Strain modulation in crumpled Si nanomembranes: Light detection beyond the Si absorption limit

Abstract

Although Si is extensively used in micro-nano electronics, its inherent optical absorption cutoff at 1100-nm limits its photonic and optoelectronic applications in visible to partly near infrared (NIR) spectral range. Recently, strain engineering has emerged as a promising approach for extending device functionality via tuning the material properties, including change in optical bandgap. In this study, the reduction in bandgap with applied strain was used for extending the absorption limit of crystalline Si up to 1310 nm beyond its intrinsic bandgap, which was achieved by creating the crumpled structures in Si nanomembranes (NMs). The concept was used to develop a prototype NIR image sensor by organizing metal-semiconductor-metal–configured crumpled Si NM photosensing pixels in 6 × 6 array. The geometry-controlled, self-sustained strain induction in Si NMs provided an exclusive photon management with shortening of optical bandgap and enhanced photoresponse beyond the conventional Si absorption limit.

The optical absorption limit of crystalline Si was extended toward SWIR range by strain modulation in crumpled Si nanomembranes.

INTRODUCTION

Recently, emergence of Si nanomembranes (NMs) with a pristine single crystalline Si thinned through silicon on insulator (SOI)–based fabrication techniques has promoted integration of Si-based electronic/optoelectronic devices in mechanically flexible and stretchable platforms (1–3). These Si NMs are easily transferrable on various mechanically flexible and stretchable substrates (1). Si NMs with thickness below 100 nm are widely used in fabrication of flexible devices as they exhibit stable electrical characteristics under mechanical bending and stretching. Along with conventional electronics, the need of photonic integrated circuits in future optical communications demands the development of near infrared (NIR) photodetectors (PDs) on Si platforms. However, the NIR photoresponse of Si-based devices has been limited to 1100 nm owing to its intrinsic indirect bandgap of 1.12 eV. Various approaches, such as alloying with Ge and preparing ultrathin Si-Ge quantum well structure stacked on a lattice matched NM, have been explored for extending the photoresponsivity of bulk Si beyond 1100 nm (4, 5). Nevertheless, practical implementation of such systems is constrained by the limited growth thickness, complex fabrication techniques, and indirect bandgap nature of such Si-Ge alloys. Other alternatives, such as introduction of surface-plasmon-polariton–based nano-groove arrays in Si NMs and creation of sub-bandgap energy level via hyper doping with chalcogen-or-metal atoms, have also been attempted (6–9). However, these approaches have serious disadvantages of processing complexities and compromise in complementary metal-oxide semiconductor (CMOS) compatibilities. Reportedly, the band structure of Si can be substantially modulated under the influence of strain (10–14); consequently, the optoelectronic properties of Si can be tuned considerably by strain engineering. The strain-induced bandgap modulation in Si NMs appears as a promising approach of tuning the photosensing in the NIR wavelength beyond its fundamental optical absorption cutoff.

Strain engineering has been applied to various forms of low-dimensional Si such as nanowires (10, 11), nanocrystals (15, 16), and NMs (13, 17), for investigating its effect on optical properties of Si. We have demonstrated the strain-induced tuning in the optical absorption of Si for detecting the short-wavelength infrared light (17). We externally applied strain using a uniquely designed bulge test setup. Although a high level of biaxial strain (~3.5%) was successfully applied to Si NMs, the gas pressure induced bulging was a complex approach, which provides a nonsustainable strain into PD pixels and thus lacks device practicality for flexible and stretchable device applications in a reversible manner. Utilization of compressive buckling or wrinkling in Si NMs via an elastomeric substrate is a unique CMOS compatible strategy for circumventing the aforementioned issues and can apply substantial amount of strain in Si NMs for modulating its bandgap (18). In addition, the use of compliant elastomeric material as a substrate containing Si NMs allows the implementation of fabricated devices into flexible and stretchable applications (19). In this approach, the applied strain can be modulated depending on the pre-strain of the compliant substrate; once induced, the buckled or crumpled structure retains the strain. By modulating the pre-strain, the periodicity, amplitude, wavelength, and, consequently, the applied strain within the crumpled structure could be tuned according to the requirements. In addition, the strain into the Si NMs can also be tuned by controlled stretching of the elastomeric substrate containing the crumpled structures.

