- Email: [email protected]
Laser-induced aluminium-assisted crystallization of Ge-rich SixGe1-x epitaxy on Si
Thin Solid Films 679 (2019) 55–57
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Laser-induced aluminium-assisted crystallization of Ge-rich SixGe1-x epitaxy on Si
T
⁎
Ziheng Liu , Xiaojing Hao, Jialiang Huang, Anita Ho-Baillie, Martin A. Green School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Aluminium-assisted crystallization Diode laser Germanium-rich silicon germanium alloy Silicon content
Single crystalline Ge-rich SixGe1-x epitaxially grown on a Si substrate is attracting great attention for its potential in optical and electronic device applications. One method of achieving SixGe1-x epitaxy is by aluminium-assisted crystallization although the challenge of controlling the Si content in the SixGe1-x is limited by the furnace annealing process. Herein, we report the use of a diode laser to induce aluminium-assisted crystallization of SixGe1-x on Si with minimal Si content in the SixGe1-x layer. By replacing furnace heat treatment with laser treatment, the reaction time is shortened from minutes to milliseconds which can limit the amount of Si incorporation into the SixGe1-x films. X-ray diffraction, Raman spectroscopy, transmission electron microscopy and X-ray spectroscopy analyses are conducted on the fabricated films revealing the achievement of Ge-rich SixGe1-x on Si through laser-induced aluminium-assisted crystallization. The higher laser dose may slightly increase the Si content and improve the crystallinity of the SixGe1-x films.
1. Introduction Single crystalline Ge-rich SixGe1-x epitaxially grown on a Si substrate is attracting great attention for its potential in optoelectronic device applications. It combines the superior optical and electrical properties of Ge-rich SixGe1-x and the compatibility with existing Si technology. The single-crystalline Ge-rich SixGe1-x/Si could be used in field-effect transistor channels due to its high mobility [1,2], in optoelectronic devices taking advantage of its high optical absorption coefficient in the near infrared [3,4], and as a template for high efficiency III-V solar cells owing to its small lattice mismatch with GaAs [5,6]. The SixGe1-x films are normally fabricated using the traditional chemical vapor deposition (CVD). CVD is relatively expensive requiring high vacuum and toxic gases such as germane and silane. For the sake of cost reduction and elimination of toxic gases in the fabrication process, we have demonstrated the epitaxial growth of SixGe1-x on Si through aluminium-assisted crystallization by using economic sputtering and furnace annealing at relatively low temperatures [7]. This method involves the sputter deposition of Ge/Al/Si stacked structure at room temperature followed by a heat treatment in the conventional furnace to induce the layer exchange resulting in Al/SixGe1-x/Si. However, we found it is difficult to limit the Si content to obtain Ge-rich SixGe1-x by this furnace-induced aluminium-assisted crystallization method since that the Si content keeps increasing with annealing
⁎
duration throughout the process according to our previous in-situ XRD study [8]. In addition, the increase in the Si content is rather rapid during the initial layer exchange stage and turns gradual when entering the diffusion stage. As a result, the reduction of annealing time particularly in the initial layer exchange stage seems to be critical for limiting the Si content. In this work, we report the use of diode laser replacing conventional furnace to induce the aluminium-assisted crystallization which can reduce the reaction time from minutes to milliseconds. Diode laser annealing has been used for lattice reordering and electrical activation of ion-implanted Si [9]. The short exposure time prevents significant Si diffusion that usually occurs in furnace annealing. The diode laser also has the advantage of selective heating, allowing it to be an ideal replacement of rapid thermal annealing which has been used for thin film defect reductions by overcoming the drawback of heating the substrate [10,11]. In this work, both advantages of short exposure time and selective heating might help to reduce the Si content in SixGe1-x films. The sputter-deposited stacked Ge/Al/Si samples are laser treated at different laser doses and characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM) and EnergyDispersive X-ray spectroscopy (EDS). Ge-rich SixGe1-x epitaxy on Si has been obtained by laser-induced aluminium-assisted crystallization.
