Subscriber access provided by READING UNIV
Communication
Controlled Formation of Radial Core-Shell Si/ Metal Silicide Crystalline Heterostructures Alon Kosloff, Eran Granot, Zahava Barkay, and Fernando Patolsky Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03237 • Publication Date (Web): 03 Dec 2017 Downloaded from http://pubs.acs.org on December 3, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 26
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
Controlled Formation of Radial Core-Shell Si/Metal Silicide Crystalline Heterostructures Alon Kosloff1, Eran Granot1, Zahava Barkay3 and Fernando Patolsky1,2*
1
School of Chemistry, the Raymond and Beverly Sackler Faculty of Exact Sciences Tel-Aviv University, Tel Aviv 69978, Israel
2
Department of Materials Science and Engineering, the Iby and Aladar Fleischman Faculty of Engineering, Tel Aviv University, Tel Aviv 69978, Israel 3
Wolfson Applied Materials Research Center, Tel-Aviv University, Israel
* To whom correspondence should be addressed:
[email protected] Keywords:
Nanowires,
Heterostructures,
Interface,
Single
Semiconductor.
1 ACS Paragon Plus Environment
Crystal,
Nickel
Silicide,
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Abstract The highly-controlled formation of ‘radial’ silicon/NiSi core-shell nanowire heterostructures has been demonstrated for the first time. Here, we investigated the ‘radial’ diffusion of nickel atoms into crystalline nanoscale silicon pillar cores, followed by nickel silicide phase formation and the creation of a well-defined shell structures. The described approach is based on a two-step thermal process, which involves metal diffusion at low temperatures in the range of 200-400oC, followed by a thermal curing step at a higher temperature of 400oC. In-depth crystallographic analysis was achieved by nanosectioning the resulting silicide-shelled silicon nanopillar heterostructures, giving us the ability to study in detail the newly formed silicide shells. Remarkably, it was observed that the resulting silicide shell thickness has a self-limiting behavior, and can be tightly controlled by the modulation of the initial diffusion-step temperature. In addition, electrical measurements of the core-shell structures revealed that the resulting shells can serve as an embedded conductive layer in future optoelectronic application. This research provides a broad insight into the Ni silicide-‘radial’ diffusion process at the nanoscale regime and offers a simple approach to form thickness-controlled metal silicide shells, in the range of 5-100nm, around semiconductor nanowire core structures, regardless the diameter of the nanowire cores. These high quality Si/NiSi core-shell nanowire structures will be applied in the near future as building blocks for the creation of utra-thin highly conductive optically transparent top electrodes, over vertical nanopillars-based solar cell devices, which may subsequently lead to significant performance improvements of these devices in terms of charge collection and reduced recombination.
2 ACS Paragon Plus Environment
Page 2 of 26
Page 3 of 26
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
Introduction The integration of different compound materials into nanowire structures opens up new opportunities in the formation of novel multi-functional heterostructures. These structures may display unique properties, which are derived both from their consisting individual materials as well as from newly-formed interface that separates them.
The formation of nanowire
heterostructures has been extensively demonstrated both in the axial1–7 and in the radial direction8–12, mainly achieved through sequential growth and deposition steps. Axial nanowire heterostructures can be grown by a metal-catalyzed process, where a nanowire is grown from a seed particle, while its composition is modulated by a change of the precursor material during the growth process. This, in turn, leads to the formation of well-defined junctions5,13,14 .Alternatively, axial interfaces can be formed by means of metal diffusion, followed by a silicide phase formation, which creates an axial modification of the starting nanowire structure15. Likewise, radial core-shell heterostructures are formed by a modulation of the nanowire material along the radial direction. For instance, a distinct shell of different composition, such as doping or compound, can be formed on existing Si nanowires cores by the Chemical Vapor Deposition process16 (CVD). Interestingly, the synthetic challenge in forming nano-heterostructures is associated with peculiar phenomena17. For example, due to the high surface-to-volume ratio and low dimensionality of nanowires, they display the ability to facilitate epitaxial growth of different materials, even at high lattice mismatches. A notable example is the growth of highly strained Ge layers on Si nanowires18. This demonstrates the unique ability of nanowires to relax strain on their interface. In another case, involving a thermal oxidation process, the interface forms an increased strain, which in turn, leads to a limited oxygen diffusion and the formation of an oxide shell with a well-controlled thickness19. Besides the synthetic challenges, the importance of heterostructures is evident by their unique emerging properties, in comparison to single element nanowires or their planar bulk counterpart. The formation of novel nano-interfaces by the compositional modulation of dissimilar materials within nanowire structures results in exclusive electronic16,20–26, photonic23,27–32, magnetic33–35 and thermal36–38 properties. The possibility to obtain structures possessing these properties opens
