US20020020856A1 - Contact electrode for n-type gallium nitride-based compound semiconductor and method for forming the same - Google Patents
Contact electrode for n-type gallium nitride-based compound semiconductor and method for forming the same Download PDFInfo
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- US20020020856A1 US20020020856A1 US09/983,189 US98318901A US2002020856A1 US 20020020856 A1 US20020020856 A1 US 20020020856A1 US 98318901 A US98318901 A US 98318901A US 2002020856 A1 US2002020856 A1 US 2002020856A1
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- 229910002601 GaN Inorganic materials 0.000 title claims abstract description 50
- 239000004065 semiconductor Substances 0.000 title claims abstract description 30
- 150000001875 compounds Chemical class 0.000 title claims abstract description 25
- JMASRVWKEDWRBT-UHFFFAOYSA-N Gallium nitride Chemical compound [Ga]#N JMASRVWKEDWRBT-UHFFFAOYSA-N 0.000 title claims abstract description 24
- 238000000034 method Methods 0.000 title claims description 62
- 239000010410 layer Substances 0.000 claims abstract description 116
- 238000000137 annealing Methods 0.000 claims abstract description 62
- 229910052751 metal Inorganic materials 0.000 claims abstract description 42
- 239000002184 metal Substances 0.000 claims abstract description 42
- 239000002344 surface layer Substances 0.000 claims abstract description 36
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims abstract description 26
- 229910052760 oxygen Inorganic materials 0.000 claims abstract description 26
- 239000001301 oxygen Substances 0.000 claims abstract description 26
- 230000015572 biosynthetic process Effects 0.000 claims abstract description 5
- 238000001312 dry etching Methods 0.000 claims description 11
- 229910000838 Al alloy Inorganic materials 0.000 claims description 6
- 229910001069 Ti alloy Inorganic materials 0.000 claims description 6
- GYHNNYVSQQEPJS-UHFFFAOYSA-N Gallium Chemical compound [Ga] GYHNNYVSQQEPJS-UHFFFAOYSA-N 0.000 claims 1
- 229910052733 gallium Inorganic materials 0.000 claims 1
- 239000010936 titanium Substances 0.000 description 24
- 230000000052 comparative effect Effects 0.000 description 8
- 239000002253 acid Substances 0.000 description 7
- 229910052782 aluminium Inorganic materials 0.000 description 6
- 229910052719 titanium Inorganic materials 0.000 description 6
- 238000004380 ashing Methods 0.000 description 5
- 230000007547 defect Effects 0.000 description 5
- 239000013078 crystal Substances 0.000 description 4
- 229910045601 alloy Inorganic materials 0.000 description 3
- 239000000956 alloy Substances 0.000 description 3
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 3
- 238000010276 construction Methods 0.000 description 3
- 229910052737 gold Inorganic materials 0.000 description 3
- 238000004519 manufacturing process Methods 0.000 description 3
- 238000002844 melting Methods 0.000 description 3
- 230000008018 melting Effects 0.000 description 3
- 229910052759 nickel Inorganic materials 0.000 description 3
- 239000007787 solid Substances 0.000 description 3
- VEXZGXHMUGYJMC-UHFFFAOYSA-N Hydrochloric acid Chemical compound Cl VEXZGXHMUGYJMC-UHFFFAOYSA-N 0.000 description 2
- 238000005275 alloying Methods 0.000 description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 1
- 238000000637 aluminium metallisation Methods 0.000 description 1
- 239000007864 aqueous solution Substances 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 229910052804 chromium Inorganic materials 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 239000012776 electronic material Substances 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 238000005530 etching Methods 0.000 description 1
- 239000012212 insulator Substances 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 229910052697 platinum Inorganic materials 0.000 description 1
- 238000007781 pre-processing Methods 0.000 description 1
- 238000004151 rapid thermal annealing Methods 0.000 description 1
- 229910052721 tungsten Inorganic materials 0.000 description 1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/40—Electrodes ; Multistep manufacturing processes therefor
- H01L29/43—Electrodes ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
- H01L29/45—Ohmic electrodes
- H01L29/452—Ohmic electrodes on AIII-BV compounds
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
- H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
- H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
- H01L21/28—Manufacture of electrodes on semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/268
- H01L21/283—Deposition of conductive or insulating materials for electrodes conducting electric current
- H01L21/285—Deposition of conductive or insulating materials for electrodes conducting electric current from a gas or vapour, e.g. condensation
- H01L21/28506—Deposition of conductive or insulating materials for electrodes conducting electric current from a gas or vapour, e.g. condensation of conductive layers
- H01L21/28575—Deposition of conductive or insulating materials for electrodes conducting electric current from a gas or vapour, e.g. condensation of conductive layers on semiconductor bodies comprising AIIIBV compounds
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
- H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
- H01L29/20—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only AIIIBV compounds
- H01L29/2003—Nitride compounds
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
- H01L2924/0001—Technical content checked by a classifier
- H01L2924/0002—Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00
Definitions
- the present invention relates to a contact electrode for a gallium nitride-based compound semiconductor device and a method for forming the same, and more specifically to an n-type contact electrode having a low specific contact resistance, for a gallium nitride-based compound semiconductor device and a method for forming the same.
- An n-type contact electrode for a gallium nitride-based compound semiconductor device has realized a relatively low specific contact resistance by using a multi-layer or alloy electrode and an n-type GaN contact layer.
- Japanese Patent Application Pre-examination Publication No. JP-A-07-045867 the content of which is incorporated by reference in its entirety into this application
- U.S. Pat. 5,563,422 claiming Convention Priorities based on eight Japanese patent applications including said Japanese patent application, the content of which is incorporated by reference in its entirety into this application, disclose that an alloy of Ti (titanium) and Al (aluminum) and a multi-layer film of Ti and Al are preferred as the n-type contact electrode. This will be called a “first prior art example” hereinafter.
