Understanding the low resistivity of the amorphous phase of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ phase-change material: Experimental evidence for the key role of Cr clusters

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, ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ (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.

Figure 1

XRD patterns of as-deposited (a) and annealed (b) CrGT films.

Figure 2

The dependence of the resistivity, mobility, and carrier density on Cr content in amorphous CrGT. Error bars are comparable to symbol sizes.

Figure 3

(a) Temperature dependence of the carrier density of amorphous CrGT. The broken lines show a fit obtained using Eq. (1). Error bars are comparable to symbol sizes. (b) The location of the Fermi level relative to the mobility edge for the valence band ($E_{\mathrm{F}}–E_{\mathrm{VME}}$) as a function of Cr content.

Figure 4

Reflectance (a) and transmittance (b) spectra for amorphous CrGT as a function of wavelength.

Figure 5

Tauc plots for CrGT films with 13.7 at. % Cr (a), 15.8 at. % Cr (b), 17.3 at. % Cr (c), 18.5 at. % Cr (d), and 20.6 at. % Cr (e) as a function of phonon energy (solid lines) showing linear fits (dashed lines).

Figure 6

Optical band gap of amorphous CrGT as a function of Cr content. Error bars are comparable to symbol sizes.

Figure 7

(a) Cr2s and (b) $\mathrm{Te}3p_{3/2}$ peaks obtained from HAXPES of amorphous CrGT. The purple, yellow, and red peaks are ascribed to Cr-Te, Cr-Cr, and Ge-Te bonds, respectively.

Figure 8

Area fraction of Ge-Te (red), Cr-Cr (yellow), and Cr-Te bonds (purple) as a function of Cr content.

Figure 9

(a) Cr content dependence of the valence band-edge spectra of amorphous CrGT. The inset is a zoomed-in image of the spectra in the vicinity of the Fermi level. The black solid line indicates the Fermi level. (b) The location of the Fermi level relative to valence band maximum ($E_{\mathrm{F}}-E_{\mathrm{V}}$) as a function of Cr content. The inset represents the linear fit used to determine the valence band maximum in the CrGT film with 13.7 at. % Cr.

Figure 10

(a) Schematic image showing the evolution of the band structure in amorphous CrGT for different Cr content. $E_{\mathrm{C}}, E_{\mathrm{F}}, E_{\mathrm{V}}$, and $E_{\mathrm{VME}}$ indicate the conduction band minimum, the Fermi level, the valence band maximum, and the mobility edge for the valence band, respectively. (b) Schematic image of the band structure for amorphous CrGT. The shaded area indicated by yellow solid lines indicates the density of states ascribed to the Cr clusters.