In Chapter 2, we first studied the thermal stability of the as-deposited HfOxNy thin films on the Ge substrate by employing rapid thermal annealing. After undergoing high temperature processing, we observed several interesting physical and electrical features presented in the HfOxNy/Ge system, including a large Ge out-diffusion (> 15 at.%) into high-k films, positive VFB shift, severe charge trapping, and increased leakage current. These phenomena are closely related to the existence of GeOx defective layer and the degree of resultant GeO volatilization.
We abated these undesirable effects, especially for reducing the amount of Ge incorporation (< 5 at.%) and the sub-stoichiometric oxide at dielectric-substrate interface, through performing NH3 plasma pretreatment on the Ge surface. These improvements can be interpreted in terms of a surface nitridation process that enhanced the thermal stability of the high-k/Ge interface. Moreover, we also adopted the Si2H6 thermal pretreatment to relieve Ge out-diffusion in HfOxNy/Ge capacitors. It was found that capping an ultrathin Si layer onto a Ge substrate retarded GeO volatilization and suppressed oxide charge trapping in the MIS structures and, thus, enhanced the thermal stabilities of entire HfOxNy/Ge MIS structures. We provide herein a schematic energy band diagram to explain the resultant charge trapping behavior in these systems.
References (Chapter 2)
[1] R. Li, S. J. Lee, H. B. Yao, D. Z. Chi, M. B. Yu, and D. L. Kwong, “Pt-germanide Schottky source/drain germanium p-MOSFET with HfO2 gate dielectric and TaN gate electrode,” IEEE Electron Device Lett., vol. 27, p. 476, 2006.
[2] Y.-L. Chao and J. C. S. Woo, “Reduction of parasitic resistance of self-aligned copper germanide for germanium p-MOSFETs,” Ext. Abst. Int. Conf. on Solid State Devices and Materials (SSDM), p. 1052, 2006.
[3] A. Ritenour, S. Yu, M. L. Lee, N. Lu, W. Bai, A. Pitera, E. A. Fitzgerald, D. L. Kwong, and D. A. Antoniadis, “Epitaxial strained germanium p-MOSFETs with HfO2 gate dielectric and TaN gate electrode,” Tech. Dig. Int. Electron Devices Meet., p. 433, 2003.
[4] C. H. Huang, M. Y. Yang, A. Chin, W. J. Chen, C. X. Zhu, B. J. Cho, M.-F. Li, and D. L.
Kwong, “Very low defects and high performance Ge-on-insulator p-MOSFETs with Al2O3 gate dielectrics,” Tech. Dig. VLSI Symp., p. 119, 2003.
[5] The International Technology Roadmap for Semiconductors (ITRS), http://public.itrs.net, 2004.
[6] O. Weber, F. Andrieu, M. Cass, T. Ernst, J. Mitard, F. Ducroquet, J.-F. Damlencourt, J.-M.
Harmann, D. Lafond, A.-M. Papon, L. Militaru, L. Thevenod, K. Romanjek, C. Leroux, F.
Martin, B. Guillaumot, G. Ghibaudo, and S. Deleonibus, “Experimental determination of mobility scattering mechanisms in Si/HfO2/TiN and SiGe:C/HfO2/TiN surface channel n- and p-MOSFETs,” Tech. Dig. Int. Electron Devices Meet., p. 867, 2004.
[7] C. S. Kang, H.-J. Cho, K. Onishi, R. Choi, R. Nieh, S. Gopalan, S. Krishnan, and J. C. Lee,
“Improved thermal stability and device performance of ultra-thin (EOT<10 Å) gate dielectric MOSFETs by using hafnium oxynitride (HfOxNy),” Tech. Dig. VLSI Symp., p.
146, 2002.
[8] H. C.-H. Wang, C.-W. Tsai, S.-J. Chen, C.-T. Chan, H.-J.Lin, Y. Lin, H.-J. Tao, S.-C.
HfSiON as gate dielectric for advanced CMOS technology,” Tech. Dig. VLSI Symp., p.
170, 2005.
