Light emitting diode

A light emitting diode includes a substrate, a light emitting layer, a first cladding layer having a first conductivity type and an energy gap greater than an energy gap of the light emitting layer, a second cladding layer having a second conductivity type and an energy gap greater than an energy gap of the light emitting layer, and an intermediate barrier layer having the same conductivity type as the conductivity type of the light emitting layer but different from the conductivity type of the first or second cladding layer, and having an energy gap less than the energy gap of the first or second cladding layer but greater than the energy gap of the light emitting layer. The light emitting diode has a double heterostructure such that the light emitting layer is interposed between the first and second cladding layer. The intermediate barrier layer is disposed between the light emitting layer and the first cladding layer and/or between the light emitting layer and the second cladding layer.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a light emitting diode (hereinafter
 referred to as an "LED") having a double heterojunction structure. More
 particularly, the present invention relates to a technique for preventing
 a reduction in a light output of an LED for long-time operation.
 2. Description of the Related Art
 An LED having a so-called double heterostructure has a high level of light
 emission efficiency and a high light output and therefore is widely used
 for a display, a light source of optical communications, or the like.
 FIG. 12 is a cross-sectional view illustrating a conventional LED 800
 having a typical double heterostructure. The LED 800 is an InGaAlP based
 LED which includes layers having lattice match with a GaAs substrate and
 emits light ranging from red light to green light. In the LED 800,
 a substrate 1: made of n-type GaAs;
 a first buffer layer 2: made of n-type GaAs;
 a light reflection (DBR:Distributed Bragg Reflector) layer 3: including
 n-type (Al.sub.0.4 Ga.sub.0.6).sub.0.5 In.sub.0.5 P layers and n-type
 Al.sub.0.5 In.sub.0.5 P layers deposited in an alternative fashion;
 a first cladding layer 4: made of n-type Al.sub.0.5 In.sub.0.5 P, doped
 with Si at an impurity concentration of 5.times.10.sup.17 cm.sup.-3, 1
 .mu.m thick;
 a light emitting layer 6: made of p-type (Ga.sub.0.7 Al.sub.0.9).sub.0.5
 In.sub.0.5 P, 0.5 .mu.m thick;
 a second cladding layer 7: made of p-type Al.sub.0.5 In.sub.0.5 P, doped
 with Zn at an impurity concentration of 5.times.10.sup.17 cm.sup.-3, 1
 .mu.m thick;
 a first current diffusion layer 91: made of p-type Al.sub.0.7 Ga.sub.0.3
 Al, doped with Zn at an impurity concentration of 1.times.10.sup.18
 cm.sup.-3, 1 .mu.m thick; and
 a second current diffusion layer 92: made of p-type Al.sub.0.7 Ga.sub.0.3
 As, doped with Zn at an impurity concentration of 3.times.10.sup.18
 cm.sup.-3, 6 .mu.m thick;
 are deposited in this order.
 The first and second current diffusion layers 91 and 92 constitute a
 current diffusion layer 9.
 A film of AuGe is provided as an n-side electrode 11 on a lower surface of
 the substrate 1 by a typical deposition method. A film of AuZn is provided
 on a upper surface of the p-type current diffusion layer 9 by the same
 deposition method. The AuZn film is subjected to photolithography
 patterning so as to remain a circular portion thereof as a p-side
 electrode 10 to which a metal wire is bonded for connecting the p-side
 electrode 10 to an external conductor. Light generated in the light
 emitting layer 6 is radiated from a portion of the upper surface of the
 p-type current diffusion layer 9 from which the AuZn film has been
 removed.
 The first buffer layer 2 is used for preventing defects and contaminants of
 the substrate 1 from effecting the layers deposited the substrate 1. The
 first buffer layer 2 is not required when the substrate 1 has a
 satisfactorily treated upper surface. The DBR layer 3 reflects light
 generated in the light emitting layer 6 toward the substrate 1. This
 prevents light absorption by the substrate 1 and the reflected light goes
 in a direction away from the substrate 1, contributing to the brightness
 of the LED 800.
 The current diffusion layer 9 has low resistivity so as to make an
 approximate ohmic contact with the p-side electrode 10 and also to diffuse
 a current injected from the p-side electrode 10 into the entire light
 emitting layer 6. This is why the current diffusion layer 9 requires a
 high level of impurity concentration, in this case, to prevent impurity Zn
 from diffusing into the light emitting layer 6, the first current
 diffusion layer 91 having a low impurity concentration is provided in the
 lower part of the current diffusion layer 9.
 To obtain a high level of light emission efficiency, a conventional LED
 adopts a double heterostructure as shown in FIG. 15. FIG. 15 is a
 cross-sectional view illustrating an example of an AlGaInP based LED 900
 which have lattice match with GaAs substrate 101. A structure of each
 layer in the LED 900 is as follows:
 a substrate 101: made of n-type GaAs;
 a buffer layer 102: made of n-type GaAs;
 an n-type first cladding layer 103: made of n-type (Ga.sub.0.3
 Al.sub.0.7).sub.0.5 In.sub.0.5 P, doped with Si at an impurity
 concentration of 1.times.10.sup.18 cm.sup.-3, 1 .mu.m thick;
 a light emitting layer 104: made of p-type (Ga.sub.0.7 Al.sub.0.3).sub.0.5
 In.sub.0.5 P, 0.5 .mu.m thick;
 a p-type second cladding layer 105: made of p-type Al.sub.0.5 In.sub.0.5 P,
 doped with Zn at an impurity concentration of 5.times.10.sup.17 cm.sup.-3,
 1 .mu.m thick;
 a first current diffusion layer 61: made of p-type Ga.sub.0.3 Al.sub.0.7
 Al, doped with Zn at an impurity concentration of 1.times.10.sup.18
 cm.sup.-3, 1 .mu.m thick;
 a second current diffusion layer 62: made of p-type Ga.sub.0.3 Al.sub.0.7
 As, doped with Zn at an impurity concentration of 3.times.10.sup.18
 cm.sup.-3, 6 .mu.m thick; and
 a contact layer 108: made of p-type GaAs.
 An n-side electrode 109 and a p-side electrode 107 are provided on the
 substrate 1 and the contact layer 108, respectively.
 The AlGaInP based LED 800 in FIG. 12 generates light by injecting a
 current. In FIG. 13, a dashed line A indicates a relationship between an
 impurity concentration of the light emitting layer 6 and a light output in
 an initial period after starting light emission. The peak of the light
 output is at an impurity concentration of 1.times.10.sup.17 cm.sup.-3 in
 the initial period after starting light emission. However, the light
 output gradually decreases with time. For example, a current of 50 mA is
 supplied to the LED 800 for 1000 hours at room temperature. In FIG. 13, a
 dashed line B indicates a relationship between an impurity concentration
 of the light emitting layer 6 and a light output after the 1000-hour light
 emission. A light output after the 1000-hour light emission becomes lower
 at an impurity concentration of 1.times.10.sup.17 cm.sup.-3 while a light
 output becomes higher at an impurity concentration of 5.times.10.sup.17
 cm.sup.-3 where the light output is maximum, which is different from in
 the initial period after starting light emission.
 Our studies have found that such a change in a light output after long-time
 light emission is caused by: (1) a non-radiative recombination center
 generated at a pn junction interface between the n-type first cladding
 layer 4 and the p-type light emitting layer 6; and (2) an influence from
 diffused impurities in the light emitting layer 6.
