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Lattice recovery by rapid thermal annealing in Mg+-implanted InP assessed by Raman spectroscopy
Nuclear Instruments and Methods in Physics Research B 175±177 (2001) 252±256
www.elsevier.nl/locate/nimb
Lattice recovery by rapid thermal annealing in Mg-implanted InP assessed by Raman spectroscopy ~ez a, R. Cusc B. Marcos a, J. Ib an o a, F.L. Martõnez b, G. Gonz alez-Dõaz b, L. Art us b
a,*
a Institut Jaume Almera (CSIC), Lluõs Sol e i Sabarõs s.n., 08028 Barcelona, Spain Departamento de Fõsica Aplicada III, Facultad de Fõsica, Universidad Complutense, 28040 Madrid, Spain
Abstract Mg is the most suitable ion to produce p-type InP layers by means of ion-beam implantation. We present a Ramanscattering study of lattice recovery by rapid thermal annealing on InP implanted with Mg at 80 keV, with a dose of 1014 cm 2 . Rapid thermal annealings for 10 s at dierent temperatures between 300°C and 875°C were carried out to study the eect of annealing temperature in the recovery of the InP lattice and on implant activation. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 85.40.Ry; 78.30.)j; 63.20.)e; 63.50.+x Keywords: Ion-beam implantation; Raman scattering; Lattice damage
1. Introduction InP is a well-suited material for the fabrication of high-performance optoelectronic and microwave devices, as it exhibits a higher peak electron drift velocity and higher breakdown ®eld compared to GaAs. Since ion-beam implantation is a doping technique capable of accurate doping control and precise registration in multistep processing, several n-type and p-type dopants have been extensively investigated for ion-beam doping of InP. It follows from these studies that obtaining highly doped layers using ion implantation is more
*
Corresponding author. Tel.: +34-93-409-54-10; fax: +3493-411-00-12. E-mail address: [email protected] (L. Art us).
dicult for p-type than for n-type doping. Activation of p-type dopants in ion-beam implanted InP is usually less than 50% and the hole concentrations that can be achieved are generally below 1019 cm 3 . Devices such as high-performance junction ®eld-eect transistors fabricated on InP [1] require a p layer with a steep, well-controlled pro®le for gate formation. Be, Mg, Zn and Cd have been tried as p-type dopants in InP. Be, in addition to its toxicity, has the drawback of a large in-diusion during thermal annealing. On the other hand, Mg, Zn and Cd have less in-diusion problems but the implantation process leaves a heavily damaged surface that may result in a surface dead layer without carriers after annealing [2,3]. The lower mass of Mg should be advantageous in minimizing this damage and achieving a greater penetration depth than the heavier ions. In
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 5 3 0 - 9
B. Marcos et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 252±256
fact, hole concentrations up to about 1019 cm 3 have been reported in Mg -implanted InP after activation by rapid thermal annealing (RTA) [4], although the control of the doping pro®le remains a challenge in p-type implantations. Raman spectroscopy has proven to be a powerful tool to probe the lattice damage induced by ion-beam implantation, to monitor the crystalline recovery, and to determine the electrical activation achieved by annealing the implanted samples [5±9]. Recently, we established Raman-scattering criteria for characterization of implantation-doped n-type zinc-blende semiconductors after the RTA stage [10]. Despite its technological interest, very few Raman data on p-type implanted and annealed InP have been published. To our knowledge, only a Raman scattering study of lattice damage in Zn -implanted InP and the subsequent lattice recovery by RTA has been reported [11], and no Raman-scattering data are available on annealed Mg -implanted InP. In a previous work [12], we studied by means of Raman scattering the progressive amorphization of the InP lattice for increasing Mg ¯uences, and we showed that InP becomes fully amorphized for a dose of 1014 cm 1 . In the present work, we present a Raman-scattering study of the annealing temperature (TA ) eects on the lattice recovery and on the implant activation for Mg -implanted InP. 2. Experiment A dose of 1014 cm 2 Mg was implanted at 80 keV into (1 0 0)-oriented semi-insulating InP wafers supplied by Sumitomo. The implantations were carried out at room temperature, with the samples tilted 7° o the normal incidence to avoid channeling eects. Subsequent RTA of the implanted samples was carried out using an RTP-600 system from MPT Corp. in a graphite susceptor, face down on a Si wafer. The samples were annealed for 10 s at several temperatures in the 300± 875°C range. Raman-scattering experiments were carried out at room temperature, using a T64000 Jobin±Yvon spectrometer equipped with a charge-coupled device detector cooled with liquid nitrogen. The
253
