JP2012083238A - Infrared detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an infrared detector that induces surface plasmon to enhance the sensitivity of a wavelength to be detected.SOLUTION: An infrared detector includes a light absorption layer 200 that absorbs infrared using an inter-subband transition of a quantum dot 24 that is laminated in a multi-layer, and a lower electrode 27 and an upper electrode 28 provided so as to interpose the light absorption layer 200. The upper electrode 28 is provided with periodic holes 29 to induce surface plasmon and thereby enhance the sensitivity of a wavelength to be detected.

Description

  The present invention relates to an infrared detection device for detecting infrared rays.

  There is an increasing demand for photodetection technology in the infrared region, including heat detection. Conventionally, infrared light detection semiconductor elements that have been widely used include mercury / cadmium / tellurium (MCT), quantum well (QW) intersubband transitions, and quantum dot (QD) intersubband transitions. The one used is mentioned.

  In the MCT-based material, infrared light is absorbed, and detection is performed by converting a current generated by excitation of electrons in the valence band into a conductor or a voltage. Since light having energy larger than the band gap of the semiconductor is absorbed, in principle, the detector has a wide sensitivity region.

  However, the MCT material is a material system in which crystal growth and its process are very difficult, and there is a problem that the detector becomes very expensive. Further, since it is difficult to increase the area, there is a problem that it is very difficult to manufacture a large-area two-dimensional infrared detector array.

  As a means for solving these problems, light absorption by intersubband transition of electrons or holes having a quantum structure such as a quantum well or a quantum dot has been utilized.

  The quantum confinement structure makes the electrons or electron energy discrete. When the energy difference between the discrete levels coincides with the infrared energy to be detected, electrons or holes are excited from the ground level to the excited level by light absorption, and a current resulting therefrom is detected.

  The advantage over the MCT-based material that is the bulk material of the quantum confinement structure is that the thermal noise is low. This is because the degree of freedom in the direction of movement of carriers is three-dimensional for bulk materials, whereas it is two-dimensional or zero-dimensional for quantum confinement structures.

  Therefore, in the quantum confinement structure, compared with a bulk material such as MCT, noise reduction in the signal-to-noise ratio is realized, and higher detection sensitivity is expected.

  As a technology related to this, there is, for example, Japanese Patent Laid-Open No. 10-326906 (Patent Document 1). Patent Document 1 discloses a light detection element using a quantum structure such as a quantum well or a quantum dot.

JP-A-10-326906

  However, in the case of a quantum well, the intersubband transition absorption efficiency for light perpendicularly incident on the light detection surface, that is, incident light having an electric field component perpendicular to the stacking direction is very low.

  On the other hand, in the case of quantum dots, there is no problem of low light absorption efficiency with respect to normal incidence as in quantum wells. However, the light absorption efficiency is typically several times greater for the electric field component parallel to the stacking direction of the quantum dots than for the electric field component perpendicular to the stacking direction.

  Therefore, a mechanism for efficiently converting incident light having only an electric field component perpendicular to the stacking direction of the quantum structure into an electric field component parallel to the stacking direction is desired.

  An object of the present invention is to provide a technique for solving the above-described problems of the prior art, and to provide an infrared detection device that induces surface plasmons and increases sensitivity at a wavelength to be detected. .

An infrared detection device according to the present invention is
A light-absorbing layer that absorbs infrared rays using intersubband transitions of quantum dots stacked in multiple layers;
A lower electrode and an upper electrode provided so as to sandwich the light absorption layer;
The upper electrode is formed with periodic holes for inducing surface plasmon and increasing sensitivity at a wavelength to be detected.

In addition, another infrared detection device according to the present invention is
A light-absorbing layer that absorbs infrared rays by utilizing intersubband transitions of multilayered quantum wells;
A lower electrode and an upper electrode provided so as to sandwich the light absorption layer;
The upper electrode is formed with periodic holes for inducing surface plasmon and increasing sensitivity at a wavelength to be detected.

  According to the present invention, it is possible to induce surface plasmon and increase sensitivity at a wavelength to be detected.

It is a figure explaining the principle of the infrared absorption by the transition between subbands. It is the cross section and top view of the infrared rays detection apparatus which concern on the 1st Embodiment of this invention. It is explanatory drawing of the sensitivity spectrum of the infrared rays detection apparatus by this invention. It is a figure which shows the example of a calculation of the electric field enhancement spectrum by surface plasmon. It is a conceptual diagram of the electric field intensity distribution of surface plasmon. It is the cross section and top view of the infrared rays detection apparatus which concern on the 2nd Embodiment of this invention.

