A prototype Ge-Sb-Te thin film phase-change memory device has been fabricated and reversible threshold and phase change switching demonstrated electrically, with a threshold voltage of 1.5 – 1.7 V. The Ge-Sb-Te thin film was fabricated by chemical vapour deposition (CVD) at atmospheric pressure using GeCl4, SbCl5, and Te precursors with reactive gas H2 at reaction temperature 780 °C and substrate temperature 250 °C. The surface morphology and composition of the CVD-grown Ge-Sb-Te thin film has been characterized by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The CVD-grown Ge-Sb-Te thin film shows promise for the phase change memory applications

B. Gholipour

C.C. Huang

D.W. Hewak

J.Y. Ou

K. Knight

English

Huang, Chung-Che

Gholipour, Behrad

Ou, Jun-Yu

Knight, Kenton

Hewak, Daniel

Southampton (e-Prints Soton)

  1 Electrical phase change of CVD-grown Ge-Sb-Te thin film device  C.C. Huang, B. Gholipour, J.Y. Ou, K. Knight and D.W. Hewak Optoelectronics Research Centre, University of Southampton, Southampton,  SO17 1BJ, United Kingdom Abstract A prototype Ge-Sb-Te thin film phase-change memory device has been fabricated and reversible threshold and phase change switching demonstrated electrically, with a threshold voltage of 1.5 – 1.7 V. The Ge-Sb-Te thin film was fabricated by chemical vapour  deposition  (CVD)  at  atmospheric  pressure  using  GeCl4,  SbCl5,  and  Te precursors  with  reactive  gas  H2  at  reaction  temperature  780 oC  and  substrate temperature 250 oC. The surface morphology and composition of the CVD-grown Ge-Sb-Te thin film has been characterized by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The CVD-grown Ge-Sb-Te thin film shows promise for the phase change memory applications.  Keywords Ge-Sb-Te, thin films, chemical vapour deposition, phase-change memory         Introduction Phase change random access memory (PCRAM) has attracted considerable interest as a candidate for the next generation of non-volatile devices which will meet current and future needs of higher density and operation speed [1, 2]. Ternary Ge2Sb2Te5 (GST) compounds are widely regarded as the most commercially viable and practical phase change family of materials for this application. These materials are currently being trialled commercially and processes which deposit GST films by RF sputtering are being implemented into production lines [3]. CVD techniques are expected to play a role in device fabrication and recently metal organic CVD process has been   2 applied to deposit GST materials in sub micron cell pores [4]. There remain however many challenges [5] which include the need to control device to device variability and undesirable  changes  in  the  phase  change  material  that  can  be  induced  by  the fabrication procedure. A confined cell structure where the phase change material is formed  inside  a  contact  via  is  expected  to  be  essential  for  the  next  generation PCRAM device because it requires lower switching power [6]. This structure however requires more complex deposition of the active chalcogenide into a cell pore. We believe  that  CVD  techniques  could  provide  better  performance  and  enable  the production  of  thin  films  with  superior  quality  compared  to  those  obtained  by sputtering, especially in terms of conformality, coverage, and stoichiometry control, and  allows  implementation  of  phase-change  films  in  nanoelectronic  devices.  In addition,  CVD  deposition  is  well  known  to  provide  higher  purity  materials  and provides the scope for new phase change materials with optimized properties to be deposited.  For the past eight years, we have been developing a CVD technique based on metal halide precursors for the deposition of chalcogenide thin films [7]. In this paper, we report  on  the  deposition  of  Ge-Sb-Te  thin  film  using  atmospheric  pressure  CVD processes with metal chloride and Te precursors. The Ge-Sb-Te thin film has also been  characterized  by  SEM  and  EDX  techniques.  In  addition,  electrical characterization of a prototype Ge-Sb-Te thin film phase change memory device has been carried out and reversible threshold and phase change switching demonstrated.  Apparatus and experimental methods  The  CVD  apparatus  for  Ge-Sb-Te  thin  film  deposition  is  shown  schematically  in Figure 1a. A specially designed tube furnace with two individually controlled heating zones was used to produce a tailored gradient hot zone within a horizontal quartz tube  which  formed  the  reactor.  