Electroluminescent device of compound semiconductor

The present invention provides an electroluminescent device of a Group II-VI compound semiconductor which comprises a substrate, a light-emitting portion, and a conductive portion provided at least between the substrate and the light-emitting portion for injecting into the light-emitting portion the current to be produced in the device by the application of an external voltage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to electroluminescent devices of compound 
semiconductors, and more particulary to improvements in electroluminescent 
devices comprising a Group II-VI compound semiconductor such as zinc 
sulfide (ZnS) or zinc selenide (ZnSe). 
2. Description of the Prior Art 
Group II-VI compound semiconductors such as ZnS and ZnSe are generally used 
as materials for devices, such as blue light-emitting diodes, for 
producing light with high efficiency over the region of ultraviolet rays 
to visible rays. 
FIG. 10 shows an example of structure conventionally used for 
electroluminescent devices of such a Group II-VI compound semiconductor. 
Indicated at 71 is a low-resistance n-type ZnS single-crystal substrate 
prepared from a ZnS bulk single crystal grown by the halogen chemical 
transport process, by heat-treating the crystal in molten zinc at 
1000.degree. C. for 100 hours. A light-emitting layer 74 of n-type ZnS and 
an insulating layer 75 of insulating ZnS are successively formed 
epitaxially over the substrate 71 by molecular beam epitaxy (MBE) or 
organometallic chemical vapor deposition (MOCVD). Gold (Au) is deposited 
on the insulating layer 75 by vacuum evaporation to form a positive 
electrode 77. An ohmic electrode of indium (In) serving as a negative 
electrode 78 is formed on the rear surface of the low-resistance n-type 
substrate 71. Thus, a MIS electroluminescent device is fabricated. 
Also proposed is an electroluminescent device wherein a conductive layer is 
provided between a substrate and a light-emitting layer. 
With reference to FIG. 11, the proposed device comprises a low-resistance 
n-type ZnS substrate 83 having a resistivity of 10 to 1 ohm-cm and a 
thickness of 300 to 1000 .mu.m, a low-resistance n-type ZnS conductive 
layer 84 having a resistivity of 10.sup.-2 to 10.sup.3 ohm-cm and 
epitaxially formed over the substrate 83, for example, by MBE from ZnS 
with Al, Cl or the like added thereto, a low-resistance n-type ZnS 
light-emitting layer 85 and a ZnS high-resistance layer 86 which are 
formed successively over the layer 84, for example, by MBE, an ohmic 
electrode 82 formed on the rear surface of the substrate 83 by depositing 
In thereon by vacuum evaporation and heat-treating the deposit in a 
high-purity gas atmosphere at 450.degree. C. for several seconds to 
several minutes, an electrode 87 formed on the high-resistance ZnS layer 
86 by depositing Au thereon by vacuum evaporation, and lead wires 81 and 
88 suitably arranged. 
With these conventional electroluminescent devices, the current injected 
via the electrodes 77, 78 or 82, 87 flows through the device over a wide 
region, so that the current density in the light-emitting layer 74 or 85 
is small. This makes it difficult to obtain luminescence with high 
brightness. Furthermore, the light produced by the emitting layer 74 or 85 
radiates through the device in every direction. It is therefore likely 
that the light produced will not be taken out of the device efficiently. 
Either one of the foregoing constructions of electroluminescent devices may 
be used for fabricating a monolithic display device which comprises a 
multiplicity of minute luminescent chips having a unit size of 100 .mu.m 
and prepared by forming a minute discrete pattern on the substrate. When 
current is passed through the device, the majority of current loss occurs 
in the substrate. It is therefore likely that the device is high in the 
series resistance of the chips and has an impaired insulating property 
between the chips. 
An object of the present invention, which has been accomplished in view of 
the foregoing situation, is to provide an electroluminescent device of a 
Group II-VI compound semiconductor adapted to produce light with high 
brightness. 
