JP2011040459A - Multilayered quantum dot structure and method of manufacturing the same, and solar cell element and light emitting element using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multilayered quantum dot structure having respective layers made faster in growing speed than before by providing a normal GaAs layer having a simple structure as an intermediate layer of a quantum dot layer without growing a strain compensation layer increasing processes, and to provide a method of manufacturing the same. <P>SOLUTION: In the multilayered quantum dot structure having InGaAs quantum dot laminate structures provided on a GaAs buffer layer, an arbitrary number of InGaAs quantum dot laminate structures 6 are laminated, each of the laminate structures 6 including an InGaAs thin-film layer 3 provided with a plurality of InGaAs quantum dots 4 and a GaAs buffer layer 5 provided on the InGaAs thin-film layer 3 so as to bury the InGaAs quantum dots 4. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a semiconductor light-receiving / light-emitting element, and more particularly to a multi-layered quantum dot structure and a manufacturing method, and a solar cell element and a light-emitting element using the same.

  In a semiconductor light-receiving / light-emitting element using In (Ga) As quantum dots, higher density is essential for improving characteristics. As a method for increasing the density of quantum dots, there are a method for increasing in-plane dot density and a method for stacking multiple quantum dot layers. In particular, when considering application to a solar cell structure, it is necessary that the quantum dots are aligned vertically and in the in-plane direction.

  However, in the method of multi-stacking quantum dots, since the quantum dots use lattice-mismatched crystal growth, dislocations and defects occur due to the accumulation of lattice strains in the crystal due to multi-stacking, especially InAs quantum dots Then, it is known that the crystal characteristic will deteriorate remarkably (for example, refer nonpatent literature 1).

  When multiple layers of InAs quantum dots are stacked, if the intermediate layer between the quantum dots is made as thick as about 40 nm, multiple layers can be formed while maintaining its good optical characteristics. In this case, dots are not lined up and down, and the alignment structure necessary for the solar cell structure cannot be obtained.

As one answer to this problem, a material having a small lattice constant such as N (nitrogen) is added to the intermediate layer GaAs between InAs quantum dots, and the lattice strain is relaxed by using the GaNAs layer as an intermediate layer (strain compensation). The use of multiple layers is proposed.
However, since another element of N is required, there is a problem that it is necessary to prepare another molecular beam cell (see, for example, Non-Patent Document 2).

In addition, in order to grow a high-quality quantum dot structure, an extremely slow growth rate of 0.006 ML (molecular layer) / s (seconds) has been required (see, for example, Non-Patent Document 3). .
In that case, it takes a long time to stack a plurality of quantum dots. For example, if 100 layers are stacked, it takes 12 hours only for the dot layer, which is unrealistic.

On the other hand, for improving the quality of InAs quantum dot growth, the joint researcher of the present inventor has proposed that the crystallinity is improved by using As2 (see Patent Documents 1 and 2). However, it has been expected that it is impossible to stack the quantum dot layer to the extent that the required solar cell output can be obtained without the strain compensation layer.
Furthermore, as semiconductor light-receiving / light-emitting elements equipped with quantum dots, development for lasers has been given priority, and development as solar cells has been delayed.

JP 2007-53322 A JP 2007-194378 A

“L. Marti, N. Lopez, E. Antolin, E. Canovas, A. Luque, C. Stanley, C. Farmer, and P. Diaz, Appl. Phys. Lett. 90, (2007) 233510. “R. Oshima, T. Hashimoto, H. Shigekawa, and Y. Okada, J. Appl. Phys. 100, (2006) 083110. “K. Yamaguchi, K. Yujobo, and T. Kaizu, Jpn. J. Appl. Phys. 39, (2000) L1245. “T. Sugaya, T. Amano, and K. Komori, J. Appl. Phys. 104, (2008) 083106. “R. Oshima, A. Takata, and Y. Okada, Appl. Phys. Lett. 93, 083111 (2008).