- FIG. 1 there is shown a diagrammatic sectional view of the n-type contact electrode of the first prior art example.
- the n-type contact electrode has a construction in which a Ti layer 102 and an Al layer 103 are deposited on an n-type GaN contact layer 101 in the named order.
- an ohmic characteristics is obtained by annealing at a temperature of not less than 400° C.
- FIGS. 2A to 2 D there are shown diagrammatic sectional views for illustrating one method for forming the electrode structure of the first prior art example.
- This method is disclosed by A. T. Ping et al, “Ohmic Contacts to n-type GaN Using Pd/Al Metallization”, Journal of Electronic Materials, Vol. 25, No. 5, 1996, pp.819-824, the content of which is incorporated by reference in its entirety into this application.
- This method will be called a “second prior art example” hereinafter.
- an n-type GaN contact layer 101 is etched by a dry-etching as shown in FIG. 2A, and an ashing processing is conducted by using an oxygen plasma as shown in FIG. 2B, and thereafter, as a pre-processing, an etching is conducted by using a hydrochloric acid aqueous solution as shown in FIG. 2C, and then, the Ti layer 102 and the Al layer 103 are deposited on the n-type GaN contact layer 101 in the named order as shown in FIG. 2D. Finally, a rapid thermal annealing (abbreviated to “RTA”) is conducted at a temperature of 650° C. for 30 seconds.
- RTA rapid thermal annealing
- JP-A-07-221103 discloses an English abstract of JP-A-07-221103 is available from the Japanese Patent Office and the content of the English abstract of JP-A-07-221103 is also incorporated by reference in its entirety into this application
- an electrode structure which has improved the electrode structure of the first prior art example. This will be called a “third prior art example” hereinafter.
- the electrode structure of this third prior art example after a double layer metal film of Ti and Al is formed on an n-type semiconductor layer with Ti being in contact with the n-type semiconductor layer, or after an alloy film of Ti and Al is formed on the n-type semiconductor layer, a metal having a melting point higher than that of Al is deposited.
- the third prior art exemplifies Au, Ti, Ni, Pt, W, Mo, Cr and Cu as an the metal having the melting point higher than that of Al, and mentions that Au, Ti and Ni are particularly preferable.
- FIG. 3 there is shown a diagrammatic sectional view of the n-type contact electrode of the third prior art example.
- the n-type contact electrode has a construction having a Ti layer 102 , an Al layer 103 , an Ni layer 104 and an Au layer 105 , which are deposited on an n-type GaN contact layer 101 in the named order.
- an ohmic characteristics is obtained by annealing at a temperature of not less than 400° C., similarly to the first prior art example.
- the Ni layer 104 prevents aluminum from separating out to a surface and also suppresses oxidation of aluminum. Therefore, it is advantageous in that a bonding wiring formed onto the Au layer 105 becomes difficult to be peel off.
- FIGS. 4A and 4B there are shown diagrammatic sectional views for illustrating one method for forming the electrode structure of the third prior art example.
- This method is disclosed by Z. Fan et al, “Very low resistance multilayer ohmic contact to n-GaN”, Applied Physics Letters, Vol. 68, No. 12, Mar. 18 1996, pp. 1672-1674, the content of which is incorporated by reference in its entirety into this application.
- This method will be called a “fourth prior art example” hereinafter.
- an n-type GaN contact layer 101 is etched by a dry-etching as shown in FIG. 4A, and then, a Ti layer 102 , an Al layer 103 , an Ni layer 104 and an Au layer 105 are deposited on the n-type GaN contact layer 101 in the named order as shown in FIG. 4B. Finally, the RTA processing is conducted at a temperature of 900° C. for 30 seconds.
- the fourth prior art reported that when no annealing is conducted, the specific contact resistance of 3.3 ⁇ 10 ⁇ 6 ⁇ cm 2 is obtained.
- the inventors actually manufactured the n-type contact electrodes in the same process as the fourth prior art example, and measured a contact characteristics of the n-type contact electrodes manufactured.
- no ohmic characteristics could not be obtained when the annealing was conducted at a temperature of not greater than 400° C. or when no annealing was conducted. This is considered because of damage given on the surface of the n-type GaN contact layer 101 by the dry etching in the step shown in FIG.
- the device manufacturing process can be simplified and also the degree of freedom in the manufacturing steps can be made large.
- Another object of the present invention is to provide an n-type contact electrode having a low specific contact resistance, for a gallium nitride-based compound semiconductor and a method for forming the same with no annealing step.
- Still another object of the present invention is to provide an n-type contact electrode having a low specific contact resistance, for a gallium nitride-based compound semiconductor and a method for forming the same by performing an annealing at a low temperature.
- a contact electrode having a further low specific contact resistance including a high concentration oxygen-doped surface layer formed in a surface of an n-type contact layer of a gallium nitride-based compound semiconductor, and a metal electrode formed on the oxygen-doped surface layer.
- the high concentration oxygen-doped surface layer can be formed for example by exposing the n-type contact layer of the gallium nitride-based compound semiconductor to an oxygen plasma.
- oxygen enters in the n-type gallium nitride-based compound semiconductor contact layer, a high concentration of oxygen donors are formed in a surface of the n-type gallium nitride-based compound semiconductor contact layer. Since good donors are formed in the surface of the n-type gallium nitride-based compound semiconductor contact layer, an n-type contact electrode having a low specific contact resistance is obtained with good reproducibility, with performing no annealing.
- the metal electrode is formed of a metal multilayer film including a Ti layer in contact with the oxygen-doped surface layer and an Al layer formed on the Ti layer, or a metal multilayer film including a Ti/Al alloy layer in contact with the oxygen-doped surface layer.
- a Pt film is formed on the metal multilayer film.
- a method for forming a contact electrode including the step of exposing an n-type contact layer of a gallium nitride-based compound semiconductor to an oxygen plasma to form an oxygen-doped surface layer in a surface of the n-type contact layer, and forming an electrode metal on the oxygen-doped surface layer.
