JP2011129733A - Quantum dot structure manufacturing method and quantum dot structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a quantum dot structure having light emission wavelengths distributed uniformly in a wider wavelength range than before. <P>SOLUTION: The quantum dot structure is manufactured by the following steps of: preparing a substrate 11; providing quantum dots 13 on the substrate; depositing thin films 14 on the substrate 11 such that the thin films 14 are lower than the quantum dots 13 and surface diffusion is suppressed; and lastly removing parts of the quantum dots 13 which are exposed from the thin films 14. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a quantum dot structure using quantum dots applicable to key devices used in information communication, quantum information processing fields, and the like, and a quantum dot structure.

  The so-called low-dimensional quantum structure, in which the fine structure of a semiconductor with a narrow bandgap is surrounded in two or three dimensions by a semiconductor with a wide bandgap, is promising for higher functionality and higher performance of optical and electronic devices. In recent years, it has attracted a great deal of attention as the key to the development of the optical and electronic industries.

  In particular, quantum dots with a three-dimensional quantum confinement structure have a remarkable quantum effect due to the sharpening of the density of states based on the strong confinement effect of electrons. It is expected to be realized as a basic structure of optical and electronic devices.

  As a typical method for forming a quantum dot using a compound semiconductor, for example, there is a growth method called Stranski-Krastanov (SK) mode growth (Non-patent Document 1, Patent Document 1).

  This is because a so-called lattice mismatched material having a lattice constant different from that of a substrate is supplied onto a semiconductor substrate with a growth amount exceeding a predetermined thickness called a critical film thickness determined depending on the material. This is a method of epitaxial growth.

  As a result, island-like dot structures are formed in a self-organized manner on a thin thin film called a wetting layer. Since this method does not require lithography and is based only on crystal growth, it attracts attention as a method capable of forming high-quality crystal dots.

  In recent years, it has become active to manufacture a single quantum dot in isolation, to investigate its characteristics, and to manufacture advanced light emitting devices using the isolated quantum dot. .

  At this time, as one of the characteristics required for the quantum dots, in order to use a high-sensitivity silicon photodetector, the emission wavelength in the 1 μm band is required.

  Therefore, an indium flash method has been proposed in order to shorten the quantum dot emission wavelength (Non-patent Document 2 and Patent Document 2).

  In this method, a thin GaAs cap layer is grown on a quantum dot having InAs in the emission wavelength to 1.3 μm band formed by using a normal SK mode growth method, and then heat treatment is performed to re-evaporate the top of the quantum dot by heat. This effectively reduces the height of the quantum dots and shortens the emission wavelength of the quantum dots.

  A typical process diagram of the indium flash method is shown in FIGS.

  First, as shown in FIG. 8A, a semiconductor substrate 11 is prepared, and as shown in FIG. 8B, quantum dots 13 are formed on the semiconductor substrate 11 by a normal SK mode self-forming method. Thereafter, a thin film 14 is deposited as shown in FIGS. 8C and 9, and then heat treatment is performed to re-evaporate the upper portions of the quantum dots 13 exposed on the thin film, as shown in FIG. 8D. .

JP 2003-124574 A Japanese Patent Laid-Open No. 10-289996

D. Leonard, M. Krishnamurthy, CMReaves, SPDenbaars, and PMPetroff “Drect formation of quantum-sized dots form uniform coherent islands of InGaAs on GaAs surface”, Appl. Phys. Lett, 63 (23), 6 December 1993 , p3203-3205 Wasilewski. Z. R, Fafard. S, Mccaffrey .J.P, ”Size and shape engineering of vertically stacked self-assembled quantum dots”, Journal of Crystal Growth, volume201 / 202, 1999, p1131

  Here, in the case of a structure using quantum dots manufactured by the conventional indium flash method, the emission wavelengths of the quantum dots having a shorter wavelength are densely distributed around a certain emission wavelength.

  However, when considering application to a single photon light source, an entangled photon pair light source, etc., it is desirable that the emission wavelength is uniformly distributed over a wide wavelength region in order to increase the degree of freedom in device design.

  In particular, when single photon light sources having a plurality of emission wavelengths are integrated on the same wafer, a single quantum dot that emits light at a desired wavelength in an arbitrary region on the wafer is required. Wide area is an essential technology.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for manufacturing a quantum dot structure in which the emission wavelength is uniformly distributed in a wavelength region wider than the conventional one.

