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Photoluminescence characterization of non-radiative defect density on silicon surfaces and interfaces at room temperature
Thin Solid Films 364 (2000) 196±199 www.elsevier.com/locate/tsf
Photoluminescence characterization of non-radiative defect density on silicon surfaces and interfaces at room temperature V.Yu. Timoshenko a, b,*, A.B. Petrenko a, Th. Dittrich b, W. FuÈssel c, J. Rappich c a
M. V. Lomonosov Moscow State University, Faculty of Physics, 119899 Moscow, Russia Technische UniversitaÈt MuÈnchen, Physik Department E16, D-85747 Garching, Germany c Hahn-Meitner-Institut, Abt. Photovoltaik, Rudower Chaussee 5, D-12489 Berlin, Germany b
Abstract Room temperature photoluminescence resulting from radiative interband recombination (1.1 mm) in c-Si excited by of XeCl or N2 laser pulses is studied experimentally and theoretically. Numerical simulations show that the quantum yield of PL of passivated c-Si wafers excited with the energy density 1±3 mJ/cm 2 reaches a maximal value of 3% for c-Si wafer with the bulk lifetime of 2.5 ms. Simulated transients of PL ®t very well to the experimentally observed ones, and allow an evaluation of the density of surface non-radiative defects. The exponential part of the PL transient is sensitive to the surface defect densities in the range from 10 8 to 10 12 cm 22. The total yield of PL is practically reciprocal to the density of surface defects in the range from 10 10 to 10 14 cm 22. These inferences have been used to obtain the densities of surface non-radiative defects for c-Si wafers irradiated with the laser pulses of energy densities near the melting threshold of c-Si. q 2000 Published by Elsevier Science S.A. Keywords: Luminescence; Surface defects; Silicon
1. Introduction
2. Experimental details
The ef®ciency of room temperature interband photoluminescence (PL) of c-Si at 1.1 mm is limited by non-radiative bulk and surface defects. This allows one to measure sensitively the defect concentration by PL measurements. For example, cw laser excitation has been used for PL monitoring of the surface defect formation due to the oxidation process [1]. The bimolecular nature of the PL of c-Si causes a super linear growth of the PL quantum yield with excitation level. High carrier densities without remarkable heating are reached by PL excitation with short laser pulses. Express characterization and stroboscopic probing of Si bulk and surfaces were shown by the PL method with pulsed laser excitation [2]. The PL measurement gives a quantitative characterization of the surface defect densities using calibration procedure with results of other methods, for instance, the conventional C±V technique [3]. In the present paper we use PL for a quantitative characterization of the defect densities on different Si surfaces: native or thermally oxidized, HF-treated or irradiated by strong laser pulses.
Samples of p-type Si(100) (thickness 250 and 400 mm, bulk lifetime t 0 about 250 ms and 1 ms) with native, anodic (see [3] for details) or thin thermal (8008C, 10 nm) oxides were investigated. The PL of Si was excited by single pulses of XeCl (308 nm, 25 ns) or N2 (337 nm, 0.5 ns) lasers with the energy density W 1±3 mJ/cm 2. The PL signal at 1.1 mm was selected with a prism monochromator. A silicon avalanche photodiode with a time constant of 30 ns was used to detect the PL transient. A charge accumulating InGaAs photodiode was applied to measure the total yield of PL. The experiments were performed in vacuum (10 25 mbar) at room temperature. Note, that no effect of ambient (vacuum or air) on the PL signal was observed for the oxidized Si wafers. Some experiments were carried out using the irradiation of XeCl laser with the pulse energy density up to 900 mJ/ cm 2 to induce defect formation on Si surface. The PL signal was measured in situ by probing with N2 laser excitation. The irradiated Si surface was controlled by time resolved re¯ectivity method [4] to check the processes of laser induced heating and melting. The threshold energy density for melting of Si surface amounted to Wm 750 mJ/cm 2.
