High-performance flexible electron field emitters fabricated from doped crystalline Si pillar films on polymer substrates

Abstract

We report a new approach for the synthesis of various crystalline Si nanostructures on a polyimide (PI) substrate via microwave plasma enhanced chemical vapor deposition (MWPECVD) using SiCl₄/H₂ as precursors, and study the effects of conducting type (i.e., intrinsic, n-type, and p-type) on the electron field emission (EFE) properties of the Si nanostructures. H₂ plasma treated B-doped crystalline Si pillars (H₂: p-Si pillars) with a diameter of 50 nm and a sharp tip radius of 16 nm on a Mo-coated PI substrate reveals the best EFE performance with a low turn-on field of 5.85 V μm⁻¹, high current density of 1.37 mA cm⁻²@10 V μm⁻¹, and an extremely high field enhancement factor of 1281.13. This superior EFE performance is achieved because of its geometric features and high conductivity across the emitters. In addition, a flexible crystalline Si film-based field emission prototype device using the H₂: p-Si pillar sample as the cathode is constructed. No obvious deterioration on EFE characteristics is observed when the device is subjected to bending at a radius of curvature (R) of 10 mm. According to the lifetime test, we achieve a half-life time over 10 h when a repeating FE-on/off test of 9 times at an R of 10 mm is performed, indicating high flexibility and good stability. These results thus demonstrate important steps toward a low-cost approach for creating high-performance and flexible field emission displays.

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1. Introduction

To date, a variety of semiconductor nanostructured materials have been explored for application as electron field emitters of electron sources,¹ flat panel displays,² and microwave power amplifiers.³ It is well understood that the main requirements for field emitters are low turn-on field (E₀), high current density (J), and good current stability. Many kinds of carbon nanomaterials such as amorphous carbon,⁴ carbon nanotubes (CNTs),⁵ carbon nanosheets,⁶ and ultrananocrystalline diamond,⁷ are regarded as suitable materials for meeting these demands. They are used in cold cathodes because of their low work function, high aspect ratio, and high conductivity, which result in good electron field emission (EFE) properties. Despite these promising features, there are still technical challenges that remain to be solved, for example, degradation of EFE properties over long-term operation of CNTs,⁸ and poor adhesion of diamond films to the substrate.⁹ Moreover, high temperature is always essential for synthesis of these carbon nanomaterials, which limits the use of polymer substrates for flexible EFE-related devices.

From another perspective, developing Si-based field emitters is highly desirable because of the widely utilized Si material and process compatibility with the already existing Si integrated circuit technology.¹⁰ Unfortunately, the EFE characteristics of Si-based materials are still unsatisfactory. Intensive research efforts have been devoted to improving the EFE properties of Si materials. Besides the requirement of high-quality crystallinity for obtaining high carrier mobility, these modification techniques include nanostructure engineering,^10–14 H₂ plasma treatment,^13,14 metal implantation,¹⁵ adoption of a metal interfacial layer¹⁶ and coverage with metal particles,¹¹ carbon nanomaterial film,¹² and ferroelectric film.¹⁷ Among the abovementioned methods, two primary strategies, formation of nanostructure with small apex radius and increasing the conductivity, are employed to achieve superior EFE performance. The former involves sharp features, which increase the localized electric field owing to geometric features of the emitter. This leads to facile emission of electrons from the Si surface at low E₀ and a drastic increase in the field enhancement factor (β) in comparison with its flat surface counterpart.^18,19 On the other hand, incorporation of dopant or metal in the Si structure renders the Si-based material even more conductive and is thus a route for enhancing electron transport. It results in the EFE devices with low E₀ accompanied by high J.^11,15 Owing to the aforementioned modification techniques, Si-based EFE devises exhibiting high emission current and ballasting effect by using nanostructured Si arrays as emitters were developed to date.^20,21 Such nanostructured Si arrays were fabricated from high-quality crystalline Si wafer using top-down etching process. However, to the best of our knowledge, the integration of Si film-based field emitters into flexible device architectures has not been reported. This is most likely due to poor thermal stability of polymer substrates and an immature transfer process.

