JP7175502B2 - circularly polarized light emitting diode - Google Patents

Description

The present disclosure relates to circularly polarized light emitting diodes.

At present, the development of optical technology that deals with the polarization and phase of light is remarkable, and as a light source that directly emits circularly polarized light, a circularly polarized light emitting diode that combines a semiconductor LED structure and a ferromagnetic metal is being studied.

Even if the semiconductor LED structure and the ferromagnetic metal are simply directly bonded, the spin-aligned electrons in the ferromagnetic material cannot efficiently enter the semiconductor LED structure due to the large difference in electrical conductivity. Therefore, it is being studied to use tunnel conduction to put electrons with aligned spins in a ferromagnetic material into a semiconductor LED structure. A light-emitting diode device has been proposed that produces pure circularly polarized light at room temperature by placing a film between a semiconductor LED structure and a ferromagnetic metal [1].

The aluminum oxide layer can be formed by epitaxially growing an aluminum layer directly on the gallium arsenide layer of the semiconductor LED structure and naturally oxidizing the epitaxially grown aluminum layer. By oxidizing the lattice-aligned crystallized aluminum layer, a lattice-aligned crystallized aluminum oxide layer can be obtained. In this way, an LED that emits substantially 100% circularly polarized light can be fabricated by incorporating spin-aligned electrons in a ferromagnetic material into a semiconductor LED structure.

N. Nishizawa, K. Nishibayashi, H. Munekata, Pure circular polarization electroluminescence at room temperature with spin-polarized light-emitting diodes, PNAS 2017, 8, 1783 - 1788.

However, when purely circularly polarized light is emitted, a voltage of about 10 V is applied across the entire circularly polarized light emitting diode, and an electric field of 10 to 20 MV/cm is applied to the aluminum oxide layer of the tunnel insulating film. The magnitude of this electric field is about the same as the electric field strength that causes dielectric breakdown in sapphire, and even if the crystallinity of the aluminum oxide layer of the tunnel insulating film is high, the dielectric breakdown may cause light emission to disappear, resulting in a low yield. There was a problem.

In addition, non-radiative recombination increases due to leakage current flowing in the end face and film that become the light emitting surface of the circularly polarized light emitting diode. Large current densities were required to obtain pure circularly polarized light emission due to oxygen diffusion into the semiconductor layer adjacent to the lower external quantum efficiency.

Therefore, there is a need for circularly polarized light-emitting diodes that generate purely circularly polarized light at low current densities and have improved yields.

In order to solve the above problems, the present inventors conducted intensive research and arranged a semiconductor layer of aluminum arsenide with high insulating properties as a tunnel insulating film between the aluminum oxide layer and the gallium arsenide layer of the semiconductor LED structure. found the structure. FIG. 1 shows a schematic cross-sectional view of an example of the circularly polarized light emitting diode of the present disclosure.

The present disclosure is a circularly polarized light emitting diode having a double heterostructure, a tunnel insulating film arranged on the double heterostructure, and a ferromagnetic electrode arranged on the tunnel insulating film,
the tunnel insulating film includes an aluminum arsenide layer and an aluminum oxide layer disposed on the aluminum arsenide layer;
wherein the double heterostructure contains gallium, aluminum, and arsenic, and a gallium arsenide layer disposed in contact with the aluminum arsenide layer of the tunnel insulating film;
It is intended for circularly polarized light emitting diodes.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a circularly polarized light emitting diode which can generate purely circularly polarized light at a low current density and has an improved yield.

FIG. 1 is a cross-sectional schematic diagram of an example of a circularly polarized light emitting diode of the present disclosure. FIG. 2 is a schematic cross-sectional view of a circularly polarized light emitting diode of a comparative example. FIG. 3 is a perspective view showing the length L and width W of the circularly polarized light emitting diode element, the magnetization direction of the ferromagnetic electrode, and the light emission direction. FIG. 4 is a graph showing the relationship between photon energy and electroluminescence intensity (EL intensity) obtained with the circularly polarized light emitting diode of the present disclosure. 5 is a graph showing the relationship between the degree of circular polarization and the current density of the circularly polarized light emitting diodes produced in Examples 1 to 6 and the circularly polarized light emitting diode produced in Comparative Example 1. FIG.