* Corresponding author. 0040-6090/00/$ - see front matter q 2000 Published by Elsevier Science S.A. PII: S00 40-6090(99)0095 5-4
V.Y. Timoshenko et al. / Thin Solid Films 364 (2000) 196±199
197
3. Calculation of PL signal We study theoretically the PL response of c-Si wafer with thickness d excited by short laser pulse with the energy density W,,Wm. Bimolecular radiative recombination, non-radiative Auger recombination, Shockley± Read non-radiative recombination on the surface and in the bulk and ambipolar diffusion of non-equilibrium carriers are taken into account in the one-dimensional model [3]. The simple relation between the surface nonradiative recombination velocity (S) and the density of non-radiative defects (N) is used [3,5]: S s £ y £ N, where s and y are the recombination cross section and the thermal velocity of carriers, respectively. The room temperature of c-Si (y 107 cm/s) and the existence of the ef®cient non-radiative recombination centers like silicon dangling bonds on Si surface (s 10215 cm 22) [5] are assumed. The in¯uence of the defect density on the irradiated front surface Nf and on the back surface Nb of the cSi wafer are taken into account for the calculation of nonequilibrium carrier density n(t,x), where x is coordinate perpendicular to the wafer surface. The kinetic equation for n(t,x) is solved numerically (for details see [3]). The PL transient IPL(t) is calculated as the total rate of radiative recombination of photo-excited carriers in the whole wafer. The effect of PL reabsorption in the bulk of c-Si is neglected. The internal quantum yield of PL h PL is calculated as the number of radiative recombined carriers divided on the number of absorbed photons. An important issue in the calculation of the PL signal under pulsed excitation for Si surface characterization is to choose the level of the PL excitation. Two values of the duration t p of laser pulse (tp 0:5 and 25 ns of FWHM) as well as the laser irradiation absorbed in the near surface layer (a21 1026 cm) were used in an accordance with parameters of the laser excitation in our experiment. The surfaces of the Si wafer is assumed to be well passivated (N f N b 109 cm 22). Fig. 1 shows that the calculated dependence of h PL is proportional to W in the range up to 1 mJ/cm 2. This dependence is obviously due to the bimolecular character of interband radiative recombination in c-Si. The value of h PL max reaches the maximum h PL under the excitation with W in 2 the range 1.5±3 mJ/cm depending on the bulk lifetime t 0 of carriers. The PL of c-Si with t 0 from 25 ms to 2.5 ms is max max characterized by h PL in the region from 0.2 to 3 %. h PL decreases slightly for shorter duration of the laser pulse. max for W . 3 mJ/cm 2 This fact as well as the drop of h PL are caused by Auger recombination process. We remark that the laser induced heating of Si surface is max . not taken into account in the discussed calculation of h PL In fact, according to our estimation the laser pulse with tp 25 ns and W 10 mJ/cm 2 induces a temperature increase in the near surface layer by only about 10 K for tens of nanoseconds. Such heating is not important for the value of h PL for c-Si with the initial temperature of 300 K.
Fig. 1. Calculated quantum yield of PL of c-Si wafers (d 400 mm, N f N b 2 £ 1010 cm 22, t0 25 ms, 250 ms and 2.5 ms) vs. the energy density of the laser excitation with the pulse duration of 25 ns (solid lines) and 0.5 (dotted line).
Nevertheless, the laser induced heating should additionally reduce h PL for c-Si irradiated with higher W and (or) shorter t p. Fig. 2 shows calculated PL transients of c-Si (to 250 ms, d 400 mm). IPL(t) has a non-exponential character for times shorter than 10 ms due to carrier diffusion and Auger recombination. For larger times IPL(t) exhibits practically an exponential decay. The evaluated lifetime of the exponential part of PL transient t PL is plotted as a function of the surface defect density in the insert of Fig. 2. It can easily be seen that the value of t PL is sensitive to the surface defect density, varied from 10 9 to 10 12 cm 22 in our case. The bulk life time and the diffusion process of carrier excess limit t PL for smaller and larger value of Nf,b, respectively.