 FIGS. 14A and 14B illustrate states of energy bands of around the light
 emitting layer 6. FIG. 14A shows a state in the initial period after
 starting light emission, while FIG. 14B shows a state after the long-time
 light emission.
 The pn junction interface 40 is a heterointerface where two layers having
 largely different energy gaps As shown in FIG. 14A make contact with each
 other. There is a large internal stress at the heterointerface 40. When a
 voltage is applied between the p-side electrode 10 and the n-side
 electrode 11 in order to generate light, a high electric field level is
 applied across the heterointerface 40.
 The combination of the internal stress and the energy of light generated in
 the light emitting layer 6 causes a lattice defect at the heterointerface
 40. This lattice defect grows along the direction of the electric field
 line into the light emitting layer 6 over the long-time light emission.
 The lattice defeat leads to formation of a deep energy level 20 in the
 vicinity of the heterointerface 40 as shown in FIG. 14B. The carriers, a
 hole and an electron, combine together at the deep energy level without
 emitting light. Such a deep energy level is called a non-radiative energy
 level. Since radiative recombination 30 of the LED 800 is a spontaneous
 emission process, the non-radiative recombination 31 at the non-radiative
 energy level 20 has a shorter lifetime than that of the radiative
 recombination 30. Therefore, when the number of carriers combining at the
 non-radiative energy level 20 is increased, the light emission efficiency
 of the LED 800 decreases.
 Long-time light emission continues to cause the growth of the lattice
 defect which becomes widespread inside the light emitting layer 6. In
 other words, the light emitting layer 6 develops a lot of portions having
 the non-radiative energy level 20. Therefore, the light emission
 efficiency of the LED 800 is further decreased, i.e., the light output of
 the LED 800 is reduced compared with in the initial period of the light
 emission.
 Japanese Laid-Open Publication No. 2-151085 discloses a semiconductor light
 emitting device (hereinafter referred to as a "LED of conventional Example
 2") having structure similar to that shown in FIG. 12. The LED of
 conventional Example 2 includes an intermediate cladding layers interposed
 between the light emitting layer 6 and the first and second cladding
 layers 4 and 7. The intermediate cladding layers each have a thickness of
 more than about 10 .ANG. and less than about 200 .ANG., and an energy gap
 having a value between those of the light emitting layer 6 and the first
 and second cladding layers 4 and 7. In the LED of conventional Example 2,
 heterointerfaces are formed between the intermediate cladding layer and
 the first and second cladding layers 4 and 7, and between the intermediate
 cladding layer and the light emitting layer 6. Accordingly, the
 differences in an energy gap at the interfaces can be decreased, thereby
 reducing the internal stress. This, therefore, creates difficulty for a
 lattice defect to be generated and thus there are less non-radiative
 recombination centers in the light emitting layer 6.
 In the LED of conventional Example 2, however, a pn junction is formed at
 the interface between the light emitting layer 6 and the intermediate
 layer. A lattice defect due to light emission is generated at the
 interface where a high electric field level exists. Although a decrease in
 a light output of the LED of conventional Example 2 is effectively
 delayed, long-time light emission allows a lattice defect generated at the
 interface to develop. The growth of the lattice defect decreases a light
 output of the light emitting layer 6.
 As described in FIG. 13, after the long-time light emission, the light
 emitting layer having a higher concentration of impurity will have a
 higher light output. This phenomenon will now be described. When the light
 emitting layer 6 has a higher concentration of impurity than an optimal
 concentration, the resistivity of the light emitting layer 6 becomes low.
 Therefore, an electric field applied across the pn junction interface
 between the first cladding layer 4 and the light emitting layer 6 becomes
 small in extent, resulting in a low light output in an initial period
 after starting light emission. After long-time light emission, extra
 impurities are diffused in the light emitting layer 6 because of the
 electric field and heat generated in the vicinity of the light emitting
 layer 6. The diffusion of impurities increases the electric field and
 therefore the light output. In this case, a defect is also generated at
 the pn junction interface, and therefore, the light emission efficiency
 decreases after the long-time light emission.
 In the LED 900 shown in FIG. 15, the buffer layer 102 is used to shield the
 influence of defects and contaminants of the substrate 101. The buffer
 layer 102 is not necessary when surface treatment of the substrate 101 is
 satisfactory. The contact layer 108 is made of GaAs, which does not
 contain Al, in order to facilitate ohmic contact with the p-side electrode
 107. The contact layer 108 does not allow light generated by the light
 emitting layer 104 to pass therethrough. However, the contact layer 108 is
 provided directly under the electrode 107, adding no disadvantage to light
 radiation.
 In the LED 900 shown in FIG. 15, energy gaps of the light emitting layer
 104 and the first and second cladding layers 103 and 105 are set by a
 molar fraction of Al. The lattice constant of a III-V compound
 semiconductor is almost not variable when Al is replaced with Ga or vice
 versa. The greater the molar fraction of Al that is included, the greater
 the energy gap of the compound semiconductor. Hereinafter, the proportion
 of Al in the total amount of Al and Ga in a mixed crystal is regarded as a
 molar fraction of Al in the mixed crystal.
 To obtain a high light output of the LED 900, it is required to
 satisfactorily confine carriers within the light emitting layer 104 by
 making differences between the energy gaps of the light emitting layer 104
 and the first and second cladding layers 103 and 105 sufficiently great.
 The LED 900 has a double heterostructure in which the (Ga.sub.0.7
 Al.sub.0.3).sub.0.5 In.sub.0.5 P light emitting layer 104 is interposed
 between the n-type (Ga.sub.0.3 Al.sub.0.7).sub.0.5 In.sub.0.5 P first
 cladding layer 103 and the p-type (Ga.sub.0.3 Al.sub.0.7).sub.0.5
 In.sub.0.5 P second cladding layer 105 which have great energy gaps. A
 molar fraction of Al of the light emitting layer 104 is 0.3 while both of
 molar fractions of Al of the first and second cladding layers 103 and 105
 are 0.7.
 To obtain a high light output of the LED 900, diffusion of carriers
 injected from the electrode 107 into the entire light emitting layer 104
 is required. To this end, a decrease in the resistivity of the current
 diffusion layer 106 by increasing an impurity concentration of the current
 diffusion layer 106 to a sufficient high level is required. The substrate
 101 is typically made of an n-type semiconductor, so that a p-type
 semiconductor is used for the current diffusion layer 106. However, an
 impurity for a p-type semiconductor, such as Zn or Mg, is likely to
 diffuse. An interface between the layers having impurity concentrations
 largely different from each other has a high impurity concentration
 gradient. Therefore, in the interface, the impurity is likely to diffuse
 due to interaction of electrical energy with light energy generated by the
 light emitting layer 104.
 For example, the current diffusion layer 106 and p-type second cladding
 layer 105, as well as the p-type second cladding layer 105 and light
 emitting layer 104, have the above-described relationship therebetween.
 Therefore, impurity diffusion is likely to take place between the current
 diffusion layer 106 and p-type second cladding layer 105 and between the
 p-type second cladding layer 105 and light emitting layer 104.
