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Effect of thermal annealing on the optical and structural properties of silicon implanted with a high hydrogen fluence
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 242 (2006) 650–652 www.elsevier.com/locate/nimb
Effect of thermal annealing on the optical and structural properties of silicon implanted with a high hydrogen fluence A. Kling
a,b,*
, J.C. Soares b, A. Rodrı´guez c, T. Rodrı´guez c, M. Avella d, J. Jime´nez
d
a
Instituto Tecnolo´gico e Nuclear, Estrada Nacional 10, 2686-953 Sacave´m, Portugal Centro de Fı´sica Nuclear da Universidade de Lisboa, 1649-003 Lisboa, Portugal Departamento de Tecnologı´a Electro´nica, E.T.S.I. de Telecomunicacio´n, Universidad Polite´cnica de Madrid, 28040 Madrid, Spain d Universidad de Valladolid, Departamento de Fı´sica de la Materia Condensada, 47011 Valladolid, Spain b
c
Available online 19 September 2005
Abstract Silicon capped by thermal oxide has been implanted with 1 · 1017 H/cm2 and the implant profile peaking at the interface. Samples were subjected to thermal annealing and characterized by ERD, FTIR, RBS/channeling, UV/VIS reflectance and cathodoluminescence regarding H-content, crystalline quality and light emission. The results show that the luminescent properties are independent of the hydrogen content but are strongly related with the present damage. 2005 Elsevier B.V. All rights reserved. PACS: 61.72.Tt; 61.72.Ss; 78.60.Hk; 78.40.Fy; 61.85.+p Keywords: Ion implantation; Elastic recoil detection; Cathodoluminescence; UV/VIS reflection
1. Introduction Although hydrogen is an important and widely investigated impurity in silicon still many questions about its influence on the structural and optical properties are open. Recently, room temperature cathodoluminescence has been observed in silicon with an asymmetric hydrogen profile stemming from PIII [1]. In this work the influence of H concentration profile and lattice damage on the luminescent properties has been studied for an analogue implantation profile. 2. Experimental Thermally oxidized (1 0 0) Si wafers (SiO2 layer thickness: 330 nm) were implanted with 56 keV Hþ 2 to a fluence * Corresponding author. Address: Instituto Tecnolo´gico e Nuclear, Estrada Nacional 10, 2686-953 Sacave´m, Portugal. Tel.: +351 21 994 6154; fax: +351 21 994 1039. E-mail address: [email protected] (A. Kling).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.087
of 1 · 1017 H/cm2 at room temperature using our 210 kV Danfysik ion implanter. SRIM calculations [2] show that the maximum of the hydrogen concentration (8 · 1021 at./ cm3) is located at the interface between the SiO2 and the Si wafer. Half of the samples were etched in order to remove the oxide layer. Samples with and without the oxide layer were annealed for 30 min at 300, 400, 550, 700 and 800 C in a nitrogen atmosphere. Hydrogen depth profiles were measured by ERD using a 2.0 MeV He+ beam at the 3.1 MV ITN van-de-Graaff accelerator. Hydrogen loss in the annealed samples was also measured by FTIR spectroscopy using the asimplanted sample as reference. Damage production and its evolution during annealing was monitored by RBS/ channeling measurements in the (1 0 0) direction and by UV–VIS reflectance measurements. Light emission was studied by cathodoluminescence (CL) using 10–15 keV e-beams to ensure that all possible excitation mechanisms are covered. The CL spectra were recorded with a CCD detector in order to minimize charge effects on the cathodoluminescence intensity.
