JP2000196065A - Device equipped with quantum dot - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain the energy of quantum dots from being affected by a temperature change by a method wherein the quantum dots are formed on the surface of a substrate and covered with a first semiconductor crystal layer, and furthermore the quantum dots and the first semiconductor crystal layer are covered with a second semiconductor crystal layer. SOLUTION: A GaAs buffer layer 15 is grown on a GaAs substrate 11, and many InAs quantum dots 12 are formed on the top surface of the buffer layer 15. An InAs layer 12a thinner than the quantum dot 12 is present around the quantum dots 12, and gaseous trimethyl indium, triethyl gallium, and arsine are supplied onto the surfaces of the InAs layer 12a and the quantum dots 12 to form a first distortion relaxing layer 13 of semiconductor crystal, by which the quantum dots 12 are buried in the distortion relaxing Layer 13. Thereafter, gaseous triethyl gallium and arsine are supplied to the distortion relaxing layer 13 so as to form A GaAs coating layer 14 on the distortion relaxing layer 13.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an element having quantum dots.

[0002]

2. Description of the Related Art With the progress of semiconductor processes, nanoscale growth techniques and microfabrication techniques have been used for the production of semiconductor devices. The growth technology and microfabrication technology not only increase the degree of circuit integration, but also devices utilizing quantum mechanical effects, such as HBT (hetero-bi
polar transistor) and quantum well lasers have been put to practical use.

There is great expectation in the semiconductor field for quantum dots as the ultimate material utilizing the quantum mechanical effect.

With respect to the lasing threshold of semiconductor lasers and the temperature characteristics of the thresholds, it has been pointed out that quantum well lasers have a limit in improving their characteristics, and the use of quantum dots has been proposed as one method of avoiding this problem. . In addition, Shunichi Muto, Jpn.J. Appl. Phys. Vol. 34 (1
995) As shown in pp. L210-L212, quantum dot memories using the hole burning effect have been proposed, and researches for creating new next-generation devices using new materials have been active in recent years.

[0005] The quantum dot structure is a potential box that is extremely fine enough to give three-dimensional quantum confinement to carriers. Since the density of states of a quantum dot is a delta function, one quantum dot Only two electrons can enter the ground level of the conduction band. The use of quantum dots as the active region of a semiconductor laser has the merit that the interaction between electrons and holes and light can be made as efficient as possible, and is expected to be a next-generation device technology.

As a technique for producing such a quantum dot structure, first, there is an advancement of an artificial processing technique which is an extension of the development of a fine processing technique. Examples include lithography using an electron beam, a method using a quantum dot structure at the top of a viramid-shaped crystal stacked on a mask pattern, and a method using a tetrahedral hole etched under a mask pattern. How to make quantum dots, how to use the initial lateral growth on a vicinal substrate,
Atomic manipulation method using STM technology,
and so on. For example, the structure of a quantum dot formed at the vertex of a viramid crystal is described in US Pat. No. 5,313,484, and a method of forming a quantum dot on the inner surface of a tetrahedral hole is described in US Pat.
6,821.

[0007] These methods have an advantage that the formation position can be arbitrarily controlled because of the common feature that the quantum dot structure is artificially processed.

[0008] A new technique that has emerged in recent years is a method of self-forming quantum dots. A specific fabrication method is to vapor-phase epitaxially grow a lattice-mismatched semiconductor under certain conditions.
3D microstructure (quantum dot structure), not 3D film
Is self-formed. This method is easy to carry out, and it is possible to obtain quantum dots of extremely high quality uniformity, high number density and high quality as compared with the case of artificial processing. Such a technique is described in Istavan Daruka et al.,
PHYSICAL REVIEW LETTERS, Vol. 79, No. 19, 10 Nov. 1
997.

A semiconductor device using the self-formed quantum dot, for example, a semiconductor laser has been actually reported, and the possibility of the quantum dot device has become a reality.

FIG. 1 shows a self-formed In on a GaAs substrate 1.
This shows a state where the As dots 2 are covered by the GaAs layer 3.

[0011]

As described above, there has been remarkable progress in the technology of forming quantum dots, but along with this, the problems for practical use of devices have also become apparent. One of them is temperature dependence of quantum dot energy. In general, as the temperature rises, the energy decreases, and the temperature change affects device characteristics. For example, when the temperature is low as shown in FIG.
Compared with the state where the temperature is high as shown in FIG. 3B, the crystal lattice distortion between the quantum dot 2 and the surrounding crystals 1 and 3 is different.