 Even when the light emitting layer 104 initially has an optimal
 concentration of a p-type impurity, the concentration changes due to
 diffusion of the impurity and therefore the light emission efficiency of
 the light emitting layer 104 is decreased. Further, the p-type impurity
 entering the light emitting layer 104 by diffusion is unlikely to settle
 into a normal position of lattice, becoming a non-radiative recombination
 center which has a deep energy level.
 In the conventional LED 900 shown in FIG. 15, the current diffusion layer
 106 includes two layers. The lower layer is a first current diffusion
 layer 61 having a low impurity concentration. Therefore, the impurity
 concentration gradient between the light emitting layer 104 and the first
 current diffusion layer 61 becomes a low value, whereby diffusion of Zn is
 prevented. The first current diffusion layer 61 and the second current
 diffusion layer 62 has the same molar fraction of Al.
 Conventionally, molar fractions of Al of the first and second cladding
 layers 103 and 105 are about 0.7. The inventors have found that the
 above-described conventional technique is insufficient to prevent impurity
 diffusion when the molar fractions of Al of the first and second cladding
 layers 103 and 105 are increased up to about 1.0 in order to enhance
 carrier confinement and obtain a higher light output of the LED 900. In
 other words, the above-described p-type impurity diffusion is significant
 when a molar fraction of Al is great.
 SUMMARY OF THE INVENTION
 A light emitting diode according to the present invention includes a
 substrate; a light emitting layer: a first cladding layer having a first
 conductivity type and an energy gap greater than an energy gap of the
 light emitting layer; a second cladding layer having a second conductivity
 type and an energy gap greater than an energy gap of the light emitting
 layer; and an intermediate barrier layer having the same conductivity type
 an the conductivity type of the light emitting layer but different from
 the conductivity type of the first or second cladding layer, and having an
 energy gap less than the energy gap of the first or second cladding layer
 but greater than the energy gap of the light emitting layer. The light
 emitting diode has a double heterostructure such that the light emitting
 layer is interposed between the first and second cladding layer. The
 intermediate barrier layer is disposed between the light emitting layer
 and the first cladding layer and/or between the light emitting layer and
 the second cladding layer.
 Therefore, a crystal defect generated at a pn junction is prevented from
 affecting a light emitting layer, thereby realizing the LED in which a
 reduction in a light output is prevented even after long-time light
 emission.
 In one embodiment of the present invention, a thickness of the intermediate
 barrier layer is less than a diffusion length of a minority carrier in the
 intermediate barrier layer and is greater than a value such that a
 non-radiative recombination center generated at an interface between the
 intermediate barrier layer and the first or second cladding layer has
 substantially no influence on the light emitting layer.
 In one embodiment of the present invention, a thickness of the intermediate
 barrier layer is in the range of 0.1 .mu.m or more and 0.5 .mu.m or less.
 Therefore, a crystal defect generated at a pn junction is prevented from
 affecting a light emitting layer so that a reduction in light emission
 efficiency is prevented, thereby realizing the LED in which a reduction in
 a light output is prevented even after long-time light emission.
 In one embodiment of the present invention, the energy gap of the
 intermediate barrier layer is greater by 0.2 eV or more than the energy
 gap of the light emitting layer.
 Therefore, non-radiative recombination in the intermediate barrier layer is
 further decreased, thereby realizing the LED having a high level of light
 emission efficiency.
 In one embodiment of the present invention, the intermediate barrier layer
 is an indirect transition type semiconductor layer having a long
 non-radiative recombination lifetime.
 Therefore, non-radiative recombination in the intermediate barrier layer is
 substantially eliminated, thereby realizing the LED having a high level of
 light emission efficiency.
 In one embodiment of the present invention, the intermediate barrier layer
 includes first and second intermediate barrier layers. The first
 intermediate barrier layer is provided between the light emitting layer
 and the first cladding layer. The second intermediate barrier layer is
 provided between the light emitting layer and the second cladding layer.
 The first intermediate barrier layer having the same conductivity type as
 the conductivity type of the light emitting layer but different from the
 conductivity type of the first cladding layer adjacent to the first
 intermediate barrier layer, and having an energy gap less than the energy
 gap of the first cladding layer but greater than an energy gap of the
 light emitting layer. The second intermediate barrier layer having the
 same conductivity type as the conductivity type of the light emitting
 layer and the conductivity type of the second cladding layer adjacent to
 the second intermediate barrier layer, and having an energy gap less than
 the energy gap of the second cladding layer but greater than the energy
 gap of the light emitting layer.
 Therefore, a crystal defect generated at a pn junction is prevented from
 affecting a light emitting layer so that a reduction in light emission
 efficiency is prevented. Further, a p-type impurity which tends to be
 easily diffused is prevented from diffusing into the light emitting layer,
 thereby preventing a reduction in light emission efficiency.
 In one embodiment of the present invention, the substrate is made of GaAs
 the first cladding layer is made of (Ga.sub.1-x2 Al.sub.x2).sub.0.5
 In.sub.0.5 P (x1&lt;x2.ltoreq.1); the light emitting layer is made of a
 (Ga.sub.1-x1 Al.sub.x1).sub.0.5 In.sub.0.5 P (0.ltoreq.x1&lt;1); the
 intermediate barrier layer is made of (Ga.sub.1-x4 Al.sub.x4).sub.0.5
 In.sub.0.5 P (x1&lt;x4&lt;x2, x3); and the second cladding layer is made of
 (Ga.sub.1-x3 Al.sub.x3).sub.0.5 In.sub.0.5 P (x1&lt;x3.ltoreq.1).
 Therefore, a light output is less decreased in a spectrum from red light to
 green light even after long-time light emission.
 According to another aspect of the invention, a light emitting diode
 includes a substrate; a light emitting layer; a p-type cladding layer
 having an energy gap greater than an energy gap of the light emitting
 layer; and an n-type cladding layer having an energy gap greater than the
 energy gap of the light emitting layer. The light emitting diode is made
 of at least III-V compound semiconductor material and has a double
 heterostructure such that the light emitting layer is interposed between
 the p-type and n-type cladding layer. The p-type cladding layer includes a
 p-type second intermediate barrier layer and a p-type second cladding
 layer. The p-type second intermediate barrier layer is nearer the light
 emitting layer than the p-type second cladding layer is. The p-type second
 intermediate barrier layer has a lower molar fraction of Al and A lower
 impurity concentration than a molar fractino of Al and an impurity
 concentration of the p-type second cladding layer, respectively.
 Therefore, even when the LED is a high-intensity LED which includes a
 cladding layer having a high molar fraction of Al, a p-type impurity which
 tends to be easily diffused is prevented from diffusing from the current
 diffusion layer or the p-type second cladding layer into the light
 emitting layer even after long-time light emission, thereby preventing a
 reduction in light emission efficiency.
 In one embodiment of the present invention, a molar fraction of Al of the
 p-type second intermediate barrier layer is 0.5 or less, and a molar
 fraction of Al of the p-type second cladding layer is 0.7 or more.
 Therefore, the crystallinity of the p-type second intermediate barrier
 layer is satisfactorily maintained, thereby making it possible to prevent
 diffusion of an impurity.
 In one embodiment of the present invention, an impurity concentration of
 the p-type second intermediate barrier layer is 3.times.10.sup.17
 cm.sup.-3 or less. A thickness of the p-type second intermediate barrier
 layer is in the range of 0.1 .mu.m or more and 0.5 .mu.m or less.