(Principle of the present invention)
First, the principle of infrared absorption by intersubband transition, which is the premise of the present invention, will be described with reference to FIG. This utilizes light absorption due to intersubband transition of electrons or holes having a quantum structure such as quantum wells and quantum dots. The quantum confinement structure makes the electrons or electron energy discrete. FIG. 1 shows a band structure for electrons.

  As shown in FIG. 1, quantum dots or quantum well layers 12 are formed between the barrier layer 11 and the barrier layer 13. Under such a configuration, when the energy difference between the discrete levels coincides with the infrared energy to be detected, the electrons 16 are excited from the electron ground level 14 to the electron excitation level 15 by light absorption, resulting in this. Detect current.

  Next, embodiments of the present invention will be described in detail with reference to the drawings.

(First embodiment)
First, with reference to FIG. 2, the structure of the infrared detection apparatus according to the first embodiment of the present invention will be described. Here, FIG. 2 shows a sectional view and a top view of the infrared detecting device according to the first embodiment of the present invention.

  First, referring to a cross-sectional view, an i-type buffer layer 21, an n-type lower contact layer 22, and an i-type barrier layer 23 made of the same material as the semi-insulating substrate 20 are formed on the semi-insulating substrate 20. The lower electrode 27 is formed in contact with a part of the n-type lower contact layer 22.

  On the i-type barrier layer 23, quantum dots 24 that absorb infrared rays and i-type barrier layers 25 are alternately formed. In order to increase absorption efficiency with a large number of quantum dots 24, typically, about 10 layers of quantum dots 24 are stacked. The light absorption layer 200 is configured by the quantum dots 24 and the i-type barrier layer 25 stacked in multiple layers. The light absorption layer 200 absorbs infrared rays by utilizing intersubband transition of the quantum dots 24 stacked in multiple layers.

  An n-type upper contact layer 26 is formed on the light absorption layer 200. An upper electrode 28 is formed on the n-type upper contact layer 26.

  Under such a configuration, the photocurrent flowing between the upper electrode 28 and the lower electrode 27 is detected.

  Referring to the top view, the upper electrode 28 has circular holes 29 in a square lattice pattern. It is assumed that infrared rays that enter the circular hole 29 perpendicularly, that is, that propagate in the + z direction are detected. The period of the circular hole 29 is a in both the x and y directions, and the hole radius is r.

  Here, it is essential that the holes are periodically opened, and periodicity such as the shape of the holes and the triangular lattice is not essential.

  Thus, the periodic hole (for example, circular hole 29) for inducing the surface plasmon and increasing the sensitivity at the wavelength to be detected is formed in the upper electrode 28.

(Operational principle of the present invention)
Next, the operation principle of the present invention will be described with reference to FIGS.

  When light having an electric field component in the x or y direction propagates in the + z direction with respect to the structure of the infrared detection device shown in FIG. 2, surface plasmons are induced according to the period a. The surface plasmon has a resonance spectrum reflecting the periodic structure of the metal. The resonance wavelength is given by Equation 1.

  Here, j and k are indices representing the order of diffraction, and each is an integer of 0 or more. However, both are not 0 at the same time. .epsilon.m and .epsilon.d represent the dielectric constants of a metal and a dielectric (here, GaAs), respectively, and each have a real part and an imaginary part. For example, εm = εm ′ + iεm ″ and εd = εd ′ + iεd ″.

  When it is assumed that the metal is a perfect conductor, that is, the real part is small and the imaginary part is very large, and the imaginary part of the dielectric constant of GaAs is not so large, Equation 1 can be approximated by Equation 2.

Here, _{n d} is the real part of the refractive index of the semiconductor GaAs, corresponding to approximately √ (εd '). The λ satisfy this, the lower diffraction orders, 1 × a × _{n d,} 1 / √2 times, 1/2-fold, made 1 / √5 times ... and.

  Since the diffraction efficiency generally decreases as the diffraction order increases, it is desirable to use a low diffraction order.

  FIG. 5 conceptually shows the spatial dependence of the z-direction electric field near the resonance wavelength. The surface plasmon is generated in both directions with respect to the interface between the metal and the dielectric, but the metal side is not shown. Here, 50 indicates incident infrared rays.