Corresponding author. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.tsf.2019.04.005 Reçu le 28 novembre 2018; Reçu en forme révisée le 18 mars 2019; accepté le 2 avril 2019 Available online 03 April 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
Thin Solid Films 679 (2019) 55–57
Z. Liu, et al.
2. Materials and methods N-type Si (100) wafers as substrates for this work were cleaned using the RCA solution [12] followed by HF dip. Al films were deposited on Si using an AJA ATC2200 RF magnetron sputtering system with a 2-in. Al target (99.999% purity). The Al deposition rate was 2 nm/min at a processing pressure of 2 × 10−1 Pa. Ge films were then sputter-deposited on the Al films using a 2-in. Ge target (99.999% purity) at a deposition rate of 4 nm/min at 1.3 × 10−1 Pa. Both the Al and Ge depositions were conducted at room temperature. 120 nm thick Al and 160 nm thick Ge films were deposited in order to form continuous films after the layer exchange [13]. Lissotschenko Mikrooptick GmbH continuous-wave diode laser with line-focus optics was used to induce the aluminium-assisted crystallization. The laser wavelength is 808 nm with a beam size of 12 mm × 270 μm. The Ge/Al/Si samples were placed on a preheating stage at 200 °C to reduce the thermal stress caused by the laser treatment. The laser scanning speed was 100 mm/ min corresponding to an exposure time of 160 milliseconds. The samples were treated with laser energy densities of 300–450 J/cm2. The sample temperature during the laser process was measured using a high-speed pyrometer (LumaSense IGAR 12) at the 2-color mode which is independent of the emissivity of the object. The fabricated films were characterized using XRD (Panalytical Empyrean Thin Film XRD) at a voltage of 45 kV and a current of 40 mA, using Cu Kα radiation, Raman spectroscopy (Renishaw Ramascope), TEM and EDS (Phillips CM200 microscope equipped with an EDAX EDS system). The TEM was operating at 200 kV and the sample was prepared by focused ion beam (FIB) milling at 30 kV using Nova Nanolab 200. 3. Results and discussion
Fig. 2. Raman spectra of Ge/Al/Si samples treated by laser at 400 J/cm2 and 450 J/cm2; and by furnace annealing at 400 °C for 60 min.
The effect of laser dose on the aluminium-assisted crystallization is studied by fixing the laser scan at 100 mm/min. It is found that at the laser dose of 400 J/cm2, Ge/Al layer exchange proceeds. The temperature measured at this starting point was 440 °C which is higher than that in furnace-induced aluminium-assisted crystallization [7]. Since higher temperature helps to complete the layer exchange for a shorter duration [8], the higher temperature requirement for laser-induced aluminium-assisted crystallization is reasonable considering the reduced reaction time from minutes to milliseconds. The Ge/Al/Si sample was also treated at 450 J/cm2 with a measured temperature of 470 °C to investigate the effect of higher laser dose. The crystallinity of SixGe1-x samples was examined by XRD measurements. Fig. 1 shows the XRD 2θ-Ω profiles of the Ge/Al/Si samples
before laser treatment, after laser treatment at 400 J/cm2 and 450 J/ cm2. For the as-deposited sample, only a Si (400) peak at 69.2o from the Si wafer is present indicating the amorphous nature of the Al and Ge films deposited at room temperature. After laser treatments, both samples show a SixGe1-x (400) peak at around 66o in addition to the Si (400) peak. There is no other peak observed indicating the epitaxial growth of single-crystalline SixGe1-x on Si. The SixGe1-x peak of 400 J/ cm2 laser treated sample is at 66.18o which is quite close to the pure Ge peak at 66o suggesting that the Si