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Inelastic electron tunnelling spectroscopy in N-MOS junctions with ultra-thin gate oxide
Solid-State Electronics 47 (2003) 1663–1668 www.elsevier.com/locate/sse
Inelastic electron tunnelling spectroscopy in N-MOS junctions with ultra-thin gate oxide C. Petit a, G. Salace a
a,*
, D. Vuillaume
b
Laboratoire d’Analyse des Solides, Surfaces et Interfaces, LASSI-DTI, UMR CNRS 6107, Universit e de Reims, BP 1039, 51687 Reims Cedex 2, France b Institut d’Electronique et de Micro electronique du Nord, IEMN, UMR CNRS 8520, avenue Poincar e, BP 69, 59652 Villeneuve d’Ascq Cedex, France
Abstract The inelastic electron tunnelling spectroscopy (IETS) technique is performed to provide information concerning the vibrational and excitational modes present in MOS tunnelling junctions (molecular species present in silicon dioxide and phonon modes of both silicon substrate and silicon dioxide). The first relevant structures of silicon substrate phonons are investigated in both (1 1 1) and (1 0 0) orientations for the first time. The separated peaks obtained after deconvolution are identified and compared with literature data for both silicon orientations. The IET spectrum of ultrathin silicon dioxide is also obtained though this spectrum is more difficult to detect. Finally, two different technologies of manufacturing MOS tunnelling junctions are compared. 2003 Elsevier Ltd. All rights reserved.
1. Introduction
2. Test devices
With the drastic reduction of new gate oxide up to tunnelling thickness (3–1.5 nm), it becomes interesting to apply the inelastic electron tunnelling spectroscopy (IETS) method to the last existing oxide process of MOS generation in microelectronics. A tunnelling spectrum reveals mainly the vibration energies of molecules included between the electrodes and phonons modes for the tunnel barrier and electrodes (gate and semiconductor substrate). A vibration energy of quantum hm results in a peak at bias voltage V ¼ hm=e in the second derivative of the characteristics I–V at low temperature (typically at the liquid helium temperature T ¼ 4:2 K). In this paper, the IETS method, which has given a lot of results in MIM [1] (metal–insulator–metal junctions), has been used with tunnel MOS junctions of last technologies.
The MOS devices studied were Al–SiO2 –Si (nþ or pþ ) capacitors on highly doped Si samples provided by Siltronix (doping level typically ¼ 4.7 · 1019 cm3 ). Ultrathin silicon dioxide films were grown by two different thermal oxidations. The first one is performed in the IEMN of Lille. Ultra-thin silicon oxide films were grown by thermal oxidation in dry O2 at 600 C for 4 min (oxide thickness 2.1 nm) followed by a post-oxidation anneal (POA) in dry N2 at 600 C for 30 min in the same oven. After thermal oxidation, aluminum gates were deposited (thickness 300 nm) by e-beam evaporation (using an electron gun in a 108 Torr vacuum chamber) through shadow masks to define square electrodes with areas of 1 · 102 and 3.6 · 103 cm2 . Both the (1 1 1) and (1 0 0) silicon orientations, called A111 and A100 , have been investigated. The second thermal oxidation is performed by the LETI in Grenoble. Silicon oxide films, of thickness 1.8 nm, were grown by a rapid thermal oxidation (RTO) process, followed by deposition of the 500 nm aluminum gate. The aluminium gate has been preferred (rather than the usual nþ -polysilicon gate) in order to avoid the additional voltage drop due to quantum effects in the
*
Corresponding author. Tel.: +33-3-26-91-3364; fax: +33-326-91-3312. E-mail address: [email protected] (G. Salace).
0038-1101/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-1101(03)00179-5
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polysilicon gate. Only (1 0 0) (B100 ) silicon orientation has been investigated. The inelastic electron tunnelling (IET) spectra were performed at liquid helium temperature. These spectra were obtained by measuring the second derivatives of the tunnelling current through the junction by a lock-in amplifier detection technique, with an AC modulation signal (f ¼ 893 Hz, Vx ¼ 2 mV) and a slowly varying DC voltage applied to the junction (described in a previous paper [2]). The resistances of tunnel junctions are in the magnitude order of 100–800 X corresponding to a well-adapted impedance in our derivative setting. By pumping in cryostat, above the liquid helium phase, it is possible to lower the bath temperature from 4.2 K down to 1.9 K in order to improve the spectra resolution. But, as outlined by Klein et al. [3], usually the Vx modulation voltage is more serious limitation than the kT limitation.
