Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-08-05
2004-05-11
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S021000, C257S079000, C257S096000, C257S347000
Reexamination Certificate
active
06734503
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nitride-based semiconductor element, and more particularly, it relates to a nitride-based semiconductor element having a nitride-based semiconductor layer.
2. Description of the Background Art
A technique of growing a nitride-based semiconductor layer, consisting of a material different from that of an underlayer, on the underlayer is known in general. In crystal growth of GaN which is one of nitride-based semiconductors, for example, a nitride-based semiconductor layer is hetero-grown on a heterogeneous substrate such as a sapphire substrate due to a small number of lattice-matching substrates. In relation to this, generally known is a technique of inserting a buffer layer grown under a low temperature between the substrate and the GaN layer in order to grow GaN having excellent crystallinity with a small number of crystal defects.
Also when the aforementioned low-temperature buffer layer is employed, however, defects of about 1×10
9
cm
−2
are still present and the density of reducible defects is limited. Further, it is difficult to reduce the density of dislocations. To this end, generally proposed is a technique employing an underlayer for reducing the density of dislocations by epitaxial lateral overgrowth (ELOG) when growing GaN. This epitaxial lateral overgrowth is disclosed in Journal of Oyo Denshi Bussei Bunkakai, Vol. 4 (1998), pp. 53 to 58 and 210 to 215, for example.
In this ELOG, a mask layer is formed on a prescribed region of the underlayer for selectively growing a GaN layer from the underlayer through the mask layer, so that the GaN layer grows in the vertical direction and then grows in the lateral direction. Dislocations are laterally bent due to the lateral overgrowth, whereby the density of dislocations reaching a flattened surface of the nitride-based semiconductor layer is remarkably reduced as compared with the underlayer from about 1×10
8
cm
−2
to less than about 1×10
6
cm
−2
.
A method referred to as PENDEO, disclosed in International Workshop on Nitride Semiconductor (IWN 2000), Nagoya, p. 79, is also known as a method of reducing the density of dislocations through selective lateral growth. According to PENDEO, a nitride-based semiconductor layer is grown on a substrate and the surface of this nitride-based semiconductor layer is recessed to partially expose the substrate. The nitride-based semiconductor layer is laterally grown on projection portions of the nitride-based semiconductor layer. Further, Japanese Patent Laying-Open No. 2001-168042 or Jpn. J. Appl. Phys. 40 (2001) L583 discloses a method of laterally growing a nitride-based semiconductor layer on a substrate with recess portions.
In each of the aforementioned conventional methods employing selective lateral growth, however, the nitride-based semiconductor is frequently cracked in a subsequent process step such as a step of separation of the substrate into elements due to the difference between the thermal expansion coefficients of the mask layer and the grown nitride-based semiconductor or voids resulting from facets bonded on a portion of the mask layer.
In the conventional selective lateral growth, since the density of dislocations is reduced in the vicinity of the surface of the nitride-based semiconductor layer, it is difficult to obtain lattice relaxation by dislocations. Therefore, the nitride-based semiconductor is frequently cracked and the substrate is frequently warped.
Particularly when the nitride-based semiconductor layer has a smaller lattice constant than the substrate as in an AlGaN layer formed on a GaN substrate, for example, or the nitride-based semiconductor layer has a larger thermal expansion coefficient than the substrate such as an Si substrate or an SiC substrate, the aforementioned cracking or warpage is frequently caused.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a nitride-based semiconductor element capable of preventing a nitride-based semiconductor from cracking and reducing the degree of warpage of a substrate.
Another object of the present invention is to effectively prevent a nitride-based semiconductor layer formed by selective lateral growth from cracking in the aforementioned nitride-based semiconductor element.
In order to attain the aforementioned objects, a nitride-based semiconductor element according to a first aspect of the present invention comprises a first region formed on a prescribed region of a substrate and provided with an element including a first nitride-based semiconductor layer having a prescribed thickness and a second region formed on a region of the substrate other than the first region and provided with the first nitride-based semiconductor layer with a thickness smaller than the thickness in the first region.
The nitride-based semiconductor element according to the first aspect is formed with the second region provided with the first nitride-based semiconductor layer with a thickness smaller than the thickness in the first region provided with the element as described above, whereby strain easily concentrates to the second region provided with the first nitride-based semiconductor layer with the smaller thickness. Thus, strain of the first region provided with the element is relaxed, whereby the first nitride-based semiconductor layer provided on the first region can be effectively prevented from cracking, and the degree of warpage of the substrate can be reduced. Consequently, the nitride-based semiconductor element can be formed on the first region with an excellent yield.
In the aforementioned nitride-based semiconductor element according to the first aspect, the first region provided with the element preferably includes a first portion, formed on the substrate, having difficulty in allowing growth of the first nitride-based semiconductor layer and a second portion, formed on the substrate, easily allowing growth of the first nitride-based semiconductor layer, and the second region preferably includes a third portion, formed on the substrate, having difficulty in allowing growth of the first nitride-based semiconductor layer. According to this structure, the thickness of the portion of the first nitride-based semiconductor layer formed on the second region can be easily reduced below the thickness of the portion of the first nitride-based semiconductor layer formed on the first region.
In the aforementioned nitride-based semiconductor element including the first, second and third portions, the first portion of the first region is preferably formed with a first width, and the third portion of the second region is preferably formed with a second width larger than the first width. According to this structure, the first nitride-based semiconductor layer is more difficult to be grown on the third portion of the second region having a larger width as compared with the first portion of the first region having a smaller width, whereby the thickness of the portion of the first nitride-based semiconductor layer formed on the second region can be easily reduced below the thickness of the portion of the first nitride-based semiconductor layer formed on the first region.
In the aforementioned nitride-based semiconductor element including the first, second and third portions, the first portion of the first region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes any of a region having no buffer layer, a region having a mask layer and a region having a recess portion, the second portion of the first region easily allowing growth of the first nitride-based semiconductor layer preferably includes any of a region having a buffer layer, a region having no mask layer and a region having no recess portion, and the third portion of the second region having difficulty in allowing growth of the first nitride-based semiconductor layer preferably includes any of a region having no buffer layer, a region having a mask layer and a region hav
Hata Masayuki
Hayashi Nobuhiko
Kunisato Tatsuya
Nomura Yasuhiko
Ohbo Hiroki
McDermott & Will & Emery
Nelms David
Nguyen Dao H.
Sanyo Electric Co,. Ltd.
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