Ultra-thin Silicon Films Research Articles

Semiconductor-enhanced Raman scattering sensors via quasi-three-dimensional Au/Si/Au structures

Abstract We present a feasible way to strongly enhance Raman signals via introducing an ultra-thin dielectric film in the dual-layer plasmonic hotspots structure, which forms a quasi-three-dimensional structure. The Raman intensity was obtained with an enhancement factor of 735% for the dual-layer metal structure buffered with an ultra-thin silicon film. Moreover, the silicon layer based surface-enhanced Raman scattering (SERS) substrate provided a Raman signal two to five times larger than that of the silica buffered substrate. These distinct responses confirm that the ultra-thin high-index semiconductor film has the capability of additionally enhancing Raman scattering. Otherwise, the upper and lower metal clusters can support multiple kinds of plasmonic resonances, which produce a remarkable physical enhancement of the Raman signals. Besides these impressive optical properties, the substrates have prominent advantages on structural features, since the fabrication process can be fulfilled simply, suggesting a feasible way for a large-area and low-cost SERS platform. The findings may pave an avenue to achieve insights on the dielectric enhanced Raman scattering and hold potential applications in optoelectronics, such as environmental and health sensors.

Efficient two-level parallelization approach to evaluate spin relaxation in a strained thin silicon film

The evaluation of the spin lifetime in an ultra-thin silicon film is a massive computational challenge because of the necessity of performing appropriate double integration of the strongly scattering momentum-dependent spin relaxation rates. We have tackled the problem by dividing the whole computation range into two levels. Our scheme in each level is based on a hybrid parallelization approach, using the message passing interface MPI and OpenMP. In the first level, the algorithm precalculates the subband wave functions corresponding to fixed energies and archives the results in a file-based cache to reduce memory consumption. In the second level, we compute the spin relaxation time by using the archived data in parallel. This two-level computation approach shows an excellent parallel speedup, and most efficient ways to maximally utilize the computational resources are described. Finally, how an application of shear strain can dramatically increase the spin lifetime is shown.

Enhanced optical absorption in ultrathin silicon films using embedded silica-coated silver nanoparticles

Abstract Silver nanospheres with silica shells (Ag@SiO2) embedded into the absorber layer are explored as a plasmonic optical absorption enhancement method for thin-film photovoltaic cells. We observe dramatically enhanced absorption of the solar spectrum into a semiconductor medium by embedding a single monolayer of Ag@SiO2 nanoparticles in the central region of ultrathin mixed-phase amorphous/microcrystalline silicon (a/ μ c-Si) films. Optical absorption measurements indicate that integration of Ag@SiO2 nanoparticles increased absorption in the 400–1100 nm portion of the solar spectrum by 26% and 88% for 290 nm and 100 nm thick a/ μ c-Si thin films, respectively, in comparison to control films without any nanoparticles. This enhancement is concentrated in the longer wavelength range where the photon energy to bandgap ratio is higher, potentially yielding even larger increases in net device efficiency. Taking into account the interference of light within thin films, we show that parasitic losses due to particle absorption are minimal, consisting

Self-assembled antireflection coatings for light trapping based on SiGe random metasurfaces

We demonstrate a simple self-assembly method based on solid state dewetting of ultrathin silicon films and germanium deposition for the fabrication of efficient antireflection coatings on silicon for light trapping. We fabricate SiGe islands with a high surface density, randomly positioned and broadly varied in size. This allows one to reduce the reflectance to low values in a broad spectral range (from 500 nm to 2500 nm) and a broad angle (up to ${55}^{\ensuremath{\circ}})$ and to trap within the wafer a large portion of the impinging light $(\ensuremath{\sim}40%)$ also below the band gap, where the Si substrate is nonabsorbing. Theoretical simulations agree with the experimental results, showing that the efficient light coupling into the substrate is mediated by Mie resonances formed within the SiGe islands. This lithography-free method can be implemented on arbitrarily thick or thin ${\mathrm{SiO}}_{2}$ layers and its duration only depends on the sample thickness and on the annealing temperature.

