- Email: [email protected]
Defect-engineering in SiC by ion implantation and electron irradiation
Microelectronic Engineering 83 (2006) 146–149 www.elsevier.com/locate/mee
Defect-engineering in SiC by ion implantation and electron irradiation G. Pensl a,*, F. Ciobanu a, T. Frank a, D. Kirmse a, M. Krieger a, S. Reshanov a, F. Schmid a, M. Weidner a, T. Ohshima b, H. Itoh b, W.J. Choyke c a
Institute of Applied Physics, University of Erlangen-Nu¨rnberg, Staudtstrasse 7, D-91058 Erlangen, Germany b Japan Atomic Energy Agency, 1233 Watanuki, Takasaki, Gunma 370-12, Japan c Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA Available online 18 November 2005
Abstract Three examples are given, which show that ion implantation and electron irradiation can drastically modify the electrical properties of SiC and SiC-based MOS capacitors. (1) It is demonstrated that sulphur ions (S+) implanted into 6H-SiC act as double donors with ground states ranging from 310 to 635 meV below the conduction bandedge. (2) Co-implantation of nitrogen (N+) – and silicon (Si+) – ions into 4H-SiC leads to a strong deactivation of N donors. Additional experiments with electron (e)-irradiated 4H-SiC samples (E(e) = 200 keV) support the idea that this deactivation is due to the formation of an electrically neutral (Nx–VC, y)-complex. (3) Implantation of a surface-near Gaussian profile into n-type 4H-SiC followed by a standard oxidation process leads to a strong reduction of the density of interface traps Dit close to the conduction bandedge in n-type 4H-SiC/SiO2 MOS capacitors. 2005 Elsevier B.V. All rights reserved. Keywords: Defect centers; Electron irradiation; Ion implantation.
1. Introduction Ion implantation and electron irradiation are useful tools to manipulate the electrical properties of the widebandgap semiconductor SiC. For example, the conductivity of n-type SiC can be raised by implantation of phosphorus donors [1] and decreased by implantation of inert gases like neon or argon. In this case, implantation-induced, acceptor-like defects are formed, which act as compensation [2]. In this paper, we present three examples demonstrating that defects can be engineered in a particular way. Based on double-correlated deep level transient spectroscopy (D-DLTS) investigations, we identified a double donor in 6H-SiC, which originates from implanted S+-ions. A strong deactivation of implanted N donors is observed in 4H-SiC, when in addition Si+-ions are co-implanted. In *
Corresponding author. Tel.: +49 9131 8528426; fax: +49 9131 8528423. E-mail address: [email protected] (G. Pensl). 0167-9317/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.10.040
combination with high energy electron (E(e) = 200 keV) irradiations, there are clear indications that an electrically neutral (Nx–VC,y)-complex is responsible for the N donor deactivation [3]. The third example is related to n-type 4H-SiC/SiO2 MOS capacitors. Implantation of a Gaussian N profile and subsequent growth of an oxide to a depth greater than the implanted N profile result in a strong reduction of the density of interface traps close to the conduction bandedge of the MOS capacitor [4]. 2. Sulphur double donors Sulphur (S) box profiles with a mean concentration of 1015 cm3 were implanted into n-type 6H-SiC (epilayer with doping level [N] = 2 · 1016 cm3) to a depth of 1.3 lm. The samples were annealed at 1700 C for 30 min and subsequently analyzed with DLTS. The DLTS spectrum (open squares) in Fig. 1 shows three peaks, one sharp peak and two broader peaks. All three peaks scale with the implanted S-dose. The fit of Eq. (1) to the experimental data results in six individual DLTS peaks (dashed curves);
G. Pensl et al. / Microelectronic Engineering 83 (2006) 146–149
DLTS signal b1 (fF)
40
30
15 -3 n-6H-SiC:S [S]=1x10 cm
Tw=64ms
+/2+
S(k1,k2)
0/+
S(k2) 0/+
S(h)
20
0/+
S(k1)
+/2+
S(h)
10
0 100 120 140 160 180 200 220 240
147
Table 1 Activation energies and concentrations of S double donors obtained from the fit of Eq. (1) to the measured DLTS spectrum Defect
Activation energy (meV)
Concentration (cm3)
S(h)0/+ S(h)+/2+ S(k1)0/+ S(k1)+/2+ S(k2)0/+ S(k2)+/2+
255 405 314 520 332 541
4.2 · 1014 3.9 · 1014 2.9 · 1014 3.6 · 1014 5.6 · 1014 5.8 · 1014
Open squares in Fig. 1.
