Study on the phase change behavior of nitrogen doped Bi2Te3 films
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 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).
References (21)
- et al.
Doping effects on the thermoelectric properties of Cu-intercalated Bi2Te2.7Se0.3
Curr. Appl. Phys.
(2015) - et al.
Carbon layer application in phase change memory to reduce power consumption and atomic migration
Mater. Lett.
(2017) Phase-change materials-Towards a universal memory
Nat. Mater.
(2005)- et al.
Reducing the stochasticity of crystal nucleation to enable subnanosecond memory writing
Science
(2017) - et al.
Unique bond breaking in crystalline phase change
Adv. Mater.
(2018) Ti-sb-te Phase Change Materials: Component Optimisation, Mechanism and Applications
(2017)- et al.
Trade-off between SET and data retention performance thanks to innovative materials for phase-change memory
IEDM
(2013) - et al.
Mixed-precision in-memory computing
Nat. Electron.
(2018) - et al.
Scandium doped Ge2Sb2Te5 for high-speed and low-power-consumption phase change memory
Appl. Phys. Lett.
(2018) - et al.
Compositional tuning in sputter-grown highly-oriented Bi-Te films and their optical and electronic structures
Nanoscale
(2017)
Cited by (9)
Grain growth and phase transformation of nano-sized titanium dioxide powder during heat treatment and spark plasma sintering
2022, Journal of Materials Research and TechnologyCitation Excerpt :Hence, enhancing the ZT value to achieve high-performance materials requires a high Seebeck coefficient, low electrical resistivity, and low thermal conductivity in a material system [2,3]. Numerous theoretical analyses and experimental studies have been conducted to improve the thermoelectric performance by metal doping [4,5], nitrogen doping [6,7], chemical reduction [8] or off-diagonal composites [9]. Our group has also made several efforts to improve the thermoelectric performance of TiO2 by reduction treatment and doping to adjust the carrier density [10,11].
A comprehensive review on the effects of doping process on the thermoelectric properties of Bi<inf>2</inf>Te<inf>3</inf> based alloys
2022, Journal of Alloys and CompoundsCitation Excerpt :The results showed the donor roles of Fe atoms, the decrease of the antisite defect concentration with In doping and the important role of Mg content on the optimum value of ZT (0.8 at 350 k for Mg-doped Bi2Te3). The effective role of doping in the improvement of microstructure of fabricated Bi2Te3 materials has been confirmed in different studies [38,39]. The results showed that the doping processes based on the content, type and the fabrication procedure can influence the thermoelectric properties of doped Bi2Te3 materials.
Overview of the Role of Alloying Modifiers on the Performance of Phase Change Memory Materials
2021, Journal of Electronic MaterialsMicrostructures in thin Bi<inf>2</inf>Te<inf>3</inf> films according to transmission electron microscopy
2020, AIP Conference ProceedingsInsulator–metal transition and ultrafast crystallization of Ga<inf>40</inf>Sb<inf>60</inf>/Sn<inf>15</inf>Sb<inf>85</inf> multiple interfacial nanocomposite films
2019, Journal of Materials Science: Materials in Electronics