Reduction in crystallization time of Sb:Te films through addition of Bi

Abstract

The electrical, optical, and phase change properties of bismuth doped Sb 8 Te 2 films have been characterized. Thin films of the material, with up to 15 at. % percent Bi, have been synthesized; amorphous films were stable at room temperature with a Bi concentration of up to 13 at. %. The effect of Bi on the phase change properties of the film is shown to reduce the crystallization time by an order of magnitude while the crystallization activation energy reduction is minimal; 0.2 eV. Over the last 30 years, 1 phase change materials have been extensively investigated for optical data storage applications. Phase change recording is based on the reversible switching between the crystalline and amorphous states. The rate that data can be erased and directly overwritten is reliant on the crystallization time of the material; thus, understanding the effect of compositional variations on the crystallization times of these materials is of key importance.The effect of Bi doping, in the well researched Ge 2 Sb 2 Te 5 ͑GST͒ phase change alloy, has been reported by a number of groups. [2][3][4] Experimental results demonstrate that the use of Bi as a dopant could play an important role in the demonstration of improved data storage materials. When doped in GST, Bi is known to reduce the transition temperatures without affecting the crystal structure or its lattice parameters. 2In addition to the well known GST composition, the Sb:Te binary compositions are also known phase change materials. 5The appreciation of bismuth's mechanism in Sb:Te compounds could aid the understanding in the somewhat more complex Bi doped ternary systems. The Sb 2 Te phase shows a wide compositional tolerance with Sb content ranging from 63 to 81 at. %. 6 In this work, samples containing 80% Sb have been doped with Bi; close to the maximum Sb concentration at which the Sb 2 Te phase is formed. Since this crystal can form over a wide Sb compositional range, it is anticipated that it will show some acceptance of the Bi atom. The crystallization, optical, and electrical properties have been investigated.The Bi content was varied between 0 and 15 at. % by attaching pure Bi pieces to a Sb 8 Te 2 sputtering target. Sputtering was performed in an argon atmosphere, controlled at 0.5 Pa, with a rf power of 100 W. Films, with thickness of 100 nm, were deposited onto Si and SiO 2 substrates. The film composition was measured by x-ray fluorescence spectroscopy using a Rigaku RIX 2100 system. The crystallization temperature and its activation energy were measured by monitoring the reflection as a function of temperature, using a Linkam microscope furnace in conjunction with a broad visible source and a monochromator-detector setup. Measurements of the crystallization temperature were made at heating rates of 5, 10, 15, 20, and 25°C min −1 in an Ar atmosphere. These heating rates are many orders of magnitude lower than that of laser or joule heating. However, for the purpose of comparing samples measured in the same fashion, these heating rates were deemed suffici...

Atomistic Origin of the Enhanced Crystallization Speed and n‐Type Conductivity in Bi‐doped Ge‐Sb‐Te Phase‐Change Materials

“…Wang and co‐workers found that doping GST with 3.3 at% Bi improved the crystallization speed and lowered the crystallization temperature, which was correlated with a reduction in the crystallization activation energy. Simpson et al likewise found that Bi doping decreased the crystallization time in Sb 2 Te 8 , although it was noted that, in contrast to the work found elsewhere, the doping appeared to have little effect on the activation energy. It was speculated that the faster crystallization may be due to an increase in free volume in the amorphous material, and a corresponding decrease in viscosity, facilitating easier atomic diffusion during crystallization .…”

“…In spite of these caveats, the present simulations do seem to reproduce the trend in experimental growth‐rate enhancement imparted by Bi doping. To investigate the mechanism proposed by Simpson et al, we studied the effect of the dopant on the compressibility of the amorphous and crystalline models. To do this, the volume of each model was changed by ±500 Å 3 (corresponding to volume reductions and expansions of up to ≈10%) in eleven equal steps, with the atomic positions being scaled accordingly, and the total energy was obtained after ionic relaxation.…”

“…Bi has been shown to have a range of interesting effects, both on GST hosts, and also on other PCM systems . Wang and co‐workers found that doping GST with 3.3 at% Bi improved the crystallization speed and lowered the crystallization temperature, which was correlated with a reduction in the crystallization activation energy.…”

Effect of periodic number of [Si/Sb80Te20] x multilayer film on its laser-induced crystallization studied by coherent phonon spectroscopy

“…It is also shown that Bi doping into Sb 80 Te 20 can further reduce crystallization time but lead to a low crystallization temperature or poor archival stability [4]. In contrast, by adding Ge, Ag, In, and Ga into Sb 70 Te 30 or Sb 80 Te 20 , a good archival stability is achieved, but the crystallization rate slows [1,2,5].…”

Non-melting super-resolution near-field apertures in Sb–Te alloys

“…In contrast to conventional static testers, 6 the system monitors the transmission of light through the sample with nanosecond time resolution. To ensure that the probe beam measures the center of the pump beam's spot both laser beams were coupled into a single mode optical fiber.…”