Al passivation on the oxidation of Ge

Microelectronic Engineering 88 (2011) 407–410

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The effect of Se and Se/Al passivation on the oxidation of Ge D. Tsoutsou ⇑, Y. Panayiotatos, S. Galata, A. Sotiropoulos, G. Mavrou, E. Golias, A. Dimoulas MBE Laboratory, Institute of Materials Science, NCSR DEMOKRITOS, 153 10 Athens, Greece

a r t i c l e

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Article history: Available online 13 December 2010 Keywords: XPS High-k dielectric Se passivation Ge

a b s t r a c t The molecular and atomic oxidation of molecular beam deposited Se passivating layers on Ge substrates was in situ investigated by X-ray photoelectron spectroscopy. It turns out that while Se is efficient in suppressing Ge oxidation upon molecular oxygen exposure, an extra thin Al layer is needed to protect the Ge surface from highly reactive atomic oxygen radicals. Electrical measurements performed on the Al-covered surfaces reveal that Se is beneficial in reducing the interface state density. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Germanium (Ge) and InGaAs are considered as alternative channel materials for the end-of-the roadmap CMOS in about 10 years from now. InGaAs up to now gives the best results for nMOS while Ge is suitable for pMOS FET devices. Therefore, a possible future implementation could be a dual channel CMOS consisting of InGaAs nMOS and Ge pMOS co-integrated on the same Si substrate. To simplify processing, it is highly desirable that a common gate stack is employed for both types of FETs. It has been shown [1] that S-passivated Al2O3 yields very good symmetrical results on both InGaAs and Ge MOSCAPs, and MOSFETs implying that this gate stack could be a good candidate for the common gate stack. Selenium (Se) passivated Al2O3 could be an alternative gate stack. Se being in the same group VI A with S is expected to have similar chemical activity with S with potentially better thermal and chemical stability. While Se-passivated III–V [2–4] as well as S-passivated Ge [5–8] surfaces have been studied before, there is very limited work on the use of Se as a passivating layer for Ge. In this work, the structural and electrical Ge surface passivation is being investigated by means of molecular beam deposition (MBD) of Se. In particular, molecular and atomic oxygen exposure of the Se-treated Ge surfaces was performed, in order to examine whether Se is efficient in suppressing Ge oxidation. In addition, the behavior of the Se-treated Ge surface, integrated with high-k Al2O3 dielectric was further studied. 2. Experimental details Prior to the Se treatment, Ge substrates were annealed at 360 °C in a molecular beam epitaxy (MBE) chamber until a clean 2  1 ⇑ Corresponding author. Tel.: +30 210 650 3341. E-mail address: [email protected] (D. Tsoutsou). 0167-9317/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.11.038

reconstructed Ge surface was achieved. Subsequently, elemental Se was evaporated from a Ta effusion cell at 160 °C. Three different deposition temperatures were used (Tdep = RT, 250 °C and 400 °C), in order to study the thermal stability of the Se layer. In all cases, a (1  1) reconstruction was observed by RHEED after the Se deposition. Some samples were further deposited with an additional protective thin Al layer (0.4 nm) produced by e-beam evaporation. Finally, the Se and Se/Al modified surfaces were exposed to molecular and atomic oxygen (MO (30 min) and AO (5 min), respectively) beams, with a partial O2 pressure of 5  10 5 mbar. It should also be noted that during Al deposition and O exposure, the Ge substrate was kept at the same temperature as for Se deposition. Interface bonding was in situ studied by X-ray photoelectron spectroscopy (XPS) provided by a standard Al Ka (1486.6 eV) source with an energy resolution of 0.8 eV and a pass energy of 15 eV. The electron take-off angle relative to the sample surface normal was 37°. XPS core-level photoemission lines were decomposed by curve fitting using Lorentzian–Gaussian functions for the different components and a Shirley background. The electrical properties of the Se and Se/Al-passivated Ge surfaces were studied by room temperature capacitance and conductance versus voltage (CV, GV) as well as current density–voltage (JV) measurements on metal–insulator–semiconductor (MIS) capacitors. High-k Al2O3 (7 nm) was produced by the co-deposition of Al and AO beams, while the gate electrode was formed by patterning an in situ sputtered Pt layer (30 nm) through a shadow mask. The back ohmic contact was eutectic In–Ga alloy. Post-metallization O2 (Tann = 300 °C for 15 min) annealing was performed for the electrical characterization of the samples. It should be noted at this point that, only the AO-exposed samples were electrically tested, since the atomic oxidation method is the one commonly used in the gate oxide formation of MIS devices. Molecular oxidation has been performed for the structural XPS characterization,

