JPH0569370B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention improves the sensitivity of photoconductive infrared detectors by effectively extending the lifetime of excess minority carriers generated in crystals that absorb infrared rays. Concerning an improved photoconductive infrared detector.

[Conventional technology]

A conventional photoconductive infrared detector has electrodes 1 and 2 for passing a bias current, as shown in FIG. A potential difference proportional to the voltage was taken out from the same electrode and amplified.
Infrared Physics Volume 15 (1975)
111 pages). In FIG. 2b, 5 is an anodized film, 6 is an insulating film transparent to infrared rays, 8 is an adhesive, and 9 is a substrate. However, according to this structure, excessive minority carriers generated in the crystal 4 by infrared rays drift and diffuse to the electrodes 1 and 2, and are recombined and carriers are lost, resulting in deterioration of responsivity. In order to reduce this effect, the overlap structure shown in Figure 3a and b was devised (Infrared Physics, such as MAKinch).
Volume 17 (1977) p. 137). As can be seen from the figure, the feature of this device is that the electrodes 1 and 2 not only contact the crystal 4, but also touch a part of the insulating film 6 that is transparent to infrared rays and is attached to the crystal 4. By covering it, it also serves as a light-shielding film. By employing this structure, it is possible to increase the distance over which excess minority carriers drift and diffuse to the electrode with a high recombination rate, and therefore the effective lifetime of the carriers can be further extended. As a result, the responsivity is not degraded as much as in the devices with the structures shown in FIGS. 2a and 2b.

[Problem that the invention seeks to solve]

However, even in the device with the overlap structure shown in FIGS. 3a and 3b, there is a problem that the responsivity is still lower than the theoretical value due to the short life of carriers due to recombination of excess minority carriers on the side wall of the light receiving section.

SUMMARY OF THE INVENTION An object of the present invention is to provide a photoconductive infrared detector with improved sensitivity by reducing the frequency with which excess minority carriers recombine on the crystal sidewalls.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides bias current electrodes at both ends of a crystal that absorbs infrared rays, and a passivation film in the center, and an insulating film transparent to infrared rays deposited on the bias current electrode, an opening having an area smaller than the central portion and spaced apart from the side wall of the crystal by a carrier diffusion length or more; , a light-shielding mask is provided to overlap the bias current electrode.

[Effect]

FIG. 1a is a plan view showing an embodiment of the photoconductive infrared detector of the present invention, and FIG.
It is a sectional view at A'. In the figure, the same components as in the conventional example are given the same reference numerals as in FIGS. 2 and 3 and will be explained.

The photoconductive infrared detector of the present invention has bias electrodes 1 and 2 placed at both ends of a crystal 4 that absorbs infrared rays.
Further, an anodic oxide film 5 is provided as a passivation film in the center of the crystal, and a light-receiving part 3 and There is an opening with a smaller area than the central part, and the same electrodes 1 and 2 are formed when viewed from above the element.
It has a light shielding mask 7 that overlaps with the light shielding mask 7. Further, it is sufficient that the light-shielding mask overlaps the bias electrodes 1 and 2 even slightly.

Now, excessive minority carriers generated in the crystal 4 directly under the light receiving part 3 due to infrared radiation drift and diffuse by the electrode applied between the electrodes 1 and 2, and are recombined and lost at the same electrodes 1 and 2. be exposed. In addition to this, the carriers also diffuse in the direction perpendicular to the electric field, reach the side walls of the crystal 4, recombine with the interface states, and are lost.
In order to prevent recombination of carriers on the side walls, a light-shielding mask having an opening with a smaller area than the center of the crystal 4 may be provided. Here, the distance between the end of the opening of the light receiving section 3 and the crystal side wall must be greater than the diffusion length of the carrier. Here, the carrier diffusion length depends on the operating temperature, crystal material, etc., and is Hg _1-x Cd _x
In the case of Te (x=0.22) crystal, the temperature at 77K varies from about 30 μm for a good quality crystal to about 10 μm for a poor quality crystal. By providing such a light-shielding mask, carriers will not disappear on the side walls of the crystal. In other words, the photoconductive infrared detector having the above structure has improved responsiveness and increased sensitivity.