  In order to solve the above problems, the first aspect of the present invention is as follows: (a) a quantum dot is provided on the substrate; (b) lower than the quantum dot and surface diffusion is suppressed. And (c) removing a portion of the quantum dot exposed from the thin film.

  A second aspect of the present invention is a quantum dot structure manufactured using the quantum dot manufacturing method described in the first aspect.

  ADVANTAGE OF THE INVENTION According to this invention, the method of manufacturing the quantum dot structure in which the light emission wavelength is uniformly distributed in the wavelength range wider than before can be provided.

1 is a cross-sectional view showing a quantum dot structure 1. FIG. It is an A1 direction arrow line view of FIG. It is a figure which shows the procedure of manufacture of the quantum dot structure. It is sectional drawing which shows the quantum dot structure 1a. It is a figure which shows the procedure of manufacture of the quantum dot structure 1b. It is a figure which shows the photo-luminescence measurement result of Example 1. It is a figure which shows the photo-luminescence measurement result of a comparative example. It is a typical process drawing of the conventional indium flash method. It is an A2 direction arrow directional view of Drawing 8 (c).

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail based on the drawings.

  First, with reference to FIGS. 1-3, the outline of the structure of the quantum dot structure 1 which concerns on embodiment of this invention, and the outline of a manufacturing method are demonstrated.

  First, an outline of the configuration of the quantum dot structure 1 will be described with reference to FIGS. 1 and 2.

  As shown in FIGS. 1 and 2, the quantum dot structure 1 includes a substrate 11, quantum dots 13 formed on the substrate 11, and a thin film 14 formed on the substrate 11 (around the quantum dots 13). have.

  Here, as is clear from FIG. 1, the thin film 14 is generated on the substrate 11 in a state in which surface diffusion on the substrate 11 is suppressed, whereby each quantum dot 13 has a height from the substrate 11. Are formed differently.

  By adopting a structure in which the surface diffusion of the thin film 14 is suppressed in this way, the quantum dot structure 1 has a structure in which the emission wavelength is uniformly distributed in a wavelength region wider than the conventional one (details will be described later).

  Next, a manufacturing method of the quantum dot structure 1 will be described with reference to FIG.

  First, as shown in FIG. 3A, a substrate 11 is prepared. The substrate 11 is, for example, a substrate having a compound semiconductor containing group III and group V elements, and more specifically, a substrate of a semiconductor such as GaAs or InP.

  The substrate 11 is usually a substrate with an exposed (001) crystal plane, but a substrate with a high index plane exposed, specifically, the normal direction is the [110] direction or the [111] direction. Thus, a substrate with the (110) plane or the (111) A plane exposed may be used. This is because, as described later, this method is expected to suppress the surface diffusion of the thin film 14 (see Physical Review Letters 79, 3938 (1997)).

  Next, as shown in FIG. 3B, the quantum dots 13 are grown on the substrate 11 by the SK mode self-forming method.

As the quantum dots 13, for example, those having a compound semiconductor containing group III and group V elements are used, and more specifically, InAs and InGaAs are used.

  Next, as shown in FIG. 3C, the thin film 14 is deposited on the substrate 11 to be lower than the quantum dots 13. The thin film 14 is, for example, a film having a compound semiconductor containing group III and group V elements, and more specifically, a semiconductor film such as GaAs or InP.

  Next, as shown in FIG. 3D, the top of the quantum dots 13 exposed from the thin film 14 is re-evaporated by heat treatment, thereby completing the quantum dot structure 1.

  Here, when forming the thin film 14, the thin film 14 is formed under such conditions that surface diffusion of the thin film 14 on the substrate 11 is suppressed.

  When the thin film 14 is formed under such conditions, a local distribution occurs in the thickness of the thin film 14 in the vicinity of the quantum dots 13 as shown in FIG. 3C, and a heat treatment is performed as shown in FIG. The distribution of the height of the subsequent quantum dots 13 is caused.

  Therefore, it becomes possible to manufacture the quantum dot structure 1 in which the emission wavelength is uniformly distributed over a wide wavelength range.

  Specifically, the conditions that suppress the surface diffusion include the following conditions.

