JP4812656B2 - Quantum dot photodetector and method for manufacturing the same - Google Patents

Description

  The present invention relates to a quantum dot photodetector suitable for an infrared sensor and the like and a method for manufacturing the same.

  Conventionally, a quantum dot type photodetector is known as a kind of photodetector. FIG. 5 is a cross-sectional view showing the structure of a conventional quantum dot photodetector.

  In a conventional quantum dot photodetector, an intermediate layer 104 made of intrinsic GaAs is formed on a contact layer 103 made of n-type GaAs. On the intermediate layer 104, a plurality of quantum dots 105 made of InAs are formed. Then, another intermediate layer 104 is formed so as to cover the quantum dots 105, and about 10 combinations of such quantum dots 105 and intermediate layers 104 are stacked. A contact layer 108 made of n-type GaAs is formed on the uppermost intermediate layer 104.

  When light (infrared rays) is incident on the thus configured quantum dot photodetector, as shown in FIG. 6, electrons existing at a low ground level in the InAs quantum dots 105 absorb energy by light absorption. As a result, it is photoexcited to a high energy level (excitation level) in the dot. Then, photoexcited electrons are emitted to the outside of the InAs quantum dots 105, whereby a photocurrent is generated. At this time, the intermediate layer 104 functions as a barrier layer. Light (infrared rays) is detected based on this photocurrent. Therefore, the sensitivity of the photodetector depends on the probability of transition between levels in the quantum dot 105 when light absorption occurs. This transition probability is largely related to the overlap of wave functions between the ground level (transition source) and the excitation level (transition destination), and the stronger the overlap, the more the quantum dot infrared detection is performed. The sensitivity of the vessel is improved.

  However, in the above-described quantum dot-type photodetector, when the GaAs intermediate layer 104 covering the InAs quantum dots 105 is formed during the production thereof, as shown in FIG. The mixed crystal layer 111 is formed. When the mixed crystal layer 111 is formed, as shown in FIG. 7, the potential change (the bottom of the conduction band) from the InAs quantum dots 105 to the GaAs intermediate layer 104 serving as a barrier becomes gradual. As a result, there is a large difference in the spread of the wave function between the ground level near the bottom side in the potential well and the excitation level near the top side, and the overlap between the two is reduced. For this reason, the sensitivity of the conventional quantum dot photodetector is not sufficient.

Japanese Patent Laid-Open No. 10-256588

  An object of the present invention is to provide a quantum dot photodetector capable of improving sensitivity and a method for manufacturing the same.

  The inventors of the present application have studied to solve the problems of the prior art. As shown in FIG. 8, the AlAs layer 106 is capped between the quantum dots 105 and the intermediate layer 104 formed thereon. It was found that by forming as a layer, mixed crystallization of InAs and GaAs can be suppressed. When the AlAs layer 106 is present, the surface diffusion of In desorbed from the InAs quantum dots 105 is suppressed by Al. Further, since AlAs also acts as a high-energy barrier layer, as shown in FIG. 9, the potential gradient from InAs to the barrier becomes steep, and the wave function overlaps between the ground level and the excited level. Become stronger.

  However, as the inventors of the present application have further studied, it has been found that the AlAs layer 106 is preferentially formed on the GaAs intermediate layer 104 and is not easily formed on the InAs quantum dots 105. As a result, when an AlAs layer 106 having a thickness that prevents mixed crystallization is formed on the quantum dots 105, the thick AlAs layer 106 is formed on the intermediate layer 104. Then, as shown in FIG. 10, the thick AlAs layer 106 prevents the movement of the electrons 112 and acts as a current blocking layer for the photocurrent 113. For this reason, even if mixed crystallization can be suppressed, sufficient photocurrent cannot be obtained, and sufficient sensitivity cannot be obtained.

  Such selective growth of the AlAs layer 106 is caused by a difference in lattice constant between AlAs and InAs and a difference in lattice constant between AlAs and GaAs. That is, the lattice constant of AlAs is almost equal to the lattice constant of GaAs constituting the intermediate layer 104, but smaller than the lattice constant of InAs constituting the quantum dot 105. Between these lattice constants, “AlAs ( (Cap layer) ≈GaAs (intermediate layer 104) <InAs (quantum dot 105) ”. For this reason, the AlAs layer 106 grows preferentially on the intermediate layer 104. Therefore, it is considered that the material of the cap layer covering the quantum dots 105 made of InAs preferably has a lattice constant close to that of InAs.

