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IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 11, NOVEMBER 2007
Effects of Sulfur Passivation on Germanium MOS Capacitors With HfON Gate Dielectric Ruilong Xie and Chunxiang Zhu, Member, IEEE
Abstract—In this letter, we study the effects of sulfur (S) passivation, using aqueous ammonium sulfide ((NH4 )2 S), on germanium (Ge) MOS capacitors with sputtered HfON as gate dielectric and TaN as metal-gate electrode. Compared with control samples, the S passivation can effectively reduce both equivalent oxide thickness and interface-state density. X-ray-photoelectronspectroscopy analysis shows that (NH4 )2 S treatment can reduce the Ge–O bonds on Ge surface. The thermal stability of the S passivation under different postmetal-annealing temperatures was also examined, and it was found that samples with (NH4 )2 S treatment exhibit stable Ge/high-k interface upon 550-◦ C postmetaldeposition annealing, whereas interface quality degrades for those samples without S passivation. Index Terms—Germanium (Ge), MOS capacitor, sulfur (S) passivation.
I. I NTRODUCTION
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ARIOUS approaches have been investigated to overcome the scaling limits of modern complementary MOSFETs. High-permittivity (high-k) materials are being studied to replace the conventional silicon dioxide for gateoxide scaling [1]. At the same time, germanium (Ge) substrate is increasingly being studied for MOSFET applications because of its high intrinsic carrier mobilities [2]. The lack of a stable Ge oxide makes it difficult to fabricate a Ge MOSFET. However, the emergence of high-k dielectric gives an opportunity to fabricate devices having both merits of high-k and Ge. To have a high-quality high-k gate stack on Ge substrate, minimizing the GeOx at the surface between the high-k and substrate is a critical issue because Ge oxide is thermally unstable, water-soluble, and of poor electrical properties [3]. To minimize the GeOx , Ge-surface passivation is needed before gate-stack formation. Most works so far have concentrated on nitridation using NH3 gas treatment [4], [5] or atomic N exposure [6], where nitrogen is introduced into the interface of Ge oxide to passivate the Ge. However, they still exhibit high interface-state density (Dit > 1012 cm−2 ) [4], [6], and nitrogen involvement has potential for mobility degradation. Recently, Frank et al. [7] reported Ge MOS capacitors with sulfur (S) passivation by aqueous ammonium sulfide ((NH4 )2 S) treatment. It was found that interface-state density Manuscript received August 1, 2007. The review of this letter was arranged by Editor A. Chatterjee. The authors are with the Silicon Nano Device Laboratory, Department of Electrical and Computer Engineering, National University of Singapore, Singapore 119260 (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2007.907415
is lower than NH3 -nitridation-passivated samples. However, the gate stack did not go through any high-temperature annealing (e.g., > 400 ◦ C). In addition, there is no report on electrical properties like equivalent oxide thickness (EOT) and gateleakage current. In this letter, we show that S passivation can reduce the EOT of gate stack, whereas it can maintain the gateleakage current; moreover, it can improve the thermal stability of the Ge/high-k gate stack. II. E XPERIMENT The starting wafers for the experiment were n-type Ge wafers (Sb-doped, resistivity = 0.04 − 0.08 Ω · cm). The native oxide (GeOx ) was removed by cyclic rinsing between deionized water and diluted HF [8]. After that, the substrates were immersed into 20% aqueous (NH4 )2 S solution for 30 min at room temperature, which was followed by a water rinse and a N2 blow-dry. Control samples with only HF cyclic rinsing were also prepared. After that, HfON was then formed on both types of samples by HfON deposition with reactive sputtering and postdeposition annealing in a N2 ambient at 500 ◦ C for 1 min (the residual oxygen concentration is < 5 ppm). A 150-nm TaN-gate electrode was then sputtered, which was followed by lithography and dry-etching processes. Postmetal annealing (PMA) at different temperatures was then performed for thermal-stability investigation. The final step was forming gas anneal in H2 + N2 ambient at 420 ◦ C for 2 h. High-resolution ex situ X-ray-photoelectron-spectroscopy (XPS) analysis was performed with a standard Al X-ray source. Secondary-ion-mass-spectrometry (SIMS) analysis was used to study the Ge profile in the gate stack. Capacitance–voltage (C–V ) and J–V characteristics were measured by an Agilent 4284A LCR meter and an HP4156A semiconductor parameter analyzer, respectively. III. R ESULTS AND D ISCUSSION To confirm the S incorporation after the (NH4 )2 S treatment, the XPS data of S 2p signal are measured and shown in Fig. 1(a). The peak (∼162 eV) of the S 2p signal indicates that S was introduced on the Ge surface after (NH4 )2 S treatment. The inset of N 1s spectra shows that N would not be introduced on the Ge surface by the (NH4 )2 S treatment. The XPS data in the Ge 3d signal are shown in Fig. 1(b) which demonstrates that both HF-cleaned and (NH4 )2 S-treated surfaces are predominantly composed of metallic Ge (∼29.7 eV) from the substrate. For the samples cleaned by HF only, there is a small peak (signal from 32 to 33 eV) which represents GeOx (x ≤ 2).