Heteroepitaxy of Highly Oriented GaN Films on Non‐Single Crystal Substrates Using a Si(111) Template Layer Formed by Aluminum‐Induced Crystallization (Phys. Status Solidi RRL 3/2018)

For most light-emitting diodes (LEDs), high power electronics and solar cells applications, gallium nitride must be deposited as a thin film on the surface of another single-crystal material. By using the periodic arrangement of surface atoms as a template, gallium and nitrogen atoms can arrange themselves to form a crystalline thin film. For LED applications in particular, silicon carbide and sapphire are the most widely used single crystal substrates because they are transparent and their atomic arrangement is symmetrically matched with that of gallium nitride. While glass is cheaper than these substrates and also transparent, it lacks a regular, uniform arrangement of its atoms. This lack of a surface template pattern means that the Ga and N atoms have no template, so GaN cannot be grown directly on its surface. In article no. 1700392, this problem is overcome by forming an ultrathin (∼30 nm) silicon layer on the glass surface. The ultrathin silicon film provides a uniformly oriented surface that acts as the template for GaN growth, while being thin enough to remain transparent. This work also demonstrates that other amorphous and mismatched substrates, such as oxidized Si(001) and polycrystalline diamond, can be used in the same manner.

Introducing pinhole magnification by selective etching: application to poly-Si on ultra-thin silicon oxide films

Carrier selective junctions formed by polycrystalline silicon (poly-Si) on ultra-thin silicon oxide films are currently in the spotlight of silicon photovoltaics. We develop a simple method using selective etching and conventional optical microscopy to determine the pinhole density in interfacial oxide films of poly-Si on oxide (POLO)-junctions with excellent electrical properties. We characterize the selective etching of poly-Si versus ultra-thin silicon oxide. We use test structures with deliberately patterned openings and 3 nm thin oxide films to check the feasibility of magnification by undercutting the interfacial oxide. With the successful proof of our concept we introduce a new method to access the density of nanometer-size pinholes in POLO-junctions with excellent passivation properties.

Semiconductor meta-surface based perfect light absorber

We numerically proposed and demonstrated a semiconductor meta-surface light absorber, which consists of a silicon patches array on a silicon thin-film and an opaque silver substrate. The Mie resonances of the silicon patches and the fundamental cavity mode of the ultra-thin silicon film couple strongly to the incident optical field, leading to a multi-band perfect absorption. The maximal absorption is above 99.5% and the absorption is polarization-independent. Moreover, the absorption behavior is scalable in the frequency region via tuning the structural parameters. These features hold the absorber platform with wide applications in optoelectronics such as hot-electron excitation and photo-detection.

Zeroth Order Fabry–Perot Resonance Enabled Strong Light Absorption in Ultrathin Silicon Films on Different Metals and Its Application for Color Filters

In this work, Fabry–Perot nanocavity resonance enabled strong light absorption in ultrathin single layer silicon films on different metal surfaces was investigated. It was found that the peak absorption wavelength depends on the metal underneath the silicon film in addition to the silicon film thickness. Perfect light absorption was observed in deep subwavelength ultrathin silicon films on titanium (Ti) and chromium (Cr) metal surfaces in visible spectrum, which is caused by the zeroth order Fabry–Perot resonance mode where the round-trip phase delay in the nanocavity is zero. A perfect light absorber with an ultrathin silicon film as thin as 1/27 of the absorption wavelength on titanium metal surface has been demonstrated.

Transfer of Ultra-Thin Semi-Conductor Films onto Flexible Substrates

Applying tensile strain on an undirect band gap semiconductor crystal is a very promising way to tune it into a direct band gap [1] , [2] . This basic feature can be an outstanding progress for the use of classical semiconductor as silicon or germanium in interesting optoelectronic applications [3] . One way to apply a tensile strain is to transfer an ultra-thin of semiconductor onto a flexible substrate [4] (erg: an organic polymer). The goal of this paper is to propose a simple way to obtain an ultra-thin silicon film (<200 nm) onto an expendable polymer. It consists in transferring an Unibond™ SOI silicon thin film onto a classical tape maintain by a dicing frame. This process is mainly based on classical temporary bonding process. As shown on figure, a first process is carried out onto 200 mm wafer with a SOI silicon film of 200nm and a buried oxide box of 400nm. No default is observed on the full stack after the grinding and etching steps used to remove the SOI backside substrate. No SiO2trace is also observed after the last dioxide etching. The thin silicon film is then characterized and no crack is observed. The stack is then laminated onto a frame with a Furukawa SP-537T-230 adhesive tape. The demounting of the carrier is easy and no default appears on the silicon film. The last process consists in removing the glue from the silicon. We obtain a 200 nm silicon film bonded onto the tape. Nevertheless, this glue removal leads to the appearance of some cracks ono the silicon thin film. A second process will then be presented in order to avoid the last glue removal steps and improving the final thin film quality. XRR characterizations of the transfered thin film will be presented as well as potential amount of tensile stress achievable with this unique structure. [1] Ju Li et al., MRS Bulletin, 39, pp 108-114, 2014 [2] Feng Zhang et al., Phys. Rev. Lett. 102, 2009 [3] Diode Lasers and Photonic Integrated Circuits, Second Edition, 2012 John Wiley & Sons, chap 4 [4]& Salvatore Giovanni A.et al., ACS NANO Volume 7, Issue: 10, P8809-8815, 2013 Figure 1