[9] A. L. P. Rotondaro, M. R. Visokay, J. J. Chambers, A. Shanware, R. Khamankar, H. Bu, R. T. Laaksonen, L. Tsung, M. Douglas, R. Kuan, M. J. Bevan, T. Grider, J. Mcpherson, L. Colombo, “Advanced CMOS transistors with a novel HfSiON gate dielectric,” Tech.
Dig. VLSI Symp., p. 148, 2002.
[10] T. Yamaguchi, R. Iijima, T. Ino, A. Nishitama, H. Satake, and N. Fukushima, “Additional scattering effects for mobility degradation in Hf-silicate gate MISFETs,” Tech. Dig. Int.
Electron Devices Meet., p. 621, 2002.
[11] M. Houssa, T. Conard, F. Bellenger, G. Mavrou, Y. Panayiotatos, A. Sotiropoulos, A.
Dimoulas, M. Meuris, M. Caymax, and M. M. Heynsa, “Electrical properties of atomic-beam deposited GeO1-xNx/HfO2 gate stacks on Ge,” J. Electrochem. Soc., vol.
153, p. G1112, 2006.
[12] S. Zhu, R. Li, S. J. Lee, M. F. Li, A. Du, J. Singh, C. Zhu, A. Chin, and D. L. Kwong,
“Germanium pMOSFETs with Schottky-barrier germanide S/D, high-k gate dielectric and metal gate,” IEEE Electron Device Lett., vol. 26, p. 81, 2005.
[13] N. Lu, W. Bai, A. Ramirez, C. Mouli, A. Ritenour, M. L. Lee, D. Antoniadis, and D. L.
Kwong, “Ge diffusion in Ge metal oxide semiconductor with chemical vapor deposition HfO2 dielectric,” Appl. Phys. Lett., vol. 87, p. 051922, 2005.
[14] N. Wu, Q. Zhang, C. Zhu, D. S. H. Chan, A. Du, N. Balasubramanian, M. F. Li, A. Chin, and J. K. O. Sin, “A TaN-HfO2-Ge pMOSFET with novel SiH4 surface passivation,”
IEEE Electron Device Lett., vol. 25, p. 631, 2004.
[15] K. Prabhakaran and T. Ogino, “Selective bond-breaking and bond-making in oxynitride of Si and Ge: a case of chemical bond manipulation,” Surf. Sci., vol. 387, p. L1068, 1997.
[16] W. H. Wu, B. Y. Tsui, Y. P. Huang, F. C. Hsieh, M. C. Chen, Y. T. Hou, Y. Jin, H. J. Tao, S.
C. Chen, and M. S. Liang, “Two-frequency C-V correction using five-element circuit model for high-k gate dielectric and ultrathin oxide,” IEEE Electron Device Lett., vol. 27, p. 399, 2006.
[17] H.-T. Lue, C.-Y. Liu, and T.-Y. Tseng, “An improved two-frequency method of capacitance measurement for SrTiO3 as high-k gate dielectric,” IEEE Electron Device Lett., vol. 23, p. 553, 2002.
[18] M.-K. Lee, C.-F. Yen, J.-J. Huang, and S.-H. Lin, “Electrical characteristics of postmetallization-annealed MOCVD-TiO2 films on ammonium sulfide-treated GaAs,” J.
Electrochem. Soc., vol. 153, p. F266, 2006.
[19] D. K. Schroder, Semiconductor Material and Device Characterization, 2nd ed., pp.
368–378, John Wiley & Sons, Inc., New York, 1998.
[20] W. A. Hill and C. C. Coleman, “A single-frequency approximation for interface-state density determination,” Solid-State Electron., vol. 23, p. 987, 1980.
[21] Q. Zhang, N. Wu, D. M. Y. Lai, Y. Nikolai, L. K. Bera, and C. Zhu, “Germanium incorporation in HfO2 dielectric on germanium substrate,” J. Electrochem. Soc., vol. 153, p. G207, 2006.
