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Anomalously large band-bending for HF-treated p-Si surfaces
Applied Surface Science 216 (2003) 24–29
Anomalously large band-bending for HF-treated p-Si surfaces D. Watanabe*, A. En, S. Nakamura, M. Suhara, T. Okumura Department of Electrical Engineering, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan
Abstract Electronic properties of the HF-treated Si surfaces have been characterized by the Kelvin method combined with surface photovoltage (SPV) measurements. With the use of 340 nm ultraviolet light source, a relatively large SPV of 0.45 V was detected at a photocurrent density of 1 mA/cm2 for the diluted (e.g. 4.5%) HF-treated p-Si(0 0 1) surface. On the other hand, no SPV was induced for the HF-treated n-Si(0 0 1) wafer. This result indicates that there is anomalously large surface band-bending toward the surface, and Fermi-level position at the surface is pinned in the vicinity of the bottom of the conduction band at the HF-treated p-Si(1 0 0). It is considered that the residual fluorine responsible for an anomalously large band-bending at the pSi(1 0 0) surface treated with HF. Furthermore, the value of the built-in potential for the HF-treated p-Si(0 0 1) surface was estimated to be about 0.60 eV at the room temperature from the result of the temperature dependence of the effective saturation current. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Kelvin method; Surface photovoltage (SPV); H-terminated Si surface; Surface band-bending; Fermi level
1. Introduction A silicon-on-insulator (SOI) material is attracting much attention as an ideal substrate for high-speed, low-power and highly integrated devices. As the thickness of the top-Si layer in the SOI materials is reduced to less than 100 nm, electronic properties of the surface as well as the buried Si/SiO2 interfaces become crucial to the device performance in addition to the quality of the top-Si layer. Particularly, the electrical characterization of the SOI materials has been of great interest in virtually every step during the processing of the SOI wafers and devices. The conventional electrical techniques, such as the current– voltage (I–V), capacitance–voltage and deep-level transient spectroscopy measurements, require several *
Corresponding author. Tel.: þ81-426-77-1111; fax: þ81-426-77-2756. E-mail address: [email protected] (D. Watanabe).
electrodes (contacts) [1,2], and hence the electronic properties are inevitably affected during the formation process of electrodes. Therefore, non-destructive and contactless methods are expected for such characterization. Recently, we have demonstrated that the Kelvin method, in combination with surface photovoltage (SPV) measurements, is promising for the contactless as well as non-destructive electrical characterization of SOI materials [3,4]. In addition, we have investigated that the light-intensity dependence of the SPV gives data equivalent to familiar I–V characteristics of diodes. Thus, we call this method the contactless I–V method. Up to now, we have demonstrated only the qualitative analysis of the electrical properties of the surface as well as the buried Si/SiO2 interfaces of SOI materials. However, in order to characterize the electronic properties of the surface as well as the buried Si/ SiO2 interfaces of SOI materials, the establishment of the quantitative analysis for bulk Si surfaces is highly
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00486-0
D. Watanabe et al. / Applied Surface Science 216 (2003) 24–29
essential expected. The data on the surface of bulk Si can be utilized for discrimination of the properties at the buried interface from those of the top surface for the SOI materials. In addition, a stable and reproducible Si surface is also necessary for such characterization. The HF-treated Si surface is known to be the very stable surface due to the passivation of surface dangling bonds by hydrogen [5–7]. Therefore, with the use of the HF treatment, the bulk Si surface is considered to be identical to the SOI surface. In this paper, we present the non-contact electricalcharacterization of electronic properties of the HFtreated Si surfaces by the Kelvin-SPV method. In order to determine physical parameters, e.g. the built-in potential, from the data of the Kelvin-SPV measurements, the temperature dependence of the contactless I–V characteristics is also measured.
