Abstract:We have investigated the electronic structure of as-deposited, crystalline, and laser-reamorphized Ge2Sb2Te5 using high resolution, hard x-ray photoemission spectroscopy. A shift in the Fermi level as well as a broadening of the spectral features in the valence band and the Ge 3d level between the amorphous and crystalline state is observed. Upon amorphization, Ge 3d and Sb 4d spectra show a surprisingly small breaking of resonant bonds and changes in the bonding character as evidenced by the very similar dens… Show more
“…Figure 4(b) shows the X-ray photoemission spectra (XPS) recorded near the valence-band maximum (VBM) of the GST film with T
a between 150 °C and 190 °C. We confirm that the Fermi levels (E = 0 eV) of GST films used in field effect measurements are all located near the VBM, consistent with previous studies on phase change materials 33,34 .Figure 4( a ) Band diagram of MIS structure under applied bias voltage. ( b ) The XPS spectra of the valence band for GST films with increasing T
a between 150 °C and 190 °C.…”
The tunable disorder of vacancies upon annealing is an important character of crystalline phase-change material Ge2Sb2Te5 (GST). A variety of resistance states caused by different degrees of disorder can lead to the development of multilevel memory devices, which could bring a revolution to the memory industry by significantly increasing the storage density and inspiring the neuromorphic computing. This work focuses on the study of disorder-induced carrier localization which could result in multiple resistance levels of crystalline GST. To analyze the effect of carrier localization on multiple resistant levels, the intrinsic field effect (the change in surface conductance with an applied transverse electric field) of crystalline GST was measured, in which GST films were annealed at different temperatures. The field effect measurement is an important complement to conventional transport measurement techniques. The field effect mobility was acquired and showed temperature activation, a hallmark of carrier localization. Based on the relationship between field effect mobility and annealing temperature, we demonstrate that the annealing shifts the mobility edge towards the valence-band edge, delocalizing more carriers. The insight of carrier transport in multilevel crystalline states is of fundamental relevance for the development of multilevel phase change data storage.
“…Figure 4(b) shows the X-ray photoemission spectra (XPS) recorded near the valence-band maximum (VBM) of the GST film with T
a between 150 °C and 190 °C. We confirm that the Fermi levels (E = 0 eV) of GST films used in field effect measurements are all located near the VBM, consistent with previous studies on phase change materials 33,34 .Figure 4( a ) Band diagram of MIS structure under applied bias voltage. ( b ) The XPS spectra of the valence band for GST films with increasing T
a between 150 °C and 190 °C.…”
The tunable disorder of vacancies upon annealing is an important character of crystalline phase-change material Ge2Sb2Te5 (GST). A variety of resistance states caused by different degrees of disorder can lead to the development of multilevel memory devices, which could bring a revolution to the memory industry by significantly increasing the storage density and inspiring the neuromorphic computing. This work focuses on the study of disorder-induced carrier localization which could result in multiple resistance levels of crystalline GST. To analyze the effect of carrier localization on multiple resistant levels, the intrinsic field effect (the change in surface conductance with an applied transverse electric field) of crystalline GST was measured, in which GST films were annealed at different temperatures. The field effect measurement is an important complement to conventional transport measurement techniques. The field effect mobility was acquired and showed temperature activation, a hallmark of carrier localization. Based on the relationship between field effect mobility and annealing temperature, we demonstrate that the annealing shifts the mobility edge towards the valence-band edge, delocalizing more carriers. The insight of carrier transport in multilevel crystalline states is of fundamental relevance for the development of multilevel phase change data storage.
“…The primary peaks (63%) with a 2/3 area constraint correspond to the Sb−Ga bonds in the tetrahedral environ- This observation of defective octahedral states in Te and Sb has been well reported in GST. 36 Here, the same was observed in GaSb-based PCM alloys. The broadening is generally attributed to Sb being in a disordered state.…”
Section: ■ Results and Discussionsupporting
confidence: 77%
“…Here, the Sb 4d and Ga 3d core-orbitals are very important, as they are sensitive to the change in the chemical environment, the result of a structural change. 36 Since the change in the chemical nature is very small here, we have done the composition analysis. As discerned from Figure 9, the Sb 4d spectrum of the as-deposited sample (S1) shows welldefined spin−orbit splitting and has two different components.…”
Section: ■ Results and Discussionmentioning
confidence: 99%
“…Due to this delocalization, the valence band offset is often tuned in PCM when it changes its phase from amorphous to crystalline. Prior hard XPS studies revealed that the valence band of the crystalline state falls onto the Fermi level and thereby explaining its metallicity, whereas an offset of ∼0.4 eV in the amorphous state justifies its high resistance state. − To get more information about the valence band structure and its changes, synchrotron radiation UPS was performed in all four samples.…”
Section: Resultsmentioning
confidence: 99%
“…To reveal the origin of this increase in vacancies in S2 by chemical bonding, we performed core-level PES measurements using synchrotron radiation on S1 and S2, which are presented in Figure . Here, the Sb 4 d and Ga 3 d core-orbitals are very important, as they are sensitive to the change in the chemical environment, the result of a structural change . Since the change in the chemical nature is very small here, we have done the composition analysis.…”
For phase change materials (PCMs) to become a viable universal memory candidate and obsolete von-Neumann architecture, materials with very high crystallization speed are needed. Moreover, introducing prestructural ordering inside the material is also being touted as one of the promising techniques to reach the speed of SRAM or further down. In this aspect, GaSb alloys are showing much promise for not only having very low crystallization times by themselves, but also showing an enormous drop in the programming time while crystallizing the reamorphized material. Here, we demonstrate how the threshold switching behavior changes for the fully amorphous film in contrast with the disordered film with nucleation sites using conductive-atomic force microscopy. It is found that the required power and programming current for memory switching with nominally stoichiometric GaSb (44:56) are 23 nW and 6.2 nA, respectively. As expected for a nucleation-oriented PCM, the deposited thin film has two voltage thresholds during the local programming process, one for conduction and another for memory switching. However, when the nucleation sites are introduced inside the disordered film, the conduction and memory switching become simultaneous. This is also found to reduce the threshold power and SET current by 93% (1.5 nW) and 91% (550 pA), respectively. Furthermore, we also reveal the origin behind this behavioral change observed between these two thin films by using high-resolution transmission electron microscopy and synchrotron radiation photoelectron spectroscopy.
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