3 ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 26
up new opportunities in achieving multi-functionality. For example, the formation of an abrupt radial or axial dopant variation leads to the creation of well-defined p-n junctions. These type of junctions are fundamental for photovoltaic device applications10,12,39–41. In another paradigm, the formation of an epitaxial Si/silicide ‘axial’ interface within nanowire structures can be achieved by a metal diffusion process, followed by a silicide segment formation. As a result the contact resistivity is reduced, while shortening the active-nanowire FET channel length42. The axial silicidation process of Si nanowires was vastly demonstrated for the preparation of ‘axial’ segmented conductive-semiconductive silicide/Si nanowire heterostructures15,25,43–47. These works broadly investigated the kinetics and thermodynamics of the Ni propagating front, from metal contacts or a point source, along the axial direction of the nanowires, followed by the subsequent concomitant silicide phase formation. Additionally, this ‘axial’ solid-state reaction was investigated under a broad range of temperatures and surfaces48,49, resulting in the ability to form Si-silicide ‘axial’ junctions with atomically sharp interfaces. The described process resulted in the formation of Si nanowire-based FET devices displaying ultra-short active channels
25,50
,
thus improving the devices performance by demonstrating a higher sensitivity and lower power consumption. Currently, in contrary to extensively reported ‘axial’ diffusion process, there are no published studies attempting the controlled preparation of ‘radial’ Si/Ni-silicide core-shell nanowire heterostructures. This fact may be further reasoned by the intrinsic challenges related to the synthesis of such core-shell structures, mainly due to the difficulty in attaining the formation of thickness-controlled NiSi shell structures of ultra-thin dimensions and high morphological quality, 1-30nm, at the given Ni diffusion and silicide growth rates, without leading to the inevitable formation of completely silicidized nanowires. Few recent works15,51–53 focused on the Ni diffusion into Si nanostructures. In these reports, an angular tilted deposition of Ni was performed leading to partially covered Si nanopillars, followed by a thermal treatment resulting in a formation of irregular Ni silicide/Si structures51,53. Furthermore, the Transmission Electron Microscopy (TEM) analysis performed in these cases was limited by the nanopillars thickness, thicker than 100nm. In addition, due to fast Ni diffusion rate and short radial diffusion depth, undesirable features might occur, such as severe surface agglomeration, or the formation of fully silicided nanopillars.
4 ACS Paragon Plus Environment
Page 5 of 26
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
The solution for these issues, associated with the challenge of achieving a nanometer scale level of control over the thickness of the forming shell and its crystalline attributes, poses several process considerations including: 1) a metal deposition method that gives a conformal Ni coating; 2) the thermal treatment should be monitored in a way that prevents fast Ni diffusion leading to a full nanowire silicidation. 3) TEM analysis should be done on nanopillars radial cross-sections, which provide the ability to observe the shell silicide phase formation inside the inner-core area and its crystallographic relation to the core Si phase. Here, our proposed dual-step process involves the diffusion of Ni and NiSi phase formation in the ‘radial’ direction, its source being a conformal non-limiting Ni metal layer in direct contact with the surrounding nanowire surface. The proposed approach is based on the fabrication of Si nanopillar arrays, followed by a controlled modulation of their outer shell chemical composition by the ‘radial’ diffusion of Ni from a non-limiting conformal Ni-coated layer. The thermal treatment process is divided into two key steps. In the first step, a mixed diffused Ni/Si layer is formed, while in the second step after the removal of the unreacted residual Ni layer, thermal curing is done at a higher temperature, leading to the formation of a homogeneous NiSi shell phase. Subsequently, the formation of nanometer-thin nanopillars sections is achieved by using the nanodicing method presented before54, which in turn opens up the possibility to perform a high resolution morphological characterization of the resulting core-shell radial nanowire heterostructures, as well as their shell thickness and crystallographic characteristics. This report provides an insight into the process of Ni ‘radial’ diffusion and silicide shell formation at the nanoscale regime. It also offers a route to create radial nanowire heterostructures displaying a controlled silicide shell thickness. These structures may serve as future candidates in photovoltaic and optoelectronic applications, due to the low electrical resistance of the resulting Si/silicide radial interfaces, the optical properties associated with their nanometer-thin metal silicide shells55, and the electrical properties of the highly conductive shells likely to improve charge collection and reduce recombination.