In view of the above-mentioned problems, the present invention provides a normal GaAs layer having a simple structure as an intermediate layer of a quantum dot layer without growing a strain compensation layer that increases the number of steps. An object is to obtain a multi-stacked quantum dot structure and manufacturing method that are faster than those.
It is another object of the present invention to obtain a solar cell element and a light emitting element using the multi-stacked quantum dot structure.

In order to achieve the above object, the present invention basically uses an InGaAs quantum dot structure in which a GaAs buffer layer is provided on an InGaAs thin film layer in which InGaAs quantum dots are provided. This InGaAs quantum dot structure forms a high-speed growth quantum dot structure without a strain compensation layer. The end faces in the stacking direction in the InGaAs quantum dot structure of the InGaAs thin film layer and the GaAs buffer layer can be flat without distortion. InGaAs quantum dots are grown as quantum dots on an InGaAs thin film layer after it is formed. InGaAs thin film layers and InGaAs quantum dots are grown continuously, and InGaAs forms quantum dots in a self-forming manner.
Further, with the above InGaAs quantum dot structure as a unit, this InGaAs quantum dot structure is stacked in the required number of layers (the number of steps) to obtain a high output as an InGaAs quantum dot stacked structure. Since this InGaAs quantum dot structure is used, the number of layers as many as could not be achieved so far can be stacked.
Furthermore, a high-power solar cell can be obtained by configuring the above InGaAs quantum dot laminated structure as an i layer and providing a p-type and n-type GaAs layer sandwiching the i-layer to form a solar cell.

Since InGaAs has a smaller lattice mismatch ratio to GaAs than InAs, it can be multi-layered while maintaining good optical characteristics without a strain compensation layer.
In addition, the thickness of the buffer layer for multi-stacking can be reduced while maintaining good crystal characteristics of the quantum dots.
Specifically, a buffer layer of 40 nm or more is conventionally required for multi-layering of InAs quantum dots, but in the InGaAs system of the present invention, the thickness can be set to an arbitrary thickness within a range of 20 nm or less and exceeding 0 nm. it can. The quantum dot structure can be aligned in the growth direction by setting the intermediate layer to an arbitrary thickness within the range of 20 nm or less and exceeding 0 nm. In fact, an InGaAs quantum dot layer having an In composition of 0.4 was successfully grown with 100 layers of 20 nm intermediate layers while maintaining good optical properties. About In composition, the arbitrary values in the range of 0.3 or more and 0.5 or less are desirable.

On the other hand, InGaAs quantum dots can be grown at any growth rate within the range of 0.1 ML (molecular layer) / S (seconds) to 1 ML / S. Even in the case of As4 (that is, As ₄ ) molecules that are often used as As molecular beams, the above multi-layering can be performed by using As2 (that is, As ₂ ) molecules generated by heating them at high temperatures.
However, the optical characteristics of the multi-stacked quantum dot structure are improved by using As2. For example, when an InGaAs quantum dot is grown using a growth rate of 1 ML / S, good optical characteristics cannot be expected unless the group III atoms of In and Ga are sufficiently diffused and incorporated into an energy stable site. This is particularly necessary at high growth rates.
It has been reported by the inventors that the diffusion of group III atoms is promoted by the As2 molecular beam (see Non-Patent Document 4). Even in a multi-layered quantum dot structure, the As2 molecular beam is used for high-speed growth. When grown, the optical properties are superior.

