JP3125328B2 - Polarized electron beam generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001] The present invention relates to a polarized electron beam generating element for generating a polarized electron beam comprising a group of electrons whose spin directions are unevenly distributed in one of two types by receiving light energy.

[0002]

2. Description of the Related Art A polarized electron beam composed of a group of electrons whose spin directions are unevenly distributed in one of two types is used to investigate a magnetic structure inside a nucleus and a magnetic structure on a material surface in a field of high energy elementary particle experiments. It is used as an effective means above. When generating such a polarized electron beam, for example, the crystal surface of a compound semiconductor such as gallium-arsenic is irradiated with a circularly polarized laser, and the spin direction is biased in one direction due to a selective transition accompanying conservation of angular momentum. It is common practice to extract polarized electron beams.

[0003]

According to the above-described conventional polarized electron beam generating element, the polarization rate of the generated polarized electron beam is theoretically 50%, that is, the upspin rate is high. In addition, it is estimated that the ratio of the down spin is 1: 3 or 3: 1, and an electron beam having a sufficiently high polarization rate cannot be obtained.

On the other hand, it is conceivable to use a polarized electron beam generating element in which a unidirectional anisotropy is given to a valence band by applying a stress in a certain direction to a semiconductor crystal.
It is difficult to obtain a sufficiently large strain or to apply the strain stably, and there is a problem that an apparatus for applying a strain stress from the outside hinders the extraction of electrons.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a polarized electron capable of stably and easily obtaining an electron beam having a sufficiently high polarization rate. It is to provide a line generating element.

[0006]

SUMMARY OF THE INVENTION In order to achieve the above object, the gist of the present invention is to provide a polarized electron beam composed of a group of electrons whose spin directions are unevenly distributed in one of two types. A polarized electron beam generating element generated by receiving energy, comprising at least one pair of a first compound semiconductor layer and a second compound semiconductor layer having mutually different lattice constants and being hetero-bonded to each other. In addition, the first compound semiconductor layer is formed such that the energy level difference between the sub-band of the heavy hole and the sub-band of the light hole in the second compound semiconductor layer generating the polarized electron beam becomes larger than the energy due to the thermal noise. The difference lies in that the lattice constant difference between the layer and the second compound semiconductor layer is determined.

[0007]

In this way, since the second compound semiconductor layer having a different lattice constant is hetero-coupled to the first compound semiconductor layer, lattice distortion is imparted to the second compound semiconductor layer. And the band splitting of the valence band is generated. That is, the valence band of the second compound semiconductor layer includes a heavy-hole subband and a light-hole subband, and the energy levels of both subbands are one at the lowest energy state in the absence of lattice distortion. However, the difference in energy level is caused by the application of lattice strain. The spin directions of electrons extracted by excitation of both subbands are opposite to each other. Therefore, if light energy that excites only the subband having the higher energy level, that is, the subband having the smaller energy gap with the conduction band, is injected into the second compound semiconductor layer,
An electron group unevenly distributed in one spin direction is exclusively excited and emitted, and an electron beam having a sufficiently high polarization is obtained. Moreover, since the lattice bond is stably applied from the inside by the hetero bond between the compound semiconductor layers having different lattice constants, the polarized electron beam having a stable polarization rate is obstructed by the external strain applying device. It can be obtained easily without having to do it.

On the other hand, if the energy level difference between the heavy hole and the light hole is small, both sub-bands are excited by thermal noise, so that a sufficient effect of improving the polarization rate cannot always be obtained. Since the lattice constant difference between the two compound semiconductor layers is determined so that the energy level difference between the hole sub-band and the light hole sub-band becomes larger than the energy due to thermal noise, the sub-band having the lower energy level Excitation in the band is suppressed, and an extremely high polarization rate can be realized.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings.