Fig. 2. Calculated (lines) and measured (circles and crosses) transients of PL of a c-Si wafer (d 400 mm, t 0 250 ms) excited with XeCl laser pulses. The corresponding defect densities on the both sides (Nf Nb) label near the curves. The inset shows lifetimes of the exponential parts of the PL transients as a function of the surface defect density.
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4. Experimental results and discussion The measured PL transient for the Si wafer with a thin thermal oxide is plotted by circles in Fig. 2. The simulated PL transient ®ts very well the measured one corresponding to the surface defect density N f N b 2 £ 1010 cm 22 with an error bar of about 2 £ 109 cm 22. The oxide removal by HF treatment of the Si wafer led to a PL intensity decrease. A ®t of the PL transient for the HF-treated Si wafer gives the surface defect densities of N f N b 5 £ 1011 cm 22 with an error bar of about 2 £ 1011 cm 22. The larger error bar of the defect density determination for the HF-treated samples is caused by the limitation for smaller value of t PL (larger Nf,b) due to the carrier diffusion process discussed above. The value of h PL is found to be extremely sensitive to a variation of surface defect density for N f;b . 1010 cm 22 (see Fig. 3). The calculated dependence of h PL on N f N b (solid line) agrees very well with the measured total yield of PL for the c-Si wafers with anodic oxide on both sides. The defect density in midgap Dit at the Si/SiO2 interface was studied by conventional capacitance-voltage technique. It is assumed that N f N b Dit £ 1 eV [3] and this gives the calibration of the PL signal. The total yield of PL is practically reciprocal to Nf for the region from 10 10 to 10 14 cm 22. The saturation of the h PL(Nf) dependence for N f , 1010 cm 22 is caused by the limitation of non-radiative recombination in the bulk or (and) on the back surface for N b . 1012 cm 22. We used the procedure of the determination of the surface defect density discussed above to investigate quantitatively the laser induced defect transformation on a Si surface. The dependence of t PL on Nf and Nb (Nf Nb) obtained from the simulated transients is in good agreement with the experimental values of t PL measured for the anodic oxidized Si wafers (crosses in the insert of Fig. 4). The PL transient of the c-Si wafer (t0 1 ms, d 250 mm) with native oxide is
Fig. 3. Calculated quantum yield of PL (lines) of c-Si wafers (d 250 mm, t0 1 ms) with different defect densities on the back side as function of the defect density on the front side. Measured total yield of PL (circles) for c-Si wafers with anodic oxide are plotted as a function of Dit Dit £ 1 eV N b N f . The PL is excited by pulses of a N2 laser.
®tted by the calculated curve for N f N b 5 £ 1012 cm 22. The evaluated t PL is depicted as a solid circle in the insert of Fig. 4. The total yield of PL and t PL of the native oxidized Si wafer was found to decrease after the XeCl laser irradiation with W . 300 mJ/cm 2 (see Fig. 4). The possible reason of the diminution of PL yield and the PL lifetime after the irradiation with W , Wm is the laser induced formation of non-radiative defects located on the Si surface or in a thin (10±40 nm) surface layer [6]. We remark, that the laser induced defect formation for the thermally oxidized or HF-treated Si wafers was observed for W larger than 600 mJ/cm 2. That evidences to the important role of the surface coverage for the laser induced defect formation below the melting threshold. The laser irradiation with W ^ Wm led to a strong quenching of PL due to the fast resolidi®cation process in the near surface layers of the wafer. The defect density on the irradiated side of the wafer can be calculated taking into account the small thickness of the laser processed layer and assuming recombination parameters of the laser induced defects close to the parameters of Si dangling bonds (s 10215 cm 22). The most sensitive determination of Nf can be done by calibration from the dependence of h PL on Nf (see Fig. 3). The in¯uence of the defects on the back side of the wafer (N b 5 £ 1012 cm 22) on the total yield of PL can be neglected and therefore hPL , N f 21 . The calculated values of Nf are related to the right axis in Fig.4. In particular, the value of Nf about 7 £ 1013 cm 22 is reached after the laser irradiation with W 800 mJ/cm 2. 5. Conclusions The room temperature PL of c-Si has been applied to probe the non-radiative defect density on silicon surfaces.
