Enhanced nanoscale resistive switching memory characteristics and switching mechanism using high-Ge-content Ge0.5Se0.5 solid electrolyte
© Rahaman et al.; licensee Springer. 2012
Received: 8 September 2012
Accepted: 19 October 2012
Published: 6 November 2012
We demonstrate enhanced repeatable nanoscale bipolar resistive switching memory characteristics in Al/Cu/Ge0.5Se0.5/W, as compared with Al/Cu/Ge0.2Se0.8/W structures, including stable AC endurance (>105 cycles), larger average SET voltage (approximately 0.6 V), excellent data retention (>105 s) at 85°C, and a high resistance ratio (>104) with a current compliance of 8 μA and a small operation voltage of ±1.5 V. A small device size of 150 × 150 nm2 and a Cu nanofilament with a small diameter of 30 nm are both observed by high-resolution transmission electron microscope in the SET state. The Ge x Se1 − x solid electrolyte compositions are confirmed by both energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The switching mechanism relies on the smaller barrier heights for holes rather than for electrons; the positively charged Cuz+ ions (i.e., holes) migrate through the defects in the Ge x Se1 − x solid electrolytes during SET/RESET operations. Hence, the Cu nanofilament starts to grow at the Ge0.5Se0.5/W interface, and starts to dissolve at the Cu/Ge0.5Se0.5 interface, as illustrated in the energy band diagrams. Owing to both the higher barrier for hole injection at the Cu/Ge0.5Se0.5 interface than at the Cu/Ge0.2Se0.8 interface and greater thermal stability, the resistive switching memory characteristics of the Al/Cu/Ge0.5Se0.5/W are improved relative to the Al/Cu/Ge0.2Se0.8/W devices. The Al/Cu/Ge0.5Se0.5/W memory device can also be operated with a low current compliance of 1 nA, and hence, a low SET/RESET power of 0.61 nW/6.4 pW is achieved. In addition, a large memory size of 1,300 Pbit/in2 is achieved with a small nanofilament diameter of 0.25 Å for a small current compliance of 1 nA.
Keywordsnanoscale memory resistive switches high Ge solid electrolyte
Resistive switching random access memory (RRAM) devices have recently become promising candidates for future low-power nanoscale nonvolatile memory applications [1–3]. RRAM devices involving materials such as HfO x [4, 5], SrTiO3, TiO2[7, 8], ZrO2[9, 10], Na0.5Bi0.5TiO3, NiO x [12, 13], ZnO , TaO x [15, 16], and AlO x [17, 18] are widely reported. However, their precise switching mechanism remains unclear, despite being important for applications. On the other hand, other resistive switching memory materials exploit the migration of cations (Ag+ or Cuz+, z = 1 and 2) in solid electrolytes such as Ge x Se1 − x[19–21], GeS2, Ta2O5, SiO2, Ag2S [25, 26], ZrO2, TiO x /ZrO2, GeSe x /TaO x , HfO2, CuTe/Al2O3, Ti/TaO x , and GeO x . Resistive switching memory that uses Cu/ZnO/Pt , Ag/SiO2/Pt , and Ag/ZrO2/Pt  structures has also been reported recently. Further, recent studies also conclude that the growth of a metallic filament, which is at the heart of the conduction mechanism, is initiated at the Cu/ZnO (or Ag/SiO2 or Ag/ZrO2) interface and that its dissolution starts at the ZnO/Pt (or SiO2/Pt or ZrO2/Pt) interface, in contrast to previously reported results. Therefore, a better understanding of the switching mechanism based on the formation and dissolution of the Cu or Ag filament in solid electrolytes is required for future applications. In this regard, the Ge x Se1 − x (x = 0.2 to 0.4) solid electrolytes have attracted considerable interest. In these, mobile Cuz+ or Ag+ ions play an important role in the formation and dissolution of the metallic filament [19–21]. Furthermore, important benefits of using Ge x Se1 − x as switching materials are their 100% device yield and their ease of processing. Kund et al.  reported GeSe-based resistive switching memory in an Ag/GeSe/W structure with a current compliance (CC) of 10 μA and showing data retention up to 70°C. Jeong et al.  reported threshold switching using Pt/GeSe/Pt structures. Although Se-rich Ge0.3Se0.7 (or Ge0.2Se0.8) solid electrolytes have been extensively studied [19–21, 29, 37], there are no reports on solid electrolytes with a low Se (or, equivalently, a high Ge) content, such as Ge0.5Se0.5, showing enhanced memory performance. The melting points of Se and Ge are 220.5°C and 937.4°C, respectively, suggesting that the thermal stability and the memory characteristics can both be improved by increasing the relative Ge content. In this study, we investigated an Al/Cu/Ge0.5Se0.5/W memory