528.7-nm line of an Ar laser was used as a source of excitation. The absorption coecient of InP at this wavelength is a 1:03 105 cm 1 [13], which implies that Raman signal coming from 112 nm below the surface is attenuated by approximately a factor of 10. Taking into account the projected range and the longitudinal straggling for the 80 keV Mg implantation pro®le, which are esti respectively [12], and mated to be 1055 and 684 A, the smearing out of the doping pro®le due to Mg diusion during the annealing process, the volume probed by the Raman experiments is within the doped layer. The laser power used was about 100 mW on the sample. The Raman measurements were recorded at room temperature on a (1 0 0) face in the z xyz backscattering geometry, where x; y and z are along the [1 0 0], [0 1 0] and [0 0 1] crystallographic directions, respectively. According to the selection rules for the zinc-blende structure, the LO mode is allowed in this geometry, whereas TO modes are forbidden in backscattering from a (1 0 0) face. The spectra were recorded using the double subtractive con®guration of the spectrometer with 100 lm slits. 3. Results and discussion Fig. 1 shows the ®rst- and second-order opticalmode spectra of InP samples which were implanted with Mg under the same conditions and subsequently annealed at dierent temperatures. In this ®gure, the Raman spectra A to G correspond, respectively, to TA 300°C, 400°C, 500°C, 600°C, 700°C, 800°C and 875°C. The intense peak at 343.5 cm 1 observed in all the spectra corresponds to the LO mode, while only a weak peak is detected at the frequency of the forbidden TO mode (303.8 cm 1 ). The second-order optical region displays the three characteristic peaks of InP at 617, 650 and 682 cm 1 , corresponding to the 2TO, TO + LO and 2LO modes, respectively [14]. As can be seen from spectrum A, where the characteristic second-order optical peaks are already observed, a relatively high degree of lattice recovery is achieved for TA as low as 300°C. However, the intense peak detected at the zone-center LO frequency (343.5 cm 1 ) exhibits a large full width
254
B. Marcos et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 252±256
Fig. 1. First- and second-order optical regions of the Raman spectra obtained in the z xyz con®guration from Mg -implanted InP samples at 014 cm 2 , annealed for 10 s at dierent temperatures: A to G, 300°C, 400°C, 500°C, 600°C, 700°C, 800°C and 875°C. The intensity of the second-order region has been multiplied by a factor of 10.
at half height (FWHH) and a clear asymmetry with a low-energy tail, which is indicative of the participation of k 6 0 modes (disorder-activated longitudinal optical modes, DALO) [15] and hence of a signi®cant residual implantation damage in the InP lattice. This is also con®rmed by the presence of a broad intense band around the center-zone TO frequency, which is associated with the relaxation of the Raman selection rules and the activation by disorder of TO modes with k 6 0. The relatively high degree of crystalline recovery achieved at TA 300°C is in contrast with the previous results on Si -implanted InP, where
crystalline features were not observed at all in samples annealed at such a low annealing temperature [16]. This could be related to the lower mass of Mg and to its higher diusivity in the InP lattice. As can be seen in Fig. 1(A±E), the LO peak becomes more intense and exhibits a decrease in its FWHH for increasing TA up to 700°C. These changes in the LO peak re¯ect the gradual recovery of long-range ordering in the InP lattice that implies a more stringent veri®cation of the k 0 selection rule for Raman scattering, drastically reducing the participation of phonons with k 6 0. The peak at the TO frequency shows a parallel intensity decrease with TA , and it becomes barely visible in the sample annealed at 700°C (Fig. 1(E)), where its intensity is 10 times smaller than for the sample annealed at 300°C. The weak intensity of the TO signal is a clear evidence of the excellent crystallinity recovery preserving the original (1 0 0) crystal orientation that is achieved by RTA. The increase of phonon coherence length with TA is also con®rmed by the second-order optical peaks, which become narrower and better de®ned for increasing TA . At TA P 800°C, the intensities and widths of the second-order optical peaks are similar to those of samples annealed at lower temperatures, although the intensity of the ®rst-order LO mode decreases dramatically. In Fig. 2, we evaluate the degree of lattice recovery in the annealed samples by plotting the values of the LO and 2LO linewidths as well as the asymmetry of the LO peak. As can be seen in Fig. 2(a), the width of the LO peak rapidly decreases from 6 cm 1 for TA 300°C to 4 cm 1 for TA 500°C. The linewidth decrease is accompanied by a marked reduction of the peak asymmetry. These results suggest a fast recovery of the phonon coherence length for relatively low annealing temperatures, although for TA 500°C some degree of misorientation and/or polycrystalline regions still persist (see the intensity of the TO region in Fig. 1(C)). Only a minor decrease of the LO width and asymmetry is achieved by further increasing TA up to 700°C. Nevertheless, the forbidden TO signal is visibly reduced in the sample annealed at 700°C (Fig. 1(E)), indicating an excellent lattice recovery that preserves the original (1 0 0) orientation. For higher TA , the on-
B. Marcos et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 252±256
Fig. 2. Evolution with the annealing temperature TA of the full width at half height (FWHH) of the LO and 2LO peaks. (a) FWHH of the LO mode and ratio between the left and right half-widths of the LO peak. (b) FWHH of the 2LO mode normalized to the 2LO mode of virgin InP.
