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Electroluminescent device comprising porous siliconRelated Patent Categories: Semiconductor Device Manufacturing: Process, Making Device Or Circuit Emissive Of Nonelectrical SignalElectroluminescent device comprising porous silicon description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070004064, Electroluminescent device comprising porous silicon. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application is a Continuation of application Ser. No. 10/931,218 filed Sep. 1, 2004, which in turn is a Division of application Ser. No. 10/051,059, filed Jan. 22, 2002, which in turn is a divisional of application Ser. No. 08/913,414 filed Sep. 17, 1997, which is the U.S. national phase of international application PCT/GB96/00589, filed in English on 15 Mar. 1996, which designated the US. PCT/GB96/00589 claims priority to GB Application No. 9505569.5 filed 20 Mar. 1995. The entire contents of these applications are incorporated herein by reference. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The invention concerns an electroluminescent device and more particularly an electroluminescent device fabricated from porous silicon material. [0004] 2. Discussion of Prior Art [0005] Light emitting devices may emit light by a variety of processes. A conventional tungsten wire light bulb emits visible light when an element in the light bulb reaches a certain temperature. The emission of visible light from a substance at high temperature is termed incandescence. Luminescence is a phenomenon distinct from incandescence and is produced when electrons lose energy radiatively when moving from an excited energy state to a lower energy state which may be their ground state. Photoluminescence is luminescence from electrons which are excited into a high energy level by the absorption of photons. Photoluminescent porous silicon is described in U.S. Pat. No. 5,348,618. Electroluminescence is luminescence from electrons which are excited to higher energy levels by an electric field or an electric current. An example of electroluminescent porous silicon is described in United Kingdom Patent No. GB 2268333 B. [0006] Light emitting diodes are an important class of electroluminescent devices. Light emitting diodes are commonly fabricated from semiconducting materials in the Ga.sub.1-xAl.sub.xAs, Ga.sub.1-xIn.sub.xAs.sub.1-yP.sub.y, and GaAs.sub.1-xP.sub.x systems. A measure of the efficiency of a light emitting diode is its external quantum efficiency, which is defined as the number of photons emitted by the diode divided by the number of electrons entering the diode. Devices fabricated from these materials may have external quantum efficiencies greater than 10%. Electroluminescent devices fabricated from GaAs compounds suffer from the disadvantage that they are difficult to integrate monolithically with silicon based integrated circuit technologies. It has for many years been an important objective of workers skilled in the field of semiconductor technology to be able to produce an electroluminescent device which is compatible with silicon based integrated circuit technologies. [0007] A world-wide interest in the possible use of porous silicon as a luminescent material in an electroluminescent device was generated by a paper by L. T. Canham in Applied Physics Letters, Volume 57, Number 10, 1990, pp 1046-1048. This paper reported efficient visible photoluminescence from quantum wires in porous silicon at room temperature. A silicon quantum wire may be defined as a physically continuous column of silicon of width not greater than 10 nm, having a length which is not less than twice its width, and whose boundaries are surrounded by a suitable passivation layer. A porous silicon electroluminescent device offers the advantage of potential compatibility with conventional silicon integrated circuit fabrication techniques for use in applications such as optical displays and optoelectronic integrated circuits. [0008] As mentioned above, electroluminescent porous silicon is described in United Kingdom Patent No. GB 2268333 B. The world-wide interest in electroluminescent porous silicon has been evidenced by a large number of published scientific papers which describe electroluminescent devices incorporating porous silicon. However, the luminescence efficiencies reported for these devices have been disappointingly low. V. P. Kesan et al. in Journal of Vacuum Science and Technology A, Volume 11, Number 4, 1993, pp 1736-1738 have reported a p-n junction porous silicon electroluminescent device having an efficiency in the range 0.04% to 0.1%. The Kesan et al. device however has a threshold current density of 30 000 Am.sup.-2 before electroluminescence is observable. Such a high threshold value would seem to be incompatible with the stated efficiency values. Also, there is no indication in the paper of Kesan et al. as to whether the quoted efficiency measurement is an external quantum efficiency figure or some other efficiency, such as internal quantum efficiency. If the quoted efficiency figure is an internal quantum efficiency, the external quantum efficiency figure would be significantly lower, perhaps of the order of a factor 10 lower. [0009] F. Koziowski et al. in Sensors and Actuators A, Volume A43, No. 1-3, 1994, pp 153-156 report a light emitting device in porous silicon having a quantum efficiency of 0.01%. This paper however only provides details of the electrical characteristics of luminescent devices having quantum efficiencies in the range 10.sup.-3 to 10.sup.-4%. [0010] L. V. Belyakov et al. in Semiconductors, Volume 27, No. 11-12, 1993, pp 999-1001 have reported luminescence efficiencies of up to 0.3% for cathodically biased electroluminescent porous silicon devices incorporating a liquid electrolyte. They reported the observation of electroluminescence at a current density of 200 Am.sup.-2. A device incorporating a liquid electrolyte would be difficult to integrate with a conventional silicon based microcircuit. [0011] W. Lang et al. in Journal of Luminescence, Volume 57, 1993, pp 169-173 describe an electroluminescent device which has a thin gold top electrode. Lang et al. observes electroluminescence above a current threshold of 1.1 Am.sup.