JPH07320633A - Semiconductor spin polarized electron source - Google Patents

Abstract

PURPOSE:To provide a semiconductor spin polarized electron source from which a large quantity of electrons having high spin polarized degree can be emitted by forming a p-type conductive strained super lattice structure with no lattice relaxation and composed of reciprocally formed well layers and barrier layers as a spin polarized electron generating region on a substrate. CONSTITUTION:A block layer 4 (p-Al0.35Ga0.65As) with smaller electron affinity than that of a substrate 1 is formed on the substrate 1 (p-GaAs). As a spin polarized electron generating region, strained well layers 8 (p-In0.15Ga0.85As) having larger lattice constants than those of the substrate 1 and thickness about electron wavelength or thinner and barrier layers 6 (p-GaAs) having lower valence electron band energy than that of the strained well layers 8 and thickness thin enough to transmit electrons in conduction band can permeate by tunnel effect are reciprocally formed to compose a p-type conductive short cycle strained super lattice structure with no lattice relaxation. Moreover, a surface layer 7 (p<+>-In0.15Ga0.85As) to absorb the curve of the bands is formed on these layers.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a semiconductor spin-polarized electron source capable of extracting electrons having a large spin polarization at a large current density. The field of application of the spin-polarized electron source is, for example, an electron source attached to an electron-positron linear collision type accelerator, which is an important device in elementary particle research. In the accelerator, an electron source that can take out spin-polarized electrons is desired.

[0002]

2. Description of the Related Art As a semiconductor spin-polarized electron source for taking out spin-polarized electrons, a semiconductor spin-polarized electron source utilizing a band structure of the semiconductor itself has been disclosed by G. Lanpel et al.
solid state communication,
Vol. 16, p. 877, 1975). This semiconductor spin-polarized electron source is a p-type GaAs surface on which an alternating multilayered film of cesium (Cs) and oxygen (O) that produces a negative electron affinity is deposited.
By irradiating a circularly polarized laser beam having an energy of about s band gap, electrons having a spin polarization of 50% at the maximum can be extracted to the outside of the semiconductor. GaAs
Has a valence band structure in which bands of heavy holes and light holes are degenerate, and the ratio of the downward spin to the upward spin of the electrons excited in the conduction band from these bands is 3: 1 due to the difference in transition probability. . Therefore, a maximum polarization of 50% is obtained.

In order to obtain a high polarization degree approaching 100%, it is necessary to solve the degeneracy of the valence band. Therefore, there have been proposed one using a strained crystal and one utilizing a short-period semiconductor superlattice.

A semiconductor spin-polarized electron source using a strained crystal is, for example, Physics Letters (Phys) by Nakanishi et al.
ics Letters A, Vol. 158, p. Three
45, 1991). FIG. 2 shows the structure of a semiconductor spin-polarized electron source using a strained crystal. p-GaA
A p-GaPAs lattice relaxation layer 2 having a lattice constant larger than that of GaAs and having lattice relaxation is provided on the s substrate 1, and a strained thin p-GaAs strain layer 3 without lattice relaxation is provided thereon. Compressive stress acts in the in-plane direction on the uppermost GaAs strained layer 3 so as to match the lattice of the GaPAs lattice relaxation layer 2, and the heavy hole band and the light hole band of the GaAs valence band degenerate. Can be solved. As a result, the heavy hole band becomes energetically high, and when the energy of the excitation light is selected as the energy from this heavy hole band to the conduction band, only the electrons in the heavy hole band are excited and the spin You can get the perfect electron. Therefore, in principle, 100% polarization can be obtained. Actually, the spin polarization degree of electrons that can be taken out becomes low due to band broadening due to thermal energy and spin scattering inside the strained crystal. However, electrons coming out of the surface by laminating alternating multilayer films of Cs and O on the surface. As a result of measuring the polarization degree of the above, a high polarization degree exceeding 80% was obtained.