set of electrical activation of the dopants occurs, making the width of LO peak unreliable for the evaluation of the degree of crystallinity in the implanted samples, whereas the second-order optical modes still provide reliable criteria to assess the crystalline recovery [10]. As can be seen in Fig. 2(b), the 2LO width exhibits a decrease with TA parallel to that observed in the LO peak for TA up to 600°C, which re¯ects the progressive lattice recovery. For TA > 700°C, no appreciable change can be observed in the 2LO width, in contrast with the important intensity reduction and width increase of the ®rst-order spectrum. The assessment based on the second-order optical Raman spectrum shows that there is no signi®cant variation in the degree of crystallinity of the samples annealed at 700°C, 800°C and 875°C. The dramatic change between the ®rst-order Raman spectra of the samples annealed at 700°C (Fig. 1(E)) and those of the samples annealed at 800°C (Fig. 1(F)) and at 875°C (Fig. 1(G)) are due to the electrical activation of the Mg atoms, which results in the formation of a dense hole plasma whose collective excitations couple to the polar LO modes of the InP lattice. In contrast with n-type semiconductors, where two LO phonon±plasmon coupled modes (LOPCM) are generally observed [17], in p-type samples the larger mass of the holes
255
and the overdamped nature of the hole plasma result in the observation of a single LOPCM peak very close to the unscreened LO mode frequency [18]. To illustrate this point, we show in Fig. 3 the ®rst-order optical Raman spectrum of the sample annealed at 875°C (diamonds), where the Raman intensity has been decomposed in the contributions from the depletion zone LO mode (dashed line) and from the LOPCM (dotted line). A Lorentzian line shape (dashed line) with the frequency and width of the LO mode observed in the sample annealed at 600°C was subtracted from the Raman spectrum to account for the contribution of the depletion-zone unscreened LO mode. The resulting curve was ®tted using the LOPCM line-shape model outlined in [18], which takes into account the contributions of both, intra- and interband hole transitions to the electrical response of the hole plasma. The LOPCM line shape displayed in Fig. 3 (dotted line) was calculated for a total hole density Nh 1:3 1018 cm 3 , which is in good agreement with the activation values obtained from Hall eect measurements. The sum of
Fig. 3. Room temperature z xyz Raman spectrum of the sample annealed at 875°C (diamonds) showing the Raman signatures of dopant activation. The peak observed at 344 cm 1 is decomposed into a sum of a Lorentzian line shape (dashed line) corresponding to the depletion zone LO mode and an LOphonon±plasmon coupled-mode line shape (dotted line) corresponding to an overdamped hole plasma of density Nh 1:3 1018 cm 3 . The solid line corresponds to the sum of the two calculated line shapes.
