Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
1998-12-23
2002-12-24
Yamnitzky, Marie (Department: 1774)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S213000, C428S690000, C428S698000, C428S699000, C428S917000, C257S094000, C257S101000, C257S103000, C313S506000
Reexamination Certificate
active
06497944
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a light-emitting device made with gallium-nitride-group compound-semiconductor such as light-emitting diode, laser diode, etc.
Gallium-nitride-group compound-semiconductors have been increasingly used as the semiconductor material for the visible light-emitting devices and for use in the electronic devices of high operating temperature. The development has been significant in the field of blue and green light-emitting diodes.
In manufacturing the gallium-nitride-group compound-semiconductor devices, an insulating sapphire is generally used as the substrate for growing semiconductor film. Such devices are different from the light-emitting devices where semiconductor substrates other than gallium-nitride-group type substrates, such as, for example GaAs or InGaP, are utilized. Specifically, those using an insulating substrate like the present sapphire have the n-side and p-side electrodes formed in one side of the substrate wherein the semiconductor film has been formed, because the electrodes can not be provided from the substrate.
Meanwhile, in the recent manufacture of light-emitting devices, including those using the sapphire substrate, the growing of gallium-nitride-group semiconductor thin film by a metal organic CVD method has become a main stream procedure. In the procedure, a substrate is placed in a reaction tube, and metal organic compound gas (tri-methyl-gallium [TMG], tri-methyl-aluminum [TMA], tri-methyl-indium [TMI], etc.) are supplied therein as the material gas for the Group III element, and ammonia, hydrazine, etc. as the material gas for the Group V element, while maintaining the substrate at a high temperature 900° C.-1100° C., to have an n-type layer, a light-emitting layer and a p-type layer grown on the substrate in a stacked structure. After the layers are grown and formed, the p-type layer and the light-emitting layer are partially etched off to have the n-type layer exposed, and then an n-side electrode and a p-side electrode are formed on the surface of exposed n-type layer and the p-type layer, respectively, for example by a deposition method.
Most of the recent light-emitting devices have the above described double-hetero-structure, fabricated by stacking the thin films of gallium-nitride-group compound-semiconductor on a sapphire substrate.
FIG. 2
shows a cross sectional structure of a prior art light-emitting device of gallium-nitride-group compound- semiconductor.
In
FIG. 2
, a buffer layer
12
, an n-type layer
13
of gallium-nitride (GaN), a light-emitting layer
14
of indium-gallium-nitride (InGaN), a p-type clad layer
15
of aluminum-gallium-nitride (AlGaN) and a p-type contact layer
16
of GaN are stacked on a sapphire substrate
11
. A p-side electrode
17
is formed on the p-type contact layer
16
, and an n-side electrode
18
is formed on an exposed surface of the n-type layer
13
provided by partially removing the following three layers, p-type contact layer
16
, p-type clad layer
15
and light-emitting layer
14
. The n-type electrode
18
is normally made with aluminum (Al), titanium (Ti), gold (Au), or such other metals. The light-emitting gallium-nitride-group compound-semiconductor devices of the above structure have been disclosed in, for example, Japanese Patent Publication No. 6-268259.
In the prior art light-emitting gallium-nitride-group compound-semiconductor devices of the above structure, the n-type layer
13
is formed of a gallium-nitride-group compound-semiconductor doped with n-type impurities such as silicon (Si), germanium (Ge). More specifically, during the growth of the n-type layer of gallium-nitride-group compound-semiconductor by said metal organic CVD method, silane, mono-methyl-silane, etc. are supplied, together with the material gas, as material gas for Si, or germane, mono-methyl-germane, etc. as material gas for Ge. The carrier concentration of n-type layer
13
may be controlled by adjusting the flow rate of the material gas for n-type impurities.
In the gallium-nitride-group compound-semiconductor, the n-type layer may also be formed by intentionally not doping the n-type impurities, because it exhibits the n-type property even without the n-type impurities being doped therein.
If in the light-emitting gallium-nitride-group compound-semiconductor devices the efficiency of light-emission is to be maintained high, the operating voltage needs to be lowered. In order to reduce the operating voltage, the series resistance in respective layers of compound-semiconductor stacked on the substrate
11
and the contact resistance with electrode have to be made low.
An effective means for reducing the series resistance of n-type layer
13
and the contact resistance with the n-side electrode
18
is to increase the doping quantity of n-type impurities during growth of n-type layer
13
by metal organic CVD. However, when doping quantity of the n-type impurities is increased, a strain can be generated in the grown n-type layer
13
, which increases and readily leads to cracks at the n-type layer
13
. If there are cracks in the n-type layer
13
, an even emission of light may not be obtained over the entire surface, and the reliability of a light-emitting device may be degraded.
On the other hand, if priority is placed on suppression of cracks at n-type layer
13
, the n-type layer
13
needs to be grown and formed in a reduced doping quantity of the n-type impurities. In this case, however, it becomes difficult to reduce the contact resistance with n-side electrode
18
. If, in compensation of the above, the layer thickness of the n-type Is layer
13
is increased up-to about &mgr;m (e.g. 16, 17 &mgr;m) in order to reduce the series resistance of a light-emitting device, cracks are easily induced like in the earlier described case. In addition, it needs a longer time for growing the crystal, which is an additional disadvantage in the manufacture thereof.
As described in the above, if in a light-emitting gallium-nitride-group compound-semiconductor device the doping quantity of n-type impurities is increased for lowering the operating voltage, or the layer thickness is increased, the occurrence of cracks may be unavoidable, which leads to a degraded light-emitting capability and a deteriorated manufacturing yield rate.
The problems expected to be solved by the present invention with a light-emitting gallium-nitride-group compound-semiconductor device using an insulating substrate are; first to reduce the operating voltage, and second to suppress the occurrence of cracks during growth for an improved manufacturing yield rate.
SUMMARY OF THE INVENTION
A novel invented light-emitting gallium-nitride-group compound-semiconductor device of the present invention has a stacked structure comprising an n-type layer, a light-emitting layer and a p-type layer formed one after the other on an insulating substrate. In accordance with the present invention, the p-type layer and light-emitting layer are partially removed from the surface of the stacked structure formed on the substrate such that the n-type layer is exposed, and then an electrode is formed on the exposed surface of the n-type layer. The n-type layer contains, in order from the substrate, at least a first n-type layer and a second n-type layer whose carrier concentration is higher than that of the first n-type layer. The electrode is disposed on the second n-type layer.
With the above structure, a light-emitting gallium-nitride-group compound-semiconductor device is formed, in which the operating voltage is low and the manufacturing yield rate is high.
REFERENCES:
patent: 5587593 (1996-12-01), Koide et al.
patent: 5903017 (1999-05-01), Itaya et al.
patent: 6060730 (2000-05-01), Tsutsui
patent: 6320207 (2001-11-01), Furukawa et al.
patent: 5-283745 (1993-10-01), None
patent: 06268259 (1994-09-01), None
Kamei Hidenori
Oku Yasunari
Matsushita Electric - Industrial Co., Ltd.
Yamnitzky Marie
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