A semiconductor spin-polarized electron source using a semiconductor superlattice having a short period is described by, for example, Omori et al. In Physical Review Letters.
tters, Vol. 67, p. 3291, 1991)
It is described in. FIG. 3 shows the structure of a semiconductor spin-polarized electron source using a semiconductor superlattice. On the p-GaAs substrate 1, a p-AlGaAs block layer 4 having a wide bandgap for preventing electrons excited by the substrate from going to the superlattice layer thereon, and p-GaAs having a thickness equal to or less than the wavelength of electrons. Well layer 5
P-AlGa with a thickness that allows electrons and electrons to tunnel through
A short-period superlattice having a laminated structure of the As barrier layer 6 and a highly-doped p ⁺ -GaAs surface layer 7 that absorbs the bending of the surface band in a thin region are provided. In the superlattice structure, a miniband is formed by the quantum effect. At this time, the heavy hole band and the light hole band in the valence band have different energy positions due to a large difference in effective mass, and degeneration can be resolved. As a result, the band of heavy holes becomes energetically high as in the strained crystal, and if the energy of excitation light is selected as the energy from the band of heavy holes to the conduction band,
Only the electrons in the heavy hole band are excited, and electrons with completely uniform spins are obtained. Therefore, in principle 1
A polarization of 00% can be obtained. Actually, C on the surface
As a result of measuring the polarization of electrons emitted from the surface by stacking sO multilayer films, a high polarization exceeding 70% was obtained.

[0006]

A semiconductor spin-polarized electron source is a semiconductor spin-polarized electron source for obtaining spin-uniform electrons.
%, And a high quantum efficiency for obtaining a large current is desired. However, in the conventional GaAs crystal and superlattice structure, the quantum efficiency is relatively high, but the degree of polarization is not sufficient. In the strained crystal, although the degree of polarization is large, the existence of crystal defects and the strained layer thickness cannot be increased. Is 0.5%
It was as low as or less, and none satisfied both requirements. Therefore, it is desired to develop a semiconductor spin-polarized electron source having both high spin polarization and high quantum efficiency.

An object of the present invention is to provide a semiconductor spin-polarized electron source which has both higher spin polarization and higher quantum efficiency than ever before.

[0008]

The semiconductor spin-polarized electron source of the present invention has a lattice constant larger than the lattice constant of the substrate as an area where at least spin-polarized electrons are generated on the substrate, and the spin-polarized electron source has an electron wavelength of not more than about the electron wavelength. Strained superlattice structure of p-type conduction without lattice relaxation, which is composed of an alternating stack of a well layer having a thickness and a barrier layer having a valence band energy lower than that of the well layer and allowing conduction band electrons to pass through by a tunnel effect. It is characterized by including.

[0009]

In the semiconductor spin polarized electron source of the present invention,
By applying compressive stress to the well layer of the superlattice structure,
The energy difference between the heavy hole and light hole bands in the valence band generated in the superlattice structure is further widened. For this reason, it becomes easy to selectively excite only the electrons in the heavy hole band into the conduction band, and to extract the electrons having high quantum efficiency and higher spin polarization than the conventional spin-polarized electron source. You can

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings showing embodiments.

FIG. 1 is a schematic view showing the layer structure of an embodiment of the present invention. In FIG. 1, the same symbols as those in FIGS. 2 and 3 are the same as those in FIGS. 2 and 3 and perform the same functions. Reference numeral 8 has a lattice constant larger than that of the substrate and compressive stress in the plane is It is a strained well layer having a thickness equal to or less than the applied electron wavelength.

(First Embodiment) Regarding the operation of the semiconductor spin-polarized electron source of the first embodiment of the present invention, p-
GaAs, p-Al _0.35 Ga _0.65 As on the block layer 4,
The strained well layer 8 has p-In _0.15 Ga _0.85 As and the barrier layer 6
On the surface layer 7 and p ⁺ -In _0.15 Ga _0.85 A on the surface layer 7.
s will be described as an example.

Since the strained well layer 8 has a larger lattice constant than the substrate 1 and the barrier layer 6, a compressive stress is applied in the in-plane direction to match the lattice, and the strained well layer is distorted so that the lattice in the stacking direction becomes long. As a result, the degeneracy of the valence band is released, and the heavy hole band and the light hole band have different energies. Which energy becomes higher depends on the direction of strain, but in this case, the band of light holes comes to a lower energy position than the band of heavy holes.

In a short-period superlattice composed of the strained well layer 8 and the barrier layer 6 stacked as described above, a miniband of each band is formed by the quantum effect. The energy of the heavy hole miniband does not have a significant energy shift due to the large effective mass of the heavy hole, and is only slightly smaller than the energy of the heavy hole band of the strained well layer 8. On the other hand, the light hole miniband has a significant energy shift because of its small effective mass, and moves to the low energy side, which is larger than the light hole band energy of the strained well layer 8. As a result, the energy difference between the heavy-hole miniband and the light-hole miniband is wider than if the lattice were simply distorted.

