JP5217140B2 - Optical semiconductor device - Google Patents

Description

  The present invention relates to an optical semiconductor device having a quantum structure for confining electrons and discretizing the density of states of electrons, and is particularly suitable for application to an infrared detector.

  In recent years, it has been noted that a quantum dot structure can be easily realized by a growth mode called a Stranski-Krastanov (SK) mode, and development of an optical semiconductor device using the quantum dot structure has become active. For example, examples of materials for quantum dots that can be grown on a GaAs substrate include InAs, GaInAs, GaInNAs, and GaSb.

  Recently, a quantum dot infrared detector (QDIP: Quantum Dot Infrared Photo-detector) has attracted attention as a new infrared detector. Since QDIP uses quantum dots as an infrared absorbing material, high sensitivity characteristics are obtained by light absorption in all three dimensions due to the three-dimensional confinement effect of electrons by the quantum dots. Therefore, infrared light incident perpendicularly to the infrared absorption layer can be detected without using an optical coupler or the like, and there is an advantage that the manufacturing process can be simplified. In addition, since discrete energy levels are formed by the quantum dots, dark current excited from the quantum dots due to thermal excitation can be suppressed. Therefore, it attracts attention as an infrared detector that operates by electronic cooling (for example, see Non-Patent Document 1).

  QDIP has a structure in which both sides of an infrared absorption layer are sandwiched between electrode layers. The infrared absorption layer is configured by stacking a plurality of quantum structure layers that cover a plurality of arranged quantum dots with an intermediate layer. The intermediate layer is for recovering strain introduced into the crystal by the growth of quantum dots.

FIG. 7 shows the basic principle of infrared absorption in QDIP.
When an infrared wavelength corresponding to the energy difference between the ground level of the quantum dot conduction band and the excitation level of the quantum dot is incident on this QDIP, the ground level of the quantum dot conduction band changes to the excitation level of the quantum dot. Electrons are transitioned. This excited level is at an energy level lower than the potential of the intermediate layer and a barrier exists, but electrons flow to the continuous band by tunneling through this barrier portion. The QDIP functions as an infrared detector when the electrons reach the electrode and are detected as a current.

An example of a typical device structure of QDIP is described in Non-Patent Document 2. In this document, a buffer layer is grown on a GaAs substrate, and a lower electrode layer, an infrared absorption layer, and an upper electrode layer are grown in this order. As the infrared absorbing layer, InAs is used as a material, 2.0 ML is grown at a growth rate of 0.22 ML / sec. Here, 20 ml of Ga _0.85 In _0.15 As is grown to fill the quantum dots, and 180 ml of GaAs is grown on the intermediate layer. This structure is repeated a plurality of times to form the basic structure of QDIP.

K.W.Berryman, S.A.Lyon, and Mordechai Segav. Appl. Phys. Lett. 70, 1861 (1997). Zhengmao Ye, Joe C. Campbell, Zhonghui Chen, Eui-Tae Kim, and Anupam Madhukar; J. Appl. Phys. Vol. 92. 7462 (2002) E. Luna, J. L. Sanchez-Rojas, A. Guzman, J. M. G. Tijero, and E. Munoz,; IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 1, JANUARY 2003

  While QDIP has the above advantages, the energy barrier (tunnel film thickness) that exists between the quantum dots and the intermediate layer is relatively large, so the probability that electrons excited by infrared rays tunnel to the continuous band is low. The electrons cannot reach the electrodes. Therefore, there is a problem that it is difficult to realize a high-sensitivity infrared detector with a small photocurrent.

  In this respect, a quantum well infrared detector (QWIP) has been studied as a conventional infrared detector having a quantum well structure in which the direction of electron confinement is one-dimensional. This detector has a problem of low sensitivity. However, in order to solve this problem, Non-Patent Document 3 discloses a method of delta-doping impurities.

  However, in QWIP, the manufacturing process is relatively complicated, and dark current cannot be sufficiently suppressed as in QDIP, and it must be said that it is insufficient as an infrared detector. Further, Non-Patent Document 3 is limited to a qualitative description relating to the delta doping of impurities, and in this description, it is extremely difficult to apply to an actual infrared detector.

