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b solidi physica status pss Phys. Status Solidi B, 1–5 (2010) / DOI 10.1002/pssb.200983956 www.pss-b.com basic solid state physics Boron induced charge traps near the interface of Si/SiO2 probed by second harmonic generation ,1 1,5 1,6 1 1,2 3 Heungman Park* , Jingbo Qi , Ying Xu , Kalman Varga , Sharon M. Weiss , Bridget R. Rogers , 4 1 Gunter Lüpke , and Norman Tolk 1 Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, Tennessee 37235, USA 3 Department of Chemical and Biomolecular Engineering, Nashville, Vanderbilt University, Tennessee 37235, USA 4 Department of Applied Science, College of William and Mary, Williamsburg, Virginia 23187, USA 5 National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA 6 Zomega Terahertz Corporation, Troy, New York 12180, USA 2 Received 15 December 2009, revised 27 February 2010, accepted 12 March 2010 Published online 15 June 2010 Keywords defects, frequency conversion, silicon, surfaces and interfaces * Corresponding author: e-mail [email protected], Phone: þ1 615 343 2219, Fax: þ1 615 343 7263 We review recent second harmonic generation (SHG) measurements for highly boron-doped Si/SiO2 systems. Using electric field sensitive time-dependent SHG (TD-SHG), we determined that the direction of the initial DC electric field at the interface induced by boron induced charge traps is from oxide to silicon thus demonstrating that the boron induced charge traps in the oxide are positively charged. For a thin oxide ("2 nm) both boron traps and O2 surface oxide traps contribute. However, for a highly boron-doped Si/SiO2 sample with a thick thermally grown oxide (thickness: 12 nm), the TD-SHG signal exhibits a monotonic decrease arising from filling only the boron charge traps. By fitting our data, we show that the interface effective susceptibility jx(2)j is heavily dependent on doping concentration. ! 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction of electric field induced second harmonic generation SHG has been established as an effective method to study surface and interface electronic properties of materials. Because SHG signals vanish for inversion symmetric materials such as silicon and also amorphous SiO2 under the dipole radiation approximation [1–3], SHG signals arise primarily from surfaces and interfaces. When there is an electric field present inside these materials, the SHG signal contains an additional bulk x(3) contribution [4–8]. Electric field induced SHG (EFISH) can be described by the following equation [9, 10] ! !2 I ð2vÞ ¼ !xð2Þ þ xð3Þ Edc ! ðI ðvÞ Þ2 : (1) x(2) and x(3) are non-linear second- and third-order electric susceptibilities, respectively, and Edc is a quasi-static electric field inside these materials. For Si/SiO2 systems, if the oxide thickness is less than 10 nm, electrons from the silicon valence band can be excited to the conduction band of the oxide, usually by a multiphoton process. The excited electrons can travel through the oxide and then may be captured by oxygen molecules on the surface of the oxide [10]. The remaining holes in the silicon and the electrons in the oxygen molecules form a capacitor-like electric field. This electric field is responsible for the EFISH signals in Si/SiO2 systems. If the fundamental laser beam wavelength is 800 nm (1.55 eV), the typical EFISH measurement shows a slow monotonic increase, which saturates with time as shown in Fig. 1. For 800 nm photons, a three-photon absorption process is required to excite the electrons from the silicon valence band to the oxide conduction band [10], a situation where the electron excitation probability is very low and thus the trapping rate is also low. Therefore Edc increases slowly and the time-dependency can be easily detected by a timedependent SHG (TD-SHG) measurement. Edc in Eq. (1) is expressed as a time-dependent electric field Edc(t) across the interface of Si/SiO2. The EFISH technique has been extensively used to investigate the interface properties of semiconductor-oxide systems. In particular, it was used to study electron and hole dynamics at the interface of Si/SiO2 ! 