SUMMARY OF THE INVENTION 
The present invention provides an electroluminescent device of a compound 
semiconductor which comprises an electroluminescent device body formed on 
a substrate and providing a light-emitting portion and a conductive 
portion joined to the lower surface and/or the upper surface of the 
light-emitting portion, and a pair of electrodes for applying therethrough 
an external voltage to the body to cause electroluminescence, the 
conductive portion comprising a conductive layer formed of a Group II-VI 
compound semiconductor made to have a substantially low resistance by the 
addition of an impurity element, and a conductive layer part provided in 
the conductive layer and formed of the Group II-VI compound semiconductor 
having an impurity element added thereto at a higher concentration than 
the conductive layer, the conductive layer part having a width to restrict 
the path of current flow through the light-emitting portion. 
Thus, the present invention provides an electroluminescent device of a 
Group II-VI compound semiconductor which comprises a substrate, a 
light-emitting portion, and a conductive portion provided at least between 
the substrate and the light-emitting portion for injecting into the 
light-emitting portion the current to be produced in the device by the 
application of an external voltage. The conductive portion comprises a 
conductive layer formed of a Group II-VI compound semiconductor made to 
have a substantially low resistance by the addition of an impurity 
element, and a conductive layer part provided in the conductive layer and 
formed of the Group II-VI compound semiconductor having an impurity 
element added thereto. The conductive layer part is made to have a higher 
impurity concentration than the conductive layer so that the current to be 
injected into the light-emitting portion can be confined to the conductive 
layer part which is lower than the layer in resistivity. Consequently, 
current of high density can be injected into the light-emitting portion to 
effect luminescence with high brightness. 
The refractive index of the conductive portion having impurities added 
thereto increases with a decrease in the impurity concentration. 
Accordingly, the conductive layer and the conductive layer part are 
different in refractive index, with the result that the light produced by 
the light-emitting portion is confined to the conductive layer of the 
higher refractive index when passing through the conductive portion, 
making it possible to deliver the light from the device with a high 
efficiency.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The electroluminescent device of the present invention is prepared from a 
Group II-VI compound semiconductor. The light emitted by the 
light-emitting portion by the application of a voltage across the 
electrodes has a multiplicity of colors including blue and appears on the 
upper surface or lower surface of the device. The device is used as a 
light source having high energy and high brightness for various displays, 
printers, facsimile systems, etc. 
The greatest structural feature of the electroluminescent device of the 
invention is that the conductive portion joined to the lower surface 
and/or the upper surface of the light-emitting portion comprises a 
conductive layer (hereinafter referred to as the "first conductive layer") 
and a conductive layer part (hereinafter referred to as the "second 
conductive layer") provided in the first conductive layer, the second 
conductive layer having a higher impurity concentration than the first 
conductive layer. 
The substrate for use in the invention is preferably made of a Group II-VI 
compound semiconductor. Examples of such semiconductors are low-resistance 
n-type ZnS, low-resistance n-type ZnSe, low-resistance n-type ZnS.sub.x 
Se.sub.l-x and the like, and insulating ZnS, insulating ZnSe, insulating 
ZnS.sub.x Se.sub.l-x and the like. 
For example, the substrate of low-resistance n-type ZnS (or low-resistance 
n-type ZnSe or low-resistance n-type ZnS.sub.x Se.sub.l-x) is prepared 
from a ZnS bulk single crystal (or ZnSe bulk single crystal or ZnS.sub.x 
Se.sub.l-x bulk single crystal) grown by the halogen chemical transport 
process, by heat-treating the single crystal in molten zinc at 
1000.degree. C. for about 100 hours. The substrate thus obtained is made 
to have a low resistance. Preferably, the above materials forming the 
substrate have the following resistivities (ohm-cm). 
ZnS: 1 to 10, more preferably about 1. 
ZnSe 10.sup.-2 to 10, more preferably about 1. 
ZnS.sub.x Se.sub.l-x : 1 to 10, more preferably about 1. 
Examples of n-type impurities useful for the heat treatment in preparing 
the substrates are Al, Ga and the like, Cl and Br. Also usable are In, I 
and the like. 
For preparing the substrate of insulating ZnS (or insulating ZnSe or 
insulating ZnS.sub.x Se.sub.l-x), it is desirable to use a ZnS bulk single 
crystal (or ZnSe bulk single crystal or ZnS.sub.x Se.sub.l-x bulk single 
crystal) as is without subjecting the crystal to low-resistance treatment. 