Fig. 4. Measured dependence of the total yield of PL (left axis) of a c-Si wafer (d 250 mm, t0 1 ms) with native oxide on the energy density of XeCl laser irradiation. The PL is excited by pulses of a N2 laser. The right axis relates to the calibrated dependence of Nf (W). The arrow marks the melting threshold energy Wm. The inset shows t PL calculated (line) and measured for c-Si wafers with anodic (crosses) and native (circle) oxide as a function of the surface defect density.
V.Y. Timoshenko et al. / Thin Solid Films 364 (2000) 196±199
Numerical simulations were used to ®nd the optimal energy densities for the PL excitation and to ®t the PL transients observed experimentally. The simultaneous analysis of the PL lifetime and the PL total yield allows one to determine changes of the surface defect density in the broad range from 10 8 to 10 14 cm 22. The PL diagnostics of the surface defects for the oxidized and HF-treated c-Si wafers and for the laser irradiated samples of c-Si was demonstrated. Acknowledgements V.Yu. Timoshenko is grateful to the Alexander von Humboldt Foundation for ®nancial support.
199
References [1] T. Konishi, M. Yao, H. Tajima, H. Ohshima, T. Ito, Hattori, Jpn. J. Appl. Phys. 31 (1992) L1216. [2] V.Yu Timoshenko, J. Rappich, Th. Dittrich, Appl. Surf. Sci. 123/124 (1998) 111. [3] V.Yu Timoshenko, A.B. Petrenko, M.N. Stolyarov, Th. Dittrich, W. Fuessel, J. Rappich, J. Appl. Phys. 85 (1999) 4171. [4] G.E. Jellison Jr., D.H. Lowndes, D.N. Mashburn, R.F. Wood, Phys. Rev. B 34 (1986) 2407. [5] E. Yablonovich, D.L. Alara, C.C. Chang, T. Gmitter, T.R. Bright, Phys. Rev. Lett. 57 (1986) 249. [6] V.I. Emelyanov, P.K. Kashkarov, Appl. Phys. A 55 (1992) 161.
Photoluminescence characterization of non-radiative defect density on silicon surfaces and interfaces at room temperature V.Yu. Timoshenko a, b,*, A.B. Petrenko a, Th. Dittrich b, W. FuÈssel c, J. Rappich c a
M. V. Lomonosov Moscow State University, Faculty of Physics, 119899 Moscow, Russia Technische UniversitaÈt MuÈnchen, Physik Department E16, D-85747 Garching, Germany c Hahn-Meitner-Institut, Abt. Photovoltaik, Rudower Chaussee 5, D-12489 Berlin, Germany b
Abstract Room temperature photoluminescence resulting from radiative interband recombination (1.1 mm) in c-Si excited by of XeCl or N2 laser pulses is studied experimentally and theoretically. Numerical simulations show that the quantum yield of PL of passivated c-Si wafers excited with the energy density 1±3 mJ/cm 2 reaches a maximal value of 3% for c-Si wafer with the bulk lifetime of 2.5 ms. Simulated transients of PL ®t very well to the experimentally observed ones, and allow an evaluation of the density of surface non-radiative defects. The exponential part of the PL transient is sensitive to the surface defect densities in the range from 10 8 to 10 12 cm 22. The total yield of PL is practically reciprocal to the density of surface defects in the range from 10 10 to 10 14 cm 22. These inferences have been used to obtain the densities of surface non-radiative defects for c-Si wafers irradiated with the laser pulses of energy densities near the melting threshold of c-Si. q 2000 Published by Elsevier Science S.A. Keywords: Luminescence; Surface defects; Silicon
1. Introduction
2. Experimental details
The ef®ciency of room temperature interband photoluminescence (PL) of c-Si at 1.1 mm is limited by non-radiative bulk and surface defects. This allows one to measure sensitively the defect concentration by PL measurements. For example, cw laser excitation has been used for PL monitoring of the surface defect formation due to the oxidation process [1]. The bimolecular nature of the PL of c-Si causes a super linear growth of the PL quantum yield with excitation level. High carrier densities without remarkable heating are reached by PL excitation with short laser pulses. Express characterization and stroboscopic probing of Si bulk and surfaces were shown by the PL method with pulsed laser excitation [2]. The PL measurement gives a quantitative characterization of the surface defect densities using calibration procedure with results of other methods, for instance, the conventional C±V technique [3]. In the present paper we use PL for a quantitative characterization of the defect densities on different Si surfaces: native or thermally oxidized, HF-treated or irradiated by strong laser pulses.
