Electrical transport mechanism of the amorphous phase in Cr₂Ge₂Te₆ phase change material

Abstract

A Cr 2 Ge 2 Te 6 (CrGT) phase change material (PCM) was studied. Different from conventional PCMs, it shows an inverse resistance change between a low-resistance amorphous phase and a high-resistance crystalline phase. Moreover, the anomalous low resistivity in the amorphous CrGT is considered to be due to a large carrier density, but the mechanism of electrical transport is still not clear. In this study, the electrical transport mechanism of the amorphous CrGT was discussed based on the temperature dependence of the resistivity, carrier density, mobility, and current-voltage characteristics. Above 300 K, the conduction mechanism of the amorphous CrGT was thermally activated band conduction, which is different from the conventional Ge-Sb-Te PCMs that show Poole-Frenkel conduction in the amorphous phase. Below 300 K, the amorphous CrGT shows hopping conduction, changing from variable range hopping (Mott VRH) to Efros-Shklovskii variable range hopping (ES-VRH) with decreasing temperature. The crossover from Mott VRH to ES-VRH was observed at around 200 K. Furthermore, the Fermi level was not pinned at the center of bandgap; instead, it was located near the valence band.

Temperature‐Dependent Electronic Transport in Non‐Bulk‐Resistance‐Variation Nitrogen‐Doped Cr2Ge2Te6 Phase‐Change Material

“…Therefore, the amorphous NCrGT exhibits a hopping conduction below 300 K with a transport transition from Mott-VRH to ES-VRH at 200 K and thermally activated band conduction over 300 K. The transport behavior corresponds to that of an amorphous Cr 2 Ge 2 Te 6 thin film without doping. [24] In the crystalline NCrGT film, similar hopping conduction can be observed with a T cross value from Mott-VRH to ES-VRH around 180 K (Figure 2b). However, unlike the amorphous NCrGT, the p-value can be determined even in the hightemperature region over 300 K, indicating a hopping behavior.…”

“…The estimated Δ CG for amorphous and crystalline NCrGT was 54.7 and 38.0 meV, respectively, comparable to the amorphous pure CrGT in the previous work. [ 24 ] The DOS at the Fermi level ( N ( E F )) and the localization length ( ξ ) that describes the extension of the localized wave function in the VRH process can be estimated from the following equations [ 25,26 ] where is the static low‐temperature NCrGT dielectric constant, is the host static dielectric constant, and is the contribution due to the electron in the localized states. The host static dielectric constant of pure crystalline CrGT has been estimated to be 4.0 using the plane plate capacitor model.…”

High Contact Resistivity Enabling Low‐Energy Operation in Cr2Ge2Te6‐Based Phase‐Change Random Access Memory

“…The carrier type of crystalline CrGT is p‐type measured by both Hall effect and Seebeck coefficient ( S ) measurements, as summarized in Table 1 . [ 17,23,24 ] However, there are only reports with Hall effect measurements about the carrier type of amorphous CrGT (Table 1). [ 17,24 ] Generally, amorphous chalcogenides show p‐type behavior in the Seebeck effect but n‐type behavior in the Hall effect, i.e., a p–n anomaly.…”

“…[17] The carrier type of crystalline CrGT is p-type measured by both Hall effect and Seebeck coefficient (S) measurements, as summarized in Table 1. [17,23,24] However, there are only reports Amorphous phase p [17,24] p þ117…”

“…Crystalline phase p [17] p [23] þ214 [23] with Hall effect measurements about the carrier type of amorphous CrGT ( Table 1). [17,24] Generally, amorphous chalcogenides show p-type behavior in the Seebeck effect but n-type behavior in the Hall effect, i.e., a p-n anomaly. Thus, it is considered difficult to determine the carrier type of an amorphous PCM.…”