JP2012019083A - Quantum dot type infrared ray detection element, and quantum dot type infrared ray imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a quantum dot type infrared ray detection element and a quantum dot type infrared ray imaging apparatus to increase a photocurrent under a restriction of the number of quantum dot layers.SOLUTION: Spacer layers are provided between a quantum dot lamination structure and upper and lower electrode formation layers respectively. In the quantum dot lamination structure, a plurality of lamination structures consisting of a quantum dot layer, which consists of a plurality of quantum dots, and an interlayer, which sandwiches the quantum dot layer and has a bandgap wider than that of the quantum dot, are laminated.

Description

  The present invention relates to a quantum dot infrared detector and a quantum dot infrared imaging device having a quantum dot structure as an infrared detector.

  In recent years, an infrared detection element using a quantum structure has been used as an infrared detection element for detecting infrared rays in the 10 μm band. Among them, a quantum dot infrared detector (QDIP) that can detect infrared rays incident vertically is attracting attention.

  In this QDIP, electrons located in a quantum level on the conduction band side in an InAs quantum dot sandwiched between intermediate layers made of an AlGaAs layer are excited by infrared rays incident from the outside, and a photocurrent induced by the electrons is generated. Infrared light is detected by capturing.

  For example, since the quantum dots are formed on a GaAs substrate in a molecular beam epitaxial apparatus, they are formed as quantum dot layers in a plane perpendicular to the growth direction. Usually, a quantum dot laminated structure in which a plurality of these are laminated is sandwiched between two n-type GaAs electrode layers, and electrons excited by infrared rays are separated from the quantum dot laminated structure to reach the electrode layer, thereby inducing a photocurrent.

  FIG. 8 is a conceptual cross-sectional view of a conventional QDIP. For example, a quantum dot layer composed of a plurality of quantum dots 52 composed of InAs is included in an intermediate layer 53 composed of AlGaAs having a larger band gap than the quantum dot layer. And the electrode forming layers 54 and 55 made of n-type GaAs formed above and below the quantum dot stacked structure 51.

  In addition, electrodes are formed on the electrode forming layers 54 and 55, respectively, and when detecting infrared rays, a voltage is applied to the quantum dot stacked structure 51 by a power source via the electrode forming layers 54 and 55.

  FIG. 9 is an explanatory diagram of the principle of infrared detection, FIG. 9 (a) shows the dark current when no infrared light is incident, and FIG. 9 (b) shows the photocurrent when infrared light is incident. . First, as shown in FIG. 9A, electrons 57 are supplied to the conduction band side quantum level 56 of the quantum dot 52 from the electrode formation layers 54 and 55 and the like. As a result, in the quantum dot laminated structure 51, the average space charge is negative, and the conduction band shape is convex upward.

  The highest point at the lower end of the conduction band of the quantum dot stacked structure 51 becomes a barrier for the electrons 58 located in the electrode forming layer 54 on the cathode side, but the electrons 59 having energy higher than this barrier pass through the barrier and pass through the other anode. It reaches the electrode forming layer 55 on the side and becomes a dark current flowing between the electrode forming layers.

  As shown in FIG. 9B, when the infrared ray 60 is incident on the excited electron 57 and leaves the quantum dot stacked structure 51 and reaches the electrode forming layer 55, the quantum dot stacked structure 51 has an average space charge. Tilt positive by one electron.

  As a result, the conduction band shape becomes less convex so that the barrier becomes lower, and the number of electrons 59 and 61 that can pass therethrough increases. As a result, the current increases, and this increased current is reflected by light. It will be induced as an electric current.

  Since the performance of infrared detection elements such as QDIP is generally determined by the photocurrent, increasing the photocurrent is important for realizing a high-performance infrared detection element. Therefore, as a method of increasing the photocurrent, a method of increasing the number of quantum dot layers is often used (see, for example, Patent Document 1).

JP 2008-198677 A

  However, since the quantum dots are formed using a material that is not lattice-matched to the substrate, accumulation of strain occurs as the number of layers increases. Accumulation of strain causes undesired formation of quantum dots, generation of dislocation defects, and the like. Therefore, there is a problem that not only increase in photocurrent can be expected, but also performance deterioration due to increase in dark current occurs. This limits the upper limit of the number of layers to some extent.

  Accordingly, an object of the present invention is to increase the photocurrent within the constraint that the number of quantum dot layers is limited.

  From one disclosed aspect, a quantum dot stacked structure in which a plurality of stacked structures including a quantum dot layer composed of a plurality of quantum dots and a quantum dot layer sandwiched between the quantum dots and an intermediate layer having a wider band gap than the quantum dots, There is provided a quantum dot infrared detecting element having upper and lower spacer layers sandwiching a quantum dot stacked structure and upper and lower electrode forming layers provided outside the upper and lower spacer layers.