3. Experimental results Fig. 1 shows an IET spectrum after background removal in an energy range from 0 to 200 meV (0 to 1613 cm1 ), obtained by averaging several scans from an Al/ SiO2 /Si (nþ ) sample elaborated with the first thermal oxidation process (in Lille) on the (1 1 1) orientation. We can distinguish three regions with specific infrared vibration modes: • strong structures that correspond to phonons in the Si substrate in the first energy range (35–80 meV corresponding to 282–645 cm1 ); • a smaller band structure corresponding to the phonons of the SiO2 barrier in the range 130–180 meV (1049–1452 cm1 );
• small structures sometimes attributed to molecular vibrations present in the tunnel barrier in the other ranges of energy (80–130 meV (645–1049 cm1 ), and higher than 180 meV (1452 cm1 )). 3.1. Silicon substrate phonons For the (1 1 1) silicon orientation, we have deconvolved essentially the phonon spectrum into three dominant peaks already assigned in the paper quoted above [2] and compared, for the first time, in the Fig. 2 with an averaged spectrum obtained with the microelectronics (1 0 0) silicon orientation. The recent work of Lye et al. [4] and that of Balk et al. [5] and Bencuya [6] had previously established the assignment of these peaks in (1 0 0) orientation. 3.1.1. (1 1 1) orientation The optical phonons at exactly 65.2 mV (525.9 cm1 ) due to k ¼ 0 are the strongest structures in the inelastic spectra and the energy positions are in good agreement with the literature for (1 1 1) orientation [7]. The peak at 53.7 mV (433 cm1 ) and the peak at 47.5 mV (383 cm1 ) may be assigned respectively to the LO and LA phonons of a wave vector k at the Brillouin zone boundary in the (1 1 1) direction (L point). In our knowledge, these two last peaks are evidenced in IET spectra for the first time, and their energies are in good agreement with neutron scattering data [8]. The deconvolution of this spectrum is shown in Fig. 3. A fourth peak embedded in the vibration band structure nearly to 60 meV (484 cm1 ), may correspond to the phonon mode excitation in no specular wave vector transmission relative to the (1 0 0) direction [9]. This weak mode does not appear in our
ω
þ
Fig. 1. IET spectrum of an Al/SiO2 /Si(n ) tunnel junction for silicon orientation (1 1 1).
Fig. 2. IET spectrum of an Al/SiO2 /Si(nþ ) tunnel junction. Curves labelled A100 and A111 correspond to (1 0 0) and (1 1 1) silicon orientation.
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Fig. 3. Deconvolution of silicon phonon modes of spectrum A111 in Fig. 2 for (1 1 1) silicon orientation.
Fig. 4. Deconvolution of silicon phonon modes of spectrum A100 in Fig. 2 for (1 0 0) silicon orientation.
previous paper in reason of too large modulation. The peak appearing as a shoulder at 69.7 meV (562 cm1 ) is not identified. Table 1 shows the characteristic energy peak positions of (1 1 1) silicon phonons.
attributed to R01 or D1 LA mode. A D02 LO phonon mode is assigned to the peak according at 51.1 mV (412 cm1 ) (53.6 meV in [4]). The peak appearing as a shoulder at 64.4 meV (519 cm1 ) (63.4 meV in [4]) may be due to a rocking mode LO phonon of the SiO2 barrier [13,14], and a similar SiO2 TO phonon mode appears at 55.6 meV (448 cm1 ); this mode, no present in [4], occurs in our spectrum. The small differences (0–3.4 mV) may be accounted for the inaccuracy of deconvolution operation. The solid curve is the original data and the dashed curve is the sum of the extracted modes. The difference is not perceptible. Table 2 shows the characteristic energy peak positions of (1 0 0) silicon phonons in and some SiO2 phonons peaks occurring in the same energy range. It is worth outlining that the energy positions of some phonon modes change and shift after applying an electrical stress, whereas the main peak due to the optical phonon in the Brillouin zone centre or (1 0 0) orientation remains unaffected. These changes could reflect an alteration of the first silicon monolayers arrangement.
