Phase change memory engineering and integration with CMOS technology

ADVANCED MATERIALS AND DEVICE ENGINEERING FOR PCRAM TECHNOLOGY FANG WEIWEI LINA NATIONAL UNIVERSITY OF SINGAPORE 2011 ADVANCED MATERIALS AND DEVICE ENGINEERING FOR PCRAM TECHNOLOGY FANG WEIWEI LINA (B. ENG. (HONS.)), NATIONAL UNIVERSITY OF SINGAPORE A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2010 Acknowledgements First and foremost, I would like to express my appreciation to my research advisors, Dr. Yeo Yee Chia, Dr. Zhao Rong, and Prof. Chong Tow Chong for their support throughout these four years. I am thankful to them for sharing their knowledge and experiences, and have benefitted immensely from the valuable insights and guidance from the regular discussions with them. In addition, I am especially grateful to Dr. Yeo and Dr. Zhao, not only in the area of research and academic, but also for their understanding and patience when my mum was seriously ill. Most of my research work was performed at the Data Storage Institute (A*STAR). I would like to thank Dr. Li Minghua, for all the discussions we had and help provided during my candidature. I would like to highlight the assistance rendered by Cheng Peihwa, Tony Law, Desmond Loke, Toh Yeow Teck, Yang Hongxin and Lim Kian Guan, whom I frequently bugged. A special note of thanks goes out to Peihwa, Tony, and Desmond for the friendship and encouragements given. To Dr. Sze Jiayin, Dr. Lee Hock Koon, and Dr. Shi Luping, I would like to acknowledge the helpful pointers they have given. I am also grateful to Zhang Zheng and Dr. Pan Jisheng of Institute of Materials Research and Engineering (A*STAR), who have given me a lot of help and provided many useful discussions during the course of my research work. Special thanks to Zhang Zheng, who has tirelessly performed my many XPS requests. To the friends I‘ve met in SNDL, Rinus Lee, Koh Shao Ming, Chin Hock Chun, Tan Kian Ming, Andy Lim, Zhang Lu, Liu Fangyue, Ivana, Yang Yue and i many others, I‘m grateful that our paths have crossed. In addition, I would also like to extend my appreciation to the technical staff of SNDL, Mr Yong Yu Fu, Mr O Yan Wai Linn, Mr Patrick Tang, Mr Lau Boon Teck, and Mr Sun Zhiqiang, for their help in one way or another. Last but not least, I would like to extend my deepest gratitude to my family. To my dad, sisters Amy, Michelle and Sheryl, and Shunfu, thank you for your support throughout this journey. Finally to my mum: Even as you‘re no longer here, I still want to tell you ‗Thank you‘, the way you heard it. ii Table of Contents Acknowledgements i Table of Contents . iii Abstract . vi List of Tables viii List of Figures . ix List of Symbols . xix Chapter Introduction 1.1 Overview for Non-volatile Memory Technology . 1.2 Phase Change Memory Technology . 1.2.1 Phase change materials and memory device structures 1.2.2 Basic principles of phase change memory 1.2.3 Phase change memory integration 1.3 Objectives of Research . 11 1.4 Thesis Organization 11 1.5 References . 14 Chapter Band Alignment of Phase Change and Dielectric Materials 2.1 Introduction . 21 2.2 Methodology . 23 2.3 Experiment 26 2.4 Results and Discussion . 29 2.4.1 Alignment of Ge2Sb2Te5 and various dielectric materials 29 2.4.2 Alignment of nitrogen-doped Ge2Sb2Te5 and SiO2 34 2.4.3 Alignment of GeTe – Sb2Te3 tieline alloys and SiO2 . 38 2.5 Summary . 43 2.6 References . 45 iii Chapter Band Alignment of Phase Change and Metal Contact Materials 3.1 Introduction . 52 3.2 Experiment 54 3.3 Results and Discussion . 56 3.3.1 Amorphous Nitrogen-doped Ge2Sb2Te5 and Metals . 56 3.3.2 Crystalline Ge2Sb2Te5 and Metals 71 3.4 Summary . 75 3.5 References . 76 Chapter Dependence on the Properties of Ge2Sb2Te5 on Nitrogen Doping Concentration and Application in Phase Change Memory 4.1 Introduction . 81 4.2 Experiment 82 4.2.1 Material characterization of nitrogen-doped Ge2Sb2Te5 . 82 4.2.1 Device fabrication and testing setup . 83 4.3 Results and Discussion . 86 4.3.1 Dependence of material properties on nitrogen concentration . 86 4.3.2 Dependence of device performance on nitrogen concentration 93 4.4 Summary . 102 4.5 References . 