Results and Discussion The fabrication process of core-shell Si/NiSi nanopillar arrays is described in Figure 1. First, an array of circles of 300nm diameter and 2 µm spacing, is patterned by e-beam lithography on an
5 ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
electron-resist film, coated on a p-doped Si substrate, Figure 1A. Second, a 100nm Ni layer is ebeam evaporated, followed by resist lift-off, resulting in the formation of nickel disk arrays serving as etch masks, Figure 1B. Third, the substrate is anisotropically top-down etched by a deep reactive ion etching (DRIE) Bosch process, using SF6 and C4F8 gases, leading to the formation of Si nanopillar structures beneath the Ni-disks etch masks, having a height of 2.4 µm and a diameter of ~300nm, Figure 1C. Next, in order to clean the nanopillars surface, a thin oxide shell is formed by thermal oxidation and subsequently removed by an oxide etchant step. As a result, the pillars are smoothed and their diameter is reduced to ca. 200nm. This step is crucial for facilitating subsequent diffusion surface reactions. As a preparation for the next step, a 50-60nm Ni conformal layer is sputtered over the nanopillars array, Figure 1D. Fourth, a controlled diffusion of Ni atoms into the Si nanopillar cores is performed by a low temperature (200oC-400oC) annealing step, forming a primary shell structure composed of a thin Ni/Si mixed layer within the outer volume of the pillar core. Then, the excess unreacted Ni layer is removed by a Ni etchant process, Figure 1E. Finally, the sample is treated by an additional thermal step, done at a higher temperature (>400oC) for a longer time period. As a result, a clear highly homogeneous silicide phase is created within the Si core structure, and the formation of Si/silicide nanopillar heterostructures is achieved, Figure 1F. Later on, in order to analyze the radial shell thickness, crystalline phase structure and silicide/Si interface properties, the resulting core-shell nanopillar arrays are cross-sectioned by ultramicrotomy, Figure 1F inset, following the nanodicing method54. Figure 2 shows Scanning Electron Microscopy (SEM) images of nanopillars and their corresponding TEM section views. Figure 2.1A,B depicts the etched nanopillars structure after having their surface cleaned by thermal oxidation and oxide layer removal. Figure 2.2A,B shows the pillars structure after sputtering a 40nm conformal Ni layer. Figure 2.3A,B reveals the resulting silicide/Si heterostructure, created after the following steps: (1) Si/Ni mixed layer formation through diffusion by a rapid thermal processing (RTP) at a temperature of 250oC for 1 min (all RTP steps were applied by a 10 oC/sec heating ramp) . (2) Removal of unreacted excess Ni layer followed by structural thermal curing and phase formation, done by a second RTP annealing step at 400oC for 10min. As seen in Figure 2.3B, the formation of a 15nm Ni silicide shell is achieved. One can clearly note the difference in shell morphology between the Ni sputtered layer (Figure 2.2B) and the formed silicide phase in Figure 2.3B. As observed, the Ni 6 ACS Paragon Plus Environment
Page 6 of 26
Page 7 of 26
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
shell is characterized by an outer grain structure, while the silicide phase is embedded within the Si nanowire slice, and shows a clear border interface with the Si core. It is important to mention that these observations cannot be made through the corresponding SEM images (Figure 2.2A and 2.3A), or by SEM images taken by a backscatter detector (BSE) (See supplementary Figure S1). This emphasizes the importance of our employed nanoslicing methodology for the accurate analysis of the silicide shell orientation, thickness, and crystalline phase structure. A series of time-correlated experiments was performed with the purpose of achieving control over the final thickness of the silicide shell. First, the temperature is kept constant at 250oC during the thermal-diffusion step, while the time of diffusion is varied in the range of 10-600 seconds, followed by the second thermal-curing step at 400oC for a fixed period of 600 seconds. Figure 3A shows the results of 10, 30, 60, 600 seconds diffusion times. For each case, the resulting shell thickness was measured for multiple slices. Interestingly, a variation of diffusion depth with time is not noted, evident by the fact that after increasingly long annealing times, the resulting shell thickness reaches a plateau value of ca. 18nm (having a statistical distribution error of +/-4nm), Figure 3A.5. This indicates that the silicidation process, the nickel diffusion into silicon cores, displays a clear self-limiting behavior. It is important to emphasize that this unexpected shell-thickness self-limiting behavior occurs regardless the thickness of