  Further, from another disclosed aspect, a quantum dot stacked structure in which a plurality of stacked structures including a quantum dot layer composed of a plurality of quantum dots and an intermediate layer having a wider band gap than the quantum dots are sandwiched between the quantum dot layers And a quantum dot infrared detecting element having upper and lower spacer layers sandwiching the quantum dot stacked structure and upper and lower electrode forming layers provided outside the upper and lower spacer layers, and forming each quantum dot Quantum dot infrared detector provided with a protruding electrode on the electrode forming layer on the anode side of the infrared detecting element, and a semiconductor integrated circuit having a connection pad at a position corresponding to each protruding electrode and a signal processing circuit A quantum dot infrared imaging device having a circuit device is provided.

  According to the disclosed quantum dot infrared detection element and quantum dot infrared imaging device, spacer layers are provided on both sides of the quantum dot stacked structure, and the fluctuation of the height of the barrier against electrons due to the quantum dot stacked structure is increased. As a result, even if the number of quantum dot layers is the same, it is possible to increase the photocurrent flowing from the electrode formation layer by being induced by the photocurrent caused by the electrons excited by infrared rays.

It is a conceptual sectional view of QDIP of an embodiment of the invention. It is explanatory drawing of the increase in the barrier fluctuation | variation in embodiment of this invention. It is explanatory drawing of the thickness dependence of the spacer layer thickness / quantum dot laminated structure of a photocurrent. It is explanatory drawing of the manufacturing process to the middle of the quantum dot type | mold infrared detection element of Example 1 of this invention. It is explanatory drawing of the manufacturing process to the middle after FIG. 4 of the quantum dot type | mold infrared detection element of Example 1 of this invention. It is explanatory drawing of the manufacturing process after FIG. 5 of the quantum dot infrared rays detection element of Example 1 of this invention. It is a notional perspective view of the quantum dot type infrared imaging device of Example 2 of the present invention. It is a conceptual sectional view of conventional QDIP. It is explanatory drawing of an infrared detection principle.

  Here, with reference to FIG. 1 thru | or FIG. 3, QDIP of embodiment of this invention is demonstrated. FIG. 1 is a conceptual cross-sectional view of a QDIP according to an embodiment of the present invention. QDIP includes a quantum dot layer structure 1 composed of a quantum dot layer composed of a plurality of quantum dots 2 and an intermediate layer 3 sandwiching the quantum dot layer, and spacer layers 4, 5 formed above and below the quantum dot layer structure 1. The electrode forming layers 6 and 7 are provided above and below the spacer layers 4 and 5, respectively.

The quantum dots 2 are made of, for example, In _1-x Ga _x As (0 ≦ x <1), and the intermediate layer 3 is made of Al having a wider band gap than the semiconductor constituting the quantum dots 2 and having a thickness of 30 nm or more. _y Ga _1-y As consists (0 ≦ y ≦ 1). The spacer layers 4 and 5 are made of A _z Ga _1-z As (0 ≦ z ≦ 1), and typically z = y. The electrode forming layers 6 and 7 are made of, for example, an n-type GaAs layer, and each electrode forming layer 6 and 7 is provided with, for example, an electrode having an AuGe / Au laminated structure.

  FIG. 2 is an explanatory diagram of an increase in barrier fluctuation in the embodiment of the present invention. FIG. 2A is an explanatory diagram of the barrier fluctuation in the conventional QDIP without the spacer layer, and the fluctuation of the potential barrier due to the escape of the photoexcited electrons is ΔE1.

On the other hand, as shown in FIG. 2B, when the spacer layer is provided, the fluctuation of the potential barrier due to the escape of the photoexcited electrons is caused by the spacer layer 4 in addition to the fluctuation ΔE ₁ in the quantum dot stacked structure 1. also occur variations Delta] E ₂ in 5.

Therefore, as shown in FIG. 2C, the fluctuation of the electron potential barrier seen from the electrode forming layers 6 and 7 is ΔE ₁ + ΔE ₂ , and the fluctuation ΔE ₁ when the spacer layers 4 and 5 are not provided is ΔE ₁ . Increase by _two . As a result, the photocurrent induced by being dragged by the photocurrent due to the escape of the photoexcited electrons shown in FIG. 9B increases.

FIG. 3 is an explanatory diagram of the dependence of the photocurrent on the thickness of the spacer layer / the thickness of the quantum dot stacked structure. Here, the intermediate layer 3 is formed of Al _0.2 Ga _0.8 As and the spacer Layers 4 and 5 are also formed of Al _0.2 Ga _0.8 As.

