Light Emitting Silicon
Visible light emission from porous Si (PSi) produced via anodic etching in aqueous HF has stimulated tremendous interest over the past several years due to its potential application in opto-electronic devices and lasers and the ability to be integrated with current Si processing technology. Luminescence from porous silicon was observed over a decade ago, and since that time scientists have struggled to develop a mechanism to describe its photophysical properties. Despite the lack of a consistent physical description, a host of test devices have been produced which make use of the tunable luminescence and high surface area of porous silicon. Some of these devices have included: molecular sensors, cavity lasers, light emitting diodes, optical switches, photo-voltaic cells, and high field electron sources. Development of devices of this sort will require a detailed understanding of the luminescence properties of the nanostructured material.
We are interested in studying the electronic properties of PSi - from optical to possible device applications. From the optical standpoint we have been investigating the luminescence from the bulk films as well as single silicon grains stemming from the bulk material. Due to the high degree of structural inhomogeneity and parametric tunability of etched bulk samples, a direct correlation between chromophore size and emission wavelength has been difficult. Our approach to better describing the luminescence mechanism has been two fold. First, we obtained a clear understanding of how the morphologies of the bulk PSi films correlate to the observed optical behavior.
Figure 1. SEM images and spectra of porous Si samples. The images are examples of a low porosity (left) and high porosity (middle). The spectra (right) indicate the fluorescence tunability of porous Si.
Second, we remove the effects of spatial averaging in order to better describe the luminescence behavior of this interesting material. This is achieved by removing the Si chromophore from the bulk material and applying single molecule spectroscopic techniques.
Figure 2. Luminescence of Individual PSi nanoparticles on glass (left). Luminescence intensity of a single PSi nanoparticle as a function of time (right).
Using this method we can observe influences of surface species on the luminescence, distributions of emission wavelengths, dynamic spectral drifts, resolved vibronic structure, discrete jumps in intensity, luminescence intermittency, and irreversible photobleaching. In addition, using single particle imaging in conjuction with statistical analysis we have been able to determine the number of emitters per PSi particle.
Figure 3. A 15 x 15 mm fluorescence image (3-D, z-direction is intensity) of individual PSi nanoparticles (left). Intensity histogram of 481 low current density PSi nanoparticles showing multiple peak intensities indicative of 2-3 chromophores.
From the device application standpoint, we have been interested in creating active PSi structures for possible use in sensory devices, biomaterials, waveguides, field-effect transistors, and multi-color displays. The drive for designing PSi-based devices has pushed researchers to investigate ways of controllably patterning PSi surfaces. Our approach to directly patterning luminescent silicon involves dry removing microstructures of PSi from the Si substrate using nothing more than a bare elastomeric stamp made from poly(dimethylsiloxane) (PDMS). These PSi structures then be further transferred to a free-standing flexible polymer film.
Figure 4. A) An optical micrograph of a patterned porous silicon surface with representative fluorescence spectra (underneath) of a bright and dim line. B) AFM image of the same surface with a cross-sectional analysis (underneath) through the region represented by the white line. C) SEM image of the same sample.
The pattern design on the surface can virtually be anything. We have shown the arrays of PSi posts can be created by simply stamping the surface a second time orthogonal to the first
Figure 5. A) White light reflection image of a porous silicon sample stamped twice. B) Fluorescence image of the same sample. C) Fluorescence spectra of a bright post and dark region in (B).
Our goal is to utilize our high resolution optical microscopy and spectroscopy techniques to fully investigate the bulk PSi optical properties. We believe that studying the system on a chromophor-by-chromophore basis has led to a more detailed understanding of the emission properties of PSi. We have also shown that control over bulk morphologies through soft lithography can be a bridge for incorporating the material in silicon based devices.