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Buried ZnTe nanocrystallites in thermal SiO2 on silicon synthesized by high dose ion implantation
Nuclear Instruments and Methods in Physics Research B 178 (2001) 126±130
www.elsevier.nl/locate/nimb
Buried ZnTe nanocrystallites in thermal SiO2 on silicon synthesized by high dose ion implantation H. Karl *, I. Groûhans, W. Attenberger, M. Schmid, B. Stritzker Institut fur Physik, Universitat Augsburg, D-86135 Augsburg, Germany
Abstract Nanocrystalline precipitates of the direct bandgap II±VI compound semiconductor ZnTe were synthesized by sequential high dose ion implantation of the elements. Thermal SiO2 on (1 0 0)-silicon was chosen as target material in order to provide compatibility to silicon technology. The implantation dose was 2 1016 and 4 1016 cm 2 . Subsequent rapid thermal annealing of the as-implanted samples controls the formation of the nanocrystalline precipitates of ZnTe. The size distribution of the crystallites, their orientation and crystal structure were analyzed by thin ®lm X-ray diraction (XRD), secondary ion mass spectrometry depth pro®ling (SIMS) and Rutherford backscattering spectroscopy (RBS) as a function of implantation dose and annealing conditions. For selected samples transmission electron microscopy (TEM) cross-sections complete the structural investigations and provide direct information on the spatial size distribution of the nanocrystals. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Tellurides; ZnTe; Nanocrystals; Ion implantation; TEM; Quantum dots
1. Introduction There is intense research activity to develop synthesis technologies for nanocrystals and quantum dots. This is mainly due to a tremendous change of the electronic, optical and other physical and chemical properties with respect to the corresponding bulk materials [1±4]. In order to synthesize these materials, dierent techniques are employed, i.e. ablation techniques, plasma reac-
* Corresponding author. Tel.: +49-821-598-3408; fax: +49821-598-3425. E-mail address: [email protected] (H. Karl).
tions and growth in solvents. They have in common that they produce particles, which are chemically very reactive and will change or even deteriorate their properties due to their large surface to volume fraction. This is the reason that these particles have to be embedded into an appropriate host material or need a surface passivation. There are only a few techniques which result in buried nanocrystals. The two most common of these techniques are molecular beam epitaxy and high dose ion beam implantation [5,6]. The ®rst is ideal to grow two-dimensional arrays of quantum dots, the second allows to produce well de®ned three-dimensional buried layers with a high volume fraction of nanocrystals, which is advantageous in many technological applications of these
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 4 8 2 - 1
H. Karl et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 126±130
materials. It is the high dose ion beam synthesis of ZnTe-nanocrystals in thermal SiO2 on (1 0 0)-Si which is investigated in this paper. In contrast to elemental semiconductor nanocrystals like silicon and germanium, the synthesis of compound semiconductors requires sequential implantation of its constituents [2,5,6]. The present study uses RBS, SIMS depth pro®les, TEM cross-sectional images and thin ®lm X-ray diraction pattern to systematically investigate the formation of ZnTe-precipitates.
2. Experimental The elements Te and Zn were implanted with equal doses into a 1 lm thick layer of thermally grown SiO2 on (1 0 0)-silicon at room temperature. The energy was selected so that a projected range of approximately 80 nm for both elements was obtained. The implantation doses of each component were 2 1016 cm 2 and 4 1016 cm 2 , respectively, and Te implantation was followed by that of Zn. Subsequent thermal treatment was done in a rapid-thermal-processing furnace at 1000°C and 800°C for 20 min each. The annealing atmosphere was ¯owing Ar containing 4% H2 .
127
Cross-sectional TEM images were performed using a 300 kV TEM and standard cross section specimen preparation. Dynamic quadrupole SIMS depth pro®les were performed using 5 keV Cs ions.
3. Results and discussion Due to sputtering during implantation of Zn and Te the maximum concentration of the implanted elements shifts towards the SiO2 surface. In this work the Te implantation was always followed by that of Zn with the result that an implantation dose of 2 1016 cm 2 of each constituent shifts the Te concentration maximum 7 nm towards the specimens surface. Moreover the implanted pro®le broadens due to straggling, which is more pronounced for the lighter element Zn. TEM images before and after annealing are shown in Fig. 1. It can be seen from Fig. 1 that the formation of precipitates begins already in the as-implanted sample. Annealing at 800°C results in further growth, at 1000°C coalescence to larger crystallites occur. In addition there are nanocrystals beyond the end-of-range, in deeper parts of the SiO2 .