The present disclosure is a circularly polarized light emitting diode having a double heterostructure, a tunnel insulating film arranged on the double heterostructure, and a ferromagnetic electrode arranged on the tunnel insulating film, wherein the tunnel insulating film is , an aluminum arsenide layer and an aluminum oxide layer disposed on the aluminum arsenide layer, wherein the double heterostructure includes gallium, aluminum, and arsenic, and is disposed at a position of the tunnel insulating film in contact with the aluminum arsenide layer. A circularly polarized light-emitting diode that includes a gallium arsenide layer that has been coated.

According to the circularly polarized light emitting diode of the present disclosure, it is possible to improve the crystallinity of the aluminum oxide layer in the tunnel insulating film.

Conventionally, when an aluminum oxide layer is formed on and in contact with a gallium arsenide layer of a semiconductor LED structure, oxygen diffuses into the gallium arsenide layer to form an arsenic oxide layer on the outermost surface of the gallium arsenide layer. In this case, since arsenic oxide and aluminum oxide have a large difference in crystal structure, the crystallinity of the aluminum oxide layer is degraded.

In contrast, if an aluminum arsenide layer is to be placed over the gallium arsenide layer of the semiconductor LED structure, an aluminum layer is formed over and in contact with the aluminum arsenide layer, and the aluminum layer is oxidized to form aluminum oxide. When this aluminum layer is oxidized, the surface of the underlying aluminum arsenide layer is oxidized to form an aluminum oxide layer. Since the aluminum oxide layer formed by oxidizing the surface of the aluminum arsenide layer and the aluminum oxide layer disposed thereon have the same crystal structure, it is possible to obtain an aluminum oxide layer with high crystallinity. can.

In addition, according to the circularly polarized light emitting diode of the present disclosure, since the highly insulating aluminum arsenide layer is arranged under the tunnel insulating film, it is possible to suppress leakage current from flowing through the film and the end face.

Furthermore, according to the circularly polarized light emitting diode of the present disclosure, since it has a structure in which the aluminum arsenide layer is arranged between the aluminum oxide layer and the semiconductor LED structure, when the aluminum layer is oxidized to form the aluminum oxide layer, , the diffusion of oxygen into the semiconductor layers of the semiconductor LED structure can be suppressed.

Thus, according to the circularly polarized light emitting diode of the present disclosure, an aluminum arsenide layer with high insulating properties is arranged between the aluminum oxide layer and the gallium arsenide layer of the semiconductor LED structure, and the crystal of the aluminum oxide layer of the tunnel insulating film properties are improved, the leakage current in the film and to the edge is suppressed, and the diffusion of oxygen into the semiconductor layers of the semiconductor LED structure is suppressed. Due to these effects, the dielectric breakdown of the tunnel insulating film is suppressed, the yield is greatly improved, and purely circularly polarized light emission can be obtained with a significantly lower current density than before.

The circularly polarized light emitting diode of the present disclosure preferably exhibits a degree of circular polarization of 50% or higher, more preferably 80% or higher, even more preferably 90% or higher. In the present application, the degree of circular polarization within the preferred range is referred to as pure circular polarization. The degree of circular polarization refers to the ratio of the electroluminescence intensity of right-handed circularly polarized light or left-handed circularly polarized light to the sum of the electroluminescence intensities of right-handed circularly polarized light and left-handed circularly polarized light.

Aluminum oxide is represented by AlOx, where x is preferably between 1.0 and 1.5. Aluminum arsenide is represented by AlAs.

The circularly polarized light emitting diode of the present disclosure has a double heterostructure. A combination of the tunnel insulating film including the aluminum arsenide layer and the aluminum oxide layer and the double heterostructure can provide high luminous efficiency.

The double heterostructure is a gallium arsenide-based double heterostructure, and includes a gallium arsenide layer containing gallium, aluminum, and arsenic and located in contact with the aluminum arsenide layer of the tunnel insulating film.

The double heterostructure can be a PIN structure or an NIP structure in which the n-type semiconductor illustrated in FIG. 1 is arranged on the ferromagnetic electrode side.

The double heterostructure is preferably a PIN structure. In order to put electrons with aligned spins from the ferromagnetic electrode into the double heterostructure semiconductor LED structure, the double heterostructure layer in contact with the aluminum arsenide layer of the tunnel insulating film is made of n-type gallium arsenide, which is an n-type semiconductor. preferable. Since electrons have a longer relaxation time than holes, the electrons can reach the light-emitting layer in the double heterostructure with their spins aligned.