In this study, we report NIR photosensing beyond the fundamental absorption limit of Si through geometry-controlled self-sustained strain induction in crumpled Si NMs. Crumpled Si NMs of various thicknesses were used for fabricating PDs in metal-semiconductor-metal (MSM) configuration, and their photodetection characteristics were investigated extensively. Crumpled Si NMs (35- and 70-nm thick) exhibited broad photoresponse in wavelengths of 405 to 1310 nm, surpassing the intrinsic absorption limit of Si. A 6 × 6 matrix of MSM-type PD arrays was fabricated with crumpled Si NMs (~35 nm) on polydimethylsiloxane (PDMS) substrate for demonstrating the room-temperature NIR imaging capability. The combination of mechanical resilience of ultrathin Si NMs and geometry-controlled self-sustained strain induced in the crumpled structure provides a useful approach in development of NIR detectors for future imaging applications, such as electronic eye based on mechanically flexible and stretchable platforms.

RESULTS

Si NMs of two different thicknesses, 35 and 70 nm, were used for investigating the effect of crumpling on applied strain and photosensing characteristics. Figure 1A and fig. S1 schematically represent the fabrication of crumpled Si NM–based NIR PDs and their working in different wavelength range of incident light under reversible stretching. The scanning electron microscopy micrographs of 35- and 70-nm-thick crumpled Si NM strips, prepared using 35% pre-strain value, are shown in Fig. 1B. The images clearly reveal the formation of uniform and periodically crumpled geometry with definite peaks and valleys in Si NMs. When an ultrathin, uniform layer of a high-modulus material is strongly attached to a pre-stretched low modulus supporting substrate, a wavy crumpled structure is obtained after the strain of the supporting substrate is released (19–22). The geometrical parameters, such as wavelength (λ) and amplitude (h), of such wavy structures are affected by the pre-strain applied in the elastomeric substrate (fig. S2). With an increase in the pre-strain in PDMS, the λ and h of the obtained crumpled structures in the Si NMs decrease and increase, respectively. Since the length of the Si NM is fixed, to accommodate this excess strain, the h of the crumpled geometry increases, while λ reduces for the Si NMs of a particular thickness. To further quantitatively analyze the formation of peaks and valleys in the crumpled geometry of 35- and 70-nm-thick Si NM strips (prepared with 35% pre-strain), the three-dimensional (3D) imaging of surface topography using atomic force microscopy was performed, and the results are presented in Fig. 1C. The obtained results clearly exhibit uniform and periodic wavy geometry with a sinusoidal pattern. For 35-nm-thick Si NMs, the λ and h of the crumpled structure were found to be 3.1 and 650 nm, respectively. Whereas, for 70-nm-thick Si NMs, the values of λ and h were found to be 11.5 and 4.4 μm, respectively. These results signify an increase in both the λ and h values of the obtained crumpled structures with an increase in the thickness of Si NMs. This is attributed to the fact that the stress release of the elastomeric PDMS substrate containing thin Si NMs leads to the formation of well-defined wavy structure with higher local strain within the peaks and valleys of the crumpled geometry. In such crumpled structures, both the wavelengths and amplitudes depend linearly on the Si NM thickness for a specific pre-strain value, and, as a result, thick Si NM having higher bending stiffness creates lower number of wrinkles than thin Si NM (19). The existence of crumpled structure and strain enhances the photosensing characteristics of the Si NM strips due to the improvement of light extraction efficiency through light trapping/scattering effect and the extension of the optical absorption through band structure modulation. These characteristics will be discussed in detail later. On the basis of the distinctive benefits of the crumpled geometry, a prototype of an imaging system with crumpled Si NMs has been fabricated on PDMS substrate (Fig. 1D, see Materials and Methods).

Fig. 1. Fabrication of crumpled Si NMs and their implementation into stretchable 6 × 6 array devices for strain controlled photodetection.

(A) Schematic representation of the fabricated crumpled Si NM–based NIR PD unit working under reversible stretching. (B) SEM images of the crumpled Si NMs of 35- and 70-nm thicknesses prepared with the pre-strain of 35%. (C) 3D atomic force microscopy topographic images of crumpled Si NM samples of 35- and 70-nm thickness with corresponding surface and height profiles. (D) Digital photographs and optical images of fabricated 6 × 6 array device with crumpled Si NMs on bendable/stretchable PDMS substrate. Photo credit: Ajit K. Katiyar, Yonsei University.

