JPH07147392A - Quantum element - Google Patents

Abstract

PURPOSE:To achieve a quantum element which can efficiently modulate the correlation between electrons or between positive holes in a quantum box array according to an external electric field. CONSTITUTION:Quantum dots QD in a structure where the periphery of i-type AlxGa1-XAs layer 3 constituting a quantum well part is surrounded by i-type AlAs layer 2 constituting a barrier layer are laid out within mutually adjacently within one surface to form a quantum dot array. Each quantum dot QD is asymmetrical in terms of composition for the direction off crossing the arrangement surface. By applying an external electric field in this direction, the correlation between electrons in the quantum dot array is modulated. Each quantum dot QD may be asymmetrical in terms of shape for the direction of crossing the arrangement surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quantum device, and more particularly to a quantum device using a quantum box (also called a quantum dot).

[0002]

2. Description of the Related Art As a future image of semiconductor devices, positive use of interaction between electrons is beginning to be considered. The reason for this is that it has become possible to realize a low charge density homogeneously by improving the quality of semiconductor crystals, and it has become possible to confine charges in a narrow region by means of compound semiconductor heterojunctions. Is mentioned. From such a point of view, the coupled quantum dot array has recently attracted attention.

The present applicant has previously proposed in Japanese Patent Application No. 5-21982 a method of modulating the movement of electrons between quantum dots in such a coupled quantum dot array by an external electric field, and further, Japanese Patent Application No. In No. 5-174774, we proposed a quantum dot assembly device to which this method is applied.

[0004]

However, in the methods proposed in Japanese Patent Application Nos. 5-21982 and 5-174774, the efficiency of inter-electron correlation modulation in a quantum dot array by an external electric field is high. Not necessarily high enough.

Therefore, it is an object of the present invention to provide a quantum device capable of efficiently modulating the correlation between electrons or holes in a quantum box array by an external electric field. Another object of the present invention is to provide a transistor capable of efficiently modulating the current flowing from one end to the other end of the quantum box array by an external electric field.

[0006]

In order to achieve the above object, a quantum device according to the first invention of the present invention has a plurality of quantum boxes (QD) arranged adjacent to each other in one plane, A plurality of quantum boxes (QD) are compositionally asymmetric with respect to a direction orthogonal to the above plane.

In the quantum device according to the first aspect of the present invention, since the plurality of quantum boxes (QD) are compositionally asymmetric with respect to the above-mentioned direction, the quantum confinement barrier of the plurality of quantum boxes (QD) is high. Changes with respect to the above direction.

In the quantum device according to the first aspect of the present invention, preferably, a plurality of quantum boxes (QD) are formed by compound semiconductor heterojunctions, and the mixed crystal ratio of the compound semiconductors forming the compound semiconductor heterojunctions is in the above direction. The height of the barrier changes with respect to the above direction due to the change.

In an exemplary embodiment of the quantum device according to the first invention, the height of the barrier increases or decreases monotonically with respect to the above direction.

In a preferred embodiment of the quantum device according to the first aspect of the present invention, the electrical conductivity in the plurality of quantum boxes (QDs) is modulated by applying an external electric field in the above direction.

In a preferred embodiment of the quantum device according to the first invention, an electrode (4) for applying an external electric field.
Are provided adjacent to a plurality of quantum boxes (QDs).

A quantum device according to the second invention of the present invention is
A plurality of quantum boxes (Q
D), and a plurality of quantum boxes (QD) are geometrically asymmetric with respect to the direction orthogonal to the plane.

In the quantum device according to the second aspect of the present invention, the plurality of quantum boxes (QDs) are geometrically asymmetric with respect to the above direction, so that the quantum boxes (QDs) adjacent to each other are formed.
The distance between them changes in the above direction.

In an exemplary embodiment of the quantum device according to the second aspect of the invention, the spacing between the quantum boxes (QDs) monotonically increases or decreases with respect to the above direction.

In the quantum device according to the second aspect of the present invention, preferably, a plurality of quantum boxes (QD) are formed by a compound semiconductor heterojunction.

In a preferred embodiment of the quantum device according to the second invention, the electrical conductivity in the plurality of quantum boxes (QDs) is modulated by applying an external electric field in the above direction.

In a preferred embodiment of the quantum device according to the second invention, an electrode (2) for applying an external electric field is used.
4) is provided adjacent to the plurality of quantum boxes (QDs).

In a preferred embodiment of the quantum device according to the second aspect of the invention, the quantum boxes (QD) have a pyramidal shape so that the spacing between the quantum boxes (QD) changes in the above direction. , For example having the shape of a cone, a triangular pyramid, a quadrangular pyramid or a regular tetrahedron.

The quantum box (QD) may have a shape in which the top of a cone, a triangular pyramid, a quadrangular pyramid or a regular tetrahedron is cut off.

In the transistor according to the third aspect of the present invention, a plurality of quantum boxes (QD) arranged adjacent to each other in one plane are provided in the channel region, and a plurality of quantum boxes (QD) are provided.
D) is compositionally or geometrically asymmetric with respect to the direction orthogonal to the plane.

