Munib Wober

Multicolored Vertical Silicon Nanowires

We demonstrate that vertical silicon nanowires take on a surprising variety of colors covering the entire visible spectrum, in marked contrast to the gray color of bulk silicon. This effect is readily observable by bright-field microscopy, or even to the naked eye. The reflection spectra of the nanowires each show a dip whose position depends on the nanowire radii. We compare the experimental data to the results of finite difference time domain simulations to elucidate the physical mechanisms behind the phenomena we observe. The nanowires are fabricated as arrays, but the vivid colors arise not from scattering or diffractive effects of the array, but from the guided mode properties of the individual nanowires. Each nanowire can thus define its own color, allowing for complex spatial patterning. We anticipate that the color filter effect we demonstrate could be employed in nanoscale image sensor devices.

Filter-Free Image Sensor Pixels Comprising Silicon Nanowires with Selective Color Absorption

The organic dye filters of conventional color image sensors achieve the red/green/blue response needed for color imaging, but have disadvantages related to durability, low absorption coefficient, and fabrication complexity. Here, we report a new paradigm for color imaging based on all-silicon nanowire devices and no filters. We fabricate pixels consisting of vertical silicon nanowires with integrated photodetectors, demonstrate that their spectral sensitivities are governed by nanowire radius, and perform color imaging. Our approach is conceptually different from filter-based methods, as absorbed light is converted to photocurrent, ultimately presenting the opportunity for very high photon efficiency.

Si Microwire Solar Cells: Improved Efficiency with a Conformal SiO2 Layer

Silicon microwire arrays have attracted considerable attention recently due to the opportunity they present as highly efficient and cost-effective solar cells. In this study, we report on efficient Si microwire array solar cells with areas of 1 cm(2) and Air Mass 1.5 Global conversion efficiencies of up to 10.6%. These solar cells show an open-circuit voltage of 0.56 V, a short-circuit current density of 25.2 mA/cm(2), and a fill factor of 75.2%, with a silicon absorption region that is only 25 μm thick. In particular, the maximum overall efficiency of the champion device is improved from 8.71% to 10.6% by conformally coating the wires with a 200 nm thick SiO2 layer. Optical measurements reveal that the layer reduces reflection significantly over the entire visible range.

Fabrication techniques of high aspect ratio vertical lightpipes using a dielectric photomask

We report the development of new fabrication techniques for creating high aspect ratio optical lightpipes in SiO2 layers of 10μm thickness and above. A dielectric photo mask was used for deep reactive ion etching. Our experiments show that CF4-based reaction gases were best for deep etching with high selectivity and etch rate. Trenches with diameters or width of 1.5μm were demonstrated, with an aspect ratio of 7.2:1 and a sidewall angle of 87.4 degrees. We also present the lift-off process of the etch masks and the via-filling procedures for the lightpipes. These structures are useful for image sensors, vertical interconnect and waveguiding applications.

Versatile control of metal-assisted chemical etching for vertical silicon microwire arrays and their photovoltaic applications.

A systematic study was conducted into the use of metal-assisted chemical etching (MacEtch) to fabricate vertical Si microwire arrays, with several models being studied for the efficient redox reaction of reactants with silicon through a metal catalyst by varying such parameters as the thickness and morphology of the metal film. By optimizing the MacEtch conditions, high-quality vertical Si microwires were successfully fabricated with lengths of up to 23.2 μm, which, when applied in a solar cell, achieved a conversion efficiency of up to 13.0%. These solar cells also exhibited an open-circuit voltage of 547.7 mV, a short-circuit current density of 33.2 mA/cm2, and a fill factor of 71.3% by virtue of the enhanced light absorption and effective carrier collection provided by the Si microwires. The use of MacEtch to fabricate high-quality Si microwires therefore presents a unique opportunity to develop cost-effective and highly efficient solar cells.

