Optical and structural properties of amorphous Se x Te100-x aligned nanorods
© Al-Agel; licensee Springer. 2013
Received: 13 September 2013
Accepted: 26 November 2013
Published: 9 December 2013
In the present work, we report studies on optical and structural phenomenon in as-deposited thin films composed of aligned nanorods of amorphous Se x Te100-x (x = 3, 6, 9, and 12). In structural studies, field emission scanning electron microscopic (FESEM) images suggest that these thin films contain high yield of aligned nanorods. These nanorods show a completely amorphous nature, which is verified by X-ray diffraction patterns of these thin films. Optical studies include the measurement of spectral dependence of absorption, reflection, and transmission of these thin films, respectively. On the basis of optical absorption data, a direct optical band gap is observed. This observation of a direct optical band gap in these nanorods is interesting as chalcogenides normally show an indirect band gap, and due to this reason, these materials could not become very popular for semiconducting devices. Therefore, this is an important report and will open up new directions for the application of these materials in semiconducting devices. The value of this optical band gap is found to decrease with the increase in selenium (Se) concentration. The reflection and absorption data are employed to estimate the values of optical constants (extinction coefficient (k) and refractive index (n)). From the spectral dependence of these optical constants, it is found that the values of refractive index (n) increase, whereas the values of extinction coefficient (k) decrease with the increase in photon energy. The real and imaginary parts of dielectric constants calculated with the values of extinction coefficient (k) and refractive index (n), are found to vary with photon energy and dopant concentration.
Amorphous semiconductors have been known for years, and a lot of work on the applications of these materials is available in the literature[1, 2]. Among these materials, chalcogenides are the most studied materials. In fact, amorphous materials became popular only after the discovery of chalcogenides, and later, many interesting physical properties of these materials[3, 4] were reported. These chalcogenides have special application in optical devices due to their transparency in the IR region. They are also used in switching and memory devices, and the most popular application of these materials is in phase change recording[5, 6]. Among the chalcogen family, selenium and tellurium have been studied widely due their potential applications[7, 8]. Glassy selenium is one of the popular materials for the development of various solid-state devices such as electrophotographic and switching and memory devices. For the last few years, tellurium-rich alloys attracted a lot of attention due to their potential applications in data storage devices[10, 11]. It is well understood that the bonding between Se and Te is weaker than the Se-Se bonds due to the catalytic effect of tellurium on the crystallization of selenium. Several workers[12–14] reported that tellurium-rich glasses have good transparency in the infrared and high refractive index, which makes these glasses important for optical devices also.
Tellurium-rich glassy alloys of Se-Te are widely used for commercial, scientific, and technological purposes. Their application ranges from optical recording media to xerography[15–17]. Khan et al. studied the electrical and optical properties of thin films of a-Se x Te100-x system. They reported an indirect optical band gap and electrical transport via a thermally activated process in this system. Salah et al. studied the thin films of polycrystalline Te94Se6 nanoparticles. Further, they prepared these nanoparticles at different working gas pressures and studied the pressure dependence of optical band gap in these nanoparticles. They reported that a direct optical band gap and the values of optical band gap are found to be pressure dependent. Salah et al. deposited thin films composed of nanoparticles of polycrystalline Se x Te100-x and studied the optical properties of these nanoparticles. They reported a direct optical band gap in this system, and the values of optical band gap are found to be size and composition dependent. In the present work, we have also studied a-Se x Te100-x system and produced aligned nanorods of this alloy. The optical and structural properties of these well-aligned nanorods are studied. In our case, we found that these nanorods are aligned and their structure is completely amorphous. These amorphous nanorods show an enhanced and direct band gap as compared to the reported results on polycrystalline materials[19, 20]. These findings in the field of nanochalcogenide glasses will be interesting for applications in devices as these materials are cost-effective, and fabricating devices using these materials will also reduce the cost of devices. It is also important to understand the optical phenomenon in a-Se x Te100-x nanorods as reduction in the size of the material (nanoscale) may result in a dramatic change in the properties. Keeping the above facts in view, it is therefore extremely important to study the properties of as-prepared a-Se x Te100-x aligned nanorods.