Flexibe Carbon Nanotube/Silicon Thin Solar Cells Using Scalable Fabrication Techniques

Hybrid approaches that combine silicon (Si) and single walled carbon nanotube (SWNT) thin films could lead to the development of low cost and highly efficient flexible solar cells. Their earth abundance coupled to their intrinsic mechanical flexibility open a platform for the next generation of photovoltaic devices, which could be utilized in various applications such as portable power generators for emerging wearable electronics. However, most flexible thin-Si based solar cells are fabricated by multiple patterning steps and high diffusion/annealing temperatures which lead to high production cost and low scalability(1). In this talk, we will demonstrate a new class of flexible hybrid solar cells by combining mechanically flexible p-type SWNT thin films with planar ultra-thin flexible n-type Si wafers with a power conversion efficiency (PCE) of 8.61%. We will show that the enriched semiconducting SWNTs thin films have a dual function as a charge collector and an infrared (IR) photoconvertor. Flexible p-SWNT thin films are fabricated using a superacid sliding method developed in our lab (2, 3) and combined with flexible thin (≤50 µm) n-type Si wafers. We show flexible n-type wafers patterned to selectively expose the n-type Si windows surrounded by SiO2 insulating layers (Figure 1a). An ultrathin/flexible SWNT/Si solar cell based on 12 µm c-Si presents a Jsc (short circuit current density) of 14.1 mA/cm2, Voc (open circuit voltage) of 0.432 V, and FF (fill factor) of 0.571, resulting in a 3.48% PCE. By increasing the Si thickness to 50 µm, we illustrate that more photons are captured while the flexibility of the thin Si is still preserved with a 7.37% PCE (Jsc of 23.68 mA/cm2, Voc of 0.506 V, and FF of 0.615) (Figure 1b). We show that we can further enhance these devices to capture more photons by a simple solution coating of anti-reflective titanium oxide nanoparticles over on the SWNT/Si windows(4). This light-trapping step increases the PCE to 8.61% (Figure 1b). The added value of this new architecture provides the flexibility to tune the Si structure/thicknesses independently from the SWNT film process. Finally, we will reveal using transient absorption data that there are significant photogenerated carriers in the SWNTs films that are injected to the Si, which contributes to the large efficiency observed for an ultrathin film silicon solar cell. This means that there is significant photogenerated current in the IR, which is due to the presence of SWNTs where Si does not absorb. Indeed, the unique capability to control the functionality and thickness of SWNT thin film coupled to ultra thin c-Si gives new implications to the use of SWNTs for carbon based electronic devices. 1. S. Wang et al., Large-Area Free-Standing Ultrathin Single-Crystal Silicon as Processable Materials. Nano Letters 13, 4393-4398 (2013). 2. X. Li et al., Improved efficiency of smooth and aligned single walled carbon nanotube/silicon hybrid solar cells. Energy & Environmental Science 6, 879-887 (2013). 3. Y. Jung, X. Li, N. K. Rajan, A. D. Taylor, M. A. Reed, Record high efficiency single-walled carbon nanotube/silicon p–n junction solar cells. Nano Letters 13, 95-99 (2012). 4. X. Li, Y. Jung, J.-S. Huang, T. Goh, A. D. Taylor, Device Area Scale-Up and Improvement of SWNT/Si Solar Cells Using Silver Nanowires. Advanced Energy Materials 4, 1614-6840 (2014). Figure 1

Effect of particle clustering of silver nanoparticles on ultrathin silicon solar cell

