Active solid-state devices (e.g. – transistors – solid-state diode – Bulk effect device – Bulk effect switching in amorphous material
Patent
1995-02-28
1996-10-22
Mintel, William
Active solid-state devices (e.g., transistors, solid-state diode
Bulk effect device
Bulk effect switching in amorphous material
257 95, 257 99, 257103, 437228, 437905, 437906, 216 51, 216 56, 216 79, 216108, H01L 3300
Patent
active
055679544
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a light emitting device and to a method of making such a device. The invention is particularly directed to a device which employs luminescent semiconductor properties.
2. Discussion of Prior Art
There has been much research recently into visible electroluminescence from porous silicon, which is a sponge-like structure made by first anodising and then etching a silicon substrate. Three papers which discuss the present state of the art are as follows: L. T. Canham, "Silicon Quantum wire array fabrication by electrochemical and chemical dissolution of wafers"; Appl. Phys. Lett. 57 (10), 3rd Sep. 1990, page 1046; Nobuyoshi Koshida and Hideki Koyama, "Visible electroluminescence from porous silicon", Appl. Phys. Lett. 60 (3), 20th Jan. 1992, page 347; and Volker Lehman and Ulrich Gosele, "Porous Silicon: Quantum sponge structures grown via a self-adjusting etching process", Adv. Mater. 4 (1992) No. 2, page 114.
The basic prior art light emitting device is disclosed in the aforementioned paper by Koshida and Koyama. It consists of a diode structure made from a substrate of p-type silicon, on top of which is formed a porous layer, typically 0.2 .mu.m to 1.0 .mu.m thick. Electrodes are placed both on the porous layer and on the underside of the substrate so that an electrical bias potential can be applied to the diode. One of the electrodes, that on the porous layer, is made of semi-transparent material so that light generated within the diode structure may be emitted.
The porous layer is formed by subjecting the top surface of the substrate to anodisation. This is believed to produce a porous layer comprising an array of columns or wires of low dimensionality, vertical to the surface, and separated by holes or spaces and wherein each column comprises silicon embedded in silicon oxide. Generally the porosity of the layer is increased by subsequent etching. An etchant is used which thins the columns by chemical dissolution, with a resultant increase in the size of the spaces between the columns. Light is generated within the diode structure in response to electrical bias. There is currently some uncertainty in the scientific world as to how the structure emits light.
SUMMARY OF THE INVENTION
The present invention provides a light emitting device incorporating porous material of low dimensionality consisting at least partly of semiconductor material and produced by an etching process, upon the porous material a discontinuous layer comprising islands of electrically conducting and etch resistant material, together with contacting means for making electrical contact to the porous material and the discontinuous layer.
The expression "low dimensionality" in relation to a material means that the material has, in at least one direction, dimensions of the order of or less than the exciton diameter or the De Broglie wavelength of electrons or holes in the material. This leads to quantum confinement in the relevant direction. Quantum wells, quantum wires and quantum dots are known in the prior art and correspond to one, two and three dimensional confinement respectively. In practice, this corresponds to material feature dimensions less than 50 nm in extent, and preferably less than 25 nm.
The invention provides the advantage that it is a light emitting semiconductor device which is activated by electrical bias applied to the contacting means.
The invention also provides the advantage that the residual semiconductor material remaining in the porous layer is that located under the islands of electrically conducting and etch resistant material, which provided protection thereof during the etching process. The islands therefore define the locations of residual semiconductor material, and provide electrical contact to resulting low dimensional semiconductor material. The islands may have diameters of 5 nm to 100 nm, preferably 5 nm to 20 nm or 10 nm to 20 nm, and inter-island spacings may be in the range 10 nm to 500 nm, preferably 10 nm to 50 nm.
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Bassous et al., "Characterization of Microporous Silicon Fabricated by Immersion Scanning", Mat. Res. Soc. Symp. Proc., vol. 256, 1992, pp. 23-26.
Halimaoui et al., "Electroluminescence in the Visible Range During Anoidc Oxidation of Porous Silicon," Appl. Phys. Lett. 59(3), 15 Jul. 1991, pp. 304-306.
Namavar et al., "Visible Electroluminescence from Porous Silicon NP Heterojunction Diodes," Appl. Phys. Lett, 60(20);18 May 1992, pp. 2514-2516.
Richter et al., "Current-Induced Light Emission From a Porous Silicon Device," IEEE Electron Device Letters, vol. 12, No. 12, Dec. 1991, pp. 691-692.
Koshida et al., "Visible Electroluminescence From Porous Silicon", Appl. Phys. Lett. 60(3), 20 Jan. 1992, pp. 347-349.
Dobson Peter J.
Leigh Peter A.
Pearson Richard O.
Mintel William
The Secretary of State for Defence in Her Brittanic Majesty's Go
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