JPH1154846A - Resonant surface emitting device - Google Patents

Abstract

(57) [Problem] To improve the directivity and monochromaticity by reducing the spread of light from a light extraction surface in a resonance type surface emitting device having a quantum well type light emitting layer. SOLUTION: In a quantum well type light emitting layer 18, a band split occurs in a valence band due to a shift of a quantum effect, and a sub-band of a heavy hole and a sub-band of a light hole are generated. As a result, the emission wavelength range becomes broader on the lower wavelength side, and the light on the lower wavelength side resonates in the direction inclined from the vertical direction due to the Bragg reflection condition and exits from the light extraction surface 36. In order to prevent this, the band gap energy is smaller than that of the barrier layers 16 and 20 so that a compressive strain is applied to the light emitting layer 18 such that the energy levels of the heavy holes and light holes in the valence band become substantially equal. I-GaAs with small lattice constant
_A light emitting layer 18 made of a yP _1-y single crystal compound semiconductor was epitaxially grown on the barrier layer 16.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resonance type surface emitting device having a quantum well type light emitting layer, and more particularly, to a technique for reducing the spread of light from a light extraction surface and increasing the directivity in the vertical direction. Things.

[0002]

[Prior art]

(a) a quantum well type light-emitting layer sandwiched between barrier layers, and (b) a pair of optical resonators provided between the barrier layer and the light-emitting layer and configured to reflect light generated in the light-emitting layer There has been known a resonance type surface emitting element having a reflector and causing the light generated in the light emitting layer to resonate in the optical resonator and extract the light from a light extraction surface parallel to the light emitting layer. For example, RC-L
An example is a device called an ED (resonant cavity light emitting diode) or a VCSEL, and an optical semiconductor device described in Japanese Patent Application Laid-Open No. 4-167484.

FIG. 4 shows an example of such a conventional resonance type surface emitting device. The resonance type surface emitting device 50 includes a substrate-side reflecting mirror 54 and a first barrier layer which are sequentially grown on a substrate 52. 56, a light-emitting layer 58, a second barrier layer 60, a radiation-surface-side reflecting mirror 62, and a clad layer 64. When a drive current is applied between electrodes (not shown) provided on the upper and lower surfaces of the light-emitting layer, The light generated in the layer 58 is resonated between the pair of reflecting mirrors 54 and 62, and is emitted to the outside from a light extraction surface 66 which is the upper surface of the cladding layer 64. For example, in the case of a GaAs surface emitting device having an emission wavelength λ ₀ of about 850 nm, the barrier layers 56 and 60 are made of an AlGaAs single crystal compound semiconductor having a thickness of about λ ₀ / 4n (n is a refractive index). The light emitting layer 58 is made of a GaAs single crystal compound semiconductor having a thickness of about 10 nm.

[0004]

By the way, in a light emitting device having such a quantum well type light emitting layer, a band split occurs in a valence band of the light emitting layer due to a shift of a quantum effect, and a sub-band of a heavy hole is generated. And a light hole sub-band, the difference in the energy level causes the emission wavelength range to be broader on the shorter wavelength side than the peak wavelength λ ₀ . That is, as shown by a dotted line in FIG. 2, an emission spectrum is generated on the shorter wavelength side than the peak wavelength λ ₀ due to the sub-band of the light hole having a lower energy level, and the entire emission spectrum is shown by a solid line. It becomes broader on the shorter wavelength side. FIG. 2 shows the light extraction surface 6.
6 is an intensity distribution of an emission wavelength including an oblique direction extracted from FIG.

On the other hand, the resonance type light emitting element generally has a characteristic of emitting light in a direction perpendicular to the light extraction surface due to a so-called cavity QED effect, that is, excellent directivity. The light on the shorter wavelength side than the (peak wavelength) resonates in the direction inclined from the vertical direction due to the Bragg reflection condition as shown in FIG. 4 and exits from the light extraction surface. For this reason, in the case of a resonance type light emitting device having a quantum well type light emitting layer, the relative intensity distribution of outgoing light having the outgoing angle θ as a parameter is widened as shown by a solid line in FIG. 3, and a desired directivity can be obtained. In addition, there is a problem that monochromaticity is reduced and chromatic aberration is impaired.
Note that the emission angle θ includes the refraction of light on the light extraction surface 66.

The present invention has been made in view of the above circumstances, and an object of the present invention is to reduce the spread of light from a light extraction surface in a resonance type surface light emitting device having a quantum well type light emitting layer. To improve directivity and monochromaticity.

[0007]

Means for Solving the Problems In order to achieve the above object, the first invention comprises (a) a quantum well type light emitting layer sandwiched between barrier layers, and (b) a quantum well type light emitting layer sandwiching the barrier layer and the light emitting layer. And a pair of reflecting mirrors constituting an optical resonator for reflecting light generated in the light emitting layer, and resonating the light generated in the light emitting layer with the optical resonator to be parallel to the light emitting layer. (C)
The light emitting layer is subjected to a compressive strain in the stacking direction such that the energy levels of the heavy holes and the light holes in the valence band of the light emitting layer are substantially equal.

In a second aspect based on the first aspect, the barrier layer is a single crystal compound semiconductor of Al _x Ga _1-x As (where 0 <x <1), and the light-emitting layer is the Al _x Ga _1-x GaAs _y P _1-y having a smaller lattice constant than the As single crystal compound semiconductor (where 0 <
y <1) It is a single crystal compound semiconductor, and is characterized in that a compressive strain in a stacking direction is applied to its light emitting layer by crystal growth by an epitaxial growth technique.

[0009]

In such a resonance type surface emitting device, compressive strain in the stacking direction is applied to the light emitting layer so that the energy levels of the heavy and light holes in the valence band of the light emitting layer are substantially equal. As a result, the emission wavelengths of the two are substantially matched, and the emission wavelength width is narrowed. This allows
The resonance in the oblique direction of the optical resonator is reduced, the spread of light from the light extraction surface is reduced, the directivity is improved, and the monochromaticity is improved, and the chromatic aberration is reduced. Further, since the light from the light hole is well extracted, the light emission intensity is increased and the light extraction efficiency is improved.

[0010] In the second invention, Al _x Ga _1-x As by the lattice constant than that on the single-crystal compound semiconductor is smaller GaAs _y P _1-y single crystal compound semiconductor is epitaxially grown, the GaAs _y P Compressive strain in the stacking direction, in other words, tensile strain in the plane direction, is favorably applied to the _1-y single crystal compound semiconductor.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Here, the present invention is suitably applied to a resonance type surface emitting device having a GaAsP-based quantum well type light emitting layer. The present invention can be applied to a surface emitting device.

The compression strain of the light emitting layer in which the energy levels of the heavy hole and the light hole in the valence band of the light emitting layer become substantially equal is, for example, such that the half width of the emission spectrum including the light emitting component in the oblique direction is about 40 nm or less. Is set to
When the barrier layer and the light emitting layer are stacked by epitaxial growth technology, a compound semiconductor having a smaller band gap energy and a smaller lattice constant than the barrier layer is used as the light emitting layer. In the case of a GaAsP-based quantum well light emitting layer,
Al _x Ga _1-x As single crystal compound semiconductor
The mixed crystal ratio x of Al is, for example, in the range of 0.2 to 0.6, particularly 0.4.
The mixed crystal ratio y of As of the GaAs _y P _1-y single crystal compound semiconductor forming the light emitting layer is preferably, for example, about 0.96 in the range of 0.90 to 0.99.

The distance between the pair of reflecting mirrors of the optical resonator, that is, the total thickness of the barrier layer and the light emitting layer between them is about ｎn (n is the refractive index) of the light emission peak wavelength λ ₀ of the light emitting layer. It is desirable that As the pair of reflecting mirrors, a semiconductor multilayer film reflecting mirror is preferably used, but a dielectric thin film, a metal thin film, or the like can also be used.

As the epitaxial growth technique of the second invention, MOCVD (Metal Organic Chemistry) having excellent film thickness control is used.
Although an ical vapor deposition (organic metal chemical vapor deposition) method is preferably used, other methods such as an MBE (molecular beam epitaxy) method can also be used.

An embodiment of the present invention will be described below in detail with reference to the drawings. In the following examples, the dimensional ratios and the like of each part are not necessarily drawn accurately.

FIG. 1 is a diagram showing a configuration of a surface emitting type light emitting diode (hereinafter, simply referred to as a light emitting diode) 10 which is an embodiment of a resonance type surface emitting device of the present invention. In the figure, a light-emitting diode 10 has a substrate-side reflecting mirror 14, a first barrier layer 16, a light-emitting layer 18, a second barrier layer 20, and a radiation surface which are sequentially crystal-grown on a substrate 12 by an epitaxial growth technique such as MOCVD. It comprises a side reflector 22, a cladding layer 24, and a current control layer 26, and a lower electrode 28 and an upper electrode 30 fixed to the lower surface of the substrate 12 and the upper surface of the current control layer 26, respectively.

The substrate 12 is a compound semiconductor made of an n-GaAs single crystal having a thickness of, for example, about 350 μm. The substrate-side reflecting mirror 14 is, for example, an n-AlAs single-crystal compound semiconductor having a thickness of about 75 nm and an n-Al _x Ga _1-x As single-crystal compound semiconductor having a thickness of about 60 nm. For example, a so-called n-type distributed reflection type semiconductor multilayer film reflecting mirror (DBR) constituted by alternately stacking about 30 sets of Al _x Ga _{1 -x} As
The mixed crystal ratio x of Al of the single crystal compound semiconductor is, for example, about 0.2. The thickness of each layer constituting the substrate-side reflecting mirror 14 has an emission peak wavelength λ ₀ (about 850 nm in this embodiment).
Is determined to be about 1 / 4n (n is a refractive index).

First barrier layer 16 and second barrier layer 20
Are i-Al _x Ga ₁ -xAs single crystal compound semiconductors each having a thickness of about 55 nm, and a mixed crystal ratio x of Al is, for example, about 0.3. The thickness of each of these barrier layers 16 and 20 is determined to be about １ ／ n (n is a refractive index) of the emission peak wavelength λ ₀ (about 850 nm in this embodiment). The light emitting layer 18 is a so-called quantum well composed of an i-GaAs _{yP 1-y} single crystal compound semiconductor having a smaller band gap energy and a smaller lattice constant than the barrier layers 16 and 20.
Its thickness is about 10 nm so that the emission peak wavelength λ ₀ is about 850 nm, for example, and the mixed crystal ratio y of As is about 0.96, for example. Since the lattice constant of the light emitting layer 18 is smaller than the lattice constant of the first barrier layer 16, a compressive strain (tensile strain in a plane direction) is applied to the light emitting layer 18 in the stacking direction. The constant difference is determined so that the energy levels of the heavy hole and the light hole in the valence band of the light emitting layer 18 caused by the quantum effect are substantially equal. The first barrier layer 16 is an n ⁻ layer, that is, a ν layer, and the light emitting layer 18 and the second barrier layer 20 are p ^−, that is, a π layer.

Similarly to the substrate-side reflector 14, the radiation-surface-side reflector 22 includes, for example, a p-AlAs single-crystal compound semiconductor having a thickness of about 75 nm and a p-Al _x Ga _1- in _x as single crystal compound and a semiconductor stacked 10 sets about alternately DBR,
The mixed crystal ratio x of Al in the Al _x Ga _1-x As single crystal compound semiconductor is, for example, about 0.2, and the thickness of each layer is the emission peak wavelength λ.
₀ (about 850 nm in this embodiment) is determined to be about １ ／ n (n is a refractive index). In this embodiment, the substrate-side reflecting mirror 14 and the radiation-side reflecting mirror 22
Correspond to a pair of reflecting mirrors, and the distance between them, that is, the length of the optical resonator is about 425 nm in terms of the length in vacuum (refractive index n = 1), that is, about 1/2 of the emission peak wavelength λ ₀ . It is length. For this reason, the light generated in the light emitting layer 18 is repeatedly reflected by the substrate-side reflecting mirror 14 and the radiation surface-side reflecting mirror 22, and a standing wave is formed between them, and the light emitting layer 18 is located at the antinode of the standing wave. The substrate-side reflecting mirror 14 and the radiation-surface-side reflecting mirror 22 constitute an optical resonator that repeatedly reflects light generated in the light emitting layer 18.

The cladding layer 24 is, for example, a p-Al _x Ga _{1 -x} As single crystal compound semiconductor having a thickness of about 2 μm,
The current control layer 26 is, for example, n-Al _x Ga having a thickness of about 1 μm.
_{In a 1-x} As single crystal compound semiconductor, the mixed crystal ratio x of Al is, for example, about 0.2. In the current control layer 26, a recess 32 having a diameter of, for example, about 50 μm is provided at the center thereof by etching or the like, and impurities such as Zn, which are p-type dopants, are diffused from the upper surface side at a high concentration. As a result, the high concentration diffusion region 3
4 are formed. In the high-concentration diffusion region 34, the conductivity of the cladding layer 24 is enhanced, and the conductivity type of the current control layer 26 is reversed to be a p-type semiconductor. As a result, a current constriction structure is formed in which only the central portion of the current control layer 26 whose conductivity type has been inverted up to the boundary with the cladding layer 24, that is, only the lower portion of the recess 32 can be energized. Of these, light is mainly generated only in the lower portion of the recess 32, and
Light is extracted from the light extraction surface 36 which is the bottom surface of the recess 32.

The lower electrode 28 has a thickness of, for example, about 1 μm, and is formed by laminating an Au—Ge alloy, Ni and Au on the entire lower surface of the substrate 12 in order from the substrate 12 side. The upper electrode 30 has a thickness of, for example, about 1 μm, and an Au—Zn alloy and Au are sequentially laminated on the peripheral portion of the current control layer 26 on the outer peripheral side of the concave portion 32 from the current control layer 26 side. It was done. These lower electrodes 28
The upper electrode 30 is an ohmic electrode.

In such a light emitting diode 10, when a forward drive current is applied between the lower electrode 28 and the upper electrode 30, light is generated in the quantum well type light emitting layer 18; The generated light is resonated between the pair of reflecting mirrors 14 and 22 constituting the optical resonator, and is emitted from the light extraction surface 36 parallel to the light emitting layer 18.

By the way, in the quantum well type light emitting layer 18 as in the present embodiment, a band split occurs in the valence band due to the shift of the quantum effect, and a heavy hole subband and a light hole subband occur. Due to the difference in the energy level, the emission wavelength range becomes broader on the shorter wavelength side than the peak wavelength λ _{0 as} shown by the solid line in FIG. Further, such light on the short wavelength side resonates in a direction inclined from the vertical direction due to the Bragg reflection condition and exits from the light extraction surface 36. Therefore, as shown by a solid line in FIG. However, there is a problem in that the relative intensity distribution of the emitted light is widened, the desired directivity cannot be obtained, and the monochromaticity is reduced to deteriorate the chromatic aberration.

On the other hand, in the present embodiment, a compressive strain is applied to the light emitting layer 18, and the energy levels of the heavy hole and the light hole in the valence band of the light emitting layer 18 are made substantially equal. The wavelengths are substantially matched, and the emission wavelength width is narrowed as shown by the dashed line in FIG. Thereby, the resonance in the oblique direction of the optical resonator is reduced, and as shown by the dashed line in FIG. 3, the spread of the light from the light extraction surface 36 is reduced, and the directivity is improved, and the monochromaticity is improved. Chromatic aberration is reduced. Further, since the light from the light hole is well extracted, the light emission intensity is increased and the light extraction efficiency is improved. In addition, the half-width of the emission spectrum of all the emitted light of this embodiment shown by the one-dot chain line in FIG. 2 is about 40 nm.

In the present embodiment, the barrier layers 16 and 20
The light emitting layer 18 made of an i-GaAs _{yP 1-y} single crystal compound semiconductor having a smaller band gap energy and a smaller lattice constant than the light emitting layer 18 is epitaxially grown on the barrier layer 16 so that the light emitting layer 18 Compression strain (plane-direction tensile strain) is preferably applied.

While the embodiment of the present invention has been described in detail with reference to the drawings, the present invention can be embodied in still another embodiment.

For example, in the embodiment, the case where the present invention is applied to the light emitting diode 10 has been described.
The present invention can be similarly applied to other resonance type surface emitting devices such as L.

Further, in the embodiment, the case where the present invention is applied to the light emitting diode 10 for a point light source that extracts light from the light extraction surface 36 provided at the center of the upper surface has been described. The present invention is similarly applied to a light emitting diode of a full-surface light emitting type that extracts light from a light source.

Although not specifically exemplified, the present invention
Various changes can be made without departing from the spirit of the invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a light emitting diode which is one embodiment of a resonance type surface emitting device of the present invention.

FIG. 2 is a diagram for explaining an emission spectrum (dashed line) of the light emitting diode of FIG. 1 in comparison with a conventional case (solid line).

FIG. 3 shows the directivity of the light emitting diode shown in FIG. 1 (dashed line).
Is a diagram for explaining in comparison with the conventional case (solid line).

FIG. 4 is a diagram illustrating an example of a conventional resonance type surface emitting element.

[Description of Signs] 10: Light-emitting diode (resonant surface-emitting device) 14: Substrate-side reflector 16: First barrier layer 18: Light-emitting layer 20: Second barrier layer 22: Radiation-side reflector 36: Light extraction surface

Claims (2)

[Claims] 1. A quantum well type light-emitting layer sandwiched between barrier layers, and a pair of reflectors provided between the barrier layer and the light-emitting layer to constitute an optical resonator for reflecting light generated in the light-emitting layer. A resonant surface light-emitting device having a mirror and resonating light generated in the light-emitting layer by the optical resonator and extracting the light from a light extraction surface parallel to the light-emitting layer, wherein a heavy hole in a valence band of the light-emitting layer is provided. A compressive strain in a stacking direction is applied to the light emitting layer such that the energy levels of the light hole and the light hole are substantially equal. 2. The barrier layer is made of Al _x Ga _{1 -x} As (where 0
<X <1) single-crystal compound semiconductor, the light emitting layer is the Al _x
Ga whose lattice constant is smaller than that of Ga _1-x As single crystal compound semiconductor
As _y P _1-y (where 0 <y <1) is a single crystal compound semiconductor, in which a compressive strain in a stacking direction is applied to the light emitting layer by crystal growth by an epitaxial growth technique. A resonance type surface emitting device according to claim 1.