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim solidi status physica pss b H. Park et al.: Boron induced charge traps near the interface of Si/SiO2 probed by SHG SHG (a.u.) 2 600 400 200 0 0 200 400 600 Time (s) Figure 1 TD-SHG measurement from a Sb doped Si wafer with a native oxide (resistivity: 0.005–0.02 V cm) [11]. systems [9, 12], and also for Si/Al2O3 [13, 14], Si/HfO2 [13, 15], Si/MgO [16], Si/ZrO2 [13, 17] and SOI [18] systems. In addition to these material systems, Scheidt et al. investigated highly boron-doped Si/SiO2 systems [19]. We also reported independently similar results with a somewhat different interpretation [20]. In particular, Scheidt et al. used rotational anisotropic SHG (RA-SHG) to determine the interface electric field direction. The RA-SHG results showed a phase difference between initial and saturated SHG signals, which confirmed that the built-in boron induced charge traps and the oxygen charge traps create DC electric fields in opposite directions. In our previous paper, we focussed on the formation mechanism and stability of boron induced charge traps, and filling process by internal photo-emission. In this paper we discuss our experimental results and interpretations. 2 Experimental setup Our experimental setup is shown in Fig. 2. A Mira Ti:Sapphire laser is used to generate a P-polarized 800 nm beam. The beam is focused onto the samples at 458. The laser repetition rate is 75 MHz with a pulse width of about 150 fs. A prism is used to separate the resulting 400 nm beam. The 400 nm SHG beam is directed to a photo-multiplier-tube. The SHG intensity is measured by a photon counter. The plane-of-incidence is parallel to the h100i sample crystal plane. When the sample was rotated to λ =800 nm (1.55 eV) λ =400nm the h110i crystal orientation, the same results were observed. All measurements were performed in air at room temperature and in dark conditions. Table 1 is a description of the silicon (100) wafers used in this study, which outlines the dopant species, resistivity, and dopant concentrations, respectively. Ellipsometry was used to measure the thicknesses of the native oxide layers. All wafers were found to have a 2 nm thick oxide within experimental error. A laser power of 400 mW corresponds to an approximately 8.8 GW/cm2 peak intensity on the sample surface. 3 Boron induced charge traps An initial sharp decrease as a function of time was observed in our TD-SHG measurement for highly boron-doped silicon wafers as shown in Fig. 3. However, measurements on highly antimony-doped wafers did not show a similar decrease (Fig. 1). Only wafers B and C showed an initial decrease in these TD-SHG measurements. It was clear that wafer A would also show the initial SHG decrease if SHG data acquisition rate were fast enough. The time-dependency of the SHG measurements arises directly from the timedependent electric field across the interface of Si/SiO2. Thus we suggest that the initial decrease in the SHG measurement comes from the presence of a substantial preexisting electric field across the Si/SiO2 interface, which was created during the growth of the oxide. We attribute this electric field to the presence of B- ions in the silicon and the presence of Bþ ions in the oxide, forming a capacitor-like field across the interface. The initial SHG signal at t ¼ 0 comes from x(2) þ x(3)E(t ¼ 0) as shown in Eq. (1). The E(t ¼ 0) is the initial electric field which is present at the interface. This initial electric field comes from boron charge traps as shown by in Fig. 3. The initial electric field direction is from the oxide to the silicon substrate. During intense laser illumination, photo-excited electrons in the silicon substrate fill the boron Bþ charge traps and also fill the oxygen traps on the oxide surface. The electrons captured by the Bþ charge traps decrease the magnitude of the initial electric field. The electrons captured by the oxygen traps increase an additional electric field, which is opposite to the direction of the initial boron-induced electric field. Therefore the magnitude of the PMT Figure 2 (online color at: www.pss-b.com) Experimental setup for SHG measurements. SHG (a.u.) 400 Sample Ԙ ԙ 300 E=0 Ԝ ԛ 200 Table 1 Silicon sample description. Ԛ wafer label dopant resistivity (V cm) concentration (cm&3) wafer wafer wafer wafer boron boron boron antimony 5–15 0.01–0.02 0.001–0.009 0.005–0.020 2.7 ' 1014–9 ' 1015 3.1 ' 1018–8.4 ' 1018 9.7 ' 1018–1.3 ' 1020 1.2 ' 1018–1.1 ' 1019 A B C D E≠ 0 ! 