JPH0777270B2 - Infrared detector - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to infrared detectors.

[0002]

2. Description of the Related Art HgCdTe, which is a mixed crystal of HgTe and CdTe, has been mainly used as an infrared image sensor material. This is because the photoelectric conversion efficiency of HgCdTe is high, and because the HgTe-CdTe system is a solid solution at all rates, there is an advantage that the infrared wavelength to be detected in a wide range can be set by changing the composition ratio of HgTe and CdTe. Etc.

However, HgCdTe has problems that it is difficult to obtain a large-area and highly-uniform wafer, its mechanical strength is low, and thermal instability is caused mainly by the diffusion and escape of Hg atoms. Therefore, an infrared sensor using a type II superlattice of a III-V group compound semiconductor, in which the strength and thermal stability of the wafer are higher than that of HgCdTe, has been proposed or prototyped.

For example, Applied Physic
s Letters (Applied Physics Letters) Vol, 52, 1581 (1988) and strained InSb / InAsSb strain superlattice, and JP-A-62-1
There is an InAs / GaSb-based superlattice disclosed in Japanese Patent No. 85476. The former is to form a p / i / n structure of an InSb / InAsSb strained superlattice which is type II by lattice strain, and applies a bias voltage in the superlattice growth axis direction to operate as a photodiode. Due to the lattice strain, the spatial indirect transition energy gap between InSb and InAsSb is smaller than the band gap of InAsSb, and thus infrared rays having a longer wavelength than InAsSb can be detected. In the latter, the InAs / GaSb type II superlattice is considered as a series connection of electrons localized in the InAs electron well and electric dipoles of heavy holes localized in the GaSb hole well. . When infrared light is incident in a pulse in the superlattice growth direction, charge transfer occurs due to a spatial indirect transition between the GaSb hole well and the InAs electron well, and an electromotive force is instantaneously generated in the superlattice growth axis direction. This is detected by a pair of electrodes mounted parallel to the superlattice growth surface so as to sandwich the lattice from above and below.

[0005]

By the way, InSb /
In the InAsSb system, it is technically not easy to grow the superlattice while maintaining the lattice strain, and therefore the productivity is not high. On the other hand, the InAs / GaSb system can obtain a superlattice structure relatively easily by MBE growth, but the infrared detector extended earlier is for detecting pulsed infrared light and is continuously used. It cannot be used as an image sensor for detecting infrared light. It is considered that a bias current is applied from a pair of electrodes mounted parallel to the superlattice growth surface along the InAs / GaSb superlattice growth axis to operate as a photoconductive infrared detector. In this case, Such inconvenience occurs.

First, the top of the valence band of GaSb is InAs.
Since it is above the lower end of the conduction band, the superlattice period gradually increases, and the dark current gradually approaches the semimetal state, which causes an increase in dark current. In order to prevent this, the thickness of each layer of InAs and GaSb may be reduced to raise the conduction band subband level and lower the valence band subband level. However, when the conduction band and valence band sub-band levels are changed, the wavelength of infrared light that can be detected also changes. Therefore, by controlling the thickness of each layer of the superlattice, the dark current and the infrared wavelength of detection can be controlled simultaneously. Is difficult When the thickness of InAs / GaSb is thin (Japanese Patent Application Laid-Open No. 62-62160)
In the example of Japanese Patent Laid-Open No. 85476, InAs / GaSb = 3n
m / 5 nm) is particularly difficult, and it is impossible to mass-produce with good reproducibility.

Further, in the type in which the electrodes are mounted parallel to the superlattice growth surface, a bias current is caused to flow along the superlattice growth axis, but the conduction band band offset between InAs / GaSb is about 0.9 eV, In fact, it is difficult to pass an electric current. In addition, the high mobility of the InAs conduction band cannot be used, and the response speed does not increase.
In addition, since the read electrode is attached to the superlattice growth surface which is the light receiving surface, there is a problem that the light receiving area is reduced and the sensitivity is lowered.

An object of the present invention is to form Al _X Ga _1-X Sb / InAs (x> 0.3) composed of a III-V group compound semiconductor.
Using a superlattice, Al _x Ga _1-x Sb / InAs (x> 0.
3) By attaching an electrode to the end face of the superlattice and flowing a bias current in parallel with the superlattice face, the detected infrared wavelength can be controlled by changing the Al composition without changing the superlattice structure. An object of the present invention is to provide an infrared detector capable of high-speed response due to the high mobility of the conduction band and having a large light receiving area and high sensitivity.

