- Email: [email protected]
Investigating the formation of isotopically pure layers for quantum computers using ion implantation and layer exchange
Nuclear Inst. and Methods in Physics Research B 461 (2019) 30–36
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Investigating the formation of isotopically pure layers for quantum computers using ion implantation and layer exchange
T
Jonathan Englanda, , David Coxa, Nathan Cassidya, Bobur Mirkhaydarova, Andres Perez-Fadonb ⁎
a b
Ion Beam Centre, Advanced Technology Institute, University of Surrey, Guildford GU2 7XH, UK Kings College London Mathematics School, 80 Kennington Road, London SE11 6NJ, UK
ARTICLE INFO
ABSTRACT
Keywords: Ion-implantation Isotopically pure layers Layer exchange Quantum computing
Quantum computers have been proposed that exploit entangled quantum states between atoms that are isolated from environmental perturbations in a “semiconductor vacuum” which can be formed by cryogenically cooling an isotopically pure, defect free crystalline layer consisting of Si, or Ge. In a preliminary investigation of an implant and deposition layer exchange technique to produce such “vacuums”, a layer of aluminium was implanted with 28Si using a conventional implanter. After annealing and cross sectioning, layer exchange was observed to have produced multiple isolated crystals in a cross sectional TEM image. Further deposited Al layers were implanted with Ge using a SIMPLE (Single Ion Multispecies Positioning at Low Energy) implanter over a range of fluences. After anneals at 250 °C and Al removal, crystals of Ge (which also contained Si) were seen at areal densities that increased with implant fluence.
1. Introduction In 1998 Bruce Kane [1] proposed that quantum computation could be achieved using logic gates based on qubits that exploit hyperfine interactions between nuclei with spin, such as 31P or 209Bi, and the electrons in the lattice in which they are embedded. For such schemes to work, the qubits must be isolated from the environment (to avoid uncontrolled external disruption to delicate quantum states) and identical (to allow all the qubits to be controlled by the same NMR frequency) [2]. The spinning nuclei can be isolated from environmental perturbations by incorporating them in a “semiconductor vacuum” which can be formed by cryogenically cooling (to a few mK) an isotopically pure, defect free crystalline layer consisting of atoms that have no intrinsic electronic or nuclear spin. Layers composed of pure 28Si [2], or 74Ge [3], meet this last requirement and would be particularly useful as they could conceivably be straightforwardly integrated into conventional CMOS devices for the industrial manufacture of quantum computers systems. Naturally occurring Si is predominantly (92.2%) 28 Si, but significantly contains 4.7% 29Si atoms which can cause disruption through their nuclear spins. Although the remaining 3.1% of atoms are 30Si, which have no spin, they must be avoided because variations in lattice bond lengths (which are isotope dependent) upset the criterion that qubit environments are identical. As well as applications in quantum technologies, isotopically pure materials have been
⁎
investigated for the superior heat transfer properties of their superuniform lattices [4]. Quantum technologies have up to now often sourced isotopically pure Si from stocks prepared for standards [2]. Large quantities of isotopically pure 28Si were prepared for the Avogadro Project (production of pure 28Si spheres for the kg standard) by enriching SiF4 gas in centrifuges and using the gas to produce solid crystals by float zone techniques, or by CVD deposition [4]. Another production approach has used specially developed electromagnetic isotopic enrichment equipment [5]. Industrial ion implanters have their origins in calutrons isotope separators and so it might be thought that an ion implanter could be used as an isotope separator to selectively deposit low energy 28Si onto a natural Si substrate. An unpublished study carried out at the University of Surrey by L. Antwis, R. Webb, C. Jeynes and R. Gwilliam presented at the Ion Beam Modifications of Materials Conference 2016 investigated implants carried out in a conventional Dan Fysik implanter at 2 keV using a deceleration lens to increase available beam current (and hence throughput). A subsequent RBS analysis showed that the resultant 28Si layer was heavily oxidized in the non-UHV implanter endstation, contained beam transported contaminants such as 14N2+, and limited in isotopic enrichment through intermixing with the Si substrate. A higher energy 20 keV implant was also investigated but RBS measurements and TRIDYN calculations showed that 28Si fraction was limited to 0.95 after 2 × 1017 ions cm−2 by self-sputtering of the Si
Corresponding author. E-mail address: [email protected] (J. England).
https://doi.org/10.1016/j.nimb.2019.09.013 Received 30 May 2019; Received in revised form 7 September 2019; Accepted 9 September 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 461 (2019) 30–36
J. England, et al.
(~1.3 atoms ion−1) at these increased energies. This paper describes our first investigations of an implant layer exchange technique that could overcome the self-sputtering and oxidation limitations seen in the implant-only approach. Layer exchange [6] has been used commercially to deposit layers of poly-crystalline Si onto glass substrates for low cost solar cells [7]. The standard deposition-based process had the following steps:
implanted for systems such as N-V qubits in diamond. Wien filters in the columns allow mass selected implants. The SIMPLE end-stations are held under ultrahigh vacuum; oxidation of the sample during implantation is not a major concern. We took the opportunity to further investigate implanted layer exchange by carrying out a study using the first SIMPLE tool whilst it was being commissioned. The column was being aligned using Au ions from a AuGe LMAIS. We utilised Ge ions from this LMAIS to implant sections of Si wafer that had been stripped of their native oxide using hydrofluoric acid before depositing 100 nm of Al using a Kurt J. Lesker high vacuum evaporator. Ge was expected to behave similarly to Si and had the advantage that the implanted Ge could be easily differentiated from substrate Si during post-process analyses. The SIMPLE tool was used in a continuous (rather than pulsed) mode to implant 5 μm × 5 μm square regions with Ge beams to fluences in the 1017 ions cm−2 range within a few minutes. Because the Wien filter had a larger than normal mass selection aperture after the Wien filter, the implanted ions were not mono isotopic in these studies. A normal sized mass selection aperture would have allowed the implanted beam to be mono-isotopic. This approach allowed experiments over variable fluences to be carried out easily and did not compromise the post process SEM, TEM and AFM based analyses which all had small fields of view. All implants were carried out normal to the substrate at 25 keV, the maximum energy possible. The various experiments are summarised in Table 1.