the evaporated Ni layer, 40-80nm, which is obviously non-limiting. This is further confirmed by the fact that a pure residual Ni shell always remains after our thermal treatment process, see Supplementary Movie 1. Notably, the self-limiting diffusion phenomenon is observed even for the formation of the thinnest NiSi shell structures, at conditions where the Ni metal pool is certainly non limiting. As these experimental conditions do not allow control over the shell thickness, due to the selflimiting nature of the process, additional time-controlled experiments were done at a higher diffusion temperature of 350oC, in order to both overcome this intrinsic shell thickness limitation and shed light into the mechanism of the shell formation, Figure 3B. Notably, a thicker silicide shell is obtained at this higher temperature, yet, the silicidation process still displays the same self-limiting behavior, evident by reaching a plateau average shell thickness of ca. 35nm (with a distribution error of +/-8nm), Figure 3B.5. These results clearly demonstrate the capability to tightly control the thickness of the resulting silicide shell by the modulation of the first annealing step temperature, which determines the initial Ni diffusion depth. Figure 3C shows the results of 7 ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
a temperature-controlled series of experiments, done by performing the first diffusion step at a constant time period of 60 seconds, while varying its temperature in the range of 200oC-400oC. As before, all temperature-controlled experiments were followed by an excess residual Ni removal, and a thermal curing step performed at 400oC for 10 minutes. Notably, the results show a controlled monotonic increase in the resulting silicide shell thickness, with a growing rate of 0.17nm/deg. (Figure 3C.6). These results clearly demonstrate the capability to control the thickness of the resulting NiSi shell by the proposed temperature-dependent process, each temperature condition leading to a self-limiting well-defined shell thickness, regardless the diameter and crystal structure of the silicon nanowire core. Generally, silicidation is a process in which a metal and Si are in intimate contact at high temperatures, leading to the interdiffusion of atoms and the formation of a new metal silicide phase. For the case of nickel silicides, Ni diffuses interstitially in the Si crystal.56 After the formation of the nickel silicide phase, Ni atoms from the Ni source must diffuse through the newly formed silicide phase where its diffusion is slower.47 The NiSi-Si couple is the most favorable in forming atomically abrupt interfaces, because Si and NiSi display comparable lattice constants and atomic densities.15,47 Upon transformation to the nickel silicide, the volume of the silicon nanowire expands, leading to a considerable accumulation of stress in the confined nanowire core. As the forming nickel silicide shell becomes thicker at a given annealing temperature, the accumulated stress reaches a point where no more nickel atoms can diffuse into the silicon core, thus leading to the self-limiting shell thickness phenomenon. This is mainly attributed to the great compressive stress that develops into the core/shell nanowire structure, which retards the diffusion of nickel atoms and limits further volume expansion through silicide phase formation. These results provide a clear picture on the influence of developing lattice stress on the growth behavior of silicide phases under strongly confined nanoscale environments. Past reports have demonstrated the influence of stress on the nickel diffusion rate and silicide phase formation on silicon-silicon oxide core shell nanowires.48 The resulting interface, formed between the Si core and the silicide shell is shown in Figure 4C. Notably, this interface which borders between the Si/silicide crystal phases displays a uniform structure. In order to analyze the phase structure and its relation to the silicon core area, we applied the TEM convergent beam approach. The beam is converged on the silicide shell, or the adjacent Si core area accordingly, and convergent Beam Electron Diffraction patterns (CBED) 8 ACS Paragon Plus Environment
Page 8 of 26
Page 9 of 26
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