Samples of p-type Si(100) (thickness 250 and 400 mm, bulk lifetime t 0 about 250 ms and 1 ms) with native, anodic (see [3] for details) or thin thermal (8008C, 10 nm) oxides were investigated. The PL of Si was excited by single pulses of XeCl (308 nm, 25 ns) or N2 (337 nm, 0.5 ns) lasers with the energy density W 1±3 mJ/cm 2. The PL signal at 1.1 mm was selected with a prism monochromator. A silicon avalanche photodiode with a time constant of 30 ns was used to detect the PL transient. A charge accumulating InGaAs photodiode was applied to measure the total yield of PL. The experiments were performed in vacuum (10 25 mbar) at room temperature. Note, that no effect of ambient (vacuum or air) on the PL signal was observed for the oxidized Si wafers. Some experiments were carried out using the irradiation of XeCl laser with the pulse energy density up to 900 mJ/ cm 2 to induce defect formation on Si surface. The PL signal was measured in situ by probing with N2 laser excitation. The irradiated Si surface was controlled by time resolved re¯ectivity method [4] to check the processes of laser induced heating and melting. The threshold energy density for melting of Si surface amounted to Wm 750 mJ/cm 2.
* Corresponding author. 0040-6090/00/$ - see front matter q 2000 Published by Elsevier Science S.A. PII: S00 40-6090(99)0095 5-4
V.Y. Timoshenko et al. / Thin Solid Films 364 (2000) 196±199
197
3. Calculation of PL signal We study theoretically the PL response of c-Si wafer with thickness d excited by short laser pulse with the energy density W,,Wm. Bimolecular radiative recombination, non-radiative Auger recombination, Shockley± Read non-radiative recombination on the surface and in the bulk and ambipolar diffusion of non-equilibrium carriers are taken into account in the one-dimensional model [3]. The simple relation between the surface nonradiative recombination velocity (S) and the density of non-radiative defects (N) is used [3,5]: S s £ y £ N, where s and y are the recombination cross section and the thermal velocity of carriers, respectively. The room temperature of c-Si (y 107 cm/s) and the existence of the ef®cient non-radiative recombination centers like silicon dangling bonds on Si surface (s 10215 cm 22) [5] are assumed. The in¯uence of the defect density on the irradiated front surface Nf and on the back surface Nb of the cSi wafer are taken into account for the calculation of nonequilibrium carrier density n(t,x), where x is coordinate perpendicular to the wafer surface. The kinetic equation for n(t,x) is solved numerically (for details see [3]). The PL transient IPL(t) is calculated as the total rate of radiative recombination of photo-excited carriers in the whole wafer. The effect of PL reabsorption in the bulk of c-Si is neglected. The internal quantum yield of PL h PL is calculated as the number of radiative recombined carriers divided on the number of absorbed photons. An important issue in the calculation of the PL signal under pulsed excitation for Si surface characterization is to choose the level of the PL excitation. Two values of the duration t p of laser pulse (tp 0:5 and 25 ns of FWHM) as well as the laser irradiation absorbed in the near surface layer (a21 1026 cm) were used in an accordance with parameters of the laser excitation in our experiment. The surfaces of the Si wafer is assumed to be well passivated (N f N b 109 cm 22). Fig. 1 shows that the calculated dependence of h PL is proportional to W in the range up to 1 mJ/cm 2. This dependence is obviously due to the bimolecular character of interband radiative recombination in c-Si. The value of h PL max reaches the maximum h PL under the excitation with W in 2 the range 1.5±3 mJ/cm depending on the bulk lifetime t 0 of carriers. The PL of c-Si with t 0 from 25 ms to 2.5 ms is max max characterized by h PL in the region from 0.2 to 3 %. h PL decreases slightly for shorter duration of the laser pulse. max for W . 3 mJ/cm 2 This fact as well as the drop of h PL are caused by Auger recombination process. We remark that the laser induced heating of Si surface is max . not taken into account in the discussed calculation of h PL In fact, according to our estimation the laser pulse with tp 25 ns and W 10 mJ/cm 2 induces a temperature increase in the near surface layer by only about 10 K for tens of nanoseconds. Such heating is not important for the value of h PL for c-Si with the initial temperature of 300 K.