The double heterostructure more preferably has a PIN structure including a p-type aluminum gallium arsenide layer, a p-type gallium arsenide layer, an n-type aluminum gallium arsenide layer, and an n-type gallium arsenide layer, and the n-type arsenide of the double heterostructure An aluminum arsenide layer of a tunnel insulating film is arranged on and in contact with the gallium layer. In this case, the p-type gallium arsenide layer becomes the light emitting layer.

The aluminum gallium arsenide of the p-type aluminum gallium arsenide layer is represented by p-Al _a Ga _b As, preferably a is 0.10 to 0.45 and b is 0.55 to 0.90. The p-type aluminum gallium arsenide layer is preferably doped with C, Be, Zn, Si, or combinations thereof. The p-type aluminum gallium arsenide layer is preferably doped with a dopant of 1×10 ¹⁷ to 1×10 ¹⁸ /cm ³ .

Gallium arsenide in the p-type gallium arsenide layer is represented by p-GaAs. The p-type gallium arsenide layer is preferably doped with C, Be, Zn, Si, or combinations thereof. The p-type gallium arsenide layer is preferably doped with a dopant of 1×10 ¹⁷ to 1×10 ¹⁸ /cm ³ .

The aluminum gallium arsenide of the n-type aluminum gallium arsenide layer is represented by n-Al _c Ga _d As, preferably c is 0.10-0.45 and d is 0.55-0.90. The n-type aluminum gallium arsenide layer is preferably doped with Sn, Si, Te, or combinations thereof. The n-type aluminum gallium arsenide layer is preferably doped with a dopant of 1×10 ¹⁶ to 1×10 ¹⁸ /cm ³ .

Gallium arsenide in the n-type gallium arsenide layer is represented by n-GaAs. The n-type gallium arsenide layer is preferably doped with Sn, Si, Te, or combinations thereof. The n-type gallium arsenide layer is preferably doped with a dopant of 1×10 ¹⁸ to 5×10 ¹⁹ /cm ³ .

The double heterostructure preferably includes an anti-diffusion layer. A diffusion barrier layer may be disposed, for example, between the p-type gallium arsenide layer and the n-type aluminum gallium arsenide layer to prevent dopant diffusion. The diffusion barrier layer is preferably an undoped aluminum gallium arsenide layer.

The non-doped aluminum gallium arsenide layer is preferably represented by Al _e Ga _f As, where e is 0.10-0.45 and f is 0.55-0.90.

A double heterostructure can be formed on a single crystal substrate. As the single crystal substrate, for example, a p-type gallium arsenide substrate having a (100) plane as illustrated in FIG. 1 can be used.

The double heterostructure may include a buffer layer between the single crystal substrate and the clad layer. A clad layer with good crystallinity can be formed by epitaxially growing a buffer layer having a flat surface on a single crystal substrate and epitaxially growing a clad layer thereon. A p-type gallium arsenide layer, for example, can be used as the buffer layer arranged between the single crystal substrate and the clad layer. A p-type gallium arsenide layer as a buffer layer is also preferably represented by GaAs and is doped with the same types and amounts of dopants as above.

The thickness of the aluminum arsenide layer is preferably 2.0 nm to 2.8 nm, more preferably 2.0 nm to 2.4 nm. The thickness of the aluminum arsenide layer may be 2.2 nm or more. When the thickness of the aluminum arsenide layer is within the preferred range, the aluminum arsenide layer can be obtained with a flatter surface, and the crystallinity of the aluminum oxide layer formed thereon can be improved.

The thickness of the aluminum oxide layer is preferably 0.2 nm to 1.0 nm, more preferably 0.6 nm to 1.0 nm. The thickness of the aluminum oxide layer may be 0.8 nm or less. By setting the thickness of the aluminum oxide layer within the preferable range, the efficiency of electron injection into the semiconductor layer by tunnel conduction can be improved.

The thickness of the tunnel insulating film including the aluminum arsenide layer and the aluminum oxide layer is preferably 2.2 nm to 3.0 nm. When the thickness of the tunnel insulating film is within the preferred range, electrons can be tunneled more satisfactorily, and the electric field applied to the entire tunnel insulating film is reduced, so that dielectric breakdown of the tunnel insulating film is prevented. more restrained.