103 Chapter Silicide Electrode Contacts for Compact Integration of Phase Change Memory with CMOS Technology 5.1 Introduction . 107 5.2 Experiment 109 5.3 Results and Discussion . 111 5.3.1 Material Properties and Thermal Analysis . 111 5.3.2 Material Interfaces and Band Alignment 116 5.3.3 Device Electrical Characterization . 118 5.4 Summary . 125 5.5 References . 127 iv Chapter Silicide Metal Contact and Dielectric Interlayer for Operation Power Reduction in Phase Change Memory 6.1 Introduction . 130 6.2 Simulation of Thermal Properties . 131 6.3 Results and Discussion . 137 6.3.1 Device Fabrication 137 6.3.2 Device Electrical Characterization . 139 6.4 Summary . 147 6.5 References . 148 Chapter 7.1 Conclusion and Future Work Conclusion 152 7.1.1 Band alignment of phase change and dielectric materials 153 7.1.2 Band alignment of phase change and metal contact materials . 153 7.1.3 Dependence on the properties of Ge2Sb2Te5 on nitrogen doping concentration and application in phase change memory 154 7.1.4 Silicide electrode contacts for compact integration of phase change memory with CMOS technology 155 7.1.5 Silicide metal electrode contact and dielectric interlayer for operation power reduction in phase change memory 155 7.2 Future work . 156 7.3 References . 158 Appendix A. List of Publications . 161 v Abstract Phase change random access memory (PCRAM) is one of the strongest contenders to replace the current Flash memory technology, which is reaching its fundamental scaling limits. PCRAM is an electrically-induced thermally-activated device in which joule heating plays an important role in phase transformation and data storage. Not only does PCRAM exhibit fast switching speed and high endurance, it also has excellent scaling capability and compatibility with complementary metaloxide-semiconductor (CMOS) technology. Materials engineering may be performed to tailor the properties of the phase-change material for improvement of memory device characteristics. This thesis summarizes work on advanced materials and device engineering for PCRAM technology. The energy band alignment between Ge-Sb-Te based phase change materials and common microelectronic materials such as dielectrics and metals was first investigated. Significant Fermi level pinning at the interface between phase change materials and metals was discovered. The results are useful for calculation of leakage currents between closely spaced cells as well as the contact resistance in PCRAM. Nitrogen-doped Ge2Sb2Te5 PCRAM devices were fabricated and the dependence of the electrical characteristics on nitrogen content in Ge2Sb2Te5 was investigated next. Two regimes of the crystallization process were observed, depending on the nitrogen content in Ge2Sb2Te5. Introduction of a small amount of nitrogen in Ge2Sb2Te5 was found to be useful for optimization of device performance. The criterion for fast switching and good device performances were evaluated based on material and device characterization. vi Energy band alignment studies show that metal silicides could possibly offer better contacts than conventional heater materials. Various metal silicides were investigated to assess their suitability as a contact material in PCRAM devices. Memory cells with metal silicide contacts and optimized nitrogen-doped Ge2Sb2Te5 were fabricated. Good device performance was achieved. Further improvement to this structure was made by inserting a thin dielectric layer at the interface between the silicide and phase change layer. The low thermal conductivity dielectric layer reduces thermal diffusion, thus enabling reduction of the reset current. Exploration of advanced materials and device designs opens up new avenues for compact PCRAM device design and integration with CMOS technology. vii List of Tables Table 3.1 Effective work function m,eff of Al, W and Pt on the various nitrogen-doped GST. The vacuum work functions m,vac of the respective metals are also shown for comparison. .60 Table 3.2 Various material parameters for nitrogen-doped GST. The bandgap (Eg), electron affinity (χ), and charge neutrality level (ECNL) measured with respect to the vacuum level (Evac) are shown in units of eV. The experimentally extracted slope parameter (Sx) and the dielectric constant (ε∞) are also recorded. 