 Therefore, even when the LED includes the p-type second intermediate
 barrier layer, the p-type second cladding layer maintains a carrier
 confinement effect, thereby realizing a high light output and making it
 possible to maintain a characteristic of the LED at high temperature.
 Further, the p-type second cladding layer has a low impurity concentration
 and therefore absorbs incoming impurities to prevent them from diffusing
 into the light emitting layer.
 In one embodiment of the present invention, the substrate is made of GaAs;
 the n-type first cladding layer is made of (Ga.sub.1-x2 Al.sub.x2).sub.0.5
 In.sub.0.5 P (x1&lt;x2.ltoreq.1); the light emitting layer is made of
 (Ga.sub.1-x1 Al.sub.x1).sub.0.5 In.sub.0.5 P (0.ltoreq.x1&lt;x2, x3); the
 p-type second intermediate barrier layer is made of (Ga.sub.1-x4
 Al.sub.x4).sub.0.5 In.sub.0.5 P (x1&lt;x4&lt;x3, an impurity concentration of
 less than 5.times.10.sup.17 cm.sup.-3); and the p-type second cladding
 layer is made of (Ga.sub.1-x3 Al.sub.x3).sub.0.5 In.sub.0.5 P
 (x1&lt;x3.ltoreq.1, an impurity concentration of 5.times.10.sup.17 cm.sup.-3
 or more).
 Therefore, even when the LED emits light in a spectrum from red light to
 green light at high intensity for a long time, a p-type impurity which
 tends to be easily diffused is prevented from diffusing from the current
 diffusion layer or the p-type second cladding layer into the light
 emitting layer, thereby preventing a reduction in a light output.
 Thus, the invention described herein makes possible the advantages of (1)
 providing an intermediate barrier layer to prevent a defect generated at a
 pn junction interface from penetrating into a light emitting layer; (2)
 satisfactorily confining carriers by increasing a molar fraction of Al in
 a p-type second cladding layer; and therefore (3) providing an LED with a
 high level of reliability in which a reduction in a light output is
 prevented after long-time light emission.
 These and other advantages of the present invention will become apparent to
 those skilled in the art upon reading and understanding the following
 detailed description with reference to the accompanying figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Hereinafter, embodiments of the present invention will be described with
 reference to the accompanying drawings.
 EXAMPLE 1
 FIG. 1 is a cross-sectional view illustrating a structure of an LED 100
 according to Example 1 of the present invention. For the sake of
 simplicity, components having the same functions as those of the LED 800
 are indicated by the same reference numerals as those used therein. The
 LED 100 includes:
 a substrate 1: made of GaAs;
 a first cladding layer 4: made of (Ga.sub.1-x2 Al.sub.x2).sub.0.5
 In.sub.0.5 P (x1&lt;x2.ltoreq.1);
 a light emitting layer 6: made of (Ga.sub.1-x1 Al.sub.x2).sub.0.5
 In.sub.0.5 P (0.ltoreq.x1&lt;1);
 an intermediate barrier layer 5: made of (Ga.sub.1-x4 Al.sub.x4).sub.0.5
 In.sub.0.5 P (x1&lt;x4&lt;x2, x3); and
 a second cladding layer 7: made of (Ga.sub.1-x3 Al.sub.x3).sub.0.5
 In.sub.0.5 P (x1&lt;x3.ltoreq.1).
 With this structure, it is possible to obtain an LED such that a light
 output of the LED is less decreased in a spectrum from red light to green
 light even after long-time light emission.
 More specifically, the LED 100 includes:
 a substrate 1: made of n-type GaAs;
 a first buffer layer 2: made of n-type GaAs;
 a light reflection (DBR) layer 3: including n-type (Al.sub.0.4
 Ga.sub.0.6).sub.0.5 In.sub.0.5 P layers and n-type Al.sub.0.5 In.sub.0.5 P
 layers deposited in an alternatively fashion;
 a first cladding layer 4: made of n-type Al.sub.0.5 In.sub.0.5 P, doped
 with Si at an impurity concentration of 5.times.10.sup.17 cm.sup.-3, 1
 .mu.m thick;
 an intermediate barrier layer 5: made of p-type (Ga.sub.0.5
 Al.sub.0.5).sub.0.5 In.sub.0.5 P, doped with Zn at an impurity
 concentration of 1.times.10.sup.17 cm.sup.-3, 0.2 .mu.m thick;
 a light emitting layer 6: made of p-type (Ga.sub.0.7 Al.sub.0.3).sub.0.5
 In.sub.0.5 P, doped with Zn at an impurity concentration of
 1.times.10.sup.17 cm.sup.-3, 0.5 .mu.m thick;
 a second cladding layer 7: made of p-type Al.sub.0.5 In.sub.0.5 P, doped
 with Zn at an impurity concentration of 5.times.10.sup.17 cm.sup.-3, 1
 .mu.m thick;
 a second buffer layer 8: made of p-type (Al.sub.0.05 Ga.sub.0.95).sub.0.95
 In.sub.0.05 P, doped with Zn at an impurity concentration of
 1.times.10.sup.18 cm.sup.-3, 0.15 .mu.m thick; and
 a current diffusion layer 9: made of p-type (Al.sub.0.01
 Ga.sub.0.99).sub.0.99 In.sub.0.01 P, doped with Zn at an impurity
 concentration of 5.times.10.sup.18 cm.sup.-3, 7 .mu.m thick.
 The LED 100 in this specific example differs from the LED 800 shown in FIG.
 12 in that the intermediate barrier layer 5 is provided between the light
 emitting layer 6 and first cladding layer 4. The intermediate barrier
 layer 5 has a conductivity type which is the same as that of the light
 emitting layer 6 but different from that of the first cladding layer 4.
 The energy gap of the intermediate barrier layer 5 is greater than that of
 the light emitting layer 6 but less than that of the first cladding layer
 4. An impurity concentration of Zn in the p-type light emitting layer 6 is
 1.times.10.sup.17 cm.sup.-3, which is the optimal value for light
 efficiency in an initial period after starting light emission.
 Different from the conventional LED 800, in Example 1, the current
 diffusion layer 9 is made of InGaAlP. This is an attempt to make light
 absorption as low as possible and light output as great as possible.
 However, the current diffusion layer 9 does not have lattice match with the
 GaAs substrate 1. To decrease the resistivity of the current diffusion
 layer 9, a molar fraction of Al need to be a low value, i.e., 0.01.
 Therefore, a molar fraction of In is set to 0.01 to compensate a reduction
 in a energy gap due to the decreased molar fraction of Al. Since the
 current diffusion layer 9 contains the 0.01 molar fraction of In, an upper
 surface thereof is smoother than when made of GaP. Therefore, an electrode
 10 formed on the upper surface of the current diffusion layer 9 will be
 difficult to detach. The low In molar fraction of 0.01 does not allow the
 current diffusion layer 9 to have lattice match with any of the layers
 from the substrate 1 to the second cladding layer 7. Accordingly, the
 second buffer layer 8 is provided between the second cladding layer 7 and
 the current diffusion layer 9 to prevent generation of crystal defects due
 to mismatch of lattice constants. Specifically, the second buffer layer 8
 has a lattice constant intermediate between that of the current diffusion
 layer 9 and those of the substrate 1 and so on. The intermediate lattice
 constant of the second buffer layer 8 is obtained when both of molar
 fractions of Al and In are 0.05.