(1) Temperature In order to suppress the surface diffusion of the thin film 14, it is desirable that the temperature (of the substrate 11) when depositing the thin film 14 is lower than the temperature during the growth of the quantum dots 13. Specifically, it is desirable that the temperature be 60 ° C. or more lower than the temperature at which the quantum dots 13 are grown.

  It is desirable that the temperature at which the upper part of the quantum dots 13 is re-evaporated is also lower than the temperature at which the quantum dots 13 are grown.

(2) Partial pressure of group V element (As) When the thin film 14 is deposited, the surface diffusion of the thin film 14 can be suppressed if the surface diffusion of the group III element (eg, Ga) is largely suppressed.

  Specifically, when depositing the thin film 14, the partial pressure of the group V element (for example, As) of the thin film 14 is made larger than the partial pressure during the growth of the quantum dots 13. In general, the surface diffusion distance of a group III element increases when the partial pressure of the group V element decreases, and the surface diffusion distance decreases when the partial pressure of the group V element increases.

  Therefore, when the partial pressure of the group V element is increased, the surface diffusion distance of the thin film 14 is shortened, and as a result, the same surface diffusion suppressing effect as that in which the growth temperature is lowered is obtained.

  Specifically, the partial pressure of the group V element at the time of depositing the thin film 14 is preferably at least twice the partial pressure at the time of growth of the quantum dots 13.

  Note that a decrease in temperature and an increase in the partial pressure of the group V element when depositing the thin film 14 have a synergistic effect. If the temperature of the substrate 11 is lowered and the partial pressure of the group V element is increased, the surface diffusion suppressing effect of the group III element is remarkably exhibited. However, even if the substrate temperature is high, the group V element is exhibited. By increasing the partial pressure, the surface diffusion of the group III element is suppressed. Even when the partial pressure of the group V element is low, the surface diffusion of the group III element can be suppressed by reducing the temperature of the substrate 11.

(3) Number of atoms per unit molecule of group V element The group V element (for example, As) is usually supplied during growth by directly heating a solid raw material and sublimating from the solid. When supplied by this method, for example, when the group V element is As, it is supplied in the form of As ₄ molecules in which four atoms are bonded. On the other hand, the As ₄ molecule is thermally decomposed by reheating immediately before being supplied onto the substrate 11 to reduce the form of As ₂ , that is, the number of atoms per unit molecule of the group V element than before heating. It is also possible to supply in the state. It is known that the surface diffusion distance of Ga (group III element) is also suppressed by supplying it on the substrate 11 in the form of As ₂ , and the surface diffusion of the thin film 14 can be suppressed. (For example, see Japanese Journal of Applied Physics Vol. 36, p. 5670 (1997)). In the case group V element constituting the thin film 14 is P, using the As no P, may be supplied to decompose the P ₄ molecule to P ₂ molecules.

  Thus, according to the present embodiment, when the quantum dot structure 1 is manufactured, the thin film 14 is formed under the condition that the surface diffusion of the thin film 14 on the substrate 11 is suppressed, so that the vicinity of the quantum dot 13 is formed. Thus, a local distribution is generated in the thickness of the thin film 14, and a distribution of the quantum dot height after the heat treatment is generated.

  Therefore, the quantum dot structure 1 in which the emission wavelength is uniformly distributed over a wide wavelength range can be manufactured.

  Next, the structure and manufacturing method of the quantum dot structure 1a according to the second embodiment will be described with reference to FIG.

  In the second embodiment, a cover layer 15 is provided on the substrate 11 so as to cover the quantum dots 13 and the thin film 14 in the first embodiment.

  In the second embodiment, elements having the same functions as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

  First, the structure of the quantum dot structure 1a will be described with reference to FIG.

  As shown in FIG. 4, the quantum dot structure 1 a has a cover layer 15 formed on the substrate 11 so as to cover the quantum dots 13 and the thin film 14.

  The cover layer 15 is a layer for efficiently confining carriers in the quantum dots by embedding the quantum dots with a material having a large band gap. For example, the cover layer 15 is a layer having GaAs.

  Next, a method for manufacturing the quantum dot structure 1a will be described.

  The manufacturing method of the quantum dot structure 1a is the same as the manufacturing method of the quantum dot 1 except that the step of forming the cover layer 15 is added last. That is, first, the substrate 11 is prepared, and the quantum dots 13 are grown on the substrate 11 by the SK mode self-forming method.

  Next, a thin film 14 is deposited on the substrate 11.