3.1.2. (1 0 0) orientation The IET phonon spectrum for (1 0 0) orientation has been deconvolved after subtraction of the inelastic tunnelling background. This deconvolution in the range of 30–70 meV (242–565 cm1 ), Fig. 4, in six separated peaks, is very near that one obtained by Lye et al. [4]. These distinguishing peaks have been obtained thanks to a best resolution (Vx ¼ 2 mV). The most intense phonon mode located at 60 meV (484 cm1 ) (59.4 meV in [4]) is resultant from the TO mode associated with conduction band minima in the (1 0 0) direction. This strong mode has also been reported by Kulda et al. [10], and infrared absorption measurements performed by Balkanski and Nazarewicz [11]. Again, the comparison with neutron scattering data is in agreement with margin of error of the neutron data, which is typically a few mV [8]. First, the peak at 36.6 meV (295 cm1 ) (36.8 meV in [4]), is assigned by Lye et al. [4] to R1 acoustic and R3 optical modes toward the K point, but may also correspond to the phosphorus donor ionization energy [12]; then, the peak located at 44 mV (355 cm1 ) (43.6 mV in [4]) is
3.2. Silicon dioxide phonons Fig. 5 shows the phonon vibration band of the SiO2 (or SiOx ) tunnelling barrier in range (140–170 meV) (1129–1371 cm1 ). The deconvolution of this band in
Table 1 Characteristic energy of silicon phonons for the (1 1 1) orientation L point
C point
LA mode (meV)
LO mode (meV)
TO mode (meV)
TO mode (meV)
47.5 45 –
53.7 54 –
59.6 60 –
65.2 64.5 64.5
This work [9] [7]
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Table 2 Characteristic energy of silicon and some SiO2 phonons in the same energy range of silicon phonons for the (1 0 0) orientation Phosphorous donor or R1 accoustic and R3 optic toward K point (meV)
R01 or D1 LA mode in X point (meV)
D02 LO mode toward X point (meV)
R1 or R2 TO mode toward X point (meV)
Rocking TO1 mode in SiO2 (meV)
Rocking LO1 mode in SiO2 (meV)
36 36.8 36.8 – – – –
44 43.6 – – – – –
51.1 53.6 – – – – –
60 59.4 – – 57.8 60 –
55.6 – – 56.7 – – 56.6
64.4 63.4 – 62.8 – 64 –
This work [4,13] [12] [14] [15] [5] [21]
port studies, mean free paths in the range from 0.7 to 1.5 nm are deduced. The oxide thickness is barely larger than these mean free paths, reducing the inelastic tunnelling interaction in the silicon dioxide. It is not the case for this inelastic effect in the silicon bulk where the interaction probability is higher and not also the case for molecular vibration in the gate oxide, because the size of these molecules. For this purpose, a simple model has been presented by Langan and Hansma [17] which suggest that the dependence of peak intensity on surface concentration will vary with the size of the dopant molecules.. The ballistic transport is, in our opinion, the main reason why silicon dioxide phonons are always more difficult to detect by IETS than those of silicon substrate or gate. Fig. 5. IET spectrum and deconvolution of silicon dioxide modes.
four specific peaks (TO3 , LO4 , TO4 , LO3 vibration modes of SiO2 ) [14] and a molecular vibration (P–O mode at 165.6 mV (1335 cm1 ) due to the phosphorus doping level) had been accurately made [4] and found again in our first paper with a slight shift in energy [2]. Here, in the result shown in Fig. 5, all SiO2 (or SiOx ) phonon spectrum is shifted towards high bias of an voltage amount of about 10 mV (with an inaccuracy of 1.5 mV due to deconvolution). This additional voltage is unfortunately due to a bad back electrical contact. The phonons of the SiO2 (or SiOx ) barrier are more often present in the IET spectra of the (1 1 1) substrate orientation than in the (1 0 0) orientation. This is may be due to the low intensity of this vibration band corresponding to the small relative conductance change of about 1% for the LO3 mode relatively to 6% for the strong structure of silicon optical phonon. An another reason for the difficulty to observe the SiO2 (or SiOx ) phonon spectrum, is the very thin (1–5 nm) used in new microelectronics devices. Ballistic transport is observed in Fischetti et al. work [16]. From these ballistic trans-
3.3. Molecular vibrations What is the capability of IETS to identify specific molecular vibrations located at the critical Si–SiO2 interface? The strongly rising elastic background of the conductance limits, at high bias voltage, the observation of inelastic tunnelling peaks. Nevertheless, we can risk an identification in the first half-infrared energy range (30–250 meV corresponding to 242–2016 cm1 ). Fig. 6 shows five spectra of a tunnelling junction elaborated in Grenoble, we can see a good reproducibility of these tunnelling spectra. Deconvolved modes in the range between 40 and 140 mV (322–1129 cm1 ) are given in Fig. 7. We have interpreted the 78.6 meV (634 cm1 ) peak as a Si–C vibrational mode, the peak at about 88.8 meV (716 cm1 ) as the energy excitation of Si–H wagging vibration, the peak at 103.3 meV (833 cm1 ) as the excitation of the Si–N bending vibration and the peak at 113 meV (911 cm1 ) as the excitation of the Si3 –Si–H (or O3 –Si–H) bending vibration, this vibration is very near in energy of the bending mode of Al–O–H mode currently observed at 110 mV with Al–Al2 O3 –Pb junction in IETS, several years ago [1]. The silicon element being neighbouring of aluminium in Mendeleiff table, the
C. Petit et al. / Solid-State Electronics 47 (2003) 1663–1668
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dI / dV (millimhos)
0.35
ω
0.30
0.25
0.20
0.15 0
20
40
60
80
100
120
VG (mV)
Fig. 8. Conductance and conductance jump Dr=r of an Al/ SiO2 /Si(nþ ) tunnel junction for (1 0 0) silicon orientation.
