Competing Crystal Growth in Ge–Sb Phase‐Change Films

Abstract

Analysis of crystal growth in thin films of phase‐change materials can provide deeper insights in the extraordinary phase transformation kinetics of these materials excellently suited for data storage applications. In the present work crystal growth in GexSb100‐x thin films with x = 6, 7, 8, 9, and 10 is studied in detail, demonstrating that the crystallization temperature increases from ∼80 °C for Ge6Sb94 to ∼200 °C for Ge10Sb90 and simultaneously the activation energy for crystal growth also significantly increases from 1.7 eV to 5.5 eV. The most interesting new finding is that in the thin films containing 8, 9, and 10 at% Ge two competing growth modes occur which can have several orders of magnitude difference in growth rate at a single external temperature: an initial mode with isotropic slow growth producing circular crystals with smooth surfaces and growth fronts and a fast growth mode producing crystals with triangular shape having rough surfaces and growth fronts indicative of dendritic‐like growth. The slow‐growth mode becomes increasingly dominant for crystallization at low temperatures when the Ge concentration is increased from 8 to 10 at% Ge. For a certain Ge concentration, the slow growth mode becomes increasingly dominant at lower temperatures and the fast growth mode at higher temperatures. Latent heat produced during crystallization is considered a principal factor explaining the observations. The fast growth mode is associated with (eutectic) decomposition generating more latent heat and instable growth fronts and the slow growth mode is associated with thermodynamically less stable homogeneously alloyed crystals generating less latent heat, but stable growth fronts.

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

“…Earlier results on 200 nm thick Sb films (containing 7 atom % Ge) show crystal sizes in the order of millimeters. 12 Crystal sizes increase when lowering the Ge concentration from 8 to 6 atom %. Apparently, the optimum overall crystallization rate for the ultrathin pure Sb films occurs here for a film thickness at 4.1 nm and a T x of about 160 °C.…”

Chalcogenides by Design: Functionality through Metavalent Bonding and Confinement

“…However, for Sb it is known that under normal conditions, i.e., standard film thickness of at least several tens of nanometers, the amorphous phase is not sufficient stable against spontaneous crystallization at room temperature and can even show explosive crystallization. [ 155,156 ] This recent work now demonstrates two strategies, which help to stabilize the amorphous phase of Sb. The first one is increasing the quenching rate by which the amorphous phase (glass) is produced.…”

“…Of course an alternative, more simple route to stabilize the crystallization of Sb is adding Ge, where T X of thick films effectively increases from below room temperature to about 200 °C when going from pure Sb to Ge 10 Sb 90 . [ 156 ]…”

Mechanism of Nano-Structuring Manipulation of the Crystallization Temperature of Superlattice-like [Ge8Sb92/Ge]3 Phase-Change Films

“…For example, the GeSbTe phase-change material used currently has some drawbacks, such as a slow crystallization rate, poor thermal stability, a higher writing power, etc. Although Sb-rich Ge 1−x Sb x (x > 0.9) phase-change films were found to have a high optical reflectivity contrast between amorphous and crystalline states and a growth-dominant crystallization mechanism that led to a fast crystallization rate, they had poor thermal stability or a short-time retention of data [ 6 , 7 ]. Therefore, different techniques have been developed to improve or optimize the properties of phase-change materials in past two decades.…”