2. Experimental procedures The schematic diagram of the Kelvin-SPV measurement system is shown in Fig. 1. The Kelvin method is known as a technique to determine the workfunction of materials [8]. Due to the existence of the contact potential deference (DVCPD), periodic
25
vibration of the reference electrode induces the displacement current through the circuit. In this work, the probe electrode of a gold mesh (2.5 mm Ø) was vibrating at about 210 Hz by the piezo-actuator. The displacement current was detected by using a lock-in amplifier, as the dc bias voltage was applied to the air gap between the electrode and the backside of the wafer. In general, the energy band of the semiconductor bends either upward or downward in the vicinity of the semiconductor surfaces. Hence, when the semiconductor is illuminated with light over band gap, the SPV should be induced. The induced SPV is added to the original DVCPD, and therefore, it can be also measured by the Kelvin method. We measured the SPV as a function of the light-intensity. To the first approximation, the SPV (VSPV) increases with the logarithm of the number of photogenerated carriers. Since the rate of photogenerated carriers in unit area corresponds to the current density (JL), the relationship between VSPV and JL can be written as [9,10] JL VSPV ¼ V0 ln þ1 ; (1) J0 where V0 and J0 are constants with the dimensions of voltage and current, respectively. This relationship is
Fig. 1. Schematic diagram of the Kelvin-SPV measurement system.
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D. Watanabe et al. / Applied Surface Science 216 (2003) 24–29
essentially the same as that for solar cells. In the actual experiments, we measured SPV by changing the illumination intensity. Provided that all absorbed photons in a semiconductor contribute to the photocurrent, JL can be estimated by the following relation: JL ¼ qFð1 RÞ;
(2)
where F is the incidental photon flux, R the optical reflectivity of Si, q the electronic charge, and a the optical absorption coefficient of Si, and L the minority carrier diffusion length of Si. In Eq. (1), V0 and J0 are should be determined by the limiting process for carrier transport over the barrier region, i.e. the depletion layer. V0 and J0 of Schottky-barrier-type solar cell, for example, become kT/q and A T 2 expðqfB =kTÞ, respectively, when the thermionic emission (TE) process is dominant (k: Boltzmann’s constant, T: temperature, A: effective Richardson constant, fB: Schottky barrier height) [11]. If additional current components, such as a recombination current, are superimposed upon the TE current, Eq. (1) can be phenomenologically modified by introducing the ideality factor n. Then, we can express the light-intensity dependence of the SPV by the following empirical form: qVSPV JL ¼ J0 exp 1 : (3) nkT This equation is the same form as that for the general I–V characteristics. In this sense, the Kelvin-SPV measurements can be called a ‘‘contactless I–V method’’. Sample can be illuminated through the meshed probe to measure the SPV. A UV light source with a center wavelength of 340 nm was used and the illumination intensity was changed by using a computer controlled variable reflective neutral density filter. Temperature dependence of the contactless I– V characteristics was also performed in the temperature range from 40 to þ80 8C. We used p-Si(0 0 1) wafers with a resistivity of 16– 18 O cm and n-Si(0 0 1) wafers with a resistivity of 4– 5 O cm. Prior to the measurement, these wafers were cleaned by the RCA method [12], and then dipped into a 4.5% HF solution for 1 min and rinsed with the deionized water for 10 min. After precleaning, the annealing process in vacuum at 100 8C for 20 min was performed in order to obtain a stable Si surface. Measurement was carried out in N2 atmosphere.
3. Results and discussion 3.1. Large band-bending Fig. 2 shows the light-intensity dependence of the SPV, i.e. the contactless I–V characteristics, for the HF-treated p-Si and n-Si(0 0 1) wafers. The SPV for the HF-treated p-Si wafer increases to the negative axis with increasing illumination intensity. This negative SPV for p-Si wafer is attributed to downward band-bending in the top surface. In addition, the SPV of about 0.45 V was observed at equivalent current density of 1 mA/cm2, and no saturation of the SPV was observed still in such a range of the illumination intensity as shown in Fig. 2. This result indicates that there is a large band-bending at the HF-treated pSi(0 0 1) surface. The Fermi level of the charge– neutral region of p-Si with a carrier concentration of about 1015 cm3 is located at about 0.25 eV above the top of valence band at room temperature. Therefore, it is found that the HF-treated p-Si(0 0 1) surface is inverted. On the other hand, no SPV was observed for the HF-treated n-Si(0 0 1) wafer. Therefore, it is considered that the energy band of the HF-treated nSi(0 0 1) surface is almost flat. These results indicate that the Fermi-level position at the HF-treated Si(0 0 1) surface is pinned in the vicinity of the bottom of the conduction band. The H-terminated surface is well known as a very stable surface, and which prevents from oxidation [5–7]. In general, the surface band-bending of the H-terminated Si is thought to be very small, which is due to the
Fig. 2. Contactless I–V characteristics for the HF-treated p-Si and n-Si(0 0 1) wafers.
