Sublimation of GeTe Nanowires and Evidence of Its SizeEffect Studied by in Situ TEMJoanne W. L. Yim,†,‡Bin Xiang,†,‡and Junqiao Wu*,†,‡Department of Materials Science and Engineering, UniVersity of California, Berkeley,Berkeley, California 94720, and Materials Sciences DiVision, Lawrence Berkeley NationalLaboratory, Berkeley, California 94720Received July 13, 2009; E-mail: [email protected]: We report sublimation of crystalline GeTe nanowires at elevated temperatures in vacuum imagedby in situ transmission electron microscopy. The GeTe nanowires exhibit significant melting point suppressionin the presence of Au contamination. A nanosized effusion cell is formed by coating the GeTe core with aSiO2shell, where the core can be evaporated or sublimated from the open end of the shell at hightemperatures. By measuring the speed of the moving interface between the condensed and vapor phases,we determined the vaporization coefficient of these nanowires to be greater than or equal to ∼10-3overa wide range of temperatures. At the final stage of the nanowire vaporization, the material loss occurs ata higher rate, which is evidence of a higher vaporization coefficient for nanosized GeTe. This in situ techniqueoffers a quantitative method of investigating phase transition dynamics and kinetics of nanomaterials, animportant topic for designing nanoscale devices to be operated at high temperatures such as phase changememory.IntroductionPhase-change memory has emerged as a strong contender forreplacing flash memory for nonvolatile memory technology.Phase-change material (PCM), commonly chalcogenides andmost popularly Ge2Sb2Te5(GST), exhibiting an amorphous-crystalline transition under an applied electrical or opticalimpulse, can be probed for the existing bit state using eitherreflectivity or resistivity.1The change in optical reflectivity isexploited in rewritable CD and DVD technology. Electricallyinterrogated PCM devices sandwich the PCM between anelectrode and a resistive heating element. To drive the systemto crystallize or amorphize, a large current pulse is applied tolocally heat the PCM. Devices in excess of 8MB have beenassembled with excellent data retention and cycle life;2however,upon repeated cycling memory elements ultimately fail due tophase separation and loss of electrical contact integrity withelectrodes.3While the active PCM is routinely encapsulated,the intense local heating required for switching introducesthermal stress which can damage the encapsulant and allow thePCM to evaporate.4To prevent failure and optimize the designof high density data storage, it is important to understand thedynamic transition behavior of PCM from a condensed to vaporphase at the nanoscale.Single crystalline nanowires of many PCMs have been grown,including GeTe,5-7GST,8Sb2Te3,5and In2Se3.9High densitymemory arrays constructed from GST nanowires have beenshown to withstand >105write/rewrite cycles.8Reducing theactive PCM volume can increase the crystallization speed andlower the power consumption.10In addition, nanowires presenta convenient test-bed for investigating nanoscale phenomena,especially for in situ techniques. In situ transmission electronmicroscopy (TEM) heating experiments on metal nanoparticleshave identified surface melting and particle coalescence attemperatures much lower than in the bulk.11,12Previous in situTEM heating experiments on encapsulated Ge nanowires haveshown a significant depression of the melting point correlatedwith nanowire diameter.13Similarly, for GeTe nanowireswithout encapsulation, the PCM was observed to melt andsubsequently evaporate at temperatures lower than the bulkmelting point.14With regard to device failure analysis, thedevelopment of a void in the active material volume during in†University of California.‡Lawrence Berkeley National Laboratory.(1) Lacaita, A. L.; Wouters, D. J. Phys. Status Solidi A 2008, 205 (10),2281–2297.(2) Pirovano, A.; Redaelli, A.; Pellizzer, F.; Ottogalli, F.; Tosi, M.; Ielmini,D.; Lacaita, A. L.; Bez, R. IEEE Trans. DeVice Mater. Reliab. 2004,4 (3), 422–427.(3) Nam, S.-W.; Kim, C.; Kwon, M.-H.; Lee, H.-S.; Wi, J.-S.; Lee, D.;Lee, T.-Y.; Khang, Y.; Kim, K.-B. Appl. Phys. Lett. 2008, 92 (11),111913–3.(4) Yin, Y.; Miyachi, A.; Niida, D.; Sone, H.; Hosaka, S. Jpn. J. Appl.Phys., Part 2 2006, 45 (28), L726–L726.(5) Meister, S.; Peng, H.; McIlwrath, K.; Jarausch, K.; Zhang, X. F.; Cui,Y. Nano Lett. 2006, 6 (7), 1514–1517.(6) Yu, D.; Wu, J.; Gu, Q.; Park, H. J. Am. Chem. Soc. 2006, 128 (25),8148–8149.(7) Jennings, A. T.; Jung, Y.; Engel, J.; Agarwal, R J. Phys. Chem. C2009, 113 (17), 6898–6901.(8) Lee, S.-H.; Jung, Y.; Agarwal, R. Nat. Nanotechnol. 2007, 2 (10),626–630.(9) Sun, X.; Yu, B.; Ng, G.; Nguyen, T. D.; Meyyappan, M. Appl. Phys.Lett. 2006, 89 (23), 233121–3.(10) Milliron, D. J.; Raoux, S.; Shelby, R. M.; Jordan-Sweet, J. Nat. Mater.2007, 6 (5), 352–356.(11) Mohamed, M. B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. A1999, 103 (49), 10255–10259.(12) Wang, Z. L.; Petroski, J. M.; Green, T. C.; El-Sayed, M. A J. Phys.Chem. B 1998, 102 (32), 6145–6151.(13) Wu, Y.; Yang, P. AdV. Mater. 2001, 13 (7), 520–523.(14) Sun, X.; Yu, B.; Ng, G.; Meyyappan, M. J. Phys. Chem. C 2007, 111(6), 2421–2425.Published on Web 09/15/200910.1021/ja905808d CCC: $40.75  2009 American Chemical Society145269J. AM. CHEM. SOC. 2009, 131, 14526–14530Downloaded by U OF CALIFORNIA BERKELEY on October 7, 2009 | http://pubs.acs.org Publication Date (Web): September 15, 2009 | doi: 10.1021/ja905808dsitu electrical biasing highlights the importance of understandingand preventing PCM loss during cycling.15Here we report ona method utilizing in situ microscopy to evaluate the speed ofevaporation and sublimation of nanomaterials.Experimental SectionThe GeTe nanowires were grown using the standard Au-catalyzedvapor-liquid-solid (VLS) method.6GeTe source powder (AlfaAesar 99.999%) sublimated at the center of a 1 in. tube furnacewas transported by 120 sccm Ar downstream to a SiO2substratecovered with 40 nm diameter Au colloids (Ted Pella). A vacuumpump was run continuously to set the furnace pressure at ∼10 Torr.The source was held at 400 °C and the receiving substrates at ∼350°C for 5 h and then allowed to cool naturally. The nanowires weremanually transferred to a molybdenum TEM grid with a laceycarbon film, and the entire grid was coated with an amorphous SiO2shell using low temperature plasma