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in order to decouple the Ge–Se bonding configuration from possible radical oxidation of the Ge surface.

of the oxidized Se-treated Ge surfaces. The Se–O bonding configuration should shift to higher binding energies (59 eV) compared

3. Characterization results and discussion

Se 3d Se-Ge, Se-Al

(e)

Normalized intensity (arb. units)

Fig. 1, presents the wide Se3d/Ge3d core level spectra of various Se-covered Ge surfaces, recorded for three different Se deposition temperatures (RT, 250 °C and 400 °C). In all three cases, the Se rate was kept constant during deposition (200 s) at 0.1 Å/s, aiming in the formation of a 2 nm layer with a sticking coefficient of 1. Nevertheless, it is found that the Se/Ge ratio decreases as the temperature increases. For 400 °C, only traces of Se can be detected on the surface, while most of it has been desorbed. This finding can be justified considering the low evaporation temperature of Se (160 °C). A temperature of 250 °C was chosen as optimum representing a Secovered surface with the minimum required amount of Se. Subsequently, an ultrathin Se layer of 0.4 nm was chosen to study its passivation effect on Ge. A deep inspection into the Se bonding configurations of various oxidized Se (0.4 nm)-treated Ge surfaces at 250 °C was achieved by recording the XPS detailed Se 3d (Fig. 2) spectra. The intensity of each spectrum has been normalized to the maximum of peak (a) that corresponds to an amorphous Se layer deposited on an unreactive Si/SiON/Pt substrate. In this case, the average binding energy position of the Se 3d5/2 and 3d3/2 doublet at 55.2 eV indicates that Se atoms are bonded to form elemental Se. The Se 3d negative chemical shift of about 0.5 eV in the case of the Se-treated Ge (Fig. 2b) as compared to the amorphous Se sample, is attributed according to the second nearest neighbor effect to the formation of Se–Ge bonding configuration [10]. In addition, the signals of the three different oxidized Se-treated surfaces (Fig. 2c–e), are also indicative of Se being bonded to the Ge surface. Finally, it should be noted that no sign of Se–O bonding is evidenced in the spectra

x4.8

(d) Ge/Se/Al/AO Se-Ge

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Ge/Se/MO Se-Ge

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Ge/Se (0.4 nm)

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Se-Se

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Si/SiON/Pt/Se

58

56

54

52

50

Binding energy (eV) Fig. 2. Se 3d core level normalized spectra recorded for various MO and AO exposed Se-covered Ge surfaces. The XPS spectrum of a Si/SiON/Pt/Se (a) structure is also presented for comparison.

Se 3d / Ge 3d

Ge 3d Ge

0

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Ge-O, Ge-Se

(f) Ge/Se/Al/AO Ge-O, Ge-Se

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x10

(c) Ge/Se (400 C)

(b) Ge/Se (250 C)

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Intensity (arb. units)

Se-Ge Ge/Se/AO Ge-O

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(c) Ge/Se/MO Ge-Se

(b) Ge/Se GeO

(a) Ge/Se (RT) 56

Ge

0

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Ge/MO 52

35

30

Binding energy (eV) Fig. 1. Se3d/Ge3d core-level photoemission spectra for Ge/Se structures, illustrating the thermal stability of the Se layer.