D. Watanabe et al. / Applied Surface Science 216 (2003) 24–29
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layer. Both surface recombination current and a diffusion current, in general, contribute to the photocurrent in the SPV measurements. However, in the case of the HF-treated p-Si(0 0 1) wafers, the surface recombination current could be dominant because of its large band-bending, i.e. for the inversion surface, and very shallow penetration depth for the UV light. According to the Shockley–Read–Hall theory for carrier recombination, the surface recombination current, JSR, can be given as [11] JSR ¼ Fig. 3. Time variation of the contactless I–V curve for the HFtreated Si surface in the air at 293 K.
passivation of surface dangling bonds. In addition, Mo¨ nch et al. have reported that the H-terminated Si, which is treated in vacuum, surface band-bending is very small [13]. Therefore, it should be thought that the origin of surface large band-bending is residual atoms adsorped on the top surface. On the other hand, Schlaf et al. have recently reported that residual fluorine on the HF-treated Si surface increases surface band-bending for p-Si(1 1 1) [14]. In our experiments, it is also observed that 0.3% surface of sample was covered with fluorine revealed by XPS measurements. Fig. 3 shows the time variation of the contactless I–V curve for the HF-treated Si surface in the air at 293 K. The SPV is decreased gradually with the exposure time. With increasing the exposure time up to 3 days, the SPV varied by 0.1 Vas compared to that for the as-treated surface. It is considered from the fact of near midgap Fermi-level position for the Si/ SiO2 interface that this Si surface was oxidized and the surface band-bending decreases with exposing time. It is reported that the H-terminated surface became oxidized preferentially at the Si–F residual bond [15]. 3.2. Quantitative analysis In the present work, we used a UV light source with a center wavelength of 340 nm. At 340 nm, the optical absorption coefficient of Si is on the order of 106 cm1, and hence the penetration depth is several tens of nm. Therefore, almost all photons are absorbed within a strongly-bended surface depletion
qvth sp sn NSS ðps ns n2i Þ sp ½ps þ ni expððEi Et Þ=kTÞ
(4)
þ sn ½ns þ ni expððEi Et Þ=kTÞ where vth is the thermal velocity for holes and electrons, sp and sn the capture cross-sections for holes and electrons, respectively, NSS the surface trap density, ps and ns is hole density and electron density at the surface, ni the intrinsic carrier density, and Et the surface trap energy level. In the case of the inversion surface, the assumptions of low injection level, ns @ ps and ns ns0 are valid. In addition, the Boltzmann approximation gives that nsps can be represent as ns0 ps0 expðqVSPV =kTÞ. Therefore, Eq. (4) can be represented as qVSPV JSR ¼ qvth sNSS ps0 exp 1 ; (5) kT where capture cross-sections for holes and electrons are assumed to be identical to each other. Here, ps0 is the equilibrium hole density at the surface, and it can be represent as qVFS ps0 ¼ NV exp ; (6) kT where NV is the effective density of states in valence band, VFS is define as the position of the surface Fermi level above the top of the valence band (EFS EV). In addition, the effective saturation current in Eq. (3), J0, can be written as qVFS qVFS J0 ¼ qvth sNSS NV exp / T 2 exp ; kT kT (7) where temperature dependence of NV / T 1:5 and vth / T 0:5 are used [11]. This equation indicates that
28
D. Watanabe et al. / Applied Surface Science 216 (2003) 24–29
Fig. 4. Temperature dependence of the contactless I–V characteristics for the HF-treated p-Si(1 0 0) wafer. The inset shows the Arrhenius’ plot of temperature dependence of the effective saturation current. The dashed line indicates the result of the least-square fit to the experiment data.
