Sn Incorporation in Ultra-Thin InAs Nanowires for Next-Generation

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Sn Incorporation in Ultra-Thin InAs Nanowires for NextGeneration Transistors Characterized by Atom Probe Tomography Alexander Devin Giddings, Peter Ramvall, Tim Vasen, Aryan Afzalian, Ruey-Lian Hwang, Dr. Yee-Chia Yeo, and Matthias Passlack ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02092 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 2, 2019

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ACS Applied Nano Materials

Sn Incorporation in Ultra-Thin InAs Nanowires for Next-Generation Transistors Characterized by Atom Probe Tomography A. Devin Giddings*†, Peter Ramvall‡, Tim Vasen‡, Aryan Afzalian‡, Ruey-Lian Hwang†, Yee-Chia Yeo†, Matthias Passlack‡

† Taiwan Semiconductor Manufacturing Company, Ltd., 166, Park Ave. II, Hsinchu Science Park, Hsinchu 30075, Taiwan

‡ TSMC R&D Europe B.V., Kapeldreef 75, 3001, Leuven, Belgium

KEYWORDS. ultra-thin nanowire, InAs, VLS, MOCVD, atom probe tomography, transmission electron microscopy

ABSTRACT. Growth of ultra-thin semiconducting nanowires (NWs) and incorporation of dopants suitable for future CMOS scaling targets (diameter < 20 nm) is a challenge. Limits

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on dopant incorporation in thin NWs has led to concerns about the suitability of these structures. In this work, the atomic structure of the thinnest InAs NWs ever reported, down to 7 nm diameter, is characterized using transmission electron microscopy (TEM) and atom probe tomography (APT). It is demonstrated that there is no fundamental limit of Sn incorporation into ultra-thin InAs NWs. Additionally, the Sn distribution of the Au-catalyst particle controlling the growth is characterized.

TEXT. Semiconductor nanowires (NWs) have the potential to become key building blocks in future generations of light-emitting diodes,[1] solar cells,[2] and batteries[3] as well ultimate-scale, post-CMOS transistor architectures.[4,5] The potential of NWs stems from the high degree of control that can be exerted over their growth, providing versatility necessary for complex bottom-up designs.[6] However, creating ultra-thin NWs which have dimensions compatible with future CMOS scaling targets, for example diameter < 20 nm,[7] is a significant challenge, particularly with respect to doping of electronic or magnetic impurities.[8–11] Limitations on dopant incorporation have raised concerns


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about the suitability of these structures.[12–13] Here we demonstrate the creation of ultrathin InAs NWs and utilize atom probe tomography (APT) to investigate the Sn concentration at atomic scale. No reduction of Sn incorporation with decreasing NW diameter was found, suggesting there is no fundamental limit of Sn-doping in ultra-thin InAs NWs. This result paves the way for use of NWs for further miniaturization of electronic devices. Controlling and measuring dopant incorporation efficiency, and the resulting carrier concentration, in ultra-thin NWs are very challenging. The resistivity of InAs NWs with diameters below 40 nm is strongly influenced by surface effects.[14] For NWs grown in vapor-liquid-solid (VLS)[15] mode there are different mechanisms by which dopant incorporation can occur: axially, through the catalyst particle, or radially, by growth on the side of the NW. The situation is complicated because the behavior of the dopant depends on both its chemical species and on the diameter of the NW.[16–17] Whilst the unique geometry of NWs furnishes them with desirable electrical properties, it also means that standard characterization techniques employed for semiconductor devices are inadequate. For example, the relatively large surface-to-volume ratio of ultra-


