Study of the effect of thermal annealing on the optical and electrical properties of vacuum evaporated amorphous thin films in the system Ge₂₀Te_80-xBi_x

Abstract

We systematically studied the effect of thermal annealing on the optical and electrical properties of amorphous semiconducting thin films in the system Ge 20 Te 80−x Bi x (x = 0, 0.19, 2.93, 7.35) prepared by flash evaporation in a vacuum of 1 × 10 −6 Torr. The films are characterized by x-ray diffraction (XRD) and electron probe micro analysis. The annealing temperature is kept at 150 • C, 180 • C and 220 • C. No crystallization of the thin films is achieved on annealing up to the temperature of 150 • C. At a higher temperature of annealing, microcrystals of Te, Bi 2 Te 3 , Ge-Te, etc, are observed along with an amorphous phase as indicated by XRD analysis. The fundamental optical absorption edge and reflection spectra of as-prepared and annealed films are determined. Optical interband transitions are observed for various films (as-prepared and annealed). The presence of crystalline Bi 2 Te 3 in films annealed at 220 • C is also supported by the reflection spectrum. The optical energy gap (E g ), the slope parameter ( ) of the absorption edge and the tailing parameter (B −1 ) of the energy band tails are computed from the optical data. The dc electrical conductivity (σ dc ) of various films is studied in the temperature range of 150-450 K. We observe that two types of conduction take place: conduction through extended states in the higher temperature region, and conduction through localized states in the band tails and at the Fermi level by the hopping process assisted by phonons at lower temperatures. The data at higher temperatures have been fitted with the expression σ dc = σ 0 exp(− E/kT ) and the electrical parameters, E and σ 0 , are also determined. It is observed that the bismuth concentration and annealing temperature dependences of the optical and electrical parameters are different in the two regions of compositions, x 2.93 and x > 2.93, indicating structural differences in the two sets of compositions. It is pointed out that the bulk form of these amorphous semiconductors exhibits a carrier sign reversal at a bismuth concentration of about 3.5 at%. However, the thin-film form of these amorphous semiconductors does not show such a carrier sign reversal in the electrical transport.

Electrical and optical properties of Sn10Sb20−xBixSe70 (0⩽x⩽8) glassy films

“…The values of preexponential factor r 0 as observed from Table 1 do not show any particular trend with Bi concentration. The large value of r 0 (100-500 S cm À1 and above) indicates that conduction takes place predominantly in the extended states and the smaller value of r 0 (1-10 À2 S cm À1 and less) points out the presence of transport in the localized states [20]. As observed from Table 1, in the present system the conduction is in the extended as well as in localized states.…”

Network topology and thermal annealing dependence of some physical properties in amorphous Sn–Sb–Se films

“…The composition dependence of various physical properties of ternary chalcogenide has been extensively studied due to their potential technological applications in solid-state devices [1][2][3][4][5]. The chalcogenide glasses, IV-V-VI group, especially Ge-Sb-Se glasses have been investigated in the recent past due to their applicability as an optical fiber material in the far-infrared region.…”

Nitrogen-implanted Ge2Sb2Te5film used as multilevel storage media for phase change random access memory

“…Many studies on C-RAM using chalcogenide semiconductors such as GeTeAsSi [1,9], GeTe [10], GeSbTe [6,8,11], GeTeBi [12,13], GeSb (Cu, Ag) [14], GeTeAs [15], In-Te [16], AsSbTe [17,18], SeSbTe [19], PbGeSb [20], GeSbTe-N [21], etc have been reported. But all of the above chalcogenide semiconductors were used only as single-level storage media for C-RAM.…”