameter Rs is only effective in the downward-curvature region (non-linear region) of the forward I–V characteristics at sufficiently high applied voltage, but parameters such as the ideality factor n and barrier height Φb,0(V) are effective in both the linear and non-linear regions of these characteristics, accompanying the change of the Schottky barrier height (SBH) [4]. The electrical properties of polymeric [6] and non-polymeric organic compounds [7] have been investigated for more than the last two or three decades.
As it is known, the first reported example of a Schottky diode made with a conductive polymer was based on polyacetylene. A typical application is discharge-protection for solar cells connected to lead-acid batteries. While standard silicon diodes have a forward voltage drop of about 0.6 V, Schottky diodes have a drop of only about 0.3 V. This is due to the higher current density in the Schottky diode. The performances of these devices are dependent on various factors such as fabrication parameters, characteristics of the polymer and the metal used. The availability of material in abundance, its cost, processability, environmental stability are also deciding factors for a material to be used for device application [3], [8], [9].
Most polymers are excellent electrical insulators, and display high electrical resistance and very little conduction of an electric current [10]. One promising candidate to act as polymer host for proton-conducting biopolymer electrolyte is chitosan [11]. Chitosan is the deacetylated form of chitin, which is a linear biopolymer of acetylamino-d-glucose (Fig. 1) [12]. A monomer of chitosan consists of hydroxyl and amine functional groups, which have lone pair electrons that are suitable for the preparation of solid polymer electrolytes. The existence of lone pair electrons enables the chelation of a proton donor [13]. In the form of a hydrogel, chitosan is used in a wide range of applications such as wastewater treatment [14], separation membrane [15], food packaging [16], [17], [18], wound healing [17], [19], and a drug delivery system [20], [21]. Chitosan is found abundantly in nature. In addition, it has also hydrophilicity, biocompatibility, biodegradability and antibacterial property [12].
Gupta and Singh [22] calculated various junction parameters related to a Schottky diode fabricated by using the composite of polyaniline with polyvinyl chloride (PAn–PVC). Abthagir and Saraswathi [23] investigated electronic properties of polyindole and polycarbazole Schottky diodes. Huang et al. [24] studied electronic and junction properties of poly(2,5-dimethoxyaniline)-polyethylene oxide blend/metal Schottky diodes. Yamada and Honma [11] investigated a low-cost anhydrous proton conductor consisting of a composite of chitosan, one of the world's discarded materials, and methanediphosphonic acid (MP) having a high proton exchange capacity. In our previous study [4], it had been determined that the Ag/chitin/n-Si structure shows a rectifying contact behaviour, and electronic parameters related to this structure had been calculated. The aim of the present study is to determine whether or not the Ag/chitosan/n-Si structure shows a rectifying contact behavior and to calculate electronic parameters related to this structure. In addition, it is also aimed the comparison of electronic parameters related to Ag/chitosan/n-Si and Ag/chitin/n-Si structures.
Section snippets
Experimental
The samples were prepared using a polished n-type Si wafer with [1 0 0] orientation and ρ = 5–10 Ω cm resistivity. The wafer was chemically cleaned using the RCA cleaning procedure (i.e. a 10 min boil in NH4 + H2O2 + 6H2O followed by a 10 min boil in HCl + H2O2 + 6H2O). Then, a low-resistivity ohmic back contact to the n-type Si wafer was made by using AuSb alloy, followed by a temperature treatment at 420 °C for 3 min in N2 atmosphere. The native oxide on the front surface of the substrates was removed in HF/H2
Results and discussion
The forward and reverse bias I–V characteristics of the Ag/chitosan/n-Si contact at room temperature are given in Fig. 2. As can be seen from Fig. 2, the I–V characteristics of the device show a rectifying behavior, i.e. while the reverse current shows weak bias voltage dependence, the forward current increases exponentially with the voltage. The current–voltage (I–V) equation in respect to the thermoionic emission theory in the presence of interfacial layer is given by [25]:
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