JPH0648732B2 - Photovoltaic infrared detector - Google Patents

Description

The present invention relates to a structure of a photovoltaic infrared detector using a semiconductor having a narrow bandgap.

[Conventional technology]

It is generally known that an infrared detector using a semiconductor having a narrow bandgap has high sensitivity. As this detector, for example, there is a photoconductive photovoltaic element using Hg _1-X CD _X Te. Of these, the former is the current mainstream, but the latter is considered to be the mainstream in the future due to its features such as low power consumption.

A typical form of the photovoltaic element is a photodiode using a pn junction, and an example of this structure is shown in the sectional view of FIG. In the figure, 21 is a sapphire substrate, 4 is p
Type Hg _0.8 Cd _0.2 Te, 22 is an adhesive such as epoxy,
5 is an n ⁺ layer formed on p-type Hg _0.8 Cd _0.1 Te 4 by ion implantation or the like, 6 is a surface protective insulating film such as ZnS, 7 and 8 are p-side and n ⁺ -side electrodes, 10 Gold lead wire, 11 is incident infrared light. In this example, the n ⁺ p junction formed by ion implantation or the like operates as a photodiode, and infrared light having a wavelength of about 10 μm or less can be detected corresponding to the forbidden band width of Hg _0.8 Cd _0.2 Te.

[Problems to be solved by the invention]

Generally, a problem with such a narrow bandgap photodiode is that the dark current is large. The component of this dark current is, for example, "Semiconductors and Semimetals (Se
miconductors and semimetals) "18 volumes (1981)
As described in Chapter 6 of the above, three types of diffusion current, GR current, and tunnel current can be considered. Among them, the tunnel current is negligible in the vicinity of 77 ° K, which is the temperature at which the photovoltaic element is used, compared to others. In addition, the magazine "Journal of Upright Physics (Journal of A
pplied Physics) ", Vol. 58, P372 (198
As described in 5)), if a composite insulating film of an anodic sulfide film and ZnS is used as the surface protective insulating film 6, G-
The R current becomes negligible compared to the diffusion current. Therefore, in order to further reduce this dark current, it is important to reduce the diffusion current.

As a means for reducing this diffusion current, as described in the above-mentioned “Semiconductors and Semimetals”, the thickness of p-type Hg _0.8 Cd _0.8 Te 4 is small and the back surface (the surface on the adhesive side) is It is necessary to reduce the surface recombination rate. At this time, p-type Hg _0.8 Cd
The thickness of _0.2 Te4 needs to be smaller than the fractional carrier diffusion length, and the fractional carrier diffusion length is several tens of μm.
Since it is a degree, the thickness will be less than this.

In order to reduce the surface recombination rate of the back surface, for example, a surface protective insulating film may be formed on the back surface as described above. To further reduce the influence of this surface recombination, p near
^{For the +} layer, it is effective to use npp ⁺ coupling as a whole. As a result, the minority carriers in p-type Hg _0.8 Cd _0.8 Te are less likely to reach the back surface due to the presence of the p ⁺ layer, and the influence of surface recombination is reduced.

In order to form this p ⁺ layer, it is necessary to thermally diffuse a substance that serves as an acceptor, such as gold or phosphorus. However, Hg _0.8 C
For substances with weak atomic bonds such as d _0.2 Te, the temperature should be 1
If the temperature is raised to above 00 ° C., the constituent elements, such as mercury, are separated from the surface and lattice defects such as vacancies occur near the surface, so it is extremely difficult to control such a process using high temperature. When diffusing gold, p-type Hg _0.8 Cd _0.2
The thinner Te, the more difficult it is to form the p ⁺ layer only on the back surface. Therefore, it is difficult to reduce the diffusion current by such a method.

The object of the present invention is to solve such a problem and to provide p ⁺ on the back surface.
It is an object of the present invention to provide a porphomotive type semiconductor infrared detecting group in which the influence of surface recombination on the back surface is reduced and the diffusion current is reduced without forming a layer.

[Means for solving problems]

The photovoltaic infrared detector of the present invention has a structure in which a semimetal layer made of a semimetal, an insulating layer made of a semi-insulating semiconductor, and a semiconductor layer made of a semiconductor having a narrow band gap are sequentially formed on an epitaxially grown substrate. In addition, a photodiode is formed on the semiconductor layer having the narrow bandgap, and an electrode to which a voltage is applied is provided on the semi-metal layer.

