 | Stabilization of Photon/Electron Emission from Nanocrystalline Si Devices
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| | Nobuyoshi Koshida1), Akira Kojima1), Junichi Kadokura1) and Yoshiki Nakajima1) |
| | 1) Dept of Electrical and Electronic Engineering, Tokyo University of A&T |
| | We recently developed a nanocrystalline porous silicon (nc-PS) EL device which offers the red emission with an external power efficiency (EPE) of 0.4% [1] and an external quantum efficiency (EQE) of 1.1% [2] at an operating voltage of 5 V. On the other hand, the nc-PS diode also operates as an efficient surface-emitting ballistic cold cathode [3]. It is useful for excitation source of flat panel display [4]. These devices are basically composed of a top contact, nc-PS layer, and a single- or poly-crystalline n+-type Si substrate, and an ohmic back contact. To prolong the operation life of EL and electron emission to a practical level, the following two approaches have been pursued.
(1) Encapsulation of device surface with a water-proof film
The first one is capping of the PS layer with some transparent films to protect against penetration of oxygen and water molecules followed by current-triggered oxidation of nc-Si layer. By employing electron cyclotron resonance (ECR) sputtering technique for coverage of the active nc-PS layer with semitransparent SiO2 films, the EL operation is significantly stabilized without affect on the EL efficiency. It is shown that from thermal desorption spectra analyses, the capability of capped films as a barrier against penetration of water molecules is a key factor for stabilizing the EL operation.
(2) Chemical functionalization of nc-Si surfaces
Another approach is complete passivation of each nc-Si surfaces. As recently reported [5], chemical modification of hydride nc-Si surfaces by conjugated organic molecules is very effective to preserve the intensity and the emission band of photoluminescence. This technique was applied to EL-emissive nc-PS diodes using CH3(CH2)8CHO. By thermal treatment in it, the Si-H bonding on nc-Si surfaces should be converted into more stable Si-O bonding. Actually, the EL life time is significantly enhanced under a cw operation. |
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