Electrical and optical properties of thin film of a-Se70Te30 nanorods

https://doi.org/10.1016/j.jallcom.2009.07.049Get rights and content

Abstract

The electrical and optical properties of thin film of a-Se70Te30 nanorods are studied. Initially, the melt quenching technique is employed to prepare the glassy alloy of Se70Te30. X-ray diffraction technique is used to verify the amorphous nature of this alloy. The thin film of a-Se70Te30 is deposited on the glass substrate under an ambient gas (Ar) atmosphere using inert-gas consolidation (IGC) method. Transmission electron microscopy (TEM) is employed to study the microstructure of this film. It is found that the film contains nanorods of diameters varying from 30 to 80 nm and length of the order of few hundreds of nanometers. For electrical properties, the temperature dependence of dc conductivity is also studied over a temperature range of 450–100 K. On the basis of the temperature dependence of dc conductivity, the conduction mechanism in this film of a-Se70Te30 nanorods is elucidated. The results show that the thermally activated process is responsible for the transport of carriers for the temperature range of 450–300 K. For temperature region 300–100 K, the conduction takes place via variable range hopping (VRH). Therefore, three dimensions Mott's variable range hopping (3D VRH) is applied to explain the conduction mechanism for the transport of charge carriers for temperature region 300–100 K. In optical properties, optical absorption measurements of the thin film of a-Se70Te30 nanorods are also carried out in the wavelength range 400–900 nm. These studies indicate that the absorption mechanism is due to indirect transition. The optical band gap is estimated to be 1.18 eV.

Introduction

During last few years, nano-chalcogenides have attracted a lot of attention due to their interesting properties and great potential in different fields [1], [2], [3]. These nano-chalcogenides show a dramatic change in their optical and electronic properties on reduction of size below the excitonic size [4]. In recent years, a variety of nanomaterials have been synthesized for various applications. There are many methods [5], [6] available in the literature for growth of nanomaterials. Among these existing methods, vapor condensation of the materials is also one of the important methods for synthesis of nanoparticles [7]. This synthesis process is also called the inert-gas consolidation (IGC) method. Gleiter and coworkers [8] first introduced this method to produce the materials that demonstrated the exciting properties of nanostructured materials [9]. In this method, a material is typically vaporized into a low density gas by resistive heating and the vapors migrate from the hot source into a cooler gas by a combination of convection and diffusion. This decrease in temperature leads to a rapid decrease in the equilibrium vapor pressure and resulting in high supersaturation [9]. At high supersaturation, the vapors rapidly nucleate, thereby forming very large numbers of extremely small particles. These particles then grow by Brownian coagulation [10]. During this deposition process, the substrate is cooled using liquid nitrogen to enhance the deposition efficiency. Various types of nanomaterials have been synthesized using this method [11], [12]. In the present communication, we have also employed inert-gas consolidation (IGC) method to deposit the thin film of a-Se70Te30 nanorods.

Here, we have selected selenium-based amorphous alloy because amorphous selenium has emerged as promising material due to its potential technological importance. It is widely preferred in the fabrication of electro-photographic devices. More recently, it is reported that the selenium-based materials offer attractive advantages for switching and memory devices [13]. Selenium has some disadvantages like short lifetime and low photosensitivity. Therefore, selenium-based alloys are more useful due to greater hardness, higher photosensitivity, higher crystallization temperature and low ageing effect compared to pure amorphous Se [14], [15]. There have been several attempts to modify the electrical, thermal and optical properties of amorphous Se to some extent by alloying it with certain additives like Ga, Te, Sb, Bi, Ga, etc., [16], [17], [18]. The impurity, doping concentration and the method of alloying can be considered as an effective tool of modifying the transport properties of amorphous Se films. In the present work, we have added Te to the a-Se–Te system and deposited the thin film of this sample. Here, the substitution of Te to selenium partly breaks up the Se8 ring structure and increases the chain fraction. Also, it has been pointed out that Se–Te has some advantages over amorphous Se, as for as their use in xerography is concerned [19]. Glassy alloys of Se–Te system based on Se have become materials of considerable commercial, scientific and technological importance. They are widely used for various applications in many fields such as optical recording media due to their excellent laser writer sensitivity, xerography, electrographic applications such as photoreceptors in photocopying, laser printing, infrared spectroscopy and laser fibre techniques [20], [21]. Amorphous materials especially the chalcogenide glasses (Se and Te) are promising materials currently receiving much attention for their use in the fabrication of various solid state devices such as LEDs and image sensors. These amorphous materials are widely used as a storage media in optical recording for data storage devices. Producing the amorphous materials at nanoscale is an interesting and extremely important as the control of nanometer-scale structures could be potential research subject for glassy materials. It is well known that we can fabricate atomically flat surfaces in single crystalline materials by controlling atoms one by one. However, in case of amorphous materials such atomic manipulation is meaningless, since the amorphous structure is disordered at that scale. Therefore, the nano-chalcogenides may yield a greater variety of structures than that of crystalline nanostructures, since the bonding constraint arising from crystalline unit cells does not exist. It will be interesting to investigate these materials at nanolevel.