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thin NWs makes it difficult to use electrical methods based on capacitance measurement to accurately estimate the carrier concentration.[18] Similarly, the routine method for measuring dopant atom incorporation, secondary ion mass spectroscopy (SIMS), has a lateral resolution (~100 nm) i.e. not sufficient for monitoring NWs. A better spatial resolution can be obtained by energy dispersive X-ray spectroscopy (EDX) which can be applied to map the distribution of atomic species along a single NW. However, the EDX detection limit of about 0.1–1% is too high to detect relevant doping levels in electronic devices made from NWs. APT is an analytical characterization technique that enables the position and identity of atoms in small volumes to be determined with nanometer-scale resolution.[19–21] It has been previously demonstrated that APT is a suitable method to measure dopant distribution in semiconductor NWs at very high sensitivity levels.[22–27] In this work we extend this technique to the analysis of ultra-thin NWs. The ultra-thin InAs NWs were grown using Metal-Organic Chemical Vapour Deposition (MOCVD) at a temperature of 460–470°C at a V/III-ratio of 27–38. The Sn-dopant precursor molar fractions were varied between 0 and 3.7×10−5. The InAs crystal structure


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is pure wurtzite without stacking faults. As shown in Scheme 1(a), one of the ultra-thin NW types have a 3–4 nm thick radial ZrO2 layer deposited post-growth by atomic layer deposition (ALD). The primary purpose is to serve as a dielectric layer for a MOSFET gate stack, but it also stabilizes the ultra-thin InAs NWs during transmission electron microscopy (TEM) characterization. High-resolution TEM (HRTEM) images in Figure 1 show examples of the ultra-thin InAs NWs. To precisely control the resistivity of the NW and create sharp doping interfaces, a dopant is supplied during growth. In case of VLS growth, this means that the dopant must pass through the catalyst particle (in these experiments, an Au catalyst particle). The Au particle needs to be supersaturated with the dopant atoms before they will be expelled into the growing InAs NW. The concentration at which supersaturation occurs depends strongly on the types of dopant species and their respective solubility in Au and Au/In alloy. For example, S reaches supersaturation at low concentrations, so it can be incorporated into a NW, while it appears impossible to reach supersaturation with Si in Au making Si an unsuitable dopant for VLS grown NWs.[16]


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In Figure 2(a) a high-angle annular dark field (HAADF) image with EDX point quantification of an InAs NW grown from a 5 nm diameter Au colloid doped with Sn is shown. The Au particle on top of the 8-9 nm diameter InAs NW contains 29% Sn, about double the amount of In. Thus, the Sn incorporation to the Au particle does not seem to be significantly limited by the Gibbs-Thomson effect, whereby the surface energy of the droplet leads to an increasing chemical potential at small diameters, which would hinder the Sn from entering the Au in sufficient amount to provide the necessary supersaturation for the Sn to enter the growing InAs NW.[28] Since the detection limit of our EDX instrument is about 1%, equivalent to a doping density 3.5×1020 cm-3, it is not possible to detect any doping in the InAs NW itself, which is expected to be an order of magnitude lower. Thus, it is not possible to determine by EDX whether sufficient supersaturation of Sn in the Au catalyst is obtained for Sn dopant to be expelled into the InAs NW. To meet the requirements for APT analysis, InAs NWs were grown to a height of about 5 μm in a low-density pattern [22] consisting of squares with 4 NWs 10 μm apart as shown in Figure 2(b). The distance to neighboring groups of NWs is 130-300 m. In this design, the NWs comprised of two main segments; see Figure 1(b) provides a schematic of this


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design. The 400 nm topmost part is un-tapered with an ultra-thin ~12 nm diameter. The lower part is wider and tapered with an undoped core which is cladded by a radially grown shell with Sn doping. This geometry provided mechanical stability, allowing the NW to be transferred with a micromanipulator. It also improved thermal conductivity between the specimen apex and the instrument stage, necessary for good quality laser-assisted APT measurements.[29] The extension of topmost untapered section of the NW is an indication of the precursor diffusion length on the NW sides. Thus, a tapered base section starts to form as soon as the NWs grow longer than the precursor diffusion length. We believe that the characteristics of the untapered section is similar regardless of the length of the tapered section. Thus, the APT characterization of the untapered InAs NW section and Au particle presented here is representative for also for NWs with a total length shorter than 5 m. The result of the APT characterization is displayed in Figure 3. Spatial calibration of the reconstruction was aided by pre and post-field-evaporation scanning electron microscope (SEM) images of each individual NW, an example of which is shown in Figure 3(a). For compositional analysis the NW is divided into three parts, the un-tapered top, the tapered