the surface recombination current. In addition, extrapolation of this linear portion in a semi-logarithmic plot of the contactless I–V curve to the current axis gives the effective saturation current density. The inset shows the Arrhenius’ plot of the temperature dependence of the effective saturation current. Using a least square fit to the experiment data for the temperature dependence of the effective saturation current, the value of the Fermi-level position at the surface VFS is estimated to be about 0.85 eV. Since, the Fermi level of the charge–neutral region for p-Si with a carrier concentration of about 1015 cm3 is located at 0.25 eV above the top of valence band, the value of the built-in potential (Vbi) is estimated to be about 0.60 eV. This result indicates that there is anomalously large surface band-bending at the HF-treated p-Si(0 0 1) surface.
4. Conclusions the experimental data of the temperature dependence of the contactless I–V characteristics gives the surface Fermi-level position (VFS). Thus, value of surface band-bending, i.e. the built-in potential (Vbi), can be estimated by taking the Fermi level of the charge– neutral region. Fig. 4 shows the temperature dependence of the contactless I–V characteristics for the HF-treated pSi(1 0 0) wafer. With increasing temperature, the magnitude of SPV at the same illumination intensity decreases because of increasing the rate of the surface recombination rate due to the thermally generated carriers. As can be seen from Eq. (5), if the photocurrent is dominated by the surface recombination current, the equivalent current density should increase exponentially with increasing the SPV. However, in the region of photocurrent density lower than 5 106 A/cm2 (denoted by ‘region A’), no exponential relationship is observed. On the contrary, it is found from the linear scale plot, the equivalent current density follows the linear SPV dependence. The detail of this current component is not clear to date. On the other hand, above the photocurrent density of 5 106 A/cm2 (denoted by ‘region B’), the good linearity over several decades in a semi-logarithmic plot is observed as shown in Fig. 4. The estimated ideality factor is n ¼ 1–1.1. Therefore, it is found that this current is attributed to
We have characterized electronic properties of the HF-treated Si surfaces by the Kelvin-SPV method. With the use of 340 nm ultraviolet light source, a relatively large SPV of 0.45 V was detected at 1 mA/cm2 of the photocurrent density for the diluted (e.g. 4.5%) HF-treated p-Si(0 0 1) surface. On the other hand, no SPV was observed for the HFtreated n-Si(0 0 1) wafer. This result indicates that there is anomalously large surface band-bending at the HF-treated p-Si(0 0 1) surface, and that the Fermi-level position at the surface is pined in the vicinity of the bottom of the conduction band. It is considered that residual fluorine is responsible for an anomalously large band-bending at the p-Si(0 0 1) surface treated with HF. The value of the built-in potential for the HF-treated p-Si(0 0 1) surface was estimated to be about 0.60 eV from the result of the temperature dependence of the effective saturation current.
Acknowledgements This work was supported partly by JSPS Research for the Future Program under the project ‘Ultimate characterization technique of SOI wafer for the NanoScale LSI Devices’.
D. Watanabe et al. / Applied Surface Science 216 (2003) 24–29
References [1] H.S. Kang, C.G. Ahn, B.K. Kang, Y.K. Kwon, J. Electrochem. Soc. 145 (1998) 3581. [2] Y. Li, T.-P. Ma, IEEE Trans. ED 45 (1998) 1329. [3] T. Okumura, K. Eguchi, A. En, M. Suhara, Jpn. J. Appl. Phys. 40 (2001) 5217. [4] T. Okumura, A. En, K. Eguchi, M. Suhara, Mater. Sci. Eng. B 91–92 (2002) 182. [5] V.A. Burrows, Y.J. Chabal, G.S. Higashi, K. Raghavachari, S.B. Christman, Appl. Phys. Lett. 53 (1988) 998. [6] T. Takahagi, I. Nagai, A. Ishitani, H. Kuroda, Y. Nagasawa, J. Appl. Phys. 64 (1988) 3516. [7] D. Gra¨ f, M. Grundner, R. Schulz, L. Mu¨ hlhoff, J. Appl. Phys. 68 (1990) 5155.
