JP4583194B2 - Infrared detector - Google Patents

Description

  The present invention relates to an infrared detector, and in particular, uses a quantum dot structure in which quantum dots are embedded with a plurality of embedded layers, and detects infrared rays using electronic transitions when infrared rays are absorbed by the quantum dot structure. It relates to an infrared detector.

  Conventionally, an infrared detector has a quantum well type infrared detector (Quantum infrared detector) that uses a quantum well structure in which a quantum well layer that absorbs infrared rays and emits carriers is embedded with a barrier layer that receives carriers emitted therefrom. Well Infrared Photodetector (QWIP) was used.

  In recent years, an attempt has been made to use a quantum dot structure that can increase sensitivity as compared with a quantum well structure for an infrared detector. Such a quantum dot infrared detector (Quantum Dot Infrared Photodetector, QDIP) uses a quantum dot structure in which quantum dots that absorb infrared rays and emit carriers are embedded with a cap layer and a barrier layer. In QDIP, after the infrared rays are absorbed and carriers are released from the quantum dots, it takes a long time until the carriers are captured by other quantum dots, and the amount of current variation with respect to the variation in the number of carriers in the quantum dots increases. This indicates that the photoconductive gain is large and that the sensitivity to the number of unit photons is high.

Here, the carrier number fluctuation includes a carrier number fluctuation as a noise component, and is defined by the following equation, for example.
S = I _photo = eΦηg ... (1)
N = (4eI _photo gB) ^0.5 = 2eg (ΦηB) ^0.5 (2)
In equations (1) and (2), S and I _photo are signal values, e is the charge amount, Φ is the number of unit photons, η is the conversion efficiency from photons to carriers, g is the photoconductive gain, and N is the noise signal value. , B is the frequency.

  However, in a situation where the number of carriers fluctuates constantly due to factors other than the target infrared light, such as background light, the amount of current variation with respect to the number of carriers as a noise component is also large. As is clear from the equations (1) and (2), when the photoconductive gain g increases, not only the signal value S but also the noise signal value N increases.

  In order to reduce the noise signal value N, a cap layer having an electron energy potential lower than that of the barrier layer is formed at a position very close to the quantum dots in the barrier layer that receives carriers emitted from the quantum dots. This cap layer captures carriers having a low electron energy potential and recharges the carriers to the quantum dots. That is, the cap layer captures the carriers emitted from the quantum dots and reduces the photoconductive gain g. Here, if the carriers are sufficiently trapped, the photoconductivity gain g is reduced and the sensitivity is lowered. Usually, the quantum dot structure is repeatedly stacked about 5 to 10 times to increase the infrared absorption amount. To increase the sensitivity.

By the way, InAs quantum dots are scattered on a GaAs substrate or a GaAs layer, and InGaAs serving as a cap layer and GaAs serving as a barrier layer are epitaxially grown in order on the GaAs substrate or GaAs layer. There has been proposed a technique for forming a multilayer structure (see Patent Document 1).
JP-A-8-88345

  However, in the case where InGaAs and GaAs are used for the cap layer and the barrier layer in which the InAs quantum dots are embedded, as in the quantum dot structure disclosed in Patent Document 1, the difference between the lattice constants is relatively large. Therefore, the lattice mismatch between the cap layer and the barrier layer is increased. If such a large lattice mismatch exists in the quantum dot structure, there is a problem that crystal defects are likely to occur when these are stacked.

  This invention is made | formed in view of such a point, and it aims at providing the infrared detector which can laminate | stack many quantum dot structures more repeatedly.

In the present invention, in order to solve the above-described problem, as illustrated in FIG. 1, in an infrared detector that detects infrared rays, an AlGaAs barrier layer, a quantum dot disposed above the barrier layer, and the quantum detector infrared detector comprising a quantum dot structure having a GaAs cap layer embedding the dots are provided.

According to such an infrared detector, using AlGaAs barrier layer 1c using GaAs to caps layer 1b, using similar materials lattice constant. Thereby, since the lattice mismatch between the cap layer 1b and the barrier layer 1c is reduced, the occurrence of crystal defects can be suppressed even when the quantum dot structures are stacked.

In the present invention, quantum dots that absorb infrared rays and emit carriers are embedded by a cap layer and a barrier layer having similar lattice constants.
In this way, even when the quantum dot structures are stacked, the generation of crystal defects can be suppressed, so that more quantum dot structures can be stacked repeatedly. Thereby, a highly sensitive infrared detector can be realized.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
First, the first embodiment will be described. FIG. 1 is a schematic cross-sectional view of an essential part of an example of the light absorption part of the first embodiment, and FIG. 2 is an energy band structure diagram of the quantum dot structure.

