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N2-plasma nitridation on Si(111): Its effect on crystalline silicon nitride growth
Surface Science 606 (2012) L51–L54
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Surface Science Letters
N2-plasma nitridation on Si(111): Its effect on crystalline silicon nitride growth Chung-Lin Wu a, b,⁎, Wei-Sheng Chen a, Ying-Hung Su a a b
Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan
a r t i c l e
i n f o
Article history: Received 6 February 2012 Accepted 6 March 2012 Available online 15 March 2012 Keywords: Silicon nitride N2 plasma Scanning tunneling microscopy/spectroscopy Epitaxy
a b s t r a c t This study uses a combination of atom-resolved scanning tunneling microscopic and spectroscopic (STM/STS) techniques to investigate the effects of N2-plasma nitridation on a crystalline β-Si3N4 ultrathin film grown on the Si(111) substrate. The proposed two-step growth process, including N2-plasma nitridation followed by vacuum annealing both at high temperature (~900 °C), can substantially improve the atomic and electronic structures of the β-Si3N4 having an atomically uniform morphology and stoichiometry. The effects of nitridation and post-annealing temperatures were examined by monitoring the morphological and electronic–structural evolutions of a non-stoichiometric surface. Moreover, the two-step N2-plasma nitridation process has extremely low activation energy and thus minimizes the thermal energy from the substrate for practical growth of β-Si3N4 ultrathin film. © 2012 Elsevier B.V. All rights reserved.
Silicon nitride (Si3N4) is a promising dielectric material for applications in present metal–insulator–semiconductor devices because of its fascinating fundamental properties such as large dielectric constant, wide band gap, and excellent strength [1]. Unlike conventional dielectric SiOx/Si system, Si3N4 has a crystalline β-phase, and its (0001) surface shows a good lattice match with the Si(111) surface. This makes β-Si3N4 more attractive for a further scaling Si-based device. It has been found that an ultrathin (~1.3 nm) β-Si3N4 layer can be grown by thermal nitridation methods established on an Si(111) substrate annealed at a high temperature of about 900 °C in various precursor nitrogen molecular environments, such as NH3, NO, and N2 [2–4]. However, previous attempts have failed to achieve high homogeneity in the surface morphology and stoichiometry of β-Si3N4 ultrathin film grown by thermal nitridation. Partial nitridation areas typically appeared on the β-Si3N4 surface [2,4], leading to a device breakdown or current leaking when using a β-Si3N4 ultrathin dielectric layer. To improve its homogeneity, a nitrogen plasma source is expected because of its higher kinetic energy and activity. Recent research shows that plasma-nitrided β-Si3N4 is a potential template for graphene-based electronics [5], and for growing high quality III-nitride quantum dots and magnetic nanoclusters on an Si substrate [6–8]. Thus, β-Si3N4 grown on silicon will likely enable the integration of low-dimensional functionalities on Si wafers with high performance. Despite these works have demonstrated that the nitrogen plasma is effective for β-Si3N4 grown on an Si(111) surface, due to the absence of atomic resolution results, it remains unclear if nitrogen plasma can
⁎ Corresponding author at: Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan. E-mail address: [email protected] (C.-L. Wu). 0039-6028/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2012.03.004
achieve epitaxial growth of β-Si3N4 without the partial nitridation area. It is also unclear if N2-plasma induces surface damage, or if a method exists for eliminating such damage. Furthermore, in previous nitridation processes, the β-Si3N4 crystallization energy was provided mainly by the thermal energy from the silicon substrate at 900 °C. This high-temperature growth process might limit the use of this crystalline dielectric layer in the standard semiconductor process. In this letter, we present a two-step growth process: short nitrogen plasma exposure followed by thermal annealing both at 900 °C, can substantially improve the β-Si3N4 quality evaluated by comprehensive microscopic and spectroscopic studies on the atomic scale. We also reveal a substantial recovery of N2-plasma-beam damage on low-temperature grown non-stoichiometric silicon nitride layer after brief annealing at elevated temperatures. Moreover, without high-temperature annealing, the energetic N2-plasma can achieve relatively low-temperature (700 °C) epitaxy of β-Si3N4 on Si substrate. The in situ atom-resolved investigations of N2-plasma nitridation on Si(111) substrate was performed in an ultrahigh vacuum growth chamber (with a base pressure of 2 × 10 − 10 Torr) equipped with a radio-frequency (rf) nitrogen plasma (ADDON), which is connected to the scanning tunneling microscope (STM) chamber by a gate valve. The Si(111) substrates were cleaned to remove the native oxide in the preparation chamber by overnight degassing at ~ 600 °C followed by several cycles of flash annealing to ~1150 °C for 30 s in a UHV environment. This substrate cleaning process produced a (7 × 7)-ordered Si(111) surface. The nitrogen plasma for silicon nitride growth was held constant at a constant rf power of 500 W and a chamber pressure of 5 × 10 − 5 Torr, which was correlated to an equivalent N2 flux of 0.2 sccm. Figure 1 summarizes representative STM/STS results on the (8 × 8)-reconstructed surface of a β-Si3N4(0001) ultrathin layer