63 Table 3.3 Theoretical and experimental conduction and valence band offsets between GST and materials such as Si, SiO2, HfO2, Ta2O5, and Si3N4. The charge neutrality values ECNL of the respective materials above the valence band edge used in the calculation are also listed. 71 Table 6.1 Electrical resistivity and thermal conductivity values of various materials used in the simulation. 133 viii 6.5 [1] References D.-H. Kang, D.-H. Ahn, M.-H. Kwon, H.-S. Kwon, K.-B. Kim, K. S. Lee, and B.-K. Cheong, ―Lower Voltage Operation of a phase change memory device with a highly resistive TiON layer,‖ Japan. Jour. Appl. Phys., vol. 43, pp 5342-5244, 2004. [2] S.-M. Yoon, K.-J. Choi, N.-Y. Lee, S.-W. Jung, S.-Y. Lee, Y.-S. Park, B.-G. Yu, S.-J. Lee, and S.-G. Yoon, ―Nonvolatile memory switching behaviours of phase-change memory devices using Ti-Si-N layers,‖ Jour. Electrochem. Soc., vol. 155, no. 6, pp. H421-H425, 2008. [3] H.-Y. Cheng, Y.-C. Chen, C.-M. Lee, R.-J. Chung, and T.-S. Chin, ―Thermal stability and electrical resistivity of SiTaNx heating layer for phase-change memories,‖ Jour. Electrochem. Soc., vol. 153, no. 7, pp. G685-G691, 2006. [4] C. Xu, Z. Song, B. Liu, S. Feng, and B. Chen, ―Lower current operation of phase change memory cell with a thin TiO2 layer,‖ Appl. Phys. Lett., vol. 92, no. 6, 062103, 2008. [5] F. Rao, Z. Song, Y. Gong, L. Wu, S. Feng, and B. Chen, ―Programming voltage reduction in phase change memory cells with tungsten trioxide bottom heating layer/electrode ,‖ Nanotechnology, vol. 19, no. 44, 445706, 2008. [6] Y. Matsui, K. Kurotsuchi, O. Tonomura, T. Morikawa, M. Kinoshita, Y. Fujisaki, N. Matsuzaki, S. Hanzawa, M. Terao, N. Takaura, H. Moriya, T. Iwasaki, M. Moniwa, and T. Koga, ―Ta2O5 interfacial layer between GST and W plug enabling low power operation of phase change memories,‖ Int. Elect. Dev. Meet. Tech. Dig., pp 523-527, 2006. 148 [7] S.-Y. Lee, K.-J. Choi, S.-O. Ryu, S.-M. Yoon, N.-Y. Lee, Y.-S. Park, S.-H. Kim, S.-H. Lee, and B.-G. Yu, ―Polycrystalline silicon-germanium heating layer for phase-change memory applications,‖ Appl. Phys. Lett., vol. 89, no. 5, 053517, 2006. [8] F. Rao, Z. Song, L. Wu, M. Zhong, S. Feng, and B. Chen, ―Phase change memory cell with an upper amorphous nitride silicon germanium heating layer,‖ Appl. Phys. Lett., vol. 91, no. 7, 073505, 2007. [9] F. Rao, Z. Song, T. Zhang, Y. Gong, L. Wu, S. Feng, and B. Chen, ―Polycrystalline Si-Rich SiSbx bottom heating layer for phase change memory,‖ Electrochem. Solid-State Lett., vol. 11, no. 6, pp. H147-H149, 2008. [10] L. W.-W. Fang, R. Zhao, E.-G. Yeo, K.-G. Lim, H. Yang, L. Shi, T.-C. Chong, and Y.-C. Yeo, submitted to Jour. Electrochem. Soc. [11] U. Gottlieb, F. Nava, M. Affronte, O. Laborde, and R. Madar, Electrical transport in metallic TM silicdes, ed. K. Maex and M. Van Rossum., 1995. [12] Marc-A. Nicolet, and S. S. Lau, Formation and characterization of transition –metal silicides, ed. N. G. Einspruch and G. B. Larrabee, 1983. [13] CRC Handbook of Chemistry and Physics, 84th ed.; CRC Press: Boca Raton, FL., ed. D. R. Lide, 2003. [14] Z. L. Wu, M. Reichling, X.-Q. Hu, K. Balasubramanian, and K. H. Guenther, ―Absorption and thermal conductivity of oxide thin films measured by photothermal displacement and reflectance methods,‖ Appl. Opt., vol. 32, no. 28, pp. 5660-5665, 1993. 149 [15] C. H. Henager, Jr., and W. T. Pawlewicz, ―Thermal conductivities of thin, sputtered optical films,‖ Appl. Opt., vol. 32, no. 1, pp. 91-101, 1993. [16] J. A Carpenter, Jr., ―Review of the state-of-the-art of measurements for and the phenomena of anomalously low thermal conductivities of dielectric thin films,‖ SPIE Proc., vol. 2714, pp. 445-464, 1996. [17] J. C. Lambropoulos, M. R. Jolly, C. A. Amsden, S. E. Gilman, M. J. Sinicropi, D. Diakomihalis, and S. D. Jacobs, ―Thermal conductivity of dielectric thin films,‖ Jour. App. Phys., vol. 66, no. 9, pp. 4230-4242, 1989. [18] A. J. Griffin, Jr, F. R. Brotzen, and P. J. Loos, ―Effect of thickness on the transverse thermal conductivity of thin dielectric films,‖ Jour. App. Phys., vol. 75, no. 8, pp. 3761-3764, 1994. [19] S.