Here, the physical meaning of "transfer energy" used in the following description will be described.

Now, the quantum box 1 whose center coordinates are (r _x , r _y )
Considered center coordinates (-r _x, -r _y) quantum boxes binding system consisting of two quantum boxes and quantum box 2. Then, the dynamics of electrons in this quantum box coupled system can be calculated from the exact solution of the electronic state of the isolated hydrogen atom to the hydrogen molecular ion (H ₂
LCAO, which is known as an effective approximation of when considering the electronic states of ⁺⁾ (Linear Combination of Atomic Orb
Itals) Consider based on the approximation.

Considering this LCAO approximation, when the quantum boxes 1 and 2 that were initially isolated approach each other, the ground state | 1> of the electrons in the quantum box 1 and the quantum box 2
Width of the energy level E ₀ of the ground state | 2> of the electron
Splitting of E occurs, and two states, a bound state and an anti-bound state, are obtained. The energies and wave functions of these bound and anti-bound states are expressed as follows.

[0024]

[Equation 1]

[Equation 2] Where ΔE is called transfer energy,
As described later, it is a measure of the electron tunneling time τ between the quantum boxes.

The Hamiltonian of this quantum box coupled system is

[Equation 3] Is written,

[Equation 4] Is an eigenstate of this Hamiltonian as shown in the following equation.

[Equation 5]

Now, for example, assuming that electrons are localized in the quantum box 1, this state is

[Equation 6] Can be written. When this state is time-developed by the Schrodinger equation, the state at time t is

[Equation 7] Becomes

From this,

[Equation 8] It can be seen that at time t that satisfies, the electrons localized in the quantum box 1 reach the quantum box 2. Therefore, within this LCAO approximation range, the electron tunneling time τ from quantum box 1 to quantum box 2 is

[Equation 9] Can be considered.

More generally, this tunnel time τ is

[Equation 10] Can be written.

From the above, if the dynamics of electrons in the quantum box coupled system is simplified most, the electrons will move by tunneling depending on the magnitude of the transfer energy ΔE between the quantum boxes.

Next, the expression of the transfer energy ΔE within the range of LCAO approximation will be found.

Now, considering a single square quantum box having a side length of 2d, its potential energy is

[Equation 11] Is. Therefore, if we write the kinetic energy as K, the Hamiltonian of this system is

[Equation 12] Is. This Hamiltonian ground state

[Equation 13] And its energy is E ₀ ,

[Equation 14] Holds.

On the other hand, the Hamiltonian of a coupled quantum box system consisting of two square quantum boxes can be written as

[0033]

[Equation 15] However, if the center coordinates of one quantum box and the center coordinates of the other quantum box are written as described above,

[Equation 16] Is.

On the other hand, the wave function of the Hamiltonian ground state of the single square quantum box expressed by the equation (10) is

[Equation 17] In Although,

[Equation 18] Respectively

[Formula 19] Meets

When the above preparation is completed, the energy eigenvalue of the Hamiltonian of the coupled quantum box system expressed by Eq. (12) is calculated on the two-dimensional subspace where the eigenstates of the single square quantum box expressed by Eq. (15) extend. Ask in. Since the two eigenstates shown in Eq. (15) are not orthogonal, if an orthogonal basis is first constructed, this will be as follows, for example.

[0036]

[Equation 20]

When the Hamiltonian matrix elements are calculated with this orthogonal basis,

[Equation 21]

[Equation 22]

[Equation 23]

[Equation 24] Becomes However, in the calculation of these matrix elements,
Equation (16) and the following equation were used.

[0038]

[Equation 25]

As can be seen from the equations (20) and (21), since the non-diagonal elements of the Hamiltonian matrix are 0, this Hamiltonian matrix is actually diagonalized. Therefore, the energy eigenvalue is

[Equation 26] And their eigenvectors are

[Equation 27] Has become.

In equations (18) and (19), from the localization of the wave function,

[Equation 28] As energy,

[Equation 29] Can be thought of as.

The transfer energy (Δ
When E) is small, the electrons are localized in each quantum box, and the electron motion due to tunneling between quantum boxes is suppressed.

[0042]

According to the quantum device of the first aspect of the present invention configured as described above, a plurality of quantum boxes (QDs) are compositionally asymmetric with respect to a direction orthogonal to their array plane. Therefore, an external electric field is applied in that direction to control the distribution of the electron or hole wave function in the multiple quantum boxes (QDs), so that a quantum box array composed of multiple quantum boxes (QDs) It is possible to efficiently modulate the correlation between electrons or holes.

According to the quantum element of the second aspect of the present invention configured as described above, the plurality of quantum boxes (QDs) are geometrically asymmetric with respect to the direction orthogonal to their array plane. Therefore, an external electric field is applied in that direction to control the distribution of the wave function of electrons or holes in a plurality of quantum boxes (QDs), so that a plurality of quantum boxes can be produced. It is possible to efficiently modulate the correlation between electrons or holes in a quantum box array including quantum boxes (QD).

