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Study of the photoexcited carrier dynamics in InP:Fe using time-resolved reflection and photoluminescence spectra
Applied Surface Science 230 (2004) 158–162
Study of the photoexcited carrier dynamics in InP:Fe using time-resolved reflection and photoluminescence spectra Shihua Huang, Xi Li, Fang Lu* Surface Physics Laboratory, Fudan University, Shanghai 200433, China Received 2 December 2003; received in revised form 14 February 2004; accepted 14 February 2004 Available online 17 April 2004
Abstract The photoexcited carrier dynamics and photoluminescence of the undoped InP and Fe implanted InP was studied by timeresolved reflection and photoluminescence spectra. The decay times of reflection recovery and the radiative recombination for Fe implanted InP are shorter than those of undoped InP. Considering the surface recombination, a model was developed to simulate the reflection recovery dynamics, it agrees with the experimental results very well. Moreover, we obtained the ambipolar diffusion coefficient and the surface recombination velocity by using the model. For Fe-doped InP, the surface recombination velocity is much larger than that for the undoped InP, which is probably due to Fe2þ/3þ trapping centers and the large surface band bending. The PL decay time for Fe implanted InP is shorter than that for undoped InP, which is ascribed to the capture centers introduced by metallic precipitates. # 2004 Elsevier B.V. All rights reserved. PACS: 78.55.Cr; 78.47.þP; 68.35.FX Keywords: Fe implantation InP; Time-resolved reflection; Transient photoluminescence; Ambipolar diffusion
1. Introduction Fe-doped InP is used as a semi-insulating substrate material for high speed electronic devices, such as InxAl1xAs/InyGa1yAs high electron mobility transistors (HEMTs) and optoelectronic devices. Fe ion implantation may be used to produce semi-insulating (SI) layers in n-InP thanks to a chemical compensation mechanism [1]. An understanding of the carrier behavior of InP:Fe is essential for the improvement of these *
Corresponding author. Tel.: þ86-21-65642683; fax: þ86-21-65109395. E-mail address: [email protected] (F. Lu).
devices. Fe ion implantation in combination with a post-implantation annealing treatment is used to introduce the active Fe atoms in n-InP. It has been observed [2] that metallic inclusions are responsible for the semi-insulating behavior in Cu-doped InP. By using optical pump-probe techniques, carrier dynamics in semiconductors with metallic precipitates can be studied. Carriers are photoexcited by an above band gap pump pulse and then one measures either changes in reflection [3–5], changes in Raman spectra [6], or the stimulated luminescence [7]. In this paper, we have studied the photoexcited carrier dynamics in Fe-doped InP using time-resolved
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.02.024
S. Huang et al. / Applied Surface Science 230 (2004) 158–162
2. Experiment The samples were prepared using high-quality substrates, nominally undoped, n-type 2 1016 cm3 InP wafers. During the implantation process, the substrate holder is kept at a temperature of 250 8C. Single energy Fe ions were implanted at 270 keV with a dose of 5 1015 cm2. In order to prevent surface decomposition due to P evaporation, the post-annealing was carried out in a reactor with a phosphine flux. The annealing temperature is 600 8C, and the annealing time is about 70 min. The annealing treatment can introduce a certain concentration of active Fe in n-InP and the recovery of the damage brings the majority of Fe atoms in random interstitial positions, which probably causes the agglomeration of Fe–P precipitates [8]. In order to investigate the photoexcited dynamics of carriers in Fe-doped InP, we performed time-resolved differential reflection measurements and transient PL measurements using a Ti:sapphire laser system (Spectra Physics), delivering optical 80-fs pulses with a repetition rate of 82 MHz and an output power of 500 mW at l ¼ 800 nm. For the time-resolved differential reflection measurement, the pump pulses were normally incident and the probe pulses were incident with an angle of 58. The laser beam was split into pump and probe beams with an intensity ratio of 6:1, and two beams were focused on the sample surface with a diameter of about 20 mm. The probe pulses were delayed by a step-motor-driver delay setup. The reflection bleaching signals were detected using a Si photocell and a lock-in amplifier. For the transient PL measurement, the photoluminescence (PL) was detected using a near-infrared photomultiplier (PMT) and a high-speed digital oscilloscope. For the FT-IR absorption measurement, the detector is DTGS TEC and the beam splitter is KBr.