Fig. 6. Five IET spectra of an Al/SiO2 /Si(nþ ) tunnel junction for (1 0 0) silicon orientation.
the quantum of vibrational energy of the molecules. In Fig. 8, we shows the conductance of this device, determined by the fist derivative dI==dV vs VG of the IG –VG curve, and the variation Dr=r vs VG . It is easy to evaluate the jump Dr=r in the conductance for the silicon modes phonons. For the most intense phonon mode located at 60 mV (resultant from the TO mode in the (1 0 0) direction), the jump Dr=r amountsaround 2.7%. The quantity Dr=r is related [20] with the observed IG –VG characteristics by the relation: Z V 00 Dr 1 d2 I V dV ð1Þ r I dV 2 V0 where V 0 and V 00 are the starting and the ending voltages of the variation band under consideration. This relation may be used to quantify the strength of the vibration modes. Table 3 gives the jump Dr=r of conductance r for the vibrational modes r described above and, consequently, an idea of relative peak intensities.
Fig. 7. IET spectrum of an Al/SiO2 /Si(nþ ) tunnel junction for (1 0 0) silicon orientation.
4. Conclusion Si–O–H bending mode is expected to roughly at the same energy [1,18,19]. Both latter peaks had been previously identified [2] in the sample elaborated in Lille. Yanson et al. [20] showed that the intensity of the inelastic process is characterized by the relative jump Dr=r in conductance at voltage VG ¼ hx=e, where hx is
In this paper, the IET lattice vibration spectra were obtained for two silicon orientations. For each orientation, the phonon bands were deconvoluted in separate phonon modes with accuracy. These modes were assigned in energy in agreement with literature data. The
Table 3 Energy and jump of conductivity Dr=r for different vibrational modes of an Al/SiO2 /Si(nþ ) tunnel junction for (1 0 0) silicon orientation Vibrational modes
TO phonon mode
Si–C
Wagging Si–H
Si–N
Bending Si–OH
Energy (meV) Wave number (cm1 ) Dr=r (%)
60 484 2.7
78.6 633 0.49
89.1 718 0.6
103.3 833 1.8
113 911 0.51
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band vibration analysis of SiO2 grown on (1 1 1) silicon is very near than LyeÕs investigation on (1 0 0) orientation. Slight series resistance has shifted the peak positions. The interest of this IETS technique is to compare the IET spectra of samples elaborated according to several technologies. Our results, extending those of other authors, demonstrate that it is possible to obtain additional information as impurities due to contaminants accidentally incorporated during the fabrication process of the MOS devices. With the advent ultra-thin oxide gate, the ability to probe the good quality of these tunnel oxides in as-fabricated devices, makes IETS a powerful aid in microelectronics for study these devices. The authors are indebted to C. LEROUX for LETI sample elaboration. The RMNT Network under the ‘‘ULTIMOX’’ project has financially supported this work.
References [1] Hansma PK, editor. Tunnelling spectroscopy. New York: Plenum Press; 1982. [2] Salace G, Petit C, Vuillaume D. J Appl Phys 2002;91(9): 5896. [3] Klein J, Leger A, Belin M, Dufourneau D, Sangster MJL. Phys Rev B 1972;7:2336. [4] Lye WK, Hasegawa E, Ma TP, Barker RC, Hu Y, Kuehun J, et al. Appl Phys Lett 1997;71:2523.
