Elsevier

Applied Surface Science

Volume 518, 15 July 2020, 146227
Applied Surface Science

Full Length Article
Effects of surface oxidation on the crystallization characteristics of Ge-rich Ge-Sb-Te alloys thin films

https://doi.org/10.1016/j.apsusc.2020.146227Get rights and content

Highlights

  • An optimized Ge-rich GeSbTe alloy shows high crystallization temperature (T ~ 380 °C).

  • Oxidation in Ge-rich GeSbTe reduces the crystallization temperature by ~50 °C.

  • The phase change in GeSbTe is studied via in-situ Transmission Electron Microscopy.

  • Oxidized and non-oxidized Ge-rich GeSbTe present different nucleation mechanisms.

  • Chemical phase separation produces the formation of Ge and rocksalt GST phase.

Abstract

We have studied the effect of surface oxidation on the crystallization of Ge-rich Ge-Sb-Te materials, promising for Phase Change Memories working at high temperatures (>350 °C). For this, we have compared the structural and chemical characteristics of films left exposed to air with those shown by TiN-encapsulated films. The effect of air exposure is to lower the temperature at which the onset of crystallization starts by 50–60 °C. Instead of homogeneous nucleation observed in encapsulated films, crystallization proceeds from the surface towards the bulk of the film and results in a massive redistribution of the chemical elements, forming Ge grains which grow until Ge concentration is low enough to allow the Ge2Sb2Te5 rocksalt phase to nucleate. In the air-exposed films, Ge crystallization preferentially occurs at the film surface while the Ge2Sb2Te5 grains develop later, at higher temperature, and deeper in the film. Our results strongly suggest that “seeds” are formed in or below the oxide during the early stage of annealing, promoting the heterogeneous nucleation of the Ge cubic phase at a lower temperature than observed in encapsulated films. These seeds necessarily involve oxygen and we speculate that crystalline Sb2O3 nuclei formed in the surface layer during annealing play this role.

Introduction

After their successful exploitation as storage media for Digital Versatile Disc (DVD), Phase Change Materials (PCMs) are again attracting the interest of microelectronic industries as they are foreseen to be used for the next generation of electronic memories, beyond the technological nodes accessible to Flash devices [1], [2], [3], [4]. Recently, they have been also explored in the framework of neuromorphic computing and machine learning as synaptic elements in artificial neural networks thanks to the feasibility of multilevel data storage [5], [6], [7]. Furthermore, the potential of Phase Change Random Access Memories (PCRAMs) to store and process huge amount of data opens the route towards the development of the concept of the Internet of Things [5], [6], [7].

PCMs can reversibly transit from the amorphous to the crystalline phases by means of heating processes [2]. Most of the physical properties of these materials are very different, depending on the phase. The large change of optical reflectivity is exploited in optical data storage devices. In PCRAMs, the information bits 0/1 are stored in the high/low electrical resistivity of the respective amorphous and crystalline states. Phase switching is induced by means of electrical power pulses of adequate shapes through Joule heating [1]. Prototypical PCMs are GeTe and Ge2Sb2Te5 [8], which have been extensively studied for their rapid switching speed (time scale up to nanosecond [9] and sub-nanosecond [10]) and high resistivity contrast between the amorphous and crystalline phases (up to 104 ohm∙cm change [11], [12]), thus representing suitable candidates for PCRAMs. The most appealing characteristics of PCRAMs are their cyclability [13], their endurance and fast programming [14], while presenting an extremely easy scaling path [4]. Another important characteristic of a PCM is its crystallization temperature (Tχ), which dictates the stability of the RESET state and the reliability of the PCRAM when exposed to high working temperatures [15], [16]. In this respect, the canonical and largely studied GeTe and Ge2Sb2Te5 phases show their limit with crystallization temperatures of approximately 230 and 170 °C, respectively.

The quest for PCMs showing high Tχ (typically larger than 300 °C), as desired for specific embedded applications, has triggered the “discovery” of Ge-rich Ge-Sb-Te (GST) alloys, where Tχ is found to increase with Ge content [17], [18]. The addition of impurity elements, such as nitrogen [19], [20], carbon [21], oxygen [22], bismuth [23] and Sb [24], has also been reported to increase Tχ. Today, N doped Ge-rich GST alloys appear to be one of the PCMs of choice for such applications [18], [25], [26], [27].