Temperature (K) Fig. 1. DLTS spectrum (open squares) taken on a S-implanted 6H-SiC sample (S-dose = 1015 cm3).
the solid curve in Fig. 1 corresponds to the sum of the individual curves: 2DC Tw 2p=T w b1 ¼ exp ; ð1Þ 1 2 Tw se ð2p=T w Þ þ s2 e where Tw is the period width of the capacitance transient, se is the emission time constant and DC the amplitude of capacitance transient. The charge state indicated in Fig. 1 for each center is obtained from D-DLTS investigations, which reveal the activation energy Ea as a function of the electric field applied (Poole–Frenkel effect [5,6]). An example is given in Fig. 2 for the S(h)0/+-center showing that the experimental data (full dots) can be reproduced within the error bars by assuming a ‘‘0/+’’ charge state. The activation energies and defect concentrations obtained from the fit procedure are listed in Table 1. It turns out that the concentrations of singly charged states are equal to the corresponding concentrations of the doubly charged ones: ½SðhÞ0=þ ½SðhÞþ=2þ ; ½Sðk1 Þ0=þ ½Sðk1 Þþ=2þ and ½Sðk2 Þ0=þ ½Sðk2 Þþ=2þ .
3. Nitrogen donor deactivation For the deactivation experiments, we used p-type and ntype 4H-SiC epilayers with a doping level of [Al] = 3 · 1016 cm3 and [N] = 1 · 1016 cm3, respectively. In all cases, we implanted a box-shaped N profile to a depth of 1 lm (mean [N] = 3 · 1018 cm3, Timpl = 500 C). Some samples were co-implanted with a second species at different concentrations; these concentrations varied in a wide range: [C] = (1017–3 · 1019) cm3, [Si] = (1015–1019) cm3 or [Ne] = (1016–8 · 1018) cm3. The annealing steps were carried out for 30 min each at temperatures ranging between 300 and 1700 C. Fig. 4 summarizes the performed Hall effect investigations. The C/N- and Ne/N-co-implanted samples show the expected behavior. The electrically active N donor concentration slightly decreases with increasing concentration of the co-implanted species because of the increase of the compensation. In the case of Si/N-co-implantation, however, both the N donors and the compensation decrease with increasing co-implanted Si concentration. It seems that N donors together with an intrinsic species, which is generated by the Si implantation, form an electrically
300
n-6H-SiC:S
0/+
S(h)
280
a
Activation energy E (meV)
Further the sum over the concentrations of singly charged (or doubly charged) states corresponds roughly to the implanted S concentration. All these features give evidence
that the observed DLTS peaks are caused by S double donors residing on the three inequivalent lattice sites of 6HSiC. The assignment to the hexagonal site and to the two cubic sites is adopted from nitrogen donors and is speculative. The levels are summarized in Fig 3.
260 240
-/0 0/+
220 15
[S]=1x10 cm
-3
+/2+
200 40
60
80
100 120 140 160 180
Mean electric field F (kV/cm)
Fig. 2. Activation energy Ea of the S(h)0/+-center versus mean electric field (full dots); the Poole–Frenkel correction indicates that this defect state is singly positively charged after emission (solid curve).
Fig. 3. Energy scheme of the S double donor residing on the 3 inequivalent lattice sites of 6H-SiC (one hexagonal site (h) and two cubic sites (k1, k2)); the activation energies are corrected by the Poole–Frenkel effect.