3. Results and discussion Fig. 1 shows the typical transient PL spectrum for Fe implantation InP. We found that the radiative recombination lifetime (tR) is in the magnitude of nanosecond. By means of the simple exponential decay fit, we obtain tR to approximate 19 ns. For the undoped InP, tR is 56 ns. The PL decay time for Fe implanted InP is shorter than that of the undoped InP. Transient reflection change (DR/R) results from the increasing of the photoexcited carrier density N that is coupled to the complex refractive index ^n (¼n þ ik, where n and k are refractive index and extinction coefficient, respectively). Since DR=R ! 1 and k ! n (n ¼ 3:46 and k ¼ 0:20 at the probe wavelength of 800 nm for InP), DR/R can be described as DR 4Dn ¼ 2 R n 1
(1)
where Dn is pump laser-induced change in n. By applying the classical Drude model for the optical properties of quasi-free carriers to photoexcited electron–hole pairs, the change in the refractive index Dn is given by Dn ¼
Ne2 2nmeh e0 o2
(2)
where e is the fundamental electron charge, e0 the dielectric constant (8:85 1012 F/m), meh the reduced
150
(a) The undoped InP,τ R=56 ns 120
PL intensity (a. u.)
differential reflection measurement. Considering the influence of the ambipolar and surface recombination velocity on the carrier dynamics, the temporal evolution of the concentration of photoexcited carriers was investigated. In addition, the measurement of transient photoluminescence (PL) and the Fourier transform infrared (FT-IR) absorption measurements were employed.
159
(b) Fe implanted InP ,τ R= 19 ns
90
60
(a) 30
(b) 0 0
40
80
120
160
200
Delay time (ns)
Fig. 1. The PL dynamics of undoped InP and Fe implanted InP measured at room temperature.
160
S. Huang et al. / Applied Surface Science 230 (2004) 158–162
Transient reflectivity change (a. u.)
1.2
The undoped InP 0.8
12
14
16 ps
18
20
0.4
Fe implanted InP 0.0 0
30
60
90
120
150
Delay time(ps)
Fig. 2. The measured reflection recovery dynamics of the undoped InP and Fe implanted InP. The inset shows the decay of reflection recovery dynamics in the initial several picosecond.
effective mass of electron–hole pair (6:9 1032 kg), and o the angular frequency of probe laser (2:36 1015 s1). For Fe ion implanted InP and undoped InP, Fig. 2 shows the change of the transient reflectivity as a function of the time. We can see that the decay of the reflection signal in InP becomes much faster after Fe implantation. It is characterized by a fast decay over the first several picosecond and by a slower decay which time is more than 60 ps. For the decays of reflection recovery dynamics, they decay fast at the initial several picosecond after the delay between pump and probe beam is zero, which is probably caused by ‘‘hot carrier’’ relaxation. Immediately after excitation with above band gap energy photons, large amounts of nonequilibrium carriers are generated. Their temperatures are much higher than those of the thermal equilibrium carriers due to the excess kinetic energy received during laser excitation, therefore, the nonequilibrium carriers are regarded as ‘‘hot carriers’’. After several picoseconds, these ‘‘hot carriers’’ relax to the bottom of the conduction band by means of emitting optical phonons. As the carriers lose their excess energy, the system returns to the thermal equilibrium. However, the diffusion and the surface recombination of carriers persist over tens of picosecond or so much as hundreds of picosecond until the spatial inhomogeneity returns to equilibrium. Since the diffusion and the surface recombination of carriers are essential to the understanding of the