  Next, the upper part of the quantum dot 13 exposed from the thin film 14 is re-evaporated by performing heat treatment.

  Finally, a cover layer 15 is formed on the substrate 11 so as to cover the quantum dots 13 and the thin film 14, thereby completing the quantum dot structure 1a.

  As described above, according to the second embodiment, the quantum dot structure 1 a includes the cover layer 15, and has an effect equal to or greater than that of the first embodiment.

  Hereinafter, the present invention will be specifically described by way of examples.

Example 1
The quantum dot structure 1b shown in FIG. 5 was manufactured by the following procedure, and the light emission characteristics were evaluated.

  First, a GaAs substrate 11 was prepared (FIG. 5A).

Next, the substrate 11 is introduced into an MBE (molecular beam epitaxial) apparatus (V80H manufactured by Oxford), and the substrate temperature is raised to 600 ° C. while irradiating the As ₄ molecular beam obtained by heating and sublimating the solid As. The natural oxide film on the surface of the substrate 11 was removed, and a GaAs buffer layer 12 was grown at 580 ° C. by 300 nm (FIG. 5B).

Thereafter, the temperature of the substrate 11 is lowered to 485 ° C., and In is supplied onto the substrate 11 by an amount corresponding to 1.8 ML under a pressure of As partial pressure 6 × 10 ⁻⁶ Torr (8 × 10 ⁻⁴ Pa). The quantum dot 13 of this was manufactured (FIG.5 (c)).

  Next, the temperature of the substrate 11 was further lowered to 425 ° C., and a GaAs thin film 14 was grown by 1 nm (FIG. 5D).

  Thereafter, the temperature of the substrate 11 was raised again, and the tops of the quantum dots 13 were removed (FIG. 5E).

  As a result of this process, the GaAs deposited around the quantum dots 13 is extremely suppressed in surface diffusion because the temperature of the substrate 11 is low, and gathers in the vicinity of the quantum dots nonuniformly as shown in FIG. It became uneven.

  Thereafter, the GaAs cover layer 15 was grown to 100 nm, and the quantum dot structure 1b was completed (FIG. 5F).

  Photoluminescence measurement of the quantum dot structure 1b at a low temperature of 10K was performed. The results are shown in FIG.

  As is clear from FIG. 6, emission characteristics were obtained in which bright lines resulting from single quantum dot emission were distributed over a wide range of wavelengths.

(Example 2)
The partial pressure of As when depositing the thin film 14 was increased except that the set temperature of the evaporation source of As was raised to 1.2 × 10 ⁻⁵ Torr (1.6 × 10 ⁻³ Pa). The quantum dot structure 1b was manufactured under the same conditions as in Example 1, and the light emission characteristics were evaluated. As a result, similar to Example 1, emission characteristics in which bright lines resulting from single quantum dot emission were distributed over a wide range of wavelengths were obtained.

(Example 3)
A quantum dot structure 1b was manufactured under the same conditions as in Example 1 except that the number of atoms per unit molecular beam of the As molecular beam during deposition of the thin film 14 was reduced, and the light emission characteristics were evaluated. Specifically, the following method was used.

The number of atoms per unit molecular beam of the As molecular beam is 4 in the case where the As solid is heated and sublimated to form one molecule (that is, takes the form of As ₄ ). The heating temperature in this case is usually about 300 ° C. By heating the As ₄ molecular beam obtained by this method at a higher temperature, an As ₂ molecular beam can be obtained. The evaporation source developed for this purpose is called a cracking cell, and As ₄ molecules generated by sublimation inside the cell are again heated and decomposed by a heater provided at the cell outlet to be in an As ₂ state.

Therefore, in Example 3, the temperature of the outlet heater was set to 900 ° C. using a cracking cell, and an As ₂ molecular beam was obtained. As in the first and second examples, this As ₂ molecular beam was placed in the quantum dot wafer manufacturing process, and the As molecular beam at the time of GaAs thin film deposition was deposited under As ₂ molecular beam irradiation.

  As a result, as in Example 1, the emission characteristic in which the bright lines resulting from single quantum dot emission were distributed over a wide wavelength range was obtained.

Example 4
A quantum dot structure 1b was manufactured under the same conditions as in Example 1 except that the substrate 11 cut out with a high index surface was used as the substrate 11, and the light emission characteristics were evaluated.