1019 4H-SiC:N TA=1700˚C
reference sample ([N]=3.3x1018 cm-3)
(full symbols)
N-donor concentration (cm-3)
1019
, , ,
1018
C/N Si/N Ne/N
1018
1017 1016
1017
1018
1019
1017 1020
(open symbols)
G. Pensl et al. / Microelectronic Engineering 83 (2006) 146–149 concentration of compensation (cm-3)
148
Co-implanted C-, Si-, Ne-concentration (cm-3)
Fig. 4. Concentration of N donors and of the compensation versus concentration of co-implanted species (C, Si or Ne). The horizontal dashed straight line corresponds to the maximum concentration of electrically activated N donors.
neutral and thermally stable complex in n-type 4H-SiC. The intrinsic species could either be Si interstitials or carbon vacancies, which are also generated by the implanted surplus of Si atoms. A series of Si/N-co-implanted 4HSiC samples was annealed at different temperatures. It turned out that this particular complex was formed at annealing temperatures around 1450 C. Next step was to search with DLTS in the whole bandgap of 4H-SiC for the dominating defect center. For this reason, we implanted n-type 4H-SiC with N and coimplanted p-type 4H-SiC with Si/N and annealed the samples at temperatures between 1000 and 1700 C. The DLTS spectra taken on n-type 4H-SiC displayed the Z1/Z2-center as the dominating defect, which is thermally stable up to around 1450 C, while the P2-center [7] was observed at higher concentration in p-type SiC subsequent to anneals above 1400 C. In 4H-SiC, the energy positions of these defects are: EðZ 1 =Z 2 Þ ¼ EC 650 meV;
EðP 2 Þ ¼ EV þ 1:6 eV.
We repeated the latter experiment by substituting the Si implantation by irradiating the 4H-SiC samples with high
Fig. 6. Scheme of N-implanted, n-type 4H-SiC MOS capacitor; dmax = position of maximum of the implanted Gaussian N profile, dcons = SiC layer consumed for the oxidation.
energy electrons (E(e) = 200 keV). It is well known that electrons of 200 keV energy only damage the C-sublattice in SiC [8]. Fig. 5 discloses identical results as observed for the Si-implanted samples (not shown here). Again the Z1/Z2-center disappears subsequent to annealing steps at temperatures around 1450 C and the P2-center is generated at enhanced concentration at the same temperature. In summary, we have observed that the deactivation of N donors occurs at around 1450 C. This temperature also corresponds to the onset of the formation of the P2-center [7], which is the only dominating defect in the lower half of the bandgap of p-type 4H-SiC at such high temperatures. Because of the fact that the electron-irradiated samples cannot contain any Si-related components at extremely high concentrations, we propose at the present knowledge that a (Nx–VC,y)-complex is responsible for the deactivation of N donors. 4. Reduction of traps in n-type 4H-SiC MOS capacitors by N-implantation For the fabrication of MOS capacitors, we used two 4HSiC epilayers (n-type: [N] = 2 · 1016 cm3, p-type: [Al] = 3 · 1016 cm3). One half of each wafer was implanted with a Gaussian N profile. The implantation parameters of E(N+) = 20 kV and [N]max = 3 · 1018 cm3 were optimized for n-type SiC in a former publication [4].
1015
1.0
n-4H-SiC MOS 0.8 Si-face
P2 (p-type) 1014
0.6 0.4
0.10 T=250K
0.08
N-implanted 0.06 standard 0.04 0.02
0.2 E(e )=200 keV -
200
500
800 1100 1400 Annealing temperature TA (˚ C)
1700
Fig. 5. Z1/Z2-center (full dots) and P2-center (open dots) as a function of the annealing temperature determined by DLTS in e-irradiated (200 keV) n-type and p-type 4H-SiC, respectively.
0.0 -20
G/ω / COx
Z1/Z2 (n-type)
C/COx
Defect concentration (cm-3)
4H-SiC
0.00 -10 0 10 Gate bias (V)
Fig. 7. Normalized capacitance and normalized conductance versus gate bias taken at room temperature (probe frequency = 1 kHz, sweep rate = 0.2 V/s).
interface state density D it (cm-2eV-1)
G. Pensl et al. / Microelectronic Engineering 83 (2006) 146–149
n-type
1013
149
magnitude, while it is increased by roughly one order of magnitude in p-type MOS capacitors close to the valence bandedge. At the present, the reason for this increase is not clarified yet.
p-type
4H-SiC MOS 12
10
5. Conclusion , ,
11
10
1010 3.3
3.0
2.7
Si-face, standard Si-face, N-implanted
2.4 0.9 Eit-EV (eV)
0.6
0.3
0.0
Fig. 8. Density of interface traps Dit of a standard oxidized and of an additionally N-implanted n-/p-type 4H-SiC MOS capacitor.