content might be low. With the laser dose increasing from 400 J/cm2 to 450 J/cm2, the SixGe1-x (400) peak slightly shifts towards the Si (400) revealing more Si incorporation into the film. This might be owing to the increased diffusivity of Si in Al at elevated temperature caused by the higher laser dose [14]. The SixGe1-x peak intensity is also increased suggesting improved crystallinity when higher laser dose is used. Raman spectroscopy was employed to verify the relative Si content in the SixGe1-x films. The beam power was set to 6 mW to preclude the annealing effect during measurements. Fig. 2 shows the Raman spectra of the Ge/Al/Si samples treated by laser scanning at 400 J/cm2 and 450 J/cm2. Raman result of the SixGe1-x film fabricated by aluminiumassisted crystallization through furnace annealing at 400 °C for 60 min from our previous work is also included for comparison [7]. As shown in Fig. 2, both laser-treated samples show a strong GeeGe peak and a weak SieGe peak confirming the formation of Ge-rich SixGe1-x films. The GeeGe peak is shifted from the peak position of bulk Ge at 300 cm−1 which might be due to the increased lattice constant caused by the Al incorporation in the films [15]. The sample treated at 450 J/ cm2 exhibits a slightly stronger SieGe peak which agrees with the XRD result showing more Si incorporation when higher laser dose is used. Comparing to the furnace annealed sample which has a strong SieGe peak and a weak SieSi hump, the laser treated samples show a much weaker SieGe peak and no SieSi signal confirming much lower Si content in the laser treated SixGe1-x films by aluminium-assisted
Fig. 1. XRD 2θ-Ω diffraction profiles of Ge/Al/Si samples before laser treatment, after laser treatment at 400 J/cm2 and 450 J/cm2. 56
Thin Solid Films 679 (2019) 55–57
Z. Liu, et al.
4. Conclusions Diode laser has been introduced in the aluminium-assisted crystallization of SixGe1-x on Si wafer as a replacement of furnace annealing to reduce the reaction time and therefore the Si content in the SixGe1-x. The XRD, Raman, TEM and EDS results reveal that Ge-rich SixGe1-x on Si through laser-induced aluminium-assisted crystallization has been successfully fabricated. The reaction temperature in this laser-induced process is higher than that in the furnace-annealed method, thereby accelerating the layer exchange. The higher laser dose may slightly increase the Si content and improve the crystallinity of the SixGe1-x films. Acknowledgements This work has been supported by the Australian Government through the Australian Research Council (ARC, grant number DP160103433) and the Australian Renewable Energy Agency (ARENA, grant number 1-UFA002) and by Epistar Corporation and Shin Natural Gas Co., Ltd., Taiwan. Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government. References [1] M.L. Lee, E.A. Fitzgerald, Electron mobility characteristics of n-channel metaloxide-semiconductor field-effect transistors fabricated on Ge-rich single- and dualchannel SiGe heterostructures, J. Appl. Phys. 95 (2004) 1550–1555. [2] G. Capellini, M.D. Seta, Y. Busby, M. Pea, F. Evangelisti, G. Nicotra, C. Spinella, M. Nardone, C. Ferrari, Strain relaxation in high Ge content SiGe layers deposited on Si, J. Appl. Phys. 107 (2010) 063504. [3] D.D. Cannon, J. Liu, D.T. Danielson, S. Jongthammanurak, U.U. Enuha, K. Wada, J. Michel, L.C. Kimerling, Germanium-rich silicon-germanium films epitaxially grown by ultrahigh vacuum chemical-vapor deposition directly on silicon substrates, Appl. Phys. Lett. 91 (2007) 252111. [4] J. Aubin, J.M. Hartmann, M. Bauer, S. Moffatt, Very low temperature epitaxy of Ge and Ge rich SiGe alloys with Ge2H6 in