D. Mangaiyarkarasi

Controlled blueshift of the resonant wavelength in porous silicon microcavities using ion irradiation

High-energy focused proton beam irradiation has been used to controllably blueshift the resonant wavelength of porous silicon microcavities in heavily doped p-type wafers. Irradiation results in an increased resistivity, hence a locally reduced rate of anodization. Irradiated regions are consequently thinner and of a higher refractive index than unirradiated regions, and the microcavity blueshift arises from a net reduction in the optical thickness of each porous layer. Using this process wafers are patterned on a micrometer lateral scale with microcavities tuned to different resonant wavelengths, giving rise to high-resolution full-color reflection images over the full visible spectrum.

Controlled intensity emission from patterned porous silicon using focused proton beam irradiation

We have fabricated light emitting porous silicon micropatterns with controlled emission intensity. This has been achieved by direct write irradiation in heavily doped p-type silicon (0.02Ωcm) using a 2MeV proton beam, focused to a spot size of 200nm. After electrochemical etching in hydrofluoric acid, enhanced photoluminescence is observed from the irradiated regions. The intensity of light emission is proportional to the dose of the proton beam, so the PL intensity of the micropattern can be tuned and varied between adjacent regions on a single substrate. This behavior is in contrast to previous ion beam patterning of p-type silicon, as light is preferentially created as opposed to quenched at the irradiated regions.

Fabrication of large-area patterned porous silicon distributed Bragg reflectors

A process to fabricate porous silicon Bragg reflectors patterned on a micrometer lateral scale over wafer areas of several square centimeters is described. This process is based on a new type of projection system involving a megavolt accelerator and a quadrupole lens system to project a uniform distribution of MeV ions over a wafer surface, which is coated with a multilevel mask. In conjunction with electrochemical anodisation, this enables the rapid production of high-density arrays of a variety of optical and photonic components in silicon such as waveguides and optical microcavities for applications in high-definition reflective displays and optical communications.

Fabrication of three dimensional porous silicon distributed Bragg reflectors

Three-dimensional distributed Bragg reflectors, which reflect all incident wavelengths, have been fabricated with micrometer dimensions in porous silicon, resulting in white reflective surfaces when viewed over a wide angular range. Large area arrays of several mm2 containing many individual micrometer-size pixellated reflectors that can be tuned to reflect a narrow or wide range of wavelengths are designed to appear either as constant or changing reflective images to the naked eye. This work opens avenues in controlling the reflection of light in all directions for applications in wide-angle displays, broadband reflective surfaces for resonant white light emission from semiconductor nanocrystals, and three-dimensional microcavities.

Porous silicon Bragg reflectors with sub-micrometer lateral dimensions.

Bragg reflectors with widths down to 300 nm have been fabricated in porous silicon. This was achieved by irradiation of highly-doped p-type silicon with a focused beam of high-energy ions in a channeled alignment, in which the beam is aligned with a major crystallographic direction. The reflected colour is controllably tuned across the visible spectrum by varying the ion irradiated dose. The depth distribution of ion induced defects differs in channeled alignment compared to random beam alignment, resulting in the hole current during subsequent anodisation being more confined to narrower lateral regions, enabling different reflective wavelengths to be patterned on a sub-micron lateral scale. This work provides a means of producing high-density arrays of micron-size reflective colour pixels for uses in high-definition displays, and selectively tuning the wavelengths of porous silicon Fabry-Perot microcavities across the visible and infra-red ranges for optical communications and computing applications.

Controlled Shift in Emission Wavelength from Patterned Porous Silicon Using Focused Ion Beam Irradiation

Photoluminescence images containing several distinct color emissions, from green to red, have been obtained using high-energy focused ion beam irradiation, in conjunction with metal-aided anodization of 4 Ω cm p-type silicon. The ion irradiation increases the local resistivity in a controlled manner resulting in smaller hole currents flow through the irradiated areas. This causes a controlled redshift of up to 200 nm in the photoluminescence emission, which in terms of the quantum confinement model would correlate to larger nanocrystallites forming in the irradiated region.

Porous-silicon-based Bragg reflectors and Fabry-Perot interference filters for photonic applications

Visible light emission from the porous silicon (PSi) formed by anodic etching of Si in HF solution has raised great interest in view of possible applications of Si based devices in optoelectronics. In particular, multilayers consisting of periodic repetition of two PSi layers whose refractive indices are different can be exploited to design interference filters for controlling the emission wavelength as well as for the spectral narrowing of the wide emission band of Psi. Fabry-Perot optical microcavities with an active layer of λ\2 or λ sandwiched between two Bragg reflectors, consisting of alternating layers of high and low refractive indices are fabricated on heavily doped p-type silicon. We have investigated the optical properties of these microstructures using reflectivity and photoluminescence measurements at various temperature.

Effects of focused MeV ion beam irradiation on the roughness of electrochemically micromachined silicon surfaces

The authors compare the effects of focused and broad MeV ion beam irradiation on the surface roughness of silicon wafers after subsequent electrochemical anodization. With a focused beam, the roughness increases rapidly for low fluences and then slowly decreases for higher fluences, in contrast to broad beam irradiation where the roughness slowly increases with fluence. This effect is important as it imposes a limitation on the ability to fabricate smooth surfaces using focused ion beam irradiation. For a given fluence, small variations in the resistivity of an irradiated area may arise due to fluctuations of the focused beam current during irradiation. These small variations in resistivity then give rise to an increased roughness during the electrochemical etching. The roughness may be reduced by increasing the scan speed, which alters the way in which the fluctuations in fluence are averaged out over the irradiated surface.

Micro‐patterned porous silicon using proton beam writing

A high‐energy beam of hydrogen or helium ions focused to a small spot in a nuclear microprobe selectively damages a silicon lattice. This damage acts as an electrical barrier during subsequent formation of porous silicon by electrochemical etching of p‐type wafers, so the un‐irradiated regions are preferentially anodized. This process has opened up new research directions for the fabrication of a variety of high‐aspect ratio, multi‐level microstructures in silicon, such as gratings, photonic lattices and waveguides. The same process enables local modifications and control of the properties of the porous silicon, leading to new luminescent micro‐structures in which both the spatial location and the wavelength and intensity of emission can be carefully controlled. The reflectivity and transmission of Bragg reflectors and microcavities fabricated in porous silicon can also be controlled using this same process and examples are given of each of these applications.

Electrochemical Anodization of Silicon-on-Insulator Wafers Using an AC

Electrochemical anodization of bulk silicon has applications in many micromachining processes. However, its use for silicon photonics is limited because silicon-on-insulator (SOI) wafers cannot be anodized using a conventional process because of the buried oxide. We overcome this using an alternating potential to induce an ac across an SOI wafer, treating it as a capacitative structure. The resultant surface roughness is comparable to that obtained using conventional anodization, and uniform etching across a 6 mm exposed surface is obtained with a minimum patterned linewidth of 2.5 μm in the device layer.