Influence of thermal and compositional variations on conduction mechanisms and localized state density of amorphous Cd₅₀S_50−xSe_x thin films

Highlights

• Amorphous Cd₅₀S_50−xSe_x (30 ≤ x ≤ 50) thin-film samples of thickness 200 nm have thermally evaporated synthesized.

• The dc-electrical parameters dependency on the temperature was discussed.

• Conduction mechanisms, Mott parameters and localized state density were studied.

• New data were measured, investigated and studied

• Obtained data were strongly dependent on the temperature and composition.

Abstract

This work reports the study of dc-electrical conductivity of the amorphous chalcogenide Cd₅₀S_50−xSe_x (30 ≤ × ≤ 50 at.%) thin films and its dependent upon the temperature and composition. Thin films were prepared by the thermal evaporation process onto normal glass substrates in a vacuum about 8.2 × 10⁻⁴ Pa. The deposition rate and the film thickness were maintained constant at about 8 nm/s and 200 nm, respectively. X-ray diffraction was used to check the amorphous nature of thin-film samples. The resistance of the film samples has been measured in the temperature range 293 K to 473 K by using the two-point probe technique. DC-electrical conductivity was determined from the resistance measurements. The sheet resistance, the conduction mechanisms, activation energies, Mott parameters, barrier potential energy, trapping state energy and the density of localized states near the Fermi level, were investigated and studied. Obtained electrical data of the ternary Cd-S-Se thin films were investigated and studied in terms of Mott's variable range hopping model. All studied electrical parameters were found to be strongly dependent on the Se-content.

Introduction

Recently, many solid-state researchers have directed their efforts towards the study of group 12–16 semiconductors due to their unique and interesting properties. Among these semiconducting alloys are chalcogenide cadmium sulphoselenide systems. These ternary Cd-S-Se systems, as well as their thin films are considered promising semiconductors used on a large scale in several potential applications [[1], [2], [3]]. Owing to Cd-S-Se compositions are characterized by high sensitivity to light and high photoconductivity, they are frequently used in the optical devices, photovoltaic applications and in the field of electronics and infrared optics [2,3]. It is known that, the un-doped chalcogenide glasses have a high electrical resistivity, which specifies their extent in the opto-electronic devices and technological applications, as well as many other different uses [3,4]. Therefore, scientists focused their study on the effect of adding active elements to those materials to modify their physical properties. These effects are determined by the chemical composition and the concentrations of added atoms, as well as by the nature of the glass itself [[5], [6], [7]].

The scope of this article is the study of the dc-electrical conductivity of Cd₅₀S_50−xSe_x thin films, due to their many potential applications in the electronic and optoelectronic devices. Since, chalcogenide Cd-S-Se thin films are distinguished by their high sensitivity to light. Consequently, they are utilized in several applications. Such as light emitting diodes, temperature sensor devices, gas sensor devices, environmental sensor devices, piezoelectric devices, infrared photo-detectors, optical switching, electron-beam pumped lasers, spintronics, electro-photography, photo-electrochemical, photocatalysis, opto-electronic devices and solar cells, as well as in quantum computing, electroluminescent devices and transistors [4,6,[8], [9], [10]].

On the other hand, there are common features of non-crystalline Cd-S-Se systems, which are the existence of the structural defects and localized states in the mobility band gap. Noting that, the density of localized states controls several physical properties of the amorphous semiconductors. Therefore, the estimation of the density of states close to the Fermi level has been an important concern, taken into consideration the intensity of these states varies from a composition to another depending on its constitutive ratios [11]. This is attributed to the absence of long-range order, in addition the other defects inherent [12,13]. Amorphous cadmium sulpho-selenide thin films, like the other chalcogenide synthesis materials, exhibit continuous variations in their electrical characterizations as their chemical composition were changed, especially the transport mechanism of charge carriers [[14], [15], [16], [17], [18], [19]].