  The infrared detector has a light absorption unit 1 in which a plurality of quantum dot structures are stacked. The quantum dot structure has a configuration in which the cap layer 1b is provided so as to be in contact with the upper surface of the quantum dot 1a, and the quantum dot 1a and the cap layer 1b are embedded in the barrier layer 1c.

  Here, InAs, InGaAs, InAlAs, or InGaAlAs is used for the quantum dots 1a. GaAs or AlGaAs is used for the cap layer 1b. AlGaAs is used for the barrier layer 1c. However, when AlGaAs is used for the cap layer 1b, the barrier layer 1c uses AlGaAs in which the As composition of the barrier layer 1c is larger than the Al composition of the cap layer 1b.

  The light absorbing portion 1 having such a configuration is first formed by an molecular beam epitaxy (MBE) method, for example, on a GaAs substrate, for example, on an n-type lower portion having a thickness of about 5000 mm for electrical connection to the outside. After growing the contact layer, for example, AlGaAs having a thickness of about 300 to 500 mm is grown thereon to form the barrier layer 1c. Next, for example, 2 to 3 ML of InAs in molecular layer units is supplied at about 500 ° C., which is lower than usual, and InAs is formed in an island shape by utilizing a self-organization phenomenon. As a result, InAs quantum dots 1a having a diameter of about 200 to 400 inches and a height of about 30 to 50 inches are formed. Next, for example, GaAs having a thickness of about 30 to 50 mm is grown so as to bury the quantum dots 1a from the upper surface, thereby forming a cap layer 1b. Next, on this cap layer 1b, for example, AlGaAs having a thickness of about 300 to 500 mm is grown to form a barrier layer 1c. In this way, the light absorbing portion 1 is formed on the lower contact layer by repeatedly growing the barrier layer 1c, the quantum dots 1a, and the cap layer 1b. After the formation of the light absorbing portion 1, an n-type upper contact layer having a thickness of about 5000 mm is further grown thereon for electrical connection to the outside.

  As described above, when GaAs is used for the cap layer 1b and AlGaAs is used for the barrier layer 1c, and the above materials are used for the cap layer 1b and the barrier layer 1c, the lattice constants of both layers are similar. The difference between the constants is smaller than in the conventional case where InGaAs and GaAs are used for the cap layer and the barrier layer, respectively, and lattice mismatch between the two layers can be suppressed.

  In the quantum dot structure of the light absorbing portion 1 formed in this way, the energy levels of the conduction bands of the quantum dot 1a, the cap layer 1b, and the barrier layer 1c are respectively the level A as illustrated in FIG. , Level B and level C. Thus, the cap layer 1b has an electron energy potential higher than that of the quantum dots 1a, and the barrier layer 1c has an electron energy potential higher than that of the cap layer 1b.

Next, the operation of the schematic diagram of FIG. 1 will be described.
Carriers emitted from the quantum dots 1a flow according to an electric field applied between the upper and lower contact layers when energy that can move through the conduction bands of the quantum dots 1a, the cap layer 1b, and the barrier layer 1c is obtained. That is, carriers in the valence band of the quantum dot 1a are excited by absorption of infrared rays and exceed the level A of the quantum dot 1a, the level B of the cap layer 1b, and the level C of the barrier layer 1c. Current flows between the contact layers.

  Thereafter, in the barrier layer 1c, a carrier flows from the quantum dot 1a and a current flows according to the applied electric field. Moreover, the carrier emitted from the quantum dot 1a as a noise component is captured by the cap layer 1b, and the carrier is recharged in the quantum dot 1a.

  The infrared detector has such a light absorption unit 1 as one pixel, and is configured by arranging a large number of light absorption units 1 on a single GaAs substrate, for example. In each of the light absorption units 1, when a large amount of infrared light is absorbed, a larger amount of carriers are released, and by using this, an image corresponding to the temperature of the object is generated in the infrared detector. It becomes like this.

  As described above, in the infrared detector according to the first embodiment, since the lattice mismatch between the cap layer 1b and the barrier layer 1c of the quantum dot structure is reduced, the quantum dot structure is stacked. Crystal defects are less likely to occur when the light absorbing portion 1 is formed. If the quantum dot structure is formed using the material as described above, it can be repeatedly laminated 20 times or more. As a result, a larger number of quantum dots 1a can be present in one light absorber 1, that is, in one pixel, so that the sensitivity of the infrared detector can be increased.

