Infrared image detector utilizing Schottky barrier junctions

An infrared image detector array is constructed by depositing a plurality non-contiguous strips of infrared radiation responsive, semiconductor material on one side of a base substrate. A contiguous metal semiconductor contact in then overlaid on the plurality of strips thereby forming an individual Schottky barrier detector element wherever the metal contact crosses one of the plurality of strips.

BACKGROUND OF THE INVENTION 
This invention relates to photo-detectors and more particularly to an 
infrared image detector. 
It is well established in the infrared photodetector field that an infrared 
detector can be built utilizing epitaxial lead salt thin film technology. 
Single crystal semiconductor films of lead chalcogenides can be 
epitaxially grown on heated alkali halide and alkaline earth halide 
substrates by vacuum evaporation. The lead chalcogenides used include the 
sulfides, selenides, tellurides and mixtures thereof. The substrates are 
single crystals of infrared transparent alkali halides and alkaline earth 
halides. Examples include barium fluoride, strontium fluoride, calcium 
fluoride, lithium fluoride, sodium chloride, potassium chloride, etc. 
Next, a metal semiconductor contact such as lead or indium is deposited on 
the surface of an epitaxial layer of the lead chalcogenide thereby 
creating a non-ohmic Schottky barrier at the point of contact resulting in 
an infrared sensitive diode. A detailed description of infrared sensitive 
diode concepts, suitable materials and methods of fabrication are 
disclosed more fully in U.S. Pat. No. 4,406,050, entitled "Method for 
Fabricating Lead Halide Sensitized Infrared Photodiodes", which was issued 
to the present inventor on Sept. 27, 1983, and is herein incorporated by 
reference. 
It is also well established in the prior art that a multi-color, 
self-filtering detector can be built by depositing successive epilayers of 
lead chalcogenide on the base substrate. Each layer is normal to the 
surface of the substrate and contiguous in directions parallel to the 
surface of the substrate. Each layer is photoresponsive to infrared 
radiation based upon its chemical composition and is also a photon filter 
for the succeeding layers, i.e. each layer is used as a detection layer 
and as a filter layer. The response yielded by the detection portion of 
each layer is measured as a voltage versus the frequency of an infrared 
wave band. The wave band of each layer yielding a measurable voltage is 
determined by the chemical composition of the layer. Thus, the resulting 
infrared detector can be built to receive radiation in predetermined wave 
bands. Typical and illustrative of this technology is U.S. Pat. No. 
4,323,911 which was issued to Campbell. 
In FIG. 1, there is shown a cross-sectional view of a prior art infrared 
detector having N detector elements. A first semiconductor layer made of N 
separate materials 11A, 11B, . . . 11N and a second semiconductor layer 
made of N separate materials 12A,12B . . . ,12N is deposited on a base 
substrate 10. Layer 12 forms a detection layer made of N distinct 
materials defining N optical areas. Each optical area has different 
optical and radiation absorption properties due to differences in chemical 
composition. Each optical area will therefore respond to infrared 
radiation over a different wave band of the infrared spectrum. 
Accordingly, optical area 12A is fitted with a semiconductor metal contact 
13A and a lead wire 14A. Metal contact 13A is a non-ohmic contact. Each 
adjacent contiguous detection layer 12B, . . . ,12N is similarly fitted 
with non-ohmic contacts 13B, . . . ,13N and lead wires 14B, . . . ,14N. 
Each optical area must also be equipped with an ohmic contact or ground 
wire 15A,15B . . . ,15N which is in turn attached to an appropriate lead 
wire 16A,16B . . . ,16N. 
The aforementioned construction of prior art infrared image detectors poses 
several technical disadvantages. First, the surface of detection layer 12 
must be insulated against short circuits when electrical connections are 
made to the metal contacts 13A,13B . . . ,13N thereby adding complexity 
and cost to the fabrication process. Second, the detection layer 12 is 
optically and electrically active in the areas 17 not covered by the metal 
contacts thereby causing optical cross-talk between detector elements. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
infrared image detector array using epitaxial lead salt film technology 
that minimizes optical cross-talk between detector elements in the array. 
Another object of the present invention is to provide an infrared image 
detector array having simple and efficient electrical connection points. 
A still further object of the present invention is to provide an infrared 
image detector array having low fabrication costs. 
Other objects and advantages of this invention will become more apparent 
hereinafter in the specifications and drawings. 
In accordance with the invention, a plurality of non-contiguous strips of 
infrared light responsive semiconductor material are deposited on an 
infrared light transparent substrate. A contiguous metal semiconductor 
contact is then overlaid on the semiconductor strips. An infrared detector 
element is formed wherever the metal semiconductor contact crosses one of 
the semiconductor strips. The metal contact serves as the ohmic connection 
point for all of the formed detector elements while each of the 
semiconductor strips serve as a non-ohmic Schottky barrier connection 
point.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings and in particular to FIG. 2, there is shown a 
cross-sectional view of the infrared image detector array 1 according to 
the present invention. A series of non-contiguous parallel strips 11A, . . 
. ,11N of semiconductor material are epitaxially grown by vapor deposition 
onto an infrared transparent base substrate 10. Base substrate 10 is 
preferably single crystals of alkali halides and alkali earth halides. 
