- Email: [email protected]
Study of Si+-implanted and annealed InP by means of Raman spectroscopy
NINMI B
Boom Interactions with Materials lb Atoms
Nuclear Instruments and Methods in Physics Research B 132 ( 1997) 627-632
ELSEVIER
Study of SF-implanted
and annealed InP by means of Raman spectroscopy
R. Cusc6 a, J. Ib&iez a, N. Blanc0 b, G. Gonziilez-Diaz b, L. Art& a,* a hstitut Jaume Almera, Consell Superior d’hoestigacions Cienti$ques (CSIC). Lluis Sol’ i Saharis s.n.. 08028 Uarcelona. Spuin ’ Departamento de Electricidad y Electr‘nica, Facultud de Fisica. Vnirersidad Comphtense. 28040 Madrid, Spain Received 15 September 1997
Abstract We have carried out a Raman scattering study of the lattice damage produced by 150 keV Si + implantation in InP and the subsequent lattice recovery by rapid thermal annealing (RTA). The Raman spectra show that for implantation doses usually employed for producing n-type InP, the sample is fully amorphized by the implantation. The degree of lattice recovery achieved by RTA at different temperatures in the range between 300°C and 875°C is also studied.
The observation of a strong TO mode in the samples annealed at the lower temperatures indicates the existence of polycrystalline and/or misoriented regions. Second-order Raman scattering is used to obtain a more accurate estimation of the crystalline recovery, which is found to be highest for RTA at 875°C for 10 s. 0 1997 Elsevier Science B.V. PACS: 61.72.V~; 63.20.-e; 63.50.+x Kqworcls: Ion beam implantation; Rapid
thermal
annealing;
1. Introduction InP is an interesting material for optoelectronics and high-speed electronic devices because of its high-electron mobility and excellent lattice match with the low band gap Ino.s3 Ga0.d7 As ternary alloy. Controllable and reproducible means of introducing dopants into InP structures are essential for producing high-performance devices. Ionbeam implantation is a doping technique very
*Corresponding author. Tel.: 34 3 4900552; fax: 34 3 411 IO0I?: e-mail: [email protected].
Raman
spectroscopy
well-suited for fabrication of semiconductor device structures because it permits us to obtain well-defined doping profiles over selected areas with good lateral definition. The bombardment of energetic ions into the host material damages its crystalline structure creating clusters of disordered material and, for high-implantation doses, leads to a complete amorphization of the implanted layer. A postimplantation annealing treatment has to be carried out to restore the crystallinity and to electrically activate the implanted dopants. The thermal annealing step of implanted samples is critical in III-V semiconductors because diffusion and redistribution of atomic species and
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PlISOl68-583X(97)00512-0
638
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impurities may alter their properties and strongly influence the implant activation [l--3]. This is particularly important for InP since, due to the very different vapor pressures of In and P, the surface decomposition temperature is well below the typical annealing temperatures and conventional furnace annealings may result in an important phosphorous loss. Raman spectroscopy is a powerful nondestructive technique to monitor the degree of crystallinity of implanted samples. With increasing disorder, intensity decrease and line broadening of the Raman peaks are observed, which are associated with the reduction of phonon coherence length and hence provide a measure of the degree of disorder in the crystal. Several studies of the lattice damage by implantation of Si+ [4,5], Ga+ [6], Zn+ [7], and Be+ [4,8] into InP, and the subsequent lattice recovery by thermal annealing have been published. Apart from Ref. [5], conventional furnace annealings were performed in these studies. and the Raman results indicate a poor lattice recovery. Rapid thermal annealing (RTA) at 875°C was employed in Ref. [S], where the degree of lattice recovery obtained was higher than those previously reported. In this paper we investigate by means of Raman scattering the effects on the sample crystallinity of the ion-beam doping processing of n-type layers in InP. Both, the lattice amorphization by the ion bombardment as well as the different degree of lattice recovery and dopant activation achieved by performing RTA at different temperatures are studied. The Raman signatures of the recovery process up to the optimum annealing temperature are discussed taking into account the presence of free-charge after dopant activation.