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grown using the proposed two-step process at 900 °C beginning with a short plasma nitridation for 10 s followed by vacuum annealing for 1 min. The large filled-state STM images [Fig. 1(a) and its inset] both show that the surface is electronically identical and has no partial nitridation area. The monolayer step height of the β-Si3N4 (0001) terraces is about 3.3 ± 0.1 Å (calibrated by the Si(111)-7 × 7 step height), which is the same as that grown by thermal nitridation [2]. The reconstructed surface reveals clearly unit cells of 8/3 × 8/3 spots with a periodicity of ~10.2 Å. The zoom-in STM image in Fig. 1(b) shows that the 8/3 × 8/3 spots, and the 8 × 8 (30.7 Å × 30.7 Å) diamond-shape super-cells are visible with slightly detectable separations between the 8 × 8 cells. To quantify the electronic structures of β-Si3N4 (0001) surface in detail, we compare the tunneling-current computation with the combined tunneling spectra of the absolute current in a logarithmic display [log(I), Fig. 1(c)] and the normalized differential conductivity [(dI/dV)/(I/V), Fig. 1(d)] as a function of the sample bias. The tunneling spectra were acquired simultaneously in I(V) and AC modulated lock-in amplifier signal channels, and an average of 30–50 such measurements were taken. The log(I)-V plot accentuates the current variations to clearly reveal the gap edges and identify the origins of the tunneling current. To fit the log(I)-V plot, the tunneling current was calculated in a manner similar to prior determinations [9,10]. This calculation integrates over all states between the metal tip and the sample Fermi-level times the transmission coefficient, which is estimated using WKB approximation. To match the computations to the experiments, several main parameters were set in the calculation: a band gap energy of about 5 eV for intrinsic β-Si3N4 [11–13], a work function of 4.5 eV for the tungsten tip, a tip-sample separation of 0.6 nm, and a tunneling area of 1 nm 2. A comparison is shown in Fig. 1(c), following the calculated tunneling current curves of conduction band (IC) and valence band (IV), the log(I)-V and (dI/dV)/(I/V) curves both display a clear conduction band minimum (CBM) at
+2.5 V and a valence band maximum (VBM) at −2.7 V with respect to the Fermi edge (EF = 0 V), which matched quite well with the PES measurements and the density of state (DOS) calculations for βSi3N4(0001)/Si(111) heterojunction [14,15]. This good match supports our identification of the band edges and indicates negligible tip-induced band bending. However, to achieve a better fit with the experimental spectra, two additional current contributions were introduced at bias V b − 3 V for IN and at bias V > 1 V for Igap. The IN current contribution can be solely attributed to the nitrogen dangling bond state, which was successfully identified by previous computational and experimental results [2,12,16]. The Igap tail extending out from the CB was also found to arise from a Si dangling bond state at the rest layer [16,17]. These identifications of tunneling spectrum make it possible to examine the effects of annealing temperature on the surface states related to the crystallization of an ultrathin non-stoichiometric silicon nitride film, which was formed by briefly exposing the Si(111)-7 × 7 surface to the N2-plasma (10 s) at room temperature. As expected, this RT grown ultrathin silicon nitride layer showed the nonstoichiometric phase (SiNx) having an atom-disordered surface [Fig. 2(a)]. Compared with the computational tunneling spectrum, the experimental spectrum revealed that the SiNx had no apparent valence band edge of β-Si3N4, but strong tunneling currents at below − 3 V and above +3 eV. This could be attributed to numerous
I Si IC I Gap IV 10nm Sample Bias (eV)
I Si IN
IC I Gap
IV 10nm Sample Bias (eV)
RT-Plasma SiN x annealed at 900 °C