In FIG. 1, a polarized electron beam generating element 10
Is a well-known MOCV on the semiconductor substrate 12.
The first compound semiconductor layer 14 and the second compound semiconductor layer 16 sequentially grown by the D crystal growth method are provided.
The semiconductor substrate 12 has a thickness of about 350 μm and has a carrier concentration of about 5 × 10 ¹⁸ (cm ⁻³ ) by doping zinc Zn as an impurity, for example. p-GaAs), and the surface is a (100) plane. The first compound semiconductor layer 14 has a thickness of about 2.0 μm and has a carrier concentration of about 5 × 10 ¹⁸ (cm ⁻³ ) by doping zinc Zn as an impurity.
Arsenic-phosphorus p-type semiconductor single crystal layer (p-GaAs
_0.83 P _0.17 ). Also, the second compound semiconductor layer 16
Has a thickness of about 0.08 μm, for example, zinc Z
Gallium-arsenic p having a carrier concentration of about 5 × 10 ¹⁸ (cm ⁻³ ) by doping n as an impurity
It is a type semiconductor single crystal layer (p-GaAs). Note that no oxidation treatment film or the like is provided on the surface of the second compound semiconductor layer 16.

The thickness of the second compound semiconductor layer 16 is about 0.1.
Although it exceeds the critical film thickness for performing coherent growth at about 08 μm, the first compound semiconductor layer 14 and the second compound semiconductor layer 16 have different lattice constants by about 0.6%, so that the second compound semiconductor layer 14 and the second compound semiconductor layer 16 have different lattice constants. The semiconductor layer 16 is hetero-coupled to the first compound semiconductor layer 14 in a state of having a lattice distortion, and the lattice distortion causes the sub-bands of the heavy hole and the light hole in the valence band of the second compound semiconductor layer 16 to fall. A difference occurs in energy levels. This energy level difference is larger than the energy due to thermal noise generated when using the polarized electron beam generator 10. The energy E due to the thermal noise is given by the following equation (1) and varies depending on the temperature. In this embodiment, the energy level difference is about 40 meV, and the energy E of the thermal noise at room temperature (about 25 ° C.) is about 26 meV. Is also large enough.

E = kT...
・ (1) However, k: Boltzmann constant T: Absolute temperature

The polarized electron beam generator 10 constructed as described above is used, for example, in a polarized electron beam generator (electron gun) 20 shown in FIG. In FIG.
In addition to the polarized electron beam generator 20, a polarization rate measuring device 22 for measuring the polarization rate of electrons contained in the electron beam generated therefrom, A polarized electron transfer device 24 for transferring the polarized electron beam to the polarization rate measuring device 22 is provided.

The polarized electron beam generator 20 includes a vacuum housing 30 for forming a high vacuum chamber and a vacuum housing 3.
A turbo-molecular pump 32 and an ion pump 34 for making 0 a high vacuum of about 10 ^-9 torr, and a container holder for holding the polarized electron beam generating element 10 in a vacuum housing 30 and cooling it with liquid nitrogen. 36, a liquid nitrogen container 38 surrounding the container-shaped holder 36 to adsorb residual gas, a plurality of electrodes 40 for extracting electrons from the surface of the polarized electron beam generating element 10, and a polarized electron beam generating element A cesium emitter 42 and an oxygen emitter 44 for emitting cesium and oxygen toward the surface of the device 10, and a laser beam irradiator 46 for irradiating the surface of the polarized electron beam generator 10 with a laser beam. I have. The laser light irradiation device 46 includes a tunable laser light source 50 for selectively outputting laser light having a wavelength of 700 to 900 nm, a polarizer 52 for passing only linearly polarized light, and a quarter for converting linearly polarized light to circularly polarized light. A wavelength element 54 and a mirror 56 for irradiating circularly polarized laser light toward the surface of the polarized electron beam generating element 10 are provided.

The polarization measuring device 22 is disposed in a tank 60 filled with chlorofluorocarbon gas, supported by a high-pressure insulator 62 and applied with a voltage of 100 kV from an anode 63 (Mott scattering tank) 64; A turbo-molecular pump 66 connected to a tank 64 for ^creating a high vacuum of about 10 ^-6 therein, an accelerating electrode 68 for accelerating a polarized electron beam, a polarized electron beam supported by a disk (not shown) , A pair of surface barrier type detectors 72 for detecting electrons scattered at θ = 120 ° by colliding with gold nuclei in the gold foil 70, and a surface amplified by a preamplifier (not shown). An LED 74 that converts a signal from the barrier type detector 72 into light is provided, and a light receiver 76 that receives the light output of the LED 74 and converts it into an electric signal.