  FIG. 3 conceptually shows the sensitivity spectrum of the infrared detector near the resonance wavelength.

(A) shows a sensitivity spectrum determined only by quantum dots. This central wavelength can be selected depending on the size / height of the quantum dot, the material composition, and the like. The wavelength broadening of sensitivity corresponds to a large number of quantum dot energy level variations, that is, nonuniform spread.

(B) has shown the spectrum of the z component of the electric field induced by the surface plasmon by a metal periodic structure. Since the sensitivity of the entire photodetector is a product of (a) and (b), if both wavelengths are matched by an appropriate design, the sensitivity is finally reduced in a narrow band as shown in (c). Is realized.

  A few words about the benefits of narrowing the bandwidth. In the case of infrared detection, a broadband infrared ray due to black body radiation may be a noise factor together with a signal having a wavelength to be detected. By narrowing the band, the detection efficiency of such a wideband noise component decreases, and as a result, the signal-to-noise ratio is improved.

(Calculation of increase effect by surface plasmon)
Subsequently, the electric field due to surface plasmons was numerically calculated to enhance the effect. Assuming light detection in the 10 μm band, the refractive index of the light absorption layer made of GaAs is assumed to be 3.2.

From Equation 2, the period of the holes in the metal was set to a = 3.1 μm (= 10 / 3.2). Further, the radius of the circular hole was set to r = a × 0.3 = 0.93 μm. The thickness of the metal electrode is 90 nm.

  FIG. 4 shows the calculation result of the z component of the electric field in the vicinity of 100 nm from the electrode in the + z direction when white light is incident using the finite difference time domain (FDTD) method.

  In this calculation, the refractive index of gold (Au) used for the electrode is a value derived from the dispersion relation assuming a Drude model.

  When FIG. 4 is seen, it turns out that the z component of an electric field becomes strong at the wavelength of 10.4 micrometer vicinity. The full width at half maximum of this spectrum is about 0.2 μm. Therefore, referring to FIG. 3, a detection device having high detection sensitivity in a narrow band can be obtained.

  Whereas the design by Equation 2 is 10 μm, the FDTD calculation shows about 10.4 μm because the imaginary part of the refractive index of the dielectric (GaAs) and the metal (Au) are perfect conductors, that is, the dielectric constant is pure. This is an imaginary number, and its absolute value is quite large.

  At this time, the length Lp at which the z component of the electric field was 1 / e from the maximum value, that is, the penetration length of the plasmon was about 0.8 μm. The thickness of the light absorption layer is sufficient if it is about twice or less of Lp. Even if the light absorption layer is made thicker than this, the electric field enhancement is not expected as expected.

  This calculation is only an example, and when it is desired to operate at a desired wavelength, the period a may be appropriately selected according to the above method.

  As described above, according to the first embodiment of the present invention, as the light absorption layer, the infrared detection device using the intersubband transition of the quantum dot, the electrode for detecting the photocurrent is periodically formed. By adding a simple structure, it is possible to provide an infrared detection device that induces surface plasmons and increases the sensitivity at the wavelength to be detected. Thus, in the first embodiment of the present invention, the intersubband transition in the quantum dot is enhanced by the surface plasmon.

  Next, with reference to FIG. 2, the manufacturing method of the infrared rays detection apparatus which concerns on the 1st Embodiment of this invention is demonstrated.

(1) Crystal growth including quantum dots As the semi-insulating substrate 20, a GaAs substrate having a (001) plane orientation is prepared. While introducing this substrate into a molecular beam epitaxial (MBE) apparatus and irradiating As4 molecular beam obtained by heating and sublimating solid As, the substrate temperature is raised to 600 ° C. The oxide film is removed. Thereafter, the i-type buffer layer 21 made of the same GaAs as that of the substrate is laminated at about 300 nm at a substrate temperature of about 580 ° C.

Subsequently, an n-type lower contact layer 22 made of GaAs having a thickness of about 300 nm and an Si concentration of about 2 × 10 ¹⁸ cm ⁻³ and an i-type barrier layer 23 of GaAs having a thickness of 70 nm are sequentially stacked. .

  Thereafter, the substrate temperature is lowered to about 490 ° C., and InAs corresponding to a nominal thickness of about 2 to 3 atomic layers is supplied.

