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Blistering effects of low energy hydrogen and helium ions implanted in GaAs(1 0 0) crystals
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 242 (2006) 620–622 www.elsevier.com/locate/nimb
Blistering effects of low energy hydrogen and helium ions implanted in GaAs(1 0 0) crystals A. Gigue`re *, N. Desrosiers, B. Terreault INRS-EMT, Universite´ du Que´bec, 1650 Boul. Lionel-Boulet, Varennes, Que., Canada J3X 1S2 Available online 26 October 2005
Abstract The conditions required for blistering after annealing in n-type GaAs(1 0 0) were investigated using low energy H, D or He ion implantations at room temperature for different fluences from 1 · 1016 to 1 · 1017 ion/cm2. The threshold fluence for H+ at 5 keV is 3 · 1016 H+/cm2 and blistering is still present for fluences as high as 1 · 1017 H+/cm2, in contrast with silicon where blisters due to low energy H+ implantation appear at comparable fluence (2 · 1016 H+/cm2) but disappear at slightly higher fluence (5 · 1016 H+/cm2). The critical annealing temperature in GaAs was found to be 175 ± 25 C, which is much lower than in silicon (350 C). Blistering is also obtained with He at low fluence, contrary to Si which is impervious to He blistering in this fluence range. We have discovered that the exfoliated crater depths depend strongly on the He fluence, varying from 75 ± 12 nm (for 1.6 · 1016 He+/cm2), consistent with the most probable ion projected range determined by simulation, to a saturation value of 155 ± 10 nm for doses >4 · 1016 He+/cm2. Our results suggest that exfoliation is triggered at a local He concentration of 1.5–3.0 at.% inside GaAs. 2005 Elsevier B.V. All rights reserved. PACS: 61.82.Fk; 61.72.Vv
1. Introduction The integration of III–V type and silicon semiconductors is still an interesting scientific challenge. A promising approach to transfer GaAs thin layers onto silicon is the ion-cut technology [1,2] first developed by Bruel. The crucial point for this technology is the exfoliation and transfer of a thin single crystal layer to new substrates under controlled conditions. The main aim of this work was the establishment of these critical conditions for low energy blistering of GaAs by H, D and He implantations at room temperature (RT). For silicon, we have already demonstrated that blistering/exfoliation due to low-keV H implantation only occurs in a narrow fluence window (between 2 · 1016 and 5 · 1016 H+/cm2) and displays a giant isotope effect: blistering appears at 2–3 times higher dose for deuterium than for hydrogen [3]. The present
*
Corresponding author. E-mail address: [email protected] (A. Gigue`re).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.187
study aimed at establishing whether similar effects take place in GaAs in spite of the many differences between these semiconductors. 2. Experimental Si doped n-type 3 in. GaAs(1 0 0) wafers were implanted at RT with H, D and He at different doses from 1016 to 1017 ions/cm2. The ion flux was constant at about 1 lA/cm2 without sample heating and was normally incident on the GaAs surface. The samples were heat treated at 400 C for 30 s in nitrogen gas using an IR RTA furnace (HeatPulse 610). The temperature ramping speed was 100 C/s and overshoots of 20 C were tolerated. The blistered surfaces were examined by SEM (JEOL JSM-6300F) and AFM (Nanoscope III AFM). The evaluation of crater depths and exfoliated areas was obtained by a large number of AFM measurements, avoiding flattening or data fitting transformations. These measurements were confirmed by SEM.