The reason for the temperature change of energy is essential. The reason is that the band gap changes due to the change in the lattice constant because the lattice constant of the semiconductor crystal depends on the temperature.

That is, such a phenomenon appears not only in quantum dots but also in quantum wells. In order to overcome such phenomena, new material systems are being searched for, but they have not been successful yet.

An object of the present invention is to provide a semiconductor device having a structure in which the energy of quantum dots is hardly affected by a change in temperature.

[0015]

SUMMARY OF THE INVENTION The above-mentioned problem is solved by, as illustrated in FIG. 3, a quantum dot 12 formed on a surface of a substrate 11 and made of a semiconductor having a first lattice constant. A first semiconductor crystal layer having a second lattice constant different from the lattice constant and covering the quantum dots from a first direction; and a third lattice constant different from the second lattice constant. And the quantum dots 12 and the first semiconductor crystal layer 1 from a second direction perpendicular to the first direction.
And a second semiconductor crystal layer covering the third semiconductor crystal layer.

In the above-described device having quantum dots, the first direction is a direction parallel to the surface of the substrate 11.

In the above-described device having quantum dots, the quantum dots 12 are microcrystals generated due to lattice distortion of the substrate 11.

In the above-described device provided with quantum dots, the second lattice constant of the first semiconductor crystal layer 13 is larger than the third lattice constant of the second semiconductor crystal layer 14. It is characterized by. In this case, the second lattice constant of the first semiconductor crystal layer 13 may be larger than the third lattice constant of the second semiconductor crystal layer 14 by 1.4% or more.

In the above-described device having quantum dots, the quantum dots are made of a III-V group semiconductor or a II-VI group semiconductor. In this case, said III
The -V group semiconductor may be composed of In _x Ga _1-x As (0 <x ≦ 1).

In the above-described device having quantum dots, at least one of the first semiconductor crystal layer and the second semiconductor crystal layer is made of a group III-V semiconductor. In this case, the III-V group semiconductor may be composed of In _x Ga _1-x As (0 ≦ x <1).
In addition, the III-V group semiconductor forming the first semiconductor crystal layer 13 or the second semiconductor crystal layer 14 may be made of In _x Ga.
_1-x As (0 ≦ x <1), and the In _x Ga _1-x As
The composition ratio x of indium constituting the first semiconductor crystal layer 13 is set to 0.1 in the first semiconductor crystal layer 13 than the second semiconductor crystal layer 14.
It may be two or more.

The above-mentioned figure numbers and reference numerals are cited for easy understanding of the present invention, and the present invention is not limited to them.

Next, the operation of the present invention will be described.

According to the present invention, a part of the quantum dots is replaced by the first
By covering the quantum dots and the second semiconductor crystal layer covering the quantum dots, the lattice distortion is reduced by reducing the influence of the lattice distortion on the original energy of the quantum dots. The amount of change in the lattice strain energy of the quantum dot due to the temperature change is reduced, and as a result, the amount of change in the total energy of the quantum dot is suppressed as compared with the conventional case.

This is based on the idea that the total energy change of a quantum dot due to a temperature change is approximately equal to the sum of the change in energy as a bulk and the change in energy due to lattice distortion.

[0025]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 3 is a sectional view showing a quantum dot structure device according to an embodiment of the present invention.

The quantum dot structure element shown in FIG. 3 is an example of a structure in which quantum dots 12 made of indium arsenide (InAs) are self-formed on a crystal substrate 11 made of gallium arsenide (GaAs).

Indium (In), gallium (Ga), and arsenic (A) have compositions that cause lattice mismatch with the GaAs crystal substrate 11.
When the raw material s) is supplied, an InAs thin film is formed on the main surface of the GaAs crystal substrate 11 at the beginning of the supply, but after the thin film exceeds the film elastic limit, three-dimensional growth occurs and quantum dots are formed. 12 are formed.

After covering the InAs quantum dots 12 with a strain relaxation layer (first semiconductor crystal layer) 13 of, for example, indium gallium arsenide (InGaAs), the strain relaxation layer 13 is covered with a GaAs coating layer (second semiconductor crystal layer). Layer 14). This allows
The quantum dots 12 are embedded in the strain relief layer 13, and the strain relief layers 13 are present on the side and upper portions of the quantum dots 12.

The conventional quantum dot structure element shown in FIG. 1 does not employ a structure in which the quantum dots 2 are covered with an InGaAs strain relaxation layer, and has the same uniform semiconductor crystal (GaAs) as the underlying GaAs substrate 1. ) 13 so as to cover the quantum dots 12.