As described above, in a short-period superlattice (single-periodic strained superlattice) using strained wells, a strained crystal or a strainless short-period with a different energy difference between heavy hole band and light hole band. Since it is larger than the superlattice, the electron transition from the valence band to the conduction band by photoexcitation is limited to only from the heavy hole miniband. Therefore, almost 100% of spin-polarized electrons can be generated in the semiconductor.

The spin-polarized electrons excited in the conduction band miniband diffuse through the miniband to the surface layer 7. However, since the superlattice has a short period, the width of the miniband is sufficient. It has wide electron mobility and high electron mobility similar to bulk crystal. Therefore, the spin-polarized electrons move to the surface layer 7 in a short time without being subjected to spin scattering. In the surface layer 7, electrons are accelerated by a large internal electric field and jump out of the semiconductor. Therefore, it is possible to extract almost 100% of spin-polarized electrons.

Further, in the short period strained superlattice, since lattice relaxation does not occur from the substrate to the uppermost layer, crystal defects are scarcely generated, so that excited electrons are not recombined and strained crystal Since a thick light absorption region can be formed, excitation light can be effectively used, and high quantum efficiency is also realized.

As described above, the semiconductor spin-polarized electron source of the present invention makes it possible to extract electrons with a high spin polarization degree close to 100% with high quantum efficiency.

Next, the manufacturing method of the first embodiment of the present invention will be described using the same material as that used in the explanation of the operation.

The device structure of the present invention is produced by a molecular beam epitaxy method (MBE: Molecular) as a crystal growth method.
Beam Epitaxy) and substrate temperature 520
Performed at ° C. First, the thickness is 1 μm on the p-type GaAs substrate 1.
At a Be acceptor concentration of 5 × 10 ¹⁸ cm ^-3 p-Al _0.35 G
a _0.65 As block layer was formed. Then, Be concentration 5
× 10 ¹⁷ cm p-GaAs barrier layer having a thickness of 3.1nm at ^-3 and Be concentration ^{5 × 10 17 cm p-In} 0.15 thick 2.0nm ^-3
Ga _0.85 As Strained well layer 18
Period (91.8 nm) is formed, and finally Be concentration is 4 × 1.
0 ¹⁹ cm ^-3 and 4.8 nm thick p ⁺ -In _0.15 Ga _0.85 As
A surface layer was formed. Then, the substrate temperature was cooled to −10 ° C., and an As protective film was deposited to a thickness of about 1 μm in order to suppress the oxidation of the surface in the atmosphere to complete the device.

This device was introduced into an ultra-high vacuum polarization measuring device and then heated to 400 ° C. to evaporate and remove the As protective film on the surface to obtain a clean surface. After that, a multilayer film of Cs and O for obtaining a negative electron affinity was formed, and the preparation for measurement was completed. The spin polarization and quantum efficiency of this device were measured, and it was found that the maximum spin polarization of 87 was obtained under the irradiation conditions of the excitation laser wavelength of 915 nm and CW of 100 μW.
% And a quantum efficiency of 2% were obtained, and a large degree of polarization and a high quantum efficiency could be satisfied at the same time.

(Second Embodiment) The operation of the semiconductor spin-polarized electron source according to the second embodiment of the present invention is shown in the structural diagram in FIG.
Using p-GaAs for the substrate 1 and p-A for the blocking layer 4.
_0.35 Ga _0.65 As, p-In _0.15 Ga in the strained well layer 8.
_0.85 As, described barrier layer _{6 p-Al 0.35 Ga 0.65 As} , the surface layer 7 of p ⁺ -In _0.15 Ga _0.85 As an example.

Since AlGaAs having a wider bandgap than the substrate 1 is used for the barrier layer 6, the energy difference between the heavy-hole miniband and the light-hole miniband is larger than that in the first embodiment. The spin polarization is further improved as compared with the first embodiment.

The manufacturing method of the second embodiment of the present invention is almost the same as that of the first embodiment. As the barrier layer 6, p-Al _0.35 Ga having a Be concentration of 5 × 10 ¹⁷ cm ^-3 and a thickness of 3.1 nm is used.
_0.65 As was used, and otherwise the same as in the first embodiment.
When the spin polarization and the quantum efficiency of this device were measured, the maximum spin polarization of 90% and the quantum efficiency of 2% were obtained under the irradiation condition of the excitation laser wavelength of 830 nm and CW of 100 μW, which was higher than that of the first embodiment. A large degree of polarization was obtained and high performance was achieved.

