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Patent Issued for Nitride-Based Semiconductor Light-Emitting Device and Method for Fabricating the Same

August 6, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventors Yokogawa, Toshiya (Nara, JP); Kato, Ryou (Osaka, JP), filed on September 7, 2009, was published online on July 22, 2014.

The patent's assignee for patent number 8785965 is Panasonic Corporation (Osaka, JP).

News editors obtained the following quote from the background information supplied by the inventors: "A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting device because its bandgap is sufficiently wide. Among other things, gallium nitride-based compound semiconductors (which will be referred to herein as 'GaN-based semiconductors' and which are represented by the formula Al.sub.xGa.sub.yIn.sub.zN (where 0.ltoreq.x, y, z.ltoreq.1 and x+y+z=1)) have been researched and developed particularly extensively. As a result, blue-ray-emitting light-emitting diodes (LEDs), green-ray-emitting LEDs and semiconductor laser diodes made of GaN-based semiconductors have already been used in actual products (see Patent Documents Nos. 1 and 2, for example).

"When a semiconductor device is fabricated using GaN-based semiconductors, a sapphire wafer, an SiC wafer, an Si wafer or any other appropriate wafer is used as a wafer on which a crystal of GaN-based semiconductors needs to be grown. No matter which of these wafers is used, however, it is always difficult to achieve a sufficient degree of lattice matching between the wafer and the GaN-based semiconductor crystal (i.e., to realize a coherent growth). As a result, a lot of dislocations (including edge dislocations, spiral dislocations and mixed dislocations) will usually be produced inside the GaN-based semiconductor crystal and will have as high a density as approximately 1.times.10.sup.9 cm.sup.-2 when a sapphire wafer or an SiC wafer is used, for example. Consequently, an increase in threshold current and a decrease in reliability will be unavoidable as for a semiconductor laser diode, and an increase in power dissipation and a decrease in efficiency or reliability will be inevitable as for an LED. Also, some existent GaN wafers may certainly have a lower density of dislocations but its crystal would have a lot of residual strain. That is why even if a GaN-based semiconductor crystal is grown on such a wafer, it should be difficult to go without experiencing a similar problem.

"Thus, a so-called 'epitaxial lateral overgrowth (ELO)' technique has been proposed as a method for reducing the density of dislocations in a GaN-based semiconductor crystal. Such a method will effectively reduce the number of threading dislocations in a system with a high degree of lattice misfit. If a GaN-based semiconductor crystal is grown on each of the wafers described above by ELO, the upper part of the seed crystal will have a region in which there are a lot of dislocations at a density of approximately 1.times.10.sup.9 cm.sup.-2, but the density of locations can be reduced to around 1.times.10.sup.7 cm.sup.-2 in a laterally growing region. And if an active region, which is an electron injected region, is defined over such a region with a fewer number of dislocations, the reliability can be increased."

As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventors' summary information for this patent: "Technical Problem

"The present inventors, however, discovered that even such a GaN-based semiconductor light-emitting device, of which the crystal was grown by ELO, was not without a different kind of problem. Specifically, when a GaN-based semiconductor crystal, which had been grown by ELO, was analyzed with an X-ray micro beam, there was a non-uniform distribution of strain within a plane of the GaN-based semiconductor crystal. Such a non-uniform distribution of strain is not beneficial because it would make the emission intensity non-uniform within that plane.

"It is therefore an object of the present invention to suppress the non-uniform strain in a nitride-based semiconductor light-emitting device, of which the crystal has been grown by ELO process.

"Solution to Problem

"A semiconductor device according to the present invention is a nitride-based semiconductor light-emitting device having a nitride-based semiconductor multilayer structure. The nitride-based semiconductor multilayer structure includes: an active layer including an Al.sub.aIn.sub.bGa.sub.cN crystal layer (where a+b+c=1, a.gtoreq.0, b.gtoreq.0 and c.gtoreq.0); an Al.sub.dGa.sub.eN overflow suppressing layer (where d+e=1, d>0, and e.gtoreq.0); and an Al.sub.fGa.sub.gN layer (where f+g=1, f.gtoreq.0, g.gtoreq.0 and f
"In one preferred embodiment, the In-doped layer has an In concentration of 8.times.10.sup.18/cm.sup.3 or less.

"In another preferred embodiment, the In-doped layer forms a part of the Al.sub.dGa.sub.eN overflow suppressing layer and is arranged closest to the active layer.

"In still another preferred embodiment, the In-doped layer is a half or less as thick as the Al.sub.dGa.sub.eN overflow suppressing layer.

"In yet another preferred embodiment, the nitride-based semiconductor light-emitting device further includes a selectively grown layer. The nitride-based semiconductor multilayer structure is arranged on the selectively grown layer. And the Al.sub.dGa.sub.eN overflow suppressing layer is located on the other side of the active layer opposite to the selectively grown layer.

"In yet another preferred embodiment, the nitride-based semiconductor multilayer structure has an m plane as its principal surface.

"In another preferred embodiment, the selectively grown layer is an Al.sub.xGa.sub.yIn.sub.zN crystal layer (where x+y+z=1, x.gtoreq.0, y.gtoreq.0 and z.gtoreq.0) that has grown from a surface region of an Al.sub.uGa.sub.vIn.sub.wN layer (where u+v+w=1, u.gtoreq.0, v.gtoreq.0 and w.gtoreq.0), which is not covered with a mask layer.