a reduced pressure – chemical vapour deposition tool, J. Cryst. Growth 445 (2016) 65–72. [5] S.A. Ringel, J.A. Carlin, C.L. Andre, M.K. Hudait, M. Gonzalez, D.M. Wilt, E.B. Clark, P. Jenkins, D. Scheiman, A. Allerman, E.A. Fitzgerald, C.W. Leitz, Singlejunction InGaP/GaAs solar cells grown on Si substrates with SiGe buffer layers, Prog. Photovolt. Res. Appl. 10 (2002) 417–426. [6] M.R. Lueck, C.L. Andre, A.J. Pitera, M.L. Lee, E.A. Fitzgerald, S.A. Ringel, Dual junction GaInP/GaAs solar cells grown on metamorphic SiGe/Si substrates with high open circuit voltage, Electron Device Lett., IEEE 27 (2006) 142–144. [7] Z. Liu, X. Hao, F. Qi, A. Ho-Baillie, M.A. Green, Epitaxial growth of single-crystalline silicon–germanium on silicon by aluminium-assisted crystallization, Scr. Mater. 71 (2014) 25–28. [8] Z. Liu, X. Hao, A. Ho-Baillie, M.A. Green, In situ X-ray diffraction study on epitaxial growth of SixGe1−x on Si by aluminium-assisted crystallization, J. Alloys Compd. 695 (2017) 1672–1676. [9] J.S. Williams, W.L. Brown, H.J. Leamy, J.M. Poate, J.W. Rodgers, D. Rousseau, G.A. Rozgonyi, J.A. Shelnutt, T.T. Sheng, Solid-phase epitaxy of implanted silicon by cw Ar ion laser irradiation, Appl. Phys. Lett. 33 (1978) 542–544. [10] B. Eggleston, S. Varlamov, M. Green, Large-area diode laser defect annealing of polycrystalline silicon solar cells, Electron Devices, IEEE Trans. 59 (2012) 2838–2841. [11] Z. Liu, X. Hao, J. Huang, W. Li, A. Ho-Baillie, M.A. Green, Diode laser annealing on Ge/Si (100) epitaxial films grown by magnetron sputtering, Thin Solid Films 609 (2016) 49–52. [12] W. Kern, The evolution of silicon wafer cleaning technology, J. Electrochem. Soc. 137 (1990) 1887–1892. [13] O. Nast, S.R. Wenham, Elucidation of the layer exchange mechanism in the formation of polycrystalline silicon by aluminum-induced crystallization, J. Appl. Phys. 88 (2000) 124–132. [14] J.O. McCaldin, H. Sankur, Diffusivity and solubility of Si in the Al metallization of integrated circuits, Appl. Phys. Lett. 19 (1971) 524–527. [15] K. Toko, K. Nakazawa, N. Saitoh, N. Yoshizawa, N. Usami, T. Suemasu, Doublelayered Ge thin films on insulators formed by an Al-induced layer-exchange process, Cryst. Growth Des. 13 (2013) 3908–3912. [16] Z. Liu, X. Hao, A. Ho-Baillie, M.A. Green, One-step aluminium-assisted crystallization of Ge epitaxy on Si by magnetron sputtering, Appl. Phys. Lett. 104 (2014) 052107.
Fig. 3. Characterization of the cross-sectional structure of the 400 J/cm2 treated sample: (a) TEM image, EDS mappings of (b) Ge and (c) Si.
crystallization. The microstructure of SixGe1-x films was analysed by TEM and EDS measurements. Fig. 3 shows the cross-sectional TEM image (a), and the EDS mappings of Ge (b) and Si (c) of the sample treated at 400 J/cm2. As shown in the TEM image, the SixGe1-x film is formed on the Si substrate displacing the Al layer to the top. There is some Ge residue in the top layer since the initial Ge amount is excessive compared to Al and the layer exchange is not fully completed within the short reaction time of 160 milliseconds. Planar defects are observed in the SixGe1-x film which is similar to the film when furnace annealing is used reported in Ref. [7, 16]. The EDS mappings reveal uniform distributions of Ge and Si within the film in the current work. The gold coating was deposited before FIB milling as a capping layer to improve conductivity. The Ge and Si EDS signals of the gold coating layer might be owing to the artificial dust created during the FIB milling process. The Si intensity within the film is quite low which similar to the background level. These results further confirm that the SixGe1-x film is Ge-rich.