We used confocal Raman spectroscopy for evaluating the actual mechanical strain induced in the peaks and valleys of the crumpled Si NMs, and the results are presented in Fig. 2. Figure 2 (A and B) shows the optical images of the 35- and 70-nm-thick crumpled Si NMs samples prepared by transferring Si NMs on 35% pre-stretched PDMS. The schematic representation of Raman scattering through the crumpled Si NM samples of different thicknesses is shown in Fig. 2C. The effect of crumpling on the application of strain was observed across both of the Si NM samples (Fig. 2D). A broad Raman peak with distinguishable compressive and tensile components can be clearly observed at the peak and valley regions in the Raman spectrum for both the samples. The Raman spectrum recorded at different locations (valley, flat part, and peak are denoted by i, ii, and iii, respectively, for 35-nm-thick Si sample in Fig. 2A and 1, 2, and 3, respectively, for 70-nm-thick Si sample in Fig. 2B) reveals the existence of different strain characteristics at the peak and valley region. For example, 35-nm-thick sample mainly exhibits tensile or compressive strain at the peak or valley, respectively. On the other hand, 70-nm-thick Si NM sample exhibits the existence of both tensile and compressive components simultaneously at peak and valley regions, where tensile (compressive) components dominate at peak (valley) region. The well-ordered crumpling of Si NM provided a periodical strain distribution along pre-stretching direction (x direction) and a gradual tensile-to-compressive variation along z direction at the peak region. This inhomogeneous strain distribution along the vertical direction mainly originates from the combined effect of local bending and the shear forces existing at the interface of Si NM and elastomeric substrate. Because the penetration depth of 532-nm laser in Si is much higher than the thickness of the probed Si NM samples, the Raman signal from the total thickness can be obtained. As a result, both tensile and compressive components are obtained at the peak and valley region in 70-nm-thick Si NM sample with dominance of tensile (compressive) component at the peak (valley) region. The maximum strain value (compressive or tensile) obtained for both 35 and 70 nm samples was ~1.5%, which is enough for notably extending the optical absorption toward NIR wavelength range. The strains occur at peaks and valleys of the formed wavy structure mainly originating from the local bending of the Si NMs and can be calculated using; ɛ = t/2R_c, where t is the Si NM thickness, and R_c is the radius of the curvature at the peaks or valleys of the wave. In general, the shape of crumpled Si fabricated by releasing the pre-strain in the PDMS elastomer results in a wavy geometry analogous to a simple sine function, such as y = Asin(2π/λ)x, where A and λ represent the amplitude and the wavelength of the wave, respectively. Using the sine function approximation, we can estimate the radius of the curvature R_c and redefine the strain in the crumpled wavy geometry of Si in terms of amplitude, A, and wavelength, λ, using the equation: ɛ = 2π²At/λ² (19). Thus, the applied strain into the peaks or valleys is usually independent of Si thickness t due to the linear dependence of the λ and A values with respect to the thickness and corresponding adjustment in the radius of curvature. Raman peak position and intensity maps obtained across the highlighted part of the crumpled Si samples are presented in Fig. 2 (E to H). The peak position and the intensity maps clearly reveal a well-defined strain distribution profile with periodic variation in strain across the crumpled structure, in which mainly tensile and compressive strains occur at the peak and valley regions, respectively.

Fig. 2. Evaluation of strain distribution across the crumpled geometry in Si NMs.

(A and B) Optical images of the 35- and 70-nm-thick crumpled Si NM samples with the enlarged view of the region used for the Raman mapping. The black and white scale bars correspond to 6 and 20 μm, respectively. (C) Schematic of Raman scattering through the crumpled Si NM samples of different thicknesses. (D) Raman spectra of the 35-nm-thick (top) and 70-nm-thick (bottom) crumpled Si NM samples recorded at the different locations marked in the optical images. (E to H) Raman peak position and intensity distribution mapping images of 35-nm-thick [(E) and (F)] and 70-nm-thick [(G) and (H)] crumpled Si NM samples obtained across the magnified region in optical images and the dotted red line.