  The present invention has been made in view of the above problems, and provides an optical semiconductor device and a method for manufacturing the same that can simplify the manufacturing process, sufficiently suppress dark current, and realize high-sensitivity light detection. The purpose is to provide.

The optical semiconductor device of the present invention includes a substrate, a plurality of electrode layers formed above the substrate, and a light absorption layer having a function of absorbing light when a voltage is applied from the electrode layer. The absorption layer is formed by stacking a plurality of quantum structure layers that are made of quantum wires or quantum dots and directly cover with an intermediate layer a quantum structure that confines electrons and discretizes the density of states of electrons, and includes at least one quantum structure. layer is to have a impurity layer N-type impurities are introduced into the intermediate layer on the side a higher potential than the quantum structure while applying a voltage to the light absorbing layer, the upper surface of the quantum structure And the lower surface of the impurity layer is within 10 nm .

  A method for manufacturing an optical semiconductor device of the present invention includes a substrate, a plurality of electrode layers formed above the substrate, and a light absorption layer having a function of absorbing light by applying a voltage from the electrode layer. In manufacturing an optical semiconductor device, the light absorption layer is formed by stacking a plurality of quantum structure layers that cover an electron and confine electrons and discretize the density of states of electrons with an intermediate layer, and at least one quantum structure layer is formed. Regarding the structure layer, an N-type impurity is introduced into the intermediate layer on the side having a higher potential than the quantum structure in a state where a voltage is applied to the light absorption layer to form an impurity layer.

  According to the present invention, the manufacturing process is simple, dark current can be sufficiently suppressed, and high-sensitivity light detection can be realized.

-Basic outline of the present invention-
As a result of intensive studies to reduce the energy barrier (tunnel film thickness) existing between the quantum dots and the intermediate layer, the present inventor has introduced an N-type impurity into a predetermined portion in the intermediate layer of the light absorption layer. I came up with it.

The light absorption layer in the optical semiconductor device of the present invention is formed by stacking a plurality of quantum dot layers. An impurity layer is formed in at least one of these quantum dot layers.
FIG. 1 shows the basic structure of this quantum dot layer. In addition, in FIG. 1, although the quantum dot of the structure which confines an electron from each three-dimensional direction is illustrated as a quantum structure, the case where a quantum dot is used also as a quantum wire of the structure which confine | seals an electron from each two-dimensional direction Similar effects can be obtained.

  The quantum dot layer 100 is configured to cover a plurality of quantum dots 101 arranged side by side with an intermediate layer 102. Then, an N type is applied to a predetermined portion in the intermediate layer 102, in this case, a predetermined portion in the intermediate layer 102 on the side that is higher in potential (high bias) than the quantum dot 101 in a state where a voltage (bias) is applied to the light absorption layer. An impurity layer 103 into which impurities are introduced is formed.

The impurity layer 103 is formed to have an N-type impurity concentration and thickness that can form a desired tunnel thickness.
The N-type impurity concentration of the impurity layer 103 is set to a value within the range of 1 × 10 ¹⁶ / cm ³ to 1 × 10 ²⁰ / cm ³ . Here, if the value is smaller than 1 × 10 ¹⁶ / cm ³ , it is not considered as an impurity-doped state (in other words, considered as an intrinsic semiconductor), and is insufficient as an impurity addition amount. On the other hand, when the value is larger than 1 × 10 ²⁰ / cm ³ , the impurity is considered to be a part of the material composition rather than being in a doped state of the impurity. The value of the N-type impurity concentration depends on the applied bias and device structure, and is changed according to the design.

  The film thickness of the impurity layer 103 is such that the distance between the upper surface (the top of the quantum dot 101) and the lower surface of the impurity layer 103 is within 10 nm, and the thickness (height) of the quantum dot 101 from the thickness of the quantum dot layer 100. And a value less than the value obtained by subtracting the distance. By setting the separation distance within 10 nm, the electron tunnel probability can be improved. By forming the impurity layer 103 with the thickness, a desired depletion layer is formed in the intermediate layer 102.

In the semiconductor device provided with the quantum dot layer 100 configured as described above, the relationship between the tunnel film thickness and the tunnel probability was examined. The result is shown in FIG.
For the sake of simplicity, FIG. 2 shows a calculation result assuming that a 140 meV rectangular barrier exists between the excitation level and the intermediate layer material. From this calculation result, it is understood that the tunnel probability is increased by about one digit by reducing the tunnel film thickness (film thickness tunneling from the excitation level of the quantum dots to the continuous band) by about 3 nm, for example.