Our previous study reported the feasibility of using microwave plasma enhanced chemical vapor deposition (MWPECVD) with a SiCl₄/H₂ mixture to directly deposit highly crystalline Si film onto polyimide (PI) substrate at a temperature as low as 185 °C.²² Furthermore, a self-biased sputtering solid doping source (SSSDS) process incorporated with SiCl₄/H₂ microwave plasma was adopted to effectively dope dopant atoms into the Si network for growing doped crystalline Si films.²³ In order to continue our efforts on the development of MWPECVD grown Si nanostructures on polymer substrates toward flexible electronics, we herein present a new strategy to fabricate Si pillars using ultrathin Au film as a catalyst to induce the vapor–solid–solid (VSS) reaction in the aforementioned Si deposition system and thus to grow doped crystalline Si pillars. The VSS mechanism using Au catalyst appears to be the favorable process for the synthesis of Si nanostructure because the growth temperature can be lower than the eutectic temperature of the Au–Si system (∼363 °C). It leads to less unintentional incorporation of impurities, resulting from the reduction in atom diffusivity and solid solubility associated with the lower temperatures.²⁴ The doped crystalline Si pillars was further treated by the H₂ plasma dry etching process to generate a sharp nanostructure with a small apex radius at the pillar tips. This step makes the specific nanoscale structures ideal field emitters. In addition, a sputtered Mo interfacial layer was introduced between the crystalline Si film and PI substrate, where the Mo metal film can increase the lateral conductivity and enhance electron transport. The present study focused on the effects of the conducting type (i.e., intrinsic, n-type, and p-type) and nanostructure morphology on the EFE property of the synthesized crystalline Si film. Moreover, a flexible Si film-based EFE (Si-EFE) prototype device was fabricated by using H₂ plasma treated B-doped crystalline Si pillars on Mo-coated PI substrate as the cathode. Their EFE characteristics, lifetime stability, and bending durability were evaluated in detail.

2. Experimental

2.1 Preparation of the crystalline Si film-based field emitters

After the flexible PI substrate used in this study (150 μm in thickness; 20 mm × 20 mm dimensions) was ultrasonically cleaned and dried, a Mo film with a thickness of 50 nm was coated on the PI substrates by using a magnetron sputtering system. Si film deposition was carried out by glow discharge decomposition of a H₂–diluted SiCl₄ mixture in a MWPECVD system. The MWPECVD apparatus is described in detail elsewhere.²² After the chamber was evacuated to a base pressure of 1 × 10⁻² Torr, Ar plasma treatment was applied to clean the substrate preparatory to Si film deposition under an Ar flow rate of 50 sccm and a microwave power of 250 W for 5 min. To efficiently control the flow rate of liquid SiCl₄ precursor, a cryostat apparatus was employed to maintain the SiCl₄ temperature at −55 °C. Saturated vapor was then introduced into the vacuum chamber by H₂ at a flow rate of 10 sccm, which corresponds to a SiCl₄ flow rate of 2.9 × 10⁻² sccm. For all the crystalline Si film deposition processes, the SiCl₄ vapor and diluted H₂ of 100 sccm were channeled separately into the chamber under a fixed pressure of 5 Torr and a microwave power of 750 W for 3 min. These parameters were regarded as the optimal conditions based on our previous study.^22,23 Subsequently, the crystalline intrinsic Si film was deposited onto the Mo/PI substrate, which was designated as “i-Si film”. As the substrate holder equipped with solid doping targets, the doping atoms can effectively doped into the Si film during the SiCl₄/H₂ microwave plasma process. Through this approach, we utilized ten B- and six P-doping targets designated as B(10) and P(6), respectively, in the process because they produced doped crystalline Si films having the highest conductivity and carrier concentration.²⁷ The B- and P-doped crystalline Si films, which are known to exhibit p- and n-type characteristics, on the Mo/PI substrates were designated as “p-Si film” and “n-Si film”, respectively. To synthesize the B-doped crystalline Si pillars, the Mo/PI substrate was first precoated with an ultrathin sputtered Au-layer (∼5 nm) to facilitate the VSS reaction. This step was followed by the abovementioned procedure using the SSSDS process integrated with SiCl₄/H₂ microwave plasma. The film obtained was designated as “p-Si pillars”. The B-doped crystalline Si pillars was further subjected to H₂ plasma dry etching treatment using the same MWPECVD system for 30 min at a microwave power of 750 W, a pressure of 5 Torr, a H₂ flow rate of 100 sccm, and a negative substrate bias of 60 V. The sample thus treated by H₂ plasma was designated as “H₂: p-Si pillars”. The schematic preparation processes for the aforementioned five crystalline Si film samples are revealed in Fig. S1.† During the crystalline Si film deposition, the substrate temperature was constantly monitored by using an infrared pyrometer through the quartz window of the MWPECVD chamber.

2.2 Characterization of the crystalline Si film-based field emitters

The crystalline characteristics of the synthesized Si film samples were evaluated by X-ray diffractometry (XRD, Shimadzu XRD6000) using Cu Kα1 radiation (1.5405 Å) and by micro-Raman spectroscopy (Horiba Jobin Yvon HR800UV) using a 514 nm Ar⁺ laser as an excitation source. Raman spectra were deconvoluted into three peaks: one for the crystalline phase near 520 cm⁻¹, the other for the intermediate (nanocrystalline) phase around 500 cm⁻¹, and the last for the amorphous phase at 480 cm⁻¹. The crystalline volume fraction (X_C) of the synthesized crystalline Si film was calculated according to its integrated intensity. A field emission scanning electron microscope (FESEM, JEOL JSE-6500F) was used to examine the cross-sectional morphologies and to determine the thickness of the synthesized crystalline Si film. To characterize the detailed crystallographic structure and elemental composition of the synthesized crystalline Si films, transmission electron microscopy (TEM, JEOL JEM-ARM200F) equipped with an energy-dispersive spectrometer (EDS) was utilized to obtain cross-sectional bright-field images (BFIs), selected area electron diffraction (SAED) patterns, and line-scan elemental profiles at 200 kV. Meanwhile, high-resolution (HR) mode, fast Fourier transformation (FFT) mode, and scanning transmission electron microscopy (STEM) mode were also employed to conduct lattice imaging analysis and subsequent structural identification.

