JP6115938B2 - Method for forming quantum dots and solar cell - Google Patents

Description

  The present invention relates to a method for forming quantum dots and a solar cell to which quantum dots are applied.

  In recent years, application of quantum dots to optoelectronic devices has attracted attention, and various studies and developments have been made. The quantum dot, which is a three-dimensional carrier confinement structure, has a sharper carrier energy spectrum than the quantum well structure, which is a one-dimensional carrier confinement, and the quantum wire structure, which is a two-dimensional confinement. Become discrete.

  On the other hand, when the in-plane density of the quantum dots becomes high, the wave functions of the electrons in the quantum dots are coupled in-plane and overlap, and an intermediate band is formed in the conduction band and valence band of the quantum dots. It has been proposed to apply quantum dots to solar cells using the generation of intermediate bands. However, in the known configuration, several tens to hundreds of quantum dot layers are stacked.

For the formation of quantum dots, a technique using so-called SK (Stranski-Krastanov) mode growth that appears in the early stage of heteroepitaxial growth is generally employed. Since this method uses strain energy generated at the heterointerface, it does not require processing into a bulk material like lithography and etching. Therefore, quantum dots can be formed by a relatively simple process. As a method for forming quantum dots using the SK mode, a method in which a GaAsSb buffer layer is grown on a GaAs buffer layer and an InAs quantum dot is grown on the GaAsSb buffer layer has been proposed (for example, Patent Document 1 and Non-Patent Documents). Reference 1). Prior to the formation of the quantum dot layer, this method inserts a base buffer layer (GaAsSb buffer layer) containing antimony (Sb), thereby suppressing coalescence between quantum dots per layer. The in-plane density of the quantum dots is increased to 2 × 10 ¹¹ cm ⁻² .

An example of growing an InAs quantum dot by forming an InAsSb wetting layer is known in comparison with a method of forming a GaAsSb underlayer buffer layer prior to the growth of an InAs quantum dot layer (for example, non-industry). In-plane density of InAs quantum dots formed under these growth conditions is reported to be 5 × 10 ¹⁰ cm ⁻² .

Japanese Patent No. 4825965 (Japanese Patent Laid-Open No. 2006-80293)

K. Yamaguchi, T. Kanto, "Self-assembled InAs quantum dots on GaSb / GaAs (001) layers by molecular beam epitaxy", Journal of Grystal Growth 275 (2005) e2269-e2273 N. Kakuda, et al, "Sb-mediated growth of high-density InAs quantum dots and GaAsSb embedding growth by MBE", Applied Surface Science 254 (2008) 8050-8053 "

  Considering the effective use of solar energy, it is desirable to further improve the in-plane density of quantum dots and realize an optical device using an intermediate band. Accordingly, an object of the present invention is to provide a quantum dot forming method that realizes a quantum dot arrangement higher in density than in the past while suppressing coalescence between quantum dots, and a solar cell using the method.

In order to solve the above problems, a method for forming high-density quantum dots is provided as a first aspect of the present invention. This method
When an InAs layer is grown on a GaAs buffer layer, antimony (Sb) is supplied at a pressure of 2.0 × 10 ⁻⁷ to 3.3 at the stage of the wetting layer before the growth of the InAs layer shifts to three-dimensional growth. Introduced at 6 × 10 ⁻⁷ Torr to grow InAsSb wetting layer,
When the InAsSb wetting layer grows from 0.5 molecular layer (ML) to 1.5 ML, the introduction of the Sb is stopped and the InAs layer is subsequently grown to form InAs quantum dots.
Process.

  In a preferred embodiment, the growth temperature of the InAsSb wetting layer and InAs quantum dots is 460-470 ° C.

For example, when the growth temperature is 460 ° C., an InAsSb wetting layer is grown by 1.0 to 1.5 ML with an Sb supply pressure of 3.6 × 10 ⁻⁷ Torr, and an InAs quantum dot is grown to 8.0 × 10 ^11. It is formed with an in-plane density of cm ^{−2 to} 1.0 × 10 ¹² cm ⁻² .

