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Reduction of Threading Dislocation Density in Sputtered Ge/Si (100) Epitaxial Films by Continuous-Wave Diode Laser Induced Recrystallization Ziheng Liu, Xiaojing Hao, Jialiang Huang, Anita W. Y. Ho-Baillie, and Martin A. Green ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00130 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018
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Reduction of Threading Dislocation Density in Sputtered Ge/Si (100) Epitaxial Films by ContinuousWave Diode Laser Induced Recrystallization Ziheng Liu*, Xiaojing Hao, Jialiang Huang, Anita Ho-Baillie, and Martin A. Green School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia Keywords: Epitaxial Ge films; defect reduction; continuous-wave diode laser; recrystallization; magnetron sputtering Abstract We have developed a cost-effective, up-scalable and high-throughput method combining continuouswave (CW) diode laser and magnetron sputtering for fabricating low-defect single-crystalline Ge films for high efficiency III-V solar cells application. CW diode laser induced recrystallization is demonstrated to dramatically reduce the threading dislocation density (TDD) of sputter-deposited single-crystalline Ge/Si epitaxial films by more than three orders of magnitude. This might be due to the change of growth mechanism from initial Ge/Si hetero-epitaxy in sputtering process to Ge/Ge homo-epitaxy by the laser induced lateral recrystallization process, overcoming the typical issue of Ge/Si lattice mismatch to achieve low TDD.
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1. Introduction
Epitaxy of Ge on Si substrate has received intensive attention for its compatibility with Si process flow and the superior properties of Ge. The driving force for the integration of Ge with Si are its applications in metal-oxide-semiconductor field-effect transistors as high mobility channel 1, in Sibased optical devices as photodetector 2, and in high efficiency III-V solar cells as the growing substrates 3. Epitaxial Ge films on Si might be used as virtual Ge substrates for fabricating III-V high efficiency solar cells owing to its small lattice mismatch with GaAs. Compared with the bulk Ge substrates, virtual Ge substrates are much cheaper and have better electrical and mechanical properties. However, the large lattice mismatch (4.2%) between Ge and Si can generate high threading dislocation density (TDD) in Ge films 2, 4. Multiple approaches have been studied to reduce TDD of the epitaxial Ge films. Graded SiGe buffer is one way to gradually reduce the lattice mismatch and therefore the TDD. However, 10 µm thick buffer layer and chemical mechanical polishing are required 5. Defects blocking through substrate patterning is another method to reduce TDD which involves expensive and low-throughout nanometer level patterning 6. Thermal annealing approaches including two-step Ge deposition followed by furnace annealing 7, cycled Ge epitaxial growth and thermal annealing 8, and cyclic thermal annealing 9 have also been investigated to reduce TDD of Ge. These conventional thermal treatments require annealing at high temperature for long duration and can only obtain limited TDD reduction. In contrast to the aforementioned approaches, continuous-wave (CW) diode laser induced recrystallization offers a low-cost and fast alternative to effectively reduce the TDD of Ge films.
CW diode laser annealing has been used to achieve electrical activation and good lattice reordering in ion-implanted Si
10
and as a replacement of rapid thermal annealing for defect reduction in
polycrystalline Si solar cells
11-12
. The CW diode laser was first time employed to reduce TDD in
sputter-deposited single-crystalline Ge epitaxial films on Si in our previous work through annealing 2 ACS Paragon Plus Environment
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at a temperature below the Ge melting point 13. However, even after multiple cycles of laser scans, the TDD reduction is limited by two orders of magnitude due to the reemission of dislocations
14
.
Melting the defects away and getting the film recrystallized might be feasible to further reduce TDD. With a similar reasoning, high quality polycrystalline Si thin films with low defect density and high mobility have been obtained by laser-induced liquid-phase crystallization
15
. In this work, we
increase the laser intensity to melt the Ge films and significant TDD reduction of more than three orders of magnitude is achieved after only one laser scan.
The significant TDD reduction can be attributed to the lateral recrystallization process induced by CW diode laser scanning. The process in this work is different from the recrystallization in pulsed laser annealing
16
where the whole layer is melted and recrystallized perpendicularly from Si. By
scanning the sample with CW diode laser, the illuminated region is melted and recrystallized laterally at a high speed which follows the laser beam motion. Owing to the lateral regrowth, the Ge film itself may act as the recrystallization seed instead of Si which changes the mechanism from Ge/Si hetero-epitaxy to Ge/Ge homo-epitaxy and therefore overcomes the typical issue of Ge/Si lattice mismatch. As a result, the TDD of Ge film can be dramatically reduced by the CW diode laser induced recrystallization. Our process makes good use of not only the fast and selectively heating characteristics of the CW diode laser, but also its unique scanning scheme.