According to the transistor of the third aspect of the present invention configured as described above, the plurality of quantum boxes (QDs) provided in the channel region are compositional with respect to the direction orthogonal to their arrangement planes. Or, since they are asymmetrical in shape, a plurality of quantum boxes (QD) are controlled by applying an external electric field in that direction to control the distribution of electron or hole wavefunctions in the plurality of quantum boxes (QD). It is possible to efficiently modulate the correlation between electrons or between holes in a quantum box array made of QD). Thereby, the current flowing from one end to the other end of the quantum box array in the channel region can be efficiently modulated by the external electric field, and the transistor operation can be realized.

[0045]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a quantum device according to the first embodiment of the present invention.

As shown in FIG. 1, in the quantum device according to the first embodiment, an i-type AlAs layer 2 as a barrier layer is provided on an n-type GaAs substrate 1. In the i-type AlAs layer 2, a plurality of i-type Al _x Ga _1-x As layers 3 each having a rectangular parallelepiped shape are two-dimensionally arranged in the xy plane in a predetermined arrangement. It is embedded. Then, a structure in which the i-type AlAs layer 2 as a barrier layer surrounds the i-type Al _x Ga _1-x As layer 3 as a quantum well portion is surrounded by AlAs / Al _x Ga _1-x As.
A heterojunction quantum dot QD is formed, and a plurality of quantum dots QD are two-dimensionally arranged in the xy plane to form a two-dimensional quantum dot array. Here, the size of the quantum dots QD and the adjacent quantum dots QD
The distance between them is selected to be about 10 nm, for example.
Further, an electrode 4 made of a conductor such as a metal is provided on the i-type AlAs layer 2 as the barrier layer. In this case, the n-type GaAs substrate 1 is used as the other electrode. During operation of the quantum device according to the first embodiment, for example, the n-type GaAs substrate 1 as this electrode is used.
Is grounded and a voltage is applied to the electrode 4.

In the quantum device according to the first embodiment, the composition of each quantum dot QD changes along the z-axis direction perpendicular to the xy plane, so that each quantum dot QD has a z-direction.
It is compositionally asymmetric with respect to the axial direction. Specifically, the mixed crystal ratio (Al composition ratio) x of the i-type Al _x Ga _1-x As layer 3 as the quantum well portion is a function of z coordinate, and is typically a linear function.

FIG. 2 shows an example of the energy band structure along the line α-α (z-axis direction) in FIG. 1 at this time. As shown in FIG. 2, in this case, the mixed crystal ratio x of the i-type Al _x Ga _{1 -x} As layer 3 is equal to the i-type A of the lower portion in FIG.
From the value of 0 at the interface with the lAs layer 2 to the value of x0 (for example, 0.1) at the interface with the i-type AlAs layer 2 in the upper portion in FIG. 1, linearly in the positive direction of the z axis. The energy E _{c at} the bottom of the conduction band of the i-type Al _x Ga _{1 -x} As layer 3 monotonically increases as a result, and monotonically increases in the positive direction of the z axis.

Next, the operating principle of the quantum device according to the first embodiment constructed as described above will be explained.

First, the amount to be controlled in the quantum device according to the first embodiment will be defined. The transfer energy T (which is represented here by T instead of ΔE) is the amount that determines the ease of electron transfer when electrons move between adjacent quantum dots QD by tunneling.
Is. In addition, here, of the many-body interactions between electrons in the quantum dot array, on-site coulomb
Consider only energy U. This on-site Coulomb energy U is energy that rises due to the Coulomb force acting between two electrons when one quantum dot QD contains two electrons. When the quantum dot array is half filled, that is, when the quantum dot QD has an electron density of one electron, the electric conductivity of this electron system is R = T / U. It is known to change dramatically (Mott transition). In the quantum device according to the first embodiment, this R is effectively modulated by the external electric field as follows.

Now, when there is no potential difference between the electrode 4 and the n-type GaAs substrate 1 as the other electrode, that is, z
It is assumed that the electronic state in the quantum dot QD is as shown in FIG. 3 when the external electric field is not applied in the axial direction. As shown in FIG. 3, in this state, the electron wave function in the quantum dot QD has a negative side of the z-axis (see FIG.
(Distributed in the middle left). The effective barrier height for quantum confinement of electrons in the quantum dot QD at this time is as shown in FIG.

Next, the potential of the electrode 4 is set to the n-type GaAs substrate 1
By setting the potential higher than the potential of, specifically, for example, n
An external electric field in the negative direction of the z-axis is applied by grounding the GaAs substrate 1 and applying a positive voltage to the electrode 4. Then, as shown in FIG. 4, the distribution of the electron wave function in the quantum dot QD moves to the positive side of the z axis (the right side in FIG. 4), that is, to the high potential side. The effective barrier height of the quantum confinement of electrons in the quantum dot QD at this time is as shown in FIG. 4, which is smaller than that before the application of the external electric field. Therefore, the existence probability of electrons in the one having a lower tunneling barrier height separating the adjacent quantum dots QD, that is, in the adjacent quantum dot QD on the positive side of the z-axis becomes higher, and as a result, between the adjacent quantum dots QD. The transfer energy T will increase.

