Thickness-Dependent Crystallization of Ultrathin Antimony Thin Films for Monatomic Multilevel Reflectance and Phase Change Memory Designs

Abstract

Phase change materials, with more than one reflectance and resistance states, have been a subject of interest in the fields of phase change memories and nanophotonics. Although most current research focuses on rather complex phase change alloys, e.g. , Ge2Sb2Te5, recently, monatomic antimony thin films have aroused a lot of interest. One prominent attractive feature is its simplicity, giving fewer reliability issues like segregation and phase separation. However, phase transformation and crystallization properties of ultrathin Sb thin films must be understood to fully incorporate them into future memory and nanophotonics devices. Here, we studied the thickness-dependent crystallization behavior of pulsed laser-deposited ultrathin Sb thin films by employing dynamic ellipsometry. We show that the crystallization temperature and phase transformation speed of as-deposited amorphous Sb thin films are thickness-dependent and can be precisely tuned by controlling the film thickness. Thus, crystallization temperature tuning by thickness can be applied to future memory and nanophotonic devices. As a proof of principle, we designed a heterostructure device with three Sb layers of varying thicknesses with distinct crystallization temperatures. Measurements and simulation results show that it is possible to address these layers individually and produce distinct and multiple reflectance profiles in a single device. In addition, we show that the immiscible nature of Sb and GaSb could open up possible heterostructure device designs with high stability after melt-quench and increased crystallization temperature. Our results demonstrate that the thickness-dependent phase transformation and crystallization dynamics of ultrathin Sb thin films have attractive features for future memory and nanophotonic devices.

Characterizing optical phase-change materials with spectroscopic ellipsometry and polarimetry

“…Several authors have already used dynamic ellipsometry to establish the crystallization temperature of several PCMs such as Sb [42], Sb 2 Te 3 or Ge 2 Sb 2 Te 5 [43]. To illustrate this procedure, and as an example, Fig.…”

“…To illustrate this procedure, and as an example, Fig. 4a shows the values of Ψ monitored in time while annealing amorphous antimony (Sb) films of different thicknesses with a heating rate of 5 • C/min [42]. The evolution of Ψ with increasing temperature shows an abrupt drop associated with the crystallization of the film.…”

“…5 shows a scheme of this technique and recent results of its application to the observation of transient effects in crystalline PCM thin film. A wide range of events happens during light-induced phase transitions of PCM between time scales from 0.1 -100 ps [42][43][44][45][46][47][48][49][50][51][52][53][54] and time-resolved ellipsometry in complement with ultrafast structural methods are ideal tools for probing these dynamics.…”

Phase-change thin films growth and functionality

“…4 A reflectance tunning by a heterostructure design, consisting of phase-change alloys, showed promising results by smart engineering for resonance tuning and an additional set of reflectance profiles from the phase switching. 31,32,44,45 One other interesting aspect of phase-change alloys, used in multilevel reflectance and integrated optics, is the ability to produce partial structural states promising for future reconfigurable display devices. A nonlinear effective complex refractive index can be found by locally tunning the relative ratio between the crystalline and amorphous phases of a phase-change alloy, thus leading to multiple reflectance states.…”

“…[50][51][52] Moreover, stoichiometric GaSb is a much more stable compound than the prototypical GeSbTe alloys and thus provides an opportunity to tune the electrical and optical properties without worrying about phase separation. 45 In addition, the immiscible nature of GaSb and Sb allow for multilayer designs that do not suffer from intermixing as is the case in the widely studied GeTe-Sb2Te3 multilayers or superlattices after the seminal work by Simpson et al. 53 Finally, monatomic antimony thin films have been a research interest for their attractive properties in various research fields in the past few years.…”