Electroluminescent Devices Made from Silicon Nanocrystals Embedded in Various Host Matrices
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Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
Nanocrystal colloidal suspensions in methanol and isopropyl were prepared for fabrication of the silica sol-gel films.
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Surface layer consists of nm size Si structures, which are responsible for light emission
Either n or p-doped Si can be etched Fig. 1 Setup used for fabrication of PSi
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Fig. 2 Cross section diagram depicting Si nanocrystals in the PSi structure
RESULTS Optical Characterization Our optical characterization included absorption, photoluminescence (PL) and PL excitation (PLE) spectroscopy. Si nanocrystals of the appropriate size and shape yield efficient PL that spectrally shift with excitation wavelength. Fig. 3 shows PL spectra of Si nanocrystals in a toluene colloidal suspension that illustrates the spectral shifts. Peak wavelengths varied from about 380 nm to 430 nm when excited from 300 nm to 360 nm. Our optical analysis revealed that quantum confinement effects are responsible for the blue emission and the red peak is most likely due to traps arising from quantum confinement effects[9]. The spectral shifts in the PL are due to quantum confinement effects that result from the different size and shape distribution in the samples. The spectral shifts are crucial since this indicates that the emission wavelength can be controlled by controlling the size and shape of the nanocrystals. Colloidal suspension in various solvents, such as hexane, methanol, and toluene, all showed identical behavior thus indicating that the solvent does not interfere with the mechanism for light emission. Our experimental analysis and comparison with theoretical models [10,11] indicate that our samples range in size approximately between 1.9-2.6 nm diameters. Device Structure and Processing/Fabrication A thin single-layered electroluminescent (EL) device using Si nanocrystals embedded in a host matrix as the emitter material was achieved. Glass slides with patterned, thin film IndiumTin-Oxide (ITO) were used as the anode. Thin (- 2000 A) EL films were easily deposited on the ITO substrate by spin coating techniques, resulting in a highly transparent layer. The active media were comprised of Si nanocrystals embedded in thin films of various matrices including polyvinylcarbazole (PVK), polymethylmethacrylate (PMMA) and silica sol-gels. The cathode consisted of evaporated thin (1000 - 1500 A) films of Al or Ca.
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Wavelength (nm) Figure 3 Photoluminescence of Si nanocrystals in a toluene colloidal suspension. This figure illustrates the shifting of the PL peak; with excitation wavelength. Device Characterization The device was designed to operate via an electron-hole (e-h) recombination process. For the device to work, a transparent host matrix of larger energy gap than the Si nanocrystal 's energy gap is needed. The matrices mentioned abov
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