At this time, focusing on the single quantum dot QD, the wave function of the electron spreads in the z-axis direction by the external electric field, and the quantum confinement strength of the electron becomes small. On the other hand, the on-site Coulomb energy U generally increases as the quantum confinement strength increases, so that U decreases due to the external electric field. Therefore, it can be seen that due to the external electric field, T → increases U → decreases, and R = T / U extremely increases. By this method, the inter-electron correlation of the electron system in the quantum dot array can be efficiently modulated by the external electric field.

FIG. 5 shows an example of the result of calculation of the change in the transfer energy T due to the applied electric field using the interval d between the adjacent quantum dots as a parameter. However, in this calculation, the length of the quantum dot in the z-axis direction is 16n.
A cross section perpendicular to the m and z axis directions is a square having a side length of 6 nm. From FIG. 5, d = 3 nm, 4 nm, 5
It can be seen that in any case of nm, the transfer energy is greatly changed by applying the electric field.

Next, a method of manufacturing the quantum device according to the first embodiment constructed as described above will be described.

First, as shown in FIG. 6, the i-type AlAs layer 2a and the i-type Al _x Ga _1-x A are formed on the n-type GaAs substrate 1 by, for example, the metal organic chemical vapor deposition (MOCVD) method.
The s layer 3 and the i-type AlAs layer 2b are sequentially epitaxially grown. Here, the epitaxial growth of the i-type Al _x Ga _1-x As layer 3 is performed while changing the mixed crystal ratio x by changing the supply amount of the growth raw material.

Next, after forming a resist pattern (not shown) on the i-type AlAs layer 2b at a portion corresponding to the quantum dot QD to be formed by, for example, an electron beam lithography method, the resist pattern is used as a mask to react, for example. Anisotropic etching is performed in the direction perpendicular to the substrate surface by the reactive ion etching (RIE) method. This anisotropic etching is performed until the i-type AlAs layer 2a is exposed. After that, the resist pattern is removed. As a result, FIG.
As shown in FIG. 5, the i-type Al _x Ga _1-x As layer 3 and the i-type AlAs layer 2b are patterned into a rectangular column shape.

Next, as shown in FIG. 8, for example, MOCV
The i-type AlAs layer 2c is epitaxially grown by the D method to form a square columnar i-type Al _x Ga _1-x As layer 3 and i.
The portion between the type AlAs layers 2b is filled. In this case, the entire i-type AlAs layer 2a, 2b, 2c corresponds to the i-type AlAs layer 2 in FIG.

Next, a metal film is formed on the entire surfaces of the i-type AlAs layers 2b and 2c by, for example, a vacuum vapor deposition method or a sputtering method, and then the metal film is patterned as necessary, as shown in FIG. , The electrode 4 is formed. This completes the desired quantum device.

As described above, according to this first embodiment,
Efficiently performing inter-electron correlation modulation in a quantum dot array by an external electric field in the z-axis direction by making each quantum dot QD compositionally asymmetric with respect to the z-axis direction orthogonal to the array planes thereof. You can

In the above-described first embodiment, the quantum dots QD are compositionally asymmetric with respect to the z-axis direction perpendicular to their arrangement planes, but each quantum dot QD is perpendicular to their arrangement planes. Even when the shape is asymmetric with respect to the z-axis direction, the same effect as that of the first embodiment can be obtained. That is, as shown in FIG. 10, for example, a quantum dot array may be formed using tetrahedral quantum dots QD that are asymmetric with respect to the z-axis direction. Quantum dots QD of such tetrahedral shape, for example, as shown in FIG. 11, the entire surface, for example, SiO ₂ of the GaAs substrate (not shown in the figure)
After forming a film and selectively etching the SiO ₂ film to form a triangular opening, the AlGaAs layer 11 is selectively epitaxially grown on the GaAs substrate at the opening, and a triangular pyramid is formed thereon. It can be formed by epitaxially growing the GaAs layer 12 and then by epitaxially growing the AlGaAs layer 13 so as to cover them.

Then, next, the quantum dot Q as described above is used.
A second embodiment of the present invention utilizing the shape asymmetry of D will be described. FIG. 12 shows a quantum device according to the second embodiment of the present invention.

As shown in FIG. 12, in the quantum device according to the second embodiment, an i-type Al _y Ga _1-y As layer 22 as a barrier layer is provided on an n-type GaAs substrate 21. In the i-type Al _y Ga _1-y As layer 22, for example, an i-type G having a tetrahedral shape as a quantum well portion is formed.
A plurality of aAs layers 23 are two-dimensionally embedded in the xy plane in a predetermined arrangement. And i-type A as a barrier layer
Al _y by l _y Ga _1-y structure peripherally-surrounded the i-type GaAs layer 23 serving as a quantum well portion by As layer 22
Quantum dot Q with Ga _1-y As / GaAs heterojunction
D is formed, and the quantum dots QD are two-dimensionally arranged in the xy plane to form a two-dimensional quantum dot array. Here, the size of the quantum dots QD is selected to be, for example, about 10 nm, and the interval between the adjacent quantum dots QD is, for example, 10 n as an average value of the maximum value and the minimum value.
m is selected. Further, i-type Al _y G as a barrier layer
An electrode 24 made of a conductor such as a metal is provided on the a _1-y As layer 22. In this case, n-type GaA
The s substrate 21 is used as the other electrode. Similar to the quantum device according to the first embodiment, during operation of the quantum device according to the second embodiment, for example, the n-type GaAs substrate 21 as this electrode is grounded and a voltage is applied to the electrode 24.