Particle clustering is a major concern for uniform dispersal of nanoparticles in various deposition procedures. Well separated uniform distribution of metal nanoparticles is essential for effective coupling of surface plasmons. This work experimentally and theoretically, discusses the effect of nanoparticle clustering on the light trapping efficiency of silver nanoparticles. Pulsed laser deposition system has been used for deposition of silver nanoparticles, and substrate heating has been used to promote uniform distribution of nanoparticles. Pre-heated substrate depositions are compared with corresponding post-annealed samples. XRD, FESEM, Photoluminescence and UV-visible spectroscopy have been used to study the variations in their structural and optical properties. Mono-dispersal of silver nanoparticles for pre-heated substrates results in sharper surface plasmon resonance in comparison to post-annealed samples. Mie theory is used to estimate the particle size of the nanoparticles and findings are in accordance with quantitative analysis of FESEM images. Finite-difference time domain technique is used to discuss the effect of particle distribution on an ultrathin film silicon solar cell. Device degradation is observed as a result of clustering of silver nanoparticles. Hence, mono-dispersal of plasmonic nanostructures is important for required results and pre-heated deposition of metal nanoparticles by pulsed laser deposition can effectively solve the problem of particle clustering.

Optical absorption enhancement in 40 nm ultrathin film silicon solar cells assisted by photonic and plasmonic modes

Presently, energy problems and environmental issues have attracted the scientific community for the development of cost-effective and high-performance solar cells. Thin film solar cells are cheaper but weak light absorption in longer wavelength has demanded an efficient light trapping scheme for the better harvesting of solar radiation to a maximum possibility. In this paper, we numerically explore the design efforts of an ultrathin film silicon solar cell, integrated with top dielectric and bottom metal gratings. The proposed design is influenced by the localized surface plasmon modes, surface plasmon polariton and optical resonances which leads to the optimal harvesting of sunlight within 40nm thick absorbing layer. The optimized design of solar cell shows enhanced light absorption with cell efficiency ∼25% at normal transverse magnetic polarization condition. Our design approach assisted by photonic and plasmonic modes is promising for the realization of new generation, low-cost ultrathin film solar cells.

Zeroth order Fabry-Perot resonance enabled ultra-thin perfect light absorber using percolation aluminum and silicon nanofilms

We demonstrated perfect light absorption in optical nanocavities made of ultra-thin percolation aluminum and silicon films deposited on an aluminum surface. The total layer thickness of the aluminum and silicon films is one order of magnitude less than perfect absorption wavelength in the visible spectral range. The ratio of silicon cavity layer thickness to perfect absorption wavelength decreases as wavelength decreases due to the increased phase delays at silicon-aluminum boundaries at shorter wavelengths. It is explained that perfect light absorption is due to critical coupling of incident wave to the fundamental Fabry-Perot resonance mode of the structure where the round trip phase delay is zero. Simulations were performed and the results agree well with the measurement results.

Low thermal budget for Si and SiGe surface preparation for FD-SOI technology

Ultra thin Silicon films of Silicon-on-Insulator technology are metastable and thus cannot be submitted to high temperature treatments that may roughen or disrupt the film during the set of technological steps required for device fabrication. This paper concerns the development of an efficient low temperature cleaning process of Si and SiGe surfaces that enables a subsequent good-quality epitaxy of raised source and drain. For this purpose wet-clean, plasma-clean and several combinations of both are used. We thus propose two effective surface cleaning processes with low thermal budget optimized for FD-SOI technology.

A strain or electric field induced direct bandgap in ultrathin silicon film and its application in photovoltaics or photocatalysis.

The indirect bandgap character of silicon greatly limits its applications in electronic or optoelectronic devices, and direct bandgaps are highly desirable in all silicon allotropes. The successful synthesis of ultrathin or even monolayer silicon films experimentally has opened new opportunities to further modulate the electronic structure of silicon through external modulation. In this work, strain or electric field effects on the electronic structure of ultrathin silicon film (USF) are systematically explored. The results demonstrate that all USFs are indirect band-gap semiconductors; interestingly, tensile strain or electric field efficiently tunes the USFs into direct band gap semiconductors. The indirect to direct band gap transition in the USFs not only extends their light adsorption spectra into the visible light region but also greatly enhances the adsorption intensity. Because of this, strained USFs have great potential to be used as a high-performance photovoltaic material. Furthermore, the high stability, moderate band-gap and proper band edge positions demonstrate that monolayer and bilayer USFs can also be used as photocatalysts for water splitting.