carrier behavior in InP:Fe, they are mainly taken into account. Both the PL decay and the reflection recovery decay of the Fe implanted InP become faster than those of the undoped InP. In order to understand these results, we have developed a model to simulate the temporal evolution of the concentration of photoexcited carriers in the samples. Some previous research results indicate that the thermalization time in InP resulting from carrier–carrier and carrier–photon scattering is of the magnitude of several hundred femtoseconds [9], which is much faster than the time of reflection recovery dynamics obtained here. In addition, the Auger recombination would deplete the photoexcited carriers on a time scale of 0.1–10 ns [10]. Therefore, in our case the radiative recombination time, the thermalization time, and the Auger recombination time can be neglected. Since the optical penetration depth at the wavelength of 800 nm is less than 1 mm for the samples studied here, and the laser spot focus on the sample is about 20 mm, we can assume that the gradient of the photoexcited carrier density along z direction is much larger than that in the x–y plane. Therefore, the reflection recovery dynamics is mainly depended on the carrier transport along the z direction. Considering surface recombination, the reflection recovery dynamics following the excitation can be described by @Nðz; tÞ @ 2 Nðz; tÞ Nðz; tÞ ¼D @t @z2 tR with the boundary conditions @Nðz; tÞ ¼ SNðz; tÞ and D @z z¼0
(3)
Nðz; tÞjz¼1 ¼ 0 (4)
and the initial condition Nðz; tÞjt¼0 ¼ N0 expðazÞ
(5)
where N(z, t) is the carrier density, z the perpendicular to the sample surface, D the ambipolar diffusion coefficient, tR the bulk recombination lifetime, S the surface recombination velocity which is assumed to be independent of the carrier density, N0 the initial excess carrier density at the sample surface, and a the absorption coefficient. By using the method of
S. Huang et al. / Applied Surface Science 230 (2004) 158–162
with WðxÞ ¼ expðx2 Þ erfcðxÞ. The absorption coefficient was taken to be 3 104 cm1 for the excited wavelength 800 nm [11]. According to the penetration depth dependence of reflection by Aspnes and Frova [12], the reflection signal arises from a surface layer of depth 6102 mm in the case of a=2K ! 1, where K is the magnitude of the light momentum vector in the crystal. Taking into account of these conditions, we calculated the ambipolar diffusion coefficient and the surface recombination velocity dependence of photoexcited carrier dynamics. By making use of Eq. (6) and the parameters (a ¼ 3 104 cm1 and the penetration depth z ¼ 6 102 mm), we obtain the fitting to the experimental data, as shown in Fig. 3. We find that the experimental results agree with the theoretical model very well. For undoped InP and Fe implanted InP, the values of S and D obtained from the fitting are 1:21 104 cm/s and 2.23 cm2/s, 4:32 105 cm/s and 1.17 cm2/s, respectively. When the parameters (S and D) are optimized one after another, there is a minor change in the trace for S and D. The value of S obtained from the fit, 4:32 105 cm/s, is comparable to the largest one reported [13], but much larger than the typical values [14–17]. The surface recombination velocity of the Fe implanted InP is about 14 times larger than that of the undoped InP. But its ambipolar diffusion coefficient (1.17 cm2/s) is only the half of that of the undoped InP. Therefore, we believe that the decay time of reflection recovery dynamics in Fe
Transient reflectivity change (a. u.)
1.2
4
The undoped InP
S=1.21 10 cm/s
Fe implanted InP
S=4.32 10 cm/s
2
D=2.23 cm /s
0.8
0.4
5
2
D=1.17 cm /s
0.0 0
30
60
90
120
150
Delay time (ps)
Fig. 3. The symbols indicate the measured reflection recovery dynamics of the undoped InP and Fe implanted InP. The straight lines indicate the fitted temporal evolution by using Eq. (6). For the undoped InP and Fe implanted InP, the values of S and D obtained from the fitting of the experimental data are 1:21 104 cm/s and 2.23 cm2/s, 4:32 105 cm/s and 1.17 cm2/s, respectively.
implanted InP is mainly dominated by surface recombination. Fe implantation will introduce defects in the InP and then the smoothness of its surface becomes poor, which probably results in the large surface band bending for Fe implanted InP. In order to obtain the information of defects in Fe implanted InP and the undoped InP, we have employed FT-IR absorption measurement, and the measurement results are shown in Fig. 4. For Fe implanted InP, FT-IR spectroscopy shows an intense absorption peak located at 8
2858 cm
1
6
Absorption (a. u.)