The standard oxidation process (Tox = 1120 C, tox = 22 h, nominally dry O2) was adjusted in such a way that the SiC/ SiO2-interface is located at the trailing edge of the implanted Gaussian N profile as shown in Fig. 6. In Fig. 7, the normalized C–V and G/x–V characteristics of the not-implanted and the N-implanted n-type 4H-SiC MOS capacitor taken at 250 K are displayed. The C–V characteristic of the N-implanted MOS capacitor is shifted to negative voltages due to fixed positive charges and the corresponding G/x–V characteristic is strongly reduced in height. The density of interface states is obtained from the area below the conduction peak. In Fig. 8, Dit of the not-implanted and N-implanted MOS capacitor are compared. In n-type MOS capacitors, Dit close to the conduction bandedge is reduced by two and a half orders of
Ion implantation and electron irradiation are useful tools to adjust the electrical properties in the bulk of SiC and at the interface of SiC/SiO2 structures. Three examples are presented in this paper to demonstrate the benefit of defect-engineering. Acknowledgement The support of this work by the German Science Foundation (SiC-Forschergruppe) is gratefully acknowledged. References [1] M. Laube, F. Schmid, G. Pensl, G. Wagner, M. Linnarsson, M. Maier, J. Appl. Phys. 92 (2002) 549. [2] F. Schmid, T. Frank, G. Pensl, Mater. Sci. Forum 483–485 (2005) 641. [3] F. Schmid, Dissertation, Erlangen, 2005. [4] F. Ciobanu, G. Pensl, V. AfanasÕev, A. Scho¨ner, Mater. Sci. Forum 483–485 (2005) 693. [5] J. Frenkel, Phys. Rev. 54 (1938) 647. [6] J.L. Hartke, J. Appl. Phys. 39 (1968) 4871. [7] K. Danno, T. Kimoto, H. Matsunami, Appl. Phys. Lett. 86 (2005) 122104. [8] A.A. Rempel, W. Sprengel, K. Blaurock, K.J. Reichle, J. Major, H.E. Schaefer, Phys. Rev. Lett. 89 (2002) 185501.
Defect-engineering in SiC by ion implantation and electron irradiation G. Pensl a,*, F. Ciobanu a, T. Frank a, D. Kirmse a, M. Krieger a, S. Reshanov a, F. Schmid a, M. Weidner a, T. Ohshima b, H. Itoh b, W.J. Choyke c a
Institute of Applied Physics, University of Erlangen-Nu¨rnberg, Staudtstrasse 7, D-91058 Erlangen, Germany b Japan Atomic Energy Agency, 1233 Watanuki, Takasaki, Gunma 370-12, Japan c Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA Available online 18 November 2005
Abstract Three examples are given, which show that ion implantation and electron irradiation can drastically modify the electrical properties of SiC and SiC-based MOS capacitors. (1) It is demonstrated that sulphur ions (S+) implanted into 6H-SiC act as double donors with ground states ranging from 310 to 635 meV below the conduction bandedge. (2) Co-implantation of nitrogen (N+) – and silicon (Si+) – ions into 4H-SiC leads to a strong deactivation of N donors. Additional experiments with electron (e)-irradiated 4H-SiC samples (E(e) = 200 keV) support the idea that this deactivation is due to the formation of an electrically neutral (Nx–VC, y)-complex. (3) Implantation of a surface-near Gaussian profile into n-type 4H-SiC followed by a standard oxidation process leads to a strong reduction of the density of interface traps Dit close to the conduction bandedge in n-type 4H-SiC/SiO2 MOS capacitors. 2005 Elsevier B.V. All rights reserved. Keywords: Defect centers; Electron irradiation; Ion implantation.