Thin films of a-Se x Te100-x were deposited using a rapid thermal evaporation technique. In this method, as-prepared alloys were evaporated in an argon gas environment. Thermal evaporation was modified to rapid thermal evaporation by constructing a small sub-evaporation chamber using a quartz tube that is 30 mm in diameter and 110 mm in length. An arrangement was made in this quartz tube for the gas inlet, opening of the evaporation source, sample holder, and the gas outlet. With the quartz tube, we were able to confine the evaporated material and maintain a uniform gas pressure in the vicinity of the evaporation source. A molybdenum boat was used as an evaporation source. For depositing the thin films, the glass substrate was pasted at the top of the tube. Film thickness was measured with a quartz crystal thickness monitor (FTM 7, BOC Edwards, West Sussex, UK). After loading the glass substrate and the source material, the chamber was evacuated to 10-5 Torr. The inert gas (Ar) with 0.1 Torr pressure was injected into the sub-chamber, and the same gas pressure was maintained throughout the evaporation process. Once a thickness of 500 Å was attained, the evaporation source was covered with a shutter, which was operated from outside. After the process was over, thin films were taken out of the chamber and were analyzed for structural and optical properties. X-ray diffraction patterns of thin films of a-Se x Te100-x nanorods were obtained with the help of an Ultima-IV (Rigaku, Tokyo, Japan) diffractometer (λ = 1.5418 Å wavelength CuKα radiation at 40 kV accelerating voltage and 30 mA current), using parallel beam geometry with a multipurpose thin film attachment. X-ray diffraction (XRD) patterns for all the studied thin films were recorded in theta - 2 theta scans with a grazing incidence angle of 1°, an angular interval (20° to 80°), a step size of 0.05°, and a count time of 2 s per step. Field emission scanning electron microscopic (FESEM) images of these thin films containing aligned nanorods were obtained using a Quanta FEI SEM (FEI Co., Hillsboro, OR, USA) operated at 30 kV. A 120-kVtransmission electron microscope (TEM; JEM-1400, JEOL, Tokyo, Japan) was employed to study the microstructure of these aligned nanorods. Energy-dispersive spectroscopy (EDS) was employed to study the composition of these as-deposited films using EDAX (Ametek, Berwyn, PA, USA) operated at an accelerating voltage of 15 kV for 120 s.
To study the optical properties of these samples, we deposited the a-Se x Te100-xthin films on the glass substrates at room temperature using a modified thermal evaporation system. The thickness of the films was kept fixed at 500 Å, which was measured using the quartz crystal thickness monitor (FTM 7, BOC Edwards). The experimental data on optical absorption, reflection, and transmission was recorded using a computer-controlled JascoV-500UV/Vis/NIR spectrophotometer (Jasco Analytical Instruments, Easton, MD, USA). It is well known that we normally measure optical density with the instrument and divide this optical density by the thickness of the film to get the value of the absorption coefficient. To neutralize the absorbance of glass, we used the glass substrate as a reference as our thin films were deposited on the glass substrate. The optical absorption, reflection, and transmission were recorded as a function of incident photon energy for a wavelength range (400 to 900 nm).
Results and discussion
where OD is the optical density measured for a given film thickness (t).
From the spectral dependence of absorption coefficient (α), we found an increase in the value of absorption coefficient (α) with the increase in photon energy for the a-Se x Te100-x thin films. For this system of aligned nanorods, the calculated values of the absorption coefficient are of the order of ~105 cm-1. This is comparable with the reports of other workers presented in the literature[18–21].
where A is a constant of the order of unity and ν0 is the constant corresponding to the lowest excitonic frequency.
where ν is the frequency of the incident beam (ω = 2π ν), B is a constant, Eg is the optical band gap, and n is an exponent. This exponent can have different values, i.e., 1/2, 3/2, 2, or 3, depending on the nature of electronic transition responsible for the absorption. For allowed direct transition, we take n as 1/2 for allowed direct transition and as 3/2 for forbidden direct transition, whereas for allowed indirect transition, n is taken as 2. In our case, we observed the allowed direct transition, and we take n to be equal to 1/2[24, 25].