Silicon nitride light pipes for image sensors

Recently, there has been an increasing trend toward the use of image sensors based on complementary metal oxide semiconductor (CMOS), rather than charged coupled device (CCD), technology. Advantages of CMOS image sensors include low power usage, compatibility with CMOS logic technology, permitting random access of image data, and circuit integration [1], [2], [3]. In conventional CMOS image sensors, however, increasing circuit complexity may be detrimental to optical performance. As the number of metal interconnect layers increases, so does the distance between the microlenses and photodiodes, reducing light collection efficiency and increasing inter-pixel cross-talk. Here, we discuss a means for overcoming this through the use of “light pipes”. These comprise vertical waveguides formed in the intermetal dielectric region between the microlenses and photodiodes of a CMOS image sensor. We present experimental results on the etching of cylindrical pillars into thick silicon nitride (SixNy) layers, and on the fabrication and characterization of photodiodes. In general little work has been carried out on the demonstration of very small silicon nitride pillars. Previous work involved the use of small metal particles[4], and on the use of silicon nitride pillars for solar cell applications [5]. In this work, we describe a way of obtaining well-controlled shaped pillars with vertical side walls.

Flexible crystalline silicon radial junction photovoltaics with vertically aligned tapered microwires

Much attention has been paid to thin crystalline silicon (c-Si) photovoltaic devices due to their excellent flexibility characteristics, stable efficiency, and possibility of use as highly efficient next-generation flexible photovoltaic devices (FPVs). To fabricate thin c-Si FPVs, it is important to improve their light-absorption properties while maintaining the flexible characteristics. In this study, vertically aligned microwires (MWs) on a 50 μm-thick thin c-Si substrate are designed for novel FPVs. Increasing the length of the MWs enhances the optical properties of the thin c-Si without affecting its flexibility. To maximize the efficiency of the thin c-Si FPVs with MWs, tapered MWs and a localized back-contact structure are devised. This device shows a maximum efficiency of 18.9%. In addition, the proposed thin c-Si FPV with MWs shows high stability without any change in efficiency, even with 1000 bending cycles with a bending radius of 12 mm. Thus, we successfully demonstrate battery-free flexible electronic devices integrated with our thin c-Si FPVs with MWs.

Silicon photodetectors integrated with vertical silicon nitride waveguides as image sensor pixels: Fabrication and characterization

The current trend toward image sensors with ever-increasing pixel counts is prompting continual reductions in pixel area, leading to significant cross-talk and efficiency challenges. The realization of image sensor pixels containing waveguides presents a means for addressing these issues. The fabrication of such pixels is however not straightforward. Conventional waveguides employed in integrated optics are horizontal, but waveguides needed for the proposed sensor must be vertical and integrated with photodetectors. Here, the authors describe a fabrication process for vertical silicon nitride waveguides integrated with silicon photodetectors. The authors describe the etching, deposition, and planarization techniques that enable the formation of silicon nitride waveguides embedded in silicon dioxide. They also describe a fabrication process for silicon photodetectors, including a means for ensuring that their photosensitive areas have sizes consistent with those of photodetectors employed in conventional i...

Vertical waveguides integrated with silicon photodetectors: Towards high efficiency and low cross-talk image sensors

We describe the experimental realization of vertical silicon nitride waveguides integrated with silicon photodetectors. The waveguides are embedded in a silicon dioxide layer. Scanning photocurrent microscopy is performed on a device containing a waveguide, and on a device containing the silicon dioxide layer, but without the waveguide. The results confirm the waveguide’s ability to guide light onto the photodetector with high efficiency. We anticipate that the use of these structures in image sensors, with one waveguide per pixel, would greatly improve efficiency and significantly reduce inter-pixel crosstalk.

Fabrication of high-aspect-ratio lightpipes

The authors report the development of two fabrication processes for creating high-aspect-ratio lightpipes in a 10 μm thick SiO2 layer, with smooth, uniform, and straight vertical sidewalls. Both processes require only standard optical lithography, without the need for advanced electron beam or deep-UV lithography. One process employs a dielectric etch mask and the other uses a negative photoresist as the etch mask. The experiments show that the CF4-based reaction gases are best for deep etching with high selectivity and etch rate. Trenches with diameters or width of 1.5 μm are demonstrated, with an aspect ratio of 7.2:1 and a sidewall angle of 87.4°. The authors also achieve cylindrical lightpipes with an aspect ratio of 3.8:1 and a sidewall angle of 89.5°. They anticipate that these high-aspect-ratio lightpipe structures would be useful for complementary metal-oxide semiconductor image sensors, where they would increase the efficiency of light collection, and reduce interpixel cross-talk.