In another example, when the growth temperature is 470 ° C. and the supply pressure of Sb is in the range of 2.2 × 10 ⁻⁷ to 3.5 × 10 ⁻⁷ Torr, the InAsSb wetting layer is 0.8 ML to 1.5 ML. Growing to form InAs quantum dots with an in-plane density of 6.0 × 10 ¹¹ cm ^{−2 to} 7.6 × 10 ¹¹ cm ⁻² .

In the 2nd side surface of this invention, the solar cell using the quantum dot mentioned above is provided. The solar cell includes a transparent electrode, a first conductivity type semiconductor layer, a second conductivity type semiconductor layer, and a light absorption located between the first conductivity type semiconductor layer and the second conductivity type semiconductor layer. A layer, and an electrode located on a surface opposite to the transparent electrode,
The light absorption layer includes a GaAs buffer layer,
A quantum dot layer including an InAsSb wetting layer formed on the GaAs buffer layer and an InAs quantum dot grown on the InAsSb wetting layer;
A GaAs intermediate layer covering the quantum dot layer;
The quantum dot layer and the GaAs intermediate layer are alternately stacked one or more layers.

As a good configuration example, the in-plane density of the InAs quantum dots of the light absorption layer is 4.0 × 10 ¹¹ cm ^{−2 to} 1.0 × 10 ¹² cm ⁻² .

  The thickness of the InAsSb wetting layer is 0.5 to 1.5 ML, and the height of the InAs quantum dots is 1.6 to 3.0 nm.

  Very high density quantum dots can be efficiently formed using SK mode growth. A solar cell with high energy conversion efficiency can be realized by utilizing generation of an intermediate band due to in-plane density increase of quantum dots.

It is the schematic of the quantum dot formed by embodiment of this invention. It is a graph which shows the relationship between the growth amount of an InAsSb wetting layer at the time of forming a quantum dot by the method of 1st Embodiment, and quantum dot density. It is an AFM image in each measurement point of the graph of FIG. It is a table | surface which shows the growth conditions in each measurement point of the graph of FIG. It is a graph which shows the relationship between the growth amount of an InAsSb wetting layer, the average height of a quantum dot, and the size (diameter) of a horizontal direction. It is a graph which shows the relationship between the Sb supply amount at the time of InAsSb wetting layer formation, and a quantum dot density. 7 is an AFM image of quantum dots at each supply amount of FIG. 6. It is a table | surface which shows the other growth conditions in each supply amount of FIG. It is the schematic of the sample for the emission spectrum measurement of the quantum dot produced by the method of 1st Embodiment. It is the emission spectrum measured with the sample of FIG. It is a schematic block diagram of the solar cell manufactured using the formation method of the quantum dot of a 1st embodiment. It is the schematic which shows the energy band structure of the light absorption layer of the solar cell of FIG. It is a figure which shows the growth conditions of the quantum dot formed in 2nd Embodiment. It is a figure which shows Sb supply pressure dependence of the in-plane density of the InAs quantum dot grown at 470 degreeC in 2nd Embodiment, and Sb supply pressure dependence of dot height. It is a figure which shows the wetting layer film thickness dependence of the in-plane density of the InAs quantum dot grown at 460 degreeC and 470 degreeC in 2nd Embodiment, and the wetting layer film thickness dependence of dot height. It is a figure which shows Sb supply pressure dependence of the in-plane density of the InAs quantum dot grown at 460 degreeC and 470 degreeC in 2nd Embodiment, and Sb supply pressure dependence of dot height. It is an AFM image of InAs quantum dots when grown at 460 ° C. in the second embodiment. It is a figure which shows the effect of forming an InAs quantum dot on an InAsSb wetting layer.

A method for forming quantum dots that greatly improves the in-plane density will be described below with reference to the drawings.
<First Embodiment>
In a quantum dot structure with a high in-plane density, electron wave functions in the quantum dots are coupled in-plane, and energy level multiplexing (intermediate banding) occurs. New development of optical nanodevices using this intermediate band is expected.

In the first embodiment, by introducing antimony (Sb) in the growth stage of the InAs wetting layer when forming InAs quantum dots, the in-plane density of 6.5 × 10 ¹¹ cm ⁻² or more is unprecedented. Self-formation of InAs quantum dots is realized.