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interface and core-shell lower part. Sn was detected in all three parts of the structure. A table giving the results from the three independent measurements is provided in Figure 3(b). Taking the average of the measurements, the topmost part was found to contain an average of 500 ± 20% Sn, corresponding to a doping density of 1.8×1019 cm-3. The standard deviation amongst measurements is consistent with uncertainty based on counting statistics and background considerations from a single measurement. Sn follows In in the periodic table, as shown in Figure 3(c), leading to a quantification issue whereby the minor Sn peaks in the mass-spectrum could be masked in the tail of the dominant In peak. Optimized measurement conditions of 0.5 pJ laser pulse energy (LPE) was chosen for low background noise and high sensitivity and also to have the In ions predominately evaporate in a single charge state. This would allow Sn to be detected from the unobscured Sn++ peaks. However, the Sn charge-state ratio (CSR; usually defined as ratio of ions evaporating in double and single charge states) is also affected by measurement conditions and, as a result, the proportion of Sn that can, or cannot, be detected will vary from measurement to measurement. Modifying the LPE to change the In CSR by a decade, between ~1×10-4 to ~1×10-5, would cause the apparent Sn


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concentration to be reduced in half. Thus, the In CSR could be used to check consistency of Sn composition within and between measurements, even if the true composition could not be determined. This is a significant contributor to spread in measured composition. The middle and lower parts of the NW display average Sn compositions of 505 ± 9% ppm and 534 ± 21% ppm (1.8×1019 cm-3 and 1.9×1019 cm-3), respectively; see Figure 3(b) for the values from the individual measurements. Figure 4 shows an APT cross section of the thicker base of the NW. Revealed are an undoped core with Sn doping below the estimated detection level of 80 ppm (3×1018 cm-3) and a doped shell with a Sn composition 750 ppm (3.0×1019 cm-3). This result is commensurate with the growth times of 5 min of undoped InAs (including nucleation) and 5 min of Sn-doped InAs including 1 min with modulated doping, as described in methods. The fact that the thin central core of the NW is undoped in the lower part, when the Sn doping was off, and doped in the upper part, when Sn doping was on, conclusively shows that it is possible to introduce Sn through the Au particle of ultra-thin InAs NWs. The distribution of atomic species of the Au particle characterized by APT is shown in Figure 5. From the APT profile it is clear that the amount of Sn is increasing towards the


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lower third of the particle, close to the InAs. Such a concentration gradient with lower Sn away from the Au surface has not been observed before. The state of the Au particle is monitored after growth and thus likely to be different than during growth. After growth all precursors except AsH3 were turned off. The decrease in Sn content close to the Au surface may originate from an out diffusion where Sn is leaving from the Au surface immediately after growth when the Sn precursor (TESn) is switched off from the reactor and not maintaining the Sn level in the Au particle any longer. In summary, the growth of ultra-thin, down to 7 nm diameter, InAs NWs was demonstrated. We have also presented Sn-doping characterization by laser-assisted APT on epitaxial ultra-thin NW structures with 12 nm top diameter. A high Sn level detected in the Au particle by EDX on ultra-thin 8–9 nm diameter InAs NW suggests no appreciable Gibbs-Thomson effect present in spite of the small size. From these results we conclude that Sn is a suitable dopant to obtain doping densities above 1019 cm-3 without altering the growth morphology of the ultra-thin NWs. No reduction of dopant incorporation with decreasing diameter was found suggesting there is no fundamental limit of doping ultra-thin NWs. This is necessary for such NWs to be useful in further


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scaling of electronic devices. APT characterization of the post-growth Au catalyst particle also revealed a spatial distribution of the In and Sn, which has not been previously observed. Methods MOCVD growth All NWs were grown in a standard Aixtron CCS closed coupled showerhead 3 × 2 inch low-pressure MOVPE system with rotation of the wafers. Purified hydrogen with a total flow of 8 l/min was used as carrier gas. The precursors for growth of Sn-doped InAs were tetraethyltin (TESn), trimethylindium (TMIn), and arsine (AsH3). The NWs were grown in the (111) direction on Sn-doped (1×1018 cm-2) InAs (111) substrates in vapor-liquid-solid (VLS)[14] mode with 5 to 20 nm diameter Au particles as catalysts. In the work presented here both Au colloids and Au discs precisely positioned by electron-beam lithography were used. Au colloids dispensed in a liquid are a simple and straightforward way to prepare samples for VLS growth. The drawback is the difficulty in positioning the Au particles. From our observations we can conclude that there is no significant difference in NW growth conditions between the two Au particle deposition methods.