Parallel strips 11A, . . . ,11N are semiconductor materials that are 
optically responsive to infrared radiation. The strips are typically 
formed from lead chalcogenides and include the sulfides, selenides, 
tellurides and mixtures thereof. Alternatively, the strips may be formed 
from alloys such as lead chalcogenides and tin chalcogenides or lead 
chalcogenides and cadmium chalcogenides. The dimensions of the strips are 
limited only by the dimensions of the substrate 10 and the resolution of 
the fabrication technique. The strips may be any shape as long as they are 
non-contiguous. Typically, a strip is 0.002 inches wide, 0.3 inches long, 
1 to 10 micrometers thick and is separated from an adjacent strip by 0.002 
inches. 
A contiguous metal semiconductor contact strip 20 is vapor deposited on the 
semiconductor strips 11A,. . . ,11N and on the substrate 10. Contiguous 
strip 20 may be any metal semiconductor material such as lead or indium. 
As can be seen from a top view of the present invention shown in FIG. 3, 
the contiguous metal contact strip 20 crosses the parallel strips 11A, . . 
. ,11N in a perpendicular fashion thereby forming active detector element 
areas 21A, . . . ,21N. However, contiguous metal contact strip 20 may 
alternatively cross the parallel strips 11A, . . . ,11N at an angle other 
than 90.degree. as shown in FIG. 4. Electric contacts 13A, . . . ,13N 
deposited on the parallel strips 11A, . . . ,11N serve as the non-ohmic 
Schottky barrier contacts for connection to an electrical voltage 
measuring device. A single contact 15 is deposited at one point on the 
metal contact strip 20 and serves as the ohmic or ground contact for all 
of the detector elements 21A, . . . ,21N. 
A multi-color response may be achieved by utilizing materials of varied 
composition for each of the strips 11A, . . . ,11N that are responsive to 
distinct wavelengths in the electromagnetic spectrum. Alternatively, a 
multi-color, self-filtering detector detector array may be constructed by 
depositing multi-layer strips 110A,. . . ,110N as shown in FIG. 5. In this 
case, semiconductor layer 111 will serve as an optical filter for 
semiconductor layer 112 which serves as the detection layer. 
The examples of the present invention shown in FIG. 2, 3 and 4 are for a 
linear diode array. Two dimensional detector arrays may be constructed by 
merely repeating the linear diode pattern over the entire area of the 
substrate 10 as shown in FIG. 6. Alternatively, the strips 11A, . . . ,11N 
need not be parallel but may be deposited on the base substrate 10 in any 
non-contiguous pattern as shown in FIG. 7. 
In yet another embodiment of the present invention, semiconductor strips 
11A, . . . ,11N and contiguous metal contact 20 may be interchanged in the 
construction of the detector array. In FIG. 8 a long strip of 
semiconductor material 11 is first deposited on the substrate 10. Metal 
strips 20A, . . . ,20N are then deposited on the strip 11 and substrate 
10. Detector elements are formed at the crossings of the semiconductor 
strip 11 with the metal strips 20A, . . . ,20N. Since semiconductor strip 
11 is less conductive than metal, a higher series resistance is introduced 
into each of the detector circuits if the ohmic contact is far from the 
detector element. Therefore, multiple ohmic contacts 15A, . . . ,15N must 
be made on the semiconductor strip 11 next to, and in the spaces between, 
metal strips 20A, . . . ,20N. It should be noted that this configuration 
may be less desirable due to the added complexity of fabrication. 
In prior art detectors, cross-talk between detector elements resulted from 
the migration of charge carriers from one element to an adjacent element 
since the semiconductor material covered the entire area of the substrate. 
However, by depositing the infrared responsive semiconductor material as 
non-contiguous strips, the physical separation of the strips prevents the 
migration of charge carriers thereby reducing crosstalk between detector 
elements. Thus, any response generated at an individual detector element, 
such as 21A shown in FIG. 3, cannot interfere with the response generated 
at adjacent detector elements. 
A further advantage of the present invention lies in the fact that an 
individual detector element is formed at the crossing of a semiconductor 
strip 11A and metal contact strip 20. Accordingly, a detector element 21A 
is not surrounded by radiation absorbing semiconductor material on all 
sides as in the prior art. Prior art detector elements were subject to 
error due to a blurred optical image created when radiation was absorbed 
by the semiconductor material adjacent and contiguous to a detector 
element. However, since the present invention utilizes non-contiguous 
semiconductor strips, detector element 21A is surrounded by radiation 
absorbing semiconductor material on only two sides. Thus, radiation 
falling on areas 22 not covered by the semiconductor strips passes through 
the infrared transparent base substrate 10 and is not absorbed. The 
blurred optical image error of the present invention is thereby reduced by 
a factor of approximately two over the prior art. Furthermore, substrate 
areas 22 not covered by the semiconductor strips are also available for 
other purposes, such as electric connections that are less susceptible to 
short circuits than prior art connections. 
Finally, because the material used in the fabrication of the substrate 10 
is usually transparent to visible light, illumination of the substrate 10 
should facilitate the use of mask alignment in the deposition of the 
semiconductor materials and metal contacts. This should reduce the labor 
cost in the fabrication of the detector arrays. 
Thus, although the invention has been described relative to specific 
embodiments thereof, it is not so limited and numerous variations and 
modifications thereof will be readily apparent to those skilled in the art 
in light of the above teaching. It is therefore to be understood that 
within the scope of the appended claims the invention maybe practiced 
otherwise than as specifically described.