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[8] Lord Kelvin, Phil. Mag. 46 (1898) 82. [9] W.H. Brattain, J. Bardeen, Bell Syst. Technol. J. 32 (1953) 1. [10] V. Gerstner, A. Knoll, W. Pfeiffer, A. Thon, G. Gerber, J. Appl. Phys. 88 (2000) 4851. [11] S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley, New York, 1981. [12] W. Kem, D.A. Puotinen, RCA Rev. 187 (1970). [13] W. Mo¨ nch, P. Koke, S. Krueger, J. Vac. Sci. Technol. 19 (1981) 313. [14] R. Schlaf, R. Hinogami, M. Fujitani, S. Yae, Y. Nakato, J. Vac. Sci. Tecnol. A 17 (1999) 164. [15] A. Chatterjee, T. Iwasaki, T. Ebina, M. Kubo, A. Miyamoto, J. Phys. Chem. B 102 (1998) 9215.
Anomalously large band-bending for HF-treated p-Si surfaces D. Watanabe*, A. En, S. Nakamura, M. Suhara, T. Okumura Department of Electrical Engineering, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan
Abstract Electronic properties of the HF-treated Si surfaces have been characterized by the Kelvin method combined with surface photovoltage (SPV) measurements. With the use of 340 nm ultraviolet light source, a relatively large SPV of 0.45 V was detected at a photocurrent density of 1 mA/cm2 for the diluted (e.g. 4.5%) HF-treated p-Si(0 0 1) surface. On the other hand, no SPV was induced for the HF-treated n-Si(0 0 1) wafer. This result indicates that there is anomalously large surface band-bending toward the surface, and Fermi-level position at the surface is pinned in the vicinity of the bottom of the conduction band at the HF-treated p-Si(1 0 0). It is considered that the residual fluorine responsible for an anomalously large band-bending at the pSi(1 0 0) surface treated with HF. Furthermore, the value of the built-in potential for the HF-treated p-Si(0 0 1) surface was estimated to be about 0.60 eV at the room temperature from the result of the temperature dependence of the effective saturation current. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Kelvin method; Surface photovoltage (SPV); H-terminated Si surface; Surface band-bending; Fermi level
1. Introduction A silicon-on-insulator (SOI) material is attracting much attention as an ideal substrate for high-speed, low-power and highly integrated devices. As the thickness of the top-Si layer in the SOI materials is reduced to less than 100 nm, electronic properties of the surface as well as the buried Si/SiO2 interfaces become crucial to the device performance in addition to the quality of the top-Si layer. Particularly, the electrical characterization of the SOI materials has been of great interest in virtually every step during the processing of the SOI wafers and devices. The conventional electrical techniques, such as the current– voltage (I–V), capacitance–voltage and deep-level transient spectroscopy measurements, require several *
Corresponding author. Tel.: þ81-426-77-1111; fax: þ81-426-77-2756. E-mail address: [email protected] (D. Watanabe).
electrodes (contacts) [1,2], and hence the electronic properties are inevitably affected during the formation process of electrodes. Therefore, non-destructive and contactless methods are expected for such characterization. Recently, we have demonstrated that the Kelvin method, in combination with surface photovoltage (SPV) measurements, is promising for the contactless as well as non-destructive electrical characterization of SOI materials [3,4]. In addition, we have investigated that the light-intensity dependence of the SPV gives data equivalent to familiar I–V characteristics of diodes. Thus, we call this method the contactless I–V method. Up to now, we have demonstrated only the qualitative analysis of the electrical properties of the surface as well as the buried Si/SiO2 interfaces of SOI materials. However, in order to characterize the electronic properties of the surface as well as the buried Si/ SiO2 interfaces of SOI materials, the establishment of the quantitative analysis for bulk Si surfaces is highly
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00486-0
D. Watanabe et al. / Applied Surface Science 216 (2003) 24–29
essential expected. The data on the surface of bulk Si can be utilized for discrimination of the properties at the buried interface from those of the top surface for the SOI materials. In addition, a stable and reproducible Si surface is also necessary for such characterization. The HF-treated Si surface is known to be the very stable surface due to the passivation of surface dangling bonds by hydrogen [5–7]. Therefore, with the use of the HF treatment, the bulk Si surface is considered to be identical to the SOI surface. In this paper, we present the non-contact electricalcharacterization of electronic properties of the HFtreated Si surfaces by the Kelvin-SPV method. In order to determine physical parameters, e.g. the built-in potential, from the data of the Kelvin-SPV measurements, the temperature dependence of the contactless I–V characteristics is also measured.
