Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
2000-05-04
2002-10-15
Pham, Long (Department: 2823)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S102000, C438S603000, C438S615000
Reexamination Certificate
active
06465273
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to light emitting devices with a light emitting diode (LED) and methods of manufacturing the same and in particular to light emitting devices with a low-cost, high-performance ZnSe homoepitaxial LED and methods of manufacturing the same.
2. Description of the Background Art
Conventionally, AlGaAs and GaAsP for red color, GaP for yellow-green color, AlGaInP for orange and yellow colors, and the like have been used as materials for LEDs of high luminance. Such LEDs are formed on a conductive substrate.
FIG. 4A
shows a light emitting device
1
a
having the LED as described above. As shown in the figure, light emitting device
1
a
includes resin
2
, an LED chip
3
a
, lead frames
4
a
and
4
b
, and a wire
5
.
FIG. 4B
is an enlarged view of a region
6
shown in FIG.
4
A. As shown in the figure, LED chip
3
a
is connected to lead frame
4
a
via silver paste
18
. LED chip
3
a
includes a conductive substrate
7
a
, an epitaxial, light emitting layer
8
a
causing an emission, a first electrode
10
a
, and a second electrode
17
. The first electrode
10
a
is connected to lead frame
4
b
via wire
5
.
The LED thus configured is provided by an established, mass-production process at low cost and one such LED is produced at as low a cost as ten yen or therebelow.
However, an LED based on GaInN, a material for blue and green colors and furthermore for white color, is configured as shown in
FIGS. 5A and 5B
, since it uses as its substrate an insulating sapphire substrate.
More specifically, as shown in
FIGS. 5A and 5B
, an LED chip
3
b
has first and second electrodes
10
a
and
17
a
on an outermost surface of an epitaxial, light emitting layer
8
b
. The second electrode
17
a
is provided in a concave portion formed on a surface of epitaxial light emitting layer
8
b
. The second electrode
17
a
is connected to lead frame
4
a
via a wire
5
b
and the first electrode
10
a
is connected to lead frame
4
b
via a wire
5
a
. An insulating substrate
7
b
is fixed to lead frame
4
a
with silver paste
18
posed therebetween.
Because of the complicated structure as described above, a light emitting device having a GaInN-based LED requires a high manufacturing cost and its unit price is several times greater than the aforementioned low-cost product.
A ZnSe-based material is promising as a material for an LED for blue and green colors and white color. The present inventors have worked on developing an LED having a ZnSe-based homoepitaxial structure employing an n-type ZnSe substrate which is conductive and also transparent in the visible range. Such structure is different from a GaInN-based structure in that it has a conductive substrate. As such, it may have the low-cost LED structure as shown in
FIGS. 3A and 3B
and theoretically an LED can be manufactured at low cost.
However, it has been found that forming an ohmic electrode on an n-type ZnSe substrate having low carrier concentration requires a special technique. More specifically, for an n-type ZnSe substrate having a carrier concentration of no less than 3×10
18
cm
−3
, an ohmic electrode can be readily provided by normally depositing and then thermally annealing with Ti, Al or a similar material. It has been found that for an n-type ZnSe substrate having a carrier concentration less than 3×10
18
cm
−3
, however, an ohmic electrode cannot be readily provided by the combination of normal deposition and thermal anneal.
It has also been found that silver (Ag) readily diffuses into a ZnSe crystal and thus readily causes a defect referred to as non-luminescent center. In other words, using silver paste to fix a ZnSe crystal may degrade an LED containing the ZnSe crystal.
SUMMARY OF THE INVENTION
The present invention has been made to overcome such disadvantages as above and it contemplates providing an ohmic electrode on an n-type ZnSe substrate having low carrier concentration, and preventing an LED from the degradation as described above.
In accordance with the present invention a light emitting device includes an n-type ZnSe substrate, an electrode base, and a conductive layer. The conductive layer is formed of In or an In alloy and it fixes the n-type ZnSe substrate and the electrode base and also serves as an ohmic electrode for the n-type ZnSe substrate. The electrode base is only required to be formed of conductive material, such as a lead frame or an electrode provided on an insulating substrate.
The present inventors have studied materials for fixing an n-type ZnSe substrate and an electrode base and have found that a conductive layer of In or In alloy may be used as a material for fixing the n-type ZnSe substrate and the electrode base to obtain ohmic contact if the substrate has a carrier concentration of as low as less than 3×10
18
cm
−3
. Furthermore, unlike silver, In or the In alloy does not diffuse easily into a ZnSe crystal to cause a defect referred to as non-luminiscent center. As such, an LED can be prevented from degrading due to such diffusion.
In the present invention a light emitting device preferably includes a ZnSe homoepitaxial light emitting diode. Such light emitting diode has on an n-type ZnSe substrate an epitaxial light emitting layer of ZnSe-related compounds.
The present invention is particularly useful for a light emitting device including a ZnSe homoepitaxial light emitting diode capable of emitting blue and green lights and also white light.
The n-type ZnSe substrate has a carrier concentration preferably of at least 3×10
17
cm
−3
and less than 3×10
18
cm
−3
.
A conductive layer of In or In alloy allows an ohmic electrode to be provided for carrier concentrations as low as above.
In the present invention a light emitting diode preferably operates on a voltage of no more than 3V. Accordingly, it is useful for a backlight of an LCD (Liquid Crystal Display) incorporated for example in a mobile phone.
In the present invention a light emitting device may be manufactured by a method including the following steps: an epitaxial light emitting layer is provided on an n-type ZnSe substrate. In or an In alloy is melted on an electrode base. Directly on the melted In or In alloy the n-type ZnSe substrate is mounted and subjected to at least one of vibration and pressure. Thereafter it is thermally annealed.
In and the In alloy have a melting point as low as 155° C. or therebelow and In and the like have a superior wettability with respect to metal and also solidify at room temperature. As such, In and the like can be used as a conductive adhesive or so-called solder. The present inventors have studied whether In and the like can be used for adhering the n-type ZnSe substrate and the electrode base together and found that using the technique characteristic to the invention as described above allows In and the like to be used as a conductive adhesive in accordance with the present invention. More specifically, by mounting the n-type ZnSe substrate directly on melted In or the like and subjecting it to at least one of vibration and pressure the In or the like can diffuse into the ZnSe crystal substrate for a temperature as low as approximately 200° C., and thereafter by thermally annealing the same an eutectic alloy can be produced and ohmic contact can be obtained for the ZnSe substrate of low carrier concentration.
The vibrant applied is preferably ultrasonic vibration. The pressure applied is preferably at least 0.544 MPa (5.56×10
−2
kg/mm
2
) and less than 109 MPa (11.1 kg/mm
2
).
The vibration applied may be a scrubbing operation caused by a mechanical vibration having a frequency of at least 1 Hz and at most 1000 Hz. In this instance, a pressure of at least 0.217 MPa (2.22×10
−2
kg/mm
2
) and less than 109 MPa (11.1 kg/mm
2
) is applied simultaneously with the vibration.
If vibration and pressure are applied simultaneously, preferably the frequency is at least 10 Hz and at most 300Hz and the pressure is a
Katayama Koji
Matsubara Hideki
Saegusa Akihiko
Fasse W. F.
Fasse W. G.
Pham Long
Sumitomo Electric Industries Ltd.
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