When a ZnS.sub.0.5 Se.sub.0.5 crystal which is grown by the halogen 
chemical transport process or high-pressure melting process is used, the 
substrate obtained is colored yellow or orange and is low in transparency 
for the luminescence wavelength, so that the blue light produced, for 
example, needs to be taken out from the semiconductor side. However, if a 
ZnS.sub.0.5 Se.sub.0.5 crystal which is grown by the sublimation process 
is used, the resulting substrate is almost colorless and transparent and 
is desirable since blue light emitted can be taken out also from the 
substrate side. The substrate to be used in this case need not be 
subjected to the low-resistance treatment conventionally employed, but the 
wafer obtained from a bulk single crystal can be used as is, i.e. with its 
high resistivity (with insulating to semi-insulating property, 10.sup.6 to 
10.sup.15 ohm-cm). 
The light-emitting portion to be formed in the present invention is 
preferably an n-type ZnS light-emitting layer providing a ZnS 
electroluminescent device of the MIS (metal insulator semiconductor) type, 
or a light-emitting layer having a p-n junction provided by the 
combination of n-type ZnSe and p-type ZnSe for forming a p-n junction 
electroluminescent device of the planar structure type. 
In the case of the MIS-type electroluminescent device, the Group II-VI 
compound semiconductor for the light-emitting layer is not limited to ZnS; 
also usable is, for example, ZnSe, ZnS.sub.x Se.sub.l-x or ZnS.sub.y 
Te.sub.l-y. When the light-emitting layer is of the p-n junction type, use 
of a ZnSe p-n junction is not limitative; also usable are a ZnS p-n 
junction, a ZnS.sub.x Se.sub.l-x p-n junction, a ZnS.sub.y Te.sub.l-y p-n 
junction, p-n heterojunctions afforded by such materials, and various 
other junctions. 
According to the present invention, the first conductive layer which is 
made to have a substantially low resistance by the addition of impurity 
means one having a resistivity of 1 to 10.sup.-2 ohm-cm. This resistivity 
is controllable by setting the concentration of impurity added to 
10.sup.16 to 10.sup.18 at.cm.sup.-3, whereby the electric resistance is 
settable to the range useful for electroluminescent devices. 
On the other hand, it is desirable that the second conductive layer have a 
higher impurity concentration than the first conductive layer. More 
specifically, the impurity concentration of the second layer is greater 
than the first preferably by at least one order of magnitude. 
Examples of n-type impurities for use in preparing the first and second 
conductive layers and the light-emitting layer are elements from Group III 
such as boron (B), aluminum (Al), gallium (Ga), indium (In) and thallium 
(Tl), and elements from Group VII such as chlorine (Cl), bromine (Br), 
fluorine (F) and iodine (I). At least one of these elements is used, or at 
least one of such elements from Group III is used in combination with at 
least one of these elements from Group VII. On the other hand, examples of 
useful p-type impurities are elements from Group Ia such as lithium (Li), 
sodium (Na), potassium (K), rubidium (Rb) and cesium (Cs), the elements 
from Group Ib, i.e. copper (Cu), silver (Ag) and Gold (Au), thallium from 
Group III and elements from Group V such as nitrogen (N), phosporus (P), 
arsenic (As), antimony (Sb) and bismuth (Bi). At least one of these 
elements is used, or at least one of the above elements from Group Ia or 
Ib is used in combination with at least one element from Group V. These 
impurities are suitable for giving epitaxial films which are small in 
their degree of compensation and have a high quality. 
The first and second conductive layers and the n-type light-emitting layer 
of the invention have the impurity concentration E (cm.sup.-3), 
resistivity R (ohm-cm) and thickness (height) L (.mu.m) (see FIG. 1) given 
in Table 1 below. 
TABLE 1 
______________________________________ 
E R L 
______________________________________ 
First conductive 
10.sup.16 -10.sup.18 
1-10.sup.-2 
5-10 
layer 10.sup.18 5 .times. 10.sup.-2 
5 
Second conductive 
10.sup.17 -10.sup.19 
10.sup.-1 -10.sup.-3 
5-10 
layer 10.sup.19 5 .times. 10.sup.-3 
5 
n-Type light- 
10.sup.15 -10.sup.18 0.5-5 
emitting layer 
10.sup.17 2 
______________________________________ 
Note: The upper numerical ranges and the lower values listed in Table 1 
for each layer are preferable ranges and more preferable values, 
respectively. 