[22] C.-C. Cheng, C.-H. Chien, C.-W. Chen, S.-L. Hsu, C.-H. Yang, and C.-Y. Chang,
“Effects of postdeposition annealing on the characteristics of HfOxNy dielectrics on germanium and silicon substrates,” J. Electrochem. Soc., vol. 153, p. F160, 2006.
[23] C.-C. Cheng, C.-H. Chien, J.-H. Lin, C.-Y. Chang, G.-L. Luo, C.-H. Yang, and S.-L. Hsu,
“Thermochemical reaction of ZrOx(Ny) interfaces on Ge and Si substrate,” Appl. Phys.
Lett., vol. 89, p. 012905, 2006.
[24] P. M. Lambert, “Hafnium germanate from a hydrous hafnium germanium oxide gel,”
Inorg. Chem., vol. 37, p. 1352, 1998.
[25] V. V. Afanas’ev, Y. G. Fedorenko, and A. Stesmans, “Interface traps and dangling-bond
[26] C. O. Chui, F. Ito, and K. C. Saraswat, “Nanoscale germanium MOS dielectrics—Part I:
Germanium oxynitrides,” IEEE Trans. Electron Devices, vol. 53, p. 1501, 2006.
[27] D. Flandre, F. Campabadal, J. Esteve, E. Lora-Tamayo, and F. V. D. Wiele, “ac capacitance and conductance measurements of two-terminal metal-oxide-semiconductor-oxide-semiconductor capacitors on silicon-on-insulator substrates,” J. Appl. Phys., vol. 70, p. 5111, 1991.
[28] E. H. Nicollian and J. R. Brews, MOS (Metal Oxide Semiconductor) Physics and Technology, John Wiley & Sons, Inc., New York, 1982.
[29] Y. Fukuda, T. Ueno and S. Hirono, “Electrical behavior of germanium oxide/germanium interface prepared by electron-cyclotron-resonance plasma oxidation in capacitance and conductance measurements,” Jap. J. Appl. Phys., vol. 44, p. 7928, 2005.
[30] Y. Fukuda, T. Ueno, S. Hirono and S. Hashimoto, “Electrical characterization of germanium oxide/germanium interface prepared by electron-cyclotron-resonance plasma irradiation,” Jap. J. Appl. Phys., vol. 44, p. 6981, 2005.
[31] A. Dimoulas, G. Vellianities, G. Mavrou, E. K. Evangelou, and A. Sotiropoulos, “Intrinsic carrier effects in HfO2–Ge metal–insulator–semiconductor capacitors,” Appl. Phys. Lett., vol. 86, p. 223507, 2005.
[32] W. P. Bai, N. Lu, and D.-L. Kwong, “Si interlayer passivation on germanium MOS capacitors with high-k dielectric and metal gate,” IEEE Electron Device Lett., vol. 26, p.
378, 2005.
[33] N. Wu, Q. Zhang, D. S. H. Chan, N. Balasubramanian, and C. Zhu, “Gate-first germanium nMOSFET with CVD HfO2 gate dielectric and silicon surface passivation,”
IEEE Electron Device Lett., vol. 27, p. 479, 2006.
[34] N. Wu, Q. Zhang, C. Zhu, C. C. Yeo, S. J. Whang, D. S. H. Chan, M. F. Li, B. J. Cho, A.
Chin, D.-L. Kwong, A. Y. Du, C. H. Tung, and N. Balasubramanian, “Effect of surface NH3 anneal on the physical and electrical properties of HfO2 films on Ge substrate,” Appl.
Phys. Lett., vol. 84, p. 3741, 2004.
[35] C.-C. Cheng, C.-H. Chien, G.-L. Luo, C.-H. Yang, M.-L. Kuo, J.-H. Lin, and C.-Y. Chang,
“Ultrathin Si capping layer suppresses charge trapping in HfOxNy/Ge metal-insulator-semiconductor capacitors,” Appl. Phys. Lett., vol. 90, p. 012905, 2007.
[36] NIST Electron Inelastic-Mean-Free-Path Database 71 (v. 1.1), National Institute of Standards and Technology, 2000.