 Effects of the intermediate barrier layer 5 will be described below with
 reference to FIG. 2.
 FIGS. 2A and 2B illustrate band states of the LED 100 after long-time light
 emission. Like FIGS. 14A and 14B, FIG. 2A illustrates a state of the LED
 100 when carriers are injected to the light emitting layer 6 with a
 forward bias. The intermediate barrier layer 5 has a molar fraction of Al,
 such that an energy gap level thereof is between those of the n-type first
 cladding layer 4 and p-type light emitting layer 6. Since the intermediate
 barrier layer 5 is of a p-type, a pn junction is formed between the n-type
 first cladding layer 4 and the p-type intermediate barrier layer 5.
 Injected carriers are recombined in the vicinity of the pn junction. Both
 a hole and a electron exist in the layer which has a lower energy gap,
 i.e., the intermediate barrier layer 5. Even when a thickness of the
 intermediate barrier layer 5 is sufficiently less than the diffusion
 length of the injected minority carrier, a sufficient number of minority
 carriers are also injected to the light emitting layer 6. Since the
 radiative recombination lifetime is shorter in the light emitting layer 6
 than in the intermediate barrier layer 5, a greater proportion of carriers
 are consumed in the light emitting layer 6 than in the intermediate
 barrier layer 5. Thus, the light emitting layer 6 has a lack of carriers
 and therefore the carriers entering the intermediate barrier layer 5 are
 quickly transferred to the light emitting layer 6. Although a large number
 of holes and electrons exist in the intermediate barrier layer 5,
 radiative recombination efficiently takes place in the light emitting
 layer 6. This is because the lower energy gap leads to the shorter
 radiative recombination lifetime.
 As described above, the intermediate barrier layer 5 has substantially no
 influence on light emission and light efficiency of the LED 100.
 FIG. 2B illustrates a band state of the LED 100 including the intermediate
 barrier layer 5 after long-time light emission. An energy level 20 of
 non-radiative recombination is created in the vicinity of the pn junction.
 Nevertheless, since carriers in the intermediate barrier layer 5 quickly
 diffuse into the light emitting layer 6 as described above, the number of
 carriers which combine at this energy level 20 is small, resulting in
 prevention of a decrease in light efficiency.
 As described above, the intermediate barrier layer 5 has a conductivity
 type which is the same as that of the light emitting layer 6, but
 different from that of the first cladding layer 4. The energy gap of the
 intermediate barrier layer 5 is greater than that of the light emitting
 layer 6 but less than that of the first cladding layer 4. With the
 intermediate barrier layer 5, the LED 100 can be realized such that light
 efficiency of the light emitting layer 6 does not decrease both in an
 initial period of light emission and after long-time light emission.
 A diffusion length of an electron is typically 0.5-1.5 .mu.m. In Example 1,
 the InGaAlP based semiconductor layers whose molar fractions of In are
 around 0.5 have a diffusion length of about 0.5 .mu.m. In the LED 100,
 therefore, a thickness of the intermediate barrier layer 5 is set to 0.2
 .mu.m. After long-time light emission, a crystal defect is generated in
 the vicinity of the interface (pn junction) between the p-type
 intermediate barrier layer 5 and the n-type first cladding layer 4. To
 prevent the crystal defect from growing and therefore affecting the light
 emitting layer 6 in a light emission time of an actual use, a thickness of
 the p-type intermediate barrier layer 5 is preferably great, i.e., 0.1
 .mu.m or more.
 In the LED 100, when an energy gap of the p-type intermediate barrier layer
 5 is greater by 0.2 eV or more than that of the light emitting layer 6
 (i.e., x4-x1.gtoreq.0.15 where x4 and x1 are molar fractions of Al of-the
 p-type intermediate barrier layer 5 and light emitting layer 6,
 respectively, as described above), most of holes and electrons combine in
 the light emitting layer 6, resulting in higher light efficiency of the
 LED 100.
 When a molar fraction of Al is 0.5 or more, the intermediate barrier layer
 5 made of InGaAlP based semiconductor becomes an indirect transition type
 semiconductor layer. Therefore, almost no radiative recombination 30 takes
 place in the intermediate barrier layer 5, thereby further increasing the
 efficiency of the radiative recombination in the light emitting layer 6.
 In the LED 100, a molar fraction x4 of Al of the intermediate barrier
 layer 5 is set to 0.5, whereby the intermediate barrier layer 5 becomes an
 indirect transition type semiconductor layer. This makes it difficult for
 the intermediate barrier layer 5 to emit light, whereby substantially all
 injected holes and electrons radiatively combine in the light emitting
 layer 6.
 A number of LEDs according to Example 1 were manufactured, and then
 subjected to a long-time operation under the conditions such that a
 driving current of 50 mA was applied to the LEDs for 1000 hours at room
 temperature, like the operation in FIG. 13. Light outputs of the LEDs were
 measured after the 1000 hours. As a result, the average light output was
 650 .mu.W and a rate of change of the light output was within .+-.2%
 compared with the light output in an initial period of light emission
 where a driving current is 20 mA. The rate of change of the light output
 is sufficiently small for practical use.
 EXAMPLE 2
 FIG. 3 is a cross-sectional view illustrating an LED 200 according to
 Example 2 of the present invention. For the sake of simplicity, components
 having the same functions as those of the LED 100 of Example 1 are
 indicated by the same reference numerals as those used therein.
 The LED 200 of Example 2 differs from the LED 100 of Example 1 in that a
 light emitting layer 6 is not doped with impurity, and therefore is of an
 n-type, and an intermediate barrier layer 50 is provided between the
 n-type light emitting layer 6 and a p-type second cladding layer 7. The
 energy gap of the intermediate barrier layer 50 is greater than that of
 the light emitting layer 6, but lower than that of the second cladding
 layer 7. The n-type intermediate barrier layer 50 is:
 made of (Ga.sub.0.5 Al.sub.0.5).sub.0.5 In.sub.0.5 P, doped with Si at an
 impurity concentration of 1.times.10.sup.16 cm.sup.-3, 0.2 .mu.m thick.
 FIG. 4 illustrates a band state of the LED 200 after long-time light
 emission.
 In the case where the LED 200 is made of InGaAlP based material which has
 lattice match with a GaAs substrate 1, when the light emitting layer 6 is
 not doped with impurity, the light emitting layer 6 becomes an n-type
 conductivity type. Example 2 will be described in the case where the light
 emitting layer 6 is of an n-type.
 In the LED 200, the n-type intermediate barrier layer 50 is provided
 between the n-type light emitting layer 6 and the p-type second cladding
 layer 7. The intermediate barrier layer 50 is doped with Si. The impurity
 concentration is preferably 1.times.10.sup.17 cm.sup.-3 or less. A
 thickness of the intermediate barrier layer 50 is less than a diffusion
 length of a hole as a minority carrier. The diffusion length of a hole is
 less than that of an electron, i.e., about 0.3 .mu.m . Accordingly, a
 thickness of the intermediate barrier layer 50 is 0.2 .mu.m in the LED 200
 of Example 2. This value is greater than 0.1 .mu.m, which is the thickness
 value that prevents influence of a crystal defect caused by a pn junction
 between the intermediate barrier layer 50 and the p-type second cladding
 layer 7 from reaching the light emitting layer 6. Therefore, a decrease in
 a light output after long-time light emission is sufficiently small for
 practical use, like the LED 100 of Example 1.