256
B. Marcos et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 252±256
the depletion-zone LO Lorentzian line shape and the calculated LOPCM line shape, which is displayed in Fig. 3 as a solid line, ®ts very well the experimental data. Therefore, the high-energy tail observed in the Raman spectra of activated samples is well accounted for by the coupling between the free-hole plasma and LO phonons. This coupling is also observed in the spectra of the sample annealed at 800°C (Fig. 1(F)), although the LO intensity in spectrum F is slightly higher than in spectrum G, re¯ecting a lower hole density and hence a wider depletion depth. 4. Conclusions Raman scattering has been used to characterize the lattice recovery of Mg -implanted InP and the electrical activation of the dopants after RTA processes. By using dierent annealing temperatures we have observed the gradual recovery of the crystalline features of the ®rst- and second-order Raman spectra of InP in a previously amorphized InP sample. The evolution of the Raman spectra with TA suggests the existence of three stages in the lattice recovery process. First, for TA between 300°C and 400°C, the implanted sample starts its recrystallization process giving rise to small crystalline/polycrystalline regions. Then, for TA between 500°C and 700°C, there is an increase of the crystalline regions and the coherence length of the phonons asymptotically approaches its maximum value. At TA 700°C, the misorientations, which are present at lower TA , have been largely removed. Finally, for the highest annealing temperatures the electrical activation of the dopants takes place, as indicated by the presence of LOPCM modes in the Raman spectra. Although we already detect the existence of free charge in the samples annealed at 800°C, our results suggest that the
optimal charge activation is achieved by RTA at 875°C for 10 s.
Acknowledgements The authors acknowledge the Spanish Ministry of Science and Technology for ®nancial support.
References [1] J.B. Boos, W. Kruppa, B. Molnar, IEEE Electron Device Lett. 10 (1989) 79. [2] J.P. Donelly, C.E. Hurwitz, Appl. Phys. Lett. 31 (1977) 418. [3] T. Inada, S. Taka, Y. Yamamoto, J. Appl. Phys. 52 (1981) 6623. [4] W.H. van Berlo, G. Landgren, J. Appl. Phys. 66 (1989) 3117. [5] M. Holtz, R. Zallen, A.E. Geissberger, R.A. Sadler, J. Appl. Phys. 59 (1986) 1946. [6] J. Wagner, C. Homann, Appl. Phys. Lett. 50 (1987) 682. [7] H. Yoshida, T. Katoda, J. Appl. Phys. 67 (1990) 7281. [8] R. Cusc o, G. Talamas, L. Art us, J.M. Martin, G. Gonzalez-Dõaz, J. Appl. Phys. 79 (1996) 3927. [9] L.L. Abels, S. Sundaram, R.L. Schmidt, J. Comas, Appl. Surf. Sci. 9 (1981) 2. ~ez, J. Martin, G. Gonzalez[10] L. Art us, R. Cusc o, J. Iban Dõaz, J. Appl. Phys. 82 (1997) 3736. [11] E. Bedel, G. Landa, R. Carles, J.B. Renucci, J.M. Roquais, P.N. Favennec, J. Appl. Phys. 60 (1986) 1980. ~ez, R. Cusc [12] J. Iban o, N. Blanco, G. Gonzalez-Dõaz, L. Art us, Nucl. Instr. and Meth. B 148 (1999) 454. [13] D.E. Aspnes, A.A. Studna, Phys. Rev. B 27 (1983) 985. [14] L. Art us, R. Cusc o, J.M. Martin, G. Gonzalez-Dõaz, Phys. Rev. B 50 (1994) 11552. [15] O. Brafman, R. Manor, Phys. Rev. B 51 (1995) 6940. ~ez, N. Blanco, G. Gonzalez-Dõaz, [16] R. Cusc o, J. Iban L. Art us, Nucl. Instr. and Meth. B 132 (1997) 627. [17] See, for instance, G. Abstreiter, M. Cardona, A. Pinczuk, in: M. Cardona, G. G untherodt (Eds.), Light Scattering in Solids IV, Topics in Applied Physics Vol. 54, Springer, Berlin, 1984, and references therein. [18] K. Wan, J.F. Young, Phys. Rev. B 41 (1989) 10772.