At this time, InAs grows three-dimensionally in an island shape due to strain generated due to a difference in lattice constant between InAs and GaAs, quantum dots 24 called SK (Stranski-Krastanov) mode are formed. The typical size of a typical quantum dot is about 30 to 50 nm in diameter, about 2 to 8 nm in height, and the surface density per cm ² is about 5 × 10 ¹⁰ .

  The above-mentioned nominal thickness expressed as 2 to 3 atomic layers is a numerical value including components that re-evaporate on the substrate surface, and the actual average thickness is often smaller than this value.

  Subsequently, by stopping the supply of InAs and supplying GaAs, a GaAs i-type barrier layer 25 having a thickness of about 50 nm is grown, and the quantum dots 24 are embedded.

  Here, the i-type barrier layer 25 may be set to a thickness at which the uneven shape of the surface formed by the quantum dots 24 is substantially flat, and a thickness of about 50 nm or more is a standard.

  When 10 layers of the quantum dots 24 are desired to be stacked, the above-described stacking of the quantum dots 24 and the i-type barrier layer 25 is alternately repeated 10 times to obtain a desired structure.

Subsequently, an n-type upper contact layer 26 made of GaAs having a thickness of about 100 nm and a Si concentration of about 2 × 10 ¹⁸ cm ⁻³ is deposited.

  In the above manufacturing method, GaAs is used as the i-type barrier layer, but AlGaAs may be used instead. Since AlGaAs has a larger band gap energy than GaAs, the detection wavelength of the quantum dot infrared detector can be shortened (in other words, the energy of light can be increased).

  However, if the Al composition is large, mixing with In, which is one of the atoms constituting the quantum dot 24, may occur and the crystal characteristics may deteriorate. Therefore, it is desirable that the Al composition ratio be about 0.4 or less.

  Further, although the quantum dots 24 and their peripheral structures are formed by the MBE method, the present invention is not limited to this method, and other crystal growth methods such as a metal organic chemical vapor deposition method (MOCVD method) may be used. good.

  Further, although GaAs is used as the substrate for crystal growth, this may be InP. In this case, the barrier layer may be composed of InGaAlAs that lattice matches with InP.

(2) Detector Structure Processing and Electrode Process Subsequently, a part of the i-type GaAs barrier layer 23 is selectively etched from the n-type upper contact layer 26 using lithography and dry etching to form the n-type lower contact layer 22. Expose part of the surface.

  By this selective etching, the separated structure becomes one of the detection elements. The size of the light receiving surface of the detector varies depending on the application, but is typically about 40 μm to 200 μm.

  The infrared detection device may be composed of only one element, or may be an array in which such elements are arranged in a line or in a plane.

  Next, electrodes 27 and 28 made of AuGe / Ni / Au are formed on the upper and lower contact layers by a lift-off method. The lift-off method includes processes such as lithography, metal deposition, and resist stripping.

  Through the above steps, the basic configuration of the photodetector portion of the infrared detection device according to the first embodiment is completed.

(Second Embodiment)
Next, with reference to FIG. 6, the structure of the infrared detection apparatus according to the second embodiment of the present invention will be described. Here, FIG. 6 shows a cross-sectional view and a top view of the infrared detecting device according to the second embodiment of the present invention.

  The infrared detection device according to the second embodiment is different from the first embodiment (see FIG. 2) in that instead of the light absorption layer 200 (see FIG. 2) composed of quantum dots 24, The light-absorbing layer 600 having a multiple quantum well structure is used. The light absorption layer 600 includes a quantum well layer 60 and an i-type barrier layer 61 stacked in multiple layers. The light absorption layer 600 absorbs infrared rays by utilizing the intersubband transition of the quantum well layer 60 stacked in multiple layers.

  The other configuration is almost the same as the configuration of the infrared detecting apparatus according to the first embodiment shown in FIG.

The manufacturing method is also almost the same as that of the first embodiment shown in FIG. In the quantum well layer 60, for example, GaAs can be used as the quantum well layer, and Al _x Ga _1-x As can be used as the i-type barrier layer 61. By appropriately selecting the composition ratio x, a desired intersubband transition energy can be selected. Also in this case, it is desirable that the Al composition ratio x is about 0.4 or less.

  The other manufacturing steps are substantially the same as the manufacturing steps of the infrared detecting device according to the first embodiment shown in FIG.