The strain-driven unique optical properties of crumpled Si NMs were then used for evaluating their photoresponse characteristics by fabricating PD devices with a simple MSM architecture. This type of mechanically flexible and stretchable two terminal devices are the basic building blocks that can be further combined for developing an image sensor integrated into advanced mechanically flexible and stretchable circuits with proper functionality. Figure 3A represents the schematic of the fabricated device with its electrical measurement setup. Figure 3B shows the optical microscopic images of the flat and crumpled states of the single MSM devices fabricated using a 70-nm-thick Si NM. The device was illuminated from the top from optical fiber–coupled laser diodes of various wavelengths in the range of 405 to 1310 nm. Figure 3C schematically represents the mechanism of optical absorption enhancement in crumpled Si, which will be discussed in detail later. For comparison, photoresponse characteristics of flat Si NM devices (the same device before releasing the pre-strain) were also studied in the similar manner. The current-voltage characteristics of the devices fabricated with 35- and 70-nm-thick crumpled Si NMs showed typical MSM-type nature (fig. S3A) with a substantial change in the current under the light exposure (fig. S3B). The transient photoresponse characteristics of the devices fabricated with 70- and 35-nm-thick Si NMs were recorded at an applied electrical bias of 5 V under the irradiation of 520-, 685-, 980-, and 1310-nm wavelengths, and the results are shown in Fig. 3 (D and E, respectively). The optical sources were switched on and off for visualizing the dynamic change in the photocurrent developed in the illuminated devices. The obtained transient photocurrent plots reveal that for the incident wavelengths of 520, 685, and 980 nm, both flat and crumpled states of the devices exhibited a clear on-off trend with a higher photocurrent in crumpled state indicating the superior photosensing capability of crumpled Si. With enhanced optical absorption in crumpled Si NMs, a greater number of electron and hole pairs were generated under illumination, thus, resulting in enhanced photoresponse. When the NIR light of 1310 nm was irradiated on the flat device, no photoresponse was observed, whereas the crumpled devices exhibited a very clear light detection. The intrinsic optical bandgap of Si (1.12 eV) is appropriate for the detection of 520, 685, and 980 nm. However, for the detection of 1310 nm, the optical bandgap should be ≤0.94 eV. A wavelength of 1310 nm is well beyond the absorption range of intrinsic Si; therefore, photogenerated charge carriers, which would lead to photoresponse, were not generated in the flat Si NMs. The distinct photoresponse obtained under 1310-nm illumination for the 35- and 70-nm-thick crumpled Si NMs implied the existence of enough strain into the crumpled structure. This strain reduced the bandgap of Si and enabled the detection of 1310 nm. The 70-nm-thick crumpled Si NM device exhibited higher photocurrent compared with 35-nm-thick device, revealing the significance of the thickness of the Si NM in detecting 1310-nm wavelength. The 15-nm-thick crumpled Si NM–based device prepared with the maximum pre-strain (35%) could not detect NIR wavelength beyond 980 nm, which is possibly due to the low optical absorption cross section of thin Si NMs. In contrast, thicker Si with higher absorption cross section can provide extended photosensing; therefore, Si NM devices with thicknesses of 35 and 70 nm and 35% pre-strain were fabricated and characterized for potential detection of NIR wavelength. Here, 35% pre-strain was found as the optimized value that can be applied to PDMS of appropriate thickness.

Fig. 3. Strain-induced photoresponse of the fabricated crumpled Si NM–based MSM PD units.

(A) Schematic representation of the photodetection in crumpled Si NM–based device. (B) Optical microscopic images of the fabricated MSM PDs with flat and crumpled geometry. Scale bars, 30 μm. (C) Schematic representation of optical absorption enhancement in crumpled Si via multiple scattering of the incident light. (D and E) Photoresponse characteristics of the planar and crumpled PD devices fabricated with 70- and 35-nm-thick Si NMs, respectively. Photoresponse is recorded under the exposure of 520-, 685-, 980-, and 1310-nm illumination.