[5] Balk P, Do-Thanh L, Ewert S, Kuball M, Schmitz S. Appl Surf Sci 1987;30:304. [6] Bencuya I. Electron tunnel-oxide-degenerate silicon junctions. PhD thesis, U.M.I Dissertation information Service. 1984. [7] Wolf EL. Phys Rev Lett 1968;20(5):204. [8] Brockhouse BN. Phys Rev Lett 1959;2(6):256. [9] Kovchavtsev AP, Kurishev GL, Postnikov KO, Sokolov RA, Subbotin IM. Phys Stat Sol 1988;106:669. [10] Kulda J, Strauch D, Davoine P, Ishi€ı Y. Phys Rev B 1994;50:1347. [11] Balkanski M, Nazarewicz W. J Phys Chem Solids 1961;23: 573. [12] Fistul VI. Heavily doped semiconductors. New York: Plenum Press; 1969. [13] Lye WK, Ma TP, Barker RC, Hasegawa E, Hu Y, Kuehun J, et al. In: Seiler DG, Diebold AC, Bullis WM, Shaffner TJ, McDonald R, Walters EJ, editors. Characterization and metrology for ULSI technology: 1998 International Conference. [14] Kirk CT. Phys Rev B 1988;38:1255. [15] Asche M, Sarbei OG. Phys Stat Sol B 1981;103:11. [16] Fischetti MV, Dimaria DJ, Dori L, Bajey J, Tierney E, Stasiak J. Phys Rev B 1987;35:4404. [17] Langan JD, Hansma PK. Surf Sci 1975;52:211. [18] Jaklevic RC, Lambe J. Phys Rev Lett 1966;17(22):1139. [19] Simonsen MG, Coleman RV, Hansma PK. J Chem Phys 1974;9:3789. [20] Yanson IK, Bogatina NI, Verkin BJ, Shklyarevskii OI. Soviet Phys JETP 1972;35(3):540. [21] Lange P. J Appl Phys 1989;66:201.
Inelastic electron tunnelling spectroscopy in N-MOS junctions with ultra-thin gate oxide C. Petit a, G. Salace a
a,*
, D. Vuillaume
b
Laboratoire d’Analyse des Solides, Surfaces et Interfaces, LASSI-DTI, UMR CNRS 6107, Universit e de Reims, BP 1039, 51687 Reims Cedex 2, France b Institut d’Electronique et de Micro electronique du Nord, IEMN, UMR CNRS 8520, avenue Poincar e, BP 69, 59652 Villeneuve d’Ascq Cedex, France
Abstract The inelastic electron tunnelling spectroscopy (IETS) technique is performed to provide information concerning the vibrational and excitational modes present in MOS tunnelling junctions (molecular species present in silicon dioxide and phonon modes of both silicon substrate and silicon dioxide). The first relevant structures of silicon substrate phonons are investigated in both (1 1 1) and (1 0 0) orientations for the first time. The separated peaks obtained after deconvolution are identified and compared with literature data for both silicon orientations. The IET spectrum of ultrathin silicon dioxide is also obtained though this spectrum is more difficult to detect. Finally, two different technologies of manufacturing MOS tunnelling junctions are compared. 2003 Elsevier Ltd. All rights reserved.
1. Introduction
2. Test devices
With the drastic reduction of new gate oxide up to tunnelling thickness (3–1.5 nm), it becomes interesting to apply the inelastic electron tunnelling spectroscopy (IETS) method to the last existing oxide process of MOS generation in microelectronics. A tunnelling spectrum reveals mainly the vibration energies of molecules included between the electrodes and phonons modes for the tunnel barrier and electrodes (gate and semiconductor substrate). A vibration energy of quantum hm results in a peak at bias voltage V ¼ hm=e in the second derivative of the characteristics I–V at low temperature (typically at the liquid helium temperature T ¼ 4:2 K). In this paper, the IETS method, which has given a lot of results in MIM [1] (metal–insulator–metal junctions), has been used with tunnel MOS junctions of last technologies.
The MOS devices studied were Al–SiO2 –Si (nþ or pþ ) capacitors on highly doped Si samples provided by Siltronix (doping level typically ¼ 4.7 · 1019 cm3 ). Ultrathin silicon dioxide films were grown by two different thermal oxidations. The first one is performed in the IEMN of Lille. Ultra-thin silicon oxide films were grown by thermal oxidation in dry O2 at 600 C for 4 min (oxide thickness 2.1 nm) followed by a post-oxidation anneal (POA) in dry N2 at 600 C for 30 min in the same oven. After thermal oxidation, aluminum gates were deposited (thickness 300 nm) by e-beam evaporation (using an electron gun in a 108 Torr vacuum chamber) through shadow masks to define square electrodes with areas of 1 · 102 and 3.6 · 103 cm2 . Both the (1 1 1) and (1 0 0) silicon orientations, called A111 and A100 , have been investigated. The second thermal oxidation is performed by the LETI in Grenoble. Silicon oxide films, of thickness 1.8 nm, were grown by a rapid thermal oxidation (RTO) process, followed by deposition of the 500 nm aluminum gate. The aluminium gate has been preferred (rather than the usual nþ -polysilicon gate) in order to avoid the additional voltage drop due to quantum effects in the
*
Corresponding author. Tel.: +33-3-26-91-3364; fax: +33-326-91-3312. E-mail address: [email protected] (G. Salace).