The  first  heating  zone  (zone  1)  upstream  of  the sample provided a region of sufficient temperature for the reaction of precursors to   3 take place efficiently. The second heating zone (zone 2) was designed to keep the substrate at suitable temperature for deposition.  The entire process took place at atmospheric pressure. The  reaction  temperature  at  zone  one  was  set  at  780oC  where the Te  precursor within a quartz crucible injection tube was located. The reactive gas, 6% H2 balanced with  argon  gas  (6%H2/Ar),  and  the  carrier  argon  gases  for  GeCl4  ,SbCl5 and  Te vapours  were  delivered  through  mass  flow  controllers  (MFCs).  The  substrates, located in zone two, were kept at 250oC inside the gradient tube furnace throughout the deposition process. For the initial trials of Ge-Sb-Te thin film depositions by our CVD reaction, we used fixed gas flow rates of 50ml/min, 200ml/min, 50ml/min, and 300ml/min  for  GeCl4,  SbCl5,  Te  and  6%H2  /Ar  respectively.  The  temperatures selections  were  based  on  our  previous  work  on  Ge-Sb  thin films fabricated  by  a similar  CVD technique [8].  A prototype Ge-Sb-Te  thin film phase-change memory device,  as  shown  in  Figure  2a  has  been  fabricated  for  our  preliminary  electrical characterization. The Ge-Sb-Te thin film grown by CVD technique with a thickness of about 20nm was deposited on the SiO2/Si substrate pre-sputtered with a Mo bottom electrode slab about 500nm thick. This was followed by the top molybdenum (Mo) electrode strips with the dimensions of 1.5mm wide  and 150nm thick which were deposited  on  top  of  the  CVD-grown  Ge-Sb-Te  thin  film.  The  device  was  then completed with a sputtered 140nm ZnS-SiO2 capping layer. Among several of the devices fabricated, one was sacrificed with a diamond pen and cleaved to show the multilayer  structure  of  the  edge  profile  as  shown  in  Figure  2b.  This  SEM  image revealed  the  Ge-Sb-Te  thin  film  phase-change  memory  device  fabricated  was  in good agreement with the design in Figure 2a.          Results and Discussion Ge-Sb-Te  thin  films  have  been  successfully  deposited  on  SiO2/Si  substrates. Scanning  electron  microscopy  (SEM)  technique  has  been  applied  to  study  the   4 morphology of the Ge-Sb-Te thin films. A typical SEM micrograph of the Ge-Sb-Te thin film on SiO2/Si is shown in Figure 1b. As shown in Figure 1b, homogeneous crystallized  Ge-Sb-Te  thin  film  has  been  successfully  fabricated  by  this  CVD technique.  The  thickness  of  the  film  determined  by  the  edge  profile  of  cleaved sample (Figure 1c) revealed that the deposition rate was about 3 nm/min. The grain sizes of the 450nm thick crystallized Ge-Sb-Te thin film were in the range of 30-40nm. Energy  dispersive  X-ray  (EDX)  analysis  has  been  applied  to  determine  the composition of Ge-Sb-Te thin films. In the result shown, a composition with 32.1 ± 0.7 % Ge, 8.8 ± 0.3 % Sb and 59.1 ± 0.8 %Te was obtained in the 450nm thick Ge-Sb-Te thin film.   The CVD technique potentially offers improved film conformality, morphology, and compositional  control,  and  increased  manufacturing  throughput  without  creating surface  damage.  From  the  SEM  images  shown  in  Figure  1b  and  1c,  we  have demonstrated  the  good  surface  morphology  of  Ge-Sb  thin  films  with  our  CVD process using metal chloride precursors. Ge-Sb-Te materials have been intensively studied  in  the  phase  change  switching  memory  application  therefore  we  have designed  and  fabricated  a  prototype  phase  change  device  based  on  our  CVD deposited film for further electrical characterization. The electrical characterization of the above Ge-Sb-Te thin film device was carried out with  the  Cascade  probe  station  and  Agilent  4155C  semiconductor  parameter analyzer shown in Figure 2c. By introducing a voltage sweep between 0 – 4.5 volts, the current across the fabricated cell was measured using the source measure unit within the semiconductor parameter analyzer. It should be noted that an internal load resistance  within  the  instrument  provides  current  limitation  in  order to  protect the device.  