Optical parameters of a-Se x Te 100- x thin films at 600 nm
ε r ′
ε r ″
8.40 × 105
5.16 × 105
10.6 × 105
6.50 × 105
It is well understood that the optical absorption is dependent on both the short-range order and defect states observed in amorphous systems. We can employ Mott and Davis's ‘density of state model’ to explain this decrease in optical band gap with the increase in Se concentration. It was suggested by Mott and Davis that the degree of disorder and defects in the amorphous systems are two major factors affecting the width of the localized states near the mobility edges. For the present case of a-Se x Te100-x thin films, it is proposed that the unsaturated bonds together with some saturated bonds are produced during the deposition of atoms in the present as-deposited films. It is well known that the as-deposited chalcogenide thin films always contain a high concentration of unsaturated bonds or defects. These defects are responsible for the presence of localized states in the amorphous band gap. Therefore, these unsaturated bonds result in the formation of defects in the presently studied thin films containing aligned nanorods, thereby producing a large number of localized/defect states in the present system. Tellurium glass contains short chains, whereas selenium glass contains long chains and selenium rings. As Se concentration increases or Te concentration decreases, the number of Se rings increases and the number of long Se-Te polymeric chains and Se-Te mixed rings decreases. Therefore, the addition of selenium to tellurium increases the number of defect states, which increases further with the increase in Se concentration. As these defect states are also associated with unsaturated bonds formed during the deposition of these thin films, we may state that the number of unsaturated bonds increases with the increase in Se concentration. This increase in the defect states or unsaturated bonds with the concentration of Se results in the narrowing of optical band gap. Therefore, the optical band gap in the present system decreases with the increase in Se concentration. We can also interpret this decrease in optical band gap with respect to the shift in Fermi level. The position of Fermi level in such systems is determined by the distribution of electrons over the localized states.
Where λ is the wavelength of the incident light and α is the absorption coefficient.
In the case of compound semiconductors deposited from the vapor, we may consider the possibility of like bonds. In III-V compounds, we may consider two types of like bonds, which are taken as two possible anti-site defects. In such cases, chemical disorder produces large change in potential through the Coulombian interaction due to large ionic contribution to the bonding. Theye reported that the bonding in glassy materials is covalent and the chemical disorder results only in small changes in the local potential.
These direct band gap materials may have potential applications in optical recording media, xerography, electrographic applications, infrared spectroscopy, and laser fibers. Moreover, their transparency in the infrared region and their high refractive index are good indicators for integrated optics and detection in the mid- and thermal infrared spectral domain. The observance of a direct band gap in this material is very interesting and will open up new direction for applications in nanodevices. Since the popular direct band gap materials, e.g., GaAs, GaN, InAs, and InP, are more expensive as compared to chalcogenides and most of the industries are facing problems in reducing the cost of the devices due to the high cost of these materials, the chalcogenides being a cheap material will provide a good option for industries to produce cost-effective devices.
The amorphous nature of Se x Te100-x thin films (x = 3, 6, 9, and 12) is predicted with the help of XRD patterns. On the basis of the best fitting of optical absorption data, it is suggested that the band gap follows direct optical transitions and its value decreases on adding the Se content to the presently studied system. One of the possible reasons behind this decrease in band gap may be due to the increase in the disorderedness of the system, which results in an increase in the density of defect states. The value of refractive index increases with the increase in photon energy, whereas the value of extinction coefficient decreases with the increase in photon energy and Se concentration. The calculated values of real and imaginary parts of dielectric constants are found to decrease with the increase in Se content for the present system. On the basis of the above reported values of optical parameters, one may decide the suitability of these nanorods for optical devices.
This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant no. 81/130/1433. The author therefore acknowledges with thanks DSR technical and financial support.
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