Next, what happens when a temperature change occurs in the structure of the present invention will be described.

In the quantum dot structure element, the temperature change of the energy of the quantum dot depends on the temperature change of the lattice constant, and the energy decreases as the temperature increases.

FIGS. 4 (a) and 4 (b) schematically show changes in crystal lattice strain according to temperature changes in the quantum dot structure device of the present invention shown in FIG. On the other hand, FIGS. 2 (a) and 2 (b) schematically show a change in lattice strain of the conventional quantum dot shown in FIG. 1 according to a temperature change between a low temperature state and a high temperature state. .

In FIG. 2 and FIG. 4, one crystal lattice constituting the quantum dot and its surrounding layer is represented by one block, but the number of crystal lattices (blocks) is smaller than the actual number. it's shown. This is to make it easier to understand the change in the crystal lattice strain of the quantum dot and the crystal around it.

First, with respect to the quantum dot structure in a low temperature state, in the conventional structure, as shown in FIGS. 2A and 2B, a quantum dot 2 having a large lattice constant is formed while inducing lattice distortion. ing.

On the other hand, in the present invention, FIGS. 4 (a) and 4 (b)
As shown in (1), the amount of strain applied to the quantum dots 12 in the vertical direction is smaller than that in the related art due to the presence of the crystal of the strain relaxation layer 13 having a somewhat larger lattice constant in the horizontal direction. In FIG. 4, the strain relaxation layer 13 is shown only in the lateral direction for easy understanding of the relaxation of crystal strain. However, in FIG. Also strain relaxation layer 1
3 is formed extremely thin, so that the amount of distortion in the lateral direction is actually smaller than in the conventional case.

Incidentally, the energy change ΔE total of the quantum dot due to the temperature change is expressed by the following equation (1). In addition,
In Equation (1), ΔEbulk is an energy change amount as a bulk, ΔEstrain is an energy change amount due to lattice strain, and ΔEoffset is a change in band offset between a quantum dot and a crystal around the quantum dot. This is the amount of change in quantum confinement energy that occurs.

ΔEtotal = ΔEbulk + ΔEstrain
+ ΔEoffset (1) When the element temperature is increased, ΔEbulk, ΔEstrain, and ΔEoffset change, and ΔEtotal changes.

Considering the element structure shown in FIG. 3, InAs forming the quantum dot 12 has a lattice constant of 6.0584 °.
And the linear thermal expansion coefficient of the InAs due to heat is 5.2 × 10 ^-6
K- ¹ . In contrast, GaAs has a lattice constant of 5.6.
533 °, which is smaller than InAs, but since GaAs has a larger linear expansion coefficient due to heat than InAs, the lattice strain tends to be relaxed when the temperature rises.

Therefore, in the conventional structure shown in FIG. 1, when the temperature of the quantum dot structure rises, the lattice strain energy decreases, so that the sign of ΔEstrain becomes negative. ΔE
The sign of the bulk is also negative when the temperature rises, and the change in crystal strain promotes the temperature change in the energy of the bulk. That is, in the conventional quantum dot structure element, the temperature change of the crystal lattice causes a large difference between the relaxation of the strain and the contraction between the quantum dot and the crystal around the quantum dot as shown in FIG.

Note that ΔEoffset is a secondary effect associated with ΔEbulk, and has a negligible magnitude.

On the other hand, in the quantum dot structure device of this embodiment shown in FIG. 3, InGa having an intermediate composition between InAs and GaAs is used.
Since the strain relaxation layer 13 made of As surrounds the periphery of the quantum dot 12, the influence of the crystal strain on the energy of the original quantum dot 12 is small, so that the change in the strain energy when the temperature rises is small. , FIG.
As shown in (a) and (b), the overall energy change can be suppressed as compared with the conventional example.

From the above, a new means for producing a high-density and highly uniform quantum dot structure device is provided.
A high performance quantum dot device is realized.

Next, a more specific method of manufacturing the quantum dot structure device of the present embodiment shown in FIG. 3 will be described.

The quantum dots shown in FIG. 3 are formed on a GaAs substrate by MOVPE by a self-forming method. During the crystal growth by MOVPE, the substrate temperature was set to 525 ° C. As a source, the group III element is trimethylindium (TMI) or trimethylindium dimethylethylamine adduct (TMIDME).
A), supplied by triethyl gallium (TEG) and trimethyl gallium (TMG), V group material is supplied by arsine (AsH _3).