The ferromagnetic electrodes are preferably composed of Fe, FeCo, FeCoB or GaMnAs.

The circularly polarized light emitting diode of the present disclosure includes an anti-oxidation electrode on the ferromagnetic electrode, preferably the ferromagnetic electrode. The antioxidation electrode is preferably an Au/Ti electrode consisting of an Au layer and a Ti layer, an Au/Cr electrode consisting of an Au layer and a Cr layer, or a single layer Au electrode.

A single crystal substrate can be placed on a metal plate. The metal plate is preferably made of Cu, Al or Au, more preferably made of Cu.

An ohmic contact layer can be arranged between the single crystal substrate and the metal plate. The ohmic contact layer is preferably composed of In/Ag paste, AuZn/Ag paste, or Au/Ag paste, more preferably In/Ag paste.

Circularly polarized diodes of the present disclosure can be fabricated using molecular beam epitaxy (MBE). A layer of aluminum oxide can be formed by oxidizing an aluminum layer formed by MBE. The oxidation of the aluminum layer is preferably performed at room temperature or below in order to prevent deterioration of the crystallinity of the aluminum oxide layer to be formed. The oxidizing atmosphere is preferably dry air, and the aluminum oxide layer can be formed by natural oxidation in dry air. The natural oxidation time is preferably 10 hours or more. When the aluminum layer is naturally oxidized once, the maximum thickness for natural oxidation is 0.7 nm. Therefore, when forming an aluminum oxide layer with a thickness of more than 0.7 nm, it is possible to form an aluminum oxide layer of 0.7 nm once, form an aluminum layer on the aluminum oxide layer, and oxidize the aluminum oxide layer.

(Example 1)
(Formation of circularly polarized light-emitting layer)
An In/Ag paste was applied on a Cu substrate, and a p-type Zn-doped p-GaAs (1×10 ¹⁸ /cm ³ ) substrate was placed on the In/Ag paste. Using an electron beam epitaxy method (MBE method), a substrate temperature of 510° C. was used to form a circularly polarized light-emitting layer with a double heterostructure shown in FIG. 1 as a schematic cross-sectional view on the substrate in the following order:
1. 500 nm thick Be-doped p-type GaAs (1×10 ¹⁸ /cm ³ ) (buffer layer);
2. 500 nm thick Be-doped p-Al _0.3 Ga _0.7 As (1×10 ¹⁸ /cm ³ ) (cladding layer);
3. 500 nm thick Be-doped p-Ga ₇ As (1×10 ¹⁸ /cm ³ ) (light-emitting layer);
4. Non-doped Al _0.3 Ga _0.7 As (dopant diffusion prevention layer) with a thickness of 15 nm;
5. 300 nm thick Si-doped n-type Al _0.3 Ga _0.7 As (1×10 ¹⁷ /cm ³ ) (cladding layer);
6. 15 nm thick Si-doped n-type GaAs (5×10 ¹⁸ /cm ³ ) (buffer layer).

(Formation of tunnel insulating film)
Using an electron beam epitaxy method (MBE method), a tunnel insulating film shown as a cross-sectional schematic diagram in FIG. 1 was formed on the double heterostructure in the following order at a substrate temperature of 25° C.:
7. Deposit an AlAs layer with a thickness of 2.0 nm (tunnel insulating film);
8. Deposit an Al layer with a thickness of 0.56 nm;
9. Oxidize the Al layer by exposing it to dry air for 10 hours to form a 0.70 nm thick aluminum oxide layer (tunnel insulating film);

(Preparation of magnetic metal electrode)
On the formed tunnel insulating film, a ferromagnetic electrode (Fe) layer and an Au/Ti layer (electrode layer) were vapor-deposited in a vacuum chamber of an electron beam vapor deposition apparatus according to the following procedure:
10. Form a resist pattern with a width of 40 nm parallel to the GaAs (110) direction by photolithography;
11. On the upper surface, a Fe film with a width of 40 μm, a length of 1 mm, and a thickness of 100 nm is formed (ferromagnetic electrode);
12. Form a Ti film with a width of 40 μm, a length of 1 mm, and a thickness of 20 nm on the Fe film (oxidation protective film of the Fe film);
13. An Au film (electrode) having a width of 40 μm, a length of 1 mm, and a thickness of 50 nm was formed on the Ti film.

