Elsevier

Journal of Alloys and Compounds

Volume 754, 25 July 2018, Pages 227-231
Journal of Alloys and Compounds

Study on the phase change behavior of nitrogen doped Bi2Te3 films

https://doi.org/10.1016/j.jallcom.2018.04.294Get rights and content

Highlights

  • The stability of amorphous Bi2Te3 has been improved by N doping.

  • Grains in Bi2Te3 are limited to smaller size after N doping.

  • Volume expansion has been observed in crystallization of N doped Bi2Te3.

  • Low-power reversible phase change ability of BTN4 has been verified in PCM cell.

Abstract

Bi2Te3 has been widely used as thermoelectric material due to its overall good properties, such as low thermal conductivity, high electrical conductivity, and flexible atoms arrangement for structural optimization et al. However, its potential in phase change memory (PCM) application is not well evaluated due to its unstable amorphous state. In this work, stability of amorphous Bi2Te3 has been improved by N doping. Crystallization temperature of 175 °C and data retention of 43.6 °C have been achieved. Smaller and defective grains have been directly observed. Abnormal volume expansion of 7.2% has been observed after crystallization. Low-power consumption and good reversible phase change ability of N doped Bi2Te3 has been verified in PCM cell.

Introduction

Phase change memory (PCM) has attracted much attention for its many advantages, including fast operation speed, good data retention, high density, mature fabrication technology, etc [1,2]. The fast and reversible switching ability of phase change material between two distinguishing states - covalent bonding dominated amorphous state (high-resistive) and metavalent bonding dominated crystalline state (low-resistive) [3,4], is the cornerstone of data storage in PCM. Diversity in phase change materials allows PCM to have numerous application scenarios, such as dynamic random-access memory (DRAM)-like PCM with high speed [2], auto-mobile memory with good data retention [5], neuromorphic device with controllable cumulative crystallization and nonlinear response to stimulations [6]. Researchers are still making efforts to discover new phase change materials, as well as optimize the existing ones. A lot of the optimizations are focused on Ge2Sb2Te5 [5,7] and Sb2Te3 [2,4] host materials by doping method, as they have already shown proper phase change abilities. Bi2Te3 belongs to the same space group of R3¯m as that of Sb2Te3, consist of quintuple Te(1)-Bi-Te(2)-Bi-Te(1) layers connected by van der Waals (vdW) force [8]. As one of many excellent thermoelectric materials [[9], [10], [11]], crystalline Bi2Te3 has a low thermal conductivity of 0.6 W/mK [11], that is crucial for high thermal efficient PCM. Quintuple Te(1)-Bi-Te(2)-Bi-Te(1) layers are connected to each other by vdW force, an weak interaction that can facilitate the formation of nanostructures, such as Bi2Te3/Sb2Te3 superlattices [12], and the incorporation of foreign atoms, such as Cu incorporation to form phonon-glass electron-crystal material, making material optimization easy to implement [13]. More importantly, the fingerprint of phase change material - metavalent bonding, has been experimentally confirmed in Bi2Te3 [3]. However, there are few reports on the application of Bi2Te3 in PCM. The as-deposit polycrystalline Bi2Te3 under room temperature shows its poor stability of the amorphous phase, that is fatal for nonvolatile storage, as the bit data stored in high resistance amorphous Bi2Te3 will fade away very quickly. Thus, the first priority is to enhance its stability of the amorphous phase before its application in PCM. In phase change material, nitrogen doping will lower the nucleation and growth speed during crystallization, attributed to the reduced atomic mobility and increased topological mismatch in the local coordination [14]. Consequently, nitrogen doping is regarded as an effective way to improve thermal stability of amorphous phase change material. In this work, nitrogen doped Bi2Te3 films have been fabricated. The improved stability of the amorphous phase and low power consumption of nitrogen doped Bi2Te3 based PCM cell have been verified.

Section snippets

Experiments

The Bi2Te3 films with different nitrogen doping concentration were prepared by sputtering Bi2Te3 alloy target at a radio frequency (RF) power of 25 W, under Ar flow of 20 sccm and N2 flow of 0 sccm, 2 sccm, and 4 sccm for samples Bi2Te3 (BTN0), Bi2Te3N2 (BTN2), and Bi2Te3N4 (BTN4), respectively, under a base pressure of 2.0 × 10−4 Pa. The composition of Bi2Te3 was confirmed by energy dispersive spectrometer (EDS). 200-nm thick BTN0, BTN2, and BTN4 films were deposited on the SiO2 substrates for

Results and discussion

R-T trajectories of BTN0, BTN2, and BTN4 films during heating and cooling are shown in Fig. 1 (a). The low (∼3 × 102 Ω/sq.) and stable resistance of BTN0 sample during the whole heating process has confirmed the crystalline phase of as-deposit BTN0, indicating a poor stability of amorphous Bi2Te3. From the well crystallized as-deposit BTN0 and the partial crystalline as-deposit Sb2Te3 [15], we can tell the weaker intrinsic stability of amorphous Bi2Te3 than that of Sb2Te3, so that higher N

Conclusions

In summary, the phase change behavior of N doped Bi2Te3 has been verified. The stability of amorphous Bi2Te3 has been improved by N doping. Grains in BTN4 are consist of quintuple layers units, same as that in Bi2Te3. The abnormal volume expansion phenomenon during crystallization of BTN4 film still need further investigation. Low-power reversible phase change ability of BTN4 has been verified in PCM cell.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (2017YFA0206101, 2017YFB0701703), “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDA09020402), National Integrate Circuit Research Program of China (2009ZX02023-003), National Natural Science Foundation of China (61376006, 61401444, 61504157, 61622408), Science and Technology Council of Shanghai (17DZ2291300).

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