First, a buffer layer 15 of GaAs is grown to a thickness of 0.5 μm on the main surface of the GaAs substrate 11 using TEG and AsH ₃ as shown in FIG.

Next, as shown in FIG.
(Mono layer) TIMMEEA equivalent to 0.1 ML equivalent to TMG and AsH ₃ alternately in 14 cycles in the buffer layer 1
Feed on top of 5. As a result, a large number of quantum dots (three-dimensional long islands) 12 made of InAs and having a height of about 10 nm are formed on the upper surface of the buffer layer 15. The quantum dot 12
Is present around the InAs layer 12a which is thinner than the height of the quantum dot.

Subsequently, the gasified TMI, TEG and As
H ₃ is supplied to the surfaces of the InAs layer 12a and the quantum dots 12, whereby the strain relaxation layer 13 of In _0.3 Ga _0.7 As is formed on the InAs layer 12a and the quantum dots 12, as shown in FIG. Is formed to a thickness of 10 nm, and the quantum dots 12 are embedded by the strain relaxation layer 13. That is, the quantum dot 12
Becomes a state in which the upper and side portions are covered by the In _0.3 Ga _0.7 As strain relaxation layer 13.

After that, gaseous TEG and AsH ₃ are mixed with In _0.3 G
a _0.7 As layer to supply the In layer as shown in Fig. 5 (d).
_A covering layer 14 of GaAs is formed on the _0.3 Ga _0.7 As strain relaxation layer 13 to a thickness of 30 nm.

FIG. 6 is a photoluminescence (PL) spectrum of the sample at a measurement temperature of 20 to 200K. The sample has the quantum dot structure element of the present invention shown in FIG.

In FIG. 6, the light emission intensity (PL energy) is normalized. 1.35 μm which is the longest wavelength
The emission peak in the vicinity indicates emission from the ground level of the quantum dot 12, and the emission peak position hardly changes even with a temperature difference of 180K.

FIG. 7 shows a comparison of the change in PL energy temperature with a quantum dot prepared under several conditions.

In FIG. 7, the sample A has the above In _0.3 G
a is a device in which quantum dots are embedded with _0.7 As, sample B is a device having substantially the same structure as sample A and has quantum dots embedded in In _0.2 Ga _0.8 As, and sample C is sample A.
When a device with embedded quantum dots by In _0.15 Ga _{0. 85} As almost the same structure, D is an element embedded in GaAs created by the simultaneous supply of the growing material. That is, samples A to C have the quantum dot structure element of the present invention shown in FIG. 3, and sample D substantially has the conventional quantum dot structure element shown in FIG.

According to FIG. 7, by embedding the quantum dots with a material having a composition ratio x of indium gallium arsenide represented by In _x Ga _{1 -x} As of 0.2 or more, the energy change with respect to temperature can be remarkably suppressed. I understand. The difference between the PL energies of 0K and 200K is almost the same for Sample C and Sample D, but the difference between the PL energies of 50K and 200K is
Is smaller.

Incidentally, In _0.15 Ga _0.85 As has an intrinsic lattice constant larger than that of GaAs by about 1.4%. Therefore, according to FIG. 7, it is desirable that the lattice constant of the strain relaxation layer 13 is larger than the lattice constant of the coating layer 14 by 1.4% or more.

According to the principle of the present invention, since the quantum dots have a three-dimensional structure, the amount of strain in the vertical and horizontal crystal axis directions can be individually controlled. This characteristic is based on the InGa
The present invention is not limited to the self-assembled quantum dots of the As-based crystal, and the present invention is not limited to a material or a method of forming the quantum dots.

For example, a GaAs substrate, indium phosphide (InP)
When a substrate is used, the respective crystal materials of the quantum dots and the strain relaxation layer covering the quantum dots are selected as shown in Table 1. When a silicon carbide (SiC) substrate, a sapphire substrate, or a gallium nitride (GaN) substrate is used, the respective crystal materials of the quantum dots and the strain relief layer covering the quantum dots are selected as shown in Table 2. Further, when a silicon (Si) substrate is used, the respective crystal materials of the quantum dots and the strain relaxation layer covering the quantum dots are selected as shown in Table 2.

There are many combinations of the strain relaxation layer formed on the quantum dots, and the relationship between the lattice constant of the material and the band gap is as shown in FIGS. ing.

