Selena Chan

Nanoscale silicon microcavities for biosensing

We report the design and testing of a versatile biosensor exclusively using silicon. The device structure consists of a microcavity resonator made of various porous silicon layers. Porous silicon contains silicon nanocrystals that can luminesce efficiently in the visible, depending on the size and passivation conditions. When a luminescent porous silicon layer is inserted between two Bragg reflectors (also made of porous silicon), the broad luminescence band is altered and multiple and very narrow peaks are detected. The position of these peaks is extremely sensitive to a small change in refractive index, such as that obtained when a biological object is attached to the large internal surface of porous silicon. We have demonstrated a DNA sensor that displays appropriate sensitivity, selectivity and response speed. The device fabrication procedure and the results of extensive testing are presented. An extension of the DNA biosensor has been made to include the detection of viral DNA. This work will lead to the development of an all-silicon sensor array for the detection of biomacromolecules.

Tunable, narrow, and directional luminescence from porous silicon light emitting devices

Controlled room-temperature photoluminescence (PL) and electroluminescence (EL) from porous silicon (PSi) multilayer structures are achieved. The basic structure is composed of two PSi multilayer mirrors sandwiching a highly luminescent PSi film. This creates an active microcavity resonator, in which a significant PL and EL line narrowing is observed. EL from a microcavity resonator is shown to have a high angular concentration of the output emission (30° around the normal axis). A tunable and narrow EL device, from an all-PSi microcavity resonator is presented, which offers the possibility for high spectral purity, flat-panel displays.

Porous silicon multilayer structures: A photonic band gap analysis

A photonic model for freshly anodized porous silicon multilayer structures is presented. The photonic structures are composed of alternating high and low dielectric function porous silicon layers. The model takes into account the presence of silicon dioxide and its lattice expansion in the porous structure. We work with oxidized structures and our results fit completely the experimentally measured optical shift.

Silicon microcavity light emitting devices

Abstract We report the optical and electrical properties of all-silicon microcavity light emitting devices (LEDs) made of a thin, high-porosity porous silicon active layer sandwiched between two Bragg reflectors made of several pairs of low index/high index of refraction porous silicon layers of lower porosities. The design and passivation of the structures are discussed. The electroluminescent properties are examined in detail, including the strong narrowing of the spectrum, the widely tunable peak position, the emission directionality, the power efficiency, and the stability. The possible uses of these silicon microcavity LEDs are considered and the challenges that need to be met in terms of materials science and device design are examined.

Nanoscale microcavities for biomedical sensor applications

Porous silicon nanostructures are ideal hosts for sensor applications because of their large internal surface area, which implies strong adsorbate effects. The average pore size can be easily adjusted to accommodate either small or large molecular species. When porous silicon is fabricated into a structure consisting of two high reflectivity multilayer mirrors separated by an active layer, a microcavity is formed. Multiple narrow and visible luminescence peaks are observed with a full width at half the maximum value of 3 nm. These multiple peak microcavity resonators are very sensitive structures. Any slight change in the effective optical thickness induces a change in the reflectivity spectra, causing a shift in the interference peaks. We demonstrate the usefulness of this microcavity resonator structure as a biosensor. Biosensors are devices that exploit the powerful recognition capability of bioreceptors. We have fabricated a DNA biosensor based on a porous silicon multiple peak microcavity structure. An initial strand of DNA is first immobilized in a porous silicon substrate and then subsequently exposed to its complementary DNA strand. Shifts in the luminescence spectra are observed and detected for DNA concentrations less than 1 (mu) M. When exposed to a non- complementary DNA strand, no shifts are observed. A detailed study on the selectivity and sensitivity issues of porous silicon microcavity biosensors is presented.

Silicon Light Emitters: Preparation, Properties, Limitations, and Integration with Microelectronic Circuitry

Starting with Canham’s discovery in 1990 that porous silicon (PSi) can emit bright light in the visible range of the spectrum, there has been a strong interest in silicon light emitters. PSi and other light-emitting forms of silicon contain nanostructures or crystallites in the nanometer size range. Throughout most of the 1990’s, the intense visible luminescence from nanoscale silicon crystallites has been a source of numerous investigations and considerable debate. Today, most of the controversies have been put to rest. However, much less has been written about nanoscale Si light-emitting devices, in part because some of their characteristics are less than ideal and not well understood. This paper reviews the status of nanoscale silicon light emitters. It starts with a survey of the manufacturing methods used to produce nanoscale Si. Next, key physical, optical, electrical, and structural properties of nanoscale Si are examined. The fabrication of electroluminescent devices (LEDs) is then discussed. We focus on the stability, efficiency, speed, and spectral characteristics of nanoscale Si light emitters. Recent results obtained on microcavity PSi LEDs and 1.5μm LEDs produced by doping PSi with erbium are discussed. Finally, the integration of PSi LEDs with microelectronic circuitry is reported and the prospects for practical devices are briefly examined.