1. Introduction Ion implantation and electron irradiation are useful tools to manipulate the electrical properties of the widebandgap semiconductor SiC. For example, the conductivity of n-type SiC can be raised by implantation of phosphorus donors [1] and decreased by implantation of inert gases like neon or argon. In this case, implantation-induced, acceptor-like defects are formed, which act as compensation [2]. In this paper, we present three examples demonstrating that defects can be engineered in a particular way. Based on double-correlated deep level transient spectroscopy (D-DLTS) investigations, we identified a double donor in 6H-SiC, which originates from implanted S+-ions. A strong deactivation of implanted N donors is observed in 4H-SiC, when in addition Si+-ions are co-implanted. In *
Corresponding author. Tel.: +49 9131 8528426; fax: +49 9131 8528423. E-mail address: [email protected] (G. Pensl). 0167-9317/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.10.040
combination with high energy electron (E(e) = 200 keV) irradiations, there are clear indications that an electrically neutral (Nx–VC,y)-complex is responsible for the N donor deactivation [3]. The third example is related to n-type 4H-SiC/SiO2 MOS capacitors. Implantation of a Gaussian N profile and subsequent growth of an oxide to a depth greater than the implanted N profile result in a strong reduction of the density of interface traps close to the conduction bandedge of the MOS capacitor [4]. 2. Sulphur double donors Sulphur (S) box profiles with a mean concentration of 1015 cm3 were implanted into n-type 6H-SiC (epilayer with doping level [N] = 2 · 1016 cm3) to a depth of 1.3 lm. The samples were annealed at 1700 C for 30 min and subsequently analyzed with DLTS. The DLTS spectrum (open squares) in Fig. 1 shows three peaks, one sharp peak and two broader peaks. All three peaks scale with the implanted S-dose. The fit of Eq. (1) to the experimental data results in six individual DLTS peaks (dashed curves);
G. Pensl et al. / Microelectronic Engineering 83 (2006) 146–149
DLTS signal b1 (fF)
40
30
15 -3 n-6H-SiC:S [S]=1x10 cm
Tw=64ms
+/2+
S(k1,k2)
0/+
S(k2) 0/+
S(h)
20
0/+
S(k1)
+/2+
S(h)
10
0 100 120 140 160 180 200 220 240
147
Table 1 Activation energies and concentrations of S double donors obtained from the fit of Eq. (1) to the measured DLTS spectrum Defect
Activation energy (meV)
Concentration (cm3)
S(h)0/+ S(h)+/2+ S(k1)0/+ S(k1)+/2+ S(k2)0/+ S(k2)+/2+
255 405 314 520 332 541
4.2 · 1014 3.9 · 1014 2.9 · 1014 3.6 · 1014 5.6 · 1014 5.8 · 1014
Open squares in Fig. 1.
Temperature (K) Fig. 1. DLTS spectrum (open squares) taken on a S-implanted 6H-SiC sample (S-dose = 1015 cm3).
the solid curve in Fig. 1 corresponds to the sum of the individual curves: 2DC Tw 2p=T w b1 ¼ exp ; ð1Þ 1 2 Tw se ð2p=T w Þ þ s2 e where Tw is the period width of the capacitance transient, se is the emission time constant and DC the amplitude of capacitance transient. The charge state indicated in Fig. 1 for each center is obtained from D-DLTS investigations, which reveal the activation energy Ea as a function of the electric field applied (Poole–Frenkel effect [5,6]). An example is given in Fig. 2 for the S(h)0/+-center showing that the experimental data (full dots) can be reproduced within the error bars by assuming a ‘‘0/+’’ charge state. The activation energies and defect concentrations obtained from the fit procedure are listed in Table 1. It turns out that the concentrations of singly charged states are equal to the corresponding concentrations of the doubly charged ones: ½SðhÞ0=þ ½SðhÞþ=2þ ; ½Sðk1 Þ0=þ ½Sðk1 Þþ=2þ and ½Sðk2 Þ0=þ ½Sðk2 Þþ=2þ .
