JPH11135833A - Optical semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve external light-emitting efficiency by providing a reflecting layer, which reflects the light emitted from a light emitting element layer to the side of the light-emitting element layer between a substrate and the light- emitting element layer. SOLUTION: On a semiconductor substrate 1, a light-emitting element layer 2 having a P-N junction, an ohmic contact electrode 3 on the side of the light- emitting surface, an ohmic contact electrode 4 on the back side of the light- emitting surface and a reflecting layer 6, on which a thin layer is laminated, are formed. When a voltage is applied across the ohmic contact electrodes 3 and 4, lighting is produced at the P-N junction part formed in the light- emitting element layer 2. The emitted light is radiated towards the light-emitting surface side of the surface of the light-emitting element layer, the side surface side and the back surface side of the light-emitting surface. The light radiated towards the back-surface side is reflected at the reflecting layer 6 and discharged to the outside from the side of the light-emitting surface. Thus, the external light emitting efficiency is improved.

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 made of a compound semiconductor, and more particularly, to a structure of a light emitting device having high external luminous efficiency.

[0002]

2. Description of the Related Art Light emitting diodes are used in a wide range of applications, such as visible light emitting diodes for display and infrared light emitting diodes for various sensors or remote control. Especially recently, portable small devices,
The range of use such as remote control is expanding. 2. Description of the Related Art In general, devices used for portable small devices and the like are rapidly decreasing in operating voltage and operating current, such as CMOS LSI. On the other hand, a conventional light emitting diode formed of a compound semiconductor consumes a large amount of current, which is a major factor in shortening the life of a battery. Therefore, it is desired to reduce the current consumption of the light emitting diode.

In order to reduce the current consumption of the light emitting diode, it is necessary to improve the external light emitting efficiency.
Conventionally, in order to improve external luminous efficiency, measures such as reducing crystal defects and selecting a substrate that is transparent to the emission wavelength, and taking measures against material crystals and using dots on the light-emitting surface electrodes to block emitted light In order to reduce the occupied area or to process the light emitting surface into a dome shape, measures have been taken regarding the structure of the light emitting diode.

For example, as a measure against the structure, the present applicant has proposed a light having a structure having a reflecting surface which reflects light transmitted through the back surface of the semiconductor substrate toward the front surface (light emitting surface) at an angle radiated from the surface of the semiconductor substrate. A semiconductor device has been proposed (Japanese Patent Application No. 8-38708).

FIG. 4 is a sectional view of the optical semiconductor device proposed above. In the figure, 1 is a semiconductor substrate, 2 is a light emitting element layer having a PN junction, 3 is an ohmic contact electrode formed on the surface of the semiconductor substrate 1 which is a light emitting surface, and 4 is formed on the reflection surface side of the back surface of the semiconductor substrate 1 The formed ohmic contact electrodes 5 are reflection surfaces.

When a voltage is applied between the ohmic contact electrodes 3 and 4, light emission occurs at a PN junction formed in the light emitting element layer 2. This emitted light is emitted toward the light emitting surface side, the side surface side of the light emitting element layer surface, and the rear surface side of the light emitting surface. Of the light, the light emitted toward the back surface side of the light emitting surface is reflected by the reflecting surface 5 to the light emitting surface side at an angle to be emitted from the light emitting surface side to the outside, so that the external light emitting efficiency can be increased.

[0007]

Generally, when an optical semiconductor device having such a structure is formed of GaAs, the ohmic contact electrode is generally formed of an AuGe alloy. The ohmic contact electrodes 3 and 4 formed on the surface of the light emitting element layer 2 and the back surface of the semiconductor substrate 1 are formed so that the area of each is about 25%.

However, in the region where the ohmic contact electrode is formed, Ge absorbs a part of the emitted light,
There is a problem in that it does not function as a reflecting surface. SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and to provide an optical semiconductor device capable of improving external luminous efficiency even when an ohmic contact electrode is formed.

[0009]

According to the present invention, there is provided an optical semiconductor comprising: a light emitting element layer laminated on a semiconductor single crystal substrate; and at least one pair of electrodes connected to the light emitting element layer. The device is characterized in that a reflection layer is provided between the substrate and the light-emitting element layer, the light-emitting element layer reflecting light emitted from the light-emitting element layer.