Fig. 1. (a) Large-scale occupied-state STM image (80 nm × 80 nm) of a β-Si3N4(0001)(8 × 8) surface formed by two-step nitridation process with a monoatomic high step measured at sample bias Vs = − 4.0 eV and tunneling current It = 0.8 nA. Inset shows a zoom-out image acquired in area of 500 nm × 500 nm. (b) Zoom-in STM image shows the (8 × 8) and (8/3 × 8/3) unit cells composed of nitrogen adatoms, and the long diagonal of the unit cells align along the [11–2] direction of the Si(111) substrate. (c) Logarithmic display of the tunnel current as a function of voltage obtained at the (8 × 8)-reconstructed surface. Experimental curves are shown by open circles, and theoretical fits are shown by dash and solid lines. The valence band (IV), conduction band (IC), N-adatom (IN), and gap state (IGap) tunneling currents are indicated. The positions of the valence (EVBM) and conduction band edges (ECBM) can be observed in the spectrum. (d) Normalized differential conductivity (dI/dV)/(I/V) as a function of voltage. The spectral peak associated with the N adatoms at around − 4.2 eV is marked by Nadatom. The valence and conduction band onsets show clearly and match well with a band gap of ~ 5.0 eV in this spectrum.
IN I Gap IC
IV 10nm Sample Bias (eV)
Fig. 2. STM images (50 nm× 50 nm) (left) and the corresponding tunneling spectra (right) with fitting curves for different annealing temperature of RT-grown SiNx: (a) initial SiNx atom-disordered surface after 10 s exposure of N2-plasma on Si(111)(7× 7) surface at RT; (b) after 1 min vacuum annealing at 600 °C; (c) after 1 min vacuum annealing at 900 °C. The IV, IC, IN, IGap tunneling currents and the additional Si surface dangling bond (ISi) tunneling current are indicated in the fitting results.
N adatom (IN) and Si dangling bond (ISi) states on the surface, respectively [18–20]. After brief annealing to 600 °C (1 min) to recrystallize the SiNx layer, Fig. 2(b) reveals a slightly atom-ordered surface. The corresponding tunneling spectrum indicates that the 600 °C annealing promotes the appearance of band edge structures of β-Si3N4, but does not completely eliminate the Si surface dangling bonds (ISi). At a higher annealing temperature of 900 °C (1 min), the SiNx vanished and the surface revealed two crystalline regions with 7 × 7 and 8 × 8 surface reconstructions, distinguished by two levels of brightness [Fig. 2(c)]. The tunneling spectrum obtained over the “dark” 8 × 8 area shows an insulating nature with characteristic β-Si3N4 band edges but no Si surface dangling bonds. This is nearly identical to the characteristics of β-Si3N4 grown by two-step N2-plasma nitridation with clean (8 × 8)-reconstruction [Fig. 1(c)]. The tunneling spectra shown here are quite suitable for monitoring the N2-plasma induced damage of silicon nitride film during device fabrication. These results imply that the defects generated by a low-temperature nitridation process might be suppressed and that the crystalline quality could be significantly improved during high-temperature annealing above 600 °C. To gain quantitative insights into the N2-plasma assisted two-step growth for β-Si3N4 thin film, Fig. 3 plots the nitridation–temperature dependent mean growth rate derived from the β-Si3N4 8 × 8 reconstruction area (average over 1000 nm 2 scanning size) during 10 s nitridation and 1 min post-annealing processes. The temperaturedependent growth rate R can be described by an Arrhenius equation R = R0 exp(− EA/kBT) with an activation energy of EA = (80 ± 20) meV (~7.75 J/Mol), where T is the nitridation temperature and kB is the Boltzmann constant. This apparent activation energy represents the sum of energies required for epitaxial seeds formation and nucleation in the two-step growth process of β-Si3N4. The apparent activation energy is about two orders lower than the formation energy of β-Si3N4 (~730 kJ/Mol) [21]. This means that most of the reactive gas species were plasma, which mainly contributed to the formation of β-Si3N4 epitaxial seeds and thus minimize the thermal active process. Furthermore, without post annealing, Fig. 4(a) shows that β-Si3N4 film grown by briefly exposing Si(111) substrate to N2-plasma (10 s) at 700 °C is covered with numerous adatom clusters. The surface and electronic structures of this film are essentially identical to the two-step grown β-Si3N4 (0001) surface, as indicated by the pronounced (8× 8)-reconstruction and wide band-gap tunneling spectrum [(dI/dV)/(I/V), Fig. 4(b)]. According to the observed spectroscopic contrast of the small adatom clusters and the silicon back-band state at
R (nm2/s)
10 nm
E A = (80±20) meV
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Fig. 4. (a) Large-scale (100 nm × 50 nm) STM image of a β-Si3N4(0001)-(8 × 8) surface formed by direct plasma nitridation at 700 °C. Adsorbed Si adatoms can be observed in the image. (b) Characteristic (dI/dV)/(I/V)-V spectrum taken on the (8 × 8)-surface. (c) The (dI/dV)/(I/V)-V spectrum shows the gapless feature taken on the Si cluster region, indicated by the dashed circle in the STM image (a).