3. Nitrogen donor deactivation For the deactivation experiments, we used p-type and ntype 4H-SiC epilayers with a doping level of [Al] = 3 · 1016 cm3 and [N] = 1 · 1016 cm3, respectively. In all cases, we implanted a box-shaped N profile to a depth of 1 lm (mean [N] = 3 · 1018 cm3, Timpl = 500 C). Some samples were co-implanted with a second species at different concentrations; these concentrations varied in a wide range: [C] = (1017–3 · 1019) cm3, [Si] = (1015–1019) cm3 or [Ne] = (1016–8 · 1018) cm3. The annealing steps were carried out for 30 min each at temperatures ranging between 300 and 1700 C. Fig. 4 summarizes the performed Hall effect investigations. The C/N- and Ne/N-co-implanted samples show the expected behavior. The electrically active N donor concentration slightly decreases with increasing concentration of the co-implanted species because of the increase of the compensation. In the case of Si/N-co-implantation, however, both the N donors and the compensation decrease with increasing co-implanted Si concentration. It seems that N donors together with an intrinsic species, which is generated by the Si implantation, form an electrically
300
n-6H-SiC:S
0/+
S(h)
280
a
Activation energy E (meV)
Further the sum over the concentrations of singly charged (or doubly charged) states corresponds roughly to the implanted S concentration. All these features give evidence
that the observed DLTS peaks are caused by S double donors residing on the three inequivalent lattice sites of 6HSiC. The assignment to the hexagonal site and to the two cubic sites is adopted from nitrogen donors and is speculative. The levels are summarized in Fig 3.
260 240
-/0 0/+
220 15
[S]=1x10 cm
-3
+/2+
200 40
60
80
100 120 140 160 180
Mean electric field F (kV/cm)
Fig. 2. Activation energy Ea of the S(h)0/+-center versus mean electric field (full dots); the Poole–Frenkel correction indicates that this defect state is singly positively charged after emission (solid curve).
Fig. 3. Energy scheme of the S double donor residing on the 3 inequivalent lattice sites of 6H-SiC (one hexagonal site (h) and two cubic sites (k1, k2)); the activation energies are corrected by the Poole–Frenkel effect.
1019 4H-SiC:N TA=1700˚C
reference sample ([N]=3.3x1018 cm-3)
(full symbols)
N-donor concentration (cm-3)
1019
, , ,
1018
C/N Si/N Ne/N
1018
1017 1016
1017
1018
1019
1017 1020
(open symbols)
G. Pensl et al. / Microelectronic Engineering 83 (2006) 146–149 concentration of compensation (cm-3)
148
Co-implanted C-, Si-, Ne-concentration (cm-3)
Fig. 4. Concentration of N donors and of the compensation versus concentration of co-implanted species (C, Si or Ne). The horizontal dashed straight line corresponds to the maximum concentration of electrically activated N donors.
neutral and thermally stable complex in n-type 4H-SiC. The intrinsic species could either be Si interstitials or carbon vacancies, which are also generated by the implanted surplus of Si atoms. A series of Si/N-co-implanted 4HSiC samples was annealed at different temperatures. It turned out that this particular complex was formed at annealing temperatures around 1450 C. Next step was to search with DLTS in the whole bandgap of 4H-SiC for the dominating defect center. For this reason, we implanted n-type 4H-SiC with N and coimplanted p-type 4H-SiC with Si/N and annealed the samples at temperatures between 1000 and 1700 C. The DLTS spectra taken on n-type 4H-SiC displayed the Z1/Z2-center as the dominating defect, which is thermally stable up to around 1450 C, while the P2-center [7] was observed at higher concentration in p-type SiC subsequent to anneals above 1400 C. In 4H-SiC, the energy positions of these defects are: EðZ 1 =Z 2 Þ ¼ EC 650 meV;
EðP 2 Þ ¼ EV þ 1:6 eV.
We repeated the latter experiment by substituting the Si implantation by irradiating the 4H-SiC samples with high
Fig. 6. Scheme of N-implanted, n-type 4H-SiC MOS capacitor; dmax = position of maximum of the implanted Gaussian N profile, dcons = SiC layer consumed for the oxidation.
energy electrons (E(e) = 200 keV). It is well known that electrons of 200 keV energy only damage the C-sublattice in SiC [8]. Fig. 5 discloses identical results as observed for the Si-implanted samples (not shown here). Again the Z1/Z2-center disappears subsequent to annealing steps at temperatures around 1450 C and the P2-center is generated at enhanced concentration at the same temperature. In summary, we have observed that the deactivation of N donors occurs at around 1450 C. This temperature also corresponds to the onset of the formation of the P2-center [7], which is the only dominating defect in the lower half of the bandgap of p-type 4H-SiC at such high temperatures. Because of the fact that the electron-irradiated samples cannot contain any Si-related components at extremely high concentrations, we propose at the present knowledge that a (Nx–VC,y)-complex is responsible for the deactivation of N donors. 4. Reduction of traps in n-type 4H-SiC MOS capacitors by N-implantation For the fabrication of MOS capacitors, we used two 4HSiC epilayers (n-type: [N] = 2 · 1016 cm3, p-type: [Al] = 3 · 1016 cm3). One half of each wafer was implanted with a Gaussian N profile. The implantation parameters of E(N+) = 20 kV and [N]max = 3 · 1018 cm3 were optimized for n-type SiC in a former publication [4].