When the stacking means having a high growth rate described above is applied, the multi-stacked quantum dot structure of the present invention can grow an InGaAs quantum dot and an InGaAs thin film layer at an arbitrary growth within a range of 0.1 ML / s to 1 ML / S. The multi-stacked quantum dot structure can be grown at a maximum speed of 1 ML / S.
Furthermore, a solar cell structure or a semiconductor laser structure is constructed using the multi-stacked quantum dot structure having a high growth rate.
A method for producing a multi-stacked quantum dot structure in which an InGaAs quantum dot stacked structure is provided on a GaAs buffer layer,
In the InGaAs quantum dot stacked structure, an InGaAs thin film layer is provided, a plurality of InGaAs quantum dots are grown thereon, and a GaAs buffer layer is formed on the InGaAs quantum dots and the InGaAs thin film layer so as to embed the InGaAs quantum dots. The series of manufacturing steps are continuously performed by stacking an arbitrary number of InGaAs quantum dot structures 6 and the InGaAs quantum dots and the InGaAs thin film layer are within a range of 0.1 ML / s to 1 ML / S. The GaAs layer is laminated at an arbitrary thickness within a range of 20 nm or less and exceeding 0 nm.

In the present invention, by adopting a multi-layer InGaAs quantum dot structure composed of a GaAs layer and InGaAs quantum dots, it is possible to construct a quantum dot structure that grows at high speed without a strain compensation layer.
Furthermore, by using As2 as As, high-speed growth can be achieved at any growth rate within a range of 0.1 ML / s to 1 ML / S without a strain compensation layer, and particularly 1 ML / S is achieved. be able to.
In addition, by using the quantum dot structure, a solar cell element and a light emitting element that have never existed can be configured. In particular, it can be applied to high-power and ultra-high-efficiency solar cell structures while exhibiting excellent crystal growth characteristics with very little distortion.
Since the InGaAs quantum dot structure of the present invention has sensitivity at a wavelength of 900 nm or more, the solar cell of the present invention having a multi-stacked quantum dot structure incorporating this InGaAs quantum dot structure, the light-emitting element has an absorption sensitivity at wavelengths of 900 nm or more. And can be configured to have an emission wavelength.

It is a schematic block diagram of one Embodiment of the multi-layer InGaAs quantum dot structure of this invention. It is a cross-sectional transmission electron micrograph of actually _In 0.4 _{Ga 0.6} As quantum dots 50 layers stacked structure. It is a scanning electron micrograph of the surface of a multi-layered In _0.4 Ga _0.6 As quantum dot structure. This is a photoluminescence emission characteristic of a structure in which 1, 20, 30, and 50 In _0.4 Ga _0.6 As quantum dot layers are stacked. The effect of the molecular beam of As2 and As4 is shown. It is the schematic diagram of the Example which applied the multi-layered InGaAs quantum dot structure grown in the buffer layer 20nm using As2 to the solar cell structure. It is a characteristic view of the element produced according to this invention. It is the figure which showed the external quantum efficiency of the solar cell structure of this invention.

  Embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 illustrates a preferred embodiment of a multi-layer InGaAs quantum dot structure constructed in accordance with the present invention.
The multi-stacked quantum dot structure 1 is formed by growing a GaAs layer 2 on a GaAs substrate (not shown) and then laminating an InGaAs quantum dot structure 6 on the GaAs layer 2 by a required number of stages. Provide and configure. The InGaAs quantum dot structure 6 includes an InGaAs thin film layer 3 provided with a plurality of InGaAs quantum dots 4 and a GaAs buffer layer 5 provided on the InGaAs thin film layer 3 so as to embed the InGaAs quantum dots 4. An InGaAs thin film layer 3 is formed, an InGaAs quantum dot 4 is grown thereon, and a GaAs buffer layer 5 is formed on the InGaAs thin film layer 3 and the InGaAs quantum dot 4.