FIG. 3 shows an example of a circuit for calculating the polarization rate based on the two-channel signal from the surface barrier type detector 72. In the figure, the signal from the surface barrier type detector 72 is amplified by a preamplifier 84 and then amplified by an LED.
The light is converted into an optical signal by an optical signal 74. The light receiver 76 receives the optical signal, converts it into an electric signal, and supplies it to the arithmetic and control circuit 80 via the interface 78. The arithmetic control circuit 80 calculates the polarization rate of the electron group included in the polarized electron beam based on the input signal from an arithmetic expression stored in advance,
It is displayed on the display 82.

Returning to FIG. 2, the polarized electron transfer device 24
Is a pair of small tubes 90 having a low conductance provided in a pipe connecting the vacuum housing 30 and the high-pressure tank 64.
An ion pump 92 provided at a position sandwiched between the pair of thin tubes 90; and a spherical capacitor device 94 for electrostatically bending a polarized electron beam drawn from the polarized electron beam generating element 10 at a right angle. And a Helmholtz coil 96 for magnetically bending the polarized electron beam toward the high-pressure tank 64 at a right angle. If the vacuum housing 30 and the high-pressure tank 64 have a relative positional relationship that does not require bending of the polarized electron beam, the spherical condenser device 94 and the Helmholtz coil 96 become unnecessary.

In the case of generating a polarized electron beam in the above-described apparatus, since no oxidized film is provided on the surface of the second compound semiconductor layer 16 of the polarized electron beam generating element 10, immediately after the growth, The polarized electron beam generator 10 stored in a vacuum desiccator is used. First, after the polarized electron beam generating element 10 is fixed to the lower end of the container holder 36, the inside of the vacuum housing 30 is set to a high vacuum of about 10 ^-9 , and heated to a temperature of about 420 ° C. for about 15 minutes by a heater (not shown). Thereby, the surface of the polarized electron beam generating element 10 is cleaned. Next, the cesium emitter 42 and the oxygen emitter 44
Cesium and oxygen are alternately released from the substrate toward the surface of the polarized electron beam generating element 10 to adsorb only a small amount of cesium and oxygen. Thereby, the polarized electron beam generating element 10
In the surface of the above, the electron affinity (energy gap corresponding to the difference between the energy level of electrons at the bottom of the conduction band and the vacuum level) is negative. Circularly polarized laser light was irradiated from the laser light irradiation device 46 at room temperature without cooling the polarized electron beam generator 10 with liquid nitrogen. When the energy of the circularly polarized laser light is injected, an electron group whose spin direction is unevenly distributed to one side is generated from the surface of the polarized electron beam generating element 10, and this electron group is converted by the electrode 40 as a polarized electron beam. It is withdrawn. The polarized electron beam is applied to the gold foil 70 in the high-pressure tank 64 by the polarized electron transfer device 24, and the polarization rate is measured by the circuit shown in FIG.

According to the experiments conducted by the present inventors, a polarized electron beam generating element conventionally used (a P-GaAs layer grown on a P-GaAs substrate: polarized electron beam generating element 10) (Corresponding to the first compound semiconductor layer 14 removed) was about 43%, whereas the polarized electron beam generating element 10 of the present example clearly shows the measurement results from FIG. The wavelength of the pump laser is 855 to 870 nm
, A polarization rate of 85% or more could be achieved. The quantum efficiency (QE) in this embodiment is as shown in FIG.
At ８870 nm, it is about 3 × 10 ⁻⁴ .

