JPH077847B2 - Semiconductor light emitting element - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a semiconductor light emitting device such as a light emitting diode or a laser diode, which emits light of green to purple.

[Technical background of the invention and its problems]

Among the three primary colors of light, red, green, and blue, a light emitting element using a III-V group compound semiconductor has been developed for red and green, and mass production has already been performed. The remaining blue light-emitting element has not been commercialized despite strong demand for development.

Semiconductor materials for blue light emitting devices include ZnS, ZnSe which are II-VI group compounds and GaN and I which are III-V group compounds having a wide band gap.
SiC, which is a group V compound, has been studied. Among these materials, ZnS and ZnSe have a lattice constant close to that of GaP and GaAs of III-V group compounds, and a large-area and good-quality crystal layer can be obtained by using these GaP or GaAs crystals as a substrate. It is especially promising because it can be obtained. However, ZnS and ZnSe both have n-type conductivity, but have a problem that p-type conductivity cannot be obtained. Therefore, the inability to obtain a pn junction is the main reason for delaying the practical application of the blue light emitting element.

Although there is no definite theory as to why p-type conductivity cannot be obtained in ZnS and ZnSe, the following reasons can be considered. First, in ZnS and ZnSe,
Few of the acceptor impurities make impurity levels that are shallow enough to create vacancies in the valence band. Second, even if there are acceptor impurities that make shallow impurity levels, they exist in the crystal. If added, the so-called self-compensation effect will induce donor-type defect generation.

[Object of the Invention]

The present invention solves the above-mentioned problems and provides a II-V with a wide forbidden band.
An object of the present invention is to provide a semiconductor light-emitting device that is capable of emitting blue light with high brightness by forming a good pn junction using a group I compound semiconductor.

[Outline of Invention]

The present invention provides a p-type conductive layer forming a pn junction of a light emitting device,
Ga _x A1 _1-x P compound semiconductor (0 ≦ x ≦ 1) and ZnS _y Se _1-y compound semiconductor or Zn _y Cd _1-y S compound semiconductor (0 ≦ y ≦
It is characterized in that it is constituted by a superlattice in which 1) and are alternately laminated.

〔The invention's effect〕

According to the present invention, by introducing the superlattice structure, a p-type conductive layer made of a compound semiconductor having a wide bandgap can be obtained, whereby a light emitting device that emits light with high brightness from green to purple can be obtained.

Example of Invention

 Examples of the present invention will be described below.

FIG. 1 shows the device structure of one embodiment. In the figure, 1 is an n-type GaP crystal substrate, and 2 is an n-type ZnS _z Se _1-z layer (0 ≦ z ≦ 1) epitaxially grown on the substrate 1 to which Al is added as a donor impurity. This n-type ZnS
A p-type conductive layer 3 having a superlattice structure is formed on the _z Se _1-z layer 2. Reference numeral 4 is a p-side electrode, and 5 is an n-side electrode.

FIG. 2 shows a part of the p-type conductive layer 3 in an enlarged manner. That is, the p-type conductive layer 3 is a non-doped G that is a III-V group compound semiconductor.
aP layer 31i (i = 1, 2, ..., N) and A as an acceptor impurity
ZnS layers 32i (i = 1, 2, ..., N) as II-VI group compound semiconductors in which g is added at about 10 ¹⁶ to 10 ¹⁹ / cm ³ are alternately laminated to form a superlattice. Thickness L _B of ZnS layer 32i and GaP layer 31i
The thickness L _Z of each is about 5 to 40 Å. Such a superlattice structure is obtained, for example, by a vapor phase growth method (MOCVD) using an organometallic compound.

FIG. 3 shows the band structure of the p-type conductive layer 3 having such a superlattice structure. The top of the valence band of the ZnS layer 32i is at the GaP layer 31i.
It is located 0.9 to 1.2 eV lower than the top of the valence band of. On the other hand, the impurity level of the Ag acceptor is located 0.55 to 0.70 eV higher than the top of the valence band of the ZnS layer 32i. Therefore, the concentration of Ag impurities added to the ZnS layer 32i should be 10 ¹⁶ / c.
Once you have high as m ³ or more, the average Fermi level E _F of the superlattice structure will position lower than the top of the valence band of the GaP layer 31i as shown in Figure 3. This makes the GaP layer 3
Vacancies having the same concentration as the Ag impurity concentration are formed in 1i. The ZnS layer 32i acts as a barrier when these vacancies move in the stacking direction of the superlattice, but if the thickness L _B of the ZnS layer 32i is selected within the range of 5 to 30 Å, the probability of tunneling through the barrier is sufficient. Getting higher,
It is possible to obtain p-type conductivity having high conductivity in the superlattice stacking direction. FIG. 4 shows that the thickness L _Z of the GaP layer 31i is 20Å, the Ag addition concentration of the ZnS layer 32i is 10 ¹⁷ / cm ³ , and the ZnS layer 32i is
7 shows the conductivity characteristics of the superlattice structure when the thickness L _{B of} is changed.

