Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1999-12-21
2002-10-22
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S014000, C257S094000, C257S102000, C257S103000, C257S104000
Reexamination Certificate
active
06469314
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to light emitting devices and more particularly to a light emitting diode with improved brightness and reliability and to a method of fabricating the light emitting diode.
BACKGROUND ART
Traditional lighting sources such as fluorescent lights, incan-descent lights, and neon lights have a number of disadvantages relative to light emitting diodes (LEDs). These disadvantages include their large sizes, the lack of durability due to the use of fragile filaments, short operating lifetimes, and high operating voltages. In contrast, LEDs are small in size, durable and require low operating voltages. Furthermore, LEDs have much longer operating lifetimes. A typical LED has an operating life of 10,000 hours or more, as compared to a halogen lamp which has a mean operating life of 500 to 4,000 hours. Furthermore, unlike the traditional lighting sources which fail by filament breakage, an LED fails by a gradual reduction in light output. Therefore, many lighting applications could benefit from the advantages of LEDs. However, to effectively compete with the traditional lighting sources, LEDs must be bright and must maintain their brightness over their expected operating lifetime, i.e., they must be reliable.
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 the use of more than one light emitting layer, where these light emitting layers are commonly known as active layers. These LEDs are referred to as either multi-well (MW) LEDs if the layer thickness values are greater than 100 Angstroms, or they are referred to as multiple quantum well (MQW) LEDs if the layer thickness values are less than approximately 100 Angstroms. The distinction between these two different types of LEDs is whether the layers are thin enough for quantum confinement effects to become important, i.e., for discrete or quantized energy states to occur in the active layers. (This generally occurs for well thickness values less than approximately 100 Angstroms and depends on the energy band structure of the materials in question.) In contrast, LEDs having a single active layer will be referred to as either double heterostructure (DH) LEDs, or as single quantum well (SQW) LEDs, again depending on whether the individual active layer thickness values are greater than or less than approximately 100 Angstroms, respectively. Further improvements have been attempted by adjusting the number and/or thickness of the active layers. For example, U.S. Pat. No. 5,410,159 to Sugawara et al. describes an MQW LED having eight to nineteen active layers, preferably ten to nineteen, in which the thickness of the active layers is 10 to 100 Angstroms, and more typically 50 Angstroms. The device described by Sugawara et al. is thus restricted to the quantum regime.
A conventional MQW LED is schematically illustrated in FIG.
1
. (Note that this LED in
FIG. 1
could also represent an MW LED.) The LED 10 includes a substrate
12
of a first conductivity type, a lower confining layer
14
of the first conductivity type, the MQW active region
16
which may be of the first conductivity type, may be undoped, or may be of a second conductivity type, an upper confining layer
18
of the second conductivity type, and an optional window layer
20
of the second conductivity type. The MQW active region includes two or more active layers
22
that are separated from each other by one or more barrier layers
24
. Although the MQW active region is shown to include four active layers, the number of active layers included in the active region can be much greater.
In the most common configuration, the first conductivity type is ntype and the second conductivity type is p-type. Since this is the most common LED configuration, such a configuration will be used here as an example. In this configuration, the n-type lower confining layer
14
of the LED
10
in
FIG. 1
is electrically connected to an n-type ohmic contact
26
via the substrate
12
, and the p-type upper confining layer
18
or the optional window layer
20
is electrically connected to a p-type ohmic contact
28
. (Note that it is also possible to form an LED where the first conductivity type is p-type and the second conductivity type is n-type. Such an LED may be formed by either growing the LED on a p-type substrate, or bonding or attaching the LED to a p-type substrate or other p-type semiconductor material.)
When a potential is applied to the ohmic contacts
26
and
28
, electrons are injected into the MQW active region
16
from the n-type lower confining layer
14
and holes are injected into the MQW active region from the p-type upper confining layer
18
. The radiative recombination of electrons and holes within the active layers
22
of the active region generates light. However, when the recombination occurs within the lower confining layer, the upper confining layer, or the barrier layers of 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 layers of the LED. The multiple quantum wells formed by the active layers of the LED 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 equates to an increase in the light output of the LED.
With respect to the reliability issue, U.S. Pat. No. 5,909,051 to Stockman et al., which is assigned to the assignee of the present invention, describes a method of doping at least one conductive region adjacent to the active region with impurities to fabricate minority carrier devices, such as an AlGaInP LED, having an increased operating stability, i.e., reliability. In a preferred embodiment, the impurities that are introduced to the conductive region are oxygen dopant atoms. Note that the fabrication method of Stockman et al. asserts that the oxygen dopant atoms should be placed in a region adjacent to the active region.
A concern with conventional fabrication methods and the resulting devices, such as those described in Stockman et al., is that improvement in LED brightness and improvement in LED reliability are often in an inverse relationship. Therefore, an improvement in one parameter often results in a penalty in the other parameter. In particular, the method of Stockman et al. provides improved LED reliability. However, it often also results in a penalty in initial light output. Although the method of Stockman et al. results in a more stable light output, these devices with oxygen doping are thus initially dimmer than devices without oxygen doping.
Although improvements in either brightness (i.e., light output as measured by I
v
in units of &mgr;Cd) or in reliability of LEDs may be achieved by conventional methods, additional improvements that minimize any penalizing effects on the other parameter are desired. Therefore, what is needed is an LED fabrication method and a resulting LED that has been configured to optimally improve both the brightness and the reliability of the LED.
SUMMARY OF THE INVENTION
An LED and a method of fabricating the LED utilize controlled oxygen (O) doping to form at least one layer of the LED having an O dopant concentration which is correlated to the dominant emission wavelength of the LED. The O dopant concentration is regulated to be higher when the LED has been configured to have a longer dominant emission wavelength. Since the dominant emission wavelength is dependent on the composition of the active layer(s) of the LED, the O dopant concentration in the layer is related to the composition of the active layer(s). The controlled O doping improves the reliability while minimizing any light output penalty due to the introduction of O dopants.
In an exemplary embodiment, the LED is an AlGaInP LED that inc
Chen Eugene I.
Grillot Patrick N.
Huang Jen-Wu
Stockman Stephen A.
Flynn Nathan J.
Klivans Norman R.
LumiLeds Lighting U.S., LLC
Sefer Ahmed N.
Skjerven Morrill LLP
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