around −2 V [Fig. 4(c)], we conclude that the nonreactive silicon atoms on β-Si3N4 surface often results in clusters and would deposit at elevated substrate temperature. This thus suggests the possibility of a new process design for growing a crystalline Si3N4 ultrathin layer through low-temperature N2-plasma incorporation to avoid harmful surface reactions such as ion bombardment or ion implantation during high-temperature growth. In summary, the atomically morphological and spectroscopic results of this study show that an ultrathin crystalline β-Si3N4 layer can be uniformly grown without surface defects and clusters using two-step plasma nitridation process at high temperature. Lowtemperature nitrided non-stoichiometric silicon nitride thin films reveal numerous Si dangling bonds and no apparent valence band, which can be improved at an elevated nitridation temperature and during brief vacuum annealing. Very low activation energy is founded by observing the morphological evolution of non-stoichiometric silicon nitride films grown at different nitridation temperatures. This makes β-Si3N4 ultrathin layer possible to directly grow using N2-plasma at relatively low substrate temperature without post annealing process. Acknowledgment
50 nm
This work was supported by the National Science Council in Taiwan. References
50 nm
1000/T (K-1) Fig. 3. Mean growth rate R of β-Si3N4 ultrathin film as a function of reciprocal nitridation temperature with same post-annealing process. Characteristic STM images having low (8 × 8 reconstruction) and high (7 × 7 reconstruction) brightness regions are shown in right reflecting the change in β-Si3N4 fraction at different nitridation temperatures.
[1] T.P. Ma, IEEE Trans. Electron Devices 45 (1998) 680. [2] H. Ahn, C.-L. Wu, S. Gwo, C.M. Wei, Y.C. Chou, Phys. Rev. Lett. 86 (2001) 2818; C.-L. Wu, J.-L. Hsieh, H.-D. Hsueh, S. Gwo, Phys. Rev. B 65 (2002) 045309. [3] X.-S. Wang, G. Zhai, J. Yang, L. Wang, Y. Hu, Z. Li, J.C. Tang, X. Wang, K.K. Fung, N. Cue, Surf. Sci. 494 (2001) 83. [4] E. Bauer, Y. Wei, T. Müller, A. Pavlovska, I.S.T. Tsong, Phys. Rev. B 51 (1995) 17891. [5] Ming Yang, Chun Zhang, Shijie Wang, Yuanping Feng, Ariando, AIP Adv. 1 (2011) 032111. [6] C.-L. Wu, J.-C. Wang, M.-H. Chan, T.T. Chen, S. Gwo, Appl. Phys. Lett. 83 (2003) 4530.