0038-1101/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-1101(03)00179-5
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polysilicon gate. Only (1 0 0) (B100 ) silicon orientation has been investigated. The inelastic electron tunnelling (IET) spectra were performed at liquid helium temperature. These spectra were obtained by measuring the second derivatives of the tunnelling current through the junction by a lock-in amplifier detection technique, with an AC modulation signal (f ¼ 893 Hz, Vx ¼ 2 mV) and a slowly varying DC voltage applied to the junction (described in a previous paper [2]). The resistances of tunnel junctions are in the magnitude order of 100–800 X corresponding to a well-adapted impedance in our derivative setting. By pumping in cryostat, above the liquid helium phase, it is possible to lower the bath temperature from 4.2 K down to 1.9 K in order to improve the spectra resolution. But, as outlined by Klein et al. [3], usually the Vx modulation voltage is more serious limitation than the kT limitation.
3. Experimental results Fig. 1 shows an IET spectrum after background removal in an energy range from 0 to 200 meV (0 to 1613 cm1 ), obtained by averaging several scans from an Al/ SiO2 /Si (nþ ) sample elaborated with the first thermal oxidation process (in Lille) on the (1 1 1) orientation. We can distinguish three regions with specific infrared vibration modes: • strong structures that correspond to phonons in the Si substrate in the first energy range (35–80 meV corresponding to 282–645 cm1 ); • a smaller band structure corresponding to the phonons of the SiO2 barrier in the range 130–180 meV (1049–1452 cm1 );
• small structures sometimes attributed to molecular vibrations present in the tunnel barrier in the other ranges of energy (80–130 meV (645–1049 cm1 ), and higher than 180 meV (1452 cm1 )). 3.1. Silicon substrate phonons For the (1 1 1) silicon orientation, we have deconvolved essentially the phonon spectrum into three dominant peaks already assigned in the paper quoted above [2] and compared, for the first time, in the Fig. 2 with an averaged spectrum obtained with the microelectronics (1 0 0) silicon orientation. The recent work of Lye et al. [4] and that of Balk et al. [5] and Bencuya [6] had previously established the assignment of these peaks in (1 0 0) orientation. 3.1.1. (1 1 1) orientation The optical phonons at exactly 65.2 mV (525.9 cm1 ) due to k ¼ 0 are the strongest structures in the inelastic spectra and the energy positions are in good agreement with the literature for (1 1 1) orientation [7]. The peak at 53.7 mV (433 cm1 ) and the peak at 47.5 mV (383 cm1 ) may be assigned respectively to the LO and LA phonons of a wave vector k at the Brillouin zone boundary in the (1 1 1) direction (L point). In our knowledge, these two last peaks are evidenced in IET spectra for the first time, and their energies are in good agreement with neutron scattering data [8]. The deconvolution of this spectrum is shown in Fig. 3. A fourth peak embedded in the vibration band structure nearly to 60 meV (484 cm1 ), may correspond to the phonon mode excitation in no specular wave vector transmission relative to the (1 0 0) direction [9]. This weak mode does not appear in our
ω
þ
Fig. 1. IET spectrum of an Al/SiO2 /Si(n ) tunnel junction for silicon orientation (1 1 1).
Fig. 2. IET spectrum of an Al/SiO2 /Si(nþ ) tunnel junction. Curves labelled A100 and A111 correspond to (1 0 0) and (1 1 1) silicon orientation.
C. Petit et al. / Solid-State Electronics 47 (2003) 1663–1668
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Fig. 3. Deconvolution of silicon phonon modes of spectrum A111 in Fig. 2 for (1 1 1) silicon orientation.
Fig. 4. Deconvolution of silicon phonon modes of spectrum A100 in Fig. 2 for (1 0 0) silicon orientation.
previous paper in reason of too large modulation. The peak appearing as a shoulder at 69.7 meV (562 cm1 ) is not identified. Table 1 shows the characteristic energy peak positions of (1 1 1) silicon phonons.
attributed to R01 or D1 LA mode. A D02 LO phonon mode is assigned to the peak according at 51.1 mV (412 cm1 ) (53.6 meV in [4]). The peak appearing as a shoulder at 64.4 meV (519 cm1 ) (63.4 meV in [4]) may be due to a rocking mode LO phonon of the SiO2 barrier [13,14], and a similar SiO2 TO phonon mode appears at 55.6 meV (448 cm1 ); this mode, no present in [4], occurs in our spectrum. The small differences (0–3.4 mV) may be accounted for the inaccuracy of deconvolution operation. The solid curve is the original data and the dashed curve is the sum of the extracted modes. The difference is not perceptible. Table 2 shows the characteristic energy peak positions of (1 0 0) silicon phonons in and some SiO2 phonons peaks occurring in the same energy range. It is worth outlining that the energy positions of some phonon modes change and shift after applying an electrical stress, whereas the main peak due to the optical phonon in the Brillouin zone centre or (1 0 0) orientation remains unaffected. These changes could reflect an alteration of the first silicon monolayers arrangement.