- an electrode metal is formed on the oxygen-doped surface layer. If an acid processing were conducted after the oxygen plasma processing as in the second prior art example explained hereinbefore, oxygen is removed, with the result that the oxygen donors is reduced, and therefore, an n-type contact electrode having a low specific contact resistance is no longer obtained. On the other hand, if an electrode metal is formed on the oxygen-doped surface layer with conducting no acid processing after the oxygen plasma processing but before formation of the electrode, even if no annealing is conducted, an n-type contact electrode having an ohmic characteristics is obtained with good reproducibility.
- an annealing can be conducted.
- a temperature for this annealing is preferably 500° C. to 600° C.
- a method for forming a contact electrode including the step of dry-etching an n-type contact layer of a gallium nitride-based compound semiconductor, exposing the n-type contact layer to an oxygen plasma to form an oxygen-doped surface layer in a surface of the n-type contact layer, forming an electrode metal on the oxygen-doped surface layer, and thereafter, conducting an annealing.
- This annealing is conducted preferably at a temperature of 600° C. to 800° C.
- the ohmic characteristics cannot be obtained if the annealing is not conducted at a temperature of not less than 500° C. However, if the annealing is conducted at a temperature of not less than 600° C., it is possible to obtain a specific contact resistance which is further lower than that obtained in the method in accordance with the second aspect of the present invention. The reason for this is considered to be that because of the crystal defect generated by the dry etching, the alloying between the electrode and the contact layer is facilitated at the time of the annealing.
- the metal electrode is formed of a metal multilayer film including a Ti layer in contact with the oxygen-doped surface layer and an Al layer formed on the Ti layer, or a metal multilayer film including a Ti/Al alloy layer in contact with the oxygen-doped surface layer.
- a Pt film is formed on the metal multilayer film.
- FIG. 1 is a diagrammatic sectional view of the n-type contact electrode of the first prior art example
- FIGS. 2A to 2 D are diagrammatic sectional views for illustrating one method for forming the electrode structure of the first prior art example
- FIG. 3 is a diagrammatic sectional view of the n-type contact electrode of the third prior art example
- FIGS. 4A and 4B are diagrammatic sectional views for illustrating one method for forming the electrode structure of the third prior art example
- FIGS. 5A and 5B are diagrammatic sectional views for illustrating a first embodiment of the method in accordance with the present invention for forming the n-type electrode
- FIGS. 6A, 6B and 6 C are diagrammatic sectional views for illustrating a third embodiment of the method in accordance with the present invention for forming the n-type electrode.
- FIG. 7 is a graph illustrating a relation between the RTA annealing temperature and the specific contact resistance in n-type electrodes formed in accordance with the present invention and in accordance with the prior art.
- an oxygen plasma ashing processing is performed for a surface of an n-type GaN contact layer 1 , to form a high concentration oxygen-doped surface layer on the surface of an n-type GaN contact layer 1 , and thereafter, without performing any additional processing, a Ti layer 2 , an Al layer 3 , a Pt layer 4 and an Au layer 5 are deposited on the surface of the n-type GaN contact layer 1 (namely, on the high concentration oxygen-doped surface layer) in the named order to form a metal electrode, as shown in FIG. SB. After the metal electrode is formed, no annealing is performed. The oxygen plasma ashing processing was performed for two minutes under an oxygen pressure of b 0 . 8 torr and a plasma power of 200 W.
- Characteristics of the n-type contact electrode thus formed was measured.
- the n-type contact electrode having an ohmic characteristics could be formed with good reproducibility, with performing no annealing.
- the specific contact resistance was 5 ⁇ 10 ⁇ 5 ⁇ cm 2 .
- the metal electrode is formed on the surface of the oxygen-doped surface layer without conducting an acid processing to the n-type GaN contact layer 1 .
- the RTA processing is conducted for 30 minutes at a temperature of 500° C., after the first embodiment of the method in accordance with the present invention for forming the n-type electrode is performed.
- reproducibility of the ohmic characteristics of the n-type contact electrode obtained was further improved, and the specific contact resistance obtained was 1.4 ⁇ 10 ⁇ 6 ⁇ cm 2 , which is lower than that obtained in the first embodiment of the method in accordance with the present invention.
- a dry etching is performed to a surface of an n-type GaN contact layer 1 , and then, as shown in FIG. 6B, an oxygen plasma ashing processing is performed for the surface of an n-type GaN contact layer 1 , to form a high concentration oxygen-doped surface layer on the surface of an n-type GaN contact layer 1 .
- a Ti layer 2 , an Al layer 3 , a Pt layer 4 and an Au layer 5 are deposited on the surface of the n-type GaN contact layer 1 (namely, on the high concentration oxygen-doped surface layer) in the named order to form a metal electrode, as shown in FIG. 6C. Furthermore, an annealing is performed.
- the oxygen plasma ashing processing was performed under the same condition as that in the first embodiment.
- the annealing is an RTA processing for 30 minutes at a temperature of 600° C.
- the metal electrode is formed of a metal multilayer film including the Ti layer 2 in contact with the n-type contact layer and the Al layer formed on the Ti layer 2 .
- the metal electrode was formed of a metal multilayer film including a Ti/Al alloy layer in contact with the n-type contact layer, a similar effect could be obtained.
- the Pt film is formed on the metal multilayer film. This Pt film prevents the Al layer from diffusing on an electrode surface and from being oxidized, and also prevents the Au layer from diffusing into the Al layer and its underlying layer.
- This function is the same as that of the Ni layer 104 in the third and fourth prior art examples.
- Pt has a melting point higher than that of Ni, and the diffusion preventing function of Pt is more excellent than Ni.
- Ni itself diffuses, but Pt almost never diffuses.
- FIG. 7 there is shown a graph illustrating a relation between the RTA annealing temperature and the specific contact resistance in n-type electrodes formed in accordance with the present invention and in accordance with the prior art.