a dramatic decrease of the activation rate due to the amphoteric behavior of Si in the InP lattice [lo]. For the implantation conditions used in this work, a gaussian doping profile peaking at M 170 nm below the surface, with a projected range of = 95 nm, is obtained. After implantation, RTA for 10 s at temperatures ranging from 300°C to 875°C were performed using a RTP-600 system from MPT Corp. in a graphite susceptor, with the InP samples face down on a Si wafer. Annealing at higher temperatures results in a degradation of the sample surface. Raman-scattering measurements were taken at room temperature, using a T64000 Jobin-Yvon spectrometer equipped with a charge-coupled device detector cooled with liquid nitrogen. The 514.5 nm line of an argon-ion laser was used as a source of excitation. The absorption coefficient of InP at this wavelength is CI= 1.13 x lo5 cm-‘, so the volume probed by Raman measurements is inside the damaged layer (Raman signal coming from 100 nm below the surface is attenuated by approximately a factor of 10). The Raman measurements were recorded at room temperature in backscattering geometry on a (1 0 0) face. The first-order Raman spectra of the annealed samples were obtained using the triple additive configuration of the spectrometer with 100 urn slits, while the rest of the spectra were recorded using the double subtractive configuration with 100 urn slits. To suppress the low-frequency Raman lines arising from rotational modes of atmospheric molecules the sample area was purged with a continuous argon gas flow.
3. Results and discussion 2. Experiment Semi-insulating Fe-doped (1 0 0) wafers of liquid-encapsulated-Czochralski grown InP supplied by Sumitomo were used in the experiments. ‘*Si was implanted at 150 keV at doses of 5 x lo’?, 5 x 10”. and 5 x 10’” cm-‘. This implantation energy and range of doses are typically used for ntype doping of semi-insulating InP for device applications [9]. Higher implantation doses result in
Ion beam bombardment displaces lattice atoms and creates point defects and disordered regions in the solid. With increasing lattice disorder, the phonon coherence length is reduced and the k = 0 selection rule is relaxed, giving rise to mesurable shifts and broadenings of the Raman peaks [l 11. At high-implantation doses clusters of amorphous material develop, which give rise to an amorphous-like contribution to the Raman spectrum reflecting the phonon density of states.
R. Cusc‘ et al. I Nucl. Instr. and Meth. in Phys. Rex B 132 (1997) 627632
Fig. 1 shows the Raman spectra of Si+-implanted InP samples for several implantation doses compared with the spectrum of virgin InP. As can be seen in Fig. l(a), the spectrum of virgin InP (A) displays a sharp peak at the InP LO frequency whereas almost no Raman signal is detected at the TO frequency, which reflects the good quality of the (1 0 0) single crystal, as the TO mode is symmetry-forbidden in backscattering configuration from a (1 0 0) face. For the sample implanted with 5 x 1012cm-’ (B) although an intense, sharp LO peak is still observed, a strong TO signal appears in the Raman spectrum which indicates a relaxation of the crystal selection rules. Also, two weak broad structures appear centered around 62 and 138 cm-l, associated with disorder-activated transverse and longitudinal acoustic modes (DATA and DALA modes, respectively) [12]. A significant loss of intensity and a broadening of the first-order peaks, as well as an enhancement of the disorder-activated acoustic modes, are observed in the Raman spectrum of the sample implanted with 5 x lOI cm-’ (C), which indicates a higher degree of disorder and the presence of a significant proportion of amorphous material in this sample. In the Raman spectra of the sample im-
(a) -
I
(+5)
@I
n
k!
Raman shift (cm-‘)
Raman shift (cm-‘)
Fig. 1. (a) Raman spectra of InP samples implanted with 150 keV Si+ at different doses. A to D, respectively: virgin InP, 5 \: IO”. 5 x 10”. and 5 x lOI cm-l. (b) The corresponding second-order optical-phonon region of the Raman spectra.