A. Kling et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 650–652
651
Fig. 1. ERD spectra obtained with a 2.0 MeV He+ beam impinging on an SiO2-capped and bare sample in the as-implanted state.
related to the wagging mode (630 cm1) in the IR absorption spectra (bare samples). All values are normalized to the as-implanted sample. A low hydrogen loss in SiO2capped samples is observed for annealing temperatures up to 400 C. In the case of annealing at 800 C the bare sample and the SiO2 layer of the capped sample loose virtually all their hydrogen. On the other hand, the Si substrate of the capped sample still exhibits a significant Hretention. Fig. 3 shows the annealing-induced evolution of the lattice damage determined by channeling, the crystalline quality studied by UV–VIS reflectance and the cathodoluminescence intensity. While for RBS/channeling the ratios of damage peak area (aligned incidence) and random spectrum area have been plotted for the reflectance studies the area of the 280 nm peak has been analyzed and normalized to the virgin wafer [3]. For the channeling a minimum damage after annealing at 560 C is observed. On the other hand, the reflectance measurements show that the crystalline quality of the bare sampleÕs top surface region (around 100 nm depth) increases with annealing temperature until it nearly reaches that of virgin silicon after annealing at 700 C. These findings indicate that the damage detected by RBS is mainly due to the formation of bubbles and cavities that disturb the channeling effect but allow the surrounding material to be of good crystal quality. The CL spectra exhibit a broad visible band emission, composed of at least three sub-bands, at 2.5 eV, 2.2 eV (dominant) and 1.9 eV. A similar spectrum peaking at 2.2 eV was reported for H+-irradiated SiO2 [4] and is regarded as the dominant for dry SiO2. However, in our samples the capping oxide was removed after the implantation. Annealing left the spectral shape unaltered, but a strong influence of annealing temperature on the cathodoluminescence intensity has been found as presented in Fig. 3. The maximum CL intensity corresponds to the minimum damage measured by channeling which suggests that the CL emission is inversely related to the detected
Fig. 2. Hydrogen content of the samples in the as-implanted state and after annealing for SiO2-capped and bare samples measured by ERD and IR absorption.
Fig. 3. Evolution of the damage, crystalline quality and luminescence intensity in the bare Si sample during annealing determined by RBS/ channeling, UV/VIS reflectance and cathodoluminescence, respectively. The maximum CL yield coincides with the minimal damage.
3. Results and discussion Fig. 1 shows the ERD spectra of samples with and without SiO2 cap layer in the as-implanted state. A total implantation fluence of 9.5 · 1016 H/cm2 was derived from the spectrum for the capped sample. This value agrees very well with the nominal one (1 · 1017 H/cm2). RUMP analysis further revealed that about half of the implanted fluence (4.6 · 1016 H/cm2) is located in the Si substrate with a Gaussian profile truncated at the Si/SiO2 interface. The SiO2 layer contains 6.0 · 1016 H/cm2 in the interface region itself, 2.5 · 1016 H/cm2 between the interface and surface (constant concentration) and 1.8 · 1016 H/cm2 in a surface peak. The bare sample shows only the desired truncated Gaussian H-profile in the silicon and confirms the retained fluence of 4.6 · 1016 H/cm2. In Fig. 2 the evolution of the hydrogen content in both sample types with annealing temperature is plotted, illustrating its rapid decrease during annealing. The retained amount of hydrogen has been extracted from the ERD measurements (bare and capped samples) and the dip
652
A. Kling et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 650–652
damage. On the other hand, the crystalline quality of the Si substrate improves and the hydrogen content decreases with increasing annealing temperature indicating that the luminescence emission cannot be related to the H bubbles themselves but rather to residual SiO2. Also Liu et al. [1] claimed that SiO2 inside the cavities created during their formation is responsible for an electroluminescence emission centered at 2.2 eV. 4. Conclusions The analysis of various properties of hydrogenimplanted silicon (H-concentration, H-profile, structural properties, light emission) shows that the luminescent properties are independent of the hydrogen content but strongly related with the present damage. Further investigations using deuterium and helium implantations with
fluences that produce the same amount of damage will be used to confirm these findings. Acknowledgement The authors would like o thank to the Spanish–Portuguese Integrated Action Program for the financial support. References [1] W. Liu, S.C.H. Kwok, R.K.Y. Fu, P.K. Chu, T.F. Hung, Z. Xu, C. Lin, K.F. Li, H.L. Tam, K.W. Cheah, Semicond. Sci. Technol. 18 (2003) L55. [2] The latest version of this program can be found at. [3] K.L. Chiang, C.J. Dell-Occa, F.N. Schwettmann, J. Electrochem. Soc. 126 (1979) 2267. [4] H. Koyama, J. Appl. Phys. 51 (1980) 2228.