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The epitaxy started with a thermal annealing step at 500°C. Meanwhile the substrate was stabilized by an AsH3 flow of 20 ml/min at 100 mbar reactor pressure. After annealing, the susceptor temperature was set to growth temperature. When the correct growth temperature was attained and stabilized for about 3 minutes InAs NW growth commenced. The reactor temperature was calibrated by means of a LayTec EpiR TT insitu metrology system. The NWs designed for APT investigation were grown by first 5 min (including unknown nucleation time) undoped InAs and then 5 min of Sn-doped InAs growth. In this way the lower part of the NWs will have an undoped core and a Sn-doped doped shell while the upper part of the NWs will be entirely Sn-doped. In order to investigate if the Sn-doping can be modulated in such a way that a sharp doping interface is created, a special switching scheme was introduced during the last 60 seconds of growth, when the untapered top part of the NW is formed. By vent-run switching the TESn was turned on and off every 10 seconds, intended to create a periodic modulation in the doping, with periodicity around 50 nm. Thus, the last 1 min of InAs NW was grown with an average


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Sn-doping of about half. For investigation by TEM, shorter NWs with 5 min of undoped InAs growth only, were used. ALD deposition of ZrO2 Before TEM investigation the InAs NWs were covered by a 4-5 nm thick ZrO2 layer. A Picosun SUNALE R-series 4 inch tool was used for the deposition. Precursors were tetrakisethylmethylaminozirconium (TEMAZr) and water with N2 as carrier gas. Deposition temperature was 125°C. SEM and TEM imaging Routine characterization of the as-grown NW heterostructures was carried out using a Hitachi SU8010 SEM. To prepare for investigation by HRTEM by a JEOL JEM-3000F, copper grids covered with a lacey carbon was gently pressed on the InAs growth surface to pick up NWs. The lacey carbon the NWs are resting on during HRTEM investigation is about 5 nm thick and thus on the same order as the NW diameter and clearly visible on some of the HRTEM images. APT characterization NWs were prepared for APT measurement using a so-called “pluck-and-place” procedure whereby a single NW is identified in an SEM and picked up using a micromanipulator. It was then attached to a Si microtip using a Pt braze formed


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by electron-beam induced deposition. Although the FEI Helios 460 NanoLab with EasyLift manipulator used for this had dual-beam functionality, only the electron column was used because even a single scan from the Ga ion beam would damage the ultra-small NWs. Examples of this method are well established in literature.[22-24] The APT characterization was performed in a LEAP 4000X Si atom probe operating in laser mode with a 355 nm picosecond laser. Laser pulse energy was 0.5 pJ with a repetition rate of 250 kHz, although other energies were tested. Base temperature was set to 30 K. Direct measurement of NWs from the substrate was also successfully performed, but this was found to be less reliable, and the ensuring data noisier, than the pluck-and-place method. The primary reason for this was evaporation from nearby NWs causing noise in the measurement, and secondary the 5 μm length of the NW was too short to prevent significant interaction from the substrate.[30] Data reconstruction was performed using CAMECA IVAS 3.6. Tip profile reconstruction mode using dimensions obtained pre-run and, when available, post-run SEM images of the specific measured NW as well as TEM images of other NWs from the same wafer.


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FIGURES

Scheme 1. Schematic illustration of the fabrication steps for the (a) ultra-thin NWs which were coated with ZrO2, for TEM measurement, and (b) the ultra-thin NWs which were grown with a tapered base section, for APT measurement. Not to scale.