  FIG. 1 is a schematic diagram for explaining a method for forming high-density quantum dots. A GaAs buffer layer 12 is formed on the GaAs (001) substrate 11 to a thickness of 200 to 250 nm by molecular beam epitaxial growth (MBE). At this time, the substrate temperature is 590 ° C., and the growth rate is 0.130 ± 0.001 nm / s.

  Next, the InAs layer 15 is grown at 470 ° C. using the SK growth mode. InAs has a lattice constant different from that of GaAs of the underlying buffer layer 12. Therefore, when the two-dimensional growth by epitaxial growth reaches a certain level, the strain energy exceeds the elastic limit of the film and shifts to three-dimensional growth by self-organization. The crystal film at the stage of two-dimensional growth before the transition to three-dimensional growth is called a “wetting layer”.

  Sb is introduced under predetermined conditions in the growth stage of the InAs wet layer. For convenience, the InAs wetting layer to which Sb is added is referred to as “InAsSb wetting layer 13”. When the InAsSb wetting layer 13 has grown to a 0.5 to 1.5 molecular layer (ML), the supply of Sb is stopped and InAs is subsequently grown. As described above, the three-dimensional InAs quantum dots 14 self-grow on the InAsSb wetting layer 13 due to the strain energy of InAs caused by the difference in lattice constant from the underlying GaAs. The InAsSb wetting layer 13 and the InAs quantum dots 14 are combined to form an InAs layer (or quantum dot layer) 15.

  The total growth amount of the InAs layer 15 including the InAsSb wetting layer 13 is 1.5 to 5.0 ML. The decrease in surface energy due to Sb atoms segregated in the InAsSb wetting layer 13 suppresses coalescence between adjacent quantum dots 14 during the quantum dot growth process.

According to this formation method, a high-density InAs quantum dot array with an in-plane density of 4.0 × 10 ¹¹ cm ⁻² or more is realized. In particular, when the growth amount of the InAsSb wetting layer 13 is 1.0 ML or more, or when the supply pressure of Sb is increased even if the growth amount of the InAsSb wetting layer 13 is less than 1.0 ML, the InAs quantum dots 14 The in-plane density can be increased to 6.0 × 10 ¹¹ cm ⁻² or more. This will be described in detail below.

FIG. 2 is a graph showing the relationship between the growth amount of the InAsSn wetting layer 13 and the in-plane density of the InAs quantum dots. The common growth conditions of the samples are that the growth temperature of the GaAs buffer layer 12 is 590 ° C., the growth temperature of the InAsSb wetting layer 13 is 470 ° C., the supply amount (pressure) of As is 7.3 × 10 ⁻⁶ Torr, and the supply of Sb. The amount (pressure) was set to be substantially constant 2.1 ± 0.1 × 10 ⁻⁷ Torr, and the growth amount of the InAsSb wetting layer 13 was changed from 0 ML to 1.0 ML. From this graph, it can be seen that there is a linear correlation between the in-plane density of the InAs quantum dots and the growth amount of the InAsSb wetting layer 13. It should be noted that the growth temperature of InAsSb and the supply pressure of As include some variation due to the relationship between the devices.

  FIG. 3 is an AFM image of InAs quantum dots corresponding to each measurement point in FIG. FIG. 3A shows a case where the InAsSb wetting layer 13 is not grown, that is, the case where the InAs wetting layer (two-dimensional growth layer) 13 is not introduced at the stage of the InAs wetting layer (two-dimensional growth layer) 13 and is directly shifted to three-dimensional growth (sample 1). . 3B shows a case where the InAsSb wetting layer 13 is grown by 0.59 ML (sample 2), and FIG. 3C shows a case where the InAsSb wetting layer 13 is grown by 0.75 ML (sample 3). ) Shows a case where the InAsSb wetting layer 13 is grown by 1.0 ML (sample 4). From Samples 2 to 4, it can be seen that the in-plane density is improved while maintaining the uniform size and shape of the InAs quantum dots 14.