separation of variables, the analytical solution to Eq. (3) is N0 t z2 Nðz; tÞ ¼ exp exp tR 2 4Dt ( ! z 1=2 W aðDtÞ 2ðDtÞ1=2 !) z 2S=D 1=2 þ W aðDtÞ þ 1=2 S=D a 2ðDtÞ " ! z W aðDtÞ1=2 þ 2ðDtÞ1=2 !# S z 1=2 ðDtÞ þ W (6) D 2ðDtÞ1=2
161
Fe implanted InP 4
2
0
1000
The undoped InP 2000
3000
4000
5000
6000
7000
1
Wavenumber (cm )
Fig. 4. The FT-IR absorption spectroscopy of undoped InP and Fe implanted InP.
162
S. Huang et al. / Applied Surface Science 230 (2004) 158–162
2858 cm1 which is associated with Fe intracenter transition [18], however, the peak does not appear for the undoped InP. The Fe2þ/3þ trap has been activated in Fe implanted InP, where it acts as a recombination center capturing both electron and holes with comparable capture cross-section. This clearly indicates the high level of compensation is expected in Fe implantation InP [18]. The results of FT-IR measurement indicate that a mass of Fe2þ/3þ trapping centers exist in Fe implanted InP. Since the surface recombination velocity is strongly dependent on the surface potential and the presence of defects located within the forbidden gap (recombination), the large surface recombination velocity of Fe implantation InP is probably due to a mass of Fe2þ/3þ trapping centers and the large surface band bending. Also, a large number of Fe2þ/3þ trapping centers will result in the lower mobility of carrier. According to the Einstein relation, the diffusion coefficient of carrier is directly proportional to the mobility of carrier. Therefore, the ambipolar diffusion coefficient of Fe implanted InP is lower than that of the undoped InP. On the other hand, the PL decay time for Fe implanted InP is shorter than that for the undoped InP, which is probably due to Fe2þ/3þ trapping centers in Fe implanted InP which have great cross-section for electron–hole recombination.
4. Conclusion The decay time of reflection recovery and the radiative recombination time of Fe implanted InP and undoped InP has been investigated. Both of the decay times of the implanted InP are shorter than those of the undoped InP. Considering surface recombination, a model of reflection recovery dynamics was proposed. This model agrees with the experimental results very well. Moreover, we obtained the ambipolar diffusion coefficient and the surface recombination velocity by using the model. For Fe implanted InP, the surface recombination velocity is much larger than that of the undoped InP, which is probably due to
Fe2þ/3þ trapping centers and the large surface band bending. The PL decay time for Fe implanted InP is shorter than that for undoped InP, which is ascribed to Fe2þ/3þ trapping centers in Fe implanted InP.
Acknowledgements This work was supported by the special funds for Major State Basic Research Project No. G2001CB3095 of China, the Commission of Science and Technology of Shanghai, and the National Natural Science Foundation of China.