[0059]

[Table 1]

In Table 1, the quantum dots 12 are represented by In _y Ga
_When composed of a group III-V element such as _1-y As (0 <y ≦ 1), the strain relaxation layer 13 and the coating layer 14 are made of, for example, In _x Ga.
_When composed of a group III-V element such as _1-x As (0 <x ≦ 1), the magnitude of the composition ratio x depends on the coating layer 14.
It is preferable that the strain relieving layer 13 is larger by 0.2 or more than that.

[0061]

[Table 2]

[0062]

[Table 3]

The formation of the quantum dots is not limited to the above-described method, and may be formed, for example, on an inclined surface (off surface) of a compound semiconductor substrate. In this case, the quantum dots are formed along the inclined surface. Thus, a strain relaxation layer is formed.

[0064]

As described above, according to the present invention, by covering a part of the quantum dot with the first semiconductor crystal layer, the lattice distortion of the quantum dot and the second semiconductor crystal layer covering the quantum dot can be reduced. The effect of lattice strain on the original energy of the quantum dot is reduced, and the amount of change in the lattice distortion energy of the quantum dot due to temperature change is reduced.As a result, the change in the total energy of the quantum dot is reduced. It can be suppressed more than before.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a conventional quantum dot structure device.

FIGS. 2 (a) and 2 (b) are schematic diagrams showing a temperature change of a crystal lattice of a conventional quantum dot structure device.

FIG. 3 is a sectional view showing a quantum dot structure element according to the embodiment of the present invention.

FIGS. 4 (a) and 4 (b) are schematic diagrams illustrating a change in temperature of a crystal lattice of the quantum dot structure element according to the embodiment of the present invention.

5 (a) to 5 (d) are cross-sectional views showing steps of manufacturing a quantum dot structure device according to an embodiment of the present invention.

FIG. 6 is a diagram showing a photoluminescence spectrum in a temperature range of 20 to 200 K of the quantum dot structure device according to the embodiment of the present invention.

FIG. 7 is a diagram showing a relationship between temperature and PL energy of the quantum dot structure device of the present invention and a conventional device.

FIG. 8 is a first diagram illustrating a relationship between a band cap (wavelength) and a lattice constant.

FIG. 9 is a second diagram illustrating a relationship between a band cap and a lattice constant.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Crystal substrate 12 ... Quantum dot 13 ... Strain relaxation layer 14 ... Coating layer 15 ... Buffer layer

Claims (10)

[Claims] 1. A quantum dot comprising a semiconductor having a first lattice constant and formed on a surface of a substrate, having a second lattice constant different from the first lattice constant,
And a first semiconductor crystal layer covering the quantum dots from a first direction; and a third lattice constant different from the second lattice constant;
An element having quantum dots, comprising: the quantum dots and a second semiconductor crystal layer covering the first semiconductor crystal layer from a second direction perpendicular to the first direction. 2. The device according to claim 1, wherein the first direction is a direction parallel to the surface of the substrate. 3. The device with quantum dots according to claim 1, wherein said quantum dots are microcrystals generated due to lattice distortion with respect to said substrate. 4. The quantum according to claim 1, wherein the second lattice constant of the first semiconductor crystal layer is larger than the third lattice constant of the second semiconductor crystal layer. An element with dots. 5. The semiconductor device according to claim 4, wherein said second lattice constant of said first semiconductor crystal layer is at least 1.4% larger than said third lattice constant of said second semiconductor crystal layer. An element comprising the quantum dot according to the above. 6. The quantum dot comprises a III-V semiconductor or
2. A device comprising a quantum dot according to claim 1, wherein the device comprises a II-VI semiconductor. 7. The group III-V semiconductor comprises In _x Ga _{1 -x} As.
The device provided with the quantum dot according to claim 6, wherein (0 <x ≦ 1). 8. The device having quantum dots according to claim 1, wherein at least one of said first semiconductor crystal layer and said second semiconductor crystal layer is made of a group III-V semiconductor. 9. The method according to claim 9, wherein the III-V semiconductor is In _x Ga _1-x As.
The device provided with the quantum dot according to claim 8, wherein (0 ≦ x <1). 10. The group III-V semiconductor constituting the first semiconductor crystal layer or the second semiconductor crystal layer is In _x Ga.
_1−x As (0 ≦ x <1), and the composition ratio x of indium constituting In _x Ga _{1− x} As is larger in the first semiconductor crystal layer than in the second semiconductor crystal layer. The device provided with the quantum dot according to claim 8, wherein the value is 0.2 or more.