Nuclear Instruments and Methods in Physics Research B 242 (2006) 650–652 www.elsevier.com/locate/nimb
Effect of thermal annealing on the optical and structural properties of silicon implanted with a high hydrogen fluence A. Kling
a,b,*
, J.C. Soares b, A. Rodrı´guez c, T. Rodrı´guez c, M. Avella d, J. Jime´nez
d
a
Instituto Tecnolo´gico e Nuclear, Estrada Nacional 10, 2686-953 Sacave´m, Portugal Centro de Fı´sica Nuclear da Universidade de Lisboa, 1649-003 Lisboa, Portugal Departamento de Tecnologı´a Electro´nica, E.T.S.I. de Telecomunicacio´n, Universidad Polite´cnica de Madrid, 28040 Madrid, Spain d Universidad de Valladolid, Departamento de Fı´sica de la Materia Condensada, 47011 Valladolid, Spain b
c
Available online 19 September 2005
Abstract Silicon capped by thermal oxide has been implanted with 1 · 1017 H/cm2 and the implant profile peaking at the interface. Samples were subjected to thermal annealing and characterized by ERD, FTIR, RBS/channeling, UV/VIS reflectance and cathodoluminescence regarding H-content, crystalline quality and light emission. The results show that the luminescent properties are independent of the hydrogen content but are strongly related with the present damage. 2005 Elsevier B.V. All rights reserved. PACS: 61.72.Tt; 61.72.Ss; 78.60.Hk; 78.40.Fy; 61.85.+p Keywords: Ion implantation; Elastic recoil detection; Cathodoluminescence; UV/VIS reflection
1. Introduction Although hydrogen is an important and widely investigated impurity in silicon still many questions about its influence on the structural and optical properties are open. Recently, room temperature cathodoluminescence has been observed in silicon with an asymmetric hydrogen profile stemming from PIII [1]. In this work the influence of H concentration profile and lattice damage on the luminescent properties has been studied for an analogue implantation profile. 2. Experimental Thermally oxidized (1 0 0) Si wafers (SiO2 layer thickness: 330 nm) were implanted with 56 keV Hþ 2 to a fluence * Corresponding author. Address: Instituto Tecnolo´gico e Nuclear, Estrada Nacional 10, 2686-953 Sacave´m, Portugal. Tel.: +351 21 994 6154; fax: +351 21 994 1039. E-mail address: [email protected] (A. Kling).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.087
of 1 · 1017 H/cm2 at room temperature using our 210 kV Danfysik ion implanter. SRIM calculations [2] show that the maximum of the hydrogen concentration (8 · 1021 at./ cm3) is located at the interface between the SiO2 and the Si wafer. Half of the samples were etched in order to remove the oxide layer. Samples with and without the oxide layer were annealed for 30 min at 300, 400, 550, 700 and 800 C in a nitrogen atmosphere. Hydrogen depth profiles were measured by ERD using a 2.0 MeV He+ beam at the 3.1 MV ITN van-de-Graaff accelerator. Hydrogen loss in the annealed samples was also measured by FTIR spectroscopy using the asimplanted sample as reference. Damage production and its evolution during annealing was monitored by RBS/ channeling measurements in the (1 0 0) direction and by UV–VIS reflectance measurements. Light emission was studied by cathodoluminescence (CL) using 10–15 keV e-beams to ensure that all possible excitation mechanisms are covered. The CL spectra were recorded with a CCD detector in order to minimize charge effects on the cathodoluminescence intensity.
