Thermal stability of phase change GaSb\GeTe, SnSe\GeTe and GaSb\SnSe double stacked films revealed by X-ray reflectometry and X-ray diffraction
Introduction
Due to the evolution in information technology (IT), in the communications, as well as in the entertainment industry, there is an increasing demand for higher capacity storage solutions, along with high access-speed of the stored information. Therefore, many research groups in the world work to find improved storage solutions by new technologies and novel concepts.
Phase-change materials (PCMs) could be an excellent candidate for integration in the new random access electronic memory: Phase-change Random Access Memory (PRAM). These non-volatile memories have large cycling endurance, fast program and access times, extended scalability, low power consumption and perfect data retention capability [1]. PCMs are based on switching between the amorphous and crystalline states of material, they have low activation energy of crystallization (Ea ≤ 6 eV) and a large difference between the resistivity of the two states (their ratio exceeds several orders of magnitude). Chalcogenide PCMs [2] are the most promising materials for data storage applications. They can open the way to neuronal-like computational systems by emulating the biological synapses in nanoscale devices [3].
In order to obtain the increase of information storage capacity, storing multiple states in a single recording cell is desirable. This can be achieved by structuring the recording cell minimum as a double film separated or not by a metallic or thermal barrier film. Such recording cells can be operated to achieve intermediate resistance values, between the highest and the lowest values, due to the successive crystallizations of the films. Thus, phase-change memory devices with stacked films have shown an increased interest in the last years [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]].
Because the switching mechanism seems to be thermally activated, this kind of double stacked film must be stable (the films should not diffuse to each other) at least up to the highest crystallization temperature of the materials composing the two films. Multiple bits storage was reported in stacked chalcogenide structures [7,8] but without investigating the thermal stability of these structures. Thermal stability is an important issue for chalcogenide materials since they are known as low crystallization point materials and inter-films diffusion could lead to film mixing that might suppress multiple bits storage in such structures.
In this paper, GaSb\GeTe, GaSb\SnSe and SnSe\GeTe double stacked films are subjected to successive thermal treatments (up to 300 °C). The objective of this study is to establish how thermally stable are these stacked films, more precisely to determine the temperature at which the atomic diffusion from one film to the other is initiated. The efficiency of hafnium (Hf) as a barrier film to block atoms diffusion is also addressed. Hafnium has been chosen because it is a compact (high density) transition metal and usually, in memory device technology, the diffusion barrier film should have a high thermal stability, low electrical resistivity and low contact resistivity.
Section snippets
Materials and methods
GaSb\GeTe, SnSe\GeTe and GaSb\SnSe double stacked films (in each stacked film the second film is on top) were grown on silicon substrates (with an about 2 nm amorphous SiO2 at the surface) by pulsed laser deposition (PLD) using a KrF* laser source (λ = 248 nm, τFWHM = 25 ns), model COMPexPro 205, from Lambda Physics-Coherent. Commercially available targets (purity of 99.99%) were irradiated with a laser fluence of 1.5 J/cm2 at laser repetition rate of 3 Hz. The depositions were carried out at
GaSb, GeTe and SnSe single films
GaSb, GeTe and SnSe single films have been deposited in the same PLD conditions, in order to calibrate the thickness and the mass density of each film from the stacked films structure. In Table 1 are summarized the obtained results.
The fit of GeTe XRR diagram suggests that at the GeTe film surface a thick film (7.5 ± 0.5 nm) with a lower mean mass density (1.4 ± 0.2 g/cm3) exists (that it can be fitted by three films with different densities and thicknesses), which proved (by XPS measurement,
Conclusions
During our studies, both XRR and GIXRD measurements were used to investigate the stability against thermal diffusion of GaSb\GeTe, SnSe\GeTe and GaSb\SnSe stacked films. The most stable stacked film structure, lasting up to 300 °C, is GaSb\GeTe, which might be used in multiple state switching devices if it will also show appropriate electrical characteristics. The other two heterostructures, namely SnSe\GeTe and GaSb\SnSe, show strong atom diffusivity between the films at temperatures as low as
Acknowledgments
The authors kindly acknowledge for the financial support of the Ministry of National Education (Romania) in the frame of the Project TE 74/2015 (PN-II-RU-TE-2014-4-0498), to Dr. Gabriel Scinteie for collaboration at XRR fitting with the LEPTOS software, and to Ioana Cristina Bucur for XPS measurements.
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