Magnetron sputtering was used to deposit the Ge films in this work. Compared with chemical vapour deposition (CVD) and molecular beam epitaxy (MBE) systems which require high vacuum and may involve toxic gases
7, 17
, magnetron sputtering is a lower cost and safer alternative in fabricating
epitaxial Ge films on Si. The combination of magnetron sputtering and CW diode laser explored in this work is compatible with large scale production of templates to fabricate III-V solar cells.
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2. Experimental
The Ge films were deposited by using a radio frequency magnetron sputtering system (AJA ATC2200) in this work. The substrates used were N-type Si (100) wafers which went through the RCA cleaning
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and HF dip before loading into the deposition chamber. 3 inch intrinsic Ge target
(99.999% purity) was used to sputter-deposit the Ge films. The process pressure was 1.5×10-3 millibar and the deposition rate was 4 nm/min. Ge films with thicknesses between 100 and 200 nm were deposited on Si at a substrate temperature of 400 oC. 150 nm thick SiO2 were deposited following Ge depositions as the capping layer. The laser tool used in this work was Lissotschenko Mikrooptick GmbH (LIMO) continuous-wave diode laser with line-focus optics. The schematic diagram of the CW diode laser scanning process is shown in Figure S1 (supplementary data). Details of the laser process can be found in our previous report 13. The processing time for a 4 inch wafer is around 3 min at a scan velocity of 400 mm/min. Laser scans with energy densities of 67-95 J/cm2 were applied on the Ge samples. The crystallinity of the Ge samples before and after laser treatments was analyzed by Raman spectroscopy (Renishaw Ramascope) and transmission electron microscopy (TEM Phillips CM200 microscope) measurements.
3. Results and discussions
To identify the laser conditions for melting Ge thin film, the laser power has been varied to produce energy dose from 67 to 95 J/cm2 with a fixed exposure time of 40 milliseconds. Figure 1 shows the peak temperatures of the 200 nm Ge samples treated at different laser doses. The peak temperature rises sharply from 830 oC to 937 oC when the laser dose is increased from 67 J/cm2 to 78 J/cm2. After the raise, the peak temperature plateaus suggesting partial melting of the Ge sample and the absorption of latent heat
11
. As the laser dose is further increased above 92 J/cm2, the peak
temperature increases with the laser dose again. Three laser doses of 73 J/cm2, 78 J/cm2, and 92
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J/cm2 were chosen to investigate the effects of laser treatments at temperatures i) below, ii) at, and iii) above Ge melting point, respectively.
Figure 1 Peak temperatures of the 200 nm Ge samples as a function of the laser doses.
TEM measurements were employed to study the effects of CW diode laser treatments at different doses on TDD of the epitaxial Ge samples. The evaluation of TDD was conducted through counting the dislocations along the planes parallel to the substrate in different areas of the samples and then estimating the TDD range
19
. Figure 2 shows the cross-sectional TEM images of Ge samples (a)
before and after laser treatments at laser doses of (b) 73 J/cm2, (c) 78 J/cm2, and (d) 92 J/cm2. As shown in Figure 2(a), the epitaxial Ge film before laser annealing has a high TDD above 1010 cm-2. After laser annealing at 73 J/cm2 below the Ge melting point, the TDD of Ge is reduced to 108-109 cm-2 which is in the similar range to that obtained by conventional thermal annealing 9. In this case, the dislocations within the Ge film are thermally activated to glide and can annihilate when having opposite Burger vector and are within an annihilation radius 20. When the laser dose is increased to 78 J/cm2 that is able to melt the Ge film, the TDD is dramatically reduced by more than three orders of magnitude to 106-107 cm-2. The CW diode laser induced melting and recrystallization process 5 ACS Paragon Plus Environment
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breaks the limit of two orders of magnitude TDD reduction by conventional thermal annealing. Interestingly, it is found that the significant TDD reduction might be attributed to the lateral recrystallization of Ge which uses the Ge film as the recrystallization seed (laterally in a homoepitaxial growth mode) instead of Si (vertically in a hetero-epitaxial growth mode) and the high recrystallization speed induced vacancies supersaturation which will cause the climbing of dislocations to the lateral surface 21-22. In this way, TDD owing to the 4.2% lattice mismatch between Si and Ge can be significantly reduced. Once the laser dose is further increased to 92 J/cm2 above the Ge melting point, pronounced Ge penetration into Si and high TDD of 109-1010 cm-2 are observed in Figure 2(d). The high TDD likely originates from the Si/Ge mixture at the interface region owing to the lattice mismatch
23
. According to these observations, laser treatment at the Ge melting point
inducing recrystallization offers the most effective TDD reduction.
Figure 2 (a)-(d) Cross-sectional TEM images and (e) Raman spectra of 200 nm Ge samples before and after laser treatment: (a) no laser treatment, (b) 73 J/cm2, (c) 78 J/cm2, and (d) 92 J/cm2.
