Tuning Schottky Barrier Height of Ni Germanide for High Performance Nano-scale Ge MOSFETs Application

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1 Tuning Schottky Barrier Height of Ni Germanide for High Performance Nano-scale Ge MOSFETs Application YING-YING ZHANG, JUNG-DEUK BOK, SANG-UK PARK, BYOUNG-SOEK PARK, SE-KYUNG OH, HONG-SIK SHIN, HYUK-MIM KWON, IN-SHIK HAN, HI-DEOK LEE * Department of Electronics Engineering Chungnam National University Gung-Dong 220, Yuseong-Gu, Daejeon KOREA hdlee@cnu.ac.kr Abstract: - In this study, we tuned the Schottky barrier height of Ni germanide by using and incorporation into Ni germanide for high performance nano-scale Ge MOSFETs application. The results exhibited that the electron Schottky barrier height or work function of Ni germanide was increased about 30 mev and decreased about 90 mev by and incorporation, respectively. Hence, the proposed and incorporated Ni germanide structures are promising for high performance Ge pmosfets and nmosfets, respectively, due to the lower germanide to source/drain contact resistance. Key-Words: - Schottky barrier height, Ni germandie, incorporation, incorporation, high performance, Ge MOSFETs 1 Introduction Self-aligned silicidation (salicide) or self-aligned germanidation (salmanide) is one of key technologies in the state-of-art complementary metal oxide semiconductor field emitter transistor (CMOSFET) process to make ohmic or Schottky contact at source/drain and gate region [1]. Ni germanide is being considered as a promising salmanide material for Ge MOSFETs because of its advantages over other germanide materials, such as, low formation temperature, nickel monogermanide phase, and low sheet resistivity [2-3]. There are considerable efforts currently in the field to adjust Schottky barrier heights to reduce the contact resistance between the contact and the doped semiconductor. As a result, elements that bring the work function of the germanide/silicide closer to the band edge are studied extensively. Rare-earth elements such as and Er have work function closer to the conduction band while elements such as and Pt have work function closer to the valence band. Therefore, rare-earth metal germanides can exhibit low Schottky barrier heights to n-type devices and are therefore suitable for electrical contact formation in nmosfets either as ohmic contacts to n+ source/drain or even as Schottky source/drain, while or Pt element is suitable for pmosfets. In the silicide, there were a lot of studies on the decrease of the Schottky barrier height between Ni silicide and source/drain to improve the device performance by reducing the contact resistance, that is, incorporation of Pt or [4-6] and rare earth metals such as, ytterbium () [7-8], erbium (Er) [9], and terbium (Tb) [10] into Ni silicide can induce lower hole and electron SBH, respectively. In the germanide, there were also a lot of studies on the Schottky barrier of germanide on Ge substrate, such as Ti, Ni,, and Pt germanide, etc [11-17]. But there was little study on the comparison of different effect of or incorporation into Ni germanide on the work function change. In this paper, the tuning of the Schottky barrier height of Ni germanide was demonstrated by using and incorporation. We achieved 30 mev increase and 90 mev decrease of Schottky barrier height of Ni germanide for electron. 2 Experimental Details The process flow for experiments was shown in Fig. 1. Patterned n-type Ge-on-Si wafers were used for exact extraction of Schottky barrier height between Ni germanide and Ge substrate. After the residual oxide removal by dipping in 1% diluted HF solution, metals were deposited by using RF magnetron sputter system. The detailed splits of structures are split to /Ni/TiN, and /Ni/TiN. Pure Ni/TiN is also used for comparison. incorporation was used to increase electron barrier height and was used to decrease electron barrier height, respectively, therefore, to tune the work function of. Rapid thermal process (RTP) was carried out for germanidation for 30 sec. The un-reacted metals were ISSN: ISBN:

2 selectively etched off by using H 3 PO 4 solution at 150 o C for 30 sec. Diameter of Schottky diodes was split as 166, 119, 86, 56, and 42 µm. The I V relationship of Schottky diode was measured using HP4156C semiconductor analyzer. The uniformity of interface of Ni germanide was observed by using field emission scanning electron microscopy (FESEM, Model: S-4700, Maker: Hitach). n-type Ge-on-Si Substrate (Patterned) Residual Oxide Removal(1% HF, 10s) Metal Deposition: - Ni/TiN - /Ni/TiN - /Ni/TiN RTP for Germanidation ( 30sec ) Selective Wet Etching ( H 3PO o C, 30sec) I-V Measurement and Analysis Fig. 1. Process flow for experiments. 3 Results and Discussions Work Function n-ge 0 ev Fig. 3. Cross-sectional FESEM images of formed by (a) Ni/TiN, (b) /Ni/TiN, and (c) /Ni/TiN. The cross-sectional FESEM images of formed by Ni/TiN, /Ni/TiN, and /Ni/TiN structures are shown in Fig. 3. After RTP for germanidation, the uniform interface can be obtained from all structures ev n-ge 4.13 ev 4.63 ev 4.79 ev Ni 5.12 ev 5.15 ev Fig. 2. Sketch map of work function of metal and n-ge. The sketch map of work function of metal and n-ge was shown in Fig. 2. The electron affinity and band gap of Ge are 4.13 and 0.66 ev, respectively. The work function of pure,, and Ni is 2.60, 5.12, and 5.15 ev, respectively. The previous study reported that the work function of was about 4.63 ev [18]. has smaller work function than Ni, while has a similar work function with Ni. The work function of is larger than n-ge for the metal/n-ge contact, the contact is made via Schottky contact. The work function of was tuned by adding and into. Current Density [A/cm 2 ] n-ge_d=86µm Ni/TiN /Ni/TiN /Ni/TiN Voltage [V] Fig. 4. I-V curves of the /n-ge diodes formed by Ni/TiN, /Ni/TiN, and /Ni/TiN. Figure 4 shows I-V curves of the /n-ge diodes formed by Ni/TiN, /Ni/TiN, and /Ni/TiN structures. The extracted electron Schottky barrier height is shown in Fig. 5. The /Ni/TiN and /Ni/TiN structures show a lower and greater leakage current than pure Ni/TiN structure, respectively, possibly due to the increase and decrease of Schottky barrier height, respectively, as ISSN: ISBN:

3 shown in Fig. 5. The Schottky barrier height was extracted using the differentiation of I-V curve [19]. The results exhibit that the electron Schottky barrier height increased about 30 mev by and decreased about 90 mev by incorporation, which implies the same increase and decrease of the work function of Ni germanide. Hence, the proposed and incorporated structures are promising structures for high performance Ge pmosfets and nmosfets, respectively, due to the lower germanide to source/drain contact resistance. Electron Barrier Height [ev] Ni/TiN /Ni/TiN /Ni/TiN Diode Diameter [µm] Fig. 5. Extracted electron Schottky barrier height of using Ni/TiN, /Ni/TiN, and /Ni/TiN as a function of the diode diameter. Our previous studies shown in Fig. 6 and 7 explained the distribution of and atoms after germanidation, respectively. Figure 6 shows cross-sectional scanning transmission electron microscopy (STEM, model: D-2300A) Z-contrast image and corresponding STEM energy dispersive X-ray spectrometry (EDX) maps for Ge,, and Ni atoms for the /Ni/TiN structure [20]. Figure 7 shows secondary ion mass spectrometer (SIMS, model: Model: CAMECA IMS-6f) depth profile of for the /Ni/TiN structure [21]. atoms are piled up at the /Ge interface as well as at the surface region (Fig. 6), although atoms distribute throughout the germanide. The intensity of atoms at the /Ge interface region is several times greater than that at the center region of the film. Figure 7 also shows the existence of atoms at the surface region of. Moreover, there is an increase in elements at the interface of /Ge. This indicates that a large amount of and atoms are out-diffused to the surface region through the film during Ni germanidation. Such out-diffusion behavior could be explained by the difference in the surface tension of the elements and by the much greater reactivity of Ni than the other elements, which agrees well with previous studies. Fig. 6. Cross-sectional STEM Z-contrast image for /Ni/TiN structure. The inset shows the line depth profile of Ge,, and Ni ingredients [20]. Intensity (counts/sec) Ge Ni Sputter Depth (nm) Fig. 7. The SIMS depth profile of for /Ni/TiN. There are peaks of concentration both at the surface region of and interface of /Ge [21]. To investigate the mechanism of the adjustment of the work function, the energy band diagram of a layer formed by using and incorporation and n-type Ge before and after contact, respectively, is demonstrated in Fig. 8. According to the description shown in Fig. 6 and 7, we can get that and atoms are piled-up at /Ge interface as well as at the surface region in the /Ni/TiN and /Ni/TiN structures. Before contact, the Fermi level in n-type Ge and was above that in the pure layer, while Fermi level in was below that in the pure layer. After contact, the Fermi level becomes constant throughout the system in thermal equilibrium, and the vacuum band energies must be bended because of its continuous characteristics. Then, the work function of was increased and decreased by and, respectively, because the has higher work function and has lower work function as shown in Fig. 8(b). ISSN: ISBN:

4 (a) TiN (b) TiN layer layer Before Contact Increased by ~30 mev Decreased by ~90 mev n-ge After Contact Ge sub. Work Function 0 ev 2.60 ev 4.13 ev n-ge 4.63 ev 4.79 ev Ni 5.12 ev 5.15 ev n-ge Fig. 8. Energy band diagram of a layer formed by using and incorporation and n-type Ge (a) before and (b) after contact. 4 Conclusion Incorporation of and into Ni germanide is effective to tune the SBH for electron. It is shown that the incorporated and metals mainly distribute the /Ge interface as well as at the surface region, which results in the 30 mev increase and the 90 mev decrease of Schottky barrier height of Ni germanide by and incorporation, respectively. Therefore, reducing the contact resistance (or Schottky barrier height) between Ni germanide and source/drain using the and incorporation, is promising to improve the device performance of p-type and n-type nano-scale MOSFET, respectively. Acknowledgment This work was in part supported by grant No from the Korea Science and Engineering Foundation (KOSEF). This work was also financially supported by the Ministry of Knowledge Economy (MKE) and Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Strategic Technology. References: [1] S. Wolf, Silicon processing for the VLSI era Volume 4-Deep-submicron prcess technology, pp , Lattice Press, [2] Q. Zhang, N. Wu, T. Osipowicz, L. K. Bear, and C. Zhu, Formation and thermal stability of Nickel germanide on germanium substrate, Jpn. J. Appl. Phys., Vol. 44, No. 45, 2005, pp. L1389-L1391. [3] S. L. Hsu, C. H. Chien, M. J. Yang, R. H. Huang, C. C. Leu, S. W. Shen, and T. H. Yang, Study of thermal stability of nickel monogermanide on singleand polycrystalline germanium substrates, Appl. Phys. Lett., Vol. 86, 2005, pp [4] L. E. Terry, and J. Saltich, Schottky barrier heights of nickel-platinum silicide contacts on n-type Si, Appl. Phys. Lett., Vol. 28, No. 4, 1976, pp [5] H. K. Liou, X. Wu, U. Gennser, V. P. Kesan, S. S. Lyer, K. N, Tu, and E. S. Yang, Interfacial reactions and Schottky barriers of Pt and on epitaxial Si 1-x Ge x alloys, Appl. Phys. Lett., Vol. 60, No. 5, 1992, pp [6] L. J. Jin, K. L. Pey, W. K. Choi, D. A. Antoniadis, E. A. Fitzgerald, and D. Z. Chi, Electrical characterization of platinum and palladium effects in nickel monosilicide/n-si Schottky contacts, Thin Solid Films, Vol. 504, 2006, pp [7] J. D. Chen, H. Y. Yu, M. F. Li, D. L. Kwong, M. J. H. van Dal, J. A. Kittl, A. Lauwers, P. Absil, M. Jurczak, and S. Biesemans, -doped Ni FUSI for the n-mosfets gate electrode application, IEEE Electron Device Lett., Vol. 27, No. 3, 2006, pp [8] W. J. Lee, D. W. Kim, S. Y. Oh, Y. J. Kim, Y. Y. Zhang, Z. Zhong, S. G. Li, S. Y. Jung, I. S. Han, T. K. Gu, T. S. Bae, G. W. Lee, J. S. Wang, and H. D. Lee, Work function variation of Nickel silicide using Ytterbium buffer layer for Schottky barrier MOSFET, J. Appl. Phys., Vol. 101, 2007, pp [9] W. Huang, Y. L. Min, G. P. Ru, Y. L. Jiang, X. P. Qu, and B. Z. Li, Effect of erbium interlayer on nickel silicide for formation on Si(100), Applied Surface Science, Vol. 254, 2008, pp [10] A. E. J. Lim, R. T. P. Lee, C. H. Tung, S. Tripathy, D. L. Kwong, and Y. C. Yeo, Full silicidation of silicon gate electrodes using Nickel-Terbium alloy for MOSFET applications, J. Electronchem. Soc., Vol. 153, No. 4, 2006, pp. G337-G340. [11] D. Han, Y. Wang, D. Tian, W. Wang, X. Liu, J. Kang, and R. Han, Studies of Ti- and Ni-germanide ISSN: ISBN:

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