(Third Embodiment) FIG. 1 is a structural diagram showing the operation of a semiconductor spin-polarized electron source according to a third embodiment of the present invention.
Using p-GaAs for the substrate 1 and p-A for the blocking layer 4.
_0.35 Ga _0.65 As, p-In _0.15 Ga in the strained well layer 8.
_{0.85 As, p-GaP 0.2 As} 0. 8 to the barrier layer 6 will be described on the surface layer 7 of p ⁺ -In _0.15 Ga _0.85 As an example.

Since GaP _0.2 As _0.8 having a lattice constant smaller than that of the GaAs substrate 1 is used for the barrier layer 6,
In the short period superlattice in combination with the In _0.15 Ga _0.85 As strain well having a larger lattice constant than the s substrate, the average lattice constant is almost equal to that of the GaAs substrate, and the thickness of the short period superlattice is increased. Also does not cause lattice relaxation. Therefore, by increasing the thickness of the short period superlattice, the first
Further, the quantum efficiency can be improved as compared with the second embodiment.

The manufacturing method of the third embodiment of the present invention is the same as that of the first embodiment. Be concentration 5 × 1 as the barrier layer 6
0 ¹⁷ with p-GaP _0.2 As _0.8 thick 3.1nm in cm ^-3, the thickness of the entire short-period superlattice is as thick as 300 nm, others were the same as the first embodiment. When the spin polarization and quantum efficiency of this device were measured, the maximum spin polarization of 88% and quantum efficiency of 4% were obtained under the excitation laser wavelength of 880 nm and CW irradiation of 100 μW. Greater polarization than the example was obtained and high performance was achieved.

(Fourth Embodiment) FIG. 1 is a structural diagram showing the operation of the semiconductor spin-polarized electron source according to the fourth embodiment of the present invention.
Using p-GaAs for the substrate 1 and p-G for the block layer 4
undoped i-In _0.15 Ga in the strained well layer 8
_0.85 As, p-GaAs for the barrier layer 6, p ^{+ for the} surface layer 7
Description will be made by taking -In _0.15 Ga _0.85 As as an example.

Since the strained well layer 8 does not contain ionized impurities (acceptors), recombination due to crystal defects caused by the impurities is reduced. Since the excited spin-polarized electrons have a higher probability of existing in the strained well layer 8 than in the barrier layer 6, this effect is large and a quantum efficiency higher than that in the first embodiment can be obtained.

The manufacturing method of the fourth embodiment of the present invention is almost the same as that of the first embodiment. As the barrier layer 6, p-Al _0.35 Ga having a Be concentration of 5 × 10 ¹⁷ cm ^-3 and a thickness of 3.1 nm is used.
_0.65 As, the strained well layer 8 was undoped, and i-In _0.15 Ga _0.85 As having a thickness of 2.0 nm was used. The spin polarization and quantum efficiency of this device were measured. As a result, a spin polarization of 87% and a quantum efficiency of 3% were obtained under the irradiation condition of the excitation laser wavelength of 915 nm and CW of 100 μW, which was larger than that of the first embodiment. Efficiency was obtained and high performance was achieved.

(Fifth Embodiment) FIG. 1 is a structural diagram showing the operation of a semiconductor spin-polarized electron source according to a fifth embodiment of the present invention.
Using p-GaAs for the substrate 1 and p-A for the blocking layer 4.
_0.35 Ga _0.65 As, p-In _0.15 Ga in the strained well layer 8.
_0.85 As, explained undoped i-GaAs barrier layer 6, the surface layer 7 of p ⁺ -In _0.15 Ga _0.85 As an example.

Since only the strained well layer 8 contains the acceptor but not the barrier layer 6, band bending occurs due to space charges in the short period superlattice. The bending of the band increases the effective barrier height for holes, so that the energy difference between the minibands of heavy holes and light holes becomes larger than that in the first embodiment. A larger spin polarization is obtained.

The manufacturing method of the fifth embodiment of the present invention is almost the same as that of the first embodiment. The barrier layer 6 is undoped i-GaAs having a thickness of 3.1 nm, and the strain well layer 8 is B.
e p-In _0.15 G with a concentration of 5 × 10 ¹⁷ cm ^-3 and a thickness of 2.0 nm
a _0.85 As was used, and otherwise the same as in the first embodiment. When the spin polarization and quantum efficiency of this device were measured, the wavelength of the excitation laser was 915 nm and the CW was 100 μ.
Under the irradiation condition of W, the spin polarization degree is 89% and the quantum efficiency is 2.
%, A spin polarization degree higher than that of the first embodiment was obtained, and high performance was achieved.