A. Gigue`re et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 620–622
621
3. Results and discussion 3.1. Hydrogen in GaAs Fig. 1 shows the dose dependence of the blistered/exfoliated area for H or D implantation at 5 keV in GaAs and Si [3] after heat treatments. One can see, in Si, the absence of blistering for fluences greater than 4 · 1016 H+/cm2, in contrast with GaAs showing blistered surfaces up to 1 · 1017 H+/cm2. The hydrogen fluence needed to reach the maximum blistered area is two times higher in GaAs than in Si. We note that these fluences correspond to 5 at.% H in Si and 12 at.% H in GaAs. The nature of H trapping in Si or GaAs should be quite different. In Si, where radiation-induced interstitials and single vacancies are both mobile at RT, implanted H is mostly trapped in vacancies that it passivates [4]. In GaAs vacancy migration is hindered by charge effects since neighbouring Ga and As vacancies are not interchangeable. Moreover, H+ implantations in Si or GaAs will create different damage distributions resulting in large variations in available dangling bonds. The giant isotope effect is also observed when H and D are implanted in GaAs as already observed [3] in Si. The fact that the isotope effect occurs in GaAs in spite of the multiple differences between these semiconductors will require that its explanation be independent of the particular substrate chemistry. Fig. 2(a) shows blisters and exfoliations on the surface of a sample implanted with 4 · 1016 H+/cm2 at 10 keV and Fig. 2(b) a cross-section. Blisters are created by microcracks that are parallel to the surface and situated at a depth of the order of the projected range. Crater depth measurements (at 10 keV) give a value of 102 ± 10 nm
H - GaAs
60
D - GaAs H - Si
Blistered Area (%)
50
40
30
Fig. 2. (a) Plan view SEM image for 4 · 1016 H+/cm2 at 10 keV in GaAs (400 C: 30 s) and (b) cross-section view for the same surface.
for all doses, in excellent agreement with the mean projected range (RP) of 108 nm calculated by SRIM [5]. An interpretation of the mechanisms behind blistering/exfoliation was proposed by Ho¨chbauer et al. for H+ at 40 keV into Si [6]. They reported a slight dependence of crater depths on dose, i.e. up to 8% variation. This value is in the range of incertainty of our measurements. We cannot conclude on the crater depth dose dependence. The critical temperature for blistering/exfoliation of GaAs was determined as 175 ± 25 C for 5 · 1016 H+/cm2 in agreement with Gawlik et al. [7]. The critical temperature for H+ at 5 keV in Si is 350 ± 50 C. It is worth mentioning that the exfoliation observed after heat treatment is a sudden effect and RTA treatments of 10 s at temperatures higher than 200 C are sufficient to observe blistering in GaAs. 3.2. Helium in GaAs
20
10
0 0
2
4
6
8
10
12
14
16
18
20
Fluence (1016 ions/cm2) Fig. 1. Blister-covered area (% of total) as a function of ion fluence for H+ at 5 keV in Si (d) and GaAs (s) and D+ at 5 keV in GaAs (h).
Low dose He+ at 10 keV shows blistering in GaAs after an adequate thermal treatment contrary to Si which is impervious to He+ blistering in this fluence range. The presence of exfoliation when helium is implanted in GaAs has been already demonstrated by other research [8,9]. These observations were obtained under relatively high dose and at higher energies. Fig. 3 shows the thickness of the exfoliated GaAs surfaces (crater depth) as a function of fluence, at 10 keV. The lowest fluence for exfoliation is about 1.6 · 1016 He+/cm2, corresponding to a maximum
A. Gigue`re et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 620–622
622
He distribution in GaAs at high dose. In fact, the crater depths at the different doses all correspond to the depth where the local concentration is in the range 1.5–3.0 at.% He. Follstaedt et al. [8] have observed arrays of bubble-decorated dislocation lines forming on a particular plane at the deep end of the He-implanted layers in GaAs. This array creates a fracture path for the exfoliation that occurs under thermal treatments when dislocations start to move. The origin of these dislocation-bubbles is still unknown but our results suggest that they appear at a He concentration of 1.5–3.0 at.% He (1021 He/cm3). Correlation can be made with Fig. 4 in [8]: the TEM image of (0 0 1) GaAs implanted with 4 · 1016 He+/cm2 at 40 keV shows bubbles and dislocations-bubbles down to 360 nm. This value corresponds to a concentration of 2.0 ± 0.5 at.% He when simulations are performed by SRIM.
180
160
140
Depth (nm)
120 100
80 60
40
20
0 1
2
3
4
5
6
7
Dose (1016 He+/cm2)
Fig. 3. Fluence dependence of exfoliated crater depths for He+ implanted at 10 keV in GaAs after annealing at 400 C for 30 s.