[37] I. Barin and O. Knacke, Thermochemical Properties of Inorganic Substances (Berlin, Springer, 1977).
[38] W.-J. Zhu, T.-P. Ma, T. Tamagawa, J. Kim, and Y. Di, “Current transport in metal/hafnium oxide/silicon structure,” IEEE Electron Device Lett. vol. 23, p. 97, 2002.
[39] V. V. Afanas’ev and A. Stesmans, “Energy band alignment at the (100)Ge/HfO2
interface,” Appl. Phys. Lett. vol. 84, p. 2319, 2004, and references 3, 14, and 15 cited within.
[40] K.-L. Seo, P. C. Mclntyre, S. Sun, D.-L. Lee, P. Pianetta, and K. C. Saraswat, “Chemical states and electronic structure of a HfO2/Ge(001) interface,” Appl. Phys. Lett. vol. 87, p.
042902, 2005.
[41] M. Liu, Q. Fang, G. He, L. Li, L. Q. Zhu, G. H. Li, and L. D. Zhang, “Effect of post deposition annealing on the optical properties of HfOxNy films,” Appl. Phys. Lett. vol. 88, p. 192904, 2006.
Fig. 2.1 Cross-sectional TEM image of as-deposited Pt/HfOxNy/Ge gate stack.
Fig. 2.2 SIMS profiles of as-deposited HfOxNy/Ge gate stack after two different annealing conditions: (a) 400 °C, 30 s; (b) 500 °C, 5 min.
Fig. 2.3 Multi-frequency C–V curves of Pt/HfOxNy/n-Ge capacitors measured at 10 kHz (▲), 100 kHz (●), and 1MHz (■): (a) as-deposited; (b) 500 °C PDA, 30 s;
(c) 500 °C PMA, 30 s.
Fig. 2.4 Typical 100-kHz G–V curves of as-deposited and annealed Pt/HfOxNy/n-Ge
capacitors (solid symbols); the annealing duration is 30 s for these two thermal processes. Note that the G–V curve of as-deposited Pt/HfOxNy/n-Si capacitor (open squares) is added for comparison.
Fig. 2.5 The VFB shift (the left y-axis) and I–V characteristics (the right y-axis) of Pt/HfOxNy/n-Ge capacitors subjected to different thermal processing.
Fig. 2.6 Bidirectional sweep (1 MHz) C–V curves of Pt/HfOxNy/n-Ge capacitors prepared without (■) and with (△) NH3 nitridation. The inset displays the corresponding plots of Jg versus Vg before and after PDA at 600 °C.
Fig. 2.7 Dependence of the hysteresis width in C–V (1 MHz) sweep on the starting accumulation and inversion gate biases; the absence of the hysteresis behavior in the gate-bias ranged from 0 to 1 V.
Fig. 2.8 Ge 2p3 XPS spectra of Pt/HfOxNy/Ge capacitors prepared without and with NH nitridation. Three components were extracted: Ge, GeO , and GeO .
Fig. 2.9 Ge 3p XPS spectra of Pt/HfOxNy/Ge capacitors prepared without and with NH3
nitridation. Three components were extracted: Ge, GeOx, and GeO2.
Fig. 2.10 SIMS profiles of HfOxNy thin films deposited on Ge substrates (a) without and (b) with Si passivation. The solid lines ( — ) and solid squares ( ■ ) refer to the as-deposited samples; the dotted lines ( --- ) and open squares ( □ ) refer to the annealed samples (500 °C, 5 min).
Fig. 2.11 (a) XPS spectra displaying the Si 2p and Ge 2p core levels for Ge samples capped with a Si layer (ca. 8 Å) before ( ■ ) and after ( □ ) deposition of the HfOxNy high-k film. (b) Ge 2p spectra of Pt/HfOxNy/Ge gate stacks before (solid symbols) and after (open symbols) dielectric annealing at 500 °C for 5 min. Note that the lowest curve ( ＊ ) indicates that no Ge was incorporated into the high-k film when thermal processing—PDA and PMA—was not undertaken.
