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Lattice damage study of implanted InGaAs by means of Raman spectroscopy
Journal of Luminescence 87}89 (2000) 721}723
Lattice damage study of implanted InGaAs by means of Raman spectroscopy S. HernaH ndez , B. Marcos , R. CuscoH , N. Blanco, G. GonzaH lez-DmH az, L. ArtuH s * Institut Jaume Almera (CSIC), Llun& s Sole& i Sabarn& s s.n., 08028 Barcelona, Spain Departamento de Fn& sica Aplicada III, Facultad de Fn& sica, Universidad Complutense, 28040 Madrid, Spain
Abstract We have studied by means of Raman scattering the crystallinity loss induced by ion-beam implantation in the InV Ga\V As alloy lattice matched to InP. Si> was implanted at 150 keV with #uences in the 10 } 5;10 cm\ range. The Raman scattering signatures of implantation-induced disorder and the progressive amorphization of the InV Ga\V As crystal are discussed. With increasing implantation dose, the GaAs-like LO mode exhibits a gradual intensity reduction and an asymmetric broadening, while a broad peak emerges at low frequencies corresponding to the activation by the induced disorder of transverse acoustic modes. The Raman scattering measurements have allowed us to check the full amorphization of the In Ga As for a Si> implantation dose of 5;10 cm\. 2000 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 The ternary alloy In Ga As lattice matched to InP V \V is being actively investigated because of its potential applications in optoelectronic and high-speed devices. Electron mobility and peak drift velocity are high in In Ga As, which make this material attractive for V \V high-performance junction "eld-e!ect transistors [1,2]. Also, integrated optoelectronic devices operating in the low-loss window of silica "bers can be realized using the In Ga As/InP lattice-matched system [3]. Ion-beam implantation is a versatile method for producing the shallow doped semiconductor layers and junctions that are required for device processing. However, the implantation of high doses of dopant ions severely damages the host crystal and postimplantation annealing treatments are necessary. Raman scattering
* Corresponding author. Tel.: #34-3-409-54-10; fax: #34-3411-00-12. E-mail address: [email protected] (L. ArtuH s)
has been previously proved to be a powerful, non-destructive technique to assess the degree of lattice disorder induced by ion-beam bombardment in III}V zinc-blende semiconductors [4,5]. In semiconductor alloys, the random isoelectronic substitutions at the anion or cation sites give rise to an intrinsic disorder in the crystal. The structural relaxations and changes in masses and bond restoring forces are re#ected in the alloy vibrational properties [6]. Although Raman scattering has been used to obtain information about the LO phonons and the intrinsic disorder in III}V alloys, and, in particular, in the In Ga As alloy [7}9], very few Raman-scattering studies have addressed the issue of the disorder induced by ion-beam implantation in III}V ternary alloys [10,11]. To our knowledge only one work has been published on implantation-induced disorder in the In Ga As alloy [11], in which a single Si> im plantation dose of 10 cm\ at 300 keV was studied. While the projected range for the Si> at this energy is about 305 nm, the 514.5 nm laser line, which has a penetration depth of about 45 nm [12], was used as excitation source in Ref. [11]. Therefore, the Raman
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 7 4 - 9
722
S. Herna& ndez et al. / Journal of Luminescence 87}89 (2000) 721}723
measurements only probed a shallow region of the sample far from the peak of the Si> distribution where the implantation damage is highest. In the present work, we have carried out a Ramanscattering study of lattice damage in Si>-implanted In Ga As for several implantation doses up to the full amorphization of the crystal, using an excitation wavelength that probes deeper into the Si> implantation pro"le. This allows us to obtain a more accurate assessment of the degree of implantation-induced damage and to determine the amorphization dose for this implantation conditions.