To further evaluate the PD figure of merits of the crumpled Si devices, photocurrent density and responsivity of 70-nm-thick crumpled Si with respect to incident optical power for 1310-nm illumination was recorded (fig. S4A). As expected, with an increase in the incident optical power, the photocurrent density and responsivity were found to increase and decrease, respectively. A similar trend for the photocurrent density and responsivity has also been reported earlier for various 2D materials and thin Si-based PD devices (23, 24). The fabricated crumpled devices were observed to show a notable photocurrent under 1310-nm illumination even at a low power density of 75 mW/m². Another important parameter for MSM-structured PDs is the normalized photo–to–dark current ratio (NPDR), which is defined as NPDR = (I_ph/I_d)/P_in (25). The incident optical power dependent NPDR variation of 70-nm-thick crumpled Si NM device under 1310-nm-illumination is shown in fig. S4B. Similar to the responsivity, the NPDR also decreases with increase in incident power of the 1310-nm illumination. The minimum value of NPDR was calculated as 7.72 W⁻¹ at incident power density of 0.16 Wm⁻². The decrease in the NPDR and responsivity with increase in incident power density is attributed to the presence of trap states at the surface of Si NMs. The external quantum efficiency (EQE) is another important metric, which refers to the incident-photon–to–collected-electron conversion efficiency. We evaluated the EQE values of both flat and crumpled devices when subjected to incident light of different wavelengths (520, 685, 980, and 1310 nm). These values were estimated using the equation EQE (%) = (1240I_p/λ_LP_o) × 100, where I_p, λ_L, and P_o represent the photocurrent, wavelength of the incident light, and optical power, respectively (fig. S5). The introduction of a crumpled structure in Si NMs led to a remarkable enhancement in EQE values and the expansion of the light detection range. This phenomenon is attributed to the synergistic impact of bandgap narrowing induced by applied strain and the light-trapping effect. As demonstrated previously, the wrinkled or wavy structuring in the layered materials can enhance the light matter interaction and thereby reduce light reflection and consequently amplify the optical absorption (26, 27). The mechanism behind the absorbance enhancement in the crumpled Si NMs could be better understood with the schematic shown in Fig. 3C. The top panel of the figure shows the cross-sectional view of the flat Si NM attached to PDMS surface. In this case, the incident light interacts with Si NM following the optical path (denoted by the red arrows) limited by the two interfaces between (i) air-Si and (ii) Si-PDMS. In between these interfaces, light gets absorbed in the Si (photo-active layer) and results in the observed photocurrent via electron hole pair generation. However, as shown in the bottom panel of the figure, in case of crumpled geometry, the optical path traversed by the incident light rays within the photo-active Si layer becomes longer as a consequence of multiple scattering, internal reflections, and interferences at the interfaces (19, 20). Therefore, the probability of light absorption in the photo-active Si layer becomes higher in case of crumpled geometry compared with the flat structure. The crumpled geometry in Si NM strips of the same thickness of the flat Si NM strips has introduced unique optical properties. The crumpled geometry in the Si NMs strongly affects the optical properties and, consequently, the photoresponse of the ultrathin Si NMs. The ultralow thickness of the Si NMs provides a low-absorbance cross section in the flat state, which results in lower photoresponse. The effect of thickness of Si NM and crumpling on its absorbance was theoretically investigated for 35- and 70-nm-thick flat and crumpled Si NM samples. The corresponding results are presented in fig. S6, in which the calculated theoretical limit of light trapping (4n²) for 35- and 70-nm-thick Si NMs are also plotted for comparison (28). We observed that with increase in thickness, the absorbance also increased for both flat and crumpled geometry. This enhanced absorbance in the thicker Si NM can be explained by the equation of I = (1 − R)I₀e^−αt, where R, I₀, I, α, and t are the reflectance, incident light power, transmitted light power, absorption coefficient, and thickness of Si NM, respectively. The absorbance in Si NMs was observed to considerably increase with the introduction of crumpled geometry. Similar results were also reported previously in wrinkled and crumpled structures of various other materials (20, 26). Thus, the Si NMs of sufficient thickness can provide notable photoresponse, which can be further increased via incorporating crumpling into the fabricated devices.