3.1.2. (1 0 0) orientation The IET phonon spectrum for (1 0 0) orientation has been deconvolved after subtraction of the inelastic tunnelling background. This deconvolution in the range of 30–70 meV (242–565 cm1 ), Fig. 4, in six separated peaks, is very near that one obtained by Lye et al. [4]. These distinguishing peaks have been obtained thanks to a best resolution (Vx ¼ 2 mV). The most intense phonon mode located at 60 meV (484 cm1 ) (59.4 meV in [4]) is resultant from the TO mode associated with conduction band minima in the (1 0 0) direction. This strong mode has also been reported by Kulda et al. [10], and infrared absorption measurements performed by Balkanski and Nazarewicz [11]. Again, the comparison with neutron scattering data is in agreement with margin of error of the neutron data, which is typically a few mV [8]. First, the peak at 36.6 meV (295 cm1 ) (36.8 meV in [4]), is assigned by Lye et al. [4] to R1 acoustic and R3 optical modes toward the K point, but may also correspond to the phosphorus donor ionization energy [12]; then, the peak located at 44 mV (355 cm1 ) (43.6 mV in [4]) is
3.2. Silicon dioxide phonons Fig. 5 shows the phonon vibration band of the SiO2 (or SiOx ) tunnelling barrier in range (140–170 meV) (1129–1371 cm1 ). The deconvolution of this band in
Table 1 Characteristic energy of silicon phonons for the (1 1 1) orientation L point
C point
LA mode (meV)
LO mode (meV)
TO mode (meV)
TO mode (meV)
47.5 45 –
53.7 54 –
59.6 60 –
65.2 64.5 64.5
This work [9] [7]
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Table 2 Characteristic energy of silicon and some SiO2 phonons in the same energy range of silicon phonons for the (1 0 0) orientation Phosphorous donor or R1 accoustic and R3 optic toward K point (meV)
R01 or D1 LA mode in X point (meV)
D02 LO mode toward X point (meV)
R1 or R2 TO mode toward X point (meV)
Rocking TO1 mode in SiO2 (meV)
Rocking LO1 mode in SiO2 (meV)
36 36.8 36.8 – – – –
44 43.6 – – – – –
51.1 53.6 – – – – –
60 59.4 – – 57.8 60 –
55.6 – – 56.7 – – 56.6
64.4 63.4 – 62.8 – 64 –
This work [4,13] [12] [14] [15] [5] [21]
port studies, mean free paths in the range from 0.7 to 1.5 nm are deduced. The oxide thickness is barely larger than these mean free paths, reducing the inelastic tunnelling interaction in the silicon dioxide. It is not the case for this inelastic effect in the silicon bulk where the interaction probability is higher and not also the case for molecular vibration in the gate oxide, because the size of these molecules. For this purpose, a simple model has been presented by Langan and Hansma [17] which suggest that the dependence of peak intensity on surface concentration will vary with the size of the dopant molecules.. The ballistic transport is, in our opinion, the main reason why silicon dioxide phonons are always more difficult to detect by IETS than those of silicon substrate or gate. Fig. 5. IET spectrum and deconvolution of silicon dioxide modes.
