Note: Descriptions are shown in the official language in which they were submitted.
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BACI<GROUND OF TH~ INVENTION
Field of the Invention
This invention relates generally to a light emitting diode,
and IS directed more particularly to a green light emitting diode of a
high light emitting efficiency.
Description of the Prior Art
Light emitting diodes are known which use gallium phosphide
(GaP), and which will emit red, yellow or green light.
In general, when a green light emitting diode is employed
for displaying, the green light can be easily noticed as compared with
the other color lights, and a viewer's eyes are not fatigued as much
with the green light. In practice, however, the green light emitting
diode have not been so used as much as a red light emitting diode.
It is well known that it is required, in order to enhance the
light emitting efficiency of a green light emitting diode, that nitrogen N,
which may become the light emitting center, exist near or adjacent the
PN junction. However, even if rlitrogen N is sufficiently diffused over
the epitaxial layer having the PN junction according to the method dis-
closed in the U.S. Patent No. 3,893,875 which issued on July 8, 1975,
and whose assignee is the same as that of the present application, this
is not sufficient to provide a green light emitting diode-having high light
emitting efficiency which can be manufacturçd with higll reproducibility
and yield;
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- SUMI!~ARY OF T~ INVE:NTI ON
The inventors of the present invention have discovered
after various experiments that if the donor concentration in an
N type layer portion and the acceptor concentration gradient in a
P type layer portion near the PN junction therebetween are
selected at predetermined values respectively, a green light
emitting diode having a very high light emitting efficiency
can be produced.
Accordingly, it is an object of this invention to pro-
vide a green light emitting diode having a high light emitting
efficiency.
It is another object of the present invention to pro-
vide a green light emitting diode in which a donor concentration
and an acceptor concentration gradient are selected suitably for
making the light emitting efficiency of the green light emitting
diode high.
In accordance with the foregoing objects, there is
provided a green light emitting diode comprising: a semi-
conductor substrate of gallium phosphide; and an epitaxial layer
of gallium phosphide formed on said substrate and including
nitrogen therein; said epitaxial layer having an N type layer
portion directly formed on said substrate, a P type layer portion
formed on and interfaced with said N type layer portion, a PN
junction being formed therebetween, said N type layer portion
being doped to have a donor concentration of between 1 x 1017
and 2 x 10 cm , said P type layer portion being doped to have
an acceptor concentration gradient adjacent said PN junction
of not less than 7 x 10 cm 4.
Additional and other objects, features and advantages of
this invention will be apparent from the following description
taken in conjunction with the accompanying drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic enlarged cross-sectlonal ~iew
of an embodiment of the green light emitting diodes according
to this invention;
Figure 2 is a schematic cross-sectional view of a
reaction furnace which is employed for explaining a method for
manufacturing the green light emitting diode of this invention;
Figure 3 is a graph showing a temperature program in
the reaction furnace shown in Figure 2;
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Figure 4 is a graph of measured curves sl~owing the
relation between the concentration of H2S gas and the light emitting
efficiency of the llght emitting diode, and used for explaining the
- invention; and
Figure 5 is a graph showing the concentration distribution
curves of acceptors and donors.
DESCRIPIION OF THE PREFERRED EMBODlMENTS
An embodiment of the green light emitting diodes according
to this invention will be now described with reference to Figure 1 which
sl~ows its cross-section in enlarged scale. In Figure 1J reference
numeral 1 designates a gallium phosphide single crystal substrate of an
N type on which an epltaxial layer 2 made of gallium phosphide is
formed. This epitaxial layer 2 consists of an N type layer portion 2n
and a P type layer portion 2p, which have a P~l junction ~ formed there-
between. A donor impurity, such as sulphur S o'r tellurium Te is
doped into the substrate 1 and N type layer portion 2n, respectively,
and nitrogen N, 'which will become the light emitting centerj is further
''doped into the'N type layer portion 2n. An acceptor impurity, such as
'ZillC zn~and nitrogen N, which will become the light émitting center, are
doped into the P type layer portion 2p.
The inventors of this invention have discovered that if, in
the above construction of tbe light emitting diode, the donor concentration
in the N type layer 2n is selected at or between 1 x 1017and '2 x 1018
cm 3 and the acceptor concentration gradient in the ~ type layer 2p near
the PN junction J is selected more than 7 x 10 crn respectively, a
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green light emitting diode having a higll liglit emitting efficiency is
obtained.
In order to better understand the present invention, one of
the methods of providing the light emitting diode of this invention will
be now described. With this method, as shown in Figure 2, a reaction
furnace or open tube of ~uartz 3 is disposed such that its tube axis can
be inclined in a direction a from the horizontal direction about a fulcrum
4. A heatillg means or furnace S is provided in association with the
open tube of quartz 3 which has a region I at the temperature of about
500~700C and a region II at the temperature of about 1050~1200
along the axis of the open tube 3. ~ crucible 7, which is movably
~upported by a quartz pull rod 6, is located in the open tube 3 at the
region I and an acceptor impurity 8 such as zinc Zn is received in the
crucible 7. A boat 9 is located in the open tube 3 at the region 11.