1. Deposit Al onto glass; 2. Deposit (amorphous) Si onto the Al; 3. Heat at ~500 °C for around 1 h. During the annealing step, which was carried out below the eutectic temperature of the Si Al alloy, the amorphous Si dissolved into the Al and then diffused until it found a site where it formed a new crystal (heterogeneous nucleation was found to occur particularly on Al grain boundaries) or an earlier formed Si crystal upon which it could grow. The process was driven because crystalline Si has a lower Gibbs free energy than amorphous Si. Si dissolution continued throughout the anneal and occurred more favourably around smaller radii features; after full completion of the process, Ostwald ripening produced a continuous layer of poly crystalline Si, and the original Al had been displaced above the Si layer. The best results appeared to occur when the two layers had a similar thickness and an oxide interface between deposited Al and Si was found to be important [6]. Continuous crystalline Si layers have been grown epitaxially when this process has been carried out on Si wafers [8] where nucleation occurs on the substrate. Other metals, such as Ag, Au and Ni [6], and crystallising atoms such as Ge [9] have also been explored. This paper proposes replacing the Si deposition step with implants of isotopically pure 28Si (or 74Ge). If ultimately successful, this approach could enable the manufacture of qubit substrates using standard process equipment within a CMOS fabrication plant.
2.3. Post implant processing Post implant annealing was carried out in a Carbolite TZF 12/100 tube furnace in ambient atmosphere. Temperatures were chosen guided by previous work [9] which were all below the Ge Al eutectic temperature of 420 °C. For samples 2 and 5 in Table 1, Al was stripped by immersion in tetra methyl ammonium hydroxide (TMAH) for a few minutes.
2. Experimental method
2.4. Metrology
2.1. Si implants using a conventional implanter
Metrology of the samples usually involved cross section views of the samples post anneal. Single sided cross sections were made after protecting the sample surface by depositing Pt or C surface layers and then sputtering a trench using either Ga in a FEI Nova Nanolab 600 Dual Beam FIB or Xe using TESCAN FERA3 Xe Plasma FIB. The cross section could then be immediately examined using the SEM column built into either FIB. The trench could be extended and undercut to allow removal of a lamella, which could be thinned and examined by STEM in either FIB. This lengthy process could only be applied to selected samples.
The first evaluation of the implanted layer exchange method was carried out using a conventional beamline Dan Fysik implanter. The beam current capabilities of this implanter reach their maximum at 30 keV. An Al film was deposited onto a 50 mm diameter Si wafer in a Nordiko 2000 RF Magnetron Sputtering system prior to a 28Si/30 keV/ 0⁰/6 × 1017 cm−2 implant that took around 24 h to complete. The anneal conditions, carried out in ambient atmosphere in a rapid thermal annealer, were chosen to be 400 °C for 3 h, guided by [6], in the hope that this would allow complete diffusion and subsequent epitaxial growth of the implanted 28Si to occur. A TEM lamella was made in house of the sample post anneal using a TESCAN FERA3 Xe Plasma FIB and bright field images and EDS measurements taken using an in-house Hitachi HD-2300A STEM. Maps of elemental locations were made by false colouring the pixels of the bright field image according to the EDS measurements.
Table 1 A summary of implant and anneal conditions for the Ge layer exchange experiments. The order in the table reflects the implant sequence, but not order of annealing and metrology. Implants were carried out at a current of 11 pA, except Sample 5 which used 3.7 pA. Sample
Implant Time (min) [Fluence (1017 ions cm−2)]
Anneal Temp. (°C)/ Time (h)
Post Anneal Process and Metrology
1
6, 10, 13 [1, 1.6, 2.2] 6, 10, 13 [1, 1.6, 2.2] 12, 20, 26 [2, 3.3, 4.3] 12, 20, 26 [2, 3.3, 4.3] 9, 12, 16, 22, 24, 27, 33 [0.5, 0.6, 0.8, 1.1, 1.2, 1.4, 1.7]
250/12
X-SEM (Ge), STEM (Ge), IBA (No Ge) Al removal SEM EDX (No Ge) X-SEM (No Ge), STEM (Ge), TEM/EDX (Ge) X-SEM (No Ge)
2.2. Ge implants using a SIMPLE implanter The University of Surrey now hosts two advanced implanters that are being developed with the aim of reproducibly and deterministically placing single ions to form qubits. The SIMPLE (Single Ion Multispecies Positioning at Low Energy) implanters are essentially focused ion beam (FIB) tools optimized to deliver low intensity pulsed beams (in which there is an average of less than one ion per pulse) and very efficiently detect secondary electrons emitted when an implant event occurs. The first SIMPLE tool contains liquid metal (LMIS) or liquid metal alloy (LMAIS) ion sources for species such as P and Bi aimed at Si based qubits, whilst the second tool contains a gas source to allow N to be
2 3 4 5
31
400/12 250/1 400/1 250/12
Al removal SEM (Ge), SEM EDX (Ge)
Nuclear Inst. and Methods in Physics Research B 461 (2019) 30–36
J. England, et al.
Fig. 1. a) Bright Field TEM image of Al layer on Si after implant layer exchange. False colour EDS elemental maps of the same region show in which pixels elements have been detected: b) green for aluminium, c) red for oxygen and d) blue for silicon. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Neither FIB had EDS capability for elemental identification and so elemental identification in these images was inferred from image grey scales. Top down SEM images of samples could also be taken using and FEI Quanta ESEM which had an EDX capability, so that the presence of Ge could be tested. A lamella (from sample 3 annealed at 250 °C for 1 h in Table 1) was sent to EurofinsEAG for commercial TEM EDS (as the inhouse STEM was unavailable) where images were collected using a FEI Tecnai TF-20 FEG/TEM operated at 200 kV and EDS spectra were acquired using an Oxford INCA, Bruker Quantax EDS system. 3. Results 3.1. Si implants The TEM images (Fig. 1) confirmed that in the image the dark regions in the light Al layer were indeed primarily composed of Si. The 28 Si had totally diffused from the implanted region and had exchanged with the Al to form polycrystals below a surface Al layer. The Si layer may have formed a more continuous layer if the deposited metal thickness, which was measured to be 450 nm, had been better matched to the implant range. The guideline for efficient layer exchange of equal starting Si and Al layer thicknesses [6] suggests an Al thickness closer to 100 nm would have been more optimal. The crystals did not appear to have grown epitaxially, perhaps because the template of the substrate wafer had been covered by the native oxide that had not been stripped from the wafer prior to the Al deposition.
