Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2001-05-10
2002-05-07
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S012000, C257S086000
Reexamination Certificate
active
06384430
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light emitting diode (hereinafter referred to as an “LED”)having a double heterojunction structure. More particularly, the present invention relates to a technique for preventing a reduction in a light output of an LED for long-time operation.
2. Description of the Related Art
An LED having a so-called double heterostructure has a high level of light emission efficiency and a high light output and therefore is widely used for a display, a light source of optical communications, or the like.
FIG. 12
is a cross-sectional view illustrating a conventional LED
800
having a typical double heterostructure. The LED
800
is an InGaAlP based LED which includes layers having lattice match with a GaAs substrate and emits light ranging from red light to green light. In the LED
800
,
a substrate
1
: made of n-type GaAs;
a first buffer layer
2
: made of n-type GaAs;
a light reflection (DBR:Distributed Bragg Reflector) layer
3
: including n-type (Al
0.4
Ga
0.6
)
0.5
In
0.5
P layers and n-type Al
0.5
In
0.5
P layers deposited in an alternative fashion;
a first cladding layer
4
: made of n-type Al
0.5
In
0.5
P, doped with Si at an impurity concentration of 5×10
17
cm
−3
, 1 &mgr;m thick;
a light emitting layer
6
: made of p-type (Ga
0.7
Al
0.3
)
0.5
In
0.5
P, 0.5 &mgr;m thick;
a second cladding layer
7
: made of p-type Al
0.5
In
0.5
P, doped with Zn at an impurity concentration of 5×10
17
cm
−3
, 1 &mgr;m thick;
a first current diffusion layer
91
: made of p-type Al
0.7
Ga
0.3
As, doped with Zn at an impurity concentration of 1×10
18
cm
−3
, 1 &mgr;m thick; and
a second current diffusion layer
92
: made of p-type Al
0.7
Ga
0.3
As, doped with Zn at an impurity concentration of 3×10
18
cm
−3
, 6 &mgr;m thick; are deposited in this order.
The first and second current diffusion layers
91
and
92
constitute a current diffusion layer
9
.
A film of AuGe is provided as an n-side electrode
11
on a lower surface of the substrate
1
by a typical deposition method. A film of AuZn is provided on a upper surface of the p-type current diffusion layer
9
by the same deposition method. The AuZn film is subjected to photolithography patterning so as to remain a circular portion thereof as a p-side electrode
10
to which a metal wire is bonded for connecting the p-side electrode
10
to an external conductor. Light generated in the light emitting layer
6
is radiated from a portion of the upper surface of the p-type current diffusion layer
9
from which the AuZn film has been removed.
The first buffer layer
2
is used for preventing defects and contaminants of the substrate
1
from affecting the layers deposited the substrate
1
. The first buffer layer
2
is not required when the substrate
1
has a satisfactorily treated upper surface. The DBR layer
3
reflects light generated in the light emitting layer
6
toward the substrate
1
. This prevents light absorption by the substrate
1
and the reflected light goes in a direction away from the substrate
1
, contributing to the brightness of the LED
800
.
The current diffusion layer
9
has low resistivity so as to make an approximate ohmic contact with the p-side electrode
10
and also to diffuse a current injected from the p-side electrode
10
into the entire light emitting layer
6
. This is why the current diffusion layer
9
requires a high level of impurity concentration. In this case, to prevent impurity Zn from diffusing into the light emitting layer
6
, the first current diffusion layer
91
having a low impurity concentration is provided in the lower part of the current diffusion layer
9
.
To obtain a high level of light emission efficiency, a conventional LED adopts a double heterostructure as shown in FIG.
15
.
FIG. 15
is a cross-sectional view illustrating an example of an AlGaInP based LED
900
which have lattice match with a GaAs substrate
101
. A structure of each layer in the LED
900
is as follows:
a substrate
101
: made of n-type GaAs:
a buffer layer
102
: made of n-type GaAs;
an n-type first cladding layer
103
: made of n-type (Ga
0.3
Al
0.7
)
0.5
In
0.5
P, doped with Si at an impurity concentration of 1×10
18
cm
−3
, 1 &mgr;m thick;
a light emitting layer
104
.: made of p-type (Ga
0.7
Al
0.3
)
0.5
In
0.5
P, 0.5 &mgr;m thick;
a p-type second cladding layer
105
: made of p-type Al
0.5
In
0.5
P, doped with Zn at an impurity concentration of 5×10
17
cm
−3
, 1 &mgr;m thick;
a first current diffusion layer
61
: made of p-type Ga
0.3
Al
0.7
As, doped with Zn at an impurity concentration of 1×10
18
cm
−3
, 1 &mgr;m thick;
a second current diffusion layer
62
: made of p-type Ga
0.3
Al
0.7
As, doped with Zn at an impurity concentration of 3×10
18
cm
−3
, 6 &mgr;m thick; and
a contact layer
108
: made of p-type GaAs.
Ann-side electrode
109
and a p-side electrode
107
are provided on the substrate
1
and the contact layer
108
, respectively.
The AlGaInP based LED
800
in
FIG. 12
generates light by injecting a current. In
FIG. 13
, a dashed line A indicates a relationship between an impurity concentration of the light emitting layer
6
and a light output in an initial period after starting light emission. The peak of the light output is at an impurity concentration of 1×10
17
cm
−3
in the initial period after starting light emission. However, the light output gradually decreases with time. For example, a current of 50 mA is supplied to the LED
800
for 1000 hours at room temperature. In
FIG. 13
, a dashed line B indicates a relationship between an impurity concentration of the light emitting layer
6
and a light output after the 1000-hour light emission. A light output after the 1000-hour light emission becomes lower at an impurity concentration of 1×10
17
cm
−3
while a light output becomes higher at an impurity concentration of 5×10
17
cm
−3
where the light output is maximum, which is different from in the initial period after starting light emission.
Our studies have found that such a change in a light output after long-time light emission is caused by: (1) a non-radiative recombination center generated at a pn junction interface between the n-type first cladding layer
4
and the p-type light emitting layer
6
; and (2) an influence from diffused impurities in the light emitting layer
6
.
FIGS. 14A and 14B
illustrate states of energy bands of around the light emitting layer
6
.
FIG. 14A
shows a state in the initial period after starting light emission, while
FIG. 14B
shows a state after the long-time light emission.
The pn junction interface
40
is a heterointerface where two layers having largely different energy gaps as shown in
FIG. 14A
make contact with each other. There is a large internal stress at the heterointerface
40
. When a voltage is applied between the p-side electrode
10
and the n-side electrode
11
in order to generate light, a high electric field level is applied across the heterointerface
40
.
The combination of the internal stress and the energy of light generated in the light emitting layer
6
causes a lattice defect at the heterointerface
40
. This lattice defect grows along the direction of the electric field line into the light emitting layer
6
over the long-time light emission. The lattice defect leads to formation of a deep energy level
20
in the vicinity of the heterointerface
40
as shown in FIG.
14
B. The carriers, a hole and an electron, combine together at the deep energy level without emitting light. Such a deep energy level is called a non-radiative energy level. Since radiative recombination
30
of the LED
800
is a spontaneous emission process, the non-radiative recombination
31
at the non-radiative energy level
20
has a shorter lifetime than that of the radiative recombination
30
. Therefore, when the number of carriers combining at the non-radiative energy level
20
is increased, the light emissi
Hosoba Hiroyuki
Kurahashi Takahisa
Murakami Tetsuroh
Nakatsu Hiroshi
Meier Stephen D.
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
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