〔Example〕 Embodiments of the present invention will be described below with reference to the drawings.

In the embodiment shown in FIG. 1, the distance L and width of the electrodes 1 and 2 were both 80 μm, and the size of the opening of the light-shielding mask was 20 μm□. The material of the photoconductive infrared detector is an n-type Hg _0.8 Cd _0.2 Te single crystal 4 whose majority carrier concentration is approximately 5×10 ¹⁴ cm ^-3 at 77K. This single crystal 4 was reduced to a thickness of 13 μm by polishing and etching, and bias current electrodes 1 and 2 were formed by vapor depositing Ti/Au/Cr on both ends. In addition, to prevent excessive minority carriers from recombining at the interface,
An anodic oxide film 5 was attached to the back surface to keep the interfacial potential positive. Furthermore, there is an insulating film transparent to infrared rays on top of that.
ZnS6 was attached to a thickness of 1.2 μm by sputtering. This also serves as an anti-reflection film. The waveform was adjusted to 10.6 μm. on this insulating film 6
A light shielding mask 7 was formed by vapor depositing Au/Ti, and the shape of the opening of the light receiving section 3 was determined. Here, the distance between the end of the light receiving section 3 and the crystal side wall was set to 30 μm, which is longer than the 20 μm diffusion length of excess minority carriers. This infrared detector was bonded with an epoxy adhesive 8 onto a sapphire substrate 9 (thickness: 0.5 mm), which has good thermal conductivity and excellent electrical insulation. This detector is equipped with an aperture with an F ratio of 1.6 and a narrow band interference filter (center wavelength 10.82 μm, wavelength width 0.78 μm, transmittance 82.6
%) and cooled to liquid nitrogen temperature under conditions of photon background 1 × 10 ¹⁶ phcm ^-2 s ^-1 .
The sensitivity of the detector was measured at a bias current of 0-20 mA and a frequency of 20 KHz. The gain of the amplifier used for measurement was 1100, and the band was 500KHz. As a result, the best value of NEP of the infrared detector of the present invention is 1.5×
It was 10 ^-14 W/√Hz. On the other hand, for comparison, the third
The conventional detector shown in the figure (L=80μm, W=
When a device (20 μm) was made on the same chip as the device of the present invention and the sensitivity was determined, the best NEP value was 2.5.
×10 ^-14 W/√Hz. As can be seen by comparing both values, the photoconductive infrared detector of the present invention has improved sensitivity by about 1.7 times compared to the conventional device. In other words, the integration time for obtaining a certain signal-to-noise ratio can be reduced to about 1/3.

〔Effect of the invention〕

As described above, the present invention has the effect of extending the life of excess minority carriers and improving the sensitivity of photoconductive infrared detectors.

[Brief explanation of the drawing]

Figure 1a is a plan view showing an embodiment of the photoconductive infrared detector of the present invention, and Figure 1b is A in Figure a.
-A' is a cross-sectional view, Figure 2a is a plan view of a conventional photoconductive infrared detector, and Figure 2b is a cross-sectional view of Figure 2a.
A sectional view taken along the line B', FIG. 3a is a plan view of an improved conventional photoconductive infrared detector, and FIG.
It is a sectional view taken along CC' in FIG. DESCRIPTION OF SYMBOLS 1, 2... Bias electrode, 3... Light receiving part, 4... Crystal, 5... Anodic oxide film, 6... Insulating film transparent to infrared rays, 7... Light shielding mask, 8... Adhesive, 9... Substrate.

Claims (1)

[Claims] 1 Bias current electrodes are provided at both ends of a crystal that absorbs infrared rays, a passivation film is provided in the center, and an insulating film transparent to infrared rays is attached on the passivation film and the bias current electrode. , an opening having an area smaller than the central portion and spaced apart from the side wall of the crystal by a carrier diffusion length or more, and further comprising a light-shielding mask overlapping the bias current electrode. Conduction type infrared detector.