- the electrode metal was Ti/Al/Pt/Au
- the RTA annealing time was a constant time of 30 seconds. Therefore, the graph of FIG. 7 illustrates dependency of the specific contact resistance upon the RTA annealing temperature.
- “ ⁇ ”(solid circle) indicates the specific contact resistance when the contact electrode was formed in accordance with the electrode forming methods of the first and second embodiments.
- the specific contact resistance obtained was 5 ⁇ 10 ⁇ 5 ⁇ cm 2 in the first embodiment and 1.4 ⁇ 10 ⁇ 6 ⁇ cm 2 in the second embodiment.
- “ ⁇ ”(cross) indicates a comparative example 1 in which, in addition to the electrode forming methods of the first and second embodiments, an acid processing is performed for the surface of the n-type GaN contact layer 1 just before formation of the electrode, to reduce the oxygen donors.
- the comparative example 1 when no annealing was performed, the ohmic characteristics could not be obtained with good reproducibility.
- the annealing was performed at a temperature of 500° C., the specific contact resistance obtained was higher than that of the second embodiment by about two figures.
- a difference between the first and second embodiments and the comparative example 1 is particularly remarkable when the annealing temperature was not greater than 500° C.
- the reason for the high specific contact resistance in the comparative example 1 is considered to be that, because of the acid processing to the surface of the n-type GaN contact layer 1 , the oxygendoped surface layer formed on the surface of the n-type GaN contact layer 1 by the oxygen plasma processing was removed.
- the electrode forming method of the first embodiment it is possible to obtain the n-type contact electrode having the ohmic characteristics with high reproducibility, even if no annealing is performed.
- the specific contact resistance is relative high, but if a large contact area is ensured by contriving a device structure, a relative high specific contact resistance is no longer a problem, and on the other hand, a low temperature device process can be advantageously used.
- the electrode forming method of the second embodiment it is necessary to perform the annealing at a temperature of not less than 500° C., but it is possible to reduce the specific contact resistance by about one figure in comparison with the first embodiment. Therefore, a necessary contact area can be reduced in comparison with the first embodiment.
- a device process temperature in the order of 500° C. is permissible, it is possible to obtain a contact resistance lower than the first embodiment.
- the electrode forming method in accordance with the third embodiment of the present invention after the n-type GaN contact layer is etched by the dry etching and then exposed to the oxygen plasma, the electrode is formed on the n-type GaN contact layer without performing the acid processing to the n-type GaN contact layer. If the dry etching is conducted, crystal defect occurs in the surface region of the n-type contact layer. Therefore, differently from the method in accordance with the first embodiment of the present invention, the ohmic characteristics cannot be obtained if the annealing is not conducted at a temperature of not less than 500° C.
- the annealing is conducted at a temperature of 600° C., it is possible to obtain a specific contact resistance which is further lower than those obtained in the methods in accordance with the first and second embodiments of the present invention.
- the reason for this is considered to be that because of the crystal defect generated by the dry etching, the alloying between the electrode and the contact layer is facilitated at the time of the annealing.
- the graph of FIG. 7 also illustrates the electrode forming method in accordance with the third embodiment of the present invention, and a comparative example 2 in which, in addition to the electrode forming method in accordance with the third embodiment of the present invention, an acid processing is performed for the surface of the n-type GaN contact layer 1 after the oxygen plasma processing but before formation of the electrode.
- “ ⁇ ”(solid triangle) indicates the electrode forming method in accordance with the third embodiment of the present invention
- ⁇ ”(solid square) indicates the comparative example 2.
- the electrode forming method in accordance with the third embodiment of the present invention could obtain, with the annealing temperature of 600° C., the specific contact resistance of 1.2 ⁇ 10 ⁇ 8 ⁇ cm 2 , which is smaller than the fourth prior art example requiring the annealing temperature of 900° C., by about one figure.
- the electrode forming method in accordance with the third embodiment of the present invention can obtain, with a relatively low annealing temperature, the specific contact resistance which is further lower than the prior art examples. Accordingly, the electrode forming method in accordance with the third embodiment of the present invention can advantageously be used when a device process temperature in the order of 600° C. is permissible and when it is necessary to reduce the contact area as small as possible.
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Abstract
Description
- 1. Field of the invention
- The present invention relates to a contact electrode for a gallium nitride-based compound semiconductor device and a method for forming the same, and more specifically to an n-type contact electrode having a low specific contact resistance, for a gallium nitride-based compound semiconductor device and a method for forming the same.
- 2. Description of related art
- An n-type contact electrode for a gallium nitride-based compound semiconductor device has realized a relatively low specific contact resistance by using a multi-layer or alloy electrode and an n-type GaN contact layer. For example, Japanese Patent Application Pre-examination Publication No. JP-A-07-045867, the content of which is incorporated by reference in its entirety into this application, and U.S. Pat. 5,563,422 claiming Convention Priorities based on eight Japanese patent applications including said Japanese patent application, the content of which is incorporated by reference in its entirety into this application, disclose that an alloy of Ti (titanium) and Al (aluminum) and a multi-layer film of Ti and Al are preferred as the n-type contact electrode. This will be called a “first prior art example” hereinafter.
- Referring to FIG. 1, there is shown a diagrammatic sectional view of the n-type contact electrode of the first prior art example. As shown in FIG. 1, the n-type contact electrode has a construction in which a
Ti layer 102 and anAl layer 103 are deposited on an n-typeGaN contact layer 101 in the named order. In this construction, an ohmic characteristics is obtained by annealing at a temperature of not less than 400° C. - Referring to FIGS. 2A to2D, there are shown diagrammatic sectional views for illustrating one method for forming the electrode structure of the first prior art example. This method is disclosed by A. T. Ping et al, “Ohmic Contacts to n-type GaN Using Pd/Al Metallization”, Journal of Electronic Materials, Vol. 25, No. 5, 1996, pp.819-824, the content of which is incorporated by reference in its entirety into this application. This method will be called a “second prior art example” hereinafter.