2. Experimental procedures The schematic diagram of the Kelvin-SPV measurement system is shown in Fig. 1. The Kelvin method is known as a technique to determine the workfunction of materials [8]. Due to the existence of the contact potential deference (DVCPD), periodic
25
vibration of the reference electrode induces the displacement current through the circuit. In this work, the probe electrode of a gold mesh (2.5 mm Ø) was vibrating at about 210 Hz by the piezo-actuator. The displacement current was detected by using a lock-in amplifier, as the dc bias voltage was applied to the air gap between the electrode and the backside of the wafer. In general, the energy band of the semiconductor bends either upward or downward in the vicinity of the semiconductor surfaces. Hence, when the semiconductor is illuminated with light over band gap, the SPV should be induced. The induced SPV is added to the original DVCPD, and therefore, it can be also measured by the Kelvin method. We measured the SPV as a function of the light-intensity. To the first approximation, the SPV (VSPV) increases with the logarithm of the number of photogenerated carriers. Since the rate of photogenerated carriers in unit area corresponds to the current density (JL), the relationship between VSPV and JL can be written as [9,10] JL VSPV ¼ V0 ln þ1 ; (1) J0 where V0 and J0 are constants with the dimensions of voltage and current, respectively. This relationship is
Fig. 1. Schematic diagram of the Kelvin-SPV measurement system.
26
D. Watanabe et al. / Applied Surface Science 216 (2003) 24–29
essentially the same as that for solar cells. In the actual experiments, we measured SPV by changing the illumination intensity. Provided that all absorbed photons in a semiconductor contribute to the photocurrent, JL can be estimated by the following relation: JL ¼ qFð1 RÞ;
(2)
where F is the incidental photon flux, R the optical reflectivity of Si, q the electronic charge, and a the optical absorption coefficient of Si, and L the minority carrier diffusion length of Si. In Eq. (1), V0 and J0 are should be determined by the limiting process for carrier transport over the barrier region, i.e. the depletion layer. V0 and J0 of Schottky-barrier-type solar cell, for example, become kT/q and A T 2 expðqfB =kTÞ, respectively, when the thermionic emission (TE) process is dominant (k: Boltzmann’s constant, T: temperature, A: effective Richardson constant, fB: Schottky barrier height) [11]. If additional current components, such as a recombination current, are superimposed upon the TE current, Eq. (1) can be phenomenologically modified by introducing the ideality factor n. Then, we can express the light-intensity dependence of the SPV by the following empirical form: qVSPV JL ¼ J0 exp 1 : (3) nkT This equation is the same form as that for the general I–V characteristics. In this sense, the Kelvin-SPV measurements can be called a ‘‘contactless I–V method’’. Sample can be illuminated through the meshed probe to measure the SPV. A UV light source with a center wavelength of 340 nm was used and the illumination intensity was changed by using a computer controlled variable reflective neutral density filter. Temperature dependence of the contactless I– V characteristics was also performed in the temperature range from 40 to þ80 8C. We used p-Si(0 0 1) wafers with a resistivity of 16– 18 O cm and n-Si(0 0 1) wafers with a resistivity of 4– 5 O cm. Prior to the measurement, these wafers were cleaned by the RCA method [12], and then dipped into a 4.5% HF solution for 1 min and rinsed with the deionized water for 10 min. After precleaning, the annealing process in vacuum at 100 8C for 20 min was performed in order to obtain a stable Si surface. Measurement was carried out in N2 atmosphere.
