Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With reflector – opaque mask – or optical element integral...
Reexamination Certificate
2003-04-17
2004-11-30
Hu, Shouxiang (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With reflector, opaque mask, or optical element integral...
C257S099000
Reexamination Certificate
active
06825502
ABSTRACT:
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-200298, filed Jun. 30, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light emitting element and, more particularly, to the electrode structure of a light emitting element.
2. Description of the Related Art
The recent progress of light emitting elements is remarkable. In particular, small-sized, low-power-consumption, high-reliability light emitting diodes (LEDs) are developed and extensively used as display light sources.
Red, orange, yellow, and green LEDs currently put to practical use are made of group III-V compound semiconductors using As and P as group V elements, e.g., AlGaAs, GaAlP, GaP, and InGaAlP. On the other hand, green, blue, and ultraviolet LEDs are made of compound semiconductors such as GaN. In this way, LEDs having high emission intensity are realized.
When the luminance of these LEDs is increased, applications such as outdoor display devices and communication light sources are presumably greatly extended.
FIG. 1
shows the structure of a conventional violet LED.
A light emitting element
110
for emitting violet light is bonded on a lead frame
120
by silver paste
130
. The p- and n-electrodes of this light emitting element
110
are connected to the lead frame
120
by bonding wires
150
. The light emitting element
110
is covered with an epoxy resin
180
.
FIG. 2
shows the light emitting element shown in FIG.
1
.
On a sapphire (Al
2
O
3
) substrate
200
, an n-GaN layer
210
and a p-GaN layer
220
are formed. The n-GaN layer
210
has a recess. Since the p-GaN layer
220
is not present on this recess, the n-GaN layer
210
is exposed in this recess of the n-GaN layer
210
.
An n-side electrode
230
is formed on the recess of the n-GaN layer
210
. A transparent electrode
240
having properties of transmitting light is formed on the p-GaN layer
220
. In addition, a bonding electrode
250
for wire bonding is formed on the p-GaN layer
220
.
When a voltage is applied between the two lead frames
120
in the LED shown in
FIGS. 1 and 2
, an electric current is injected into the p-GaN layer
220
from the bonding electrode
250
and the transparent electrode
240
. This electric current flows from the p-GaN layer
220
to the n-GaN layer
210
.
In the boundary (p-n junction) between the p-GaN layer
220
and the n-GaN layer
210
, light having energy h&ugr;(h: Planck's constant, &ugr;=c/&lgr;, c: velocity of light, &lgr;: wavelength) is generated when the electric current flows. This light is emitted upward from the transparent electrode
240
.
In the transparent electrode
240
, however, the light transmittance and the conductivity have a relationship of trade-off.
That is, to increase the light transmittance, the thickness of the electrode need only be decreased. However, if the electrode thickness is decreased, the conductivity lowers. When the conductivity lowers, no electric current can be supplied to the whole p-n junction any longer, and this decreases the light generation efficiency. Also, to increase the conductivity, the thickness of the electrode need only be increased. However, if the electrode thickness is increased, the light transmittance lowers. When the light transmittance lowers, light generated in the p-n junction cannot be efficiently extracted to the outside of the chip.
As a technology by which this problem is solved, a technology of emitting light toward the sapphire substrate
200
is known.
FIG. 3
shows a light emitting element using this technology.
Since this light emitting element is bonded on a lead frame by flip chip bonding, an LED having this light emitting element is called a flip chip type LED.
A high-reflectance electrode
260
is formed on p-GaN
220
. Of light generated in the p-n junction, light traveling to a sapphire substrate
200
is directly emitted to the outside of the chip. Of light generated in the p-n junction, light heading to the electrode
260
is reflected by this electrode
260
. The reflected light travels to the sapphire substrate
200
and is emitted to the outside of the chip.
The sapphire substrate
200
will be described below.
When InGaN is used as an active layer, an LED currently put to practical use emits light within the range of blue to green. The bandgap of the sapphire substrate
200
is approximately 3.39 eV (wavelength &lgr;≈365 nm) at room temperature (300K). That is, the sapphire substrate
200
has properties of transmitting light within the range of blue to green (the wavelength &lgr; is approximately 400 to 550 nm).
A flip chip type LED is very effective as a technology of extracting light to the outside of the chip with high efficiency, but also has a problem.
That is, it is generally difficult to form an ohmic contact with the p-GaN
220
when the high-reflectance electrode
260
is used. This ohmic contact is an essential technology to reduce the contact resistance between the electrode
260
and the p-GaN
220
and thereby improve the performance of the element.
Conventionally, therefore, the electrode
260
is given a two-layered structure including an ohmic layer for forming an ohmic contact and a high-reflection layer having high reflectance. The ohmic layer improves the performance and the high-reflection layer increases the light emission efficiency at the same time.
Unfortunately, the ohmic layer obtains an ohmic contact by interdiffusion of metal atoms between this ohmic layer and the p-GaN
220
, so these metal atoms naturally diffuse from the ohmic layer to the high-reflection layer. Since this diffusion lowers the performance and reliability of the light emitting element, it must be eliminated.
FIG. 4
shows an LED made of group III-V compound semiconductors having As and P as group V elements.
This LED emits light within the range of red to green.
On an n-GaAs substrate
300
, an n-GaAs buffer layer
310
and an n-InGaAlP cladding layer
320
are formed. On this n-InGaAlP cladding layer
320
, an InGaAlP active layer
330
, a p-InGaAlP cladding layer
340
, and a p-AlGaAs current diffusing layer
350
are formed.
On the p-AlGaAs current diffusing layer
350
, a p-GaAs contact layer
360
and a p-side electrode
370
are formed. An n-side electrode
380
is formed on the back side of the n-GaAs substrate
300
.
In a light emitting element made of group III-V compound semiconductors (e.g., GaAs, AlGaAS, and InGaAlP) having As and P as group V elements, a sufficiently thick current diffusing layer (the AlGaAs current diffusing layer
350
) is formed on a p-semiconductor layer without forming any transparent electrode on a p-semiconductor layer (the InGaAlP cladding layer
340
). This sufficiently thick current diffusing layer has a function of evenly injecting an electric current into the entire InGaAlP active layer
330
. Since the AlGaAs current diffusing layer
350
increases the light generation efficiency in the vicinity of the active layer, satisfactory optical power can be assured.
In the light emitting element shown in
FIG. 4
, an electric current given to the p-side electrode
370
is injected into the InGaAlP active layer
330
via the p-AlGaAs current diffusing layer
350
. Light generated near the InGaAlP active layer
330
is emitted upward from the p-AlGaAs current diffusing layer
350
except for a region where the p-side electrode
370
exists.
The film thickness, however, of the current diffusing layer
350
must be increased to well diffuse the electric current for the reason explained below. That is, if the film thickness is small, the electric current is not diffused but injected only into the active layer
330
immediately below the p-side electrode
370
. Consequently, most of the light generated near the active layer
330
is interrupted by the p-side electrode
370
.
In the fabrication of an LED and an LD (Laser Diode), MO-CVD (Metal Organic-Chemical Vapor Deposition) or MBE (Molecular Beam Ep
Okazaki Haruhiko
Sugawara Hideto
Hogan & Hartson LLP
Hu Shouxiang
Kabushiki Kaisha Toshiba
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