Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1999-10-14
2001-11-13
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S098000, C257S094000, C257S096000, C257S103000, C257S190000
Reexamination Certificate
active
06316785
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a mismatched lattice semiconductor device, and more particularly a gallium nitride semiconductor device formed on a substrate.
2. Discussion of the Background
A bulk substrate, such as sapphire, SiC, Si or GaAs, is commonly used in a gallium nitride semiconductor device as a substitute for a GaN bulk single crystal, which is difficult to form. Organic metal chemical vapor deposition (MOCVD), or halide vapor phase epitaxy (halide VPE), has been relied upon for the epitaxial growth of GaN on the principal surface of a bulk substrate. The (0001) C face of Al
2
O
3
(sapphire) is widely used as a bulk substrate. Because a lattice mismatching as large as 13.8% exists between sapphire and GaN, a misfit dislocation is likely to occur from the stress acting upon the crystal lattice during the growth of GaN, resulting in the formation of a dislocation having a high density in the order of 10
8
to 10
10
cm
−2
between the sapphire substrate and a grown GaN layer and also resulting in the failure of GaN to grow into a layer of high quality. This dislocation propagates in the direction of growth of GaN. In a GaN semiconductor laser device that is formed on the principal surface of the substrate, the dislocation extends through the main structure of the element such as the active layer or the layer overlying it, and thereby lowers the characteristics of the laser and its reliability. The propagation or increase of the dislocation is also caused by an electric current applied for driving the device, and causes serious problems including the shortening of its life and the lowering of its reliability. In order to make a GaN semiconductor laser device of improved reliability, it is therefore necessary to form a crystal of improved quality by lowering the density of the dislocation formed between the substrate and the GaN layer, which is as high as 10
8
to 10
10
cm
−2
, or by reducing the dislocation from propagating to the active layer of the laser. The same problems occur with the use of any other bulk substrate causing a lattice mismatching with GaN, such as SiC, Si or GaAs.
Attempts have recently been made to form a striped mask of SiO
2
on the surface of a GaN layer grown on a sapphire substrate and to grow GaN again on the mask to form a GaN layer having a low dislocation density. This is known as an ELOG (epitaxially lateral overgrowth GaN substrate) [see (1) A. Usui, et al.:
J. Jpn. Appl. Phys.,
vol. 36, No. 78, pp. L899-L902 (1997), and (2) S. Nakamura, et al.:
Appl. Phys. Lett.
72, 211 (1998)]. It is, however, necessary to form a GaN layer having a thickness of at least several tens of microns to cover the SiO
2
mask completely. Moreover, voids are likely to appear above the SiO
2
mask, and it is necessary to form a GaN layer having a thickness of nearly 100 microns to fill the voids and form a perfectly flat surface. An undesirably large amount of material is, however, required for forming such a layer. Moreover, a difference in the coefficient of thermal expansion between the SiO
2
mask and GaN is likely to cause cracking along the edge of the mask. Thus, ELOG is seriously defective from a productivity standpoint.
Japanese Patent Laid-Open No. 64791/1996 discloses protrusions for controlling crystal growth, but they have amorphous sidewall surfaces, and the single crystal serving as the seed for crystal growth is at the bottom of grooves between the protrusions. Defects appearing on the single crystal surfaces extend in the direction of crystal growth, and gather on the amorphous protrusions. The defects make it difficult to form an operating area on the protrusions.
According to the present invention, such difficulty can be diminished and an operating area can also be formed on the protrusions, as will be explained in detail.
SUMMARY OF THE INVENTION
In view of the above problems, an object of the present invention is to provide a GaAlInBN systems such as GaN semiconductor device of high reliability having a core kept free from any influence of a dislocation occurring between a substrate and an epitaxially grown layer having a lattice mismatching therebetween.
The present invention provides a GaAlInBN system semiconductor device including a layer for reducing the propagation of a dislocation formed between a substrate and a GaAlInBN system semiconductor layer formed thereon, and having a protrusion, or protrusions having sidewalls which are single crystal.
When a GaAlInBN system semiconductor layer is formed on a substrate, a dislocation occurs therebetween as a crystal lattice defect due to strain, and propagates through the crystal in the direction in which it grows. In the epitaxial growth of GaN, however, it grows faster laterally than perpendicularly to the substrate, and if its lateral growth can be promoted, it is possible to direct the dislocation laterally and thereby restrain its propagation to the active area. According to the present invention, therefore, protrusions having sidewalls which are single crystal are formed at least between a substrate and a GaN layer formed thereon to promote the lateral growth of the crystal so that upon recrystallization, it may grow from the sidewalls of the protrusions to the middle area between the adjoining protrusions. Its lateral growth is by far faster than its growth perpendicular to the substrate, and the dislocation extends laterally from the sidewalls. It is, therefore, possible to lower the density of the dislocation propagating to any upper layer including the main structure of the device and thereby obtain a semiconductor device of higher quality than has hitherto been obtained. This is due to the single crystal exposed on the sidewalls.
The layer for reducing the propagation of a dislocation preferably comprises a multilayer film formed from Al
x
Ga
y
In
z
B
1−(x+y+z)
N or Al
u
Ga
v
In
w
B
1−(u+v+w)
N where (0≦x, y z, U, v, w≦1). Moreover, the layer preferably has a plurality of protrusions, each having a height d
1
of 0.1 to two microns and a width W
2
of at least one micron at its top, every two adjoining protrusions having a distance W
1
of one to 10 microns therebetween. For the convenience of manufacture, the protrusions are preferably formed in a regularly repeated pattern.
The present invention provides a AlGaInBN system semiconductor device comprising a substrate having a top surface, and at least one structure disposed on the top surface of the substrate and having a single crystal sidewall. The at least one structure is formed from the materials represented as Al
x
Ga
y
In
z
B
1−(x+y+z)
N where (0≦x, y, z≦1). The semiconductor device also comprises a layer disposed on the substrate and the structure. The layer is formed from the materials represented as Al
u
Ga
v
In
w
B
1−(u+v+w)
N where (0≦u, v, w≦1). The semiconductor device also comprises a main semiconductor element disposed on the layer.
The present invention provides a AlGaInBN system semiconductor device comprising a substrate having a top surface and a layer disposed on the top surface of the substrate. The layer has a lower and an upper portion having a boundary on which a protrusion is formed. The protrusion has a sidewall which is single crystal. The lower and the upper portion are formed from the materials represented as Al
x
Ga
y
In
z
B
1−(x+y+z)
N where (0≦x, y, z≦1) and Al
u
Ga
v
In
w
B
1−(u+v+w )
N where (0≦u, v, w≦1). The semiconductor device also comprises a main semiconductor element disposed on the layer.
The present invention provides a semiconductor device comprising a substrate having a top surface, a layer formed on the top surface of the substrate, and a semiconductor main element formed on the layer. The layer has a first portion with a substantial portion of a plurality of dislocations oriented in a direction perpendicular to the top surface and has a second portion with a substantial portion of a
Nunoue Shin-ya
Yamamoto Masahiro
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Tran Minh Loan
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