The  I-V  characteristics  of  the  CVD  grown  Ge-Sb-Te  thin  film  device  are shown in Figure 3a and 3b. These were obtained both using a voltage sweep whilst measuring the current as well as using a current sweep whilst measuring the voltage. This  revealed  three  states  of  the  film  with  a  threshold  of  around  1.5  -1.7  Volts.   5 Starting at a high resistance state and applying the sweep, which at the threshold gives  way  to the stable  phase  change  (memory  switching)  by  virtue  of the  Joule heating to above the crystallization point of the material. Subsequent sweeps reveal that the low resistance crystalline state is stable unless switched back to the high resistance  through  electrically  melt  quenching  the  layer.  This  electrical  switching between amorphous and crystalline phases, is reproducible at least ten times. The resistance contrast between the two phases is in the order of 102 Ω making this result very promising for PCRAM application.     Conclusion A prototype Ge-Sb-Te thin film phase-change memory device has been fabricated and reversible threshold and phase change switching demonstrated electrically, with a  threshold  voltage  of  1.5  –  1.7  V.  Ge-Sb-Te  thin  films  deposited  on  SiO2/Si substrates  have  been  successfully  fabricated  by  CVD  using  metal  chloride precursors.  The  Ge-Sb-Te  films  have  been  characterized  by  the  SEM  and  EDX techniques.  Currently,  we  are  working  on  composition  optimization,  further characterization  and  nano-scale  device  fabrication  based  on  these  Ge-Sb-Te  thin films.  The  growth  of  phase-change  materials  into  nanoelectronic  devices  is  very essential  and  our  CVD  process  has  demonstrated  a  potential  solution  for  this application.         Acknowledgements The authors would like to acknowledge the technical assistance of Mr. John Tucknott, Mr. Neil Fagan, Mr Ed Weatherby, Mr Mark Lessey and Mr. Trevor Austin. This work was  funded  by  the  Engineering  Physical  Sciences  Research  Council  through  our Portfolio Grant EP/C515668/1.     6 References 1.  M.H.R. Lankhorst, B. W.S.M.M. Ketelaars and R.A.M. Wolters, Natural materials, 4 (2005) 347-352. 2.  C.W. Jeong, S.J. Ahn, Y.N. Hwang, Y.J. Song, J.H. Oh, S.Y. Lee, S.H. Lee, K.C. Ryoo, J.H. Park, J.H. Park, J.M. Shin, F. Yeung, W.C. Jeong, J.I. Kim, G.H. Koh, G.T. Jeong, H.S. Jeong, and K. Kim, Jpn. J. Appl. Phys., 45 (2006) 3233-3237. 3.  R. Bez and F. Pellizzer, E*PCOS 2007, Zermatt, Switzerland, September 2007. 4. J. Bae, H. Shin, D. Im, H.G. An, J. Lee, S. Cho, D. Ahn, Y. Kim, H. Horii, M. Kang, Y. Ha, S. Park, U.I. Chung, J.T. Moon, and W.S. Lee, E*PCOS 2008, Prague, Czech Republic, September 2008. 5. G.W. Burr, M.J. Breitwisch, M. Franceschini, D. Garetto, K. Gopalakrishnan, B. Jackson, B. Kurdi, C. Lam, L.A. Lastras, A. Padilla, B. Rajendran, S. Raoux, R.S. Shenoy, J. Vac. Sci. Technol. B, 28 (2010) 223-262. 6.  Y.S. Park, K.J. Choi, N.Y. Lee, S.M. Yoon, S.Y. Lee, S.O. Ryu, and B.G. Yu, Jpn. J. Appl. Phys., 45 (2006) L516-L518. 7.  (a)  C.C.  Huang,  D.W  Hewak,  Electronics  Letters,  40  (2004)  863-865.  (b)  C.C Huang, D.W Hewak, and J.V. Badding, Optics Express, 12 (2004) 2501-2505. (C) C.C. Huang, K. Knight, and D.W. Hewak, Optical Materials, 29 (2007) 1344-1347. 8. C.C. Huang, B. Gholipour, K. Knight, J.Y. Ou, and D.W. Hewak, submitted to Thin Solid Films.           7    List of figure captions Fig. 1 (a) Schematic diagram of CVD system used for Ge-Sb-Te thin film deposition (b) SEM top-view image of Ge-Sb-Te thin films deposited SiO2/Si substrate (c) SEM cross-sectional image of 450nm thick Ge-Sb-Te thin film deposited on SiO2/Si substrate   Fig. 2 (a )A designed  Ge-Sb-Te thin film phase-change memory device (b) SEM image of the fabricated Ge-Sb-Te thin film device (c) setup for electrical characterization of Ge-Sb-Te thin film device     Fig. 3 (a)I-V characteristics of the Ge-Sb-Te thin film phase-change memory device using a voltage sweeps whilst measuring the current.(b) I-V characteristics of the Ge-Sb-Te thin film phase-change memory device using a current sweep whilst measuring the voltage.                    8                                      Fig. 1                   9                                      Fig. 2                 10             Fig. 3 

Electrical phase change of CVD-grown Ge-Sb-Te thin film device

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