Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With reflector – opaque mask – or optical element integral...
Reexamination Certificate
2001-10-31
2003-04-22
Whitehead, Jr., Carl (Department: 2813)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With reflector, opaque mask, or optical element integral...
C257S103000
Reexamination Certificate
active
06552369
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a light emitting diode (LED) chip structure and a fabricating method thereof, and more particularly relates to a LED chip structure using a Bragg reflector layer with high reflectivity for increasing the brightness of an LED.
BACKGROUND OF THE INVENTION
The conventional AlGaInP light emitting diode, as shown in
FIG. 1
, has a double heterostructure (DH), and is composed of an n-type (Al
x
Ga
1−x
)
0.5
In
0.5
P lower cladding layer
4
having an Al composition of about 0.7 to about 1.0 and is formed on an n-type GaAs substrate
3
; an (Al
x
Ga
1−x
)
0.5
In
0.5
P active layer
5
; a p-type (Al
x
Ga
1−x
)ln0.5P upper cladding layer
6
having an Al composition of about 0.7 to about 1.0; and a p-type high energy gap GaP or AlGaAs window layer
7
. The emitting wavelength from the conventional LED structure can be varied by changing the composition of the active layer
5
, and the wavelength thereof can be changed from 650 nm of red light to 555 nm of pure green light. However, one disadvantage of the conventional light emitting diode is that, when the light generated in the active layer is emitted deep into the GaAs substrate, the light is easily absorbed by the GaAs substrate since the GaAs substrate has a smaller energy gap. Accordingly, an LED with high efficiency cannot be fabricated.
Several conventional techniques about light emitting diodes have been disclosed for preventing the light from being absorbed by the substrate. However, these conventional techniques still have some disadvantages and limitations. For example, the method disclosed by Sugawara et al. and published in Appl. Phys. Lett., vol. 61, 1775-1777 (1992), is to add a distributed Bragg reflector (DBR) layer to a GaAs substrate for reflecting the light entering the GaAs substrate thereby decreasing the light absorbed by the GaAs substrate. However, because the DBR layer can only effectively reflect the light approximately vertical to the GaAs substrate, the efficiency thereof is not very good.
Kish et al. disclosed a wafer-bonded transparent-substrate (TS) (Al
x
Ga
1−x
)
0.5
In
0.5
P/Gap light emitting diode [Appl. Phys Lett. vol. 64, no. 21, 2839 (1994);” Very High-Efficiency Semiconductor Wafer-Bonded Transparent-Substrate (Al
x
Ga
1−x
)
0.5
In
0.5
P/Gap”], wherein the TS AlGaInP LED is fabricated by growing a very thick (about 50 &mgr;m) p-type GaP window layer with the method of hydride vapor phase epitaxy (HVPE) at a temperature above the eutectic point of AuSn solder. After bonding, the n-type GaAs substrate is selectively removed by using conventional chemical etching techniques. The exposed n-type layers are subsequently bonded to an n-type GaP substrate of 8-10 mil in thickness. The resulting TS AlGahiP LED exhibits a two-fold improvement in light output in comparison with the absorbing substrate (AS) AlGaInP LED. However, the fabrication process of TS AlGahnP LED is too complicated. Therefore, it is difficult to manufacture the TS AlGaInP LED with high yield and low cost.
Horng, et al. reported a mirror-substrate (MS) AlGaInP/metal/SiO
2
/Si light emitting diode fabricated by a wafer-fused technique [Appl. Phys. Lett. Vol. 75, No. 20, 3054 (1999); “AlGaInP Light-Emitting Diodes with Mirror Substrates Fabricated by Wafer Bonding”], wherein AuBe/Au is used as the adhesive to bond the Si substrate and light emitting diode epilayers. However, the luminous intensity of the MS AlGaInP LED is only about 90 mcd with 20 mA injection current, which is 40% lower than that of the TS AlGaInP LED. Besides, both the p-electrode and the n-electrode are formed on the same side, so that the chip size cannot be decreased. Therefore, the chip size is larger than the conventional LED chip that has a p-electrode on one side and a n-electrode on the other side. Thus, this type of LED chip can hardly satisfy the trend of package size being moved toward miniaturization.