2. Experiment In this work we have used (100)-oriented In Ga As wafers grown by the liquid-phase epitaxy (LPE) method on InP substrates. The samples were Si>implanted at 150 keV with doses of 10, 5;10, 10, 5;10, 10, and 5;10 cm\. Raman-scattering measurements were carried out at room temperature using a T64000 Jobin-Yvon spectrometer equipped with a charge-coupled device detector (CCD) cooled with liquid nitrogen. The Raman spectra were recorded in backscattering con"guration using the 528.7 nm line of an Ar> laser. For this excitation wavelength we estimate that approximately 10% of the incident light is still acting as a probing light at a depth of about 60 nm. We selected the Ar> laser line with the smallest absorption in order to probe well into the implanted Si> distribution. To suppress the low-frequency Raman lines associated with interactions with atmospheric molecules the samples were kept in vacuum inside an optical cryostat.
3. Results and discussion Fig. 1 shows the unpolarized Raman spectra of the Si>-implanted In Ga As samples as a function of ion implantation dose, compared with the spectrum of the unprocessed material. The In Ga As alloy exhibits V \V a two-mode behaviour [8], and two LO modes can be observed in Fig. 1 (A) at 232 and 269.5 cm\, corresponding to the InAs- and GaAs-like modes, respectively. The Raman spectrum of the alloy is dominated by the GaAs-like LO mode, owing to the higher polarizability of the GaAs bonds as compared to the InAs bonds in In Ga As [9].The GaAs-like LO peak is slightly asymmetric with a tail towards low energies due to the intrinsic disorder of the alloy [6]. A broad Raman peak is observed between the InAs- and GaAs-like LO modes, which has been previously assigned to a disorder mode [7,8]. The broad band observed between 100 and 200 cm\ corresponds to second-order transverseacoustic modes (2TA) and longitudinal acoustic modes
Fig. 1. Room-temperature unpolarized Raman spectra of In Ga As samples, Si> implanted with doses of 10 (B), 5;10 (C), 10 (D), 5;10 (E), 10 (F), and 5;10 cm\ (G), compared with the spectrum of virgin In Ga As (A).
activated by the intrinsic alloy disorder (DALA) [9]. The second-order optical Raman spectrum is softened by alloying and it shows up as a broad band between 430 and 550 cm\, with a sharper feature at 535 cm\ that corresponds to the GaAs-like 2LO mode (inset of Fig. 1). According to TRIM simulations [13] for the implantation conditions of our samples, the projected range of the implanted Si> is 149 nm, with a longitudinal straggling of 87 nm. Taking into account the absorption of the 528.7 nm line, we estimate that we are probing well into the implanted Si> pro"le, with an attenuation of the Raman signal of +10% occurring at 60% of the peak of the implanted Si> pro"le. Implanted samples [Fig. 1(B}G)] clearly display a progressive intensity reduction and asymmetric broadening of the GaAs-like LO peak, which re#ects the relaxation of the q"0 selection rule associated with the loss of crystallinity in the implanted lattice. The ratio C /C between left and right half* 0 widths of the GaAs-like LO peak increases from 1.07 for the sample implanted with 10 cm\ to 1.72 for the sample implanted with 10 cm\, while its frequency decreases from 269.4 to 267.6 cm\ indicating the activation of qO0 modes as a result of the implantationinduced structural disorder in the alloy. The peak related to the intrinsic alloy disorder between the InAs- and GaAs-like LO frequencies is clearly observed in Fig. 1(B}F). For higher implantation doses, this peak broadens signi"cantly and it merges with the InAs- and GaAs-like optical modes, which are also broadened by the implantation-induced structural disorder and give rise to a #at plateau between 220 and 270 cm\. The InAs- and GaAs-like TO modes, which, in principle, should be activated due to the relaxation of the q"0
S. Herna& ndez et al. / Journal of Luminescence 87}89 (2000) 721}723
selection rule, are not resolved probably because of the large width of these modes and their interferences with the overlapping disorder mode. A new broad peak, whose intensity increases with the implantation dose, emerges centered at about 50 cm\ in the samples implanted with doses higher than 5;10 cm\, and has its maximum intensity in the sample implanted with the highest dose [Fig. 1(G)]. This peak is due to the activation of transverse acoustic modes and its intensity is an indication of the degree of lattice damage introduced by the implantation. As can be seen in the inset of Fig. 1, the second-order optical band, although very weak, can be detected even in samples with high implantation doses. However, its most distinct feature, the 2 LO peak, is strongly attenuated with increasing implantation doses. Contrary to the case of III}V binary crystals [5], second-order Raman scattering is a less sensitive probe for crystal damage in alloys because of the softening of these modes by the alloy disorder. Whereas the GaAs-like peak can still be observed in the sample implanted with 10 cm\, crystalline features are totally absent from the spectrum of the sample implanted with 5;10 cm\. The GaAs- and the InAslike LO peaks have vanished and the intensity of the plateau in the optical-frequency region of the spectrum has drastically reduced. Only the low-frequency band, reminiscent of the tetrahedral short-range bonding of the amorphized crystal, remains in spectrum of Fig. 1 (G). This clearly con"rms the full amorphization of the alloy for an implantation dose of 5;10 cm\.