The electrical conductivity (σ) of the synthesized crystalline Si film samples was measured by an electrometer (Keithley 2400) using a sputtered Al film (∼100 nm) as ohmic contact between Si film and electrode. To investigate the effect of structural characteristics on the transport behavior of electrons, two configurations were utilized: (i) a pair of electrodes located on the surface of the crystalline Si film for measuring the conductivity along the Si film surface (designated as σ_∥) and (ii) a setup with one electrode located on the crystalline Si film surface and the other located on the Mo interfacial layer for measuring the conductivity across the crystalline Si film thickness (designated as σ_⊥).

The EFE property of the synthesized crystalline Si films was measured by using a tunable parallel-plate capacitor setup, in which a Mo hemispherical probe with 2 mm diameter was used as anode and crystalline Si film samples were used as cathode. The cathode-to-anode distance was controlled by using a micrometer. The current–voltage characteristics were acquired under high vacuum at a pressure below 1 × 10⁻⁶ Torr by using an electrometer (Keithley 2410). On the basis of the Fowler–Nordheim (F–N) theory,²⁵ the EFE characteristics were modeled as shown in eqn (1),

where A = 1.54 × 10⁻⁶ (A eV V⁻²), B = 6.83 × 10⁹ (eV^−3/2 V m⁻¹), β is the field-enhancement factor, E (V μm⁻¹) is the applied field calculated from the ratio of the applied voltage to the cathode-to-anode distance, J (mA cm⁻²) is the current density determined by dividing the total current by the area of the anode (Mo probe), and ϕ is the work function of the emitting materials measured on a photoelectron spectrometer (Riken Keiki AC-2). Thus, the EFE parameters in terms of E₀ and β were respectively extracted from the macroscopic electric field required to reach a J value of 10 μA cm⁻² and from the obtained J–E curve using the F–N plot (namely, ln(J/E²) vs. 1/E plot).

2.3 Preparation and characterization of the flexible crystalline Si-EFE prototype devices

To visualize the application of the EFE characteristics, a flexible Si-EFE prototype devices with diode structure using H₂: p-Si pillar sample, was constructed, where the components are schematically explained in Fig. S2.† The Si-EFE prototype device mainly consisted of two indium tin oxide-coated polyethylene terephthalate (ITO/PET) faceplates, each of which comprised the anode faceplate with a screen-printed green phosphor film and the cathode faceplate with H₂: p-Si pillar sample bonded by Cu tape. The cathode-to-anode separation was fixed by a PI double-sided tape of 100 μm thickness used as spacer to create the cathode–anode distance (e.g., 100 μm). The anode faceplate, spacer, and cathode faceplate were assembled to form the flexible crystalline Si-EFE prototype device in sandwich configuration, which was employed in this study as shown in Fig. 1. Also shown in the figure is a photograph of a bent crystalline Si-EFE prototype device. The cathode area of 3.14 cm² was measured.

Fig. 1 Photograph and schematic cross-sectional image of the fabricated flexible crystalline Si-EFE prototype device.

The EFE characteristics of the crystalline Si-EFE prototype device were measured by using an electrometer (Keithley 2410) under high vacuum with a pressure below 1 × 10⁻⁶ Torr. In addition, the flexibility was evaluated by mechanical static bending tests, in which the flexible Si-FED was bent to generate a tensile strain at a designated radius of curvature (R) of 10 mm. Moreover, lifetime measurement was also performed to evaluate the stability and durability of the crystalline Si-EFE prototype device by repeating FE-on and FF-off pulses separated by a duration of 1 h under a bended state. The current density–time (J–t) behavior was recorded during the test.

3. Results and discussion

During crystalline Si film deposition and post H₂ plasma treatment, the substrate temperature instantly increased, and reached a plateau at a temperature of around 210 °C after 10 min. The increase in temperature was due to microwave irradiation and continuous ion bombardment of the substrate under high-density plasma. In this work, deposition of crystalline Si film was completed within 3 min at the highest substrate temperature of 185 °C, and no obvious variation in substrate temperature was detected when different Si structures were grown. On other hand, although the treatment time of the H₂ plasma dry etching process was as long as 30 min, the substrate temperature was kept at relatively low (218 °C). The substrate temperatures, however, were far below the glass transition temperature (∼260 °C) and melting temperature (∼350 °C) of the PI polymer.²⁶ Thus, the PI polymer did not suffer degradation by thermal exposure during crystalline Si film deposition and post H₂ plasma treatment.

