JPH08181386A - Semiconductor optical element - Google Patents

Abstract

PURPOSE: To remarkably reduce the state density of holes, and improve the efficiency of a light emitting element, by constituting at least an active layer as zincblende structure. CONSTITUTION: A surface nitride layer is formed on a (001) GaAs substrate 1 by high temperature annealing in, e.g. an ammonia atmosphere. After that, a first clad layer 3 of N-Al0.2 Ga0.8 N, a multiquantum well active layer 4 of GaN/Al0.2 Ga0.8 N, a second clad layer 5 of P-Al0.2 Ga0.8 N, and a P-GaN contact layer 6 are continuously formed by an organometal vapor deposition method. All of the GaAs substrate 1-GaN contact layer 6 are constituted of zincblende type crystal. In AlGaInN system, zincblende type crystal is used in the active layer 4, and further quantum effect, 2-axial type strain, etc., are controlled. Thereby a short wavelength semiconductor light emitting element whose luminous efficiency is higher than the conventional element can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting device used in the fields of optical communication, optical information processing, etc., and more particularly to an AlGaInN type short wavelength semiconductor optical device.

[0002]

2. Description of the Related Art In recent years, the demand for semiconductor lasers has increased in many fields, and research and development have been actively promoted centering on GaAs and InP. Along with this, the crystal growth technology of semiconductor lasers such as MBE and MOVPE has made great progress, and high-quality compound semiconductor crystals can be produced with good reproducibility. At present, GaInP / AlGaInP-based semiconductor lasers with an oscillation wavelength of about 630 nm have been commercialized.
The development of semiconductor light emitting devices is progressing with various materials aiming at shorter wavelengths. Among these, ZnCdSe-based materials have recently achieved room temperature continuous oscillation of short-wavelength semiconductor lasers with an oscillation wavelength of around 500 nm. Attention has been paid to this world and development for practical use is currently underway.

However, since the operating voltage is high and the proliferation of lattice defects in the crystal during current injection is intense, it is difficult to obtain a practical level of reliability and it takes a lot of time for practical use. It is considered.

On the other hand, another promising material for a short-wavelength laser is a wurtzite GaN-based material. Since there is no high-quality lattice-matched substrate for epitaxial growth in this material system, the development of semiconductor light-emitting devices has been delayed compared to ZnCdSe systems, but in recent years high-luminance light emission exceeding 1 cd that can be used outdoors Diodes have been realized, and semiconductor lasers are expected to be realized in the future. As a light emitting diode, the device life is
Compared with other practically used semiconductor light emitting device materials for about 105 hours, it seems that it can be applied to semiconductor lasers.

[0005]

However, in order to use such a light emitting diode for outdoor display or to reduce power consumption, further improvement in efficiency is desired. Further, in order to realize a semiconductor laser that is currently being researched, it cannot be said that the current materials have sufficient luminous efficiency, and higher luminous efficiency is required.

An object of the present invention is to provide a short wavelength semiconductor light emitting device having higher luminous efficiency than ever before.

[0007]

In order to achieve the above object, the present invention provides a sphalerite structure having a small hole density of states,
For example, a GaN-based material or the like is used for the light emitting layer to realize a short wavelength semiconductor light emitting device having significantly higher light emission efficiency than the conventional wurtzite type crystal.

[0008]

[Function] In the AlGaInN-based material, by using a zinc blende type crystal for the active layer and further controlling the quantum effect and biaxial strain, it is possible to greatly reduce the density of states of holes, and thus the light emitting element The efficiency of can be improved.

[0009]

EXAMPLES Currently, wurtzite structure crystals are used for all the GaN-based light-emitting devices that are practically used. In addition to the wurtzite type for GaN-based crystals, conventional I such as GaAs
There is a zinc blende type which has the same crystal structure as the II-V group compound semiconductor.

However, wurtzite type crystals have a large state density in terms of materials, particularly in the valence band. When the density of states is high, carriers having a very high density must be injected into the active layer in order to operate as a light emitting device. As a result, the light emission efficiency is low, and particularly when light emission with a single energy is used as in a semiconductor laser, the efficiency is significantly reduced.

Therefore, in this embodiment, a semiconductor light emitting device using a zinc blende type different from such a wurtzite type GaN-based material will be described.

(Embodiment 1) FIG. 1 is a sectional view of an element of a zinc blende type GaN / AlGaN quantum well semiconductor laser. Introduction (00
1) A surface nitride layer 2 is formed on a GaAs substrate 1 by annealing at a high temperature in an ammonia atmosphere, for example. After that, by n-Al0.2Ga0.8N first cladding layer 3, GaN / Al0.2Ga0.8N multiple quantum well active layer 4, p-Al by metalorganic vapor phase epitaxy.
A 0.2Ga0.8N second cladding layer 5 and a p-GaN contact layer 6 are continuously formed. Subsequently, a SiO 2 insulating film 7 is deposited, a stripe-shaped opening for current injection is formed by etching, and finally an anode electrode 8 and a cathode electrode 9 are formed.

Here, the GaN-based crystal grown on the GaAs substrate 1 becomes a zinc blende type. That is, also in the semiconductor laser of FIG. 1, the GaAs substrate 1 to the GaN contact layer 6 are all made of zinc blende type crystal.