When the substrate is a III-V compound semiconductor, the reflective layer is a laminated film including a thin film of a II-VI compound semiconductor, and the thickness H of each thin film constituting the laminated film is determined by a light emitting element. When the emission wavelength of the layer is A and the refractive index of the thin film is B, by satisfying H = A / 4B, the external luminous efficiency can be improved.

Particularly, the substrate and the light emitting element layer are made of GaAs.
When s, the reflective layer is a laminated film of a GaAs layer and a ZnSe layer, and the GaAs layer and the ZnSe layer each have at least four layers alternately laminated, or a laminated film of a ZnSe layer and a ZnS layer, Or ZnSSe
The external luminous efficiency can be improved by employing a structure in which a layer and a ZnCdS layer are stacked and each layer is alternately stacked by at least 20 layers.

[0012]

Embodiments of the present invention will be described below. FIG. 1 shows a cross-sectional view of the optical semiconductor device of the present invention. In the figure, 1 is a semiconductor substrate, 2 is a light emitting element layer having a PN junction, 3 is an ohmic contact electrode on the light emitting surface side,
Is an ohmic contact electrode on the back side of the light emitting surface, and 6 is a reflective layer having a structure in which thin layers are stacked. Ohmic contact electrodes 3, 4
When a voltage is applied therebetween, light emission occurs at the PN junction formed in the light emitting element layer 2. This emitted light is emitted toward the light emitting surface side, the side surface side of the light emitting element layer surface, and the rear surface side of the light emitting surface.

[0013] Of the emitted light, the light emitted toward the back surface side of the light emitting surface is reflected by the reflective layer 6 and emitted to the light emitting surface side. Due to the reflection layer 6, almost no light reaches the back surface of the light emitting surface, and the external light emission efficiency is not affected by the shape of the ohmic contact electrode 4. Therefore, the light absorbed by the ohmic contact electrode and not emitted to the outside is emitted to the outside, and the external luminous efficiency is increased. In addition, there is an advantage that the area of the ohmic contact electrode 4 can be formed wider than before, and the contact resistance can be reduced.

Here, it is necessary to select a material for the reflective layer 6 which has no lattice mismatch with the semiconductor substrate 1 and is less affected by band discontinuity. Further, in order to increase the reflectance, it is necessary to stack thin layers having different refractive indices, and to set the thickness of each layer to a quarter of the emission wavelength. Furthermore, in order to prevent the forward voltage from rising more than necessary, it is desirable that the film has a low resistance.

Specifically, the optical semiconductor device of the present invention is formed as follows. Impurity concentration is 1017 atom / c
On an n-type GaAs substrate of the order of m3, a GaAs thin film and Z-VI compound semiconductor
The nSe thin films are alternately laminated. FIG. 2 shows an enlarged view of the reflection layer portion. In the figure, 7 is a GaAs film, 8 is ZnS
e film. ZnSe film formed by MBE method, GaAs
In the film, Ga and Se serve as donor dopants for each layer, and the reflection layer 6 forms a low-resistance N-type layer.

The thicknesses of the GaAs film 7 and the ZnSe film 8 are set as follows in order to form a reflection layer. That is, when the emission wavelength is A and the refractive index of the film is B,
The thickness H of the layer is set as H = A / 4B.

GaAs and ZnSe have refractive indices of 3.62 and 2.50, respectively, and have a large difference in refractive index. Therefore, even if the number of films to be laminated is relatively small, the reflectance is large. FIG.
2 is a graph showing the relationship between the number of laminations and the reflectance. As shown in FIG. 3A, when the number of laminations exceeds four (each layer is alternately laminated by four layers), the reflectivity exceeds 90%, indicating that a good reflection layer is obtained. Here, when the light emitting layer is made of GaAs, the light emission wavelength becomes 850 nm. In order to form an effective reflection layer, Ga is calculated according to the above equation.
The thickness of each of As and ZnSe is 58.7 nm, 8
It becomes 5.0 nm.