[0009]

In order to achieve the above object, an infrared detector of the present invention is an infrared detector having a mesa structure and a pair of electrodes, wherein the mesa structure is epitaxially grown on an insulating substrate. Al _X Ga _1-X Sb / I
The mesa structure is an nAs (x> 0.3) superlattice, and the pair of electrodes is attached to the side surface of the mesa structure and applies a bias current in the superlattice plane direction of the mesa structure.

InAs / which is a type II superlattice
Infrared radiation is detected by the spatial indirect transition from the Al _X Ga _1-X Sb valence band subband to the InAs conduction band subband of the Al _X Ga _1-X Sb superlattice.

[0011]

[Operation] By adopting such a structure, firstly,
The AlSb / InAs superlattice has a spatial indirect transition energy gap of 230 meV from the top of the AlSb valence band to the bottom of the InAs conduction band, and the GaSb / InAs superlattice has -150 meV from the top of the GaSb valence band to the bottom of the InAs conduction band. Since it has a spatial indirect transition energy gap of, by changing the Al composition, a superlattice having a spatial indirect transition energy gap in the range of 0 to 230 meV can be produced when the superlattice period is sufficiently long (however, , The upper limit of the spatial indirect transition energy gap is larger than 230 meV when the superlattice period is short).

Therefore, the superlattice period must be controlled within a small range because the GaSb / InAs superlattice has a negative spatial indirect transition energy gap from the upper end of the GaSb valence band to the lower end of the InAs conduction band. The restriction that it does not happen is relaxed.

FIG. 1 shows a superlattice structure of Al _X Ga _1-X Sb / I.
The results of calculating the difference between the ground states of the conduction band, the light hole band, and the heavy hole band subbands with respect to the composition x when nAs = 5 nm / 10 nm are shown. In FIG. 1, h
h ₁ -e ₁ indicates the space indirect transition energy gap between the heavy hole and the conduction band, and 1 h ₁ -e ₁ indicates the space indirect transition energy gap between the light hole and the conduction band. As can be seen from FIG. 1, the superlattice period, or more accurately, Al _X Ga _1-X S
The detected infrared wavelength can be controlled by changing the Al composition without changing the thickness of each of the b and InAs layers. Small Al composition, Al _X Ga _1-X Sb /
When the InAs superlattice system approaches a semi-metallic state, the dark current when used as an infrared detector increases. Therefore, in order to prevent the dark current from increasing, the heavy hole band subband basis of Al _x Ga ₁ -x Sb is used. When the lower limit of the energy gap from the level to the InAs conduction band subband ground level is 100 meV, the allowable Al composition x range is x> 0.3.

Secondly, since the infrared detector having the structure of the present invention is of a type in which a voltage change is read from a pair of electrodes attached so that a bias current can be applied in parallel with the superlattice plane, a pulsed infrared ray is detected. It can detect not only light but continuous infrared light, and is suitable for infrared image sensors. Further, with such a structure, the high mobility of the InAs conduction band can be utilized, so that an infrared detector with a fast response can be obtained.

Thirdly, the bias current application and signal readout electrodes are epitaxially grown Al _X Ga _{1 -X} Sb.
Since the / InAs superlattice is attached to the side surface of the mesa structure, shielding of the superlattice surface, which is the infrared light incident surface, is minimized, and a highly sensitive infrared detector can be obtained.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. As shown in FIG. 2, Al _0.5 Ga _0.5 Sb 5 nm
/ InAs 10nm superlattice 3 is an insulating GaAs substrate 1
15 through the GaSb buffer layer 2 having a thickness of 1 μm
Epitaxial growth is performed for 0 period. This superlattice layer 3
2, is processed into a mesa shape, and a pair of Au electrodes 4 are attached to the side surfaces of the mesa.

When detecting infrared rays, if a bias current is applied from both ends of the electrode 4 in the layer direction of the Al _0.5 Ga _0.5 Sb 5 nm / InAs 10 nm superlattice 3, the carriers generated by the irradiation of infrared light will be generated. As a result, a voltage change occurs across the electrode 4, which is read.