10 1 x1016 1.6 x1016 8
2 x1016 3 x1016
4. Conclusion We have established the existence of the isotope effect and narrow fluence windows for exfoliation/blistering in GaAs for low energy H and D ions, as already observed for Si, in spite of the multitude of differences between these semiconductors. The crater depth in H-implanted GaAs corresponds to the peak of the H implantation distribution evaluated by SRIM. On the other hand, crater depths in He-implanted GaAs are strongly dependent on He+ fluence and correspond to the depth where the local concentration reaches 1.5–3.0 at.% He.
4 x1016
6
at.% He
Acknowledgements Many thanks to Christian Dubuc and Pierre-Paul Mercier for their advice concerning the RTA treatments. This work was made possible by grants from NSERC and a FQRNT post-doctoral fellowship to A. Gigue`re.
4
2
References 0 50
100
150
Depth (nm)
Fig. 4. Calculated concentrations profiles of He+ implanted at 10 keV in GaAs for different doses. Circles indicate local He concentrations for the corresponding measured crater depths.
concentration of only 3 ± 0.5 at.% He in GaAs by SRIM simulation (Fig. 4). The mean projected range (RP) at 10 keV is 78 ± 5 nm and corresponds to the exfoliated layer thickness for the lowest critical fluence. The saturated value at 155 ± 5 nm coincides instead with the deep end of the
[1] M. Bruel, Electron. Lett. 31 (1995) 1201. [2] I. Radu, I. Szafraniak, R. Scholz, M. Alexe, U. Go¨sele, J. Appl. Phys. 94 (2003) 7820. [3] O. Moutanabbir, A. Gigue`re, B. Terreault, Appl. Phys. Lett. 84 (2004) 3286. [4] O. Moutanabbir, B. Terreault, J. Chem. Phys. 121 (2004) 7973. [5] J.F. Ziegler, J.P. Biersack, SRIM_2003.10. Available from:. [6] T. Ho¨chbauer, A. Misra, M. Nastasi, J.W. Mayer, J. Appl. Phys. 92 (2002) 2335. [7] G. Gawlik, R. Ratajczak, A. Turos, J. Jagielski, S. Bedell, W.L. Lanford, Vacuum 63 (2001) 697. [8] D.M. Follstaedt, S.M. Myers, J.C. Barbour, G.A. Petersen, J.L. Reno, L.R. Dawson, S.R. Lee, Nucl. Instr. and Meth. B 160 (2000) 476. [9] G. Gawlik, J. Jagielski, B. Piatkowski, Vacuum 70 (2003) 103.
Nuclear Instruments and Methods in Physics Research B 242 (2006) 620–622 www.elsevier.com/locate/nimb
Blistering effects of low energy hydrogen and helium ions implanted in GaAs(1 0 0) crystals A. Gigue`re *, N. Desrosiers, B. Terreault INRS-EMT, Universite´ du Que´bec, 1650 Boul. Lionel-Boulet, Varennes, Que., Canada J3X 1S2 Available online 26 October 2005
Abstract The conditions required for blistering after annealing in n-type GaAs(1 0 0) were investigated using low energy H, D or He ion implantations at room temperature for different fluences from 1 · 1016 to 1 · 1017 ion/cm2. The threshold fluence for H+ at 5 keV is 3 · 1016 H+/cm2 and blistering is still present for fluences as high as 1 · 1017 H+/cm2, in contrast with silicon where blisters due to low energy H+ implantation appear at comparable fluence (2 · 1016 H+/cm2) but disappear at slightly higher fluence (5 · 1016 H+/cm2). The critical annealing temperature in GaAs was found to be 175 ± 25 C, which is much lower than in silicon (350 C). Blistering is also obtained with He at low fluence, contrary to Si which is impervious to He blistering in this fluence range. We have discovered that the exfoliated crater depths depend strongly on the He fluence, varying from 75 ± 12 nm (for 1.6 · 1016 He+/cm2), consistent with the most probable ion projected range determined by simulation, to a saturation value of 155 ± 10 nm for doses >4 · 1016 He+/cm2. Our results suggest that exfoliation is triggered at a local He concentration of 1.5–3.0 at.% inside GaAs. 2005 Elsevier B.V. All rights reserved. PACS: 61.82.Fk; 61.72.Vv
1. Introduction The integration of III–V type and silicon semiconductors is still an interesting scientific challenge. A promising approach to transfer GaAs thin layers onto silicon is the ion-cut technology [1,2] first developed by Bruel. The crucial point for this technology is the exfoliation and transfer of a thin single crystal layer to new substrates under controlled conditions. The main aim of this work was the establishment of these critical conditions for low energy blistering of GaAs by H, D and He implantations at room temperature (RT). For silicon, we have already demonstrated that blistering/exfoliation due to low-keV H implantation only occurs in a narrow fluence window (between 2 · 1016 and 5 · 1016 H+/cm2) and displays a giant isotope effect: blistering appears at 2–3 times higher dose for deuterium than for hydrogen [3]. The present