629
planted with 5 x lOI cm-? (D), long-wavelength first-order optical-phonon peaks have been replaced by a broad band centered around 300 cm-’ containing the whole density of states. This confirms the complete amorphization of the material. A new broad structure develops at 436 cm-’ which was also observed in Ref. [7] and has been assigned to phosphorous-phosphorous vibrations associated with the formation of phosphorous clusters [ 131. Fig. l(b) shows the corresponding second-order optical-mode regions of the implanted samples. In the spectrum of virgin InP (A) three peaks can be clearly observed at 617, 650, and 682 cm-‘, corresponding to 2T0, TO + LO and 2L0 modes, respectively [14]. As can be seen from Fig. l(b), these second-order peaks are very sensitive to lattice disorder. Their intensity already shows a noticeable decrease in the sample implanted with 5 x lOI cmm2(B), and it is dramatically reduced in the sample implanted with 5 x 10” cm-’ (C). No signal at all is detected in this spectral region for the sample implanted with 5 x 1014cm -’ (D). Spectra D from Fig. l(a) and (b) show that after Si+ implantation at 5 x 1014 cm-? InP samples are fully amorphized. In Fig. 2(a) we display the first-order Raman spectra obtained from InP samples implanted with 5 x lOI cm-? after annealing for 10 s at different temperatures in the 300875°C range. As can be seen from the broad band detected in the opticalmode region of the spectrum (A), the sample annealed at 300°C is still amorphous. Even for the low annealing temperature of 400°C a significant degree of lattice recovery is achieved, as reflected by the presence of well-defined peaks in spectrum B. However, the high-intensity of the TO peak reveals the existence of polycrystalline and/or misoriented regions in the material in the early stages of recrystallization. Increasing the annealing temperature to 500°C improves crystallinity, as shown by the noticeable narrowing of the TO and LO peaks of spectrum C. but the presence of the strong TO signal indicates that misorientation problems persist. A further increase of the annealing temperature to 600°C produces a significant decrease of the LO intensity which we attribute to the onset of electrical activation of the implanted dopants.
630
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Fig. 2. (a) Raman spectra of InP samples implanted with 150 keV Si+ at a dose of 5 x IO” cm-? after RTA at different temperatures. A to G, respectively: 300°C. JOO”C, 5OO”C, 600°C. 7OO”C, 800°C. and 875°C. (b) The corresponding second-order optical-phonon region of the Raman spectra.
The free-electron gas generated by the activation of the Si atoms couples with the bulk polar LO mode giving rise to LO-plasmon coupled modes (LOPCM) [5.15], and therefore the small Raman signal detected at the LO frequency is due only to Raman scattering by the unscreened LO mode in the surface depletion zone. It should be remarked that, among all the first- and second-order Raman peaks shown in Fig. 2(a) and (b), only the first-order LO peak exhibits a substantial change of intensity between spectra C and D, which supports our conclusion that the onset of charge activation occurs for annealing temperatures around 600°C. Electrical measurements on ion-beam doped InP reported by Nadella et al. [lo] show an important activation-rate drop of about 40% when the annealing temperature is reduced from 875°C to 825°C [lo]. Activations for lower annealing temperatures are not reported, presumably due to difficulties in carrying out the electrical measurements, but the trend is consistent with our estimation of the temperature at which electrical activation sets in. The intense TO mode observed in spectrum D shows that polycrystalline and/or misorientated regions still remain in the implanted
layer. As the annealing temperature is risen to 700°C (E) and 800°C (F), the intensity of the TO mode decreases, reflecting the gradual recovery of the (1 0 0)-oriented single crystal. The highest degree of crystallinity and electrical activation is achieved for the sample annealed at 875°C (G). In fact, a less intense, narrow and symmetric LO peak is observed in spectrum G, and the intensity of the TO peak has decreased considerably. The Raman peak detected at the TO frequency contains a significant contribution from the _ branch of the LOPCM, whose frequency, for high-density, high-mobility free-electron plasmas, asymptotically approaches the TO frequency [5.15]. The small shoulder visible on the high-energy side of this peak may be due to an overtone of the longitudinal acoustic branch [7]. It has been recently pointed out that in zincblende compounds first-order Raman scattering can be misleading to assess the lattice recovery of implanted samples after annealing, and second-order Raman scattering was proposed as a more reliable criterion [16]. In fact, when charge activation sets in the observed LO peak is due only to the surface depletion zone LO mode, and the _ LOPCM can mask the forbidden TO mode. In Fig. 2(b) the second-order optical-phonon spectra of the annealed samples are shown. The second-order peaks are already recovered after RTA at 400°C and display intensities around 70% of those of the virgin InP for all the annealing temperatures above 4OO”C, up to 875°C. However, as the annealing temperature is increased from 400°C (B) to 875°C (G) a progressive sharpening of the second-order peaks is clearly observed. This reflects the gradual improvement in the long-range order of the restored InP crystal with increasing annealing temperature. Among the three second-order optical peaks of InP, the TO + LO peak displays the smallest width. In Fig. 3 we have plotted the full width at half height of the TO + LO peaks for samples annealed at different temperatures. A substantial decrease in the TO + LO width is observed when going from 400°C to .5OO”C, and as the annealing temperature is further increased the TO + LO width steadily approaches its value in virgin InP. The width of the TO + LO peak for the sample
R. Cusc’ et al. I Nucl. Instr. and Meth. in Phys. Res. B I32 (1997) 627432
631
ing [3]. The large width of the surface depletion zone LO peak suggests a lower degree of ordering which could be due to stoichiometric disturbances near the surface.