Fig. 2. Comparison of TRIDYN predictions for 28Si (solid lines) and 74Ge implants (broken lines) into Al once self-sputtering has limited the maximum concentrations of the implanted species. This occurred after 6 × 1017 ions cm−2 for the Si and 1 × 1017 ions cm−2 for the Ge implant.
significantly. The Ge profile did not change after a fluence of 1 × 1017 ions cm−2 (that shown in the figure) with a maximum Ge fraction at the surface of only 0.4, which quickly decayed away over 40 nm. Fig. 3 shows a set of Ge implants carried out over a range of fluences (Sample 5 in Table 1), decreasing from left to right. The Wien Filter had been tuned to transmit 72Ge (the central row of spots in Fig. 3) but the wider than normal aperture had let through 70Ge (bottom row), (and tentatively) 73Ge (not visible) and 74Ge (top row). The second column from the left was aborted part way through and should be ignored. Although the abundance of these isotopes in naturally occurring Ge is 20.7%, 27.5%, 7.7% and 36.4% respectively, the intensity in the
3.2. Ge implants Fig. 2 compares the characteristics of 28Si and 74Ge implants predicted using TRIDYN. These suggest that Al had not been sputtered significantly by the Si. At 30 keV net sputtering almost exactly balanced the number of incoming ions. The Si profile reached steady state after a fluence of ~6 × 1017 ions cm−2 (the fluence shown in the figure) with an atomic fraction of Si in the Al saturated at ~0.85 over a depth of 50 nm. In contrast, the heavier Ge did sputter away the Al layer 32
Nuclear Inst. and Methods in Physics Research B 461 (2019) 30–36
J. England, et al.
cross section SEM image (Fig. 5b) indeed showed dark regions, interpreted to be Z contrast from the expected Ge precipitates. In the meantime, the 250 °C/1 h sample was imaged by STEM in the Xe FIB. The image in the region of the implanted layer (Fig. 5c) clearly shows areas with large differences in contrast. The dark areas were assumed to be Ge (high Z leading to low e-beam transmission), and the light regions, Al (low Z leading to high transmission). The less extreme contrasting regions in the non-implanted region (Fig. 5d) were interpreted to be related to changes in transmission related to different orientations of deposited Al crystals. This interpretation of diffraction contrast [10], rather than Z contrast, was confirmed by TEM EDS measurements which failed to find Ge in the non-implanted regions. The thermal budget was also increased by raising the anneal temperature for two cases (Samples 2 and 4 in Table 1), neither of which showed any sign of retained Ge. Sample 4 annealed at 400 °C/1 h showed no dark regions in a cross section SEM image. Sample 2 annealed at 400 °C/12 h was imaged after the Al strip (Fig. 6). SEM EDS confirmed that there was no Ge left on this sample. Taken together this suggests that epitaxial Ge crystal formation did not occur at this high temperature so that the implanted Ge was removed with the Al rather than being retained on the substrate. Alarmingly, pits were seen in several implanted regions, but intriguingly not where the highest implant dose was expected (72Ge region in the centre of the image). Pit formation has been observed in Al annealed on Si (1 1 1) wafers [11]. It is apparent that low temperatures (i.e. relative to the eutectic point) should be chosen to allow good crystallisation and avoid substrate dissolution. These results suggested that the 250 °C/12 h anneal was the closest we had tried to an optimum thermal budget and so a final sample was implanted with a range of fluences and annealed under these conditions (sample 5 in Table 1). A top down SEM image (not shown) showed roughness under the regions implanted with 72Ge and a subsequent Al strip showed these regions to be covered with “white crystallites” (assumed to be Ge) whose densities increased with fluence (Fig. 7). Top down SEM EDS spot measurements confirmed the presence of Ge before the Al strip (not shown) and afterwards (inset in Fig. 7). Evidence of pitting (minor compared to the post 400 °C anneal case) was also observed. Higher resolution top down SEM images of the Ge crystal regions in Fig. 7 were converted to black/white pixels using ImageJ software and the areal coverage estimated from the ratio of number of white (“Ge”) to black (“substrate”) pixels in a 6 µm by 6 µm square around each implant region. The trend of coverage, plotted in Fig. 4, suggests that full coverage would have occurred for an implant fluence of 2.7 × 1017 ions cm−2. The TRIDYN model prediction that the atomic fraction of Ge in the Al saturated above 1 × 1017 ions cm−2 suggests that sputtering of Al layer by the implant to optimise the film thickness was important. Full Ge coverage could presumable have been achieved at a fluence 1 × 1017 ions cm−2 with the correct choice of initial Al film thickness (perhaps 60 nm after implant by considering Fig. 4). The composition of the white “crystals” being Ge had not been definitively shown and so a cross section was made from sample 3 and sent for TEM/EDS imaging. Fig. 8a) shows cross sectional TEM images of regions around a dark region similar to that shown in Fig. 5b. The EDS maps (Fig. 8b–e) confirmed that these regions were Ge rich. The nanobeam diffraction patterns Fig. 8f) and g) showed that the Ge rich regions were poly-crystalline and did have some components aligned to the substrate Fig. 8i). The EDS maps suggested the presence of oxide at the Si substrate/Al layer interface; perhaps reduction in this surface oxide would have improved the crystal quality and alignment. The diffraction patterns from the Si substrate (not shown) and interface Fig. 8i) were identical. Knowing the Si lattice constant (5.431 Å) allowed the camera length for the diffraction measurements to be determined. The lattice constant extracted from the diffraction pattern for the dense Ge region f) was ~5.6 Å the value expected for Ge (5.658 Å) and SixGe1-x, which has values between Ge and Si depending on
Fig. 3. Regions implanted over a sequence of fluences (decreasing left to right) and subsequently imaged in the SIMPLE tool using a low intensity primary beam and secondary electron detectors. Each column above contains three spots: the top spot was attributed to 74Ge, the middle to 72Ge and the bottom spot to 70Ge.
Fig. 4. Comparison of Al sputter depths measured for the sample shown in Fig. 3 to those predicted by TRIDYN. Also shown is the areal coverage of Ge crystals (□) measured after the Al was stripped (see Fig. 7). The solid line is a guide for the eye only.
respective beam spots may have been different due to varying beam transmission away from the central path through the Wien Filter. Fig. 4 compares the predicted Al sputter depths against those measured by AFM (NT-Solver) for Sample 5 in Table 1. The measured sputter yields were around a factor of 2 lower than the model which may be partly accounted for because the beam intensity, measured before the Wien Filter, included all the isotopes, but only a fraction of this was implanted into the 72Ge spot. Fig. 5a shows a X-SEM image of a sample 3 in Table 1 after a 250 °C/ 1 h anneal. The section seemed to have been made correctly across the location of the implant because a thinning of the Al layer can be seen but there is no evidence of Ge precipitates which would be expected to appear dark against the lighter Al layer. This suggested that a higher thermal budget was required to progress the layer exchange. A second sample was annealed at the same temperature (250 °C) but for 12 h. A 33
Nuclear Inst. and Methods in Physics Research B 461 (2019) 30–36
J. England, et al.
Fig. 5. Cross sectional SEM images of samples after 250 °C anneals for a) 1 h and b) 12 h. c) and d) are STEM images of the implanted and non-implanted regions of the sample shown in a). In all images, the bottom layer is the Si substrate, the middle layer is Al and the top layer Pt deposited during FIB sectioning.
