Direct bandgap materials based on the thin films of Se x Te100 − x nanoparticles
© Salah et al.; licensee Springer. 2012
Received: 31 July 2012
Accepted: 31 August 2012
Published: 15 September 2012
In this study, we fabricated thin films of Se x Te100 − x (x = 0, 3, 6, 9, 12, and 24) nanoparticles using thermal evaporation technique. The results obtained by X-ray diffraction show that the as-synthesized nanoparticles have polycrystalline structure, but their crystallinity decreases by increasing the concentration of Se. They were found to have direct bandgap (Eg), whose value increases by increasing the Se content. These results are completely different than those obtained in the films of Se x Te100 − x microstructure counterparts. Photoluminescence and Raman spectra for these films were also demonstrated. The remarkable results obtained in these nanoparticles specially their controlled direct bandgap might be useful for the development of optical disks and other semiconductor devices.
In the last two decades, much research work is focused on the synthesis and characterization of semiconducting nanomaterials . Among these nanostructures, nanochalcogenides are important materials for various applications such as nanoelectronic devices, nanomemory devices, optical memory devices, etc. Recently, we have produced different nanochalcogenides and studied their structural, optical, and electrical properties [2–7]. However, these studies are still at the early stage and need to be further extended to cover more chalcogenide, that is, due to the primary remarkable results obtained in their nanostructure forms.
Several studies were focused on fabricating different nanostructures of different amorphous alloys/crystalline materials and studying their properties. For example, Tripathi et al.  have studied the optical properties of Se100 − xTe x (x = 4, 8, and 16) nanostructured thin films grown by thermal evaporation. Chawla et al.  have synthesized Zn1 − xCd x S:Cu nanoparticles and tuned the bandgap by increasing the Cd content. El-Nahass et al.  have studied the influence of heat treatment and gamma ray irradiation on the structural and optical characterizations of nanocrystalline cobalt phathalocyanine thin films. Gracin et al.  have analyzed the amorphous-nanocrystalline multilayer structures by optical, photodeflection, and photocurrent spectroscopies.
The studies on the preparation and characterization of nanocrystalline thin films are also available in the literature by various workers [12–14]. Keeping in view the scope of nanostructure materials, we have communicated several reports on synthesis and characterization of nanochalcogenides in both the amorphous and crystalline forms by different techniques [2–7]. Therefore, the preparation and characterization of nanochalcogenide materials are extremely important for application in optical devices. This work reports on thin films of Se x Te100 − x (x = 0, 3, 6, 9, 12, and 24) nanoparticles synthesized using thermal evaporation method. The as-synthesized nanoparticle films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), absorption spectrum, photoluminescence (PL), and Raman spectroscopy.
Glassy alloy of Se x Te100 − x (x = 0, 3, 6, 9, 12, and 24) in powder form have been prepared by the method adopted from Khan et al. . In this method, high purity (99.999%) materials are weighed in appropriate proportions according to their atomic percentages (at.%) and sealed into quartz ampoules under vacuum of about 10−5 Torr. The sealed ampoules are then kept in a muffle furnace, where the temperature is raised up to 950 K at the rate of 3 K/min. Once the desired temperature of 950 K is reached, the sealed ampoules are kept at this temperature for 14 h with rocking. Through the heating process, ampoules are rotated in clockwise and anticlockwise directions with the help of the motor to ensure homogeneity of the composition within the samples. Once this process is over, the melt is rapidly quenched in ice water to make it amorphous.
Thin films of Se x Te100 − x (x = 0, 3, 6, 9, 12, and 24) nanoparticles were fabricated using physical vapor condensation method. Initially, a small quantity of glassy alloy of Se x Te100 − x in powder form is kept in a molybdenum boat, and the chamber is evacuated to a vacuum of the order of 10−6 Torr. After reaching this vacuum level, the argon gas is purged inside the chamber. The pressure of the gas was kept constant at 5 Torr. The glassy alloy is then evaporated in the presence of the ambient argon gas atmosphere in the chamber to get the nanostructures. The substrate is cooled with liquid nitrogen, and this evaporated material is deposited on a glass substrate pasted on this cooled substrate. The thicknesses of the films were measured using a quartz crystal monitor Edward model FTM 7 (Edwards BOC, England, UK). The thickness is fixed at 30 nm for all the films. This value was confirmed by the surface profiler AS 500 Tencor AlphaStep (Brumley South, Inc. Mooresville, NC, USA).
