JP4829809B2 - Quantum well infrared detector - Google Patents

Description

  The present invention relates to a quantum well infrared detector (QWIP) including, for example, a multiple quantum well (MQW) layer.

  Conventionally, as shown in FIG. 5, the quantum well infrared detector (QWIP) has an element structure in which a quantum well composed of a lower contact layer 105, a well layer 101, and a barrier layer 102 is stacked in multiple layers on a substrate 100. The multi-quantum well (MQW) layer 103 (here, the barrier layer and the well layer are repeated 50 times) and the upper contact layer 104 are sequentially stacked. The structure is sandwiched between the lower contact layers 105.

Such a quantum well infrared detector is used with a bias voltage applied between the upper and lower contact layers 104 and 105 as shown in FIG.
Here, FIG. 6 shows an energy band diagram on the conduction band edge side when a bias voltage is applied between the upper and lower contact layers 104 and 105 in the quantum well infrared detector having such a structure.

  In such a quantum well infrared detector, as shown in FIG. 6, electrons (carriers) located at the ground level in the well layer 101 are excited to the excitation level by infrared rays incident from the outside. . Since a bias voltage is applied between the upper and lower contact layers 104 and 105, the excited electrons flow between the contact layers 104 and 105 and become a current (hereinafter referred to as a photocurrent). That is, when infrared rays are incident from the outside, photoexcited carriers are extracted from the quantum well composed of the well layer 101 and the barrier layer 102, and a photocurrent is obtained. Infrared light is detected by detecting this photocurrent.

In addition, as a prior art document regarding such a quantum well type infrared detector, for example, there is Non-Patent Document 1.
BF Levine, "Quantum-well infrared photodetectors", J. Appl. Phys. 74 (8), 15 October 1993

However, in the quantum well infrared detector as described above, as shown in FIG. 6, thermally excited carriers (electrons) are also taken out of the quantum well and become dark current. Ideally, it is desirable to reduce the dark current and increase the photocurrent.
For example, in order to extract a large amount of photocurrent, as shown in FIGS. 7A and 7B, the bias voltage is increased from the low bias state to the high bias state and formed at the conduction band edge of the barrier layer 102. It is conceivable to make the triangular potential barrier thinner.

However, in this case, the probability that thermally excited carriers also pass through the barrier layer 102 increases, and the dark current also increases.
The present invention was devised in view of such a problem, and an object thereof is to provide a quantum well infrared detector capable of increasing a photocurrent while suppressing an increase in dark current. .

Therefore, the quantum well infrared detector of the present invention includes a quantum well layer and a contact layer that is provided so as to sandwich the quantum well layer and applies an operation bias, and the quantum well layer is a well made of InGaAs. A first barrier layer in contact with the well layer, and a second barrier layer in contact with the first barrier layer, the first barrier being positioned on the positive bias side with respect to the well layer when an operation bias is applied. The first barrier layer is made of AlGaSb having an effective electron mass smaller than that of InGaAlAs constituting the second barrier layer .
The quantum well infrared detector of the present invention includes a quantum well layer and a contact layer that is provided so as to sandwich the quantum well layer and applies an operation bias, and the quantum well layer is a well layer made of GaAs. And a first barrier layer in contact with the well layer and a second barrier layer in contact with the first barrier layer, the first barrier layer being positioned on the positive bias side with respect to the well layer when an operation bias is applied. The second barrier layer is provided, and the first barrier layer is made of InAlAs, which has a smaller effective mass of electrons than AlGaAs constituting the second barrier layer.

  Therefore, according to the quantum well infrared detector of the present invention, there is an advantage that the photocurrent can be increased while suppressing an increase in dark current.

Hereinafter, a quantum well infrared detector according to an embodiment of the present invention will be described with reference to FIGS.
A quantum well infrared detector (QWIP) according to the present embodiment includes, for example, a lower contact layer 7, a barrier layer 8, a plurality of well layers 2, on a substrate 1, as shown in FIG. The first barrier layer 3, the second barrier layer 4, and the upper contact layer 6 are sequentially stacked, and the first barrier layer 3 in contact with the well layer 2, the well layer 2, and the second barrier layer 3 in contact with the first barrier layer 3. A multi-quantum well (MQW) layer 5 (here, the well layer 2, the first barrier layer 3, and the second barrier layer 4 are repeated 50 times) in which a quantum well composed of the barrier layer 4 is stacked in multiple layers is a lower contact layer. 7 and the upper contact layer 6.