57
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Laser-induced aluminium-assisted crystallization of Ge-rich SixGe1-x epitaxy on Si
T
⁎
Ziheng Liu , Xiaojing Hao, Jialiang Huang, Anita Ho-Baillie, Martin A. Green School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Aluminium-assisted crystallization Diode laser Germanium-rich silicon germanium alloy Silicon content
Single crystalline Ge-rich SixGe1-x epitaxially grown on a Si substrate is attracting great attention for its potential in optical and electronic device applications. One method of achieving SixGe1-x epitaxy is by aluminium-assisted crystallization although the challenge of controlling the Si content in the SixGe1-x is limited by the furnace annealing process. Herein, we report the use of a diode laser to induce aluminium-assisted crystallization of SixGe1-x on Si with minimal Si content in the SixGe1-x layer. By replacing furnace heat treatment with laser treatment, the reaction time is shortened from minutes to milliseconds which can limit the amount of Si incorporation into the SixGe1-x films. X-ray diffraction, Raman spectroscopy, transmission electron microscopy and X-ray spectroscopy analyses are conducted on the fabricated films revealing the achievement of Ge-rich SixGe1-x on Si through laser-induced aluminium-assisted crystallization. The higher laser dose may slightly increase the Si content and improve the crystallinity of the SixGe1-x films.
1. Introduction Single crystalline Ge-rich SixGe1-x epitaxially grown on a Si substrate is attracting great attention for its potential in optoelectronic device applications. It combines the superior optical and electrical properties of Ge-rich SixGe1-x and the compatibility with existing Si technology. The single-crystalline Ge-rich SixGe1-x/Si could be used in field-effect transistor channels due to its high mobility [1,2], in optoelectronic devices taking advantage of its high optical absorption coefficient in the near infrared [3,4], and as a template for high efficiency III-V solar cells owing to its small lattice mismatch with GaAs [5,6]. The SixGe1-x films are normally fabricated using the traditional chemical vapor deposition (CVD). CVD is relatively expensive requiring high vacuum and toxic gases such as germane and silane. For the sake of cost reduction and elimination of toxic gases in the fabrication process, we have demonstrated the epitaxial growth of SixGe1-x on Si through aluminium-assisted crystallization by using economic sputtering and furnace annealing at relatively low temperatures [7]. This method involves the sputter deposition of Ge/Al/Si stacked structure at room temperature followed by a heat treatment in the conventional furnace to induce the layer exchange resulting in Al/SixGe1-x/Si. However, we found it is difficult to limit the Si content to obtain Ge-rich SixGe1-x by this furnace-induced aluminium-assisted crystallization method since that the Si content keeps increasing with annealing
⁎
duration throughout the process according to our previous in-situ XRD study [8]. In addition, the increase in the Si content is rather rapid during the initial layer exchange stage and turns gradual when entering the diffusion stage. As a result, the reduction of annealing time particularly in the initial layer exchange stage seems to be critical for limiting the Si content. In this work, we report the use of diode laser replacing conventional furnace to induce the aluminium-assisted crystallization which can reduce the reaction time from minutes to milliseconds. Diode laser annealing has been used for lattice reordering and electrical activation of ion-implanted Si [9]. The short exposure time prevents significant Si diffusion that usually occurs in furnace annealing. The diode laser also has the advantage of selective heating, allowing it to be an ideal replacement of rapid thermal annealing which has been used for thin film defect reductions by overcoming the drawback of heating the substrate [10,11]. In this work, both advantages of short exposure time and selective heating might help to reduce the Si content in SixGe1-x films. The sputter-deposited stacked Ge/Al/Si samples are laser treated at different laser doses and characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM) and EnergyDispersive X-ray spectroscopy (EDS). Ge-rich SixGe1-x epitaxy on Si has been obtained by laser-induced aluminium-assisted crystallization.
