JP4673398B2 - Quantum dot infrared detector - Google Patents

Description

  The present invention relates to a quantum dot infrared detecting element having a quantum dot layer as a photoelectric conversion layer.

  Quantum Dot Infrared Photodetector (QDIP), in which a plurality of quantum dot layers are stacked to form a photoelectric conversion layer, has sensitivity to infrared rays that are perpendicularly incident on the substrate. For this reason, quantum dot infrared detectors (also called quantum dot infrared detectors) can be better detectors than quantum well infrared detectors (QWIP), which are difficult to detect normal incident light. Expected. The quantum dot infrared detection element is also superior in that the dark current is smaller than that of the quantum well infrared detection element (Non-Patent Document 1).

  FIG. 1 is a cross-sectional view of a main part for explaining a configuration example of the quantum dot infrared detection element 2.

  1 includes an n-type GaAs first electrode layer 4, a photoelectric conversion layer 6, an n-type GaAs second electrode layer 8, and a metal reflective metal layer. 10 is provided.

  A large number of InAs quantum dots 12 are formed on the same plane inside the photoelectric conversion layer 6, and an intermediate layer 14 made of i-type AlGaAs covers the quantum dots 12. A plurality of quantum dot layers 16 formed by the large number of quantum dots 12 and the intermediate layer 14 are laminated to form the photoelectric conversion layer 6. Here, the quantum dots 12 are formed by a self-organization forming method such as Stranski-Krastanov (SK) mode using a molecular beam epitaxial apparatus or the like.

  Each of the first and second electrode layers 4 and 8 is provided with an electrode (not shown), and an external circuit (not shown) is connected to the electrode. The external circuit applies a voltage to the first and second electrode layers 4 and 8 to extract and detect a photocurrent from the quantum dot infrared detection element 2.

  The infrared ray 20 to be detected is incident on the quantum dot infrared detection element 2 from the back surface of the substrate 21 made of n-type GaAs. A part of the infrared ray 20 incident from the back surface is absorbed by the quantum dots 12 of the photoelectric conversion layer 6 and excites the electrons 22 to the excitation level of the quantum dots. The excited electrons 22 are pulled away from the quantum dots 12 by the electric field formed inside the photoelectric conversion layer 6 by the voltage applied between the first and second electrode layers 4 and 8, and a positive potential is applied. Flow toward the formed electrode layer.

  The remaining infrared rays 20 ′ not absorbed by the quantum dots 12 are reflected by the reflective metal layer 10 and reenter the photoelectric conversion layer 6. A part of the reflected infrared ray 20 ′ is absorbed by the photoelectric conversion layer 6. The quantum efficiency of the quantum dot infrared detection element 2 is increased by the re-incidence of infrared rays to the light absorption layer 6.

  FIG. 2 is a band diagram for explaining the state of the conduction band of the quantum dots 12. A ground level 24 and an excitation level 26 are formed in the conduction band of the quantum dot 12. The quantum dot infrared detection element 2 is formed so that the ground level 24 is filled with electrons. On the other hand, the quantum dot infrared detection element 2 is formed so that the excitation level 26 is empty.

When an infrared ray 20 having a wavelength corresponding to the energy difference between the ground level 24 and the excitation level 26 is incident on the quantum dot 12, electrons that satisfy the ground level 24 are excited to the excitation level 26 (so-called sub-levels). Interband transition). The excited electrons are caused to transition from the excitation level 28 to the conduction band of the intermediate layer 14 by the electric field 28 applied to the intermediate layer 6, and further toward the electrode layer to which a positive potential is applied by receiving a force from the electric field. Go away. FIG. 2 shows a case where the excitation level 26 is formed with lower energy than the conduction band edge 30 of the intermediate layer 6. However, the excitation level 26 is not necessarily formed at a lower energy than the conduction band edge of the intermediate layer, and may be formed at a higher energy than the conduction band edge 30 of the intermediate layer 6.
KW Berryman, SA Lyon, and Mordechai Segev.Appl.Phys.Lett. 70, 1861 (1977).

  The quantum efficiency per layer of the quantum dot layer 16 is low. For this reason, in the quantum dot infrared detection element 2, a plurality of quantum dot layers 16 are stacked, and the reflective metal layer 10 is provided on the surface opposite to the infrared incident surface to enhance quantum efficiency.

  The number of electrons per unit time emitted from the excitation level 26 to the intermediate layer 14 decreases as the electric field 28 decreases, and increases as the electric field 28 increases. Therefore, in the quantum dot infrared detection element 2, the thickness of the intermediate layer 14 is limited to a thickness of, for example, about 50 nm so that the intermediate layer 14 is not thickened more than necessary and the electric field 28 is not reduced.

  The semiconductor (for example, InAs) used as the quantum dot 12 and the semiconductor (for example, AlGaAs) which forms the intermediate | middle layer 14 used as a base | substrate differ. Due to the difference in lattice constant, lattice strain is accumulated in the semiconductor layer that becomes the quantum dot, and the accumulated lattice strain energy serves as a driving force to form the quantum dot 12.

  On the other hand, the intermediate layer 14 in contact with the quantum dots 12 is also distorted in the direction opposite to that of the quantum dots 12. This lattice distortion is greatest at a position in direct contact with the quantum dots 12 and becomes weaker as the distance from the quantum dots 12 in the film forming direction is increased. The preferable thickness 50 nm as the intermediate layer 14 described above is a thickness at which the lattice strain (lattice strain of the intermediate layer) substantially disappears on the surface of the intermediate layer 14.

  However, even on the surface of the intermediate layer formed to such a thickness, the lattice strain is not completely lost. For this reason, when the number of quantum dot layers 16 to be stacked increases, crystal defects (transition) occur in the photoelectric conversion layer due to accumulated lattice distortion, and finally the performance of the quantum dot infrared detection element 2 significantly decreases. End up.

  For this reason, the thickness of the quantum dot layer 16 which can be laminated | stacked is about 20 layers at most, and the quantum efficiency of the quantum dot infrared detection element 2 is only about 1% at most.

  Accordingly, the present inventors have developed a quantum dot infrared detection element in which a strain buffer layer 32 formed of the same semiconductor material (for example, AlGaAs) as that of the intermediate layer 14 is inserted into the photoelectric conversion layer 6.

  FIG. 3 is a cross-sectional view of a main part for explaining the configuration of the quantum dot infrared detecting element 34 provided with the strain buffer layer 32. As shown in FIG. 3, the quantum dot infrared detection element 34 developed by the present inventors has been described with reference to FIG. 1 in that a strain buffer layer 32 is inserted inside the photoelectric conversion layer 6. This is different from the quantum dot infrared detecting element 2 of FIG.

  As shown in FIG. 3, the photoelectric conversion layer 6 has, for example, a first photoelectric conversion layer 36 in which 15 quantum dot layers 16 are stacked, and a 720 nm-thickness formed of the same semiconductor material as the intermediate layer 14. A strain buffer layer 32 and a second photoelectric conversion layer 38 in which 15 quantum dot layers 16 are stacked are sequentially stacked.

  As described above, the strain generated in the first photoelectric conversion layer 36 is substantially reduced by sandwiching the strain buffer layer 32 in which the same semiconductor material as that of the intermediate layer 16 is deposited between the first and second photoelectric conversion layers. After completely disappearing, the second photoelectric conversion layer 38 can be grown.

  Therefore, the total number of quantum dot layers including the first and second photoelectric conversion layers can be increased to, for example, 30 layers. For this reason, it is expected that the quantum efficiency of the quantum dot infrared detection element 34 is improved.

  By the way, in the quantum dot infrared detection element 34 shown in FIG. 3, the photoelectric conversion layer 6 is thickened by the amount of the strain buffer layer 32. For this reason, the electric field (hereinafter referred to as the internal electric field) inside the photoelectric conversion layer, that is, the electric field applied to the quantum dots is reduced. For this reason, the number of electrons emitted from the excitation level 26 to the intermediate layer 14 decreases, and the quantum efficiency does not increase as expected.

  If the voltage applied to the quantum dot infrared detection element 34 is increased, the internal electric field reduced by the introduction of the strain buffer layer 32 can be increased. However, the quantum dot infrared detection elements are arranged in a two-dimensional array and each element is driven by a CMOS circuit in a normal use state. A voltage generated by a complementary metal oxide semiconductor (CMOS) circuit is about 3V at most. Therefore, there is a limit to increasing the voltage applied to the quantum dot infrared detection element 34 to improve the quantum efficiency.

  Accordingly, an object of the present invention is to provide a quantum dot infrared detection element 34 with high quantum efficiency by suppressing or avoiding the reduction of the internal electric field caused by providing the strain buffer layer 32 in the photoelectric conversion layer 6.

  In order to achieve the above object, a disclosed quantum dot infrared detecting element includes a semiconductor substrate, a plurality of quantum dot layers each having a plurality of quantum dots and an intermediate layer covering the plurality of quantum dots. A plurality of photoelectric conversion layers stacked on the semiconductor substrate; and a strain buffer layer in which a p-type semiconductor layer and an n-type semiconductor layer are stacked in alignment with the semiconductor substrate, wherein the strain buffer layer includes the plurality of photoelectric conversion layers. Arranged between the layers.

  According to the disclosed quantum dot infrared detection element, the number of quantum dot layers that can be stacked is increased in the photoelectric conversion layer by providing the strain buffer layer. Furthermore, since the strain buffer layer is formed by stacking the p-type semiconductor layer and the n-type semiconductor layer, it is possible to suppress or avoid the reduction of the internal electric field due to the provision of the strain buffer layer.

  For this reason, the photocurrent generated by the quantum dot infrared detection element according to the present invention increases (that is, the quantum efficiency increases).

  According to the disclosed quantum dot infrared detection element, the quantum efficiency of the quantum dot infrared detection element in which the photoelectric conversion layer is formed by the quantum dot layer is increased.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited to these embodiments, but extends to the matters described in the claims and equivalents thereof.

(Embodiment 1)
(1) Configuration and Manufacturing Method FIG. 4 is a cross-sectional view of the main part for explaining the configuration of the quantum dot infrared detection element according to the present embodiment.

  With reference to FIG. 4, the structure of the quantum dot infrared detection element according to this Embodiment is demonstrated for every step according to a manufacture procedure.

(I) Formation of the 1st electrode layer 4 In this step, the GaAs layer used as the 1st electrode layer 4 is formed.

An n-type GaAs layer having a thickness of 1000 nm is grown on the n-type GaAs substrate 21 by molecular beam epitaxy. The growth temperature is 600 ° C. The impurity doped in this GaAs layer is Si, and its concentration is 2 × 10 ¹⁸ / cm ³ .

  Although not particularly mentioned, the semiconductor layer described in the subsequent steps is also grown by molecular beam epitaxy as in this step.

(Ii) Formation of first photoelectric conversion layer 36 In this step, a semiconductor stacked structure is formed in which a plurality of quantum dots 12 formed by a plurality of quantum dots 12 and an intermediate layer 14 covering the quantum dots 12 are stacked. The This semiconductor stacked structure becomes the first photoelectric conversion layer.

First, an i-type Al _0.2 Ga _0.8 As layer 42 (insulating Al _0.2 Ga _0.8 As layer) having a thickness of 50 nm is grown while the growth temperature is lowered from 600 ° C. to 500 ° C. It is done. This i-type Al _0.2 Ga _0.8 As layer is the first intermediate layer.

  Next, InAs raw materials (In molecular beam and As molecular beam) corresponding to 2 ML (two atomic layers) are supplied in a state where the growth temperature is maintained at 500 ° C. The supply rate of the InAs raw material is a supply rate necessary for the growth of the InAs layer at a growth rate of 0.2 ML / s (0.2 atomic layer / second), assuming that the growth film grows in a layer form.

Since InAs has a lattice constant larger than that of the i-type Al _0.2 Ga _0.8 As layer 42 (lattice-matched to the GaAs substrate 21), it grows under a compressive strain. In the early stage of growth, InAs grows in layers and forms a layer called a wetting layer (not shown). Thereafter, in order to relax the strain energy accumulated in the InAs film, InAs starts three-dimensional growth, and a large number of quantum dots 12 'are formed. That is, the quantum dots of the present embodiment are self-organized and formed by the SK mode.

An i-type Al _0.2 Ga _0.8 As layer having a thickness of 50 nm is stacked on the quantum dots 12 ′ thus formed. This i-type Al _0.2 Ga _0.8 As becomes the intermediate layer 14 ′, and forms the first quantum dot layer 16 ′ together with the quantum dots 12 ′.

  Thereafter, the same steps (formation of quantum dots and intermediate layers) as those described above are repeated, so that a total of 15 quantum dot layers 16 are formed.

(Iii) Formation of Strain Buffer Layer In this step, the n-type semiconductor layer 44 and the p-type semiconductor layer 46 are stacked to form a semiconductor stacked structure that becomes the strain buffer layer 48. Here, the n-type semiconductor layer 44 and the p-type semiconductor layer 46 are lattice-matched to the substrate 21, respectively.

First, an n-type Al _0.2 Ga _0.8 As layer having a thickness of 360 nm is grown on the first photoelectric conversion layer 36. The impurity doped in the Al _0.2 Ga _0.8 As layer is Si, and its concentration is 5.2 × 10 ¹⁵ / cm ³ . This n-type Al _0.2 Ga _0.8 As layer becomes the n-type semiconductor layer 44.

Next, a p-type Al _0.2 Ga _0.8 As layer having a thickness of 360 nm is grown on the n-type semiconductor layer 44. The impurity doped in the p-type Al _0.2 Ga _0.8 As layer is Be, and its concentration is 5.2 × 10 ¹⁵ / cm ³ . This p-type Al _0.2 Ga _0.8 As layer becomes the p-type semiconductor layer 46.

(IV) Growth of Second Photoelectric Conversion Layer 38 In this step, a plurality of quantum dot layers 16 formed by the plurality of quantum dots 12 and the intermediate layer 14 covering the quantum dots 12 are stacked, and the second photoelectric conversion layer is formed. A semiconductor stacked structure to be 38 is formed.

  The procedure in this step is substantially the same as the procedure described above in “(ii) Formation of first photoelectric conversion layer 36”. However, the growth temperature is 500 ° C. until the uppermost quantum dot 52 is grown from the lowermost intermediate layer 50, and increases from 500 ° C. to 600 ° C. during the growth of the uppermost intermediate layer 54. Be warmed.

(V) Formation of Second Electrode Layer 8 In this step, a GaAs layer to be the second electrode layer 8 is formed.

  The procedure in this step is substantially the same as the procedure in “(i) Formation of first electrode layer 4”. However, it differs from the procedure in “(i) Formation of first electrode layer 4” in that the thickness of the grown n-type GaAs layer is 360 nm instead of 1000 nm. Further, the n-type GaAs layer is different in that it is grown not on the GaAs substrate 21 but on the second photoelectric conversion layer 38.

(Vi) Formation of Mesa Next, the semiconductor stacked structure formed by the above “(i) formation of the first electrode layer 4” to “(V) formation of the second electrode layer 8” is obtained by photolithography and A cylindrical mesa is processed using dry etching technology. Here, the dry etching is performed until the first electrode layer 4 is exposed.

(Vii) Formation of first and second electrodes 56 and 58 Next, first and second electrodes 56 and 58 made of AuGe / Au are formed on the first and second electrode layers 4 and 8, respectively. Is done.

(Viii) Formation of Reflective Metal Layer 10 Next, the reflective metal layer 10 in which Au is laminated on the Ti layer is formed on the entire surface of the second electrode layer 8 excluding the region where the second electrode 58 is formed. It is formed.

  Through the steps “(i) formation of the first electrode layer 4” to “(Viii) formation of the reflective metal layer 10” described above, the quantum dot infrared detection element 40 according to the present embodiment is formed.

  That is, the quantum dot infrared detection element 40 according to the present embodiment includes a semiconductor substrate 21 as shown in FIG.

  In addition, the quantum dot infrared detection element 40 has a plurality of quantum dots 12 formed on the same plane and a plurality of quantum dots 16 having an intermediate layer 14 covering the quantum dots 12 stacked on the semiconductor substrate 21. A plurality of photoelectric conversion layers (for example, first and second photoelectric conversion layers 36 and 38) are provided.

  Further, the quantum dot infrared detection element 40 includes a strain buffer layer 48 in which a p-type semiconductor layer 46 and an n-type semiconductor layer 44 are laminated, which are aligned with the semiconductor substrate 21.

  In the quantum dot infrared detection element 40 according to the present embodiment, a structure in which a plurality of the photoelectric conversion layers (for example, the first and second photoelectric conversion layers 36 and 38) are stacked via the strain buffer layer 48. Is formed.

  Furthermore, in the quantum dot infrared detection element 40 of the present embodiment, the structure is sandwiched between the first and second electrode layers 4 and 8 on which the electrodes 56 and 58 are formed, respectively.

  In the example described with reference to FIG. 4, the photoelectric conversion layers stacked via the strain buffer layer 48 are two layers, but the number of stacked photoelectric conversion layers may be three or more. Good. However, even in that case, it is preferable that a strain buffer layer 48 in which a p-type semiconductor layer 46 and an n-type semiconductor layer 44 are stacked (in the same order) is provided between the photoelectric conversion layers.

  In the present embodiment, both the quantum dots 12 and the intermediate layer 14 are formed of a so-called non-doped semiconductor (also called an i-type semiconductor or an insulating semiconductor) that is not intentionally doped. However, the quantum dot layer 16 formed by the quantum dots 12 and the intermediate layer 14 does not necessarily need to be non-doped, and may be n-type, for example. However, it is preferable that all the quantum dot layers 16 have the same conductivity type.

(2) Operation (i) Enhancement of Internal Electric Field Next, the operation of the quantum dot infrared detection element 40 according to the present embodiment will be described.

FIG. 5 is a diagram illustrating a state in which external circuit 60 is connected to quantum dot infrared detection element 40 according to the present embodiment. The external circuit 60 has a voltage generation function and a current detection function, and detects the photocurrent generated by the quantum dot infrared detection element 40. In the example shown in FIG. 5, the external circuit 60 is formed by a constant voltage source 61 and a current detector 63 connected in series. Here, the voltage generated by the constant voltage source is V ₀ .

  The positive electrode 62 of the external circuit 60 is connected to the second electrode 58 of the quantum dot infrared detection element 40. On the other hand, the negative electrode 64 of the external circuit 60 is connected to the first electrode 56 of the quantum dot infrared detection element 40.

  That is, the positive electrode 62 of the external circuit 60 is electrically connected to the second electrode layer 8 disposed on the p-type semiconductor layer 46 side. On the other hand, the negative electrode 56 of the external circuit 60 is electrically connected to the first electrode layer 4 disposed on the n-type semiconductor layer 44 side.

  FIG. 6 illustrates the change in the position of the energy at the conduction band edge (bottom of the conduction band) from the first electrode layer 4 to the second electrode layer 8 inside the quantum dot infrared detection element 40. It is. The horizontal position in FIG. 6 corresponds to the position inside the quantum dot infrared detection element. The left end in FIG. 6 corresponds to the first electrode layer 4, and the right end corresponds to the second electrode layer 8. The vertical position in FIG. 6 corresponds to the energy of electrons. The higher you go, the higher the energy.

  The diagram shown by the solid line in FIG. 6 shows the positional change of the conduction band edge energy of quantum dot infrared detection element 40 according to the present embodiment. On the other hand, a diagram indicated by a broken line shows a change in the position of the conduction band edge energy of the quantum dot infrared detection element 34 described with reference to FIG. 3 and in which the strain buffer layer 32 is not doped with impurities. The structures of the two quantum dot infrared detection elements 34 and 40 are the same except for the presence or absence of impurity doping in the strain buffer layer. In FIG. 6, the change in the conduction band edge energy in the quantum dots 12 is omitted.

  In the quantum dot infrared detecting element 34 described with reference to FIG. 3, the energy at the conduction band edge is changed from the first electrode 4 on the low potential side to the second electrode on the high potential side, as shown by the broken line in FIG. It decreases linearly toward 8.

  By the way, in the quantum dot infrared detecting element 40 according to the present embodiment, the n-type and p-type semiconductor layers 44 and 46 forming the strain buffer layer 48 form pn junctions, and generate positive and negative space charges, respectively. .

  For this reason, the potential of the n-type semiconductor layer 44 that generates positive space charge rises. Accordingly, the conduction band edge energy of the n-type semiconductor layer 44 decreases. Therefore, as indicated by the solid line in FIG. 6, the conduction band edge energy at the right end (position in contact with the n-type semiconductor layer 44) of the first photoelectric conversion layer 36 also decreases.

  On the other hand, the potential of the p-type semiconductor layer 46 that generates negative space charge drops. Accordingly, the conduction band edge energy of the p-type semiconductor layer 46 increases. For this reason, as indicated by the solid line in FIG. 6, the conduction band edge energy at the left end of the second photoelectric conversion layer 38 (position in contact with the p-type semiconductor layer 46) increases.

  Therefore, as shown in FIG. 6, the change in the conduction band edge energy in each of the first and second photoelectric conversion layers 36 and 38 becomes sharper than that in the case where the strain buffer layer is not doped with impurities. Therefore, the electric field strength (value obtained by differentiating the potential with respect to the position coordinates) in the first and second photoelectric conversion layers 36 and 38 is increased.

  That is, according to the present embodiment, the decrease in the strength of the electric field (hereinafter referred to as the internal electric field) in the first and second photoelectric conversion layers 36 and 38 due to the provision of the strain buffer layer is suppressed or reduced. Avoided.

  In the quantum dot infrared detecting element 40 according to the present embodiment, the process of detecting infrared rays will be described as follows.

  First, as described above, infrared rays (not shown) to be detected are incident from the back surface of the n-type GaAs substrate 21 to the quantum dot infrared detection element 40 in which the reduction of the internal electric field is suppressed or avoided. (See FIG. 5). Infrared rays incident from the back surface reach the first and second photoelectric conversion layers 36 and 38 and are absorbed by the quantum dots 12. The infrared light absorbed by the quantum dots 12 excites electrons in the ground level 24 to the excitation level 26 (see FIG. 2). The excited electrons make a transition from the excitation level 26 to the intermediate layer 14 by the internal electric field applied to the quantum dots 12, and then flow toward the second electrode layer 8 to which a positive potential is applied. The flow of electrons that has reached the second electrode layer 8 is extracted as a photocurrent to the external circuit 60 and detected.

  As described above, in the quantum dot infrared detection element 40 according to the present embodiment, the decrease in the strength of the internal electric field due to the provision of the strain buffer layer is suppressed or avoided. Therefore, the phenomenon of reduction of electrons transitioning from the excitation level 26 to the intermediate layer 14 which is a problem in the quantum dot infrared detection element 34 (see FIG. 3) in which no impurity is doped in the strain buffer layer is described in the present embodiment. Is mitigated or avoided in the quantum dot infrared sensing element 40 according to

  On the other hand, the provision of the strain buffer layer 48 increases the number of stackable quantum dot layers 16 from about 20 to 30 or more. Therefore, the quantum dot infrared detection element 40 according to the present embodiment has a quantum efficiency, that is, a photocurrent of 1.5 times that of the quantum dot infrared detection element 2 having no strain buffer layer described with reference to FIG. (30 layers / 20 layers).

Incidentally, in the example shown in FIG. 6, the conduction band edge energy inside the strain buffer layer 48 is almost flat. Therefore, almost all the voltage V ₀ applied between the first and second electrode layers 4 and 8 is applied to the first and second photoelectric conversion layers 36 and 38. For this reason, in the example shown in FIG. 6, the decrease in the strength of the internal electric field due to the provision of the strain buffer layer is almost completely avoided.

  Next, as in the example shown in FIG. 6, specific conditions under which the decrease in the strength of the internal electric field is almost completely avoided will be examined.

First, assume that both n-type and p-type semiconductor layers 44, 46 are fully depleted and have the same thickness d _i / 2. Further, it is assumed that the n-type and p-type semiconductor layers 44 and 46 have the same impurity concentration N. At this time, the diffusion potential difference V _pn appearing at both ends of the strain buffer layers 44 and 46 is expressed by the following equation.

Here, e represents the charge of electrons, ε ₀ represents the dielectric constant of vacuum, and ε _s represents the relative dielectric constant of the semiconductor forming the strain buffer layer.

By the way, if the total thickness of the photoelectric conversion layer 6 formed by the first and second photoelectric conversion layers 36 and 38 and the strain buffer layer 48 is _dt , the potential becomes substantially flat inside the strain buffer layer 48. These conditions can be expressed by the following equation.

  From the equations (1) and (2), the following equation is derived.

  That is, in order to substantially completely avoid a decrease in the strength of the internal electric field, the impurity concentration of the n-type and p-type semiconductor layers 44 and 46 only needs to be a value N obtained according to the equation (3).

The impurity concentration of 5.2 × 10 ¹⁵ / cm ³ of the n-type and p-type semiconductor layers 44 and 46 described in the above “(1) Configuration and manufacturing method” is V ₀ = 3 V, d _i = 720 nm, d _t = 2320 nm and ε _s = 13 are values determined based on Equation (3).

(Ii) Effective Use of Incident Light As described above, part of infrared light (not shown) incident from the back surface of the substrate 21 passes through the first and second photoelectric conversion layers 36 and 38, and the reflective metal layer 10 Is reflected by. For this reason, incident light and reflected light interfere with each other, and a standing wave is formed inside the quantum dot infrared detection element 40.

  FIG. 7 is a conceptual diagram for explaining a standing wave state formed inside the quantum dot infrared detection element 40. FIG. 7 illustrates a standing wave envelope 66 (standing wave electric field envelope) generated by the interference of the incident wave 68 and the reflected wave 70.

  Since the reflective metal layer 10 is made of metal, the intensity of the standing wave is zero on the surface. Therefore, infrared standing waves are formed with the element-side surface of the reflective metal layer 10 as a node. The period in which this section appears is expressed by the following equation.

Here, λ _p represents the wavelength of the infrared rays to be detected in vacuum, and n represents the refractive index of the semiconductor forming the quantum dot infrared detecting element 40.

In the quantum dot infrared detecting element 40 according to the present embodiment, a point located at 1.52 μm (= 10 μm / (2 × 3.3) = λ _p / 2n) from the element surface is the center of the strain buffer layer 48 (element It is formed so as to be 1.52 μm from the surface (= 360 nm + 50 nm × 16 layers + 360 nm) (see FIG. 7). In addition, λ _p was 10μm, n is 3.3.

  Therefore, in the quantum dot infrared detection element 40 according to the present embodiment, the node where the standing wave intensity is weak is excluded from the first and second photoelectric conversion layers 36 and 38, and the antinode with strong standing wave intensity is the first. The first and second photoelectric conversion layers 36 and 38 are unevenly distributed. For this reason, since the number of electrons excited from the ground level of the quantum dot to the excitation level (per unit time) increases, the quantum efficiency of the quantum dot infrared detector 40 according to the present embodiment increases.

In order to make the node of the standing wave come to the center of the strain buffer layer 48, the thickness of the second electrode layer 8 may be adjusted as appropriate. Here, since the impurity concentration of the second electrode layer 8 is as high as 2 × 10 ¹⁸ / cm ³ , the width of the depletion layer is at most several tens of nm. For this reason, when the second electrode layer 8 needs to be thinned, the second electrode layer 8 can be thinned to about several tens of nm.

  FIG. 8 is a conceptual diagram for explaining the state of the standing wave formed inside the quantum dot infrared detecting element 34 described with reference to FIG.

  In the quantum dot infrared detection element 34 described with reference to FIG. 3, the position adjustment of the strain buffer layer 32 as described above is not performed. Accordingly, the node of the standing wave does not always come inside the strain buffer layer 32, and the node of the standing wave comes inside the first or second photoelectric conversion layer 36, 38 as shown in FIG. May end up. In such a case, the quantum efficiency of the quantum dot infrared detection element 34 becomes low.

  As is clear from the above description, the quantum dot infrared detection element 40 according to the present embodiment described with reference to FIG. 4 has a surface 74 (first surface) that faces the light incident surface 72 (the back surface of the substrate 21). 2 has a reflective metal layer 10 formed on the surface of the second electrode layer 8.

  The quantum dot infrared detection element 40 is formed such that the inside of the strain buffer layer 48 is located at a position where the distance from the surface 74 is half the wavelength of the light to be detected.

  However, the strain buffer layer 48 may be formed so that the distance from the surface 74 is a natural number multiple of half the wavelength of the light to be detected. Also in this case, since at least one of the plurality of nodes formed between the first and second electrode layers 4 and 8 is formed inside the strain buffer layer, the quantum efficiency is improved.

(Embodiment 2)
The present embodiment relates to an infrared detection device including the quantum dot infrared detection element 40 according to the first embodiment.

  The main part of the quantum dot infrared detection element 40 according to the first embodiment is the same as the configuration described in FIG. 5 referred to in order to explain the operation of the quantum dot infrared detection element 40 in the first embodiment.

  That is, infrared detection device 76 according to the present embodiment has quantum dot infrared detection element 40 according to the first embodiment and external circuit 60 (voltage generation source).

  Here, the external circuit 60 (voltage generation source) supplies a positive potential to the photoelectric conversion layer (second photoelectric conversion layer 38) in contact with the p-type semiconductor layer 46, and the photoelectric conversion layer (in contact with the n-type semiconductor layer 44). A negative potential is supplied to the first photoelectric conversion layer 36).

  That is, the positive electrode 62 of the external power supply 60 is electrically connected to the electrode layer disposed on the p-type semiconductor layer 46 side of the first and second electrode layers 4, 8, that is, the second electrode layer 8. . The negative electrode 64 of the external power supply 60 is electrically connected to the electrode layer disposed on the n-type semiconductor layer 44 side of the first and second electrode layers 4, 8, that is, the first electrode layer 4. Yes.

  Here, external circuit 60 according to the present embodiment is formed of a CMOS circuit. The external circuit 60 has a current detection function and a voltage generation function, and is formed so as to supply a constant voltage of 3 V to the quantum dot infrared detection element 40.

(Modification)
In the example described above, the thickness of the strain buffer layer 48 is 720 nm (= 360 nm + 360 nm), but the thickness of the strain buffer layer 48 is not limited to this value. However, the thickness of the strain buffer layer 48 is preferably thicker than 50 nm at which the lattice strain substantially disappears, and is preferably 5 μm or less which does not hinder mesa processing by dry etching.

  Further, the thickness of the intermediate layer is not limited to 50 nm described above. The thickness of the intermediate layer is preferably 30 nm or more so that columnar quantum dots are formed and the quantum state does not change, and is preferably 50 nm or less so that the internal electric field is not reduced unnecessarily.

  In the above example, the quantum dots are formed of InAs, but may be formed of other semiconductor materials such as InGaAs. Furthermore, in the above example, the intermediate layer is formed of AlGaAs, but may be formed of other semiconductor materials such as GaAs.

  In the above example, each semiconductor layer is grown by MBE (Molecular Beam Epitaxy) method, but may be grown by other methods such as metal organic vapor phase epitaxy.

It is principal part sectional drawing explaining the example of 1 structure of a quantum dot infrared rays detection element. It is a band figure explaining the state of the conduction band of a quantum dot. It is principal part sectional drawing explaining the structure of the quantum dot infrared rays detection element provided with the distortion buffer layer. It is principal part sectional drawing explaining the structure of the quantum dot infrared rays detection element according to Embodiment 1. FIG. It is a figure explaining the state by which the external circuit for detecting a photocurrent was connected to the quantum dot infrared detection element. FIG. 5 is a diagram for explaining energy change at the conduction band edge from the first electrode layer to the second electrode layer in the quantum dot infrared detection element according to the first embodiment. It is a conceptual diagram explaining the state of the infrared standing wave formed in the inside of the quantum dot infrared detection element according to Embodiment 1. FIG. It is a conceptual diagram explaining the state of the infrared standing wave formed in the inside of the quantum dot infrared detection element described in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 2 ... Quantum dot infrared detection element 4 ... 1st electrode layer 6 ... Photoelectric conversion layer 8 ... 2nd electrode layer 10 ... Reflective metal layer 12, 12 '... Quantum dot 14, 14 '... Intermediate layer 16 ... Quantum dot layer 20, 20' ... Infrared ray 21 ... Substrate 22 ... Electron 24 ... Ground level 26 ... Excitation level 28 .... Electric field 30 ... conduction band edge 32 ... strain buffer layer 34 ... quantum dot infrared detector 36 ... first photoelectric conversion layer 38 ... second photoelectric conversion layer 40 ... Quantum dot infrared detection element (Embodiment 1)
42 ... i-type Al _0.2 Ga _0.8 As layer 44 ... n-type semiconductor layer 46 ... p-type semiconductor layer 48 ... strain buffer layer (Embodiment 1)
50... Lowermost intermediate layer 52... Uppermost quantum dot 54... Uppermost intermediate layer 56 .. First electrode 58... Second electrode 60. ..Constant voltage source 62 ... Positive electrode
63: current detector 64 ... negative electrode 66 ... standing wave envelope 68 ... incident wave 70 ... reflected wave 72 ... incident surface 74 ... (quantum dot infrared detection) Surface 76 of element (infrared detector)

Claims (7)

A semiconductor substrate;
A plurality of quantum dot layers having a plurality of quantum dots and an intermediate layer covering the plurality of quantum dots, a plurality of photoelectric conversion layers laminated on the semiconductor substrate;
A strain buffer layer in which a p-type semiconductor layer and an n-type semiconductor layer are laminated, which is aligned with the semiconductor substrate;
The quantum dot infrared detecting element, wherein the strain buffer layer is disposed between the plurality of photoelectric conversion layers. In the quantum dot type infrared sensing element according to claim 1,
A quantum dot infrared detecting element, comprising a reflective metal layer that reflects infrared rays, formed on a surface facing a light incident surface. In the quantum dot type infrared sensing element according to claim 2,
A quantum dot infrared detecting element, wherein the inside of the strain buffer layer is positioned at a position where the distance from the surface is a natural number multiple of half the wavelength of light to be detected . In the quantum dot type infrared sensing element according to claim 1 or 2,
The strain buffer layer has a thickness of 50 nm or more and 5 μm or less,
A quantum dot infrared detection element, wherein the intermediate layer has a thickness of 30 nm or more and less than 50 nm. In the quantum dot type infrared sensing element according to any one of claims 1 to 3,
The quantum dots are formed of either InAs or InGaAs;
The quantum dot infrared detecting element, wherein the intermediate layer is formed of one of GaAs and AlGaAs. A plurality of quantum dot layers having a semiconductor substrate and a plurality of quantum dots and an intermediate layer covering the plurality of quantum dots are aligned with the plurality of photoelectric conversion layers stacked on the semiconductor substrate, and the semiconductor substrate, comprising a strain buffer layer in which a p-type semiconductor layer and an n-type semiconductor layer are laminated, the strain buffer layer being disposed between the plurality of photoelectric conversion layers;
Supplying a positive potential to the photoelectric conversion layer in contact with the p-type semiconductor layer;
An infrared detector comprising a voltage generation source for supplying a negative potential to the photoelectric conversion layer in contact with the n-type semiconductor layer. A plurality of quantum dot layers having a semiconductor substrate and a plurality of quantum dots and an intermediate layer covering the plurality of quantum dots are aligned with the plurality of photoelectric conversion layers stacked on the semiconductor substrate, and the semiconductor substrate, a strain buffer layer in which a p-type semiconductor layer and an n-type semiconductor layer are stacked; and a reflective metal layer that reflects infrared light and is formed on a surface facing the light incident surface. A quantum dot infrared detector disposed between the photoelectric conversion layers of
Supplying a positive potential to the photoelectric conversion layer in contact with the p-type semiconductor layer;
An infrared detector comprising a voltage generation source for supplying a negative potential to the photoelectric conversion layer in contact with the n-type semiconductor layer.

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