-M. Lee, and D. G. Cahill, ―Heat transport in thin dielectric films,‖ Jour. App. Phys., vol. 81, no. 6, pp. 2590-2595, 1997. [20] A. J. Griffin, Jr, F. R. Brotzen, and P. J. Loos, ―The effective transverse thermal conductivity of amorphpous Si3N4 thin films,‖ Jour. App. Phys., vol. 76, no. 7, pp. 4007-4011, 1994. [21] M. Dieckmann, D. Ristau, U. Willamowski, and H. Schmidt, ―Measurement of thermal conductivity in dielectric films by the thermal pulse method,‖ SPIE Proc., vol. 2253, pp. 712-719, 1994. [22] M. L. Grilli, T. Aumann, D. Ristau, M. Dieckmann, F. V. Alvensleben, ―Thermal conductivity of E-Beam and lBS coatings,‖ SPIE Proc., vol. 2775, pp. 409-421, 1996. 150 [23] F. Rao, Z. Song, L. Wu, Y. Gong, S. Feng, and B. Chen, ―Phase change memory cell based on Sb2Te3/TiN/Ge2Sb2Te5 sandwich-structure,‖ Solid State Electron., vol. 53, no. 3, pp. 276-278, 2009. [24] S.-H. Kim, S.-S. Yim, D.-J. Lee, K.-S. Kim, H.-M. Kim, K.-B. Kim, and H. Sohn, ―Diffusion barriers between Al and Cu for the Cu interconnect of memory devices,‖ Electrochem. Solid-State Lett., vol. 11, no. 5, pp. H127H130, 2008. 151 Chapter Conclusion and Future work 7.1 Conclusion PCRAM is a promising candidate to become the mainstream non-volatile memory technology to replace Flash technology, due to their large cycling endurance capability, as well as fast programming and access times with good potential for scaling beyond 15 nm [1] – [4]. Various leading memory companies have been developing phase change memory technology. Intel and STMicroelectronics demonstrated prototype PCRAM chips with a 128 Mb storage capacity in 2008 [5], while Samsung has announced plans to integrate PCRAMs into their smartphones in early 2010 [6]. To create an entirely new memory technology platform which deviates from the traditional form of memory storage requires significant efforts and continuous developments. This thesis has studied several areas in phase change memory technology that possibly provides guidelines for engineering phase change memory devices. The main contributions of this thesis are summarized in the following pages. 152 7.1.1 Band alignment of phase change and dielectric materials In Chapter 2, a high-resolution XPS technique was used to determine the energy band alignment between phase change materials and various dielectric materials [7] – [9]. Having knowledge of the energy band alignment is important since it affects contact resistance and carrier transport across materials interface, and is essential for clear understanding of device physics and operation. The electronic properties of GST in relation to potential dielectric materials that are commonly used in CMOS technology and may be used in a memory device were thus investigated. The dielectric materials studied were SiO2, HfO2, Ta2O5, and Si3N4 [7]. SiO2 is the prevailing dielectric material currently used in PCRAMs, while nitrogen doping into the phase change material is frequently employed to improve device performances. The impact of nitrogen doping in GST on the energy band alignment with SiO2 was thus studied, and shown to have a linear dependence on the doping concentration in amorphous GST [8]. Finally, it was demonstrated that the choice of phase change material (GeTe – Sb2Te3 pseudobinary line) clearly affects the energy band alignment with a dielectric material, such as SiO2. The information would therefore be important in the selection of materials for design and optimization of materials in PCRAM devices [9]. 7.1.2 Band alignment of phase change and metal contact materials In Chapter 3, the barrier height between amorphous N-GST and metals with varying vacuum workfunction (i.e. Al, W, Pt) was investigated. It was observed that the metals studied form an ohmic contact with GST, doped with various nitrogen 153 content. Significant Fermi level pinning was found to occur at the interface between the phase change materials and metals [10] – [11]. The charge neutrality level was observed to be located near the valence band of the phase change materials, which could account for the generally good contact between these phase change and metal materials. Experimental extraction of various intrinsic parameters of the phase change materials was performed [11]. These parameters are the bandgap, dielectric constant and electron affinity of each N-GST film. The experimental results were compared with that obtained by the charge neutrality model and were found to be in good agreement with each other. Finally, the barrier height of crystalline GST and some metals were determined, which was observed to be larger than that in the amorphous state. 