4. Conclusions We have studied the lattice damage induced by Si> implantation in the In Ga As alloy lattice matched to V \V InP. Raman scattering is a non-destructive technique that can be used to probe the degree of lattice damage of the implanted layers. We have discussed the features in the Raman spectra of In Ga As that are associated with lattice disorder. Some of these are related to the intrinsic alloy disorder and are already present in the
723
pristine In As Ga sample. By studying a set of In As Ga samples implanted with increasing doses we have identi"ed the Raman-scattering signatures of the structural lattice damage induced by implantation. This has allowed us to characterize the degree of lattice damage and to determine that the Si> amorphization dose for the alloy is +5;10 cm\.
Acknowledgements The authors wish to acknowledge the Spanish Ministerio de EducacioH n y Cultura for "nancial support. One of us (S.H.) acknowledges a scholarship awarded by the CIRIT.
References [1] K.W. Wang, C.L. Cheng, J. Long, D. Mitcham, IEEE Electron Device Lett. 9 (1988) 205. [2] S. Koumetz, J. Marcon, K. Ketata, M. Ketata, C. DubonChevallier, P. Launay, J.L. Benchimol, Appl. Phys. Lett. 67 (1995) 2161. [3] T.P. Pearsall, R.W. Hopson Jr., J. Electron. Mater. 7 (1978) 133. [4] R. CuscoH , G. TalamaH s, L. ArtuH s, J.M. Martin, G. GonzaH lez-DmH az, J. Appl. Phys. 79 (1996) 3927. [5] J. IbaH n ez, R. CuscoH , N. Blanco, G. GonzaH lez-DmH az, L. ArtuH s, Nucl. Instr. and Meth. B 148 (1999) 454. [6] O. Brafman, R. Manor, Phys. Rev. B 51 (1995) 6940. [7] T.P. Pearsall, R. Carles, J.C. Portal, Appl. Phys. Lett. 42 (1983) 436. [8] J.P. Estrera, P.D. Stevens, R. Glosser, W.M. Duncan, Y.C. Kao, H.Y. Liu, E.A. Beam III, Appl. Phys. Lett. 61 (1992) 1927. [9] G. Landa, R. Carles, J.B. Renucci, Solid State Commun. 86 (1993) 351. [10] K. Kamimoto, T. Katoda, J. Appl. Phys. 59 (1986) 1477. [11] K. Kamimoto, T. Katoda, Appl. Phys. Lett. 42 (1983) 811. [12] J.E. Maslar, J.F. Dorsten, P.W. Bohn, S. Agarwala, I. Adesida, C. Caneau, R. Bhat, Phys. Rev. B 50 (1994) 17 143. [13] J.F. Ziegler, J.P. Biersak, U. Littmark, in: The Stopping and Range of Ions in Solids, Pergamon, New York, 1985.
