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SiO2 structures
ARTICLE IN PRESS
Vacuum 76 (2004) 303–306 www.elsevier.com/locate/vacuum
Defects induced by hydrogen implantation in n-Si/SiO2 structures S. Simeonov, A. Gushterov, A. Szekeres, E. Kafedjiiska Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee, 1784 Sofia, Bulgaria
Abstract Capacitance–voltage measurements at 77 and 300 K have shown that 11 keV hydrogen ion implantation of n-Si/SiO2 structure generates defects in the SiO2 film and at the Si/SiO2 interface. Deep-Level Transient Spectroscopy spectra reveal the creation of deep levels in the Si substrate. In the accumulation mode tunnelling type conduction through the 120 nm thick SiO2 film is observed. r 2004 Elsevier Ltd. All rights reserved. Keywords: Silicon; Hydrogen implantation; Thermal SiO2; Oxide and interface traps; Deep levels
1. Introduction Ion implantation is in widespread use for selective doping in semiconductor devices and integrated circuits. Hydrogen ion implantation with an energy of 5–10 keV provides a source of atomic hydrogen, which passivates electron traps at the Si/SiO2 interface [1]. Also, implantation of hydrogen ions with energy of 10 keV leads to radiation-stimulated diffusion of boron in Si [2]. It has also been shown that more uniform SiO2 with lower intrinsic stress and better interface structure can be grown on preliminary hydrogenated Si
Corresponding author. Fax:+359-2-975-36-32.
E-mail address: [email protected] (S. Simeonov).
substrate [3]. These examples emphasise the beneficial effect of hydrogen implantation on the properties of Si/SiO2 structures. Ion implantation, however, generates electrically active defects in semiconductor materials and structures. The study of these defects is indispensable for the characterisation of the interaction of implanted ions with substrates and establishment of conditions for particular ion implantation. In this work, defects induced by hydrogen implantation in Si/SiO2 structures are studied. Information about the nature and concentration of electrically active defects in these structures is obtained from the analysis of the capacitance–voltage (C–V) and current–voltage (I–V) characteristics and the spectra of the deep-level transient spectroscopy (DLTS).
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.036
ARTICLE IN PRESS S. Simeonov et al. / Vacuum 76 (2004) 303–306
304
2. Experimental Phosphorous-doped 5–10 O cm Si(1 0 0) wafers were thermally oxidised at 1050 1C in dry oxygen (H2Oo3 ppm) till 120 nm thick SiO2 layer was formed. Through the SiO2 layer, 11 keV hydrogen ions were implanted with doses of 1013 and 1014 cm2. With this oxide thickness and the H+ ion energy the maximum concentration of implanted ions is at the Si/SiO2 interface. The MOS capacitors were prepared by evaporation of Al contacts. The oxide and interface charges were estimated from the 1 MHz C–V and I–V measurements at 77 and 300 K. The deep levels in the Si bulk were detected by DLTS measurements with a Semi-trap DLTS 81 apparatus with lock-in detection.
3. Results and discussion The 1 MHz C–V characteristics of the MOS capacitors, implanted with H+ ions with different doses are given in Fig. 1. The regions of accumulation, depletion and strong inversion are clearly seen on these C–V plots. The substrate doping level, ND, given in Table 1, is calculated from the ratio of 300 K capacitances in accumulation and strong inver-
300K 300K 77K 77K
1.0
C/Cox
0.8
0.6
0.4
0.2
-20
-10
0
10
20
30
Voltage (V) Fig. 1. Normalised 1 MHz C–V characteristics of MOS structures implanted with H+ ions with dose of 1013 cm2 (circles) and 1014 cm2 (triangles).