device with improved resistive switching memory characteristics compared to those of an Al/Cu/Ge0.2Se0.8/W device. They include the repeatability of switching cycles (>103), a larger SET voltage (VSET) of approximately 0.6 V (due to the greater barrier height for holes or Cuz+ ions), stability, and long AC endurance (>105 cycles) when operated with a small voltage of ±1.5 V. The composition of the solid electrolytes was confirmed by energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The barrier height for hole injection at the Cu/Ge0.5Se0.5 interface (0.75 eV) is lower than that for electron injection at the Ge0.5Se0.5/W interface (0.91 eV), so hole injection dominates. The positively charged Cuz+ ions (i.e., holes) migrate and start to grow at the Ge0.5Se0.5/W interface and then start to dissolve at the Cu/Ge0.5Se0.5 interface. We investigated this process in terms of energy band diagrams. The barrier height for hole injection at the Cu/Ge0.5Se0.5 interface is also greater than at the Cu/Ge0.2Se0.8 interface (0.75 vs. 0 eV). This affects the migration of Cuz+ ions via defects as well as the formation and dissolution of Cu filaments in the SET and RESET operations in the Cu/Ge0.5Se0.5 solid electrolyte. Furthermore, we observe better stability in data retention in both the high-resistance state (HRS) and the low-resistance state (LRS) (over >105 s) at 85°C in the Al/Cu/Ge0.5Se0.5/W memory device compared to the Al/Cu/Ge0.5Se0.5/W device. This results from the better thermal stability of the Ge0.5Se0.5 switching material. A Cu nanofilament diameter of 30 nm is also observed by high-resolution transmission electron microscopy (HRTEM) under SET conditions in the 150 × 150 nm2 memory device. The Al/Cu/Ge0.5Se0.5/W memory device can be operated with a low CC of 1 nA, an appropriate value for future atomic-scale devices on the scale of 0.25 Å.
The thicknesses of the resistive switching material and of the memory device were evaluated from a HRTEM image. HRTEM was carried out using a FEI Tecnai G2 F-20 field-emission system (FEI Co., Hillsboro, OR, USA) with an operating voltage of 200 kV and a resolution of 0.17 nm. A molybdenum (Mo) grid was used for TEM observations. Memory characteristics, such as current–voltage (I-V) relations, endurance, and data retention were measured using an HP4156C semiconductor parameter analyzer (Agilent Technologies Inc., Santa Clara, CA, USA). Charge-trapping phenomena were observed by capacitance-voltage (C-V) measurements using the HP4284A LCR meter (Agilent Technologies Inc.). The frequency applied during the C-V measurement was 1 MHz. The capacitance was measured in parallel capacitance-conductance mode. For electrical measurements, the bias was applied to the TE while the BE was grounded. More than 100 devices were measured at random to assess the uniformity of the memory characteristics.
Results and discussion
We investigated the superior and repeatable bipolar resistive switching memory characteristics of an Al/Cu/Ge0.5Se0.5/W structure, as compared to an Al/Cu/Ge0.2Se0.8/W structure, with a small operating voltage of ±1.5 V. The composition of the switching materials was confirmed using both EDX and XPS. We demonstrated a nanoscale memory device with a size of 150 × 150 nm2, as confirmed by HRTEM. This Al/Cu/Ge0.5Se0.5/W memory device has a higher VSET of approximately 0.6 V, a stable endurance over >105 cycles, and shows excellent data retention characteristics over a time of >105 s at 85°C and a large resistance ratio of >104. A lower barrier height for hole injection than for electron injection helps the propagation of the Cuz+ ions and the initiation of growth and dissolution at the Ge0.5Se0.5/W and Cu/Ge0.5Se0.5 interfaces, respectively. The migration of Cuz+ ions, via defects, into the Ge x Se1 − x solid electrolyte explains the basic switching mechanism. The Cu nanofilament with a diameter of 30 nm is also observed by HRTEM under SET. The Al/Cu/Ge0.5Se0.5/W device can be operated with a current as low as 1 nA. Furthermore, the SET and RESET powers are small at 0.61 nW and 6.4 pW, respectively. This suggests that the solid electrolyte Ge0.5Se0.5, with a higher Ge content, in an Al/Cu/Ge0.5Se0.5/W structure paves the way to future atomic scale (<1 Å) nonvolatile memory applications.
This work was supported by the National Science Council (NSC), Taiwan, under contract numbers NSC-98-2923-E-182-001-MY3 and NSC-101-2221-E-182-061. The authors are also grateful to MA-tek, Hsinchu for their HRTEM support.