- In the method of the second prior art example, first, an n-type
GaN contact layer 101 is etched by a dry-etching as shown in FIG. 2A, and an ashing processing is conducted by using an oxygen plasma as shown in FIG. 2B, and thereafter, as a pre-processing, an etching is conducted by using a hydrochloric acid aqueous solution as shown in FIG. 2C, and then, theTi layer 102 and theAl layer 103 are deposited on the n-typeGaN contact layer 101 in the named order as shown in FIG. 2D. Finally, a rapid thermal annealing (abbreviated to “RTA”) is conducted at a temperature of 650° C. for 30 seconds. - In this case, a specific contact resistance of 6×10−6Ωcm2 is obtained. This specific contact resistance does not greatly change even if the RTA temperature changes in the range of 550° C. to 750° C. However, if the RTA temperature is less than 550° C. or if no annealing is conducted, the ohmic characteristics cannot be obtained.
- Furthermore, Japanese Patent Application Pre-examination Publication No. JP-A-07-221103, the content of which is incorporated by reference in its entirety into this application (an English abstract of JP-A-07-221103 is available from the Japanese Patent Office and the content of the English abstract of JP-A-07-221103 is also incorporated by reference in its entirety into this application), discloses an electrode structure which has improved the electrode structure of the first prior art example. This will be called a “third prior art example” hereinafter.
- In the electrode structure of this third prior art example, after a double layer metal film of Ti and Al is formed on an n-type semiconductor layer with Ti being in contact with the n-type semiconductor layer, or after an alloy film of Ti and Al is formed on the n-type semiconductor layer, a metal having a melting point higher than that of Al is deposited. The third prior art exemplifies Au, Ti, Ni, Pt, W, Mo, Cr and Cu as an the metal having the melting point higher than that of Al, and mentions that Au, Ti and Ni are particularly preferable.
- Referring to FIG. 3, there is shown a diagrammatic sectional view of the n-type contact electrode of the third prior art example. As shown in FIG. 3, the n-type contact electrode has a construction having a
Ti layer 102, anAl layer 103, anNi layer 104 and anAu layer 105, which are deposited on an n-typeGaN contact layer 101 in the named order. In this example, an ohmic characteristics is obtained by annealing at a temperature of not less than 400° C., similarly to the first prior art example. - In the contact electrode structure of the third prior art example, the
Ni layer 104 prevents aluminum from separating out to a surface and also suppresses oxidation of aluminum. Therefore, it is advantageous in that a bonding wiring formed onto theAu layer 105 becomes difficult to be peel off. - Referring to FIGS. 4A and 4B, there are shown diagrammatic sectional views for illustrating one method for forming the electrode structure of the third prior art example. This method is disclosed by Z. Fan et al, “Very low resistance multilayer ohmic contact to n-GaN”, Applied Physics Letters, Vol. 68, No. 12, Mar. 18 1996, pp. 1672-1674, the content of which is incorporated by reference in its entirety into this application. This method will be called a “fourth prior art example” hereinafter.
- In the method of the fourth prior art example, first, an n-type
GaN contact layer 101 is etched by a dry-etching as shown in FIG. 4A, and then, aTi layer 102, anAl layer 103, anNi layer 104 and anAu layer 105 are deposited on the n-typeGaN contact layer 101 in the named order as shown in FIG. 4B. Finally, the RTA processing is conducted at a temperature of 900° C. for 30 seconds. - In this electrode structure, a specific contact resistance of 8.9×10−8Ωcm2 is obtained, which is remarkably lower than the value obtained in the second prior art example. In this case, it is important that the
Ni layer 104 and theAl layer 103 are thick. In addition, it is an indispensable condition for obtaining a low specific contact resistance that Ni and Au never diffuse into the n-typeGaN contact layer 101. On the other hand, it was reported that when no annealing is conducted, the specific contact resistance is 3.3×10−6Ωcm2. - In the above mentioned prior art examples, a minimum specific contact resistance of 8.9×10−8Ωcm2 is obtained in the fourth prior art example. However, the annealing at as a high temperature as 900° C. deteriorates other electrodes, semiconductor films and insulator films in the case that a semiconductor device is manufactured, and therefore, resultantly remarkably restricts a device manufacturing process. Accordingly, it is necessary to lower a necessary annealing temperature.
- In addition, the fourth prior art reported that when no annealing is conducted, the specific contact resistance of 3.3×10−6Ωcm2 is obtained. The inventors actually manufactured the n-type contact electrodes in the same process as the fourth prior art example, and measured a contact characteristics of the n-type contact electrodes manufactured. However, no ohmic characteristics could not be obtained when the annealing was conducted at a temperature of not greater than 400° C. or when no annealing was conducted. This is considered because of damage given on the surface of the n-type
GaN contact layer 101 by the dry etching in the step shown in FIG. 4A, with the result that it is difficult to obtain the ohmic characteristics with good reproducibility, when the annealing is conducted at a temperature of not greater than 400° C. or when no annealing is conducted. Furthermore, it is not preferable to a device characteristics that much damage remains given on the contact layer without conducting the annealing. - Furthermore, in the electrode structures of the first and third prior art examples and in the electrode forming method of the second prior art example, no ohmic characteristics was obtained when the annealing was conducted at a temperature of not greater than 400° C. or when no annealing was conducted.
- Therefore, a technology for forming a low resistance n-contact electrode at a low annealing temperature is demanded.
- In addition, when a large area for the n-contact electrode can be ensured, even if the specific contact resistance is not so low, if the ohmic characteristics of the n-contact can be obtained with good reproducibility with no annealing, the device manufacturing process can be simplified and also the degree of freedom in the manufacturing steps can be made large.
- Accordingly, it is an object of the present invention to provide an n-type contact electrode having a low specific contact resistance and a method for forming the same, which have overcome the above mentioned defect of the conventional one.