When Sb is not introduced into the wet layer of the InAs layer 15 in the sample 1, the in-plane density of the InAs quantum dots is 2.2 × 10 ¹¹ cm ⁻² . In contrast, sample 2 has an in-plane density of 4.2 × 10 ¹¹ cm ⁻² , sample 3 has an in-plane density of 4.5 × 10 ¹¹ cm ⁻² , and sample 4 has an in-plane density of 6.0 × Improve to 10 ¹¹ cm ^-2 . From the graph of FIG.
(1) By increasing the growth amount of the InAsSb wetting layer 13 to 0.5 ML or more, the in-plane density of the InAs quantum dots can be increased to 4.0 × 10 ¹¹ cm ⁻² or more, and (2) InAsSb wetting The in-plane density of the InAs quantum dots can be 6.0 × 10 ¹¹ cm ⁻² or more by making the growth amount of the layer 13 about 1.0 ML or more.
I understand.

  FIG. 4 is a table showing detailed growth conditions at each measurement point in FIG. The growth temperature and growth rate of the GaAs buffer layer 12, the growth temperature of the InAsSb wetting layer 13, the growth rate of the InAs layer 15, and the As supply amount are constant within an error range. The growth amount of the InAs layer after stopping the supply of Sb is adjusted so that the total growth amount of the InAs layer 15 including the wetting layer 13 falls within the range of 2.18 to 2.21 ML.

  Based on the Sb supply condition of FIG. 4 and the graph of FIG. 2, when the growth amount of the InAsSb wetting layer 13 is fixed to about 1.0 ML, the InAs quantum dot 14 is adjusted by adjusting the supply amount (pressure) of Sb. It is reasonably predicted that the in-plane density will be higher. In other words, even when the growth amount of the InAsSb wetting layer 13 is smaller than 1.0 ML, the in-plane density of the InAs quantum dots can be reduced by appropriately adjusting the Sb supply amount (pressure). It is predicted that the growth can be maintained as high as when the growth amount is 1.0 ML. The measurement result will be described later with reference to FIG.

  FIG. 5 is a graph plotting the relationship between the growth amount of the InAsSb wetting layer 13, the average height of the InAs quantum dots 14 (left vertical axis), and the horizontal size of the InAs quantum dots 14 (right vertical axis). It is. The growth conditions are as shown in FIG. When the growth amount of the InAsSb wetting layer 13 is increased, the average height of the InAs quantum dots 14 decreases, but is constant between 2.5 nm and 3.0 nm, more specifically between 2.8 nm and 3.0 nm. It is thought that it becomes. On the other hand, the horizontal size (diameter) of the InAs quantum dots 14 decreases as the growth amount increases in the range where the growth amount of the InAsSb wetting layer 13 is up to 1.0 ML. However, the horizontal size is expected to be constant at certain points.

FIG. 6 is a graph plotting the relationship between the Sb supply amount (pressure) and the density of the InAs quantum dots 14 while maintaining the growth amount of the InAsSb wetting layer 13 in the range of 0.75 ML to 0.79 ML. It can be seen that by increasing the Sb supply pressure, the in-plane density of the InAs quantum dots 14 is increased to around 6.5 × 10 ¹¹ cm ⁻² even when the growth amount of the InAsSb wetting layer 13 is smaller than 1.0 ML.

7 is an AFM image of the InAs quantum dots 14 at each measurement point in FIG. 6, and FIG. 8 is a table showing the growth conditions at each measurement point in FIG. 7A shows a case where the supply pressure of Sb is 2.2 × 10 ⁻⁷ Torr (sample 11), and FIG. 7B shows a case where the supply pressure of Sb is 2.3 × 10 ⁻⁷ Torr ( Sample 12) and FIG. 7C show the case where the supply pressure of Sb is 2.8 × 10 ⁻⁷ Torr (sample 13). The growth temperature of InAsSb is 470 ° C., and the supply pressure of As is 7.3 × 10 ⁻⁶ Torr.

When comparing the sample 12 in FIG. 7B and the sample 13 in FIG. 7C with reference to FIG. 8, even if the InAsSb wetting layer 13 has the same growth amount (ML), the Sb supply pressure is increased. It can be seen that the in-plane density and the quantum dot size can be improved while maintaining the uniformity of the InAs quantum dots 14. When the result of FIG. 8 and the result of FIG. 4 are combined, the supply pressure of Sb is adjusted in the range of 2.2 × 10 ^{−7 to} 2.8 × 10 ⁻⁷ Torr, and the InAsSb wetting layer is adjusted to 0.7 ML to 1 It can be seen that when grown to 0.0 ML, the InAs quantum dots 14 grow at an in-plane density of 6.0 × 10 ¹¹ cm ^{−2 to} 6.5 × 10 ¹¹ cm ⁻² .