4. Conclusions This work has demonstrated implanted layer exchange for Si and Ge. A continuous epitaxial mono-crystalline layer has yet to be formed but results suggest that a continuous layer might be achieved for Ge/ 25 keV implants at a fluence of 1 × 1017 cm−2 with an initial Al layer thickness of 60 nm. Some removal of the oxide layer which had blocked epitaxial growth in the Si case had improved the alignment of Ge growth to the substrate, but further reduction in the oxide layer at the substrate/Al interface would be required to improve epitaxial growth onto the substrate and allow mono-crystalline growth. Crystallisation occurred at annealing temperatures below the GeAl eutectic temperature. It was shown that high anneal temperatures should be avoided to minimise pitting caused by dissolution of the Si substrate. Surprisingly, atomic diffusion between the substrate and Ge layer exchange crystals was observed. Further study is required to see if lower annealing temperatures can eliminate this diffusion, otherwise pure Ge layers may not be formed and the highest isotopic enrichment achievable for 28Si layers could be compromised. It is recognized that the as-formed layers will contain some Al, governed by its solubility limit at the annealing temperature; 40 ppm Al has been measured after deposition-only layer exchange [6]. The perturbing effect of an Al contaminant atom in a quantum device (related to its electronic spin) will be ~2000 times stronger than that of a 29Si atom (related to its nuclear spin) [1]. It is hoped that chemical clean-up of the layer using thermal gettering processes, such as those used in the solar industry [12], or other approaches, including gettering implants [13], will successfully reduce chemical contamination to acceptable levels. Crystal defects present in the as-formed 28Si layer could be removed by amorphising the layer with a second implant of 28Si (or unretained species such as an inert gas) followed by a solid phase epitaxy regrowth anneal. These early experiments show that there is much investigation required to optimise the deposition, implant and annealing conditions for the implanted layer exchange process and to determine the highest isotopic enrichment and lowest defect and metal contamination levels possible.
Fig. 6. A top down SEM image of Sample 2 after annealing at 400 °C for 12 h and then removing the Al. The three columns are for total Ge implant fluences of 1, 1.6 and 2.2 x1017 ions cm−2. The vertical lines were milled by the FIB before Al removal to aid later location of the implanted regions. The largest of the “pits” in the top (74Ge) row was over 1 μm in diameter.
composition. The intermixing of the Ge into the substrate and Si into the crystals above the substrate were unanticipated observations. Further TEM EDS measurements which were not within in the scope of this study will be required to quantify these effects. Indeed, in regions where Ge was observed but crystals had not formed, Ge was observed at the surface of the Al and in the Si substrate. In retrospect, the dark regions observed below the implanted Al layer in the STEM images (Fig. 5c) had indicated Ge rich regions.
34
Nuclear Inst. and Methods in Physics Research B 461 (2019) 30–36
J. England, et al.
Fig. 7. An SEM image of the same region of Fig. 3 after annealing and Al strip revealing the “white” Ge crystals where 72Ge was implanted. Several square artefacts caused by C deposition during earlier SEM imaging are visible, particularly on the spot of highest Ge density. The inset shows a top down SEM EDS spectrum from a region within the white box. Two EDS peaks show the presence of Ge.
Fig. 8. a) HAADF TEM image of a cross section from an implanted region of Sample 3 in Table 1. EDS maps are shown for b) germanium, c) aluminium, d) oxygen and e) silicon which reveal the “white” crystals to be composed of Si and Ge. The open circles on the image show the locations of the diffraction pattern measurements f) left, g) middle, h) top right and i) bottom right. 35
Nuclear Inst. and Methods in Physics Research B 461 (2019) 30–36
J. England, et al.
Acknowledgments
References
APF was participating in a Nuffield Summer Student Internship. JE and BM were supported through Surrey Ion Beam Centre which is funded by United Kingdom Engineering and Physical Sciences Research Council (UK EPSRC) and industrial revenue. The SIMPLE tools were funded by UK EPSRC through grant EP/N015215/1. DC was funded through Surrey University Advanced Technology Institute and National Physical Laboratory. NC was an Eng.D. student with IonOptika and Surrey University funded by UK EPSRC and Ion Optika. The authors would like to thank Luke Antwis, Alex Royle and Adrian Cansell at the Surrey Ion Beam Centre for carrying out the Al deposition, annealing and beamline implants for the Si experiment, Vlad Stolojan (Advanced Technology Institute) for TEM imaging of the Si sample and Sukanta Biswas and Yifei Meng of Eurofins EAG for TEM measurements of the Ge sample.
[1] B. Kane, Nature 393 (1998) 133. [2] K.M. Itoh, H. Watanabe, MRS Commun. 4 (2014) 143. [3] N. Hendrickx, D. Franke, A. Sammak, M. Kouwenhoven, D. Sabbagh, L. Yeoh, R. Li, M.L. Tagliaferri, M. Virgilio, G. Capellini, G. Scappucci, M. Veldhorst, Nature Commun. 9 (2018) 2835. [4] A.V. Inyushkin, A.N. Taldenkov, J.W. Ager III, E.E. Haller, H. Riemann, N.V. Abrosimov, H.J. Pohl, P. Becker, J. Appl. Phys. 123 (2018) 095112. [5] K.J. Dwyer, H.S. Kim, D.S. Simons, J.M. Pomeroy, Phys. Rev. Mater. 1 (2017) 064603. [6] O. Nast, The Aluminium-induced Layer Exchange Forming Polycrystalline Silicon on Glass for Thin-film Solar Cells, Ph.D. thesis University of Marburg, 2000. [7] M.A. Green, P.A. Basore, N. Chang, D. Clugston, R. Egan, R. Evans, D. Hogg, S. Jarnason, M. Keevers, P. Lasswell, J.O. Sullivan, U. Schubert, A. Turner, S.R. Wenham, T. Young, Solar Energy 77 (2004) 857. [8] G. Majni, G. Ottaviani, Appl. Phys. Lett. 31 (1977) 125. [9] S. Hu, A.F. Marshall, P.C. McIntyre, Appl. Phys. Lett. 97 (2010) 082104. [10] B. Fultz, J. Howe, Transmission Electron Microscopy and Diffractometry of Materials, Springer, Berlin, Heidelberg, 2008, p. 337. [11] N. Fujimura, H. Kurosaki, T. Ito, Y. Nakayama, J. Appl. Phys. 64 (1988) 4499. [12] A. Istratov, H. Hieslmair, E.R. Weber, Appl. Phys. A 70 (2000) 489. [13] “Ion Implantation – Science and Technology”, edited J. F. Ziegler, Ion Implantation Technology Company (1996) 65.