References [1] S.J. Pearton, Mater. Sci. Rep. 4 (1990) 313. [2] R. Leon, M. Kaminska, K. Yu, E.R. Weber, Phys. Rev. B 46 (12) (1992) 460. [3] D.H. Auston, S. McAffe, C.V. Shank, E.P. Ippen, O. Teschke, Solid State Electron. 21 (1978) 147. [4] C.A. Hoffman, K. Jarasiunas, H.J. Gerritsen, A.V. Nurmikko, Appl. Phys. Lett. 33 (1978) 536. [5] Y.-G. Zhao, Y.-D. Qin, X.-L. Huang, J.-J. Wang, Y.-H. Zou, R.A. Masut, M. Beaudion, Solid State Commun. 105 (1998) 393. [6] J.F. Young, K. Wan, Phys. Rev. B 35 (1987) 2544. [7] K. Kash, J. Shah, Appl. Phys. Lett. 45 (1984) 401; K. Mettler, Appl. Phys. 12 (1977) 75. [8] E. Wendler, T. Opfermann, P.I. Gaiduk, J. Appl. Phys. 82 (1997) 5965. [9] U. Hohenester, P. Supancic, P. Kocevar, X.Q. Zhou, W. Kutt, H. Kurz, Phys. Rev. B 47 (1993) 13233. [10] W. Rosenthal, Solid State Commun. 13 (1973) 1215. [11] D.E. Aspnes, A.A. Studna, Phys. Rev. B 27 (1983) 985. [12] D.E. Aspnes, A. Frova, Solid State Commun. 7 (1969) 155. [13] C.A. Hoffman, H.J. Gerritsen, A.V. Nurmikko, J. Appl. Phys. 51 (1980) 1603. [14] D. Aspnes, Surf. Sci. 132 (1983) 406. [15] H.C. Casey, E. Buehler, Appl. Phys. Lett. 30 (1977) 247. [16] K.T. Tsen, G. Halama, O.F. Sankey, S.C. Tsen, H. Morkoc, Phys. Rev. B 40 (1989) 8103. [17] T. Nakamura, T. Katoda, J. Appl. Phys. 55 (1984) 3064. [18] S.G. Bishop, in: S. Pantelides (Ed.), Deep Centers in Semiconductors, Gordon & Breach, New York, 1986, p. 541.
Study of the photoexcited carrier dynamics in InP:Fe using time-resolved reflection and photoluminescence spectra Shihua Huang, Xi Li, Fang Lu* Surface Physics Laboratory, Fudan University, Shanghai 200433, China Received 2 December 2003; received in revised form 14 February 2004; accepted 14 February 2004 Available online 17 April 2004
Abstract The photoexcited carrier dynamics and photoluminescence of the undoped InP and Fe implanted InP was studied by timeresolved reflection and photoluminescence spectra. The decay times of reflection recovery and the radiative recombination for Fe implanted InP are shorter than those of undoped InP. Considering the surface recombination, a model was developed to simulate the reflection recovery dynamics, it agrees with the experimental results very well. Moreover, we obtained the ambipolar diffusion coefficient and the surface recombination velocity by using the model. For Fe-doped InP, the surface recombination velocity is much larger than that for the undoped InP, which is probably due to Fe2þ/3þ trapping centers and the large surface band bending. The PL decay time for Fe implanted InP is shorter than that for undoped InP, which is ascribed to the capture centers introduced by metallic precipitates. # 2004 Elsevier B.V. All rights reserved. PACS: 78.55.Cr; 78.47.þP; 68.35.FX Keywords: Fe implantation InP; Time-resolved reflection; Transient photoluminescence; Ambipolar diffusion
1. Introduction Fe-doped InP is used as a semi-insulating substrate material for high speed electronic devices, such as InxAl1xAs/InyGa1yAs high electron mobility transistors (HEMTs) and optoelectronic devices. Fe ion implantation may be used to produce semi-insulating (SI) layers in n-InP thanks to a chemical compensation mechanism [1]. An understanding of the carrier behavior of InP:Fe is essential for the improvement of these *
Corresponding author. Tel.: þ86-21-65642683; fax: þ86-21-65109395. E-mail address: [email protected] (F. Lu).