[Action]

According to the configuration of the present invention, since the semiconductor layer serving as the photodiode portion is formed on the epitaxially grown substrate via the semi-metal semiconductor layer and the semi-insulating semiconductor layer, the MIS is substantially formed on the back surface of the photodiode. (Metal-insulator-
Since the (semiconductor) structure is formed, by applying an appropriate voltage to the semi-metal semiconductor layer, the back surface of the photodiode can be put into an accumulation state. In this case, the photodiode is equivalent to taking an n ⁺ pp ⁺ junction, so that the diffusion current is reduced and the photodiode operates with a small dark current.

〔Example〕

 Embodiments of the present invention will now be described with reference to the drawings and drawings.

FIG. 1 is a sectional view showing an embodiment of the present invention. In the figure, 1 is a substrate of epitaxially grown CdTe or the like, 2
Is a Hg _0.8 Cd _0.8 Te layer, 3 is a semi-insulating CdTe layer,
4 is p-type Hg _0.8 Cd _0.2 Te layer, 5 is p-type Hg _0.8 C
An n ⁺ layer formed by a method such as ion implantation on the d _0.2 Te layer 4, 6 is a surface protective insulating film such as ZnS, 7 and 8 are p-side and n ⁺ -side electrodes, and 9 is Hg _0.9 Cd _0.1. T
Electrodes for voltage application to the e-layer 2 are gold lead wires, 11 is incident infrared light. In this example, the infrared detecting section is composed of a photodiode with an n ⁺ p junction formed in Hg _0.8 Cd _0.1 Te and can detect infrared rays with a maximum wavelength of about 10 μm.

The roles of the Hg _0.8 Cd _0.1 Te layer 2 and the semi-insulating CdTe layer 3 in this example will be described below. The forbidden band width of Hg _1-X Cd _X Te, which is a mixed crystal semiconductor, changes depending on its X value, and at 77 ° K, which is the operating temperature of a general sensing element,
As described in the above-mentioned magazine "Semiconductors and Semimetals", Vol. 18, when X is about 0.13, the forbidden band width becomes zero, and when X is less than that, it becomes a semimetal.

Therefore, the Hg _0.9 Cd _0.1 Te layer 2 can be regarded as a semi-metal layer, whereby the Hg _0.9 Cd _0.1 Te layer 2, the semi-insulating CdTe layer 3, and the p-type Hg _0.8 Cd _0.2 Te layer 4 are MIS (metal It can be regarded as an (insulator-semiconductor) structure. At this time, the physical properties of CdTe, such as its lattice constant, are very close to those of Hg _0.8 Cd _0.2 Te, so the interface characteristics between them are extremely good.

Therefore, the electrode 7 is used as the reference potential, and the electrode 9 or Hg
By applying a negative voltage to the _0.9 Cd _0.1 Te layer 2, the lower surface of the p-type Hg _0.8 Cd _0.1 Te layer 4 can be put into an accumulation state. Therefore, the same effect as that in which the p ⁺ layer is formed on the surface opposite to the one in which the photodiode is formed is obtained, and the photodiode is substantially an n ⁺ pp ⁺ junction, so that this photodiode flows. The diffusion current is reduced, and the device operates as an infrared detection element with a small dark current.

〔The invention's effect〕

As described above, according to the present invention, the dark current of the photovoltaic infrared detector can be reduced, and a high-performance infrared detector can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view of a photovoltaic infrared detector according to an embodiment of the present invention, and FIG. 2 is a sectional view of an example of a conventional photovoltaic infrared detector. 1 ... CdTe substrate, 2 ... Hg _0.9 Cd _0.1 Te layer,
3 ... Semi-insulating CdTe layer, 4 ... p-type Hg _0.8 Cd
_0.2 Te layer, 5 ... n ⁺ layer, 6 ... Surface protection insulating film, 7
...... p side electrode, 8 ...... n ⁺ side electrode, 9 ...... voltage application electrode, 10 ...... gold lead wire, 11 ...... incident infrared light, 21 ...
... Sapphire substrate, 22 ... Adhesive.

Claims (1)

[Claims] 1. A semimetal layer made of a semimetal, an insulating layer made of a semi-insulating semiconductor, and a semiconductor layer made of a semiconductor having a narrow bandgap are sequentially formed on an epitaxially grown substrate. A photovoltaic infrared detector characterized in that a photodiode is formed on the layer, and an electrode to which a voltage is applied is provided on the semi-metal layer.