are obtained from these selected areas at different specimen tilt angles. Figure 4A.1-5 shows CBED patterns obtained from the Si core area (Figures 4A.1, 2, 3) and the silicide shell (Figures 4A.4, 5). The specific areas at which the diffraction patterns were sampled are shown in the corresponding low magnification section image (Figure 4A). The distances and angle measurements of the diffracted circles shows a correlation to the NiSi phase (Pnma, a-5.17A, b3.33A, c-5.60A), evident by its [101] and [102] Zone Axis Patterns (ZAP), Figures 4A.4 and 4A.5 correspondingly, while the detection of the silicon phase structure is evident by its [001],[114] and [103] ZAP’s, Figures 4A.1, 4A.2 and 4A.3 correspondingly. To further assess this result, the same slice is analyzed by a Scanning Transmission Electron Microscope (STEM), in order to obtain vertical and horizontal EDX quantification line scans. A high-angle annular dark-field (HAADF) image is shown in Figure 4H, along with the corresponding vertical/horizontal line scans, Figure 4I,J. This elemental analysis indeed differentiates between two regions, namely the shell area having a composition of 50/50 [%] Ni:Si, and the core area, evident by a sudden decrease of the Ni signal, consisting of a Si-only atomic content. These results, along with high resolution TEM characterization of the Si/silicide border area, Figure 4C, undoubtedly demonstrate the formation of a clear Si/NiSi interface structure. However, further examination through beam convergent analysis at different shell regions showed that the shell morphological structure is not uniform, but rather composed from multiple grains. Figures 4 E, F, G shows bright field images, taken at a different specimen tilt angles. The sample orientation was chosen according to the NiSi [101] zones axis position of each grain (the specific location of each specific grain is specified in Figures 4 E, F, G, accordingly, by yellow arrows). It is noted that the grains have different sizes, while the border between themselves is depicted by an abrupt termination, which is seen in the high resolution image, Figure 4D, or by an intermixed termination noted from the observation of moire fringes in their mixing area, Figure 4B. To further understand the grain orientation, the data of the angular tilts that was used to obtain the CBED patterns in Figure 4A.1-5 are plotted on a stereographic projection along with the {111}, {110}, simulated planes/directions of Si, and the {211}, {202}, {103}, {112} planes of the NiSi phase, (See supplementary Figure S7). This was done according former reports57,58 which studied the texture of poly crystalline NiSi films, formed by a solid state reaction between a thin Ni film and a Si (001) substrate. These studies concluded that NiSi grains tend to form an axiotaxial alignment, where their (211) or (202) planes are 9 ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 26
oriented parallel to Si (110) planes of the substrate. The alignment occurs due to a plane matching of NiSi (211) and (202) to Si (220) plane, at an angle of 45 degrees to the surface normal. In addition, other preferred orientations that occur due to interfacial plane alignment were also found, for example, the alignment between NiSi (103) or (112) to Si(110) at angles of 40.8 or 46.5 degrees, respectively. Nonetheless, in our study, the resulting projection corresponding to one specific grain does not show a preferred aligned-orientation with its adjacent Si core. In addition, phase and grain orientation mapping were obtained by using the transmission Kikuchi diffraction (TKD) method in scanning electron microscopy (SEM). TKD patterns were measured for the core-shell crosssections using Oxford Nordlys II detector and Aztec software, installed on Quanta 200FEG ESEM (Environmental SEM) (See supplementary Figure S2). Here again, the results do not show a specific grain orientation relation. Furthermore, in order to observe the events that occur beyond the described process window, few further experiments were done. The effect of the initial diffusion step was investigated by performing “diffusion-only” experiments. For this purpose, a similar Ni-coated pillars sample was thermally treated by RTP at 300oC c for 1 min. The unreacted residual layer is removed, however, in contrast to the two-step silicide process, here, the sample is not treated with an additional curing step but undergoes the sectioning process directly. A corresponding section is analyzed by low magnification TEM, Figure 5A. As noted, the slice displays a non-uniform core-to-shell border, having a mixture of cured/non cured phase regions, which can be seen by the corresponding high resolution TEM image, Figure 5B. Phase variation is also indicated by the difference in the selected reduced-FFT patterns of these regions, Figure 5B insets. Also, it is important to mention that an attempt to converge the beam on the same area (in order to obtain CBED) resulted in a subsequent variation of the shell structure (See Supplementary Information Section Figure S3). Therefore, we conclude that the sample is not stable under the electron beam due to the intrinsic properties of the non-cured mixed layer. Additionally, the same “diffusion-only” concept was applied in another sample. Here, the thermal-diffusion step was done at a higher temperature of 500oC for 5 minutes, Figure 5C. Surprisingly, at these conditions the core-shell structure is still observed. This indicates again on the self-limiting nature of shell structure formation, as observed before, Figure 3. Furthermore, 10 ACS Paragon Plus Environment