In the above-described embodiments of the present invention, G is used as the substrate.
Although only aAs was shown, InP, InAs, GaS
Obviously, a compound semiconductor such as b or GaP, an element semiconductor such as Si or Ge, or another single crystal semiconductor substrate or a single crystal metal substrate may be used. Further, as a constituent semiconductor of the strained superlattice, GaAs, InGaAs, AlG
Although only aAs and GaPAs are shown, any combination of semiconductors that satisfies the conditions of the present invention is possible, and InP, InAlAs, I
nAlGaAs, AlGaPAs, GaSb, AlGa
It is obvious that typical compound semiconductors such as Sb, InAs, GaP, GaN, AsGaN, and other semiconductors may be used.

Although only AlGaAs is shown as the block layer, any semiconductor having an electron affinity smaller than that of the substrate may be used. Although only InGaAs, which is the material of the strained well layer, has been shown as the surface layer, it may be a material that serves as a barrier layer or other semiconductor material whose electron affinity is not extremely small as compared with the short period strained superlattice. Good. Although only As has been shown as a protective film against oxidation in the atmosphere, Sb or InAs may be used as long as it vaporizes at a temperature at which the superlattice structure is not broken.

Although the first to fifth embodiments have been shown as the embodiments of the present invention, it is clear that the semiconductor spin-polarized electron source of the present invention can be constituted by a combination thereof.
For example, in the second to fifth embodiments, two of the three elements of the composition of the barrier layer, the presence / absence of strain in the barrier layer, and the impurity doping position are fixed in the same manner as in the first embodiment, and only one element is fixed. However, even if two or more elements are changed, it is clear that higher performance can be achieved as compared with the first embodiment.

It is also clear that the strained well layer and the barrier layer may be further divided into a plurality of layers to have different compositions and dopings.

[0038]

According to the semiconductor spin-polarized electron source of the present invention, it is possible to extract a large amount of electrons having a large spin polarization, and the operation life is extended because it operates with weak excitation light intensity.

[Brief description of drawings]

FIG. 1 is a structural diagram of a short period strained superlattice showing first to fifth embodiments of the present invention.

FIG. 2 is a structural diagram of a conventional example using a strained crystal.

FIG. 3 is a structural diagram of a conventional example using a short period superlattice.

[Explanation of symbols]

 1 Substrate 2 Lattice Relaxation Layer 3 Strained Layer 4 Block Layer 5 Well Layer 6 Barrier Layer 7 Surface Layer 8 Strained Well Layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshimasa Kurihara 403, No. 403, 4-25, Matsushiro, Tsukuba City, Ibaraki Prefecture No. 403 No. 404 (72) Inventor Nakanishi, No. 4, No. 128, Kawanayama-cho, Showa-ku, Nagoya, Aichi Prefecture Building No. 43

Claims (6)

[Claims] 1. A well layer having a lattice constant larger than that of the substrate and having a thickness equal to or less than an electron wavelength as a region for generating spin-polarized electrons on the substrate, and valence band energy higher than that of the well layer. 1. A semiconductor spin-polarized electron source, comprising a strained superlattice structure of p-type conduction without lattice relaxation, which is composed of alternating layers with a barrier layer having a low thickness and a thickness that allows electrons in the conduction band to be transmitted by a tunnel effect. 2. The semiconductor spin-polarized electron source according to claim 1, wherein at least the barrier layer is made of a semiconductor having substantially the same lattice constant as the substrate. 3. The semiconductor spin polarized electron source according to claim 1, wherein at least the barrier layer is made of a semiconductor having a lattice constant smaller than that of the substrate, and the average lattice constant of the well layer and the barrier layer is substantially the same as that of the substrate. 4. The semiconductor spin polarized electron source according to claim 1, 2 or 3, wherein the well layer and the barrier layer contain p-type impurities in the same amount. 5. The semiconductor spin-polarized electron source according to claim 1, 2 or 3, wherein at least a part of the barrier layer contains p-type impurities, and the well layer contains almost no impurities. 6. The semiconductor spin-polarized electron source according to claim 1, wherein at least a part of the well layer contains a p-type impurity, and the barrier layer contains almost no impurity.