36
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Investigating the formation of isotopically pure layers for quantum computers using ion implantation and layer exchange
T
Jonathan Englanda, , David Coxa, Nathan Cassidya, Bobur Mirkhaydarova, Andres Perez-Fadonb ⁎
a b
Ion Beam Centre, Advanced Technology Institute, University of Surrey, Guildford GU2 7XH, UK Kings College London Mathematics School, 80 Kennington Road, London SE11 6NJ, UK
ARTICLE INFO
ABSTRACT
Keywords: Ion-implantation Isotopically pure layers Layer exchange Quantum computing
Quantum computers have been proposed that exploit entangled quantum states between atoms that are isolated from environmental perturbations in a “semiconductor vacuum” which can be formed by cryogenically cooling an isotopically pure, defect free crystalline layer consisting of Si, or Ge. In a preliminary investigation of an implant and deposition layer exchange technique to produce such “vacuums”, a layer of aluminium was implanted with 28Si using a conventional implanter. After annealing and cross sectioning, layer exchange was observed to have produced multiple isolated crystals in a cross sectional TEM image. Further deposited Al layers were implanted with Ge using a SIMPLE (Single Ion Multispecies Positioning at Low Energy) implanter over a range of fluences. After anneals at 250 °C and Al removal, crystals of Ge (which also contained Si) were seen at areal densities that increased with implant fluence.
1. Introduction In 1998 Bruce Kane [1] proposed that quantum computation could be achieved using logic gates based on qubits that exploit hyperfine interactions between nuclei with spin, such as 31P or 209Bi, and the electrons in the lattice in which they are embedded. For such schemes to work, the qubits must be isolated from the environment (to avoid uncontrolled external disruption to delicate quantum states) and identical (to allow all the qubits to be controlled by the same NMR frequency) [2]. The spinning nuclei can be isolated from environmental perturbations by incorporating them in a “semiconductor vacuum” which can be formed by cryogenically cooling (to a few mK) an isotopically pure, defect free crystalline layer consisting of atoms that have no intrinsic electronic or nuclear spin. Layers composed of pure 28Si [2], or 74Ge [3], meet this last requirement and would be particularly useful as they could conceivably be straightforwardly integrated into conventional CMOS devices for the industrial manufacture of quantum computers systems. Naturally occurring Si is predominantly (92.2%) 28 Si, but significantly contains 4.7% 29Si atoms which can cause disruption through their nuclear spins. Although the remaining 3.1% of atoms are 30Si, which have no spin, they must be avoided because variations in lattice bond lengths (which are isotope dependent) upset the criterion that qubit environments are identical. As well as applications in quantum technologies, isotopically pure materials have been
⁎
investigated for the superior heat transfer properties of their superuniform lattices [4]. Quantum technologies have up to now often sourced isotopically pure Si from stocks prepared for standards [2]. Large quantities of isotopically pure 28Si were prepared for the Avogadro Project (production of pure 28Si spheres for the kg standard) by enriching SiF4 gas in centrifuges and using the gas to produce solid crystals by float zone techniques, or by CVD deposition [4]. Another production approach has used specially developed electromagnetic isotopic enrichment equipment [5]. Industrial ion implanters have their origins in calutrons isotope separators and so it might be thought that an ion implanter could be used as an isotope separator to selectively deposit low energy 28Si onto a natural Si substrate. An unpublished study carried out at the University of Surrey by L. Antwis, R. Webb, C. Jeynes and R. Gwilliam presented at the Ion Beam Modifications of Materials Conference 2016 investigated implants carried out in a conventional Dan Fysik implanter at 2 keV using a deceleration lens to increase available beam current (and hence throughput). A subsequent RBS analysis showed that the resultant 28Si layer was heavily oxidized in the non-UHV implanter endstation, contained beam transported contaminants such as 14N2+, and limited in isotopic enrichment through intermixing with the Si substrate. A higher energy 20 keV implant was also investigated but RBS measurements and TRIDYN calculations showed that 28Si fraction was limited to 0.95 after 2 × 1017 ions cm−2 by self-sputtering of the Si
Corresponding author. E-mail address: [email protected] (J. England).
https://doi.org/10.1016/j.nimb.2019.09.013 Received 30 May 2019; Received in revised form 7 September 2019; Accepted 9 September 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 461 (2019) 30–36
J. England, et al.
(~1.3 atoms ion−1) at these increased energies. This paper describes our first investigations of an implant layer exchange technique that could overcome the self-sputtering and oxidation limitations seen in the implant-only approach. Layer exchange [6] has been used commercially to deposit layers of poly-crystalline Si onto glass substrates for low cost solar cells [7]. The standard deposition-based process had the following steps:
implanted for systems such as N-V qubits in diamond. Wien filters in the columns allow mass selected implants. The SIMPLE end-stations are held under ultrahigh vacuum; oxidation of the sample during implantation is not a major concern. We took the opportunity to further investigate implanted layer exchange by carrying out a study using the first SIMPLE tool whilst it was being commissioned. The column was being aligned using Au ions from a AuGe LMAIS. We utilised Ge ions from this LMAIS to implant sections of Si wafer that had been stripped of their native oxide using hydrofluoric acid before depositing 100 nm of Al using a Kurt J. Lesker high vacuum evaporator. Ge was expected to behave similarly to Si and had the advantage that the implanted Ge could be easily differentiated from substrate Si during post-process analyses. The SIMPLE tool was used in a continuous (rather than pulsed) mode to implant 5 μm × 5 μm square regions with Ge beams to fluences in the 1017 ions cm−2 range within a few minutes. Because the Wien filter had a larger than normal mass selection aperture after the Wien filter, the implanted ions were not mono isotopic in these studies. A normal sized mass selection aperture would have allowed the implanted beam to be mono-isotopic. This approach allowed experiments over variable fluences to be carried out easily and did not compromise the post process SEM, TEM and AFM based analyses which all had small fields of view. All implants were carried out normal to the substrate at 25 keV, the maximum energy possible. The various experiments are summarised in Table 1.
