JP2008187022A - Infrared detector - Google Patents

Abstract

Light detection performance is improved by supplying a sufficient number of electrons to a conduction band quantum level in a quantum dot by excitation from a valence band.
A quantum dot stacked structure in which a plurality of layers made of quantum dots are stacked, a p-type electrode layer, an n-type electrode layer, and a quantum well structure sandwiched between the p-type electrode layer and the n-type electrode layer Infrared detection comprising: a semiconductor element comprising: a semiconductor element, wherein the semiconductor element is formed at a position in contact with the quantum dot stacked structure.
[Selection] Figure 3

Description

  The present invention relates to an infrared detector having a quantum dot structure as an infrared absorbing portion.

Quantum Dot Infrared Photodetector (QDIP) is an i-type (
(Intrinsic) It has InAs quantum dots sandwiched between GaAs layers. In general, a state in which the moving direction of electrons in a material is constrained in a one-dimensional direction is called a quantum well, and a state constrained in all three-dimensional directions is called a quantum dot. Within the quantum dot, electrons exist in a discrete energy state.

When infrared rays are incident on the quantum dot infrared detector having the above structure, electrons located in the conduction band quantum level in the InAs quantum dots sandwiched between the i-type GaAs layers are excited between subbands. As a result, the electrons leave the quantum dot binding. The quantum dot infrared detector detects infrared rays by capturing electrons excited in the conduction band in this way as current (hereinafter referred to as photocurrent).

  Since quantum dots are formed by a self-forming method in a molecular beam epitaxial apparatus, they are formed in a plane (hereinafter referred to as a layer) perpendicular to the growth direction. In order to increase the efficiency of detecting infrared rays, a device in which a plurality of layers are usually laminated is used as a device.

FIG. 1 shows a laminated structure of a conventional quantum dot infrared detector. Conventionally, this laminated structure is sandwiched between n-type GaAs electrodes, and a potential difference is given between the electrodes. Due to this potential difference, electrons excited between subbands by infrared rays flow between the electrodes and become a photocurrent. FIG. 2 shows a band profile when a photocurrent flows.

As shown in FIG. 2, since a potential difference is applied to the n-type GaAs electrode, the band profile is inclined downward substantially to the right. In FIG. 2, a bulge (negative potential) portion of the conduction band edge occurs between the left n-type GaAs electrode and the central quantum dot stacked structure (i-type GaAs layer portion). The rise of the conduction band edge is formed by electrons diffused from the left n-type GaAs electrode to the i-type GaAs layer portion and electrons bound to the quantum dots. As shown in FIG. 2, the band profile of the quantum dot portion is a structure in which a depression is formed in the downward direction (positive direction) at the boundary between the conduction band and the forbidden band (the lower end of the conduction band) to bind electrons. It has become. In addition, at the boundary of the valence band with the forbidden band (the upper end of the valence band), a protrusion is formed in the upward direction (negative direction), and the hole is bound. When excitation of electrons from the valence band to the conduction band occurs in the quantum dot, holes are left in the protruding part at the valence band edge, and electrons are accumulated in the depression at the conduction band edge.

  When infrared rays are incident while electrons are accumulated in the conduction band of the quantum dots, the bound electrons are disengaged beyond the subband depression range beyond the subband depression.

By the way, even in the state where infrared rays are not incident, electrons located in the low potential side electrode layer (described as 0 V in FIG. 2) flow between the electrode layers due to the potential difference applied between the electrode layers. Become. Since the performance of the detector is generally expressed by a photocurrent / dark current ratio, increasing the photocurrent without increasing the dark current is one problem in improving the performance of the detector.
Eui-Tae Kim et al., High detectivity InAs quantum dot infraredphotodetectors, Applied physics letters, USA, American Institute of Physics, April 26, 2004, vol.84, number17, p.3277-3279

  In order to increase the photocurrent, it is necessary that a sufficient number of electrons exist in the conduction band quantum level in the quantum dot in advance. Conventionally, attempts have been made to supply electrons to quantum levels by adding n-type impurities to quantum dots or i-type GaAs layers sandwiching quantum dots. However, electrons once released outside the quantum dot due to heat or the like are not easily relaxed (not easily transitioned) to the quantum level in the quantum dot whose level has been discretized. As a result, it was difficult to supply a sufficient amount of electrons to the conduction band quantum level in the quantum dot. Therefore, it is a problem to directly supply a sufficient number of electrons to the conduction band quantum level.

  An object of the present invention is to provide a quantum dot infrared detector with improved light detection performance by directly supplying a sufficient number of electrons to the conduction band quantum level in the quantum dot.

  The present invention employs the following means in order to solve the above problems. That is, the present invention provides a quantum dot stacked structure in which a plurality of layers composed of quantum dots are stacked, a p-type electrode layer, an n-type electrode layer, and a quantum sandwiched between the p-type electrode layer and the n-type electrode layer. Infrared detection comprising a semiconductor element having a well structure. Here, the semiconductor element is formed at a position in contact with the quantum dot stacked structure.

  According to the present invention, by applying a voltage to the quantum well structure, light having energy between the levels can be emitted by transition of electrons between the valence band quantum level and the quantum level in the conduction band. it can. Further, the light enters the quantum dot stacked structure, and can excite electrons between energy levels. As a result, constrained electrons can be accumulated in the conduction band in the quantum dot stacked structure. In this state, when infrared rays are incident on the infrared detection, the bound electrons are released from the binding of the quantum dots and contribute to an increase in photocurrent.

  According to the present invention, the light detection performance can be improved by supplying a sufficient number of electrons to the conduction band quantum level in the quantum dot by excitation from the valence band.

  Hereinafter, an infrared detector according to the best mode for carrying out the present invention (hereinafter referred to as an embodiment) will be described with reference to the drawings. The configuration of the following embodiment is an exemplification, and the present invention is not limited to the configuration of the embodiment.

<Outline of the invention>
FIG. 3 shows the basic configuration of the infrared detector. In this infrared detector, a semiconductor element having a structure in which a semiconductor having a quantum well structure is further sandwiched between p-type and n-type electrode layers is formed beside the quantum dot stacked structure.

  That is, in FIG. 3, an intrinsic (i-type) indium gallium arsenide compound (InGaAs) layer is sandwiched between i-type GaAs layers to form a quantum well structure. In this case, a one-dimensional well is formed in the normal direction of the substrate surface (direction perpendicular to the layer surface). Further, this quantum well structure is sandwiched between a p-type gallium arsenide compound (GaAs) electrode and an n-type GaAs electrode layer.

On the other hand, the layer containing quantum dots is formed by incorporating indium arsenic compound (InAs) into the i-type GaAs layer. In this case, the InAs portion in the i-type GaAs layer forms a quantum dot, which is a well that confines electrons in a three-dimensional direction. A stacked structure is formed by stacking a plurality of layers including such quantum dots. Further, the quantum dot stacked structure is sandwiched between n-type GaAs electrode layers.

  Furthermore, the quantum well structure has an energy difference between the conduction band quantum level of the quantum well structure and the valence band quantum level, and the energy between the conduction band quantum level of the quantum dot and the valence band quantum level. It shall be located within the energy range of ± 50 meV with respect to the difference.

In order to satisfy this energy difference condition, for example, the quantum dot material is InAs, and the layer material sandwiching it is a GaAs layer, and the conduction band quantum level and valence band quantum level of the quantum dot derived from them are When the energy difference between them is 1.4 eV at a temperature of about the liquid nitrogen temperature that is the operating temperature of the detector, the quantum well structure has, for example, a thickness of 10 nm with an In composition included in the range 0.05 to 0.13. A one-dimensional quantum well structure having InGaAs as a well layer and GaAs as a barrier layer is employed. The composition ratio means that x is 0.05 to 0.13 when indium gallium arsenide is expressed by the formula In _X Ga _1-X As.

  FIG. 4 shows band profiles at the conduction band edge and the valence band edge in the structure of this embodiment. When a positive potential difference is applied between the p-type GaAs electrode layer and the n-type GaAs electrode layer sandwiching the quantum well structure, electrons (e) are generated from the n-type GaAs electrode layer and holes are generated from the p-type GaAs electrode layer. (H) is supplied to the conduction band and valence band quantum levels of the quantum well structure, respectively.

  Electrons and holes relax between bands in the quantum well structure and emit light. When the energy difference between the quantum levels in the quantum well structure and that in the quantum dot substantially coincide, the light emitted from the quantum well structure becomes excitation light, and the intersubband excitation occurs between the quantum levels in the quantum dot. Then, electrons are generated in the conduction band quantum level and holes are generated in the valence band quantum level.

  As a result, the excited electrons are accumulated in the conduction band quantum level (the depression at the conduction band edge). In such a state, when infrared rays are incident on the quantum dot stacked structure, electrons that are constrained by the conduction band quantum level are released from the constrained state, and compared with the case where there is no excitation by light from the quantum well structure, The detected photocurrent will increase. In this way, according to the structure shown in FIGS. 3 and 4, electrons serving as the basis of the photocurrent are supplied to the conduction band quantum level in the quantum dot by excitation from the valence band without addition of impurities. Therefore, the photocurrent generated when infrared rays are incident is increased.

The energy difference between quantum levels of quantum dots has an energy distribution of approximately ± 50 meV.
In other words, the energy difference between quantum levels of quantum dots has a spread of ± 50 meV, so that the energy difference between quantum levels in the quantum well structure is distributed within the energy range of ± 50 meV with respect to the center energy of this energy difference. However, it can be said that the same phenomenon as described above occurs.

That is, when the energy difference between the quantum levels of the quantum well structure has a spread, the photocurrent increases as a result. On the other hand, in a one-dimensional quantum well structure in which InGaAs with an In composition of 0.05 and 0.13 is used as a well layer and a GaAs layer as a barrier layer, the energy difference between quantum levels is 1.45 at a temperature of about liquid nitrogen temperature. eV, 1.35eV. Therefore, by setting the In composition within these ranges, it is possible to position the energy between quantum levels within an energy range of ± 50 meV with respect to 1.4 eV. In addition, the difference between the energy level between the valence band and the conduction band in the quantum dot and the difference between the energy levels between the valence band and the conduction band in the one-dimensional quantum well structure is approximately ± 50 meV. Become. As a result, the photocurrent at the quantum dots can be increased by the light emitted from the quantum well structure.

By the way, the dark current flowing through the quantum dot infrared detector is mainly caused by the conduction band edge of the quantum dot laminated structure portion with respect to the low potential side electrode layer (described as 0 V in FIG. 4) (conduction band which is a boundary with the forbidden band). Depends on the rise of the bottom edge. This is because this bulge becomes a barrier for electrons located in the low potential side electrode layer, so that the larger the bulge amount, the higher the barrier energy and the smaller the dark current.

The conventional structure and the quantum dot stacked structure portion in this embodiment are i-type semiconductors, and can be regarded as an ni junction formed by the low potential side electrode layer and the quantum dot stacked structure portion. Therefore, in these structures, the amount of rise of the conduction band edge of the quantum dot stacked structure portion with respect to the low potential side electrode layer can be said to be substantially equal, so the dark current amount is also equal in the conventional structure and the structure of the present embodiment.

On the other hand, when an n-type impurity is added to the quantum dots or the i-type GaAs layer sandwiching the quantum dots, the quantum dot stacked structure is an n-type semiconductor on average. Since the junction is an nn junction, the amount of rise of the conduction band edge of the quantum dot stacked structure portion with respect to the low potential side electrode layer is smaller than that of the ni junction, and the dark current amount is larger. The ni junction has the property of being a weak np junction, and n
Diffusion of electrons from the mold region to the i-type region forms a bulge in the negative direction of the conduction band edge. On the other hand, in the nn junction, the bulge at the conduction band edge is smaller than that of the ni junction. Because it becomes.

Therefore, by supplying n-type impurities in the quantum dot and its vicinity, the electron supply to the conduction band quantum level in the quantum dot is increased and the photocurrent is expected to increase, but the dark current also increases at the same time. In this method, the improvement of the photocurrent / dark current ratio as the detector performance is not sufficient.

However, in the structure of this embodiment, such an n-type impurity addition is unnecessary,
Only the photocurrent can be increased without increasing the amount of dark current. For this reason, a sufficient improvement in the photocurrent / dark current ratio is realized as compared with the conventional structure and other means.

<Embodiment 1>
FIG. 5 shows the structure of the quantum dot of the infrared detector according to Embodiment 1 of the present invention. For example, a p-type GaAs layer is grown on a GaAs substrate by a molecular beam epitaxial method at a substrate temperature, for example, 600 degrees Celsius. The thickness is 1000 nm, for example, Be is used as the p-type impurity, and its concentration is 2 × 10 ¹⁸ / cm ³ , for example.

  Subsequently, an i-type GaAs layer, an i-type InGaAs layer, and an i-type GaAs layer are grown in this order at a substrate temperature of 600 degrees Celsius. The thickness of the GaAs layer is 50 nm, for example, and the thickness of the InGaAs layer is 10 nm, for example. The In composition of the InGaAs layer is set to, for example, 0.09 so as to be included in the range of 0.05 to 0.13.

Subsequently, an n-type GaAs layer is grown at a substrate temperature of 600 degrees Celsius. The thickness is 1000 nm, for example, Si is used as the n-type impurity, and its concentration is 2 × 10 ¹⁸ / cm ³ . As described above, the substrate temperature may be controlled to a temperature suitable for crystal growth by molecular beam epitaxy, for example, 600 degrees Celsius.

Subsequently, an i-type GaAs layer is grown. The thickness is, for example, 50 nm, and the substrate temperature is reduced from 600 degrees Celsius to, for example, 500 degrees Celsius during the growth of this layer.