According to the invention, the expression that the second conductive layer 
has a width to restrict the path of current flow through the 
light-emitting portion means that the second conductive layer confines the 
current flowing into the light-emitting layer, such that the path of 
current flow through the light-emitting layer is limited to the portion 
thereof immediately above or below the second conductive layer, i.e., to a 
small region having a diameter (e.g. length d.sub.1 in FIG. 1) in the 
widthwise direction (e.g. direction X indicated by arrow B in FIG. 1) 
perpendicular to the direction of height of the second conductive layer 
(e.g. direction Y indicated by arrow A in FIG. 1). 
For example when the device (chip) measures 300 .mu.m.times.300 .mu.m, the 
second conductive layer may be in the form of a solid cylinder having a 
diameter d1 of about 30 to about 100 .mu.m (more preferably 50 .mu.m) as 
seen in FIG. 1, or in the form of an annular high-concentration impurity 
layer having an inside diameter d2 of 15 to 50 .mu.m and an outside 
diameter d3 of 50 to 150 .mu.m as shown in FIG. 2, or may comprise a 
plurality of minute conductive layer portions, each measuring about 10 
.mu.m.times.10 .mu.m, having a rectangular section and arranged laterally 
in a row or in the form of a two-dimensional matrix. 
According to the invention, epitaxial compound semiconductor layers are 
formed one over another on the compound semiconductor substrate to form 
the body of an electroluminescent device. To be suitable, the device body 
usually measures 50 to 500 .mu.m in width, 80 to 800 .mu.m in length and 2 
to 15 .mu.m in thickness. The device body may be formed singly on the 
substrate, or a multiplicity of device bodies can be formed in a scattered 
arrangement to provide a monolithic electroluminescent display or a 
large-area electroluminescent display. FIG. 9 shows a monolithic 
electroluminescent display including compound semiconductor 
electroluminescent devices 58 each having an insulating ZnSe substrate 51, 
an electroluminescent device body formed on the substrate 51 and 
comprising a p-type ZnSe conductive layer 52, a p-type ZnSe light-emitting 
layer 53, an n-type ZnSe light-emitting layer 54 and an n-type ZnSe 
conductive layer 55 (a conductive layer portion provided in the conductive 
layer 55 and having a higher n-type impurity concentration than the layer 
55 is not shown, but is the same as described hereinafter) which are 
formed one over another, an Au positive electrode 56 being provided on the 
conductive layer 52, and an In negative electrode 57 being provided on the 
conductive layer 55 forming the uppermost layer of the device body. 
Indicated at 56a and 57a are lead wires. The device 58 on the substrate 
measures 450 .mu.m in width H and 750 .mu.m in length M. The display has 
24.times.24 electroluminescent devices as arranged separately from one 
another in the form of a matrix. An arrangement of such displays provides 
a large-area electroluminescent display apparatus. 
FIG. 5 shows an MIS-type electroluminescent device 60 having a 
high-resistance layer formed on a light-emitting layer. More specifically, 
the device 60 comprises an insulating ZnS substrate 61, a device body 
formed on the substrate and having an n-type ZnS conductive layer 62 and 
an n-type ZnS light-emitting layer 63 on the layer 62, a ZnS 
high-resistance layer 64 formed on the light-emitting layer 63, an Au 
positive electrode 65 provided on the layer 64, and an In negative 
electrode 66 provided on the conductive layer 62. Indicated at 65a and 66a 
are lead wires. 
FIG. 6 shows an electroluminescent device 90 of the p-n junction type. The 
device 90 comprises an insulating ZnSe substrate 91, a device body formed 
on the substrate and comprising an n-type ZnSe conductive layer 92, an 
n-type ZnSe light-emitting layer 93, a p-type ZnSe light-emitting layer 94 
and a p-type ZnSe conductive layer 95 which are formed one over another, 
an Au positive electrode 96 provided on the conductive layer 95, and an In 
negative electrode 97 provided on the conductive layer 92. Indicated at 
96a and 97a are lead wires. 
