Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – Including change in a growth-influencing parameter
Reexamination Certificate
2000-07-24
2001-10-30
Utech, Benjamin L. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
Including change in a growth-influencing parameter
Reexamination Certificate
active
06309459
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a GaN semiconductor element used as a light source, for example, of an indicating lamp, full color display or DVD (Digital Versatile Disc)system, and its manufacturing method.
GaN and other nitride semiconductors are practically useful as semiconductors to make blue-color diodes, blue-color laser diodes, and so forth. In
x
Ga
1−x
N, among them, can be changed in band gap in the range from 2 eV to 3.4 eV by adjusting the mole fraction x of In, and is especially remarked as a hopeful material of light emitting elements for light in the visible band.
In the present application, “nitride semiconductors” involve III-V compound semiconductors expressed by B
x
In
y
Al
z
Ga
(1−x−y−z)
N (0≦x≦1, 0≦y≦1, 0≦z≦1), and involves mixed crystals including phosphorus (P) or arsenic (As) in addition to N as the group V element.
Double-heterostructures using an In
x
Ga
1−x
N layer, such as structures sandwiching an In
x
Ga
1−x
N active layer between AlGaN cladding layers, are effective for confinement of injected carriers and light, and are employed to made light emitting elements for a high luminance or a short wavelength.
Explained below are a semiconductor light emitting element with a double-heterostructure using an InGaN layer as its active layer and a method for manufacturing same.
FIG. 12
is a cross-sectional view schematically showing the semiconductor light emitting element of a double-heterostructure including a InGaN active layer and AlGaN cladding layers.
FIGS. 13A through 13D
are cross-sectional views schematically showing the semiconductor light emitting element of
FIG. 12
under different steps of its manufacturing process.
As shown in
FIG. 12
, sequentially stacked on a sapphire substrate
21
are a GaN buffer layer
22
, n-type Al
y
Ga
1−x
N cladding layer
23
(0≦y≦1), In
x
Ga
1−x
N active layer
25
(0≦x≦1), p-type Al
z
Ga
1−z
N cladding layer
27
(0≦z≦1) and p-type GaN layer
28
which are stacked sequentially on a sapphire substrate
21
.
These layers are typically grown by MOCVD (Metal-Organic Chemical Vapor Deposition) in the following process. In the explanation made below, the sapphire substrate
21
is named substrate
21
, and mole fractions x, y and z of layers are omitted.
The substrate
21
is first introduced into a MOCVD reactor, and annealed in a flow of hydrogen gas at 1100° C. for 10 minutes. After that, the temperature of substrate
21
is decreased to 520° C., and the GaN buffer layer
22
is grown to 50 nm on the surface of the substrate
21
(see FIG.
13
A).
Then, the substrate
21
is heated to 1100° C., and the n-type AlGaN cladding layer
23
is grown to 4 &mgr;m, maintaining the temperature at 1100° C. (see FIG.
13
B).
Thereafter, the temperature of the substrate
21
is decreased to 750° C., and the InGaN active layer
25
is grown to 0.1 &mgr;m, maintaining the temperature at 750° C. (see FIG.
13
C).
Then, the substrate
21
is heated to 1100° C., and the p-type AlGaN cladding layer
27
of 0.15 &mgr;m and the p-type GaN contact layer
28
of 0.3 &mgr;m are grown under the constant temperature of 1100° C. Thus the light emitting element with a double-heterostructure is formed (see FIG.
13
D).
For growth of the cladding layer
23
and
27
, the temperature must be set at about 1000° C. which is higher by 200 through 350° C. than the growth temperature for the InGaN active layer
25
. That is, before and after the growth of the InGaN active layer
25
, the temperature of the substrate
21
had to be increased and decreased. Because of GaN having the melting temperature of about 1000° C. and InN having the melting point of about 500° C., the process involved the following problems.
1. Decomposition of InN occurs in the InGaN active layer
25
in the process of increasing the temperature. GaN, however, is very unlikely to be decomposed. Therefore, the InGaN active layer
25
is partly replaced by GaN, and its crystallographic property deteriorates.
2. Decomposition of InN in InGaN in the process of increasing the temperature results in decreasing the thickness of the InGaN active layer
25
.
3. The hetero-interface between the cladding layer
23
and the InGaN active layer
25
degrades due to the problems 1 and 2 indicated above in the process of increasing the temperature.
4. In the process of decreasing the temperature, an unintentional InGaN layer not having the predetermined mole fraction of In may be grown, and the hetero-interface between the active layer
25
and the cladding layer
27
may degrade.
Deterioration of the crystallographic property of the active layer
25
and degradation of the hetero-interface invite a decrease in emission efficiency of light emitting elements and an increase in threshold value of laser elements. Further, the decrease in thickness of the InGaN active layer
25
means a structural deviation of the element from a designed value, and it invited degradation of initial characteristics and reliability of light emitting elements.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a unique SCH structure (Separate-Confinement Heterostructure) promising a good crystallographic property of the active layer and good hetero-interfaces on and under the active layer, and preventing changes in thickness of the active layer, to realize compound semiconductor light emitting elements with a high emission efficiency and reliability or laser elements with a high slope efficiency and reliability.
According to the invention, there is provided a compound semiconductor element comprising:
a first cladding layer made of a nitride semiconductor having a first band gap;
a first graded layer formed on the first cladding layer;
an active layer formed on the first graded layer and having a second band gap narrower than the first band gap;
a second graded layer formed on the active layer; and
a second cladding layer formed on the second graded layer and made of a nitride semiconductor having a third band, gap wider than the second band gap,
the first graded layer being made of a nitride semiconductor to become gradually narrower in band gap from a portion nearest to the first cladding layer toward a portion nearest to the active layer within the range from the first band gap to this second band gap, and
the second graded layer being made of a nitride semiconductor to become gradually wider in band gap from a portion nearest to the active layer toward a portion nearest to the second cladding layer within the range from the second band gap to the third b and gap.
As a preferred embodiment of the invention, said first cladding layer is made of Al
y
Ga
1−y
N (0≦y≦1), said first graded layer is made of In
u
Ga
1−u
N (0≦u≦1), said active layer is made of In
x
Ga
1−x
N (0≦x≦1), said second graded layer is made of In
w
Ga
1−w
N (0≦w≦1), and said second cladding layer is made of Al
z
Ga
1−z
N (0≦z≦1).
As an another preferred embodiment of the invention, said first cladding layer is made of Al
y
Ga
1−y
N (0≦y≦1), said first graded layer is made of GaN
u
P
1−u
(0≦u≦1), said active layer is made of GaN
x
P
1−x
(0≦x≦1), said second graded layer is made of GaN
w
P
1−w
(0≦w≦1), and said second cladding layer is made of Al
z
Ga
1−z
N (0≦z≦1).
The method for manufacturing a compound semiconductor element comprising a step of growing a compound semiconductor layer having a gradually changing content of indium along the growth direction by changing the growth temperature while maintaining the supplying ratio of a source material of indium relative to the source materials of the other group III elements in a constant value.
As a preferred embodiment of the invention, said compound semiconductor layer is grown by increasing the growth temperature to gradually decrease the content of indium along the growth direction.
Also it is preferred that a source material of a gro
Anderson Matthew
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Utech Benjamin L.
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