Page 11 of 26
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
at this higher temperature some of the slices show evidence of almost complete silicidation of the Si cores (See Supplementary Information Section Figure S4). To have an additional overview on the radial silicide reaction, a sectioned array of Ni-coated nanopillars was placed on a nitride membrane-TEM grid. The sample was analyzed by TEM using an in-situ TEM heating holder, and the temperature was raised at a rate of 10oC/min, starting at a temperature of 190oC, and heating paused for a 5 minutes period. In that time period, the thermal drift is stabilized while the formation of a self-limited shell structure at each temperature is formed and recorded by low magnification TEM analysis (for a movie of the temperature-correlated self-limited shell formation process see Supplementary Information Section). The resulting movie shows the silicide reaction process of a single slice, where the process starts at a low temperature of ~200oC. Initially a mixed layer is formed, which is evident by a low contrast diffused layer, having a non-clear border definition. Later, the Si/Ni mixed layer re-structures as evident by the appearance of silicide grains formation, reaching its maximum thickness at a temperature of ~400oC. Also, in order to observe the effect of the second thermal curing-step applied temperature on the silicide shell morphology and crystal structure, the normal two-step process is repeated at higher thermal curing temperatures. The process begins with a thermal-diffusion step done at a temperature of 300oC for 1 minute, followed by the Ni residual layer removal. Finally, the second thermal-curing step is performed at higher temperatures of 600oC or 700oC for 10 minutes. Figure 5D shows the result of the sample treated at a thermal-curing temperature of 600oC. Notably, a core-shell structure still exists. In contrast, the sample thermally-cured at 700oC (Figure 5E) resulted in a fully silicided Si cores array. In order to examine the phase structure of these samples, as well as the one described before (one step of diffusion, done at 500oC for 5 min), multiple slices of the samples were analyzed by the TKD-SEM method (See Supplementary Information Section Figure S5). The results show that for the sample treated at 500oC, the silicide is composed of the NiSi phase only. However, for the 600oC and 700oC samples, although most sections consisted of the NiSi phase only, there is a lower occurrence of an additional NiSi2 silicide phase. This phase was scarcely found in small areas of the silicide shell in the sample treated at 600oC and at a larger extent for the fully silicided sample cured at 700oC.
11 ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 12 of 26
Next, in order to investigate the electrical properties of resulting core-shell nanopillars, the substrate, containing the thermally processed pillar array was sonicated in ethanol, resulting in the formation of a nanopillars suspension. This enables to transfer individual nanopillar elements onto a silicon acceptor substrate, and a subsequent electrical connection by Ti/Pd/Ti metal contacts, using e-beam lithography followed by a metal lift-off process. Through this process, two different electrically-connected pillars-based devices were prepared: the first, taken as a reference, consisted of a single crystal Si nanopillar connected by a two-probe configuration, Figure 5F inset. The second device consists of a nanopillar that was treated by the radial silicidation process (diffusion step at 300oC for 1 min and thermal post-curing at 400oC for 10 minutes), connected through a 4-probe configuration, Figure 5G inset. The measured I-V curves are presented in Figure 5F, G. As evident from these results, a clear cut difference is observed between the Si reference pillars sample to the Si/Silicide core-shell pillar structures. The first type of devices shows semiconductor rectifying properties, having a measurable current in range of nA, while the second type demonstrates ohmic highly conductive properties, with a measurable current in the range of mA. The silicide shell resistivity calculated from the IV curve and the device geometry is 99.5 μΩ ∗ cm (See Supplementary Information Section). Our measured resistivity value is ca. 10-fold larger than the expected resistivity values reported for the single-crystalline NiSi phase15,46. However, it is important to emphasize that our shell structure is composed of NiSi large multi-grains, rather than a single crystalline structure.