Thus, in the ZnS or ZnSe bulk crystal, the impurities that form a deep impurity level and do not generate carriers effectively act as a shallow acceptor impurity level in the superlattice structure of the present example, generating positive carriers. As a result, a p-type conductive layer that cannot be obtained with a bulk crystal is obtained. In this embodiment, in order to effectively perform the radiative recombination, it is desirable that the thickness of the p-type conductive layer 3 having a superlattice structure is about 0.2 to 3 μm.

When a forward bias is applied to the pn junction of the light emitting device of FIG. 1 thus configured, electrons are mainly injected from the n-type ZnS _z Se ₁ -z layer 2 to the p _- type conductive layer 3. The injected electrons cause radiative recombination with holes in the GaP layer 31i in the p-type conductive layer 3. Light emission in the GaP layer is due to the heavy hole band (hea
vy hole band and light hole ban
Since d) separates in the superlattice, it has a two-peak structure. The relationship between each peak wavelength and the GaP layer thickness L _Z
Shown in the figure. 51 is heavy hole band, 52 is light
This is due to the hole band. As L _Z becomes smaller, the light emission shifts to the shorter wavelength side due to the quantum effect, and the emission color becomes an average wavelength of two peaks, and when L _Z is about 15Å, it becomes blue, and at about 10Å, it becomes purple. On the other hand, the GaP crystal is an indirect transition type in the bulk state, but by adopting a superlattice structure, the Brillouin zone bending effect (zone follo
ding effect) will be a direct transition type. Therefore, the transition probability of light emission increases, and the external quantum efficiency shows a high value of 1%.

Here, a reference example will be described. Although the basic device structure is the same as that in FIG. 1, in this reference example, the p-type conductive layer 3 is formed as shown in FIG. That is, the p-type conductive layer 3 is constituted by a superlattice structure in which a GaP layer 31i 'to which Zn is added as an acceptor impurity in an amount of about 10 ¹⁶ to 10 ¹⁹ / cm ³ and a non-doped ZnS layer 32i' are alternately laminated.

The band structure of the p-type conductive layer 3 in the device of this reference example is shown in FIG. Zn impurities form a p-type conductive layer as a shallow acceptor impurity in a bulk GaP crystal. It is considered that such a bulk crystal that forms an impurity level at a shallow position still forms a shallow level near the band edge even in the superlattice even if the level position changes to some extent. Therefore, in FIG. 7, Fermi
The level is located near the top of the valence band of the GaP layer, and vacancies are formed in the GaP layer to the same extent as Zn impurities. Similar to the previous embodiment, these holes can be tunneled out by selecting the thickness of each layer, which contributes to p-type conductivity. The band gap of this superlattice structure is wider than that of GaP crystal due to the quantum effect, and by changing the thickness of the GaP layer, high efficiency is achieved from green to purple by the same principle as in the previous example. The light emission characteristics are shown.

FIG. 8 shows an element structure of an embodiment using a p-type substrate as a starting substrate. That is, using a p-type GaP crystal substrate 81, a p-type conductive layer 82 having a superlattice structure similar to that of the previous embodiment was formed thereon, and an n-type ZnS _z Se _1-z layer 83 was formed thereon. It is a thing. In order to extract light from the GaP substrate 81 side, the GaP substrate 81 is etched from the back surface to provide a light extraction window. 84 is p
The side electrode, 85 is an n-side electrode.

This example also exhibits excellent light emission characteristics similar to those of the previous example.

FIG. 9 shows a device structure of still another embodiment. In this embodiment, an n-type ZnS _z Se _1-z substrate 91 is used and a p-type conductive layer 92 having a superlattice structure is formed on the n-type ZnS _z Se ₁ -z substrate 91. 93 is a p-side electrode and 94 is an n-side electrode.