l6 [o
4. Conclusion
400
500
600
700
800
900
Annealing temperature (“C) Fig. 3. Plot of the widths of the TO + LO (squares) and LO (circles) peaks of Si’-implanted and annealed InP as a function of annealing temperature. On the right-hand-side vertical axis we have indicated the widths of the LO and TO + LO peaks in virgin InP.
annealed at 875°C deviates only by about 7% from its value in virgin InP, which confirms the excellent lattice recovery achieved by RTA at this temperature. For comparison, we have also plotted the width of the first-order LO as a function of annealing temperature. It also shows a significant decrease when going from 400°C to 5OO”C,parallel to that of second-order TO + LO. However the LO width increases again for the sample annealed at 6OO”C, where, as discussed above, the dopant activation sets in and the observed LO mode originates in the surface depletion zone. A steady reduction of the LO peak is also observed for higher annealing temperatures. However. the LO width measured in the sample annealed at 875°C is roughly twice its value in virgin InP, in contrast with the TO + LO width, which, as mentioned above, lies within 7% of the virgin InP value. Stoichiometric disturbances are likely to be present in the implanted, annealed InP samples due to the implantation/annealing processing. During the implantation, the different recoiling range of the elements of the compound results in a indium excess near the surface [17]. This stoichiometric imbalance is further modified by diffusion processes and phosphorous loss occurring during the anneal-
We have studied by means of Raman spectroscopy the lattice recovery of Si+-implanted InP by RTA at different annealing temperatures of samples fully amorphized by the implantation. The combined use of first- and second-order Raman scattering allows us to assess the crystallinity recovery of the implanted layer as well as the activation of the dopants. Our Raman measurements show that a fairly high degree of crystallinity recovery is achieved at relatively low annealing temperatures, but a considerable proportion of polycrystalline and/or misoriented regions are still present, and the electrical activation is very low. For annealing temperatures above 600°C dopant activation and a steady recovery of the (1 0 0) InP single crystal with increasing temperature are observed. The optimum single-crystal recovery is obtained for samples annealed at 875°C for 10 s, for which the widths of the second-order Raman peaks approach the values in virgin InP.
Acknowledgements
This work has been supported by the Spanish Ministerio de Education y Ciencia. One of the authors (J.I.) acknowledges CSIC for the award of a post graduate scholarship.
References Ul M. Gauneau,
R. Chapllain. A. Rupert, E.V.K. Rao, N. Duhamel, J. Appl. Phys. 57 (1985) 1029. 121A.K. Chin, I. Camlibel, T.T. Sheng, W.A. Bonner, Appl. Phys. Lett. 43 (1983) 495. [31 C.C.D. Wong, R.H. Bube, J. Appl. Phys. 55 (1984) 3804. R.L. Schmidt, J. Comas. Appl. 141 L.L. Abels. S. Sundaram, Surf. Sci. 9 (1981) 2. G. TalamBs, L. Artus, J.M. Martin, G. [51 R. Cusco. Gonzalez-Diaz. J. Appl. Phys. 79 (1996) 3927.
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[6] S.J. Yu. H. Asahi. S. Emma, H. Sumida, S. Gonda, H. Tanoue, J. Appl. Phys. 66 (1989) 856. [7] E. Bedel, G. Landa, R. Caries. J.B. Renucci, J.M. Roquais. P.N. Favennec. J. Appl. Phys. 60 (1986) 1980. [S] C.S. Rao. S. Sundaram, R.L. Schmidth. J. Comas. J. Appl. Phys. 54 (1983) 1808. [9] W.S. Lee, S.A. Kitching. S.W. Bland. Electron. Lett. 25 (1989) 522. [IO] R.K. Nadella. M.V. Rao. D.S. Simons. P.H. Chi. M. Fatemi, H.B. Dietrich. J. Appl. Phys. 70 (1991) 1750. [l l] K. Tiong, P.M. Amirtaraj, F. Pollak. D.E. Aspnes, Appl. Phys. Lett. 44 (1984) 122. [I’] H. Kawamura, R. Tsu. L. Esaki. Phys. Rev. Lett. 29 (1972) 1397.