To examine the effect of crumpling on the optical absorption, both flat and crumpled Si NMs were further investigated using finite element method (FEM) performed with COMSOL Multiphysics platform, and the results are presented in Fig. 4 (A and B). Dimensions of the crumpled geometry, such as wavelength and amplitude, of both of the thicknesses of Si NMs were extracted from experimental results and used in the simulation. The planar geometry of both the thicknesses of Si NMs was also considered for comparison. The light absorption scheme with the electric-field intensity distribution profile for the crumpled and planer geometry of Si NMs bonded on PDMS substrate was solved by a theoretical model based on Fresnel’s equation using the wave optics module of COMSOL Multiphysics software. The modeled structures of the crumpled Si NMs are shown in fig. S7. To evaluate the optical absorption characteristic of different crumpled Si samples and their planar counterparts, the cross-sectional electric field intensity distribution (|E|²) above and within the crumpled Si samples including the PDMS substrate was simulated for an incident light of wavelength varying between 400 and 980 nm (fig. S8). The electromagnetic waves, which propagate downward as plane waves, were considered to be perpendicularly incident on the samples from the top to bottom (fig. S7). The incident electric field interacts with Si NM surfaces and subsequently partially transmitted, scattered back, and absorbed in the Si NMs. Figure 4 (A and B) shows the color-coded electric field distribution obtained for planar and crumpled Si NM samples of 35- and 75-nm thicknesses, respectively. The field distribution for planar Si NMs of both the thickness showed ordered fringing patterns, which originated from the superposition between the incident and reflected wave trains (29–31). The field distribution for crumpled geometry is notably different than that of the planar one, where the noticeable difference is the absence of fringing patterns. A substantial light scattering occurs at the uneven morphology of the wavy geometry, and it modifies the propagation directions of the reflected light. Therefore, the interference was not so pronounced in the case of crumpled samples owing to the weak reflected wave trains; thus, a nonuniform distribution of relatively weak electric field was obtained across the medium above the samples. Notably, the light scattering in the crumpled NM sample of thin Si was more pronounced than that of the thick one. This can be attributed to the existence of increased number of crumples and high radius of curvature at the peak and valley of each crumple in thin Si NM sample. Furthermore, the mechanical strain distribution in the crumpled structure of 35- and 70-nm-thick Si NMs was also studied through simulation using COMSOL Multiphysics software. The simulated 3D strain distribution in the 35- and 70-nm-thick crumpled Si NM models is presented in Fig. 4C and fig. S9. The numerical values and 3D distribution of the strain induced in the modeled crumpled Si NM system were observed to approximate the experimental strain values and distribution. A similar tendency of strain distribution was obtained with tensile and compressive strains at the peaks and valleys, respectively. These simulated mechanical strain results matched well with the strain evaluation and distribution obtained from confocal Raman spectroscopy (Fig. 2).

Fig. 4. Effect of crumpled geometry and strain on the optical absorption enhancement and tuning through bandgap modulation.

(A and B) Theoretical calculation of the electric field distribution across the crumpled and planar Si NMs surfaces of 35- and 70-nm thicknesses demonstrating the optical absorbance enhancement in crumpled structure. (C) Theoretical calculation of strain distribution for 35-nm-thick (left) and 70-nm-thick (right) crumpled Si NM samples prepared with 30% pre-strain. Color coded scale bar corresponds to the strain values in percent. (D) Electronic band structure estimation of unstrained flat (left) and crumpled (right) Si NM through DFT calculation. The strain induced bandgap modulation in crumpled Si provides the photodetection of 1310-nm light. (E) DFT calculated variation of optical bandgap in crumpled Si NMs as a simultaneous effect of tensile (x) and compressive (z) strains.

The photodetection characteristics of 35- and 70-nm crumpled Si NM devices, beyond the conventional absorption limit of Si (1100 nm), can be realized using the band structure calculations of Si in unstrained and strained conditions. The band structures of Si NMs for flat and crumpled states were modeled using density functional theory (DFT) calculation, and the results are presented in Fig. 4D. The induction of strain into Si can considerably alter the atomic arrangements of the lattice, consequently reducing the bandgap (11–13, 32, 33). It was predicted that for the unstrained case, the conduction band minimum was located at X point, while the valence band maximum was located at Γ point, implying indirect optical transition in Si with a bandgap of 1.12 eV (Fig. 4E). With this band structure, detecting photons of 1310-nm wavelength is difficult for Si NMs. Whereas when sufficient strain was induced in Si NMs by creating crumpled structures, the valance band maxima and conduction band minima approximated each other, and the bandgap was reduced. Specifically, as approximated from the DFT calculation results, the strain-induced reduction in the bandgap (ΔE ~ 0.18 eV) was due to the collective effect of tensile and compressive strains (1.5% ϵ_x and −1% ϵ_x) induced in the crumpled structure. When thin materials were transformed into such wavy structures via crumpling, the peak and valley of the individual wave experienced tensile and compressive strains, respectively. The crumpling induced strain into Si reduced its bandgap to ~0.94 eV, which was sufficient for detecting the photons of 1310-nm wavelength. Thus, bandgap of crumpled Si NMs was reduced from 1.12 to ~0.94 eV through the strain induction and, consequently, phonon-assisted photon absorption process helped the detection of photons of 1310-nm wavelength.