In the first embodiment, since the cap layer 1b captures the carrier as the noise component, it is possible to suppress the current amount fluctuation with respect to the carrier number fluctuation.
In the first embodiment, the case where the cap layer 1b is formed of one kind of material has been described as an example. However, if the cap layer 1b is configured by laminating a plurality of materials at appropriate levels. The sensitivity of the infrared detector can be adjusted by finely adjusting the trapped amount of carriers emitted from the quantum dots 1a.

  In the first embodiment, the case where the cap layer 1b is provided in contact with the upper surface of the quantum dot 1a is described as an example. However, the cap layer 1b may be provided in contact with the lower surface of the quantum dot 1a. . In that case, infrared detection is performed in the same manner as described above.

Further, an n-type epitaxial layer for supplying carriers may be further provided between the layers constituting the light absorption unit 1.
Next, a second embodiment will be described. FIG. 3 is a schematic cross-sectional view of an essential part of an example of the light absorption unit according to the second embodiment.

  The infrared detector has a light absorbing portion 2 in which a plurality of quantum dot structures are stacked. The quantum dot structure has a configuration in which the cap layer 2b is provided in contact with the upper surface of the quantum dot 2a, and the quantum dot 2a and the cap layer 2b are embedded with the barrier layer 2c. In addition, it has a current outlet 2d for taking out the generated current from the light absorbing portion 2 and a current inlet 2e for charging the light absorbing portion 2 with carriers. In addition, a via 2f that connects cap layers as a plurality of channels in series is provided.

  Here, the material used in the first embodiment is also used in the second embodiment. The principle of current generation is the same as in the first embodiment, and the direction of the current path is parallel to the cap layer with the cap layer including quantum dots as a channel. Moreover, when the name of a component is the same, the function is also the same. In addition, in the quantum dots 2a, the cap layer 2b, and the barrier layer 2c, the energy band structure is the same as that in the first embodiment.

  After the formation process similar to that of the first embodiment, the light absorption unit 2 having such a configuration is subjected to etching gas of SiCl4, Cl2, and Ar, and a pressure of about 0.5 Pa is applied. The current outlet 2d, the current inlet 2e, and the via 2f are formed by dry etching using a 100 W high-frequency power source.

  As described herein, when GaAs is used for the cap layer 2b and AlGaAs is used for the barrier layer 2c, and when the above materials are used for the cap layer 2b and the barrier layer 2c, the lattice constants of both layers are similar. The difference between the constants is smaller than in the conventional case where InGaAs and GaAs are used for the cap layer and the barrier layer, respectively, and lattice mismatch between the two layers can be suppressed.

Next, the operation of the schematic diagram of FIG. 3 will be described.
Carriers emitted from the quantum dots 2a flow according to an applied electric field when energy that can move through the conduction bands of the quantum dots 2a, the cap layer 2b, and the barrier layer 2c is obtained. That is, carriers in the valence band of the quantum dot 2a are excited by absorption of infrared rays, and a current flows when the level A of the quantum dot 2a, the level B of the cap layer 2b, and the level C of the barrier layer 2c are exceeded. become.

Thereafter, in the cap layer 2b, current flows from the carriers emitted from the quantum dots 2a in accordance with the electric field applied in the direction parallel to the cap layer.
Thereafter, the generated current is taken out from the light absorption unit 2 by the current outlet 2d. Further, the carrier is charged in the light absorption unit 2 by the current inlet 2e.

  The infrared detector has such a light absorbing portion 2 as one pixel, and is configured by arranging a large number of light absorbing portions 2 on a single GaAs substrate, for example. In each light absorbing unit 2, when a lot of infrared rays are absorbed, a larger amount of carriers are emitted, and by using this, an image corresponding to the temperature of the object is generated in the infrared detector. It becomes like this.

  As described above, in the infrared detector according to the second embodiment, since the lattice mismatch between the cap layer 2b and the barrier layer 2c of the quantum dot structure is reduced, the quantum dot structure is stacked. Crystal defects are less likely to occur when the light absorbing portion 2 is formed. If the quantum dot structure is formed using the material as described above, it can be repeatedly laminated 20 times or more. As a result, more quantum dots 2a can be present in one light absorber 2, that is, in one pixel, so that the sensitivity of the infrared detector can be increased. Further, the damage to the light absorbing portion 2 is small at the time of processing the via 2f and the like of the light absorbing portion 2.