 It is well known that long-time light emission leads to diffusion of Zn
 which is a p-type impurity. As Zn diffuses in the light emitting layer 6,
 the light emission efficiency of the light emitting layer 6 is decreased.
 Particularly for a high-output LED, the current diffusion layer 9 and the
 p-type second cladding layer 7 are doped with a large amount of Zn in
 order to decrease the resistivity thereof. In this case, the Zn diffusion
 to the light emitting layer 6 is significant, resulting in a decrease in
 the light emission efficiency.
 In the LED 200 of Example 2, however, the intermediate barrier layer 50
 having a low impurity concentration is provided between the p-type second
 cladding layer 7 and the light emitting layer 6, whereby diffusion of Zn
 to the light emitting layer 6 is prevented and therefore a decrease in the
 light emission efficiency is prevented. In particular, since the
 intermediate barrier layer 50 has a low molar fraction of Al, the
 diffusion of Zn is subsequently low. Therefore, the intermediate barrier
 layer 50 is effective to prevention of Zn diffusion. As a result, the LED
 200 can have light emission efficiency which is not decreased after
 long-time light emission even when an impurity concentration of the
 current diffusion layer 9 is increased so as to decrease an operational
 voltage.
 In the LED 200 of Example 2, a light output is substantially not decreased
 since there is substantially no diffusion of Zn from the p-type current
 diffusion layer 9 and the p-type second cladding layer 7. For example,
 after long-time operation under such conditions that a driving current of
 50 mA is applied to the LEDs for 1000 hours at room temperature like the
 operation in FIG. 2, a light output of the LED 200 is within .+-.2% from
 450 .mu.W obtained in an initial period of light emission where a driving
 current is 20 mA. Note that the driving voltage is decreased by 10%
 compared with that of Example 1.
 EXAMPLE 3
 FIG. 5 is a cross-sectional view illustrating an LED 300 according to
 Example 3 of the present invention. For the sake of simplicity, components
 having the same functions as those of the LED 100 of Example 1 are
 indicated by the same reference numerals as those used therein.
 The LED 300 of Example 3 differs from the LED 100 of Example 1 in that in
 addition to a p-type first intermediate barrier layer 51 (indicated by
 reference numeral 5 in Example 1) provided between an n-type first
 cladding layer 4 and a p-type light emitting layer 6, a p-type second
 intermediate barrier layer 52 is provided between the n-type light
 emitting layer 6 and a p-type second cladding layer 7. The p-type
 intermediate barrier layer 52 is:
 made of (Ga.sub.0.5 Al.sub.0.5).sub.0.51 In.sub.0.49 P, doped with Zn at an
 impurity concentration of 1.times.10.sup.17 cm.sup.-3, 0.2 .mu.m thick.
 FIG. 6 illustrates band states of the LED 300 in an initial period of light
 emission and after long-time light emission, respectively.
 In addition to a configuration of the LED 100 of Example 1, the p-type
 second intermediate barrier layer 52 is provided between the p-type light
 emitting layer 6 and the p-type second cladding layer 7 which do not form
 a pn junction interface. The p-type second intermediate barrier layer 52
 prevents generation of a crystal defect due to an energy gap between the
 light emitting layer 6 and the second cladding layer 7 as described in
 Example 2. The prevention of the reduction in the light output is more
 effective than that of the LED 100 of Example 1.
 Furthermore, the p-type second intermediate barrier layer 52 has a low
 impurity concentration and a low molar fraction of Al. Therefore, the
 p-type second intermediate barrier layer 52 prevents diffusion of Zn from
 the p-type current diffusion layer 9 and the p-type second cladding layer
 7 into the light emitting layer 6, thereby preventing a decrease in light
 emission efficiency.
 As a result, the LED 300 of Example 3 prevents a decrease in light emission
 efficiency of the light emitting layer 6 over a longer time period of
 light emission than the LED 100 of Example 1.
 Moreover, as compared with the LED 100 of Example 1, the LED 300 of Example
 3 has a low level of crystal defect generation and thus a higher light
 output. Specifically, a light output of the LED 300 was measured after
 1000-hour operation where a driving current of 50 mA was applied to the
 LED 300 at room temperature. This condition is similar to that used in the
 operation of FIG. 13. As a result, a light output of the LED 300 was 720
 .mu.W. This value was within .+-.2% from a light output obtained in an
 initial period of light emission where a driving current is 20 mA.
 EXAMPLE 4
 FIG. 7 is a cross-sectional view illustrating an LED 400 according to
 Example 4 of the present invention. For the sake of simplicity, components
 having the same functions as those of the LED 100 of Example 1 are
 indicated by the same reference numerals as those used therein.
 The LED 400 of Example 4 differs from the LED 100 of Example 1 in that a
 light emitting layer 60 has multi-quantum well (MQW) structure and an
 intermediate barrier layer 5 is provided between the light emitting layer
 60 and the n-type first cladding layer 4. The light emitting layer 60 is
 composed by alternation of Ga.sub.0.51 In.sub.0.49 P layers and
 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 In.sub.0.5 P layers which have a thickness
 loss than the de Broglie wavelength. An energy gap of the intermediate
 barrier layer 5 has a value between those of the light emitting layer 60
 and n-type first cladding layer 4. The intermediate barrier layer 5 is
 provided with a thin thickness, i.e., 0.05 .mu.m.
 With the MQW structure, generation of a crystal defect in the light
 emitting layer 60 is hindered. Therefore, any crystal defect generated in
 a pn junction between the n-type first cladding layer 4 and the
 intermediate barrier layer 5 hardly continues to spread its growth into
 the light emitting layer 60. It is possible to prevent reduction of a
 light output when a thickness of the intermediate barrier layer 5 is of a
 value up to 0.02 .mu.m.
 EXAMPLE 5
 FIG. 8 is a cross-sectional view illustrating an LED 500 according to
 Example 5 of the present invention. For the sake of simplicity, components
 having the same functions as those of the LED 100 of Example 1 are
 indicated by the same reference numerals as those used therein.
 Each of the LEDs of the foregoing Examples has an n-type substrate. In
 Example 5, the LED 500 has a p-type substrate. In this case, the same
 effects as those of the LEDs of the foregoing Examples are also obtained.
 The LED 500 of Example 5 includes:
 a substrate 1: made of p-type GaAs;
 a first buffer layer 2: made of p-type GaAs;
 a light reflection (DBR) layer 3: including p-type (Al.sub.0.4
 Ga.sub.0.6).sub.0.5 In.sub.0.5 P layers and p-type Al.sub.0.5 In.sub.0.5 P
 layers deposited in an alternatively fashion;
 a first cladding layer 4: made of p-type Al.sub.0.5 In.sub.0.5 P, doped
 with Zn at an impurity concentration of 5.times.10.sup.17 cm.sup.-3, 1
 .mu.m thick;
 an intermediate barrier layer 5: made of n-type (Ga.sub.0.5
 Al.sub.0.5).sub.0.5 In.sub.0.5 P, doped with Si at an impurity
 concentration of 5.times.10.sup.17 cm.sup.-3, 0.1 .mu.m thick;
 a light emitting layer 6: made of n-type (Ga.sub.0.7 Al.sub.0.3).sub.0.5
 In.sub.0.5 P, 0.5 .mu.m thick;
 a second cladding layer 7: made of n-type Al.sub.0.5 In.sub.0.5 P, doped
 with Si at an impurity concentration of 5.times.10.sup.17 cm.sup.-3, 1
 .mu.m thick;
 a second buffer layer 8: made of n-type (Al.sub.0.05 Ga.sub.0.95).sub.0.95
 In.sub.0.05 P, doped with Si at an impurity concentration of
 1.times.10.sup.18 cm.sup.-3 1, 0.15 .mu.m thick; and
 a current diffusion layer 9: made of n-type (Al.sub.0.1
 Ga.sub.0.99).sub.0.99 In.sub.0.01 P, doped with Si at an impurity
 concentration of 1.times.10.sup.18 cm.sup.-3, 7 .mu.m thick.