7.1.3 Dependence on the properties of Ge2Sb2Te5 on nitrogen doping concentration and application in phase change memory In Chapter 4, the macroscopic material properties of N-GST were investigated and their implications on PCRAM device performances were evaluated. Increasing the nitrogen doping concentration in GST raises the crystallization temperature, and leads to direct transformation to the stable HCP crystalline structure [12]. The sheet resistances also increase, confirming two regimes of the crystallization process. While low amounts of nitrogen results in reduction of the programming current, higher amounts of nitrogen required a much higher programming current. It was found that transformation to the metastable FCC phase appears to be an essential 154 criterion to achieve low programming currents and maintain its fast switching properties in PCRAM devices [12]. 7.1.4 Silicide electrode contacts for compact integration of phase change memory with CMOS technology In Chapter 5, a new electrode material was implemented in a phase change memory device. The suitability of NiSi and PtSi as electrode contacts and heaters were assessed. The thermal stability and good contact of these films with the phase change layer were confirmed. Characteristic memory behaviour of these devices was observed, including the snap-back behaviour expected of phase change memory devices. A sufficient resistance window of at least one order of magnitude between the SET and RESET state was achieved [13]. This work also highlights the choice of electrode contact on the switching behaviour. This work therefore enables the direct integration of PCRAM directly on the silicided drain regions of MOSFETs, facilitating compact integration with reduced process complexity and cost. 7.1.5 Silicide metal electrode contact and dielectric interlayer for operation power reduction in phase change memory In Chapter 6, a low thermal conductivity dielectric was incorporated into the phase change memory device employing a silicide bottom electrode material. The dielectric functions as both a thermal insulator, as well as an adhesive layer. Low programming currents and good memory device characteristics were achieved with 155 this combination [14]. These results provide a method for reducing operating power in PCRAM devices. 7.2 Future work This thesis has presented some initial results and more detailed investigation and characterization will be required for evaluation of novel material integration into PCRAMs. Possible future work is highlighted in this section. Chapters and deal with the energy band alignment of amorphous GST with various semiconductor materials, dielectrics and metal materials. The energy band alignment of amorphous N- GST was also investigated. The heart of a PCRAM device lies in its ability to switch between the amorphous and crystalline phase. Understanding the energy band lineup when the phase change material is in the crystalline phase would also be important. Although some initial results were presented, several issues such as material desorption under high vacuum and temperature needs to be resolved for such a study to be successful. Moreover, besides the commonly investigated GST composition, a significant number of other phase change materials are also available for similar investigations to attain the requirements of a wide range of applications. Integration of PCRAM with silicides as the metal contact was successfully demonstrated in Chapter and 6. Addition of a thin dielectric interlayer between the silicide electrode and phase change layer reduces the programming power. This work only investigated the effect