FIG. 9 is a schematic diagram showing a sample structure for measuring the photoluminescence emission spectrum of the InAs quantum dots 14 of the first embodiment. A GaAs buffer layer 12 was formed on a GaAs (001) substrate 11 to a film thickness of 210 nm at a growth temperature of 590 ° C. and a growth rate of 0.130 nm / second. Thereafter, an InAsSb wetting layer 13 was grown by 0.77 ML at an Sb supply pressure of 2.2 × 10 ⁻⁷ Torr, an As supply pressure of 7.3 × 10 ⁻⁶ Torr, and a growth temperature of 470 ° C. The InAs quantum dots 14 were formed by stopping the supply of Sb and further growing InAs by 1.41 ML (the total growth amount of the InAs layer 15 was 2.18 ML). At this time, the in-plane density of the InAs quantum dots 14 is 4.5 × 10 ¹¹ cm ⁻² . The InAs quantum dots 14 were embedded with a GaAs cap layer 16 having a thickness of 60 nm.

  FIG. 10 is a photoluminescence emission spectrum of the sample of FIG. The emission spectrum does not have a sharp peak and extends over a wide wavelength range. The peak wavelength is 955 to 960 nm, and the FWHM (Full Width at Half Maximum) is 86.8 meV. This is because the InAs quantum dots 14 are grown at a very high density, and the wave function of the confined electrons is coupled in-plane (multiplexed) between adjacent quantum dots, and a plurality of energy levels are generated. it is conceivable that.

  In a conventional single-junction (PN junction) solar cell, only light having a frequency exceeding the band gap energy of the light absorption layer is absorbed, and thus light having a band gap energy or less cannot be used. In contrast, when the high-density InAs quantum dots of the first embodiment of the present invention are applied to the light absorption layer of the solar cell, the energy levels are multiplexed to form a band-like level. It is possible to absorb light in a band not conventionally used in the sunlight spectrum, thereby reducing transmission loss and improving energy conversion efficiency.

FIG. 11A is a schematic configuration diagram of a solar cell 20 using the InAs quantum dots 14 of the first embodiment. The solar cell 20 includes a transparent electrode 29, a p-type semiconductor layer 28, a light absorption layer 27, an n-type semiconductor layer 22, and a back electrode 21. In the light absorption layer 27, a plurality of InAs layers 15 including InAs quantum dots 14 are stacked via a GaAs intermediate layer 26. The InAs layer 15 includes an InAsSb wetting layer 13 and InAs quantum dots 14. The in-plane density of the InAs quantum dots 14 is 4.0 × 10 ^{11 to} 6.5 × 10 ¹¹ cm ⁻² . The height of the InAs quantum dots 14 is 2.8 to 3.0 nm. The growth amount of the GaAs intermediate layer 26 is 0.25 to 1.5 ML. Since InAs quantum dots 14 are located at a very high density in the horizontal plane, an intermediate band is formed in the plane of the quantum dot layer by electronic coupling between adjacent InAs quantum dots as shown in FIG. To do. In the conventional technique, more than 100 quantum dot layers are stacked in the vertical direction and wave functions are coupled in the vertical direction, but the InAs quantum dots of the first embodiment can be coupled in the lateral direction in a wide range. An intermediate band can be generated efficiently without stacking a great number of layers.

When manufacturing such a solar cell, for example, an n-type semiconductor layer (not shown) is grown on the n-type GaAs substrate 22. The n-type semiconductor layer is, for example, a GaAs layer doped with any n-type impurity atom. Thereafter, the InAsSb wetting layer 13 is grown at 470 ° C. by 0.5 to 1.5 ML, more preferably 0.6 to 1.0 ML, the supply of Sb is stopped, and InAs is subsequently grown. Thereby, InAs quantum dots 14 are formed. InAs quantum dots are embedded with an (In) GaAs (SbN) intermediate layer 26. The process from the formation of the InAsSb wetting layer to the formation of the (In) GaAs (SbN) intermediate layer 26 is repeated a desired number of times. Accordingly, the InAs quantum dot layer is stacked in a self-aligned manner in the vertical direction while maintaining the in-plane density of the quantum dots of 4.0 × 10 ^{11 to} 6.5 × 10 ¹¹ cm ⁻² . A p-type semiconductor layer 28 is formed thereon. The p-type semiconductor layer 28 is a GaAs layer, for example, and is doped with an arbitrary p-type impurity atom. An ITO transparent electrode 29 is formed on the p-type semiconductor layer 28. Although not shown, the transparent electrode may be previously filled with a metal material such as Ag or Au at a predetermined position. An electrode 21 is formed on the back surface of the n-type GaAs substrate by vapor deposition or the like.