Subband levels corresponding to the superlattice structure of FIG. 2 are shown in FIG. At the Al composition x = 0.5, the lower end of the conduction band of InAs is 55 meV above the upper end of the valence band of Al _0.5 Ga _0.5 Sb, and a spatial indirect transition energy gap of 55 meV for a superlattice with a sufficiently large period. Hold. In the example of FIG. 2, since the superlattice period is 15 nm, which is relatively short, the conduction band, the heavy hole band, and the light hole band form subbands as shown in FIG. Is located above the bottom of the original conduction band for the conduction band and below the top of the original valence band for the hole band. In FIG. 3, for the conduction band and the hole band, the permissible bands corresponding to the ground level and the first excitation level are shown by hatching. e ₁ and e ₂ are the ground level and the first excitation level of the conduction band subband, and hh ₁ and hh ₂ are the ground level and the first excitation level of the heavy hole band subband, respectively, 1h ₁ , 1
h ₂ represents the ground level and the first excitation level of the light hole band subband, respectively. Since hh ₁ and hh ₂ are narrow in width and cannot draw diagonal lines, they are shown only by solid lines. E _C and E _V indicate the bottom of the conduction band and the top of the valence band, respectively. From FIG. 3, it can be seen that the heavy hole band has a very narrow subband width and little overlap with the valence band wavefunction, whereas the light hole band has a wide subband width and has a valence band wavefunction. It can be seen that there is a large overlap. Therefore, the infrared detector of FIG.
_It operates as an infrared detector with a cut-off wavelength of 4.7 μm corresponding to an energy of ₁ −1 h ₁ = 265 meV.

In the above embodiments, the superlattice structure is made of Al _0.5.
Ga _0.5 Sb 5 nm / InAs 10 nm was used.
The l composition x is not limited to x = 0.5, but x (> 0.
The cut-off wavelength of the infrared detector can be controlled by changing the value of 3). Also, the superlattice period is Al
Not limited to _X Ga _1-X Sb 5 nm / InAs 10 nm, but change the cut-off wavelength of the infrared detector by changing the thickness of the Al _X Ga _1-X Sb layer and InAs layer after determining the Al composition x. You can also let it.

[0020]

As described above, according to the present invention, the detected infrared wavelength can be controlled by changing the Al composition without changing the superlattice period, and the high mobility of the InAs conduction band can be obtained. An infrared detector capable of high-speed response and having a large light-receiving area and high sensitivity can be obtained.

[Brief description of drawings]

FIG. 1 shows a superlattice structure of Al _x Ga ₁ -x Sb / InAs = 5.
It is a figure which shows the result of having calculated with respect to Al composition x the difference between the ground levels of a conduction band, a light hole band, and a heavy hole band subband in the case of nm / 10 nm.

FIG. 2 is a diagram showing an example of the present invention.

FIG. 3 is a subband diagram corresponding to the Al _0.5 Ga _0.5 Sb / InAs superlattice of FIG.

[Explanation of symbols]

1 Insulating GaAs substrate 2 GaSb buffer layer 3 Al _0.5 Ga _0.5 Sb / InAs superlattice 4 Au electrode hh ₁ -e ₁ Heavy hole-space indirect transition energy gap between conduction bands 1h ₁ -e ₁ Light hole-conduction Spatial indirect transition energy gap between bands E _C bottom of conduction band E _V top of valence band e ₁ conduction band subband ground level e ₂ conduction band subband first excitation level 1h ₁ light hole band subband ground level 1h ₂ light hole band subband first excitation level hh ₁ heavy hole band subband ground level hh ₂ heavy hole band subband first excitation level

Claims (2)

[Claims] 1. An infrared detector having a mesa structure and a pair of electrodes, wherein the mesa structure is epitaxially grown on an insulating substrate.
l _X Ga _1-X Sb / InAs (x> 0.3) superlattice mesa structure, a pair of electrodes is attached to the side surface of the mesa structure, and a bias current is applied in the direction of the superlattice surface of the mesa structure. An infrared detector characterized in that 2. A type II superlattice, InAs / Al.
_The infrared ray according to claim 1, wherein the infrared ray is detected by a spatial indirect transition from the Al _X Ga _1-X Sb valence band subband to the InAs conduction band subband of the _X Ga _1-X Sb superlattice. Detector.