  In FIG. 3, the photocurrent is increased in the cases (b) and (c) where the spacer layers 4 and 5 are provided, as compared with the conventional example shown in FIG. 3A where the spacer layers 4 and 5 are not provided. . Therefore, within the range confirmed by the experiment, if the ratio of the thickness of the spacer layers 4 and 5 to the thickness of the quantum dot stacked structure is set to 0.5 to 1.4, the photocurrent can be reliably increased. Become.

  On the other hand, in the experimental example in which the ratio of the thickness of the spacer layers 4 and 5 to the thickness of the quantum dot laminated structure is 1.0 and the thickness of the intermediate layer is 20 nm, the photocurrent is higher than that of the conventional example of (a). The result was decreased. In general quantum dot growth, if the thickness of the intermediate layer is less than 30 nm, it becomes impossible to grow a desired quantum dot due to the influence of the lower quantum dot layer. Is thought to have been reduced.

  On the other hand, since this effect is not seen in the experimental example in which the thickness of the intermediate layer is 30 nm or more, in addition to the ratio of the spacer layer thickness to the thickness of the quantum dot stacked structure, The thickness of 30 nm or more makes the present invention effective. In addition, although the quantum dot layers at the upper and lower ends are sandwiched between the spacer layer and the intermediate layer, the thickness of the “quantum dot stacked structure” in the present invention is effectively the total thickness of the intermediate layers.

Based on the above, the quantum dot infrared detection element of Example 1 of the present invention will be described next with reference to FIGS. First, as shown in FIG. 4A, the thickness of the lower electrode formation layer is set to, for example, 1 μm on the semi-insulating GaAs substrate 11 at a substrate temperature of, eg, 600 ° C. by, for example, molecular beam epitaxy. An n-type GaAs layer 12 is grown. In this case, for example, Si is used as the n-type dopant, and its concentration is set to 2 × 10 ¹⁸ cm ⁻³ , for example.

Subsequently, as shown in FIG. 4B, an i-type Al _0.2 Ga _0.8 As spacer layer having a thickness of, for example, 140 nm and an Al composition ratio of 0.2 on the n-type GaAs layer 12. Grow 13

  Next, after the substrate temperature is lowered from 600 ° C. to a temperature at which self-organization formation can occur, for example, 500 ° C., as shown in FIG. 4C, the growth rate is set to 0.2 atomic layer / second, for example. InAs quantum dots 14 are formed by supplying 2.0 atomic layers. At this time, in the process of supplying InAs, compressive strain applied to InAs increases by supplying a certain amount, InAs grows three-dimensionally, and InAs quantum dots 14 are self-organized and formed. The aggregate of dots 14 becomes a quantum dot layer.

Next, as shown in FIG. 4D, for example, an i-type Al _0.2 Ga _0.8 As intermediate layer 15 having an Al composition ratio of 0.2 and a thickness of 30 nm is grown.

Next, as shown in FIG. 5E, the process of forming InAs quantum dots 14 and the process of forming the i-type Al _0.2 Ga _0.8 As intermediate layer 15 are repeated alternately, for example, 8 times. As a result, a quantum dot stacked structure 16 having nine quantum dot layers is formed.

Subsequently, as shown in FIG. 5 (f), while the substrate temperature is raised from 500 ° C. to 600 ° C., the thickness on the quantum dot stacked structure 16 is, for example, 140 nm and the Al composition ratio is 0.2. The i-type Al _0.2 Ga _0.8 As spacer layer 17 is grown.

Subsequently, as shown in FIG. 6G, with the substrate temperature maintained at 600 ° C., the thickness of the upper electrode formation layer is 1 μm, for example, and 2 × 10 ¹⁸ cm ⁻³ of Si-doped An n-type GaAs layer 18 is formed.

  Next, as shown in FIG. 6H, the element part is dug down to a predetermined size by a standard lithography process and a dry etching process to expose the n-type GaAs layer 12 serving as a lower electrode formation layer.

  Next, an AuGe layer and an Au layer are sequentially deposited on the exposed portion of the n-type GaAs layer 12 and the surface of the n-type GaAs layer 18 by a standard metal vapor deposition method to form a cathode 19 and an anode 20 having an AuGe / Au structure. By doing so, the basic structure of the quantum dot infrared detection element is completed.

  The cathode 19 and the anode 20 are connected to a CMOS circuit or the like, a potential difference is applied between the n-type GaAs layer 12 and the n-type GaAs layer 18, and the current flowing between them is measured by the detector 21. The photocurrent flowing as a response of the quantum dot can be observed. Note that the power source 22 in FIG. 6H is an equivalent power source such as a CMOS circuit.

In Example 1 of the present invention, i-type Al _0.2 Ga _0.8 As spacer layers 13 and 17 are formed above and below a quantum dot stacked structure 16 having an intermediate layer with a thickness of 30 nm, and a quantum dot stacked structure is formed. Since the ratio of the thickness of the i-type Al _0.2 Ga _0.8 As spacer layers 13 and 17 to the thickness of 16 (270 nm) is about 0.5 (140/270), the layers of the same quantum dot layer Higher photocurrent can be obtained compared to several QDIPs.