Fig. 1. Cross-sectional TEM images for an implantation dose of 2 1016 cm
2
Zn and Te each.
128
H. Karl et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 126±130
These ®ndings are veri®ed by RBS spectra and SIMS depth pro®les (Fig. 2). For the dose of 2 1016 cm 2 the diusion region of Zn and Te can be seen in the SIMS pro®les with logarithmic concentration scale. No diusion to deeper parts of the SiO2 layer could be found for 800°C, consistent with the TEM images. From these data it cannot be concluded whether there is residual Zn and Te left in between the precipitates depicted in the TEM images.
Bright and dark ®eld TEM images for the higher dose and annealing condition of 1000°C for 20 min are shown in Fig. 3, again there are precipitates in greater depths than the end-of-range. RBS measurements (Fig. 2) are showing that the concentration pro®les of Zn and Te are smeared out toward. In addition the dark ®eld TEM images show crystallites in any depth, which ful®ll the imposed diraction condition. This result suggests that the precipitates in greater depths are also
Fig. 2. First row shows the SIMS pro®les of Zn, second row those of Te, and RBS are depicted in the last row. From these measurements a shift of the Zn concentration distribution towards the surface is obvious. Moreover, a diusion tail towards greater depths for both elements is identi®able.
H. Karl et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 126±130
129
Fig. 3. Dark and bright ®eld cross-sectional TEM images and high energy electron diraction pattern (from right to the left).
crystalline and either consist of Zn and Te, only Zn or Zn-silicate [7] or even other compounds. But if only Zn-silicate is formed, Te must be distributed in between these crystallites in order to explain the RBS measurements. Another interesting feature of the SIMS depth pro®les is that the Zn pro®le is shifted after annealing towards that of Te (see Fig. 2), whereas the Te pro®le remains at its initial as-implanted location. This holds for both doses and all annealing conditions described here. Moreover, the formation of ZnTe results in a very sharp interface in close vicinity to the Te endof-range for a dose of 4 1016 cm 2 and annealing at 1000°C for 20 min. From X-ray diraction measurements (not shown) it can be concluded that predominantly ZnTe nanocrystallites with cubic rocksalt structure were formed in the implantation region.
that Zn diusion dominates that of Te and therefore Te determines the location of the band of highest ZnTe nanocrystal concentration. This is in contradiction to an extended diusion region to greater depths for both Zn and Te. Although precipitates were found at greater depth than the end-of-range by cross-sectional TEM images, further investigations have to be performed in order to determine to which compounds or elements the crystallites in deeper parts of the sample correspond to and whether there is residual Zn- or Te- material in the SiO2 -matrix left. Acknowledgements The authors acknowledge support from their colleagues, especially J. Lindner and W. Reiber for their assistance during our TEM investigations and many helpful discussions.
4. Conclusion The formation of ZnTe nanocrystals with cubic structure in thermally grown SiO2 on (1 0 0)-Si was achieved by sequential ion implantation of the elements and subsequent annealing. The resulting spatial distribution of the nanocrystals was investigated using the complementary techniques TEM, SIMS, XRD and RBS. There is a strong evidence
References [1] P. Lefebvre, H. Mathieu, J. Allegre, T. Richard, A. Combettes-Roos, M. Pauthe, W. Granier, Semicond. Sci. Technol. 12 (1997) 958. [2] C.W. White, J.D. Budai, S.P. Withrow, J.G. Zhur, E. Sonder, R.A. Zuhr, A. Meldrum, D.M. Hembree Jr., D.O. Henderson, S. Prawer, Nucl. Instr. and Meth. B 141 (1998) 228±240.
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H. Karl et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 126±130
[3] C. Delerue, G. Allan, M. Lannoo, Phys. Rev. B 48 (15) (1993) 11024. [4] A.P. Alivisatos, Science 271 (1996) 933. [5] A. Meldrum, C.W. White, L.A. Boatner, I.M. Anderson, R.A. Zuhr, E. Sonder, J.D. Budai, D.O. Henderson, Nucl. Instr. and Meth. B 148 (1999) 957.