*
Corresponding author. E-mail address: [email protected] (A. Gigue`re).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.187
study aimed at establishing whether similar effects take place in GaAs in spite of the many differences between these semiconductors. 2. Experimental Si doped n-type 3 in. GaAs(1 0 0) wafers were implanted at RT with H, D and He at different doses from 1016 to 1017 ions/cm2. The ion flux was constant at about 1 lA/cm2 without sample heating and was normally incident on the GaAs surface. The samples were heat treated at 400 C for 30 s in nitrogen gas using an IR RTA furnace (HeatPulse 610). The temperature ramping speed was 100 C/s and overshoots of 20 C were tolerated. The blistered surfaces were examined by SEM (JEOL JSM-6300F) and AFM (Nanoscope III AFM). The evaluation of crater depths and exfoliated areas was obtained by a large number of AFM measurements, avoiding flattening or data fitting transformations. These measurements were confirmed by SEM.
A. Gigue`re et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 620–622
621
3. Results and discussion 3.1. Hydrogen in GaAs Fig. 1 shows the dose dependence of the blistered/exfoliated area for H or D implantation at 5 keV in GaAs and Si [3] after heat treatments. One can see, in Si, the absence of blistering for fluences greater than 4 · 1016 H+/cm2, in contrast with GaAs showing blistered surfaces up to 1 · 1017 H+/cm2. The hydrogen fluence needed to reach the maximum blistered area is two times higher in GaAs than in Si. We note that these fluences correspond to 5 at.% H in Si and 12 at.% H in GaAs. The nature of H trapping in Si or GaAs should be quite different. In Si, where radiation-induced interstitials and single vacancies are both mobile at RT, implanted H is mostly trapped in vacancies that it passivates [4]. In GaAs vacancy migration is hindered by charge effects since neighbouring Ga and As vacancies are not interchangeable. Moreover, H+ implantations in Si or GaAs will create different damage distributions resulting in large variations in available dangling bonds. The giant isotope effect is also observed when H and D are implanted in GaAs as already observed [3] in Si. The fact that the isotope effect occurs in GaAs in spite of the multiple differences between these semiconductors will require that its explanation be independent of the particular substrate chemistry. Fig. 2(a) shows blisters and exfoliations on the surface of a sample implanted with 4 · 1016 H+/cm2 at 10 keV and Fig. 2(b) a cross-section. Blisters are created by microcracks that are parallel to the surface and situated at a depth of the order of the projected range. Crater depth measurements (at 10 keV) give a value of 102 ± 10 nm
H - GaAs
60
D - GaAs H - Si
Blistered Area (%)
50
40
30
Fig. 2. (a) Plan view SEM image for 4 · 1016 H+/cm2 at 10 keV in GaAs (400 C: 30 s) and (b) cross-section view for the same surface.
for all doses, in excellent agreement with the mean projected range (RP) of 108 nm calculated by SRIM [5]. An interpretation of the mechanisms behind blistering/exfoliation was proposed by Ho¨chbauer et al. for H+ at 40 keV into Si [6]. They reported a slight dependence of crater depths on dose, i.e. up to 8% variation. This value is in the range of incertainty of our measurements. We cannot conclude on the crater depth dose dependence. The critical temperature for blistering/exfoliation of GaAs was determined as 175 ± 25 C for 5 · 1016 H+/cm2 in agreement with Gawlik et al. [7]. The critical temperature for H+ at 5 keV in Si is 350 ± 50 C. It is worth mentioning that the exfoliation observed after heat treatment is a sudden effect and RTA treatments of 10 s at temperatures higher than 200 C are sufficient to observe blistering in GaAs. 3.2. Helium in GaAs
20
10
0 0
2
4
6
8
10
12
14
16
18
20
Fluence (1016 ions/cm2) Fig. 1. Blister-covered area (% of total) as a function of ion fluence for H+ at 5 keV in Si (d) and GaAs (s) and D+ at 5 keV in GaAs (h).