  As a result of intensive studies based on such knowledge, the inventors of the present application have come up with the following aspects of the invention.

  The quantum dot photodetector according to the present invention includes a first intermediate layer, a quantum dot formed on the first intermediate layer, and at least the first intermediate layer among side surfaces of the quantum dot. A first cap layer that covers a portion in contact with the first cap layer; a second cap layer that covers at least a portion of the quantum dot that is not covered by the first cap layer; and the first and second cap layers. To a second intermediate layer covering the quantum dots. The difference in lattice constant between the first cap layer and the first intermediate layer is smaller than the difference in lattice constant between the first cap layer and the quantum dot. Further, the difference in lattice constant between the second cap layer and the first intermediate layer is larger than the difference in lattice constant between the second cap layer and the quantum dots.

  In the method for manufacturing a quantum dot photodetector according to the present invention, quantum dots are formed on the first intermediate layer, and then, at least a portion of the side surface of the quantum dot that is in contact with the first intermediate layer is covered. A first cap layer is formed. Next, a second cap layer that covers at least a portion of the quantum dot that is not covered by the first cap layer is formed. Then, a second intermediate layer covering the quantum dots is formed from above the first and second cap layers. In addition, as a material of the first intermediate layer, quantum dots, first cap layer, second cap layer, and second intermediate layer, a gap between the first cap layer and the first intermediate layer is used. The difference in lattice constant is smaller than the difference in lattice constant between the first cap layer and the quantum dot, and the difference in lattice constant between the second cap layer and the first intermediate layer. However, a material having a larger lattice constant difference between the second cap layer and the quantum dots is used.

  According to the present invention, since the lattice constants of the first cap layer and the second cap layer are appropriately defined, the mixed crystal between the quantum dots and the second intermediate layer can be obtained without disturbing the photocurrent. Can be suppressed. For this reason, high sensitivity can be obtained.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the accompanying drawings. FIG. 1 is a cross-sectional view showing the structure of a quantum dot photodetector (infrared sensor) according to an embodiment of the present invention.

In the present embodiment, for example, a buffer layer 2 made of GaAs, for example, is formed on a substrate 1 made of GaAs having a mirror index of (001) on the surface. A contact layer 3 made of, for example, n-type GaAs is formed on the buffer layer 2. The n-type impurity concentration in the contact layer 3 is, for example, 1 × 10 ¹⁸ cm ⁻³ . Further, an intermediate layer 4 made of intrinsic GaAs is formed on the contact layer 3. The thickness of the intermediate layer 4 is, for example, about 50 nm.

A plurality of quantum dots 5 made of InAs are formed on the intermediate layer 4. For example, the quantum dots 5 have a diameter of about 25 nm and a height of about 4 nm. The quantum dots 5 are dispersed on the intermediate layer 4 at a density of about 5 × 10 ¹⁰ pieces / cm ⁻² . An AlAs layer 6 made of intrinsic AlAs is formed as a first cap layer in the region between the quantum dots 5 on the intermediate layer 4. That is, at least a part in contact with the intermediate layer 4 on the side surface of the quantum dot 5 is covered with the AlAs layer 6. Further, a region of the surface of the intermediate layer 4 where the quantum dots 5 are not formed is covered with the AlAs layer 6. The thickness of the AlAs layer 6 is smaller than the height of the quantum dots 5 and is, for example, about 0.5 nm. Furthermore, an AlSb layer 7 made of intrinsic AlSb is formed on the quantum dots 5 as a second cap layer, following the surface of the quantum dots 5. The thickness of the AlSb layer 7 is, for example, about 0.5 nm. Further, an intermediate layer 4 that covers the AlAs layer 6 and the AlSb layer 7 is formed. The intermediate layer 4 is also made of intrinsic GaAs and has a thickness of, for example, about 50 nm. A light absorption unit layer is composed of the quantum dots 5, the AlAs layer 6, the AlSb layer 7, and the intermediate layer 4 covering them. Further, nine such light absorption unit layers are laminated. Therefore, in the present embodiment, a total of 10 light absorption unit layers are provided. A thin AlAs layer 6 may be interposed between the quantum dots 5 and the AlSb layer 7.