A. Kling et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 650–652
651
Fig. 1. ERD spectra obtained with a 2.0 MeV He+ beam impinging on an SiO2-capped and bare sample in the as-implanted state.
related to the wagging mode (630 cm1) in the IR absorption spectra (bare samples). All values are normalized to the as-implanted sample. A low hydrogen loss in SiO2capped samples is observed for annealing temperatures up to 400 C. In the case of annealing at 800 C the bare sample and the SiO2 layer of the capped sample loose virtually all their hydrogen. On the other hand, the Si substrate of the capped sample still exhibits a significant Hretention. Fig. 3 shows the annealing-induced evolution of the lattice damage determined by channeling, the crystalline quality studied by UV–VIS reflectance and the cathodoluminescence intensity. While for RBS/channeling the ratios of damage peak area (aligned incidence) and random spectrum area have been plotted for the reflectance studies the area of the 280 nm peak has been analyzed and normalized to the virgin wafer [3]. For the channeling a minimum damage after annealing at 560 C is observed. On the other hand, the reflectance measurements show that the crystalline quality of the bare sampleÕs top surface region (around 100 nm depth) increases with annealing temperature until it nearly reaches that of virgin silicon after annealing at 700 C. These findings indicate that the damage detected by RBS is mainly due to the formation of bubbles and cavities that disturb the channeling effect but allow the surrounding material to be of good crystal quality. The CL spectra exhibit a broad visible band emission, composed of at least three sub-bands, at 2.5 eV, 2.2 eV (dominant) and 1.9 eV. A similar spectrum peaking at 2.2 eV was reported for H+-irradiated SiO2 [4] and is regarded as the dominant for dry SiO2. However, in our samples the capping oxide was removed after the implantation. Annealing left the spectral shape unaltered, but a strong influence of annealing temperature on the cathodoluminescence intensity has been found as presented in Fig. 3. The maximum CL intensity corresponds to the minimum damage measured by channeling which suggests that the CL emission is inversely related to the detected
Fig. 2. Hydrogen content of the samples in the as-implanted state and after annealing for SiO2-capped and bare samples measured by ERD and IR absorption.
Fig. 3. Evolution of the damage, crystalline quality and luminescence intensity in the bare Si sample during annealing determined by RBS/ channeling, UV/VIS reflectance and cathodoluminescence, respectively. The maximum CL yield coincides with the minimal damage.
3. Results and discussion Fig. 1 shows the ERD spectra of samples with and without SiO2 cap layer in the as-implanted state. A total implantation fluence of 9.5 · 1016 H/cm2 was derived from the spectrum for the capped sample. This value agrees very well with the nominal one (1 · 1017 H/cm2). RUMP analysis further revealed that about half of the implanted fluence (4.6 · 1016 H/cm2) is located in the Si substrate with a Gaussian profile truncated at the Si/SiO2 interface. The SiO2 layer contains 6.0 · 1016 H/cm2 in the interface region itself, 2.5 · 1016 H/cm2 between the interface and surface (constant concentration) and 1.8 · 1016 H/cm2 in a surface peak. The bare sample shows only the desired truncated Gaussian H-profile in the silicon and confirms the retained fluence of 4.6 · 1016 H/cm2. In Fig. 2 the evolution of the hydrogen content in both sample types with annealing temperature is plotted, illustrating its rapid decrease during annealing. The retained amount of hydrogen has been extracted from the ERD measurements (bare and capped samples) and the dip
652
A. Kling et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 650–652
damage. On the other hand, the crystalline quality of the Si substrate improves and the hydrogen content decreases with increasing annealing temperature indicating that the luminescence emission cannot be related to the H bubbles themselves but rather to residual SiO2. Also Liu et al. [1] claimed that SiO2 inside the cavities created during their formation is responsible for an electroluminescence emission centered at 2.2 eV. 4. Conclusions The analysis of various properties of hydrogenimplanted silicon (H-concentration, H-profile, structural properties, light emission) shows that the luminescent properties are independent of the hydrogen content but strongly related with the present damage. Further investigations using deuterium and helium implantations with
fluences that produce the same amount of damage will be used to confirm these findings. Acknowledgement The authors would like o thank to the Spanish–Portuguese Integrated Action Program for the financial support. References [1] W. Liu, S.C.H. Kwok, R.K.Y. Fu, P.K. Chu, T.F. Hung, Z. Xu, C. Lin, K.F. Li, H.L. Tam, K.W. Cheah, Semicond. Sci. Technol. 18 (2003) L55. [2] The latest version of this program can be found at