[13] M. Wihl. M. Cardona. J. Taut. J. Non-Cryst. Solids 8-10 (1972) 172. [14] L. Artus. R. Cusco. J.M. Martin. G. Gonzalez-Diaz, Phys. Rev. B 50 (1994) 11552. [I51 G. Abstreiter. M. Cardona. A. Pinczuk, Light Scattering in Solids LV, in: M. Cardona, G. Giintherodt (Eds.). Topics in Applied Physics, vol. 54, Springer. Berlin, 1984 and references therein. [I61 L. Artus. R. Cusco, J. Ibaiiez. J.M. Martin. G. GonzalezDiaz. J. Appl. Phys., in press. [I71 L.A. Christel, J.F. Gibbons, J. Appl. Phys. 52 (1981) 5050.
Boom Interactions with Materials lb Atoms
Nuclear Instruments and Methods in Physics Research B 132 ( 1997) 627-632
ELSEVIER
Study of SF-implanted
and annealed InP by means of Raman spectroscopy
R. Cusc6 a, J. Ib&iez a, N. Blanc0 b, G. Gonziilez-Diaz b, L. Art& a,* a hstitut Jaume Almera, Consell Superior d’hoestigacions Cienti$ques (CSIC). Lluis Sol’ i Saharis s.n.. 08028 Uarcelona. Spuin ’ Departamento de Electricidad y Electr‘nica, Facultud de Fisica. Vnirersidad Comphtense. 28040 Madrid, Spain Received 15 September 1997
Abstract We have carried out a Raman scattering study of the lattice damage produced by 150 keV Si + implantation in InP and the subsequent lattice recovery by rapid thermal annealing (RTA). The Raman spectra show that for implantation doses usually employed for producing n-type InP, the sample is fully amorphized by the implantation. The degree of lattice recovery achieved by RTA at different temperatures in the range between 300°C and 875°C is also studied.
The observation of a strong TO mode in the samples annealed at the lower temperatures indicates the existence of polycrystalline and/or misoriented regions. Second-order Raman scattering is used to obtain a more accurate estimation of the crystalline recovery, which is found to be highest for RTA at 875°C for 10 s. 0 1997 Elsevier Science B.V. PACS: 61.72.V~; 63.20.-e; 63.50.+x Kqworcls: Ion beam implantation; Rapid
thermal
annealing;
1. Introduction InP is an interesting material for optoelectronics and high-speed electronic devices because of its high-electron mobility and excellent lattice match with the low band gap Ino.s3 Ga0.d7 As ternary alloy. Controllable and reproducible means of introducing dopants into InP structures are essential for producing high-performance devices. Ionbeam implantation is a doping technique very
*Corresponding author. Tel.: 34 3 4900552; fax: 34 3 411 IO0I?: e-mail: [email protected].