With the devices shown in FIGS. 5 and 6, the electrodes 66, 65 or 97, 96, 
like the electrodes 56, 57 of FIG. 9, are arranged respectively in an 
electrode forming space on the conductive layer and on the uppermost layer 
of the device body (on the conductive layer 55 in FIG. 9, on the 
high-resistance layer 64 in FIG. 5, or on the conductive layer 95 in FIG. 
6) for applying a voltage therethrough to the device body. 
When the electrodes are thus provided on the device body, the voltage which 
would otherwise be applied via the substrate can be efficiently applied to 
the device body. This precludes loss due to the substrate and results in 
the following advantages. 
(1) The reduced resistance between the electrodes makes it possible to 
provide a compound semiconductor electroluminescent device free of such 
loss and having high brightness. 
(2) Monolithic electroluminescent displays and large-area 
electroluminescent displays can be provided which comprise a multiplicity 
of minute electroluminescent chips arranged on a substrate with 
satisfactory insulation between the chips and reduced series resistance 
for producing multicolor light including blue light. 
Such a compound semiconductor electroluminescent device is fabricated by 
placing a mask having a specified rectangular aperture on a 
high-resistance compound semiconductor substrate, for example of 10.sup.6 
to 10.sup.15 ohm-cm, forming a low-resistance semiconductor conductive 
layer as the lowermost layer first on the substrate, and forming a 
compound semiconductor light-emitting layer, and a compound semiconductor 
high-resistance layer or a compound semiconductor conductive layer as 
superposed layers on the resulting conductive layer except at an electrode 
forming site. The layers are formed, for example, by MBE. Depending on how 
the mask is handled, the device is prepared in this way by one of the 
following two processes. 
The first of the processes comprises placing a mask of thin metal film 
having at least one specified rectangular aperture on the compound 
semiconductor substrate, epitaxially growing a compound semiconductor 
conductive layer first as the lowermost layer, then shifting the mask so 
as to cover the electrode forming site on the layer with the mask, forming 
a compound semiconductor light-emitting layer on the conductive layer, 
further epitaxially growing a compound semiconductor high-resistance layer 
or compound semiconductor conductive layer on the light-emitting layer to 
obtain a device body, removing the mask and thereafter providing 
electrodes at the electrode forming sites on the lowermost conductive 
layer and on the surface of the uppermost high-resistance layer or 
conductive layer. 
The second process comprises placing a mask of thin metal film having at 
least one specified rectangular aperture over the compound semiconductor 
substrate at a specified distance (which is usually 10 to 500 .mu.m to be 
suitable) away therefrom, epitaxially growing a compound semiconductor 
conductive layer first, then altering the angle of inclination of the 
substrate provided with the mask (generally suitably by 5 to 45 degrees) 
with respect to the direction of projection of the molecular beam to mask 
an electrode forming site on the conductive layer, forming a compound 
semiconductor light-emitting layer on the conductive layer, further 
epitaxially growing a compound semiconductor high-resistance layer or 
compound semiconductor conductive layer on the resulting layer to obtain a 
device body, removing the mask and providing electrodes at the electrode 
forming sites on the lowermost conductive layer and on the surface of the 
uppermost high-resistance layer or conductive layer. 
The electroluminescent device body thus obtained comprises epitaxial 
compound semiconductor layers which are all identical in shape when seen 
from above. It is suitable that the electrode forming site on the 
lowermost layer be usually 5 to 80% of the surface of the lowermost layer 
in area. 
The processes described above are very useful for epitaxially forming 
compound semiconductor layers. Moreover, the use of a high-resistance 
compound semiconductor, especially bulk single crystal, as is for the 
compound semiconductor substrate as already described and shown in FIGS. 
9, 5 and 6 facilitates separation of unit electroluminescent devices which 
are to be obtained collectively in the form of a single chip. 