Corresponding author. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.tsf.2019.04.005 Reçu le 28 novembre 2018; Reçu en forme révisée le 18 mars 2019; accepté le 2 avril 2019 Available online 03 April 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
Thin Solid Films 679 (2019) 55–57
Z. Liu, et al.
2. Materials and methods N-type Si (100) wafers as substrates for this work were cleaned using the RCA solution [12] followed by HF dip. Al films were deposited on Si using an AJA ATC2200 RF magnetron sputtering system with a 2-in. Al target (99.999% purity). The Al deposition rate was 2 nm/min at a processing pressure of 2 × 10−1 Pa. Ge films were then sputter-deposited on the Al films using a 2-in. Ge target (99.999% purity) at a deposition rate of 4 nm/min at 1.3 × 10−1 Pa. Both the Al and Ge depositions were conducted at room temperature. 120 nm thick Al and 160 nm thick Ge films were deposited in order to form continuous films after the layer exchange [13]. Lissotschenko Mikrooptick GmbH continuous-wave diode laser with line-focus optics was used to induce the aluminium-assisted crystallization. The laser wavelength is 808 nm with a beam size of 12 mm × 270 μm. The Ge/Al/Si samples were placed on a preheating stage at 200 °C to reduce the thermal stress caused by the laser treatment. The laser scanning speed was 100 mm/ min corresponding to an exposure time of 160 milliseconds. The samples were treated with laser energy densities of 300–450 J/cm2. The sample temperature during the laser process was measured using a high-speed pyrometer (LumaSense IGAR 12) at the 2-color mode which is independent of the emissivity of the object. The fabricated films were characterized using XRD (Panalytical Empyrean Thin Film XRD) at a voltage of 45 kV and a current of 40 mA, using Cu Kα radiation, Raman spectroscopy (Renishaw Ramascope), TEM and EDS (Phillips CM200 microscope equipped with an EDAX EDS system). The TEM was operating at 200 kV and the sample was prepared by focused ion beam (FIB) milling at 30 kV using Nova Nanolab 200. 3. Results and discussion
Fig. 2. Raman spectra of Ge/Al/Si samples treated by laser at 400 J/cm2 and 450 J/cm2; and by furnace annealing at 400 °C for 60 min.
The effect of laser dose on the aluminium-assisted crystallization is studied by fixing the laser scan at 100 mm/min. It is found that at the laser dose of 400 J/cm2, Ge/Al layer exchange proceeds. The temperature measured at this starting point was 440 °C which is higher than that in furnace-induced aluminium-assisted crystallization [7]. Since higher temperature helps to complete the layer exchange for a shorter duration [8], the higher temperature requirement for laser-induced aluminium-assisted crystallization is reasonable considering the reduced reaction time from minutes to milliseconds. The Ge/Al/Si sample was also treated at 450 J/cm2 with a measured temperature of 470 °C to investigate the effect of higher laser dose. The crystallinity of SixGe1-x samples was examined by XRD measurements. Fig. 1 shows the XRD 2θ-Ω profiles of the Ge/Al/Si samples
before laser treatment, after laser treatment at 400 J/cm2 and 450 J/ cm2. For the as-deposited sample, only a Si (400) peak at 69.2o from the Si wafer is present indicating the amorphous nature of the Al and Ge films deposited at room temperature. After laser treatments, both samples show a SixGe1-x (400) peak at around 66o in addition to the Si (400) peak. There is no other peak observed indicating the epitaxial growth of single-crystalline SixGe1-x on Si. The SixGe1-x peak of 400 J/ cm2 laser treated sample is at 66.18o which is quite close to the pure Ge peak at 66o suggesting that the Si