By setting the thickness of the GaAs buffer layer 5 to an arbitrary thickness within the range of 20 nm or less and exceeding 0 nm, an InGaAs quantum dot stacked structure aligned in the growth direction can be formed.
The InGaAs thin film layer 3, the InGaAs quantum dot layer 4, and the GaAs buffer layer 5 and the InGaAs quantum dot structure 6 are repeatedly stacked on the GaAs buffer layer 5, thereby forming a multi-layered structure. Even when 100 or more layers are stacked, in the case of an InGaAs quantum dot layer, no transition or crystal defect occurs. When the same stacking is performed with InAs quantum dots, dislocations and defects occur, and a good multi-layered structure cannot be formed.
In the manufacturing method of a multi-stacked quantum dot structure in which an InGaAs quantum dot stacked structure is provided on a GaAs buffer layer during manufacturing,
In the InGaAs quantum dot stacked structure, an InGaAs thin film layer is provided, a plurality of InGaAs quantum dots are grown thereon, and a GaAs buffer layer is formed on the InGaAs quantum dots and the InGaAs thin film layer so as to embed the InGaAs quantum dots. The series of manufacturing steps are continuously performed by stacking an arbitrary number of InGaAs quantum dot structures 6 and the InGaAs quantum dots and the InGaAs thin film layer are within a range of 0.1 ML / s to 1 ML / S. The GaAs layer is laminated at an arbitrary thickness within a range of 20 nm or less and exceeding 0 nm.

FIG. 2 shows a cross-sectional transmission electron micrograph of a multi-layered quantum dot structure in which 50 layers of In _0.4 Ga _0.6 As quantum dot structures are actually stacked. 2A is a cross-sectional transmission electron micrograph of a structure in which 50 layers are stacked, and FIG. 2B is an enlarged view of a main part in which a part of FIG. 2A is enlarged. A white portion in FIG. 2 represents a quantum dot portion. In FIG. 2, it can be seen that the buffer layer is 20 nm, and an InGaAs quantum dot structure having a height of 5 nm and a width of 30 nm is grown in alignment in the growth direction. From this, it can be said that the multi-layered quantum dot structure is regularly formed without any transition or crystal defect.

FIG. 3 is a scanning electron micrograph of the surface of In _0.4 Ga _0.6 As quantum dots. FIG. 3A is a scanning electron micrograph of the surface of an InGaAs quantum dot structure in which one layer and 50 layers are stacked, respectively. 4a and 4b in FIG. 3 show quantum dots. In the case of the single layer in FIG. 3A, the dots grow irregularly, but by growing 50 layers as shown in FIG. 3B, there is a tendency to align in the [1-10] direction. I understand that.

  As shown in FIGS. 2 and 3, by stacking multiple InGaAs quantum dot structures with As2 molecular beams, a structure in which InGaAs quantum dots are aligned in the growth direction and in the in-plane direction can be fabricated. Recognize.

FIG. 4 shows the photoluminescence emission characteristics of a structure in which 1, 20, 30, and 50 In _0.4 Ga _0.6 As quantum dot structures are stacked. The horizontal axis represents WAVELENGTH (wavelength) (nm), and the vertical axis represents PL INTENSITY (luminescence intensity) (au). The buffer layer is 20 nm. 4A is measured at room temperature (25 ° C.), the half-value width is 41.6 meV, FIG. 4B is measured at 10 K (low temperature), and the half-value width is 37.0 meV. In the figure, QD is an abbreviation for quantum dot.
The peak value of each characteristic in FIG. 4A is as shown in Table 1 below.
The peak value of each characteristic in FIG. 4B is as shown in Table 2 below.
It can be seen that the emission intensity increases with the number of layers of the InGaAs quantum dot structure. From this result, it can be seen that a multi-layer InGaAs quantum dot structure with a buffer layer of 20 nm having excellent optical characteristics has been grown.

FIG. 5 shows the effect of As2 molecular beam.
Four layers of In _0.4 Ga _0.6 As quantum dots were stacked. The buffer layer was 20 nm.
The peak value of each characteristic in FIG. 5 is as shown in Table 3 below.
The InGaAs quantum dot stacked structure grown with As2 has higher photoluminescence emission intensity than that grown with As4. Crystal quality is improved by promoting the diffusion of group III elements during MBE growth by the As2 molecular beam. This is remarkable when InGaAs quantum dots are grown at a high speed of about 1 ML / S.