The uniquely designed PD devices on crumpled Si NMs offer the tuning in the optical absorption and wavelength detection through change in applied strain level via either changing the pre-strain level or controlled stretching of elastomeric substrate. To systematically investigate the effect of crumpling and strain on the photoresponse, the photocurrent of the crumpled Si NM was recorded under different stretching levels, and the results are presented in Fig. 5A. The obtained dynamic photocurrent spectra reveal a systematic decrease in the photocurrent with an increase in the stretching level, with almost zero photocurrent at a stretching level of 30%. The photoresponse returned to its initial level after releasing stretch to recrumple Si NMs. The gradual stretching of crumpled structure resulted in the flattening of the structure along with decrease in the applied strain value. The images shown in the inset of Fig. 5A clearly reveal such stretching-induced flattening and the reappearance of the crumpled structure after removal of stretching force. The observation of the decrease in the photocurrent with dynamic stretching was in correlation with the enhancement in optical absorption with increased crumpling in Si NMs. When the stretching level was increased from 0 to 30%, the wavy nature of the Si NM device decreased, which decreased the optical absorption and consequently provided lower photoresponse. After analyzing the NIR photosensing capability of a single MSM device fabricated with crumpled Si, we demonstrated the NIR imaging capability by fabricating a prototype of a 6 × 6 array matrix with 70-nm-thick Si NM PD pixels. Figure 5B shows the schematic representation for demonstrating the imaging capability of the fabricated crumpled Si-based array device. Corresponding digital image of the shadow mask for creating a representative alphabet image of “Y” on the array device with the highlighted area of incident light beam is shown in the Fig. 5C. The obtained photocurrent mapping image clearly reveals the imaging of Y alphabet under 1310-nm light (Fig. 5C). To further demonstrate the combined effect of strain and crumpling on the photosensing and imaging in the Si NM PD pixel–based array device, the fabricated device was stretched and relaxed dynamically. Similar to the single-pixel device, the array device also exhibited a reversible change in the photoresponse of each unit pixel with almost zero photocurrent at 30% stretching (Fig. 5D). The obtained results with the demonstration of NIR photosensing using geometry-controlled and self-sustained strain modulation in crumpled Si NMs are very encouraging and have potential in future mechanically flexible and stretchable optoelectronic devices on Si platform.

Fig. 5. Strain-modulated dynamic photoresponse and imaging characteristics of the crumpled Si NM–based PD arrays.

(A) Dynamic photoresponse (measured with 1310-nm illumination) of the crumpled Si NM–based PD device recorded under the different stretching levels in a reversible manner. Inset shows the corresponding optical images of the crumpled Si device with different stretching levels. Scale bars, 30 μm. (B) Schematic demonstration of the imaging capability of fabricated crumpled Si NM PD pixel-based array device. (C) Obtained photocurrent map under the incident light projected through a shadow mask having an image of a representative alphabet “Y.” Scale bar, 2 mm. (D) Mapping image of the normalized photocurrent (under 1310-nm illumination) recorded with the fabricated 6 × 6 matrix of crumpled Si NMs subjected to the different stretching levels.