content might be low. With the laser dose increasing from 400 J/cm2 to 450 J/cm2, the SixGe1-x (400) peak slightly shifts towards the Si (400) revealing more Si incorporation into the film. This might be owing to the increased diffusivity of Si in Al at elevated temperature caused by the higher laser dose [14]. The SixGe1-x peak intensity is also increased suggesting improved crystallinity when higher laser dose is used. Raman spectroscopy was employed to verify the relative Si content in the SixGe1-x films. The beam power was set to 6 mW to preclude the annealing effect during measurements. Fig. 2 shows the Raman spectra of the Ge/Al/Si samples treated by laser scanning at 400 J/cm2 and 450 J/cm2. Raman result of the SixGe1-x film fabricated by aluminiumassisted crystallization through furnace annealing at 400 °C for 60 min from our previous work is also included for comparison [7]. As shown in Fig. 2, both laser-treated samples show a strong GeeGe peak and a weak SieGe peak confirming the formation of Ge-rich SixGe1-x films. The GeeGe peak is shifted from the peak position of bulk Ge at 300 cm−1 which might be due to the increased lattice constant caused by the Al incorporation in the films [15]. The sample treated at 450 J/ cm2 exhibits a slightly stronger SieGe peak which agrees with the XRD result showing more Si incorporation when higher laser dose is used. Comparing to the furnace annealed sample which has a strong SieGe peak and a weak SieSi hump, the laser treated samples show a much weaker SieGe peak and no SieSi signal confirming much lower Si content in the laser treated SixGe1-x films by aluminium-assisted
Fig. 1. XRD 2θ-Ω diffraction profiles of Ge/Al/Si samples before laser treatment, after laser treatment at 400 J/cm2 and 450 J/cm2. 56
Thin Solid Films 679 (2019) 55–57
Z. Liu, et al.
4. Conclusions Diode laser has been introduced in the aluminium-assisted crystallization of SixGe1-x on Si wafer as a replacement of furnace annealing to reduce the reaction time and therefore the Si content in the SixGe1-x. The XRD, Raman, TEM and EDS results reveal that Ge-rich SixGe1-x on Si through laser-induced aluminium-assisted crystallization has been successfully fabricated. The reaction temperature in this laser-induced process is higher than that in the furnace-annealed method, thereby accelerating the layer exchange. The higher laser dose may slightly increase the Si content and improve the crystallinity of the SixGe1-x films. Acknowledgements This work has been supported by the Australian Government through the Australian Research Council (ARC, grant number DP160103433) and the Australian Renewable Energy Agency (ARENA, grant number 1-UFA002) and by Epistar Corporation and Shin Natural Gas Co., Ltd., Taiwan. Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government. References [1] M.L. Lee, E.A. Fitzgerald, Electron mobility characteristics of n-channel metaloxide-semiconductor field-effect transistors fabricated on Ge-rich single- and dualchannel SiGe heterostructures, J. Appl. Phys. 95 (2004) 1550–1555. [2] G. Capellini, M.D. Seta, Y. Busby, M. Pea, F. Evangelisti, G. Nicotra, C. Spinella, M. Nardone, C. Ferrari, Strain relaxation in high Ge content SiGe layers deposited on Si, J. Appl. Phys. 107 (2010) 063504. [3] D.D. Cannon, J. Liu, D.T. Danielson, S. Jongthammanurak, U.U. Enuha, K. Wada, J. Michel, L.C. Kimerling, Germanium-rich silicon-germanium films epitaxially grown by ultrahigh vacuum chemical-vapor deposition directly on silicon substrates, Appl. Phys. Lett. 91 (2007) 252111. [4] J. Aubin, J.M. Hartmann, M. Bauer, S. Moffatt, Very low temperature epitaxy of Ge and Ge rich SiGe alloys with Ge2H6 in