- Another object of the present invention is to provide an n-type contact electrode having a low specific contact resistance, for a gallium nitride-based compound semiconductor and a method for forming the same with no annealing step.
- Still another object of the present invention is to provide an n-type contact electrode having a low specific contact resistance, for a gallium nitride-based compound semiconductor and a method for forming the same by performing an annealing at a low temperature.
- The above and other objects of the present invention are achieved in accordance with a first aspect of the present invention by a contact electrode having a further low specific contact resistance, including a high concentration oxygen-doped surface layer formed in a surface of an n-type contact layer of a gallium nitride-based compound semiconductor, and a metal electrode formed on the oxygen-doped surface layer.
- The high concentration oxygen-doped surface layer can be formed for example by exposing the n-type contact layer of the gallium nitride-based compound semiconductor to an oxygen plasma. In this case, oxygen enters in the n-type gallium nitride-based compound semiconductor contact layer, a high concentration of oxygen donors are formed in a surface of the n-type gallium nitride-based compound semiconductor contact layer. Since good donors are formed in the surface of the n-type gallium nitride-based compound semiconductor contact layer, an n-type contact electrode having a low specific contact resistance is obtained with good reproducibility, with performing no annealing.
- In an embodiment of the n-type contact electrode, the metal electrode is formed of a metal multilayer film including a Ti layer in contact with the oxygen-doped surface layer and an Al layer formed on the Ti layer, or a metal multilayer film including a Ti/Al alloy layer in contact with the oxygen-doped surface layer. Preferably, a Pt film is formed on the metal multilayer film.
- According to a second aspect of the present invention, there is provided a method for forming a contact electrode, including the step of exposing an n-type contact layer of a gallium nitride-based compound semiconductor to an oxygen plasma to form an oxygen-doped surface layer in a surface of the n-type contact layer, and forming an electrode metal on the oxygen-doped surface layer.
- In the method in accordance with the second aspect of the present invention, by exposing an n-type contact layer of a gallium nitride-based compound semiconductor to an oxygen plasma, oxygen donors are formed in a surface of the n-type gallium nitride-based compound semiconductor contact layer. By this processing, not only the oxygen-doped surface layer is formed in the surface of the n-type gallium nitride-based compound semiconductor contact layer, but also carbon is removed from the surface of the n-type gallium nitride-based compound semiconductor contact layer to which an electrode is to be formed.
- Therefore, an n-type contact electrode having a low specific contact resistance is obtained, and a good contact interface is obtained.
- In the method in accordance with the second aspect of the present invention, after the oxygen-doped surface layer is formed by the oxygen plasma processing, an electrode metal is formed on the oxygen-doped surface layer. If an acid processing were conducted after the oxygen plasma processing as in the second prior art example explained hereinbefore, oxygen is removed, with the result that the oxygen donors is reduced, and therefore, an n-type contact electrode having a low specific contact resistance is no longer obtained. On the other hand, if an electrode metal is formed on the oxygen-doped surface layer with conducting no acid processing after the oxygen plasma processing but before formation of the electrode, even if no annealing is conducted, an n-type contact electrode having an ohmic characteristics is obtained with good reproducibility.
- In one embodiment of the method in accordance with the second aspect of the present invention, after the electrode metal is formed on the oxygen-doped surface layer, an annealing can be conducted. By conducting the annealing, it is possible to obtain a specific contact resistance which is lower than that obtained when no annealing is conducted. A temperature for this annealing is preferably 500° C. to 600° C.
- According to a third aspect of the present invention, there is provided a method for forming a contact electrode, including the step of dry-etching an n-type contact layer of a gallium nitride-based compound semiconductor, exposing the n-type contact layer to an oxygen plasma to form an oxygen-doped surface layer in a surface of the n-type contact layer, forming an electrode metal on the oxygen-doped surface layer, and thereafter, conducting an annealing. This annealing is conducted preferably at a temperature of 600° C. to 800° C.
- In the method in accordance with the third aspect of the present invention, since the dry etching is conducted, crystal defect occurs in the surface region of the n-type contact layer. Therefore, differently from the method in accordance with the second aspect of the present invention, the ohmic characteristics cannot be obtained if the annealing is not conducted at a temperature of not less than 500° C. However, if the annealing is conducted at a temperature of not less than 600° C., it is possible to obtain a specific contact resistance which is further lower than that obtained in the method in accordance with the second aspect of the present invention. The reason for this is considered to be that because of the crystal defect generated by the dry etching, the alloying between the electrode and the contact layer is facilitated at the time of the annealing.
- In an embodiment of the methods in accordance with the second and third aspects of the present invention, the metal electrode is formed of a metal multilayer film including a Ti layer in contact with the oxygen-doped surface layer and an Al layer formed on the Ti layer, or a metal multilayer film including a Ti/Al alloy layer in contact with the oxygen-doped surface layer. Preferably, a Pt film is formed on the metal multilayer film.
- The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.
- FIG. 1 is a diagrammatic sectional view of the n-type contact electrode of the first prior art example;
- FIGS. 2A to2D are diagrammatic sectional views for illustrating one method for forming the electrode structure of the first prior art example;
- FIG. 3 is a diagrammatic sectional view of the n-type contact electrode of the third prior art example;
- FIGS. 4A and 4B are diagrammatic sectional views for illustrating one method for forming the electrode structure of the third prior art example;
- FIGS. 5A and 5B are diagrammatic sectional views for illustrating a first embodiment of the method in accordance with the present invention for forming the n-type electrode;
- FIGS. 6A, 6B and6C are diagrammatic sectional views for illustrating a third embodiment of the method in accordance with the present invention for forming the n-type electrode; and
- FIG. 7 is a graph illustrating a relation between the RTA annealing temperature and the specific contact resistance in n-type electrodes formed in accordance with the present invention and in accordance with the prior art.