FIG. 2 shows a band diagram near the GaN / Al0.2Ga0.8N multiple quantum well 4. GaN / Al0.2Ga0.8N multiple quantum well 4 is composed of GaN quantum well layer 13 and Al0.2Ga0.8N barrier layer 1.
It has a structure in which four layers are alternately stacked. Electrons and holes injected from the outside are confined in the GaN quantum well layer 13, where light is emitted by recombination. Luminous efficiency is determined by the probability of this recombination. The recombination probability strongly depends on the energy distributions of electrons and holes, and the smaller the density of states, the higher the probability of recombination per carrier density. In general, the density of states of holes is larger than that of electrons, so that it can be considered that the size of the density of states of holes greatly affects the luminous efficiency.

FIG. 3 shows the band structures of kz in the valence band of zinc blende type and wurtzite type crystals. kz is the wave number with respect to the growth direction, and it is assumed that the sphalerite type is the (001) direction and the wurtzite type is the (0001) direction.

In the case of the zinc blende type of FIG. 3 (a), the heavy hole 21 and the light hole 22 are degenerated at kz = 0. When the bands are close to each other in this manner, they are mixed with each other, and as a result, the density of states of the valence band, that is, the curvature of the band is increased.

However, if a quantum well perpendicular to the z direction is used, the heavy hole 21 and the light hole 22 can be separated by the quantum effect as shown in FIG. 3 (c). Therefore, mixing between bands can be reduced, the density of states of holes can be reduced, and light emission can be achieved at a low threshold value. By the way, FIG. 3 (c) shows the case where the quantum well structure is used for the active layer, and the horizontal axis shows the wave number k〃 in the plane of the quantum well layer. As shown in (c),
It can be seen that when the quantum well structure is used, the narrow band is melted and the heavy hole and the light hole can be separated, and in addition, the curvature of the heavy hole 41 becomes small and the density of states can be made considerably small.

(Embodiment 2) As another embodiment, even if the active layer has a zinc-blende structure and a biaxial strain parallel to the plane is used instead of the quantum well structure, the active layer is The same effect as using the quantum well structure can be obtained. Here, the biaxial strain means that the amounts of strain in two directions that are perpendicular to each other in the plane are equal. In a quantum well structure, strain is equal in a plane perpendicular to the Z direction (growth direction) of the quantum well layer, for example, in a predetermined direction (x direction) and in a direction perpendicular to that direction (y direction). is there. AlGaIn currently in practical use
In a semiconductor laser using a P-based III-V group compound semiconductor, high performance has been achieved by the method based on this biaxial strain. If at least the amount of biaxial strain in the quantum well layer in the quantum well structure is equal, the density of states becomes small, and the greatest effect is obtained as the characteristics of the laser.

On the other hand, in the case of the wurtzite type currently used in GaN-based light emitting devices, it is as shown in FIG. 3 (b). It can be seen that the two bands from the band edge, the first hole 31 and the second hole 32, have almost the same curvature in the z direction. Since the energy shift due to the quantum effect is proportional to the curvature of the z-direction band,
The energy interval between the hole 31 and the second hole 32 cannot be expanded. Therefore, the state density cannot be reduced.

Even if biaxial strain is used for the active layer, since the two bands have almost the same energy shift, the energy interval is not changed. Therefore,
The first hole 31 and the second hole 32 maintain a strongly mixed state and have a very high density of states.

As is clear from the above description, the wurtzite GaN-based material has a large hole state density, and it is difficult to reduce this state density even if the structure is changed.
Therefore, when used as a semiconductor light emitting element, only an element having low efficiency can be obtained in principle. However, if a zincblende-type GaN-based material is used as in this example, the efficiency of the light emitting device can be improved by controlling the quantum effect, biaxial strain, and the like.

Although the GaN quantum well layer is used in this embodiment, the same effect can be obtained by using a material of any composition of AlGaInN mixed crystal system other than GaN crystal. Even if the active layer is not a quantum well structure but a bulk crystal, the same effect as that of using a quantum well structure can be obtained due to biaxial strain and the like.

[0023]

According to the present invention, the luminous efficiency can be remarkably increased as compared with the conventional wurtzite type crystal, and a high-performance semiconductor optical device for blue to ultraviolet light can be realized.

[Brief description of drawings]

Fig. 1 Device cross section of zinc blende type GaN / AlGaN quantum well semiconductor laser

Fig. 2 Band diagram near GaN / Al0.2Ga0.8N multiple quantum well 4

FIG. 3 Band structure diagram of valence band of sphalerite and wurtzite crystals

[Explanation of symbols]

 1 (001) GaAs substrate 2 surface nitrided layer 3 n-Al0.2Ga0.8N first clad layer 4 GaN / Al0.2Ga0.8N multiple quantum well active layer 5 p-Al0.2Ga0.8N second clad layer 6 p- GaN contact layer 7 SiO2 insulating film 8 Anode electrode 9 Cathode electrode 11 Conduction band 12 Valence band 13 GaN quantum well layer 14 Al0.2Ga0.8N barrier layer 21 Heavy hole 22 Light hole 23 Spin-orbit separation hole 31 First hole 32 Second hole 33 Third hole

Claims (6)

[Claims] 1. A light emitting device made of an AlGaInN-based compound semiconductor, wherein at least the active layer has a zinc blende structure. 2. The semiconductor optical device according to claim 1, wherein the active layer is a single crystal. 3. A GaAs single crystal as a substrate, and a compound semiconductor is formed on the substrate by epitaxial growth.
3. The semiconductor optical device according to item 1. 4. The semiconductor optical device according to claim 1, wherein a cubic SiC single crystal substrate is used, and a compound semiconductor is formed on the substrate by epitaxial growth. 5. The semiconductor optical device according to claim 1, wherein the active layer has a single or a plurality of InGaN quantum well layers. 6. The semiconductor optical device according to claim 1, wherein the active layer has biaxial strain.