a reduced pressure – chemical vapour deposition tool, J. Cryst. Growth 445 (2016) 65–72. [5] S.A. Ringel, J.A. Carlin, C.L. Andre, M.K. Hudait, M. Gonzalez, D.M. Wilt, E.B. Clark, P. Jenkins, D. Scheiman, A. Allerman, E.A. Fitzgerald, C.W. Leitz, Singlejunction InGaP/GaAs solar cells grown on Si substrates with SiGe buffer layers, Prog. Photovolt. Res. Appl. 10 (2002) 417–426. [6] M.R. Lueck, C.L. Andre, A.J. Pitera, M.L. Lee, E.A. Fitzgerald, S.A. Ringel, Dual junction GaInP/GaAs solar cells grown on metamorphic SiGe/Si substrates with high open circuit voltage, Electron Device Lett., IEEE 27 (2006) 142–144. [7] Z. Liu, X. Hao, F. Qi, A. Ho-Baillie, M.A. Green, Epitaxial growth of single-crystalline silicon–germanium on silicon by aluminium-assisted crystallization, Scr. Mater. 71 (2014) 25–28. [8] Z. Liu, X. Hao, A. Ho-Baillie, M.A. Green, In situ X-ray diffraction study on epitaxial growth of SixGe1−x on Si by aluminium-assisted crystallization, J. Alloys Compd. 695 (2017) 1672–1676. [9] J.S. Williams, W.L. Brown, H.J. Leamy, J.M. Poate, J.W. Rodgers, D. Rousseau, G.A. Rozgonyi, J.A. Shelnutt, T.T. Sheng, Solid-phase epitaxy of implanted silicon by cw Ar ion laser irradiation, Appl. Phys. Lett. 33 (1978) 542–544. [10] B. Eggleston, S. Varlamov, M. Green, Large-area diode laser defect annealing of polycrystalline silicon solar cells, Electron Devices, IEEE Trans. 59 (2012) 2838–2841. [11] Z. Liu, X. Hao, J. Huang, W. Li, A. Ho-Baillie, M.A. Green, Diode laser annealing on Ge/Si (100) epitaxial films grown by magnetron sputtering, Thin Solid Films 609 (2016) 49–52. [12] W. Kern, The evolution of silicon wafer cleaning technology, J. Electrochem. Soc. 137 (1990) 1887–1892. [13] O. Nast, S.R. Wenham, Elucidation of the layer exchange mechanism in the formation of polycrystalline silicon by aluminum-induced crystallization, J. Appl. Phys. 88 (2000) 124–132. [14] J.O. McCaldin, H. Sankur, Diffusivity and solubility of Si in the Al metallization of integrated circuits, Appl. Phys. Lett. 19 (1971) 524–527. [15] K. Toko, K. Nakazawa, N. Saitoh, N. Yoshizawa, N. Usami, T. Suemasu, Doublelayered Ge thin films on insulators formed by an Al-induced layer-exchange process, Cryst. Growth Des. 13 (2013) 3908–3912. [16] Z. Liu, X. Hao, A. Ho-Baillie, M.A. Green, One-step aluminium-assisted crystallization of Ge epitaxy on Si by magnetron sputtering, Appl. Phys. Lett. 104 (2014) 052107.
Fig. 3. Characterization of the cross-sectional structure of the 400 J/cm2 treated sample: (a) TEM image, EDS mappings of (b) Ge and (c) Si.
crystallization. The microstructure of SixGe1-x films was analysed by TEM and EDS measurements. Fig. 3 shows the cross-sectional TEM image (a), and the EDS mappings of Ge (b) and Si (c) of the sample treated at 400 J/cm2. As shown in the TEM image, the SixGe1-x film is formed on the Si substrate displacing the Al layer to the top. There is some Ge residue in the top layer since the initial Ge amount is excessive compared to Al and the layer exchange is not fully completed within the short reaction time of 160 milliseconds. Planar defects are observed in the SixGe1-x film which is similar to the film when furnace annealing is used reported in Ref. [7, 16]. The EDS mappings reveal uniform distributions of Ge and Si within the film in the current work. The gold coating was deposited before FIB milling as a capping layer to improve conductivity. The Ge and Si EDS signals of the gold coating layer might be owing to the artificial dust created during the FIB milling process. The Si intensity within the film is quite low which similar to the background level. These results further confirm that the SixGe1-x film is Ge-rich.
57