3.1 Microstructure characteristics of the crystalline Si film-based field emitters

Fig. 2 shows XRD patterns and Raman spectra of the crystalline Si films possessing different structures. Characteristic peaks of crystalline Si were observed for all the samples synthesized in this study, which demonstrated the feasibility of one-step synthesis of the crystalline Si film onto thermal-sensitive polymer PI substrates at low temperature using a SiCl₄/H₂ mixture as the precursor and a MWPECVD as the processing system. The high crystallinity arose from the synergistic effects of preferential etching, self-cleaning, and chemical annealing by strongly etching species of H and Cl radicals in the high-density plasma process accompanied by the ion bombardment effect.^22,23

Fig. 2 (a) XRD patterns and (b) Raman spectra of the synthesized crystalline Si films possessing different structures: (i) i-Si film, (ii) n-Si film, (iii) p-Si film, (iv) p-Si pillars, and (iv) H₂: p-Si pillars. The integrated X_C value of each crystalline Si film is also presented in the inset of (b).

Detailed observation of XRD patterns in Fig. 2(a) shows that a slight decrease in intensity and slight broadening of the full width at half maximum were observed in the diffraction peaks of the four doped crystalline Si film samples, i.e., n-Si film, p-Si film, p-Si pillars, and H₂: p-Si pillars, as compared with that of i-Si film. This finding implies the decreases of crystallinity and crystal size after introduction of the dopants into the Si films.

Similar tendencies of crystallization characteristics may also be seen in the Raman spectra shown in Fig. 2(b). Raman spectrum of the i-Si film reveals a sharp peak centered at 520.4 cm⁻¹ and a high X_C value of 98.5%. By comparison, the doped crystalline Si films, regardless of their structure, exhibited a dominant peak close to 517–518 cm⁻¹ and a weak peak around 480 cm⁻¹. The X_C in these doped crystalline Si film samples are over 90% even they contained a small amount of amorphous Si phases. The results shown in the Raman spectra also confirm that both X_C and crystallite size decreased upon doping the crystalline Si film with the impurity. These results agree with amorphization of the doped Si network with local deformation caused by dopant atoms.²⁷ However, it is interesting to note that X_C in the H₂: p-Si pillars was improved after treatment of the p-Si pillars with H₂ plasma. This improvement is most likely due to removal of the amorphous phase, disordered structure, and native oxide through the in situ H₂ plasma etching process.^14,28

Cross-sectional morphologies of crystalline Si films having different structures are illustrated in Fig. 3. A remarkable dense columnar morphologies are found in i-Si film, n-Si film, and p-Si film samples, which agree very well with the findings of high-crystallinity characteristics from XRD and Raman analyses (Fig. 2). The high growth rate of these films under microwave glow discharge decomposition of a SiCl₄/H₂ mixture could also be observed in Fig. 3(a)–(c). After deposition for 3 min, the i-Si, n-Si, and p-Si films reached thicknesses of 739, 794, and 786 nm, respectively. These thicknesses are substantially higher than those obtained from many other PECVD systems using SiH₄/H₂ or SiH₂Cl₂/H₂ mixtures as precursors with or without gaseous doping.^29–31 The enhancement in growth rate of the doped crystalline Si films may be due to the incorporation of dopants in the Si films. After the VSS reaction using an ultrathin Au film as catalyst in SiCl₄/H₂ microwave plasma with the SSSDS process, the morphology markedly changed from a continuous columnar morphology to a featured isolated columnar morphology, as shown in Fig. 3(d). In our case, the crystalline p-Si pillars exhibited grains with 785 nm length, approximate 50 nm diameter, and 20 nm separation (Fig. 3(d)); therefore, the density of the Si-pillars is around 10⁹ # per cm². After the growth of the crystalline p-Si pillars, the nanostructured morphology was subsequently modified by a H₂ plasma process where the B-doped crystalline Si pillars was physically sputtered off, particularly the amorphous phase, native oxide, and edge-side structure. Consequently, a geometrically sharp nanostructure with a small apex radius around 16 nm formed at the tip. In addition, based on the statistic calculation, most of the diameter and the tip curvature of the H₂ plasma treated Si pillars are in the range of 50–65 nm and 14–17 nm, respectively, as shown in Fig. S3.† This indicates a well control of the Si nanostructure by using the VSS mechanism in SiCl₄/H₂ microwave plasma as proposed in this study. Morphological evidence can be found in Fig. 3(e) and (f).

Fig. 3 FESEM cross-sectional morphology of the synthesized crystalline Si films possessing different structures: (a) i-Si film, (b) n-Si film, (c) p-Si film, (d) p-Si pillars, and (e) H₂: p-Si pillars. A high-magnification image of the top surface of (e) H₂: p-Si pillars is also shown (f).