devices. Fe ion implantation in combination with a post-implantation annealing treatment is used to introduce the active Fe atoms in n-InP. It has been observed [2] that metallic inclusions are responsible for the semi-insulating behavior in Cu-doped InP. By using optical pump-probe techniques, carrier dynamics in semiconductors with metallic precipitates can be studied. Carriers are photoexcited by an above band gap pump pulse and then one measures either changes in reflection [3–5], changes in Raman spectra [6], or the stimulated luminescence [7]. In this paper, we have studied the photoexcited carrier dynamics in Fe-doped InP using time-resolved
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.02.024
S. Huang et al. / Applied Surface Science 230 (2004) 158–162
2. Experiment The samples were prepared using high-quality substrates, nominally undoped, n-type 2 1016 cm3 InP wafers. During the implantation process, the substrate holder is kept at a temperature of 250 8C. Single energy Fe ions were implanted at 270 keV with a dose of 5 1015 cm2. In order to prevent surface decomposition due to P evaporation, the post-annealing was carried out in a reactor with a phosphine flux. The annealing temperature is 600 8C, and the annealing time is about 70 min. The annealing treatment can introduce a certain concentration of active Fe in n-InP and the recovery of the damage brings the majority of Fe atoms in random interstitial positions, which probably causes the agglomeration of Fe–P precipitates [8]. In order to investigate the photoexcited dynamics of carriers in Fe-doped InP, we performed time-resolved differential reflection measurements and transient PL measurements using a Ti:sapphire laser system (Spectra Physics), delivering optical 80-fs pulses with a repetition rate of 82 MHz and an output power of 500 mW at l ¼ 800 nm. For the time-resolved differential reflection measurement, the pump pulses were normally incident and the probe pulses were incident with an angle of 58. The laser beam was split into pump and probe beams with an intensity ratio of 6:1, and two beams were focused on the sample surface with a diameter of about 20 mm. The probe pulses were delayed by a step-motor-driver delay setup. The reflection bleaching signals were detected using a Si photocell and a lock-in amplifier. For the transient PL measurement, the photoluminescence (PL) was detected using a near-infrared photomultiplier (PMT) and a high-speed digital oscilloscope. For the FT-IR absorption measurement, the detector is DTGS TEC and the beam splitter is KBr.
3. Results and discussion Fig. 1 shows the typical transient PL spectrum for Fe implantation InP. We found that the radiative recombination lifetime (tR) is in the magnitude of nanosecond. By means of the simple exponential decay fit, we obtain tR to approximate 19 ns. For the undoped InP, tR is 56 ns. The PL decay time for Fe implanted InP is shorter than that of the undoped InP. Transient reflection change (DR/R) results from the increasing of the photoexcited carrier density N that is coupled to the complex refractive index ^n (¼n þ ik, where n and k are refractive index and extinction coefficient, respectively). Since DR=R ! 1 and k ! n (n ¼ 3:46 and k ¼ 0:20 at the probe wavelength of 800 nm for InP), DR/R can be described as DR 4Dn ¼ 2 R n 1
(1)
where Dn is pump laser-induced change in n. By applying the classical Drude model for the optical properties of quasi-free carriers to photoexcited electron–hole pairs, the change in the refractive index Dn is given by Dn ¼
Ne2 2nmeh e0 o2
(2)
where e is the fundamental electron charge, e0 the dielectric constant (8:85 1012 F/m), meh the reduced
150
(a) The undoped InP,τ R=56 ns 120
PL intensity (a. u.)
differential reflection measurement. Considering the influence of the ambipolar and surface recombination velocity on the carrier dynamics, the temporal evolution of the concentration of photoexcited carriers was investigated. In addition, the measurement of transient photoluminescence (PL) and the Fourier transform infrared (FT-IR) absorption measurements were employed.
159
(b) Fe implanted InP ,τ R= 19 ns
90
60
(a) 30
(b) 0 0
40
80
120
160
200
Delay time (ns)
Fig. 1. The PL dynamics of undoped InP and Fe implanted InP measured at room temperature.
160
S. Huang et al. / Applied Surface Science 230 (2004) 158–162
Transient reflectivity change (a. u.)
1.2
The undoped InP 0.8
12
14
16 ps
18
20
0.4
Fe implanted InP 0.0 0
30
60
90
120
150
Delay time(ps)
Fig. 2. The measured reflection recovery dynamics of the undoped InP and Fe implanted InP. The inset shows the decay of reflection recovery dynamics in the initial several picosecond.