Thereafter, an N-type GaAs layer having an impurity concentration of the order of 1018 atoms / cm 3 and a P-type GaAs layer having an impurity concentration substantially equal to that of the light-emitting element layer 3 are sequentially stacked. If necessary, a contact layer for reducing contact resistance is formed, electrodes for making ohmic contact with the GaAs substrate and the P-type GaAs layer are formed, and an optical semiconductor device is formed.

ZnSe has a zinc blende type crystal structure, has a small lattice mismatch with GaAs of 0.3%, and is hardly affected by band discontinuity. In addition, since the band gap is sufficiently wide as compared with GaAs, there is no absorption of emitted light and there is no influence on luminous efficiency, so that it is suitable as a material forming a reflective layer.

Further, a ZnSe thin film and G
Instead of the laminated film of the aAs thin film, a laminated film of a ZnSe thin layer (refractive index: 2.5) and a ZnS thin layer (refractive index: 2.3) or a ZnSSe thin layer (refractive index: 2.5) and a ZnCdS thin film (refractive index) A laminated film having a ratio of 2.3) may be used. The combination of these thin films has a small difference in refractive index.
As shown in (B), when the number of laminations exceeds 20, the reflectance exceeds 90%, and it can be seen that a good reflection layer is obtained.

Like ZnSe, ZnS, ZnSSe or ZnCdS has a zinc-blende type crystal structure.
The lattice mismatch with As is small, and there is almost no influence of band discontinuity. Further, since the band gap is sufficiently wider than that of GaAs, it does not absorb emitted light and does not affect luminous efficiency, so that it is suitable as a material forming a reflective layer.

The optical semiconductor device made of GaAs has been described above. However, the present invention is not limited to this and can be applied to other optical semiconductor devices. In particular, when the substrate is a group III-V compound semiconductor, the reflective layer
When a laminated film including a group I compound semiconductor thin film is selected, a reflective layer having excellent characteristics can be formed relatively easily.

[0023]

As described above, according to the present invention, an optical semiconductor device having high external luminous efficiency can be provided irrespective of the shape of the ohmic contact electrode formed on the back surface side of the light emitting surface. The reflective layer of the present invention can be formed by an ordinary MBE method, and can be formed easily.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an embodiment of the present invention.

FIG. 2 is a diagram illustrating an embodiment of the present invention.

FIG. 3 is a diagram illustrating an embodiment of the present invention.

FIG. 4 is a sectional view of a conventional optical semiconductor device of this type.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor substrate 2 Light emitting element layer 3, 4 Ohmic contact electrode 5 Reflection surface 6 Reflection layer 7 GaAs layer 8 ZnSe layer

Claims (5)

[Claims] 1. An optical semiconductor device comprising: a light emitting element layer laminated on a semiconductor single crystal substrate; and at least one pair of electrodes connected to the light emitting element layer, wherein between the substrate and the light emitting element layer: An optical semiconductor device, comprising: a reflection layer that reflects light emitted from the light emitting element layer toward the light emitting element layer. 2. The optical semiconductor device according to claim 1, wherein
The substrate is a III-V compound semiconductor, the reflective layer is a laminated film including a II-VI compound semiconductor thin film, and the thickness H of each thin film constituting the laminated film is light emission. When the emission wavelength of the element layer is A and the refractive index of the thin film is B,
An optical semiconductor device substantially satisfying H = A / 4B. 3. The optical semiconductor device according to claim 2, wherein
The substrate and the light-emitting element layer are made of GaAs, the reflective layer is a laminated film of a GaAs layer and a ZnSe layer, and the GaAs layer and the ZnSe layer are each alternately laminated in at least four layers. An optical semiconductor device characterized by the following. 4. The optical semiconductor device according to claim 2, wherein
The substrate and the light emitting element layer are made of GaAs, the reflective layer is a laminated film of a ZnSe layer and a ZnS layer, and the ZnSe layer and the ZnS layer are alternately laminated at least 20 layers each. An optical semiconductor device characterized by the following. 5. The optical semiconductor device according to claim 2, wherein
The substrate and the light emitting element layer are made of GaAs;
An optical semiconductor device, wherein the reflection layer is a laminated film of a ZnSSe layer and a ZnCdS layer, and at least 20 layers of the ZnSSe layer and the ZnCdS layer are alternately laminated.