The boat 9 contains the N type single crystal substrate l of GaP into
which a donor impurity such as sulphur S or tellurium Te is doped and
a melt lO of gallium Ga and gallium phosphide GaP, respectively. The
open tube 3 is provided with an inlet 3a throug'n which a carrier gas
such as hydrogen H2 gas is supplied into the open tube 3. Then, the
open tube 3 is heated along a predetermined temperature program.
That is, the region I in the open tube 3 is heated at a constant tempera-
ture such, for example, as 500C, 550"C, 600C, 630C or the like, to
restrict the vaporization amount of zinc Zn as the acceptor, while the
region II in the open tube 3 is heated up to 1100C at the heating speed
of, for example, 500C/hour along the heating program shown in the
graph of Figure 3. In this case, at the time when the region II is
heated at about 82SC, the mixture of hydrogen gas H2 and an impurity
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source such as hydrogen sulphicle gas H2S which contains, for example,
sulphur S as a donor impunty is supplied into the open tube 3 through
the inlet 3a. In this case, the hydrogen gas H2 can be supplied at the
flow rate of about 195 cc/min and the hydrogen sulphide gas H2S can be
supplied at the flow rate of about 5 cc/min, respectively. Thereafter,
when the region 11 is heated at about 1100C, the open tube 3 is inclined
about the fulcrum 4 in the direction a to flow the melt 10 of Ga + GaP
to the GaP substrate 1 in the boat 9 and at the same time the NH3 gas
which is a source of nitrogen N, which will become the light emitting
center, is supplied into the open tube 3. At this time, the hydrogen
gas H2 can be supplied at the flow rate of 355 cc/min, the hydrogen
sulphide gas H2S at the flow rate of 50 cc/min, and the NH3 gas at
the flow rate of 145 cc/min, respectively. After the above state is kept
f~r about 10 minutes, the temperature in the region II is lowered at the
heat decreasing speed of about 240C/min. After about 10 minutes have
elapsed from the start of lowering the temperature in the region Il,
Zn 8 is inserted in the crucible 7 in the region I to supply the acceptor
impurity in the form Zn vapor to the melt 10 of Ga + GaP by flowing it
over the substrate 1 of GaP. In this case, the amount of Zn to be
doped into the meIt 10 is set by controlling the amount of vaporization
of Zn which is produced as a result of the temperature in the region 1.
After about 15 minutes have elapsed with the Zn vapor being supplied
to the region II, the supply of H2S gas as the donor impurity source and
the supply of NH3 gas which will become the light emitting center are
stopped.
Thus, as the temperature in the region II is lowered, an
epitaxial layer 2 of GaP is grown on the substrate 1. In this case, as
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shown in ~igure I, the epitaxial layer 2 of GaP forlIled on the substrate
1 consists of a lower N type epitaxial layer portion 2n which has been
mainly doped with the donor impurity S and with nitrogen N which will
become the light emitting center and an upper P type epitaxial layer
portion 2p which has been mainly doped by the acceptor impurities Zn
and N. In this case, a PN junction J is formed at the interface between
the upper and lower epitaxial layer portions 2p and 2n.
The reason why the supply of the NH3 gas or source of
nitrogen N which will become the light emitting center is stopped during
the growth of the epitaxial layer 2 is that it has been discovered that if
the supply of the NH3 gas is continued, the surface of the epitaxial layer
2 is roughened.
The light emitting efficiency of the thus formed green light
emitting diode is affected by the H2S gas concentration and the vaporizing
amount of Zn.
Figure 4 shows the plots which are the measured curves of
the light emitting efficiency of the green light emitting diodes versus
the H2S gas concentration. In the plots of Figure 4, curves 11, 12, 13
and 14 show the case where the vaporizing amounts of Zn, which is a
parameter, is selected at 1. 6~ 2. 2 mg/min, 3.1~4.1 mg/min,
8.3~8.9 mg/min and 12.7~14.7 mg/min, respectively.
Generally speaking, the visual sensitivity of green is greater
than that of red by 30 times, so that if the light emitting efficiency of
the green light emitting diode is higher than about 0.01~, it can be
used practically as various display elements. Therefore, as may be
apparent from the measured curves ll to 14, in order to provide a
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green light emitting diode which has the light emitting efficiency higher
than 0. 017o7 necessary for the diode to have a practical use, the H2S
gas concentration must be in the range of 0.5~10 ppm although this
depends upon the vaporizing amount of Zn.
The relationship between the H2S gas concentration and the
concentration of donors which have been doped into the epitaxial layer 2,
and also the relationship between the vaporizing amount of Zn and the
concentration of acceptors which are doped into the epitaxial layer 2
will be now considered.