[6] C.W. White, J.D. Budai, A.L. Meldrum, S.P. Withrow, R.A. Zuhr, E. Sonder, A. Purezky, D.B. Geohegan, J.G. Zhu, D.O. Henderson, MRS Proc. 504 (1998) 399. [7] A. Meldrum, E. Sonder, R.A. Zuhr, I.M. Anderson, J.D. Budai, C.W. White, L.A. Boatner, D.O. Henderson, J. Mater. Res. 14 (12) (1999) 4489.
www.elsevier.nl/locate/nimb
Buried ZnTe nanocrystallites in thermal SiO2 on silicon synthesized by high dose ion implantation H. Karl *, I. Groûhans, W. Attenberger, M. Schmid, B. Stritzker Institut fur Physik, Universitat Augsburg, D-86135 Augsburg, Germany
Abstract Nanocrystalline precipitates of the direct bandgap II±VI compound semiconductor ZnTe were synthesized by sequential high dose ion implantation of the elements. Thermal SiO2 on (1 0 0)-silicon was chosen as target material in order to provide compatibility to silicon technology. The implantation dose was 2 1016 and 4 1016 cm 2 . Subsequent rapid thermal annealing of the as-implanted samples controls the formation of the nanocrystalline precipitates of ZnTe. The size distribution of the crystallites, their orientation and crystal structure were analyzed by thin ®lm X-ray diraction (XRD), secondary ion mass spectrometry depth pro®ling (SIMS) and Rutherford backscattering spectroscopy (RBS) as a function of implantation dose and annealing conditions. For selected samples transmission electron microscopy (TEM) cross-sections complete the structural investigations and provide direct information on the spatial size distribution of the nanocrystals. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Tellurides; ZnTe; Nanocrystals; Ion implantation; TEM; Quantum dots
1. Introduction There is intense research activity to develop synthesis technologies for nanocrystals and quantum dots. This is mainly due to a tremendous change of the electronic, optical and other physical and chemical properties with respect to the corresponding bulk materials [1±4]. In order to synthesize these materials, dierent techniques are employed, i.e. ablation techniques, plasma reac-
* Corresponding author. Tel.: +49-821-598-3408; fax: +49821-598-3425. E-mail address: [email protected] (H. Karl).
tions and growth in solvents. They have in common that they produce particles, which are chemically very reactive and will change or even deteriorate their properties due to their large surface to volume fraction. This is the reason that these particles have to be embedded into an appropriate host material or need a surface passivation. There are only a few techniques which result in buried nanocrystals. The two most common of these techniques are molecular beam epitaxy and high dose ion beam implantation [5,6]. The ®rst is ideal to grow two-dimensional arrays of quantum dots, the second allows to produce well de®ned three-dimensional buried layers with a high volume fraction of nanocrystals, which is advantageous in many technological applications of these
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 4 8 2 - 1
H. Karl et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 126±130
materials. It is the high dose ion beam synthesis of ZnTe-nanocrystals in thermal SiO2 on (1 0 0)-Si which is investigated in this paper. In contrast to elemental semiconductor nanocrystals like silicon and germanium, the synthesis of compound semiconductors requires sequential implantation of its constituents [2,5,6]. The present study uses RBS, SIMS depth pro®les, TEM cross-sectional images and thin ®lm X-ray diraction pattern to systematically investigate the formation of ZnTe-precipitates.
2. Experimental The elements Te and Zn were implanted with equal doses into a 1 lm thick layer of thermally grown SiO2 on (1 0 0)-silicon at room temperature. The energy was selected so that a projected range of approximately 80 nm for both elements was obtained. The implantation doses of each component were 2 1016 cm 2 and 4 1016 cm 2 , respectively, and Te implantation was followed by that of Zn. Subsequent thermal treatment was done in a rapid-thermal-processing furnace at 1000°C and 800°C for 20 min each. The annealing atmosphere was ¯owing Ar containing 4% H2 .
127
Cross-sectional TEM images were performed using a 300 kV TEM and standard cross section specimen preparation. Dynamic quadrupole SIMS depth pro®les were performed using 5 keV Cs ions.
3. Results and discussion Due to sputtering during implantation of Zn and Te the maximum concentration of the implanted elements shifts towards the SiO2 surface. In this work the Te implantation was always followed by that of Zn with the result that an implantation dose of 2 1016 cm 2 of each constituent shifts the Te concentration maximum 7 nm towards the specimens surface. Moreover the implanted pro®le broadens due to straggling, which is more pronounced for the lighter element Zn. TEM images before and after annealing are shown in Fig. 1. It can be seen from Fig. 1 that the formation of precipitates begins already in the as-implanted sample. Annealing at 800°C results in further growth, at 1000°C coalescence to larger crystallites occur. In addition there are nanocrystals beyond the end-of-range, in deeper parts of the SiO2 .
Fig. 1. Cross-sectional TEM images for an implantation dose of 2 1016 cm
2
Zn and Te each.