1. Deposit Al onto glass; 2. Deposit (amorphous) Si onto the Al; 3. Heat at ~500 °C for around 1 h. During the annealing step, which was carried out below the eutectic temperature of the Si Al alloy, the amorphous Si dissolved into the Al and then diffused until it found a site where it formed a new crystal (heterogeneous nucleation was found to occur particularly on Al grain boundaries) or an earlier formed Si crystal upon which it could grow. The process was driven because crystalline Si has a lower Gibbs free energy than amorphous Si. Si dissolution continued throughout the anneal and occurred more favourably around smaller radii features; after full completion of the process, Ostwald ripening produced a continuous layer of poly crystalline Si, and the original Al had been displaced above the Si layer. The best results appeared to occur when the two layers had a similar thickness and an oxide interface between deposited Al and Si was found to be important [6]. Continuous crystalline Si layers have been grown epitaxially when this process has been carried out on Si wafers [8] where nucleation occurs on the substrate. Other metals, such as Ag, Au and Ni [6], and crystallising atoms such as Ge [9] have also been explored. This paper proposes replacing the Si deposition step with implants of isotopically pure 28Si (or 74Ge). If ultimately successful, this approach could enable the manufacture of qubit substrates using standard process equipment within a CMOS fabrication plant.
2.3. Post implant processing Post implant annealing was carried out in a Carbolite TZF 12/100 tube furnace in ambient atmosphere. Temperatures were chosen guided by previous work [9] which were all below the Ge Al eutectic temperature of 420 °C. For samples 2 and 5 in Table 1, Al was stripped by immersion in tetra methyl ammonium hydroxide (TMAH) for a few minutes.
2. Experimental method
2.4. Metrology
2.1. Si implants using a conventional implanter
Metrology of the samples usually involved cross section views of the samples post anneal. Single sided cross sections were made after protecting the sample surface by depositing Pt or C surface layers and then sputtering a trench using either Ga in a FEI Nova Nanolab 600 Dual Beam FIB or Xe using TESCAN FERA3 Xe Plasma FIB. The cross section could then be immediately examined using the SEM column built into either FIB. The trench could be extended and undercut to allow removal of a lamella, which could be thinned and examined by STEM in either FIB. This lengthy process could only be applied to selected samples.
The first evaluation of the implanted layer exchange method was carried out using a conventional beamline Dan Fysik implanter. The beam current capabilities of this implanter reach their maximum at 30 keV. An Al film was deposited onto a 50 mm diameter Si wafer in a Nordiko 2000 RF Magnetron Sputtering system prior to a 28Si/30 keV/ 0⁰/6 × 1017 cm−2 implant that took around 24 h to complete. The anneal conditions, carried out in ambient atmosphere in a rapid thermal annealer, were chosen to be 400 °C for 3 h, guided by [6], in the hope that this would allow complete diffusion and subsequent epitaxial growth of the implanted 28Si to occur. A TEM lamella was made in house of the sample post anneal using a TESCAN FERA3 Xe Plasma FIB and bright field images and EDS measurements taken using an in-house Hitachi HD-2300A STEM. Maps of elemental locations were made by false colouring the pixels of the bright field image according to the EDS measurements.
Table 1 A summary of implant and anneal conditions for the Ge layer exchange experiments. The order in the table reflects the implant sequence, but not order of annealing and metrology. Implants were carried out at a current of 11 pA, except Sample 5 which used 3.7 pA. Sample
Implant Time (min) [Fluence (1017 ions cm−2)]
Anneal Temp. (°C)/ Time (h)
Post Anneal Process and Metrology
1
6, 10, 13 [1, 1.6, 2.2] 6, 10, 13 [1, 1.6, 2.2] 12, 20, 26 [2, 3.3, 4.3] 12, 20, 26 [2, 3.3, 4.3] 9, 12, 16, 22, 24, 27, 33 [0.5, 0.6, 0.8, 1.1, 1.2, 1.4, 1.7]
250/12
X-SEM (Ge), STEM (Ge), IBA (No Ge) Al removal SEM EDX (No Ge) X-SEM (No Ge), STEM (Ge), TEM/EDX (Ge) X-SEM (No Ge)
2.2. Ge implants using a SIMPLE implanter The University of Surrey now hosts two advanced implanters that are being developed with the aim of reproducibly and deterministically placing single ions to form qubits. The SIMPLE (Single Ion Multispecies Positioning at Low Energy) implanters are essentially focused ion beam (FIB) tools optimized to deliver low intensity pulsed beams (in which there is an average of less than one ion per pulse) and very efficiently detect secondary electrons emitted when an implant event occurs. The first SIMPLE tool contains liquid metal (LMIS) or liquid metal alloy (LMAIS) ion sources for species such as P and Bi aimed at Si based qubits, whilst the second tool contains a gas source to allow N to be
2 3 4 5
31
400/12 250/1 400/1 250/12
Al removal SEM (Ge), SEM EDX (Ge)
Nuclear Inst. and Methods in Physics Research B 461 (2019) 30–36
J. England, et al.
Fig. 1. a) Bright Field TEM image of Al layer on Si after implant layer exchange. False colour EDS elemental maps of the same region show in which pixels elements have been detected: b) green for aluminium, c) red for oxygen and d) blue for silicon. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Neither FIB had EDS capability for elemental identification and so elemental identification in these images was inferred from image grey scales. Top down SEM images of samples could also be taken using and FEI Quanta ESEM which had an EDX capability, so that the presence of Ge could be tested. A lamella (from sample 3 annealed at 250 °C for 1 h in Table 1) was sent to EurofinsEAG for commercial TEM EDS (as the inhouse STEM was unavailable) where images were collected using a FEI Tecnai TF-20 FEG/TEM operated at 200 kV and EDS spectra were acquired using an Oxford INCA, Bruker Quantax EDS system. 3. Results 3.1. Si implants The TEM images (Fig. 1) confirmed that in the image the dark regions in the light Al layer were indeed primarily composed of Si. The 28 Si had totally diffused from the implanted region and had exchanged with the Al to form polycrystals below a surface Al layer. The Si layer may have formed a more continuous layer if the deposited metal thickness, which was measured to be 450 nm, had been better matched to the implant range. The guideline for efficient layer exchange of equal starting Si and Al layer thicknesses [6] suggests an Al thickness closer to 100 nm would have been more optimal. The crystals did not appear to have grown epitaxially, perhaps because the template of the substrate wafer had been covered by the native oxide that had not been stripped from the wafer prior to the Al deposition.