A contact layer 8 made of, for example, n-type GaAs is formed on the uppermost intermediate layer 4. The n-type impurity concentration in the contact layer 8 is, for example, 1 × 10 ¹⁸ cm ⁻³ . Then, the electrode 11 is formed on the contact layer 3, the electrode 12 is formed on the contact layer 8, and a DC voltage is applied between the electrode 11 and the electrode 12.

  In this embodiment, the lattice constants of the substances constituting the intermediate layer 4, the quantum dots 5, the AlAs layer 6 (first cap layer), and the AlSb layer 7 are set to “AlAs (first cap layer)”. ≈GaAs (intermediate layer 4) <InAs (quantum dot 5) <AlSb (second cap layer) ”is established. In other words, “the difference in lattice constant between AlAs (first cap layer) and GaAs (intermediate layer 4) is the lattice constant between AlAs (first cap layer) and InAs (quantum dot 5). The condition that “the difference is smaller than the difference” and “the difference in lattice constant between AlSb (second cap layer) and GaAs (intermediate layer 4) is AlSb (second cap layer) and InAs (quantum dot 5). Is larger than the lattice constant difference between the two. For this reason, the AlAs layer 6 tends to grow on the intermediate layer 4, and the AlSb layer 7 tends to grow on the quantum dots 5. Further, the AlAs layer 6 hardly grows on the quantum dots 5, and the AlSb layer 7 hardly grows on the intermediate layer 4. For this reason, the lower part of the side surface of the quantum dot 5 and the surface of the intermediate layer 4 are covered with the AlAs layer 6, and the upper part of the quantum dot 5 is covered with the AlSb layer 7.

  For this reason, the AlAs layer 6 suppresses the surface diffusion of In which is going to be desorbed from the quantum dots 5 and suppresses mixed crystallization due to this surface diffusion. Moreover, since the thickness (for example, 0.5 nm) of the AlAs layer 6 is lower than the height (for example, 4 nm) of the quantum dot 5 and is, for example, about 1/8 of the height of the quantum dot 5, the thickness of the AlAs layer 6 Almost no inhibition of photocurrent occurs.

  In addition, the AlSb layer 7 also suppresses the mixing of InAs and GaAs. Further, as shown in FIGS. 2 and 3, similar to AlAs, AlSb also has a high potential on the conduction band side with respect to InAs. An enclosed energy band is obtained. Therefore, according to this embodiment, the overlap of wave functions between the ground level and the excited level can be strengthened, and the sensitivity can be improved. Further, as described above, since the photocurrent is hardly inhibited by the AlAs layer 6, the sensitivity is hardly lowered.

  Next, a method for manufacturing the above quantum dot photodetector will be described. 4A to 4H are cross-sectional views illustrating a method of manufacturing a quantum dot photodetection device according to an embodiment of the present invention in the order of steps. Here, a method for manufacturing an imaging device in which a plurality of photodetecting elements are arranged in an array as a quantum dot photodetecting device will be described.

  First, as shown in FIG. 4A, a buffer layer 2 and a contact layer 3 are sequentially formed on a substrate 1. Next, an intermediate layer 4 is formed on the contact layer 3, and a light absorption unit layer including the quantum dots 5, the AlAs layer 6, the AlSb layer 7, and the intermediate layer 4 is further formed on the intermediate layer 4, for example, 10 The light absorption part 21 is formed by layering. Next, the contact layer 8 is formed on the light absorbing portion 21. Each of these semiconductor layers can be formed by, for example, a molecular beam epitaxy (MBE) method or a metal-organic vapor phase epitaxy (MOVPE) method. For example, the temperature of the substrate 1 may be set to 500 ° C. or lower, and molecular beam irradiation of In and As may be performed. Through this process, the quantum dots 5 grow by self-assembly.

  Thereafter, as shown in FIG. 4B, a diffraction grating 61 is formed in the contact layer 8 for each photodetecting element to be formed.