- Description of the Preferred embodiments
-
Embodiment 1 - First, a first embodiment of the method in accordance with the present invention for forming the n-type electrode will be described with reference to FIGS. 5A and 5B.
- As shown in FIG. 5A, an oxygen plasma ashing processing is performed for a surface of an n-type
GaN contact layer 1, to form a high concentration oxygen-doped surface layer on the surface of an n-typeGaN contact layer 1, and thereafter, without performing any additional processing, aTi layer 2, an Al layer 3, aPt layer 4 and anAu layer 5 are deposited on the surface of the n-type GaN contact layer 1 (namely, on the high concentration oxygen-doped surface layer) in the named order to form a metal electrode, as shown in FIG. SB. After the metal electrode is formed, no annealing is performed. The oxygen plasma ashing processing was performed for two minutes under an oxygen pressure of b 0.8 torr and a plasma power of 200 W. - Characteristics of the n-type contact electrode thus formed was measured. The n-type contact electrode having an ohmic characteristics could be formed with good reproducibility, with performing no annealing. The specific contact resistance was 5×10−5 Ωcm2.
- In the first embodiment, after the oxygen plasma processing, the metal electrode is formed on the surface of the oxygen-doped surface layer without conducting an acid processing to the n-type
GaN contact layer 1. With this method, even if no annealing is performed, the n-type contact electrode having the ohmic characteristics can be obtained with good reproducibility. -
Embodiment 2 - In a second embodiment of the method in accordance with the present invention for forming the n-type electrode, the RTA processing is conducted for 30 minutes at a temperature of 500° C., after the first embodiment of the method in accordance with the present invention for forming the n-type electrode is performed. As a result, reproducibility of the ohmic characteristics of the n-type contact electrode obtained was further improved, and the specific contact resistance obtained was 1.4×10−6 Ωcm2, which is lower than that obtained in the first embodiment of the method in accordance with the present invention.
- Embodiment 3
- Now, a third embodiment of the method in accordance with the present invention will be described with reference to FIGS. 6A to6C.
- First, as shown in FIG. 6A, a dry etching is performed to a surface of an n-type
GaN contact layer 1, and then, as shown in FIG. 6B, an oxygen plasma ashing processing is performed for the surface of an n-typeGaN contact layer 1, to form a high concentration oxygen-doped surface layer on the surface of an n-typeGaN contact layer 1. - Thereafter, without performing any additional processing, a
Ti layer 2, an Al layer 3, aPt layer 4 and anAu layer 5 are deposited on the surface of the n-type GaN contact layer 1 (namely, on the high concentration oxygen-doped surface layer) in the named order to form a metal electrode, as shown in FIG. 6C. Furthermore, an annealing is performed. - The oxygen plasma ashing processing was performed under the same condition as that in the first embodiment. The annealing is an RTA processing for 30 minutes at a temperature of 600° C.
- In the third embodiment, most excellent reproducibility of the ohmic characteristics was obtained, and the specific contact resistance obtained was 1.2×10−8 Ωcm2, which is lower than that obtained in the fourth prior art example.
- In the first to third embodiments, the metal electrode is formed of a metal multilayer film including the
Ti layer 2 in contact with the n-type contact layer and the Al layer formed on theTi layer 2. However, when the metal electrode was formed of a metal multilayer film including a Ti/Al alloy layer in contact with the n-type contact layer, a similar effect could be obtained. - Also in the electrode structure of the first to third embodiments, the Pt film is formed on the metal multilayer film. This Pt film prevents the Al layer from diffusing on an electrode surface and from being oxidized, and also prevents the Au layer from diffusing into the Al layer and its underlying layer.
- This function is the same as that of the
Ni layer 104 in the third and fourth prior art examples. However, Pt has a melting point higher than that of Ni, and the diffusion preventing function of Pt is more excellent than Ni. In addition, at a high temperature annealing, Ni itself diffuses, but Pt almost never diffuses. - Referring to FIG. 7, there is shown a graph illustrating a relation between the RTA annealing temperature and the specific contact resistance in n-type electrodes formed in accordance with the present invention and in accordance with the prior art. In all examples, the electrode metal was Ti/Al/Pt/Au, and the RTA annealing time was a constant time of 30 seconds. Therefore, the graph of FIG. 7 illustrates dependency of the specific contact resistance upon the RTA annealing temperature.
- In the graph of FIG. 7, “”(solid circle) indicates the specific contact resistance when the contact electrode was formed in accordance with the electrode forming methods of the first and second embodiments. The situation that the annealing temperature was 20° C., namely, no annealing was performed, corresponds to the first embodiment, and the situation that the annealing temperature was 500° C., corresponds to the second embodiment. The specific contact resistance obtained was 5×10−5 Ωcm2 in the first embodiment and 1.4×10−6 Ωcm2 in the second embodiment.
- In the graph of FIG. 7, “×”(cross) indicates a comparative example1 in which, in addition to the electrode forming methods of the first and second embodiments, an acid processing is performed for the surface of the n-type
GaN contact layer 1 just before formation of the electrode, to reduce the oxygen donors. In the comparative example 1, when no annealing was performed, the ohmic characteristics could not be obtained with good reproducibility. In addition, although the annealing was performed at a temperature of 500° C., the specific contact resistance obtained was higher than that of the second embodiment by about two figures. - A difference between the first and second embodiments and the comparative example1 is particularly remarkable when the annealing temperature was not greater than 500° C.
- The reason for the high specific contact resistance in the comparative example1 is considered to be that, because of the acid processing to the surface of the n-type
GaN contact layer 1, the oxygendoped surface layer formed on the surface of the n-typeGaN contact layer 1 by the oxygen plasma processing was removed. - As seen from the above, according to the electrode forming method of the first embodiment, it is possible to obtain the n-type contact electrode having the ohmic characteristics with high reproducibility, even if no annealing is performed. In this case, the specific contact resistance is relative high, but if a large contact area is ensured by contriving a device structure, a relative high specific contact resistance is no longer a problem, and on the other hand, a low temperature device process can be advantageously used.