In the quantum device according to the second embodiment, since the i-type GaAs layer 23 as the quantum well portion has a tetrahedral shape, each quantum dot QD has a geometrical shape in the z-axis direction. Is asymmetrical. in this case,
The cross-sectional area of each quantum dot QD when cut in parallel to the xy plane monotonically decreases in the positive direction of the z axis. And
The spacing between the adjacent quantum dots QD monotonically increases in the positive direction of the z-axis, as shown in FIG.

Next, the operating principle of the quantum device according to the second embodiment constructed as described above will be explained.

First, the potential of the electrode 24 is set lower than that of the n-type GaAs substrate 21 serving as the other electrode, specifically, for example, the n-type GaAs substrate 21 is grounded,
By applying a negative voltage to the electrode 24, an external electric field in the positive direction of the z axis is applied. Then, the electrons in each quantum dot QD receive a force in the negative direction of the z-axis and are attracted to the lower part of each quantum dot QD in FIG. That is, the wave function of the electron in each quantum dot QD is distributed biased to the negative side of the z axis. At this time, the interval between the adjacent quantum dots QD is effectively reduced, and thus the width of the barrier between the adjacent quantum dots QD is effectively reduced. Therefore, the electrons easily move between the adjacent quantum dots QD by tunneling, and the transfer energy T increases.

On the other hand, when the external electric field is applied in the opposite direction, that is, in the negative direction of the z-axis, the electrons in each quantum dot QD receive a force in the positive direction of the z-axis, and the electrons in FIG. It is pushed to the top of the quantum dot QD, that is, the top of the tetrahedron. That is, the wave function of the electron in each quantum dot QD is distributed biased to the positive side of the z axis. At this time, the interval between the adjacent quantum dots QD effectively increases, and therefore the width of the barrier between the adjacent quantum dots QD effectively increases. For this reason, it becomes difficult for electrons to move between adjacent quantum dots QD due to tunneling, and
Energy T decreases.

As described above, the transfer energy T between the quantum dots QD can be efficiently modulated by the external electric field in the z-axis direction.

As described in the first embodiment, when the quantum dot array is half-filled, that is, when the quantum dot QD has an electron density of one electron, the electricity of this electron system is changed. The conductivity changes dramatically with changes in R = T / U. That is, an external electric field in the z-axis direction can efficiently modulate the inter-electron correlation of the electron system in the quantum dot array, whereby R = T
/ U can be efficiently modulated.

When the quantum dots QD forming the quantum dot array are arranged periodically, the width of the subband formed by the periodicity is
Proportional to energy T. Therefore, in this case, it is understood that the width of the subband of the quantum dot array in which the quantum dots QD are periodically arranged can be efficiently modulated. Such a sub-bandwidth modulation method is disclosed in, for example, a velocity modulation transistor (Jpn. J. Appl. Phys. 21,
L381 (1982)) can be applied.

Next, a method of manufacturing the quantum device according to the second embodiment having the above structure will be described.

First, as shown in FIG. 14, for example, MOC
By the VD method, i-type Al _{y is formed} on the n-type GaAs substrate 21.
The Ga _1-y As layer 22a is epitaxially grown.

Next, for example, the method proposed by Fukui et al. (T. Fukui, S. Ando, and H. Saito, Science and Techno
logy of Mesoscopic Structures, p.353, ed. S. Namba,
C. Hamaguchi, and T. Ando (Springer-Verlag, Tokyo, 1
By 992)), as shown in FIG. 15, i-type Al _y Ga
A tetrahedral i-type GaAs layer 23 serving as a quantum well layer is formed in a two-dimensional array on the _1-y As layer 22a. That is, the CV is formed on the entire surface of the i-type Al _y Ga _1-y As layer 22a.
For example, a SiO ₂ film (not shown) is formed by the D method, and the SiO ₂ film is selectively etched to form equilateral triangular openings in a two-dimensional array.
The As layer is epitaxially grown. Thus, as shown in FIG. 15, i-type Al _y G portion of these openings
The tetrahedral i-type GaAs layer 2 is formed on the a _1-y As layer 22a.
3 is selectively epitaxially grown.

Next, after removing the SiO ₂ film used for the selective epitaxial growth by etching, as shown in FIG. 16, for example, i-type Al _y Ga _{1-y is formed} by MOCVD.
The As layer 22b is epitaxially grown to fill the portion between the tetrahedral i-type GaAs layers 23. in this case,
The i-type Al _y Ga _1-y As layers 22a and 22b are entirely shown in FIG.
2 corresponds to the i-type Al _y Ga _1-y As layer 22 in FIG.