Raman
spectroscopy
well-suited for fabrication of semiconductor device structures because it permits us to obtain well-defined doping profiles over selected areas with good lateral definition. The bombardment of energetic ions into the host material damages its crystalline structure creating clusters of disordered material and, for high-implantation doses, leads to a complete amorphization of the implanted layer. A postimplantation annealing treatment has to be carried out to restore the crystallinity and to electrically activate the implanted dopants. The thermal annealing step of implanted samples is critical in III-V semiconductors because diffusion and redistribution of atomic species and
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PlISOl68-583X(97)00512-0
638
R. C‘trsc vt ul. I Nwl. Itlstr. rrrd Met/j. in Phw. Rex B 132 (19971 627432
impurities may alter their properties and strongly influence the implant activation [l--3]. This is particularly important for InP since, due to the very different vapor pressures of In and P, the surface decomposition temperature is well below the typical annealing temperatures and conventional furnace annealings may result in an important phosphorous loss. Raman spectroscopy is a powerful nondestructive technique to monitor the degree of crystallinity of implanted samples. With increasing disorder, intensity decrease and line broadening of the Raman peaks are observed, which are associated with the reduction of phonon coherence length and hence provide a measure of the degree of disorder in the crystal. Several studies of the lattice damage by implantation of Si+ [4,5], Ga+ [6], Zn+ [7], and Be+ [4,8] into InP, and the subsequent lattice recovery by thermal annealing have been published. Apart from Ref. [5], conventional furnace annealings were performed in these studies. and the Raman results indicate a poor lattice recovery. Rapid thermal annealing (RTA) at 875°C was employed in Ref. [S], where the degree of lattice recovery obtained was higher than those previously reported. In this paper we investigate by means of Raman scattering the effects on the sample crystallinity of the ion-beam doping processing of n-type layers in InP. Both, the lattice amorphization by the ion bombardment as well as the different degree of lattice recovery and dopant activation achieved by performing RTA at different temperatures are studied. The Raman signatures of the recovery process up to the optimum annealing temperature are discussed taking into account the presence of free-charge after dopant activation.
a dramatic decrease of the activation rate due to the amphoteric behavior of Si in the InP lattice [lo]. For the implantation conditions used in this work, a gaussian doping profile peaking at M 170 nm below the surface, with a projected range of = 95 nm, is obtained. After implantation, RTA for 10 s at temperatures ranging from 300°C to 875°C were performed using a RTP-600 system from MPT Corp. in a graphite susceptor, with the InP samples face down on a Si wafer. Annealing at higher temperatures results in a degradation of the sample surface. Raman-scattering measurements were taken at room temperature, using a T64000 Jobin-Yvon spectrometer equipped with a charge-coupled device detector cooled with liquid nitrogen. The 514.5 nm line of an argon-ion laser was used as a source of excitation. The absorption coefficient of InP at this wavelength is CI= 1.13 x lo5 cm-‘, so the volume probed by Raman measurements is inside the damaged layer (Raman signal coming from 100 nm below the surface is attenuated by approximately a factor of 10). The Raman measurements were recorded at room temperature in backscattering geometry on a (1 0 0) face. The first-order Raman spectra of the annealed samples were obtained using the triple additive configuration of the spectrometer with 100 urn slits, while the rest of the spectra were recorded using the double subtractive configuration with 100 urn slits. To suppress the low-frequency Raman lines arising from rotational modes of atmospheric molecules the sample area was purged with a continuous argon gas flow.
3. Results and discussion 2. Experiment Semi-insulating Fe-doped (1 0 0) wafers of liquid-encapsulated-Czochralski grown InP supplied by Sumitomo were used in the experiments. ‘*Si was implanted at 150 keV at doses of 5 x lo’?, 5 x 10”. and 5 x 10’” cm-‘. This implantation energy and range of doses are typically used for ntype doping of semi-insulating InP for device applications [9]. Higher implantation doses result in
Ion beam bombardment displaces lattice atoms and creates point defects and disordered regions in the solid. With increasing lattice disorder, the phonon coherence length is reduced and the k = 0 selection rule is relaxed, giving rise to mesurable shifts and broadenings of the Raman peaks [l 11. At high-implantation doses clusters of amorphous material develop, which give rise to an amorphous-like contribution to the Raman spectrum reflecting the phonon density of states.
R. Cusc‘ et al. I Nucl. Instr. and Meth. in Phys. Rex B 132 (1997) 627632
Fig. 1 shows the Raman spectra of Si+-implanted InP samples for several implantation doses compared with the spectrum of virgin InP. As can be seen in Fig. l(a), the spectrum of virgin InP (A) displays a sharp peak at the InP LO frequency whereas almost no Raman signal is detected at the TO frequency, which reflects the good quality of the (1 0 0) single crystal, as the TO mode is symmetry-forbidden in backscattering configuration from a (1 0 0) face. For the sample implanted with 5 x 1012cm-’ (B) although an intense, sharp LO peak is still observed, a strong TO signal appears in the Raman spectrum which indicates a relaxation of the crystal selection rules. Also, two weak broad structures appear centered around 62 and 138 cm-l, associated with disorder-activated transverse and longitudinal acoustic modes (DATA and DALA modes, respectively) [12]. A significant loss of intensity and a broadening of the first-order peaks, as well as an enhancement of the disorder-activated acoustic modes, are observed in the Raman spectrum of the sample implanted with 5 x lOI cm-’ (C), which indicates a higher degree of disorder and the presence of a significant proportion of amorphous material in this sample. In the Raman spectra of the sample im-
(a) -
I
(+5)
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n
k!