Table 1 Oxide and interface traps densities in hydrogen-implanted Si/ SiO2 structures H+ dose (cm2) ND ( 1015 cm3) V ifb ðVÞ V ini ðVÞ V exp fb ðVÞ V exp ni ðVÞ Nox ( 1012 cm2) Nit ( 1011 cm2) Dit ( 1012 cm1 eV1)
1014 1.5 0.308 1.04 7.76 12.14 1.95 6.42 2.13
1013 1.3 0.312 0.94 4.14 6.24 1.13 3.15 1.05
sion. Using the corresponding ideal C–V plots, the flat-band, V ifb , and mid-band, V ini , voltages and the capacitance values at these voltages are calculated. The corresponding gate voltages V exp fb and V exp ni have been determined from the experimental C–V plots. All these voltages are given also in Table 1. It is known that electron traps at the Si/SiO2 interface above Si mid-band are acceptors [4]. Below the Fermi level these traps are negatively charged and above it — neutral. Due to this, the C–V plot shifts toward more positive voltages. In these circumstances it is more appropriate for an estimation of the oxide trap density, Nox, to use i expression qN ox ¼ C ox ðV exp ni V ni Þ. In the case 14 2 of 10 cm ion dose, the Nox values (Table 1) are higher approximately by 50–60% than those for exp 1013 cm2 dose. The difference ðV exp fb V ni Þ is exp more extended than the difference ðV fb V exp ni Þ for ideal MOS structure, which indicates that electron traps at the Si/SiO2 interface contribute to the MOS capacitance. The trapped charge density at the interface, qNit, is given by exp i i qN it ¼ C ox ½ðV exp V Þ ðV V Þ. The enni ni fb fb ergy density of electron traps at the interface, Dit, is obtained by dividing qNit with the difference of mid-band and flat-band surface potentials (0.26–0.56 eV below the Si conduction band). For the 1014 cm2 ion dose, the Dit value is approximately two times higher than that for 1013 cm2 ion dose (Table 1). The 77 K C–V characteristics of MOS structure, implanted with 1013 cm2 H+ dose, is shifted by over 20 V toward positive voltages in comparison
ARTICLE IN PRESS S. Simeonov et al. / Vacuum 76 (2004) 303–306
-13.0
-13.5
lnj (A/cm2)
to the 300 K C–V plot indicating the presence of negative charges. The inversion capacitance saturates at higher values than that calculated for the corresponding ideal MOS structure at deep inversion. Due to this, it is not possible to estimate independently the densities of fixed oxide and interface charges. Using the V exp value the fb combined density of fixed and interface charges is estimated as (4–4.5) 1012 cm2. The Dit density in the energy range of 0.05–0.15 eV below the Si conduction band is (3–5) 1012 cm2 eV1. These Dit values are 3–5 times higher than those of traps near the Si mid-band determined from the 300 K C–V measurements. These estimations are in accordance with the observations that Dit density is minimal around Si mid-band and increases toward Si conduction band. The measured Cox capacitance in accumulation for the MOS structures, implanted with 1014 cm2 ion dose, exceeds the estimated maximal capacitance of the 120 nm thick oxide by approximately 30%. In accumulation mode the parallel conductivity is about 10 and 22 mS cm2 at 300 and 77 K, respectively. As it is explained below, this conductivity is connected with the inter-trap tunnelling. The reactive component of this inter-trap tunnelling appears as an additional capacitance in the C–V characteristics. The density of these traps, estimated from this additional capacitance, is in the range of (0.5–1.5) 1011 cm2 eV1. The DLTS spectra with one broad peak are detected at reverse bias Vs 0.03, 10 and 20 V and pulse voltage 0.15 V. Due to the low pulse voltage only the traps around the quasi-Fermi level are responsible for the observed DLTS spectra. The energy positions of traps qet, measured at Vs 0.03, 10 and 20 V, are 0.34, 0.43 and 0.62 eV, respectively. For all reverse biases the qet values are lower than the corresponding quasi-Fermi level position at the Si/SiO2 interface. This is an evidence that the traps responsible for these DLTS spectra are situated in the Si substrate instead of the Si/SiO2 interface. The forward I–V characteristics for the MOS structures implanted with an ion dose of 1014 cm2 are given in Fig. 2. The conductivity changes a little with changing the temperature from 77 to 300 K. This shows
305
-14.0
-14.5 300 K 77 K
-15.0
-15.5
0
5
10
15
20
25
Voltage (V) Fig. 2. Current density vs applied voltage for hydrogen implanted n-Si/SiO2 structure with ion dose of 1014 cm2.