of one thickness of NiSi and Ta2O5, while more optimization work could be performed. In addition, exploiting the low thermal 156 conductivity of thin dielectric films can also be employed for other thin dielectric materials. Further work could involve directly implementing this device structure with CMOS technology on the silicided drain of a selection device, such as a transistor. The effect of the high temperature involved during the RESET process on the transistor should also be considered. Achieving cost-effective and higher density devices for greater storage capability has been the driving force for technology development. Besides engineering binary storage memory devices, the mulitbit storage concept can also be explored to so. The excellent thermal properties of the thin dielectric film could be a possible option to achieve a multibit memory device, by alternating the phase change film with dielectric material over several layers. This might enable manipulation of the thermal properties during the phase transformation process. Finally, most phase change memory devices require a selection device in an array, thereby increasing the cell area. On the other hand, p-type phase change materials could be integrated with a n-type semiconductor material to form a p-n heterojuction, which could function as both the selection device as well as the memory component. This could potentially reduce the footprint required, hence achieving higher density. 157 7.3 [1] References A. Pirovano, A. Redaelli, F. Pellizzer, F. Ottogalli, M. Tosi, D. Ielmini, A. L. Lacaita, and R. Bez, ―Reliability study of phase-change nonvolatile memories,‖ IEEE Trans. Dev. Mater. Reliability, vol. 4, no. 3, pp. 422-427, 2004. [2] K. Kim, and S. J. Ahn, ―Reliability investigations for manufacturable high density PRAM,‖ Int. Rel. Phys. Symp. Tech. Dig., pp. 157–162, 2005. [3] S. Lai, ―Current status of the phase change memory and its future‖, Int. Elect. Dev. Meet. Tech. Dig., pp 255-258, 2003. [4] A. Pirovano, A. L. Lacaita, A. Benvenuti, F. Pellizzer, S. Hudgens, and R. Bez, ―Scaling analysis of phase-change memory technology,‖ Int. Elect. Dev. Meet. Tech. Dig., pp 699-702, 2003. [5] S. Deffree, (6 Feb 2008), ―Intel, ST claim phase change memory prototypes,‖ http://www.edn.com/article/469754Intel_ST_claim_phase_change_memory_prototypes.php [6] A. Shah, (29 Apr 2010), ―Samsung to put PCM for smartphones in chip package,‖ http://www.pcworld.com/article/195168/samsung_to_put_pcm_for_smartpho nes_in_chip_package.html [7] L. W.-W. Fang, J.-S. Pan, R. Zhao, L. Shi, T.-C. Chong, G. Samudra, and Y.C. Yeo, "Band alignment between amorphous Ge2Sb2Te5 and prevalent CMOS materials," Appl. Phys. Lett., vol. 92, no. 3, 032107, 2008. [8] L. W.-W. Fang, Z. Zheng, J.-S. Pan, M. Li, R. Zhao, L. Shi, T.-C. Chong, and Y.-C. Yeo, "Dependence of energy band offsets at Ge2Sb2Te5/SiO2 interface 158 on nitrogen concentration," Appl. Phys. Lett., vol. 94, no. 6, 062101, 2009. [9] L. W.-W. Fang, R. Zhao, Z. Zhang, J. Pan, L. Shi, T.-C. Chong, and Y.-C. Yeo, "Band offsets between SiO2 and phase change materials in the (GeTe)x(Sb2Te3)1-x pseudobinary system," (submitted to Appl. Phys. Lett.). [10] L. W.-W. Fang, R. Zhao, J. Pan, Z. Zhang, L. Shi, T.-C. Chong, and Y.-C. Yeo, "Fermi-level pinning at the interface between metals and nitrogen-doped Ge2Sb2Te5 examined by x-ray photoelectron spectroscopy," Appl. Phys. Lett., vol. 95, no. 19, 192109, 2009. [11] L. W.-W. Fang, Z. Zhang, R. Zhao, J. Pan, M. Li, L. Shi, T.-C. Chong, and Y.-C. Yeo, "Fermi-level pinning and charge neutrality level in nitrogen-doped Ge2Sb2Te5: Characterization and application in phase change memory devices," Jour. Appl. Phys., vol. 108, no. 5, 053708, 2010. [12] L. W.-W. Fang, R. Zhao, M. Li, K.-G. Lim, L. Shi, T.-C. Chong, and Y.-C. Yeo, "Dependence of the properties of phase change random access memory on nitrogen doping concentration in Ge2Sb2Te5," Jour. Appl. Phys., vol. 107, no. 10, 104506, 2010. [13] L. W.-W. Fang, R. Zhao, E.-G. Yeo, K.-G. Lim, H. Yang, L. Shi, T.-C. Chong, and Y.-C. Yeo, ―Nickel silicide and platinum silicide electrode contacts for compact integration of phase change random access memory with CMOS technology,‖ (to be published in Jour. Electrochem. Soc.). [14] L. W.