Raman shift (cm-‘)
Raman shift (cm-‘)
Fig. 1. (a) Raman spectra of InP samples implanted with 150 keV Si+ at different doses. A to D, respectively: virgin InP, 5 \: IO”. 5 x 10”. and 5 x lOI cm-l. (b) The corresponding second-order optical-phonon region of the Raman spectra.
629
planted with 5 x lOI cm-? (D), long-wavelength first-order optical-phonon peaks have been replaced by a broad band centered around 300 cm-’ containing the whole density of states. This confirms the complete amorphization of the material. A new broad structure develops at 436 cm-’ which was also observed in Ref. [7] and has been assigned to phosphorous-phosphorous vibrations associated with the formation of phosphorous clusters [ 131. Fig. l(b) shows the corresponding second-order optical-mode regions of the implanted samples. In the spectrum of virgin InP (A) three peaks can be clearly observed at 617, 650, and 682 cm-‘, corresponding to 2T0, TO + LO and 2L0 modes, respectively [14]. As can be seen from Fig. l(b), these second-order peaks are very sensitive to lattice disorder. Their intensity already shows a noticeable decrease in the sample implanted with 5 x lOI cmm2(B), and it is dramatically reduced in the sample implanted with 5 x 10” cm-’ (C). No signal at all is detected in this spectral region for the sample implanted with 5 x 1014cm -’ (D). Spectra D from Fig. l(a) and (b) show that after Si+ implantation at 5 x 1014 cm-? InP samples are fully amorphized. In Fig. 2(a) we display the first-order Raman spectra obtained from InP samples implanted with 5 x lOI cm-? after annealing for 10 s at different temperatures in the 300875°C range. As can be seen from the broad band detected in the opticalmode region of the spectrum (A), the sample annealed at 300°C is still amorphous. Even for the low annealing temperature of 400°C a significant degree of lattice recovery is achieved, as reflected by the presence of well-defined peaks in spectrum B. However, the high-intensity of the TO peak reveals the existence of polycrystalline and/or misoriented regions in the material in the early stages of recrystallization. Increasing the annealing temperature to 500°C improves crystallinity, as shown by the noticeable narrowing of the TO and LO peaks of spectrum C. but the presence of the strong TO signal indicates that misorientation problems persist. A further increase of the annealing temperature to 600°C produces a significant decrease of the LO intensity which we attribute to the onset of electrical activation of the implanted dopants.
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300
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600
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Fig. 2. (a) Raman spectra of InP samples implanted with 150 keV Si+ at a dose of 5 x IO” cm-? after RTA at different temperatures. A to G, respectively: 300°C. JOO”C, 5OO”C, 600°C. 7OO”C, 800°C. and 875°C. (b) The corresponding second-order optical-phonon region of the Raman spectra.