Colloidal transfer printing method for periodically textured thin films in flexible media with greatly enhanced solar energy harvesting

The successful fabrication of high-performance flexible thin film solar cells (TFSCs) directly on diverse substrates is intrinsically limited by the processing temperature and substrate property. In this work, a colloidal transfer-printing (CTP) method is developed to fabricate large-area flexible thin-film absorbers with an antireflection coating and periodic configurations. Compared with a planar film, such structures exhibit much lower reflectance due to the antireflection introduced by the textured polydimethylsiloxane and the enhanced scattering introduced by the periodic back-scattering reflector. Optical simulation using the finite-element method indicates the structural periodicity for maximum light absorption is of 300 nm for an ultrathin amorphous silicon (a-Si) film with a thickness of 160 nm. The patterned a-Si film yields an overall absorption of 64.8%, which is much larger than the planar counterpart of 38.5%. This new approach to thin-film transfer can be readily extended to other material systems and device structures, opening up an effective alternative to traditional fabrication of the low-cost and high-performance optoelectronic devices.

Permittivity of Oxidized Ultra-Thin Silicon Films From Atomistic Simulations

We establish the dependence of the permittivity of oxidized ultra-thin silicon films on the film thickness by means of atomistic simulations within the density-functional-based tight-binding (DFTB) theory. This is of utmost importance for modeling ultra-thin and extremely thin silicon-on-insulator MOSFETs, and for evaluating their scaling potential. We demonstrate that electronic contribution to the dielectric response naturally emerges from the DFTB Hamiltonian when coupled to Poisson equation solved in vacuum, without phenomenological parameters, and obtain good agreement with the available experimental data. Comparison with calculations of H-passivated Si films reveals much weaker dependence of permittivity on film thickness for the SiO2-passivated Si, with less than 18% reduction in the case of 0.9-nm silicon-on-insulator.

BIMOS transistor solutions for ESD protection in FD-SOI UTBB CMOS technology

We evaluate the Electro-Static Discharge (ESD) protection capability of BIpolar MOS (BIMOS) transistors integrated in ultrathin silicon film for 28nm Fully Depleted SOI (FD-SOI) Ultra Thin Body and BOX (UTBB) high-k metal gate technology. Using as a reference our measurements in hybrid bulk-SOI structures, we extend the BIMOS design towards the ultrathin silicon film. Detailed study and pragmatic evaluations are done based on 3D TCAD simulation with standard physical models using Average Current Slope (ACS) method and quasi-static DC stress (Average Voltage Slope AVS method). These preliminary 3D TACD results are very encouraging in terms of ESD protection efficiency in advanced FD-SOI CMOS.

Directed dewetting of amorphous silicon film by a donut-shaped laser pulse

Irradiation of a thin film with a beam-shaped laser is proposed to achieve site-selectively controlled dewetting of the film into nanoscale structures. As a proof of concept, the laser-directed dewetting of an amorphous silicon thin film on a glass substrate is demonstrated using a donut-shaped laser beam. Upon irradiation of a single laser pulse, the silicon film melts and dewets on the substrate surface. The irradiation with the donut beam induces an unconventional lateral temperature profile in the film, leading to thermocapillary-induced transport of the molten silicon to the center of the beam spot. Upon solidification, the ultrathin amorphous silicon film is transformed to a crystalline silicon nanodome of increased height. This morphological change enables further dimensional reduction of the nanodome as well as removal of the surrounding film material by isotropic silicon etching. These results suggest that laser-based dewetting of thin films can be an effective way for scalable manufacturing of patterned nanostructures.

Electron mobility and spin lifetime enhancement in strained ultra-thin silicon films

Spintronics attracts much attention because of the potential to build novel spin-based devices which are superior to nowadays charge-based microelectronic devices. Silicon, the main element of microelectronics, is promising for spin-driven applications. Understanding the details of the spin propagation in silicon structures is a key for building novel spin-based nanoelectronic devices. We investigate the surface roughness- and phonon-limited electron mobility and spin relaxation in ultra-thin silicon films. We show that the spin relaxation rate due to surface roughness and phonon scattering is efficiently suppressed by an order of magnitude by applying tensile stress. We also demonstrate an almost twofold mobility increase in ultra-thin (001) SOI films under tensile [110] stress, which is due to the usually neglected strain dependence of the scattering matrix elements.