FIG. 6 is a schematic diagram of an example in which a multi-layered In _0.4 Ga _0.6 As quantum dot stacked structure grown with a buffer layer of 20 nm using As 2 is applied to a solar cell structure.
In the solar cell 10, an n layer 12 b is grown on an n-type GaAs substrate 12 a in the order of an n + -GaAs layer and an n-GaAs layer, and an InGaAs quantum dot structure 16 is formed on the 20, 20, or 30 layers as desired. Several layers are laminated to form the i layer 13.
The InGaAs quantum dot structure 16 is formed by forming an InGaAs thin film layer 13b having a plurality of InGaAs quantum dots 13a and growing a GaAs buffer layer 13c in which the InGaAs quantum dots 13a are embedded. The buffer layer was laminated at 20 nm.

The InGaAs quantum dot structure layer 13 is not doped and is semi-insulating. On the InGaAs quantum dot structure layer 13, a p layer 14 composed of a p-GaAs layer and a p + -GaAs layer GaAs layer 14 a is grown in order, and a Ti / Au electrode 15 is used as a surface contact and AuGe / is used as a back electrode. Ni / Au electrodes 11 are provided respectively. However, an antireflection film such as an AR coat is not deposited on the surface.
The growth method of the laminated structure was performed at a growth rate of 1 ML / S using As2 as described above.
In this example,
The p + -GaAs layer is 1 × 10 ¹⁹ / cm ³ : 50 nm,
The p-GaAs layer is 2 × 10 ¹⁸ / cm ³ : 150 nm,
The i layer (quantum dot + GaAs layer 20 nm) is laminated with 10, 20, 30 periods,
The n-GaAs layer is 1 × 10 ¹⁷ / cm ³ : 1000 nm,
The n + -GaAs layer is 1 × 10 ¹⁸ / cm ³ : 250 nm,
The distribution of AuGe / Ni / Au of the electrode 11 is 80 nm / 20 nm / 350 nm,
The distribution of Ti / Au of the electrode 15 is 50 nm / 500 nm.

Thus, when various characteristics of the solar cell element (multi-layered quantum dot solar cell) using the multi-layered quantum dot structure of the present invention were taken, desirable characteristics can be obtained as shown in FIG. It was.
In FIG. 7, the vertical axis represents Current Density (mA / cm ² ), the horizontal axis represents Voltage (v), the zero current density point is Voc (open voltage) (v), and the zero voltage point is Jsc. (Short circuit current) (mA / cm 2), and FF represents the shape of the characteristic. In the figure, the characteristics of the GaAs solar cell not having the multi-stacked quantum dot structure of the present invention are also shown for reference.

The characteristics of the multi-stacked quantum dot solar cell of FIG.
In the figure, the horizontal axis represents the voltage V applied to the solar cell, and the vertical axis represents the current I at that time. The conversion efficiency of this solar cell derived from these characteristics was 8.9, 8.0, and 7.0% for InGaAs quantum dot structure layers of 10, 20, and 30 layers, respectively. In Non-Patent Document 5, the structure in which 10 or 20 InAs quantum dot layers are stacked with the antireflection film deposited on the surface is 8.5% and 5.7%, respectively. This element achieves better efficiency than those values, although no antireflection film is deposited. The conversion efficiency is expected to be further improved by about 1% by depositing the antireflection film.

FIG. 8 is a diagram showing the external quantum efficiency of the present solar cell structure.
The characteristics of FIG. 8 are shown in Table 5 below.
It can be seen that absorption by the InGaAs quantum dot layer is observed on the longer wavelength side (870 nm or more) than the absorption band of the GaAs solar cell, and the quantum efficiency increases according to the number of stacked quantum dots. From these results, the multi-layered quantum dot solar cell of the present invention (solar cell using a multi-layered InGaAs quantum dot structure) can absorb long-wavelength light that cannot be absorbed by GaAs or Si, and can produce a current. .