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Figure 1. HRTEM images of InAs NW with a 3–4 nm thick radial layer of ZrO2. InAs NW diameters are (a) 14 nm, (b) 10 nm, and (c) 7 nm, respectively. (d) The crystalline InAs core reconstructed from a Fourier transform of the HRTEM image in (c). The InAs core diameter is confirmed to be about 7 nm.


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Figure 2. (a) HAADF image with EDX point quantification of In, As, Au, and Sn content of a VLS-grown InAs/Au NW with a diameter of 8–9 nm. Spectrum 1, acquired at the Au particle, reveals a substantial amount of Sn, while the amount of Sn detected at the InAs NW (Spectrum 2) is below the detection limit. The precision of the Sn detection is about 1%. The detected amounts, In 45% and As 55% of the InAs NW, instead of the expected 50/50 is an artifact of non-validated calibration because of channeling effect occurring when the EDX probe electron beam is aligned to a zone axis. (b) SEM image captured at 30 degrees tilt angle showing the InAs NWs grown 10 μm apart to meet the requirements for APT analysis.


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Figure 3. APT characterization of an InAs NW with undoped core and Sn-doped shell and top part. (a) Pre- and post-run SEM images of a single NW, along with the reconstructed 3D atom map (pink). SEM images are at 52° viewing angle. The atom


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map has been scaled accordingly. (b) Table of the Sn composition in the three parts of the structure. To improve sensitivity only the two major Sn peaks are counted, but the values have been adjusted assuming natural isotopic abundance. For the three datasets, uncertainty estimates, ±σ, are based on counting statistics and background noise. For the average value the uncertainty estimate is the numerical standard deviation. (c) Mass-spectrum from the top region of a NW, with the Sn++ peaks highlighted. Vertical bars show expected peak heights relative to the 120Sn isotope


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(a)

abundance ratio.


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Figure 4. (a) 2D color map of Sn concentration from a 600 nm length of the tapered lower part of the NW. Color scale ranges from 0 to 1000 ppm, showing the weakly doped core and the highly doped shell. A hot spot on the detector causes an apparent increase in Sn in the center of the image. The dashed green box indicated the volume used for the 1D profile in (b).

(b) 1D profile of Sn composition laterally across the lower part of the NW. The position of the 10 nm × 40 nm × 600 nm sampling volume is marked by the dashed green box in (a). Each sampling step is 1 nm. Error bars indicate ±2σ.


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Figure 5. (a) APT atom map of the Au catalyst and top of the NW. For clarity only 10% of the total counts of the O, Au, Sn, and In species are shown. (b) APT 1D composition profile from the In-rich native oxide through the Au particle and into the InAs NW. The lower third of the Au particle is rich in Sn. Error bars on the Sn composition indicate


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estimate of ±2σ deviation in composition due to statistical fluctuations and background noise. On average each measurement step contains around 3300 atoms in Au rich part and 2400 atoms in the InAs part. As such, the fundamental smallest detectable unit, being a single atom, would be equivalent to a concentration of 0.03 at.% or 0.04 at.% whilst the effective noise floor is around 5 atoms or 0.2 at.%.


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AUTHOR INFORMATION

Corresponding Author * A. D. Giddings: [email protected]

Author Contributions P. R. performed the NW sample preparation, MOCVD growth, TEM, and EDX. A. D. G. made the APT characterization. T. V. provided the electrical characterization to estimate doping concentration and doping incorporation. A. A. calculated the doping incorporation and resulting carrier concentration. P. R. and A. D. G. wrote the manuscript with contributions from all authors. R.-L. H., Y.-C. Y. and M. P. provided management support and advice for improvement of the paper.

Funding sources

All authors received funding from TSMC only.

ACKNOWLEDGMENT


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The authors would like to thank Y. C. Sun and C. H. Diaz of TSMC for support.

Supporting Information Available: further information on detection levels and error estimates, complementary version Figure 5 with a repeat dataset, and effect of laser pulse energy on mass-spectrum and Sn composition.

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ToC image 397x212mm (96 x 96 DPI)


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Sn Incorporation in Ultra-Thin InAs Nanowires for Next-Generation

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