To better understand the microstructure of the H₂ plasma treated B-doped crystalline Si pillar structure, TEM studies were carried out. The left micrograph in Fig. 4(a) shows a cross-sectional BFI of the H₂: p-Si pillar sample, and the right-hand side displays its SAED pattern. HRTEM images captured at positions labeled with squares (I) and (II), corresponding respectively to the interface and the top regions of the synthesized H₂: p-Si pillars, are shown beside the BFI image in Fig. 4(b) and (c), respectively. FFT patterns at positions (i) to (iii) in Fig. 4(b) and (iv) in Fig. 4(c) are also presented. The cross-sectional BFI reveals a completely crystalline pillar structure with grain sizes of around 50 nm throughout the thickness. The SAED pattern consists of distinct spots without a broad diffuse ring, indicating a well-defined polycrystalline Si structure with a large grain size; in addition, signals for the Au and Mo interlayer can also be seen in the SAED pattern. HRTEM microstructure of the synthesized H₂: p-Si pillar sample at the interface between the Au catalyst and Si pillar structure (region (I) of Fig. 4(a) and also Fig. 4(b)) revealed a ultrathin Au-silicide layer with a depth of around 3 nm on the Au catalyst surface and a lattice spacing of approximately 0.26 nm (image (i)). These can be indexed to the (660) planes of the Au₂Si phase.¹¹ Subsequently, crystalline Si with (111) plane, confirmed from the lattice spacing (image (ii)), was originated vertically from the top surface of the Au-catalyst and then preceded the development of an almost entirely crystalline Si pillar structure containing predominantly (111) and (220) planes (image (iii)). Elemental composition analysis of the interface between the Au catalyst and Si pillar structure are also validated by EDS line-scan elemental profiles as shown in Fig. 4(d). No amorphous incubation layer surrounding the Au catalyst and no residual contamination from the Au in the Si pillar structure were found after careful HRTEM examinations. These results suggest that the as-grown crystalline Si pillars follows the VSS growth mechanism under the base growth scheme.

Fig. 4 (a) Cross-sectional BFI of the H₂: p-Si pillar sample, with its SAED pattern shown on the right side. (b) and (c) The HRTEM images at the positions labeled with squares (I) and (II) in the BFI of (a), which respectively correspond to the interface and top region of the synthesized H₂: p-Si pillar structure. FFT patterns of the magnified areas (i) to (iv) are also displayed on the right side of (b) and (c). (d) STEM image with EDS line-scan elemental profiles across the interface of the Au catalyst and Si pillar structure.

One important point to emphasize is the formation of sharp features at the topmost layer consisting of well-defined Si(111) planes in region (II) of Fig. 4(a), as shown in Fig. 4(c). According to our previous study on the growth mechanisms of crystalline Si films using SiCl₄/H₂ as reactants,²² the stronger etching ability of H and Cl radicals on the Si network facilitate the crystallization of the Si film at low temperatures through the effects of surface etching, self-cleaning, and chemical annealing. The anisotropic etching rate of Si depends on the surface crystal orientation in the order (100) ≥ (110) ≫ (111). Therefore, the chemical stability of the (111) grain is much higher than that of other grains which are thus preferentially etched away. As a result, retention of the Si(111) grain in the Si film with nearly single crystalline Si(111) at the topmost layer is expected. Therefore, the Si pillars growth presented in this study also meets such a criterion. Upon close inspection of HRTEM images, we observed no deterioration of crystal structure at the top region of the pillar structure even when it was subjected to H₂ plasma dry etching.

It is known that the formation of metal silicide at the initial reaction is the key point in the Si-based VSS growth mechanism.³² Up to now, growth of Si nanostructures through VSS mechanism using Al,³³ Au²⁴ and Cu³² as catalyst at temperature of 430 °C, 350 °C and 510 °C, respectively had been reported. These synthesis temperature are below their eutectic temperature. In addition, the requirement of thermal energy is strongly depended on the decomposition of Si-based precursor and then facilitate the metal-silicide formation.²⁴ In this study, the growth temperature of crystalline Si film of 185 °C for a deposition time of 3 min were estimated by using an infrared pyrometer aiming at the substrate holder. It is of interest how Au film could interact with Si to form a Si pillar structure in SiCl₄/H₂ microwave plasma at such low temperature. Plasma dry etching of Au film using F- or Cl-containing reactants, e.g., CF₄–CCl₄ mixtures³⁴ and pure Cl₂,³⁵ is a well-known technique. Fortunately, the etching species H, Cl and SiCl_x (1 ≤ x ≤ 3) radicals existed in microwave plasma when SiCl₄ and H₂ were used as the reactants. We believe that the Cl-based radicals had etching ability upon reaching the Au surface, and thus formed the catalyst with nanosized Au particles from the initial Au film. Additionally, MWPECVD exhibiting high-density plasma and high ionization rate^22,23 allows a further decrease in synthesis temperature by using plasma activation. Because of these two reasons, developing a crystalline Si pillar structure even under a low substrate temperature (lower than 185 °C) became possible. This was achieved by formation of nanosized Au-catalyst particles using plasma dry etching of Cl-based radicals followed by preferential growth of Si nanostructures from Au-catalyst particles via VSS mechanism through a metal-silicide (Au₂Si) formation in SiCl₄/H₂ microwave plasma. According to the abovementioned results, the suggested growth mechanism of the crystalline Si pillars synthesized using SiCl₄/H₂ microwave plasma via Au-catalyzed VSS mechanism is proposed in Fig. 5.