1015
1.0
n-4H-SiC MOS 0.8 Si-face
P2 (p-type) 1014
0.6 0.4
0.10 T=250K
0.08
N-implanted 0.06 standard 0.04 0.02
0.2 E(e )=200 keV -
200
500
800 1100 1400 Annealing temperature TA (˚ C)
1700
Fig. 5. Z1/Z2-center (full dots) and P2-center (open dots) as a function of the annealing temperature determined by DLTS in e-irradiated (200 keV) n-type and p-type 4H-SiC, respectively.
0.0 -20
G/ω / COx
Z1/Z2 (n-type)
C/COx
Defect concentration (cm-3)
4H-SiC
0.00 -10 0 10 Gate bias (V)
Fig. 7. Normalized capacitance and normalized conductance versus gate bias taken at room temperature (probe frequency = 1 kHz, sweep rate = 0.2 V/s).
interface state density D it (cm-2eV-1)
G. Pensl et al. / Microelectronic Engineering 83 (2006) 146–149
n-type
1013
149
magnitude, while it is increased by roughly one order of magnitude in p-type MOS capacitors close to the valence bandedge. At the present, the reason for this increase is not clarified yet.
p-type
4H-SiC MOS 12
10
5. Conclusion , ,
11
10
1010 3.3
3.0
2.7
Si-face, standard Si-face, N-implanted
2.4 0.9 Eit-EV (eV)
0.6
0.3
0.0
Fig. 8. Density of interface traps Dit of a standard oxidized and of an additionally N-implanted n-/p-type 4H-SiC MOS capacitor.
The standard oxidation process (Tox = 1120 C, tox = 22 h, nominally dry O2) was adjusted in such a way that the SiC/ SiO2-interface is located at the trailing edge of the implanted Gaussian N profile as shown in Fig. 6. In Fig. 7, the normalized C–V and G/x–V characteristics of the not-implanted and the N-implanted n-type 4H-SiC MOS capacitor taken at 250 K are displayed. The C–V characteristic of the N-implanted MOS capacitor is shifted to negative voltages due to fixed positive charges and the corresponding G/x–V characteristic is strongly reduced in height. The density of interface states is obtained from the area below the conduction peak. In Fig. 8, Dit of the not-implanted and N-implanted MOS capacitor are compared. In n-type MOS capacitors, Dit close to the conduction bandedge is reduced by two and a half orders of
Ion implantation and electron irradiation are useful tools to adjust the electrical properties in the bulk of SiC and at the interface of SiC/SiO2 structures. Three examples are presented in this paper to demonstrate the benefit of defect-engineering. Acknowledgement The support of this work by the German Science Foundation (SiC-Forschergruppe) is gratefully acknowledged. References [1] M. Laube, F. Schmid, G. Pensl, G. Wagner, M. Linnarsson, M. Maier, J. Appl. Phys. 92 (2002) 549. [2] F. Schmid, T. Frank, G. Pensl, Mater. Sci. Forum 483–485 (2005) 641. [3] F. Schmid, Dissertation, Erlangen, 2005. [4] F. Ciobanu, G. Pensl, V. AfanasÕev, A. Scho¨ner, Mater. Sci. Forum 483–485 (2005) 693. [5] J. Frenkel, Phys. Rev. 54 (1938) 647. [6] J.L. Hartke, J. Appl. Phys. 39 (1968) 4871. [7] K. Danno, T. Kimoto, H. Matsunami, Appl. Phys. Lett. 86 (2005) 122104. [8] A.A. Rempel, W. Sprengel, K. Blaurock, K.J. Reichle, J. Major, H.E. Schaefer, Phys. Rev. Lett. 89 (2002) 185501.