  Subsequently, as shown in FIG. 4C, the contact layer 8 and the light absorbing portion 21 are selectively wet-etched in a mesa shape to expose a part of the contact layer 3.

  Next, as shown in FIG. 4D, a protective film 62 made of, for example, SiON is formed on the entire surface, and a first opening reaching the contact layer 8 for each photodetecting element to be formed in the protective film 62. Forming part. A second opening reaching the contact layer 3 is also formed in the protective film 62. These openings can be formed by dry etching, for example. Next, an ohmic electrode 63 is formed in each opening. In forming the ohmic electrode 63, for example, an AuGe film is formed on the contact layers 3 and 8, and then an Au film is formed thereon. The AuGe film and the Au film can be formed by, for example, a lift-off method.

  Thereafter, as shown in FIG. 4E, a third opening that reaches the contact layer 8 is formed in the protective film 62 so as to be larger than the first opening for each photodetecting element to be formed. In addition, the fourth opening reaching the contact layer 3 is also formed in the protective film 62 larger than the second opening. These openings can be formed by dry etching, for example. Subsequently, a mirror electrode 64 is formed in each opening. In forming the mirror electrode 64, for example, a TiW film is formed on the contact layers 3 and 8 and the ohmic electrode 63, and then an Au film is formed thereon. The TiW film and the Au film can be formed by, for example, a lift-off method.

  Next, as shown in FIG. 4F, separation grooves 65 that separate the light detection elements to be formed from each other are formed. The separation groove 65 is deeper than the bottom surface of the light absorbing portion 21 but shallower than the bottom surface of the contact layer 3. The separation groove 65 can be formed by, for example, dry etching for the protective film 62 and dry etching for the semiconductor layer.

  Next, as shown in FIG. 4G, a protective film 66 made of, for example, SiON is formed on the entire surface, and a fifth opening reaching the mirror electrode 64 for each photodetecting element to be formed in the protective film 66. Form. A sixth opening reaching the mirror electrode 64 on the contact layer 3 is also formed in the protective film 66. Thereafter, a base electrode 67 is formed in each opening. In forming the base electrode 67, for example, a Ti film is formed on the mirror electrode 64, and then a Pt film is formed thereon. The Ti film and the Pt film can be formed by, for example, a lift-off method.

  Subsequently, as shown in FIG. 4H, a bump electrode 68 made of, for example, In is formed on the base electrode 67. The bump electrode can be formed by, for example, a lift-off method.

  In the quantum dot photodetection device thus formed, the electrode on the contact layer 8 is used for each photodetection element, and the electrode on the contact layer 3 is used as a common element for each photodetection element. The A readout circuit is connected to each electrode.

  According to such a manufacturing method, as described above, the AlSb layer 7 can form mixed crystals between the quantum dots 5 and the intermediate layer 4 without forming the AlAs layer 6 (first cap layer) thick. Can be suppressed.

In the above embodiment, GaAs is used as the material of the substrate 1, but InP or the like may be used. Further, although InAs is used as the material of the quantum dots 5, InAlAs, InGaAs, or the like may be used. Further, the materials of the intermediate layer 4, the first cap layer, and the second cap layer are not limited to GaAs, AlAs, and AlSb, respectively. For example, InAs, GaAs, AlAs, InP, GaP, AlP, InSb, GaSb, AlSb, InN, GaN, or AlN may be used, or a mixed crystal thereof may be used. When the material of the quantum dots 5 is InGaAs or InAlAs, for example, the material of the first cap layer is AlGaAs and the material of the second cap layer is AlGaSb. However, no matter which material is used, between the lattice constants of the materials constituting each layer,
(1) The condition that “the difference in lattice constant between the first cap layer and the intermediate layer is smaller than the difference in lattice constant between the first cap layer and the quantum dot”, and (2) “ The condition that the difference in lattice constant between the second cap layer and the intermediate layer is larger than the difference in lattice constant between the second cap layer and the quantum dot needs to be satisfied.

  Further, it is preferable that the condition that “the potential with respect to charges (electrons or holes) is larger than that of the intermediate layer in both the first and second cap layers” is also satisfied. This is to obtain a sufficient quantum effect.

  Hereinafter, various aspects of the present invention will be collectively described as supplementary notes.