Fig. 5 The suggested growth mechanism of the crystalline Si pillars synthesized using SiCl₄/H₂ microwave plasma via Au-catalyzed VSS mechanism. (a) The initial Au film was etched by the Cl-based radicals to form (b) nanosized Au particles, followed by (c and d) preferential growth of Si nanostructures from Au-catalyst particles via VSS mechanism through a metal-silicide (Au₂Si) formation in SiCl₄/H₂ microwave plasma.

3.2 Electrical properties of the crystalline Si films

To investigate the effect of structural characteristics on the transport behavior of electrons, the electrical conductivity in terms of σ_∥ and σ_⊥ was measured, as shown in Fig. 6. In our previous work, σ of the crystalline Si film was determined by crystallinity and carrier concentration, with σ_∥ of i-Si film, n-Si film, and p-Si film being 3.5 × 10⁻⁹, 9.48, and 7.83 S cm⁻¹, respectively.²³ The drastic increase in conductivity of the doped crystalline Si film is due to the rise in the carrier concentration resulting from active dopants integrated into the Si network. After the Mo interfacial layer between the crystalline Si film and PI substrate was introduced, σ_∥ of the i-Si film sample was still low because of its semiconductor characteristics; however, a high σ_∥ values for the n-Si film sample (15.28 S cm⁻¹) and for the p-Si film sample (14.73 S cm⁻¹) were obtained. Further increase in σ_∥ for both the doped crystalline Si films is mainly attributed to the enhancement in the lateral electron-conducting path by the Mo interfacial layer. However, the synthesized discontinuous nanostructures in the p-Si pillar sample and H₂: p-Si pillar sample create an uneven conducting channel for electron transport, resulting in a decrease in σ_∥ of both samples.

Fig. 6 Electrical conductivity of the synthesized crystalline Si films possessing different structures: (i) i-Si film, (ii) n-Si film, (iii) p-Si film, (iv) p-Si pillars, and (v) H₂: p-Si pillars. Electron transport (I) along the crystalline Si film surface (σ_∥) and (II) across the crystalline Si film thickness (σ_⊥) were measured. Schematics of the two configurations are also presented.

Electron transport is known to proceed from the bottom to the top of the films during EFE behavior. Consequently, evaluation of electron transport ability across the Si film (i.e., σ_⊥) is important, as it significantly affects EFE performance. According to the results shown in Fig. 6, the σ_⊥ values of all crystalline Si film samples are higher than σ_∥. A probable explanation for these results is the microstructure feature, viz., columnar structure for i-Si, n-Si, and p-Si films, as well as pillar structure for p-Si and H₂: p-Si pillars. Such structures feature an electrical transport path along the length of the columnar grains, which are perpendicular to the substrate and thus bypass the clustered defects and grain boundaries between the columns. Therefore, a high performance in EFE characteristics was anticipated.

3.3 EFE characteristics of the crystalline Si films

Fig. 7(a) and (b) reveal the EFE characteristics as J–E curves and corresponding F–N plots of crystalline Si films possessing different structures. EFE analysis results in terms of E₀, J@10 V μm⁻¹ and β based on the F–N theory were summarized in Fig. 7(c) and (d). Near-linear behavior at the high-E region in the F–N plots in Fig. 7(b) indicates that the EFE characteristics from all of the crystalline Si samples follow closely the F–N tunneling mechanism.

Fig. 7 EFE characteristics in terms of (a) J–E curves and (b) corresponding F–N plots of the synthesized crystalline Si films possessing different structures: (i) i-Si film, (ii) n-Si film, (iii) p-Si film, (iv) p-Si pillars, and (v) H₂: p-Si pillars. The calculated E₀, J@10 V μm⁻¹ and β from parts (a) and (b) are summarized in parts (c) and (d).

As expected, the negligible EFE characteristics of the i-Si film with an E₀ of 23.22 V μm⁻¹ (here, defined as the E needed to extract a J of 10 μA cm⁻²) and a low J of 2.86 × 10⁻⁴ mA cm⁻² at an applied E of 10 V μm⁻¹ were obtained. The high applied E and extremely low J value indicate that emission of electrons from the i-Si film surface is difficult. In contrast, the n-Si film could be switched on at an E₀ of 11.93 V μm⁻¹ to attain a J value of 4.64 × 10⁻³ mA cm⁻²@10 V μm⁻¹; whereas p-Si film needed a lower E₀ (10.31 V μm⁻¹) to turn on the EFE behavior and achieve a higher EFE capacity (J = 9.34 × 10⁻³ mA cm⁻²@10 V μm⁻¹). β, which is a parameter dependent on the geometry, electron transport ability, crystal structure, and density of emitter of the emission sites, may be estimated from the F–N equation to assess the degree of EFE characteristics of the materials.³⁶ The β value can be derived from eqn (1) and expressed as follows (eqn (2)):

where m is the slope of the F–N plots in the high-E region. β values of the n-Si and p-Si films are 587.73 and 686.67 respectively, which are much higher than that of the i-Si film (319.39), indicating a strong effect of σ on EFE performance of the n- and p-Si films. The high conducting characteristics of both doped crystalline Si films facilitate electron transport through the material and emission from the surface, resulting in a decrease in E₀ accompanied by high J and large β.

