Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2000-01-24
2003-01-07
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S013000, C257S015000, C257S017000, C257S022000, C257S086000, C372S017000, C372S045013, C372S050121, C438S022000
Reexamination Certificate
active
06504171
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to LED structures and more particularly to the active region of an LED.
BACKGROUND ART
Light emitting diodes (LEDs) are widely accepted in many applications that require low power consumption, small dimensions, and high reliability. However, the use of LEDs in new applications is limited by their external quantum efficiency or their brightness. Therefore, many attempts have been made to improve the brightness of LEDs through various design changes. For example, improvements in LED brightness have been achieved by using multi-well active layer devices in which multiple light emitting active layers are included in the LED active region. Additional light output gains have been observed by decreasing the thickness of the individual light emitting active layers, and in the extreme case, the thickness of the individual active layers is reduced to the point where quantum confinement effects are observed (i.e., discrete or quantized energy states occur within the active layers). In such a case, the active layer thicknesses are said to have been reduced below the quantum thickness and such devices are said to operate in the quantum regime or quantum size regime. This quantum thickness depends on certain material parameters such as the electron or hole effective mass, and is therefore different for different materials. For AlGalnP LEDs, the quantum thickness is about 100 Angstroms, while for AlGalnN LEDs, the quantum thickness is about 60 Angstroms. In the context of this disclosure, we define the aforementioned LEDs as multi-well (MW) LEDs, regardless of individual active layer thickness, i.e., regardless of whether the individual active layers are thinner than the quantum thickness or thicker than the quantum thickness. Examples of such MW active layer LEDs and laser diodes are provided in U.S. Pat. No. 4,318,059 to Lang et al., U.S. Pat. No. 5,410,159 to Sugawara et al. and U.S. Pat. No. 5,661,742 to Huang et al.
In contrast to the MW LEDs discussed above, LEDs having a single active layer will be referred to either as double heterostructure (DH) LEDs, or as single quantum well (SQW) LEDs, depending on whether the individual active layer thickness values are greater than or less than the quantum thickness, respectively.
A conventional MW LED is schematically illustrated in FIG.
1
. The LED
10
includes a substrate
12
of first conductivity type, a lower confining layer
14
of first conductivity type, the active region
16
which may be of first conductivity type, may be undoped, or may be of second conductivity type, an upper confining layer
18
of second conductivity type, and an optional window layer
20
of second conductivity type. The active region includes two or more thin active layers
22
that are separated from each other by one or more barrier layers
24
. Although the active region is shown to include four active layers, the number of active layers can be anywhere from two to forty or more. In the most common configuration, the lower confining layer is made of an n-type semiconductor material, while the upper confining layer is made of a p-type semiconductor material. In this case, the n-type lower confining layer is electrically connected to an n-type ohmic contact
26
via the substrate
12
, and the p-type upper confining layer
18
or optional p-type window layer
20
is electrically connected to a p-type ohmic contact
28
. (It is also possible to grow or bond or otherwise attach the LED to a p-type substrate or other material such as metal, glass, etc., such that the lower confining layer is p-type and the upper confining layer and optional window layer are n-type. Since the most common LED configuration includes an n-type substrate, we use this case as an example here. Hence, in these examples, the first conductivity type is n-type, and the second conductivity type is p-type.)
When a potential is applied to the ohmic contacts
26
and
28
, electrons are injected into the active region
16
from the n-type lower confining layer
14
and holes are injected into the active region from the p-type upper confining layer
18
. The radiative recombination of electrons and holes within the active layers
22
generates light. However, if the recombination occurs within a layer other than one of the active layers, such as the lower confining layer, the upper confining layer, or a barrier layer within the active region, no light is generated. Thus, it is desirable to increase the probability that the electrons and holes recombine within the active layers, as opposed to recombining within some other layer of the device. The multiple wells formed by the active layers
22
of the LED
10
increase the radiative recombination probability by allowing holes or electrons that did not recombine in one of the active layers a chance to recombine in another active layer. The increase in radiative recombination of electrons and holes within the active layers of the LED equates to an increase in the light output of the LED.
Although light output gains can be realized by implementing a multi-well structure, additional light output gains are desired to achieve more widespread use of LEDs. A concern with conventional LED designs is that light is not emitted equally from all wells, and in some extreme cases, especially for AlGalnN devices (as will be illustrated in FIG.
7
), most of the light is emitted only from one or two wells in the structure. Thus, some of the wells in a conventional LED do not contribute effectively to the brightness of the LED. This problem is compounded in a transparent substrate LED in that some fraction of the light that is generated within one active layer may be absorbed in another active layer of the active region. Thus, an active layer that does not contribute efficiently to light generation limits the light output of the device in two ways. First of course, it does not generate light efficiently. Second, it may absorb some fraction of the light generated by other active layers within the active region.
In light of the above concern, what is needed is a method for increasing the light output or light generation efficiency of each well in a multi-well LED.
SUMMARY OF THE INVENTION
A light emitting device and a method of improving the light output of the device utilize a chirped multi-well active region to increase the probability of radiative recombination of electrons and holes within the light emitting active layers of the active region by altering the distribution of electrons and holes within the light emitting active layers of the active region (i.e., across the active region).
In an exemplary embodiment, the LED is an AlGaInP LED that includes a substrate of first conductivity type, an optional distributed Bragg reflector layer of first conductivity type, a lower confining layer of first conductivity type, an optional lower set-back layer of first conductivity type, the chirped multi-well active region which may be of first conductivity type, may be undoped, or may be of second conductivity type, an optional upper set-back layer of second conductivity type, an upper confining layer of second conductivity type, and an optional window layer of second conductivity type. The substrate is made of a semiconductor material, such as GaAs or GaP. In a preferred embodiment, the lower confining layer is composed of an n-type (Al
x
Ga
1−x
)
y
In
1−y
P material, where x≧0.6 and y=0.5±0.1, while the upper confining layer is composed of a p-type (Al
x
Ga
1−x
)
y
In
1−y
P material, where x≧0.6 and y=0.5±0.1. The optional upper set-back layer is formed of an undoped (Al
x
Ga
1−x
)
y
In
1−y
P material, where x≧0.6 and y=0.5±0.1. The optional upper set-back layer may be used to help control the diffusion of p-type dopants from the upper confining layer into the active region during high temperature processing steps. The optional lower set-back layer may also be formed of an undoped or n-type (Al
x
Ga
1−x
)
y
In
1−y
P materi
Chang Ying-Lan
Chen Eugene I.
Grillot Patrick N.
Kocot Christopher P.
Krames Michael R.
Flynn Nathan J.
Forde Remmon R.
Leiterman Rachel V.
Lumileds Lighting U.S. LLC
Patent Law Group LLP
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