Lattice damage study of implanted InGaAs by means of Raman spectroscopy S. HernaH ndez , B. Marcos , R. CuscoH , N. Blanco, G. GonzaH lez-DmH az, L. ArtuH s * Institut Jaume Almera (CSIC), Llun& s Sole& i Sabarn& s s.n., 08028 Barcelona, Spain Departamento de Fn& sica Aplicada III, Facultad de Fn& sica, Universidad Complutense, 28040 Madrid, Spain
Abstract We have studied by means of Raman scattering the crystallinity loss induced by ion-beam implantation in the InV Ga\V As alloy lattice matched to InP. Si> was implanted at 150 keV with #uences in the 10 } 5;10 cm\ range. The Raman scattering signatures of implantation-induced disorder and the progressive amorphization of the InV Ga\V As crystal are discussed. With increasing implantation dose, the GaAs-like LO mode exhibits a gradual intensity reduction and an asymmetric broadening, while a broad peak emerges at low frequencies corresponding to the activation by the induced disorder of transverse acoustic modes. The Raman scattering measurements have allowed us to check the full amorphization of the In Ga As for a Si> implantation dose of 5;10 cm\. 2000 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 The ternary alloy In Ga As lattice matched to InP V \V is being actively investigated because of its potential applications in optoelectronic and high-speed devices. Electron mobility and peak drift velocity are high in In Ga As, which make this material attractive for V \V high-performance junction "eld-e!ect transistors [1,2]. Also, integrated optoelectronic devices operating in the low-loss window of silica "bers can be realized using the In Ga As/InP lattice-matched system [3]. Ion-beam implantation is a versatile method for producing the shallow doped semiconductor layers and junctions that are required for device processing. However, the implantation of high doses of dopant ions severely damages the host crystal and postimplantation annealing treatments are necessary. Raman scattering
* Corresponding author. Tel.: #34-3-409-54-10; fax: #34-3411-00-12. E-mail address: [email protected] (L. ArtuH s)
has been previously proved to be a powerful, non-destructive technique to assess the degree of lattice disorder induced by ion-beam bombardment in III}V zinc-blende semiconductors [4,5]. In semiconductor alloys, the random isoelectronic substitutions at the anion or cation sites give rise to an intrinsic disorder in the crystal. The structural relaxations and changes in masses and bond restoring forces are re#ected in the alloy vibrational properties [6]. Although Raman scattering has been used to obtain information about the LO phonons and the intrinsic disorder in III}V alloys, and, in particular, in the In Ga As alloy [7}9], very few Raman-scattering studies have addressed the issue of the disorder induced by ion-beam implantation in III}V ternary alloys [10,11]. To our knowledge only one work has been published on implantation-induced disorder in the In Ga As alloy [11], in which a single Si> im plantation dose of 10 cm\ at 300 keV was studied. While the projected range for the Si> at this energy is about 305 nm, the 514.5 nm laser line, which has a penetration depth of about 45 nm [12], was used as excitation source in Ref. [11]. Therefore, the Raman
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 7 4 - 9
722
S. Herna& ndez et al. / Journal of Luminescence 87}89 (2000) 721}723
measurements only probed a shallow region of the sample far from the peak of the Si> distribution where the implantation damage is highest. In the present work, we have carried out a Ramanscattering study of lattice damage in Si>-implanted In Ga As for several implantation doses up to the full amorphization of the crystal, using an excitation wavelength that probes deeper into the Si> implantation pro"le. This allows us to obtain a more accurate assessment of the degree of implantation-induced damage and to determine the amorphization dose for this implantation conditions.