FIG. 3 shows a conduction band structure when an N-type impurity is introduced in the vicinity of a quantum dot.
By introducing the N-type impurity, a depletion layer is formed in the vicinity of the interface between the impurity layer and the intermediate layer. This shows that the tunnel film thickness decreases.

In FIG. 4, the calculation result about the relationship between the position from the top of the quantum dot of an impurity layer and a tunnel film thickness is shown.
In the calculated structure, the N impurity concentration of the impurity layer was 1 × 10 ¹⁸ / cm ³ and the film thickness was 5 nm. In addition, the material of the intermediate layer is i-GaAs, for simplicity, the intermediate layer is a P-type impurity layer, and the P-type impurity concentration is 1 × 10 ¹⁸ / cm ³ . The calculation was performed on the assumption that an internal electric field of 1 × 10 ⁷ V / m was applied to this structure. The height of the barrier from the excitation level of the quantum dot is 140 meV.

  As a result of the depletion layer being formed in the intermediate layer and the band potential changing, the distance from the excitation level of the quantum dot to the continuous band is reduced. As a result of the reduction of the tunnel film thickness due to the effect of the arrangement of the impurity layers, for example, if the tunnel film thickness is reduced by about 3 nm, the tunnel probability increases by about one digit. As a result, the photocurrent increases by an order of magnitude, and the sensitivity of the detector also improves by an order of magnitude.

-Preferred embodiment to which the present invention is applied-
FIG. 5 is a schematic cross-sectional view illustrating the method of manufacturing the quantum dot infrared detector (QDIP) according to the present embodiment in the order of steps. Hereinafter, the configuration of this QDIP will be described together with its manufacturing process.

First, as shown in FIG. 5A, a buffer layer 2 having a thickness of about 500 nm is formed on an insulating GaAs substrate 1 using GaAs or AlGaAs as a material.
Next, a lower electrode layer 3, an infrared absorption layer 4, and an upper electrode layer 5 made of N-type GaAs are sequentially grown on the buffer layer 2. These are formed as epitaxial layers by MBE (molecular beam epitaxy) or MOCVD (metal organic chemical vapor deposition).

The lower electrode layer 3 is for applying a bias to the infrared absorption layer 4, has a thickness of about 1.0 μm, for example, and has an impurity concentration of about 1 × 10 ¹⁸ / cm ³ (for example, an N-type impurity). Si) is included.

The infrared absorption layer 4 has a function of absorbing infrared rays, and the quantum dots 11 that confine electrons in three-dimensional directions and discretize the density of states of electrons recover the distortion of the quantum dots 11. The quantum dot layer 10 covered with the intermediate layer 12 is formed by laminating a plurality of layers, for example, about 10 to 20 layers.

  Here, the quantum dots 11 are uniformly formed with their growth conditions fixed. As the material of the quantum dots 11, InAs is used. This quantum dot 11 is grown by using, for example, molecular beam epitaxy (MBE), 2.0 ML at a substrate temperature of 500 ° C. and a growth rate of 0.22 ML / sec, and self-assembled quantum dots are formed using the SK mode. Formed by law.

FIG. 6 shows how the intermediate layer 12 is formed in the present embodiment.
As shown in FIG. 6A, first, when forming the intermediate layer 12, i-GaAs, which is the same material as the GaAs substrate 1, is used, and first, i-GaAs is grown so that the quantum dots 11 are completely embedded. .

Next, an N-type impurity, here Si, is introduced into the grown i-GaAs to form the impurity layer 13. For example, an impurity layer 13 having an Si impurity concentration of about 1 × 10 ¹⁸ / cm ³ is grown to a thickness of about 5 nm, for example, at a site spaced about 6 nm from the top of the quantum dot 11.

Next, i-GaAs is grown again on the impurity layer 13. At this time, i-GaAs is grown so that the film thickness of the entire intermediate layer 12 is sufficient to sufficiently recover the strain of the quantum dots 11, for example, about 60 nm, and the intermediate layer having the impurity layer 13 therein. 12 is formed.
In the present embodiment, as shown in FIG. 6B, for example, ten layers of the quantum dot layer 10 having the above structure are grown to constitute the infrared absorption layer 4.