  In addition, the quantum dot structure can be stacked and each channel can be connected in series to lengthen the current path, and the quantum dot 2a can easily capture carriers as a noise component. The fluctuation is small.

  Further, a groove is provided for the light absorbing portion 2. FIG. 4 is a schematic plan view of a main part of an example of the light absorption unit according to the second embodiment. Here, grooves 2g and 2h are provided in the light absorber 2, and the current path is bypassed to lengthen the current path. In this way, the current path can be lengthened, and the quantum dots 2a can easily capture carriers as a noise component, so that the current amount fluctuation with respect to the carrier number fluctuation as the noise component is small.

  In the second embodiment, the case where the cap layer 2b is formed of one kind of material has been described as an example. However, if the cap layer 2b is formed by laminating a plurality of materials at appropriate levels. The sensitivity of the infrared detector can be adjusted by finely adjusting the trapped amount of carriers emitted from the quantum dots 2a.

  In the second embodiment, the case where the cap layer 2b is provided so as to be in contact with the upper surface of the quantum dot 2a has been described as an example. However, the cap layer 2b may be provided so as to be in contact with the lower surface of the quantum dot 2a. . In that case, infrared detection is performed in the same manner as described above.

Further, an n-type epitaxial layer for supplying carriers may be further provided between the layers constituting the light absorption unit 2.
(Supplementary note 1) In an infrared detector for detecting infrared rays,
An infrared detector, comprising: a quantum dot structure in which a quantum dot that absorbs infrared rays and emits carriers is embedded with a plurality of buried layers having similar lattice constants.

  (Supplementary Note 2) Among the plurality of embedded layers, one embedded layer having an electron energy potential higher than that of the quantum dots is provided in contact with an upper surface or a lower surface of the quantum dots, and the electron energy potential is higher than that of the one embedded layer. 2. The infrared detector according to claim 1, wherein the quantum dot and the one embedded layer are embedded by another higher embedded layer.

(Supplementary note 3) The infrared detector according to supplementary note 2, wherein the one buried layer is thinner than a sum of thicknesses of the plurality of buried layers other than the one buried layer.
(Supplementary note 4) The quantum dot is formed of InAs, InGaAs, InAlAs, or InGaAlAs, the one embedded layer is formed of GaAs or AlGaAs, and the other embedded layer is formed of AlGaAs. Infrared detector as described.

  (Supplementary Note 5) When the one buried layer is made of AlGaAs, the other buried layer is made of AlGaAs in which the composition of As in the other buried layer is larger than the composition of Al in the one buried layer. The infrared detector according to supplementary note 4, wherein

(Supplementary note 6) The infrared detector according to supplementary note 1, wherein the light absorption unit is configured by stacking a plurality of the quantum dot structures.
(Supplementary note 7) The infrared detector according to supplementary note 6, wherein the number of stacked quantum dot structures is 20 or more.

(Additional remark 8) The said light absorption part has an electric current path of a parallel direction with respect to the surface in which the said 1 embedded layer is formed, The infrared detector of Additional remark 2 characterized by the above-mentioned.
(Additional remark 9) When the said light absorption part is comprised by laminating | stacking the said some quantum dot structure, the said each current path is connected in series, The infrared detector of Additional remark 8 characterized by the above-mentioned.

  (Supplementary note 10) The infrared detector according to supplementary note 8, wherein the current path is lengthened by removing a part of the quantum dot structure.

It is a principal part cross-section schematic diagram of an example of the light absorption part of 1st Embodiment. It is an energy band structure figure of a quantum dot structure. It is a principal part cross-sectional schematic diagram of an example of the light absorption part of 2nd Embodiment. It is a principal part plane schematic diagram of an example of the light absorption part of 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light absorption part 1a Quantum dot 1b Cap layer 1c Barrier layer

Claims (5)

An AlGaAs barrier layer;
Quantum dots disposed above the barrier layer;
A cap layer of GaAs embedding the quantum dots;
Infrared detectors, characterized in that it comprises a quantum dot structure with.   The cap layer has an electron energy potential higher than that of the quantum dots,
  The infrared detector according to claim 1, wherein an electron energy potential of the barrier layer is higher than that of the cap layer.   The infrared detector according to claim 2, wherein the cap layer is thinner than the barrier layer. Infrared detector according to claim 1, wherein the quantum dot structure of the double speed is made by laminating.   The infrared detector according to claim 4, wherein the number of stacked quantum dot structures is 20 or more.

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