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0 200 400 Time (s) 600 800 E≠ 0 Si Ԙ ԙ Ԛ ԛ Ԝ SiO2 Figure 3 Left: TD-SHG measurement from highly boron doped silicon (resistivity: 0.01–0.02 V cm), right: interpretation of the SHG measurement regarding interface electric fields (white arrow: preexisting built-in electric filed due to boron charge traps, dark arrow: induced electric field due to surface oxygen charge traps) [11]. www.pss-b.com Original Paper Phys. Status Solidi B (2010) 3 4000 b) Wafer B 3000 396mW 2000 307mW 1000 SHG (a.u.) SHG (a.u.) a) 600 500 SHG (a.u.) total electric field decreases as measured by the decreasing SHG signal, indicated by in Fig. 3. At the minimum of the SHG signal, the net total electric field becomes zero, though the two separate electric fields may not be zero, as shown by in Fig. 3. After reaching a minimum, the SHG signal increases as more electrons are transferred from silicon into the oxygen charge traps on the oxide surface and into the Bþ charge traps until both traps are saturated, as shown in and in Fig. 3. Because the native oxide layer is relatively thin, electrons can be easily detrapped from the oxygen charge traps due to quantum mechanical tunneling. When the laser beam is blocked, the electrons captured by the oxygen molecules on the surface of the oxide can tunnel back to empty states in the valence band of the silicon substrate, which results in a decrease of the electric field at the interface [9, 12]. We may expect that during the blocking the boron charge traps also release electrons to silicon substrate. However when the beam was unblocked after being blocked for a short period, no initial SHG decrease was observed as shown in Fig. 4. Even the beam was blocked for about 1 h, no significant recovery of the initial SHG decrease was observed. From the observation we can infer that the detrapping rate for the boron charge traps is much less than the detrapping rate for the surface oxygen traps or the filled Bþ charge traps become stable after filled by electrons. This interpretation is supported by a density functional calculation which shows that neutral boron (B) is more stable in amorphous SiO2 than positively ionized boron (Bþ) [21]. One paper suggested that B atoms diffuse into SiO2 as Bþ ions [22]. Even more recently, another group showed that boron has various stable and meta-stable forms in crystalline and defective SiO2 [23, 24]. This group also showed that Bþ is one of these stable forms. They showed results in 400 300 200 initial laser blocking 100 0 0 600 800 1000 1200 1400 Time (s) agreement with the previous research [21] in amorphous SiO2. 4 Boron doping concentration dependent second order electric susceptibility: x(2) The magnitude of the initial decrease in our SHG measurements correlates with boron doping concentration, as shown in Fig. 5. When laser power is sufficiently high, the initial SHG decrease is not observed for wafer B, because the oxygen charge traps are filled very quickly by the excited electrons (Fig. 5(a): 396 mW). However, when the laser power is very low, the oxygen charge traps are filled very slowly due to the fact that a three-photon excitation process is required (the three-photon excitation probability is proportional to the cube of laser power). This is illustrated in Fig. 5(a): 154 mW and Fig. 5(b): 155 mW. Thus if laser power is low enough, 4000 Wafer C 3000 397mW 2000 305mW 1000 155mW 20 40 60 80 100 0 20 1400 40 60 80 100 Time (s) Time (s) d) Wafer B (307mW) 1200 SHG (a.u.) SHG (a.u.) 400 0 0 c) 200 Figure 4 TD-SHG measurement from highly boron doped silicon (resistivity: 0.01–0.02 V cm). During the measurement the laser beam is blocked [11]. 154mW 0 second laser blocking 1000 800 600 2400 Wafer C (305mW) 2000 1600 1200 400 0 www.pss-b.com 20 40 60 Time (s) 80 100 0 20 40 60 Time (s) 80 100 Figure 5 (online color at: www.pssb.com) (a) SHG results from boron doped silicon (resistivity: 0.01–0.02 V cm) by three different laser powers [11]. (b) SHG results boron doped silicon (resistivity: 0.001–0.009 V cm) by three different laser powers [11]. (c) Fitting result of 307 mW data of (a). (d) Fitting result of 305 mW data of (b). All data were taken using identical experimental conditions. ! 