JP6635462B2 - Method for manufacturing semiconductor quantum dot device - Google Patents

Description

  The present invention relates to a method for manufacturing a semiconductor quantum dot device, and more particularly, to a novel manufacturing method capable of forming a semiconductor quantum dot device with high uniformity.

  Quantum dots are attracting attention as basic nanostructures of next-generation semiconductor devices, mainly lasers. For example, researches on application to various semiconductor optical and electronic devices such as a quantum dot laser in which quantum dots are introduced into an active layer of a semiconductor laser are being actively conducted. Of the quantum dots, InAs quantum dots formed on GaAs are particularly important.

  As a typical technique for forming quantum dots using a compound semiconductor, a technique of forming quantum dots by self-organizing growth is known. This uses a phenomenon in which quantum dots having a three-dimensional fine structure are formed by vapor phase epitaxial growth of a semiconductor having a different lattice constant on a substrate under certain conditions. Several growth modes are known for the self-assembled growth of such quantum dots, and the most well-known is a growth mode called a Transki-Krastanow (SK) growth mode ( For example, Patent Document 1, Non-Patent Document 1, etc.).

  The SK growth mode is a mode in which a semiconductor crystal to be epitaxially grown grows two-dimensionally (film growth) at the beginning of growth, but exceeds a predetermined thickness called a critical thickness determined depending on a supply material. This is a mode for shifting to three-dimensional growth. By epitaxially growing a film having a larger lattice constant than the material of the underlying buffer layer, an island-shaped dot structure is formed in a self-organizing manner. Since this method does not require lithography and uses only crystal growth, it is used as a method capable of forming high-quality quantum dots. Conventionally, the process of forming quantum dots in the SK growth mode is usually performed at a constant temperature.

  However, in the conventional method using such an SK growth mode, variation in the size of the formed quantum dots poses a problem, and it is desired to develop a method capable of forming quantum dots with higher uniformity. I was

JP 2005-79183 A

Goldstein et al., Appl. Phys. Lett. , 47, 1099, 1985

  Accordingly, an object of the present invention is to provide a novel method capable of forming a uniform and high quality quantum dot as compared with the conventional method.

  As a result of intensive studies to solve the above problem, the present inventors have found that a compound semiconductor having a specific thickness is deposited on a buffer layer by two-dimensional epitaxial growth under a high temperature condition that does not cause three-dimensional epitaxial growth. By forming the quantum dots by three-dimensional epitaxial growth under the condition where the temperature is lowered, a quantum dot with high uniformity can be obtained, and by changing the growth time in the two-dimensional growth step, the quantum dots can be formed. The inventors have found that the density can be controlled, and have completed the present invention.

That is, the present invention, in one aspect,
(1) a) a step of forming a buffer layer laminated on a semiconductor substrate by crystal growth, and b) a step of forming a quantum dot layer on the buffer layer by self-organizing growth, wherein a temperature T ₁ Forming a quantum dot by performing three-dimensional epitaxial growth at a temperature T ₂ lower than the temperature T ₁ , and then forming a quantum dot by performing three-dimensional epitaxial growth at a temperature T ₂ lower than the temperature T _1. Method;
(2) The method for manufacturing a semiconductor quantum dot device according to (1), wherein the buffer layer is formed of a material containing a III-V compound semiconductor;
(3) The method for producing a semiconductor quantum dot device according to (2), wherein the III-V compound semiconductor is GaAs;
(4) The method for manufacturing a semiconductor quantum dot device according to any one of (1) to (3), wherein the quantum dot layer is formed of a material including a III-V compound semiconductor;
(5) The method for producing a semiconductor quantum dot device according to (4), wherein the III-V compound semiconductor is InAs;
(6) T ₁ is a temperature which does not cause the three-dimensional epitaxial growth of the material forming the quantum dot layer, a method of manufacturing a semiconductor quantum dot device according to any one of the above (1) to (5);
(7) wherein _{T 1} is from 2 to 10% higher temperature than _{T 2,} the (1) - (5
The method of manufacturing a semiconductor quantum dot device according to any one of; a and (8) the range wherein _{T 1} is a 500-550 ° C., wherein _{T 2} is in the range of 470-490 ° C., above (1) to (5) A method for manufacturing a semiconductor quantum dot device according to any one of (5).