Also in this example, similarly good blue emission characteristics can be exhibited.

FIG. 10 shows an element structure of an embodiment using a double heterojunction structure. This a ZnS _z Se _1-z layer 2 of the portion in FIG. 1, as two layers of ZnS layer 2 ₁ and the ZnSe layer 2 _2, p-type layer 3 and the p-type Z
constitute a double heterojunction between nS layer 2 _1.

This example also exhibits good blue emission characteristics as in the previous examples.

The present invention is not limited to the above embodiments, but can be modified in various ways as listed below.

As a II-VI group compound semiconductor semiconductor with a superlattice structure,
Generally, ZnS _y Se _1-y (0 ≦ y ≦ 1) can be used. Also Zn _y
Cd _1-y S (0 ≦ y ≦ 1) can also be used. Further, Ga ^x Al _1-x P (0 ≦ x ≦ 1) can be generally used as the III-V compound semiconductor forming the superlattice structure.

As acceptor impurities in the II-VI group compound semiconductor layer having a superlattice structure, other than Ag, a group I element such as Li, Na, or Cu, or
Group V elements such as P and As can be added. Also III-
In addition to Zn, Be,
Group II elements or group IV elements such as Mg may be added.

Further, in the examples, acceptor impurities were added to the II-VI group of the superlattice, but the III-V group compound semiconductor or II- of the superlattice was added.
Acceptor impurities may be added to both of the group VI compound semiconductors.

The acceptor impurity level in the superlattice structure can also be formed by crystal growth such that Zn vacancies are present at a high concentration regardless of the addition of an impurity element from the outside.
This is because the Zn vacancy is located in the middle of the top of the valence band of the GaP crystal and the ZnS crystal in the superlattice structure. As a crystal growth condition for forming Zn vacancies at a high concentration, for example, MO
Taking the CVD method as an example, the molar ratio of the gas for supplying the group VI element to the gas for supplying the group II element is sufficiently large (for example,
10 times or more) should be set.

[Brief description of drawings]

FIG. 1 is a diagram showing the structure of a light emitting device according to one embodiment of the present invention, FIG. 2 is an enlarged view of the p-type conductive layer portion thereof, and FIG. 3 is a view showing the band structure of the p-type conductive layer portion, FIG. 4 is a diagram showing the conductivity characteristics of the p-type conductive layer portion, FIG. 5 is a diagram showing the relationship between the emission peak position of this light emitting device and the thickness of the GaP layer in the superlattice structure, and FIG. 6 is a reference. FIG. 7 is a diagram showing a structure of an example p-type conductive layer portion, FIG. 7 is a diagram showing a band structure of the p-type conductive layer portion, FIG. 8 to FIG. 10 are light emitting element structures of still another embodiment. FIG. 1 ...... n-type GaP crystal substrate, 2 ...... n-type ZnS _z Se _1-z layer, 2 ₁ ...
... n-type ZnS layer, 2 ₂ ... ZnSe layer, 3 ... p-type conductive layer (superlattice structure part), 4 ... p-side electrode (Au), 5 ... n-side electrode (Au-Ge), 31i …… GaP layer, 32i …… Ag-doped ZnS layer, 31i…
… Zn-doped GaP layer, 32i ′ …… ZnS layer, 81 …… p-type GaP substrate,
82: p-type conductive layer (superlattice structure part), 83: n-type ZnS _z Se
_1-z layer, 84 …… p side electrode (Au-Zn), 85 …… n side electrode (In
-Ga), 91 ... n-type ZnS _z Se _1-z substrate, 92 ... p-type conductive layer (superlattice structure part), 93 ... p-side electrode (Au), 94 ... n-side electrode (In- Ga).

Claims (1)

[Claims] 1. A semiconductor light emitting device having a pn junction formed of a compound semiconductor, wherein a p-type conductive layer is formed of Ga _x A1 _1-x P compound semiconductor (0 ≦ x ≦ 1) and an I or V group as an acceptor impurity. A semiconductor light emitting device comprising a superlattice in which ZnS _y Se _1-y compound semiconductors or Zn _y Cd _1-y S compound semiconductors (0 ≦ y ≦ 1) to which a group element is added are alternately laminated.