Fig. 2. Comparison of TRIDYN predictions for 28Si (solid lines) and 74Ge implants (broken lines) into Al once self-sputtering has limited the maximum concentrations of the implanted species. This occurred after 6 × 1017 ions cm−2 for the Si and 1 × 1017 ions cm−2 for the Ge implant.
significantly. The Ge profile did not change after a fluence of 1 × 1017 ions cm−2 (that shown in the figure) with a maximum Ge fraction at the surface of only 0.4, which quickly decayed away over 40 nm. Fig. 3 shows a set of Ge implants carried out over a range of fluences (Sample 5 in Table 1), decreasing from left to right. The Wien Filter had been tuned to transmit 72Ge (the central row of spots in Fig. 3) but the wider than normal aperture had let through 70Ge (bottom row), (and tentatively) 73Ge (not visible) and 74Ge (top row). The second column from the left was aborted part way through and should be ignored. Although the abundance of these isotopes in naturally occurring Ge is 20.7%, 27.5%, 7.7% and 36.4% respectively, the intensity in the
3.2. Ge implants Fig. 2 compares the characteristics of 28Si and 74Ge implants predicted using TRIDYN. These suggest that Al had not been sputtered significantly by the Si. At 30 keV net sputtering almost exactly balanced the number of incoming ions. The Si profile reached steady state after a fluence of ~6 × 1017 ions cm−2 (the fluence shown in the figure) with an atomic fraction of Si in the Al saturated at ~0.85 over a depth of 50 nm. In contrast, the heavier Ge did sputter away the Al layer 32
Nuclear Inst. and Methods in Physics Research B 461 (2019) 30–36
J. England, et al.
cross section SEM image (Fig. 5b) indeed showed dark regions, interpreted to be Z contrast from the expected Ge precipitates. In the meantime, the 250 °C/1 h sample was imaged by STEM in the Xe FIB. The image in the region of the implanted layer (Fig. 5c) clearly shows areas with large differences in contrast. The dark areas were assumed to be Ge (high Z leading to low e-beam transmission), and the light regions, Al (low Z leading to high transmission). The less extreme contrasting regions in the non-implanted region (Fig. 5d) were interpreted to be related to changes in transmission related to different orientations of deposited Al crystals. This interpretation of diffraction contrast [10], rather than Z contrast, was confirmed by TEM EDS measurements which failed to find Ge in the non-implanted regions. The thermal budget was also increased by raising the anneal temperature for two cases (Samples 2 and 4 in Table 1), neither of which showed any sign of retained Ge. Sample 4 annealed at 400 °C/1 h showed no dark regions in a cross section SEM image. Sample 2 annealed at 400 °C/12 h was imaged after the Al strip (Fig. 6). SEM EDS confirmed that there was no Ge left on this sample. Taken together this suggests that epitaxial Ge crystal formation did not occur at this high temperature so that the implanted Ge was removed with the Al rather than being retained on the substrate. Alarmingly, pits were seen in several implanted regions, but intriguingly not where the highest implant dose was expected (72Ge region in the centre of the image). Pit formation has been observed in Al annealed on Si (1 1 1) wafers [11]. It is apparent that low temperatures (i.e. relative to the eutectic point) should be chosen to allow good crystallisation and avoid substrate dissolution. These results suggested that the 250 °C/12 h anneal was the closest we had tried to an optimum thermal budget and so a final sample was implanted with a range of fluences and annealed under these conditions (sample 5 in Table 1). A top down SEM image (not shown) showed roughness under the regions implanted with 72Ge and a subsequent Al strip showed these regions to be covered with “white crystallites” (assumed to be Ge) whose densities increased with fluence (Fig. 7). Top down SEM EDS spot measurements confirmed the presence of Ge before the Al strip (not shown) and afterwards (inset in Fig. 7). Evidence of pitting (minor compared to the post 400 °C anneal case) was also observed. Higher resolution top down SEM images of the Ge crystal regions in Fig. 7 were converted to black/white pixels using ImageJ software and the areal coverage estimated from the ratio of number of white (“Ge”) to black (“substrate”) pixels in a 6 µm by 6 µm square around each implant region. The trend of coverage, plotted in Fig. 4, suggests that full coverage would have occurred for an implant fluence of 2.7 × 1017 ions cm−2. The TRIDYN model prediction that the atomic fraction of Ge in the Al saturated above 1 × 1017 ions cm−2 suggests that sputtering of Al layer by the implant to optimise the film thickness was important. Full Ge coverage could presumable have been achieved at a fluence 1 × 1017 ions cm−2 with the correct choice of initial Al film thickness (perhaps 60 nm after implant by considering Fig. 4). The composition of the white “crystals” being Ge had not been definitively shown and so a cross section was made from sample 3 and sent for TEM/EDS imaging. Fig. 8a) shows cross sectional TEM images of regions around a dark region similar to that shown in Fig. 5b. The EDS maps (Fig. 8b–e) confirmed that these regions were Ge rich. The nanobeam diffraction patterns Fig. 8f) and g) showed that the Ge rich regions were poly-crystalline and did have some components aligned to the substrate Fig. 8i). The EDS maps suggested the presence of oxide at the Si substrate/Al layer interface; perhaps reduction in this surface oxide would have improved the crystal quality and alignment. The diffraction patterns from the Si substrate (not shown) and interface Fig. 8i) were identical. Knowing the Si lattice constant (5.431 Å) allowed the camera length for the diffraction measurements to be determined. The lattice constant extracted from the diffraction pattern for the dense Ge region f) was ~5.6 Å the value expected for Ge (5.658 Å) and SixGe1-x, which has values between Ge and Si depending on
Fig. 3. Regions implanted over a sequence of fluences (decreasing left to right) and subsequently imaged in the SIMPLE tool using a low intensity primary beam and secondary electron detectors. Each column above contains three spots: the top spot was attributed to 74Ge, the middle to 72Ge and the bottom spot to 70Ge.
Fig. 4. Comparison of Al sputter depths measured for the sample shown in Fig. 3 to those predicted by TRIDYN. Also shown is the areal coverage of Ge crystals (□) measured after the Al was stripped (see Fig. 7). The solid line is a guide for the eye only.
respective beam spots may have been different due to varying beam transmission away from the central path through the Wien Filter. Fig. 4 compares the predicted Al sputter depths against those measured by AFM (NT-Solver) for Sample 5 in Table 1. The measured sputter yields were around a factor of 2 lower than the model which may be partly accounted for because the beam intensity, measured before the Wien Filter, included all the isotopes, but only a fraction of this was implanted into the 72Ge spot. Fig. 5a shows a X-SEM image of a sample 3 in Table 1 after a 250 °C/ 1 h anneal. The section seemed to have been made correctly across the location of the implant because a thinning of the Al layer can be seen but there is no evidence of Ge precipitates which would be expected to appear dark against the lighter Al layer. This suggested that a higher thermal budget was required to progress the layer exchange. A second sample was annealed at the same temperature (250 °C) but for 12 h. A 33
Nuclear Inst. and Methods in Physics Research B 461 (2019) 30–36
J. England, et al.
Fig. 5. Cross sectional SEM images of samples after 250 °C anneals for a) 1 h and b) 12 h. c) and d) are STEM images of the implanted and non-implanted regions of the sample shown in a). In all images, the bottom layer is the Si substrate, the middle layer is Al and the top layer Pt deposited during FIB sectioning.