Using the electrical conductivity measurements and its dependence upon temperatures, one can get good data on structural defects and the localized state density of amorphous chalcogenides [12,13]. Furthermore, to identify the roughness nature of the surface of films as well as the homogeneity and the smoothness of the film thickness, some researcher resort to measure and study the sheet resistance of film samples. This technique used also on a wide range to describe the electrical properties of doped semiconducting films. Moreover, this technique measures also the resistance of thin-film samples across a definite square area. The sheet resistance (R_Sh) can be obtained experimentally using either two-points or four-points probe technique. It is worth to mention that R_Sh of thin-film samples is implying that the electric current must pass through the surface of the sheet plane parallel to the surface of the film not perpendicular to it. On the other hand, to study the activation energies along the different temperature ranges and according to Mott and Davis, dc-electrical conductivity for the chalcogenide materials (bulk or films) is substantially dependent upon the temperature of these materials. The various conduction mechanisms can be estimated by using the Arrhenius equation [13,14]:

$${\mathrm{\sigma }}_{\mathrm{dc}}={\mathrm{\sigma }}_{\mathrm{o}}exp\left(\frac{-{\mathrm{E}}_{\mathrm{o}}}{K_B\mathrm{T}}\right)+{\mathrm{\sigma }}_{1}exp\left(\frac{-{\mathrm{E}}_1}{K_B\mathrm{T}}\right)+{\mathrm{\sigma }}_{2}exp\left(\frac{-{\mathrm{E}}_2}{K_B\mathrm{T}}\right)$$

where, the first term is corresponding to the region of the highest temperature, HT. Whereas, the second and the third terms represent the intermediate, IMT and the lowest temperature regions, LT, respectively. Noting that, each region obeys a distinguished conduction mechanism. The pre-exponential constants, σ_o, σ₁ and σ₂ are powerfully dependent upon the chalcogenide composition. The values of these constants are contributing to recognize the conduction mechanism type [[13], [14], [15]]. E_o, E₁ and E₂ are the activation energies corresponding to three different conduction regions, T is the absolute temperature and K_B is Boltzmann's constant.

Although there are various research works were devoted to study the crystalline CdSSe compositions and their thin films, but the researchers did not give their attention to study the electrical properties of the non-crystalline phases. To our knowledge, no devotion has been allocated to study of the electrical conductivity and Mott's parameters of the amorphous CdSSe thin films in the previous literature. Consequently, the authors of this work found a dearth in the previous articles concerned with the study of the amorphous films of cadmium sulphoselenide compositions, especially, which study the different conduction mechanisms and their activation energies, Mott's parameters, localized state densities, energy of both the trapping states and of the potential barriers and others.

This research work is a complementation of precedent researches, which carried out by the authors on amorphous and crystalline Cd-S-Se bulk systems and their thin films to study their physical, microstructural, optical and electrical properties. The physical and electrical properties of the bulk ternary glassy samples of Cd₅₀S_50−xSe_x systems where (30 ≤ x ≤ 50 at.%) were investigated and discussed in references [20,21]. While, the effect of composition upon the optical characterizations, optical parameters and the dispersion of the refractive index of the thermally evaporated amorphous Cd₅₀S_50−xSe_x thin films were also studied in references [6,22]. Besides on, the influence of Se-addition on the microstructural properties and the crystal imperfections as well as, the optical and electrical properties of the polycrystalline CdS_xSe_1−x thin films were also studied and published before [4,23].

The aim of the present article is to study the influence of thermal and compositional changes upon the conduction mechanisms and the localized state densities of the amorphous Cd₅₀S_50−xSe_x thin films (30 ≤ x ≤ 50 at. %) in the temperature range of 293–473 K. The authors also aimed to investigate and study the possible electrical conduction mechanisms of the ternary chalcogenide Cd₅₀S_50−xSe_x thin films. In addition, to discuss and study Mott's parameters, localized state densities, trapping energy states, the energy of the barrier potential of these amorphous Cd₅₀S_50−xSe_x thin films.