In addition, the EFE performance from p-Si film occurring at a lower E and a higher J were also found. The p-Si and the n-Si films with substantial similarities in microstructure characteristics and σ suggest that localized field enhancement and ability for electron transport play insignificant roles in this situation. According to the literature, the difference in EFE performance is ascribed to band bending induced by surface states.^18,19,36 For the n-type cathode, mid-band-gap surface states become negatively charged, resulting in the formation of a potential barrier. This potential barrier prevents electrons from reaching the apex of the n-type cathode. As band-bending acts opposite in a p-type cathode, there exists no surface potential barrier for the electrons (the minority carriers) transport to the emission site.^18,19,36 This is the reason why the p-Si film exhibited better EFE characteristics than the n-Si film.

The effect of nanostructures on EFE performance was also investigated. The p-Si pillar sample possessed superior EFE properties, viz., the EFE process could be turned on at E₀ of 7.97 V μm⁻¹ with a J of 5.10 × 10⁻¹ mA cm⁻²@10 V μm⁻¹, in contrast to the p-Si film. The corresponding β is as high as 940.92. Furthermore, E₀ further decreased to 5.85 V μm⁻¹, with J sharply increasing to 1.37 mA cm⁻²@10 V μm⁻¹ after the H₂ plasma dry etching process. Hence, the H₂: p-Si pillar sample exhibited an outstanding β value of 1281.13, which is almost four times higher than that of the i-Si film. EFE characteristics are well understood to be a result of combined effects of both electrical and geometric contributions. Because of similar results of σ for the p-Si film, p-Si pillar, and H₂: p-Si pillar samples (Fig. 6), the main factor leading to enhancement of EFE performance for the latter two these samples is ascribed to the formation of nanostructures. Nanostructures with sharp configuration features and nanosized diameter increase the localized electric field, leading to facile emission of electrons from the surface of the material.^18,19 Therefore, the H₂: p-Si pillar sample with high σ and consisting of a B-doped crystalline Si pillar structure with 50 nm diameter and a sharp feature with a small apex radius at its tip is a promising material for an ideal field emitter. An additional benefit for higher EFE performance is resulting from the reduction of the barrier layer due to removal of the native oxide surrounding the as-synthesized p-Si pillar structure via H₂ plasma treatment.¹⁴

As compared with the published data on EFE characteristics of nanostructured Si films (Fig. 8) prepared on heterogeneous substrate using various bottom-up approaches,^{10,14,16,37–40} this study demonstrate that H₂ plasma treated B-doped crystalline Si pillar sample possessed high EFE performance with E₀ = 5.85 V μm⁻¹, J = 1.37 mA cm⁻²@10 V μm⁻¹, and β = 1281.13.

Fig. 8 Comparison of EFE performance of nanostructured Si film samples prepared on a heterogeneous substrate by using various bottom-up approaches.

3.4 Performance of the flexible crystalline Si-EFE prototype devices

Because the H₂: p-Si pillar sample showed the best EFE performance; the sample was therefore used as a cathode to construct the flexible Si-EFE prototype devices. The EFE characteristics of the device under mechanical static bending at R values of ∞ (plane surface) and 10 mm are shown in Fig. 9. According to the phosphorescent images of the luminous Si-EFE prototype device under bending conditions in parts (i)–(iv) of Fig. 9, the light intensity increased monotonically with the applied voltage as the EFE behavior was triggered above 500 V (i.e., E of 5 V μm⁻¹). Additionally, the uniform emission pattern from the entire cathode area may be observed clearly. This indicates that such a high-quality H₂: p-Si pillars has uniform microstructure characteristics, electrical properties, and EFE performance. With careful examining the J–E curves (Fig. 9), no obvious deterioration in EFE characteristics could be detected after the Si-EFE prototype device was bent to an R value of 10 mm, indicating a flexible and robust cathode material. E₀ and J@10 V μm⁻¹ were, respectively, 5.88 V μm⁻¹ and 1.39 mA cm⁻² for the unbent Si-EFE prototype device, and 5.88 V μm⁻¹ and 1.32 mA cm⁻² for the bent Si-FED. These results are consistent with the critical R value of 8.0 mm for crystalline Si films reported in our previous study,²² which is the minimum bending radius of curvature (or the largest bending strain) that the flexible Si film can tolerate.