  In the above example, the case where the impurity layer 13 is formed in each intermediate layer 12 for all the quantum dot layers 10 constituting the infrared absorption layer 4 has been described. It is not always necessary to form 13. In the present invention, it is essential to form the impurity layer 13 in at least one quantum dot layer 10 according to the device structure or the like.

The upper electrode layer 5 is for applying a bias to the infrared absorption layer 4, has a thickness of, for example, about 1.0 μm, and has an impurity concentration of about 1 × 10 ¹⁸ / cm ³ (for example, an N-type impurity). Si) is included.

  Subsequently, as shown in FIG. 5B, after a resist mask 21 having a predetermined pattern is formed on the upper electrode layer 5 by lithography, the resist mask 21 is used to cover the resist mask 21. The part which is not present is dry etched from the upper electrode layer 5 to the middle of the lower electrode layer 3.

Subsequently, after removing the resist mask 21 by ashing or the like, as shown in FIG. 5C, an electrode material, for example, AuGe / Ag / Au, is deposited on the entire surface by, for example, a vacuum metal deposition method, and by a lift-off method. The lower electrode 6 connected to the lower electrode layer 3 and the upper electrode 7 connected to the upper electrode layer 5 are formed.
As described above, the QDIP according to the present embodiment is completed.

As a material of the intermediate layer 12 in which the quantum dots 11 are embedded, a material having a band gap different from the substrate material (for example, GaInAs) as in Non-Patent Document 2 may be used. In this case, for example, at the interface between the GaInAs layer in which the quantum dots 11 are embedded and the GaAs layer thereabove, for example, the thickness is about 5 nm and the N-type impurity (Si) amount is about 1 × 10 ¹⁸ / cm ^3. 13 can be grown.

In this embodiment, an example in which only i-GaAs is used as the material of the intermediate layer 12 has been described. However, the quantum layer 11 and the intermediate layer 12 in which the quantum dot 11 is embedded may be made P-type to widen the depletion layer. It is effective. For example, the quantum dot 11 is in an intrinsic semiconductor state, and the P-type impurity concentration of the intermediate layer 12 in which the quantum dot 11 is embedded is set to about 1 × 10 ¹⁸ / cm ^3. Impurity layer 13 having a thickness of about 1 × 10 ¹⁸ / cm ³ is grown. Thereafter, the i-GaAs layer is grown to such a thickness that the strain is recovered, so that the film thickness of the intermediate layer 12 is about 60 nm.

  In the present embodiment, the case where the quantum dots 11 made of InAs are formed on the GaAs substrate 1 is exemplified. However, for example, quantum dots made of SiGe are formed on the Si substrate, or InAs or GaInAs are formed on the InP substrate. You may form the quantum dot which becomes.

  As described above, according to the present embodiment, it is possible to provide a QDIP that has a simple manufacturing process, can sufficiently suppress dark current, and realizes highly sensitive infrared detection.

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

(Appendix 1) a substrate;
A plurality of electrode layers formed above the substrate;
A voltage is applied from the electrode layer, and includes a light absorption layer having a function of absorbing light,
The light absorption layer is formed by laminating a plurality of quantum structure layers that cover the quantum structure for confining electrons and discretizing the density of states of electrons with an intermediate layer,
At least one of the quantum structure layers has an impurity layer in which an N-type impurity is introduced into the intermediate layer on a side having a higher potential than the quantum structure in a state where a voltage is applied to the light absorption layer. An optical semiconductor device.

  (Supplementary note 2) The optical semiconductor device according to supplementary note 1, wherein the quantum structure includes a quantum wire or a quantum dot.

  (Additional remark 3) The optical semiconductor device of Additional remark 1 or 2 characterized by the separation distance of the upper surface of the said quantum structure, and the lower surface of the said impurity layer being less than 10 nm.

  (Supplementary note 4) The optical semiconductor device according to supplementary note 3, wherein the thickness of the impurity layer is equal to or less than a value obtained by subtracting the thickness of the quantum structure and the separation distance from the thickness of the quantum structure layer.