- Rainer W: Nanoelectronics and Information Technology: Advanced Electronic Materials and Novel Devices. 3rd edition. Wiley-VCH, Weinheim; 2012.
- Waser R, Aono M: Nanoionics-based resistive switching memories. Nat Mater 2007, 6: 833. 10.1038/nmat2023View Article
- Sawa A: Resistive switching in transition metal oxides. Mater Today 2008, 11: 28.View Article
- Lee HY, Chen PS, Wang CC, Maikap S, Tzeng PJ, Lin CH, Lee LS, Tsai MJ: Low power switching of nonvolatile resistive memory using hafnium oxide. Jpn J Appl Phys 2007, 46: 2175. 10.1143/JJAP.46.2175View Article
- Afanas’ev VV, Stesmans A, Pantisano L, Cimino S, Adelmann C, Goux L, Chen YY, Kittl JA, Wouters D, Jurczak M: TiNx/HfO2interface dipole induced by oxygen scavenging. Appl Phys Lett 2011, 98: 132901. 10.1063/1.3570647View Article
- Sun X, Li G, Chen L, Shi Z, Zhang W: Bipolar resistance switching characteristics with opposite polarity of Au/SrTiO3/Ti memory cells. Nanoscale Res Lett 2011, 6: 599. 10.1186/1556-276X-6-599View Article
- Jeong DS, Schroeder H, Waser R: Impedance spectroscopy of TiO2 thin films showing resistive switching. Appl Phys Lett 2006, 89: 082909. 10.1063/1.2336621View Article
- Kwon DH, Kim KM, Jang JH, Jeon JM, Lee MH, Kim GH, Li XS, Park GS, Lee B, Han S, Kim M, Hwang CS: Atomic structure of conducting nanofilaments in TiO2 resistive switching memory. Nat Nanotechnol 2010, 5: 148. 10.1038/nnano.2009.456View Article
- Lin CC, Chang YP, Lin HB, Lin CH: Effect of non-lattice oxygen on ZrO2-based resistive switching memory. Nanoscale Res Lett 2012, 7: 187. 10.1186/1556-276X-7-187View Article
- Lin CY, Wu CY, Wu CY, Lee TC, Yang FL, Hu C, Tseng TY: Effect of top electrode material on resistive switching properties of ZrO2 film memory devices. IEEE Electron Device Lett 2007, 28: 366.View Article
- Zhang T, Zhang X, Ding L, Zhang W: Study on resistance switching properties of Na0.5Bi0.5TiO3 thin films using impedance spectroscopy. Nanoscale Res Lett 2009, 4: 1309. 10.1007/s11671-009-9397-4View Article
- Kim DC, Seo S, Ahn SE, Suh DS, Lee MJ, Park BH, Yoo IK, Baek IG, Kim HJ, Yim EK, Lee JE, Park SO, Kim HS, Chung UI, Moon JT, Ryu BI: Electrical observations of filamentary conductions for the resistive memory switching in NiO films. Appl Phys Lett 2006, 88: 202102. 10.1063/1.2204649View Article
- Panda D, Dhar A, Ray SK: Nonvolatile and unipolar resistive switching characteristics of pulsed laser ablated NiO films. J Appl Phys 2010, 108: 104513. 10.1063/1.3514036View Article
- Chiu FC, Li PW, Chang WY: Reliability characteristics and conduction mechanisms in resistive switching memory devices using ZnO thin films. Nanoscale Res Lett 2012, 7: 178. 10.1186/1556-276X-7-178View Article
- Torrezan AC, Strachan JP, Medeiros-Ribeiro G, Williams RS: Sub-nanosecond switching of a tantalum oxide memristor. Nanotechnology 2011, 22: 485203. 10.1088/0957-4484/22/48/485203View Article
- Prakash A, Maikap S, Lai CS, Lee HY, Chen WS, Chen FT, Kao MJ, Tsai MJ: Improvement of uniformity of resistive switching parameters by selecting the electroformation polarity in IrOx/TaOx/WOx/W structure. Jpn J Appl Phys 2012, 51: 04DD06.View Article
- Wu Y, Lee B, Wong HSP: Al2O3-based RRAM using atomic layer deposition (ALD) with 1-μA RESET current. IEEE Electron Device Lett 2010, 31: 1449.View Article
- Banerjee W, Maikap S, Lai CS, Chen YY, Tien TC, Lee HY, Chen WS, Chen FT, Kao MJ, Tsai MJ, Yang JR: Formation polarity dependent improved resistive switching memory characteristics using nanoscale (1.3 nm) core-shell IrOx nano-dots. Nanoscale Res Lett 2012, 7: 194. 10.1186/1556-276X-7-194View Article
- Kozicki MN, Mitkova M: Memory devices based on mass transport in solid electrolytes. In Nanotechnology. Volume 3. Edited by: Waser R. Wiley-VCH, Weinheim; 2008.