2. Experiment In this work we have used (100)-oriented In Ga As wafers grown by the liquid-phase epitaxy (LPE) method on InP substrates. The samples were Si>implanted at 150 keV with doses of 10, 5;10, 10, 5;10, 10, and 5;10 cm\. Raman-scattering measurements were carried out at room temperature using a T64000 Jobin-Yvon spectrometer equipped with a charge-coupled device detector (CCD) cooled with liquid nitrogen. The Raman spectra were recorded in backscattering con"guration using the 528.7 nm line of an Ar> laser. For this excitation wavelength we estimate that approximately 10% of the incident light is still acting as a probing light at a depth of about 60 nm. We selected the Ar> laser line with the smallest absorption in order to probe well into the implanted Si> distribution. To suppress the low-frequency Raman lines associated with interactions with atmospheric molecules the samples were kept in vacuum inside an optical cryostat.
3. Results and discussion Fig. 1 shows the unpolarized Raman spectra of the Si>-implanted In Ga As samples as a function of ion implantation dose, compared with the spectrum of the unprocessed material. The In Ga As alloy exhibits V \V a two-mode behaviour [8], and two LO modes can be observed in Fig. 1 (A) at 232 and 269.5 cm\, corresponding to the InAs- and GaAs-like modes, respectively. The Raman spectrum of the alloy is dominated by the GaAs-like LO mode, owing to the higher polarizability of the GaAs bonds as compared to the InAs bonds in In Ga As [9].The GaAs-like LO peak is slightly asymmetric with a tail towards low energies due to the intrinsic disorder of the alloy [6]. A broad Raman peak is observed between the InAs- and GaAs-like LO modes, which has been previously assigned to a disorder mode [7,8]. The broad band observed between 100 and 200 cm\ corresponds to second-order transverseacoustic modes (2TA) and longitudinal acoustic modes
Fig. 1. Room-temperature unpolarized Raman spectra of In Ga As samples, Si> implanted with doses of 10 (B), 5;10 (C), 10 (D), 5;10 (E), 10 (F), and 5;10 cm\ (G), compared with the spectrum of virgin In Ga As (A).
activated by the intrinsic alloy disorder (DALA) [9]. The second-order optical Raman spectrum is softened by alloying and it shows up as a broad band between 430 and 550 cm\, with a sharper feature at 535 cm\ that corresponds to the GaAs-like 2LO mode (inset of Fig. 1). According to TRIM simulations [13] for the implantation conditions of our samples, the projected range of the implanted Si> is 149 nm, with a longitudinal straggling of 87 nm. Taking into account the absorption of the 528.7 nm line, we estimate that we are probing well into the implanted Si> pro"le, with an attenuation of the Raman signal of +10% occurring at 60% of the peak of the implanted Si> pro"le. Implanted samples [Fig. 1(B}G)] clearly display a progressive intensity reduction and asymmetric broadening of the GaAs-like LO peak, which re#ects the relaxation of the q"0 selection rule associated with the loss of crystallinity in the implanted lattice. The ratio C /C between left and right half* 0 widths of the GaAs-like LO peak increases from 1.07 for the sample implanted with 10 cm\ to 1.72 for the sample implanted with 10 cm\, while its frequency decreases from 269.4 to 267.6 cm\ indicating the activation of qO0 modes as a result of the implantationinduced structural disorder in the alloy. The peak related to the intrinsic alloy disorder between the InAs- and GaAs-like LO frequencies is clearly observed in Fig. 1(B}F). For higher implantation doses, this peak broadens signi"cantly and it merges with the InAs- and GaAs-like optical modes, which are also broadened by the implantation-induced structural disorder and give rise to a #at plateau between 220 and 270 cm\. The InAs- and GaAs-like TO modes, which, in principle, should be activated due to the relaxation of the q"0
S. Herna& ndez et al. / Journal of Luminescence 87}89 (2000) 721}723
selection rule, are not resolved probably because of the large width of these modes and their interferences with the overlapping disorder mode. A new broad peak, whose intensity increases with the implantation dose, emerges centered at about 50 cm\ in the samples implanted with doses higher than 5;10 cm\, and has its maximum intensity in the sample implanted with the highest dose [Fig. 1(G)]. This peak is due to the activation of transverse acoustic modes and its intensity is an indication of the degree of lattice damage introduced by the implantation. As can be seen in the inset of Fig. 1, the second-order optical band, although very weak, can be detected even in samples with high implantation doses. However, its most distinct feature, the 2 LO peak, is strongly attenuated with increasing implantation doses. Contrary to the case of III}V binary crystals [5], second-order Raman scattering is a less sensitive probe for crystal damage in alloys because of the softening of these modes by the alloy disorder. Whereas the GaAs-like peak can still be observed in the sample implanted with 10 cm\, crystalline features are totally absent from the spectrum of the sample implanted with 5;10 cm\. The GaAs- and the InAslike LO peaks have vanished and the intensity of the plateau in the optical-frequency region of the spectrum has drastically reduced. Only the low-frequency band, reminiscent of the tetrahedral short-range bonding of the amorphized crystal, remains in spectrum of Fig. 1 (G). This clearly con"rms the full amorphization of the alloy for an implantation dose of 5;10 cm\.
