TY - JOUR
T1 - Understanding the low resistivity of the amorphous phase of Cr2Ge2Te6 phase-change material
T2 - Experimental evidence for the key role of Cr clusters
AU - Hatayama, Shogo
AU - Kobayashi, Keisuke
AU - Saito, Yuta
AU - Fons, Paul
AU - Shuang, Yi
AU - Mori, Shunsuke
AU - Kolobov, Alexander V.
AU - Sutou, Yuji
N1 - Funding Information:
This work was supported by KAKENHI (Grants No. 18H02053 and No. 19H02619). A.V.K. would like to acknowledge partial support of this work by the Ministry of Education of the Russian Federation (Project No. FSZN-2020-0026). The HAXPES measurement was performed at beamline BL01B1 at SPring-8, Japan as part of Proposal No. 2020A1240.
Publisher Copyright:
© 2021 American Physical Society
PY - 2021/8
Y1 - 2021/8
N2 - Different from the prototypical elemental semiconductors such as Si and Ge, chalcogenide-based phase-change materials (PCMs) generally show very high resistivity contrast between the amorphous and crystalline phases. In contrast to conventional PCMs, such as Ge-Sb-Te alloys, where the amorphous phase possesses higher resistivity, Cr2Ge2Te6 (CrGT) exhibits the opposite dependence. Namely, the amorphous phase is characterized by a lower resistivity than the crystalline phase. Although density functional theory calculations suggest that Cr clusters are responsible for the low resistivity of amorphous CrGT, the effects of composition on the electrical properties have yet to be investigated. In this work, the dependence of the electrical properties on Cr content and the role of the Cr clusters were investigated experimentally using Hall effect, hard x-ray photoelectron spectroscopy (HAXPES), and optical property measurements. The electrical properties were found to be dependent on the Cr content. From a HAXPES core-level spectra analysis, it was found that the increased carrier density correlated with the extent of Cr clusters, indicating that the hole carriers present likely originated from Cr clusters. The increased concentration of Cr clusters was also found to lead to a shift of the valence band edge toward the Fermi level as well as to a decrease in the optical band gap. It has been suggested that the Cr clusters may induce the formation of new energy states close to the valence band edge. These results indicate that the Cr clusters play an essential role in determining the electrical properties of amorphous CrGT, and that tuning the film composition is an effective way to optimize device properties for nonvolatile memory applications.
AB - Different from the prototypical elemental semiconductors such as Si and Ge, chalcogenide-based phase-change materials (PCMs) generally show very high resistivity contrast between the amorphous and crystalline phases. In contrast to conventional PCMs, such as Ge-Sb-Te alloys, where the amorphous phase possesses higher resistivity, Cr2Ge2Te6 (CrGT) exhibits the opposite dependence. Namely, the amorphous phase is characterized by a lower resistivity than the crystalline phase. Although density functional theory calculations suggest that Cr clusters are responsible for the low resistivity of amorphous CrGT, the effects of composition on the electrical properties have yet to be investigated. In this work, the dependence of the electrical properties on Cr content and the role of the Cr clusters were investigated experimentally using Hall effect, hard x-ray photoelectron spectroscopy (HAXPES), and optical property measurements. The electrical properties were found to be dependent on the Cr content. From a HAXPES core-level spectra analysis, it was found that the increased carrier density correlated with the extent of Cr clusters, indicating that the hole carriers present likely originated from Cr clusters. The increased concentration of Cr clusters was also found to lead to a shift of the valence band edge toward the Fermi level as well as to a decrease in the optical band gap. It has been suggested that the Cr clusters may induce the formation of new energy states close to the valence band edge. These results indicate that the Cr clusters play an essential role in determining the electrical properties of amorphous CrGT, and that tuning the film composition is an effective way to optimize device properties for nonvolatile memory applications.
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U2 - 10.1103/PhysRevMaterials.5.085601
DO - 10.1103/PhysRevMaterials.5.085601
M3 - Article
AN - SCOPUS:85113595612
SN - 2475-9953
VL - 5
JO - Physical Review Materials
JF - Physical Review Materials
IS - 8
M1 - 085601
ER -