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Method of fabricating light-emitting semiconductor deviceRelated Patent Categories: Semiconductor Device Manufacturing: Process, Making Device Or Circuit Emissive Of Nonelectrical SignalMethod of fabricating light-emitting semiconductor device description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060275937, Method of fabricating light-emitting semiconductor device. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a Divisional of U.S. application Ser. No. 10/994,922 filed Nov. 22, 2004 (Atty. Docket No. TAK-0404), which claims priority to Japanese Patent Application No.2003-396151, filed Nov. 26, 2003. FIELD OF THE INVENTION [0002] This invention relates to a light-emitting diode or like semiconductor device made from chemical compounds, particularly those of Groups III-V elements, and to a method of making such a light-emitting device. BACKGROUND OF THE INVENTION [0003] A typical conventional compound semiconductor light-emitting device is a lamination of a baseplate of sapphire or silicon carbide or silicon, a buffer layer, an n-type semiconductor layer, an active layer, a p-type semiconductor layer, and a contact layer. Another example, taught by the U.S. Pat. No. 5,008,718 to Fletcher et al., is a lamination of a substrate, an n-type semiconductor layer, an active layer, a p-type semiconductor layer, and a current spreading layer which is also referred to as a transparent window layer. [0004] The light generated at the active layer is emitted through the topmost contact layer or current spreading layer. An anode or top electrical contact is provided in the form of a circular metal pad mounted centrally on the surface of the contact layer or current spreading layer, leaving the rest of the surface area of the contact layer uncovered for light emission. It layer, leaving the rest of the surface area of the contact layer uncovered for light emission. It has also been known to mount a metal pad on the contact layer or current spreading layer via a transparent electrode. [0005] Being impervious to light, the metal pad itself blocks the light from the corresponding part of the active layer. The current flowing in this part of the active layer was wasted, not contributing to the net efficiency of the light emitting device. It has therefore been required for higher efficiency light production to reduce the amount of current flowing in that part of the active layer which is opposed to the pad, and to increase the amount of the current, and make the current distribution more uniform, in the rest of the active layer. [0006] A conventional attempt to meet these requirements is found in Japanese Unexamined Patent Publication No. 2003-197965, which suggests a provision of a metal annulus or rim loosely surrounding, and electrically coupled to, the metal pad on the surface of the contact layer, and of a cur-rent spreading layer between active layer and contact layer. Itself impervious to light, however, the annulus reduced by as much as 30 to 40 percent the surface area from which light was emitted, causing a corresponding drop in the efficiency of light production. Additionally, the current spreading layer was not nearly so pervious to light as could be desired, absorbing so much light as to invite a significant diminution in the efficiency of light production. [0007] It might be contemplated to make the electrode itself pervious to light. As heretofore fabricated, however, such an electrode could pass only 70 to 90 percent of the incident light, blocking, in other words, 10 to 30 percent. [0008] The relatively thick current spreading layer proposed by the U.S. patent cited above is also objectionable from the standpoint of maximal light emission. This layer has proved to absorb so much light as to cause a significant decrease in the efficiency of light emission. SUMMARY OF THE INVENTION [0009] The present invention seeks to realize a substantive improvement in the efficiency of the light-emitting device of the kind defined. [0010] Briefly, the invention may be summarized as a light-emitting semiconductor device of improved efficiency. Included is a lamination of: (a) a first compound semiconductor layer of a first conductivity type; (b) an active layer of a compound semiconductor on the first compound semi-conductor layer; (c) a transparent second compound semiconductor layer of a second conductivity type, opposite to the first conductivity type, on the active layer; and (d) a transparent current spreading layer on the second compound semiconductor layer. An electrode is electrically coupled to the current spreading layer so as to permit emission of light radiated from the active layer through the current spreading layer. Another electrode is electrically coupled to the first compound semiconductor layer. [0011] The invention particularly features the current spreading layer in the form of a lamination of alternating first and second sublayers of different compound semiconductors. The first sublayers of the current spreading layers should be made from a compound semiconductor capable of generating therein the