Efficient light emitting diodes with modified window layers

A light emitting diode includes a first conductivity type semiconductor substrate, a basic AlGaInP double heterostructure and two window layers of second conductivity type semiconductor. A layer of first conductivity type AlGaInP an undoped AlGaInP layer and a layer of second conductivity type AlGaInP form the double heterostructure. The AlGaInP layers are epitaxially grown above the substrate sequentially. The window layers contain one layer of GaAs and the other layer of GaP. The first window layer is formed by epitaxially growing GaAs over the AlGaInP heterostructure. The second window layer is formed by growing GaP directly on the first window layer using either OMVPE or vapor phase epitaxy (VPE) technology. The inclusion of a GaAs window layer increases current spreading and, hence, the efficiency of the device. The yield rate of manufacturing the light emitting diode is also increased because the quality of GaP layer surface is improved.

FIELD OF THE INVENTION 
The invention relates to a semiconductor light emitting device, and more 
specifically to an AlGaInP double heterostructure light emitting device. 
BACKGROUND OF THE INVENTION 
Light emitting diodes using a double heterostructure AlGaInP have been 
demonstrated in recent years. A typical double heterostructure AlGaInP 
device has a GaAs n-type absorbing substrate on which several epitaxial 
layers are grown to form the light emitting device. An n-type confining 
layer of AlGaInP is first grown on the GaAs substrate. An active layer of 
undoped AlGaInP is then grown on the confining layer. The next layer is a 
top confining layer of p-type AlGaInP. The efficiency of such a light 
emitting device depends on the current spread in the top layer. Because of 
the high resistivity in the top p-type AlGaInP layer, the spread of 
current is generally small. Such a layer is typically very resistive and 
the light output is relatively low. Increase of the thickness in the top 
confining layer can widen the current spread and improve the efficiency of 
the light output. Nevertheless, it has been difficult to grow a thick 
AlGaInP layer. 
Several techniques have been presented to improve the efficiency of double 
heterostructure AlGaInP light emitting devices. One technique involves 
forming a current blocking layer of high band gap and more electrically 
conductive material, such as GaAlAs, with various shapes of current 
blocking regions and structures above the top p-type AlGaInP layer to 
increase the current density. This technique has been known to be suitable 
for relatively long wavelength light such as red or orange. However, it 
does not work well in the shorter wavelength range, such as green and 
yellow. Another technique uses a lattice mismatched GaP window layer on 
top of the upper AlGaInP confining layer. The technique requires growing a 
fairly thick layer of GaP above the AlGaInP layer. Threading dislocations 
and stacking faults often occur near the mismatched GaP and AlGaInP 
interface due to the difficulty in growing GaP layer directly on top of 
the AlGaInP layer. The long term reliability and stability remain as a big 
concern in such a light emitting diode because of the generally rough 
interface structure. 
SUMMARY OF THE INVENTION 
The present invention has been made to provide a high efficiency double 
heterostructure light emitting diode. According to the invention, the 
light emitting diode uses a conventional double heterostructure which has 
a n-type GaAs substrate, a n-type AlGaInP lower confining layer grown on 
the substrate, an undoped active AlGaInP layer and a p-type AlGaInP upper 
confining layer. A double-layer window structure having a first thin layer 
of GaAs and a second layer of GaP is formed on top of the upper confining 
layer. 
In the present invention, the first layer GaAs of the double-layer window 
can be made lattice-matched to the AlGaInP. The second layer GaP of the 
window can then be grown with high quality surface structure between the 
GaAs and GaP interface. GaAs is opaque material that has a smaller band 
gap than the band gap of the active layer. GaP is transparent material 
that has a higher band gap than the active layer band gap. The smaller 
band gap GaAs layer has a higher conductivity. Therefore, the inclusion of 
the GaAs layer not only improves the interface structure of the device but 
also increases the current spreading. The thickness of the window 
structure can also be reduced because of the higher overall conductivity 
in the window structure according to this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The schematic cross-sectional view of the present invention is shown in 
FIG. 1. The substrate of the light emitting device is n-type GaAs 
material. The thickness of the substrate is in the range of 250 to 300 
.mu.m. A layer of n-type AlGaInP is epitaxially grown directly upon the 
GaAs substrate using the technology of organometallic vapor phase epitaxy 
(OMVPE). The n-type AlGaInP layer has a thickness in the range of 0.1 to 2 
.mu.m. Above the n-type AlGaInP layer is an active layer of undoped 
AlGaInP. A p-type AlGaInP layer is then grown on top of the active region. 
Both the active region and the p-type layer are sequentially grown with 
OMVPE method. Their thicknesses range from 0.1 to 2 .mu.m typically. 
To form the window layers of the light emitting diode in this invention, a 
p-type thin layer of lattice-matched material, such as GaAs that has 
smaller band gap than that of the active layer, is firstly grown on top of 
the p-type AlGaInP layer using an OMVPE method. The thin GaAs layer is 
opaque and has a thickness from 0.01 to 0.1 .mu.m. After the above 
mentioned layers have been epitaxially grown on the wafer substrate 
sequentially, the second window layer of p-type GaP is grown directly 
above the p-type GaAs layer using either OMVPE or vapor phase epitaxy 
(VPE) technology. The p-type GaP layer of the window layers is transparent 
and has a higher band gap than that of the active layer and a thickness 
ranging from 5 to 15 .mu.m. An n-type electrode is deposited on the back 
side surface of the GaAs wafer substrate. A p-type electrode is deposited 
on the surface of the p-type GaP window layer. After the completion of the 
semiconductor fabrication process, the wafer is cut into cubic chips to 
form light emitting diodes. 
It has been known that the growth of a GaP layer directly on a lattice 
mismatched AlGaInP layer is very difficult and tends to introduce defects 
in the interface layer that may propagate downward into the active layer. 
With careful composition control, the p-type GaAs thin window layer can be 
accurately lattice matched to the p-type AlGaInP layer. It is also easier 
to grow a high quality transparent GaP window layer on the GaAs window 
layer. Therefore, the overall yield rate is greatly improved. The smaller 
band gap GaAs material also has a higher conductivity than AlGaInP and 
GaP. Because of the higher conductivity GaAs window layer, the overall 
resistivity and forward voltage of the semiconductor device are decreased. 
The current spreading is increased and, hence, the efficiency of the light 
emitting device is also raised. Another advantage of the additional GaAs 
window layer is that the thickness of the overall window layers can be 
reduced due to the higher conductivity in the GaAs layer. 
Although only a preferred embodiment of this invention has been described 
and illustrated, many modifications and variations according to the 
principle of this invention can be made. It is requested that all changes 
and modifications that come within the spirit of this invention are to be 
protected.