Conclusions In conclusion, this report describes the systematic investigation of the Ni-radial diffusion process occurring from a conformal sputtered Ni layer on Si nanopillar core structures. All past reports demonstrated the silicidation process of silicon nanowire structures along their 'axial' direction, and full silicidation, along the nanowire radial cross-section, was always observed in these cases, leading to fully silicidated nanowires. These reports mostly focused on the formation of improved electrical contacts between the silicon nanowire structures and the metal contacts deposited on them. For the purpose of creating metal-like contacts to nanowire structures for solar cell applications, one needs to carefully convert a thin-shell portion of the nanowire element into a transparent, conductive and continuous conformal silicide layer, such as to keep the active silicon core elements intact. This challenging task requires the development of new 12 ACS Paragon Plus Environment
Page 13 of 26
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
approaches in order to control the metal silicidation process along the highly confined 'radial' dimension of the silicon nanostructures, without leading to their uncontrolled or complete silicidation. The process presented here is based on a two-step thermal treatment. The first step involves the diffusion of Ni atoms, accompanied by the formation of a Ni/Si mixed layer. The second step, at a higher temperature of 400oC, is performed with the purpose of morphology restructuring accompanied by a definable silicide phase formation. Interestingly, in comparison to the case of Ni-axial diffusion on Si nanowires, our results clearly display surprisingly sharp self-limiting radial diffusion depth, which occurs in the first thermal-diffusion step, and attributed to internal radial stress formation and nickel diffusion hindering. Accordingly, the ability to tightly control over the resulting shell thickness was achieved by modulating the temperature conditions of the first thermal-diffusion step. The observed diffusion depth increases by a rate of 0.17nm/deg. The resulting highly crystalline shell structures were investigated in detail showing the formation of a clear and defined Si/silicide interface border. The silicide layer is composed of a highly crystalline NiSi grain structure. Here, in contrast to the bulk case57, a preferred grain orientation was not found. However, one should keep in mind that the formation of the radial silicide shell is accompanied by a much higher stress in comparison to bulk silicide formation. In order to investigate the morphology of the resulting heterostructured nanowires, we presented here a new method that facilitates the preparation of nanopillars cross-sections with thicknesses in the nanometer scale. Consequently, this gives us the ability to additionally characterize different shell of other radial diffusing metals or under different diffusion conditions such as into other substrate crystal facets. These ideas can lead to the development of novel heterostructures that contain a controlled and highly conductive layers. This research provides a broad insight into the Ni silicide-radial diffusion process at the nanoscale regime. This method offers a simple approach to form thickness-controlled metal silicide shells, in the range of 5-100nm, around semiconductor nanowire core structures, regardless the diameter of the nanowire cores. Although the vast majority of reports focused on devices based on individual nanostructures, many technological applications require the use of arrays of nanostructures. The conventional deposition of metals on such nanostructures arrays will only contact the top layer of the arrays, demanding hopping between the individual nanopillars to effectively transport the injected current. Our approach represents a superior approach to overcome this challenge. Furthermore, transparent contacts are another central 13 ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 14 of 26
technological field, being of critical importance for solar cell applications. Even though nanomaterials have been broadly investigated as a replacement for current transparent contact materials, a more critical question deals on how to create transparent contacts to the active area of solar cells made of a vertical nanowires arrays. In this context, considerable research and development efforts are still required in order to understand the structural properties of such contacts and their band alignment, as well as to develop approaches to achieve high quality contacts. Thus, metallic contacts to nanostructures arrays represent both challenges and opportunities. The high quality Si/NiSi core-shell nanowire structures achieved through this approach may serve in the future as building blocks for the creation of utra-thin highly conductive optically transparent top electrodes, over vertical nanopillars solar cell devices, which may subsequently lead to significant performance improvement of these devices in terms of charge collection and reduced recombination. Acknowledgements This work was in part financially supported by the Legacy Fund, Israel Science Foundation (ISF).
Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Materials and Methods section describing all experimental procedures in detail. The observation of core-shell structures through SEM using BSE and SE detectors (Figure S1). Phase and grain orientation mapping by the transmission Kikuchi diffraction (TKD) method (Figure S2). Nucleation of non-cured silicide shell under the TEM convergent beam (Figure S3). Diffusion depth and grain size in nanopillars sample, thermally-treated at a temperature of 500oC for 5 minutes (Figure S4). Silicide phase characterization at thermal conditions above the temperature of 500Co (Figure S5). Calculation of the silicide layer resistivity. Video showing the real-time temperature-correlated self-limited shell formation process (Movie 1).
14 ACS Paragon Plus Environment
Page 15 of 26
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
Figure Captions
Figure 1. Fabrication Process Steps for the Formation of Si/Silicide Radial Core-Shell Heterostructures. (A) A Si substrate is spin-coated with PMMA electron resist, followed by e-beam lithography patterning of circles array. (B) Deposition of 100nm Ni layer followed by lift-off, formation of metal disks that serve as etching masks. (C) Anisotropic substrate dry-etching, forming nanopillars structures. (D) 40nm conformal Ni sputtering. (E) Ni diffusion by low temperature annealing (200oC