  Next, the quantum dot infrared imaging device according to the second embodiment of the present invention will be described with reference to FIG. 7, but the basic manufacturing process is exactly the same as the first embodiment, and the manufacturing process is illustrated. Is omitted. FIG. 7 is a conceptual perspective view of a quantum dot infrared imaging device according to Embodiment 2 of the present invention. A quantum dot infrared detector 31 is flip-chip bonded onto a Si integrated circuit device 32 having a signal processing circuit through bumps 33 with the semi-insulating GaAs substrate side facing up.

  In this case, the quantum dot infrared detector 31 separates the elements in a two-dimensional matrix array in the step of FIG. 6H in the first embodiment, and forms a common electrode on the n-type GaAs layer 12 on the cathode side. In addition, an individual electrode is provided on the n-type GaAs layer 18 on the anode side.

  The bumps 33 are provided so as to be connected to individual electrodes provided on the n-type GaAs layer 18 on the anode side. In this case, the number of pixels is arbitrary, but for example, the number of pixels is several hundreds × several hundreds or more.

  Thus, in Example 2 of the present invention, since the spacer layers are formed above and below the quantum dot stacked structure, it is possible to obtain a high photocurrent and to increase the sensitivity of the infrared imaging device. Become.

  The embodiments of the present invention have been described above, but the present invention is not limited to the conditions shown in the embodiments. For example, although the molecular beam epitaxial method is used as the growth method, the method is not limited to the molecular beam epitaxial method, and for example, an MOCVD method (metal organic chemical vapor deposition method) may be used.

  In the above embodiment, the intermediate layer and the spacer layer are formed of AlGaAs having the same composition ratio. However, the composition ratio is not necessarily the same. For example, the Al composition ratio of the spacer layer is set to the intermediate layer. The Al composition ratio may be lower. In this case, the potential barrier due to the spacer layer as seen from the electrons present in the electrode formation layer is lowered.

DESCRIPTION OF SYMBOLS 1 Quantum dot laminated structure 2 Quantum dot 3 Intermediate layer 4, 5 Spacer layer 6, 7 Electrode formation layer 11 Semi-insulating GaAs substrate 12 n-type GaAs layer 13 i-type Al _0.2 Ga _0.8 As spacer layer 14 InAs quantum Dot 15 i-type Al _0.2 Ga _0.8 As intermediate layer 16 quantum dot layered structure 17 i-type Al _0.2 Ga _0.8 As spacer layer 18 n-type GaAs layer 19 cathode 20 anode 21 detector 22 power supply 31 quantum Dot type infrared detector 32 Si integrated circuit device 33 Bump 51 Quantum dot laminated structure 52 Quantum dot 53 Intermediate layer 54, 55 Electrode forming layer 56 Conduction band side quantum levels 57, 58, 59, 61 Electron 60 Infrared

Claims (5)

  A quantum dot stacked structure in which a plurality of stacked structures including a quantum dot layer composed of a plurality of quantum dots and the quantum dot layer and an intermediate layer having a wider band gap than the quantum dots are stacked, and upper and lower layers sandwiching the quantum dot stacked structure A quantum dot infrared detecting element having a spacer layer and upper and lower electrode forming layers provided outside the upper and lower spacer layers.   The quantum dot infrared detection element according to claim 1, wherein the spacer layer is a layer made of the same material as the intermediate layer.   The ratio of the thickness of each of the upper and lower spacer layers to the thickness of the quantum dot stacked structure is 0.5 to 1.4, and the thickness of the intermediate layer is 30 nm or more. Item 3. The quantum dot infrared detection element according to Item 2. The quantum dots are made of In _1-x Ga _x As (0 ≦ x <1), the intermediate layer is made of Al _y Ga _1-y As (0 ≦ y ≦ 1), and the spacer layer is made of Al _1. The quantum dot infrared detection element according to any one of claims 1 to 3, wherein the quantum dot infrared detection element is made of _{-zGa1 -} zAs (0≤z≤1). A quantum dot stacked structure in which a plurality of stacked structures including a quantum dot layer composed of a plurality of quantum dots and the quantum dot layer and an intermediate layer having a wider band gap than the quantum dots are stacked, and upper and lower layers sandwiching the quantum dot stacked structure A quantum dot infrared detecting element having a spacer layer and upper and lower electrode forming layers provided outside the upper and lower spacer layers is formed in a matrix, and an electrode forming layer on the anode side of each quantum dot infrared detecting element Quantum dot infrared imaging device having a quantum dot infrared detector provided with a protruding electrode and a semiconductor integrated circuit device having a connection pad at a position corresponding to each protruding electrode and having a signal processing circuit .

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