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XIE AND ZHU: EFFECTS OF SULFUR PASSIVATION ON Ge MOS CAPACITORS WITH HfON GATE DIELECTRIC
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Fig. 1. XPS data in (a) S 2p, (b) Ge 3d, and (c) Ge 2p regions from Ge(100) after HF cleaning compared with HF + (NH4 )2 S treatment. The inset of (a) is the XPS data in N 1s region. (d) SIMS profiles for HfON/Ge gate stack after 500-◦ C PDA in N2 ambient for 30 s.
Fig. 2. Capacitance–voltage characteristics of TaN/HfON/Ge capacitors (a) with (NH4 )2 S treatment and (b) without (NH4 )2 S treatment, after a 550-◦ C PMA, in N2 ambient for 30 s.
For the samples treated by (NH4 )2 S, it is found that this peak shifts to the right slightly (signal from 31.5 to 32.5 eV) toward the metallic-Ge peak; possibly, it is because the Ge–S peak (29.5 to 30.5 eV) overlaps with GeO2 peak (∼32.5 eV). To further investigate the surface chemical states after the pregate cleaning/treatment, the Ge 2p core-level spectrum is shown in Fig. 1(c). The main peak located at 1217.6 eV is attributed to metallic-Ge spectrum from the substrate. The shoulder, for the samples cleaned by HF only, ranging from 1219 to 1221 eV, is attributed to GeOx (x ≤ 2) bonds [9], which are believed to be introduced during the sample transportation [4]. For the sample treated by (NH4 )2 S, the shoulder is smaller. Considering the possible bonds that the Ge atoms may have, it is reasonable to infer that the shoulder after the (NH4 )2 S treatment consists of two types of Ge bonds: Ge–O and Ge–S. Because the shoulder of the (NH4 )2 S-treated substrate consists of both Ge–O and Ge–S and that the size of the shoulder is smaller than the control sample, the (NH4 )2 S treatment can reduce Ge–O bonds on the surface.
Fig. 3. EOT values with different surface treatments and PMA temperatures. S-passivated samples show about 0.7-nm thinner EOT.
The interface-state density was measured using frequencydependent conductance method. For the samples with (NH4 )2 S treatment and 450-◦ C PMA, Dit of 4.8 × 1011 cm−2 · eV−1 at midgap is obtained, which is lower than those reported using
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IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 11, NOVEMBER 2007
Fig. 4. (a) Gate-leakage-current density as a function of EOT with different surface treatments and PMA temperatures together with published data. (b) Typical Ig –Vg curves of Ge MOS capacitors with different surface treatments and PMA temperatures.
surface nitridation [4], [7], whereas for the samples without (NH4 )2 S treatment, Dit is 1.4 × 1012 cm−2 · eV−1 . The improved interface properties in S-treated samples can be possibly explained as the GeOS interfacial layer which may suppress the Ge outdiffusion due to less Ge–O bonds and provide more stable interface properties, whereas for the samples without S passivation, the surface consists of GeOx , and it may enhance Ge outdiffusion [10] and results in poor interface property. To prove this hypothesis, SIMS was performed on HfON/Ge samples. Fig. 1(d) shows the Ge depth profiles for the samples with or without (NH4 )2 S treatment. It can be seen that more Ge outdiffusion was detected for the samples without (NH4 )2 S treatment, whereas the samples with (NH4 )2 S treatment show sharper Ge profiles. To understand thermal stability of S passivation, PMA at different temperature was carried out for samples with and without S passivation. Fig. 2(a) shows the C–V characteristics of the TaN/HfON/Ge MOS capacitors with (NH4 )2 S treatment after 550-◦ C PMA at N2 ambient for 30 s. For comparison, the C–V characteristics of the Ge MOS capacitor without (NH4 )2 S treatment are shown in Fig. 2(b). The large frequency dispersion in accumulation for samples without S passivation may be attributed to the Ge outdiffusion into high-k dielectric, which would increase both the interface and bulk traps near the interface. The frequency dispersion in inversion may be attributed to the diffusion of impurities from dielectric into the substrate, which could increase minoritycarrier generation [13], [14], or, perhaps, to the interaction of interface slow states [13]. EOT values for the samples were extracted by fitting the C–V data, using low-frequency curves (1 kHz) in accumulation which are the least affected by shunt resistance [11], while taking into