While maintaining the substrate temperature at 500 degrees Celsius, InAs is supplied so that the growth rate becomes, for example, 0.2 ML / s (atomic layer / second). By supplying a certain amount, the compressive strain applied to InAs increases, and InAs grows three-dimensionally to form quantum dots (self-forming method). Here, the reason for lowering the substrate temperature is to facilitate the formation of the quantum dot structure. That is, when the substrate temperature is high, the movement distance of the InAs material supplied on the substrate surface becomes long, so that the phenomenon that InAs locally gathers and grows three-dimensionally does not easily occur. By setting the substrate temperature to about 500 degrees Celsius, it is possible to promote the three-dimensional growth by InAs while reducing the movement distance of the InAs material supplied on the substrate surface.

Subsequently, an i-type GaAs layer is grown to a thickness of 50 nm, for example, while maintaining the substrate temperature at 500 degrees Celsius. In order to set the number of stacked quantum dot layers to 10 for example, the supply of InAs and the growth of the i-type GaAs layer are repeated 9 times thereafter. Here, the substrate temperature is raised from 500 degrees Celsius to 600 degrees Celsius while the last i-type GaAs layer is grown. Here, since the quantum dot stacked structure is already formed, the temperature is more suitable for the crystal growth of the i-type GaAs layer. Subsequently, an n-type GaAs layer is grown at a substrate temperature of 600 degrees Celsius. The thickness is, for example, 1000 nm, Si is used as the n-type impurity, and the concentration is 2 × 10 ¹⁸ / cm ³ .

Excavation to the n-type GaAs electrode layer located in the center and excavation to the p-type GaAs electrode layer are performed by photolithography and dry etching. Subsequently, AuGe / Ni / Au electrodes are formed on the two n-type GaAs electrode layers and AuZn electrodes are formed on the p-type GaAs electrode layers by metal vapor deposition. Here, AuGe / Ni / Au refers to a laminate of a gold germanium alloy, nickel, and gold.

FIG. 6 shows a quantum dot infrared detector structure in which an electrode layer is formed. By applying a potential difference between the n-type GaAs electrode layers and measuring the current flowing between them, it is possible to observe the current change at the time of the incidence of infrared rays and to function as an infrared detector.

  Then, by applying a positive potential difference to the p-type GaAs electrode layer with respect to the n-type GaAs electrode layer located at the center, excitation light emission due to interband relaxation can be caused in the quantum well structure.

<Effect of embodiment>
According to the structure of the present embodiment, the quantum well structure has an energy difference between the conduction band quantum level of the quantum well structure and the valence band quantum level. Same energy difference between levels. Here, the difference value between the two energy differences is the extent of spread in which the energy difference between the conduction band quantum level and the valence band quantum level of the quantum dot is distributed.

Specifically, the energy difference in the quantum well structure is within an energy range of ± 50 meV with respect to the energy difference in the quantum dots. By making such an energy difference, the wavelength of light generated by passing a current through the quantum well structure effectively excites electrons in the valence band quantum level in the quantum dot to the conduction band quantum level. it can.

Therefore, sufficient electrons can be supplied to the conduction band quantum levels in the quantum dots without using a conventional method of injecting n-type impurities into the quantum dot stacked structure. As a result, the generated photocurrent value with respect to the amount of incident infrared rays can be increased without increasing the dark current, and as a result, the detection sensitivity can be improved.

It is a structural diagram of a quantum dot infrared detector in a conventional structure. It is a band profile of a quantum dot type infrared detector in a conventional structure. Structure diagram of quantum dot infrared detector It is a band profile of a quantum dot type infrared detector. It is sectional drawing which shows the structure of the quantum dot which concerns on one Embodiment of this invention. It is a figure which shows the infrared detector structure in which the electrode layer by Embodiment 1 was formed.

Claims (3)

A quantum dot stacked structure in which a plurality of layers made of quantum dots are stacked;
a p-type electrode layer, an n-type electrode layer, and a semiconductor element having a quantum well structure sandwiched between the p-type electrode layer and the n-type electrode layer, and
An infrared detector, wherein the semiconductor element is formed at a position in contact with the quantum dot stacked structure. 2. The infrared detector according to claim 1, wherein an energy difference between a conduction band quantum level and a valence band quantum level of the quantum well structure is between the conduction band quantum level and the valence band quantum level of the quantum dot. An infrared detector characterized by being located within an energy range of ± 50 meV with respect to the energy difference of. 2. The infrared detector according to claim 1, wherein in the quantum dot stacked structure, a material of quantum dots is indium arsenide (InAs), and a layer material sandwiching the quantum dots and forming a plurality of layers is gallium arsenide (GaAs). Yes, when the energy difference between the conduction band quantum level and the valence band quantum level formed when the quantum dot stacked structure is in a temperature range equivalent to the liquid nitrogen temperature is 1.4 eV, The well structure is a one-dimensional quantum well structure in which indium gallium arsenide (InGaAs) with a thickness of 10 nm having an In composition in the range of 0.05 to 0.13 is used as a well layer and gallium arsenide (GaAs) is used as a barrier layer. Infrared detector.

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