Luminescence from nano - Si Group I : Maria Szlek Maksymilian Schmidt Michal Jablonski Karol Kyziol

Luminescence (cold light, annealing) – it’s ability to emit light waves by solid states. Generated another reason than heating. There is a few kind of luminescence e.g. Photoluminescence (PL), electroluminiescence (EL). PL – exited by photons beam. EL - exited by electric field

Photoluminescence

Porous Silicon Porous silicon was discovered by accident. It was produced by non-uniform etching during the electropolishing of silicon with an electrolyte containing hydrofluoric acid. The etching resulted in a system of disordered pores with nanocrystals remaining in the inter-pore regions. Porous silicon is still manufactured by electrochemical etching of silicon in hydrofluoric acid (HF) solutions. Aqueous HF is unsuitable for the etching process because the silicon surface is hydrophobic. The porous layer can be made more structurally uniform if an ethanoic solution is used - this increases the wettability of the silicon and allows better surface penetration by the acid. Ethanoic etch solutions also reduce the formation of hydrogen gas bubbles as ethanol acts as a surfactant and prevents bubbles sticking to the silicon surface.

Scheme of produce PS

Porous Silicon What is porous silicon? In the most basic sense, porous silicon is a network of air holes within an interconnected silicon matrix.  The size of these air holes, called pores, can vary from a few nanometers to a few microns depending on the conditions of formation and the characteristics of the silicon.   The SEM image typical porous silicon sample.

The silicon nanocrystals in PS that emits visible light vary in size from 10-15Å. Raman spectroscopy gives indirect information about the microstructure of PS and has shown that the nanocrystals alter the selection rules relating to the interaction of optical phonons with incident photons.

Photoluminescence of a nanoporous silicon sample The nanoporous structures have dimensions in the low nm-range. If the structure size reaches a value below, say 3 nm, quantum effects can occur and therefore nanoporous samples can exhibit strong visible photoluminescence and electroluminscence, as can be seen in the picture below. Photoluminescence of a nanoporous silicon sample

SEM images and spectra of porous Si samples 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.

Field-effect electroluminescence In the silicon field-effect LED, a tunneling process sequentially charges the nanocrystals embedded in the gate oxide with electrons and then with holes. The electron-hole pairs radiatively recombine to yield light at approximately 750 nm.

Schematic of the field-effect electroluminescence mechanism in a silicon nanocrystal floating-gate tranisistor structure.

PL and EL emission spectra The emission spectra are inhomogeneously broadened due to the distribution of luminescent nanocrystal sizes.

The nanocrystal field-effect light-emitting device (FELED) could be used to integrate light sources on computer chips. This would allow the light sources and control circuits of display and communications device to be fabricated together, making for a faster, cheaper manufacturing process. The device is energy efficient; a prototype that generates several microwatts of optical power could be built in an area as small as a few hundred square microns, according to the researchers. The color light the transistor emits depends on the size of its nanocrystals.

Light emission

Possible mechanisms that can lead to radiative light emission in Si QDs.

A few words about silicon-based lasers… Generally silicon is not used for light sources because of the lack of efficient light emitters but there are some an optimistic note on silicon lasing.

Schematic of injection laser based on simple p-n junction

Cross-section of the silicon laser

Conclusion The prospects for a Si laser are quite good. Besides the approaches other directions of active research consider the use of Si-Ge alloys, quantum confinement, alloying effects, or nanocrystal formation. The expectations of realizing a Si-based injection laser in the near future are well founded. The variety of approaches that are now being followed, if successful, will make Si generate a rainbow of colors.

Conclusion Silicon is the material of choice for making most electronic devices. In its natural crystalline form, however, silicon has a very low optical radiative efficiency and produces light only outside the visible range. If the optical property of crystalline silicon could be modified to increase the frequency of emitted light, silicon would have even more device applications, such as use in lasers or solar cells.

References: Materialstoday January 2005; Lorenzo Pavesi Materialstoday January 2005; Philippe M. Fauchet Advanced materials 1992; Volker Lehmann, Urlich Gösele Nature materials February 2005; Robert J. Walters, George I.Bourianoff, Harry A.Atwater Nature, February 2005; Jerome Faist http://www.chem.ucsb.edu/~buratto_group/PorousSilicon.htm http://www.ece.rochester.edu/~weiss/Porous_silicon.html http://www.photonics.com http://www.trnmag.com/Stories/2005/020905/Silicon_nanocrystal_transistor_shines_Brief_020905.html http://www.theledlight.com/led-specs.html

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