(Appendix 1)
A first intermediate layer;
Quantum dots formed on the first intermediate layer;
A first cap layer that covers at least a portion of the side surface of the quantum dot that is in contact with the first intermediate layer;
A second cap layer that covers at least a portion of the quantum dot that is not covered by the first cap layer;
A second intermediate layer covering the quantum dots from above the first and second cap layers;
Have
A difference in lattice constant between the first cap layer and the first intermediate layer is smaller than a difference in lattice constant between the first cap layer and the quantum dots;
The quantum constant difference between the second cap layer and the first intermediate layer is larger than the lattice constant difference between the second cap layer and the quantum dot. Dot-type photo detector.

(Appendix 2)
2. The quantum dot photodetector according to claim 1, wherein the first and second cap layers have a charge potential with respect to that of the first and second intermediate layers.

(Appendix 3)
The quantum dot photodetector according to appendix 1 or 2, wherein a thickness of the first cap layer is smaller than a height of the quantum dots.

(Appendix 4)
4. The quantum dot photodetector according to claim 1, wherein a plurality of light absorption unit layers including the quantum dots and the second cap layer are stacked on each other.

(Appendix 5)
The first and second intermediate layers are one type of semiconductor selected from the group consisting of InAs, GaAs, AlAs, InP, GaP, AlP, InSb, GaSb, AlSb, InN, GaN, and AlN, or the group The quantum dot photodetector according to any one of appendices 1 to 4, wherein the quantum dot photodetector is composed of a mixed crystal of two or more semiconductors selected from the group consisting of:

(Appendix 6)
The quantum dot is made of InGaAs or InAlAs,
The first cap layer is made of AlGaAs,
The quantum dot photodetector according to appendix 5, wherein the second cap layer is made of AlGaSb.

(Appendix 7)
The quantum dot type according to any one of appendices 1 to 6, wherein the first cap layer covers a region of the surface of the first intermediate layer where the quantum dots are not formed. Photo detector.

(Appendix 8)
Forming quantum dots on the first intermediate layer;
Forming a first cap layer that covers at least a portion of the side surface of the quantum dot that is in contact with the first intermediate layer;
Forming a second cap layer that covers at least a portion of the quantum dot that is not covered by the first cap layer;
Forming a second intermediate layer covering the quantum dots from above the first and second cap layers;
Have
As materials for the first intermediate layer, quantum dots, first cap layer, second cap layer, and second intermediate layer,
The difference in lattice constant between the first cap layer and the first intermediate layer is smaller than the difference in lattice constant between the first cap layer and the quantum dots;
The difference in lattice constant between the second cap layer and the first intermediate layer is larger than the difference in lattice constant between the second cap layer and the quantum dots. A method for producing a featured quantum dot photodetector.

(Appendix 9)
As materials for the first intermediate layer, the first cap layer, the second cap layer, and the second intermediate layer,
The quantum dot photodetector according to appendix 8, wherein the first and second cap layers have a potential with respect to electric charges larger than that of the first and second intermediate layers. Production method.

(Appendix 10)
The method of manufacturing a quantum dot photodetector according to appendix 8 or 9, wherein a thickness of the first cap layer is made smaller than a height of the quantum dot.

(Appendix 11)
11. The method of manufacturing a quantum dot photodetector according to any one of appendices 8 to 10, wherein a plurality of light absorption unit layers including the quantum dots and the second cap layer are stacked on each other.

(Appendix 12)
As the material of the first and second intermediate layers, one kind of semiconductor selected from the group consisting of InAs, GaAs, AlAs, InP, GaP, AlP, InSb, GaSb, AlSb, InN, GaN, and AlN, or 12. The method for manufacturing a quantum dot photodetector according to any one of appendices 8 to 11, wherein a mixed crystal of two or more semiconductors selected from the group is used.

(Appendix 13)
As the material of the quantum dot, InGaAs or InAlAs is used,
As the material of the first cap layer, AlGaAs is used,
The method of manufacturing a quantum dot photodetector according to appendix 12, wherein AlGaSb is used as a material of the second cap layer.