www.elsevier.nl/locate/nimb
Lattice recovery by rapid thermal annealing in Mg-implanted InP assessed by Raman spectroscopy ~ez a, R. Cusc B. Marcos a, J. Ib an o a, F.L. Martõnez b, G. Gonz alez-Dõaz b, L. Art us b
a,*
a Institut Jaume Almera (CSIC), Lluõs Sol e i Sabarõs s.n., 08028 Barcelona, Spain Departamento de Fõsica Aplicada III, Facultad de Fõsica, Universidad Complutense, 28040 Madrid, Spain
Abstract Mg is the most suitable ion to produce p-type InP layers by means of ion-beam implantation. We present a Ramanscattering study of lattice recovery by rapid thermal annealing on InP implanted with Mg at 80 keV, with a dose of 1014 cm 2 . Rapid thermal annealings for 10 s at dierent temperatures between 300°C and 875°C were carried out to study the eect of annealing temperature in the recovery of the InP lattice and on implant activation. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 85.40.Ry; 78.30.)j; 63.20.)e; 63.50.+x Keywords: Ion-beam implantation; Raman scattering; Lattice damage
1. Introduction InP is a well-suited material for the fabrication of high-performance optoelectronic and microwave devices, as it exhibits a higher peak electron drift velocity and higher breakdown ®eld compared to GaAs. Since ion-beam implantation is a doping technique capable of accurate doping control and precise registration in multistep processing, several n-type and p-type dopants have been extensively investigated for ion-beam doping of InP. It follows from these studies that obtaining highly doped layers using ion implantation is more
*
Corresponding author. Tel.: +34-93-409-54-10; fax: +3493-411-00-12. E-mail address: [email protected] (L. Art us).
dicult for p-type than for n-type doping. Activation of p-type dopants in ion-beam implanted InP is usually less than 50% and the hole concentrations that can be achieved are generally below 1019 cm 3 . Devices such as high-performance junction ®eld-eect transistors fabricated on InP [1] require a p layer with a steep, well-controlled pro®le for gate formation. Be, Mg, Zn and Cd have been tried as p-type dopants in InP. Be, in addition to its toxicity, has the drawback of a large in-diusion during thermal annealing. On the other hand, Mg, Zn and Cd have less in-diusion problems but the implantation process leaves a heavily damaged surface that may result in a surface dead layer without carriers after annealing [2,3]. The lower mass of Mg should be advantageous in minimizing this damage and achieving a greater penetration depth than the heavier ions. In
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 5 3 0 - 9
B. Marcos et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 252±256
fact, hole concentrations up to about 1019 cm 3 have been reported in Mg -implanted InP after activation by rapid thermal annealing (RTA) [4], although the control of the doping pro®le remains a challenge in p-type implantations. Raman spectroscopy has proven to be a powerful tool to probe the lattice damage induced by ion-beam implantation, to monitor the crystalline recovery, and to determine the electrical activation achieved by annealing the implanted samples [5±9]. Recently, we established Raman-scattering criteria for characterization of implantation-doped n-type zinc-blende semiconductors after the RTA stage [10]. Despite its technological interest, very few Raman data on p-type implanted and annealed InP have been published. To our knowledge, only a Raman scattering study of lattice damage in Zn -implanted InP and the subsequent lattice recovery by RTA has been reported [11], and no Raman-scattering data are available on annealed Mg -implanted InP. In a previous work [12], we studied by means of Raman scattering the progressive amorphization of the InP lattice for increasing Mg ¯uences, and we showed that InP becomes fully amorphized for a dose of 1014 cm 1 . In the present work, we present a Raman-scattering study of the annealing temperature (TA ) eects on the lattice recovery and on the implant activation for Mg -implanted InP. 2. Experiment A dose of 1014 cm 2 Mg was implanted at 80 keV into (1 0 0)-oriented semi-insulating InP wafers supplied by Sumitomo. The implantations were carried out at room temperature, with the samples tilted 7° o the normal incidence to avoid channeling eects. Subsequent RTA of the implanted samples was carried out using an RTP-600 system from MPT Corp. in a graphite susceptor, face down on a Si wafer. The samples were annealed for 10 s at several temperatures in the 300± 875°C range. Raman-scattering experiments were carried out at room temperature, using a T64000 Jobin±Yvon spectrometer equipped with a charge-coupled device detector cooled with liquid nitrogen. The
253