(Supplementary Note 5) The concentration of the N-type impurity in the impurity layer, the Appendix 1 to 4, characterized in that it is a value within a range of ^{1 × 10 16 / cm 3 ~1} × 10 20 / cm 3 The optical semiconductor device according to any one of claims.

  (Appendix 6) The optical semiconductor device according to any one of appendices 1 to 5, wherein the optical semiconductor device is an infrared detector that detects and absorbs infrared rays with the light absorption layer.

(Appendix 7) a substrate;
A plurality of electrode layers formed above the substrate;
When manufacturing an optical semiconductor device including a light absorption layer having a function of absorbing light by applying a voltage from the electrode layer,
The light absorption layer is formed by laminating a plurality of quantum structure layers that cover the quantum structure that confines electrons and discretizes the density of states of electrons with an intermediate layer,
For at least one quantum structure layer, an N-type impurity is introduced into the intermediate layer on the side having a higher potential than the quantum structure in a state where a voltage is applied to the light absorption layer, thereby forming an impurity layer. A method for manufacturing an optical semiconductor device.

  (Additional remark 8) The said quantum structure consists of a quantum wire or a quantum dot, The manufacturing method of the optical semiconductor device of Additional remark 7 characterized by the above-mentioned.

  (Supplementary note 9) The manufacturing of an optical semiconductor device according to supplementary note 7 or 8, wherein the impurity layer is formed so that a separation distance between an upper surface of the quantum structure and a lower surface of the impurity layer is within 10 nm. Method.

  (Supplementary note 10) The optical semiconductor according to supplementary note 9, wherein the impurity layer is formed to have a thickness equal to or less than a value obtained by subtracting a thickness of the quantum structure layer and the separation distance from a thickness of the quantum structure layer. Device manufacturing method.

Note the (Supplementary Note 11) the impurity layer, wherein the concentration of the said N-type impurity is formed to a value in the range of ^{1 × 10 16 / cm 3 ~1} × 10 20 / cm 3 7 The manufacturing method of the optical semiconductor device of any one of 10-10.

It is a schematic sectional drawing which shows the basic structure of the quantum dot layer which comprises the light absorption layer in the optical semiconductor device of this invention. In the optical semiconductor device of this invention, it is a characteristic view which shows the relationship between a tunnel film thickness and a tunnel probability. In the optical semiconductor device of this invention, it is a schematic diagram which shows a conduction band structure when an N-type impurity is introduce | transduced to the vicinity of a quantum dot. In the optical semiconductor device of this invention, it is a characteristic view which shows the relationship between the position from the top of the quantum dot of an impurity layer, and a tunnel film thickness. It is a schematic sectional drawing which shows the manufacturing method of the quantum dot type | mold infrared detector (QDIP) by this embodiment to process order. It is a schematic sectional drawing which shows a mode that the intermediate | middle layer of QDIP in this embodiment is formed. It is a schematic diagram which shows the basic principle of the infrared absorption in QDIP.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 GaAs substrate 2 Buffer layer 3 Lower electrode layer 4 Infrared absorption layer 5 Upper electrode layer 6 Lower electrode 7 Upper electrode 10 Quantum dot layer 11 Quantum dot 12 Intermediate layer 13 Impurity layer 21 Resist mask

Claims (3)

A substrate,
A plurality of electrode layers formed above the substrate;
A voltage is applied from the electrode layer, and includes a light absorption layer having a function of absorbing light,
The light absorption layer is formed by stacking a plurality of quantum structure layers that are made of quantum wires or quantum dots and directly cover an intermediate layer with a quantum structure that confines electrons and discretizes the density of states of electrons,
At least one layer wherein the quantum structure layer, and organic impurities layer N-type impurities are introduced into the intermediate layer on the side a higher potential than the quantum structure while applying a voltage to said light absorbing layer And
An optical semiconductor device , wherein a separation distance between an upper surface of the quantum structure and a lower surface of the impurity layer is within 10 nm . The thickness of the impurity layer, an optical semiconductor device according to claim 1, characterized in that the thickness of the quantum structure layer is not more than a value obtained by subtracting the thickness and the distance between the quantum structure. The concentration of the N-type impurity in the impurity layer, the light according to claim 1 or 2, characterized in that it is a value within a range of ^{1 × 10 16 / cm 3 ~1} × 10 20 / cm 3 Semiconductor device.

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