four specific peaks (TO3 , LO4 , TO4 , LO3 vibration modes of SiO2 ) [14] and a molecular vibration (P–O mode at 165.6 mV (1335 cm1 ) due to the phosphorus doping level) had been accurately made [4] and found again in our first paper with a slight shift in energy [2]. Here, in the result shown in Fig. 5, all SiO2 (or SiOx ) phonon spectrum is shifted towards high bias of an voltage amount of about 10 mV (with an inaccuracy of 1.5 mV due to deconvolution). This additional voltage is unfortunately due to a bad back electrical contact. The phonons of the SiO2 (or SiOx ) barrier are more often present in the IET spectra of the (1 1 1) substrate orientation than in the (1 0 0) orientation. This is may be due to the low intensity of this vibration band corresponding to the small relative conductance change of about 1% for the LO3 mode relatively to 6% for the strong structure of silicon optical phonon. An another reason for the difficulty to observe the SiO2 (or SiOx ) phonon spectrum, is the very thin (1–5 nm) used in new microelectronics devices. Ballistic transport is observed in Fischetti et al. work [16]. From these ballistic trans-
3.3. Molecular vibrations What is the capability of IETS to identify specific molecular vibrations located at the critical Si–SiO2 interface? The strongly rising elastic background of the conductance limits, at high bias voltage, the observation of inelastic tunnelling peaks. Nevertheless, we can risk an identification in the first half-infrared energy range (30–250 meV corresponding to 242–2016 cm1 ). Fig. 6 shows five spectra of a tunnelling junction elaborated in Grenoble, we can see a good reproducibility of these tunnelling spectra. Deconvolved modes in the range between 40 and 140 mV (322–1129 cm1 ) are given in Fig. 7. We have interpreted the 78.6 meV (634 cm1 ) peak as a Si–C vibrational mode, the peak at about 88.8 meV (716 cm1 ) as the energy excitation of Si–H wagging vibration, the peak at 103.3 meV (833 cm1 ) as the excitation of the Si–N bending vibration and the peak at 113 meV (911 cm1 ) as the excitation of the Si3 –Si–H (or O3 –Si–H) bending vibration, this vibration is very near in energy of the bending mode of Al–O–H mode currently observed at 110 mV with Al–Al2 O3 –Pb junction in IETS, several years ago [1]. The silicon element being neighbouring of aluminium in Mendeleiff table, the
C. Petit et al. / Solid-State Electronics 47 (2003) 1663–1668
1667
dI / dV (millimhos)
0.35
ω
0.30
0.25
0.20
0.15 0
20
40
60
80
100
120
VG (mV)
Fig. 8. Conductance and conductance jump Dr=r of an Al/ SiO2 /Si(nþ ) tunnel junction for (1 0 0) silicon orientation.
Fig. 6. Five IET spectra of an Al/SiO2 /Si(nþ ) tunnel junction for (1 0 0) silicon orientation.
the quantum of vibrational energy of the molecules. In Fig. 8, we shows the conductance of this device, determined by the fist derivative dI==dV vs VG of the IG –VG curve, and the variation Dr=r vs VG . It is easy to evaluate the jump Dr=r in the conductance for the silicon modes phonons. For the most intense phonon mode located at 60 mV (resultant from the TO mode in the (1 0 0) direction), the jump Dr=r amountsaround 2.7%. The quantity Dr=r is related [20] with the observed IG –VG characteristics by the relation: Z V 00 Dr 1 d2 I V dV ð1Þ r I dV 2 V0 where V 0 and V 00 are the starting and the ending voltages of the variation band under consideration. This relation may be used to quantify the strength of the vibration modes. Table 3 gives the jump Dr=r of conductance r for the vibrational modes r described above and, consequently, an idea of relative peak intensities.
Fig. 7. IET spectrum of an Al/SiO2 /Si(nþ ) tunnel junction for (1 0 0) silicon orientation.
4. Conclusion Si–O–H bending mode is expected to roughly at the same energy [1,18,19]. Both latter peaks had been previously identified [2] in the sample elaborated in Lille. Yanson et al. [20] showed that the intensity of the inelastic process is characterized by the relative jump Dr=r in conductance at voltage VG ¼ hx=e, where hx is
In this paper, the IET lattice vibration spectra were obtained for two silicon orientations. For each orientation, the phonon bands were deconvoluted in separate phonon modes with accuracy. These modes were assigned in energy in agreement with literature data. The
Table 3 Energy and jump of conductivity Dr=r for different vibrational modes of an Al/SiO2 /Si(nþ ) tunnel junction for (1 0 0) silicon orientation Vibrational modes
TO phonon mode
Si–C
Wagging Si–H
Si–N
Bending Si–OH
Energy (meV) Wave number (cm1 ) Dr=r (%)
60 484 2.7
78.6 633 0.49
89.1 718 0.6
103.3 833 1.8
113 911 0.51
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band vibration analysis of SiO2 grown on (1 1 1) silicon is very near than LyeÕs investigation on (1 0 0) orientation. Slight series resistance has shifted the peak positions. The interest of this IETS technique is to compare the IET spectra of samples elaborated according to several technologies. Our results, extending those of other authors, demonstrate that it is possible to obtain additional information as impurities due to contaminants accidentally incorporated during the fabrication process of the MOS devices. With the advent ultra-thin oxide gate, the ability to probe the good quality of these tunnel oxides in as-fabricated devices, makes IETS a powerful aid in microelectronics for study these devices. The authors are indebted to C. LEROUX for LETI sample elaboration. The RMNT Network under the ‘‘ULTIMOX’’ project has financially supported this work.
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