Fig. 2.12 Bidirectional sweep (1 MHz) C–V curves of Pt/HfOxNy/Ge gate stacks lacking and containing a Si capping layer. HfOxNy was the as-deposited thin film and the capacitors were only subjected to 400 °C PMA.
Fig. 2.13 Schematic energy band diagram displaying the charge trapping model for a Pt/HfOxNy/GeOx/Ge gate stack upon (a) sweeping from the inversion bias (Vg
= VFB – 1 V) and (b) sweeping from the accumulation bias (Vg = VFB + 1 V).
Note that the value of VFB was ca. 0.5 V.
Chapter 3
Atomic-Layer-Deposited Al
2O
3Dielectric Films on Bulk Ge Substrates
3.1 Introduction
High-k Al2O3 is considered a potential alternative gate dielectric material on Si substrates for application to metal oxide semiconductor field effect transistors (MOSFETs) because of its wide bandgap energy (ca. 8.8 eV), large conduction/valance band offsets, and high thermodynamic stability. Nevertheless, there are several drawbacks affecting the deposition of Al2O3 on Si, relative to SiO2, including the low crystallization temperature, the high thermal expansion coefficient, and the high densities of negative fixed charges and interface states (Dit). Recent progress in the deposition of high-k materials—e.g., the use of deposition techniques such as atomic layer deposition (ALD)—has meant that some of these problems can be overcome [1]. Several reports have described the characteristics of ALD-Al2O3
dielectric films grown on Si using trimethylaluminum [TMA, Al(CH3)3] as the precursor and H2O as the oxidant because of the excellent ALD mechanism and broad process window [2], [3]. In recent years, many investigators have studied the deposition of ALD-HfO2 high-k layers on high-mobility Ge substrates [4] using a variety of oxidants [5] in efforts aimed at enhancing the driving current in MOSFETs. Meanwhile, various surface preparation techniques—e.g., the incorporation of ultra-thin Si [6], [7], AlNx, and GeON dielectric interlayers [8], [9], plasma treatment with NH3 or PH3 [10], [11], and chemical passivation with (NH4)2S [12]—have been developed to enhance the electrical performance of Ge capacitors and transistors [13] because the primary obstacle affecting the fabrication of high-k/Ge structures is the presence of GeO native oxides and their thermal desorption,
which debase the Ge device characteristics, e.g., to provide a high value of Dit, a large gate leakage current, and severe charge trapping.
In this chapter, we analyzed the composition and interfacial chemistry of ALD-Al2O3
dielectrics deposited on bulk Ge substrates at temperatures in the range of 50–300 °C;
subsequent thermal processing strongly influenced the structural stability and electrical properties of the Al2O3/Ge system. Interestingly, an uncommon electrical characteristic—fast minority carrier response—has been manifested during electrical measurements of Ge MOS capacitors; its variation with respect to the interface state response is also a noteworthy issue that requires clarification. In fact, many years ago, Nicollian and Brews [14] foresaw that it would be possible to observe a low-frequency (LF) C–V curve for a narrow-gap material, such as Ge, at the standard high-frequency (HF) used for traditional Si because of the higher intrinsic carrier concentration (ni). Such minority carrier characteristics for capacitors deposited on Si substrates have been established, correlated to the effects of the temperature, doping concentration, and impurity contamination, but similar studies using Ge substrates remain rare [15]–[17]. The influences of the doping concentration and the substrate type on this anomalous C–V behavior have been discovered in high-k/Ge MOS capacitors [9], [18];
the fundamental mechanisms were clarified through fitting of MOS equivalent circuit models [15], [16] and measurements of Arrhenius-dependent conductance properties [17], respectively. In the second part, we utilized a two-dimensional MEDICI numerical simulator, accompanied by theoretical calculations, to investigate the minority carrier response comprehensively—as functions of the doping concentration, measured frequency, and temperature—in terms of the electrical properties of Ge and Si MOS capacitors. We have also examined the underlying mechanistic differences through practical admittance measurements of ALD-Al2O3 thin films on both types of Ge substrates.