Low dose He+ at 10 keV shows blistering in GaAs after an adequate thermal treatment contrary to Si which is impervious to He+ blistering in this fluence range. The presence of exfoliation when helium is implanted in GaAs has been already demonstrated by other research [8,9]. These observations were obtained under relatively high dose and at higher energies. Fig. 3 shows the thickness of the exfoliated GaAs surfaces (crater depth) as a function of fluence, at 10 keV. The lowest fluence for exfoliation is about 1.6 · 1016 He+/cm2, corresponding to a maximum
A. Gigue`re et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 620–622
622
He distribution in GaAs at high dose. In fact, the crater depths at the different doses all correspond to the depth where the local concentration is in the range 1.5–3.0 at.% He. Follstaedt et al. [8] have observed arrays of bubble-decorated dislocation lines forming on a particular plane at the deep end of the He-implanted layers in GaAs. This array creates a fracture path for the exfoliation that occurs under thermal treatments when dislocations start to move. The origin of these dislocation-bubbles is still unknown but our results suggest that they appear at a He concentration of 1.5–3.0 at.% He (1021 He/cm3). Correlation can be made with Fig. 4 in [8]: the TEM image of (0 0 1) GaAs implanted with 4 · 1016 He+/cm2 at 40 keV shows bubbles and dislocations-bubbles down to 360 nm. This value corresponds to a concentration of 2.0 ± 0.5 at.% He when simulations are performed by SRIM.
180
160
140
Depth (nm)
120 100
80 60
40
20
0 1
2
3
4
5
6
7
Dose (1016 He+/cm2)
Fig. 3. Fluence dependence of exfoliated crater depths for He+ implanted at 10 keV in GaAs after annealing at 400 C for 30 s.
10 1 x1016 1.6 x1016 8
2 x1016 3 x1016
4. Conclusion We have established the existence of the isotope effect and narrow fluence windows for exfoliation/blistering in GaAs for low energy H and D ions, as already observed for Si, in spite of the multitude of differences between these semiconductors. The crater depth in H-implanted GaAs corresponds to the peak of the H implantation distribution evaluated by SRIM. On the other hand, crater depths in He-implanted GaAs are strongly dependent on He+ fluence and correspond to the depth where the local concentration reaches 1.5–3.0 at.% He.
4 x1016
6
at.% He
Acknowledgements Many thanks to Christian Dubuc and Pierre-Paul Mercier for their advice concerning the RTA treatments. This work was made possible by grants from NSERC and a FQRNT post-doctoral fellowship to A. Gigue`re.
4
2
References 0 50
100
150
Depth (nm)
Fig. 4. Calculated concentrations profiles of He+ implanted at 10 keV in GaAs for different doses. Circles indicate local He concentrations for the corresponding measured crater depths.
concentration of only 3 ± 0.5 at.% He in GaAs by SRIM simulation (Fig. 4). The mean projected range (RP) at 10 keV is 78 ± 5 nm and corresponds to the exfoliated layer thickness for the lowest critical fluence. The saturated value at 155 ± 5 nm coincides instead with the deep end of the
[1] M. Bruel, Electron. Lett. 31 (1995) 1201. [2] I. Radu, I. Szafraniak, R. Scholz, M. Alexe, U. Go¨sele, J. Appl. Phys. 94 (2003) 7820. [3] O. Moutanabbir, A. Gigue`re, B. Terreault, Appl. Phys. Lett. 84 (2004) 3286. [4] O. Moutanabbir, B. Terreault, J. Chem. Phys. 121 (2004) 7973. [5] J.F. Ziegler, J.P. Biersack, SRIM_2003.10. Available from: