JP2000174327A - Vertical micro resonator light-emitting diode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a vertical micro resonator light-emitting diode having high external quantum efficiency. SOLUTION: A vertical micro resonator hose length of half the wavelength of light-mission wavelength is formed by a pair of multilayer reflection layers 3 and 7, a quantum well layer 5 is provided at its center so that it is present at an antinode position of standing wave of optical wave, and a resonator QED(cavity quantum electrodynamics) effect enhances natural release. As the quantum well layer 5, an inclined quantum well layer 5 comprising an inclined band gap is used. This configuration localizes electrons and positive holes at the same place in the quantum well layer, causing conspicuous light emission through the recombination of electrons and positive holes, resulting in further conspicuous enhancement in natural release due to the resonator QED effect.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vertical microcavity type light emitting diode used for a print head or the like.

[0002]

2. Description of the Related Art Conventionally, a vertical microcavity type light emitting diode having a structure in which a light emitting layer is sandwiched in the center and a pair of multilayer reflective layers are provided on both sides thereof can be integrated at a high density as a one-dimensional array or a two-dimensional array. Therefore, it is known as one suitable for a printer head or the like. Typically, a quantum well layer is used as a light emitting layer, and a pair of multilayer reflective layers are symmetrically arranged on both sides of the light emitting layer to form a vertical microresonator having a resonator length of a half wavelength of the emission wavelength (for example, See JP-A-9-260783).

[0003]

According to such a vertical microcavity type light emitting diode, the resonator QED (cavityq
Although the spontaneous emission of electrons in the quantum well layer due to recombination with holes is enhanced only in the resonance mode due to the (uantum elctrodynamics) effect, an essentially high directivity and high external quantum efficiency can be obtained. In the application of high-density mounting, further higher efficiency is required. The present invention focuses on the quantum well layer to improve external quantum efficiency, and the degree of recombination of electrons with holes depends on the probability of existence of electrons and holes in the quantum well. Also, attention is paid to the fact that the degree of enhancement of spontaneous emission due to the resonator QED effect is also limited by the quantum well.

[0004]

SUMMARY OF THE INVENTION The present invention relates to a vertical microcavity light emitting diode. In one embodiment of the present invention, in a vertical microcavity light emitting diode having a light emitting layer including at least one quantum well layer and a multilayer reflective layer provided on both sides of the light emitting layer, The well layer has an inclined or step-shaped band gap that decreases from the electron injection side to the hole injection side or from the hole injection side to the electron injection side. According to this configuration, electrons and holes are localized at the same place in the quantum well, and light emission due to recombination of electrons and holes occurs more remarkably. The enhancement is more pronounced, resulting in LEDs with higher external quantum efficiency. In another aspect of the present invention, in a vertical microcavity light emitting diode having a light emitting layer having a super lattice structure including a plurality of quantum well layers and a multilayer reflective layer provided on both sides of the light emitting layer, The quantum well layer has an inclined or stepped band gap that decreases from the electron injection side to the hole injection side or from the hole injection side to the electron injection side. According to this configuration, the concentration of carriers during energization over the entire superlattice structure light-emitting layer can be suppressed, and broadening of the spectrum can be prevented. As a result, the resonator QED effect can be more efficiently obtained.
By localizing electrons and holes at the same location, the probability of radiative recombination can be increased, so that an LED with higher external quantum efficiency can be obtained. According to another aspect of the present invention, there is provided a vertical micro-resonance having a light emitting layer including at least three quantum well layers stacked with a barrier layer interposed therebetween, and a multilayer reflective layer provided on both sides of the light emitting layer. In the organic light emitting diode, at least two quantum well layers have an inclined shape or a stepped shape in which the size decreases from the electron injection side toward the hole injection side or from the hole injection side toward the electron injection side. It has a band gap and different inclined shapes or staircase shapes. According to this configuration, since the localization of the carriers is made different, the radiative recombination in each quantum well layer can be made equal, and the band-filling effect can be prevented. LED with higher external quantum efficiency with enhanced spontaneous emission due to QED effect
Is obtained.

[0005]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view of a vertical microcavity type light emitting diode (hereinafter, referred to as an LED) showing a first embodiment of the present invention. The LED shown in FIG.
MOCVD (Metal Organic Chemical Vapor)
An n-GaAs buffer layer 2, a substrate-side multilayer reflection layer 3, a substrate-side cladding layer 4, a tilted quantum well layer 5, and an emission-side cladding layer are sequentially formed by epitaxial growth techniques such as por Deposition (metal organic chemical vapor deposition). 6, an emission-side multilayer reflective layer 7, an ohmic cap layer 8, and an ohmic layer 9 are formed. Further, after the crystal is grown to the ohmic cap layer 8 and the ohmic layer 9 on the emission-side multilayer reflection layer 7, only the necessary portions are removed to a portion substantially corresponding to the substrate-side multilayer reflection layer 3 by mesa etching or the like. A film 10 is formed, and a p-electrode 11 is formed so as to make electrical contact with the ohmic layer 9.
On the other hand, on the back surface of the n-GaAs substrate 1, an n-electrode 12 is formed. p electrode 11 is Ti / Pt / Au, n electrode 12
Used AuGeNi / Au.

In FIG. 1, the substrate-side multilayer reflective layer 3 has n
It comprises a plurality of pairs of -AlAs layer 3a and n-Al0.2Ga0.8As layer 3b, and the thickness of each layer is determined so that the emission wavelength in each layer is about 1/4 wavelength. Similarly, the emission-side multilayer reflection layer 7 also includes p-AlAs layers 7a, p-
It consists of a plurality of pairs of Al0.2Ga0.8As layers 7b, and the thickness of each layer is also determined so that the emission wavelength in each layer is about 1/4 wavelength. However, the number of pairs of the emission-side multilayer reflection layer 7 must be smaller. The number of pairs is a matter of design, and the former is selected from the conditions that maximize the spontaneous emission within the range of about 100% in reflectance and the latter in the range of about 60 to 98%. Interposed between the multilayer reflective layers 3 and 7 are the substrate-side cladding layer 4, the tilted quantum well layer 5, and the emission-side cladding layer 6, and the emission wavelength in these three layers is one.
The thickness of the cladding layers 4 and 6 is adjusted so as to be about half the wavelength, and the inclined quantum well layer 5 is present at the antinode of the standing wave of the light wave. The composition of the substrate side cladding layer 4 and the emission side cladding layer 6 is i-Al0.4Ga0.6A.
s. The graded quantum well layer 5 has a composition of i-Al0.13
Ga 0.87 As is continuously changed from i-Al 0.11 Ga 0.89 As, and has an inclined band gap which becomes smaller from the electron injection side to the hole injection side. Also, since the inclined quantum well layer 5 must form a quantum well, the thickness is about 10 nm. The emission peak wavelength is 760 nm.

Next, the operation of the LED shown in FIG. 1 will be described. In FIG. 1, when a predetermined current flows from the p-electrode 11 to the n-electrode 12, light emission occurs due to recombination of electrons and holes in the inclined quantum well layer 5. In addition, the multilayer reflective layers 3, 7
As shown in FIG. 1A, a standing wave of a light wave is generated by the resonator structure, and electrons and holes are further radiatively recombined by the standing wave, and spontaneous emission is enhanced by the resonator QED effect. Light is emitted near the edge of the p-electrode 11. Here, in the case of a quantum well layer having a constant band gap, electrons are drawn to the p-electrode side and holes are drawn to the n-electrode side. Because of the layer 5, the standing wave of electrons and holes becomes a standing wave localized in the region B of FIG. 2 showing the band gap structure, that is, the p-electrode side, and the existence probability of electrons and holes is Both are localized on the p-electrode side, and the concentration of the electrons and holes makes it easier for light-emitting recombination to occur. Also, the resonator QE
The spontaneous emission enhancement by the D effect also becomes remarkable because the probability of existence of both electrons and holes increases in the same place, and as a result,
External quantum efficiency can be increased. The electron and hole levels do not tilt in the quantum well layer, even if they are tilted, so that the half-width of the spectrum may increase even if recombination occurs in any part of the quantum well. There is no.

Next, a second embodiment of the present invention will be described.
In the LED of the second embodiment, a stepped quantum well layer 20 is used in place of the inclined quantum well layer 5 in the configuration of FIG. Stepwise quantum well layer 20 of the second embodiment
Changes the composition from i-Al0.13Ga0.87As to i-Al0.11G
It has a stepwise change to a0.89As, and has a step-shaped band gap that decreases from the electron injection side to the hole injection side. The thickness is about 5 nm on both the n-electrode side and the p-electrode side. The emission peak wavelength is 760 nm. In the case of the step-type quantum well layer 20, as in the first embodiment, the standing wave of electrons and holes is localized in the region c of FIG. Since electrons and holes are localized at the same place in the quantum well layer as a wave, light emission due to recombination of electrons and holes occurs more remarkably, and spontaneous emission enhancement due to the resonator QED effect also becomes more remarkable. , LE with higher external quantum efficiency
D can be obtained. In addition, in the case of the step-type quantum well, the composition can be easily adjusted as compared with the inclined quantum well, so that the crystal growth becomes easy. As a result, there is an effect that the variation between lots can be reduced. In the first and second embodiments, an LED having a single quantum well layer has been described. However, in an LED having a double or quadruple multilayer quantum well layer configuration, an inclined type or a step type may be used. A quantum well layer can be used, and similar effects can be expected.

Next, a third embodiment of the present invention will be described.
As shown in FIG. 4, the LED according to the third embodiment has an n-GaAs substrate 1, an n-GaAs buffer layer 2, a substrate-side multilayer reflection layer 3, a substrate-side cladding layer 4, an emission port, as in the first embodiment. It has a side cladding layer 6, an emission-side multilayer reflective layer 7, an ohmic cap layer 8, an ohmic layer 9, a SiNx film 10, a p-electrode 11, and an n-electrode 12, and has a superlattice layer 30 as a light-emitting layer. This is different from the first embodiment. The superlattice layer 30 is composed of graded quantum well layers 31a, 31b, 31
c, 31d and barrier layers 32a, 32b, 32c. The number of laminations is a matter of design, but it is preferable that the number be three or more, and at most about ten. Note that the emission wavelength in three layers including the substrate-side cladding layer 4, the superlattice layer 30, and the emission-side cladding layer 6 is set to be about １ ／ wavelength. Graded quantum well layers 31a to 31
d means that the composition is from i-Al0.13Ga0.87As to i-Al0.11
It is changed continuously to Ga 0.89 As, and has an inclined band gap that decreases from the electron injection side to the hole injection side. The composition of the barrier layers 32a to 32c is i-Al0.4Ga0.6As. Also, each of the inclined quantum well layers 31a to 31
The thickness of d is about 10 nm, and the thickness of the barrier layers 32a to 32c is 8 nm.
And a superlattice structure in which carriers are coupled between quantum wells. The emission peak wavelength is 760 nm.

Next, the operation of the LED shown in FIG. 4 will be described. In FIG. 4, when a predetermined current flows from the p-electrode 11 to the n-electrode 12, as in the first embodiment, light emission occurs due to recombination of electrons and holes in the superlattice layer 30. , 7 generate a standing wave of a light wave as shown in FIG.
Further, electrons and holes recombine radiatively, and spontaneous emission is enhanced by the cavity QED effect. Here, when the quantum well layer is not of the tilt type, since there is no localization in each quantum well and the interlayer movement is free, when a current flows, electrons flow to the p-electrode side and holes flow to the p-electrode side. The concentration of the carriers on the n-electrode side causes the generation of carriers that are not at the same level, causing a broadening of the spectrum, and the cavity QE.
This has the adverse effect of reducing the D effect. On the other hand, even if it has a superlattice structure, if each of the quantum well layers 31a to 31d is of an inclined type as in the third embodiment, electrons and electrons are generated in a region e of FIG. Since localization occurs in both holes, the concentration of carriers over the entire superlattice layer 30 can be considerably reduced as compared with the case of a quantum well that is not tilted. That is, since localization of carriers occurs in each of the quantum well layers 31a to 31d, the carriers at this location lose the freedom of movement in the superlattice layer, and as a result, generation of carriers that are not at the same level occurs. Thus, the broadening of the spectrum can be suppressed, and the probability of radiative recombination increases due to the increase in the probability of electrons and holes existing in the same place, thereby improving external quantum efficiency. The quantum well layers 31a to 31a
Since the levels of electrons and holes in 1d do not become inclined,
The spectral purity does not decrease.

Next, a fourth embodiment of the present invention will be described.
The LED of the fourth embodiment uses the superlattice layer 40 having the step-type quantum well layers 41a to 41d instead of the superlattice layer 30 having the inclined quantum well layers 31a to 31d in the configuration of FIG. Things. The step-type quantum well layers 41a to 41d of the superlattice layer 40 have a composition of i-Al0.13Ga0.87.
It has a stepwise change from As to i-Al0.11Ga0.89As, and has a step-shaped band gap that decreases from the electron injection side to the hole injection side. In addition, each of the step-type quantum well layers 41a-41
The thickness of d is about 10 nm, and the thickness of the barrier layers 32a to 32c is 8 nm.
As a degree, a superlattice structure in which carriers are coupled between the quantum well layers is adopted. The emission peak wavelength is 760 nm. Also in the case of the step-type quantum well layers 41a to 41d, as in the third embodiment, both electrons and holes are localized in the region of FIG. 6 showing the bandgap structure. Freedom of movement in the superlattice layer is lost, and as a result, generation of non-identical carriers can be prevented, spectrum broadening can be suppressed, and the probability of electrons and holes existing at the same location increases. As a result, the probability of radiative recombination increases, and the external quantum efficiency can be improved.

Next, a fifth embodiment of the present invention will be described.
As shown in FIG. 7, the LED according to the fifth embodiment has an n-GaAs substrate 1, an n-GaAs buffer layer 2, a substrate-side multilayer reflective layer 3, a substrate-side cladding layer 4, and an emission port, as in the first embodiment. It has a side cladding layer 6, an emission-side multilayer reflection layer 7, an ohmic cap layer 8, an ohmic layer 9, a SiNx film 10, a p-electrode 11, and an n-electrode 12, and is laminated as a light-emitting layer via barrier layers 51a and 51b. Three quantum well layers 5
0a, 50b, and 50c are different from the first embodiment. The quantum well layer 50c has a composition of i-Al0.12
Ga 0.88 As was constant. Quantum well layer 5
0b means that the composition is from i-Al0.13Ga0.87As to i-Al0.
The band gap is continuously changed to 11Ga0.89As, and has an inclined band gap which becomes smaller from the electron injection side to the hole injection side. The quantum well layer 50a has a composition of i-Al0.14Ga0.86As
It has a bandgap of an inclined shape that decreases continuously from the electron injection side to the hole injection side, and has a slope larger than that of the quantum well layer 50b. Have The thickness of each of the quantum well layers 50a to 50c is about 10 nm. The emission peak wavelength is equal to the three quantum well layers 50a to 50c.
All are 760 nm. The thickness of the barrier layers 51a and 51b is set to 20 nm so that carriers are not coupled between the quantum wells. The emission wavelength in the layer composed of the substrate-side cladding layer 4, the quantum well layers 50a to 50c, the barrier layers 51a and 51b, and the emission-side cladding layer 6 is 1 /.
It is determined to be about two wavelengths.

Next, the operation of the LED shown in FIG. 7 will be described. Now, considering a configuration in which the same quantum well layer is stacked in multiple layers, the existence probability of carriers differs for each quantum well layer, and the existence probability of carriers decreases in order from the p-electrode 11 side. Bonding is likely to occur. Therefore, the spectrum half-width increases due to the band-filling effect in this order, and in the case of an optical resonator structure expecting the resonator QED effect, there is an inherent adverse effect of lowering the spectral purity. On the other hand, in the LED according to the fifth embodiment, the p-electrode 11 side is a normal quantum well layer 50c which is not a tilt type, and the quantum well layers 50a and 50
b is an inclined type. By using a graded quantum well layer, carrier localization occurs in the region of FIG. 8 showing a band gap structure. However, this quantum well layer 50
Since the levels of electrons and holes in b do not become inclined, the spectral purity does not decrease. That is, the carrier localization prevents the probability of the existence of carriers from being reduced without lowering the spectral purity.
It becomes possible to cause radiative recombination of the same intensity as a. Further, the inclined quantum well layer 50a located on the n-electrode side
In FIG. 8, in the region (h) of FIG. 8 showing the bandgap structure, the slope is steeper, so that greater carrier localization occurs. As described above, by setting the inclination of the inclined quantum well layers 50a to 50c so that the same light emitting recombination occurs in the three quantum well layers 50a to 50c, the light emitting recombination of the three layers can be performed. Can be distributed. This means that the band filling effect can be prevented, and as a result, the spectral purity can be increased. If the spectral purity goes up, of course,
The ratio of coupling to the resonance mode is also increased, and the spontaneous emission can be increased by the resonator QED effect.
An LED with higher external quantum efficiency can be obtained.

Next, a sixth embodiment of the present invention will be described.
The LED of the sixth embodiment uses the step-type quantum well layers 60a, 60b, and 60c instead of the inclined quantum well layers 50a, 50b, and 50c in the configuration of FIG. The quantum well layer 60c is made of IA as in the fifth embodiment.
It has a constant composition of 0.12Ga0.88As. The quantum well layer 60b has a composition on the n-electrode side of i-Al0.13Ga0.87.
As and the p-electrode side is stepwise changed to i-Al0.11Ga0.89As, and has a step-shaped band gap that decreases from the electron injection side to the hole injection side. . The quantum well layer 60a has n
The electrode side is made of i-Al0.14Ga0.86As and the p-electrode side is made of i-Al0.1Ga0.9As, and a step-shaped band gap that decreases from the electron injection side toward the hole injection side is formed. It has a larger step than the quantum well layer 50b. Even in the configuration using such a stepped quantum well layer, localization of carriers occurs in the regions l and nu in FIG. 9 showing the band gap structure. By increasing the step of the band gap in order from the p-electrode 11 side, the three quantum well layers 60a to 60
The emission recombination of 60c can be controlled in the same way, the band filling effect can be prevented, and as a result, the spontaneous emission enhancement by the resonator QED effect becomes more remarkable, and the LED having higher external quantum efficiency Can be obtained. Can be

In the fifth and sixth embodiments, it has been described that the radiative recombination is more remarkable on the p-electrode side, but this is not always the case depending on the device.
Conversely, the luminescence recombination may be more remarkable on the n side, or the luminescence recombination may be less or more at the center.
Various cases are conceivable. Even in such various cases, the arrangement may be performed in an order corresponding thereto, and the arrangement order is not limited to the present embodiment. The same applies to the number of quantum wells, and the present invention is also effective with two or more layers. Further, in the fifth and sixth embodiments, one quantum well layer is a non-graded quantum well layer or a non-stepped quantum well layer. Different quantum well layers.

[0016]

As described above, according to the present invention, a quantum well layer having an inclined or stepped band gap which decreases from the electron injection side toward the hole injection side is used as the light emitting layer. And holes are localized in the same place in the quantum well, so that light emission due to recombination of electrons and holes occurs more prominently, and has an effect of obtaining an LED with higher external quantum efficiency. .

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a vertical microcavity light emitting diode showing a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of a band gap structure in the vertical microcavity light emitting diode according to the first embodiment of the present invention.

FIG. 3 is an explanatory diagram of a band gap structure in a vertical microcavity light emitting diode according to a second embodiment of the present invention.

FIG. 4 is a cross-sectional view of a vertical microcavity light emitting diode showing a third embodiment of the present invention.

FIG. 5 is an explanatory view of a band gap structure in a vertical microcavity light emitting diode according to a third embodiment of the present invention.

FIG. 6 is an explanatory diagram of a bandgap structure in a vertical microcavity light emitting diode according to a fourth embodiment of the present invention.

FIG. 7 is a cross-sectional view of a vertical microcavity light-emitting diode showing a fifth embodiment of the present invention.

FIG. 8 is an explanatory diagram of a band gap structure in a vertical microcavity light emitting diode according to a fifth embodiment of the present invention.

FIG. 9 is an explanatory diagram of a band gap structure in a vertical microcavity light emitting diode according to a sixth embodiment of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 n-GaAs substrate 2 n-GaAs buffer layer 3 substrate-side multilayer reflection layer 4 substrate-side cladding layer 5 tilted quantum well layer 6 emission-side cladding layer 7 emission-side multilayer reflection layer 8 ohmic cap layer 9 ohmic layer 10 SiNx film 11 p electrode 12 n electrode

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Shoji Tomon 1-7-12 Toranomon, Minato-ku, Tokyo Oki Electric Industry Co., Ltd. (72) Inventor Masumi Koizumi 1-7-12 Toranomon, Minato-ku, Tokyo Offshore F term (reference) in Electric Industry Co., Ltd. 5F041 AA03 CA04 CA05 CA12 CA34 CA36 CA60 CA65 CB15 FF13 5F073 AA51 AA55 AA71 AA74 AB17 BA07 CA05 CB07 CB08 DA05

Claims (6)

[Claims] 1. A vertical microcavity type light emitting diode having a light emitting layer including at least one quantum well layer, and a multilayer reflective layer provided on both sides of the light emitting layer, wherein the quantum well layer is A vertical micro-resonance having an inclined band gap that decreases from the electron injection side to the hole injection side or from the hole injection side to the electron injection side. Type light emitting diode. 2. A vertical microcavity type light emitting diode having a light emitting layer including at least one quantum well layer and a multilayer reflective layer provided on both sides of the light emitting layer, wherein the quantum well layer is A vertical microresonance having a step-shaped band gap that decreases from the electron injection side to the hole injection side or from the hole injection side to the electron injection side. Type light emitting diode. 3. A vertical microcavity type light emitting diode comprising a light emitting layer having a superlattice structure including a plurality of quantum well layers, and a multilayer reflective layer provided on both sides of the light emitting layer. Has a band gap of an inclined shape that decreases from the electron injection side to the hole injection side or from the hole injection side to the electron injection side. Resonator type light emitting diode. 4. A vertical microcavity light emitting diode having a light emitting layer having a superlattice structure including a plurality of quantum well layers and a multilayer reflective layer provided on both sides of the light emitting layer, wherein the quantum well layer Have a step-shaped band gap that decreases from the electron injection side to the hole injection side or from the hole injection side to the electron injection side. Resonator type light emitting diode. 5. A vertical microcavity light emitting diode having a light emitting layer including at least three quantum well layers stacked with a barrier layer interposed therebetween, and a multi-layer reflecting layer provided on both sides of the light emitting layer. Wherein at least two of the quantum well layers have an inclined band gap that decreases from the electron injection side to the hole injection side or from the hole injection side to the electron injection side. And a vertical microcavity type light emitting diode characterized by having different inclined shapes from each other. 6. A vertical microcavity type light emitting diode having a light emitting layer including at least three quantum well layers stacked with a barrier layer interposed therebetween, and a multi-layer reflecting layer provided on both sides of the light emitting layer. Wherein the at least two quantum well layers have a step-shaped band gap that decreases from the electron injection side to the hole injection side or from the hole injection side to the electron injection side. And a vertical microcavity type light emitting diode characterized by having different step shapes from each other.