Fig. 9 J–E curves of the flexible Si-EFE prototype device using H₂: p-Si pillar sample as emitter under mechanical static bending at R values of ∞ and 10 mm. Calculated EFE parameters in terms of E₀ and J@10 V μm⁻¹ are provided. The insets show phosphorescent images of the luminous Si-EFE device in the bent state at applied voltages of (i) 500 V, (ii) 700 V, (iii) 900 V, and (iv) 1100 V, which correspond to E values of 5, 7, 9, and 11 V μm⁻¹ respectively. The inset in (v) shows a photograph of the bent Si-EFE device at an R value of 10 mm.

Lifetime measurements of the flexible Si-EFE prototype device were carried out because the stability and durability is an important parameter related to prospective applications of the field emitters. During the measurement, the Si-EFE prototype device was bent under an R value of 10 mm, and then a constant voltage of 900 V (i.e., E of 9 V μm⁻¹) with a pulse width of 1 h was applied to generate periodical FE-on/FE-off tests for simulating practical operations. The results obtained are plotted as J–t curves (Fig. 10). The J@9 V μm⁻¹ of the bent Si-EFE prototype device remained stable over a period of 13 h which consists of a FE-on time of 7 h and FE-off of 6 h. After the lifetime measurement was extended to 19 h, the J@9 V μm⁻¹ still retain near 58% of its initial current, indicating a half-life time (τ_EFE) of over 10 h. The decrease in J is believed to be due to the formation of an oxide layer from the adsorption of oxygen under a vacuum environment with a pressure below 1 × 10⁻⁶ Torr, which results in an increase in the potential barrier for electron tunneling.³⁷ Therefore, the synthesized sample possessing very high stability, as compared with that made of carbon nanomaterials or other Si-based materials, demonstrates the feasibility of application in flexible devices.

Fig. 10 Lifetime measurement under an applied voltage of 900 V (i.e., E of 9 V μm⁻¹) performed by repeating FE-on and FF-off pulses with a width of 1 h for the bent Si-EFE prototype device using H₂: p-Si pillar sample as emitter at an R value of 10 mm. The insets show phosphorescent images of the luminous Si-EFE prototype device after testing for (i) 1 h, (ii) 7 h, (iii) 13 h, and (iv) 19 h.

In summary, a flexible Si-EFE prototype device based on H₂ plasma treated B-doped crystalline Si pillars on Mo/PI substrate as field emitter was reported. To synthesize the Si pillar nanostructure, we carried out microwave glow discharge decomposition of H₂–diluted SiCl₄ mixture incorporate with the SSSDS process using Au-catalyzed VSS mechanism followed by H₂ plasma dry etching process. The H₂: p-Si pillar sample exhibited high EFE performance with E₀ = 5.85 V μm⁻¹, J = 1.37 mA cm⁻²@10 V μm⁻¹, and β = 1281.13. Furthermore, the high flexibility and stability of the Si-EFE prototype device was also demonstrated. According to the results, it is suggested that the flexible H₂: p-Si pillars may hold promise for the development of a next-generation cathode material for high-performance EFE devices. Linking the flexible crystalline Si-thin film transistor²² acted as active-matrix driver electronics, a flat panel display integrated with fully Si material on a flexible polymer substrate can now be envisaged.

4. Conclusions

High EFE performance H₂ plasma treated B-doped crystalline Si pillars are synthesized on a Mo/PI substrate at a low temperature by using microwave glow discharge decomposition of H₂–diluted SiCl₄ mixture integrated with the SSSDS process via Au-catalyzed VSS mechanism, which was followed by H₂ plasma dry etching process. The mechanism for the development of crystalline Si pillar structures at low temperature of 185 °C is based on nanosized Au-catalyst particle formation from the initial Au film by plasma dry etching of Cl-based radicals. Subsequently, the preferential growth of Si nanostructures from the Au catalyst particle through an Au-silicide formation in a high-density plasma system (MWPECVD) is triggered. In this study, the H₂: p-Si pillars exhibits a grain with ∼50 nm diameter and a sharp apex curvature of 16 nm. The synergistic effects of geometric and electrical contributions led to the superior EFE performance of the H₂: p-Si pillar sample (E₀ = 5.85 V μm⁻¹, J = 1.37 mA cm⁻²@10 V μm⁻¹, and β = 1281.13). The flexible Si-EFE prototype device using the H₂: p-Si pillar sample as cathode showed high flexibility and stability, viz., the bent Si-EFE prototype device possessed a τ_EFE of over 10 h with a repeated FE-on/off tests for 9 times at a R of 10 mm. This device thus has the potential to be use in a high-performance field emitter array for nanoelectronic devices.

Acknowledgements

The authors are grateful to the Ministry of Science and Technology of the Republic of China, Taiwan, for financial support under contract No. NSC 101-2221-E-007-064-MY3 and MOST 104-2221-E-007-029-MY3.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10255g

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