  Even when the multi-stacked InGaAs quantum dot structure is applied as a laser, the layer structure is the same as in FIG. The oscillation wavelength can be changed by changing the In composition and the growth time of the quantum dots.

In the present invention, the In _0.4 Ga _0.6 As quantum dot layer was directly grown on the GaAs buffer layer, but for example, an In _0.2 Ga _0.8 As layer with a thickness of about 2 nm does not occur during that period. By interposing a thickness of about a certain degree, the distortion of the quantum dot layer is relaxed, and there is a possibility that a higher quality laminated structure can be produced. In this case, due to the strain relaxation effect, the solar cell has a light response characteristic even in a longer wavelength region, and improvement of the solar cell characteristic is expected.

Further, by introducing a thin GaAsP layer in the GaAs buffer layer, it is possible to compensate for the distortion of the InGaAs quantum dot layer. It seems that the quality of the laminated structure can be improved by this distortion compensation technique.
The quantum dot structure exhibiting excellent crystal growth characteristics described above can be expected to be applied to a semiconductor laser having a low threshold (threshold value) in addition to an ultra-high efficiency solar cell structure.

DESCRIPTION OF SYMBOLS 1 Multi-stacked quantum dot structure 2 GaAs layer 3 InGaAs thin film layer 4 InGaAs quantum dot 5 GaAs layer 6 InGaAs quantum dot structure

Claims (8)

A multi-stacked quantum dot structure in which an InGaAs quantum dot stacked structure is provided on a GaAs buffer layer,
The InGaAs quantum dot stacked structure is composed of an InGaAs thin film layer 3 provided with a plurality of InGaAs quantum dots 4 and an GaAs buffer layer 5 provided on the InGaAs thin film layer 3 so as to embed the InGaAs quantum dots 4. A multi-stacked quantum dot structure comprising an arbitrary number of layers of dot structures 6 stacked. The multi-layered quantum dot structure according to claim 1, wherein the source material of As is As ₂ . 3. The multi-layered quantum dot structure according to claim 1, wherein the thickness of the GaAs layer is an arbitrary thickness within a range of 20 nm or less and exceeding 0 nm. 4. The multi-layered quantum dot structure according to claim 1, wherein four or more layers of the InGaAs quantum dot structure are stacked. 5. 4. A plurality of InGaAs quantum dots and an InGaAs thin film layer are grown at an arbitrary growth rate within a range of a growth rate of 0.1 ML / S to 1 ML / S. The multi-layered quantum dot structure according to any one of the above. A multi-stacked quantum dot solar cell comprising a multi-stacked quantum dot structure according to any one of claims 1 to 5 having a pin structure sandwiched between a p-type GaAs layer and an n-type GaAs layer. Battery element. 5. Multi-stacked quantum dot light emission characterized in that the multi-stacked quantum dot structure according to any one of claims 1 to 4 has a pin structure sandwiched between a p-type GaAs layer and an n-type GaAs layer. element. A method for producing a multi-stacked quantum dot structure in which an InGaAs quantum dot stacked structure is provided on a GaAs buffer layer,
In the InGaAs quantum dot stacked structure, an InGaAs thin film layer is provided, a plurality of InGaAs quantum dots are grown thereon, and a GaAs buffer layer is formed on the InGaAs quantum dots and the InGaAs thin film layer so as to embed the InGaAs quantum dots. The series of manufacturing steps are continuously performed by stacking an arbitrary number of InGaAs quantum dot structures 6 and the InGaAs quantum dots and the InGaAs thin film layer are within a range of 0.1 ML / s to 1 ML / S. A method for producing a multi-stacked quantum dot structure, comprising growing the GaAs layer at an arbitrary thickness within a range of 20 nm or less and exceeding 0 nm.