  FIG. 12 is a schematic diagram illustrating the band structure of the solar cell 20 of FIG. Due to the presence of the quantum dots 14 in the pn junction semiconductor layer, for example, an intermediate band is formed in the conduction band of the quantum dots, and between the valence band and the conduction band (forbidden band (Eg)) of the pn junction semiconductor, The intermediate band is localized. When sunlight enters the light absorption layer 27 of the solar cell 20, for example, electrons are excited to the conduction band with respect to light in three types of wavelength bands of λ1, λ2, and λ3 (holes are generated in the valence band). Generated).

  For light λ1 having a frequency exceeding the energy gap of the forbidden band, electrons are directly excited from the valence band to the conduction band. For longer wavelengths of light λ2, electrons are excited from the valence band to the intermediate band of quantum dots. For light λ3 having a longer wavelength, electrons can jump out from the intermediate band of the quantum dot to the conduction band. When a large number of quantum wells are close to each other, wave functions are combined between adjacent quantum wells, and originally discrete energy levels are multiplexed to form a band (intermediate band). Due to the presence of the intermediate band, the loss due to the band gap difference is reduced, and the energy conversion efficiency of the solar cell 20 can be increased.

  When 10 quantum dot layers 15 having an in-plane density of the present invention are stacked, a quantum dot density comparable to that achieved by stacking 100 conventional layers in the vertical direction is one tenth stack. It means that it can be realized with numbers. Therefore, it is possible to greatly reduce the manufacturing process of the solar cell.

As described above, according to the quantum dot forming method of the present invention, uniform quantum dots can be formed with an in-plane high density of 6.0 × 10 ¹¹ cm ⁻² or more. Solar cells to which such quantum dots are applied can improve energy conversion efficiency with a small number of layers.
Second Embodiment
In the second embodiment, the in-plane density of 1.0 × 10 ¹² cm ⁻² is reduced by further changing the thickness of the InAsSb wetting layer, the Sb supply pressure, and lowering the substrate temperature during the growth of InAs quantum dots. Realize.

  FIG. 13 is a diagram showing growth conditions for InAs quantum dots of the second embodiment. A GaAs buffer layer 12 is grown on a GaAs (001) substrate 11 by a molecular beam epitaxy (MBE) method at a substrate temperature of 590 ° C. with the same configuration as in FIG. The growth conditions of the buffer layer 12 are a growth rate of 0.13 nm / second and a film thickness of 200 nm. Thereafter, the InAs quantum dots 14 are grown under two temperature conditions.

  Under the growth condition A, an InAsSb wetting layer 13 was grown on the GaAs buffer layer 12 by 0.6 to 1.5 ML, and then an InAs quantum dot 14 was grown at 470 ° C. At this time, the total thickness of the InAs quantum dots 14 and the InAsSb wetting layer 13 is 2.2 to 2.6 ML.

  Under the growth condition B, the InAsSb wetting layer 13 was grown on the GaAs buffer layer 12 by 1.0 to 1.5 ML, and then the InAs quantum dots 14 were grown at 460 ° C. At this time, the total thickness of the InAs quantum dots 14 and the InAsSb wetting layer 13 is 2.2 ML.

  As in the first embodiment, the InAs quantum dot 14 continuously supplies In and As onto the GaAs buffer layer 12 and supplies Sb only to the wetting layer stage during quantum dot growth. A wetting layer 13 is formed. Therefore, for convenience, the InAsSb wetting layer 13 and the InAs quantum dots 14 are collectively referred to as an InAs layer (or quantum dot layer) 15.