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[7] S. Gwo, C.-P. Chou, C.-L. Wu, Y.-J. Ye, S.-J. Tsai, W.-C. Lin, M.-T. Lin, Phys. Rev. Lett. 90 (2003) 185506. [8] C.-L. Wu, L.-J. Chou, S. Gwo, Appl. Phys. Lett. 85 (2004) 2071. [9] R.M. Feenstra, J.A. Stroscio, J. Vac. Sci. Technol., B 5 (1987) 923. [10] L. Ivanova, S. Borisova, H. Eisele, M. Dähne, A. Laubsch, Ph. Ebert, Appl. Phys. Lett. 93 (2008) 192110. [11] S.Y. Ren, W.Y. Ching, Phys. Rev. B 23 (1981) 5454. [12] Y.-N. Xu, W.Y. Ching, Phys. Rev. B 51 (1995) 17379. [13] M.-E. Grillo, S.D. Elliott, C. Freysoldt, Phys. Rev. B 83 (2011) 085208. [14] H.-M. Lee, C.-T. Kuo, H.-W. Shiu, C.-H. Chen, S. Gwo, Appl. Phys. Lett. 95 (2009) 222104.
[15] M. Yang, R.Q. Wu, W.S. Deng, L. Shen, Z.D. Sha, Y.Q. Cai, Y.P. Feng, S.J. Wang, J. Appl. Phys. 105 (2009) 024108. [16] J.W. Kim, H.W. Yeom, Phys. Rev. B 67 (2003) 035304. [17] J. Robertson, M.J. Powell, Appl. Phys. Lett. 44 (1984) 415. [18] J. Robertson, J. Appl. Phys. 54 (1983) 4490. [19] R. Kärcher, L. Ley, R.L. Johnson, Phys. Rev. B 30 (1984) 1896. [20] F. Bozso, Ph. Avouris, Phys. Rev. B 38 (1988) 3937. [21] C.M. Wang, X. Pan, M. Rühle, F.L. Riley, M. Mitomo, J. Mater. Sci. 31 (1996) 5281.
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Surface Science Letters
N2-plasma nitridation on Si(111): Its effect on crystalline silicon nitride growth Chung-Lin Wu a, b,⁎, Wei-Sheng Chen a, Ying-Hung Su a a b
Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan
a r t i c l e
i n f o
Article history: Received 6 February 2012 Accepted 6 March 2012 Available online 15 March 2012 Keywords: Silicon nitride N2 plasma Scanning tunneling microscopy/spectroscopy Epitaxy
a b s t r a c t This study uses a combination of atom-resolved scanning tunneling microscopic and spectroscopic (STM/STS) techniques to investigate the effects of N2-plasma nitridation on a crystalline β-Si3N4 ultrathin film grown on the Si(111) substrate. The proposed two-step growth process, including N2-plasma nitridation followed by vacuum annealing both at high temperature (~900 °C), can substantially improve the atomic and electronic structures of the β-Si3N4 having an atomically uniform morphology and stoichiometry. The effects of nitridation and post-annealing temperatures were examined by monitoring the morphological and electronic–structural evolutions of a non-stoichiometric surface. Moreover, the two-step N2-plasma nitridation process has extremely low activation energy and thus minimizes the thermal energy from the substrate for practical growth of β-Si3N4 ultrathin film. © 2012 Elsevier B.V. All rights reserved.
Silicon nitride (Si3N4) is a promising dielectric material for applications in present metal–insulator–semiconductor devices because of its fascinating fundamental properties such as large dielectric constant, wide band gap, and excellent strength [1]. Unlike conventional dielectric SiOx/Si system, Si3N4 has a crystalline β-phase, and its (0001) surface shows a good lattice match with the Si(111) surface. This makes β-Si3N4 more attractive for a further scaling Si-based device. It has been found that an ultrathin (~1.3 nm) β-Si3N4 layer can be grown by thermal nitridation methods established on an Si(111) substrate annealed at a high temperature of about 900 °C in various precursor nitrogen molecular environments, such as NH3, NO, and N2 [2–4]. However, previous attempts have failed to achieve high homogeneity in the surface morphology and stoichiometry of β-Si3N4 ultrathin film grown by thermal nitridation. Partial nitridation areas typically appeared on the β-Si3N4 surface [2,4], leading to a device breakdown or current leaking when using a β-Si3N4 ultrathin dielectric layer. To improve its homogeneity, a nitrogen plasma source is expected because of its higher kinetic energy and activity. Recent research shows that plasma-nitrided β-Si3N4 is a potential template for graphene-based electronics [5], and for growing high quality III-nitride quantum dots and magnetic nanoclusters on an Si substrate [6–8]. Thus, β-Si3N4 grown on silicon will likely enable the integration of low-dimensional functionalities on Si wafers with high performance. Despite these works have demonstrated that the nitrogen plasma is effective for β-Si3N4 grown on an Si(111) surface, due to the absence of atomic resolution results, it remains unclear if nitrogen plasma can