4. Conclusions This work has demonstrated implanted layer exchange for Si and Ge. A continuous epitaxial mono-crystalline layer has yet to be formed but results suggest that a continuous layer might be achieved for Ge/ 25 keV implants at a fluence of 1 × 1017 cm−2 with an initial Al layer thickness of 60 nm. Some removal of the oxide layer which had blocked epitaxial growth in the Si case had improved the alignment of Ge growth to the substrate, but further reduction in the oxide layer at the substrate/Al interface would be required to improve epitaxial growth onto the substrate and allow mono-crystalline growth. Crystallisation occurred at annealing temperatures below the GeAl eutectic temperature. It was shown that high anneal temperatures should be avoided to minimise pitting caused by dissolution of the Si substrate. Surprisingly, atomic diffusion between the substrate and Ge layer exchange crystals was observed. Further study is required to see if lower annealing temperatures can eliminate this diffusion, otherwise pure Ge layers may not be formed and the highest isotopic enrichment achievable for 28Si layers could be compromised. It is recognized that the as-formed layers will contain some Al, governed by its solubility limit at the annealing temperature; 40 ppm Al has been measured after deposition-only layer exchange [6]. The perturbing effect of an Al contaminant atom in a quantum device (related to its electronic spin) will be ~2000 times stronger than that of a 29Si atom (related to its nuclear spin) [1]. It is hoped that chemical clean-up of the layer using thermal gettering processes, such as those used in the solar industry [12], or other approaches, including gettering implants [13], will successfully reduce chemical contamination to acceptable levels. Crystal defects present in the as-formed 28Si layer could be removed by amorphising the layer with a second implant of 28Si (or unretained species such as an inert gas) followed by a solid phase epitaxy regrowth anneal. These early experiments show that there is much investigation required to optimise the deposition, implant and annealing conditions for the implanted layer exchange process and to determine the highest isotopic enrichment and lowest defect and metal contamination levels possible.
Fig. 6. A top down SEM image of Sample 2 after annealing at 400 °C for 12 h and then removing the Al. The three columns are for total Ge implant fluences of 1, 1.6 and 2.2 x1017 ions cm−2. The vertical lines were milled by the FIB before Al removal to aid later location of the implanted regions. The largest of the “pits” in the top (74Ge) row was over 1 μm in diameter.
composition. The intermixing of the Ge into the substrate and Si into the crystals above the substrate were unanticipated observations. Further TEM EDS measurements which were not within in the scope of this study will be required to quantify these effects. Indeed, in regions where Ge was observed but crystals had not formed, Ge was observed at the surface of the Al and in the Si substrate. In retrospect, the dark regions observed below the implanted Al layer in the STEM images (Fig. 5c) had indicated Ge rich regions.
34
Nuclear Inst. and Methods in Physics Research B 461 (2019) 30–36
J. England, et al.
Fig. 7. An SEM image of the same region of Fig. 3 after annealing and Al strip revealing the “white” Ge crystals where 72Ge was implanted. Several square artefacts caused by C deposition during earlier SEM imaging are visible, particularly on the spot of highest Ge density. The inset shows a top down SEM EDS spectrum from a region within the white box. Two EDS peaks show the presence of Ge.
Fig. 8. a) HAADF TEM image of a cross section from an implanted region of Sample 3 in Table 1. EDS maps are shown for b) germanium, c) aluminium, d) oxygen and e) silicon which reveal the “white” crystals to be composed of Si and Ge. The open circles on the image show the locations of the diffraction pattern measurements f) left, g) middle, h) top right and i) bottom right. 35
Nuclear Inst. and Methods in Physics Research B 461 (2019) 30–36
J. England, et al.
Acknowledgments
References
APF was participating in a Nuffield Summer Student Internship. JE and BM were supported through Surrey Ion Beam Centre which is funded by United Kingdom Engineering and Physical Sciences Research Council (UK EPSRC) and industrial revenue. The SIMPLE tools were funded by UK EPSRC through grant EP/N015215/1. DC was funded through Surrey University Advanced Technology Institute and National Physical Laboratory. NC was an Eng.D. student with IonOptika and Surrey University funded by UK EPSRC and Ion Optika. The authors would like to thank Luke Antwis, Alex Royle and Adrian Cansell at the Surrey Ion Beam Centre for carrying out the Al deposition, annealing and beamline implants for the Si experiment, Vlad Stolojan (Advanced Technology Institute) for TEM imaging of the Si sample and Sukanta Biswas and Yifei Meng of Eurofins EAG for TEM measurements of the Ge sample.
[1] B. Kane, Nature 393 (1998) 133. [2] K.M. Itoh, H. Watanabe, MRS Commun. 4 (2014) 143. [3] N. Hendrickx, D. Franke, A. Sammak, M. Kouwenhoven, D. Sabbagh, L. Yeoh, R. Li, M.L. Tagliaferri, M. Virgilio, G. Capellini, G. Scappucci, M. Veldhorst, Nature Commun. 9 (2018) 2835. [4] A.V. Inyushkin, A.N. Taldenkov, J.W. Ager III, E.E. Haller, H. Riemann, N.V. Abrosimov, H.J. Pohl, P. Becker, J. Appl. Phys. 123 (2018) 095112. [5] K.J. Dwyer, H.S. Kim, D.S. Simons, J.M. Pomeroy, Phys. Rev. Mater. 1 (2017) 064603. [6] O. Nast, The Aluminium-induced Layer Exchange Forming Polycrystalline Silicon on Glass for Thin-film Solar Cells, Ph.D. thesis University of Marburg, 2000. [7] M.A. Green, P.A. Basore, N. Chang, D. Clugston, R. Egan, R. Evans, D. Hogg, S. Jarnason, M. Keevers, P. Lasswell, J.O. Sullivan, U. Schubert, A. Turner, S.R. Wenham, T. Young, Solar Energy 77 (2004) 857. [8] G. Majni, G. Ottaviani, Appl. Phys. Lett. 31 (1977) 125. [9] S. Hu, A.F. Marshall, P.C. McIntyre, Appl. Phys. Lett. 97 (2010) 082104. [10] B. Fultz, J. Howe, Transmission Electron Microscopy and Diffractometry of Materials, Springer, Berlin, Heidelberg, 2008, p. 337. [11] N. Fujimura, H. Kurosaki, T. Ito, Y. Nakayama, J. Appl. Phys. 64 (1988) 4499. [12] A. Istratov, H. Hieslmair, E.R. Weber, Appl. Phys. A 70 (2000) 489. [13] “Ion Implantation – Science and Technology”, edited J. F. Ziegler, Ion Implantation Technology Company (1996) 65.
36