However, canonical PCM materials show degraded properties and reduced Tχ when the films are left exposed for some time to air [8], [11], [28], [29], which may cause serious issues for the manufacturing of devices. If the composition of the “natural” oxide layers which form at the surface of these materials has been studied in detail [29], [30], it is only recently that the mechanisms at the origin of this reduced Tχ have been elucidated, guided by convincing TEM observations [8], [11], [12]. When GeTe is exposed to air, Ge gets selectively oxidized at the surface while Te segregates below this surface. Following this redistribution, Te may crystallize below the oxide layer, at a much lower temperature than GeTe (180 °C instead of 230 °C), and provide seeds for the subsequent heterogeneous nucleation of the remaining amorphous GeTe towards the bulk. More generally, Noé et al. [8] have shown that Tχ and, beyond the value, the crystallization mechanisms which are activated at this temperature, depend on the capping or encapsulating layer, mainly through the possible reaction of the film surface with oxygen, and on the resulting elemental redistribution which will follow this reaction [11], [12], [31]. In Ge2Sb2Te5 films, Sb can also get oxidized and Te is repelled below the oxide layer [30]. While not as clearly demonstrated, it is reasonable to think that, also in this case, the origin of the lowering of Tχ observed after air exposure relies on the elemental redistribution, resulting from selective oxidation, and on the formation of Te-rich regions which are able to crystallize at low temperature [32] and provide seeds for the subsequent heterogeneous crystallization of GST phases. Interestingly, the evidence of a two-step crystallization process, developing from the surface and propagating towards the depth of the films with different kinetics and length scales, imposes a different reading of previous reports on the influence of the film thickness on the crystallization temperature of GST films, notably when not properly capped [33], [34], [35].

Studying the impact of surface oxidation and of the aging of the film left under air exposure, in films of ~ 100 nm, is a preliminary step in order to disentangle natural oxidation, interface and size reduction effects while planning the scaling down to sub-10 nm of confined memory cells. While an extensive literature is available about these subjects for GeTe and Ge2Sb2Te5 [8], [11], [12], there is no such analogous report for the newly emerging PCMs based on Ge-rich GST alloys.

In this paper, we report on the effect of air exposure on the crystallization characteristics, onset temperature and mechanisms, of highly Ge-enriched and N-doped GST films. To this purpose, we compare the structural and chemical characteristics of 100 nm-thick films, non-encapsulated and encapsulated by a TiN layer, submitted to annealing at various temperatures and times. Ex-situ characterizations, structural and chemical, of the films after annealing were performed by X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) based techniques. We first show that the observed crystallization temperature is much lower, about 50–60 °C less, in the films left exposed to air than in those encapsulated. The observation in real time of the crystallization of the films during in-situ annealing in the TEM has been decisive in assessing the different mechanisms by which the full crystallization of the films is obtained. We show that the films left exposed to air suffer some immediate oxidation at their surface and that this oxide layer hosts the seeds which allow the heterogeneous nucleation of the first Ge crystals at the film surface during annealing. This heterogeneous crystallization from the surface, in contrast to the homogeneous crystallization observed in encapsulated films, is accompanied by a massive redistribution of the chemical elements in the depth of the films.

Section snippets

Material and methods

100 nm-thick N-doped highly Ge-enriched (E-GST, with [Ge] > 30%) films were deposited on naturally oxidized Si (1 0 0) wafers by physical vapor deposition in an industrial cluster tool. These E-GST films were either encapsulated by a 20 nm-thick TiN capping layer or left uncapped in the atmosphere for several months. To encapsulate the E-GST layer, an ultra-thin (~14 Å) Ti-rich layer was deposited on its surface prior to ∼20 nm-thick TiN deposition to favor its adhesion. Samples were annealed

Physico-chemical characteristics of as-deposited layers

Fig. 1 shows the Bright Field (BF) TEM images of the as-deposited E-GST films. As seen in the images, both films have approximately the same thickness (95–99 nm), look homogeneous in density and are structurally amorphous, as evidenced by the associated diffraction patterns inserted in the images. At the interface between the Si substrate and the film, the native Si oxide layer is detected in both samples. In the image of the non-encapsulated E-GST sample, a very thin (2–3 nm) amorphous oxide

Discussion

This experimental study has evidenced the dramatic impact of air exposure on the crystallization characteristics of amorphous Ge-rich GST alloys. Firstly, the onset of crystallization, i.e. the temperature at which Ge starts to crystallize, is typically 50–60 °C lower in the films left to air exposure than in encapsulated, air protected, films. This temperature shift is found again when comparing the structure of the samples after different annealing. Secondly, while the complete

Conclusion

We have studied the effect of air exposure on the crystallization behavior of amorphous Ge-rich GST films. For this, we have compared the structural and chemical characteristics of films left several months exposed to air at room temperature with those shown by TiN-encapsulated, i.e. air-protected films. As observed in many GST alloys, the effect of air exposure is to significantly lower the “crystallization temperature”, actually the temperature at which the onset of crystallization starts, by

CRediT authorship contribution statement

Marta Agati: Conceptualization, Data curation, Formal analysis, Investigation, Visualization, Writing - original draft. Clément Gay: Data curation, Formal analysis, Visualization. Daniel Benoit: Resources, Project administration, Validation. Alain Claverie: Conceptualization, Methodology, Validation, Funding acquisition, Project administration, Resources, Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank the people in charge of TEM and FIB facilities at CEMES, in particular S. Joulié and R. Cours for their precious technical support. We thank also Dr. Antonio Mio for his suggestions. This work has been partially funded by Minefi through the Nano2017 initiative and by the grant Labex NEXT no. ANR-10-LABX-0037 in the framework of the “Programme des Investissements d’Avenir”.

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