As indicated by the arrow in FIG. 3, a magnetic field of 5 kOe was applied to the ferromagnetic electrode made of Fe to magnetize it in the in-plane direction (light emission direction).

As described above, a circularly polarized light emitting diode device having a length L of 1 mm and a width W of 1 mm was produced. FIG. 1 shows a schematic cross-sectional view of the circularly polarized light-emitting diode element produced in Example 1. As shown in FIG. FIG. 3 shows a perspective view showing the length L and width W of the circularly polarized light emitting diode element, the magnetization direction of the ferromagnetic electrode, and the light emitting direction.

(Examples 2-6)
Circularly polarized light emitting diodes were manufactured with the combinations of length L and width W in FIG. 3 shown in Table 1. At that time, the Fe of the ferromagnetic electrode had a constant width of 40 nm and a thickness of 100 mm, and the length was made equal to the length L of the circularly polarized light emitting diode element.

(Comparative example 1)
Without forming an AlAs layer, Al was epitaxially grown (0.56 nm) on GaAs, naturally oxidized (0.70 nm) in dry air, and then further Al was grown (0.24 nm) and oxidized in dry air. Same as Example 1 except that natural oxidation (0.30 nm) was performed to form a crystalline aluminum oxide layer (1.0 nm in total), and the length L was 1.00 mm and the width W was 1.80 mm. A circularly polarized light-emitting diode shown as a cross-sectional schematic diagram in FIG. 2 was manufactured by the method.

FIG. 4 shows a graph representing the relationship between photon energy and electroluminescence intensity (EL intensity) obtained in the circularly polarized light emitting diode of the present disclosure. Right-handed circularly polarized light and left-handed circularly polarized light were separately measured using a λ/4 plate and a linear polarizer. The EL intensity was measured at room temperature with a current value of I=4.0 mA and a current density of J=10 A/cm ² . The current density J is a value obtained by dividing the current value by the area of the ferromagnetic electrode.

In FIG. 4, σ+ indicates the EL intensity of right-handed circularly polarized light, and σ− indicates the EL intensity of left-handed circularly polarized light. The ratio of the EL intensity of right-handed circularly polarized light to the total EL intensity was up to 91.7%, resulting in substantially pure circularly polarized light emission.

Table 3 shows the yield of the circularly polarized light emitting diodes produced in Examples 1-6. Table 4 shows the yield of circularly polarized light emitting diodes produced in Comparative Example 1. Dielectric breakdown occurred in many of the circularly polarized light emitting diodes produced in Comparative Example 1, and the ratio of substantially pure circularly polarized light emission was 5%. On the other hand, the circularly polarized light emitting diodes produced in Examples 1 to 6 exhibited substantially pure circularly polarized light emission in 67% of the cases, which greatly improved the yield.

FIG. 5 is a graph showing the relationship between the degree of circular polarization and the current density of the circularly polarized light emitting diodes produced in Examples 1 to 6 and the circularly polarized light emitting diode produced in Comparative Example 1. In FIG. The circularly polarized light emitting diodes fabricated in Examples 1 to 6 exhibited pure circularly polarized light emission at current densities about 1/10 that of the circularly polarized light emitting diodes fabricated in Comparative Example 1.

100 circularly polarized light emitting diode 10 ferromagnetic electrode

Claims (4)

A circularly polarized light emitting diode having a double heterostructure, a tunnel insulating film arranged on the double heterostructure, and a ferromagnetic electrode arranged on the tunnel insulating film,
the tunnel insulating film includes an aluminum arsenide layer and an aluminum oxide layer disposed on the aluminum arsenide layer;
wherein the double heterostructure contains gallium, aluminum, and arsenic, and a gallium arsenide layer disposed in contact with the aluminum arsenide layer of the tunnel insulating film;
Circularly polarized light emitting diode. 2. The circularly polarized light emitting diode according to claim 1, wherein the aluminum arsenide layer has a thickness of 2.0 nm to 2.8 nm. 3. The circularly polarized light emitting diode according to claim 1, wherein the aluminum oxide layer has a thickness of 0.2 nm to 1.0 nm. Circularly polarized light emitting diode according to any one of claims 1 to 3, wherein the ferromagnetic electrodes are Fe, FeCo, FeCoB or GaMnAs.

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