As described above, according to the polarized electron beam generator 10 of the present embodiment, the second compound semiconductor layer 16 having a different lattice constant is hetero-coupled to the first compound semiconductor layer 14, Since lattice strain is imparted to the compound semiconductor layer 16 to cause a difference in the energy level between the heavy band and the light hole sub-band in the valence band, the sub-band having the higher energy level. In the example, by injecting into the second compound semiconductor layer 16 an optical laser that excites only the sub-band of the heavy hole, that is, an excitation laser having a wavelength of 855 to 870 nm,
An electron group unevenly distributed in one spin direction is exclusively excited and emitted. In particular, although the thickness of the second compound semiconductor layer 16 exceeds the critical thickness, both compound semiconductor layers 14 and
Since the lattice constant difference of the second compound semiconductor layer 16 is relatively large, the lattice distortion of the second compound semiconductor layer 16 is large, and the energy level difference between the heavy hole subband and the light hole subband becomes larger than the energy due to thermal noise. As a result, excitation in the sub-band of the light hole having a low energy level was suppressed well, and an extremely high polarization rate of 85% or more could be realized.

The polarized electron beam generating element 10 of this embodiment
According to the method, since the hetero bond between the first compound semiconductor layer 14 and the second compound semiconductor layer 16 whose lattice constants are different from each other, the strain is stably applied from the inside, so that a stable polarization rate is provided. The polarized electron beam can be easily obtained without being disturbed by the external strain applying device.

Here, in the polarized electron beam generating element 10, the strain is internally generated due to the hetero bond between the two layers of the first compound semiconductor layer 14 and the second compound semiconductor layer 16. The difference in lattice constant between three or more layers may be used.

In the polarized electron beam generator 10, the first compound semiconductor layer 14 is made of GaAs _0.83 P _0.17 and the second compound semiconductor layer 16 is made of GaAs.
The composition of GaAs _1-x P _x can be changed as appropriate,
For example, Ga _1-x Al _x As, Ga _x In _1-x As
_{1-y Py} system, In _1-xy Ga _x Al _y P system, Ga _1-x I
Other types of compound semiconductors such as n _x P system may be used if necessary.

In the above embodiment, the energy level difference between the heavy hole sub-band and the light hole sub-band is made larger than the thermal noise at room temperature. It is sufficient that the noise is larger than the thermal noise when the line generating element 10 is used.

In the above-described embodiment, the second compound semiconductor layer 16 has a larger lattice constant than the first compound semiconductor layer 14, but the second compound semiconductor layer that generates polarized electron beams has a larger lattice constant. It does not matter if the lattice constant is smaller than that of the semiconductor layer. In that case, the light hole has a higher energy level than the heavy hole.

The above is merely an example of the present invention, and the present invention can be variously modified without departing from the gist of the present invention.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view illustrating a configuration of a polarized electron beam generating element according to one embodiment of the present invention.

FIG. 2 is a diagram showing a polarized electron beam generator and a polarization rate measuring device provided with the polarized electron beam generator of FIG. 1;

FIG. 3 is a diagram showing a measuring circuit of the polarization rate measuring device of FIG. 2;

FIG. 4 is a diagram showing a measurement result of a polarization rate measured by the polarization rate measuring device of FIG. 2;

5 is a diagram showing a measurement result of quantum efficiency when a polarized electron beam is generated by the polarized electron beam generator of FIG. 2;

[Explanation of symbols]

 10: polarized electron beam generating element 14: first compound semiconductor layer 16: second compound semiconductor layer

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-60-185145 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01J 1/34 H01J 37/075

Claims (1)

(57) [Claims] 1. A polarized electron beam generating element for generating a polarized electron beam comprising a group of electrons whose spin directions are localized in one of two types by receiving light energy, wherein the polarized electron beams are different from each other. At least one pair of the first compound semiconductor layer and the second compound semiconductor layer having a lattice constant and hetero-bonded to each other, and the sub-portion of the heavy hole in the second compound semiconductor layer that generates the polarized electron beam The difference in lattice constant between the first compound semiconductor layer and the second compound semiconductor layer is determined so that the energy level difference between the band and the sub-band of the light hole is larger than the energy due to thermal noise. Polarized electron beam generator.