effective mass of electron–hole pair (6:9 1032 kg), and o the angular frequency of probe laser (2:36 1015 s1). For Fe ion implanted InP and undoped InP, Fig. 2 shows the change of the transient reflectivity as a function of the time. We can see that the decay of the reflection signal in InP becomes much faster after Fe implantation. It is characterized by a fast decay over the first several picosecond and by a slower decay which time is more than 60 ps. For the decays of reflection recovery dynamics, they decay fast at the initial several picosecond after the delay between pump and probe beam is zero, which is probably caused by ‘‘hot carrier’’ relaxation. Immediately after excitation with above band gap energy photons, large amounts of nonequilibrium carriers are generated. Their temperatures are much higher than those of the thermal equilibrium carriers due to the excess kinetic energy received during laser excitation, therefore, the nonequilibrium carriers are regarded as ‘‘hot carriers’’. After several picoseconds, these ‘‘hot carriers’’ relax to the bottom of the conduction band by means of emitting optical phonons. As the carriers lose their excess energy, the system returns to the thermal equilibrium. However, the diffusion and the surface recombination of carriers persist over tens of picosecond or so much as hundreds of picosecond until the spatial inhomogeneity returns to equilibrium. Since the diffusion and the surface recombination of carriers are essential to the understanding of the
carrier behavior in InP:Fe, they are mainly taken into account. Both the PL decay and the reflection recovery decay of the Fe implanted InP become faster than those of the undoped InP. In order to understand these results, we have developed a model to simulate the temporal evolution of the concentration of photoexcited carriers in the samples. Some previous research results indicate that the thermalization time in InP resulting from carrier–carrier and carrier–photon scattering is of the magnitude of several hundred femtoseconds [9], which is much faster than the time of reflection recovery dynamics obtained here. In addition, the Auger recombination would deplete the photoexcited carriers on a time scale of 0.1–10 ns [10]. Therefore, in our case the radiative recombination time, the thermalization time, and the Auger recombination time can be neglected. Since the optical penetration depth at the wavelength of 800 nm is less than 1 mm for the samples studied here, and the laser spot focus on the sample is about 20 mm, we can assume that the gradient of the photoexcited carrier density along z direction is much larger than that in the x–y plane. Therefore, the reflection recovery dynamics is mainly depended on the carrier transport along the z direction. Considering surface recombination, the reflection recovery dynamics following the excitation can be described by @Nðz; tÞ @ 2 Nðz; tÞ Nðz; tÞ ¼D @t @z2 tR with the boundary conditions @Nðz; tÞ ¼ SNðz; tÞ and D @z z¼0
(3)
Nðz; tÞjz¼1 ¼ 0 (4)
and the initial condition Nðz; tÞjt¼0 ¼ N0 expðazÞ
(5)
where N(z, t) is the carrier density, z the perpendicular to the sample surface, D the ambipolar diffusion coefficient, tR the bulk recombination lifetime, S the surface recombination velocity which is assumed to be independent of the carrier density, N0 the initial excess carrier density at the sample surface, and a the absorption coefficient. By using the method of
S. Huang et al. / Applied Surface Science 230 (2004) 158–162
with WðxÞ ¼ expðx2 Þ erfcðxÞ. The absorption coefficient was taken to be 3 104 cm1 for the excited wavelength 800 nm [11]. According to the penetration depth dependence of reflection by Aspnes and Frova [12], the reflection signal arises from a surface layer of depth 6102 mm in the case of a=2K ! 1, where K is the magnitude of the light momentum vector in the crystal. Taking into account of these conditions, we calculated the ambipolar diffusion coefficient and the surface recombination velocity dependence of photoexcited carrier dynamics. By making use of Eq. (6) and the parameters (a ¼ 3 104 cm1 and the penetration depth z ¼ 6 102 mm), we obtain the fitting to the experimental data, as shown in Fig. 3. We find that the experimental results agree with the theoretical model very well. For undoped InP and Fe implanted InP, the values of S and D obtained from the fitting are 1:21 104 cm/s and 2.23 cm2/s, 4:32 105 cm/s and 1.17 cm2/s, respectively. When the parameters (S and D) are optimized one after another, there is a minor change in the trace for S and D. The value of S obtained from the fit, 4:32 105 cm/s, is comparable to the largest one reported [13], but much larger than the typical values [14–17]. The surface recombination velocity of the Fe implanted InP is about 14 times larger than that of the undoped InP. But its ambipolar diffusion coefficient (1.17 cm2/s) is only the half of that of the undoped InP. Therefore, we believe that the decay time of reflection recovery dynamics in Fe
Transient reflectivity change (a. u.)