⁎ Corresponding author at: Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan. E-mail address: [email protected] (C.-L. Wu). 0039-6028/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2012.03.004
achieve epitaxial growth of β-Si3N4 without the partial nitridation area. It is also unclear if N2-plasma induces surface damage, or if a method exists for eliminating such damage. Furthermore, in previous nitridation processes, the β-Si3N4 crystallization energy was provided mainly by the thermal energy from the silicon substrate at 900 °C. This high-temperature growth process might limit the use of this crystalline dielectric layer in the standard semiconductor process. In this letter, we present a two-step growth process: short nitrogen plasma exposure followed by thermal annealing both at 900 °C, can substantially improve the β-Si3N4 quality evaluated by comprehensive microscopic and spectroscopic studies on the atomic scale. We also reveal a substantial recovery of N2-plasma-beam damage on low-temperature grown non-stoichiometric silicon nitride layer after brief annealing at elevated temperatures. Moreover, without high-temperature annealing, the energetic N2-plasma can achieve relatively low-temperature (700 °C) epitaxy of β-Si3N4 on Si substrate. The in situ atom-resolved investigations of N2-plasma nitridation on Si(111) substrate was performed in an ultrahigh vacuum growth chamber (with a base pressure of 2 × 10 − 10 Torr) equipped with a radio-frequency (rf) nitrogen plasma (ADDON), which is connected to the scanning tunneling microscope (STM) chamber by a gate valve. The Si(111) substrates were cleaned to remove the native oxide in the preparation chamber by overnight degassing at ~ 600 °C followed by several cycles of flash annealing to ~1150 °C for 30 s in a UHV environment. This substrate cleaning process produced a (7 × 7)-ordered Si(111) surface. The nitrogen plasma for silicon nitride growth was held constant at a constant rf power of 500 W and a chamber pressure of 5 × 10 − 5 Torr, which was correlated to an equivalent N2 flux of 0.2 sccm. Figure 1 summarizes representative STM/STS results on the (8 × 8)-reconstructed surface of a β-Si3N4(0001) ultrathin layer
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grown using the proposed two-step process at 900 °C beginning with a short plasma nitridation for 10 s followed by vacuum annealing for 1 min. The large filled-state STM images [Fig. 1(a) and its inset] both show that the surface is electronically identical and has no partial nitridation area. The monolayer step height of the β-Si3N4 (0001) terraces is about 3.3 ± 0.1 Å (calibrated by the Si(111)-7 × 7 step height), which is the same as that grown by thermal nitridation [2]. The reconstructed surface reveals clearly unit cells of 8/3 × 8/3 spots with a periodicity of ~10.2 Å. The zoom-in STM image in Fig. 1(b) shows that the 8/3 × 8/3 spots, and the 8 × 8 (30.7 Å × 30.7 Å) diamond-shape super-cells are visible with slightly detectable separations between the 8 × 8 cells. To quantify the electronic structures of β-Si3N4 (0001) surface in detail, we compare the tunneling-current computation with the combined tunneling spectra of the absolute current in a logarithmic display [log(I), Fig. 1(c)] and the normalized differential conductivity [(dI/dV)/(I/V), Fig. 1(d)] as a function of the sample bias. The tunneling spectra were acquired simultaneously in I(V) and AC modulated lock-in amplifier signal channels, and an average of 30–50 such measurements were taken. The log(I)-V plot accentuates the current variations to clearly reveal the gap edges and identify the origins of the tunneling current. To fit the log(I)-V plot, the tunneling current was calculated in a manner similar to prior determinations [9,10]. This