Finally, FIGS. 7 and 8 show a ZnS multicolor electroluminescent device of 
the MIS type similar to the one shown in FIG. 5. With reference to FIGS. 7 
and 8, the device comprises a semi-insulating (high-resistance) ZnS 
substrate 31, a low-resistance n-type epitaxial ZnS conductive layer 32, 
low-resistance n-type epitaxial ZnS light-emitting layers 33a, 33b, 33c, 
hole injecting epitaxial ZnS high-resistance layers 34a, 34b, 34c, an 
ohmic metal (In) electrode 35 formed on the conductive layer 32, metal 
(Au) electrodes 36a, 36b, 36c formed on the respective high-resistance 
layers 34a, 34b, 34c, and metal lead wires 37, 38a, 38b, 38c. This device 
is prepared by placing a mask of thin metal plate having a rectangular 
aperture on the semi-insulating (high-resistance) ZnS substrate 31, 
growing the conductive layer 32 by MBE, thereafter growing the 
light-emitting layer 33a and the high-resistance layer 34a in overlapping 
relation with the conductive layer 32 by MBE with the mask shifted to form 
a light-emitting portion A, similarly forming light-emitting portions B 
and C in succession with the mask further shifted, and providing the 
electrodes 35, 36a, 36b and 36c. The device thus formed includes the three 
light-emitting portions A, B and C having the conductive layer 32 in 
common. The light-emitting layers of the respective light-emitting 
portions have added thereto as impurities Al and Ag at about one-tenth the 
concentration of Al, Al and Cu at about one-tenth the concentration of Al, 
and Al and cadmium (Cd) at one-half the concentration of Al, respectively. 
When voltage was applied to the respective light-emitting portions of the 
device obtained, blue, green and red luminescences were observed with high 
brightness. Multicolor light was produced by controlling the voltage 
applied to the three light-emitting portions. 
According to the present invention, the second conductive layer is formed 
during the growth of the first conductive layer by locally irradiating the 
grown layer, for example with an ArF excimer laser beam at a wavelength of 
193 nm, whereby impurities are added at a high concentration to the 
irradiated portion of the grown layer, consequently forming a region 
having an impurity concentration which is greater than that of the 
non-irradiated portion by about one order of magnitude. This region 
provides the second conductive layer. 
With reference to FIG. 1, indicated at 1 is a low-resistance n-type ZnS 
substrate having a resistivity of 1 ohm-cm, at 2 a first conductive layer 
of low-resistance n-type ZnS, at 3 a second conductive layer of 
low-resistance n-type ZnS having a higher impurity concentration than the 
first conductive layer 2, at 4 an n-type ZnS light-emitting layer, at 5 a 
hole injecting insulating layer of insulating ZnS, at 7 a positive 
electrode and at 8 a negative electrode. 
The first conductive layer 2, the light-emitting layer 4 and the hole 
injecting insulating layer 5 of this device are single-crystal 
semiconductor layers which are epitaxially grown one over another on the 
substrate 1 by MBE. 
The second conductive layer 3 having a higher impurity concentration than 
the first conductive layer 2 is formed during the growth of the first 
conductive layer 2 by locally irradiating the surface of the layer 2 being 
grown with a beam (for example an ArF excimer laser beam at a wavelength 
of 193 nm). When the layer is grown while being irradiated with light, 
impurity elements are added to the irradiated portion to a high 
concentration, whereby a region is formed with an impurity concentration 
which is about one order of magnitude higher than that of the 
non-irradiated portion. The first conductive layer 2 was 5 .mu.m in 
thickness and 10.sup.18 cm.sup.-3 in impurity concentration. The second 
conductive layer 3 was formed over almost the entire thickness of the 
first conductive layer 2 and was given by the above growth method an 
impurity concentration of about 10.sup.19 cm.sup.-3, i.e. about 10 times 
that of the first conductive layer 2. In this case, the first conductive 
layer 2 was 5.times.10.sup.-2 ohm-cm, and the second conductive layer 
3.5.times.10.sup.-3 ohm-cm in resistivity. 
For the second conductive layer 3 to fully confine the current, the layer 3 
is made to have a diameter d1 of about 50 .mu.m when the device (chip) is, 
for example, 300 .mu.m.times.300 .mu.m in size. The light-emitting layer 4 
was given a thickness of 2 .mu.m and an impurity concentration of 
10.sup.17 cm.sup.--3. The hole injecting insulating layer 5 was formed to 
a thickness of 20 to 700 angstroms. 
