Influence of annealing temperature on the structural and optical properties of As₃₀Te₇₀ thin films

Abstract

Chalcogenide glasses have attracted much attention largely due to their interesting physical and chemical properties. Though few published articles exist on the As-Te system, little is known about the optical properties of eutectic or near eutectic composition of As-Te system upon heat treatment. Therefore, this paper reports the effects of annealing temperature on the structural and optical parameters of As 30 Te 70 thin films. The bulk and thin films of 150 nm thick As 30 Te 70 chalcogenide glasses were prepared by melt-quenching and thermal evaporation techniques, respectively. The glass transition and crystallization reactions of the bulk samples were investigated using differential scanning calorimetry (DSC). The influence of annealing temperature on the transformation of the crystal structure was studied by X-ray diffraction (XRD), while the surface morphology of the annealed samples was examined using scanning electron microscope (SEM). The optical band gap, refractive index and extinction coefficient were also calculated. The DSC scans showed that the melting temperature remains constant at 636.56 K. In addition, other characteristic temperatures such as the glass transition temperature, the onset crystallization temperature, and the crystallization peak temperature increase with increasing the heating rate. The crystalline phases for the as-prepared and annealed films consist of orthorhombic As, hexagonal Te, and monoclinic As 2 Te 3 phases. Furthermore, the average crystallite size, strain, and dislocation density depend on the annealing temperature. The optical absorption results revealed that the investigated films have a direct transition, and their optical energy gap decreases from 1.82 eV to 1.49 eV as the annealing temperature increases up to 433 K. However, the refractive index, extinction coefficient, dielectric constant and the ratio of free carrier concentration to its effective mass, increase with increasing the annealing temperature.

Structural and optical characterization of annealed As30Te60Ga10 thin films prepared by thermal evaporation technique

“…1 reveal that the bulk As 30 Te 60 Ga 10 glass exhibits an endothermic glass transition, one distinct exothermic crystallization reaction, and endothermic melting peaks at 402.98 K, 495.22 K, 515.98 K, and 638.09 K, respectively. The observed glass transition temperature of the studied glass is higher than that of As 30 Te 70 composition, which can be attributed to the increased connectivity due to the addition of Ga [6]. This observation agrees with other ternary systems, such as As-Te-In and As-Te-Al [12,34].…”

Optical properties of annealed As30Te67Ga3 thin films grown by thermal evaporation

“…The observed melting point, 622 ± 0.5 K, is in agreement with the melting point of As 2 Te 3 phase reported elsewhere [9]. Also, the existence of Ga shifts other characteristic temperatures by about 5 K compared to the As 30 Te 70 system [12]. The values of T g , T c1 , T c2 , T p1 , and T p2 increased with the heating rate.…”

“…According to Table 1, while average crystallite size increased with annealing temperature, the dislocation density and strain decreased accordingly. This observation indicates improved crystallinity due to the heat treatment which agrees with studies reported on some other chalcogenide thin films [11][12][13]. Figure 3a shows the DSC curves of As 30 Te 67 Ga 3 glass at different heating rates (from 5 to 25 K/min).…”

“…It is clearly observed that T decreased with annealing temperature until 433 K, and increased thereafter. The decrease in the optical transmittance as a result of the thermal annealing may be due to the amorphous-crystalline transformation as observed from the XRD data and reported elsewhere [5,12,13,23]. Conversely, reflectance increased with annealing temperature at higher wavelengths.…”

Thermo‐structural characterization of (As2Se3)100‐x‐(As2Te3)x glasses for infrared optics

“…The low‐temperature phase (formed during the first crystallization peak) could not be identified by XRD, whereas the high‐temperature phase (analyzed after the second crystallization peak) was a mixture of low‐temperature phase and C2/m As 2 Te 3 —indicating phase separation and recrystallization into a more stable monoclinic phase. Regarding the present data, it is worth reminding that the fully crystalline samples exhibited monoclinic structure but showed no presence of identifiable phase‐separated phases (As 2 Te 3 , As, Se, or Te), which by the way rules out one of the alternative theories attributing the second crystallization peak to precipitation of phase‐separated Te. The most probable explanation is therefore that the first crystallization peak in the Te‐rich (As 2 Se 3 ) 100− x (As 2 Te 3 ) x glasses (see Figure ) is initiated by formation of the cubic AsSe(Te) phase that immediately partially recrystallizes into a monoclinic As 2 Se(Te) 3 .…”