(Appendix 14)
The quantum according to any one of appendices 8 to 13, wherein the first cap layer is formed so as to cover a region where the quantum dots are not formed on the surface of the first intermediate layer. Manufacturing method of dot-type photodetector.

It is sectional drawing which shows the structure of the quantum dot type | mold photodetector (infrared sensor) which concerns on embodiment of this invention. It is a band diagram which shows the energy structure in embodiment of this invention. It is a figure which shows the wave function in the potential well in embodiment of this invention. It is sectional drawing which shows the method of manufacturing the quantum dot type photodetector which concerns on embodiment of this invention. It is sectional drawing which shows the method of manufacturing the quantum dot photodetector which concerns on FIG. 4A. It is sectional drawing which shows the method of manufacturing the quantum dot photodetector which concerns on FIG. 4B. It is sectional drawing which shows the method of manufacturing the quantum dot photodetector which concerns on FIG. 4C. It is sectional drawing which shows the method of manufacturing the quantum dot photodetector which concerns on FIG. 4D. It is sectional drawing which shows the method of manufacturing the quantum dot type photodetector which concerns on FIG. 4E. It is sectional drawing which shows the method of manufacturing the quantum dot photodetector which concerns on FIG. 4F. It is sectional drawing which shows the method of manufacturing the quantum dot photodetector which concerns on FIG. 4G. It is sectional drawing which shows the structure of the conventional quantum dot type | mold photodetector. It is a band diagram which shows the energy structure in the conventional quantum dot type | mold photodetector. It is a figure which shows the wave function in the potential well in the conventional quantum dot type | mold photodetector. 3 is a cross-sectional view showing the structure of a quantum dot photodetector having an AlSb layer 106. FIG. It is a figure which shows the wave function in the potential well in the quantum dot type | mold photodetector which has the AlSb layer 106. FIG. It is sectional drawing which shows the fault of the quantum dot type | mold photodetector which has the AlSb layer 106. FIG.

Explanation of symbols

1: Substrate 2: Buffer layer 3, 8: Contact layer 4: Intermediate layer 5: Quantum dot 6: AlAs layer 7: AlSb layer 11, 12: Electrode 21: Light absorption part

Claims (6)

A first intermediate layer;
Quantum dots formed on the first intermediate layer;
A first cap layer that covers at least a portion of the side surface of the quantum dot that is in contact with the first intermediate layer;
A second cap layer that covers at least a portion of the quantum dot that is not covered by the first cap layer;
A second intermediate layer covering the quantum dots from above the first and second cap layers;
Have
A difference in lattice constant between the first cap layer and the first intermediate layer is smaller than a difference in lattice constant between the first cap layer and the quantum dots;
The quantum constant difference between the second cap layer and the first intermediate layer is larger than the lattice constant difference between the second cap layer and the quantum dot. Dot-type photo detector.   2. The quantum dot photodetector according to claim 1, wherein a potential with respect to electric charges of the first and second cap layers is larger than that of the first and second intermediate layers.   The quantum dot photodetector according to claim 1, wherein a thickness of the first cap layer is smaller than a height of the quantum dots. Forming quantum dots on the first intermediate layer;
Forming a first cap layer that covers at least a portion of the side surface of the quantum dot that is in contact with the first intermediate layer;
Forming a second cap layer that covers at least a portion of the quantum dot that is not covered by the first cap layer;
Forming a second intermediate layer covering the quantum dots from above the first and second cap layers;
Have
As materials for the first intermediate layer, quantum dots, first cap layer, second cap layer, and second intermediate layer,
The difference in lattice constant between the first cap layer and the first intermediate layer is smaller than the difference in lattice constant between the first cap layer and the quantum dots;
The difference in lattice constant between the second cap layer and the first intermediate layer is larger than the difference in lattice constant between the second cap layer and the quantum dots. A method for producing a featured quantum dot photodetector. As materials for the first intermediate layer, the first cap layer, the second cap layer, and the second intermediate layer,
5. The quantum dot photodetector according to claim 4, wherein the first and second cap layers have a potential with respect to electric charges larger than that of the first and second intermediate layers. Manufacturing method.   6. The method of manufacturing a quantum dot photodetector according to claim 4, wherein a thickness of the first cap layer is made smaller than a height of the quantum dots.

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