Such a quantum well infrared detector is used in a state where a bias voltage (operation bias) is applied between the upper and lower contact layers 6 and 7, as shown in FIG.
Here, as shown in FIG. 1, the first barrier layer 3 and the second barrier layer 4 are provided so as to be positioned on the positive bias side with respect to the well layer 2 when an operation bias is applied.
In particular, the first barrier layer 3 and the second barrier layer 4 are made of different materials. That is, as shown in FIG. 2, the first barrier layer 3 is made of a material having an effective electron mass smaller than that of the second barrier layer 4.

  In particular, the thickness of the first barrier layer 3 is preferably as thin as possible in order to suppress an increase in dark current. Here, as shown in the energy band diagram of FIG. 2, the energy level at the conduction band edge of the first barrier layer 3 becomes higher than the excitation level in the well layer 2 in the state where the operation bias is applied, and The thickness of the first barrier layer 3 is set so that the energy level at the conduction band edge of the second barrier layer 4 is lower than the excitation level in the well layer 2.

The first barrier layer 3 is made of a material having the same barrier energy height as that of the second barrier layer 4. That is, as shown in FIG. 2, the energy level at the conduction band edge of the first barrier layer 3 and the energy level at the conduction band edge of the second barrier layer 4 are not discontinuous (no conduction band discontinuity). The material of the first barrier layer 3 is selected.
Specifically, the material of the well layer 2 is preferably InGaAs, the material of the first barrier layer 3 is preferably AlGaSb, and the material of the second barrier layer 4 is preferably InGaAlAs. A specific example in which such a material is employed will be described in detail in Example 1 described later.

It is also preferable that the material of the well layer 2 is GaAs, the material of the first barrier layer 3 is InAlAs, and the material of the second barrier layer 4 is AlGaAs. A specific example in which such a material is employed will be described in detail in Example 2 described later.
Here, the probability that electrons pass through the triangular potential barrier formed at the conduction band edge of the barrier layer (transmission probability of the triangular potential) is expressed by the following formula (1) (SM Semiconductor original, “Semiconductor Device physics (1) ", Corona, page 152).

In the present embodiment, since it is configured as described above, carriers (electrons) excited by light pass through the first barrier layer 3 having a small effective mass as shown in FIG. While it increases and increases, the thermally excited carriers (electrons) do not increase so much because they pass through the first barrier layer 3 and the second barrier layer 4.
Therefore, according to the quantum well infrared detector according to the present embodiment, there is an advantage that the photocurrent can be increased while the dark current hardly increases.

  The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.
[Example 1]
In Example 1, as a quantum well infrared detector (QWIP), the peak sensitivity wavelength is set to about 8.5 μm excellent in atmospheric transmission characteristics. For example, InGaAs (In composition˜0 .5), AlGaSb is used as the material of the first barrier layer 3, and InGaAlAs is used as the material of the second barrier layer 4.

The thickness of the InGaAs well layer 2 is 50 mm, the thickness of the AlGaSb first barrier layer 3 is 138 mm, and the total thickness of the AlGaSb first barrier layer 3 and the InGaAlAs second barrier layer 4 is 500 mm. Furthermore, the number of stacked layers of the multiple quantum well layer 5 is 25, and the bias voltage is 2V.
Here, FIG. 3 shows an energy band diagram on the conduction band edge side when a bias voltage is applied between the upper and lower contact layers 6 and 7. In FIG. 3, E ₁ represents an excitation level (here 240 meV), E _f represents a Fermi level, and E ₀ represents a ground level (here 80 meV).

Assuming an epitaxial crystal lattice-matched to the InP substrate 1, in order to make the peak sensitivity wavelength about 8.5 μm in the well layer 2 of InGaAs (In composition˜0.5), In _1-xy Ga _x The composition of Al _y As is suitably about x = 0.235 and y = 0.240.
_{Here, In 1-xy Ga x Al} y bandgap and electron effective mass ratio of As, the following formula (2) is expressed by (3) [D. Olego et al . "Compositional dependence of band-gap energy and conduction-band effective mass of In _1-xy Ga _x Al _y As lattice matched to InP ", Appl. Phys. Lett., 41 (5), p.477 (1982)].

Therefore, in the case of the above composition in Example 1, the band gap is 1.1246 (eV), and the effective mass ratio of electrons is 0.06339.
By the way, in Example 1, since Al _x Ga _1-x Sb is used as the material of the first barrier layer 3, the band gap of Al _x Ga _1-x Sb is expressed by the following formula (4) [A .Bignazzi et al. "Direct- and indirect-energy-gap dependence on Al concentration in Al _x Ga _1-x Sb (x≤0.41)", Phys. Rev. B, 57 (4), p.2295 (1998) reference].