128
H. Karl et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 126±130
These ®ndings are veri®ed by RBS spectra and SIMS depth pro®les (Fig. 2). For the dose of 2 1016 cm 2 the diusion region of Zn and Te can be seen in the SIMS pro®les with logarithmic concentration scale. No diusion to deeper parts of the SiO2 layer could be found for 800°C, consistent with the TEM images. From these data it cannot be concluded whether there is residual Zn and Te left in between the precipitates depicted in the TEM images.
Bright and dark ®eld TEM images for the higher dose and annealing condition of 1000°C for 20 min are shown in Fig. 3, again there are precipitates in greater depths than the end-of-range. RBS measurements (Fig. 2) are showing that the concentration pro®les of Zn and Te are smeared out toward. In addition the dark ®eld TEM images show crystallites in any depth, which ful®ll the imposed diraction condition. This result suggests that the precipitates in greater depths are also
Fig. 2. First row shows the SIMS pro®les of Zn, second row those of Te, and RBS are depicted in the last row. From these measurements a shift of the Zn concentration distribution towards the surface is obvious. Moreover, a diusion tail towards greater depths for both elements is identi®able.
H. Karl et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 126±130
129
Fig. 3. Dark and bright ®eld cross-sectional TEM images and high energy electron diraction pattern (from right to the left).
crystalline and either consist of Zn and Te, only Zn or Zn-silicate [7] or even other compounds. But if only Zn-silicate is formed, Te must be distributed in between these crystallites in order to explain the RBS measurements. Another interesting feature of the SIMS depth pro®les is that the Zn pro®le is shifted after annealing towards that of Te (see Fig. 2), whereas the Te pro®le remains at its initial as-implanted location. This holds for both doses and all annealing conditions described here. Moreover, the formation of ZnTe results in a very sharp interface in close vicinity to the Te endof-range for a dose of 4 1016 cm 2 and annealing at 1000°C for 20 min. From X-ray diraction measurements (not shown) it can be concluded that predominantly ZnTe nanocrystallites with cubic rocksalt structure were formed in the implantation region.
that Zn diusion dominates that of Te and therefore Te determines the location of the band of highest ZnTe nanocrystal concentration. This is in contradiction to an extended diusion region to greater depths for both Zn and Te. Although precipitates were found at greater depth than the end-of-range by cross-sectional TEM images, further investigations have to be performed in order to determine to which compounds or elements the crystallites in deeper parts of the sample correspond to and whether there is residual Zn- or Te- material in the SiO2 -matrix left. Acknowledgements The authors acknowledge support from their colleagues, especially J. Lindner and W. Reiber for their assistance during our TEM investigations and many helpful discussions.
4. Conclusion The formation of ZnTe nanocrystals with cubic structure in thermally grown SiO2 on (1 0 0)-Si was achieved by sequential ion implantation of the elements and subsequent annealing. The resulting spatial distribution of the nanocrystals was investigated using the complementary techniques TEM, SIMS, XRD and RBS. There is a strong evidence
References [1] P. Lefebvre, H. Mathieu, J. Allegre, T. Richard, A. Combettes-Roos, M. Pauthe, W. Granier, Semicond. Sci. Technol. 12 (1997) 958. [2] C.W. White, J.D. Budai, S.P. Withrow, J.G. Zhur, E. Sonder, R.A. Zuhr, A. Meldrum, D.M. Hembree Jr., D.O. Henderson, S. Prawer, Nucl. Instr. and Meth. B 141 (1998) 228±240.
130
H. Karl et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 126±130
[3] C. Delerue, G. Allan, M. Lannoo, Phys. Rev. B 48 (15) (1993) 11024. [4] A.P. Alivisatos, Science 271 (1996) 933. [5] A. Meldrum, C.W. White, L.A. Boatner, I.M. Anderson, R.A. Zuhr, E. Sonder, J.D. Budai, D.O. Henderson, Nucl. Instr. and Meth. B 148 (1999) 957.
[6] C.W. White, J.D. Budai, A.L. Meldrum, S.P. Withrow, R.A. Zuhr, E. Sonder, A. Purezky, D.B. Geohegan, J.G. Zhu, D.O. Henderson, MRS Proc. 504 (1998) 399. [7] A. Meldrum, E. Sonder, R.A. Zuhr, I.M. Anderson, J.D. Budai, C.W. White, L.A. Boatner, D.O. Henderson, J. Mater. Res. 14 (12) (1999) 4489.