Figure 5 shows plots which are measured curves of the
impurities as the donor and acceptor, respectively. In the plots of
Figure 5, curves 15, 16, 17, 18 and 19 represent the concentration dis-
tributions of donors which are doped into the epitaxial layer of GaP
grown on the sùbstrate of GaP under the condition that the open tube 3
contains no Zn vapor therein and the concentration of H2S gas as the
donor impurity is changed from 0.5 ppm through 1 ppm, 1. 8 ppm and
6 ppm to 10 ppm, respectively, while curves 20, 21, 22 and 23 repre-
sent the concentration distributions of acceptors under the condition that
the open tube 3 contains no H2S gas therein and the vaporizing amount
of Zn is selected at 1. 6 mg/lllin, 3. 8 mg/min, 5.0 mg/min and 9.7
mg/min, respectively. In the plot of Figure 5, the abscissa represents
the distance d in ,~ (microns) from the interface between the substrate 1
and the epitaxial layer 2. In this case, the measurement of the donor
and acceptor concentrations is carried out by the C-V measuring method
using Schottky barrier diodes.
As may be understood from the curves in the plot of Figure
5, when the concentration of H2S gas is increased, the donor concentration
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in the epitaxial layer is increased, and when the vaporizing amount of
Zn is increased, the acceptor concentratioll in the epitaxial layer is
increased and also the gradient of the acceptor concentration is increased.
In this case, it has been discovered that the PN junction is formed at the
intersecting points of the donor and acceptor concentration distribution
curves or, in the plots of Figure 5, the intersecting points between the
curves 15 to 19 and the curves 20 to 23, or in other words, at the
positions where the donor and acceptor concentrations are the same. A
light emitting efficiency higher than 0. 01~ is obtained in some of the PN
junctions having the respective concentration distributions or in PN
junctions formed at the respective intersecting points between the curve
23 and the curves 15 to 19.
The relationship of the acceptor concentration gradient at
the P type layer side near the respective PN junctions to the vaporizing
amount of Zn is shown in the following Table I.
Table I
Vaporizing ~mount _
of Zn Acceptor Concentration Gradient
1.6 mg/min 2.2 x 102 cm 4 (For H2S
of 0.5 ppm)
3. 8 " 3. 5 ~ 6. 5 x10 cm (For H2S
of 1.8~0.5 ppm)
5.0 " 5.5~9.5 x102 cm (For H2S
of 1. 8,--0.5 ppm)
9.7 " 7~15 x102 cm 4 (For H2S
of 6~0.5 ppm)
16.5 " 7~ 15 x102 cm 4 (For H2S
_ of 6~ 0. 5 ppm)
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From the Table 1, it will be understood that the fact that
the vaporizing amount of Zn is more than 9.7 mg/min where a green
light emitting diode of a high light emitting efficiency means that the
acceptor concentration gradient near the PN Iunction is selectednot less
than 7 x 102 cm~4.
The donor concentration of the N type layer near the PN
junction, which is formed under the Zn vaporizing amount of 9. 7 mg/min
required for forming the PN junction to obtain high light emitting
efficiency, is about 1~2 x 1017 cm 3 under the condition of H2S gas
concentration being 0.5 ppm, and about 2 x 1018 cm~3 under the
condition of H2S gas concentration being 10 ppm respectively. Thus,
the donor concentration for obtaining the light emitting efficiency higher
than 0.01~ shown in ~igure 4 is in the range of 1 x 1017 cm~3 to
2 x 1ol8 cm~3
Accordingly, it will be understood that if in a green light
emitting diode, a PN junction is formed in its epitaxial layer, the donor
concentration in its N type layer portion which will form the PN junction
is selected in the range of 1 x 1017~ 2 x 1018 cm~3 and the acceptor
concentration gradient in its P type layer portion at the PN junction is
selected not less than 7 x102 cm~ 4, respectively7 the green light
emitting efficiency of the green light emitting diode can be made high
positively.
In the above example, the variation of light emitting
efficiency of the green light emitting diode due to the concentration of
nitrogen N in the epitaxial layer 2 is neglected, but if the concentration
of nitrogen N is higher than 1 x 1018 cm 3, the above mentioned
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condition is satisfied. If the concentration of N is further increased,
the light emitting efficiency is improved further. ~lowever, the maximum
concentration of N is about 6 x 1018 cm 3. If the concentration of N
is increased to about 1 x 1019 cm 3, the light emitting efficiency is,
on the contrary, lowered and the surface of the epitaxial layer is
roughened Therefore, it is undesirable that nitrogen be doped to more
than a concentration of 1 x 1019 cm~3. If the concentration of N falls
within the above range near the PN junction, the light emitting
efficiency is less affected by the nitrogen in the other portions although
its concentration in the other portions is lower or higher than the above
range.
Further, in the above example, the N type and P type
layers are formed by one epitaxial process, but it is also possible that
tlley are formed by two epitaxial processes with the same effects,
It will be apparent that maly modifications and variations
could be effected by one skilled in the art without departing from the
spirit and scope of the novel concepts of this invention, so that the
scope should be determined by the appended claims.