-2 and measured an external quantum efficiency of 0.01%. They estimate that their device had an internal efficiency which was greater than 0.1%. An external efficiency value is a measure of the efficiency of generating photons external to a device and is distinct from internal efficiency values which are measures of the efficiency of generating photons within the device. The internal efficiency value will be higher than the external efficiency value because of internal absorption and scattering mechanisms. [0012] Virtually all scientific papers published on porous silicon light emitting diodes have been concerned with device performance during operation in ambient air. An exception to this is a paper by Badoz et al. published in Proceedings 7th International Symposium on Silicon Materials Science and Technology, Electrochemical Society Inc. Pennington, New Jersey, Proc. Volume 94-10, pages 569-574D, 1994. They demonstrate that the stability of inefficient (external quantum efficiency 10.sup.-4%) porous silicon light emitting diodes is dramatically improved when operated in dry nitrogen gas rather than ambient air. They suggest that degradation arises from electrically enhanced oxidation of the silicon skeleton. [0013] Scientific papers have been published which suggest that when p-type silicon is anodized n-type porous silicon is produced. N. J. Pulsford et al. in Journal of Luminescence, Volume 57, 1993, pp 181-184 reported the anodization of 25 .OMEGA.cm p-type silicon substrates to produce photoluminescent porous silicon. From measurements of the electrical characteristics of the porous silicon, they came to the conclusion that their results were consistent with the porous silicon being n-type. Amisola et al. in Applied Physics Letters, Volume 61, Number 21, 1992, pp 2595-2597 reported scanning tunnelling microscopy measurements of porous silicon produced from p-type silicon which showed that at least the surface of the porous silicon behaved like n-type material. [0014] Measurements of the spreading resistance of a layer of porous silicon having a porosity of 30% produced from heavily doped p-type silicon, using a method described in U.S. Pat. No. 5,348,618, show that the spreading resistance of the porous silicon increases with increasing depth. This corresponds to an increase in resistivity with increasing depth. This is opposite to the behaviour of porous silicon produced from heavily doped n-type silicon, and is indicative of a n-p junction being formed at the porous silicon-silicon interface. It is concluded that previously published work describing the production of electroluminescent devices by the anodization of p-n silicon structures does not result in a p-n junction being formed within the porous silicon at a position corresponding to the original p-n interface but instead results in a heterojunction between the porous silicon and the bulk silicon. SUMMARY OF THE INVENTION [0015] It is an object of the invention to provide an alternative electroluminescent device. [0016] The present invention provides an electroluminescent device biasable to produce electroluminescence and comprising a porous silicon region and electrical connections to the porous silicon region, characterized in that electroluminescence is detectable when the device is biased such that a current having a current density of less than 1.0 Am.sup.-2 flows through the device. [0017] The invention provides the advantage that a low threshold current is required to produce electroluminescence. A low threshold current is advantageous in applications where power conservation is at a premium, for example battery powered electronics. [0018] The devices of the present invention may be fabricated by a method which includes anodizing a silicon wafer after it has received a dopant implant but without the wafer being annealed after the implantation. In general, silicon wafers are annealed after they have received a dopant implantation in order to activate electrically the dopant species and to anneal any damage to the crystal structure caused by the implantation process. Anodizing a wafer after a dopant implantation with no intervening anneal stage would be considered surprising to those familiar with silicon processing techniques. [0019] The device may exhibit electroluminescence when biased such that an electrical current having a current density of less than 0.1 Am.sup.-2 flows through the device. Electroluminescence from the device may be visible to an unaided human eye when the current density is less than 0.1 Am.sup.-2. Electroluminescence may be detectable when the current density is less than 0.01 Am.sup.-2 and as low as 0.0001 Am.sup.-2. The external quantum efficiency of the electroluminescence may be greater than 0.1%. External quantum efficiencies as high as 0.4% have been measured for devices operating at 200 K (-73.degree. C.). The combination of high efficiency and low threshold current are particularly advantageous. A device of area 1 mm.sup.2 operating under an applied bias current density of 0.0001 Am.sup.-2 would require a bias current of only 10.sup.-10 amps, or 0.1 nA, to produce detectable luminescence. [0020] In another aspect, the invention provides a solid state electroluminescent device comprising a porous silicon region and electrical connections to the porous silicon region, characterized in that the device is biasable to produce electroluminescence with an external quantum efficiency greater than 0.01%. [0021] A high external quantum efficiency is advantageous since for a given luminescent intensity, the more efficient a device, the less power it requires. [0022] The solid state device of the invention may exhibit electroluminescence with an external quantum efficiency greater than 0.1%. The external quantum efficiency may be in the range 0.01% to 0.18% and may be at least 0.4%. Continue reading about Electroluminescent device comprising porous silicon... 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