- Furthermore, according to the electrode forming method of the second embodiment, it is necessary to perform the annealing at a temperature of not less than 500° C., but it is possible to reduce the specific contact resistance by about one figure in comparison with the first embodiment. Therefore, a necessary contact area can be reduced in comparison with the first embodiment. In addition, if a device process temperature in the order of 500° C. is permissible, it is possible to obtain a contact resistance lower than the first embodiment.
- In the electrode forming method in accordance with the third embodiment of the present invention, after the n-type GaN contact layer is etched by the dry etching and then exposed to the oxygen plasma, the electrode is formed on the n-type GaN contact layer without performing the acid processing to the n-type GaN contact layer. If the dry etching is conducted, crystal defect occurs in the surface region of the n-type contact layer. Therefore, differently from the method in accordance with the first embodiment of the present invention, the ohmic characteristics cannot be obtained if the annealing is not conducted at a temperature of not less than 500° C. However, if the annealing is conducted at a temperature of 600° C., it is possible to obtain a specific contact resistance which is further lower than those obtained in the methods in accordance with the first and second embodiments of the present invention. The reason for this is considered to be that because of the crystal defect generated by the dry etching, the alloying between the electrode and the contact layer is facilitated at the time of the annealing.
- The graph of FIG. 7 also illustrates the electrode forming method in accordance with the third embodiment of the present invention, and a comparative example 2 in which, in addition to the electrode forming method in accordance with the third embodiment of the present invention, an acid processing is performed for the surface of the n-type
GaN contact layer 1 after the oxygen plasma processing but before formation of the electrode. In the graph of FIG. 7, “▴”(solid triangle) indicates the electrode forming method in accordance with the third embodiment of the present invention, and “▪”(solid square) indicates the comparative example 2. - In both of the electrode forming method in accordance with the third embodiment of the present invention and the comparative example 2, no n-type contact electrode having the ohmic characteristics could be obtained either when no annealing was conducted or when the annealing was performed at a temperature of less than 500° C., but a n-type contact electrode having the ohmic characteristics could be obtained when the annealing was performed at a temperature of not less than 500° C. In the annealing temperature range of 600° C. to 800° C., when the electrode forming method in accordance with the third embodiment of the present invention is used, the specific contact resistance can be reduced by about one figure, in comparison with the comparative example 2.
- In particular, the electrode forming method in accordance with the third embodiment of the present invention could obtain, with the annealing temperature of 600° C., the specific contact resistance of 1.2×10−8 Ωcm2, which is smaller than the fourth prior art example requiring the annealing temperature of 900° C., by about one figure.
- Thus, the electrode forming method in accordance with the third embodiment of the present invention can obtain, with a relatively low annealing temperature, the specific contact resistance which is further lower than the prior art examples. Accordingly, the electrode forming method in accordance with the third embodiment of the present invention can advantageously be used when a device process temperature in the order of 600° C. is permissible and when it is necessary to reduce the contact area as small as possible.
- The invention has thus been shown and described with reference to the specific embodiments. However, it should be noted that the present invention is in no way limited to the details of the illustrated structures but changes and modifications may be made within the scope of the appended claims.
Claims (12)
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US09/983,189 US6423562B1 (en) | 1997-01-14 | 2001-10-23 | Contact electrode for n-type gallium nitride-based compound semiconductor and method for forming the same |
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US09/983,189 US6423562B1 (en) | 1997-01-14 | 2001-10-23 | Contact electrode for n-type gallium nitride-based compound semiconductor and method for forming the same |
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US09/983,189 Expired - Lifetime US6423562B1 (en) | 1997-01-14 | 2001-10-23 | Contact electrode for n-type gallium nitride-based compound semiconductor and method for forming the same |
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1998
- 1998-01-14 US US09/006,937 patent/US20010054763A1/en active Granted
- 1998-01-14 US US09/006,937 patent/US6329716B1/en not_active Expired - Lifetime
-
2001
- 2001-10-23 US US09/983,189 patent/US6423562B1/en not_active Expired - Lifetime
Cited By (9)
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WO2008120094A2 (en) * | 2007-03-30 | 2008-10-09 | Picogiga International | Electronic device with improved ohmic contact |
WO2008120094A3 (en) * | 2007-03-30 | 2008-12-04 | Picogiga Internat | Electronic device with improved ohmic contact |
US20100038682A1 (en) * | 2007-03-30 | 2010-02-18 | Lahreche Hacene | Electronic devices with improved ohmic contact |
US7968390B2 (en) | 2007-03-30 | 2011-06-28 | S.O.I.Tec Silicon On Insulator Technologies | Electronic devices with improved ohmic contact |
US20110215380A1 (en) * | 2007-03-30 | 2011-09-08 | S.O.I.Tec Silicon On Insulator Technologies | Electronic devices with improved ohmic contact |
US8253170B2 (en) | 2007-03-30 | 2012-08-28 | Soitec SA & Soitec USA, Inc. | Electronic devices with improved OHMIC contact |
US20120028475A1 (en) * | 2010-07-30 | 2012-02-02 | Sumitomo Electric Device Innovations, Inc. | Method for fabricating semiconductor device |
US8524619B2 (en) * | 2010-07-30 | 2013-09-03 | Sumitomo Electric Device Innovations, Inc. | Method for fabricating semiconductor device including performing oxygen plasma treatment |
CN103178180A (en) * | 2013-03-26 | 2013-06-26 | 合肥彩虹蓝光科技有限公司 | Novel metal electrode gasket with low cost and high conductivity, and preparation method of gasket |
Also Published As
Publication number | Publication date |
---|---|
JPH10200161A (en) | 1998-07-31 |
US20010054763A1 (en) | 2001-12-27 |
US6423562B1 (en) | 2002-07-23 |
JP2967743B2 (en) | 1999-10-25 |
US6329716B1 (en) | 2001-12-11 |
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