tunnelling type conduction in the SiO2 layer. Since the oxide is considerably thick (120 nm) Fowler– Nordheim tunnelling through the oxide is excluded. Therefore, the observed conductivity is due to trap-assisted tunnelling of electrons through the SiO2 layer. Inter-trap electron tunnelling from occupied traps to nearest unoccupied ones is considered as a conduction mechanism in these structures. In this case the current density is given by " # 1=2 1 2ð2m qÞ1=2 jt w j ¼ qg 2 exp w _ " # ð2m qÞ1=2 w2 E sinh ; (1) 1=2 _jt where g is electron attempt to escape frequency, w is the distance between nearest traps, qjt is the energy position of traps in SiO2 energy gap, E is the electrical field in SiO2, all other notations have their common meaning [5]. The electrical field, E, in the SiO2 film is equal to (VVfb)/d, where d is the SiO2 thickness. By plotting ln(j) versus V from the slope of the plot and the intersection with the current density axis at V=0 the values of qjt and w are calculated. These are 4.5 eV and 2.06 107 cm at 300 K and 4.74 eV and 2.02 107 cm at 77 K, respectively. The values of trap energy position,
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qjt are reasonable for the SiO2 energy gap Eg=9 V. The trap density, Nt, estimated by Nt 1/w3, is (1.14–1.2) 1020 cm3. Similar trap concentrations in the order of 3 1019 and 1020 cm3 are considered for analysis of temperature-dependent stress-induced leakage current [6] and trap-assisted inelastic tunnelling [7] in MOS structures.
levels in the SiO2 layer with a concentration of (1.14–1.2) 1020 cm3. These results show that in order to obtain the beneficial effects of hydrogen implantation the ion dose should be chosen properly.
References 4. Conclusions Hydrogen implantation with an ion energy of 11 keV through 120 nm thick SiO2 generates positive oxide charges, Nox, with a density of order of 1012 cm3 and deep levels in the Si substrate. H+ implantation with a dose of 1014 cm2 leads to about a 30% increase of the oxide capacitance in the accumulation mode over that estimated for the 120 nm SiO2 layer. This effect is due to deep levels generated in the oxide. H+ implantation with a dose of 1013 cm2 creates acceptor type deep levels in the SiO2 layer and/or at the Si/SiO2 interface. The conduction through the structures is by electron tunnelling via deep
[1] Shang H, White MN, Guarini KW, Solomon P, Cartier E, McFeely, Yurkas J, Lee WC. Appl Phys Lett 2001;38:3139–41. [2] Lukas Ch, Gailliard JP, Loualiche S, Baruch P, Pfister JP, Truche R.. In: Albany JH, editor. Defects and radiation effects in semiconductors. London: The Institute of Physics; 1979. p. 551. [3] Szekeres A, Paneva A, Alexandrova S, Lisovskyy I, Litovchenko V, Mazunov D. Vacuum 2003;69:355–60. [4] Lenahan PM, Dressendorfer PV. J Appl Phys 1984;55:3495–9. [5] Simeonov SS, Yurukov I, Kafedjiiska E, Szekeres A. Proceedings of International Conference on Semiconductors, CAS 2002, Sinaja, Romania, 2001. p. 399–402. [6] DeSalvo B, Gibaudo G, Pananakis G, Guillaumot B, Reimbold G. Solid State Electron 2000;44:895–903. [7] Jimenes-Moliuos F, Palma A, Gamiz F, Baugueri J, LopezVillanueva JA. J Appl Phys 2001;90:3396–406.