Fig. 1. Calculated quantum yield of PL of c-Si wafers (d 400 mm, N f N b 2 £ 1010 cm 22, t0 25 ms, 250 ms and 2.5 ms) vs. the energy density of the laser excitation with the pulse duration of 25 ns (solid lines) and 0.5 (dotted line).
Nevertheless, the laser induced heating should additionally reduce h PL for c-Si irradiated with higher W and (or) shorter t p. Fig. 2 shows calculated PL transients of c-Si (to 250 ms, d 400 mm). IPL(t) has a non-exponential character for times shorter than 10 ms due to carrier diffusion and Auger recombination. For larger times IPL(t) exhibits practically an exponential decay. The evaluated lifetime of the exponential part of PL transient t PL is plotted as a function of the surface defect density in the insert of Fig. 2. It can easily be seen that the value of t PL is sensitive to the surface defect density, varied from 10 9 to 10 12 cm 22 in our case. The bulk life time and the diffusion process of carrier excess limit t PL for smaller and larger value of Nf,b, respectively.
Fig. 2. Calculated (lines) and measured (circles and crosses) transients of PL of a c-Si wafer (d 400 mm, t 0 250 ms) excited with XeCl laser pulses. The corresponding defect densities on the both sides (Nf Nb) label near the curves. The inset shows lifetimes of the exponential parts of the PL transients as a function of the surface defect density.
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V.Y. Timoshenko et al. / Thin Solid Films 364 (2000) 196±199
4. Experimental results and discussion The measured PL transient for the Si wafer with a thin thermal oxide is plotted by circles in Fig. 2. The simulated PL transient ®ts very well the measured one corresponding to the surface defect density N f N b 2 £ 1010 cm 22 with an error bar of about 2 £ 109 cm 22. The oxide removal by HF treatment of the Si wafer led to a PL intensity decrease. A ®t of the PL transient for the HF-treated Si wafer gives the surface defect densities of N f N b 5 £ 1011 cm 22 with an error bar of about 2 £ 1011 cm 22. The larger error bar of the defect density determination for the HF-treated samples is caused by the limitation for smaller value of t PL (larger Nf,b) due to the carrier diffusion process discussed above. The value of h PL is found to be extremely sensitive to a variation of surface defect density for N f;b . 1010 cm 22 (see Fig. 3). The calculated dependence of h PL on N f N b (solid line) agrees very well with the measured total yield of PL for the c-Si wafers with anodic oxide on both sides. The defect density in midgap Dit at the Si/SiO2 interface was studied by conventional capacitance-voltage technique. It is assumed that N f N b Dit £ 1 eV [3] and this gives the calibration of the PL signal. The total yield of PL is practically reciprocal to Nf for the region from 10 10 to 10 14 cm 22. The saturation of the h PL(Nf) dependence for N f , 1010 cm 22 is caused by the limitation of non-radiative recombination in the bulk or (and) on the back surface for N b . 1012 cm 22. We used the procedure of the determination of the surface defect density discussed above to investigate quantitatively the laser induced defect transformation on a Si surface. The dependence of t PL on Nf and Nb (Nf Nb) obtained from the simulated transients is in good agreement with the experimental values of t PL measured for the anodic oxidized Si wafers (crosses in the insert of Fig. 4). The PL transient of the c-Si wafer (t0 1 ms, d 250 mm) with native oxide is
Fig. 3. Calculated quantum yield of PL (lines) of c-Si wafers (d 250 mm, t0 1 ms) with different defect densities on the back side as function of the defect density on the front side. Measured total yield of PL (circles) for c-Si wafers with anodic oxide are plotted as a function of Dit Dit £ 1 eV N b N f . The PL is excited by pulses of a N2 laser.