two-dimensional carrier layers (i.e. those of two-dimensional electron gas or hole gas) due to heterojunctions between the first and the second sublayers. The two-dimensional carrier layers provide lateral current channels extending parallel to the major surfaces of the first sublayers. Reducing the resistivity of the current spreading layer, the two-dimensional carrier layers contribute toward the greater uniformity of current distribution in the active layer. [0012] Preferably, the second compound semiconductor layer may have a bandgap grater than that of the active layer and a thickness greater than that of each first or second current spreading sublayer. With such a thickness the second compound semiconductor layer will serve as a cladding, limiting carrier overflow. The device as a whole will improve in efficiency as the carriers are effectively contained in the active layer. [0013] Concerning the current spreading layer, the first sublayers may have a bandgap greater than that of the active layer and be fabricated from a nitride semiconductor which may, or may not, contain aluminum. The second sublayers may also have a bandgap greater than that of the active layer and be fabricated from a nitride semiconductor containing a greater proportion of aluminum than the first sublayers possibly do. The two nitride semiconductors containing aluminum in different percentages are conducive to the creation of two-dimensional carrier layers due to heterojunctions between the first and the second current spreading sublayers. [0014] It is recommended that the first compound semiconductor layer be of p-type conductivity, and that the second compound semiconductor layer and current spreading layer be of n-type conductivity. The n-type current spreading layer will generate two-dimensional electron gas, rather than two-dimensional hole gas, due to the heterojunctions contained therein. Higher in mobility than holes, the electrons will make the device better in response. [0015] The invention also provides two slightly different methods of making the light emitting device of the foregoing possible constructions. Ac-cording to one of the methods the first compound semiconductor layer, active layer, and second compound semiconductor layer are successively grown, in that order, on a substrate in a gaseous phase. Then the first and second sublayers of the current spreading layer are alternately grown a required number of times on the second compound semiconductor layer in a gaseous phase. The substrate serves as a mechanical support in the completed device. [0016] The other method is such that there is first prepared what is herein termed a growth substrate, a substrate used solely for growing the required layers thereon. There is first grown in a gaseous phase on this growth substrate the current spreading layer of the alternating first and second sublayers. Then the two compound semiconductor layers with the active layer sandwiched therebetween are successively grown in a gaseous phase on the current spreading layer. Then a support substrate, different from the growth substrate, is joined to the topmost compound semiconductor layer via a bond layer of electrically conducting material. The growth substrate is removed from the current spreading layer, either before or after the bonding of the support substrate. [0017] The first described method offers a simpler manufacturing process, demanding but one substrate for both growth and support. The second method is nevertheless advantageous in that the current spreading layer serves additionally as a buffer in gaseous phase growth of the compound semiconductor layers and active layer. [0018] Generally, the advantages gained by the instant invention may be recapitulated as follows: [0019] 1. Constituted of several or many alternations of two sublayers of different compositions, the current spreading layer contains just as many heterojunctions adjacent of which there are created layers of two-dimensional carriers, that is those of two-dimensional electrons or holes. As a consequence, the first sublayers of the current spreading layer are very low in lateral resistivity, promoting the lateral spreading of current in the current spreading, hence the uniformity of lateral current distribution in the active layer, and, ultimately, a higher efficiency of light production by the device. [0020] 2. If the multiple heterojunction current spreading layer according to the invention is to offer the same degree of uniformity in lateral current distribution as heretofore, this layer can be significantly less in thickness than its conventional counterpart. Such a thinner current spreading layer will less hinder the passage of light therethrough and incur less power loss. Continue reading about Method of fabricating light-emitting semiconductor device... Full patent description for Method of fabricating light-emitting semiconductor device Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Method of fabricating light-emitting semiconductor device patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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