 Since a p-type substrate is more difficult to manufacture than an n-type
 substrate, the n-type substrate is used for most LEDs. Since the LED 500
 includes the p-type substrate, the current diffusion layer 9 is of an
 n-type. The n-type current diffusion layer 9 can have the same current
 diffusion effect as that of a p-type current diffusion layer even when the
 impurity concentration is lower than that of the p-type current diffusion
 layer. This leads, advantageously, to a decrease in diffused impurity to
 the light emitting layer 6 and thus to substantial prevention of a
 reduction in light emission efficiency. Further, the contact resistance
 between the n-type current diffusion layer 9 and an n-side electrode can
 be lowered.
 In all the foregoing descriptions, each of the LEDs is made of InGaAlP
 based semiconductor which has lattice match with a GaAs substrate. As is
 obvious from the above descriptions, the LEDs of the present invention may
 be made of other materials such as III-V compound semiconductor (e.g.,
 AlGaAs, AlGaInSb, InGaAsP, AlGaInN, and GaInNSb) and II-VI compound
 semiconductor. Thickness and carrier concentration of layers made of these
 materials may be also modified in the LEDs of the present invention.
 EXAMPLE 6
 FIG. 9 is a cross-sectional view illustrating an LED 600 according to
 Example 6 of the present invention. For the sake of simplicity, components
 having the same functions as those of the conventional LED 900 shown in
 FIG. 15 are indicated by the same reference numerals as those used
 therein.
 The LED 600 of Example 6 differs from the conventional LED 900 in that a
 p-type second cladding layer 105 is composed of a p-type second
 intermediate barrier layer 53 and a p-type second cladding layer 54. The
 n-type second intermediate barrier layer 53 has a molar fraction of Al
 greater than that of a light emitting layer 104 and less than that of the
 p-type second cladding layer 105 of the LED 900 and is provided at the
 lower portion of the p-type second cladding layer 105. The p-type second
 cladding layer 54 has a sufficient molar fraction of Al to confine
 carriers and is provided at the upper portion of the p-type second
 cladding layer 105.
 The LED 600 of Example 6 is provided with:
 the p-type second intermediate barrier layer 53: made of p-type (Ga.sub.0.5
 Al.sub.0.5).sub.0.5 In.sub.0.5 P (a molar fraction of Al is 0.5), doped
 with Zn at an impurity concentration of 2.times.10.sup.17 cm.sup.-3, 0.3
 .mu.m thick;
 the p-type second cladding layer 54: made of p-type Al.sub.0.5 In.sub.0.5 P
 (a molar fraction of Al is 1.0), doped with Zn at an impurity
 concentration of 5.times.10.sup.17 cm.sup.-3, 1.0 .mu.m thick; and
 a current diffusion layer 106: a mono layer.
 In the above-described structure, the p-type second cladding layer 54 has a
 sufficient molar fraction of Al to confine carriers, thereby making it
 possible to obtain a high light output of the LED 600. A molar fraction x
 of Al of the p-type second cladding layer 54 is preferably in the range of
 0.7.ltoreq.x.ltoreq.1. The p-type second intermediate barrier layer 53 has
 a low impurity concentration and a low molar fraction of Al. This leads to
 prevention of impurity diffusion due to light emission, thereby preventing
 a decrease in a light output after long-time light emission.
 Note that in the p-type second intermediate barrier layer 53, the low molar
 fraction of Al as well as the low impurity concentration are responsible
 for improvement of crystallinity and thus prevention of impurity
 diffusion. The high oxidation of Al causes oxygen included in a material
 of a layer to be brought into the crystal. Therefore, a high molar
 fraction of Al makes the structure of the crystal imperfect compared with
 an ideal structure of the crystal. A crystal having such imperfect
 structure often has a vacant lattice point, which is a lattice point of an
 ideal crystal where an atom is expected to be positioned, but does not
 contain an atom; and also has a void greater than what is expected in an
 ideal crystal. This facilitates diffusion of impurity as compared with the
 diffusion in an ideal crystal.
 Preferably, a molar fraction x of Al of the p-type second cladding layer 54
 is as great as possible in order to confine carriers satisfactorily.
 Generally, x is equal to 1.0. Also, the resistivity of the p-type second
 cladding layer 54 is preferably as low as possible in order to diffuse
 carriers in the entire light emitting layer 104. To this end, an impurity
 concentration of the p-type second cladding layer 54 is preferably as high
 as possible.
 In the p-type second intermediate barrier layer 53, a molar fraction of Al
 should be set to a low value, e.g., 0.5. This improves crystallinity,
 thereby making it possible to prevent impurity diffusion in spite of a low
 thickness of the p-type second intermediate barrier layer 53.
 When a thickness of the p-type second intermediate barrier layer 53 is
 great, carriers overflowing from the light emitting layer 104 emit light
 in the p-type second intermediate barrier layer 53 or are eliminated by
 combining non-radiatively, thereby reducing light emission efficiency of
 the light emitting layer 104. To prevent this phenomenon, a thickness of
 the p-type second intermediate barrier layer 53 is preferably one half of
 a diffusion length of an electron which is a minority carrier in the
 p-type second intermediate barrier layer 53. InGaAlP crystal having a
 molar fraction of Al of 0.5 normally has an electron diffusion length of
 about 0.5 .mu.m. Accordingly, a thickness of the p-type second
 intermediate barrier layer 53 is preferably 0.3 .mu.m or less.
 Note that an impurity concentration as well as a thickness of the p-type
 second intermediate barrier layer 53 are more preferably optimized.
 FIG. 10A illustrates a relationship between an impurity concentration, and
 a ratio of a light output after 1000-hour light emission to a light output
 in an initial period of light emission for the p-type second intermediate
 barrier layer 53. FIG. 10B illustrates a relationship between a thickness,
 and a ratio of a light output after 1000-hour light emission to a light
 output in an initial period of light emission for the p-type second
 intermediate barrier layer 53. The light emission was performed under such
 conditions that a driving current of 50 mA was applied to the p-type
 second intermediate barrier layer 53 at room temperature.
 When an impurity concentration of the p-type second intermediate barrier
 layer 53 is high, light emission efficiency is decreased. This may be
 because impurities diffused from a current diffusion layer 106 and the
 p-type second cladding layer 54 depose impurities in p-type second
 intermediate barrier layer 53 to the light emitting layer 104.
 On the other hand, when a thickness of the p-type second intermediate
 barrier layer 53 is too low, some of the impurities diffuse into the light
 emitting layer 104, causing a decrease in a light output.