SUMMARY OF THE INVENTION
As described above, conventional light emitting diodes have many disadvantages. Therefore, the present invention provides a light emitting diode structure and a fabricating method thereof to solve the disadvantages of the conventional light emitting diodes.
It is an object of this invention to provide a light emitting diode structure and a fabricating method thereof. This invention can prevent emitted light from being absorbed by a substrate by using a high reflectivity Bragg reflector layer.
It is another object of this invention to provide a light emitting diode structure and a fabricating method thereof. The present invention provides a Bragg reflector layer by forming a high aluminum-containing AlGaAs/AlGaInP layer or a high aluminum-containing AlGaAs/low aluminum-containing AlGaAs layer on the substrate before the epitaxial structure of the light emitting diode is formed. Because the high aluminum-containing AlGaAs has the featuree of easy oxidation, the high aluminum-containing AlGaAs can be oxidized to form an oxide having a lower refraction index. Thus, the reflectivity and high reflection zones of the oxidized Bragg reflector can be much larger than those of the conventional DBR.
It is another object of this invention to provide a light emitting diode structure and a fabricating method thereof. Since the oxidized AlGaAs layer is electrically insulated, the Bragg reflector layer thus will limit the current within the non-oxidized regions of high aluminum-containing AlGaAs layer. Furthermore, the thickness of the lower cladding layer must be greater than 0.5 &mgr;m to achieve uniform current density in the active layer, whereby the light intensity in this chip is uniform and not restricted in the center region. Comparing to the conventional LED, the light intensity can be drastically improved because most of light emitted backward can be reflected by the oxidized Bragg reflector having high reflectivity.
In accordance with all aspects of this invention, the invention provides a structure of a light emitting diode, comprising: a substrate having a first electrode on a first surface, an epitaxial structure forming on a second surface, wherein the epitaxial structure is formed by a plurality of III-V compound semiconductor epitaxial layers including an active layer and a Bragg reflector layer sandwiched between the active layer and the substrate, and a portion of the Bragg reflector layer is oxidized, and a second electrode formed on the epitaxial structure.
In accordance with all aspects of this invention, this invention provides a method for forming a light emitting diode, comprising the steps of: providing a substrate; forming an epitaxial structure on a first surface of the substrate, wherein the epitaxial structure is formed by a plurality of III-V compound semiconductor epitaxial layers including an active layer and a Bragg reflector layer sandwiched between the active layer and the substrate; conducting a treatment of oxidation for forming a high reflectivity and a current insulating layer in a portion of the Bragg reflector layer; forming a first electrode on a second surface; and forming a second electrode on the epitaxial structure.
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patent: 6040590 (2000-03-01), OBrien et al.
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Wafer-Bonded AlGaInP/Au/AuBe/SiO2/Si Light Emitting Diodes: Horng et al., Jpn. J. App. Phys., vol. 39 (2000) Pt. 1, No. 4B, pp. 2357-2359.
AlGalnP Light-Emitting Diodes with Mirror Substrates Fabricated by Wafer Bonding, R. H. Horng et al., Applied Physics Letters, vol. 75, No. 20, Nov. 15, 1999, pp. 3054-3056.
AlGaInP/AuBe/glass Light-Emitting Diodes Fabricated by Wafer Bonding Technology R.H. Horng et al., Applied Physics Letters, vol. 75, No. 2, Jul. 12, 1999, pp. 154-156.
High-Brightness InGaAIP Green Light-Emitting Diodes H. Sugawara et al., Appl. Phys. Lett. 61 (15, Oc
Chang Chih-Sung
Chang Holin
Chen Tzer-Perng
Chiou Shu-Woei
Jr. Carl Whitehead
United Epitaxy Company LTD
Vesperman William C.
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