account the quantumconfinement effects. The results are shown in Fig. 3. It can be seen that samples with S passivation have a thinner EOT, which indicates a larger k value of dielectric or the thinner interfacial layer than those without S passivation. The decrease of EOT values after higher temperature PMA (550 ◦ C) is due to the high-k densification. Furthermore, Dit values for the samples with different surface treatments and different PMA temperatures were extracted. The Dit degrades significantly (Dit ∼ 3 × 1012 cm−2 · eV−1 ) for the samples without S passivation after PMA increased to 550 ◦ C, whereas it shows little difference for the sample with S passivation after 550 ◦ C PMA (Dit ∼ 5.0 × 1011 cm−2 · eV−1 ). The Dit values of samples
without S passivation increase after higher temperature PMA due to more Ge outdiffusion at the Ge/high-k interface [12]. Thus, S passivation improves the thermal stability of Ge gate stack. Fig. 4(a) shows the gate-leakage-current density as a function of EOT together with published data [4], [6], [8], [13], [14]. Fig. 4(b) shows the typical Jg –Vg curves of Ge MOS capacitors with different surface treatments or PMA temperatures. Both samples with or without S passivation have low gate-leakagecurrent densities of about 1 × 10−6 A/cm2 at Vg − Vfb = 1 V. Although samples without S passivation have larger EOT values, this does not improve the gate-leakage current. This may due to the poor quality of Ge oxide interfacial layer. It was also found that the gate-leakage current decreases and that the distribution becomes more uniform after the higher temperature (550 ◦ C) annealing, particularly for the S-passivated samples. This is because higher temperature annealing can densify the gate dielectric and can reduce the bulk traps in the gate oxide as well as the weak points. The improvement in gate leakage after higher temperature annealing is less effective for samples without S passivation because of more Ge outdiffusion at elevated temperature. IV. C ONCLUSION The effects of the S passivation of Ge using (NH4 )2 S have been investigated. It is found that the (NH4 )2 S treatment can reduce the interface-state density and can improve the electrical properties in terms of EOT and gate-leakage current. Moreover, it is found that samples with the (NH4 )2 S treatment show better thermal stability at Ge/high-k interface. This is possibly due to the less Ge diffusion into the high-k dielectric. R EFERENCES [1] G. D. Wilk, R. M. Wallace, and J. M. Anthony, “High-κ gate dielectrics: Current status and materials properties considerations,” J. Appl. Phys., vol. 89, no. 10, pp. 5243–5275, May 2001. [2] H. Shang, H. Okorn-Schmidt, K. K. Chan, M. Copel, J. A. Ott, P. M. Kozlowski, S. E. Steen, S. A. Cordes, H.-S. P. Wong, E. C. Jones, and W. E. Haensch, “High mobility p-channel germanium MOSFETs with a thin Ge oxynitride gate dielectric,” in IEDM Tech. Dig., 2002, pp. 441–444. [3] K. Prabhakaran and T. Ogino, “Oxidation of Ge(100) and Ge(111) surfaces: An UPS and XPS study,” Surf. Sci., vol. 325, no. 3, pp. 263– 271, Mar. 1995.
XIE AND ZHU: EFFECTS OF SULFUR PASSIVATION ON Ge MOS CAPACITORS WITH HfON GATE DIELECTRIC
[4] 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 physical and electrical properties of HfO2 films on Ge substrate,” Appl. Phys. Lett., vol. 84, no. 19, pp. 3741–3743, May 2004. [5] H. Kim, P. C. McIntyre, C. O. Chui, K. C. Saraswat, and M.-H. Cho, “Interfacial characteristics of HfO2 grown on nitride Ge(100) substrates by atomic-layer deposition,” Appl. Phys. Lett., vol. 85, no. 14, pp. 2902– 2904, Oct. 2004. [6] J. J.-H. Chen, N. A. Bojarezuk, H. Shang, M. Copel, J. B. Hannon, J. Karasinski, E. Preisler, S. K. Banerjee, and S. Guha, “Ultrathin Al2 O3 and HfO2 gate dielectrics on surface-nitrided Ge,” IEEE Trans. Electron Devices, vol. 51, no. 9, pp. 1441–1447, Sep. 2004. [7] M. M. Frank, S. J. Koester, M. Copel, J. A. Ott, V. K. Paruchuri, H. Shang, and R. Loesing, “Hafnium oxide gate dielectrics on sulfur-passivated germanium,” Appl. Phys. Lett., vol. 89, no. 11, p. 112 905, Sep. 2006. [8] C. O. Chui, H. Kim, P. C. McIntyre, and K. C. Saraswat, “Atomic layer deposition of high-κ dielectric for germanium MOS applications— Substrate surface preparation,” IEEE Electron Device Lett., vol. 25, no. 5, pp. 274–276, May 2004.
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