The invention provides, in another aspect,
(9) A semiconductor quantum dot device obtained by the method for manufacturing a semiconductor quantum dot device according to any one of (1) to (8);
(10) A semiconductor quantum dot device comprising: (a) a buffer layer formed by crystal growth on a semiconductor substrate; and (b) a quantum dot layer formed by self-organization growth on the buffer layer. The semiconductor quantum dot device, wherein the standard deviation of the height of the quantum dots in the layer is in the range of 0.3 to 1 nm, and the quantum dot density is 1 × 10 ^{8 to} 1 × 10 ¹¹ per square centimeter;
(11) The semiconductor quantum dot device according to the above (10), wherein the material forming the buffer layer is GaAs, and the material forming the quantum dot layer is InAs; and (12) the above (9) to ( A semiconductor optical device having the semiconductor quantum dot according to any one of 11) is provided.

  ADVANTAGE OF THE INVENTION According to this invention, the more uniform and high quality quantum dot can be formed conventionally. By obtaining such highly uniform quantum dots, it becomes possible to improve characteristics such as threshold current, modulation speed, and spectral purity of the quantum dot laser, and to increase the efficiency of the quantum dot solar cell. In addition, the possibility of realizing a 1.5-micron band quantum dot laser, which was difficult to form high-quality quantum dots in the past, can be enhanced, and it can be applied to optical communication, sensor applications, and LSI optical wiring systems. Be expected. Further, according to the present invention, there is an effect that the final density of quantum dots can be controlled by controlling the growth time in the two-dimensional growth stage.

FIG. 1 is a graph showing the relationship between substrate temperature and quantum dot growth in the manufacturing method of the present invention. FIG. 2 is an AFM image of the GaAs / InAs quantum dots obtained in Example 1. FIG. 3 is an AFM image of the GaAs / InAs quantum dots obtained in Comparative Example 1. FIG. 4 is an AFM image of GaAs / InAs quantum dots obtained when the growth time (InAs supply amount) in the high-temperature growth step was changed. FIG. 5 is a graph showing the dependency between the GaAs / InAs quantum dot density and the growth time (InAs supply amount) in the high-temperature growth step. FIG. 6 is an AFM image of the GaAs / InAs quantum dots when the supply rate of the As source is changed, and a graph showing the dependency between the quantum dot density and the supply rate. FIG. 6 is an emission spectrum of the GaAs / InAs quantum dots obtained in Example 1.

  Hereinafter, embodiments of the present invention will be described. The scope of the present invention is not limited by these descriptions, and can be appropriately modified and implemented without departing from the scope of the present invention, except for the following examples.

1. The method for manufacturing a semiconductor quantum dot device The method for manufacturing a semiconductor quantum dot device according to the present invention includes the steps of: forming a quantum dot on a buffer layer in forming a quantum dot in an SK growth mode;
i) depositing a compound semiconductor of a specific thickness by two-dimensional growth under high temperature conditions;
ii) Thereafter, quantum dots are formed by three-dimensional epitaxial growth under lower temperature conditions. This makes it possible to form high-quality quantum dots with high uniformity. Further, by controlling the growth time in the two-dimensional growth stage of i), the density of the final quantum dots obtained in ii) can be controlled.

  As the semiconductor crystal growth apparatus used in the manufacturing method of the present invention, for example, a molecular beam epitaxy (MBE) apparatus or a metal organic chemical vapor deposition (MOCVD) apparatus can be used. .

  Hereinafter, each step in the manufacturing method of the present invention will be described.

  First, a buffer layer made of a compound semiconductor is formed on a substrate. Here, the material forming the buffer layer is preferably a material containing a group III-V compound semiconductor, and is typically GaAs, InP, InAlAs, InAlGaAs or the like. More preferably, it is GaAs. Also, the substrate can be the same compound semiconductor as the material forming the buffer layer.

  The case where the MBE apparatus is used is as follows.

The processing for forming such a buffer layer may be performed by a method generally used in the technical field. Typically, when the substrate is GaAs, the substrate temperature is set to, for example, 550 to 600 ° C., and gallium and arsenic are used. The molecular beam is supplied into the ultra-high vacuum growth chamber to grow a GaAs buffer layer on the GaAs substrate. The thickness of the buffer layer is not limited to this, but is preferably 200 to 400 nm.