Vacuum 76 (2004) 303–306 www.elsevier.com/locate/vacuum
Defects induced by hydrogen implantation in n-Si/SiO2 structures S. Simeonov, A. Gushterov, A. Szekeres, E. Kafedjiiska Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee, 1784 Sofia, Bulgaria
Abstract Capacitance–voltage measurements at 77 and 300 K have shown that 11 keV hydrogen ion implantation of n-Si/SiO2 structure generates defects in the SiO2 film and at the Si/SiO2 interface. Deep-Level Transient Spectroscopy spectra reveal the creation of deep levels in the Si substrate. In the accumulation mode tunnelling type conduction through the 120 nm thick SiO2 film is observed. r 2004 Elsevier Ltd. All rights reserved. Keywords: Silicon; Hydrogen implantation; Thermal SiO2; Oxide and interface traps; Deep levels
1. Introduction Ion implantation is in widespread use for selective doping in semiconductor devices and integrated circuits. Hydrogen ion implantation with an energy of 5–10 keV provides a source of atomic hydrogen, which passivates electron traps at the Si/SiO2 interface [1]. Also, implantation of hydrogen ions with energy of 10 keV leads to radiation-stimulated diffusion of boron in Si [2]. It has also been shown that more uniform SiO2 with lower intrinsic stress and better interface structure can be grown on preliminary hydrogenated Si
Corresponding author. Fax:+359-2-975-36-32.
E-mail address: [email protected] (S. Simeonov).
substrate [3]. These examples emphasise the beneficial effect of hydrogen implantation on the properties of Si/SiO2 structures. Ion implantation, however, generates electrically active defects in semiconductor materials and structures. The study of these defects is indispensable for the characterisation of the interaction of implanted ions with substrates and establishment of conditions for particular ion implantation. In this work, defects induced by hydrogen implantation in Si/SiO2 structures are studied. Information about the nature and concentration of electrically active defects in these structures is obtained from the analysis of the capacitance–voltage (C–V) and current–voltage (I–V) characteristics and the spectra of the deep-level transient spectroscopy (DLTS).
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.036
ARTICLE IN PRESS S. Simeonov et al. / Vacuum 76 (2004) 303–306
304
2. Experimental Phosphorous-doped 5–10 O cm Si(1 0 0) wafers were thermally oxidised at 1050 1C in dry oxygen (H2Oo3 ppm) till 120 nm thick SiO2 layer was formed. Through the SiO2 layer, 11 keV hydrogen ions were implanted with doses of 1013 and 1014 cm2. With this oxide thickness and the H+ ion energy the maximum concentration of implanted ions is at the Si/SiO2 interface. The MOS capacitors were prepared by evaporation of Al contacts. The oxide and interface charges were estimated from the 1 MHz C–V and I–V measurements at 77 and 300 K. The deep levels in the Si bulk were detected by DLTS measurements with a Semi-trap DLTS 81 apparatus with lock-in detection.
3. Results and discussion The 1 MHz C–V characteristics of the MOS capacitors, implanted with H+ ions with different doses are given in Fig. 1. The regions of accumulation, depletion and strong inversion are clearly seen on these C–V plots. The substrate doping level, ND, given in Table 1, is calculated from the ratio of 300 K capacitances in accumulation and strong inver-
300K 300K 77K 77K
1.0
C/Cox
0.8
0.6
0.4
0.2
-20
-10
0
10
20
30
Voltage (V) Fig. 1. Normalised 1 MHz C–V characteristics of MOS structures implanted with H+ ions with dose of 1013 cm2 (circles) and 1014 cm2 (triangles).