"In this particular preferred embodiment, the nitride-based semiconductor light-emitting device includes: a substrate; and the Al.sub.uGa.sub.vIn.sub.wN layer, which has been formed on the substrate and which is partially covered with the mask layer. The selectively grown layer is in contact with the surface region of the Al.sub.uGa.sub.vIn.sub.wN layer that is not covered with the mask layer.

"In a specific preferred embodiment, another surface region of the Al.sub.uGa.sub.vIn.sub.wN layer that is covered with the mask layer defines a recess, and the selectively grown layer is out of contact with the mask layer.

"In still another preferred embodiment, the selectively grown layer forms at least a part of a GaN substrate.

"In yet another preferred embodiment, the more distant from the active layer, the lower the In concentration of the In-doped layer.

"In yet another preferred embodiment, an undoped GaN layer is arranged between the active layer and the Al.sub.dGa.sub.eN overflow suppressing layer.

"A method according to the present invention is a method for fabricating a nitride-based semiconductor light-emitting device having a nitride-based semiconductor multilayer structure and includes the steps of: (a) forming an active layer including an Al.sub.aIn.sub.bGa.sub.cN crystal layer (where a+b+c=1, a.gtoreq.0, b.gtoreq.0 and c.gtoreq.0) as a portion of the nitride-based semiconductor multilayer structure; (b) forming an Al.sub.dGa.sub.eN overflow suppressing layer (where d+e=1, d>0, and e.gtoreq.0) as another portion of the nitride-based semiconductor multilayer structure; and forming an Al.sub.fGa.sub.gN layer (where f+g=1, f.gtoreq.0, g.gtoreq.0 and f
"In one preferred embodiment, the In-doped layer has an In concentration of 8.times.10.sup.18/cm.sup.3 or less.

"In another preferred embodiment, the In-doped layer forms a part of the Al.sub.dGa.sub.eN overflow suppressing layer and is arranged closest to the active layer.

"In still another preferred embodiment, the In-doped layer is a half or less as thick as the Al.sub.dGa.sub.eN overflow suppressing layer.

"In yet another preferred embodiment, the method further includes the step of forming a selectively grown layer, and the step (b) includes forming the Al.sub.dGa.sub.eN overflow suppressing layer on the other side of the active layer opposite to the selectively grown layer.

"In yet another preferred embodiment, the nitride-based semiconductor multilayer structure has an m plane as its principal surface.

"In yet another preferred embodiment, the method further includes, before the step (a), the steps of: forming an Al.sub.uGa.sub.vIn.sub.wN layer (where u+v+w=1, u.gtoreq.0, v.gtoreq.0 and w.gtoreq.0) on a substrate; (d) forming a mask layer on a portion of the Al.sub.uGa.sub.vIn.sub.wN layer; and (e) supplying a source material to the Al.sub.uGa.sub.vIn.sub.wN layer with the mask layer, thereby growing an Al.sub.xGa.sub.yIn.sub.zN crystal layer (where x+y+z=1, x.gtoreq.0, y.gtoreq.0 and z.gtoreq.0) to be the selectively grown layer using, as a seed crystal, another portion of the Al.sub.uGa.sub.vIn.sub.wN layer that is not covered with the mask layer. The step (e) includes forming the selectively grown layer that covers the mask layer by growing laterally at least a part of the Al.sub.xGa.sub.yIn.sub.zN crystal layer.

"In this particular preferred embodiment, the step (d) includes cutting a recess through the Al.sub.uGa.sub.vIn.sub.wN layer and forming a mask layer at the bottom of the recess, and the step (e) includes growing the selectively grown layer over the mask layer with an air gap left between them.

"In a specific preferred embodiment, the method further includes, after the step (b), the step of removing at least a part of the substrate.

"In yet another preferred embodiment, the method further includes the steps of: providing a GaN substrate as the selectively grown layer; and forming an Al.sub.uGa.sub.vIn.sub.wN layer (where u+v+w=1, u.gtoreq.0, v.gtoreq.0 and w.gtoreq.0), as a portion of the nitride-based semiconductor multilayer structure, on the GaN substrate.

"Advantageous Effects of Invention

"According to the present invention, a layer including In at a concentration of 1.times.10.sup.16 atms/cm.sup.3 to 1.times.10.sup.19 atms/cm.sup.3 is formed in an Al.sub.dGa.sub.eN layer, thereby minimizing the occurrence of non-uniform strain in a nitride-based semiconductor light-emitting device. As a result, it is possible to prevent the nitride-based semiconductor light-emitting device from having a non-uniform in-plane distribution of emission."

For additional information on this patent, see: Yokogawa, Toshiya; Kato, Ryou. Nitride-Based Semiconductor Light-Emitting Device and Method for Fabricating the Same. U.S. Patent Number 8785965, filed September 7, 2009, and published online on July 22, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8785965.PN.&OS=PN/8785965RS=PN/8785965

Keywords for this news article include: Electronics, Semiconductor, Panasonic Corporation.

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