Silicon interference filters and bragg reflectors for active and passive integrated optoelectronic components

Porous silicon (PSi) multilayer structures are used in Si- based optoelectronic devices which exhibit interesting optical and electrical properties. Integration of these structure can be achieved either passively or actively. Passive elements include high reflectivity mirrors and optical filters with a maximum reflectivity peak approximately 100%. The benefit of adding a multilayer mirror below a luminescent PSi film is to reduce the amount of light absorbed by the silicon substrate and increase the light output. Placing two multilayer mirrors in between a highly luminescent PSi film creates an active microcavity resonator structure in which a significant photoluminescence and electroluminescence line narrowing (FWHM less than or equal to 20 nm) is observed. A detailed study on the passive and active roles of PSi multilayer structures is presented in a device configuration.

Nanoscale Silicon Microcavity Optical Sensors for Biological Applications

The large surface area of porous silicon provides numerous sites for many potential species to attach, which makes it an ideal host for sensing applications. The average pore size can be easily adjusted to accommodate either small or large molecular species. When porous silicon is fabricated into a structure consisting of two high reflectivity multilayer mirrors separated by an active layer, a microcavity is formed. Multiple narrow and visible luminescence peaks are observed with a full width at half maximum value of 3 nm. The position of these peaks is extremely sensitive to small changes in refractive index, such as that obtained when a biological object is attached to the large internal surface of porous silicon. We demonstrate the usefulness of this microcavity resonator structure as a DNA optical biosensor which displays appropriate sensitivity, selectivity, and response speed. A probing strand of DNA is initially immobilized in the porous silicon matrix, and then subsequently exposed to its sensing complementary DNA strand. Red-shifts in the luminescence spectra are observed and detected for various DNA concentrations. The spectral shifts confirm successful recognition and binding of DNA molecules within the porous structure. Detailed device fabrication procedures and the results of extensive testing will be presented. The detection scheme has also been extended to include the detection of viral DNA, proteins, and potentially bacteria. This work will lead to the development of a “smart bandage”, where the detection of bacteria or viruses can be diagnosed and an antibiotic treatment can be recommended.

Porous Silicon Multilayer Mirrors and Microcavity Resonators for Optoelectronic Applications

Porous silicon multilayer structures are easily manufactured using a periodic current density square pulse during the electrochemical dissolution process. The difference in porosity profile, corresponding to a variation in current density, is attributed to a difference in refractive index. Manipulating the difference in refractive index, high quality optical filters can be made with a maximum reflectivity peak ˜ 100%. The next logical step to further exploit these optical mirrors is to incorporate them into an LED device. The benefit of adding a multilayer mirror below a luminescent film of porous silicon is to significantly reduce the amount of light loss to the silicon substrate and increase the light output. However, oxidation is required to stabilize the as-anodized porous silicon film. This disrupts the overall index profile of the multilayer stack, causing the peak reflectance to blue shift. This phenomenon must be quantified and accounted before device implementation. We present a detailed study on the effects of oxidation temperature, gas environment, and annealing time of porous silicon multilayer structures in a device configuration.

Integration of Multilayers in Er-Doped Porous Silicon Structures and Advances in 1.5 μm Optoelectronic Devices

Infrared photoluminescence (PL) and electroluminescence (EL) from erbium-doped porous silicon (PSi) structures are studied. The PL and EL from the Er-doped PSi structures and the absence of silicon band edge recombination, point defect, and dislocation luminescence bands suggest that the Er-complex centers are the most efficient recombination sites. PSi multilayers with very high reflectivity (R ≥ 90%) in the 1.5 gim range have been incorporated in the structures resulting in a PL enhancement of over 100%. Stable and intense EL is obtained from the Er-doped structures. The EL spectrum is similar to that of the PL, but shifted towards higher energy. The unexpected shift in emission opens up the possibility for erbium related luminescence to encompass a larger part of the optimal wavelength window for fiber optic communications.