The hole injecting insulating layer 5 was prepared from insulating ZnS 
which was grown in the absence of any impurity element or with the 
addition of both the above-mentioned n-type impurity elements and a p-type 
impurity element from Group V such as nitrogen (N), phosphorus (P), 
arsenic (As) or antimony (Sb). 
The positive electrode 7 was formed by depositing gold (Au) on the 
insulating layer 5 at the position immediately above the second conductive 
layer 3 to a thickness of 500 to 1000 angstroms by vacuum evaporation. The 
negative electrode 8 is an ohmic electrode formed by coating the entire 
rear surface of the substrate 1 with indium-mercury (In-Hg) and heating 
the coating in high-purity hydrogen (H.sub.2) at about 400.degree. C. for 
30 seconds. 
With the ZnS electroluminescent device of the MIS type described above, the 
path of current flow through the first conductive layer 2 is restricted to 
the second conductive layer 3 having a higher impurity concentration and 
lower resistivity than the layer 2, permitting injection of the current 
into the light-emitting layer 4 at a high density. A blue luminescence 
with 5 to 10 times the brightness achieved by conventional MIS-type ZnS 
electroluminescent devices was observed through the positive electrode 7, 
insulating layer 5 and device end face. 
FIG. 2 shows another ZnS electroluminescent device of the MIS type as a 
second embodiment which has substantially the same structure as the above 
embodiment except that the second conductive layer of high impurity 
concentration is in the form of a ring. With reference to the drawing, the 
device includes a substrate 1, first conductive layer 2, light-emitting 
layer 4 and insulating layer 5 which are similar to those of the above 
embodiment. The second conductive layer, indicated at 13, is an annular 
layer having a high impurity concentration, measuring 30 .mu.m in inside 
diameter d2 and 100 .mu.m in outside diameter d3, and formed by 
irradiating the first conductive layer 2 with an ArF laser beam in the 
form of a ring while the layer 2 is being grown. This embodiment is 
generally the same as the first embodiment described above in respect of 
the thicknesses of the layers and impurity concentration. As the positive 
electrode 7, an Au electrode, 30 to 100 .mu.m in diameter, was formed by 
vacuum evaporation on the insulating layer 5 at the position immediately 
above the second conductive layer 13. An ohmic electrode of In-Hg was 
formed in an annular shape on the rear side of the substrate 1 as the 
negative electrode 8. 
The path of current flow into the light-emitting layer 4 is restricted by 
the second conductive layer 13 also in this case to give a blue 
luminescence with high brightness. Since the second conductive layer 13 
with a high impurity concentration is lower than the first conductive 
layer 2 in refractive index, the light produced by the light-emitting 
portion positioned between the second conductive layer 13 and the positive 
electrode 7 on the light-emitting layer 4 was confined in the portion of 
the first conductive layer 2 surrounded by the annular second conductive 
layer 13 and having a higher refractive index. Consequently, the light was 
delivered efficiently from the device through the rear surface of the 
substrate 1 without leaking from the end faces of the device. 
Thus, the present embodiment realizes an MIS-type ZnS electroluminescent 
device which delivers light with a high efficiency and high brightness. 
FIG. 3 shows another electroluminescent device as a third embodiment of the 
invention. With reference to the drawing, indicated at 21 is an insulating 
ZnSe substrate, at 22 a first conductive layer of low-resistance n-type 
ZnSe, at 23 a second conductive layer of n-type ZnSe having a higher 
impurity concentration than the first conductive layer 22, at 24 an n-type 
ZnSe light-emitting layer, at 25 a p-type ZnSe light-emitting layer, at 26 
a conductive layer of low-resistance p-type ZnSe, at 7 a positive 
electrode, and at 8 a negative electrode. 