34

32

30

28

26

Binding energy (eV) Fig. 3. Ge 3d core level spectra showing oxidation state differences following MO/ AO exposure of various Se-covered Ge surfaces.

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nGe/Al (0.4 nm)/AO/Al2O3 (7nm)

(b)

Tdep(stack)=250 C

1.0x10

12

0.3 0.2

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Tdep(stack)=RT 1 MHz 100 KHz 10 KHz 1 KHz

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G/(Aω q) (eV cm )

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nGe/Se (0.4nm)/Al (0.4nm)/AO/Al2O3 (7nm) nGe/Se (0.4nm)/Al (0.4nm)/AO/Al2O3 (7nm)

0

2

1

2

Gate bias Vg (V)

Fig. 4. Multi-frequency CV curves of various Ge/Al/AO/Al2O3 structures, before (a) and after (b, c) Se passivation. The insets show the normalized conductance characteristics. ‘Tdep (stack)’ refers to the deposition temperature of Se, Al, AO and Al2O3.

0.01

(c)

1E-3 1E-4 1E-5

(a)

1E-6 2

J (A/cm )

to the Se–Ge configuration, due to the more electronegative nature of O atoms. The corresponding Ge 3d core-level photoemission spectra of the various oxidized Se (0.4 nm)-treated Ge surfaces are shown in Fig. 3. A fixed spin–orbit splitting and area ratio of 0.59 eV and 2:3, respectively, were used for the fit. A weak oxidation is clearly inferred upon MO exposure of the bare Ge surface (Fig. 3a), where only a minor GeO compound (as identified by its chemical shift of 1.8 eV [9]) is needed in order to satisfactorily fit the spectrum. Depositing a thin protective Se layer on top of the clean Ge surface leads to the formation of a small Ge–Se component (Fig. 3b). The Ge-reacted peak evident after MO exposure of the Se-treated Ge surface (Fig. 3c), is attributed to the existence of both Ge–Se and Ge–O bonding configurations. The relative Gereacted/Gebulk intensity ratio decreases upon Se deposition (from 0.043 for the Ge/MO stack to 0.026 for the Ge/Se/MO as well as the Ge/Se stacks). It should also be pointed out that the intensity ratios of both Ge/Se and Ge/Se/MO stacks were found to be 0.026, suggesting that Se totally protects the Ge surface from molecular oxidation. On the other hand, highly reactive oxidizing species like AO radicals, produce a significant amount of GeOx components in the bare Ge surface (Gereacted/Gebulk intensity ratio = 0.3), chemically shifted by +2.7 eV (Fig. 3d). Again, the nature of the high binding energy peak in the case of the atomic oxidized Se-treated surface (Fig. 3e), can be attributed to the overlap of different Ge chemical states, which were assigned to the formation of Ge–Se as well as Ge–O species. Finally, the presence of an extra thin (0.4 nm) Al layer reduces the intensity of the Ge-reacted peak, revealing aluminum’s efficiency in the oxidation reduction of a Se-treated surface (Fig. 3f). Indeed the relative Gereacted/Gebulk intensity ratio decreases upon Al deposition (from 0.36 for spectrum (e) to 0.22 for spectrum (f)). Based on the XPS findings that revealed an oxidation reduction of the Ge surface, by combining thin Al and Se layers, electrical characterization was subsequently performed on such stacks. Fig. 4 depicts capacitance–voltage (C–V) curves for MIS capacitors from the O2 post-metallization annealed (a) nGe/Al/AO/Al2O3 and (b) nGe/Se/Al/AO/Al2O3 stacks, deposited at 250 °C. Stack (c) has the same structure as (b), but having been deposited at lower temperature (RT), or as argued in Fig. 1, exhibiting a higher Se–Ge bonding intensity. Electrical characterization was performed on the Al-covered samples, since as already demonstrated previously Al is beneficial in suppressing the atomic oxidation of the clean Ge surface. It is observed that all three capacitors have good electrical characteristics in terms of hysteresis and stretch-out, even though some frequency dispersion is present. The Se-passivated samples (Fig. 4b and c) exhibit a larger modulation of the capacitance compared to the non-Se-passivated Ge samples in Fig. 4a, indicating a