-W. Fang, R. Zhao, K.-G. Lim, H. Yang, L. Shi, T.-C. Chong, and Y.-C. Yeo, ―Phase change random access memory featuring silicide metal contact and dielectric interlayer for operation power reduction,‖ (submitted to Jour. 159 Vac. Sci. Technol. B). 160 Appendix A. List of Publications Journal Publications [18] L. W.-W. Fang, J.-S. Pan, R. Zhao, L. Shi, T.-C. Chong, G. Samudra, and Y.C. Yeo, "Band alignment between amorphous Ge2Sb2Te5 and prevalent CMOS materials," Applied Physics Letters, vol. 92, no. 3, 032107, 2008. [19] L. W.-W. Fang, Z. Zheng, J.-S. Pan, M. Li, R. Zhao, L. Shi, T.-C. Chong, and Y.-C. Yeo, "Dependence of energy band offsets at Ge2Sb2Te5/SiO2 interface on nitrogen concentration," Applied Physics Letters, vol. 94, no. 6, 062101, 2009. [20] L. W.-W. Fang, R. Zhao, J. Pan, Z. Zhang, L. Shi, T.-C. Chong, and Y.-C. Yeo, "Fermi-level pinning at the interface between metals and nitrogen-doped Ge2Sb2Te5 examined by x-ray photoelectron spectroscopy," Applied Physics Letters, vol. 95, no. 19, 192109, 2009. [21] L. W.-W. Fang, R. Zhao, M. Li, K.-G. Lim, L. Shi, T.-C. Chong, and Y.-C. Yeo, "Dependence of the properties of phase change random access memory on nitrogen doping concentration in Ge2Sb2Te5," Journal of Applied Physics, vol. 107, no. 10, 104506, 2010. [22] L. W.-W. Fang, Z. Zhang, R. Zhao, J. Pan, M. Li, L. Shi, T.-C. Chong, and Y.-C. Yeo, "Fermi-level pinning and charge neutrality level in nitrogen-doped Ge2Sb2Te5: Characterization and application in phase change memory devices," Journal of Applied Physics, vol. 108, no. 5, 053708, 2010. 161 [23] L. W.-W. Fang, R. Zhao, E.-G. Yeo, K.-G. Lim, H. Yang, L. Shi, T.-C. Chong, and Y.-C. Yeo, ―Phase change random access memory devices with nickel silicide and platinum silicide electrode contacts for integration with CMOS technology,‖ Journal of The Electrochemical Society, vol. 158, no. 3, H232, 2011. [24] L. W.-W. Fang, R. Zhao, K.-G. Lim, H. Yang, L. Shi, T.-C. Chong, and Y.-C. Yeo, ―Phase change random access memory featuring silicide metal contact and high-k interlayer for operation power reduction,‖ Journal of Vacuum Science and Technology B, vol. 29, no. 3, 032207, 2011. [25] L. W.-W. Fang, R. Zhao, Z. Zhang, J. Pan, L. Shi, T.-C. Chong, and Y.-C. Yeo, "Band offsets between SiO2 and phase change materials in the (GeTe)x(Sb2Te3)1-x pseudobinary system," Applied Physics Letters, vol. 98, no. 13, 132103, 2011. Conference Publications [26] L. W.-W. Fang, J.-S. Pan, A. E.-J. Lim, R. T. P. Lee, M. Li, R. Zhao, L. Shi, T.-C. Chong, and Y.-C. Yeo, ―Photoemission study of energy band alignment of amorphous Ge2Sb2Te5 phase-change material and common CMOS materials,‖ Materials Research Society Spring Meeting, San Francisco CA, Mar. 24-28, 2008. [27] L. W.-W. Fang, Z. Zhang, J.-S. Pan, M. Li, R. Zhao, L. Shi, T.-C. Chong, and Y.-C. Yeo, ―Energy band offset of amorphous Ge2Sb2Te5 on dielectric or metal and the impact of nitrogen incorporation on its alignment,‖ Materials 162 Research Society Spring Meeting, Apr. 13-17, 2009. [28] L. W.-W. Fang, Z. Zhang, J.-S. Pan, M. Li, R. Zhao, L. Shi, T.-C. Chong, and Y.-C. Yeo, ―Fermi-level pinning effect at the interface between phase change materials and metals,‖ Materials Research Society Spring Meeting, Apr. - 9, 2010. [1] L. W.-W. Fang, R. Zhao, J. Pan, Z. Zhang, L. Shi, T.-C. Chong, and Y.-C. Yeo, ―Silicides as new electrode/heater for compact integration of phase change memory with CMOS,‖ International Symposium on VLSI Technology, Systems and Applications (VLSI-TSA), Hsinchu, Taiwan, Apr. 26-28, 2010. 163 [...]