calculation integrates over all states between the metal tip and the sample Fermi-level times the transmission coefficient, which is estimated using WKB approximation. To match the computations to the experiments, several main parameters were set in the calculation: a band gap energy of about 5 eV for intrinsic β-Si3N4 [11–13], a work function of 4.5 eV for the tungsten tip, a tip-sample separation of 0.6 nm, and a tunneling area of 1 nm 2. A comparison is shown in Fig. 1(c), following the calculated tunneling current curves of conduction band (IC) and valence band (IV), the log(I)-V and (dI/dV)/(I/V) curves both display a clear conduction band minimum (CBM) at
+2.5 V and a valence band maximum (VBM) at −2.7 V with respect to the Fermi edge (EF = 0 V), which matched quite well with the PES measurements and the density of state (DOS) calculations for βSi3N4(0001)/Si(111) heterojunction [14,15]. This good match supports our identification of the band edges and indicates negligible tip-induced band bending. However, to achieve a better fit with the experimental spectra, two additional current contributions were introduced at bias V b − 3 V for IN and at bias V > 1 V for Igap. The IN current contribution can be solely attributed to the nitrogen dangling bond state, which was successfully identified by previous computational and experimental results [2,12,16]. The Igap tail extending out from the CB was also found to arise from a Si dangling bond state at the rest layer [16,17]. These identifications of tunneling spectrum make it possible to examine the effects of annealing temperature on the surface states related to the crystallization of an ultrathin non-stoichiometric silicon nitride film, which was formed by briefly exposing the Si(111)-7 × 7 surface to the N2-plasma (10 s) at room temperature. As expected, this RT grown ultrathin silicon nitride layer showed the nonstoichiometric phase (SiNx) having an atom-disordered surface [Fig. 2(a)]. Compared with the computational tunneling spectrum, the experimental spectrum revealed that the SiNx had no apparent valence band edge of β-Si3N4, but strong tunneling currents at below − 3 V and above +3 eV. This could be attributed to numerous
I Si IC I Gap IV 10nm Sample Bias (eV)
I Si IN
IC I Gap
IV 10nm Sample Bias (eV)
RT-Plasma SiN x annealed at 900 °C
Fig. 1. (a) Large-scale occupied-state STM image (80 nm × 80 nm) of a β-Si3N4(0001)(8 × 8) surface formed by two-step nitridation process with a monoatomic high step measured at sample bias Vs = − 4.0 eV and tunneling current It = 0.8 nA. Inset shows a zoom-out image acquired in area of 500 nm × 500 nm. (b) Zoom-in STM image shows the (8 × 8) and (8/3 × 8/3) unit cells composed of nitrogen adatoms, and the long diagonal of the unit cells align along the [11–2] direction of the Si(111) substrate. (c) Logarithmic display of the tunnel current as a function of voltage obtained at the (8 × 8)-reconstructed surface. Experimental curves are shown by open circles, and theoretical fits are shown by dash and solid lines. The valence band (IV), conduction band (IC), N-adatom (IN), and gap state (IGap) tunneling currents are indicated. The positions of the valence (EVBM) and conduction band edges (ECBM) can be observed in the spectrum. (d) Normalized differential conductivity (dI/dV)/(I/V) as a function of voltage. The spectral peak associated with the N adatoms at around − 4.2 eV is marked by Nadatom. The valence and conduction band onsets show clearly and match well with a band gap of ~ 5.0 eV in this spectrum.
IN I Gap IC
IV 10nm Sample Bias (eV)
Fig. 2. STM images (50 nm× 50 nm) (left) and the corresponding tunneling spectra (right) with fitting curves for different annealing temperature of RT-grown SiNx: (a) initial SiNx atom-disordered surface after 10 s exposure of N2-plasma on Si(111)(7× 7) surface at RT; (b) after 1 min vacuum annealing at 600 °C; (c) after 1 min vacuum annealing at 900 °C. The IV, IC, IN, IGap tunneling currents and the additional Si surface dangling bond (ISi) tunneling current are indicated in the fitting results.