Table 1 Oxide and interface traps densities in hydrogen-implanted Si/ SiO2 structures H+ dose (cm2) ND ( 1015 cm3) V ifb ðVÞ V ini ðVÞ V exp fb ðVÞ V exp ni ðVÞ Nox ( 1012 cm2) Nit ( 1011 cm2) Dit ( 1012 cm1 eV1)
1014 1.5 0.308 1.04 7.76 12.14 1.95 6.42 2.13
1013 1.3 0.312 0.94 4.14 6.24 1.13 3.15 1.05
sion. Using the corresponding ideal C–V plots, the flat-band, V ifb , and mid-band, V ini , voltages and the capacitance values at these voltages are calculated. The corresponding gate voltages V exp fb and V exp ni have been determined from the experimental C–V plots. All these voltages are given also in Table 1. It is known that electron traps at the Si/SiO2 interface above Si mid-band are acceptors [4]. Below the Fermi level these traps are negatively charged and above it — neutral. Due to this, the C–V plot shifts toward more positive voltages. In these circumstances it is more appropriate for an estimation of the oxide trap density, Nox, to use i expression qN ox ¼ C ox ðV exp ni V ni Þ. In the case 14 2 of 10 cm ion dose, the Nox values (Table 1) are higher approximately by 50–60% than those for exp 1013 cm2 dose. The difference ðV exp fb V ni Þ is exp more extended than the difference ðV fb V exp ni Þ for ideal MOS structure, which indicates that electron traps at the Si/SiO2 interface contribute to the MOS capacitance. The trapped charge density at the interface, qNit, is given by exp i i qN it ¼ C ox ½ðV exp V Þ ðV V Þ. The enni ni fb fb ergy density of electron traps at the interface, Dit, is obtained by dividing qNit with the difference of mid-band and flat-band surface potentials (0.26–0.56 eV below the Si conduction band). For the 1014 cm2 ion dose, the Dit value is approximately two times higher than that for 1013 cm2 ion dose (Table 1). The 77 K C–V characteristics of MOS structure, implanted with 1013 cm2 H+ dose, is shifted by over 20 V toward positive voltages in comparison
ARTICLE IN PRESS S. Simeonov et al. / Vacuum 76 (2004) 303–306
-13.0
-13.5
lnj (A/cm2)
to the 300 K C–V plot indicating the presence of negative charges. The inversion capacitance saturates at higher values than that calculated for the corresponding ideal MOS structure at deep inversion. Due to this, it is not possible to estimate independently the densities of fixed oxide and interface charges. Using the V exp value the fb combined density of fixed and interface charges is estimated as (4–4.5) 1012 cm2. The Dit density in the energy range of 0.05–0.15 eV below the Si conduction band is (3–5) 1012 cm2 eV1. These Dit values are 3–5 times higher than those of traps near the Si mid-band determined from the 300 K C–V measurements. These estimations are in accordance with the observations that Dit density is minimal around Si mid-band and increases toward Si conduction band. The measured Cox capacitance in accumulation for the MOS structures, implanted with 1014 cm2 ion dose, exceeds the estimated maximal capacitance of the 120 nm thick oxide by approximately 30%. In accumulation mode the parallel conductivity is about 10 and 22 mS cm2 at 300 and 77 K, respectively. As it is explained below, this conductivity is connected with the inter-trap tunnelling. The reactive component of this inter-trap tunnelling appears as an additional capacitance in the C–V characteristics. The density of these traps, estimated from this additional capacitance, is in the range of (0.5–1.5) 1011 cm2 eV1. The DLTS spectra with one broad peak are detected at reverse bias Vs 0.03, 10 and 20 V and pulse voltage 0.15 V. Due to the low pulse voltage only the traps around the quasi-Fermi level are responsible for the observed DLTS spectra. The energy positions of traps qet, measured at Vs 0.03, 10 and 20 V, are 0.34, 0.43 and 0.62 eV, respectively. For all reverse biases the qet values are lower than the corresponding quasi-Fermi level position at the Si/SiO2 interface. This is an evidence that the traps responsible for these DLTS spectra are situated in the Si substrate instead of the Si/SiO2 interface. The forward I–V characteristics for the MOS structures implanted with an ion dose of 1014 cm2 are given in Fig. 2. The conductivity changes a little with changing the temperature from 77 to 300 K. This shows
305
-14.0
-14.5 300 K 77 K
-15.0
-15.5
0
5
10
15
20
25
Voltage (V) Fig. 2. Current density vs applied voltage for hydrogen implanted n-Si/SiO2 structure with ion dose of 1014 cm2.