Next, a metal film is formed on the entire surface of the i-type Al _y Ga _1-y As layer 22b by, for example, a vacuum vapor deposition method or a sputtering method, and then the metal film is patterned if necessary, and the pattern shown in FIG. The electrode 24 is formed as shown in FIG. This completes the desired quantum device.

As described above, according to the second embodiment,
By making each quantum dot QD geometrically asymmetric with respect to the z-axis direction orthogonal to their array plane, the quantum dots by an external electric field in the z-axis direction can be formed in the same manner as in the first embodiment.
Correlation modulation between electrons in the array can be efficiently performed.

FIG. 17 shows a quantum device according to the third embodiment of the present invention. As shown in FIG. 17, in the quantum device according to the third embodiment, the i-type GaAs layer 23 as the quantum well part of the quantum dot QD is cut off, for example, at the top of the quadrangular pyramid in a plane parallel to the bottom face opposite thereto. It has a convex shape. Since the i-type GaAs layer 23 as the quantum well portion has such a shape, each quantum dot QD is z-shaped.
The shape is asymmetric with respect to the axial direction. And
In this case, the cross-sectional area of each quantum dot QD when cut parallel to the xy plane monotonically decreases in the positive direction of the z-axis, and the interval between adjacent quantum dots QD is the positive z-axis. Direction is increasing monotonically.

The structure of the quantum device according to the third embodiment other than the above is the same as that of the quantum device according to the second embodiment, and the description thereof will be omitted. Further, the operating principle of the quantum device according to the third embodiment is also the same as that of the quantum device according to the second embodiment, and the description thereof will be omitted.

Next, a method of manufacturing the quantum device according to the third embodiment will be described. First, as shown in FIG.
For example, the i-type Al _y Ga _1-y As layer 22a, the i-type GaAs layer 23, and the i-type Al _y Ga _1-y As layer 22b are sequentially epitaxially grown on the n-type GaAs substrate 21 by the MOCVD method. Next, this i-type Al _y Ga _1-y
A mask 25 having a shape corresponding to the quantum dots QD to be formed is formed on the As layer 22b. The mask 25 is formed of, for example, a resist.

Next, using the mask 25, the i-type Al _y Ga _1-y As layers 22b and i are formed by the wet etching method under the condition that the side etching becomes large.
The type GaAs layer 23 is sequentially etched. As a result, as shown in FIG. 19, these i-type Al _y Ga _{1-y are formed.}
The As layer 22b and the i-type GaAs layer 23 are patterned into a quadrangular pyramid shape with the top cut off.

Next, after removing the mask 25, for example, M
By the OCVD method, as shown in FIG. 20, i-type Al _y G
a _1-y, As layer 22c is epitaxially grown, the pyramidal shape in which the top portion is cut i-type Al _y Ga _1-y As
The portion between the layer 22b and the i-type GaAs layer 23 is filled. In this case, the i-type Al _y Ga _1-y As layers 22a, 22
b, i overall 22c is shown in FIG. 17 type Al _y Ga _1-y
It corresponds to the As layer 22.

Next, after a metal film is formed on the i-type Al _y Ga _1-y As layers 22b and 22c by, for example, a vacuum vapor deposition method or a sputtering method, the metal film is patterned as necessary, As shown in 17, an electrode 24 is formed. This completes the desired quantum device.
Also in the third embodiment, the same advantages as those in the second embodiment can be obtained.

Next explained is the fourth embodiment of the invention. The fourth embodiment is an embodiment in which the present invention is applied to a field effect transistor. FIG. 21 shows the fourth embodiment of the present invention.
3 shows a field effect transistor according to an example.

As shown in FIG. 21, in the field effect transistor according to the fourth embodiment, for example, semi-insulating G
n-type GaA used as an electrode in the aAs substrate 31
The s layer 32 is provided. These semi-insulating GaAs
An i-type Al _y Ga _1-y As layer 33 as a channel region is provided on the substrate 31 and the n-type GaAs layer 32. Similar to the second embodiment, this i-type Al _y Ga _1-y A
In the s layer 33, a plurality of i-type GaAs layers 34 having a tetrahedral shape, for example, as quantum well portions are two-dimensionally embedded in the xy plane in a predetermined arrangement. Then, a quantum dot formed by an Al _y Ga _1-y As / GaAs heterojunction is formed by a structure in which the i-type Al _y Ga _1-y As layer 33 as a barrier layer surrounds the i-type GaAs layer 34 as a quantum well portion. QDs are formed, and the quantum dots QDs are two-dimensionally arranged in the xy plane to form a two-dimensional quantum dot array. In addition, this i-type Al
_A gate electrode 35 made of a conductor such as a metal is provided on the _y Ga _{1 -y} As layer 33. Further, a source electrode 36 and a drain electrode 37 made of a conductor such as metal are provided on both ends of the i-type Al _y Ga _1-y As layer 33, respectively.