- Rahaman SZ, Maikap S, Chiu HC, Lin CH, Wu TY, Chen YS, Tzeng PJ, Chen F, Kao MJ, Tsai MJ: Bipolar resistive switching memory using Cu metallic filament in Ge0.4Se0.6 solid-electrolyte. Electrochem Solid-State Lett 2010, 13: H159. 10.1149/1.3339449View Article
- Yu S, Wong HSP: Compact modeling of conducting-bridge random-access memory (CBRAM). IEEE Trans Electron Dev 2011, 58: 1352.View Article
- Jameson JR, Gilbert N, Koushan F, Saenz J, Wang J, Hollmer S, Kozicki MN: One-dimensional model of the programming kinetics of conductive-bridge memory cells. Appl Phys Lett 2011, 99: 063506. 10.1063/1.3623485View Article
- Sakamoto T, Lister K, Banno N, Hasegawa T, Terabe K, Aono M: Electronic transport in Ta2O5 resistive switch. Appl Phys Lett 2007, 91: 092110. 10.1063/1.2777170View Article
- Schindler C, Thermadam SCP, Waser R, Kozicki MN: Bipolar and unipolar resistive switching in Cu-doped SiO2. IEEE Trans Electron Dev 2007, 54: 2762.View Article
- Wang D, Liu L, Kim Y, Huang Z, Pantel D, Hesse D, Alexe M: Fabrication and characterization of extended arrays of Ag2S/Ag nanodot resistive switches. Appl Phys Lett 2011, 98: 243109. 10.1063/1.3595944View Article
- Terabe K, Hasegawa T, Nakayama T, Aono M: Quantized conductance atomic switch. Nature 2005, 433: 47. 10.1038/nature03190View Article
- Liu Q, Long S, Lv H, Wang W, Niu J, Huo Z, Chen J, Liu M: Controllable growth of nanoscale conductive filaments in solid-electrolyte-based ReRAM by using a metal nanocrystal covered bottom electrode. ACS Nano 2010, 4: 6162. 10.1021/nn1017582View Article
- Li Y, Long S, Lv H, Liu Q, Wang Y, Zhang S, Lian W, Wang M, Zhang K, Xie H, Liu S, Liu M: Improvement of resistive switching characteristics in ZrO2 film by embedding a thin TiOx layer. Nanotechnology 2011, 22: 254028. 10.1088/0957-4484/22/25/254028View Article
- Rahaman SZ, Maikap S, Chen WS, Lee HY, Chen FT, Tien TC, Tsai MJ: Impact of TaOx nanolayer at the GeSex/W interface on resistive switching memory performance and investigation of Cu nanofilament. J Appl Phys 2012, 111: 063710. 10.1063/1.3696972View Article
- Nagata T, Haemori M, Yamashita Y, Yoshikawa H, Iwashita Y, Kobayashi K, Chikyow T: Bias application hard x-ray photoelectron spectroscopy study of forming process of Cu/HfO2/Pt resistive random access memory structure. Appl Phys Lett 2011, 99: 223517. 10.1063/1.3664781View Article
- Goux L, Opsomer K, Degraeve R, Muller R, Detavernier C, Wouters DJ, Jurczak M, Altimime L, Kittl JA: Influence of the Cu-Te composition and microstructure on the resistive switching of Cu-Te/Al2O3/Si cells. Appl Phys Lett 2011, 99: 053502. 10.1063/1.3621835View Article
- Rahaman SZ, Maikap S, Tien TC, Lee HY, Chen WS, Chen F, Kao MJ, Tsai MJ: Excellent resistive memory characteristics and switching mechanism using a Ti nanolayer at the Cu/TaOxinterface. Nanoscale Res Lett 2012, 7: 345. 10.1186/1556-276X-7-345View Article
- Rahaman SZ, Maikap S, Chen WS, Lee HY, Chen FT, Kao MJ, Tsai MJ: Repeatable unipolar/bipolar resistive memory characteristics and switching mechanism using a Cu nanofilament in a GeOx film. Appl Phys Lett 2012, 101: 073106. 10.1063/1.4745783View Article
- Peng S, Zhuge F, Chen X, Zhu X, Hu B, Pan L, Chen B, Li RW: Mechanism for resistive switching in an oxide-based electrochemical metallization memory. Appl Phy Lett 2012, 100: 072101. 10.1063/1.3683523View Article
- Yang Y, Gao P, Gaba S, Chang T, Pan X, Lu W: Observation of conducting filament growth in nanoscale resistive memories. Nat Commun 2012, 3: 1737.
