Study of the effect of thermal annealing on the optical and electrical properties of vacuum evaporated amorphous thin films in the system Ge20Te80-xBix
Study of the effect of thermal annealing on the optical and electrical properties of vacuum evaporated amorphous thin films in the system Ge20Te80-xBix
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DOI：
10.1088/0268-1242/17/3/302
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发表时间：
2002-03
影响因子：
1.9
通讯作者：
K. Bhatia;N. Kishore;Jitender Malik;Mahender Singh;R. Kundu;Ashwani K. Sharma;B. K. Srivastav
中科院分区：
文献类型：
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作者：
K. Bhatia;N. Kishore;Jitender Malik;Mahender Singh;R. Kundu;Ashwani K. Sharma;B. K. Srivastav
本文系统地研究了热退火对Ge_（20）Te_（80）-xBix（x = 0，0.19，2.93，7.35）非晶半导体薄膜光学和电学性质的影响。用X射线衍射（XRD）和电子探针对薄膜进行了表征。退火温度保持在150 ° C、180 ° C和220 ° C。在高达150 ° C的温度下退火时，没有实现薄膜的结晶。XRD分析表明，在较高的退火温度下，除了沿着非晶相外，还观察到Te、Bi_2Te_3、Ge-Te等微晶。测量了薄膜的基本光吸收边和反射谱。光学带间跃迁观察到各种薄膜（制备和退火）。在220 ° C下退火的薄膜中晶体Bi2Te3的存在也得到反射光谱的支持。从光学数据计算光学能隙（Eg）、吸收边的斜率参数（Δ）和能带尾部的拖尾参数（B-1）。在150 - 450 K温度范围内研究了各种薄膜的直流电导率（σ dc）。我们观察到，两种类型的传导发生：通过在较高的温度区域中的扩展状态的传导，和通过在带尾和费米能级的本地化状态的传导，在较低的温度下由声子辅助的跳跃过程。在较高温度下的数据已经用表达式σ dc = σ 0exp（− Δ E/kT）拟合，并且还确定了电参数Δ E和σ 0。在x ≤ 2.93和x> 2.93的两个组分区域中，观察到铋浓度和退火温度对光学和电学参数的依赖性是不同的，表明两组组分中的结构差异。据指出，这些非晶半导体的块体形式在铋浓度为约3.5at%时表现出载流子符号反转。然而，这些非晶半导体的薄膜形式在电输运中不显示这种载流子符号反转。
We systematically studied the effect of thermal annealing on the optical and electrical properties of amorphous semiconducting thin films in the system Ge20Te80−xBix (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, Bi2Te3, 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 Bi2Te3 in films annealed at 220 °C is also supported by the reflection spectrum. The optical energy gap (Eg), the slope parameter (Δ) 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 (σdc) 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 σdc = σ0exp(−ΔE/kT) and the electrical parameters, ΔE and σ0, 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.