Next, the operating principle of the field effect transistor according to the fourth embodiment constructed as described above will be described. Here, the source electrode 36 is grounded and a predetermined positive voltage is applied to the drain electrode 37. Also, the n-type GaAs layer 32 used as an electrode
Shall be grounded.

First, by applying a negative voltage to the gate electrode 35, an i-type Al _y Ga _1-y as a channel region is formed.
When applying a positive direction of the external electric field in the z-axis As layer 33, in the same manner as described in the second embodiment, the quantum dots embedded in the i-type Al _y Ga _1-y As layer 33 serving as the channel region The electrons in the QD receive a force in the negative direction of the z-axis and are attracted to the bottom of each quantum dot QD. At this time, the interval between the adjacent quantum dots QD is effectively reduced, and thus the width of the barrier between the adjacent quantum dots QD is effectively reduced. Therefore, the electrons easily move between the adjacent quantum dots QD by tunneling, and the transfer energy T increases. At this time, a drain current easily flows between the source electrode 36 and the drain electrode 37.

On the other hand, assuming that the external electric field is applied in the opposite direction, that is, in the negative direction of the z-axis, each quantum dot QD
The electrons inside receive a force in the positive direction of the z-axis, and each quantum dot Q
Pushed to the top of D, the top of the tetrahedron. At this time, the interval between the adjacent quantum dots QD effectively increases, and therefore the width of the barrier between the adjacent quantum dots QD effectively increases. For this reason, the electrons are
It becomes difficult to move between D by tunneling, and the transfer energy T decreases. At this time, it is difficult for a drain current to flow between the source electrode 36 and the drain electrode 37.

As described above, an external electric field in the z-axis direction is applied by applying a voltage to the gate electrode 35, and the transfer energy T between the quantum dots QD can be efficiently modulated by this external electric field. . Thereby, the drain current flowing between the source electrode 36 and the drain electrode 37 can be efficiently modulated, and the transistor operation can be realized.

Next, a method of manufacturing the field effect transistor according to the fourth embodiment having the above structure will be described.

First, the n-type GaAs layer 32 is formed by selectively doping the semi-insulating GaAs substrate 31 with an n-type impurity by, for example, an ion implantation method. Next, on the semi-insulating GaAs substrate 31 and the n-type GaAs layer 32, i-type Al _y Ga is formed by, for example, the MOCVD method.
After epitaxially growing the _1-y As layer 33, this i
Type Al _y Ga _1-y As layer 33 is patterned into the shape of the channel region by etching. After this, this i-type A
A gate electrode 35 is formed on the l _y Ga _1-y As layer 33, and a source electrode 36 and a drain electrode 37 are formed on both ends of the i-type Al _y Ga _1-y As layer 33, respectively. As a result, the intended field effect transistor is completed.

As described above, according to the fourth embodiment,
Since there is provided a quantum dot array in the x-y plane in the i-type Al _y Ga _1-y As layer 33 as a channel region,
The drain current can be efficiently modulated by the external electric field applied in the z-axis direction orthogonal to the array surface of the quantum dots QD forming the quantum dot array.

Although the embodiments of the present invention have been specifically described above, the present invention is not limited to the above-mentioned embodiments, and various modifications can be made based on the technical idea of the present invention.

For example, in the above-described first to fourth embodiments, the case where the two-dimensional quantum dot array in which the quantum dots are two-dimensionally arranged is used has been described.
The present invention can be similarly applied to the case of using a one-dimensional or three-dimensional quantum dot array in which quantum dots are arranged one-dimensionally or three-dimensionally.

In the fourth embodiment described above, the quantum dots QD forming the quantum dot array provided in the channel region have the shape asymmetry in the z-axis direction. May have composition asymmetry in the z-axis direction as used in the first embodiment, for example.

In the first embodiment, the i-type Al _x Ga _1-x forming the quantum well portion of the quantum dot QD is used.
Although the mixed crystal ratio x of the As layer 3 is changed in the z-axis direction, the quantum well portion of the quantum dot QD has a multiple quantum well structure in the z-axis direction and the effective mass of electrons is changed by changing the width of the quantum well portion. It is possible to obtain the same effect as that of the first embodiment by changing the value in the z-axis direction.

[0096]

As described above, according to the quantum device of the present invention, a plurality of quantum boxes are compositionally or geometrically asymmetric with respect to the direction orthogonal to the array plane, It is possible to efficiently modulate the correlation between electrons or holes in the quantum box array by the electric field. Further, according to the transistor of the present invention, a plurality of quantum boxes arranged adjacent to each other in one surface are provided in the channel region, and the plurality of quantum boxes are compositionally or perpendicular to the direction orthogonal to the surface. Since the shapes are asymmetric, the current flowing from one end to the other end of the quantum box array can be efficiently modulated by the external electric field.

[Brief description of drawings]

FIG. 1 is a perspective view showing a quantum device according to a first embodiment of the present invention.

FIG. 2 is an energy band diagram in a direction along a line α-α of FIG.

FIG. 3 is an energy band diagram for explaining the operating principle of the quantum device according to the first embodiment of the present invention.

