Chalcogenide glasses are one of the most general amorphous semiconductors and have the peculiar properties such as electrical conductivity, IR transmission and photo-induced changes. In this research, $Ge_xTe_{1-x}$ (0 ≤ x ≤ 50.98%) and $(GeTe)_{1-x}Bi_x$ (0 ≤ x ≤ 29.13%) thin films are fabricated on glass substrates by co-sputtering of Ge, Te and GeTe, Bi targets, respectively. The composition of thin films is controlled by changing the relative power applied to each target.
Then the optical and electrical properties of the films are investigated with variation of the composition and the deposition condition. The thickness and composition of the films are measured by α-step and ICP method, respectively. The reflectance and transmittance of the films are measured by UV/VIS/NIR spectrometer. Then the absorption coefficient(α), refractive index (n) and extinction coefficient(k) are calculated from R, T% spectra. The optical gap($E_{opt}$) of the films is determined from the UV absorption edge. In $Ge_xTe_{1-x}$ system, the optical gap shows the maximum value at x = 33.33%. The slop B of $(\alpha\hbar\omega)^{1/2}$us. $\hbar\omega$ (i.e. Taucls plot), representing the degree of structural orderness shows the similar compositional dependency with that of the optical gap. But in case of pure Te thin film, the slope B of it have the very high value because it is crystalline thin film as-deposited. And the refractive index(n) and extinction coefficient(k) corresponding to wavelength of 633 nm, show the opposite tendency to $E_{opt}$or B, i.e. have the minimum value at x = 33.33% according to the law of Kramers-Kronig. And in $(GeTe)_{1-x}Bi_x$ system, as Bi content increases, $E_{opt}$ and B decreases linearly whereas n, k increase smoothly.
And the current-voltage curves of thin films of $Ge_xTe_{1-x}$ and $(GeTe)_{1-x}Bi_x$ are measured by 4-point probe method, respectively. The resistivities of thin films are calculated from the slope of I-V curve. The maximum of resistivity exists at x = 33.33% in $Ge_xTe_{1-x}$ system and the resistivity decreases linearly with increasing Bi content in $(GeTe)_{1-x}Bi_x$ system. So the experimental similarities between the compositional dependencies of resistivity and optical gap in each system prove the assumption that the activated conduction occurs by the intrinsic excitation in the amorphous semiconductors. In addition, the switching phenomenon is only observed at the films having over about 25% Bi content in $(GeTe)_{1-x}Bi_x$ system.
The phase identification of crystalline thin films obtained by heat treatment is done by XRD measurement. In $Ge_xTe_{1-x}$ system, the thin films are deposited as amorphous ones at R.T. except for pure Te. And the amorphous thin films are completely crystallized after heat treatment(300℃, 5hr). So the main crystal phases of this system are those of Te and GeTe crystal. As a result, the crystalline peaks of Te appear dominantly in the range of 0 ≤ x ≤ 14.70(%) and those of Te and GeTe exist together in the range of 14.70 ≤ x ≤ 50.98(%). The intensity of GeTe crystalline peaks increases with the increase of Ge content in this range. In $(GeTe)_{1-x}Bi_x$ crystalline thin films obtained by heat treatment(350℃, 24hr), $Ge_{50}Te_{50}$ film show the same peaks with those of $Ge_{50.98}Te_{49.02}$ in $Ge_xTe_{1-x}$ system. And the peaks of $GeTe_4$ appear temporarily with those of GeTe crystal in only $(GeTe)_{94.72}Bi_{5.28}$ film and the peaks of GeTe and $Bi_2Ge_3Te_6$ coexist in the range of 10.89 ≤ x ≤ 17.22(%). Finally the peak position of $Bi_2Ge_3Te_6, Bi_4GeTe_7$ and $Bi_2Te_3$ appear overlapped each other in the range of 21.98 ≤ x ≤ 29.13(%).
Also the changes of optical spectra and constants of $Ge_{33.33}Te_{66.67}$ thin film caused by annealing and crystallization effects are investigated with increasing the temperature of heat treatment. The optical gap increases and n, k decrease by annealing and also shows the abrupt and opposite changes by crystallization. And the phase of the film is analyzed from XRD data measured at each heating temperature.
And the compositional dependencies of crystallization (i.e. activation energy of crystallization, crystallization temperature) are analyzed in the base of the optical spectrum changes and XRD data of the corresponding system. In $Ge_xTe_{1-x}$ system, the diffusionless crystallization occurs at x = 0, 50.98% and the nucleation & growth crystallization does in the rest range. And the compositional dependency of the activation energy of crystallization shows the same tendency as that of the optical gap and resistivity in this system, showing the maximum value at x = 33.33%. In $(GeTe)_{1-x}Bi_x$ system, the increase of Bi content causes the decrease of nucleation temperature and the rate of crystal growth simultaneously, which can be confirmed from the optical spectrum changes and XRD data.