  FIG. 14A shows the Sb supply pressure dependence of the in-plane density of the InAs quantum dots 14 under the growth condition A. FIG. FIG. 14B shows the Sb supply pressure dependence of the height of the InAs quantum dots 14 under the same growth condition A.

  In FIGS. 14A and 14B, the circles indicate data when the thickness of the InAsSb wetting layer 13 is 1.25 ML, and the square marks indicate data when the thickness of the InAsSb wetting layer 13 is 0.75 ML. It is. As can be seen from FIG. 14A, under the growth condition of 470 ° C., when the InAsSb wetting layer 13 is set to 1.25 ML, the in-plane density of the InAs quantum dots increases as the Sb supply pressure increases.

When the thickness of the InAsSb wetting layer 13 is 1.25 ML, the in-plane density of the InAs quantum dots is 6.4 × 10 ¹¹ cm ⁻² and the dot height when the Sb supply pressure is 2.2 × 10 ⁻⁷ Torr. Is 2.1 nm. This is substantially the same as the result obtained when the Sb supply pressure is 2.8 × 10 ⁻⁷ Torr when the thickness of the InAsSb wetting layer 13 is 0.75 ML.

When the thickness of the InAsSb wetting layer 13 is 1.25 ML and the Sb supply pressure is 2.8 × 10 ⁻⁷ Torr, the in-plane density of the InAs quantum dots is 7.13 × 10 ¹¹ cm ⁻² . When the Sb supply pressure is 3.5 × 10 ⁻⁷ Torr, the in-plane density of InAs quantum dots is 7.6 × 10 ¹¹ cm ⁻² . It is expected that the in-plane density of InAs quantum dots will be further increased by further increasing the Sb supply pressure.

Even when the InAsSb wetting layer 13 is 0.75 ML, the in-plane density of the InAs quantum dots is 6 × 10 ¹¹ cm ⁻² or more by setting the Sb supply pressure to 2.3 × 10 ⁻⁷ Torr or more. be able to.

  In FIG. 14B, it can be seen that the height of the InAs quantum dots decreases with increasing Sb supply pressure, but tends to saturate near 1.6 nm.

FIG. 15A and FIG. 15B show the in-plane density and dot height of InAs quantum dots when the thickness of the InAsSb wetting layer 14 is changed in the range of 0.5 ML to 1.5 ML, respectively. Show. White circles, white squares, and white rhombuses indicate the in-plane density and dot height according to the Sb supply pressure when the growth temperature of the InAs quantum dots is 470 ° C. The black circles indicate the in-plane density and dot height of InAs quantum dots when the growth temperature of InAs quantum dots is 460 ° C. and the Sb supply pressure is 3.6 × 10 ⁻⁷ Torr.

When the growth temperature is 470 ° C., increasing the thickness of the InAsSb wetting layer increases the in-plane density of InAs quantum dots, but the in-plane density of InAs quantum dots is 6.0 × 10 ¹¹ cm ^{−2 to} 7. It ^{remains in} the range of 5 × 10 ¹¹ cm ^{−2 and} no further increase in density is expected.

Therefore, as a new parameter, when the substrate temperature during the growth of InAs quantum dots is lowered to 460 ° C. and the Sb supply pressure is set to 3.6 × 10 ⁻⁷ Torr, the in-plane density of the InAs quantum dots is 8.0 ×. It was found to exceed 10 ¹¹ cm ^-2 . In particular, when the InAsSb wetting layer 13 is grown to 1.25 ML, an in-plane density of 1.0 × 10 ¹² cm ⁻² can be realized. At this time, the total thickness of the InAs layer (quantum dot layer) 15 including the InAsSb wetting layer 13 and the InAs quantum dots 14 is 2.2 ML.

  FIGS. 16A and 16B show the in-plane density of InAs quantum dots when the thickness of the InAsSb wetting layer 13 is fixed at 1.25 ML and the growth temperatures (substrate temperatures) are 470 ° C. and 460 ° C. It is a figure which compares dot height, respectively.

  14 to 16, when the substrate temperature is 460 ° C., the Sb supply pressure is further increased, the total growth amount of the InAs layer 15 is made smaller than 2.2 ML, or the substrate temperature during the growth of InAs quantum dots is increased. The in-plane density of the InAs quantum dots is expected to be further improved by lowering the temperature below 460 ° C.

