A comparative study on electrical transport properties of thin films of Ge1Sb2Te4 and Ge2Sb2Te5 phase-change materials

Abstract

We have investigated the electrical properties of Ge1Sb2Te4 and Ge2Sb2Te5 thin films in the temperature range of 27 to 200 °C. The optical bandgap values obtained from the measured absorption spectra for amorphous Ge1Sb2Te4 and Ge2Sb2Te5 are 0.70 and 0.90 eV, respectively. The results of the in situ temperature dependence of the sheet resistance demonstrate that both Ge1Sb2Te4 and Ge2Sb2Te5 thin films in the amorphous phase exhibit a temperature-activated electrical conductivity with activation energy values of 0.42 and 0.45 eV, respectively. These results show that our experimental results agree well with the predictions of Mott’s model. The temperature dependent carrier mobility was measured accurately using the surface acoustic wave method, which is an appropriate method for materials with low conductivities and low mobility. Typical values of drift mobility for amorphous Ge1Sb2Te4 and Ge2Sb2Te5 thin films at room temperature are round 10−3 cm2/(Vs) and 10−2 cm2/(Vs), respectively. The values of drift mobility remain small below 140 °C when the films are in an amorphous state. The drift mobility increases gradually as the temperature increases in the temperature range of 140–160 °C and shows dramatic changes above 160 °C. In general, the mobility of amorphous Ge2Sb2Te5 is 3 to 5 times higher than that of amorphous Ge1Sb2Te4. The smaller mobility for Ge1Sb2Te4 films might suggest that amorphous Ge1Sb2Te4 film has more vacancies, which results in more disordering as compared to that of Ge2Sb2Te5.

Far-Infrared and Raman Spectroscopy Investigation of Phonon Modes in Amorphous and Crystalline Epitaxial GeTe-Sb2Te3 Alloys

“…Please note that a full disordered c-GST326 is obtained at 130 °C 13 and it is ordered at about 183 °C 6, while t-GST is present already at 225 °C. From the slope of the curves in the linear blue region an activation energy for the conduction of c-GST326 is extracted, this giving E A  = 0.43 eV, a value which compares well with literature2122. In the red region (t-GST) the slope of P 2 has an activation energy of E A  = 0.08 eV, indication of an enhanced metallic behavior.…”

Notes on Reproducibility of Ge2Sb2Te5 Properties

“…They also assume that the σ change at the amorphous‐to‐cubic transformation is governed by N changes, reflecting shifts of the Fermi level, which originate from some bond exchanges in short‐range structures, the idea being followed by other researchers . In contrast, Kato and Tanaka proposed through conductivity and thermopower measurements that μ ≈ 10 −2 –10 −3 cm 2 /(Vs) in a‐GST, consistent with Xu's results, and the σ increase at the amorphous‐to‐cubic transformation is governed by dramatic mobility enhancements to ≈10 2 cm 2 /(Vs) with mostly fixed N of ≈10 16 cm −3 . (Also, the cubic to hexagonal is by an increase in N to ≈10 20 cm −3 with μ ≈ 10–10 2 cm 2 /(Vs) …”

“…These new applications use multilevel operations with subtle optical or electrical changes, whereas what is surprising is that related fundamental properties have scarcely been reproduced. For instance, in contrast to more‐or‐less fixed optical‐gap energies of 0.7–0.9 eV in amorphous films, refractive indices reported so far scatter over 3.3–4.5 (at λ ≈ 1–2 μm) and electrical conductivities at room temperatures extend over two orders of magnitude (see Figure ).…”

Correlation between optical absorption redshift and carrier density in phase change materials

“…2(b) for all films considered which is consistent with previous reports. [10][11][12][13][14][15][16][17] From Fig. 2(b), the carrier densities increase with increasing Ge concentrations.…”

“…Apart from optical properties, GST charge transport properties such as the carrier density, have also been investigated. [8][9][10][11][12][13][14][15][16][17] These alloys are reported to be p-type semiconductors with a very high hole concentrations 10 of over 10 20 cm À3 . It has been pointed out that carrier concentrations are not a consequence of the small gaps and thermal excitation, but are induced by a pronounced down-shift of the Fermi level, or valence band maximum up-shift.…”