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim solidi status physica pss b 4 H. Park et al.: Boron induced charge traps near the interface of Si/SiO2 probed by SHG the three-photon excitation probability can be considered negligible. In this case, only a monotonic decrease in our TDSHG measurement is observed which indicates that the boron-induced electric field decreases monotonically. This observation implies that it may require two-photon or onephoton excitation to fill the boron charge traps. If the oxide thickness is greater than 10 nm, excited electrons from the silicon valence band cannot reach the surface of the oxide. In this case oxygen charge traps do not play a role in the measured TD-SHG signals [10]. In particular, the excited electrons fill only the boron charge traps, which results in a decreasing built-in DC electric field as the traps are filled. Therefore for highly boron-doped Si/ SiO2 with a thick oxide layer, it is expected that only the boron charge traps are involved in a TD-SHG measurement and a monotonic decrease should be shown in the measurement. An experimental result is shown in Fig. 6 using wafer C with a thermally grown oxide layer (thickness: 12 nm). It showed only a monotonic decrease even with high laser beam intensity (526 mW) as expected. Relative information on the susceptibilities is obtained by fitting Eq. (1) to our data. In general, x(2) and x(3) are complex numbers therefore Eq. (1) can be modified as ! !2 (2) I ð2vÞ ¼ !jxð2Þ j þ jxð3Þ jeiu EðtÞ! ðI ðvÞ Þ2 ; where u is the relative phase between the x(2) and x(3). The time-dependent electric field E(t) can be obtained by a rate equation which can be expressed by EðtÞ ¼ E0 e&t=t1 & E1 ð1 & e&t=t2 Þ; SHG (a.u.) 1500 1250 750 500 250 526 mW Wafer C Thermally grown oxide Thickness = 12 nm 0 100 200 Time (s) 300 Figure 6 SHG results measured in highly boron-doped Si/SiO2 with a thermally grown oxide layer (oxide thickness: 12 nm, resistivity: 0.001–0.009 V cm). ! 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 5 Summary We observed an initial SHG amplitude decrease in TD-SHG measurements on highly boron-doped silicon wafers indicating a time-dependent decrease in an electric field at the Si/SiO2 interface. This effect is attributed to a pre-existing static electric field at the interface of Si/ SiO2, which is postulated to be induced by boron charge traps created during the growth of the oxide. Using a tunable ultrafast laser system, the boron charge traps were filled by an internal photoelectron emission process. We determined the direction of the electric field to be from the oxide to the silicon substrate. No significant recovery was observed on the boron charge traps after blocking the laser beam, which indicates that the boron charge traps are meta-stable or require a significant recovery time, longer than several hours. From a fitting of our SHG data, we find that the effective second order susceptibility increases in a non-linear manner as the boron concentration increases. (3) where E0 is the magnitude of the initial electric field arising from boron charge traps, E1 the magnitude of the electric field from oxygen charge traps, t1 the detrapping time constant of the boron induced charge trap, and t2 is the trapping time constant of the oxygen charge traps. One should also take into account an electron detrapping constant for the oxygen traps for accurate fitting. However, the effective value of jx(2)j is not affected by the detrapping constant for the oxygen traps and the value of jx(3)j cannot be separated from the fitting because it is combined with the electric fields, E0 and E1. In Fig. 5(c, d), 1000 Eqs. (2) and (3) are fit to the two data sets (307 mW and 305 mW). The fitting result shows that jx(2)j increases as the doping concentration increases. Van der Pauw four-point probe measurements were used to determine the resistivities of wafers B and C, which gave 0.0165 V cm and 0.00519 V cm, respectively. We compared the relative ratios of jx(2)j and doping concentration which result in jx(2)jwafer C / jx(2)jwafer B ¼ 1.55, nwafer C / nwafer B ¼ 4.72. The results indicate that the effective jx(2)j increased non-linearly with respect to boron concentration. Acknowledgements This work was supported by the Department of Energy (DOE), Basic Energy Sciences, Grant No. DE-FGO2-99ER45781. References [1] N. Bloembergen, R. K. Chang, and C. H. 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