  The case where the MOCVD apparatus is used is as follows.

The processing for forming such a buffer layer can be performed by a method generally used in the art. Typically, when the substrate is GaAs, the substrate temperature is set to, for example, 600 to 700 ° C., and TBA, trimethylgallium is used. (Trimethylgalium, TMG) can be supplied into the reaction chamber to grow a buffer layer made of GaAs on the GaAs substrate. The thickness of the buffer layer is not limited to this, but is preferably 200 to 400 nm.

Next, a quantum dot layer is formed on the buffer layer by self-organizing growth in the SK growth mode. Specifically, as shown in FIG. 1, the compound semiconductor is subjected to two-dimensional epitaxial growth with the substrate temperature set to the temperature T ₁ , and then the substrate temperature is decreased to three-dimensional at the temperature T ₂ lower than the temperature T _1. Quantum dots are formed by performing epitaxial growth of. The supply of the compound semiconductor to the substrate can be performed by a method generally used in self-organized growth in the SK growth mode.

  The material forming the quantum dot layer is preferably a material containing a group III-V compound semiconductor, and is typically InAs, InGaAs or the like. More preferably, it is InAs. Therefore, the combination of the material forming the buffer layer and the material forming the quantum dot layer is preferably GaAs / InAs.

The two-dimensional temperature T ₁ of performing epitaxial growth, in the art, at a temperature higher than the temperature normally used in the formation of quantum dots by self-organization growth S-K growth mode. Preferably, the temperature is such that the three-dimensional epitaxial growth of the material forming the quantum dot layer does not occur. On the other hand, T ₂ may be the in the art, a temperature in the range normally used in the formation of quantum dots by self-organization growth S-K growth mode.

From another aspect, _{T 1} is 1-20% higher temperature than _{T 2,} preferably 2 to 10% higher temperatures.

In a typical case, _{T 1} is in the range of 500-550 ° C., _{T 2} is in the range of 470-490 ° C.. In such a temperature range, the combination of the material forming the buffer layer and the material forming the quantum dot layer is optimal for GaAs / InAs. That is, in general, when InAs quantum dots are formed on GaAs, they are performed at a constant temperature in the range of 470 to 490 ° C. In the present invention, under the condition of a temperature T ₁ higher than such a temperature, This is characteristic in that two-dimensional epitaxial growth is performed. As a result, quantum dots with high uniformity can be obtained.

When performing the epitaxial growth of the two-dimensional at a temperature _{T 1,} a compound semiconductor growth time (nominal supply amount) is preferably from 1,000 to 3,000 seconds, preferably from 1500 to 2000 seconds. By changing the growth time, the density of the finally obtained quantum dots can be controlled. The quantum dot density is preferably in the range of 1 × 10 ^{8 to} 1 × 10 ¹¹ per square centimeter.

  As other conditions, when forming quantum dots by self-assembly growth in the SK growth mode, conditions usually used in the technical field can be used.

2. Semiconductor quantum dot device As described above, the quantum dot device obtained by the manufacturing method of the present invention is characterized by having high uniformity. Specifically, the standard deviation of the height of the quantum dots is in the range of 0.3 to 1 nm. The standard deviation of the height can be calculated by a known method, for example, measured by using an atomic force microscope and obtained by using surface particle analysis software.

  Further, the quantum dot device of the present invention can be applied to an optical device capable of emitting and absorbing light in the range of 800 to 1600 nm.

  Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.

1. Formation of InAs / GaAs quantum dots InAs / GaAs quantum dots were formed by the procedure described below. After a general pre-treatment, a 400 nm thick GaAs buffer layer was grown, followed by quantum dot growth: In supply rate 0.004 ML / s, As ₄ flux 2.0 × 10 ⁻⁶ Torr. The substrate temperature is 500 ° C. (based on pyrometer). The surface was observed using a reflection high-energy electron diffraction (RHEED) during the growth and an atomic force microscope (AFM) after the growth. First, the growth was performed while increasing the supply time (nominal supply amount) of In to 1800 s (7.2 ML), 3600 s (14.4 ML), and 7200 s (28.8 ML). Not confirmed. On the other hand, with respect to the sample grown at a substrate temperature of 500 ° C. for 1800 s, the substrate temperature was lowered to 490 ° C., and then 0.4 ML of InAs was supplied. As a result, it was confirmed that quantum dots were grown. FIG. 2 shows the obtained AFM image.