3. Results and discussion 3.1. Large band-bending Fig. 2 shows the light-intensity dependence of the SPV, i.e. the contactless I–V characteristics, for the HF-treated p-Si and n-Si(0 0 1) wafers. The SPV for the HF-treated p-Si wafer increases to the negative axis with increasing illumination intensity. This negative SPV for p-Si wafer is attributed to downward band-bending in the top surface. In addition, the SPV of about 0.45 V was observed at equivalent current density of 1 mA/cm2, and no saturation of the SPV was observed still in such a range of the illumination intensity as shown in Fig. 2. This result indicates that there is a large band-bending at the HF-treated pSi(0 0 1) surface. The Fermi level of the charge– neutral region of p-Si with a carrier concentration of about 1015 cm3 is located at about 0.25 eV above the top of valence band at room temperature. Therefore, it is found that the HF-treated p-Si(0 0 1) surface is inverted. On the other hand, no SPV was observed for the HF-treated n-Si(0 0 1) wafer. Therefore, it is considered that the energy band of the HF-treated nSi(0 0 1) surface is almost flat. These results indicate that the Fermi-level position at the HF-treated Si(0 0 1) surface is pinned in the vicinity of the bottom of the conduction band. The H-terminated surface is well known as a very stable surface, and which prevents from oxidation [5–7]. In general, the surface band-bending of the H-terminated Si is thought to be very small, which is due to the
Fig. 2. Contactless I–V characteristics for the HF-treated p-Si and n-Si(0 0 1) wafers.
D. Watanabe et al. / Applied Surface Science 216 (2003) 24–29
27
layer. Both surface recombination current and a diffusion current, in general, contribute to the photocurrent in the SPV measurements. However, in the case of the HF-treated p-Si(0 0 1) wafers, the surface recombination current could be dominant because of its large band-bending, i.e. for the inversion surface, and very shallow penetration depth for the UV light. According to the Shockley–Read–Hall theory for carrier recombination, the surface recombination current, JSR, can be given as [11] JSR ¼ Fig. 3. Time variation of the contactless I–V curve for the HFtreated Si surface in the air at 293 K.
passivation of surface dangling bonds. In addition, Mo¨ nch et al. have reported that the H-terminated Si, which is treated in vacuum, surface band-bending is very small [13]. Therefore, it should be thought that the origin of surface large band-bending is residual atoms adsorped on the top surface. On the other hand, Schlaf et al. have recently reported that residual fluorine on the HF-treated Si surface increases surface band-bending for p-Si(1 1 1) [14]. In our experiments, it is also observed that 0.3% surface of sample was covered with fluorine revealed by XPS measurements. Fig. 3 shows the time variation of the contactless I–V curve for the HF-treated Si surface in the air at 293 K. The SPV is decreased gradually with the exposure time. With increasing the exposure time up to 3 days, the SPV varied by 0.1 Vas compared to that for the as-treated surface. It is considered from the fact of near midgap Fermi-level position for the Si/ SiO2 interface that this Si surface was oxidized and the surface band-bending decreases with exposing time. It is reported that the H-terminated surface became oxidized preferentially at the Si–F residual bond [15]. 3.2. Quantitative analysis In the present work, we used a UV light source with a center wavelength of 340 nm. At 340 nm, the optical absorption coefficient of Si is on the order of 106 cm1, and hence the penetration depth is several tens of nm. Therefore, almost all photons are absorbed within a strongly-bended surface depletion
qvth sp sn NSS ðps ns n2i Þ sp ½ps þ ni expððEi Et Þ=kTÞ
(4)
þ sn ½ns þ ni expððEi Et Þ=kTÞ where vth is the thermal velocity for holes and electrons, sp and sn the capture cross-sections for holes and electrons, respectively, NSS the surface trap density, ps and ns is hole density and electron density at the surface, ni the intrinsic carrier density, and Et the surface trap energy level. In the case of the inversion surface, the assumptions of low injection level, ns @ ps and ns ns0 are valid. In addition, the Boltzmann approximation gives that nsps can be represent as ns0 ps0 expðqVSPV =kTÞ. Therefore, Eq. (4) can be represented as qVSPV JSR ¼ qvth sNSS ps0 exp 1 ; (5) kT where capture cross-sections for holes and electrons are assumed to be identical to each other. Here, ps0 is the equilibrium hole density at the surface, and it can be represent as qVFS ps0 ¼ NV exp ; (6) kT where NV is the effective density of states in valence band, VFS is define as the position of the surface Fermi level above the top of the valence band (EFS EV). In addition, the effective saturation current in Eq. (3), J0, can be written as qVFS qVFS J0 ¼ qvth sNSS NV exp / T 2 exp ; kT kT (7) where temperature dependence of NV / T 1:5 and vth / T 0:5 are used [11]. This equation indicates that
28
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Fig. 4. Temperature dependence of the contactless I–V characteristics for the HF-treated p-Si(1 0 0) wafer. The inset shows the Arrhenius’ plot of temperature dependence of the effective saturation current. The dashed line indicates the result of the least-square fit to the experiment data.
