Role of Ge Switch in Phase Transition: Approach using Atomically Controlled GeTe/Sb₂Te₃ Superlattice

Abstract

Germanium-antimony-tellurite (GST) is a very attractive material not only for rewritable optical media but also for realizing solid state devices. Recently, the study of the switching mechanism between the amorphous and crystal states has actively been carried out experimentally and theoretically. Now, the role of the flip-flop transition of a Ge atom in a distorted simplecubic unit cell is the center of discussion. Turning our viewpoint towards a much wider region beyond a unit cell, we can understand that GeSbTe consists of two units: one is a Sb 2 Te 3 layer and the other is a Ge 2 Te 2 layer. On the based of this simple model, we fabricated the superlattice of GST alloys and estimated their thermal properties by differential scanning calorimetry (DSC). In this paper, we discuss the proof of the Ge switch on the basis of thermo-histories.

Ultrafast dephasing of coherent optical phonons in atomically controlledGeTe/Sb2Te3superlattices

“…The annealing of the as-grown GeTe/ Sb 2 Te 3 SL films at 503 K ͑230°C͒ for 10 min changed the amorphous states into the crystalline state. 19 The TEM measurements confirmed that the GeTe/ Sb 2 Te 3 SLs have layered structures with clear interfaces. In addition, the structural change of the GeTe/ Sb 2 Te 3 SLs was evident from the increase in the reflectivity ͑ϳ10%͒ after the annealing.…”

“…Our motivation for the use of the atomically controlled GeTe/ Sb 2 Te 3 SLs is based on the new structural model that Ge 2 Sb 2 Te 5 is considered as superlattice, which consists of two units: one is a Ge 2 Te 2 layer and the other is a Sb 2 Te 3 layer. 19,20 That indicates understanding the role of the flipflop transition of the Ge atom in the distorted simple-cubic unit cell will be a promising strategy toward the reversible transition by means of the irradiation of ultrashort laser pulses. Our experiments show that the frequency of the coherent A 1 optical modes decreases with increasing the lattice temperature.…”

Bond-Selective Excitation and Following Displacement of Ge Atoms in GeTe/Sb2Te3Superlattice

“…The sample investigated was [GeTe/Sb 2 Te 3 ] 20 consists of alternatively deposited 5 nm thick GeTe sublayers and 5 nm thick Sb 2 Te 3 sublayers, which has been described in detail elsewhere [10]. Time-resolved pump-probe reflectivity measurements were performed utilizing a near--infrared optical pulse with 20 fs duration with a wavelength of 850 nm generated by a Ti:sapphire laser oscillator.…”

“…Chong et al reported that phase-change electrical memory based upon superlattice (SL)-like (GeTe)/ (Sb 2 Te 3 ) provided better performance with respect to the switching speed and programing current as well as durability [9]. Subsequently, crystallographically strongly--oriented GST-SL with superlattice wavelengths only a few monolayers thick were developed [10] based on the phase-change model of Kolobov and workers [11] and were shown to greatly reduce the energy requirements for switching due to a reduction in dimensionality. Furthermore, some of the current authors demonstrated ultrafast phase change from the disordered to crystalline phases within 1 ps by manipulation of the local vibrational modes originating from local GeTe 4 units in GST--SL [7].…”

“…Figure 1 illustrates the schematic view of our experimental setup. In our crystallization model based on earlier experimental and theoretical results [7,10,11], a Ge atom coordinated by four Te atoms is displaced to a vacancy site leading to the formation of three bonds at the vacancy side and the breaking of one bond on the opposite side. In that sense, irradiation of GST-SL with an optical pulse linearly polarized along the [111] direction would selectively excite a weaker bond [8], because the weaker bond in GST-SL is aligned along the [111] direction.…”