DISCUSSION

The NIR photodetection (1310 nm) has been demonstrated in Si by extending the optical absorption range through strain-induced bandgap modulation. The sustainable and tunable strain into Si was induced through creating crumpled structure by transferring Si NMs on pre-stretched PDMS followed by releasing of pre-strain. The effect of crumpled geometry and thickness on the optical absorption of Si NMs was studied both experimentally and through FEM simulation. The PD devices were fabricated with 35- and 70-nm-thick Si NMs in a simple MSM configuration, which exhibited a steady photoresponse to 1310-nm illumination under crumpled state. The enhanced photoresponse in crumpled geometry was attributed to the enhanced optical absorption obtained through the unique photon management phenomena occurring in the air, Si, and PDMS interfaces. The detection of NIR light of 1310 nm was realized owing to the combined effect of strain induced bandgap modulation and increased absorption cross section in the 35- and 70-nm-thick crumpled Si NMs devices. Furthermore, to demonstrate the use of crumpled Si into an NIR image sensor, the MSM PD pixels were organized in a form of 6 × 6 matrix, and the photocurrent patterns were recorded while mapping the representative images of an alphabet under the exposure of NIR light. The demonstration of NIR photosensing using geometry-controlled and self-sustained strain modulation in crumpled Si NMs is very encouraging for the development of mechanically flexible and stretchable optoelectronic devices.

MATERIALS AND METHODS

Fabrication of crumpled Si NM strips and stretchable 6 × 6 array devices

The fabrication process of crumpled Si NM strips along with their flat counterparts on stretchable PDMS substrate is schematically presented in fig. S1. The process begins with the cleaning of p-doped SOI wafers (SOI Tech: top Si, 70 nm; buried oxide, 2000 nm; handle Si wafer, 725 μm) using acetone, isopropyl alcohol, and deionized water. The thickness of the top Si layer was reduced to 35 nm with conventional oxidation process at 1000°C in an ambient atmosphere using a tube furnace, followed by wet chemical etching of the top oxide using buffered oxide etch. The thinning process of top Si was well optimized for achieving the desired thickness and has been described in detail in the previous report (17). The Si NM thickness was evaluated with “alpha SE” spectroscopic ellipsometer (Woollam Co.). For the fabrication of 70-nm-thick Si NM–based devices, the top 70-nm-thick Si of SOI wafer was directly transferred onto pre-stretched PDMS without any thickness reduction. A 6 × 6 array matrix system of crumpled Si NMs as a photosensing pixel with a size of 140 μm by 200 μm was fabricated using conventional photolithography and reactive ion etching process (Fig. 1D). The underneath BOX layer was then dissolved by dipping the samples in hydrofluoric acid with the Si NMs now lying on the handle Si wafer. The Si NM strips of the 6 × 6 matrix were then stamped on a pre-stretched PDMS substrate of ~1-mm thickness (component ratio A: B = 1:10, Sylgard 184, Dow Corning). The PDMS substrates of a particular length (L) were pre-stretched using a customized stretching machine that can stretch the PDMS precisely to a length of L + ΔL (ΔL, change in length). Before stamping, the PDMS substrates were exposed to ultraviolet light for 6 min in air for intensifying the bonding between the Si and PDMS. After stamping, the handle wafer was peeled off for releasing the Si NM strips on the PDMS. This formed the flat Si NM strips on the pre-stretched PDMS substrate. To complete the fabrication of the device in MSM configuration, Cr/Au (Cr: 5 nm/ Au: 50 nm) metal electrodes were deposited on the Si NM strips using a thermal evaporator. Last, the mechanical strain on the pre-stretched PDMS was released to form the crumpled Si NM strips, consequently forming the wavy geometry on the Si NM strips.

Strain and photoresponse measurement on crumpled Si NM samples

The strain values at the peaks and valleys of the crumpled geometry for different pre-strain values and different thicknesses of Si NMs were evaluated using confocal Raman spectroscopy method. Raman mapping and point spectra were recorded using Andor Tech spectrometer equipped with a liquid-nitrogen-cooled CCD and a 532-nm laser (Coherent, EN60825-1) as an excitation source. A low laser power of approximately 0.2 mW with a spot size of ∼1 μm was adopted. The photoresponse characteristics of the fabricated crumpled and flat Si NM–based MSM PDs were studied by irradiating collimated light beams using an optical fiber coupled with the laser diodes of various wavelengths in the range of 405 to 1310 nm. Corresponding current-voltage characteristics were recorded using a Keithley 4200 SCS parameter analyzer (Keithley Instruments Inc.).

Supplementary Materials

This PDF file includes:

Figs. S1 to S9