The Si/Ge inter-diffusion after laser treatment was examined by Raman spectra as shown in Figure 2(e). 514 nm Ar+ laser was used as the excitation source which penetrates the top 20 nm of the Ge 6 ACS Paragon Plus Environment
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film 24-25. The laser beam power was limited to 6 mW to prevent the locally annealing effect during measurements. As shown in Figure 2(e), all the epitaxial Ge samples exhibit peaks centered around 300 cm-1 corresponding to the Ge-Ge optical vibration modes
26
. The reduced full width at half
maximum (FWHM) and increased intensity of the Ge-Ge peak after laser treatment indicates improved crystallinity which agrees well with the TEM results. In addition, after laser treatment at 78 J/cm2 at the Ge melting point, a tiny Si-Ge peak emerges suggesting small Si diffusion into the Ge film. When the laser dose is further increased to 92 J/cm2 above the Ge melting point, a strong Si-Ge peak is detected revealing that high Si content Si1-xGex alloy is formed, which agrees with the TEM observation of Ge penetration into Si.
Figure 3 (a)-(c) Cross-sectional TEM images and (d) Raman spectra of Ge samples with different thicknesses after laser treatments at Ge melting point: (a) 200 nm, (b) 150 nm, and (c) 100 nm.
Since thin Ge layer is preferred in the application of virtual Ge substrates, the CW diode laser treatment at the Ge melting point has also been applied to Ge films of 100 nm and 150 nm to demonstrate its effective TDD reduction at different Ge thicknesses. Figure 3 shows the (a)-(c) crosssectional TEM images and (d) Raman spectra of Ge samples with thicknesses of 200 nm, 150 nm, and 100 nm after laser treatments at the Ge melting point. As shown in the TEM images, the Ge films exhibit similar TDD which lies within the range of 106 cm-2-107 cm-2. Effective TDD reduction 7 ACS Paragon Plus Environment
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has been successfully achieved for all the Ge thicknesses by the laser induced recrystallization. The Raman spectra in Figure 3(d) reveal that all the Ge samples have a strong Ge-Ge peak and a very weak Si-Ge peak. The Si-Ge peak intensity increases slightly with the reducing Ge thickness suggesting more Si diffusion in the thinner Ge sample. The penetration depth of 808 nm laser in the Ge layer is more than 200 nm
24-25
. The thinner Ge allows more absorption for the Si substrate and
therefore elevates the Si temperature which induces more Si diffusion into the Ge film.
The CW diode laser induced recrystallization is much more effective in TDD reduction than solid phase thermal annealing owing to the different mechanisms. In solid phase thermal annealing, the dislocations are thermally activated to move for annihilation and coalescence
20
. However, the
reemission accompanied with the coalescence of dislocations and generation of dislocations during the thermal process limit the extent of TDD reduction 14. In the CW diode laser treatment at the Ge melting point, Ge films are melted and recrystallized laterally following the motion of scanning laser. The lateral regrowth may allow the use of Ge film itself as the epitaxial recrystallization seed. This could change the growth mechanism from Ge/Si hetero-epitaxy to Ge/Ge homo-epitaxy and therefore overcome the typical issue of 4.2% Ge/Si lattice mismatch. In addition, the high scanning speed and selective heating of the CW diode laser lead to high thermal gradient and fast regrowth rate which are the two major factors to produce vacancy supersaturation
21-22
. The possibly induced
vacancy supersaturation might in turn cause the climb of dislocations to the lateral crystal surface. Thanks to the lateral regrowth and possibly vacancy supersaturation, the TDD of Ge can be significantly reduced by more than three orders of magnitude by the fast scanning CW diode laser induced recrystallization. The obtained Ge films with TDD of 106 cm-2-107 cm-2 may be suitable for fabrication of III–V solar cells 27-28. Future optimization of the sputter-deposition and laser processes might be able to further reduce the TDD and therefore improve the cell efficiency.
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4. Conclusions
CW diode laser treatments have been employed to reduce the TDD of epitaxial Ge films on Si grown by magnetron sputtering. The effects of laser treatments at temperatures below, at, and above Ge melting point have been investigated. For laser treatment below Ge melting point, the TDD of Ge can be reduced by only one order of magnitude which is similar to that in thermal annealing. When the laser dose is raised too high above the Ge melting point, high Si content Si1-xGex alloy with high TDD is formed. Laser treatment at the Ge melting point offers the most effective TDD reduction by more than three orders of magnitude through recrystallization. The effective TDD reduction by the laser treatment has been demonstrated on Ge films with different thicknesses. The CW diode laser induced recrystallization may change the growth mechanism from Ge/Si hetero-epitaxy to Ge/Ge homo-epitaxy and therefore overcomes the Ge/Si lattice mismatch to achieve low TDD. The method reported in this work employing magnetron sputtering and CW diode laser is compatible with large scale production of templates to fabricate III-V solar cells.
Author information
Corresponding Author
*E-mail:
[email protected]
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
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(ARENA) and by Epistar Corporation and Shin Shin Natural Gas Co., Ltd., Taiwan. Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government.
Supporting Information Available: Schematic diagram of the diode laser annealing process.
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