N adatom (IN) and Si dangling bond (ISi) states on the surface, respectively [18–20]. After brief annealing to 600 °C (1 min) to recrystallize the SiNx layer, Fig. 2(b) reveals a slightly atom-ordered surface. The corresponding tunneling spectrum indicates that the 600 °C annealing promotes the appearance of band edge structures of β-Si3N4, but does not completely eliminate the Si surface dangling bonds (ISi). At a higher annealing temperature of 900 °C (1 min), the SiNx vanished and the surface revealed two crystalline regions with 7 × 7 and 8 × 8 surface reconstructions, distinguished by two levels of brightness [Fig. 2(c)]. The tunneling spectrum obtained over the “dark” 8 × 8 area shows an insulating nature with characteristic β-Si3N4 band edges but no Si surface dangling bonds. This is nearly identical to the characteristics of β-Si3N4 grown by two-step N2-plasma nitridation with clean (8 × 8)-reconstruction [Fig. 1(c)]. The tunneling spectra shown here are quite suitable for monitoring the N2-plasma induced damage of silicon nitride film during device fabrication. These results imply that the defects generated by a low-temperature nitridation process might be suppressed and that the crystalline quality could be significantly improved during high-temperature annealing above 600 °C. To gain quantitative insights into the N2-plasma assisted two-step growth for β-Si3N4 thin film, Fig. 3 plots the nitridation–temperature dependent mean growth rate derived from the β-Si3N4 8 × 8 reconstruction area (average over 1000 nm 2 scanning size) during 10 s nitridation and 1 min post-annealing processes. The temperaturedependent growth rate R can be described by an Arrhenius equation R = R0 exp(− EA/kBT) with an activation energy of EA = (80 ± 20) meV (~7.75 J/Mol), where T is the nitridation temperature and kB is the Boltzmann constant. This apparent activation energy represents the sum of energies required for epitaxial seeds formation and nucleation in the two-step growth process of β-Si3N4. The apparent activation energy is about two orders lower than the formation energy of β-Si3N4 (~730 kJ/Mol) [21]. This means that most of the reactive gas species were plasma, which mainly contributed to the formation of β-Si3N4 epitaxial seeds and thus minimize the thermal active process. Furthermore, without post annealing, Fig. 4(a) shows that β-Si3N4 film grown by briefly exposing Si(111) substrate to N2-plasma (10 s) at 700 °C is covered with numerous adatom clusters. The surface and electronic structures of this film are essentially identical to the two-step grown β-Si3N4 (0001) surface, as indicated by the pronounced (8× 8)-reconstruction and wide band-gap tunneling spectrum [(dI/dV)/(I/V), Fig. 4(b)]. According to the observed spectroscopic contrast of the small adatom clusters and the silicon back-band state at
R (nm2/s)
10 nm
E A = (80±20) meV
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Fig. 4. (a) Large-scale (100 nm × 50 nm) STM image of a β-Si3N4(0001)-(8 × 8) surface formed by direct plasma nitridation at 700 °C. Adsorbed Si adatoms can be observed in the image. (b) Characteristic (dI/dV)/(I/V)-V spectrum taken on the (8 × 8)-surface. (c) The (dI/dV)/(I/V)-V spectrum shows the gapless feature taken on the Si cluster region, indicated by the dashed circle in the STM image (a).
around −2 V [Fig. 4(c)], we conclude that the nonreactive silicon atoms on β-Si3N4 surface often results in clusters and would deposit at elevated substrate temperature. This thus suggests the possibility of a new process design for growing a crystalline Si3N4 ultrathin layer through low-temperature N2-plasma incorporation to avoid harmful surface reactions such as ion bombardment or ion implantation during high-temperature growth. In summary, the atomically morphological and spectroscopic results of this study show that an ultrathin crystalline β-Si3N4 layer can be uniformly grown without surface defects and clusters using two-step plasma nitridation process at high temperature. Lowtemperature nitrided non-stoichiometric silicon nitride thin films reveal numerous Si dangling bonds and no apparent valence band, which can be improved at an elevated nitridation temperature and during brief vacuum annealing. Very low activation energy is founded by observing the morphological evolution of non-stoichiometric silicon nitride films grown at different nitridation temperatures. This makes β-Si3N4 ultrathin layer possible to directly grow using N2-plasma at relatively low substrate temperature without post annealing process. Acknowledgment
50 nm
This work was supported by the National Science Council in Taiwan. References
50 nm
1000/T (K-1) Fig. 3. Mean growth rate R of β-Si3N4 ultrathin film as a function of reciprocal nitridation temperature with same post-annealing process. Characteristic STM images having low (8 × 8 reconstruction) and high (7 × 7 reconstruction) brightness regions are shown in right reflecting the change in β-Si3N4 fraction at different nitridation temperatures.
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