the surface recombination current. In addition, extrapolation of this linear portion in a semi-logarithmic plot of the contactless I–V curve to the current axis gives the effective saturation current density. The inset shows the Arrhenius’ plot of the temperature dependence of the effective saturation current. Using a least square fit to the experiment data for the temperature dependence of the effective saturation current, the value of the Fermi-level position at the surface VFS is estimated to be about 0.85 eV. Since, the Fermi level of the charge–neutral region for p-Si with a carrier concentration of about 1015 cm3 is located at 0.25 eV above the top of valence band, the value of the built-in potential (Vbi) is estimated to be about 0.60 eV. This result indicates that there is anomalously large surface band-bending at the HF-treated p-Si(0 0 1) surface.
4. Conclusions the experimental data of the temperature dependence of the contactless I–V characteristics gives the surface Fermi-level position (VFS). Thus, value of surface band-bending, i.e. the built-in potential (Vbi), can be estimated by taking the Fermi level of the charge– neutral region. Fig. 4 shows the temperature dependence of the contactless I–V characteristics for the HF-treated pSi(1 0 0) wafer. With increasing temperature, the magnitude of SPV at the same illumination intensity decreases because of increasing the rate of the surface recombination rate due to the thermally generated carriers. As can be seen from Eq. (5), if the photocurrent is dominated by the surface recombination current, the equivalent current density should increase exponentially with increasing the SPV. However, in the region of photocurrent density lower than 5 106 A/cm2 (denoted by ‘region A’), no exponential relationship is observed. On the contrary, it is found from the linear scale plot, the equivalent current density follows the linear SPV dependence. The detail of this current component is not clear to date. On the other hand, above the photocurrent density of 5 106 A/cm2 (denoted by ‘region B’), the good linearity over several decades in a semi-logarithmic plot is observed as shown in Fig. 4. The estimated ideality factor is n ¼ 1–1.1. Therefore, it is found that this current is attributed to
We have characterized electronic properties of the HF-treated Si surfaces by the Kelvin-SPV method. With the use of 340 nm ultraviolet light source, a relatively large SPV of 0.45 V was detected at 1 mA/cm2 of the photocurrent density for the diluted (e.g. 4.5%) HF-treated p-Si(0 0 1) surface. On the other hand, no SPV was observed for the HFtreated n-Si(0 0 1) wafer. This result indicates that there is anomalously large surface band-bending at the HF-treated p-Si(0 0 1) surface, and that the Fermi-level position at the surface is pined in the vicinity of the bottom of the conduction band. It is considered that residual fluorine is responsible for an anomalously large band-bending at the p-Si(0 0 1) surface treated with HF. The value of the built-in potential for the HF-treated p-Si(0 0 1) surface was estimated to be about 0.60 eV from the result of the temperature dependence of the effective saturation current.
Acknowledgements This work was supported partly by JSPS Research for the Future Program under the project ‘Ultimate characterization technique of SOI wafer for the NanoScale LSI Devices’.
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