528.7-nm line of an Ar laser was used as a source of excitation. The absorption coecient of InP at this wavelength is a 1:03 105 cm 1 [13], which implies that Raman signal coming from 112 nm below the surface is attenuated by approximately a factor of 10. Taking into account the projected range and the longitudinal straggling for the 80 keV Mg implantation pro®le, which are esti respectively [12], and mated to be 1055 and 684 A, the smearing out of the doping pro®le due to Mg diusion during the annealing process, the volume probed by the Raman experiments is within the doped layer. The laser power used was about 100 mW on the sample. The Raman measurements were recorded at room temperature on a (1 0 0) face in the z xyz backscattering geometry, where x; y and z are along the [1 0 0], [0 1 0] and [0 0 1] crystallographic directions, respectively. According to the selection rules for the zinc-blende structure, the LO mode is allowed in this geometry, whereas TO modes are forbidden in backscattering from a (1 0 0) face. The spectra were recorded using the double subtractive con®guration of the spectrometer with 100 lm slits. 3. Results and discussion Fig. 1 shows the ®rst- and second-order opticalmode spectra of InP samples which were implanted with Mg under the same conditions and subsequently annealed at dierent temperatures. In this ®gure, the Raman spectra A to G correspond, respectively, to TA 300°C, 400°C, 500°C, 600°C, 700°C, 800°C and 875°C. The intense peak at 343.5 cm 1 observed in all the spectra corresponds to the LO mode, while only a weak peak is detected at the frequency of the forbidden TO mode (303.8 cm 1 ). The second-order optical region displays the three characteristic peaks of InP at 617, 650 and 682 cm 1 , corresponding to the 2TO, TO + LO and 2LO modes, respectively [14]. As can be seen from spectrum A, where the characteristic second-order optical peaks are already observed, a relatively high degree of lattice recovery is achieved for TA as low as 300°C. However, the intense peak detected at the zone-center LO frequency (343.5 cm 1 ) exhibits a large full width
254
B. Marcos et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 252±256
Fig. 1. First- and second-order optical regions of the Raman spectra obtained in the z xyz con®guration from Mg -implanted InP samples at 014 cm 2 , annealed for 10 s at dierent temperatures: A to G, 300°C, 400°C, 500°C, 600°C, 700°C, 800°C and 875°C. The intensity of the second-order region has been multiplied by a factor of 10.
at half height (FWHH) and a clear asymmetry with a low-energy tail, which is indicative of the participation of k 6 0 modes (disorder-activated longitudinal optical modes, DALO) [15] and hence of a signi®cant residual implantation damage in the InP lattice. This is also con®rmed by the presence of a broad intense band around the center-zone TO frequency, which is associated with the relaxation of the Raman selection rules and the activation by disorder of TO modes with k 6 0. The relatively high degree of crystalline recovery achieved at TA 300°C is in contrast with the previous results on Si -implanted InP, where
crystalline features were not observed at all in samples annealed at such a low annealing temperature [16]. This could be related to the lower mass of Mg and to its higher diusivity in the InP lattice. As can be seen in Fig. 1(A±E), the LO peak becomes more intense and exhibits a decrease in its FWHH for increasing TA up to 700°C. These changes in the LO peak re¯ect the gradual recovery of long-range ordering in the InP lattice that implies a more stringent veri®cation of the k 0 selection rule for Raman scattering, drastically reducing the participation of phonons with k 6 0. The peak at the TO frequency shows a parallel intensity decrease with TA , and it becomes barely visible in the sample annealed at 700°C (Fig. 1(E)), where its intensity is 10 times smaller than for the sample annealed at 300°C. The weak intensity of the TO signal is a clear evidence of the excellent crystallinity recovery preserving the original (1 0 0) crystal orientation that is achieved by RTA. The increase of phonon coherence length with TA is also con®rmed by the second-order optical peaks, which become narrower and better de®ned for increasing TA . At TA P 800°C, the intensities and widths of the second-order optical peaks are similar to those of samples annealed at lower temperatures, although the intensity of the ®rst-order LO mode decreases dramatically. In Fig. 2, we evaluate the degree of lattice recovery in the annealed samples by plotting the values of the LO and 2LO linewidths as well as the asymmetry of the LO peak. As can be seen in Fig. 2(a), the width of the LO peak rapidly decreases from 6 cm 1 for TA 300°C to 4 cm 1 for TA 500°C. The linewidth decrease is accompanied by a marked reduction of the peak asymmetry. These results suggest a fast recovery of the phonon coherence length for relatively low annealing temperatures, although for TA 500°C some degree of misorientation and/or polycrystalline regions still persist (see the intensity of the TO region in Fig. 1(C)). Only a minor decrease of the LO width and asymmetry is achieved by further increasing TA up to 700°C. Nevertheless, the forbidden TO signal is visibly reduced in the sample annealed at 700°C (Fig. 1(E)), indicating an excellent lattice recovery that preserves the original (1 0 0) orientation. For higher TA , the on-
B. Marcos et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 252±256
Fig. 2. Evolution with the annealing temperature TA of the full width at half height (FWHH) of the LO and 2LO peaks. (a) FWHH of the LO mode and ratio between the left and right half-widths of the LO peak. (b) FWHH of the 2LO mode normalized to the 2LO mode of virgin InP.