Thus, in x = 0.249, the band gap is _{In 1-xy Ga x Al y} As (x = 0.235, y = 0.240) and becomes comparable, discontinuity of the conduction band is reduced.
The effective mass ratio of electrons is expressed by the following formula (5) [H. Mathieu et al. “Electronic energy levels in Ga _1-x Al _x Sb alloys”, Phys. Rev. B, 12 (12) , p. 5846 (1975)].

Therefore, the effective mass ratio of electrons is 0.05294.
That is, the effective mass ratio of electrons is 0.06339 in InGaAlAs as the second barrier layer 4 and 0.05294 in AlGaSb as the first barrier layer 3.
Then, from a typical element structure and element bias, a value of internal electric field F = 1.45 × 10 ⁶ (V / m) = 0.145 (mV / Å) is adopted, and the triangular potential height is about 20 ( Assuming meV), when calculating the left side of the above equation (1), when the triangular potential barrier is formed by a barrier layer made of InGaAlAs, the transmission probability of the triangular potential is 0.035341, whereas this embodiment As in Example 1, when the triangular potential barrier is formed by the barrier layer 3 made of AlGaSb, the transmission probability of the triangular potential is 0.047130, which is improved by about 33%. This improvement is considered to be an increase in photocurrent.

Next, the change in dark current is estimated. The dark current I _d is approximately represented by the following formula (6).

Since the band structure is as shown in FIG. 3, when the right side of the above equation (6) is calculated, the dark current I _d is 4.57 × 10 ^{−20 in the} absence of the first barrier layer 3 made of AlGaSb. On the other hand, when there is the first barrier layer 3 made of AlGaSb, the dark current I _d is 4.90 × 10 ⁻²⁰ , which is an increase of about 1.07 times. Here, since the noise is proportional to the square root of the dark current, the increase is only 1.03 times.

Since changes in photocurrent and dark current can be estimated in this way, by inserting the first barrier layer 3 made of AlGaSb, the S / N is increased by 1.29 times and improved by 29%. Become.
[Example 2]
In Example 2, as the quantum well infrared detector (QWIP), for example, GaAs is used as the material of the well layer 2, InAlAs is used as the material of the first barrier layer 3, and AlGaAs is used as the material of the second barrier layer 4. Use.

According to such a configuration, an effect can be expected when the peak sensitivity wavelength is about 5.6 μm. When the peak sensitivity wavelength is about 5.6 μm, the atmospheric transmission characteristics are not good, but an effect can be expected when used outside the atmosphere.
In Example 2, the thickness of the GaAs well layer 2 is 48 mm, the thickness of the InAlAs first barrier layer 3 is 116 mm, and the total thickness of the InAlAs first barrier layer 3 and the AlGaAs second barrier layer 4 is 400 mm. It is said. Furthermore, the number of stacked layers of the multiple quantum well layer 5 is 25, and the bias voltage is 2V.

Here, FIG. 4 shows an energy band diagram on the conduction band edge side when a bias voltage is applied between the upper and lower contact layers 6, 7. In FIG. 4, E ₁ represents an excitation level (here, 332 meV), E _f represents a Fermi level, and E ₀ represents a ground level (here, 110.6 meV).
When the well layer 2 made of GaAs is QWIP having a width of about 50 mm and the peak sensitivity wavelength is about 5.6 μm, the composition of Al _x Ga _1-x As as the second barrier layer 4 is about x = 0.38. It becomes.

Here, the effective mass ratio of the band gap and electrons of Al _x Ga _1-x As is expressed by the following formula (7) [Sadao Adachi, “GaAs, AlAs, and Al _x Ga _1-x As: Material parameters for use in research. and device applications ", J. Appl. Phys., 58 (3), R1 (1985)] and the following formula (8) [M. Zachau et al." Electronic transport in molecular-beam-epitaxy-grown Al _x Ga. " _1-x As ", Phys. Rev. B, 33 (12), p.8564 (1986)].

Therefore, in the case of the above composition in Example 2, the band gap is 1.80979 (eV), and the effective mass ratio of electrons is 0.09884.
By the way, in Example 2, since In _y Al _1-y As is used as the material of the first barrier layer 3, the band gap of In _y Al _1-y As is expressed by the following formula (9) [I Nurgaftman et al. "Band parameters for III-V compound semiconductors and their alloys" J. Appl. Phys., 89 (11), p.5815 (2001)].