  Specifically, as the substrate 11, a (110) plane or (111) A plane cut out from an ingot of a GaAs semiconductor crystal so that the normal direction of the substrate is the [110] direction or the [111] direction. A GaAs (semiconductor) substrate was used.

  As a result, similar to Example 1, emission characteristics in which bright lines resulting from single quantum dot emission were distributed over a wide range of wavelengths were obtained.

(Comparative Example 1)
A quantum dot structure was manufactured under the same conditions as in Example 1 except that the temperature at which the thin film 14 was formed was the same as that at which the quantum dots 13 were formed, and the light emission characteristics were evaluated.

  As a result, as shown in FIG. 7, the emission wavelengths of the quantum dots were densely distributed according to a normal distribution on both sides of the long and short wavelengths with a certain wavelength (in this case, about 975 nm) as the peak center.

  As described above, by forming the thin film 14 under the condition that the surface diffusion of the thin film 14 on the substrate 11 is suppressed, it is possible to obtain the light emission characteristics in which the manufactured quantum dot structure is distributed over a wider wavelength range than before. I understood.

  Although the embodiments and examples of the present invention have been specifically described above, the present invention is not limited to the above-described embodiments and examples, and various modifications based on the technical idea of the present invention are possible. . For example, although the example using the MBE method has been described for the crystal growth method, other growth methods such as an organometallic pyrolysis method may be used, and the numerical values, substances, and materials described in the above embodiments and examples may be used. The raw materials are merely examples, and those different from these may be used as necessary.

DESCRIPTION OF SYMBOLS 1 ......... Quantum dot structure 1a ...... Quantum dot structure 1b ...... Quantum dot structure 11 ... Substrate 12 ... Buffer layer 13 ... Quantum dot 14 ... Thin film 15 ... Cover layer

Claims (14)

(A) providing quantum dots on the substrate;
(B) depositing a thin film on the substrate so as to be lower than the quantum dots and suppress surface diffusion;
(C) removing a portion of the quantum dot exposed from the thin film;
A method for producing a quantum dot structure.   2. The method of manufacturing a quantum dot structure according to claim 1, wherein the material constituting the quantum dots, the substrate, and the thin film includes a compound semiconductor containing a group III element and a group V element.   3. The quantum dot structure manufacturing method according to claim 2, wherein a material having InAs or InGaAs is used as the material of the quantum dots.   4. The method for manufacturing a quantum dot structure according to claim 2, wherein a material containing GaAs or InP is used as a material for the substrate and / or the thin film.   (B) is characterized in that the surface temperature of the group III element of the thin film is suppressed by lowering the substrate temperature when depositing the thin film below the temperature when the quantum dots are provided in (a). The quantum dot structure manufacturing method according to any one of claims 2 to 4.   (B) is characterized in that the substrate temperature when depositing the thin film is such that the deposition temperature of the thin film according to claim 4 is at least 60 degrees lower than the temperature during quantum dot growth in (a). The method for producing a quantum dot structure according to claim 5.   (B) increases the surface pressure of the group III element of the thin film by making the partial pressure of the group V element when depositing the thin film higher than the partial pressure during the quantum dot growth in (a). It suppresses, The quantum dot structure manufacturing method as described in any one of Claims 2-6 characterized by the above-mentioned.   8. The method according to claim 7, wherein (b) sets the partial pressure of the group V element when depositing the thin film to be not less than twice the partial pressure during quantum dot growth in the (a). Quantum dot structure manufacturing method.   The said V group element which reduced the number of atoms per unit molecule from before heating is used by heating the said V group element before supply, The any one of Claims 2-8 characterized by the above-mentioned. Quantum dot structure manufacturing method. The method for producing a quantum dot structure according to claim 9, wherein the group V element in which the number of atoms per unit molecule is reduced as compared with that before heating is As ₂ or P ₂ .   The quantum dot structure manufacturing method according to claim 1, wherein (a) includes providing the quantum dots on a high index surface of the substrate.   12. The method of manufacturing a quantum dot structure according to claim 11, wherein the high index plane is a (110) plane or a (111) A plane.   The step (c) removes a portion of the quantum dot exposed from the thin film by continuously performing a heat treatment to reduce an effective height of the quantum dot. The quantum dot structure manufacturing method as described in any one of these.   The quantum dot structure manufactured using the quantum dot structure manufacturing method as described in any one of Claims 1-13.

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