set of electrical activation of the dopants occurs, making the width of LO peak unreliable for the evaluation of the degree of crystallinity in the implanted samples, whereas the second-order optical modes still provide reliable criteria to assess the crystalline recovery [10]. As can be seen in Fig. 2(b), the 2LO width exhibits a decrease with TA parallel to that observed in the LO peak for TA up to 600°C, which re¯ects the progressive lattice recovery. For TA > 700°C, no appreciable change can be observed in the 2LO width, in contrast with the important intensity reduction and width increase of the ®rst-order spectrum. The assessment based on the second-order optical Raman spectrum shows that there is no signi®cant variation in the degree of crystallinity of the samples annealed at 700°C, 800°C and 875°C. The dramatic change between the ®rst-order Raman spectra of the samples annealed at 700°C (Fig. 1(E)) and those of the samples annealed at 800°C (Fig. 1(F)) and at 875°C (Fig. 1(G)) are due to the electrical activation of the Mg atoms, which results in the formation of a dense hole plasma whose collective excitations couple to the polar LO modes of the InP lattice. In contrast with n-type semiconductors, where two LO phonon±plasmon coupled modes (LOPCM) are generally observed [17], in p-type samples the larger mass of the holes
255
and the overdamped nature of the hole plasma result in the observation of a single LOPCM peak very close to the unscreened LO mode frequency [18]. To illustrate this point, we show in Fig. 3 the ®rst-order optical Raman spectrum of the sample annealed at 875°C (diamonds), where the Raman intensity has been decomposed in the contributions from the depletion zone LO mode (dashed line) and from the LOPCM (dotted line). A Lorentzian line shape (dashed line) with the frequency and width of the LO mode observed in the sample annealed at 600°C was subtracted from the Raman spectrum to account for the contribution of the depletion-zone unscreened LO mode. The resulting curve was ®tted using the LOPCM line-shape model outlined in [18], which takes into account the contributions of both, intra- and interband hole transitions to the electrical response of the hole plasma. The LOPCM line shape displayed in Fig. 3 (dotted line) was calculated for a total hole density Nh 1:3 1018 cm 3 , which is in good agreement with the activation values obtained from Hall eect measurements. The sum of
Fig. 3. Room temperature z xyz Raman spectrum of the sample annealed at 875°C (diamonds) showing the Raman signatures of dopant activation. The peak observed at 344 cm 1 is decomposed into a sum of a Lorentzian line shape (dashed line) corresponding to the depletion zone LO mode and an LOphonon±plasmon coupled-mode line shape (dotted line) corresponding to an overdamped hole plasma of density Nh 1:3 1018 cm 3 . The solid line corresponds to the sum of the two calculated line shapes.
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the depletion-zone LO Lorentzian line shape and the calculated LOPCM line shape, which is displayed in Fig. 3 as a solid line, ®ts very well the experimental data. Therefore, the high-energy tail observed in the Raman spectra of activated samples is well accounted for by the coupling between the free-hole plasma and LO phonons. This coupling is also observed in the spectra of the sample annealed at 800°C (Fig. 1(F)), although the LO intensity in spectrum F is slightly higher than in spectrum G, re¯ecting a lower hole density and hence a wider depletion depth. 4. Conclusions Raman scattering has been used to characterize the lattice recovery of Mg -implanted InP and the electrical activation of the dopants after RTA processes. By using dierent annealing temperatures we have observed the gradual recovery of the crystalline features of the ®rst- and second-order Raman spectra of InP in a previously amorphized InP sample. The evolution of the Raman spectra with TA suggests the existence of three stages in the lattice recovery process. First, for TA between 300°C and 400°C, the implanted sample starts its recrystallization process giving rise to small crystalline/polycrystalline regions. Then, for TA between 500°C and 700°C, there is an increase of the crystalline regions and the coherence length of the phonons asymptotically approaches its maximum value. At TA 700°C, the misorientations, which are present at lower TA , have been largely removed. Finally, for the highest annealing temperatures the electrical activation of the dopants takes place, as indicated by the presence of LOPCM modes in the Raman spectra. Although we already detect the existence of free charge in the samples annealed at 800°C, our results suggest that the
optimal charge activation is achieved by RTA at 875°C for 10 s.
Acknowledgements The authors acknowledge the Spanish Ministry of Science and Technology for ®nancial support.
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