The insulating ZnSe substrate 21 used for the device was prepared from a 
ZnSe bulk single crystal grown by the halogen chemical transport process, 
sublimation process or high-pressure melting process without treating the 
crystal for a reduction of resistance. The semiconductor layers were 
epitaxially grown on the substrate 21 in succession by MBE. The conductive 
layers 22, 23 and light-emitting layer 24 of n-type ZnSe were generally 
the same as those of the first embodiment in shape, size and 
characteristics. Over the layer 24 were formed the light-emitting layer 25 
of p-type ZnSe with a thickness of 2 .mu.m and hole concentration of 
5.times.10.sup.16 cm.sup.-3, and the conductive layer 26 of low-resistance 
p-type ZnSe having a thickness of 5 .mu.m and hole concentration of 
5.times.10.sup.17 cm.sup.-3. The positive electrode 7 was formed by 
depositing Au on an end portion of the p-type ZnSe conductive layer 26 by 
vacuum evaporation. The negative electrode 8 was formed by locally 
removing the grown layers, for example by chemical etching or reactive ion 
beam etching (RIE) and depositing In by vacuum evaporation on the exposed 
surface of the first conductive layer 22 of n-type ZnSe to obtain a ZnSe 
p-n junction electroluminescent device having a planar structure. 
Examples of p-type impurity elements useful for the light-emitting layer 25 
and the conductive layer 26 which are made of p-type ZnSe are elements 
from Group Ia such as lithium (Li), sodium (Na) and potassium (K), copper 
(Cu), silver (Ag) and gold (Au) from Group Ib, thallium (Tl) in Group III, 
elements in Group V such as nitrogen (N), phosphorus (P), arsenic (As), 
antimony (Sb) and bismuth (Bi), etc. 
With the positive electrode 7 formed at the end of the p-type ZnSe 
conductive layer 26, light was efficiently delivered from the upper 
portion of the device. When the substrate 21 used was made of a colorless 
transparent insulating ZnSe single crystal grown by the sublimation 
process, it was possible to deliver the light also from the substrate side 
with high brightness. 
In the case of the present device also, the path of current flow through 
the light-emitting layers 24, 25 is restricted to the portion thereof 
immediately above the second conductive layer 23 having a high impurity 
concentration and low resistivity. The device produced light with 
brightness 5 to 10 times as high as is the case with conventional devices 
at approximately the same current value. 
The planar structure including the insulating substrate 21 obviates the 
need for low-resistance treatment of the substrate crystal, greatly 
simplifies the device fabrication process and decreases the device 
resistance to diminish the loss involved. 
The present embodiment realizes high-brightness p-n junction 
electroluminescent devices. 
FIG. 4 shows another ZnSe p-n junction electroluminescent device as a 
fourth embodiment of the invention. 
Basically, this embodiment has the same construction as the third 
embodiment except that the first conductive layer 22 of low-resistance 
n-type ZnSe has formed therein a plurality of second conductive layers 23 
made of low-resistance n-type ZnSe having a higher impurity concentration 
than the first conductive layer 22. 
In this case, the flow of current is restricted at the positions of the 
individual second conductive layers 23 to produce light with high 
brightness. A plurality of rectangular second conductive layers 23, as 
small as about 10 .mu.m.times.10 .mu.m, were provided in the desired 
arrangement, whereby a high-brightness luminescence was obtained in a 
pattern corresponding to the arrangement of the second conductive layers 
23. 
Using the p-n junction electroluminescent device of monolithic structure 
according to the present embodiment, it was possible to produce light with 
high brightness in a desired configuration by altering the distribution of 
second conductive layers having different impurity concentrations. 
According to the first to fourth embodiments described above in detail, the 
semiconductor conductive layer of the p-n junction or MIS 
electroluminescent device comprises conductive layer portions with 
different impurity concentrations, so that the current through the device 
is confined in the conductive layer portion having the higher impurity 
concentration and lower resistivity, permitting injection of the current 
into the light-emitting layer at a high density. Furthermore, a difference 
in the impurity concentration produces a difference in the refractive 
index of the conductive layer. As will be apparent from the foregoing 
second and fourth embodiment, therefore, the light produced by the light 
emitting layer can be efficiently delivered from the device by confining 
the light in the first conductive layer having a higher refractive index 
than the second conductive layer when these layers are suitably desired. 
This realizes compound semiconductor electroluminescent devices which are 
adapted to deliver light with high efficiency and high brightness and 
which are very useful as light sources for various optoelectronic 
apparatus such as high-brightness blue electroluminescent displays.