(b)

1E-7 1E-8 1E-9 1E-10 1E-11 -2

-1

0

1

2

Gate bias Vg (V) Fig. 5. Typical diode current density–voltage (J–V) characteristics of the devices presented in Fig. 4.

lower density of interface states (Dit). The insets of Fig. 4, show the normalized (G/Axq, at 100 kHz) conductance–voltage (G–V) characteristics for the three samples. Dit was roughly estimated by considering the peak values of the normalized G–V data. The Se-treated samples (Fig. 4b and c) exhibit lower Dit values (7.5  1011 and 6  1011 eV 1 cm 2, respectively) compared to the solely Al-treated surface (1.2  1012 eV 1 cm 2), indicating a better Ge surface passivation. It should be emphasized that these Dit values should be used only for a comparative study between the different Se passivating configurations to determine the trend. Fig. 5 depicts typical diode current density–voltage (J–V) characteristics of the aforementioned Al-covered stacks. The Se-treated sample deposited at 250 °C (stack (b)), exhibits the lower leakage current in both forward and reverse bias, which in combination with its low Dit value, makes it the best candidate out of the three studied in this work for Ge passivation. 4. Conclusions In this work, we have studied the Se beam treatment of Ge surfaces, as an atomic passivation approach. We demonstrated that a Se-covered Ge surface is resistive to oxidation upon MO exposure, whereas the use of highly reactive AO radicals does lead to the formation of Ge oxide species. In the latter case, an additional ultrathin Al layer was proven to be effective in suppressing the oxide formation. Electrical measurements performed on the Al-covered surfaces reveal a reduced Dit value for the Se-treated surfaces.

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References [1] D. Lin, G. Brammertz, S. Sioncke, C. Fleischmann, A. Delabie, K. Martens, H. Bender, T. Conard, W.H. Tseng, J.C. Lin, W.E. Wang, K. Temst, A. Vatomme, J. Mitard, M. Caymax, M. Meuris, M. Heyns, T. Hoffmann, IEDM09, Tech. Dig., p. 327. [2] S. Takatani, T. Kikawa, M. Nakazawa, Phys. Rev. B 45 (1991) 8498. [3] T. Scimeca, Y. Watanabe, R. Berrigan, M. Oshima, Phys. Rev. B 46 (1992) 10201. [4] C. Gonzalez, I. Benito, J. Ortega, L. Jurczyszyn, J.M. Blanco, R. Perez, F. Flores, T.U. Kampen, D.R.T. Zahn, W. Braun, J. Phys. Condens. Matter 16 (2004) 2187. [5] T. Weser, A. Bogen, B. Konrad, R.D. Schnell, C.A. Schug, W. Steinmann, Phys. Rev. B 35 (1987) 8184.

[6] J. Roche, P. Ryan, G.J. Hughes, Appl. Surf. Sci. 174 (2001) 271. [7] C. Fleischmann, S. Sionke, K. Schouteden, K. Paredis, B. Beckhoff, M. Muller, M. Kolbe, M. Meuris, C. Van Haesendonck, K. Temst, A. Vantomme, ECS Trans. 25 (2009) 421. [8] M. Houssa, D. Nelis, D. Hellin, G. Pourtois, T. Conard, K. Paredis, K. Vanormelingen, A. Vantomme, M.K. Van Bael, J. Mullens, M. Caymax, M. Meuris, M.M. Heyns, Appl. Phys. Lett. 90 (2007) 22105. [9] D. Schmeisser, R.D.D. Schnell, A. Bogen, F.J. Himpsel, D. Rieger, G. Landgren, J.F. Morar, Surf. Sci. 172 (1986) 455. [10] R.L. Opila, G.D. Wilk, M.A. Alam, R.B. van Dover, B.W. Busch, Appl. Phys. Lett. 81 (2002) 1788.