Origin of the Optical Contrast in Phase-Change Materials

Abstract

Several chalcogenide alloys exhibit a pronounced contrast between the optical absorption in the metastable rocksalt and in the amorphous phase. This phenomenon is the basis for their application in optical data storage. Here we present ab initio calculations of the optical properties of GeTe and Ge1Sb2Te4 in the two phases. The analysis of our computations and experimental data reveal the correlation between local structural changes and optical properties as well as the origin of the optical contrast in these materials. We find that the change in optical properties cannot be attributed to a smearing of transition energies as commonly assumed for amorphous semiconductors: the optical contrast between the two phases can only be explained by significant changes in the transition matrix elements.

Structure of amorphousGe8Sb2Te11:<

“…Second, GeTe-rich alloys show larger changes in electronic and optical properties that correlate with the increasing concentration of tetrahedral sp 3 -bonded Ge atoms in these materials. 32 For a-GST-8,2,11, 42% of Ge atoms belong to this category so that 16% of all atoms are not bound octahedrally ͑p 3 ͒. This should increase further for x Ͻ 1 9 .…”

Temperature dependence of thermal properties of Ag8In14Sb55Te23 phase-change memory materials

“…When the laser beam is switched off, the melted marks quench into an amorphous state due to the high thermal conductivity of the substrate underneath the phase-change materials. Conversely, the recorded marks are erased when the materials are heated above the glass transition temperature but yet below the melting threshold so that the materials return to the crystalline state [7][8][9][10][11][12][13][14][15][16][17]. For the nonvolatile electrical storage, applying a higher voltage pulse (reset pulse) to the crystalline state with low resistivity leads to the local melting and consequently the formation of an amorphous information bit with high resistivity due to the rapid quenching.…”

“…With the development of the phase-change memory, various materials were examined, and the materials of the best performances in terms of speed and stability were found to be GeSbTe (abbreviated as GST) family, such as Ge 2 Sb 2 Te 5 and Ge 1 Sb 4 Te 7 [4,9,16,17,[21][22][23], and AgInSbTe (abbreviated as AIST) family, such as Ag 3.4 [2,3,7,[11][12][13][14][15][24][25][26][27][28]. The thermal properties of GST family were measured and studied in details, and the studies showed that there were common characteristics for different members in the GST family due to the distorted NaCl-type rock structures [21,22].…”

“…Chalcongenide phase-change materials are extensively used as the optical and nonvolatile electrical data storage media due to the apparent difference in optical reflectivity and electrical resistivity between the crystalline and amorphous state [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. For optical data storage, the recording is achieved by forming amorphous marks on a crystalline base with a focused laser beam.…”

Focused ion beam milled pattern structures induced by laser pulse on AgInSbTe phase change films

“…The structures in above mentioned are tapered or dome shaped, or even jet shaped, due to volume expansion of the material under laser irradiation. Recently, chalcogenide phase change materials, usually used as optical and electronic data storage media because of the differences in electrical conducting between the crystalline and amorphous states [11], were also found to exhibit similar tapered morphologies upon laser irradiation [12]. Different models have been proposed to explain the formation mechanism of the structures, such as Marangoni convection flow of the melted material [13], material evaporation of the thin film [14], thermoplastic deformation of the film [15], and photomechanical spallation of the upper film [16].…”