... MOSFET selection device and a PCRAM memory element .10 Fig 2.1 A generic phase change memory cell with dashed boxes depicting the various interfaces present in a memory device These include the phase change/ metal and phase change/ dielectric interfaces 22 Fig 2.2 Schematic flatband diagram for the band line-up (a) at the interfaces between two semiconductor materials, X and Y and (b) at a metal/... come up with disruptive memory technologies, which move away from the practical limits of charge storing Alternative memory technologies such as ferroelectric random access memory (FeRAM), magnetic random access memory (MRAM), and phase- change random access memory (PCRAM) have been poised as potential candidates for the next-generation non-volatile memory FeRAM exhibits high speed, low power and voltage,... being researched 1.2 Phase Change Memory Technology 1.2.1 Phase change materials and memory device structures Chalcogenide phase change materials are used in PCRAMs due to their phase reversible switching properties Phase change materials can exist in two stable states, i.e the amorphous (RESET) and crystalline (SET) state In the amorphous state, the 3 atoms have short range order and exhibits high electrical... nitrogen doping were also examined Finally, the band lineups of (GeTe)x(Sb2Te3)1-x phase change alloys were investigated This work enables a comprehensive understanding of the band 11 alignment of phase change materials with its surrounding materials for future device integration and optimization Due to the simple and effective method to improve phase change memory device performances, the electronic properties... the energy band alignment in phase change research was introduced The energy band alignment of phase change material with various dielectric materials was thus investigated Dielectric materials that are prevalently used in the CMOS technology fabrication process were studied The phase change material featured here is the most commonly exploited Ge2Sb2Te5 in PCRAM devices Changes in the band lineups... A thorough investigation on the properties of phase change materials and integration with materials frequently exploited in CMOS transistor technology into phase change memory devices will be furnished in this work The results achieved will provide a systematic guideline in the selection of suitable materials for implementation into future phase change memory devices 1.4 Thesis Organization The main... selection device and a PCRAM memory element 10 1.3 Objectives of Research The objective of this thesis aims to explore potential issues accompanying novel material integration into phase change memory devices by examination of the energy band alignment of phase change materials with the surrounding materials contacting it This enables the screening and evaluation of potential material candidates, even... properties of the phase change materials compares with well-established values of various semiconductor materials (open symbols) 65 Fig 3.7 Schematic energy band diagrams when a metal (Al) and phase change material (GST) are (a) not in contact, and (b) in contact with each other Φdipole is almost constant as nitrogen content in the phase change material increases, while it increases with the work... N-GST and SiO2 Energy values shown are in units of eV, and are taken with respect to the valence band of the undoped N-GST Top and bottom edges of each rectangle represent EC and EV, respectively.37 Fig 2.11 The Te 3d5/2 core-level and valence band spectra for 100 nm amorphous alloys lying along the tieline of GeTe – Sb2Te3 The solid and open symbols represent the core-level (left) and valence band (right)... as integration with complementary-metal-oxide-semiconductor (CMOS) technology [8] However, FeRAM still face issues such as degradation of remnant polarization with time, loss of polarization as well as the lost of signal with scaling The advantages of MRAM are its fast speed, straightforward integration with CMOS at the back-end-of-line and its high endurance cycles [9] Moreover, a cross-point memory . Non-volatile Memory Technology 1 1.2 Phase Change Memory Technology 3 1.2.1 Phase change materials and memory device structures 3 1.2.2 Basic principles of phase change memory 7 1.2.3 Phase change memory. nitrogen doping concentration and application in phase change memory 154 7.1.4 Silicide electrode contacts for compact integration of phase change memory with CMOS technology 155 7.1.5 Silicide. 148 Chapter 7 Conclusion and Future Work 7.1 Conclusion 152 7.1.1 Band alignment of phase change and dielectric materials 153 7.1.2 Band alignment of phase change and metal contact materials