A lot of work on electrical, optical, dielectric properties of glassy chalcogenides of Se–Te is available in the literature. Khan et al. [22] reported the electrical properties especially the dc conductivity and photoconductivity measurements of vacuum evaporated thin films of a-(Se70Te30)100−x(Se98Bi2)x system in the temperature range 308–355 K. They observed that dc conductivity and activation energy depend on the Bi concentration. Photocurrent dependence on incident radiation followed the power law (Iph  Fν). Transient photocurrent exhibited the non-exponential decay time. All these parameters showed that the recombination within the localized states is predominant. In crystallization kinetics, the heating rate dependence of glass transition and crystallization temperatures is studied to calculate the activation energy for thermal relaxation and activation energy for crystallization. The composition dependence of the activation energy for thermal relaxation and activation energy for crystallization is discussed in terms of the structure of Se–Te–Bi glassy system. Kumar et al. [23] investigated the dc conductivity and dielectric parameters of the glassy system of a-Se96−xTe4Gax (x = 0, 2, 4, 6 and 10) glasses. They have studied the frequency and temperature dependence of the dielectric constants (ɛ′) and dielectric loss (ɛ′′) in the frequency range 120–100 kHz and in the temperature range 300–450 K. They have observed dielectric dispersion on incorporating Ga in the a-Se–Te system. The results are explained on the basis of dipolar type dielectric dispersion. It is also observed that the dc conductivity and the activation energy increase on increasing the Ga content in this system. It is worth mentioning here that all these studies have been made in thin film form. The thin films are deposited using thermal evaporation method without any ambient gas atmosphere. Normally, these films contain micron size structures. Whereas, we have tried to produce these chalcogenides (Se–Te) in nanostructured form by introducing an ambient gas (Ar) and depositing these nanostructures at liquid nitrogen cooled substrate using IGC method. In the present communication, we have successfully synthesized the nanorods of a-Se70Te30 in thin film form using IGC method and studied the electrical and optical properties of this thin film.

Section snippets

Experimental details

Melt-quenching technique is employed to prepare the glassy alloy of Se70Te30. High pure materials (99.999%) are weighed according to the composition and are kept in a quartz ampoule (length ≈12 cm, internal diameter ≈0.8 cm). Finally this ampoule is sealed in a vacuum of about 10−6 Torr. The sealed ampoule is kept inside a furnace where the temperature is raised slowly (5 K min−1) to 900 °C. The ampoule is rocked frequently for 10 h at the maximum temperature to make the melt homogeneous. After

Structural studies

The scanning electron microscope (SEM) image of the thin film of a-Se70Te30 is presented in Fig. 1. SEM image reveals the typical morphologies of as grown a-Se70Te30 nanorods. The microstructure of this film is studied using a transmission electron microscope (TEM) (JEOL “JEM 2000 EX”) operated at 100 kV. Fig. 2(a)–(d) represents the transmission electron microscopy images of a-Se70Te30. It is clear from this image that this sample of a-Se70Te30 contains nanorods of diameters varying from 30 to

Conclusions

From the above results and discussion, it may be concluded that there are two types of conduction mechanisms that are responsible for the transport of carriers for the entire temperature range of 450–100 K in the present sample of a-Se70Te30 nanorods. The thermally activated process takes place for the temperature range of 450–300 K, whereas the variable range hopping is responsible for the temperature range of 300–100 K. For the thermally activated process, the calculated values of activation

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