4. Conclusions We have studied the lattice damage induced by Si> implantation in the In Ga As alloy lattice matched to V \V InP. Raman scattering is a non-destructive technique that can be used to probe the degree of lattice damage of the implanted layers. We have discussed the features in the Raman spectra of In Ga As that are associated with lattice disorder. Some of these are related to the intrinsic alloy disorder and are already present in the
723
pristine In As Ga sample. By studying a set of In As Ga samples implanted with increasing doses we have identi"ed the Raman-scattering signatures of the structural lattice damage induced by implantation. This has allowed us to characterize the degree of lattice damage and to determine that the Si> amorphization dose for the alloy is +5;10 cm\.
Acknowledgements The authors wish to acknowledge the Spanish Ministerio de EducacioH n y Cultura for "nancial support. One of us (S.H.) acknowledges a scholarship awarded by the CIRIT.
References [1] K.W. Wang, C.L. Cheng, J. Long, D. Mitcham, IEEE Electron Device Lett. 9 (1988) 205. [2] S. Koumetz, J. Marcon, K. Ketata, M. Ketata, C. DubonChevallier, P. Launay, J.L. Benchimol, Appl. Phys. Lett. 67 (1995) 2161. [3] T.P. Pearsall, R.W. Hopson Jr., J. Electron. Mater. 7 (1978) 133. [4] R. CuscoH , G. TalamaH s, L. ArtuH s, J.M. Martin, G. GonzaH lez-DmH az, J. Appl. Phys. 79 (1996) 3927. [5] J. IbaH n ez, R. CuscoH , N. Blanco, G. GonzaH lez-DmH az, L. ArtuH s, Nucl. Instr. and Meth. B 148 (1999) 454. [6] O. Brafman, R. Manor, Phys. Rev. B 51 (1995) 6940. [7] T.P. Pearsall, R. Carles, J.C. Portal, Appl. Phys. Lett. 42 (1983) 436. [8] J.P. Estrera, P.D. Stevens, R. Glosser, W.M. Duncan, Y.C. Kao, H.Y. Liu, E.A. Beam III, Appl. Phys. Lett. 61 (1992) 1927. [9] G. Landa, R. Carles, J.B. Renucci, Solid State Commun. 86 (1993) 351. [10] K. Kamimoto, T. Katoda, J. Appl. Phys. 59 (1986) 1477. [11] K. Kamimoto, T. Katoda, Appl. Phys. Lett. 42 (1983) 811. [12] J.E. Maslar, J.F. Dorsten, P.W. Bohn, S. Agarwala, I. Adesida, C. Caneau, R. Bhat, Phys. Rev. B 50 (1994) 17 143. [13] J.F. Ziegler, J.P. Biersak, U. Littmark, in: The Stopping and Range of Ions in Solids, Pergamon, New York, 1985.