Therefore, at y = 0.386, the band gap is almost the same as Al _x Ga _1-x As (x = 0.38), and the conduction band discontinuity is reduced.
The effective mass ratio of electrons is expressed by the following formula (10) [I. Nurgaftman et al. “Band parameters for III-V compound semiconductors and their alloys” J. Appl. Phys., 89 (11), p.5815 (2001)].

Therefore, the effective mass ratio of electrons is 0.09052.
That is, the effective mass ratio of electrons is 0.09884 in AlGaAs as the second barrier layer 4 and 0.09052 in InAlAs as the first barrier layer 3.
Then, from a typical element structure and element bias, a value of internal electric field F = 1.72 × 10 ⁶ (V / m) = 0.172 (mV / Å) is adopted, and the triangular potential height is about 20 ( Assuming meV), when calculating the left side of the above equation (1), when the triangular potential barrier is formed by a barrier layer made of AlGaAs, the transmission probability of the triangular potential is 0.029559. When the triangular potential barrier is formed by the first barrier layer 3 made of InAlAs as in Example 2, the transmission probability of the triangular potential is 0.034394, which is improved by about 16%. This improvement is considered to be an increase in photocurrent.

Next, the change in dark current is estimated.
Since the dark current I _d is expressed by the above formula (6), the band structure is as shown in FIG. 4. Therefore, when the right side of the above formula (6) is calculated, there is no first barrier layer 3 made of InAlAs. In this case, the dark current I _d is 9.28 × 10 ⁻²⁷ , whereas when the first barrier layer 3 made of InAlAs is present, the dark current I _d is 9.88 × 10 ⁻²⁷ , which is approximately 1.06. It is a double increase. Here, since the noise is proportional to the square root of the dark current, the increase is only 1.03 times.

  Since changes in photocurrent and dark current can be estimated in this way, by inserting the first barrier layer made of InAlAs, the S / N is increased 1.13 times, which is improved by 13%. .

It is typical sectional drawing which shows the structure (element structure) of the quantum well type | mold infrared detector concerning one Embodiment of this invention. It is an energy band diagram for demonstrating the principle of operation of the quantum well type | mold infrared detector concerning one Embodiment of this invention. It is an energy band diagram of the quantum well type | mold infrared detector concerning Example 1 of this invention. It is an energy band diagram of the quantum well type | mold infrared detector concerning Example 2 of this invention. It is typical sectional drawing which shows the structure (element structure) of the conventional quantum well type | mold infrared detector. It is an energy band diagram for demonstrating the principle of operation of the conventional quantum well type | mold infrared detector. (A), (B) is an energy band diagram for demonstrating the subject of the conventional quantum well type | mold infrared detector.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Well layer 3 First barrier layer 4 Second barrier layer 5 Multiple quantum well (MQW) layer 6 Upper contact layer 7 Lower contact layer 8 Barrier layer

Claims (4)

A quantum well layer;
A contact layer provided so as to sandwich the quantum well layer, and for applying an operation bias,
The quantum well layer includes a well layer made of InGaAs , a first barrier layer in contact with the well layer, and a second barrier layer in contact with the first barrier layer, and the well layer when the operation bias is applied The first barrier layer and the second barrier layer are provided so as to be positioned on the positive bias side with respect to the first barrier layer, and the first barrier layer is more effective in electrons than InGaAlAs constituting the second barrier layer. A quantum well infrared detector characterized by being composed of AlGaSb having a small mass . A quantum well layer;
A contact layer provided so as to sandwich the quantum well layer, and for applying an operation bias,
The quantum well layer includes a well layer made of GaAs, a first barrier layer in contact with the well layer, and a second barrier layer in contact with the first barrier layer, and the well layer when the operation bias is applied The first barrier layer and the second barrier layer are provided so as to be positioned on the positive bias side with respect to the first barrier layer, and the first barrier layer is more effective than the AlGaAs that constitutes the second barrier layer. A quantum well infrared detector characterized by being composed of InAlAs having a small mass. 3. The quantum well infrared detector according to claim 1, wherein the first barrier layer is made of a material having a barrier energy height equal to that of the material of the second barrier layer . 4. With the operating bias applied, the energy level of the conduction band edge of the first barrier layer is higher than the excitation level in the well layer, and the energy level of the conduction band edge of the second barrier layer is The quantum well type according to any one of claims 1 to 3, wherein a thickness of the first barrier layer is set so as to be lower than an excitation level in the well layer. Infrared detector .

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