  The advantage of quantum wells over quantum dots is that band broadening due to non-uniform spread can be suppressed. In the case of quantum dots, since they are formed by self-formation, a certain degree of variation in size is inevitable, and as a result, it is difficult to narrow the bandwidth.

  On the other hand, in the case of a quantum well, the bandwidth of intersubband transition energy is determined by the magnitude of fluctuations in the thickness of the quantum well and the barrier layer. This is in principle smaller than the variation in the size of the quantum dots, and as a result, the bandwidth without plasmon enhancement is narrower than that of the quantum dots.

  As described above, according to the embodiment of the present invention, as a light absorption layer, an infrared detection device using intersubband transition in a quantum dot or quantum well, which is used as an electrode for detecting photocurrent By adding a periodic structure, it is possible to provide an infrared detection device that induces surface plasmons and increases the sensitivity at a wavelength to be detected. As described above, in the embodiment of the present invention, the absorption process using the intersubband transition in the quantum well or the quantum dot is enhanced by the surface plasmon.

  As mentioned above, although embodiment and Example of this invention were described concretely, this invention is not limited to the above-mentioned embodiment and Example, Based on the technical idea of this invention, various deformation | transformation are carried out. Is possible.

  INDUSTRIAL APPLICABILITY The present invention can be used for an infrared detection device used for environmental measurement such as weather that requires spectral analysis on the short wavelength side (about 3 to 20 μm) of mid-infrared and far-infrared.

DESCRIPTION OF SYMBOLS 11 Barrier layer 12 Quantum dot or quantum well layer 13 Barrier layer 14 Electron ground level 15 Electron excitation level 16 Electron 20 Semi-insulating substrate 21 i-type buffer layer 22 n-type lower contact layer 23 i-type barrier layer 24 Quantum dot 25 i Type barrier layer 26 n-type upper contact layer 27 lower electrode 28 upper electrode 29 circular hole 50 incident infrared ray 60 quantum well layer 61 i-type barrier layer 200 light absorption layer 600 light absorption layer

Claims (10)

A light-absorbing layer that absorbs infrared rays using intersubband transitions of quantum dots stacked in multiple layers;
A lower electrode and an upper electrode provided so as to sandwich the light absorption layer;
An infrared detecting device, wherein the upper electrode is formed with periodic holes for inducing surface plasmon to increase sensitivity at a wavelength to be detected.   The periodic hole formed in the upper electrode is formed with a period a, and when the refractive index of the light absorption layer is n, the light detection efficiency in the vicinity of the wavelength n × a is increased. The infrared detection device according to claim 1.   The periodic holes formed in the upper electrode are formed with a period a, and when the refractive index of the light absorption layer is n, 1 × √2 times, 1/2 times of n × a, or The infrared detection device according to claim 1, wherein the light detection efficiency near a wavelength of 1 / √5 times increases.   When the penetration length of the surface plasmon generated by forming the periodic holes in the upper electrode is Lp, the thickness of the light absorption layer is about twice Lp or less. The infrared detection device according to claim 1. The lower electrode is provided on a lower contact layer formed on a substrate,
The infrared detection device according to claim 1, wherein the upper electrode is provided on an upper contact layer formed on the light absorption layer. A light-absorbing layer that absorbs infrared rays by utilizing intersubband transitions of multilayered quantum wells;
A lower electrode and an upper electrode provided so as to sandwich the light absorption layer;
An infrared detecting device, wherein the upper electrode is formed with periodic holes for inducing surface plasmon to increase sensitivity at a wavelength to be detected.   The periodic hole formed in the upper electrode is formed with a period a, and when the refractive index of the light absorption layer is n, the light detection efficiency in the vicinity of the wavelength n × a is increased. The infrared detection device according to claim 6.   The periodic holes formed in the upper electrode are formed with a period a, and when the refractive index of the light absorption layer is n, n × a is 1 / √2 times, 1/2 times or The infrared detection device according to claim 6, wherein the light detection efficiency near 1 / √5 times the wavelength increases.   When the penetration length of the surface plasmon generated by forming the periodic holes in the upper electrode is Lp, the thickness of the light absorption layer is about twice Lp or less. The infrared detection device according to claim 6. The lower electrode is provided on a lower contact layer formed on a substrate,
The infrared detection device according to claim 6, wherein the upper electrode is provided on an upper contact layer formed on the light absorption layer.

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