The as-synthesized samples were characterized by X-ray diffraction, using an Ultima-IV (Rigaku Corporation, Tokyo, Japan) diffractometer with Cu Kα radiation, while the morphology of these nanostructures is studied by SEM using Quanta, FEI (Eindhoven, The Netherlands). The chemical compositions of the deposited films were measured by the EDS technique using EDAX, Ametec. The optical absorption of thin films of Se x Te100 − x (x = 3, 6, 9, and 12) have been measured by a UV-visible computerized spectrophotometer (model ‘UV-1650PC,’ Shimadzu Corporation, Tokyo, Japan) in the wavelength region of 400 to 1,100 nm. Here, we have kept the samples (films) and reference (glass substrate) in the chamber to neutralize the absorption of glass. The absorption has been measured in terms of optical density. The optical absorption is measured as a function of incidence photon energy. PL emission spectra for the thin films of Se x Te100 − x were recorded at room temperature at excitation wavelength of 325 nm using a fluorescence spectrofluorophotometer, model RF-5301 PC, Shimadzu, Japan, while Raman spectra were measured using DXR Raman microscope, Thermo Scientific Inc. (Waltham, MA, USA) using the 532-nm laser as excitation source at 6-mW power.
Results and discussion
Figure 4b shows the variation of (α hν)2 with photon energy (hν) for the thin films of Se x Te100 − x nanoparticles (curves a2, b2, c2, d2, e2, and df). The value of direct optical bandgap (Eg) is calculated by taking the intercept on the x-axis. The calculated values of Eg for the thin films of Te, Se3Te97, Se6Te94, Se9Te91, Se12Te88, and Se24Te76 are 1.08, 1.17, 1.22, 1.28, 1.34, and 1.39 eV, respectively. It is clear that there is a significant increase in the value of the optical bandgap by increasing the Se concentration in this system. These values are comparable with those of Se x Te100 − x microsize alloys presented by Khan et al. . These films of Se x Te100 − x microsize particles were reported to have indirect bandgaps with values decreasing by increasing the concentration of Se . They have attributed this decrease to the increase of the localized states in the amorphous systems due to Se addition.
Tripathi et al.  synthesized nanostructures of selenium-rich samples (Se100 − xTe x , x = 4, 8,16) in the presence of oxygen and argon, but their results show indirect bandgap in their nanoparticle films (their particle size is in the range of 40 to 100 nm). This might be due to the size of their nanoparticles, which is much bigger than that of the quantum dots. The other possibility is that tellurium atom (0.140 nm) as dopants, which is bigger than Se atom (0.115 nm), could not induce defects/localized states that have the ability to arrange themselves to provide direct bandgap. In the present case, it is the remarkable results for tellurium-rich compounds in nanostructure form to have direct bandgaps. This might be attributed to the reorganization and modifications induced in positions of the localized states of the nanostructure materials as explained above.
Since the optical absorption depends on short-range order in the amorphous states and defects associated with it, the change in optical bandgap of the films of microsize particles of Se x Te100 − x was explained by Khan et al.  on the basis of ‘density of state model’ proposed by Mott and Davis . According to this model, the width of the localized states near the mobility edges depends on the degree of disorder and defects present in the amorphous structure. In particular, it is known that unsaturated bonds together with some saturated bonds are produced as the result of an insufficient number of atoms deposited in the amorphous film . The unsaturated bonds are responsible for the formation of some of the defects in the films, producing localized/defect states in the amorphous solids. However, this model  has not taken into consideration the effect of particle size on these localized states. In addition to this model, it is expected that at the nanoscale level these localized/defect states might have reorganized their positions in the band structure, resulting in the formation of direct bandgap (Figure 5). Crystal field effect might be another factor affecting the localized states formed in the nanoparticles, which might be smaller than that of the microsize particles, resulting in the widening of the bandgap. This increase in optical bandgap may also be due to the shift in Fermi level whose position is determined by the distribution of electrons over the localized states .