The free-electron gas generated by the activation of the Si atoms couples with the bulk polar LO mode giving rise to LO-plasmon coupled modes (LOPCM) [5.15], and therefore the small Raman signal detected at the LO frequency is due only to Raman scattering by the unscreened LO mode in the surface depletion zone. It should be remarked that, among all the first- and second-order Raman peaks shown in Fig. 2(a) and (b), only the first-order LO peak exhibits a substantial change of intensity between spectra C and D, which supports our conclusion that the onset of charge activation occurs for annealing temperatures around 600°C. Electrical measurements on ion-beam doped InP reported by Nadella et al. [lo] show an important activation-rate drop of about 40% when the annealing temperature is reduced from 875°C to 825°C [lo]. Activations for lower annealing temperatures are not reported, presumably due to difficulties in carrying out the electrical measurements, but the trend is consistent with our estimation of the temperature at which electrical activation sets in. The intense TO mode observed in spectrum D shows that polycrystalline and/or misorientated regions still remain in the implanted
layer. As the annealing temperature is risen to 700°C (E) and 800°C (F), the intensity of the TO mode decreases, reflecting the gradual recovery of the (1 0 0)-oriented single crystal. The highest degree of crystallinity and electrical activation is achieved for the sample annealed at 875°C (G). In fact, a less intense, narrow and symmetric LO peak is observed in spectrum G, and the intensity of the TO peak has decreased considerably. The Raman peak detected at the TO frequency contains a significant contribution from the _ branch of the LOPCM, whose frequency, for high-density, high-mobility free-electron plasmas, asymptotically approaches the TO frequency [5.15]. The small shoulder visible on the high-energy side of this peak may be due to an overtone of the longitudinal acoustic branch [7]. It has been recently pointed out that in zincblende compounds first-order Raman scattering can be misleading to assess the lattice recovery of implanted samples after annealing, and second-order Raman scattering was proposed as a more reliable criterion [16]. In fact, when charge activation sets in the observed LO peak is due only to the surface depletion zone LO mode, and the _ LOPCM can mask the forbidden TO mode. In Fig. 2(b) the second-order optical-phonon spectra of the annealed samples are shown. The second-order peaks are already recovered after RTA at 400°C and display intensities around 70% of those of the virgin InP for all the annealing temperatures above 4OO”C, up to 875°C. However, as the annealing temperature is increased from 400°C (B) to 875°C (G) a progressive sharpening of the second-order peaks is clearly observed. This reflects the gradual improvement in the long-range order of the restored InP crystal with increasing annealing temperature. Among the three second-order optical peaks of InP, the TO + LO peak displays the smallest width. In Fig. 3 we have plotted the full width at half height of the TO + LO peaks for samples annealed at different temperatures. A substantial decrease in the TO + LO width is observed when going from 400°C to .5OO”C, and as the annealing temperature is further increased the TO + LO width steadily approaches its value in virgin InP. The width of the TO + LO peak for the sample
R. Cusc’ et al. I Nucl. Instr. and Meth. in Phys. Res. B I32 (1997) 627432
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ing [3]. The large width of the surface depletion zone LO peak suggests a lower degree of ordering which could be due to stoichiometric disturbances near the surface.
l6 [o
4. Conclusion
400
500
600
700
800
900
Annealing temperature (“C) Fig. 3. Plot of the widths of the TO + LO (squares) and LO (circles) peaks of Si’-implanted and annealed InP as a function of annealing temperature. On the right-hand-side vertical axis we have indicated the widths of the LO and TO + LO peaks in virgin InP.
annealed at 875°C deviates only by about 7% from its value in virgin InP, which confirms the excellent lattice recovery achieved by RTA at this temperature. For comparison, we have also plotted the width of the first-order LO as a function of annealing temperature. It also shows a significant decrease when going from 400°C to 5OO”C,parallel to that of second-order TO + LO. However the LO width increases again for the sample annealed at 6OO”C, where, as discussed above, the dopant activation sets in and the observed LO mode originates in the surface depletion zone. A steady reduction of the LO peak is also observed for higher annealing temperatures. However. the LO width measured in the sample annealed at 875°C is roughly twice its value in virgin InP, in contrast with the TO + LO width, which, as mentioned above, lies within 7% of the virgin InP value. Stoichiometric disturbances are likely to be present in the implanted, annealed InP samples due to the implantation/annealing processing. During the implantation, the different recoiling range of the elements of the compound results in a indium excess near the surface [17]. This stoichiometric imbalance is further modified by diffusion processes and phosphorous loss occurring during the anneal-
We have studied by means of Raman spectroscopy the lattice recovery of Si+-implanted InP by RTA at different annealing temperatures of samples fully amorphized by the implantation. The combined use of first- and second-order Raman scattering allows us to assess the crystallinity recovery of the implanted layer as well as the activation of the dopants. Our Raman measurements show that a fairly high degree of crystallinity recovery is achieved at relatively low annealing temperatures, but a considerable proportion of polycrystalline and/or misoriented regions are still present, and the electrical activation is very low. For annealing temperatures above 600°C dopant activation and a steady recovery of the (1 0 0) InP single crystal with increasing temperature are observed. The optimum single-crystal recovery is obtained for samples annealed at 875°C for 10 s, for which the widths of the second-order Raman peaks approach the values in virgin InP.
Acknowledgements
This work has been supported by the Spanish Ministerio de Education y Ciencia. One of the authors (J.I.) acknowledges CSIC for the award of a post graduate scholarship.
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