 As can be seen from FIG. 10A, the p-type second intermediate barrier layer
 53 having a sufficiently low impurity concentration of 3.times.10.sup.17
 cm.sup.-3 or less prevents the p-type impurities diffused from the current
 diffusion layer 106 and the p-type second cladding layer 54 from further
 diffusing into the light emitting layer 104. In this case, a light output
 after 1000-hour light emission is 80% or more of a light output in an
 initial period of light emission, which is a level of light intensity
 sufficient for practical use.
 FIG. 10B shows a result where an impurity concentration of the light
 emitting layer 104 is 1.times.10.sup.17 cm.sup.-3. As can be seen from
 FIG. 10B, the p-type second intermediate barrier layer 53 having a
 thickness of 0.1 .mu.m or more enables realization of an LED which has a
 light output after 1000-hour light emission that is 90% or more of a light
 output in an initial period of light emission. Such an LED is practically
 useful.
 When the p-type second intermediate barrier layer 53 has a great thickness,
 the p-type second cladding layer 54 loses a carrier confinement effect.
 Such a p-type second intermediate barrier layer 53 serves as a p-type
 cladding layer having an original function. In this case, carriers are
 consumed by radiative recombination and the like in the p-type second
 intermediate barrier layer 53. Therefore, a thickness of the p-type second
 intermediate barrier layer 53 is required to be equal to or less than a
 diffusion length of an electron which is a minority carrier of a p-type
 layer. A diffusion length of an electron is 0.5-1.5 .mu.m in AlGaInP based
 compound semiconductor which has lattice match with GaAs. When a molar
 fraction of Al is 0.5 in the AlGaInP based compound semiconductor, a
 diffusion length of an electron is about 0.5 .mu.m. Therefore, a thickness
 of the p-type second intermediate barrier layer 53 is required to be 0.5
 .mu.m or less, more preferably 0.3 .mu.m or less.
 EXAMPLE 7
 FIG. 11 is a cross-sectional view illustrating an LED according to Example
 7 of the present invention. On an n-type GaAs substrate 201, an n-type
 (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P first cladding layer 21 (x=1.0,
 y=0.5, an Si carrier concentration of 5.times.10.sup.17 cm.sup.-3, 1 .mu.m
 thick); an n-type (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P first
 intermediate barrier layer 22 (x=0.5, y=0.5, an Si carrier concentration
 of 2.times.10.sup.17 cm.sup.-3, 0.5 .mu.m thick); a (Al.sub.x
 Ga.sub.1-x).sub.y In.sub.1-y P light emitting layer 203 (x=0.3, y=0.5, 0.5
 .mu.m thick): a p-type (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P second
 intermediate barrier layer 41 (x=0.5, y=0.5, a Zn carrier concentration of
 2.times.10.sup.17 cm.sup.-3, 0.5 .mu.m thick); a p-type (Al.sub.x
 Ga.sub.1-x).sub.y In.sub.1-y P second cladding layer 42 (x=1.0, y=0.5, a
 Zn carrier concentration of 5.times.10.sup.17 cm.sup.-3, 1 .mu.m thick);
 and a (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P current diffusion layer 205
 (x=0.05, y=0.90, a Zn carrier concentration of 1.times.10.sup.18
 cm.sup.-3, 7 .mu.m thick), are successively deposited. Further, a p-side
 electrode 207 is formed on an upper surface of the current diffusion layer
 205. An n-side electrode 209 is formed on a lower surface of the substrate
 201. Subsequently, a light emitting diode 700 is completely fabricated.
 The p-type second intermediate barrier layer 41 has a low molar fraction of
 Al and a low impurity concentration and further has a great thickness of
 0.5 .mu.m. Therefore, the p-type second intermediate barrier layer 41
 prevents diffusion of impurities from the current diffusion layer 205 and
 the p-type second cladding layer 42 into the light emitting layer 203,
 thereby preventing reduction of light emission efficiency. With the n-type
 first intermediate barrier layer 22, the light emitting layer 203 having a
 low molar fraction of Al can be formed thereon. The low molar fraction of
 Al leads to satisfactory crystallinity in the light emitting layer 203.
 The pn junction between the light emitting layer 203 having such
 satisfactory crystallinity and the n-type first intermediate barrier layer
 22 can improve light emission efficiency.
 In all the foregoing descriptions, each of the LEDs is made of InGaAlP
 based semiconductor which has lattice match with a GaAs substrate. As is
 obvious from the above descriptions, the LEDs of the present invention may
 be made of other materials such as III-V compound semiconductor (e.g.,
 AlGaAs, AlGaInSb, InGaAsP, and AlGaInN) in which an energy gap can be set
 by a molar fraction of Al. Thickness and carrier concentration of layers
 made of these materials may also be modified in the LEDs of the present
 invention.
 As described above, in the LED of the present invention, a light output is
 less decreased even after long-time light emission.
 According to one aspect of the present invention, a crystal defect
 generated at a pn junction is prevented from affecting a light emitting
 layer, thereby realizing the LED in which a reduction in a light output is
 prevented even after long-time light emission.
 According to another aspect of the present invention, a crystal defect
 generated at a pn junction is prevented from affecting a light emitting
 layer so that a reduction in light emission efficiency is prevented,
 thereby realizing the LED in which a reduction in a light output is
 prevented even after long-time light emission.
 According to still another aspect of the present invention, non-radiative
 recombination in the intermediate barrier layer is substantially
 eliminated, thereby realizing the LED having a high level of light
 emission efficiency.
 According to still another aspect of the present invention, a p-type
 impurity which tends to be easily diffused is prevented from diffusing
 into the light emitting layer, thereby preventing a reduction in light
 emission efficiency.
 According to still another aspect of the present invention, a light output
 is less decreased in a spectrum from red light to green light even after
 long-time light emission.
 According to still another aspect of the present invention, even when the
 LED is a high-intensity LED which includes a cladding layer having a high
 molar fraction of Al, a p-type impurity which tends to be easily diffused
 is prevented from diffusing from the current diffusion layer or the p-type
 second cladding layer into the light emitting layer even after long-time
 light emission, thereby preventing a reduction in a light output.
 According to still another aspect of the present invention, a molar
 fraction of Al of the p-type second intermediate barrier layer is 0.5 or
 less, and a molar fraction of Al of the p-type second cladding layer is
 0.7 or more, thereby satisfactorily maintaining the crystallinity of the
 p-type second intermediate barrier layer and therefore making it possible
 to prevent diffusion of an impurity.
 According to still another aspect of the present invention, even when the
 LED includes the p-type second intermediate barrier layer, the p-type
 second cladding layer maintains a carrier confinement effect, thereby
 realizing a high light output and making it possible to maintain a
 characteristic of the LED at high temperature. Further, the p-type second
 cladding layer has a low impurity concentration and therefore absorbs
 incoming impurities to prevent them from diffusing into the light emitting
 layer.
 According to still another aspect of the present invention, even when the
 LED emits light in a spectrum from red light to green light at high
 intensity for a long time, a p-type impurity which tends to be easily
 diffused is prevented from diffusing from the current diffusion layer or
 the p-type second cladding layer into the light emitting layer, thereby
 preventing a reduction in a light output.
 Various other modifications will be apparent to and can be readily made by
 those stilled in the art without departing from the scope and spirit of
 this invention. Accordingly, it is not intended that the scope of the
 claims appended hereto be limited to the description as set forth herein,
 but rather that the claims be broadly construed.