tunnelling type conduction in the SiO2 layer. Since the oxide is considerably thick (120 nm) Fowler– Nordheim tunnelling through the oxide is excluded. Therefore, the observed conductivity is due to trap-assisted tunnelling of electrons through the SiO2 layer. Inter-trap electron tunnelling from occupied traps to nearest unoccupied ones is considered as a conduction mechanism in these structures. In this case the current density is given by " # 1=2 1 2ð2m qÞ1=2 jt w j ¼ qg 2 exp w _ " # ð2m qÞ1=2 w2 E sinh ; (1) 1=2 _jt where g is electron attempt to escape frequency, w is the distance between nearest traps, qjt is the energy position of traps in SiO2 energy gap, E is the electrical field in SiO2, all other notations have their common meaning [5]. The electrical field, E, in the SiO2 film is equal to (VVfb)/d, where d is the SiO2 thickness. By plotting ln(j) versus V from the slope of the plot and the intersection with the current density axis at V=0 the values of qjt and w are calculated. These are 4.5 eV and 2.06 107 cm at 300 K and 4.74 eV and 2.02 107 cm at 77 K, respectively. The values of trap energy position,
ARTICLE IN PRESS 306
S. Simeonov et al. / Vacuum 76 (2004) 303–306
qjt are reasonable for the SiO2 energy gap Eg=9 V. The trap density, Nt, estimated by Nt 1/w3, is (1.14–1.2) 1020 cm3. Similar trap concentrations in the order of 3 1019 and 1020 cm3 are considered for analysis of temperature-dependent stress-induced leakage current [6] and trap-assisted inelastic tunnelling [7] in MOS structures.
levels in the SiO2 layer with a concentration of (1.14–1.2) 1020 cm3. These results show that in order to obtain the beneficial effects of hydrogen implantation the ion dose should be chosen properly.
References 4. Conclusions Hydrogen implantation with an ion energy of 11 keV through 120 nm thick SiO2 generates positive oxide charges, Nox, with a density of order of 1012 cm3 and deep levels in the Si substrate. H+ implantation with a dose of 1014 cm2 leads to about a 30% increase of the oxide capacitance in the accumulation mode over that estimated for the 120 nm SiO2 layer. This effect is due to deep levels generated in the oxide. H+ implantation with a dose of 1013 cm2 creates acceptor type deep levels in the SiO2 layer and/or at the Si/SiO2 interface. The conduction through the structures is by electron tunnelling via deep
[1] Shang H, White MN, Guarini KW, Solomon P, Cartier E, McFeely, Yurkas J, Lee WC. Appl Phys Lett 2001;38:3139–41. [2] Lukas Ch, Gailliard JP, Loualiche S, Baruch P, Pfister JP, Truche R.. In: Albany JH, editor. Defects and radiation effects in semiconductors. London: The Institute of Physics; 1979. p. 551. [3] Szekeres A, Paneva A, Alexandrova S, Lisovskyy I, Litovchenko V, Mazunov D. Vacuum 2003;69:355–60. [4] Lenahan PM, Dressendorfer PV. J Appl Phys 1984;55:3495–9. [5] Simeonov SS, Yurukov I, Kafedjiiska E, Szekeres A. Proceedings of International Conference on Semiconductors, CAS 2002, Sinaja, Romania, 2001. p. 399–402. [6] DeSalvo B, Gibaudo G, Pananakis G, Guillaumot B, Reimbold G. Solid State Electron 2000;44:895–903. [7] Jimenes-Moliuos F, Palma A, Gamiz F, Baugueri J, LopezVillanueva JA. J Appl Phys 2001;90:3396–406.