Comparative Example 1

  On the other hand, FIG. 3 shows an AFM image when quantum dots are formed by growing InAs while keeping the substrate temperature constant at 490 ° C.

  From a comparison between FIG. 2 and FIG. 3, it can be seen that the height of the InAs / GaAs quantum dots created in Example 1 is more uniform than in the comparative example.

  As a result of calculating the standard deviation of the height of the InAs / GaAs quantum dots obtained in Example 1, 0.36 nm was obtained.

2. Change in Density Due to Growth Time at High Temperature According to the method of Example 1, while maintaining the growth conditions (0.4 ML) after the substrate temperature was reduced to 490 ° C., the growth time t _{0 at} a substrate temperature of 500 ° C. (nominal supply Was varied. Each _{t 0} at high temperature conditions, 900s (3.6ML), 1000s ( 4.0ML), 1100s (4.4ML), 1200s (4.8ML), obtained when a 1800 s (7.2 mL) An AFM image is shown in FIG. As a result, it was found that the finally obtained InAs / GaAs quantum dot density changes depending on the growth time under high temperature conditions. The results are shown in FIG. 5 (here, ML in the figure is growth rate × growth time). The growth rates under the low temperature condition of 490 ° C. and the high temperature condition of 500 ° C. were both 0.004 ML / s. As shown in FIG. 5, the growth at the substrate temperature in two stages was adopted, and the density of the quantum dots could be increased by lengthening the growth time at a high temperature. It is considered that the reason why the density increased when the growth time was lengthened was that the amount of attached atoms of indium was increased and nuclei for forming quantum dots were easily generated.

3. Dependence of Quantum Dot Density on Source Supply Rate Varying the supply rate (supply amount per unit time) of the As source (As ₄ ) was performed, and the density change of the obtained quantum dots was verified. By changing the partial pressure of the supplied As ₄ to 2.0 × 10 ⁻⁶ Torr, 1.0 × 10 ⁻⁵ Torr, and 1.3 × 10 ⁻³ Torr, the quantum dots were formed in the same manner as in the method of Example 1. Was formed. FIG. 6 shows an AFM image of the quantum dots obtained in each case and a graph showing the relationship between the supply rate of the As source and the quantum dot density. As a result, it was found that increasing the supply rate of the As source increased the quantum dot density. The resulting density was 10 ¹⁰ front and rear, were those considered to be preferred in a quantum dot laser applications.

4. Evaluation of Optical Properties Photoluminescence measurement of the InAs / GaAs quantum dots obtained in Example 1 was performed at a low temperature. FIG. 7 shows the result. From the GaAs capped sample, light emission having a narrow half width of about 15 μeV was confirmed. Further, by observing the light emission from the neutral exciton (X) and the exciton molecule (XX), it was confirmed that a high-quality quantum dot was obtained by this method.

Claims (8)

a) on a semiconductor substrate, forming a buffer layer laminated with the crystal growth, and b) on the buffer layer, and forming a quantum dot layer by self-organization growth, 2D at temperatures T ₁ the epitaxial growth was performed, then, the to form a quantum dot by performing epitaxial growth of three-dimensional at a lower temperature T ₂ temperature T _1, characterized by having the step, a method of manufacturing a semiconductor quantum dot device. The method according to claim 1, wherein the buffer layer is formed of a material containing a group III-V compound semiconductor. 3. The method according to claim 2, wherein the III-V compound semiconductor is GaAs. The method for manufacturing a semiconductor quantum dot device according to claim 1, wherein the quantum dot layer is formed of a material containing a III-V compound semiconductor. The method for manufacturing a semiconductor quantum dot device according to claim 4, wherein the III-V compound semiconductor is InAs. The method for manufacturing a semiconductor quantum dot device according to claim ₁ , wherein T ₁ is a temperature at which three-dimensional epitaxial growth of a material forming the quantum dot layer does not occur. The method for manufacturing a semiconductor quantum dot device according to claim 1, wherein said T ₁ is a temperature that is 2 to 10% higher than T ₂ . The method for manufacturing a semiconductor quantum dot device according to claim 1, wherein the T ₁ is in a range of 500 to 550 ° C., and the T ₂ is in a range of 470 to 490 ° C. 7.

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