Raman spectra of the thin films of Se x Te100 − x nanoparticles are presented in Figure 6b (curves a2, b2, c2, d2, e2, and d2). The most noticeable features in the Raman spectra of these films are the three bands observed at around 123, 143, and 169 cm−1. The first band at 123 cm−1 is the most prominent one. Intensities of the first two bands are observed to decrease by increasing the concentration of Se, while it is vice versa for the band observed at 169 cm−1. The band at 123 cm−1 might be assigned to the A1 mode corresponding to symmetric stretching of a triangle of three Te atoms [25, 26], while that at 143 cm−1 might be ascribed to the amorphous Te-Te stretching mode [27, 28]. The later one has also been assigned to t-Se crystals . The band at around 169 cm−1 might be assigned to the Se-Se bonds . The intensity of this band is significantly increased by increasing the concentration of Se. These results show that the progressive introduction of selenium to the structure of tellurium in the alloys of S x Te100 − x induces the breaking of Te-Te bonds (especially those of the crystalline phase). These results are consistent with those of the XRD (Figure 1). The most intense band at 123 cm−1 is significantly reduced by adding Se as a result of decreasing the crystallinity of the system.
From the application point of view, the obtained results in the present tellurium rich nanomaterials (Se x Te100 − x, x = 0, 3, 6, 9, 12, and 24) are remarkable. The results show the controlled direct bandgap, which might be useful for different applications. This direct bandgap gives more favorable optoelectronic properties than the indirect bandgap . Direct bandgap means that electrons at the minimum of the conduction band have the same momentum as electrons at the maximum of the valence band, and for an indirect bandgap, the electrons do not have the same momentum. The recombination of an electron near the bottom of the conduction band with a hole near the top of the valence band requires the exchange of energy and momentum. For indirect bandgap recombination, the energy may be carried off by a photon, but one or more phonons are required to conserve momentum (Figure 5). This multiparticle interaction is improbable, and the recombination efficiency in the indirect bandgap material is lower than that in the case of direct bandgap material. The majority part of semiconductors is indirect bandgap material; compared with them, direct bandgap materials are preferred for several applications such as laser diodes. Direct bandgap structures maximize the tendency of electrons and holes to recombine by stimulated emission, thus increasing the laser efficiency. Moreover, the material with controlled bandgap (by increasing the concentration of Se in the present nanomaterial) expected to have better properties. They can be alloyed to ternary and quaternary compositions, with adjustable bandgap width. Here in the present nanomaterials, i.e., Se x Te100 − x, there are no changes in positions of the emitted wavelengths (Figure 6a) due to changing the bandgap values. The only change that could be observed is the intensity of these emissions. Thus, controlling the intensity of these emissions might be useful in optoelectronic devices. Furthermore, providing materials with wide bandgaps allows operation of power devices at higher temperatures and gives lower thermal noise to low-power devices at room temperature .
Thin films of Se x Te100 − x (x = 0, 3, 6, 9, 12, and 24) nanoparticles with particle size in the range of 15 to 20 nm have been synthesized using thermal evaporation technique. The as-grown nanoparticles have polycrystalline structure, but their crystallinity decreases by increasing the concentration of Se. These nanomaterials are found to have direct bandgap, which differ from their microstructure counterparts. Moreover, this bandgap could be tuned by changing the concentration of Se. PL emission spectra for these films showed three bands at 666, 718, and 760 nm, while Raman spectra display three bands at 123, 143, and 169 cm−1. The intensities of PL and Raman bands were decreased by increasing the concentration of Se except that of the last band of Raman wherein it is increased. These results might be useful for the development of optical disks and other semiconducting devices based on these controlled direct bandgap nanomaterials.
This paper was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant number 1-903-D1432. The authors thank DSR for the technical and financial supports.
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