®tted by the calculated curve for N f N b 5 £ 1012 cm 22. The evaluated t PL is depicted as a solid circle in the insert of Fig. 4. The total yield of PL and t PL of the native oxidized Si wafer was found to decrease after the XeCl laser irradiation with W . 300 mJ/cm 2 (see Fig. 4). The possible reason of the diminution of PL yield and the PL lifetime after the irradiation with W , Wm is the laser induced formation of non-radiative defects located on the Si surface or in a thin (10±40 nm) surface layer [6]. We remark, that the laser induced defect formation for the thermally oxidized or HF-treated Si wafers was observed for W larger than 600 mJ/cm 2. That evidences to the important role of the surface coverage for the laser induced defect formation below the melting threshold. The laser irradiation with W ^ Wm led to a strong quenching of PL due to the fast resolidi®cation process in the near surface layers of the wafer. The defect density on the irradiated side of the wafer can be calculated taking into account the small thickness of the laser processed layer and assuming recombination parameters of the laser induced defects close to the parameters of Si dangling bonds (s 10215 cm 22). The most sensitive determination of Nf can be done by calibration from the dependence of h PL on Nf (see Fig. 3). The in¯uence of the defects on the back side of the wafer (N b 5 £ 1012 cm 22) on the total yield of PL can be neglected and therefore hPL , N f 21 . The calculated values of Nf are related to the right axis in Fig.4. In particular, the value of Nf about 7 £ 1013 cm 22 is reached after the laser irradiation with W 800 mJ/cm 2. 5. Conclusions The room temperature PL of c-Si has been applied to probe the non-radiative defect density on silicon surfaces.
Fig. 4. Measured dependence of the total yield of PL (left axis) of a c-Si wafer (d 250 mm, t0 1 ms) with native oxide on the energy density of XeCl laser irradiation. The PL is excited by pulses of a N2 laser. The right axis relates to the calibrated dependence of Nf (W). The arrow marks the melting threshold energy Wm. The inset shows t PL calculated (line) and measured for c-Si wafers with anodic (crosses) and native (circle) oxide as a function of the surface defect density.
V.Y. Timoshenko et al. / Thin Solid Films 364 (2000) 196±199
Numerical simulations were used to ®nd the optimal energy densities for the PL excitation and to ®t the PL transients observed experimentally. The simultaneous analysis of the PL lifetime and the PL total yield allows one to determine changes of the surface defect density in the broad range from 10 8 to 10 14 cm 22. The PL diagnostics of the surface defects for the oxidized and HF-treated c-Si wafers and for the laser irradiated samples of c-Si was demonstrated. Acknowledgements V.Yu. Timoshenko is grateful to the Alexander von Humboldt Foundation for ®nancial support.
199
References [1] T. Konishi, M. Yao, H. Tajima, H. Ohshima, T. Ito, Hattori, Jpn. J. Appl. Phys. 31 (1992) L1216. [2] V.Yu Timoshenko, J. Rappich, Th. Dittrich, Appl. Surf. Sci. 123/124 (1998) 111. [3] V.Yu Timoshenko, A.B. Petrenko, M.N. Stolyarov, Th. Dittrich, W. Fuessel, J. Rappich, J. Appl. Phys. 85 (1999) 4171. [4] G.E. Jellison Jr., D.H. Lowndes, D.N. Mashburn, R.F. Wood, Phys. Rev. B 34 (1986) 2407. [5] E. Yablonovich, D.L. Alara, C.C. Chang, T. Gmitter, T.R. Bright, Phys. Rev. Lett. 57 (1986) 249. [6] V.I. Emelyanov, P.K. Kashkarov, Appl. Phys. A 55 (1992) 161.