FIG. 4 is an energy band diagram for explaining the operating principle of the quantum device according to the first embodiment of the present invention.

FIG. 5 is a graph showing an example of a calculation result of applied electric field dependence of transfer energy.

FIG. 6 is a perspective view for explaining the method of manufacturing the quantum device according to the first embodiment of the present invention.

FIG. 7 is a perspective view for explaining the method of manufacturing the quantum device according to the first embodiment of the present invention.

FIG. 8 is a perspective view for explaining the method of manufacturing the quantum device according to the first embodiment of the present invention.

FIG. 9 is a perspective view for explaining the method of manufacturing the quantum device according to the first embodiment of the present invention.

FIG. 10 is a perspective view showing a tetrahedral quantum dot.

FIG. 11 is a cross-sectional view showing a specific structural example of a tetrahedral quantum dot.

FIG. 12 is a perspective view showing a quantum device according to a second embodiment of the present invention.

FIG. 13 is a schematic diagram for explaining the operation principle of the quantum device according to the second embodiment of the present invention.

FIG. 14 is a perspective view for explaining the method of manufacturing the quantum device according to the second embodiment of the present invention.

FIG. 15 is a perspective view for explaining the method of manufacturing the quantum device according to the second embodiment of the present invention.

FIG. 16 is a perspective view for explaining the method of manufacturing the quantum device according to the second embodiment of the present invention.

FIG. 17 is a perspective view showing a quantum device according to a third embodiment of the present invention.

FIG. 18 is a schematic diagram for explaining the method of manufacturing the quantum element according to the third embodiment of the present invention.

FIG. 19 is a perspective view for explaining the method of manufacturing the quantum device according to the third embodiment of the present invention.

FIG. 20 is a perspective view for explaining the method of manufacturing the quantum device according to the third embodiment of the present invention.

FIG. 21 is a sectional view showing a field effect transistor according to a fourth embodiment of the present invention.

[Explanation of symbols]

1, 21 n-type GaAs substrate 2, 2a, 2b, 2c i-type AlAs layer 3 i-type Al _x Ga _1-x As layer 4, 24 electrode 22, 22a, 22b, 22c, 33 i-type Al _y Ga
_1-y As layer 23, 34 i-type GaAs layer 31 semi-insulating GaAs substrate 32 n-type GaAs layer 35 gate electrode 36 source electrode 37 drain electrode QD quantum dot

Claims (17)

[Claims] 1. A plurality of quantum boxes arranged adjacent to each other in one plane, wherein the plurality of quantum boxes are compositionally asymmetric with respect to a direction orthogonal to the plane. Quantum element to do. 2. The height of barriers for quantum confinement of the plurality of quantum boxes changes with respect to the direction because the plurality of quantum boxes are compositionally asymmetric with respect to the direction. The quantum device according to claim 1, wherein: 3. The quantum device according to claim 1, wherein the plurality of quantum boxes are formed by a compound semiconductor heterojunction. 4. The height of the barrier changes with respect to the direction because the mixed crystal ratio of the compound semiconductor forming the compound semiconductor heterojunction changes with respect to the direction. The quantum device according to claim 3. 5. The height of the barrier increases or decreases monotonically with respect to the direction.
3. The quantum device according to 3 or 4. 6. The quantum device according to claim 2, wherein the electrical conductivity in the quantum boxes is modulated by applying an external electric field in the direction. 7. The quantum device according to claim 6, wherein an electrode for applying the external electric field is provided adjacent to the plurality of quantum boxes. 8. A plurality of quantum boxes arranged adjacent to each other in one plane, wherein the plurality of quantum boxes are geometrically asymmetric with respect to a direction orthogonal to the plane. Quantum element to do. 9. The plurality of quantum boxes are geometrically asymmetric with respect to the direction, so that the interval between the quantum boxes adjacent to each other changes with respect to the direction. The quantum device according to claim 8. 10. The quantum device according to claim 9, wherein the spacing between the quantum boxes monotonically increases or decreases with respect to the direction. 11. The quantum device according to claim 8, wherein the plurality of quantum boxes are formed by a compound semiconductor heterojunction. 12. The quantum device according to claim 9, 10 or 11, wherein the electrical conductivity in each of the quantum boxes is modulated by applying an external electric field in the direction. 13. The quantum device according to claim 12, wherein an electrode for applying the external electric field is provided adjacent to the plurality of quantum boxes. 14. The quantum box has a pyramidal shape, and the quantum box has a conical shape.
3. The quantum device according to 3. 15. The quantum device according to claim 8, wherein the quantum box has a conical shape, a triangular pyramid shape, a quadrangular pyramid shape, or a regular tetrahedron shape. 16. The quantum device according to claim 8, wherein the quantum box has a conical shape, a triangular pyramid shape, a quadrangular pyramid shape, or a regular tetrahedron shape with the top portion cut off. . 17. A plurality of quantum boxes arranged adjacent to each other in a plane are provided in a channel region, and the plurality of quantum boxes are compositionally or geometrically asymmetric with respect to a direction orthogonal to the plane. Transistor.