- Liu Q, Sun J, Lv H, Long S, Yin K, Wan N, Li Y, Sun L, Liu M: Real-time observation on dynamic growth/dissolution of conductive filaments in oxide-electrolyte-based ReRAM. Adv Mater 1844, 2012: 24.
- Kund M, Beitel G, Pinnow CU, Röhr T, Schumann J, Symanczyk R, Ufert KD, Müller G: Conductive bridging RAM (CBRAM): an emerging non-volatile memory technology scalable to sub 20 nm. IEDM Tech Dig 2005. 10.1109/IEDM.2005.1609463
- Jeong DS, Lim H, Park GH, Hwang CS, Lee S, Cheong BK: Threshold resistive and capacitive switching behavior in binary amorphous GeSe. J Appl Phys 2012, 111: 102807. 10.1063/1.4714705View Article
- Ueno T, Odajima A: Study of photo-induced effect in obliquely-deposited amorphous Ge-Se films by XPS. Jpn J Appl Phys 1980, 19: L519. 10.1143/JJAP.19.L519View Article
- Ueno T, Odajima A: X-ray photoelectron spectroscopy of Ag-and Cu-doped amorphous As2Se3and GeSe2. Jpn J Appl Phys 1982, 21: 230. 10.1143/JJAP.21.230View Article
- Grubbs ME, Deal M, Nishi Y, Clemens BM: The effect of oxygen on the work function of tungsten gate electrodes in MOS devices. IEEE Electron Dev Lett 2009, 30: 925.View Article
- Anderson PA: The work function of copper. Phys Rev 1949, 76: 388. 10.1103/PhysRev.76.388View Article
- Vegard L: Die Konstitution der Mischkristalle und die Raumfüllung der Atome. Zeitschrift für Physik 1921, 5: 17. 10.1007/BF01349680View Article
- Cardarelli F: Materials Handbook. Springer, London; 2000.View Article
- Jeong HY, Kim SK, Lee JY, Choi SY: Role of interface reaction on resistive switching of metal/amorphous TiO2/Al RRAM devices. J Electrochem Soc 2011, 158: H979. 10.1149/1.3622295View Article
- Kim SY, Lee JL: Enhancement of optical properties in organic light emitting diodes using the Mg-Al alloy cathode and IrOx-coated indium tin oxide anode. Appl Phys Lett 2006, 88: 112106. 10.1063/1.2179108View Article
- Edwards TG, Sen S: Structure and relaxation in germanium selenide glasses and supercooled liquids: a Raman spectroscopic study. J Phys Chem B 2011, 115: 4307.View Article
- Boolchand P, Bresser WJ: The structural origin of broken chemical order in GeSe2 glass. Philosophical Magazine B: Physics of Condensed Matter; Statistical Mechanics, Electronic, Optical and Magnetic Properties 2000, 80: 1757.View Article
- Bakr N, Aziz M, Hammam M: Structural properties of GexSe1-x thin films prepared by semi-closed space technique. Egypt J Sol 2000, 23: 45.
- Li X, Li Y, Li S, Zhou W, Chu H, Chen W, Li IL, Tang Z: Single crystalline trigonal selenium nanotubes and nanowires synthesized by sonochemical process. Crystal Growth & Design 2005, 5: 911. 10.1021/cg049681qView Article
- Zhou GW: TEM investigation of interfaces during cuprous island growth. Acta Mater 2009, 57: 4432. 10.1016/j.actamat.2009.06.005View Article
- McHardy C, Fitzgerald A, Moir P, Flynn M: The dissolution of metals in amorphous chalcogenides and the effects of electron and ultraviolet radiation. J Phys C: Solid State Phys 1987, 20: 4055. 10.1088/0022-3719/20/26/010View Article
- Phillips JC: Structural principles of alpha-AgI and related double salts. J Electrochem Soc 1976, 123: 934. 10.1149/1.2132971View Article
- Bruchhaus R, Honal M, Symanczyk R, Kund M: Selection of optimized materials for CBRAM based on HT-XRD and electrical test results. J Electrochem Soc 2009, 156: H729. 10.1149/1.3160570View Article
- Kinoshita K, Tsunoda K, Sato Y, Noshiro H, Yagaki S, Aoki M, Sugiyama Y: Reduction in the reset current in a resistive random access memory consisting of NiOx brought about by reducing a parasitic capacitance. Appl Phy Lett 2008, 93: 033506. 10.1063/1.2959065View Article
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