Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
1999-02-16
2002-09-03
Font, Frank G. (Department: 2877)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S498000, C313S509000
Reexamination Certificate
active
06445127
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a light-emitting device comprising a gallium-nitride-group compound-semiconductor, used in optical devices such as a light-emitting diode, a laser diode, etc. More specifically, a semiconductor light-emitting device, in which the efficiency of light emission is maintained high and the color purity is remarkably improved over prior art devices.
Gallium-nitride-group compound-semiconductors have been increasingly used as the semiconductor material for the visible light emitting devices and for electronic devices having high operating temperature. The development has been significant in the field of blue light-emitting diodes.
A basic method of manufacturing the gallium-nitride-group compound-semiconductors is growing a gallium-nitride-group (GaN group) semiconductor film on the surface of an insulating sapphire substrate by means of metal organic CVD. In a practical process of forming a compound-semiconductor layer of GaN group, a substrate is placed in a reaction tube, metal organic compound gases(tri-methyl-gallium, tri-methyl-aluminum, tri-methyl-indium, etc.) are supplied therein as the material gas for the Group III element, and ammonia, hydrazine, etc. are supplied as the material gas for the Group V element, while the substrate is maintained at a high temperature 900° C.-1100° C., so as to have an n-type layer, a light-emitting layer and a p-type layer grown on the substrate in a stacked layer structure.
As described above, a light-emitting device using the GaN group compound semiconductor is useful as a device for emitting a blue light. However, with regard to technical advancement, the blue light-emitting device is slightly behind when compared with devices emitting red or green light. A reason for the delay in technical advancement is to have been caused by the difficulty of finding an appropriate GaN group compound-semiconductor material. Accordingly, blue light-emitting devices need to show an improvement in the luminance and the color purity, which have been inferior relative to those of the red and green devices.
One effort for improving the luminance, for example, is a blue light-emitting diode having a MIS structure; where, a high resistance i-type GaN group compound-semiconductor layer doped with a p-type impurity is provided on an n-type GaN group compound-semiconductor. In a device having the MIS structure, however, both the luminance and the light-emitting output tend to be insufficient for practical applications.
FIG. 4
is a cross sectional side view of a conventional light-emitting device using GaN group compound-semiconductor.
Referring to
FIG. 4
, a buffer layer
2
is formed on a sapphire substrate
1
, and an n-type clad layer
3
, a light-emitting layer
4
, a p-type clad layer
5
and a p-type contact layer
6
are formed, in order from the bottom, on the buffer layer
2
by a metal organic CVD method. A p-side electrode
7
is formed on the p-type contact layer
6
, while an n-side electrode
8
is formed on an exposed surface of the n-type clad layer
3
. The exposed portion of the n-type clad layer
3
is the result of etching-off a part of the p-type clad layer
5
and the light-emitting layer
4
.
The GaN group compound-semiconductor light-emitting device in general has a structure comprising a pn junction formed by crystallographically connecting the p-type region and the n-type region of a semiconductor. Namely, a p-type layer of semiconductor for the p-type region and an n-type layer of semiconductor for the n-type region are stacked. By applying a voltage of positive polarity on the p-type layer and a voltage of negative polarity on the n-type layer, a hole is injected from the p-type layer into the n-type layer via the pn junction, and an electron is injected from the n-type layer into the p-type layer. As a result of re-combination of the electron and the hole at the vicinity of the pn junction, a light having an energy identical to the band gap energy of semiconductor in the pn junction is emitted.
Japanese Laid-open Patent Publication No.6-260680, for example, proposes a GaN group compound-semiconductor light-emitting device having a light-emitting layer of n-type InGaN layer doped simultaneously with a p-type impurity, Zn, and an n-type impurity, Si. The Publication discloses that the strength of blue light emission increases as a result of an increase in the number of the donor-acceptor pair light emissions. According to the Publication, the efficiency of light emission and the strength of light emission have been significantly improved as compared with the so-called MIS structured light-emitting devices.
Japanese Laid-open Patent Publication No.846240 discloses a light-emitting device in which a p-type light-emitting layer is formed by doping an acceptor impurity, which is a p-type impurity, and a donor impurity, which is an n-type impurity. According to the Publication, the light-emitting layer may have holes at a high concentration and the quantity of electron injection that reaches deep into the light-emitting layer may be increased; which increases the number of electrons making the re-combination with the holes, leading to an increased luminance.
Furthermore, Japanese Laid-open Patent Publication No.9-186362 proposes a light-emitting device, in which the light-emitting layer is doped with a donor impurity and an acceptor impurity together. The light is emitted as the result of the electron transition between donor impurity level and valence band, conduction band and acceptor level, or conduction band and valence band.
The light-emitting devices disclosed in the above Publications are different from each other in terms of the structure, whether the light-emitting layer has p-type conduction or n-type conduction. Apart from the structural differences, the above disclosed light-emitting devices exhibit an improved luminous intensity as compared with the so-called MIS structured light-emitting devices having a junction of metal-insulation layer-n-type semiconductor layer, in place of a pn junction.
In the light-emitting devices for use in an outdoor display, the sun light, among other things, readily causes interference with the emission of light. Therefore, a further increase in the luminous intensity of the light-emitting devices is required for reproducing a clear image that offers a high recognition capability.
Each of the light-emitting devices disclosed in the above Publications make use of the light emission related to the impurities level, such as the D-A (donor-acceptor) pair light emission in which a p-type impurity, being an acceptor impurity, and a donor impurity emit the light in pairs.
The light emission related to the impurity level, however, generally exhibits a light having a broad spectrum. In addition, the peak wavelength of the above light-emitting device tends to shift toward the short wave side along with an increase in operating current. The broad spectrum affects the purity of color reproduction. When the light-emitting devices are used for a full-color display, range of the color reproduction is narrowed. If the peak wavelength shifts towards a shorter wavelength, the color reproduction is degraded.
As described above, the light emission characteristics of a GaN group compound-semiconductor light-emitting device using the light emitting principle related to the impurity level remain unsatisfactory, especially when it is used in a full-color display application because the color purity can be degraded, in addition to the insufficiency in the luminance.
Assuming the light-emitting layer
4
of an exemplary device of
FIG. 4
is an n-type layer, a pn junction is formed by the n-type light-emitting layer
4
and the p-type clad layer
5
stacked on the surface of the n-type light-emitting layer
4
. Namely, the re-combination of electrons and holes that contribute to the light emission takes place at the vicinity of a junction between the light-emitting layer
4
and the p-type clad layer
5
. Therefore, it is difficult to increase the efficiency of l
Kamei Hidenori
Oku Yasunari
Matsushita Electric - Industrial Co., Ltd.
Punnoose Roy M.
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