1.2
4
The undoped InP
S=1.21 10 cm/s
Fe implanted InP
S=4.32 10 cm/s
2
D=2.23 cm /s
0.8
0.4
5
2
D=1.17 cm /s
0.0 0
30
60
90
120
150
Delay time (ps)
Fig. 3. The symbols indicate the measured reflection recovery dynamics of the undoped InP and Fe implanted InP. The straight lines indicate the fitted temporal evolution by using Eq. (6). For the undoped InP and Fe implanted InP, the values of S and D obtained from the fitting of the experimental data are 1:21 104 cm/s and 2.23 cm2/s, 4:32 105 cm/s and 1.17 cm2/s, respectively.
implanted InP is mainly dominated by surface recombination. Fe implantation will introduce defects in the InP and then the smoothness of its surface becomes poor, which probably results in the large surface band bending for Fe implanted InP. In order to obtain the information of defects in Fe implanted InP and the undoped InP, we have employed FT-IR absorption measurement, and the measurement results are shown in Fig. 4. For Fe implanted InP, FT-IR spectroscopy shows an intense absorption peak located at 8
2858 cm
1
6
Absorption (a. u.)
separation of variables, the analytical solution to Eq. (3) is N0 t z2 Nðz; tÞ ¼ exp exp tR 2 4Dt ( ! z 1=2 W aðDtÞ 2ðDtÞ1=2 !) z 2S=D 1=2 þ W aðDtÞ þ 1=2 S=D a 2ðDtÞ " ! z W aðDtÞ1=2 þ 2ðDtÞ1=2 !# S z 1=2 ðDtÞ þ W (6) D 2ðDtÞ1=2
161
Fe implanted InP 4
2
0
1000
The undoped InP 2000
3000
4000
5000
6000
7000
1
Wavenumber (cm )
Fig. 4. The FT-IR absorption spectroscopy of undoped InP and Fe implanted InP.
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S. Huang et al. / Applied Surface Science 230 (2004) 158–162
2858 cm1 which is associated with Fe intracenter transition [18], however, the peak does not appear for the undoped InP. The Fe2þ/3þ trap has been activated in Fe implanted InP, where it acts as a recombination center capturing both electron and holes with comparable capture cross-section. This clearly indicates the high level of compensation is expected in Fe implantation InP [18]. The results of FT-IR measurement indicate that a mass of Fe2þ/3þ trapping centers exist in Fe implanted InP. Since the surface recombination velocity is strongly dependent on the surface potential and the presence of defects located within the forbidden gap (recombination), the large surface recombination velocity of Fe implantation InP is probably due to a mass of Fe2þ/3þ trapping centers and the large surface band bending. Also, a large number of Fe2þ/3þ trapping centers will result in the lower mobility of carrier. According to the Einstein relation, the diffusion coefficient of carrier is directly proportional to the mobility of carrier. Therefore, the ambipolar diffusion coefficient of Fe implanted InP is lower than that of the undoped InP. On the other hand, the PL decay time for Fe implanted InP is shorter than that for the undoped InP, which is probably due to Fe2þ/3þ trapping centers in Fe implanted InP which have great cross-section for electron–hole recombination.
4. Conclusion The decay time of reflection recovery and the radiative recombination time of Fe implanted InP and undoped InP has been investigated. Both of the decay times of the implanted InP are shorter than those of the undoped InP. Considering surface recombination, a model of reflection recovery dynamics was proposed. This model agrees with the experimental results very well. Moreover, we obtained the ambipolar diffusion coefficient and the surface recombination velocity by using the model. For Fe implanted InP, the surface recombination velocity is much larger than that of the undoped InP, which is probably due to
Fe2þ/3þ trapping centers and the large surface band bending. The PL decay time for Fe implanted InP is shorter than that for undoped InP, which is ascribed to Fe2þ/3þ trapping centers in Fe implanted InP.
Acknowledgements This work was supported by the special funds for Major State Basic Research Project No. G2001CB3095 of China, the Commission of Science and Technology of Shanghai, and the National Natural Science Foundation of China.
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