FIG. 17 is an AFM image of InAs quantum dots when the in-plane density is 1.0 × 10 ¹² cm ⁻² . Even if the in-plane density is increased to the order of 10 ¹² , uniform InAs quantum dots can be obtained.

  FIG. 18 is a diagram showing the relationship between the dot size (diameter) in the horizontal direction of the InAs quantum dots and the in-plane density. When the InAs quantum dots were grown on the InAsSb wetting layer 13 and grown on the GaAsSb buffer layer. Comparing when.

  Compared to the GaAsSb buffer layer, the introduction of the InAsSb wetting layer 13 increases the formation density of minute two-dimensional islands and promotes the formation of three-dimensional nuclei, thereby increasing the density of InAs quantum dots. . It can be understood from FIG. 17 that the uniformity of the quantum dots is maintained despite the increase in the density of the InAs quantum dots.

  By applying such uniform and high-density quantum dots to the solar cell, the energy conversion efficiency can be improved with a small number of stacks as in the first embodiment.

  Application to optical nanodevices, particularly solar cells, is expected.

11 GaAs substrate 12 GaAs buffer layer 13 InAsSb wetting layer 14 InAs quantum dot 15 InAs layer (quantum dot layer)
20 solar cell 21 electrode 22 n-type semiconductor layer 26 GaAs intermediate layer 27 light absorption layer 28 p-type semiconductor layer 29 transparent electrode

Claims (6)

When an InAs layer is grown on a GaAs buffer layer, antimony (Sb) is supplied at a pressure of 2.0 × 10 ⁻⁷ to 3.3 at the stage of the wetting layer before the growth of the InAs layer shifts to three-dimensional growth. Introduced at 6 × 10 ⁻⁷ Torr to grow InAsSb wetting layer,
When the InAsSb wetting layer grows from 0.5 ML to 1.5 ML, the introduction of Sb is stopped, and the InAs layer is subsequently grown to form InAs quantum dots.
A method of forming a quantum dot characterized by the above.   The method for forming quantum dots according to claim 1, wherein the growth temperature of the InAsSb wetting layer and the InAs quantum dots is 460 ° C to 470 ° C. When the growth temperature is 460 ° C., the Sb supply pressure is 3.6 × 10 ⁻⁷ Torr, and the InAsSb wetting layer is grown by 1.0 to 1.5 ML, and the InAs quantum dots are 8.0. The method for forming quantum dots according to claim 2 , wherein the quantum dots are formed with an in-plane density of × 10 ¹¹ cm ^{-2 to} 1.0 × 10 ¹² cm ^-2 . When the growth temperature is 470 ° C., the InAsSb wetting layer is grown to 0.8 ML to 1.5 ML in the range of 2.2 × 10 ^{−7 to} 3.5 × 10 ⁻⁷ Torr of the Sb supply pressure. The method of forming quantum dots according to claim 2 , wherein the InAs quantum dots are formed at an in-plane density of 6.0 × 10 ¹¹ cm ^{−2 to} 7.6 × 10 ¹¹ cm ⁻² . 5. The quantum dot forming method according to claim 1, wherein, in the growth of the InAsSb wetting layer, the supply pressure of As is 7.3 × 10 ⁻⁶ Torr. A transparent electrode;
A first conductivity type semiconductor layer;
A second conductivity type semiconductor layer;
A light absorbing layer located between the first conductive type semiconductor layer and the second conductive type semiconductor layer;
An electrode located on a surface opposite to the transparent electrode;
Including
The light absorption layer includes a GaAs buffer layer,
A quantum dot layer including an InAsSb wetting layer formed on the GaAs buffer layer and an InAs quantum dot grown on the InAsSb wetting layer;
A GaAs intermediate layer covering the quantum dot layer;
The quantum dot layer and the GaAs intermediate layer are alternately stacked one or more layers ,
The in-plane density of the InAs quantum dots is 4.0 × 10 ¹¹ cm ^{−2 to} 1.0 × 10 ¹² cm ⁻² , the growth amount of the InAsSb wetting layer is 0.5 to 1.5 ML, and the InAs quantum dots The solar cell is characterized by having a height of 1.6 to 3.0 nm .